Monosodium glutamate-induced arcuate nucleus damage affects both naturaltorpor and 2DG-induced torpor-like hypothermia in Siberian hamsters

Kimberly M. Pelz,1 David Routman,1 Joseph R. Driscoll,1 Lance J. Kriegsfeld,1,2 and John Dark1

1Department of Psychology and 2Helen Wills Neuroscience Institute, University of California, Berkeley, California

Submitted 2 June 2007; accepted in final form 15 October 2007

Pelz KM, Routman D, Driscoll JR, Kriegsfeld LJ, Dark J.Monosodium glutamate-induced arcuate nucleus damage affects bothnatural torpor and 2DG-induced torpor-like hypothermia in Siberianhamsters. Am J Physiol Regul Integr Comp Physiol 294: R255–R265,2008. First published March 1, 2007; doi:10.1152/ajpregu.00387.2007.—Siberian hamsters (Phodopus sungorus) have the ability to expressdaily torpor and decrease their body temperature to �15°C, providinga significant savings in energy expenditure. Daily torpor in hamstersis cued by winterlike photoperiods and occurs coincident with theannual nadirs in body fat reserves and chronic leptin concentrations.To better understand the neural mechanisms underlying torpor, Sibe-rian hamster pups were postnatally treated with saline or MSG toablate arcuate nucleus neurons that likely possess leptin receptors.Body temperature was studied telemetrically in cold-acclimated(10°C) male and female hamsters moved to a winterlike photoperiod(10:14-h light-dark cycle) (experiments 1 and 2) or that remained in asummerlike photoperiod (14:10-h light-dark cycle) (experiment 3). Inexperiment 1, even though other photoperiodic responses persisted,MSG-induced arcuate nucleus ablations prevented the photoperiod-dependent torpor observed in saline-treated Siberian hamsters. MSG-treated hamsters tended to possess greater fat reserves. To determinewhether reductions in body fat would increase frequency of photope-riod-induced torpor after MSG treatment, hamsters underwent 2 wk offood restriction (70% of ad libitum) in experiment 2. Although foodrestriction did increase the frequency of torpor in both MSG- andsaline-treated hamsters, it failed to normalize the proportion of MSG-treated hamsters undergoing photoperiod-dependent torpor. In exper-iment 3, postnatal MSG treatments reduced the proportion of hamstersentering 2DG-induced torpor-like hypothermia by �50% comparedwith saline-treated hamsters (38 vs. 72%). In those MSG-treatedhamsters that did become hypothermic, their minimum temperatureduring hypothermia was significantly greater than comparable saline-treated hamsters. We conclude that 1) arcuate nucleus mechanismsmediate photoperiod-induced torpor, 2) food-restriction-induced tor-por may also be reduced by MSG treatments, and 3) arcuate nucleusneurons make an important, albeit partial, contribution to 2DG-induced torpor-like hypothermia.

thermoregulation; leptin; neuropeptide Y; body mass; fat

SIBERIAN HAMSTERS UNDERGO numerous physiological and behav-ioral changes when experiencing a winterlike photoperiod witha short photophase (SP) and low ambient temperatures (Ta),including bouts of shallow, daily torpor (e.g., Ref. 23). Dailytorpor is a form of reversible hypothermia that occurs duringthe rest/sleep phase of the circadian cycle, coincident with thetime of the circadian minimum in body temperature (Tb). Infact, it is accepted that this represents an exaggeration of theusual 1–3°C sleep-dependent decrease in Tb (13, 52). Duringdaily torpor, however, Tb may decrease by as much as �20°C

for 5–8 h (27, 43). Siberian hamsters expressing torpor typi-cally do so 2–3 times a week (43). Expression of daily torporgenerates a significant savings in energy expenditure, thusdecreasing energy intake requirements in winter (21, 41).

Although most photoperiodic responses (e.g., reduced foodintake and reproductive regression) are initiated within severalweeks of the onset of a winterlike photoperiod, torpor is notinitiated for �12 wk (e.g., Ref. 23). This time coincides withthe nadir in body mass (4, 6, 42) and minimal white adiposetissue reserves (5). Reduced fat reserves appear necessary forphotoperiod-dependent daily torpor because high leptin con-centrations suppress torpor expression (19, 20, 22). Only Si-berian hamsters with markedly reduced serum leptin concen-trations (�2.6 ng/ml) enter torpor, suggesting that chronicallylow leptin concentrations are a permissive factor for torporonset (19).

Reduced leptin concentrations are required for the exagger-ated circadian decrease in Tb during daily torpor, and ob/obmice, which are leptin deficient, undergo a significantly greaterdecrease in circadian Tb min than do lean mice. Even thoughfood restriction decreases Tb min in both lean and ob/ob mice, itis reduced to a greater degree in ob/ob mice (28). Exogenousleptin treatment, on the other hand, prevents food restrictionfrom decreasing Tb min in lean mice (14). Suckling-age rat pupsundergo similar decreases in light/rest phase Tb and metabolicrate. This decrease is due to a temporary shutdown of sympa-thetic mediated nonshivering thermogenesis in brown adiposetissue (BAT) (33, 38) and, as in adult animals, is blocked byleptin treatments (48). Repeated postnatal monosodium gluta-mate (MSG) treatments produce a specific pattern of neuraldegeneration primarily focused in the hypothalamic arcuatenucleus (ARC) (e.g., Refs. 7 and 26), especially targeting ARCneuropeptide Y and proopiomelanocortin neurons, which bothcolocalize leptin receptors but have opposing effects on me-tabolism (9, 26, 31). MSG ablation of the ARC eliminates“torpor-like” circadian decreases in Tb in suckling rats (45).

The purpose of the present series of studies was to determinewhether MSG-induced ablation of ARC mechanisms wouldeliminate photoperiod-dependent torpor, whose onset appearsdependent upon chronically reduced leptin feedback as a per-missive factor. ARC MSG lesions have been successfullyproduced in Siberian hamsters, demonstrating that SP-inducedchanges in hamster body mass, food intake, fur color, and testismass persist after ARC ablation (16). If MSG ablations inSiberian hamsters mimic the effect on circadian Tb min insuckling rat pups (45), then ARC MSG ablations shouldeliminate photoperiod-dependent torpor in hamsters. Differ-

Address for reprint requests and other correspondence: J. Dark, PsychologyDept., Box 1650, Univ. of California, Berkeley, CA 94720-1650, USA (e-mail:johndark@berkeley.edu).

The costs of publication of this article were defrayed in part by the paymentof page charges. The article must therefore be hereby marked “advertisement”in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Am J Physiol Regul Integr Comp Physiol 294: R255–R265, 2008.First published March 1, 2007; doi:10.1152/ajpregu.00387.2007.

