Effects of age and photoperiod on reproduction and the spleen in the marsh rice rat (Oryzomys palustris)

To examine the interactions between age and photoperiod on reproduction and spleen weights, we exposed adult male and female rice rats of various ages to photoperiods of 16:8-h light-dark photoperiods (16L:8D) or 12L:12D. After 10 wk, animals were killed and the following data were recorded: weights of testes, seminal vesicles, uterus, ovaries, body, and spleen and, in addition, vaginal patency. Young adult males displayed a greater degree of testicular and seminal vesicle regression in short photoperiods than did older males; the testes of most older males did not regress in response to short photoperiods. Spleen weight was unresponsive to short photoperiods in all males, but was affected by age. Females, however, exhibited reproductive organ regression and decreased vaginal patency in response to short photoperiods at all ages examined. Body weights were affected by photoperiod in young females, and, as in males, photoperiod had no effect on spleen weights. These data suggest that the reproductive response to photoperiod in adult male rice rats declines with age, whereas in adult females it does not.     

Influence of photoperiod, nutrition and water availability on reproduction of male California voles (Microtus californicus)

Male California voles were maintained in long (14L:10D) or short photoperiods (10L:14D) for 10 weeks and fed a standard diet of rabbit chow and water ad libitum. One additional group in each photoperiod received the standard diet plus supplements of spinach 3 times weekly. A fifth group was housed in 14L:10D and fed the standard diet, but for 10 weeks water availability was restricted to several hours each morning. Testes and seminal vesicles were heaviest in long-day voles fed spinach supplements and lightest in short-day voles fed only the standard diet; the latter animals manifested reduced testicular spermatogenesis. Testicular weights were also depressed in voles with restricted access to water. It is suggested that photoperiods that simulate those of winter induce regression of the reproductive organs of male California voles but the availability of green vegetation counteracts the inhibitory effects of short daylengths.     

Metabolic and thermoregulatory effects of photoperiod and melatonin on Peromyscus maniculatus acclimatization

A photoperiod-related seasonal rhythm in active period (scotophase), metabolic rate and core temperature was documented for animals held at 21.0 +/- 0.1 degrees C ambient; animals that were habituated to long nights (10:14LD) had a greater metabolic reserve than those held in summer photoperiods (14:10LD). While relative weights of gonads and sex accessory tissues of mice show typical "winter" regression, interscapular brown adipose tissue mass was unaffected by photoperiod; moreover, IBAT beta adrenergic responses under "winter" photoperiods did not differ from "summer" photoperiods in the absence of cold stimulus. Thermogenic efficiency, measured as the increment of active temperature level achieved per increment of active period metabolic effort, was highest for animals exposed to short photoperiods. Thermal conductance was reduced in animals exposed to short (10:14LD) photoperiods. Heat conservation and thermogenic response capacity was enhanced by melatonin treatment and short photoperiod.     

Photoperiod influences reproduction in the prairie vole (Microtus ochrogaster)

Four experiments examined the role of photoperiod in the regulation of seasonal breeding in the prairie vole. Adult male voles maintained in short (8L:16D) as compared to long (16L:8D) photoperiods for 10 wk had reduced testicular and seminal vesicle weights, but fertility was not impaired. Male prairie voles reared from birth until 35 days of age in short as compared to long photoperiods also had reduced testicular and seminal vesicle weights, as well as diminished fertility. The incidence of pregnancy did not differ between long- and short-day female voles paired for 6 days with long- or short-day males (93%, 86%, 89%, and 88%, respectively). Photoperiod did not affect the incidence or the timing of postpartum pregnancies in long- or short-day females paired with long-day males through the birth of several litters. Adult male prairie voles retain only marginal sensitivity to short photoperiods, maturing males are highly responsive to short days, and adult females are insensitive to photoperiod. These data suggest that termination of the breeding season in the autumn may be due to the lack of fecund males in the population.     

