Journal o/ Cerebral Blood Flow and Metabolism 14:132-144 © 1994 The International Society of Cerebral Blood Flow and Metabolism Published by Raven Press, Ltd., New York

Functional Metabolic Mapping of the Rat Brain During Unilateral Electrical Stimulation of the

Subthalamic Nucleus

Minas Tzagournissakis, Catherine R. Dermon, and Helen E. Savaki

Laboratory of Physiology, Department of Basic Sciences, Division of Medicine, School of Health Sciences,

University of Crete, Crete, Greece

Summary: The alterations in local metabolic activity of several anatomically distinct brain areas were investi­gated by means of the quantitative autoradiographic 2-deoxY-D-[l-14C]glucose method in awake rats during uni­lateral electrical stimulation of the subthalamic nucleus (STH). Unilateral electrical stimulation of the STH in­duced local metabolic activation (by 70% as compared with the control group), as well as distal metabolic acti­vations in the substantia nigra reticulata (by 34%), globus pallidus (by 19%), entopeduncular nucleus (by 18%), deep layers of the superior colliculi (by 15%), and para­fascicular thalamic nucleus (by 18%), ipsilaterally to the stimulated side. The ventrolateral motor thalamic nucleus as well as the limbic components, posterior cingulate cor­tex, and anteroventral thalamic nucleus displayed bilat­eral metabolic activations (by 20--28%). These results in­dicate that, in addition to its known ipsilateral motor con-

The subthalamic nucleus (STH) is generally con­sidered to exert control over motor functions. The association of STH hypoactivity with the clinical syndrome of hemiballismus has long been docu­mented (Whittier, 1947; Whittier and Mettler, 1949; Carpenter and Carpenter, 1951; Hammond et ai., 1979; Mitchel et ai., 1985a; Crossman, 1987). The STH receives projections predominantly from the globus pallidus (Carpenter et ai., 1981; Van der Kooy et ai., 1981; Kita et ai., 1983a; Morizumi et ai., 1987; Groenewegen and Berendse, 1990) and

Received February 26, 1993; final revision received July 22, 1993; accepted August 9, 1993.

Address correspondence and reprint requests to Dr. H. E. Savaki at Laboratory of Physiology, Department of Basic Sci­ences, Division of Medicine, School of Health Sciences, Univer­sity of Crete, 711 10 Iraklion, Crete, Greece.

Abbreviations used: DG, 2-deoxY-D-glucose; S TH, subtha­lamic nucleus.

132

nections, each STH is functionally related to the limbic system bilaterally. It is suggested that the STH is a site where the central motor information is accessible to the limbic system. Quantitative image analysis of individual serial sections in the STH, substantia nigra, and globus pallidus revealed a consistent dorsoventral pattern of to­pographic interrelations. Stimulation of either the dorsal or the ventral subdivision of the STH induced always stronger activation in the dorsal compartment of the sub­stantia nigra and in the ventral compartment of the globus pallidus. These results suggest that the earlier-described inversion of the dorsoventral functional correspondence between the substantia nigra and globus pallidus may be partly mediated via the subthalamic nerve cells projecting collateral axons to both these areas. Key Words: Basal ganglia-Globus pallidus-Limbic system-Substantia nigra-Subthalamic nucleus.

the cerebral cortex (Hartmann-Von Monakow et ai., 1978; Romansky et ai., 1979; Kitai and Deniau, 1981; Afsharpour 1985; Rouzaire-Dubois and Scar­nati, 1985; Canteras et ai., 1988). It also receives moderate projections from the parafascicular tha­lamic nucleus (Sugimoto et ai., 1983; Canteras et ai., 1990; Groenewegen and Berendse, 1990) and the pedunculopontine tegmental nucleus (Nomura et ai. , 1980; Hammond et ai., 1983a; Morizumi et ai., 1987). The STH neurons project branched ax­ons to the substantia nigra pars reticulata, globus pallidus, and entopeduncular nucleus (Deniau et ai., 1978; Ricardo, 1980; Van der Kooy and Hattori, 1980; Hammond and Yelnik, 1983; Kita et ai., 1983b). Moreover, the STH sends sparse projec­tions to the mesencephalic pedunculopontine tegmental nucleus, neostriatum, substantia nigra compacta, and cerebral cortex (Jackson and Cross­man, 1981a,b; Saper and Loewy, 1982; Kita and

EFFECTS OF SUBTHALAMIC NUCLEUS STIMULATION 133

Kitai, 1987; Takada et aI., 1988; Granata and Kitai, 1989; Groenewegen and Berendse, 1990).