0363-6119/08 $8.00 Copyright © 2008 the American Physiological Societyhttp://www.ajpregu.org R255

ences in fat reserves between MSG-and saline-treated hamstersraised in SPs were previously nonsignificant (16). Neverthe-less, to eliminate the possibility that any reduced frequency oftorpor in MSG-treated hamsters may be due to enlarged fatreserves and, hence, greater leptin concentrations, a secondexperiment was undertaken. Occurrence of torpor in SP-ex-posed Siberian hamsters was also tested after food restriction todetermine whether reducing body fat would normalize torporfrequency.

The nonmetabolizable glucose analog, 2-deoxy-D-glucose(2DG), disrupts cellular glucose oxidation, producing gluco-privation (e.g., Ref. 54). Large doses of 2DG (2,500 mg/kgbody mass) induce torpor-like hypothermia in both Siberianhamsters, a placental mammal (11), and Cercarteus nanus, ahibernating marsupial mammal (53). In both species, hypother-mia is a regulated decrease in Tb setpoint whose depth andduration are characteristic of daily torpor (10, 53), even thoughthe latter species is a hibernator. Because 2DG-induced hypo-thermia and natural torpor are both regulated changes in Tb

with numerous common characteristics, it is likely they share acommon mechanism despite possessing different triggeringmechanisms. For this reason, hypothermia was examined fol-lowing systemic 2DG injections in MSG- and saline-treatedSiberian hamsters in a third experiment.

MATERIALS AND METHODS

This research was performed in research facilities approved by theAssociation for the Assessment and Accreditation of LaboratoryAnimal Care. All experimental procedures were reviewed and ap-proved by the University of California Animal Care and Use Com-mittee and conform to guidelines established by the National Institutesof Health.

Animals

Ninety-two adult male and female Siberian hamsters reared in anLP at Ta � 22 � 1°C were used. All hamsters were singly housed atweaning in polypropylene tub cages with Care Fresh bedding (Harlan,San Diego, CA) and were provided food (Purina Rodent Chow #5015)and water ad libitum.

Monosodium Glutamate-Induced Arcuate Nucleus Ablation

Siberian hamster dams and their newborn litters were assigned tothe study, and individual pups were randomly assigned to the MSG orsaline treatment groups and differentially marked with permanent inkfor identification. As litters were assigned, an attempt was made tokeep the number of males and females balanced.

MSG solution (L-glutamic acid monohydrate sodium, Sigma-Al-drich, St. Louis, MO; 80 mg/ml sterile saline) was used as describedby Ebling et al. (16) for Siberian hamsters. MSG-treated hamsterswere injected subcutaneously with 4 mg MSG/g body mass, andsaline-treated hamsters were injected with 0.05 ml sterile saline/gbody mass on postnatal days 4, 5, 6, and 7. At weaning, pups werehoused individually and remained in an LP at 22°C.

2DG Treatment

2DG (Sigma-Aldrich) was used at a dose of 2,500 mg/kg bodymass. Adult male and female Siberian hamsters (n � 59) receivedintraperitoneal injections of 2.5 mg 2DG/g body mass as a solution of2.5 mg 2DG/0.02 ml sterile saline. They also received control injec-tions of 0.02 ml saline/g body mass in a counterbalanced design.

Measuring Tb and Criterion for Torpor

To measure Tb, hamsters underwent a brief surgical procedureunder a ketamine based anesthesia [0.34 ml solution/100 g body mass(21 mg ketamine � 2.4 mg xylazine � 0.3 mg acepromazine/ml)].The midline abdomen was shaved and incised and a radiotransmitterfor telemetric recording of Tb (Model VM-FH-LT, Minimitter, Sun-river OR) was inserted into the abdomen. The peritoneum and skinwere closed with sterile sutures. At the end of surgery, hamsters wereinjected with 0.1 ml buprenorphine (0.015 mg/ml) as an analgesic toalleviate postsurgical discomfort.

Each animal’s cage was placed on a separate receiver board, and Tb

was averaged and recorded every 10 min (Dataquest, St. Paul, MN).Individual Tb records were examined after treatments and minimumTb (Tb min) was defined as the lowest Tb observed during the imme-diate 2 h after treatment or the absolute minimum Tb observed iftorporlike hypothermia occurred. Torpor was defined as a Tb �32.0°C for 30 consecutive min (36, 42), and torpor duration wasdefined as total time from 1st Tb � 32.0°C to 1st Tb � 32.0°C.

Fur Color Index

To provide verification of photoperiodic responding in MSG- andsaline-treated Siberian hamsters, fur (pelage) was rated every 1–2 wkusing a four-stage color scale developed for moulting Siberian ham-sters (15).

Histological Analysis

For immunohistochemical analysis of neuropeptide Y (NPY), ham-sters were given a lethal dose of pentobarbital sodium, and whendeeply anesthetized, they were transcardially perfused with 0.1 MPBS followed by 4% paraformaldehyde buffered with PBS. Brainswere postfixed in 4% paraformaldehyde in PBS for 2 h before beingcryoprotected overnight in a 25% sucrose solution in PBS. Because ofthe excessive number of hamsters involved in this research (90�) andthe well-established outcome of postnatal MSG treatments (e.g., Refs.16 and 55), immunohistochemical procedures were undertaken ononly a sampling of the MSG-treated (n � 28) and saline-treated (n �8) hamsters. Brains were sectioned in the coronal plane at 40 �musing a cryostat. Alternate sections were collected into PBS (pH 7.4)to be processed for NPY. Tissue was submersed in 0.5% hydrogenperoxide to reduce endogenous peroxidase activity. Subsequently,samples were incubated in 10% normal goat serum (Vector Labora-tories, Burlingame, CA) at room temperature for 1 h. Followingpreincubation in normal goat serum, the sections were incubated for48 h at 4°C in rabbit anti-NPY (ImmunoStar, Hudson, WI) and diluted1:8,000 in PBS with 0.3% Triton X-100 (Sigma). Sections were thensequentially incubated in biotinylated goat anti-rabbit IgG (1:250;Vector Laboratories) and avidin-biotin-horseradish peroxidase (HRP)complex (Vector Laboratories). HRP label was demonstrated usingnickel-intensified 0.04% diaminobenzidine (Polysciences, Warring-ton, PA) in PBS as the chromogen and 0.01% hydrogen peroxide asthe substrate. Alternate sections were mounted onto gelatin-coatedslides, dehydrated in a graded series of ethanol solutions (30, 70, 95,and 100%), and cleared in Xylenes (Fisher Scientific, Pittsburgh, PA)before the application of coverslips. Slides were examined for NPYimmunoreactivity using a Zeiss Z1 microscope.

