Testicular regression in response to food restriction and short photoperiod in white-footed mice (Peromyscus leucopus) is mediated by apoptosis

Short day lengths or reduced food availability are salient cues for small mammals that breed seasonally. Photoperiod-mediated gonadal regression in white-footed mice (Peromyscus leucopus) is a slow, orderly process that involves testicular apoptosis. Testicular regression in response to restricted caloric intake is relatively rapid, and it is generally reversed quickly by ad libitum (ad lib) feeding. To determine the contribution of apoptotic cell death during food restriction, and to examine possible interactions with photoperiod, mice housed in long (16L:8D) or short (8L:16D) photoperiods were fed either ad lib or 70% of their average ad lib intake. Testes were removed at 2, 4, 6, or 8 wk of experimental treatment. Apoptotic activity was determined by in situ TUNEL labeling and assessment of DNA laddering. Significant (P < 0.05) gonadal regression in response to short days was first detected at 8 weeks in mice fed ad lib. Food-restricted, long-day mice also showed significant testicular regression at 8 wk. Combined exposure to short day lengths and food restriction resulted in significant testicular regression at 6 wk (P < 0.05). TUNEL labeling was slightly, though significantly, elevated in germ cells at 2 and 4 wk in long-day food-restricted mice (P < 0.05). TUNEL labeling was also elevated in short-day food-restricted males at these early times but then increased nearly 5-fold at 6 and 8 wk in these mice (P < 0.001). DNA laddering confirmed elevated apoptosis. Overall apoptotic activity negatively correlated with paired testis mass, plasma testosterone, and spermatogenic index measurements in both ad lib and food-restricted males. Few histological markers of necrotic cell death were observed in any group. Taken together, these results suggest that testicular regression in response to limited caloric intake or short days is mediated by apoptosis.     

Short photoperiods reduce vascular endothelial growth factor in the testes of Peromyscus leucopus

Testicular regression in rodents occurs after short-day exposure. Vascular support is withdrawn during regression, and, presumably, new angiogenesis is inhibited. Blood vessel growth and maintenance are regulated by paracrine factors, including vascular endothelial growth factor (VEGF). Reduced angiogenesis may contribute to the onset of photoperiod-induced regression; i.e., reduction of VEGF protein would be detected early during gonadal atrophy. Alternatively, loss of blood vessel maintenance may reflect reduced testicular volume. If true, VEGF would not be expected to decline until significant regression occurred. To discriminate between these hypotheses, white-footed mice (Peromyscus leucopus) were maintained in either long (LD 16:8) or short (LD 8:16) photoperiod. Immunohistochemical and Western analyses revealed high VEGF expression in Leydig and Sertoli cells in long-day housed males and reduced VEGF expression in short-day housed males. Reductions in VEGF preceded decreases in both seminiferous tubule diameter and spermatogenic activity by 6 wk and reduced testis mass by 8 wk, suggesting that changes in VEGF are not a consequence of gonadal regression and that VEGF may play a critical role in photoperiodic regulation of testicular function.     

Photoperiodic regulation of gonadal growth and pulsatile luteinizing hormone secretion in male ferrets