The well described neuroanatomical hardware does not necessarily represent the functionally im­porta,nt neuronal connections of the STH. The 2-deoxY-D-[1-14C]glucose ([14C]DG) metabolic map­ping is determined by dynamic properties of neu­rons related to functional activity, beyond their static properties related to the anatomically traced connections (Sokoloff, 1977; Sokoloff et aI., 1977). This method has been proven useful in delineating the functional organization and topographic interre­lations of the basal ganglia components in the rat (Savaki et aI., 1983a,b, 1984, 1988, 1992; Dermon et aI., 1990, 1992). Within the general framework of obtaining deeper understanding of the sophisticated neuronal network regulating extrapyramidal mech­anisms of motor function, the specific aim of the present e4C]DG metabolic study was to investigate the rat brain areas functionally associated with the STH and furthermore to reveal any possible spe­cific topographic interrelation between functional compartments of the STH and its major target ar­eas, the substantia nigra and globus pallidus.

MATERIALS AND METHODS

Animals Experiments were carried out on 16 adult male

Sprague-Dawley albino rats (Charles River) weighing be­tween 350 and 400 g. Animals were separated into two groups. The experimental group consisted of 11 rats, which were subjected to electrical stimulation of the STH in the right hemisphere. The control group consisted of five rats, which were simply implanted with an electrode in the STH of the right hemisphere. Experiments con­ducted over several days were randomized alternatively to measure the local cerebral glucose utilization in the rats of the two groups.

Surgical preparation Each rat was anesthetized with a mixture of halothane/

nitrous oxide/oxygen (1 %, 67:33 vol/vol), and polyethyl­ene catheters were inserted in one femoral artery and one femoral vein. A bipolar stimulating electrode was verti­cally implanted into the right STH according to coordi­nates that had been previously determined experimen­tally (A: 5, L: 2.3, H: 2. 2). The electrode was sealed on the skull with dental cement, and the animals encased in a loose-fitting plaster cast around the lower abdomen were allowed to recover from anesthesia for 2-3 h.

Electrical stimulation of STH Electrical stimulation of the right STH started 2 min

before intravenous injections of the [14C]DG and contin­ued during the first 15 min of the measurement of local cerebral glucose consumption. A bipolar stimulating elec­trode and 0.2 mm external diameter was used for the STH stimulation. The distance between the two poles of the electrode was 0.2 mm and the polarity was changing ev­ery half-minute during the 17 -min stimulating period. The

electrical stimulation consisted of trains (0.25-Hz fre­quency and tOO-ms duration) of 30 square pulses (300-Hz frequency, 0. 5-ms duration, and 100-f1A intensity). Stim­ulation with lower frequency and lower intensity did not induce any metabolic changes in pilot experiments.

Measurement of local cerebral glucose utilization The measurement of local cerebral glucose consump­

tion was initiated by the intravenous injection of 100 f1Ci/ kg [14C]DG, of specific activity 50 mCi/mmol (New En­gland Nuclear). Arterial blood samples were obtained during the succeeding 45 min and the glucose and [14C]DG plasma concentrations were measured. The rats were then decapitated and the brains were removed and frozen in isopentane cooled to - 40°C. Coronal sections 20 f1m thick were cut on a cryostat microtome at - 20°C. Autoradiograms were prepared from these sections by apposing the slides, together with precalibrated 14C_ standards, to Kodak OMI x-ray film in x-ray cassettes. Local cerebral tissue concentrations of 14C were deter­mined from the auto radiographs by quantitative densi­tometry using a computer-based micro densitometer (Quanti met 970; Cambridge Instruments). The glucose consumption in discrete brain areas was calculated ac­cording to the operational equation of Sokoloff et al. (1977).

Statistical analyses Local cerebral glucose utilization values from the STH­

stimulated experimental group of rats were compared with those from the control group by the Scheffe F test extension of the one-way analysis of variance. The per­cent increases of the significantly affected average value of local cerebral glucose utilization in the experimental group (E) over its respective value in the control group (C) were calculated as [(E - C)/C] x 100. Furthermore, ipsilateral-to-contralateral to the stimulated STH differ­ences in glucose consumption were calculated within each one of the two groups using the paired two-tailed t test.

RESULTS

Following unilateral STH stimulation, ipsiversive head turning, accompanied by intermittent sniffing, exploring, and jaw movements, was observed in the experimental rats. The implantation of an electrode within the STH in the control rats did not induce any behavioral effect. The exact position of the stimulating electrode tract and tip was confirmed by overlaying the autoradiograms on the correspond­ing stained histological brain sections, and the met­abolic activity within the stimulated area was deter­mined by quantitative densitometry of the dorsal and ventral subdivisions of the STH, on computer­generated magnifications of the autoradiographic sections of interest.

Metabolic effects within STH

The auto radiograms in all STH-stimulated rats demonstrated always a well limited area of mark­edly increased optical density around the tip of the electrode, attesting to increased glucose con sump-

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134 M. TZAGOURNISSAKIS ET AL.

tion within the STH. Enhancement of optical den­sity was not observed in any area adjacent to the STH (zona incerta, cerebral peduncle, or internal capsule). Quantitative analysis of the autoradio­graphic sections at the level of the STH revealed a 70% significant (by one-way analysis of variance) increase of local cerebral glucose utilization in the electrically stimulated STH of the experimental group as compared with the respective side of the control group. Right-to-left side comparison (by the two-tailed paired t test) demonstrated a major in­crease in metabolic activity in the electrically stim­ulated STH as compared with the intact one in the experimental group. The decrease (5%) in activity within the right STH implanted with an electrode in the control group of rats demonstrates that the elec­trode itself had a minor local depressive effect on metabolic activity (Table 1).