Effect of different photoperiods on gonadal maintenance and development in Mongolian gerbils (Meriones unguiculatus)

The role of photoperiod in adult testicular maintenance and body weight and juvenile development was assessed in male Mongolian gerbils (Meriones unguiculatus). Gerbils were raised on a 14L (14 hr of light) photoperiod. In the first study, adult gerbils with functional testes were transferred to thirteen different photoperiods (0L, 2L, 4L, 6L, 8L, 10L, 12L, 14L, 16L, 18L, 20L, 22L, or 24L) and body weights and testicular size were measured every week for 10 weeks. Body weights were similar in all groups. Testicular regression had occurred in animals housed on 0L, 2L, 4L, 6L, 8L, and 24L by week 10. In the second study, 14L-born prepubertal gerbils were transferred to thirteen different photoperiods as in the first study. Body weights and testicular development were examined for 10 weeks. At the end of 10 weeks the body weights of animals in all groups except 24L were similar to those of adults. Animals in 24L had a lower body weight gain. Exposure to 0L, 2L, and 24L inhibited testicular development and testes weights were significantly different from those of the other groups. These results demonstrate that maintenance of body weight in adult gerbils appears to be independent of photoperiodic signal. Exposure to very long (24L) and short photoperiods (< 10 hr) causes testicular regression in adult gerbils. Moreover, different photoperiods experienced in early life can influence prepubertal testis growth and body weight gain.     

Carry-over effects of long and short photoperiods on body fattening and gonadal weight of blackheaded bunting (Emberiza melanocephala)

A study was made of the carry-over effect of long photoperiods followed by short photoperiods ou the fattening and gonadal response in a photoperiodic migratory species, the Blackheaded Bunting (Emberiza melanocephala). The effect was studied in photosensitive and then in photostimulated birds. Two experiments were performed: experiment I with photosensitive males and experiment II with photosensitive males exposed to 30 long photoperiods. In both of them, five groups were submitted to the following treatments for 30 days: Group S, a short daily photoperiod; Group L, a long daily photoperiod; Group LS, alternating long and short daily photoperiods; Group L 2S, one long with two short daily photoperiods; Group L 3S, one long with three short daily photoperiods. The results showed that the inductive effect of long days or the inhibitory effect of short days was affected by intervening reversed daylengths.     

The effect of brief light pulses on the photoperiodic reaction in the Djungarian hamsterPhodopus sungorus

Summary The effects of brief light pulses on photoperiodic reaction were examined in male Djungarian hamsters (Phodopus sungorus). Half of the animals came from long photoperiods and, accordingly, were in physiological summer state with large and functional testes. The other half had lived in short natural photoperiods before and was in physiological winter condition with involuted testes. These animals were exposed to long (LD 168) or to short (LD 816) photoperiods, or to short photoperiods in which the 16 h of darktime were interrupted by 1 min of light each night, either 4, 8 or 12 h after light off.Light pulses at midnight had the same effect as long photoperiods, they maintained gonadal size and activity or caused rapid recrudescence. If the light pulses were applied 4 or 12 h after light-off, however, a marked difference in effect ensued, depending on previous state and light regime. In animals coming from long photoperiods, these schedules induced regression, while in hamsters coming from short photoperiods the same schedules stimulated recrudescence, though significantly less than full long photoperiods or schedules with light-interruptions at midnight. Changes in body weight and pelage colour closely paralleled the changes in gonadal state.     

Photoperiodic response in the male laboratory rat

Normally photoperiodic laboratory rats can be induced to respond reproductively to a change in the length of the day by various experimental manipulations. One such paradigm that results in significant gonadal regression involves the treatment of rats with exogenous testosterone during exposure to short days. Studies were undertaken to assess various aspects of this model system including 1) the testicular response of testosterone-treated rats exposed to various photoperiods, 2) the time course for testicular regression under a short photoperiod, and 3) the role of the pineal gland as a mediator of the effects of day length on the neuroendocrine-gonadal axis. Photoperiods ranging in length from 2 to 22 h/24 h had no effect on testicular size in untreated rats. In contrast, while near normal testicular weights were maintained in laboratory rats treated with testosterone and exposed to 10 or more h of light per day, testicular regression occurred in rats implanted with testosterone-filled capsules and exposed to photoperiods of 8 or fewer h of light per day. Maximal testicular regression was reached in about 9 wk in testosterone-treated rats exposed to 6L:18D. Removal of the pineal gland totally blocked the inhibitory effects of exposure to short day lengths in testosterone-treated rats. These studies define some of the characteristics of an extant, but dormant, system for photoperiodic time measurement in the common laboratory rat and implicate a role for the pineal gland in this system. These experiments offer evidence that neuroendocrine factors that regulate continuous vs. seasonal reproductive patterns are malleable. Such flexibility in the photoperiodic response may also contribute to the evolution of seasonal to non-seasonal species and vice versa.     

