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We examined variation in plasma glucocorticoid concentrations of free-living Belding's ground squirrels (Spermophilus beldingi) and captive golden-mantled ground squirrels (Spermophilus lateralis) housed in constant environmental conditions. Plasma corticosterone concentrations were not significantly correlated with time held captive in traps prior to collection of blood samples in males or non-breeding females, but these variables were significantly correlated in breeding females during the pre-mating, lactation, and post-lactation periods. Among male S. beldingi, plasma corticosterone concentrations increased over the course of the active season, and were significantly higher in non-breeding than breeding individuals. Corticosterone concentrations also increased in non-breeding females throughout the active period. In breeding females, baseline (non-stress) corticosterone concentrations, determined from blood samples collected within 3 min of capture, increased during gestation and declined during lactation, whereas stress values of corticosterone, determined from blood samples collected within 4-11 min of capture, increased during gestation and then again during the period after young emerged from the natal burrow. Changes in plasma corticosterone concentrations of S. beldingi paralleled changes in body mass. Among S. lateralis, plasma concentrations of cortisol were elevated when males and females were in reproductive condition and lower in reproductively quiescent squirrels. The annual peak in plasma cortisol concentrations occurred just prior to increases in body mass associated with pre-hibernation fattening. Collectively, these results suggest that concentrations of circulating glucocorticoids fluctuate with circannual rhythmicity in conjunction with annual cycles of change in body mass and activity; within these circannual cycles glucocorticoid concentrations are influenced by stress and breeding status.

In golden-mantled ground squirrels, phase angles of entrainment of circadian locomotor activity to a fixed light-dark cycle differ markedly between subjective summer and winter. A change in ambient temperature affects entrainment only during subjective winter when it also produces pronounced effects on body temperature (Tb). It was previously proposed that variations in Tb are causally related to the circannual rhythm in circadian entrainment. To test this hypothesis, wheel-running activity and Tb were monitored for 12 to 14 months in castrated male ground squirrels housed in a 14:10 LD photocycle at 21 degrees C. Animals were treated with testosterone implants that eliminated hibernation and prevented the marked winter decline in Tb; these squirrels manifested circannual changes in circadian entrainment indistinguishable from those of untreated animals. Both groups exhibited pronounced changes in phase angle and alpha of circadian wheel-running and Tb rhythms. Seasonal variation in Tb is not necessary for circannual changes in circadian organization of golden-mantled ground squirrels.

Entrainment of circannual rhythms of body mass and reproduction was monitored for 3 years in female golden-mantled ground squirrels maintained in a simulated natural photoperiod. Both pinealectomized and pineal-intact squirrels generated circannual rhythms of body mass and estrus, but only the intact animals entrained these rhythms to a period of 365 days. In the second and third years after treatment, the period of the body mass rhythm was significantly shorter than 365 days for pinealectomized squirrels, and variance in tau among these animals was significantly greater than for intact squirrels. A similar pattern was evident in the rhythm of reproduction, which was phase-disrupted in pinealectomized squirrels but entrained in intacts. Seasonal changes in duration of nocturnal melatonin secretion by the pineal appear to be necessary to produce phase-delays required to entrain the circannual clock to a period of 12 months.

Gonadectomized male golden-mantled ground squirrels (Spermophilus lateralis) were implanted with estradiol benzoate (EB)-filled or empty capsules. Body mass was monitored before, during, and for at least 1 yr after hormone treatment. EB treatment during the mass-gain phase of the annual cycle significantly decelerated increases in body mass; the period of the circannual rhythm (CAR) of body mass was 54 days longer in EB- than blank-treated squirrels. Hormone treatment during the mass-loss phase accelerated mass loss; although this effect only approached statistical significance, some phase markers of the CAR were significantly advanced in subsequent cycles. We conclude that, as in females, estradiol affects the waveform of the CAR of males differently at different phases of the circannual cycle. Sexual differentiation does not eliminate responsiveness of CARs of squirrels to estradiol; sex differences, if any, are subtle rather than absolute and, in this respect, differ from circadian rhythms.