Metabolic effects in basal ganglia components

Unilateral stimulation of the STH induced signif­icant elevation in local cerebral glucose utilization within the ipsilateral substantia nigra pars reticu­lata and the ipsilateral globus pallidus in the exper­imental group as compared with the contralateral homologous structures and the corresponding areas of the control group of animals (Table 1). Visual inspection of the magnified autoradiographic sec­tions indicated that in all cases when (a) mostly the dorsal (Fig. 1), (b) mostly the ventral (Fig. 2), or (c) the whole dorsoventral (Fig. 3) extent of the STH was stimulated, the pattern of substantia nigra pars reticulata and globus pallidus activations was not homogeneous. Indeed, when individual local cere­bral glucose utilization values separately in the dor­sal and ventral subregions of the STH, substantia nigra pars reticulata, and globus pallidus (Fig. 4)

were analyzed along their rostrocaudal extent, a consistent dorsoventral pattern of topographic in­terrelations was revealed. Stronger stimulation of either the dorsal or the ventral subdivisions of the STH always induced a stronger activation in the dorsal compartment of the substantia nigra and in the ventral compartment of the globus pallidus. Unilateral stimulation of the STH also induced an enhancement in glucose utilization within the en­topeduncular nucleus, the ventromedial part of the posterior striatum, and the substantia nigra pars compacta.

Metabolic effects in motor areas

Significant elevation of glucose utilization was measured in the deep layers of the superior colliculi ipsilaterally to the stimulated STH. The ventrolat­eral nucleus displayed bilateral increases in glucose consumption (Table 2). The motor cortical areas demonstrated a small ipsilateral decrease in meta­bolic activity in the experimental as well as in the control group, which apparently reflects an elec­trode tract effect. There is no indication that the behavior (ipsiversive head turning) influenced the metabolic activity, because the single motor area activated (the ventrolateral thalamus) was bilater­ally affected.

Metabolic effects in sensory areas

Within the sensory system, the vestibular nu­cleus, the nucleus of the solitary tract, the nucleus ambiguus, and the superficial layers of the superior colliculi remained bilaterally unaffected. Side-to­side statistical comparison demonstrated that the ipsilateral lateral geniculate body, lateral posterior thalamic nucleus, ventrobasal thalamic complex, as well as the ipsilateral cortical parietal and somato-

TABLE 1. Bilateral effects of unilateral subthalamic nucleus (STH) stimulation on local cerebral glucose utilization in basal ganglia components

Control STH stimulation % increase -------

Ipsi Contra Ipsi Contra Ipsi Contra

Substantia nigra compacta 60 ± 10 63 ± 11 70 ± 12a 64±1O Substantia nigra reticulata 50 ± 4 51 ± 4 67 ± 17a,b 56 ± 9 34 STH 74 ± 8 78 ± 7a 126 ± 26a,b 83 ± 10 70 Posterior striatum (dorsolateral part) 80 ± 13 83 ± 14 86 ± 8 88 ± 10 Posterior striatum (ventromedial part) 82 ± 12 81 ± 11 87 ± 8a 81 ± 9 Anterior striatum (central part) 90 ± 12 92±13 92 ± 12 91 ± 9 Globus pallidus 52 ± 5 52 ± 5 62 ± lOa,b 54 ± 7 19 Entopeduncular nucleus 51 ± 5 52 ± 8 60 ± 12a 55 ± 8 18 Ventral pallidum 55 ± 5 55 ± 5 55 ± 2 55 ± 2 Accumbens nucleus 81 ± 12 80 ± 9 83 ± 10 83 ± 10

Values represent means ± SD of glucose utilization expressed in fLmol 100 g-t min-t, obtained in 5 conscious control and I I conscious STH-stimulated rats,

a Side-to-side significant differences at the p < 0, 05 level, calculated by the paired t test, within each of the two groups, b Significances at p < 0,05 level, estimated by one-way analysis of variance (Scheffe F test), between control and STH-stimulated

groups,

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EFFECTS OF SUBTHALAMIC NUCLEUS STIMULATION 135

fl· . .-iJ.- � �\: ... '\, " , "'t;' ; . \ , ' " - �

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s ubstantia nig ra

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globus pallidus

FIG. 1. C 4C]Deoxyglucose autoradiograms of coronal brain sections from a rat subjected to right subthalamic nucleus (STH) electrical stimulation. The activated substantia nigra pars reticulata (left) and globus paliidus (right) ipsilateral to the stimulated STH (middle) are displayed at three different anteroposterior levels (top, middle, bottom). Local cerebral glucose utilization values are proportional to relative optical densities.

sensory areas were less active than the contralateral homologous structures (Table 3). These small met­abolic depressions are considered to reflect an elec­trode tract effect because they were equally de­tected in the experimental as well as in the control animals.