Testicular growth was monitored in male ferrets subjected to one of the following photoperiodic treatments begun at weaning (8 weeks of age): 8 hr light/day (short days), 18 hr light/day (long days), or short days followed by transition to long days at either 10, 12, or 14 weeks of age. Mean ages to achieve adult testis width of greater than or equal to 12 mm were 27.5 +/- 1.3, 25.0 +/- 1.5, 23.6 +/- 2.9, 20.0 +/- 0.8, and 21.2 +/- 1.0 weeks in ferrets raised from weaning in long days, raised from weaning in short days, and transferred from short to long days at 10, 12, or 14 weeks, respectively. This criterion was met significantly earlier by ferrets experiencing the photoperiod transition at 12 or 14 weeks of age than by ferrets housed in long days from weaning. At the end of the experiment (30 weeks of age), mean testis width was significantly smaller in ferrets raised in long days from weaning or transferred to long days at 10 weeks of age, compared to that of the other three groups (p less than 0.05). In a second experiment, photoperiod experience with long or short days was begun at birth, and testicular size was monitored for a longer period of time. The time courses for testicular maturation were similar to that obtained when these treatments began at weaning. By 40 weeks of age, mean testis width of ferrets raised in long days was comparable to that of ferrets raised in short days. A third study determined that the retarded testicular growth observed in ferrets exposed to long days from weaning was correlated with diminished pulsatile luteinizing hormone (LH) secretion. At 28 weeks of age, mean LH pulse frequency was 0.86 +/- 0.09 pulses/hr in ferrets undergoing spontaneous puberty in short days or photoinduced puberty after a short-to-long-day transition; pulse frequency was significantly lower (0.46 +/- 0.26 pulses/hr; p less than 0.05) in ferrets raised in long days. These results indicate that gonadal growth can be precociously induced in male ferrets by exposure to a sequence of short days followed by long days, and that the absence of sufficient prepubertal exposure to short days compromises pulsatile LH secretion and rate of gonadal growth. Experience with short days during development may be necessary for manifestation of stimulatory responses to long days.     

Effect of photoperiod on the size of the Leydig cell population and the rate of recruitment of Leydig cells in adult Syrian hamsters

The number of Leydig cells was determined by stereologic procedures in adult Syrian hamsters housed in long days (14L:10D) to maintain testicular activity (active), in short days (5L:19D) for 12-13 wk to induce testicular regression (photoperiod-induced regressed), or in short days for a period of 21 wk or more to allow spontaneous gonadal recrudescence (spontaneously recrudesced). Testes were removed, sliced, fixed, embedded in Epon 812, and observed by bright-field microscopy. Testicular and seminal vesicle weights, plasma testosterone concentration, total Leydig cell volume per testis, and volume of single Leydig cell were greater (p less than 0.01) in active and recrudesced animals than in regressed animals. The density of Leydig cells was greater in the regressed testes, but the total number per testis was not influenced by photoperiod. In Experiment 2, the rate of recruitment of Leydig cells was determined in 5 adult hamsters exposed to long days (active) or 5 hamsters whose testes were regressed by exposure of animals to short days for 13 wk followed by long-day exposure to initiate testicular growth (photoperiod-induced recrudescing). Hamsters were injected for 3 days/wk for 3 wk with tritiated thymidine, 0.5 or 1 microCi/g body weight. Testes were fixed and tissues prepared, as above, and processed for autoradiography. Again, the photoperiod did not influence the number of Leydig cells per testis. Labeling of Leydig cell nuclei revealed that recruitment of new Leydig cells occurred at approximately 1.3% per day in recrudescing testes but also occurred at approximately 0.6% per day in active testes. Without change in the total number of Leydig cells, new Leydig cells were added continually to the existing population in adult hamsters with either recrudescing or active testes.     

Maternal photoperiodic history affects offspring development in Syrian hamsters

During the first 7 weeks of postnatal life, short day lengths inhibit the onset of puberty in many photoperiodic rodents, but not in Syrian hamsters. In this species, timing of puberty and fecundity are independent of the early postnatal photoperiod. Gestational day length affects postnatal reproductive development in several rodents; its role in Syrian hamsters has not been assessed. We tested the hypothesis that cumulative effects of pre- and postnatal short day lengths would restrain gonadal development in male Syrian hamsters. Males with prenatal short day exposure were generated by dams transferred to short day lengths 6 weeks, 3 weeks, and 0 weeks prior to mating. Additional groups were gestated in long day lengths and transferred to short days at birth, at 4 weeks of age, or not transferred (control hamsters). In pups of dams exposed to short day treatment throughout gestation, decreased testis growth was apparent by 3 weeks and persisted through 9 weeks of age, at which time maximum testis size was attained. A subset of males (14%), whose dams had been in short days for 3 to 6 weeks prior to mating displayed pronounced delays in testicular development, similar to those of other photoperiodic rodents. This treatment also increased the percentage of male offspring that underwent little or no gonadal regression postnatally (39%). By 19 weeks of age, males housed in short days completed spontaneous gonadal development. After prolonged long day treatment to break refractoriness, hamsters that initially were classified as nonregressors underwent testicular regression in response to a 2nd sequence of short day lengths. The combined action of prenatal and early postnatal short day lengths diminishes testicular growth of prepubertal Syrian hamsters no later than the 3rd week of postnatal life, albeit to a lesser extent than in other photoperiodic rodents.     