Neuroimmunology: modulation of the hamster immune system by photoperiod

Groups of adult male Syrian hamsters were kept in a long photoperiod (LD 14:10) or a short photoperiod (LD 10:14). After 12 weeks, half of the animals in each light:dark cycle were immunized with an immunogenic amino acid polymer. Exposure to short photoperiod was associated with a significant reduction in testicular, accessory sex organ, splenic and brown fat weights. However, photoperiod length did not influence whole body, thymic, adrenal or kidney weights. Spleens of immunized animals in the long photoperiod were significantly heavier than those of unimmunized animals in the long photoperiod, and both were heavier than spleens from immunized or unimmuized animals in the short photoperiod. This reflected increased splenic lymphocyte and macrophage counts. However, there was no difference in antibody production between animals kept in different photoperiods. These results demonstrate that the daily photoperiod length affects both hamster reproductive competence as well as selected immune parameters (splenic weight and mononuclear cell hyperplasia) but does not alter antibody production.     

Social cues attenuate photoresponsiveness of the male reproductive system in Siberian hamsters (Phodopus sungorus)

Transfer of adult Siberian hamsters (Phodopus sungorus) from long (16 h light and 8 h dark, 16L:8D) to short (8L:16D) daily photoperiods induces an involution of the gonads and a cessation of reproductive behavior 8 to 10 weeks later. However, when male and female long-day hamsters were paired on transfer to short photoperiods, the males' gonads did not undergo the typical short-day response. Similarly, when male long-day hamsters were paired with refractory females (i.e., females housed in short photoperiods for at least 28 weeks so that they became unresponsive to short photoperiods), the response of the males' reproductive system to short photoperiods also was attenuated. Thus, social cues can override or delay the effects of photoperiod on the testes of this species. These results suggest that the inhibitory effects of long durations of melatonin secretion (in response to short photoperiods) on the male hypothalamic-pituitary-gonadal axis may be attenuated by social cues such as contact with the opposite sex.     

The time of determination of gynoparae and males in the bird cherry–oat aphid, Rhopalosiphum padi

Apterous exules of Rhopalosiphum padi which experienced short photoperiods only during the first half of their nymphal life all produced gynoparae which made up 57% of the offspring produced in the first 7 days of the mother's reproductive life. Short photoperiods during the second half of a mother's nymphal life did not induce the production of gynoparae. However, when short photoperiods were experienced throughout a mother's nymphal life significantly more gynoparae (82%) were produced. Ten per cent of the offspring of mothers that experienced short photoperiods only during their adult life developed into gynoparae. Of the offspring reared in short photoperiods, but born to mothers reared and kept in long photoperiods, 30% developed into gynoparae. When mothers were exposed to short photoperiods in their adult life and their offspring so treated, then 90% of the offspring developed into gynoparae. Males only occurred amongst the last offspring of mothers that experienced short photoperiods in either the first or the second half of their nymphal life.     

Effect of long and short photoperiod on vasotocin neurons of paraventricular nuclei and adrenal function of water deprived Japanese quail

The responses of magnocellular neurons of paraventricular nuclei (PVN) and changes to adrenal activity to water deprivation in Japanese quail maintained under gonado-inhibitory and stimulatory photoperiods were examined. Water deprivation of 4 days resulted in a 12% decrease in body weight of sexually regressed short day (SD, 6L:18D) quail, while the decrease was more (18%) in sexually stimulated long day (LD, 16L:8D) quail. The increase in plasma osmolality following water deprivation was also more (47%) in LD than to SD quail (36%). Under the LD condition, quail had increased numbers, sizes and immunostaining of ir-AVT neurons of PVN compared to SD condition. A significant increase in the number of ir-AVT neurons was observed following 4 days of water deprivation in both SD and LD quail compared to their respective fully hydrated controls. However, the degree of response was more under the LD compared to the SD condition suggesting that gonado-stimulatory long days increase the activity/response of the AVT system. Increased adrenal ascorbic acid content (i.e., activity) was also observed to quail of LD when compared to SD treatment. However, osmotic stress led to adrenal hypertrophy and hyperactivity of quail of both of the photoperiodic regimes. Our findings indicate that not only osmotic stress but also photo-gonadal stimulation upregulates the expression of hypothalamic AVT genes and increases the localization of ir-AVT in many neurons of PVN. The above results support the existence of a parallel adrenal-gonad relationship and increase in adrenal function during osmotic stress, which also leads to simultaneous increase in AVT system. We conclude that photo-sexual conditions alter hypothalamic vasotocinergic and adrenal activity in Japanese quail and the degree of stimulation of the two systems following osmotic stress is higher under gonado-stimulatory LD conditions.     