Metabolic effects in reticular and

intralaminar nuclei At the diencephalic level, the intralaminar, cen­

tromedian, and centrolateral thalamic nuclei showed a minor metabolic activation in the STH­stimulated rats as compared with the control ones, which, however, was not significant. Statistically significant elevation of glucose consumption was measured only in the parafascicular thalamic nu­cleus ipsilaterally to the stimulated STH in the ex­perimental as compared with the control group (Ta­ble 4).

Metabolic effects in limbic and prefrontal systems

The only limbic diencephalic area that displayed significant bilateral enhancement in glucose con­sumption in the experimental as compared with the control group was the anteroventral thalamic nu­cleus. Among the cortical limbic areas examined, only the posterior cingulate cortex displayed signif­icant metabolic activation bilaterally. Concerning the prefrontal system, only the prefrontal (frontal medial) cortex was less active in the ipsilateral than contralateral side in both control and experimental groups (Table 5).

DISCUSSION

General considerations

The implantation of an electrode within the right STH in the control animals induced metabolic de­pressions ranging between 4 and 9% along its tract,

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136 M. TZAGOURN1SSAKIS ET AL.

substantia nig ra

s ubthalamic nucleus

globus pallidus

FIG. 2. Autoradiograms of coronal rat brain sections displaying the electrically stimulated right subthalamic nucleus (middle) and the consequent activations in the ipsilateral substantia nigra pars reticulata (left) and ipsilateral globus pallidus (right) at three different anteroposterior levels. A specific topographic interrelation between the activated subregions is illustrated.

i.e., in thalamic and cortical areas of the ipsilateral hemisphere. The present study demonstrates that unilateral electrical stimulation of the STH induces increases in local cerebral glucose utilization in very few specific brain areas. Given that the fiber systems traversing the STH are the striatal and as­cending reticular and nigral projections, our results do not indicate any major involvement of the fibers of passage (Lindvall and Bjorklund, 1974; Beck­stead et al., 1979; Ricardo, 1980). However, the [14C]DG method does not allow for discrimination between orthodromic and antidromic or direct monosynaptic and indirect polysynaptic events.

Effects elicited in substantia nigra pars reticulata The metabolic activation of the ipsilateral sub­

stantia nigra pars reticulata induced by unilateral STH stimulation in the present study is complemen­tary to previous anatomical (Kanazawa et al., 1976; Van der Kooy and Hattori, 1980; Gerfen et al.,

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1982; Hammond and Yelnik, 1983; Kita et al., 1983b; Chang et al. , 1984; Loopuijt and Van der Kooy, 1985) and electrophysiological (Hammond et al., 1978; Rouzaire-Dubois et al., 1984) studies, which demonstrate that the substantia nigra pars reticulata is a major target of the STH in the rat. Since there is no evidence for the existence of a nigrosubthalamic projection in the rat (Canteras et al., 1990), the well established subthalamonigral projection that has an excitatory nature (Hammond et aI. , 1978; Kitai and Kita, 1987; Nakanishi et al., 1987; Robledo and Feger, 1990; Brotchie and Cross­man, 1991; Feger and Robledo, 1991) orthodromi­cally mediates the metabolic activation of the sub­stantia nigra.

Effects elicited in pallidum and

entopeduncular nucleus

The massive ipsilateral subthalamopallidal pro­jection established in the rat by means of neuroana-

EFFECTS OF SUBTHALAMIC NUCLEUS STIMULATION 137

s ubs tantia nigra

s ubthalamic nucleus

globus pallidus

FIG. 3. Autoradiograms of coronal rat brain sections displaying a specific pattern of topographical organization of activated nigral (left) and pallidal (right) subregions, following ipsilateral electrical stimulation of the subthalamic nucleus (middle). The three rows correspond to three different anteroposterior levels.

tomical (Ricardo, 1980; Van der Kooy and Hattori, 1980; Groenewegen and Berendse, 1990) and elec­trophysiological (Perkins and Stone, 1980; Ham­mond et aI., 1983b; Rouzaire-Dubois et aI., 1983; Kita and Kitai, 1987; Robledo and Feger, 1990) methods can explain the metabolic activation of the ipsilateral globus pallidus elicited by unilateral STH stimulation in the present study. This activation of the globus pallidus is in agreement with recent ob­servations demonstrating that the nature of the sub­thalamopallidal projections is excitatory (Kita and Kitai, 1987, 1991; Robledo and Feger, 1990; Brotchie and Crossman, 1991) and not inhibitory as reported earlier (Perkins and Stone, 1980, 1981; Hammond et aI., 1983b; Rouzaire-Dubois et aI., 1983). Furthermore, the metabolic data presented here, suggestive of excitatory subthalamic inputs to the nigra and pallidum, are compatible with previ­ously reported metabolic findings in the rat as well

as in the monkey (Mitchell et aI., 1985a; Feger and Robledo, 1991). The metabolic activation observed in the globus pallidus was smaller than that dis­played in the substantia nigra pars reticulata. The antidromic excitation of the inhibitory pallidosub­thalamic fibers (Hattori et aI., 1973; Carter and Fibiger, 1978; Rouzaire-Dubois et aI., 1980; Van der Kooy et aI., 1981; Kita et aI., 1983a; Groenewegen and Berendse, 1990) may be responsible for mask­ing part of the metabolic activation resulting in the pallidum from the orthodromic excitation of the subthalamopallidal neurons. The lack of metabolic activation in the ventral pallidum, ipsilateral to the stimulated STH, is surprising, given the anatomi­cally described reciprocal connections between the STH and the ventral pallidum, which is under the influence of limbic structures (Ricardo, 1980; Haber et aI., 1985; Woolf and Butcher, 1986; Kita and Ki­tai, 1987; Canteras et aI., 1990; Groenewegen and