Female golden-mantled ground squirrels that sustained complete ablation of the suprachiasmatic nucleus (SCNx) were housed pre- and post-operatively at 23 degrees C and then at 6.5 degrees C for 5-7 yr. SCNx and control animals held at the higher temperature manifested circannual rhythms (CARs) in body mass. In contrast, body mass CARs were not expressed in 50% of SCNx squirrels during cold exposure; rhythm amplitude was reduced to 25-40% of pre-operative values and the interval between successive peaks in body mass fell outside the circannual range. Unlike normal squirrels that hibernate for about 6 months during each circannual cycle, these SCNx squirrels expressed bouts of torpor nearly continuously throughout 2.5 yr of cold exposure. Body mass increases were often observed during hibernation--a phenomenon never observed in control animals. The remaining SCNx squirrels that did not hibernate continuously displayed CARs in body mass within the normal range. The effects of SCN ablation on body mass rhythms presumably are related to disrupted patterns of hibernation, food intake, and metabolism. The SCN, which sustains neural and metabolic activity at low tissue temperatures, may exert greater influence on thermoregulation and metabolism during the hibernation season than at other times of year, thereby accounting for the greater effect of SCN ablation in squirrels maintained at low ambient temperatures.

Hibernation patterns were monitored continuously for 2.5 years in female squirrels that were neurologically intact or in which the hypothalamic suprachiasmatic nucleus (SCN) was completely ablated (SCNx). The number of hibernation bouts in SCNx squirrels increased by 159%, total hibernation time increased by 58%, and periodic arousals from hibernation were 47% longer in SCNx than in control squirrels; the duration of individual torpor bouts was 2 days shorter and far more variable in SCNx than in control animals. Some SCNx squirrels cycled through bouts of torpor continuously for nearly 2 years. The SCN appears to be part of the mechanism that controls the duration of the hibernation season and the temporal structure of individual torpor bouts.

We tested the hypothesis that continued secretion of gonadal steroids is necessary to suppress hibernation in male golden-mantled ground squirrels in the weeks after the terminal arousal in spring. Juvenile and adult males were gonadectomized or sham gonadectomized 1 wk after the terminal arousal; 64% of castrated and none of the shamcastrated animals resumed hibernation. Latency to resumption of torpor was 9 +/- 2 days from the time of castration, and squirrels underwent 4.3 +/- 0.9 bouts before permanently regaining euthermia. Among squirrels that resumed hibernation, bout duration was significantly shorter and torpor was shallower after castration. Castration as late as 3 wk after the terminal arousal reinstated hibernation. We suggest that the terminal arousal of male squirrels in the spring is provoked by a steroid-independent mechanism similar to that operating earlier in the hibernation season; abandonment of hibernation is contingent on concomitant sustained increases in androgen secretion during the first few weeks of euthermia.

Circadian wheel-running activity was monitored in intact and gonadectomized female and male golden-mantled ground squirrels maintained in a 14L:10D photoperiod at 21 degrees C. Marked advances in phase angle of activity onset and increases in the duration of the daily active phase occurred each year in intact animals coincident with the phase of reproductive activity. Surprisingly, long-term gonadectomized squirrels of both sexes manifested similar seasonal changes in the circadian system. Seasonal circadian variations in locomotor activity are generated by a circannual clock independent from and not influenced by concurrent changes in secretion of gonadal hormones.

Little is known about the distribution of binding sites for the pineal hormone melatonin in non-myomorph rodents. We used 2-[125I]iodomelatonin (IMEL) to analyze the distribution, affinity, and specificity of binding sites in the golden-mantled ground squirrel, a sciurid rodent that reportedly lacks IMEL binding sites in the brain. Specific binding was found not only in the pars tuberalis, but also in several telencephalic and diencephalic areas including the hypothalamic suprachiasmatic region. The affinity and specificity of IMEL binding are comparable to those reported in other rodents. IMEL binding studies in a hystricomorph rodent, the guinea pig, revealed high concentrations of receptor in the nucleus accumbens and dorsolateral thalamus. Central melatonin binding sites have now been demonstrated in species of all three rodent families. The heterogeneous distribution of melatonin receptors appears similar in the species studied, and no evidence is found to link IMEL binding sites at any particular locus to photoperiodic, circannual, or non-seasonal breeding patterns.