Optical Density Measurement

The optical density of NPY fiber staining in the ARC and para-ventricular nucleus of the hypothalamus (PVN) was measured. Eachpixel in the grayscale image capture has a measurable specific inten-sity, with each pixel having a value ranging from 0 (white) to 256(black). The average value for all pixels in an outlined area is taken asthe mean intensity of staining for a given region of the image. Opticaldensity measures were normalized to minimize differences betweenreplications of immunohistochemistry. First, a background measure-

R256 ARCUATE NUCLEUS LESIONS AND TORPOR

AJP-Regul Integr Comp Physiol • VOL 294 • JANUARY 2008 • www.ajpregu.org

ment was taken by placing a square outline, 4 times, on nonoverlap-ping, unstained areas of each section. The mean of these 4 measuresprovided the background optical density for each section. The ARC orPVN was outlined, and optical density of these regions was obtained.All mean background optical density measures were subtracted fromthe mean value for ARC or PVN staining.

Statistical Analyses

Proportions (%) entering torpor were analyzed with Chi-square orFisher exact test. Correlations were determined with Pearson productmoment. Tb min (all animals) during 2DG testing was analyzed byone-way repeated measures ANOVA. All other comparisons weremade with one- or two-way ANOVA (or ANOVA on ranks) withappropriate post hoc pairwise comparisons or by t-test (or rank sumtest). Actual values for most statistical comparisons have been omittedfor the sake of clarity. All comparisons were two-tailed and consid-ered statistically significant if P � 0.05.

Procedures

Experiment 1: Photoperiod-dependent torpor and MSG ablation ofarcuate nucleus. (Note: These animals participated in the 2DG exper-iment prior to being moved to the SP.) Adult Siberian hamsterspostnatally treated with MSG (n � 33) or saline (n � 19) andpreviously acclimated to Ta � 10°C were switched to a photoperiodwith an SP (10:14-h light-dark cycle). Four days before the change inphotoperiod (week 0) and every 1–2 wk thereafter, body mass and FurColor Index were measured. Hamsters underwent an additional sur-gical procedure, if necessary, to replace a malfunctioning transmitteror an exhausted battery. Tb records were closely watched for the firstappearance of spontaneous torpor, which typically appears after �12wk of SPs (23). Torpor bouts were analyzed for Tb min and duration oftorpor.

At week 18, the study was terminated. The hamsters were deeplyanesthetized before the gonadal and retroperitoneal white adiposetissue depots were excised and weighed, and the hamsters weretranscardially perfused for histological procedures. The experimentwas conducted using two cohorts of hamsters (n � 14 and n � 38).Although Tb records are available for all 52, fat depot mass data areonly available for the latter 38.

Experiment 2: Food-restriction-induced torpor and MSG ablationsof arcuate nucleus. At weaning, saline-treated (n � 12) and MSG-treated (n � 22) pups were housed individually and moved to a roomwith an SP as before but Ta � 22°C. Fur Color Index was evaluatedat this time (week 0) and every 2 wk until week 8 then every weekthereafter. The hamsters were anesthetized and underwent a briefsurgical procedure during week 9 to place a transmitter in the abdo-

men for recording Tb as described above. At the end of 10-wkadaptation to SPs, the hamsters were moved to an environmentalchamber with the same SP but Ta � 10°C.

At week 14 (4 wk in the cold), weekly food intake of all hamsterswas determined (from week 14 to 15), and average daily food intakewas calculated. All hamsters were provided the daily allotment offood, representing 70% of their previous average daily intake (30%reduction) beginning at week 16. If food restriction severely reducedbody mass (body mass �25 g for females and body mass � 30 g formales), food-restriction was changed to 85% of ad libitum or discon-tinued. Food-restriction lasted 10–14 days.

At week 18, body mass and Fur Color Index were recorded. Allhamsters were then given a lethal injection of pentobarbital sodiumand when deeply anesthetized gonadal and retroperitoneal fat depotswere excised and weighed before the hamsters were transcardiallyperfused for immunohistochemical verification of MSG ablations. Tb

data were analyzed with particular emphasis on week 16–18.Experiment 3: 2DG-induced hypothermia and MSG ablation of

arcuate nucleus. At �30 days of age, the saline- and MSG-treatedhamsters subsequently used in experiment 1 underwent a brief surgeryto implant radiotransmitters while under ketamine anesthesia. Afterrecovery from surgery, the hamsters were housed in an environmentalchamber with an identical LP but Tb � 10°C.

After 3 wk acclimation to Ta � 10°C, the MSG-treated (n � 37)and saline-treated (n � 21) hamsters were each randomly divided intotwo groups. During the first week, one group was tested with 2DG,and the other group with sterile saline vehicle; treatments werereversed in the 2nd week for a counterbalanced design in which eachhamster receives each treatment. Tb records were examined for thepresence or absence of torporlike hypothermia. Because male andfemale Siberian hamsters respond comparably to 2DG in LPs (10),their data were combined for analysis.

RESULTS

Histology

MSG ablations were verified by NPY immunohistochemis-try in all MSG-treated Siberian hamsters entering torpor (n �15), a randomly selected comparable number of MSG-treatedhamsters never entering torpor (n � 13), and a random sampleof saline-treated hamsters (n � 7). Brain sections evidencingNPY-positive fibers and cells in the ARC (Fig. 1A) and fibersin the PVN (Fig. 1D) are shown in a representative saline-treated control hamster, an MSG-treated hamster expressingtorpor (Fig. 1, B and E), and an MSG-treated hamster never

Fig. 1. Photomicrographs of brain sections demon-strating labeled NPY fibers and cell bodies in the Arcand NPY fibers in the PVN of a representative saline-treated hamster (A and D, respectively), a representa-tive MSG-treated hamster expressing torpor (B and E,respectively), and a representative MSG-treated ham-ster never entering torpor (C and F, respectively). Scalebar � 100 �m. (Arc, arcuate nucleus of the hypothala-mus; PVN, paraventricular nucleus of the hypothalamus;VMH, ventromedial nucleus of the hypothalamus; V3,3rd ventricle.)

R257ARCUATE NUCLEUS LESIONS AND TORPOR

AJP-Regul Integr Comp Physiol • VOL 294 • JANUARY 2008 • www.ajpregu.org

entering torpor (Fig. 1, C and F). There was a �22% diminu-tion of NPY-positive cells and fibers in the ARC of MSG-treated vs. saline-treated hamsters, as evidenced by densitom-etry measurements (Fig. 2A; P � 0.05). Within MSG-treatedhamsters, there were no differences in NPY immunoreactivitybetween those hamsters entering torpor and those never under-going torpor (Fig. 2A; P � 0.05). Further, within MSG-treatedhamsters entering torpor, there was no correlation betweendensity of surviving ARC NPY immunoreactivity and thefrequency of torpor (r � �0.31, P � 0.05). Similar outcomeswere observed for NPY immunoreactivity in the PVN, aprimary target of ARC NPY-ergic projections (Fig. 2B).