Effect of photoperiod on pineal gland volume and pinealocyte size in the Chinese hamster, Cricetulus griseus

Male adult (200-day-old) Chinese hamsters (Cricetulus griseus) raised from weaning under either LD 16:8 or LD 8:16 were used. The pineal gland of the Chinese hamster consists of superficial (major) and deep (minor) components and a continuous, or interrupted, narrow parenchymal stalk interposed between them. The volume of the superficial pineal including the parenchymal stalk is greater under LD 16:8 than under LD 8:16. Under both photoperiods, pinealocytes in the superficial pineal have larger nuclei and more abundant cytoplasm than those in the deep pineal. Nuclei in the superficial pineal appear pale and usually have irregular profiles, whereas those in the deep pineal appear dark and have round profiles. In the superficial pineal, pinealocyte nuclei are larger, paler, and more irregular; and, in addition, nuclear density is lower under LD 16:8 than under LD 8:16. Similar, but less prominent, photoperiod-induced changes occur in the volume of the deep pineal, the size of pinealocytes, and pinealocyte nuclear morphology in the deep pineal. The results indicate that the development and differentiation of pinealocytes in both pineal portions may be advanced under long photoperiods and delayed under short photoperiods, although pinealocytes in the deep pineal may remain not fully differentiated even in adults. Since testicular weights and body weights are similar under both photoperiods, the photoperiod may exert marked influences on the development of the pineal gland without affecting reproductive activity and growth rates of animals.     

Role of photoperiod in reproductive maturation and peripubertal hormone concentrations in male collared lemmings (Dicrostonyx groenlandicus).

Onset of sexual maturation was determined in weanling male collared lemmings exposed to one of three experimental regimens of different photoperiods before and after weaning. Animals gestated in photoperiods of either 16 h light:8 h dark or 8 h light:16 h dark. Those from 16 h light:8 h dark were transferred at 19 days of age to either 20 h light:4 h dark or 8 h light:16 h dark; those gestated under 8 h light: 16 h dark remained in that photoperiod throughout the experiment. After exposure for 15, 20, 25 or 30 days to the postweaning photoperiod, animals were killed and the following parameters assessed: body weight, testes weight, seminal vesicle weight, the presence or absence of epididymal spermatozoa and serum concentrations of prolactin, testosterone and corticosterone. All parameters except serum testosterone were significantly influenced by photoperiod. Animals housed under 8 h light:16 h dark had significantly greater body weights than those housed under 20 h light:4 h dark, a response that differs from that reported for other arvicoline rodents. The group gestated on 16 h light:8 h dark and transferred on day 19 to 8 h light:16 h dark had lower testes and seminal vesicle weights than the other two groups, and mature spermatozoa in the epididymides appeared 5 days later than in the 20 h light:4 h dark group. Serum prolactin was largely undetectable in animals from both 8 h light:16 h dark groups, but all males housed in 20 h light:4 h dark had 2.0-15.0 ng prolactin ml-1. Concentration of serum corticosterone was higher in animals weaned into long photoperiod, and decreased with age. These data indicate that weanling male D. groenlandicus are reproductively photoresponsive, but use a decrease in photoperiod, not static short-photoperiod exposure, to alter the rate of development. Prolactin was largely undetectable in animals exposed to short photoperiod, indicating that high concentrations of this hormone are not important for maturation. Low prolactin concentrations in animals in short photoperiods may mediate the annual moult to white pelage. The short-photoperiod-mediated decrease in corticosterone may play a role in seasonal changes in body weight and composition.     