Removal of the olfactory bulbs lengthened the period of circannual rhythms (CARs) of body mass and plasma testosterone (T) concentrations in male golden-mantled ground squirrels, but did not otherwise alter their expression. The period of the CAR was approximately 2 months longer in bulbectomized than in sham-operated animals. Peak values of body mass and T were unaffected by bulbectomy. All neurologically intact, but only 50% of bulbectomized, squirrels displayed normal hibernation patterns. We conclude that the olfactory bulbs are not the site of essential circannual oscillators. Circannual reproductive and hibernation cycles of ground squirrels are less subject to modulation by the olfactory bulbs than are the corresponding rhythms of several non-circannual hamster species.

The efficacy of photoperiod as a zeitgeber for entrainment of circannual body weight and estrous rhythms was tested in female golden-mantled ground squirrels maintained for 3 or more years in either a simulated natural photoperiod (SNP) or a fixed LD 14:10 photoperiod (FP). The role of the retinohypothalamic tract--suprachiasmatic nucleus (RHT-SCN) projection in photic entrainment was assessed in animals that sustained destruction of the SCN (SCNX). Circannual rhythms were lengthened by the SNP as compared to the FP. Mean periods (tau's) for neurologically intact animals in the third year of testing were 49.6 +/- 0.3 weeks and 43.1 +/- 1.2 weeks (p less than 0.001) for the SNP and FP groups, respectively; furthermore, 56% and 7% of animals in these groups had tau's not significantly different from 365 days (p less than 0.005), and within-group variability was lower for SNP than for FP squirrels (p less than 0.01). SCNX squirrels differed from animals with the SCN intact (SCNC), as evidenced by higher within-group variability (p less than 0.001); only 29% of SCNX squirrels had tau's not different from 365 days (p less than 0.03 compared to the SCNC group). The coupling between estrous and body weight rhythms that was evident in SCN-intact SNP and FP squirrels was disrupted in SCNX animals. The RHT-SCN pathway is implicated in entrainment and in maintenance of normal phase relations among the several circannual rhythms. In a second experiment, female squirrels were maintained for 2.5 years in an accelerated SNP that compressed two normal annual photocycles into each calendar year. Of 12 squirrels, 3 had tau's that did not differ significantly from 6 months; 6 had tau's equivalent to 12 months; and 3 had tau's significantly different from both 6 months and 12 months. The data suggest that photoperiod is a major zeitgeber for entrainment of golden-mantled ground squirrels circannual rhythms.

At different phases of the hibernation season, castrated male golden-mantled ground squirrels were implanted with capsules that either were filled with testosterone (T) or left empty (blank). Blank-treated animals hibernated normally when housed at 5 degrees C. Entry into hibernation was prevented in the majority of squirrels treated with T several days before the initial cold challenge. T concentrations that inhibited torpor (greater than 1.2 ng/ml) were comparable with those of intact males at the end of the hibernation season. In some squirrels, moderate T concentrations were compatible with hibernation, but torpor bout duration was shorter than normal. The inhibitory effect of T on hibernation did not appear to require aromatization of T to estradiol. We suggest that a steroid-independent mechanism triggers arousal from hibernation and that T-dependent processes determine whether hibernation is resumed at the end of an arousal period.

We tested the hypothesis that the free-running period and phase angles of entrainment of circadian locomotor activity rhythms of golden-mantled ground squirrels (Spermophilus lateralis) vary as a function of body temperature (Tb). Animals were maintained in a light-dark cycle (LD 14:10), and at 6-week intervals ambient temperature (Ta) was changed from 20 degrees to 30 degrees C, or vice versa. Data were collected during the animals' homeothermic and heterothermic phases. Subsequently, squirrels were housed in dim constant illumination with the same alternating temperature sequence. In heterothermic ground squirrels (those capable of Tb less than 34 degrees C), a decrease in Ta from 30 degrees to 20 degrees C caused phase delays in activity onset, phase advances in activity termination, and a decrease in duration of the active phase; increases in Ta from 20 degrees to 30 degrees C produced the opposite effect on each of these parameters. The free-running period of the activity rhythm of heterothermic squirrels increased and decreased in response to 10 degrees C decreases and increases in Ta, respectively. Changes in Ta did not affect circadian parameters in homeothermic squirrels. Seasonal variations in circadian organization previously observed in this species appear to reflect the influence of endogenous fluctuations in body temperature. Tissue temperature has a major influence on circadian rhythms of golden-mantled ground squirrels.

