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Chapter

7 Neurotransmitters and neuromodulatorsEric Herlenius and Hugo Lagercrantz

IntroductionNeuronal communication is mainly mediated bymyriads of synapses via neurotransmitters, althoughthere are also electrical synapses. Neurotransmitterscan be defined as chemicals released from neuronsthat act on specific receptors. However, neuromodu-lators released from other cells, such as adenosinetriphosphate (ATP), adenosine, and prostaglandin,can also affect neuronal signaling. Neurotransmittersand their receptor subtypes are expressed in highamounts during certain stages of development, butthen persist in only a few synapses (Parnavelas &Cavanagh, 1988).

The transient increase in expression of a transmit-ter and receptor subtype in the central nervous system(CNS) may occur during a susceptible developmentaltime window. (A transient increase occurs, but itmay not always occur during a critical window.)Development is characterized by timing and spacingor critical periods when some kind of stimulation isessential for correct development. The transmittersand modulators affect formation of synaptic contacts,maturation of synapses, and structural refinement ofconnectivity by regulating electrical activity, excitabi-lity, and release of neurotrophins (Zhang & Poo,2001). In particular, at birth a cascade of neurotrans-mitters and transcriptional factors is activated. Forexample, the norepinephrine surge at birth may beimportant for initiating the bonding of the infant tothe mother by increasing the ability to sense odors(Sullivan et al., 1994). Imprinting at birth and visualinput to form the ocular dominance columns also occurduring critical periods, and are probably dependent onthe switching on and off of neurotransmitters. However,no critical period ends suddenly; rather it tapers offgradually. Expression of transmitters and receptor

subtypes is critical for the neuronal differentiation,development of synapses, and formation of neuronalnetworks underlying behavior in the fetus as well as inthe growing child and adult human.

The general role of neurotransmitters in themaking of the brain is far from clear. Several studiessupport the importance of and an early role forneurotransmitter signaling before synaptogenesis.Although it can be postulated that neurotransmittersare involved in the detailed wiring of the neuronalcircuits, it has been demonstrated that knocking outof all synaptic trafficking in the mouse does notprevent normal brain assembly, including formationof layered structures, fiber pathways, and morpho-logically defined synapses (Verhage et al., 2000).However, transmitters and synaptic activity are nec-essary for survival of synaptic contacts, as withoutvesicle release of transmitters neurons undergoapoptosis after formation of synapses. This isbecause their maintenance depends on neurotrans-mitter secretion. In addition, it was recently shownthat early release of transmitter is unconventional innot requiring action potentials, Ca2+ entry, or vesiclefusion, which may explain how early synapses and fibertracts are formed (Demarque et al., 2002; Owens &Kriegstein, 2002a). Intercellular connections, via gapjunctions, and release of neuromodulators such as ATPthrough unopposed gap junctional channels, “connex-ins,” have emerged as central components in intercel-lular signaling networks vital for the embryo and thedeveloping CNS. This chapter focuses on the chemicaltransmitters and modulators with membrane recep-tors. For reviews of the connexins see, for example,Trosko (2007), Andang and Lendahl (2008), and Eliasand Kriegstein (2008).

Markers for neurotransmitters and neuromodula-tors during CNS development generally appear first in

The Newborn Brain: Neuroscience and Clinical Applications, 2nd edn., eds. Hugo Lagercrantz, Mark A. Hanson, Laura R. Ment,and Donald M. Peebles. Published by Cambridge University Press. © Cambridge University Press 2010.

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the caudal and phylogenetically older part of the brain,probably due to earlier neurogenesis (see Semba,2004). The neurotransmitters or modulators can acton either metabotropic or ionotropic receptors (see,e.g., Cooper et al., 2003). The action of the metabo-tropic receptors is based on their effects on G- orN-proteins and their associated enzymes and channelsin the lipid bilayer of the membrane. These effects areslower (tens of milliseconds) than for the ionotropicreceptors. Epinephrinergic, muscarinic, and peptider-gic receptors are often metabotropic and have a moremodulatory role in the mature CNS. The ionotropicreceptors respond rapidly and are also termed classI receptors. They act on ion gates, which they can openor close in less than a millisecond. The ion channelsconsist of transmembrane proteins, which can beselective for cations (activatory receptors) or anions(inhibitory). The nicotinic acetylcholine ligand-gatedion channel is a prototype (Changeux & Edelstein,2005). The binding of a ligand cause an allostericchange in the ion channel pore. The ionotropic nicotin-ic acetylcholine receptor (nAcR), the γ-aminobutyricacid (GABA)AR, the glycine receptor (GlyR), and the5-hydroxytryptamine (5-HT)3 receptor are members ofthe same evolutionary superfamily and have a similarstructure.

The 7-transmembrane receptors (7-TMRs) are themost important metabotropic receptors. They are alsoallosteric and G-protein-coupled (GPCR) and repre-sent by far the largest family of receptors. These recep-tors account for more than 1% of the human genomeand almost a third of all drugs on the market act viathese receptors. The β2-adrenoreceptor is a kind ofprototype. It has been cloned and recently also crystal-lized (Rasmussen et al., 2007).

Small lipophilic or gaseous molecules that pene-trate the cell membrane have during the past decadeproved to be important neuroactive agents. Theseunconventional transmitters are not stored in synap-tic vesicles and do not act at conventional receptorson the surface of adjacent neurons. Rather they mayinteract with nuclear receptors, e.g., retinoids, vita-min D (Mangelsdorf et al., 1995), or enzymes in thecytosol (guanylyl cyclase), e.g., nitric oxide (NO) andcarbon monoxide (CO). However, their roles duringdevelopment of the CNS are still under investigationand they are not discussed here. Properties of these“atypical” neural modulators have recently beenreviewed by Boehning and Snyder (2003) and Li andMoore (2007).

Ontogeny of neurotransmittersystemsThe choice of neurotransmitter of a precursor neurondepends on the environment. In a series of remarkableexperiments Nicole Le Douarin demonstrated thatwhen the crest of the sympathetic trunk from a quailwas transplanted into the vagal region of a chick host,the nerves became cholinergic (Le Douarin, 1981).Conversely, when vagal neurons were transplantedinto the sympathetic trunk, the nerves became epi-nephrinergic. The type of neurotransmitter expressedseemed to be dependent on a tissue factor. When cellsfrom the sympathetic ganglia were cultivated in amedium from a heart cell culture, the epinephrinergicneurons became cholinergic (Patterson & Chun,1977). The choice of transmitter could also be affectedby the presence of corticosteroids in the medium.Thus, environmental factors are important for differ-entiation and may have an inductive role during crit-ical stages of development.

The catecholaminesThe catecholamine-synthesizing enzyme tyrosinehydroxylase has been detected on the first day of/afterincubation of the chicken, dopamine has been detectedon the second day and norepinephrine and epinephrineon the third day. High concentrations of catecholamineshave been detected in Hensen’s node, correspondingwith the notochord of the mammalian embryo (seePendleton et al., 1998). Catecholaminergic neurons aregenerated at the time of telencephalic vesicle formationin rodents as well as in primates. The monoaminergicneurons reach the cerebral wall as cortical neurogenesisbegins. Catecholamines have a crucial role in early devel-opment, which has been demonstrated by deleting thegenes encoding for tyrosine hydroxylase (Zhou et al.,1995) and dopamine β-hydroxylase (DBH) (Thomaset al., 1995).