Spontaneous "regression" of enhanced immune function in a photoperiodic rodent Peromyscus maniculatus.

Short days inhibit reproduction and enhance immune function in deer mice (Peromyscus maniculatus). Their reproductive inhibition is sustained by an endogenous timing mechanism: after ca. 20 weeks in short days, reproductive photorefractoriness develops, followed by spontaneous recrudescence of the reproductive system. It is unknown whether analogous seasonal timing mechanisms regulate their immune function or whether enhanced immune function is sustained indefinitely under short days. In order to test this hypothesis, we housed adult male deer mice under long (16 h light day(-1)) or short (8 h light day(-1)) day conditions for 32 weeks or under long day conditions for 20 weeks followed by 12 weeks of short days. Mice under the long day conditions remained photostimulated over the 32 weeks, whereas mice housed under the short day conditions exhibited gonadal regression followed by photorefractoriness and spontaneous recrudescence. Mice transferred to short days at week 20 were reproductively photoregressed at week 32. Total splenocytes, relative splenic mass and mitogen-activated splenocyte proliferation were greater in those mice transferred to short days at week 20 than in those mice housed under either long or short day conditions for 32 consecutive weeks, and immune function in mice exposed to short days for 32 weeks was comparable with that of long day animals. These data suggest that short day enhancement of immune function is not indefinite. With prolonged (< or = 32 weeks) exposure to short days, several measures of immune function exhibit "spontaneous" regression, restoring long day-like immunocompetence. The results suggest that formal similarities and, possibly, common substrates exist among the photoperiodic timekeeping mechanisms that regulate seasonal transitions in reproductive and immune function.     

Reproductive photorefractoriness in rams and accompanying changes in the patterns of melatonin and prolactin secretion

Exposure of rams to alternating 16-week cycles of long and short days (16L:8D and 8L:16D) results in periods of testicular regression followed by testicular redevelopment, and there is an inverse relationship between the blood levels of prolactin and testis activity. In this study, two groups of rams were held under long or short day lengths for a period of 94 weeks. When held under either long or short days for more than 16 weeks, the animals showed spontaneous changes in gonadal activity and in the secretion of prolactin, both of which were no longer correlated with the prevailing photoperiod, i.e., they became photorefractory. The photorefractoriness was characterized by cyclical changes in testis function which were independent of day length. The period of these spontaneous cycles was similar during both treatment regimens (long days: 40.9 +/- 1.5 weeks; short days: 38.1 +/- 0.33 weeks), suggesting the presence of an endogenous pacemaker for the reproductive system. The changes in blood prolactin levels during photorefractoriness were no longer correlated with testis activity, and though cyclical, the period lengths differed under the two regimens (long days: 31.8 +/- 1.4 weeks; short days: 48.6 +/- 0.3 weeks). The rates of change in testis function and prolactin secretion were slower during the refractory state than during the photosensitive state. Upon switching the rams to a different photoperiod after the 94 weeks of exposure to fixed day lengths, the rams showed relatively rapid testicular and prolactin responses. Photoperiodic information appears to be relayed to the endocrine system through the daily pattern of melatonin secretion by the pineal. We measured the daily blood levels of melatonin on several occasions during phases of photosensitivity and photorefractoriness in the same group of rams. During the first 21 weeks under both lighting treatments, the rams showed synchronized daily patterns in their blood levels of melatonin, with elevated levels occurring mainly during the daily period of darkness. Similar synchronized daily rhythms were also seen when the rams were switched to a different photoperiod following 94 weeks of exposure to either long or short days. Between Weeks 21 and 94, the daily rhythms of melatonin did not occur consistently in all rams; often, the patterns differed markedly between individual rams held under the same day length and peak levels of melatonin were not always confined to periods of darkness.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of photoperiod on the reproductive condition of Nile grass rats (Arvicanthis niloticus) from an equatorial population