Hibernation and body mass rhythms were studied in 13 golden-mantled ground squirrels maintained in an LD 12:12 photoperiod at 5 degrees C. Complete or partial ablation of the suprachiasmatic nuclei (SCN) disrupted normal hibernation rhythms. Over the course of 2 years, several animals progressed through 4 hibernation cycles, one squirrel manifested two abnormally long hibernation seasons, and another failed to hibernate. Squirrels with intact SCN exhibited normal circannual hibernation rhythms at intervals of 11.5 +/- 0.3 months. Hibernation coincided with the weight loss phase of the body mass cycle in control squirrels, but these two rhythms were dissociated in animals with lesions of the SCN. The annual plasma testosterone rhythm was normal or slightly phase-delayed in squirrels with SCN lesions maintained at 23 degrees C. The SCN may be part of a neural circuit that mediates circannual organization of hibernation rhythms.

Testis size and spermatogenesis were monitored serially in individual golden-mantled ground squirrels before, during, and after the hibernation season. During hibernation, animals spent 81% of days in torpor at body temperatures of 3-4 degrees C. Torpor bouts of 6 days duration were interspersed with brief arousals from torpor during which animals were normothermic. In the 5 mo between December (when animals entered hibernation) and April (when torpor was spontaneously terminated), the estimated mass of testes increased gradually from 500 to 1100 mg, but spermatogenesis did not advance beyond pachytene spermatocytes, which were present before hibernation began. In contrast, during the month after torpor was terminated, testes increased rapidly to 3500 mg and after 31 days, spermatozoa were found in the epididymides. We suggest that the limited testis growth that occurred during the hibernation season was restricted to intervals during which squirrels were aroused from torpor. The major portion of gonadal growth and spermatogenesis in the laboratory, and presumably in the field, occurs after ground squirrels have regained the normothermic state. Since males are reproductively mature when first trapped in spring, these findings suggest that males are normothermic for several weeks before they emerge from their hibernacula in the spring.

Male golden-mantled ground squirrels subjected to surgical removal of substantial portions of inguinal, retroperitoneal, and epididymal adipose tissue [lipectomy (Lipx)] or to sham lipectomy (Sham) during the early prehibernatory fattening phase did not differ in body mass 4 mo later at the time of their annual body weight peaks. Body mass restoration by Lipx squirrels was achieved without compensatory increases in food intake. There was little or no regeneration of epididymal adipose tissue, complete regeneration of the retroperitoneal depot, and an increase in mass of subcutaneous fat of Lipx animals. Epididymal and retroperitoneal adipocytes were equivalent in size in Sham and Lipx squirrels; adipocytes in subcutaneous fat were 20% larger in Lipx than in Sham animals. Generation of the annual body mass cycle of this ground squirrel involves active regulation of the lipid mass.

Female golden-mantled ground squirrels were pinealectomized (Pinx) or Sham-Pinx at 70 days of age and maintained for more than 2 yr at 23 degrees C in a 10:14 light-dark photoperiod. Reproductive condition and body weight were recorded weekly and bi-weekly, respectively. Pinx and Sham-Pinx squirrels displayed circannual rhythms in body weight; the groups did not differ with respect to absolute magnitudes of annual peaks and troughs in body mass. The period of the circannual body weight cycle (measured peak to peak) was 27 and 58 days shorter in Pinx than in Sham-Pinx squirrels during the 1st and 2nd yr, respectively, after surgery. The onset of estrus during the second postoperative cycle occurred significantly earlier in Pinx than in Sham-Pinx squirrels. The pineal gland is not essential for the generation or expression of circannual or circadian cycles but may affect the period of some circannual rhythms.