Experiment 1: photoperiod-dependent torpor and MSG ab-lations of the ARC. Eighteen weeks of SPs significantly in-creased Fur Color Index in both male and female hamsters

(P � 0.05, for both; Fig. 3A). Although MSG treatmentmoderated the degree of color change in males (P � 0.05),female hamsters did not differ in fur color responsivenessbetween postnatal treatments (Fig. 3A).

The proportion of hamsters undergoing torpor-like hypo-thermia was significantly reduced in MSG-treated Siberianhamsters vs. those postnatally treated with saline (2 � 11.05,P � 0.05; Fig. 4A). The incidence of photoperiod-dependenttorpor in Siberian hamsters was more than 3 times more likelyto occur in saline-treated (68%) than MSG-treated hamsters(18%). In addition, among only those hamsters entering torporand with uninterrupted data during the 6-wk observation pe-riod, Tb min during torpor was significantly lower in saline-treated than MSG-treated hamsters (P � 0.05; Table 1). Torporduration was correspondingly twice as long in saline- vs.MSG-treated hamsters (P � 0.05), but torpor frequency did notdiffer statistically between groups (Table 1).

Fig. 2. A: comparison of densitometry measurements in the arcuate nucleus ofNPY fiber/cell immunoreactivity between Siberian hamsters treated postna-tally with saline vs. MSG, and a comparison within MSG-treated hamsters ofthe NPY immunoreactivity of those hamsters exhibiting torpor (Torpid) andthose never entering torpor (Non-Torpid). B: comparison of densitometrymeasurements of NPY fiber immunoreactivity in the hypothalamic paraven-tricular nucleus after postnatal saline vs. MSG treatment, and within the MSGtreatment group, a contrast of the NPY immunoreactivity of hamsters express-ing torpor (Torpid) and those remaining euthermic (Non-Torpid). *Signifi-cantly different from saline-treated group.

Fig. 3. A: both MSG- and saline-treated Siberian hamsters demonstrate sig-nificant increases in Fur Color Index characteristic of exposure to SPs.B: longitudinal changes in body mass of the groups during 18 wk of SPs.Although there was a significant effect of sex (male vs. female) within eachtreatment over time, there was not a significant effect of treatment (saline vs.MSG) within either sex over time.

R258 ARCUATE NUCLEUS LESIONS AND TORPOR

AJP-Regul Integr Comp Physiol • VOL 294 • JANUARY 2008 • www.ajpregu.org

There was a significant main effect of sex (male vs. female)for body mass, gonadal fat mass, and retroperitoneal fat mass(ANOVA, P � 0.05, for all; Fig. 5, A–C). Neither body massnor gonadal fat mass evidenced a significant main effect of

Fig. 5. Within sex, there was no significant difference between MSG-treatedvs. saline-treated Siberian hamsters in either body mass (A), gonadal fat mass(B), or retroperitoneal fat mass (C). *Significant main effect of treatment(ANOVA).

Fig. 4. A: a markedly greater proportion of saline-treated hamsters (68%)entered photoperiod-dependent torpor than MSG-ablated hamsters (18%).B: two weeks of food-restriction and consequent weight loss failed to normal-ize the frequency of photoperiod-dependent torpor in MSG-treated hamsters(41%) to that of saline-treated hamsters (92%). (*Significantly different fromMSG-treated group).

Table 1. Torpor characteristics of Siberian hamsterspostnatally treated with saline vs. MSG and enteringphotoperiod-dependent torpor between Week 12 and Week18 of SPs

PostnatalTreatment n

Mean # ofTorpor Bouts Torpid Tb min, °C

Duration ofTorpor, min

Saline 11 11.3�3.0 21.3�0.4 342.7�24.1MSG 5 6.0�1.8 23.5�0.6* 170.1�15.1*

All values are expressed as means � SE. *Statistically significant fromsaline-treated.

R259ARCUATE NUCLEUS LESIONS AND TORPOR

AJP-Regul Integr Comp Physiol • VOL 294 • JANUARY 2008 • www.ajpregu.org

treatment (saline vs. MSG) (P � 0.05, for both; Fig. 5, A andB). There was, however, a main effect of saline vs. MSGtreatment on retroperitoneal fat mass (P � 0.05; Fig. 5C).Despite the significant main effect, there was no effect oftreatment on retroperitoneal fat mass within either sex.

Experiment 2: food restriction-induced torpor and MSGablations of arcuate nucleus. Exposure to 15 wk of SPssignificantly increased Fur Color Index of both male andfemale Siberian hamsters postnatally treated with saline orMSG (P � 0.05, for both; Fig. 6). In neither sex did treatment(saline vs. MSG) affect fur color.

The proportion of hamsters entering torpor after 18 wk ofSPs and 10–14 days of food restriction to 70% of ad libitumintake was significantly reduced in Siberian hamsters postna-tally treated with MSG (Fisher’s exact test, P � 0.05; Fig. 4B).Forty-one percent of MSG-treated hamsters underwent torporduring the interval of food restriction compared with 92% ofsaline-treated hamsters. Food restriction during SPs increasedthe proportion of both MSG- and saline-treated hamsters en-tering torpor compared with similarly treated groups exposedto 18 wk of SPs alone (18% vs. 68%, respectively; experiment1). A few hamsters (n � 9) were unable to complete the entireinterval of food restriction because of excessive weight loss butin those hamsters with uninterrupted data throughout the 14days of food-restriction, torpor frequency, Tb min during torporand duration of torpor did not differ between treatments (P �0.05, for all; Table 2).

Although there was the usual significant sex difference inbody mass and gonadal fat mass in food restriction/SP Siberian

Fig. 7. There were sex differences in body mass (A) and gonadal fat mass (B),but not in retroperitoneal fat mass (C) in food-restricted/SP Siberian hamsters.Postnatal treatment (MSG vs. saline) failed to affect either overall body mass(A) or gonadal (B) or retroperitoneal (C) fat depot mass in SP hamsters after 2wk of food restriction.

Fig. 6. MSG- and saline-treated Siberian hamsters both significantly increaseFur Color Index during 15 wk of SP exposure in experiment 2.

Table 2. Torpor characteristics of Siberian hamsterspostnatally treated with saline vs. MSG and entering torporduring the interval of food restriction

PostnatalTreatment n

Mean No. ofTorpor Bouts Torpid Tb min, °C

Duration ofTorpor, min

Saline 4 2.8�1.2 23.8�1.4 137.3�26.7MSG 10 2.1�0.3 25.1�0.9 113.8�15.0

All values are expressed as means � SE. The data presented here excludehamsters not completing the entire food-restriction interval.