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138 M. TZAGOURN1SSAKIS ET AL.

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FIG. 4. Schematic representation of three autoradiographic coronal rat brain sections, magnified around the nigra, sub­thalamic nucleus, and pallidum. Top: substantia nigra pars reticulata level; middle: subthalamic nucleus level; bottom: globus pallidus level. AM, amygdala; SST, bed nucleus of the stria terminalis; cp, cerebral peduncle; d, dorsal subregion; DG, dentate gyrus; f, fornix; GP, globus pallid us; HIP, hippo­campus; iC, internal capsule; LH, lateral hypothalamus; LV, lateral ventricle; ml, medial lemniscus; SNC, substantia nigra pars compacta; SNR, substantia nigra pars reticulata; st, stria terminalis; STH, subthalamic nucleus; STR, striatum; RN, red nucleus; v, ventral subregion; VB, ventrobasal tha­lamic complex; VP, ventral pallidum; VTA, ventral tegmental nucleus; ZI, zona incerta.

Berendse, 1990). The relatively small metabolic ac­tivation induced in the entopeduncular nucleus by STH stimulation in our study may be due to the convergent (a) antidromically activated, inhibitory pallidosubthalamic projection (which gives

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branches to the entopeduncular nucleus) and (b) orthodromically activated, excitatory subthalamo­entopeduncular projection (Nakanishi et aI., 1991).

Topographic functional interrelations between

substantia nigra and globus pallidus

We have demonstrated in the past that electrical stimulation of precise subregions of the substantia nigra pars reticulata results in region-specific met­abolic activations within the ipsilateral globus pal­lidus, reflecting a specific topographic organization of the functionally interrelated compartments of these two basal ganglia components (Savaki et aI., 1983b, 1988). This specific interrelation between ni­gral and pallidal functional subregions was charac­terized by inverted dorsoventral topography. It was then hypothesized that the activation of the pallidal subregions induced by nigral electrical stimulation was mediated either via the activated STH or via the similarly activated striatum. This hypothesis was supported by anatomical data demonstrating that the STH branched efferents to the pallidum and nigra invert their dorsoventral topography (Kita and Kitai, 1987). The present study demonstrates that stimulation of either the dorsal or the ventral sub­division of the STH induces always stronger acti­vation in the dorsal compartment of the substantia nigra pars reticulata and the ventral compartment of the globus pallidus. This finding confirms our ear­lier hypothesis, that the inversion of the dorsoven­tral functional correspondence between the sub­stantia nigra pars reticulata and globus pallidus is at least partly mediated via the subthalamic nerve cells projecting collateral axons to both these areas.

Effects elicited in substantia nigra pars compacta

and neostriatum

The minor metabolic activations observed in our study within the ipsilateral compacta and striatum may have been mediated via the reported moderate subthalamic efferent projections to the substantia nigra pars compacta and neostriatum, respectively (Kita and Kitai, 1987; Takada et aI., 1988; Groe­newegen and Berendse, 1990). Of interest is that the ventromedial striatal region activated at present, which is also known as the "limbic striatum," re­ceives substantial input from limbic-associated structures and innervates the ventral pallidum (Haber et aI., 1985). The small activation observed within the pars compacta at present is in agreement with the enhancement of dopamine release detected in the ipsilateral compacta during unilateral electri­cal stimulation of the STH (Mintz et aI., 1986). The small striatal activation is in agreement with the ex-

EFFECTS OF SUBTHALAMIC NUCLEUS STIMULATION 139

TABLE 2. Bilateral effects of unilateral subthalamic nucleus (STH) stimulation on local cerebral glucose utilization in motor areas

Control

Ipsi

Cerebelium Hemispheres

Molecular layer 53 ± 6 Granular layer 64 ± 14

Vermis posterior (median) Molecular 71 ± 10 Granular 87 ± 7

Vermis anterior (median) Molecular 78 ± 10 Granular 88 ± I I

Fastigial nucleus 84 ± 11 Dentate nucleus 82 ± 8 Interpositus nucleus 89 ± 10

Inferior olives 73 ± 5 Red nucleus 72 ± 7 Pedunculopontine nucleus 56 ± I I Oculomotor complex (Edinger-