We evaluated the effects of photoperiod on the reproductive condition of male and female Nile grass rats (Arvicanthis niloticus) descended from members of an equatorial population trapped 2°S of the equator. Study animals housed in 12:12 light:dark (LD) cycles were transferred either to short photoperiod (9:15) or long photoperiod (15:9) for 9 weeks (males) or 11 weeks (females), and various reproductive parameters were assessed. We observed no differences between short‐ and long‐day males with respect to plasma concentration of testosterone, testicular mass, seminal vesicle mass, or spermatogenesis. Similarly, we observed no differences between short‐ and long‐day females with respect to oestrous cycles, uterine mass, follicle size, or presence of corpora lutea. Reproductive parameters of male and female A. niloticus housed in short‐ and long photoperiods were similar to those typically observed among animals descended from the same equatorial population and housed in LD 12:12. Thus, photoperiod appears not to elicit changes in reproductive condition among A. niloticus from populations whose native habitat lies within 2° of the equator. These data contrast with the results of other studies indicating that photoperiod alters reproductive condition in A. niloticus populations living >10° from the equator.     

Reproductive photoresponsiveness in Aethomys ineptus and A. namaquensis (Rodentia: Muridae) from southern Africa

Male Tete veld rats Aethomys ineptus and Namaqua rock mice A. namaquensis were independently subjected to a long photoperiod of 16L:8D and a short photoperiod of 8L:16D for 3 months in order to assess the effect of photoperiod on testicular function. Testicular parameters examined included testicular mass relative to body mass, testicular volume, seminiferous tubule diameters and changes in circulating plasma testosterone concentration. On a long day (LD) length, both species exhibited significantly higher testicular mass relative to body mass and testicular volume, and larger seminiferous tubule diameters. Circulating plasma testosterone concentrations in A. ineptus were not significantly different between the two photoperiodic treatments, but A. namaquensis showed significantly higher testosterone concentration in males on a LD length than on a short day (SD) length. This suggests that both A. ineptus and A. namaquensis are photoperiodically responsive to LDs.     

Photoperiodic regulation of glutamatergic stimulation of secretion of luteinizing hormone in male Syrian hamsters.

It has been suggested that changes in endogenous glutamatergic stimulation of secretion of luteinizing hormone (LH) induced by photoperiod play a role in regulating seasonal cycles of reproductive activity. The aim of this study was to test the hypothesis that the glutamatergic control of the secretion of LH in the male Syrian hamster is sensitive to photoperiod, by determining whether the glutamate agonist N-methyl-D-aspartate (NMDA) could stimulate LH secretion in this species and, if so, to determine whether the response varied among animals exposed to different daylengths. In the first experiment, adult male hamsters were housed in either short day (8 h light: 16 h dark) for 6 weeks to induce testicular regression, or long days (16 h light: 8 h dark) to maintain testicular function, and the effects of systemic administration of NMDA on serum LH concentrations were determined. In the short-day hamsters, all s.c. doses of NMDA (25-75 mg kg-1 body weight) produced a robust rise in serum LH concentrations within 15 min. In the long-day hamsters, basal LH concentrations were higher than in short-day hamsters, but only the highest dose of NMDA produced a significant increase in LH concentrations, and the magnitude of this increment was less than those observed in short days. In hamsters in long days, the low doses of NMDA that did not significantly alter LH concentrations nevertheless significantly suppressed serum prolactin concentrations, demonstrating the efficacy of the drug. In hamsters in short days, serum prolactin concentrations were at the limit of detection of the assay, so no inhibitory effect of NMDA on prolactin secretion could be determined on this photoperiod. In the second experiment, the effects of a fixed dose of NMDA (50 mg kg-1 body weight) was tested at intervals in hamsters exposed to short days for a prolonged period such that their testes initially regressed, but then became scotorefractory and testicular recrudescence occurred. After 6 and 12 weeks in short days, NMDA stimulated LH secretion. However, after 24 weeks in short days when testicular recrudescence was complete, the response to NMDA was lost. A third experiment determined whether the reduced response to NMDA in hamsters on long days relative to those in short days might result from higher concentrations of circulating testosterone. Hamsters in long days were castrated to remove the influence of gonadal feedback, and the response to NMDA tested 3 weeks later when endogenous LH concentrations had risen to levels characteristic of the chronically castrated condition.(ABSTRACT TRUNCATED AT 400 WORDS)     