NorepinephrineNorepinephrine is assumed to be involved in arousaland attention, fear and anxiety, and learning andmemory. The cell bodies of the norepinephrinergicneurons are concentrated in the brainstem, particu-larly in the locus ceruleus (A6) within the caudal pons(Fig. 7.1). From this structure five major norepineph-rinergic tracts originate which project to virtually allregions of the brain (Goridis & Rohrer, 2002). There

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are also clusters of norepinephrinergic cell bodies in,for example, the nucleus tractus solitarius (A2), and inthe lateral ventral tegmental field (A5). Fibers fromthese nuclei intermingle with those from the locusceruleus. Norepinephrinergic neurons appear at anearly stage in the CNS, at 12–14 days in the rat(Olson & Seiger, 1972) (total gestational age 21 days),and 5–6 weeks in the human (Sundstrom et al., 1993)(see Fig. 7.1).

Norepinephrine is essential for normal brain devel-opment. The norepinephrinergic system regulates thedevelopment of the Cajal-Retzius cells, which are thefirst neurons to be generated in the cortex and areproposed to be instrumental in neuronal migrationand laminar formation (Naqui et al., 1999).Furthermore, α2A receptors are expressed by migratingneurons in the intermediate zone, characterized by aspindlelike shape, radial alignment, and close associa-tion with radial glia (Wang & Lidow, 1997). The radial

glia participate in key steps of brain development, cort-ical neurogenesis, and migration (Noctor et al., 2001,2004). Thus, epinephrinergic transmission may beinvolved in regulating the generation, migration, andmaturation of cerebral cortical cells.

Administration of 6-hydroxyl-dopamine preventsthe natural programmed cell death of the newbornneurons and delays the formation of cortical layers.Lesioning the norepinephrinergic projections or block-ing neurotransmission with receptor antagonists pre-vents astrogliosis and glial cell proliferation. Depletionof norepinephrine during the perinatal period results insubtle dendritic changes and possibly also alterations incortical differentiation (see Berger-Sweeney Hohmann,1997).

The role of norepinephrine has been investigated bytargeted disruption of the DBH gene (Thomas et al.,1995). This resulted in fetal death, probably due to cardio-vascular failure. Only about 5% of the homozygotic

ACh

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Fig. 7.1 Arbitrary levels of monoamines and acetylcholine in (a) rat and (b) humans versus age (10-logarithmic scale). Sagittal illustrationsof cell bodies and projections of monoamine neurotransmitter systems: acetylcholinergic (ACh), dopaminergic (DA), serotonergic (5-HT),and norepinephrinergic (NE) pathways in the rat and human brain, and epinephrinergic pathways in the rat. Embryonic age (E) expressed indays for rats and weeks for humans, postnatal age in weeks (rats) and years (humans). (Data from Olson & Seiger, 1972; Herregodts et al., 1990;Naeff et al., 1992; Sundstrom et al., 1993; Almqvist et al., 1996). (Brain maps modified with permission from Heimer, L. (1988). The Human Brain andSpinal Cord. New York: Springer Verlag; and Zigmond, M. J., Bloom, F. E., Landis, S. C., Roberts, J. L., & Squire, L. R., eds (1999). FundamentalNeuroscience. San Diego: Academic Press.)

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mice survived until adulthood, presumably due to someplacental transfer of norepinephrine. Most of the micecould be rescued to birth by providing them withdihydroxyphenylserine (DDPS), a precursor that can beconverted to norepinephrine in the absence of DBH.These mice showed reduced ability for acquisition andretention for some tasks. Interestingly, female miceseemed to be deficient in their ability to take care oftheir offspring. Thus, there seems to be a critical windowduring early development when norepinephrine isinvolved in forming the pathways responsible formaternal behavior (Thomas & Palmiter, 1997).Norepinephrine is probably also involved in the olfac-tory learning of the newborn, which is of importancefor maternal recognition (Insel & Young, 2001). Theepinephrinergic receptors are metabotropic and aresubdivided into three families, α1, α2, and β. α2 and β1dominate in the brain. There is a transient overex-pression of α2 receptors in the white matter and manybrainstem nuclei during the perinatal period, sug-gesting a developmental role (Happe et al., 2004).

DopamineDopamine has a very important role in motor andcognitive programs. The cell bodies of the dopaminer-gic neurons are concentrated in the substantia nigra,the ventral tegmental area, and the retrorubral field,and they project to the basal ganglia, the olfactorybulbs, the limbic regions, and the hippocampus andcortex (Fig. 7.1). The prefrontal cortex in particular isrich in dopamine content, which is interesting withregard to its important role in reasoning, planning,problem solving, and coordinating performance inhumans (Diamond et al., 2004).

Dopaminergic neurons appear early during deve-lopment, at a gestational age of 10–15 days in the rat(Olson & Seiger, 1972) and 6–8 weeks in the human(Sundstrom et al., 1993) (Fig. 7.1), earlier in femalesthan in males. The dopamine turnover is relativelyhigh during the perinatal period when comparedwith that in adulthood.

There are twomain types of dopamine receptors: D1

and D2, but there are also D3, D4, and D5 receptors.Stimulation of the D1 receptors results in an increase incAMP formation and phosphorylation of DARPP-32,whereas D2 receptors mediate a decrease in cAMP for-mation. Extremely high levels of D1 receptors have beenreported in the pallidum during the perinatal period(Boyson & Adams, 1997). D1 receptor stimulation reg-ulates transcription of other genes, and it is possible that

abnormal perinatal stimulation can have long-termconsequences (see below).

Disturbances of the development of the dopami-nergic system may lead to dyskinesia, dystonia, tics,obsessive-compulsive disorders, and abnormal eyemovements. This has been observed in dopamine-depleted rats after 6-hydroxyl-dopamine treatmentbut with preserved norepinephrine effect. Mice withdeletion of the tyrosine hydroxylase gene have beenshown to be hypoactive and develop adipsia andaphagia, which can be treated with l-dopa (Zhou &Palmiter, 1995).

D1 receptors are involved in working memoryperformance (Williams & Goldman-Rakic, 1995).A disturbance of the development of the dopaminergicsystem has been postulated to contribute to the devel-opment of attention deficit hyperactivity disorder(ADHD), in which a deficient working memory isan important component of the syndrome. Perinatalexposure to a very low dose of methyl mercury(MeHg) results in neurotoxic effects, e.g., on memoryretention, and such alterations of brain function per-sist into adult life (Dare et al., 2003).

Infants with phenylketonuria and probably deficientdopaminergic innervation of the prefrontal cortex havebeen found to have an impaired working memory(Diamond, 1996). The catechol-O-methyltransferase(COMT) gene affects how long dopamine acts in theprefrontal cortex. It was recently shown that genotypicdifferences in COMT, inducing differences in break-down of prefrontal dopamine, are related to differencesin specific cognitive performance in normal-developingchildren (Diamond et al., 2004; Diamond, 2007).