Effects of water deprivation on corticosterone production in the male Brattleboro rat genetically deprived of vasopressin

Effects of 72 h water-deprivation on plasma corticosterone concentration have been investigated in male Brattleboro rats homozygous for hypothalamic diabetes insipidus (DI) and in male Long-Evans rats (LE), as controls. To determine the global effect of water deprivation, drinking water deprived rats were compared with hydrated animals. Because water deprived rats showed a depressed food intake, to elucidate the specific effect of dehydration alone, drinking water deprived rats were compared with similar food-restricted but water supplied animals. Increases in adrenal weights and in plasma corticosterone content, following 72 h water-deprivation, were greater in DI than in LE rats. In LE rats, they seemed to be the result of both dehydration and denutrition. Conversely in DI rats lacking vasopressin, dehydration alone increased neither adrenal weights nor plasma concentration of corticosterone; the whole plasma corticosterone content was reduced. So, in DI rats, the global response to drinking water deprivation was essentially due to food restriction, whose effect was partly suppressed by dehydration. Whatever the circumstances, plasma concentrations of corticosterone were higher in DI than in LE rats. Interrelationships between water deprivation, stress, vasopressin and glucocorticoids are discussed.     

中华通草蛉自然越冬成虫在两种光周期下滞育解除过程中的生理生化变化

摘要: 光周期在昆虫滞育中起重要作用, 而昆虫光敏感性差异导致了光在昆虫滞育解除过程中作用机制的多样性。为明确光周期在中华通草蛉Chrysoperla sinica (Tjeder)滞育解除过程中的调控机制, 本研究测定了长光周期（15L∶9D）和短光周期（9L∶15D）条件下, 中华通草蛉自然越冬种群成虫在滞育解除过程中的干重、 含水量、 过冷却点（supercooling point, SCP）、 结冰点（freezing point, FP）及蛋白质和糖原含量变化。结果表明： 雌成虫干重在长光照下处理30 d低于短光照下, 其他处理时间的干重均高于短光照下, 但处理间差异不显著（P>0.05）; 雄成虫在长光照下处理5 d的干重高于短光照下, 其余处理时间的干重均低于短光照下, 其中处理30 d 时差异显著（P<0.05）。雌、 雄成虫含水量总体呈上升趋势, 长光照下上升显著（P<0.05）, 短光照下上升不显著（P>0.05）。雌、 雄成虫的SCP、 FP在短光照下处理不同时间均无显著性差异（P>0.05）, 而长光照下表现出先降后升的趋势且处理间差异显著（P<0.05）。成虫蛋白含量在短光照条件下雄虫处理10 d以及雌虫处理15 d均显著低于长光照（P<0.05）, 其他处理之间没有显著性差异。短光照处理成虫体内糖原含量均高于长光照, 且有显著性差异（P<0.05）。多因素方差分析表明, 不同光周期对中华通草蛉成虫滞育解除过程中的SCP、 FP、 糖原变化均有显著性影响（P<0.05）, 而对干重、 含水量及蛋白质含量影响不显著;性别只对成虫FP与糖原有显著影响（P<0.05）; 处理时间对干重、 含水量、 SCP、 FP、 蛋白及糖原含量均有显著影响（P<0.01）。3种因素的交互作用对SCP、 FP、 蛋白及糖原含量均有显著影响, 对干重及含水量影响不显著。初产卵成虫体内物质含量变化测定发现, 长光照（处理10 d）下雌、 雄成虫体内蛋白和糖原含量极显著低于短光照（处理25 d）下（P<0.01）, 雄成虫体内含水量显著高于短光照下（P<0.05）。结果说明, 中华通草蛉自然越冬成虫在长、 短两种光周期下均逐渐解除滞育, 但在滞育解除过程中呈现出不同的生理生化变化。     

Functional maturation of the gonads of Turkish hamsters under various photoperiods

The golden hamster, Mesocricetus auratus, is the only photoperiodic rodent to date that has been shown to fail to respond to inhibitory (i.e., short, less than 12.5 h/day) photoperiods until after pubertal onset. In other photoperiodic hamsters, mice, and voles, short photoperiods greatly retard gonadal maturation. The Turkish hamster, Mesocricetus brandti, is a photoperiodic rodent that as an adult is reproductively competent only on photoperiods of 15-17 h of light per day; photoperiods of less than 15 or greater than 17 h of light promote gonadal regression. In this report we addressed two questions: a) are prepubertal M. brandti photoperiodic, and b) if so, is gonadal maturation enhanced or suppressed by exposure to photoperiods of greater than 17 h of light per day? Turkish hamsters were raised on photoperiods of 12, 16, 20, or 24 (= LL) h of light per day. Testicular growth was retarded for 16 wk by 12L:12D. Very long days, 20L:4D, or LL did not retard testicular development. In females, pubertal onset, as indicated by first vaginal estrus, was delayed in young raised on 12L:12D and in 2 of 18 and 4 of 19 young raised on 20L:4D and LL, respectively. These results demonstrate that prepubertal Turkish hamsters are photoperiodic, but respond differently from adults to photoperiods greater than 17 h of light per day.     