Differential activity of matrix metalloproteinases (MMPs) during photoperiod induced uterine regression and recrudescence in Siberian hamsters (Phodopus sungorus)

Siberian hamsters adapt to seasonal changes by reducing their reproductive function during short days (SD). SD exposure reduces uterine mass and reproductive capacity, but underlying cellular mechanisms remain unknown. Because matrix metalloproteinases (MMPs) and their tissue inhibitors (TIMPs) are important in uterine development, parturition, and postpartum remodeling, their expression in uterine tissue from Siberian hamsters undergoing photoperiod-mediated reproductive regression and recrudescence was investigated. Female hamsters were exposed to long day (LD, 16L:8D, controls) or SD (8L:16D) for 3-12 weeks (regression); a second group was exposed to SD or LD for 14 weeks and then transferred to LD for 0-8 weeks (recrudescence). Hamsters were euthanized, uteri collected, and homogenates analyzed by gelatin zymography or Western blotting for MMP and TIMP protein levels. Uterine weight decreased (67-75%) at SD weeks 12-14 and increased post-LD transfer (PT) reaching LD values by PT week 2. MMP-2, but not MMP-9 activity was reduced by SD week 12 or 14 but increased to LD levels at PT week 2. MMP-3 expression increased at SD week 9 compared to other SD and LD groups. MMP-14 and -13 protein levels decreased at SD week 3 but returned to LD levels by SD week 6. During recrudescence, MMP-3 (PT weeks 0-2), MMP-13 (PT week 4), and MMP-14 (PT weeks 2, 4) protein levels were higher than LD. TIMP-1 and 2 were present at low levels. Significant and differential variations in uterine MMP activity/expression during photoperiod-induced regression and recrudescence were observed. These changes likely reflect increases in tissue remodeling during both the adaptation to SD and the restoration of reproductive function.     

Reproductive effects of olfactory bulbectomy in the Syrian hamster

The effects of olfactory bulbectomy on circulating gonadotropin, prolactin and testosterone levels and on the testicular and pituitary responses to shortening of day length were studied in Syrian hamsters. Adult animals maintained on a 14L:10D cycle were sham-operated or sustained bilateral radical olfactory bulbectomies by aspiration to remove the main and accessory olfactory bulbs and the adjacent regions of the anterior olfactory nucleus. They were then maintained either on the long photoperiod or housed on a 10L:14D cycle. Testicular length was measured at weekly intervals over a 5-mo period. Sham-operated controls exhibited the normal pattern of testicular regression and eventual recrudescence on the short photoperiod. Testicular regression was significantly reduced in bulbectomized animals. Many of these animals showed no regression; others exhibited a reduced degree and/or shortened duration of regression. Serum levels of follicle-stimulating hormone (FSH) were substantially elevated in bulbectomized males maintained in long days. Their serum levels of luteinizing hormone (LH), prolactin and testosterone remained within the range for shams on long photoperiod. In short days, the bulbectomized animals showed the normal, pronounced decline in circulating prolactin levels. Serum FSH and LH levels also showed substantial declines, but the FSH levels were not reduced below the range for controls in long days, and the decline in LH levels was not as great as that for controls in short days.(ABSTRACT TRUNCATED AT 250 WORDS)     