EpinephrineThe existence of epinephrine in the brain was notaccepted until the epinephrine-synthesizing enzymephenylethanolamine-N-methyl transferase (PNMT)was detected by immunohistochemical methods.This enzyme was localized in the lower brainstem(Fig. 7.1) intermingled with norepinephrinergic neu-rons. Epinephrine in the brain is probably involved inneuroendocrine and blood pressure control. Duringearly development epinephrine contributes to enhancethe fetal respiratory rhythm in rat medulla while duringfetal maturation it acquires inhibitory actions on locusceruleus and brainstem respiratory rhythm (Fujii et al.,2006). PNMT occurs predominantly before birth inthe rat CNS; after birth, there is a decline in PNMT-containing structures (see Foster, 1992).

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SerotoninSerotonin (5-HT) and serotonergic neurons are loca-lized in the midbrain, the pineal gland, the substantianigra, the hypothalamus, and the raphe nuclei of thebrainstem (Fig. 7.1). The 5-HT neurons have wide-spread projections making it possible to coordinatecomplex sensory and motor patterns during variousbehavioral states.

There exist a multitude of heterogeneous 5-HTreceptors, classified into sevenmain receptor subtypes,5-HT1–7, with more than 15 molecularly identified 5-HT receptor subtypes so far. The majority of the 5-HTreceptors belong to the G-protein receptor family,except for 5-HT3 receptors, which are ligand-gatedion channel receptors (for a review see, Hoyer et al.,2002). 5-HT enhances motor neuron excitability.Serotonergic tonic activity is highest during wakingand arousal and absent during active or rapid eyemovement sleep. If the gene encoding for 5-HT1B

receptors is knocked out the proportion of activesleep is increased (Boutrel et al., 1999).

Serotonin can already be detected in the fertilizedegg and is involved in early morphogenesis of theheart, the craniofacial epithelia, and other structures.If embryos are cultured in the presence of serotoninuptake inhibitors or receptor ligands, specific cranio-facial malformations occur. Serotonergic cells in theraphe are among the earliest to be generated in thebrain (about embryonal day [E]10–E12 in the mouse).After their generation in the raphe, they start to pro-ject diffusely into the spinal cord and the cortex.Serotonergic cells appear in the fifth to twelfth gesta-tional week in the human (Fig. 7.1). These cells sendaxons to the forebrain and may be of importance inthe differentiation of neuronal progenitors (Gasparet al., 2003). Moreover, serotonin has been reportedto affect neuronal proliferation, differentiation, migra-tion, and synaptogenesis, although knocking out sero-tonin receptors or genes involved in its metabolismdid not seem to cause marked alterations in brainhistology (Gaspar et al., 2003). Excess serotonin pre-vents the normal development of the somatosensorycortex, which has been demonstrated in monoamineoxidase knockout mice (Cases et al., 1996). At birth,serotonergic-containing axons penetrate all corticallayers, but then decline markedly after about threeweeks. Depletion of serotonin after birth seems tohave little effect on cortical development.

Maternal serotonin may be critically involved infetal morphogenesis (Cote et al., 2007). This was

demonstrated in a mouse line deficient in peripheralserotonin biosynthesis by disruption of the tryptophanhydroxylase gene (tph1). The tph1-null females werebred with tph wild-type males. The offsprings dis-played marked brain abnormalities. The phenotypeof the mothers and not of the embryo seemed to bemost important. These findings may have some impli-cations with regard to autism, which may be related tohyposerotonism during fetal life (see below).

Disturbed development of the cerebral serotoninsystem has also been observed in victims of suddeninfant death syndrome (SIDS). Paterson et al. (2006)found fewer 5-HT receptor-binding sites in regionsof the medulla involved in homeostatic functions inSIDS victims than in controls. On the other hand,there were a higher number of serotonin neurons inthe former group, possibly as a compensatory mecha-nism. A recent mouse model might help explain howaltered serotonin homeostasis can be related to suddenunexpected death in infants. Using mice with reversibleoverexpression of 5-HT1A receptors and thus excessiveserotonin autoinhibition, it was shown that altered sero-tonin homeostasis is associated with failure to respondto environmental changes, catastrophic autonomic dys-regulation, and sudden death (Audero et al., 2008).

A transient uptake and storage of serotonin indeveloping thalamic neurons occurs during formationof somatosensory cortex in mouse because of the tem-porary expression of the high-affinity serotonin trans-porter (SERT) (Lebrand et al., 1996). This 5-HTuptake and possibly the use of 5-HT as a “borrowedtransmitter” seem necessary for the normal develop-ment and the fine tuning of cortical sensory mapsduring their critical period of development in rodents(Gaspar et al., 2003). Human fetuses have a similarrestricted time period of SERT expression (gestationalweeks 12–14) when thalamocortical fiber tractsdevelop and fine tuning of cortical sensory mapsoccurs (Verney et al., 2002). The fetal human brain,especially cortex and hippocampus, exhibits a prenatalpeak (weeks 16–22) in the density of serotonin 5-HT1A

receptors (Bar-Peled et al., 1991). Activation of the5-HT1A receptor is associated with increased neuro-genesis, neural differentiation, and dendritic matura-tion in the hippocampus. Whether 5-HT has a directeffect on neural progenitors or an indirect effect viathe glia, which express 5-HT1A receptors and releaseS-100B (an astroglial-derived growth factor) when5-HT1A receptors are activated, is currently not known(Gaspar et al., 2003).

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The serotonin concentration must be neither toohigh nor too low during the critical period of synapto-genesis and formation of brain connections. Miswiringproblems due to excess or inadequate activation ofspecific 5-HT receptors during development may beinvolved in the genesis of psychiatric disorders suchas anxiety disorders, drug addiction, and autism (fora review, see Gaspar et al., 2003).

Autism has been suggested to be related not only tohyposerotonism during fetal life but also to hyper-serotonism postnatally (Chugani, 2002). Serotonin istransiently synthesized in high levels in young chil-dren. This overactivity declines in normal but not inautistic children. Patients with a point mutation ofthe gene encoding for monoamine oxidase has beenfound to be related to antisocial behavior (Gasparet al., 2003). Selective serotonin reuptake inhibitors(SSRIs) are used by 2%–4% of pregnant women, butanimal and human studies are inconclusive regardingtheir eventual adverse effects on CNS development attherapeutic doses, although high doses may cause ana-tomical and behavioral changes (Simons et al., 2002).

Fluoxetine, the prototype SSRI, crosses the pla-centa and enters the fetal brain. The limbic system inparticular may be affected in utero. There may beuntoward subtle effects on the fetus and the newborn.Lower Apgar score, withdrawal symptoms, and lowerBayley psychomotor index scores have been reported(Lattimore et al. 2005).

AcetylcholineAcetylcholine (ACh) is one of the major neurotrans-mitters in the brain of importance for cortical activa-tion, attention, memory and learning, reward, andpain. It has a major role in the control of motor toneand movement and probably counterbalances theeffect of dopamine (see Johnston & Silverstein, 1998;Cooper et al., 2003). It is of major importance for thedevelopment and control of autonomic functions. “If asingle neurotransmitter is critical for consciousness,then it must be acetylcholine” according to Koch(2004) (see Chapter 23).