Photoperiod and melatonin influence seasonal gonadal cycles in the grasshopper mouse (Onychomys leucogaster)

Development of the reproductive apparatus was delayed in grasshopper mice maintained from birth in short photoperiods (10 h light/day). The inhibitory effects of short photoperiods on sexual maturation eventually waned and mice in 10L:14D became reproductively active. Adult mice transferred from long (14 h light/day) to short photoperiods underwent testicular regression after 10 weeks and complete gonadal redevelopment after 30 weeks. A similar phenomenon was observed in adult female mice; oestrous cycles ceased within 3 weeks and resumed after 13 weeks in the short photoperiod. The regressive effects of short photoperiods on the male reproductive system were mimicked by daily injections of melatonin administered to mice housed in 14L:10D. Responsiveness of the female reproductive system to melatonin was reduced among photorefractory as compared to photosensitive mice. We suggest that the initial rate of sexual maturation and the timing of seasonal breeding in adult mice are regulated by photoperiod; effects of short daylengths on the neuroendocrine-reproductive axis appear to be mediated by the pineal gland.     

PHOTOPERIODISM IN AMARANTHUS CAUDATUS. I. A RE-EXAMINATION OF THE PHOTOPERIODIC RESPONSE

Zabka , George G. (State U. Iowa, Iowa City.) Photoperiodism in Amaranthus caudatus. I. A re-examination of the photoperiodic response. Amer. Jour. Bot. 48(1): 21–28. Illus. 1961.—Under the conditions described in this study, Amaranthus caudatus is not subject to inductive short days until it has reached its “sensitive period” or age which is approximately 30 days after the time of germination. Beyond this sensitive period, 2 days are sufficient to initiate inflorescence primordia. Macroscopic identification of this response is possible 2–3 days later, if the plants are retained on short photoperiods. Continued development of the inflorescence is also promoted by short days. This species will also initiate inflorescence primordia on long days of 18 hours duration approximately 60 days beyond germination. Consequently, this is not an obligate short-day plant as previously described. Although A. caudatus will initiate primordia on long days, subsequent normal development of the inflorescence proceeds only under short photoperiods. Plants initiating primordia on long or short photoperiods and then placed on long photoperiods will produce inflorescences which are stubby, generally recurved and spread at the apices. Subsequent flowering and seeding is also delayed. Plants initiating primordia on long days and then placed on short days develop mature inflorescences rapidly but they are divided at the apices and exhibit numerous basal branches.     

Twenty-four-hour profiles of serum leptin in siberian and golden hamsters: photoperiodic and diurnal variations

Serum leptin concentrations were obtained from male Siberian hamsters (Phodopus sungorus) and golden hamsters (a.k.a. Syrian, Mesocricetus auratus) housed on long [light:dark (LD) 16:8] and short (LD 6:18) photoperiods for 10-11 weeks. Blood samples were collected at 45-min intervals for 24 h from individual animals using an in-dwelling atrial catheter. In Siberian hamsters, exposure to short photoperiods as compared to long photoperiods reduced body weight (32.5 +/- 1.5 vs 47.7 +/- 1.1 g) and leptin (24-h mean: 5.3 +/- 0.4 ng/ml vs 18.6 +/- 2.1 ng/ml). Although photoperiod influenced the temporal distribution of leptin in golden hamsters, the main effect of photoperiod on leptin levels in golden hamsters did not reach significance (24-h mean: 7.1 +/- 1.0 ng/ml vs 5.1 +/- 0.8 ng/ml.). Body weights of golden hamsters did not vary significantly following exposure to short photoperiod for 11 weeks (178.3 +/- 3.6 g in LD 6:18 vs 177.8 +/- 7.3 g in LD 16:8). There was no nocturnal increase in serum leptin in either species. Marked interindividual differences were apparent in individual leptin profiles. Periodogram analysis revealed that only a few animals exhibited 24-h periodicities; the presence of a significant 24-h periodicity was more common in hamsters exposed to short days. Photoperiod-associated differences in the 24-hour profile of leptin secretion may be the result of photoperiod-associated changes in feeding behavior or metabolism. A full understanding of the regulation of leptin secretion in multiple time domains may enhance our understanding of the function of leptin.