R260 ARCUATE NUCLEUS LESIONS AND TORPOR

AJP-Regul Integr Comp Physiol • VOL 294 • JANUARY 2008 • www.ajpregu.org

hamsters (P � 0.05; Fig. 7, A and B), the typical sex differencein retroperitoneal fat mass was absent (P � 0.05; Fig. 7C).There was no treatment effect on body mass, gonadal fat mass,or retroperitoneal fat mass between MSG- and saline-treatedfood-restricted/SP hamsters (Fig. 7, A–C). In addition, therewas a marked negative correlation between body mass andtorpor occurrence in food-restricted saline-treated male ham-sters (r � �0.99, P � 0.05), but there was no correlationbetween body mass and torpor in male MSG-treated hamsters(r � �0.37, P � 0.05). (An insufficient number of saline-treated females completed the entire interval of food-restrictionfor statistical comparisons.) This lack of correlation is evidentin the Tb records of comparable low body mass (Fig. 8, A andB) and high body mass (Fig. 8, C and D) saline- vs. MSG-treated hamsters.

Experiment 3: 2DG-induced hypothermia and MSG abla-tions of arcuate nucleus. Postnatal MSG treatment significantlyreduced the proportion of hamsters entering hypothermia after2DG injections (2 � 4.78, P � 0.05; Fig. 9). The proportionof MSG-treated hamsters entering 2DG-induced torpor-likehypothermia (38%) was nearly half that of saline-treated con-trols (72%) (Fig. 9).

There was a statistically significant main effect (ANOVA) of2DG vs. vehicle injections on Tb min; 2DG injections markedlydecreased Tb with comparison to vehicle injections (P � 0.05;Table 3). In addition, there was a significant main effect ofpostnatal saline vs. MSG treatment (P � 0.05; Table 3), as wellas a significant interaction effect between treatments (P �0.05). Among all hamsters, 2DG injections decreased Tb min inboth postnatal treatment groups (saline vs. MSG); nevertheless,

Tb min in saline-treated hamsters was significantly lower than inMSG-treated hamsters (P � 0.05; Table 3). Among only2DG-injected hamsters entering torpor-like hypothermia,Tb min was significantly higher in MSG than in saline-treatedhamsters (P � 0.05; Table 3 and Fig. 10), but comparisons oftorpor duration between treatments did not reach statisticalsignificance (P � 0.05; Table 3 and Fig. 10).

Fig. 8. Individual Tb records of four food-restricted/SP Siberian hamsters. In saline-treated hamsters, food-restricted-inducedtorpor is typically seen in light to moderatebody mass hamsters A: torpor was neverobserved in saline-treated hamsters weigh-ing �40 g (C) but was routinely observedin food-restricted MSG-ablated hamstersweighing �40 g (D).

Fig. 9. Postnatal MSG treatments significantly reduced the proportion ofSiberian hamsters undergoing 2DG-induced torpor-like hypothermia (38%)over the proportion of saline-treated hamsters induced by 2DG-injec-tions (72%). *Significantly different from MSG-treated group.

R261ARCUATE NUCLEUS LESIONS AND TORPOR

AJP-Regul Integr Comp Physiol • VOL 294 • JANUARY 2008 • www.ajpregu.org

DISCUSSION

Postnatal MSG treatments markedly reduced the occurrenceof photoperiod-dependent torpor in Siberian hamsters. In thosefew MSG-treated hamsters expressing torpor, torpor was sig-nificantly shallower and of a shorter duration than in saline-treated hamsters. The reduced frequency of torpor in MSG-treated hamsters was not a consequence of increased adipositybecause 2 wk of food restriction that reduced fat mass did notnormalize torpor occurrence to that of saline-treated hamsters.MSG-treatments, thus, appear likely to compromise food re-striction-induced torpor as well. MSG-induced destruction ofARC neurons is the most parsimonious explanation of thedisruption of photoperiod-dependent and food-restriction-in-duced torpor in Siberian hamsters. A conclusion supported bythe similar elimination of food deprivation-induced torpor inNational Institutes of Health Swiss laboratory mice after com-parable ARC ablations due to perinatal MSG treatments (24).

MSG treatments primarily target ARC neurons colocalizingNPY/agouti-related peptide (AgRP) or proopiomelanocortin(POMC)/cocaine and amphetamine-related transcript, both ofwhich possess leptin receptors (9, 26, 31). MSG-induced ARCablations are thus likely to eliminate the leptin receptorsnecessary for the permissive effects of chronically reducedleptin concentrations in photoperiod-dependent torpor. Highserum concentrations of exogenous leptin inhibit the occur-rence of torpor and photoperiod-dependent torpor only occursif leptin concentrations are markedly reduced (�2.6 ng/ml)(19). Postnatal MSG treatments may be ablating both theneuroendocrine transducer translating reduced leptin feedbackinto a permissive signal and effector mechanisms, for example,ARC NPY-ergic pathways, mediating torpor onset. Althoughreduced leptin feedback may be permissive for seasonal onsetof photoperiod-dependent torpor, it nevertheless remains un-clear at this time what specifically triggers this exaggeratedcircadian Tb minimum on one day and not on another. Lowleptin concentrations may be necessary, but they are notsufficient for initiating photoperiod-dependent torpor; leptinconcentrations are comparable on torpid and nontorpid days(19). It also is evidently unrelated to food availability becauseSiberian hamsters routinely undergo photoperiod-dependenttorpor in the laboratory with surplus food in their cages (e.g.,experiment 1).

Elimination of photoperiod-dependent torpor (and possiblyfood restriction-induced torpor as well) is most likely dueto MSG-induced damage to the hypothalamic arcuate nucleus.As mentioned before, the ARC possesses several peptidergicmechanisms that could potentially alter thermoregulation, themost prominent being NPY and POMC, which have opposingactions. POMC is a precursor for -MSH, which inhibits foodintake and enhances energy expenditure by activating themelanocortin-4-receptor (2, 55). AgRP is an endogenous mela-nocortin-4-receptor antagonist, increasing food intake and de-creasing BAT heat production (55). NPY is widely known as apowerful orexigen, but it also reduces energy expenditure and,more importantly, Tb. In fact, intracerebroventricular NPYinjections in homeothermic rats completely suppress sympa-thetic nerve input to BAT (17) and concomitantly decrease Tb

by 1–3°C (30). Comparable intracerebroventricular injectionsof NPY (or NPY Y1 receptor agonist) in heterothermic Sibe-rian hamsters, on the other hand, decrease Tb by as much as20°C, inducing hypothermia resembling natural torpor (36,37). The marked enhancement of food deprivation-induced

Fig. 10. 2DG-induced torpor-like hypothermia in an MSG-ablated Siberianhamster (A) and a saline-treated hamster (B) whose Tb mins are representativeof each respective group’s mean during hypothermia.