The cholinergic neurons in the brain are organ-ized in local circuit cells, for example in the caudate-putamen nucleus, and in longer projection neuronsto the cortex, the basal forebrain and the mesopon-tine tegmentum (see Semba, 1992, 2004) and Fig. 7.1.The development of cholinergic systems has beenstudied by analyzing markers such as ACh, the syn-thesizing enzyme choline acetyltransferase (ChAT)

and acetylcholinesterase. The cholinergic innervationof the cortex occurs later than the monoaminergic,about E19 in the mouse and the rat and around week20 in the human fetus. Mature levels in rodents arenot reached until after eight weeks postnatally (seeBerger-Sweeney & Hohmann, 1997). The concentra-tions of ACh reach about 20% of the adult levels atE15 in the whole brain of the rat and about 40% at dayP7 (Fig. 7.2). The levels of ChAT are much lower (1%and 8%) at the corresponding ages, indicating lowfiring rates of the cholinergic neurons. Conversely, thereceptors reach adult levels earlier. The cholinergicmarkers appear sooner in the pons–medulla, probablydue to earlier neurogenesis in the caudal and phyloge-netically older part of the brain (see Semba, 2004).

The classic cholinergic receptors –muscarinic andnicotinic receptors – undergo important changes dur-ing development (see Dwyer et al., 2008). A fetal sub-unit of the muscarinic receptor (γ-AChR) is replacedby an adult type (ε-AChR) in the muscle endplate toincrease conductance (Herlitze et al., 1996). The nic-otinic acetylcholine receptors (nACRs) consist of fivesubunits centered around a central pore. Theα4β2nAChR and α7nAChRs predominate in thebrain. These receptors have been detected at E12–13in the rat brain. Increasing levels of mRNA encodingfor the subunits have also been identified during thefirst trimester of human fetuses (Hellstrom-Lindahlet al., 1998). The nicotine receptors have been shownto be important for the proliferation and/or survival ofneuroblasts. Excessive stimulation of nicotinic recep-tors seems to enhance neuronal cell death (seeChangeux & Edelstein, 2005).

Nicotinic acetylcholine receptors (nAChRs) mayplay important roles during development and plas-ticity. Activation of nicotinic acetylcholine recep-tors promotes synaptic contacts and the wiringduring a critical period of postnatal development(Maggi et al., 2003). This has been demonstrated inthe hippocampus but may also apply for otherparts of the brain. The arousal response is lowerin mice lacking the β2-subunit of the nAChRs(Cohen et al., 2002).

Nicotinic exposure during fetal life seems to affectβ2-containing nAChRs, and explains some of theadverse effects of maternal smoking on the offspring(Weitzman et al., 1992). Newborn mice exposed tonicotine in doses corresponding to the levels humanfetuses sustain duringmoderatematernal smokingwerefound to breathe irregularly and had impaired arousal

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responses and catecholamine biosynthesis (Cohen et al.,2002). Remarkably similar effects were seen in trans-genic pups lacking the β2-subunit (Cohen et al., 2005).Thus, autonomic and catecholamine biosynthesis dis-turbances may contribute to the many-fold increase inrisk of SIDS with maternal smoking. The developmentof the dopaminergic system and increase in the risk of

attention deficit disorders in the offspring may also beaffected (Pauly & Slotkin, 2008).

Amino acid transmittersAmino acid transmitters are the most abundant trans-mitters in the CNS. However, they were recognized as

AMPA/kainate receptors–cortex

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Fig. 7.2 Expression and arbitrary levels of receptors of amino acid transmitter versus age in (a) rat and (b) humans (10-logarithmic scale). Thedegree of shading reflects relative concentration. KCC2 is a neuron-specific cotransporter of Cl– ions and is responsible for the switch of γ-aminobutyric acid (GABA) as an excitatory to an inhibitory neurotransmitter (see Fig. 7.1), N-methyl-D-aspartate (NMDA) receptors are expressedrelatively earlier than the kainate and α-amino-5-hydroxy-3-methyl-4-isoxazole propionic acid (AMPA) receptors. It is assumed that the NMDAreceptors are more involved in the wiring of the brain while the kainate and AMPA receptors are responsible for the fast traffic in the moremature brain. Embryonic age (E) expressed in days for rats and weeks for humans, postnatal age in weeks (rats) and years (humans). (Data fromHagberg et al., 1997; Herschkowitz et al., 1997; Johnston & Silverstein, 1998; Rivera et al., 1999).

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neurotransmitters in the mammalian brain muchlater than the monoamines and acetylcholine. Thiswas probably due to the fact that they are involvedin intermediate metabolism and constitute the build-ing blocks of the proteins.

The amino acids are involved in the main nervousprocesses in the brain such as sensory input, encodingof memories, and mediating movements. In thedeveloping brain they seem to play an important rolein the wiring of neuronal networks and building of theCNS cytoarchitecture (Ben-Ari et al., 1997; Wang &Kriegstein, 2008) (Fig. 7.2).

Glutamate and aspartateGlutamate and aspartate are the dominating excitatoryamino acids (EAA) and the primary neurotransmitterin about half of all the synapses in the mammalianforebrain. They constitute the major transmitters ofthe pyramidal cells, the dominating neurons in thecortex. This has been demonstrated by injection ofradioactive labeled d-[3H]glutamate into the appropri-ate projection areas (see Cavanagh & Parnavelas,1988). EAA pathways undergo striking developmentalchanges, involving transient overshoots, especiallyduring critical periods as evidenced in the visual cortexand hippocampus. EAA terminals are overproducedduring the early postnatal period, for example after7–14 days in the rat cortex and after 1–2 years in thehuman cortex, which may be related to the high gen-eration of synapses during those periods (Bourgeois,2002; Benitez-Diaz et al., 2003) (see Fig. 7.2).

Glutamate acts on at least five types of receptor.The slower-acting metabotropic receptors, of whicheight subclasses are hitherto known, are expressed ata relatively early stage. Of the ionotropic receptors, theN-methyl-d-aspartate (NMDA) receptors dominate inthe immature brain when synaptic transmission isweak and extremely plastic (Fig. 7.2). The NMDAreceptors permit entry of Na+ and Ca2+ when opened.NMDA channels seem to be crucially involved inthe appearance of long-term potentiation (LTP) andsynaptic plasticity underlying learning and memorystorage throughout life. During critical periods ofdevelopment and synaptogenesis NMDA receptorsplay an essential role in activity-dependent plasticityand synaptic refinement (for reviews see McDonald &Johnston, 1990; Qu et al., 2003; Wang & Kriegstein,2008). Dark rearing or blocking the activity withtetrodotoxin results in preservation of the NMDA recep-tors in the visual cortex. Dark rearing also preserves

the immature form of the NMDA receptors contain-ing the NR2B subunit, and the expression of NR2A isdelayed. This subunit switch is essential for develop-ment of rapid synaptic transmission (Fox et al., 1999).During maturation, the AMPA and kainate ionotropicreceptors predominate and carry most of the fast neu-ronal traffic in the brain. NMDA receptors are moreactive during early life, due to the expression of diffe-rent receptor subunits, e.g., the more immatureNR2B receptor subunit, allowing enhanced activationof the channel, increasing its capability to strengthensynapses and to learn (Tang et al., 1999). Alas,this dominant role of NMDA and increased Ca2+

influx/activation also causes the brain to be moresensitive to excitoxicity (excessive release of gluta-mate) caused by pre- and perinatal asphyxia (seeChapter 17). NMDA receptor stimulation by excessiveglutamate release leads to Ca2+ influx, which mayinduce subsequent neuronal apoptosis. Excess activa-tion of NMDA and non-NMDA receptors is impli-cated in the pathophysiology of brain injury in severalclinical disorders to which the developing brain issusceptible, including hypoxia–ischemia and seizures(McDonald & Johnston, 1990; Qu et al., 2003). Fetalrats exposed to NMDA antagonists have been foundto have excessive apoptosis in the same way as theasphyxiated perinatal brain. Ethanol is an NMDAantagonist and excessive inhibition of NMDA recep-tors causes apoptosis that may play an important role infetal alcohol syndrome – this is also discussed below(Olney et al., 2002). Thus, either too much or too littleNMDA receptor activity can be life-threatening todeveloping neurons (Lipton & Nakanishi, 1999).