Westphal + III nucleus) 87 ± 8 Superior colliculus

(deep layers) 79 ± 5 Anterior pretectal area 97 ± 19 Posterior pretectal area

Medial 71 ± 8 Lateral 75 ± 8

Thalamic ventromedial nucleus 104 ± 10 Thalamic ventrolateral nucleus 86 ± 8 Frontal cortex (motor) 85 ± I I Sensorimotor cortex (motor) 86 ± 8

Contra

53 ± 8 62 ± I I

85 ± 10 85 ± 12 90 ± 9 73 ± 7 74 ± 6 58 ± 10

87 ± 8

81 ± 5 100 ± 18

73 ± 8 76 ± 8

106 ± 9 91 ± lOb 89 ± 9b 93 ± 9b

STH stimulation

Ipsi Contra

55 ± 8 56 ± 7 63 ± 10 63 ± 10

78 ± 12 88 ± 13

86 ± 14 94 ± 14

92 ± 17 90 ± 16 88 ± 17 90 ± 17 93 ± 18 94 ± 16 75 ± 12 73 ± I I 81 ± 16 79 ± 16 56 ± 7 55 ± 7

95 ± 17 96 ± 17

91 ± 12Q,b 87 ± 12 102 ± 16 101 ± 17

83 ± 14 81 ± 13 88 ± 14 86 ± 13

113 ± 20 115 ± 19 103 ± 18Q I I I ± 19Q,b

86 ± 12 92 ± 12b 90 ± 8 97 ± lOb

% increase

Ipsi Contra

15

20 22

Values represent means ± SD of glucose utilization expressed in �mol 100 g � 1 min � I. obtained in 5 conscious control and 11 conscious STH-stimulated rats.

Q Significances at p < 0 . 05 level, estimated by one-way analysis of variance (Scheffe F test), between control and STH-stimulated groups.

b Side-to-side significant differences at the p < 0. 05 level, calculated by the paired t test, within each of the two groups.

citatory effect recorded in the striatum following chemical stimulation of the STH (Robledo and Feger, 1990).

Effects elicited in motor areas

The lack of metabolic activation in the peribra­chial pedunculopontine tegmental nucleus after STH stimulation in the present study is surprising, given that reciprocal connections between the STH and the pedunculopontine nucleus have been dem­onstrated in the rat (Beckstead et al., 1979; Jackson and Crossman, 1981b; Hammond et al., 1983a; Rye et al., 1987; Takada et al., 1988; Granata and Kitai, 1989; Canteras et al., 1990). If the ventral part of the zona incerta was partially stimulated in our exper­iments, the reciprocal connections between the zona incerta and the deep layers of the superior colliculi could have contributed to the collicular ac­tivation observed at present. However, this possi­bility is not very likely, because the medullary, pon­tine, mesencephalic, and thalamic reticular areas, the precerebellar, cerebellar, peri oculomotor , pre-

tectal, and hypothalamic regions, the ventromedial and anterolateral thalamic nuclei, and the primary motor and somatosensory cortical structures, all ar­eas connected with the zona incerta, remained un­affected in our study (Ricardo, 1981; Roger and Ca­dusseau, 1985; Shammah-Lagnado et al., 1985; Nicoledis et al., 1992). Since no direct sub­thalamocollicular projection has been described, the collicular metabolic activation may be a second­order effect, attributed to an excitation of the nigro­tectal pathway (Beckstead et al., 1979; Chevalier et al., 1981). This metabolic activation is similar to that induced by nigroreticulata stimulation (Savaki et al., 1983b) and most probably reflects the acti­vated nigrotectal terminals rather than any in�ibited tectal cells (Schwarz et al., 1979; Mata et al., 1980; Kadekaro et al., 1985; Savaki, 1989). The lack of effect in the ipsilateral ventromedial thalamic nu­cleus, which is also a major nigral projecting area in the rat (Clavier et aI., 1976; Herkenham, 1979), could be explained by the fact that the nigrotha­lamic and the nigrotectal projections originate from different subregions of the substantia nigra reticu-

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140 M. TZAGOURNISSAKIS ET AL.

TABLE 3. Bilateral effects of unilateral subthalamic nucleus (STH) stimulation on local cerebral glucose utilization in sensory and white matter structures

Control STH stimulation % increase

Ipsi Contra Ipsi Contra Ipsi Contra

Sensory system Vestibular nucleus 110 ± 18 109 ± 18 116 ± 18 114 ± 18 Nucleus solitary tract 57 ± 4 56 ± 4 59 ± 8 59 ± 8 Nucleus ambiguus 61 ± 6 61 ± 6 59 ± 8 60 ± 8 Superior colliculus (superficial layers) 81 ± 12 82 ± \3 91 ± 16 91 ± 18 Lateral geniculate 82 ± 7 88 ± 6a 90 ± 18 95 ± 20a Visual cortex (layer IV) 89 ± 10 88 ± 8 86 ± 15 91 ± 14a Thalamic ventrobasal complex 81 ± 5 88 ± 7a 90 ± 15 102 ± 17a Thalamic lateroposterior nucleus 87 ± 9 98 ± lOa 99 ± 17 115 ± 17a Parietal cortex (IV) 83 ± 9 86 ± 9" 89 ± 10 95 ± lOa Sensorimotor cortex (sensory IV) 89 ± I I 96 ± 7" 94 ± 15 104 ± 14a