Short days and exogenous melatonin increase aggression of male Syrian hamsters (Mesocricetus auratus)

Many nontropical rodent species rely on photoperiod as a primary cue to coordinate seasonally appropriate changes in physiology and behavior. Among these changes, some species of rodents demonstrate increased aggression in short, "winter-like" compared with long "summer-like" day lengths. The precise neuroendocrine mechanisms mediating changes in aggression, however, remain largely unknown. The goal of the present study was to examine the effects of photoperiod and exogenous melatonin on resident-intruder aggression in male Syrian hamsters (Mesocricetus auratus). In Experiment 1, male Syrian hamsters were housed in long (LD 14:10) or short (LD 10:14) days for 10 weeks. In Experiment 2, hamsters were housed in long days and half of the animals were given daily subcutaneous melatonin injections (15 microg/day in 0.1 ml saline) 2 h before lights out for 10 consecutive days to simulate a short-day pattern of melatonin secretion, while the remaining animals received injections of the vehicle alone. Animals in both experiments were then tested using a resident-intruder model of aggression and the number of attacks, duration of attacks, and latency to initial attack were recorded. In Experiment 1, short-day hamsters underwent gonadal regression and displayed increased aggression compared with long-day animals. In Experiment 2, melatonin treatment also increased aggression compared with control hamsters without affecting circulating testosterone. Collectively, the results of the present study demonstrate that exposure to short days or short day-like patterns of melatonin increase aggression in male Syrian hamsters. In addition, these results suggest that photoperiodic changes in aggression provide an important, ecologically relevant model with which to study the neuroendocrine mechanisms underlying aggression in rodents.     

Testicular recrudescence in intermediate day lengths reflects loss of photoperiodic memory in Siberian hamsters

Siberian hamsters transferred from a long (16 h light/day [16 L]) to an intermediate (13.5 L) day length (DL) undergo testicular regression within 2 months followed approximately 2 months later by "spontaneous" testicular recrudescence. Recovery of gonadal function after prolonged exposure to intermediate DLs is thought to reflect development of neuroendocrine refractoriness to intermediate-duration melatonin signals. The authors tested the alternative hypothesis that testicular recrudescence in 13.5 L occurs when the "memory" for the 16-L photoperiod fades and hamsters can no longer compare the 13.5-L to the prior 16-L day length. Adult hamsters transferred from 16 L to 13.5 L that underwent testicular involution were either maintained continuously in 13.5 L for 41 weeks or given a supplementary 2-week treatment of 16 L before being returned to 13.5 L. The supplementary treatment was administered either after hamsters had been in 13.5 L for 10 weeks and had involuted testes, or after 24 weeks, when the gonads had undergone recrudescence. The authors found that 16 L treatment administered at week 10 delayed final gonadal recrudescence by approximately 12 weeks; similar 16-L treatment at week 24 induced a second gonadal regression when animals were returned to 13.5 L. The most parsimonious hypothesis to account for these findings is that gonadal recrudescence in intermediate DLs reflects fading of the "memory" for prior long DLs rather than induction of refractoriness to melatonin signals generated in intermediate DLs.     

Effect of long or short photoperiod on pineal melatonin content in the white-footed mouse,Peromyscus leucopus

Adult white-footed mice were maintained under either a long photoperiod (LP, LD 16:8, lights out at 2100) or a short photoperiod (SP, LD 8:16, lights out at 1700) for six weeks. Subgroups from each lighting regime were killed at specific times over a 24 hour period. Pineal radioimmunoassayable melatonin levels were significantly elevated at night compared to daytime values. Pineal melatonin content appears to be elevated for a longer period of time in the SP mice than in the LP animals. The apparent increased melatonin production observed in white-footed mice maintained under short and reproductively repressive daylengths may help to explain the ability of chronically available exogenous melatonin to cause gonadal atrophy in this species.     