Table 3. Minimum body temperature (Tb min) in all animalsafter vehicle vs. 2DG injections in Siberian hamsterspostnatally treated with saline or MSG, and Tb min andtorpor duration in only those saline- and MSG-treatedhamsters entering torpor after 2DG injections

Treatment nTb min, °C

(All Animals)Tb min, °C

(Torpid Only)Duration ofTorpor, min

Saline-control*Vehicle* 21 35.5�0.12DG 22 27.9�1.4† 25.3�1.6 193.3�91.5

MSG-ablatedVehicle 37 35.5�0.12DG 37 31.9�0.4†‡ 29.4�0.5‡ 103.6�16.9

All values are expressed as means � SE. *Statistically significant maintreatments (postnatal treatment and pharmacological treatment; two-wayANOVA). †Statistically different from vehicle control; ‡Statistically differentfrom 2DG group in saline-controls.

R262 ARCUATE NUCLEUS LESIONS AND TORPOR

AJP-Regul Integr Comp Physiol • VOL 294 • JANUARY 2008 • www.ajpregu.org

torpor in mice by ghrelin is eliminated in NPY knockout mice,also suggesting a role for NPY in torpor (24). Although NPYmechanisms and ARC mechanisms may be involved in torporinitiation, a specific role for ARC NPY-ergic pathways isspeculative at best.

Photoperiod-dependent torpor persisted in a small percent-age of ARC-ablated MSG-treated Siberian hamsters. The mostlikely cause may be a small, but critical, population of surviv-ing ARC neurons that mediate torpor. There was no differencein density measurements of ARC NPY cell/fiber immunoreac-tivity between MSG-treated hamsters never entering torpor andMSG-treated hamsters entering torpor, and there was also nocorrelation between torpor prevalence and density of NPYimmunoreactivity within those MSG-treated hamsters becom-ing torpid. The problem with MSG ablations in Siberianhamsters, as detailed by Ebling et al. (16), is the fine linebetween maximizing ARC damage and minimizing pup mor-tality. The MSG dose and injection schedule developed byEbling et al. (16) for Siberian hamsters produces extensiveARC damage but �20% of ARC NPY-positive cells inevitablysurvive MSG treatment. In the present research, ARC NPY celland fiber immunoreactivity of MSG-treated hamsters was sim-ilarly �22% that of saline-treated hamsters (see Figs. 1 and 2).The ARC has repeatedly been shown to be the most sensitivestructure to the neurotoxic effects of MSG (e.g., Refs. 24 and49); nevertheless, damage to other sites has been described,including the area postrema (AP) (49). Even though AP abla-tions are without effect on photoperiod-dependent torpor (1),we cannot definitively rule out extra-ARC damage elsewhere,possibly contributing to the effects of MSG treatments ontorpor.

It might be argued that MSG treatments prevented photope-riod-dependent torpor because of the somewhat greater fatreserves that are typical of MSG-ablated animals (e.g., Refs. 32and 51), thus elevating circulating leptin concentrations suffi-ciently to inhibit torpor. Even though ARC leptin receptorsmay be ablated, there are also leptin receptors in the caudalbrain stem, including the AP, nucleus of the solitary tract(NTS), and the dorsal motor nucleus of the vagal nerve (25).Because ablations of the AP/NTS complex have no effect onexpression of photoperiod-dependent torpor in Siberian ham-sters (1), it is unlikely that adiposity-dependent leptin concen-trations inhibit torpor in MSG-treated hamsters. In addition,food availability restricted to 70% of ad libitum (and reducedfat reserves) in the SP in the present research increased thefrequency of torpor in both groups of food-restricted hamsters,but it did not increase the frequency of torpor in MSG-treatedhamsters to that of those treated with saline. It is unlikely,therefore, that reduced torpor frequency after MSG treatmentwas due to increased adiposity.

While the effect of food restriction cannot be parsed fromthat of photoperiod, it is clear that food restriction increasedtorpor frequency in both treatment groups. The findings alsoindicate that MSG-induced ARC ablations failed to increasethe frequency of food restriction-induced torpor to that ob-served in saline-treated hamsters. In addition, there was inter-estingly no correlation between body mass and torpor inci-dence in MSG-ablated hamsters. Perhaps, this is becauseMSG-induced ARC ablations destroy the primary leptin recep-tor necessary for body fat feedback (12, 24, 50), and anappropriate energetic challenge can initiate torpor regardless of

body fat reserves. Torpor can also be induced by food restric-tion in LPs, and even though it demonstrates several mecha-nistic differences from photoperiod-induced torpor, body massin LP ARC-intact hamsters must still be markedly decreasedbefore torpor is triggered (40).

Daily torpor is triggered in many small mammals, especiallymice, by food deprivation (complete unavailability of food)(see Refs. 29 and 34). Food deprivation can induce torpor inmice in 24 h or less (20, 24, 34). Postnatal MSG treatmentseliminate food deprivation-induced torpor in laboratory mice(24). Whether similar MSG ablations prevent food restriction-induced torpor in Siberian hamsters cannot be determinedconclusively from the present study. Hamster species appear torespond differently to food deprivation and food restrictionthan other small mammal species. For example, neither Sibe-rian (3) nor Syrian hamsters (47) show compensatory increasesin food intake after food-deprivation. Siberian hamsters, sim-ilarly, fail to enter torpor after up to 48 h of food deprivation(J. Dark and K. M. Pelz, unpublished observations). Theidiosyncratic nature of the neural mechanisms underlying re-sponses to food deprivation and food restriction in Siberianhamsters, especially with relation to torpor, remains enigmatic.

MSG ablations significantly decreased the frequency of2DG-induced torporlike hypothermia. This effect is most likelydue to ablation of forebrain, and not hindbrain, mechanisms.Although the AP/NTS is necessary for 2DG-induced foodintake in rats (39) and estrous behavior disruption in Syrianhamsters (44), 2DG-induced torpor-like hypothermia fails tostimulate Fos-like immunoreactivity in the AP/NTS (35) andAP/NTS ablation fails to prevent 2DG-induced torporlike hy-pothermia (1). 2DG treatment, on the other hand, increasesNPY mRNA and NPY turnover in the ARC of rats (46). Theintermediate effect of MSG ablations on 2DG-induced torpor-like hypothermia may reflect torpor initiation based upon awide base of neural systems to which ARC NPY pathwaysmake a substantial contribution. 2DG-induced torpor-like hy-pothermia is a regulated decrease in preferred Tb (10, 53).Because both photoperiod-dependent and food restriction-in-duced torpor also alter Tb setpoint (21), it is likely that at somepoint they all share a common mechanism. This is unlikely, onthe other hand, with regard to torpor initiation in each case.Photoperiod-dependent and food restriction-induced torpor isclearly mediated by peripheral energy shortages, whereas2DG-induced torpor-like hypothermia may more likely repre-sent severe central glucoprivation (e.g., Refs. 8, 18, 35).