Plasma luteinizing hormone (LH) levels were determined at monthly intervals in intact and ovariectomized squirrels maintained in a constant 14L:10D photoperiod at a temperature of 23 +/- 2 degrees C. LH was undetectable (less than 0.9 ng/ml) in plasma of intact females at all times of year. Females ovariectomized (OVX) at 9.5 months of age in March showed substantial increases in plasma LH in May and June but LH was undetectable between July and November. Females ovariectomized at 13 months of age in July first manifested detectable LH levels the following January and February (6-7 months post-ovariectomy). Very few adult females trapped in May and ovariectomized in August had detectable LH levels within 2 months of ovariectomy; however, females ovariectomized the following February had detectable LH titers 1 month later. Long-term studies of individual OVX squirrels indicated peak LH levels between March and June, 1980, relatively low or undetectable titers between August and December and elevated LH levels between January and March, 1981. The results are suggestive of a circannual rhythm of LH secretion which appears restricted to one season of the year and occurs independently of steroid feedback from the ovaries; ovarian steroids only modulate the levels of plasma LH during the brief annual period of hypothalamo-hypophysial activity. We suggest that onset and termination of LH release are mediated by central nervous system circannual clocks.

There was a circannual rhythm in the times of activity onset (psi o) and activity termination (psi T) and in the duration of the active phase (alpha) in female ground squirrels (Spermophilus lateralis) maintained in a light-dark (LD) 14:10 photoperiod for 22 mo. Activity onset occurred 2.3 h after light onset in the 4 mo before estrus, 0.5 h before light onset during the month encompassing estrus, and progressively later during each of the ensuing 4 mo. Termination of activity occurred later during the month of estrus than in the 4 mo preceding estrus; alpha nearly doubled during the month of estrus and decreased gradually over the next 3 mo. The psi o and psi T remained entrained to the daily illumination cycle, but the phase angles of activity onset and offset and the duration of the active phase varied seasonally and recurred with a circannual period of 9-11 mo. The LD 14:10 photoperiod entrained the circadian activity rhythm throughout the year; however, circannual variations in activity as well as in estrus and body mass were not entrained by this photoperiod but free-ran with periods of less than 12 mo.

Plasma luteinizing hormone (LH) levels were determined at monthly intervals in intact and orchidectomized ground squirrels maintained in a constant 14L:10D photoperiod at a temperature of 23 +/- 2 degrees C. Animals were orchidectomized or sham operated at different ages and at different stages of the annual reproductive cycle. LH levels were elevated in orchidectomized as compared to intact males within 1 month, and often within 48 h, after orchidectomies performed either before or after the normal breeding season. LH levels tended to remain chronically elevated in long-term orchidectomized males. Although negative feedback regulation of LH by the testes was evident at most stages of the reproductive cycle, effects of orchidectomy on LH were most pronounced during the nonbreeding season when plasma LH titers of intact males were at their annual nadirs, and less pronounced or absent during the breeding season when levels were normally elevated in intact males. For most of the year male gonadal hormones exert negative feedback on LH release. We hypothesize that under the influence of a circannual timing mechanism, sensitivity to steroid feedback is reduced as the breeding season approaches and LH secretion increases despite the attendant rise in plasma androgen levels. These results contrast with data for female squirrels in which there is a seasonally restricted and characteristically brief elevation of plasma LH after ovariectomy; in females, but not in males, onset and termination of LH secretion appear to be regulated by a circannual clock that functions independently of gonadal secretions.

A substantial proportion of total adipose tissue mass was surgically removed from female ground squirrels during weight gain or weight loss phases of the circannual body weight cycle. Within 2 months of fat removal, squirrels had restored body mass to levels appropriate to the stage of the annual body weight cycle. These findings suggest that ground squirrels may use feedback from body lipids in controlling body weight.

Male squirrels, born approximately June 1 and kept under constant conditions of photoperiod (14 h of light per day) and temperature (23 +/- 2 degrees C) from birth, manifested circannual rhythms in plasma testosterone (T) and luteinizing hormone (LH) levels. Blood samples were collected once per month for 22 months and hormone levels determined by radioimmunoassay. Testosterone levels were low or undetectable ( less than or equal to 0.05 ng/ml) between October and January of the first year, and began increasing in February and peaked during April and May (approx. 4.5 ng/ml). By August of the second year T had declined to low levels which were maintained through January (0.05 ng/ml). The cycle then repeated, with T titers elevated during April and May of the second year. LH levels were undetectable (less than or equal to ng/ml) throughout most of the first 8 months of life; LH peaks were reached by individual animals between March and July. There was a close correspondence between seasonal peaks and troughs of testosterone and those in LH. The circannual pattern of plasma T and LH also was documented for a group of field-trapped males (less than or equal to 11 months old) maintained under similar conditions and bled at monthly intervals for 14 consecutive months. Rhythms of plasma T and LH persisted with a period of 11 to 12 months under constant conditions and thus qualify as circannual cycles.