Photoperiod and temperature affect reproductive and nonreproductive functions in male prairie voles (Microtus ochrogaster)

Adult male prairie voles (Microtus ochrogaster) were housed for 10 wk and exposed to long (16L:8D) or short (8L:16D) photoperiods at 21 degrees or 5 degrees C. Maintenance in short day lengths reduced testicular, epididymal, and seminal vesicle mass and also significantly depressed spermatogenic activity. Cold ambient temperature further suppressed gonadal size in voles exposed to short days. Several pelage characteristics were affected by photoperiod, but not by temperature. Increased fur density, fur depth, and length of guard hair and underhair were observed in voles exposed to short days. Intrascapular brown fat and gonadal fat pad mass as well as body mass were significantly less in voles housed in cold temperatures than in voles exposed to warm ambient temperatures; photoperiod did not affect these parameters. Approximately 30% of the male voles exposed to short days maintained their reproductive systems, yet they clearly processed photoperiodic information; all short-day males, regardless of reproductive condition, had comparable winter pelage development. Our results suggest that in prairie voles, photoperiod may be a predictive cue for reproductive function in nature; however, it appears that pelage development is a more obligatory response to photoperiod than is reproduction.     

Photoperiod-peptide interactions in the energy intake of Siberian hamsters

Siberian hamsters (Phodopus sungorous sungorous) decrease their food intake when exposed to short (“winter-like”) photoperiods. The cause of this naturally-occurring hypophagia is unknown, but it may be due to a heightened sensitivity to the factors that normally terminate food intake in long photoperiods, such as the putative satiety peptides. The purpose of the present investigation was to test whether there would be an enhanced sensitivity to the inhibitory effects of some of these peptides on food intake in short relative to long days. Ad lib-fed, adult female Siberian hamsters were housed in a long photoperiod (LD 14:10) and injected with bombesin, glucagon, cholecystokinin octapeptide (CCK-8) and calcitonin (CT). Food intake was monitored 1, 2, 4, 6, and 24 hr post-injection. Bombesin and glucagon had no effect on food intake in long day-housed hamsters. CCK-8 and CT inhibited food intake; however, CCK-8 did so without any apparent behavioral disruption, while CT produced a marked and prolonged depression of behavior. After 10 weeks of exposure to a short photoperiod (LD 8:16) the hamsters were tested again. The previously ineffective dose of bombesin greatly inhibited food intake following short photoperiod exposure. In addition, an increased inhibition of food intake by CCK-8 was also found. In contrast, glucagon did not decrease food intake and CT still produced its non-specific, behaviorally disruptive effects. To our knowledge, this is the first demonstration that the effectiveness of a putative satiety peptide can be dependent upon a change in the photoperiod. This heightened responsiveness of short photoperiod-exposed Siberian hamsters to the inhibitory effects of bombesin and cholecystokinin may account for the reduction in food intake that accompanies short day exposure in this species.     

Photoperiod and temperature interact to affect the GnRH neuronal system of male prairie voles (Microtus ochrogaster)

Individuals of numerous species limit energy expenditure during winter by inhibiting reproduction and other nonessential functions. To time these adaptations appropriately with the annual cycle, animals rely on environmental cues that predict, well in advance, the onset of winter. The most commonly studied environmental factor that animals use to time reproduction is photoperiod. Rodents housed in short photoperiods in the laboratory or in naturally declining day lengths exhibit pronounced alterations in reproductive function concomitant with alterations in the hypothalamic gonadotropin-releasing hormone neuronal system. Because animals in their natural environment use factors in addition to photoperiod to time reproduction, the present study sought to determine the independent effects of photoperiod and temperature, as well as the interaction between these factors, on reproductive parameters and the GnRH neuronal system. Male prairie voles were housed in either long (LD 16:8) or short (LD 8:16) day lengths for 10 weeks. Animals in each photoperiod were further subdivided into groups housed in either mild (i.e., 20 degrees C) or low (i.e., 8 degrees C) temperatures. As shown with immunohistochemistry, voles that underwent gonadal regression in response to short photoperiods and long-day voles housed in low temperatures (and maintained large gonads) exhibit higher GnRH-immunoreactive (GnRH-ir) neuron numbers in the preoptic area/anterior hypothalamus (POA/AH) relative to all other groups. In addition, voles that underwent gonadal regression in response to both short days and low temperatures did not exhibit an increase in GnRH-ir neuron numbers compared to long-day, mild-temperature controls. These data suggest that photoperiod and temperature interact to influence reproductive function potentially by alterations of the GnRH neuronal system.     