ACKNOWLEDGMENTS

We are grateful to Christiana Tuthill, Dennis Kim, and Elanor E. Schoomerfor excellent technical assistance, and we especially thank David Piekarski forhis valuable comments on the manuscript.

GRANTS

This research was supported by National Institutes of Health Grant, NS-30816.

REFERENCES

1. Bae HH, Stamper JL, Heydorn EC, Zucker I, Dark J. Role of the areapostrema in control of torpor in Siberian hamsters. Am J Physiol RegulIntegr Comp Physiol 279: R591–R598, 2000.

2. Balthasar N, Dalgaard LT, Lee CE, Yu J, Funuhashi H, Williams T,Ferreira M, Tang V, McGovern RA, Kenny CD, Christiansen LM,Edelstein E, Choi B, Boss O, Aschkenasi C, Zhang CY, Mountjoy K,

R263ARCUATE NUCLEUS LESIONS AND TORPOR

AJP-Regul Integr Comp Physiol • VOL 294 • JANUARY 2008 • www.ajpregu.org

Kishi T, Elmquist JK, Lowell BB. Divergence of melanocortin pathwaysin control of food intake and energy expenditure. Cell 123: 493–505, 2005.

3. Bartness TJ, Clein MR. Effects of food deprivation and restriction, andmetabolic blockers on food hoarding in Siberian hamsters. Am J PhysiolRegul Integr Comp Physiol 266: R1111–R1117, 1994.

4. Bartness TJ, Elliott JA, Goldman BD. Control of torpor and bodyweight patterns by a seasonal timer in Siberian hamsters. Am J PhysiolRegul Integr Comp Physiol 257: R142–R149, 1989.

5. Bartness TJ, Hamilton JM, Wade GN, Goldman BD. Regional differ-ences in fat pad responses to short days in Siberian hamsters. Am J PhysiolRegul Integr Comp Physiol 257: R1533–R1540, 1989.

6. Bartness TJ, Wade GN. Photoperiodic control of seasonal body weightcycles in hamsters. Neurosci Biobehav Rev 9: 599–612, 1985.

7. Broberger C, Johansen J, Johansson C, Schalling M, Hokfelt T. Theneuropeptide Y/agouti gene-related protein (AGRP) brain circuitry innormal, anorectic, and monosodium glutamate-treated mice. Proc NatlAcad Sci USA 95: 15043–15048, 1998.

8. Buchanan TA, Cane P, Eng CC, Sipos GF, Lee C. Hypothermia iscritical for survival during prolonged insulin-induced hypoglycemia inrats. Metabolism 40: 330–334, 1991.

9. Cheung CC, Clifton DK, Steiner RA. Proopiomelanocortin neurons aredirect targets for leptin in the hypothalamus. Endocrinology 138: 4489–4492, 1997.

10. Dark J, Miller DR, Licht P, Zucker I. Glucoprivation counteracts effectsof testosterone on daily torpor in Siberian hamsters. Am J Physiol RegulIntegr Comp Physiol 270: R398–R403, 1996.

11. Dark J, Miller DR, Zucker I. Reduced glucose availability inducestorpor in Siberian hamsters. Am J Physiol Regul Integr Comp Physiol 267:R496–R501, 1994.

12. Dawson R, Pelleymounter MA, Millard WJ, Liu S, Eppler B. Atten-uation of leptin-mediated effects of monosodium glutamate-induced arcu-ate nucleus damage. Am J Physiol Endocrinol Metab 273: E202–E206,1997.

13. Deboer T, Tobler I. Temperature dependence of EEG frequencies duringnatural hypothermia. Brain Res 670: 153–156, 1995.

14. Doring H, Schwarzer K, Nuesslein-Hildesheim, Schmidt I. Leptinselectively increases energy expenditure of food-restricted lean mice. Int JObes Relat Metab Disord 22: 83–88, 1998.

15. Duncan MJ, Goldman BD. Hormonal regulation of the annual pelagecolor cycle in the Djungarian hamster, Phodopus sungorus. I. Role of thegonads and the pituitary. J Exp Zool 230: 89–95, 1984.

16. Ebling FJP, Arthurs OJ, Turney BW, Cronin AS. Seasonal neuroen-docrine rhythms in the male Siberian hamster persist after monosodiumglutamate-induced lesions of the arcuate nucleus in the neonatal period.J Neuroendocrinol 10: 701–712, 1998.

17. Egawa M, Yoshimatsu H, Bray GA. Neuropeptide Y suppresses sym-pathetic activity to interscapular brown adipose tissue in rats. Am J PhysiolRegul Integr Comp Physiol 260: R328–R334, 1991.

18. Francesconi R, Mager M. 5-Thio-D-glucose: hypothermic responses inmice. Am J Physiol Regul Integr Comp Physiol 239: R214–R218, 1980.

19. Freeman DA, Lewis DA, Kauffman AS, Blum RM, Dark J. Reducedleptin concentrations are permissive for display of torpor in Siberianhamsters. Am J Physiol Regul Integr Comp Physiol 287: R97–R103, 2004.

20. Gavrilova O, Leon LR, Marcus-Samuels B, Mason MM, Castle AL,Refetoff S, Vinson C, Reitman ML. Torpor in mice is induced by bothleptin-dependent and -independent mechanisms. Proc Natl Acad Sci USA96: 14623–14628, 1999.

21. Geiser F. Metabolic rate and body temperature reduction during hiberna-tion and daily torpor. Annu Rev Physiol 66: 239–274, 2004.

22. Geiser F, Kortner G, Schmidt I. Leptin increases energy expenditure ofa marsupial by inhibition of daily torpor. Am J Physiol Regul Integr CompPhysiol 275: R1627–R1632, 1998.

23. Goldman BD, Darrow JM, Duncan MJ, Yogev L. Photoperiod, movereproductive hormones, and winter torpor in three hamster species. In:Living in the Cold: Physiological and Biochemical Adaptations, edited byHeller HC, Musacchia XJ, and Wang LCH. New York: Elsevier, 1986.

24. Gluck EF, Stephens N, Swoap SJ. Peripheral ghrelin deepens torporbouts in mice through arcuate nucleus neuropeptide Y signaling pathway.Am J Physiol Regul Integr Comp Physiol 291: R1303–R1309, 2006.

25. Grill HJ, Kaplan JM. Interoceptive and integrative contributions offorebrain and brainstem to energy balance control. Int J Obes 25 Suppl 5:S73–S77, 2001.

26. Hahn TM, Breininger JF, Baskin DG, Schwartz MW. Coexpression ofAgRP and NPY in fasting-activated hypothalamic neurons. Nat Neurosci1: 271–272, 1998.