Ad libitum fed golden-mantled ground squirrels undergo marked annual fluctuations in body mass; during the first cycle, peak and trough masses were approximately 291 and approximately 192 g, respectively. Peak masses were significantly higher (9%) during the second than the first cycle, reflecting a 15% increase in fat-free dry mass and a 12% increase in lipid reserves. The reduction in body mass during the weight loss phase was almost entirely due to an 84% decrease in total lipid reserves and reflected a decrease in adipocyte size but not number. All measured fat depots decreased at similar rates, and there was no evidence of preferential utilization or sparing of individual depots. Fat cell size decreased progressively in each depot during the transition from peak to trough masses and did not vary among the several depots at any stage of annual cycle. Adipocyte number increased in parametrial and retroperitoneal depots but not in the subcutaneous fat between the first and second body mass peaks; fat cell size in these two depots decreased by 28 and 20%, respectively, from the first to the second peak in body mass. Reduced lipid filling of adipocytes at the second peak mass, concomitant with adipocyte hyperplasia, suggests that total lipid mass, not simply fat cell size, is regulated during the annual body mass cycle. Maturation of the subcutaneous fat depot before the initial entry into hibernation may be adaptive in providing insulation during dormancy.

Golden-mantled ground squirrels (Spermophilus lateralis) sustained complete ablation of the suprachiasmatic nuclei (SCN) while being maintained under constant conditions of photoperiod, temperature and food availability. After SCN ablation, most squirrels observed for up to 3 years postsurgically underwent normal circannual cycles in body mass. Other squirrels with similar SCN damage evidenced various anomalies in their circannual body weight cycles. Response of squirrels to SCN ablation was not related to the phase of the annual cycle in which lesions were produced. The variable nature of the effect of SCN ablation on circannual body mass cycles suggests that the SCN are non-essential components of the system underlying the generation or expression of circannual rhythms.

Golden-mantled ground squirrels were ovariectomized (OVX) or Sham-OVX during the weight gain phase of the circannual body weight cycle. Other squirrels, OVX or Sham-OVX during the weight loss phase, were subcutaneously implanted with estradiol-filled or empty Silastic capsules. The mass of several fat pads as well as adipose tissue lipoprotein lipase (LPL) activity were determined at autopsy. Ovariectomy at either phase of the annual cycle was without effect on body weight. However, LPL activity of the parametrial fat pad was increased in OVX as compared to Sham-OVX squirrels. Fourteen days of estradiol treatment during the weight loss phase decreased the mass and LPL activity of the retroperitoneal fat depot but did not affect these parameters in perirenal adipose tissue. Although estradiol exerts different or opposite effects on body mass and food intake of rats and ground squirrels, ovariectomy and estrogen treatment affect LPL activity in a similar fashion in both species.

Female squirrels were injected at birth with 50 or 1000 micrograms testosterone propionate (TP); control males and females were treated with oil vehicle. Squirrels were gonadectomized at 47 days of age. Body mass was recorded weekly and plasma luteinizing hormone (LH) was determined once monthly over the next year. Marked annual cycles in body mass were manifested by 30 out of 31 squirrels. Peak body mass and peak-to-trough differences were greater for control male and TP-female squirrels than for control female squirrels. Trough body weights did not differ among the groups. Luteinizing hormone was detectable in all male and most androgenized females but not in any control female squirrels during the first 4 mo after gonadectomy. Peak LH values were significantly greater for control male than for control female squirrels and were not influenced by neonatal androgenization in females. Testosterone propionate treatment also did not affect sex differences in timing of LH peaks or the total number of months in which LH was detectable. We conclude that testicular hormones secreted during the early postnatal period induce sex differences in the circannual pattern of weight change and some aspects of LH secretion. Complete masculinization, however, either requires more extensive action of gonadal hormones, perhaps both pre- and postnatally, or occurs through some androgen-independent mechanism.