Effects of photoperiod and temperature on the termination of diapause in the univoltine seed wasp Eurytoma plotnikovi

Abstract Mummified pistachios containing fully grown diapause larvae of Eurytoma plotnikovi Nikol'skaya (Hym., Eurytomidae) were collected in early August and late September in coastal northern Greece and subjected to various photoperiod and temperature treatments, then maintained at 19 or 26°C and a long-day (LD 16:8 h), a changing, or a short-day (LD 10:14 h) photoperiod until pupation. In larvae of early August (beginning of diapause) subjected for 20 weeks to 19°C under a long, a changing, or a short photophase, followed by 19°C and a long photophase, 50% of the larvae pupated after 24, 18 and 13 weeks respectively. After exposure for 20 or even 12 weeks to a short photophase and low temperatures (10 or 4°C), pupation occurred after only 7–8 weeks and was more synchronous. The ranges of temperature for diapause development and post-diapause morphogenesis overlap. After exposure for 12 weeks to short days and low temperature, larvae of late September pupated much sooner under long days than under short days and sooner at 26° than at 19°C. E.plotnikovi depends on both temperature and photoperiod for diapause development, low temperature having a strong favourable effect on the earlier part and long day on the later part of diapause. In a few larvae of another pistachio seed wasp, Megastigmus pistaciae Walker, after a long enough period of low temperatures, diapause was terminated normally at 26°C and long days, or at 19°C and long or short days.     

Responses to stepwise photoperiodic changes for the larval diapause of the Indian meal moth Plodia interpunctella

Abstract The Indian meal moth Plodia interpunctella Hübner (Lepidoptera: Pyralidae) diapauses as a last‐instar (fifth) larva. At 30 °C, no larvae enter diapause under any photoperiodic conditions; at 25 °C, the photoperiodic response curve is a long‐day type with a critical length of approximately 13 h light; at 20 °C, diapause is induced moderately even under long days (> 13 h). Cumulative effects of short days or long days on diapause induction are determined by alternate, stepwise and gradually changing regimes of photoperiod at 25 °C. When the larvae are repeatedly exposed to LD 16 : 8 h and LD 12 : 12 h photoperiods every other day, the incidence of diapause is 37%. When the larvae are placed under an LD 16 : 8 h photoperiod for 2 days and then under an LD 12 : 12 h photoperiod for 1 day, it is 38 %. Exposure to an LD 16 : 8 h photoperiod for 1 day and then to an LD 12 : 12 h photoperiod for 2 days induces only 15% diapause. This may indicate that the photoperiodic information is not accumulated in a simple fashion despite the generally accepted hypothesis (i.e. photoperiodic counter). Larvae exposed to an LD 16 : 8 h photoperiod for 5 days after oviposition express a very high incidence of diapause even under short days between an LD 2 : 22 h and LD 12 : 12 h photoperiod. After 10 days exposure to an LD 16 : 8 h photoperiod, however, the short day does not induce diapause strongly. On the other hand, an LD 12 : 12 h photoperiod in the early larval life is highly effective in the induction of diapause. A gradual increase or decrease of photoperiod (2 min day^?1) shows that the direction of photoperiodic change does not affect the diapause determination.