White matter Cerebellar white 34 ± 5 33 ± 5 33 ± 4 33 ± 4 Internal capsule 37 ± 3 37 ± 5 35 ± 4 35 ± 4 Corpus callosum 30 ± 2 29 ± 2 29 ± 4 29 ± 4

Values represent means ± SD of glucose utilization expressed in j.Lmol 100 g - I min - I, obtained in 5 conscious control and 11 conscious STH-stimulated rats.

a Side-to-side significant differences at the p < 0.05 level, calculated by the paired t test, within each of the two groups. b Significances at p < 0.05 level, estimated by one-way analysis of variance (Scheffe F test), between control and STH-stimulated

groups.

lata (Kernel et aI., 1988; Takada et aI., 1988). The activation of the ventrolateral motor thalamic nu­cleus bilaterally, after unilateral STH stimulation in our study, is particularly noteworthy. Since no di­rect connection has been observed between the STH and the ventrolateral thalamic nucleus in the rat, the significant metabolic activation estimated within this thalamic nucleus may have been medi­ated indirectly, via the activated pallidum (Severin et aI., 1976; Carter and Fibiger, 1978; Van der Kooy and Carter, 1981). This finding complements previ­ous semiquantitative data from a metabolic study on experimental ballism in the monkey (Mitchell et aI., 1985b) and is similar to that observed following

unilateral pharmacological manipulation of the sub­stantia nigra pars reticulata in the rat (Dermon et aI. , 1990, 1992; Savaki et aI., 1992). It is suggested that the ventrolateral motor thalamic nucleus may play some important role in transferring information to the contralateral hemisphere. The asymmetries observed in the motor cortex were the same in both control and experimental groups, reflecting a non­specific effect of the electrode that passes through the same structures in all stimulated and sham­operated animals. Given the well established ros­trocortical projections to the STH (Afsharpour, 1985; Rouzaire-Dubois and Scarnati, 1985; Can­teras et aI., 1988; Ryan and Clark, 1991), the failure

TABLE 4. Bilateral effects of unilateral subthalamic nucleus (STH) stimulation on local cerebral glucose utilization in reticular and intra laminar nuclei

Control

Ipsi Contra

Medullary reticular formation 56 ± 3 54 ± 3 Pontine reticular formation 56 ± 4 57 ± 5 Locus ceruleus 68 ± 7 70 ± 9 Raphe median 88 ± 12 90 ± 12 Raphe dorsal lateral (median) 79 ± I I Dorsal tegmental nucleus 99 ± 11 100 ± 10 Thalamic reticular nucleus 70 ± 8 76 ± loa Thalamic centromedian (median) 85 ± 7 Thalamic parafascicular nucleus 79 ± 5 80 ± 4 Thalamic centrolateral nucleus 90 ± 10 92 ± 9

STH stimulation

Ipsi Contra

59 ± 7 58 ± 7 62 ± 10 62 ± 10 68 ± 7 69 ± 10 90 ± 16 91 ± 16

89 ± 15 105 ± 18 106 ± 18

79 ± 22 85 ± 21a 95 ± 19

93 ± 14b 90 ± 14 101 ± 19 105 ± 19a

% increase

Ipsi Contra

18

Values represent means ± SD of glucose utilization expressed in j.Lmol 100 g-I min-I, obtained in 5 conscious control and 11 conscious STH-stimulated rats.

a Side-to-side significant differences at the p < 0.05 level, calculated by the paired t test, within each of the two groups. b Significances at p < 0.05 level, estimated by one-way analysis of variance (Scheffe F test), between control and STH-stimulated

groups.

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EFFECTS OF SUBTHALAMIC NUCLEUS STIMULATION 141

TABLE S. Bilateral effects of unilateral subthalamic nucleus (STH) stimulation on local cerebral glucose utilization in limbic and prefrontal nuclei

Hippocampus (moL layers) Dentate gyrus Amygdaloid complex Mamillary complex (median) Interpeduncular nucleus (median) Septum medial Lateral habenula Medial habenula Hypothalamus (posterior) Ventral tegmental area Thalamic anteromedial nucleus Thalamic anteroventral nucleus Thalamic intraanteromedian

nucleus (median) Thalamic mediodorsal nucleus Prefrontal medial cortex (IV) Anterior cingulate cortex Posterior cingulate cortex Entorhinal cortex Pyriform cortex