γ-Aminobutyric acidGABA is the dominating neurotransmitter in thenonpyramidal cells, as demonstrated by uptake of [3H]-GABA and immunochemical labeling of the GABA-synthesizing enzyme glutamic acid (GAD). Perhaps25%–40% of all nerve terminals contain GABA. Inlower mammals, the vast majority of the GABAergicinterneurons arise in the ganglionic eminence, a sub-cortical area, and then migrate tangentially to theirtarget areas in neocortex (Marin & Rubenstein, 2001).In humans, the majority of neocortical GABAergicneurons arise locally in the ventricular and subventric-ular zone, while proportionally fewer GABAergicneurons originate from the ganglionic eminence ofthe ventral forebrain (Letinic et al., 2002). GABA isregarded as the main inhibitory transmitter in the

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mature animal, but has a different and critical roleduring early development.

Before synapse formation the action of GABA ismediated through paracrine/autocrine signaling andthen through classic synaptic signaling. GABA recep-tor function regulates embryonic and neural stem cellproliferation and differentiation (Andang et al., 2008;Andang & Lendahl, 2008). During early brain develop-ment it acts as a trophic factor to influence eventssuch as proliferation, migration, differentiation, syn-apse maturation, and cell death (Owens & Kriegstein,2002b). Moreover, GABA-induced depolarization isnecessary for proper excitatory synapse formationand dendritic development of newborn cortical neu-rons and provides an activity-dependent mechanismfor achieving the balance between excitation andinhibition in the developing cortex (Wang &Kriegstein, 2008).

GABA is also a crucial transmitter in the humaninfant. When vitamin B6 was excluded from infantformula by mistake, it resulted in a disastrous seriesof deaths mainly due to GABA deficiency, whichresulted in fatal seizures (Frimpter et al., 1969).There are two types of GABA receptor: GABAA andGABAB. The GABAA receptor (GABAA-R) is an iono-tropic receptor that gates a chloride channel. It isa transmembrane protein built of several subunitswhere, for example, benzodiazepines, barbiturates,and ethanol can bind to specific sites and modulatethe opening properties of the chloride channel.Depending on their subunit composition, these recep-tors exhibit distinct pharmacological and electrophy-siological properties (Sieghart & Sperk, 2002) TheGABAB-R is coupled to a G-protein, is present inlower levels in the CNS than the GABAA receptor,and starts to function late in CNS development.

During early development the Cl– concentrationis high in the nerve cells. When GABA opens theCl– channels, a depolarization (i.e., excitation) occurs.During maturation the Cl– concentration decreaseswhich results in an opposite effect of GABA, i.e.,Cl– ions are pumped out and the cell becomes hyper-polarized (Fig. 7.2). In this way GABA switches froman excitatory to an inhibitory neurotransmitter(Miles, 1999). This switch is due to the expression ofthe K+/Cl– cotransporter (KCC2) – reported to beexpressed around birth in the brainstem, one weekafter birth in the hippocampus and between oneand two weeks in the cortex of the rat (Miles, 1999;Rivera et al., 1999; Blaesse et al., 2009) (Fig. 7.2).

Thus, GABA operates mainly as an excitatory trans-mitter on immature neurons. As described above,glutaminergic synapses initially lack functional α-amino-5-hydroxy-3-methyl-4-isoxazole propionic acid (AMPA)receptors and the NMDA channels are blocked by Mg2+

at resting membrane potentials. GABA depolarizesimmature neurons, which may result in Ca2+ influxby removing the Mg2+ blockage of NMDA channels.Thus, GABAA receptors play the role conferred toAMPA receptors in the more mature CNS (Ben-Ariet al., 1997; Onimaru et al., 1999). An increase in theintracellular Ca2+ concentration activates a wide rangeof intracellular cascades and is involved in neuronalgrowth and differentiation. Furthermore, GABA excita-tion and Ca2+ influx may act as triggers for plasticityof synaptic connections and for establishing andpatterning of neural networks. GABA-stimulated up-regulation of the expression of KCC2 may be the mech-anism underlying this synaptic switch (Kriegstein &Owens, 2001). This switch and expression of KCC2can be modulated by visual experience in the retina(Sernagor et al., 2003); thus both developmentally setcues and sensory experience may turn on this crucialswitch from excitation to inhibition. The oppositeeffect – downregulation of KCC2 expression – mayoccur with traumatic brain injury and possibly asphyxia,inducing epileptic activity due to dysfunction ofGABAergic inhibition (Rivera et al., 2002). Also thesubunit composition of GABAA and GABAB receptorschanges during postnatal development, suggesting theexistence of molecularly distinct immature and adultforms of GABAA receptors in CNS (Fritschy et al.,1994; Zheng et al., 1994; Benke et al., 2002).

The GABAA receptors have strong affinity forbenzodiazepines. Several anxiolytic and anticonvul-sant drugs increase the ability for GABA to openchloride channels. In neonatal neurons, GABA cur-rents are potentiated by barbiturates but are insensi-tive to benzodiazepines (Cherubini et al., 1991).Considering the fundamental role of GABA in thedifferent stages of cell development during embryonic,fetal, and postnatal life, and that it has a trophic roleduring early brain development, interference with thefunction of GABAergic transmission during thisperiod may affect the development of neuronal wiringand the plasticity of neuronal networks and also have aprofound influence on neural organization. For a moreextensive recent review of how GABAergic drugs, suchas ethanol, anesthetics, and anticonvulsants, may affectbrain development, see Henschel et al. (2008).

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Ethanol, which is misused by some women duringpregnancy, interacts with the GABAA receptor. Thesensitive time window in rat cerebral cortex for etha-nol exposure occurs between postnatal day (P)3 andP10. It is worth noting that GABA during this sameperiod seems to have mainly depolarizing and trophiceffects on developing cortical neurons through effectson cell proliferation and migration (Belhage et al.,1998). In humans, the intellectual deficit producedby abnormalities of brain growth is the most impor-tant component of the fetal alcohol syndrome (FAS;Kopecky & Koren, 1998). Craniofacial abnormalitiesin human fetuses related to first trimester alcoholexposure are similar to the facial defects seen inGABAA subunit receptor knockout mice (Condieet al., 1997). Children with FAS have often beenexposed repeatedly to ethanol in utero, but it is note-worthy that research in infant rodents has demonstra-ted increased apoptotic neurodegeneration followingbrief exposures. Raising blood ethanol to 50 mg/dl(a serum concentration that may be achieved easilyduring social drinking) for only 30–45 minutes can besufficient to trigger a significant neuroapoptosis dur-ing synaptogenesis (Young & Olney, 2006).