Golden-mantled ground squirrels undergo marked fattening and fat depletion during the prehibernatory and hibernatory phases, respectively, of their annual body weight cycle. Fat regulation was studied by surgical removal (lipectomy) of most of the inguinal-subcutaneous, retroperitoneal, and parametrial fat depots from female squirrels during the weight-gain phase of the annual cycle. Seven weeks after surgery, body weights of lipectomized (Lipx) and Sham-Lipx squirrels were equivalent, although white adipose tissue was not completely recovered in Lipx animals. Lipoprotein lipase (LPL) activity was markedly elevated in subcutaneous adipose tissue of Lipx squirrels, but not in other depots. Plasma triglyceride (TG) levels also were higher in Lipx squirrels 7 weeks after surgery. Changes in LPL activity and plasma TG may contribute to body mass and lipid restoration following lipectomy.

The influence of ovarian and testicular hormones on circannual body weight cycles was assessed in golden-mantled ground squirrels (Spermophilus lateralis), maintained in a LD 14:10 photoperiod at 23 degrees C. Animals were gonadectomized or sham operated at 47 days of age and implanted with empty capsules or capsules filled with estradiol benzoate (EB) or testosterone propionate (TP). Body weight, food intake, and reproductive condition were recorded at weekly intervals. Gonadectomized and intact squirrels of both sexes manifested robust circannual rhythms of body weight; cyclic secretion of gonadal hormones was not necessary for the expression of this circannual cycle. Intact and gonadectomized males weighed substantially more than their female counterparts but only during part of the annual cycle. This sex difference waxed and waned on a circannual basis in the absence of concurrent gonadal hormone secretions. EB phase-delayed attainment of peak body weights in ovariectomized squirrels but did not affect timing of trough body weights. EB increased the amount of food consumed by ovariectomized squirrels particularly during the weight gain phase. TP accelerated attainment of trough body weights in orchidectomized animals. Gonadal hormones modify the magnitude and timing of body weight peaks and troughs but are not necessary for the generation or expression of the circannual body weight cycle or for the sex difference in this cycle.

The role of white adipose tissue in development of the reproductive apparatus of male golden-mantled ground squirrels was assessed by surgical removal of fat (lipectomy) immediately prior to onset of hibernation or by manipulation of fat levels through food restriction for 5 wk preceding hibernation. Animals then were maintained without food at 6 degrees C, and they hibernated from November 1983 until April 1984. At that time, blood plasma was assayed for hormone levels, and body mass, body composition, and masses of the testes and seminal vesicle-prostate complex were determined. At autopsy, testes and sexual accessory organs were heavier in Control squirrels than in Food-Restricted or Lipectomized (LIPX) animals. Paired-testes mass was positively correlated with body mass. Testosterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) levels were uniformly low in animals sampled during torpor, but were elevated in 2 animals that had regained the euthermic state. The Food-Restricted animals spent more time in torpor than did the other two groups, and the percentage of decrease in body mass over the hibernation season was less for Food-Restricted than for LIPX or Control animals. Adequate lipid stores may be essential for the normal development of the reproductive apparatus of male ground squirrels. In the field, animals with insufficient fat stores may forego reproduction and thereby conserve energy for survival.

Golden-mantled ground squirrels, maintained under constant conditions of photoperiod and temperature sustained lesions of the suprachiasmatic nuclei (SCN) or of the medial basal hypothalamus. Destruction of the SCN eliminated or disrupted circadian activity rhythms and shortened the period of the circannual reproductive cycle. Circannual body weight cycles were eliminated or disrupted in several SCN-lesioned animals and one squirrel had a 3- to 5-mo body weight rhythm; however, most SCN-lesioned squirrels with disrupted circadian activity cycles manifested normal circannual body weight rhythms. The SCN are important for circadian organization of locomotor activity of this diurnal rodent, but the generation and expression of circannual body weight and reproductive rhythms can proceed in the absence of coherent circadian organization. The SCN are less essential for the generation and expression of circannual than of circadian cycles.