27. Heldmaier G, Klingenspor M, Werneyer M, Lampi BJ, Brooks SPJ,Storey KB. Metabolic adjustments during daily torpor in the Djungarianhamster. Am J Physiol Endocrinol Metab 276: E896–E906, 1999.

28. Himms-Hagen J. Food restrictions increases torpor and improves brownadipose tissue thermogenesis in ob/ob mice. Am J Physiol EndocrinolMetab 248: E531–E539, 1985.

29. Hudson JW. Shallow daily torpor: a thermoregulatory adaptation. In:Strategies in the Cold, edited by Hudson JW, and Wang LCH. New York:Academic, 1978.

30. Jolicoeur FB, Bouali SM, Fournier A, St-Pierre S. Mapping of hypo-thalamic sites involved in the effects of NPY on body temperature andfood intake. Brain Res Bull 36: 125–129, 1995.

31. Mercer JG, Hoggard N, Williams LM, Lawrence CB, Hannah LT,Morgan PJ, Trayhurn P. Co-expression of leptin receptor and prepro-neuropeptide Y mRNA in arcuate nucleus of mouse hypothalamus. J Neu-roendocrinol 8: 733–735, 1996.

32. Morris MJ, Tortelli CF, Filippis A, Proietto J. Reduced BAT functionas a mechanism for obesity in the hypophagic, neuropeptide Y deficientmonosodium glutamate-treated rat. Regul Pept 75–76: 441–447, 1998.

33. Nuesslein-Hildesheim B, Schmidt I. Is the circadian core temperaturerhythm of juvenile rats do to a periodic blockade of thermoregulatorythermogenesis? Pflugers Arch 427: 450–454, 1994.

34. Overton JM, Williams TM. Behavioral and physiologic responses tocaloric restriction in mice. Physiol Behav 81: 749–754, 2004.

35. Park JH, Dark J. Fos-like immunoreactivity in Siberian hamster brainduring initiation of torpor-like hypothermia induced by 2DG. Brain Res1161: 38–45, 2007.

36. Paul MJ, Freeman DA, Park JH, Dark J. Neuropeptide Y inducestorpor-like hypothermia in Siberian hamster. Brain Res 1055: 83–92,2005.

37. Pelz KM, Dark J. ICV NPY Y1 receptor agonist but not Y5 agonistinduces torpor-like hypothermia in cold-acclimated Siberian hamsters.Am J Physiol Regul Integr Comp Physiol 292: R2299–R2311, 2007.

38. Redlin U, Nuesslein B, Schmidt I. Circadian changes in brown adiposetissue thermogenesis in juvenile rats. Am J Physiol Regul Integr CompPhysiol 262: R504–R508, 1992.

39. Ritter S, Taylor JS. Vagal sensory neurons are required for lipoprivic butnot glucoprivic feeding in the rat. Am J Physiol Regul Integr Comp Physiol258: R1395–R1401, 1990.

40. Ruby NF, Nelson RJ, Licht P, Zucker I. Prolactin and testosteroneinhibit torpor in Siberian hamsters. Am J Physiol Regul Integr CompPhysiol 264: R123–R128, 1993.

41. Ruf T, Heldmaier G. The impact of daily torpor on energy requirementsin the Djungarian hamster, Phodopus sungorus. Physiol Zool 65: 994–1010, 1992.

42. Ruf T, Steiglitz A, Steinlechner S, Blank JL, Heldmaier G. Coldexposure and food restriction facilitate physiological responses to shortphotoperiod in Djungarian hamsters (Phodopus sungorus). J Exp Zool267: 104–113, 1993.

43. Ruf T, Steinlechner S, Heldmaier G. Rhythmicity of body temperatureand torpor in the Djungarian hamster, Phodopus sungorus. In: Living inthe Cold II, edited by Malan A, and Canguilhem B. London: John LibbeyEurotext, 1989.

44. Schneider JE, Zhu Y. Caudal brain stem plays a role in metabolic controlof estrous cycles in Syrian hamsters. Brain Res 661: 70–74, 1994.

45. Schoelch C, Hubschle T, Schmidt I, Nuesslein-Hildesheim B. MSGlesions decrease body mass of suckling-age rats by attenuating circadiandecreases of energy expenditure. Am J Physiol Endocrinol Metab 283:E604–E611, 2002.

46. Sergeyev V, Broberger C, Gorbatyuk O, Hokfelt T. Effect of 2-mer-captoacetate and 2-deoxy-D-glucose administration on the expression ofNPY, AGRP, POMC, MCH, and hypocretin/orexin in the rat hypothala-mus. Neuroreport 11: 117–121, 2000.

47. Silverman HJ, Zucker I. Absence of post-fast food compensation in thegolden hamster (Mesocricetus auratus). Physiol Behav 17: 271–285, 1976.

48. Stehling O, Doring H, Schmidt I. Leptin reduces juvenile fat stores byaltering the circadian cycle of energy expenditure. Am J Physiol RegulIntegr Comp Physiol 271: R1770–R1774, 1996.

49. Takasaki Y. Studies on brain lesion by administration of monosodiumL-glutamate to mice. 1. Brain lesions in infant mice caused by adminis-tration of monosodium L-glutamate. Toxicology 9: 493–305, 1978.

R264 ARCUATE NUCLEUS LESIONS AND TORPOR

AJP-Regul Integr Comp Physiol • VOL 294 • JANUARY 2008 • www.ajpregu.org

50. Tang-Christensen M, Holst JJ, Hartmann B, Vrang N. The arcuatenucleus is pivotal in mediating the anorectic effects of centrally adminis-tered leptin. Neuroreport 10: 1183–1187, 1999.

51. Tokuyama K, and Himms-Hagen J. Brown adipose tissue thermogen-esis, torpor, and obesity of glutamate-treated mice. Am J Physiol Endo-crinol Metab 251: E407–E415, 1986.

52. Walker JM, Garber A, Berger RJ, Heller HC. Sleep and estivation(shallow torpor): continuous processes of energy conservation. Science204: 1098–1100, 1979.

53. Westman W, Geiser F. The effect of metabolic fuel unavailability onthermoregulation and torpor in a marsupial hibernator. J Comp Physiol [B]174: 49–57, 2003.

54. Wick AN, Drury DR, Nakada HI, Wolfe JB. Localization of the primarymetabolic and in block produced by 2-deoxy-glucose. J Biol Chem 224:963–969, 1957.

55. Yasuda T, Masaki T, Kakuma T, Yoshimatsu H. Hypothalamic mela-nocortin system regulates sympathetic nerve activity in brown adiposetissue. Exp Biol Med 229: 235–239, 2004.

R265ARCUATE NUCLEUS LESIONS AND TORPOR

AJP-Regul Integr Comp Physiol • VOL 294 • JANUARY 2008 • www.ajpregu.org