GlycineGlycine has both excitatory and inhibitory actions andcan be regarded as the phylogenetically older inhibi-tory transmitter restricted to the brainstem and spinalcord in the adult. A similar switch as regarding theGABAA receptors from excitatory to inhibitory effectsseems to occur with maturation (Miles, 1999; Gallo &Haydar, 2003). The NMDA receptor has a modulatorysite where glycine in submicromolar concentrationsincreases the frequency of NMDA receptor channelopening. Conditions that alter the extracellular con-centration of glycine can markedly alter NMDA-receptor–mediated responses (see Corsi et al., 1996;Chapter 17).

The maturation of the inhibitory functions ofGABAergic and glycinergic interneurons may playrole in the disappearance of neonatal reflexes such asgrasping (Fitzgerald, 1991).

NeuropeptidesMore than 50 neuropeptides have been identified.In contrast to most of the other neurotransmitters/modulators, the neuropeptides are synthesized andpackaged in large dense-core vesicles in the cell somaand are carried to the nerve terminals by axonal

transport at a rate of 1.5mm/h. It is obvious thatowing to this relatively slow process the neuropepti-des cannot act as fast-switching neurotransmitters.Rather, they have a neuromodulatory role. They areoften stored together with other neurotransmitters,i.e., monoamines or EAA, and it is possible that theyplay a role in setting of the sensitivity. Some of them areprobably of less physiological importance and occur inthe body mainly as evolutionary residues (Bowers,1994). Still they are of great neuropharmacologicalinterest and their analogs or antagonists can be usedas drugs. The most well-known examples are theopioids and naloxone.

OpioidsBesides pain perception, endogenous opioids areinvolved in blood pressure and temperature regula-tion, feeding, sexual activity, and memory storage.Three major classes of opioid receptors, μ, δ, and κ,are currently known and have been characterized andcloned, all with putative receptor subtypes. All areseven-transmembrane proteins and members of theG-protein-coupled receptor superfamily. Endogenousopioid peptides with distinctive selectivity profiles arethe enkephalin (μ), endorphin (δ), and dynorphin (κ)groups.

Neurons containing β-endorphin have long projec-tions and primarily occur in the pituitary, whereasthose containing proenkephalin- and prodynorphin-derived peptides generally have moderate to shortprojections (Morita, 1992). β-Endorphin exists intwo main forms with different production sites andeffects on the brain. The nonacetylated form is foundin the anterior pituitary, is involved in fetal growth,and is expressed early during fetal brain development(E14 in the rat). The acetylated form is present in theintermediate lobe of the pituitary and is involved inpostnatal development (Wang et al., 1992). μ-Receptorbinding sites are present during mid-fetal lifeand have a high density in cardiorespiratory-relatedbrainstem nuclei, whereas the δ-opioid receptorsprimarily appear during the postnatal period in rats(Gaveriaux-Ruff & Kieffer, 2002).

Although opioid-binding sites progressively incre-ase in the developing brain, the effect of opioidsappears to be dependent on the status of neuronalmaturation. In addition, many neuronal populationsexhibit transient expression of one or the other opioidgenes but the physiological role of this is not clear.Opioid agonists inhibit mitosis and DNA synthesis in

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the developing brain and endogenous opioids exertpotent regulatory effects on brain development andmorphogenesis, as demonstrated by the administra-tion of exogenous opioid agonist and antagonist dur-ing the fetal period (Lichtensteiger, 1998). Humanneonates who have been exposed in utero to opioidssuch as heroine have a smaller head circumference andreduced body weight due to a decrease in cell number(Kopecky & Koren, 1998).

Substance P and other tachykininsSubstance P is a primary sensory transmitter media-ting pain sensations via the thin C fibers. SubstanceP is also involved in the transmission of chemorecep-tor and barometric input from the carotid and aorticchemo- and baroreceptors. Immunocytochemicalstudies have demonstrated that substance P appearsin the rat brainstem at a gestational age of 14 days andreaches a maximum at a postnatal age of 21 days, andthereafter there is a successive decrease (Sakanaka,1992). In humans there is an increase towards birthand then a leveling off during the first six months(Bergstrom et al., 1984).

Substance P may play a role in neurogenesis. Itseems to counteract damage induced by neurotoxinsand accelerates regeneration of cortical catecholaminefibers (see Sakanaka, 1992). Increased expression ofmRNA coding for pre-protachykinin A, the substanceP precursor, has been recorded in respiratory-relatednuclei in both the rabbit (Lagercrantz, 1996) and therat (Wickstrom et al., 1999). Increased expression ofPPT-A mRNA has also been detected in patches in thecaudate and putamen nuclei of the human newbornbrain (Brana et al., 1995). Thus, there are suggestionsthat substance P is involved in the resetting and adap-tation of the organism to extrauterine life.

Increased levels of substance P have been found inthe brainstem of infants dying of SIDS. Lower concen-trations of substance P were detected in the brainstemof children dying of Rett syndrome. They were alsofound in reduced concentrations in the cerebrospinalfluid (Matsuishi et al., 1997).

NPY-related peptidesNeuropeptide Y is probably the most important of thepancreatic polypeptide family in the brain. The pep-tides in the family are peptide YY (PYY), avian pan-creatic polypeptide (APP), and human pancreaticpolypeptide (HPP). NPY is released together with nor-epinephrine or epinephrine (Hokfelt et al., 2003;

Ubink et al., 2003). It is a strong vasoconstrictor andincreases the sensitivity of sympathetically innervatedsmooth muscle. In the brain NPY has been reported tobe anxiolytic and may play an important role in damp-ening excitotoxicity during seizures. It also has a rolein the control of food intake. However, transgenicmice deficient in NPY seem to develop normally andexhibit normal food intake and body weight (Barabanet al., 1997).

GalaninGalanin is involved in cognition, nociception, feeding,and sexual behavior (Bedecs et al., 1995). Of the nor-epinephrinergic neurons in the locus ceruleus, 80%contain galanin. Galanin hyperpolarizes these neuronsand inhibits the release of norepinephrine. It can bedetected at E19 in the rat fetus and is then upregulatedat birth, whereas the galanin receptors seem to bedownregulated (Wickström et al., 1999). It may possi-bly modulate the effects of the norepinephrine surge atbirth. Furthermore, it inhibits excessive glutamaterelease during perinatal asphyxia (Ubink et al., 2003).NPY has recently been shown to induce neuronalprecursor proliferation via Y1 receptors and also tohave a trophic effect on blood vessels in the CNS(Hansel et al., 2001; Hokfelt et al., 2003).

PurinesPurines are not only fundamental components in theenergy turnover of all cells but they also modulateneuronal activity through synaptic or nonsynapticrelease and interaction with specific receptors. Thepurinergic receptors are divided into type 1 receptors(P1), sensitive to adenosine and AMP, and type 2 (P2),sensitive to ATP and ADP. The actions of purinesare related as a rapid breakdown of ATP increasesthe levels of adenosine.

Purinergic mechanisms and specific receptor sub-types have been shown to be involved in variouspathological conditions in the fetus, child, and adultincluding ischemia, brain trauma, and neurodegener-ative diseases involving neuroimmune and neuro-inflammatory reactions, as well as in neuropsychiatricdiseases (for a recent review regarding purines, seeBurnstock, 2008).