Control

Ipsi Contra

73 ± 7 72 ± 7 65 ± 5 64 ± 3 43 ± 4 43 ± 4

102 ± 15 96 ± 14

59 ± 8 60 ± 8 109 ± 22 114 ± 26

67 ± loa 61 ± 6 46 ± 9 46 ± 10 52 ± 8 51 ± 8

123 ± 19 124 ± 21 106 ± 22 107 ± 24

120 ± 23 106 ± 15 106 ± 15

91 ± 12 95 ± 13a 99 ± 18 101 ± 18 91 ± 14 97 ± 16a 72 ± 8 72 ± 6 62 ± 9 63 ± 7

STH stimulation

Ipsi Contra

73 ± 10 74 ± 10 65 ± 12 63 ± 9 46 ± 5 46 ± 5

114 ± 18 94 ± 13

64 ± 8 63 ± 8 119 ± 23 120 ± 22

69 ± 7a 61 ± 6 46 ± 3 45 ± 3 47 ± 8 47 ± 8

133 ± 23 133 ± 23 136 ± 22b 136 ± 24b

129 ± 24 125 ± 25 125 ± 25

91 ± 11 96 ± 11a 99 ± 12 104 ± 12a

113 ± 16b 117 ± 16a,b

72 ± 9 73 ± 9 62 ± 7 62 ± 6

% increase

Ipsi Contra

28 27

24 21

Values represent means ± SD of glucose utilization expressed in fLmol 100 g-I min -I, obtained in 5 conscious control and 11 conscious STH-stimulated rats,

a Side-to-side significant differences at the p < 0,05 level, calculated by the paired t test, within each of the two groups, b Significances at p < 0,05 level, estimated by one-way analysis of variance (Scheffe F test), between control and STH-stimulated

groups,

of the STH stimulation to induce any metabolic change in motor cortical areas indicates that the antidromic ally excited widespread projections (e. g. , corticosubthalamic) are mapped by the e4C]DG method to a much smaller degree than the orthodromic ally excited well focused projections (e.g., subthalamonigral).

Effects elicited in sensory, reticular, and

prefrontal areas

The sensory, reticular, and prefrontal areas ex­amined in the STH -stimulated group of rats did not display any significant changes in metabolic activity when compared with the respective areas in the control group. The fact that the STH stimulation did not induce any metabolic changes in sensory and reticular regions indicates further that the electrical current did not spread in the adjacent zona incerta, which is known to be a major reticular and somato­sensory relay area in the rat (Roger and Cadusseau, 1985; Shammah-Lagnado et aI., 1985; Nikoledis et aI., 1992).

Effects elicited in intralaminar and limbic areas

The metabolic activation observed in the parafas­cicular thalamic nucleus may have been caused by the antidromic excitation of the direct parafascicu­losubthalamic projection (Sugimoto et aI., 1983; Canteras et aI., 1990; Groenewegen and Berendse, 1990). However, since the strongly activated sub-

stantia nigra is known to project to the parafascic­ular thalamic nucleus (Clavier et aI., 1976; Beck­stead et aI., 1979; Gerfen et aI., 1982; Cornwall and Phillipson, 1988), the indirect orthodromic excita­tion of the subthalamonigroparafascicular polysyn­aptic pathway may have participated. The partici­pation of the parafascicular thalamic nucleus in the limbic-related functions of the basal ganglia has been postulated in the past (Sadikot et aI., 1992). Furthermore, a direct relation between the STH and the limbic system is indicated by our finding that the anteroventral thalamic nucleus and the re­ciprocally connected with its posterior cingulate cortical area (Domesick, 1969, 1972; Jones and Leavitt, 1974; Seki and Zyo, 1984) were bilaterally activated after electrical stimulation of the STH. These effects are similar to those elicited by unilat­eral pharmacological manipulation of the substantia nigra pars reticulata (Dermon et aI., 1992; Savaki et aI., 1992). This limbic thalamocortical activation in­duced by the STH stimulation at present supports the hypothesis that the STH is associated with lim­bic processes (Canteras et aI., 1990; Groenewegen and Berendse, 1990).

CONCLUSIONS

The present results confirm the STH control on basal ganglia components and the specific topo­graphic organization of the subthalamic projections

J Cereb Blood Flow Metab. Vol, 14. No, 1. 1994

142 M. TZAGOURNISSAKIS ET AL.

to the substantia nigra pars reticulata and the globus pallidus. Based on the metabolic activations in­duced in the nigrocollicular system as well as in the globus pallidus and the ventrolateral thalamic nu­cleus, we have further evidence to support the role of the STH in the realm of the motor system. Based on the metabolic activations induced in the limbic­innervated parts of the basal ganglia, entopeduncu­lar nucleus and ventromedial striatum, as well as in the limbic-related areas, parafascicular and an­teroventral thalamic nucleus and posterior cingulate cortex, we have strong evidence to suggest that the function of the STH relates to the limbic sphere of the brain. In conclusion, the present study indicates that (a) the motor nigrocollicular and pallidotha­lamic projecting systems and (b) the limbic thalamo­cortical paths share the STH as a common relay. Consequently, it can be suggested that the STH is a site where the central motor information is accessi­ble to the limbic system.

Acknowledgment: This work was supported by the Re­gional Operational Program of Crete, European Commu­nity Support Framework 1989-93, and by a grant from the Greek Ministry of Health. We thank Mr. Philip Georgo­poulos, Mr. Vassilis Raos, and Mrs. Patricia Pizarro for technical assistance.

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