ATPThe purine nucleotide ATP is the main source ofenergy in cells, and is also stored in synaptic vesicles

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and released together with classic transmitters such asnorepinephrine and acetylcholine. The ratio betweenATP and catecholamines in chromaffin granules hasbeen found to be higher during early life than latersuggesting that ATP is a very early phylogenetic andontogenetic signaling substance (O’Brien et al.,1972). During the past decades, evidence for ATP asa neural signaling substance has emerged by examin-ing sites of storage, release, and hydrolysis, as well aspotential actions and targets. A variety of receptorsfor extracellular ATP have been identified. Some areinvolved in fast neuronal transmission and operate asligand-gated ion channels (P2T, X, and Z). Others areinvolved in the paracrine or autocrine modulation ofcell function (P2U and Y). Many receptors of this typeare coupled to phosphoinositide-specific phospholi-pase C (Fredholm, 1997). Intracellular ATP levelsdirectly change the excitability of neurons by ATP-dependent potassium channels which may hyperpo-larize cells, thus decreasing neuronal activity whenenergy resources are scarce. Moreover, the release ofATP through unopposed gap junction channels, “con-nexins” (Elias & Kriegstein, 2008), or “pannexins”(Iglesias et al., 2009), as well as intercellular ATP signal-ing are essential for the migration of neural progenitorcells and the proper formation of the subventricularzone. Interference with ATP signaling or abnormalcalcium fluctuations in basal or intermediate neuronalprogenitors may play a significant role in a varietyof genetic and acquired cortical malformations (Liuet al., 2008).

AdenosineAdenosine is a constituent of all body fluids, includingthe extracellular space of the CNS. It has multipleeffects on organs and cells of the body. Thus, its levelsare tightly regulated by a series of enzymatic steps(Fredholm, 1997). Adenosine can be regarded moreas a neuromodulator, in that it does not seem to bestored in vesicles. Adenosine is produced by dephos-phorylation of adenosine monophosphate (AMP)by 5-nucleotidase, an enzyme occurring in bothmembrane-bound and cytosolic forms. Degradationof intra- and extracellular ATP is the main source ofextracellular adenosine. Specific bidirectional trans-porters maintain intra- and extracellular concentra-tions of adenosine at similar levels. During basalconditions adenosine levels are 30–300 nM andcan rise following stimuli that cause an imbalance

between ATP synthesis and ATP breakdown. Thus,the levels during ischemia or hypoxia can rise100-fold (Winn et al., 1981b; Fredholm, 1997). Theextracellular concentrations of adenosine might behigher in the fetal brain than postnatally, since fetalPaO2 can decrease below the level (30 mmHg) whena marked increase in extracellular adenosine canbe expected (Winn et al., 1981a). Overall, adenosinedecreases oxygen consumption and has neuropro-tective effects (Arslan et al., 1997). However, hypoxiaalso induces a decrease in neonatal respiration.Theophylline and caffeine are adenosine antagoniststhat cause ventilation to increase and that decrease theincidence of neonatal apneas when given systemically,mainly due to the antagonistic effect of theophyllineon adenosine A1 receptors in the medulla oblongata(Herlenius & Lagercrantz, 1999). A variety of recep-tors for extracellular ATP have been identified.Specific adenosine receptors interact with G-proteins,adenosine A1, expressed pre- and post-synaptically inneurons ubiquitously, highest in hippocampus, and A3

receptors mainly interact with G(i/o) proteins. A2a andA2b receptors mainly interact with G(s) proteins. A2a

receptors are enriched in basal ganglia and are closelyassociated and functionally interact with dopamine D2

receptors (Fredholm et al., 2001; Stevens et al., 2002;Fredholm & Svenningsson, 2003). Oligodendrocyteprogenitor cells (OPCs) express functional adenosinereceptors, which are activated in response to actionpotential firing. Adenosine acts as a potent neuron–glial transmitter to inhibit OPC proliferation, stimu-late their differentiation, and promote the formationof myelin (Stevens et al., 2002).

The general level of neuronal activity and met-abolic processes that support it may be unusuallyhigh in the human cortex, and upregulation ofseveral genes involved in synaptic transmission isa characteristic of the human compared with non-human primate brain (Caceres et al., 2003). Themetabolic control of brain activity by adenosinethus could be even more important in humansthan in other mammals. Some caution againstextensive use of adenosine receptor antagonistssuch as caffeine has been recommended in preg-nant women and preterm infants (Schmidt, 1999;Herlenius et al., 2002). On the other hand, it hasrecently been demonstrated that infants who havebeen treated with caffeine show improved neonataloutcome as compared with controls (Schmidtet al., 2006, 2007).

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Perinatal transition

Before birthThe levels of most neurotransmitters and neuromo-dulators increase concomitantly with synapse forma-tion. Some of them surge during the perinatal period,such as glutamate, catecholamines, and some neuro-peptides, and then level off. The interesting questionis to what extent the expression of neuroactive agentsis related to the functional state of the fetus and thenewborn. On the one hand, there is an intense firingand wiring in the fetal brain, particularly duringactive sleep. The inhibitory neurotransmitter GABAis mainly excitatory in the fetal period (see above).Amino acid transmitters also act via NMDA recep-tors, which are important for the wiring and plasticityof the immature brain, although the main excitatoryfast-switching receptors (AMPA) are expressed later.On the other hand, activities such as respiratorymovements are suppressed. The fetus seldom ornever becomes aroused or wakes up. The sympathetictone is low. Furthermore, the fetus is adapted to the lowoxygen level in the womb – “Mt. Everest in utero.”If a fetus is challenged by asphyxia it is not excitedas an adult responding with a flight or fight reaction,but rather it becomes immobilized, stops breathing,and becomes bradycardic (see Lagercrantz, 1996).This paralytic state of the fetus can be caused byinhibition of the chemical neurotransmission.Adenosine is such a neuromodulator that might beinvolved in this suppression of the fetal brain. It has ageneral sedative effect. Its concentration increasesduring energy failure and hypoxia, and it has beensuggested that it can act as a modulator to cope withthe hypoxic situation (Berne, 1986). Adenosine A1

receptor activation depresses breathing substantiallyin the fetus and the neonate by inhibiting synaptictransmission and hyperpolarizing certain neurons(Herlenius & Lagercrantz, 1999). PGE2, releasedfrom the placenta, also contributes to the inhibitionof the fetus, and although the decrease of thisplacental inhibitor is not crucial for establishing con-tinuous breathing movements at birth, its removalallows the newborn baby to be vigilant after birth(Alvaro et al., 2004). In addition, release of PGE2and inhibition of the brain activity at birth alsooccur by hypoxia-induced activation of brain micro-somal prostaglandin synthase-1 (mPGES-1) and sub-sequent release of endogenous PGE2 (Hofstetter et al.,2007).

Inhibition of fetal activity may also be mediated bythe maternal oxytocin released in high concentrationsduring labor. Oxytocin has been found to reduce theintracellular concentration of chloride and thus switchthe effect of GABA from an excitatory to an inhibitoryneurotransmitter (Tyzio et al., 2006).

Thus, high levels of adenosine and prostaglandinstogether with the birth-related switch of GABAinto an inhibitory neurotransmitter all contribute toallow the stressful birth to be endured withoutdamage to the temporarily inhibited, and thus less-energyconsuming, brain.

Neuropeptides that might be involved in the sup-pression of fetal activity are NPY, somatostatin, andendogenous opioids. The levels of NPY are relativelyhigh in the fetal brain and decline after birth. Plasmalevels of endorphins and enkephalins are increased inthe umbilical cord at birth (Ramanathan et al., 1989;Aurich et al., 1990).

BirthThe healthy newborn baby is aroused and awakethe first two hours after birth and starts continuousbreathing movements. Factors such as squeezingand squashing of the fetus, increased sensoryinput, and cooling are probably important. We canhypothesize that there is a surge of excitatory neuro-transmitters and downregulation of inhibitory onesin the brain.

The increased neuronal activity is indicated bythe increased expression of immediate early genes(Ringstedt et al., 1995). The arousal and vigilanceof the newborn seem to be related to activation of thenorepinephrinergic system in the brain, particularlythe locus ceruleus, from where norepinephrinergicneurons are distributed in the whole brain (see above).The norepinephrine turnover as indicated by the ratioof the metabolite 3-methoxy-4-hydroxyphenylglycol(MHPG) to norepinephrine was increased twofoldto threefold in the newborn rat (Lagercrantz, 1996).There are indirect indications that there is also a norepi-nephrine surge in the human brain, by the finding ofhigh level of plasma catecholamines after birth.

A rapid decrease of the inhibitory neuromodulatoradenosine in the brain occurs as partial pressure ofoxygen in arterial blood rapidly increases after birth,probably contributing to the increased activity in thenewborn infant compared with the fetus. In addition, adecreased sensitivity during the first postnatal days foradenosine seems to contribute to the maintenance of

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continuous breathing (Herlenius & Lagercrantz, 1999;Herlenius et al., 2002).

Prenatal and perinatal programmingThe concept of fetal and neonatal programming dis-covered by Barker (Sayer et al., 1997) also applies tothe ontogeny of neurotransmitters and neuromodula-tors, i.e., an early stimulus or insult at a critical periodcan result in long-term changes in the structure andthe function of the organism (see Chapter 22). Forexample, it can be postulated that prenatal or perinatalstress can disturb the timetable of the expression ofneurotransmitters and neuromodulators and theirreceptors. Disruption of the normal timing or inten-sity of neurotransmitter signaling can lead to per-manent changes in proliferation, differentiation, andgrowth of their target cells during critical phases ofdevelopment of the nervous system, thereby possiblyproviding the underlying mechanisms for neurobeha-vioral or neurophysiological abnormalities associatedwith developmental exposure to neuroactive drugs andenvironmental toxins.

Hydrocortisone given to neonatal rats has beenfound to enhance the maturation of the monoaminer-gic systems in the brain (Kurosawa et al., 1980).Administration of extra glucocorticosteroids to therat fetus induces alterations of dopamine receptorresponses, which affects the spontaneous motor con-trol both in short- and long-term perspectives (Diazet al., 1997). Chronic high endogenous corticosteroidlevels can be induced by stress to the mother beforebirth, or to the child after birth. Exogenous cortico-steroids are also administered by physicians to thegrowing infant (and brain) in the management of awide spectrum of pre- and postnatal conditions. Thelong-term effects of corticosteroids on the developinghuman CNS as well as the long-lasting effects arenot well known. However, corticosteroids have beenshown to have deleterious effects on the developingbrain and behavior in several animals including pri-mates, i.e., inhibition of neural stem cells, neurogene-sis, andmigration leading to irreversible decrease in brainweight (Edwards & Burnham, 2001; Matthews, 2001).

Chronic prenatal hypoxia alters the monoamineturnover in the locus ceruleus and nucleus tractussolitarius in the adolescent rat (Peyronnet et al.,2002). This was related to disturbed control of respi-ratory behavior. Human handling of newborn rats for15 minutes during the first weeks of life affects ascen-ding serotonergic projections into the hippocampus

and causes a long-lasting increase in glucocorticoidreceptors (Sapolsky, 1997).

There are also clinical studies indicating that pre-natal stress is associated with attention deficit disor-ders in children (Weinstock, 1997). Birth insult andstress alter dopamine transporter binding in rat, pos-sibly also leading to hyper-locomotion (El-Khodor &Boksa, 2002). People with schizophrenia seem to haveexperienced more pregnancy and birth complicationsthan their healthy siblings (Stefan & Murray, 1997).For example mothers of schizophrenic patients moreoften had severe infections during pregnancy, possibleaffecting cytokines (such as IL1β, IL6, and TNFα) andindirectly the development of monoaminergic circuitsin the fetal brain (Gilmore & Jarskog, 1997; Jarskoget al., 1997; Meyer & Feldon, 2009).

AcknowledgmentsPart of this chapter contains updated and revised textfrom Lagercrantz and Herlenius (2002) and Herleniusand Lagercrantz (2004). This study was supported bygrants from the Swedish Medical Research Council, theKarolinska Institutet, and the Wallenberg Foundation.

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BibliographyValenstein, E. S. (2005). TheWar of the Soups and the Sparks.New York: Columbia University Press.

Historic box 4 Soups and sparks: on the history of nervous communicationHugo Lagercrantz

In 1877, the German physiologist Emil Du Bois-Reymond wrote that there were only two ways a nerve can causemuscle contraction, either by a chemical substance or electrically. This was the beginning of a longlasting debate onchemical versus electrical neurotransmission – ”The war of the soups and the sparks” (Valenstein, 2005).

Thomas Elliott is usually credited with the discovery of chemical neurotransmission. As a student he hypothesizedthat sympathetic nerves secrete adrenaline (epinephrine) when innervating visceral organs (1905). CharlesSherrington coined the term synapse (1897) based on Ramon y Cajal’s neuron theory. He used the Greek wordsynapse meaning “to clasp” to refer to the junction point between neurons. Sherrington was an electrophysiologistfavoring the electrical theory. John Eccles was his prominent pupil and originally the strongest opponent of chemicaltransmission. Henry Dale, who discovered the bradycardic effects of acetylcholine, became the leader of the “soupteam.”

Otto Loewi did the crucial experiment demonstrating chemical neurotransmission in 1920 in Graz, Austria.According to Loewi, the idea came up in a dream one night. However, he forgot the dream but it came back thefollowing night. He immediately woke up and scribbled down the dream and went to the laboratory and performedthe experiment. By transferring the fluid of a stimulated heart preparation from a frog onto another isolated frogheart, he found that the second heart’s rate slowed down when it was perfused with the fluid from the first heart. Thisdemonstrated that a chemical had been released from the nerve endings of the vagus – ”vagusstoff,” which wasfound to be identical with acetylcholine.

However, it took a long time before John Eccles and other electrophysiologists accepted the idea of chemicaltransmission. Eccles argued vigorously in stand-up fights against Henry Dale at the Physiological Society. It is said thatEccles changed his mind first when one of his collaborators in Sydney cut his lawn with an electric lawnmower.Unfortunately, the young guest scientist (Bernard Katz) succeeded in cutting the electric wire to the lawnmower,which is why Eccles decided to buy a lawnmower based on chemical transmission, i.e., a petrol-driven one.

Chapter 7: Neurotransmitters and neuromodulators

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