Effects of constant darkness and constant light on circadian organization and reproductive responses in the ram

The relationship between circadian rhythms in the blood plasma concentrations of melatonin and rhythms in locomotor activity was studied in adult male sheep (Soay rams) exposed to 16-week periods of short days (8 hr of light and 16 hr of darkness; LD 8:16) or long days (LD 16:8) followed by 16-week periods of constant darkness (dim red light; DD) or constant light (LL). Under both LD 8:16 and LD 16:8, there was a clearly defined 24-hr rhythm in plasma concentrations of melatonin, with high levels throughout the dark phase. Periodogram analysis revealed a 24-hr rhythm in locomotor activity under LD 8:16 and LD 16:8. The main bouts of activity occurred during the light phase. A change from LD 8:16 to LD 16:8 resulted in a decrease in the duration of elevated melatonin secretion (melatonin peak) and an increase in the duration of activity corresponding to the changes in the ratio of light to darkness. In all rams, a significant circadian rhythm of activity persisted over the first 2 weeks following transfer from an entraining photoperiod to DD, with a mean period of 23.77 hr. However, the activity rhythms subsequently became disorganized, as did the 24-hr melatonin rhythms. The introduction of a 1-hr light pulse every 24 hr (LD 1:23) for 2 weeks after 8 weeks under DD reinduced a rhythm in both melatonin secretion and activity: the end of the 1-hr light period acted as the dusk signal, producing a normal temporal association of the two rhythms. Under LL, the 24-hr melatonin rhythms were disrupted, though several rams still showed periods of elevated melatonin secretion. Significant activity rhythms were either absent or a weak component occurred with a period of 24 hr. The introduction of a 1-hr dark period every 24 hr for 2 weeks after 8 weeks under LL (LD 23:1) failed to induce or entrain rhythms in either of the parameters. The occurrence of 24-hr activity rhythm in some rams under LL may indicate nonphotoperiodic entrainment signals in our experimental facility. Reproductive responses to the changes in photoperiod were also monitored. After pretreatment with LD 8:16, the rams were sexually active; exposure to LD 16:8, DD, or LL resulted in a decline in all measures of reproductive function. The decline was slower under DD than LD 16:8 or LL.(ABSTRACT TRUNCATED AT 400 WORDS)     

Phase shifts in the circadian rhythm in plasma concentrations of melatonin in rams induced by a 1-hour light pulse

The effect of a 1-hr light pulse, given at night, on the timing of the circadian rhythm in the plasma concentration of melatonin was examined in Soay rams to investigate the mechanisms involved in determining the duration of the nocturnal peak in melatonin secretion. Animals (n = 8) were housed under short days (LD 8:16) or long days (LD 16:8) and received a light pulse at various times of night. They were released into constant dim red light (DD) on day 1. Blood samples were collected hourly for 30 hr from 1000 hr on day 3, and the plasma concentration of melatonin was determined by radioimmunoassay to assess the timing of the melatonin peak. Control animals (n = 8) were maintained under the same conditions but received no light pulse. Under short days, a light pulse given early in the night caused a phase delay in the melatonin peak, and a light pulse given in the late night caused a phase advance. The mean duration of the melatonin peak was slightly reduced following a light pulse in the early or late night, and slightly increased following a pulse given near the middle of the night. Under long days, both light-pulse treatments given at night caused a phase delay in the melatonin peak, but there was no significant change in duration of the melatonin peak. The duration of the melatonin peak at day 3 under DD in the control animals was similar for all treatments, regardless of the previous entraining photoperiod (mean duration: 12.6-14.8 hr) and was similar to that under short days (14.6 hr), but was significantly longer than that under long days (8.2 hr). Information on the phase response curve in the Soay ram and on the period of the circadian oscillator governing the melatonin rhythm (c 23.0 hr under DD) predicts a close phase relationship between the end of the light phase and the onset of the melatonin peak as observed under normal 24-hr LD cycles. The current results also indicate that light acts to entrain the circadian rhythm influencing the onset and offset of melatonin secretion, and thus dictates the duration of the melatonin peak.     

Amplitude modulation of the nightly melatonin rise in the neonatal lamb and the subsequent timing of puberty

Spring-born female lambs require a decrease in day length for the normal timing of puberty the following autumn. If this decrease occurs early in postnatal life (i.e. 0-10 weeks), puberty is delayed. This study tested the hypothesis that failure of the neonatal lamb to respond to the critical long-day to short-day signal is due to inadequate nocturnal melatonin secretion. The approach was to artificially increase, to adult levels, the low nighttime rises of melatonin during the early postnatal period. Eight female lambs served as controls; they were raised on short days until 17 wk of age, and then exposed to 5 wk of long days, after which they were returned to short days. This alternating sequence of photoperiods during mid-development would be expected to induce normal puberty. Sixteen experimental females were exposed to the critical block of long days much earlier; they were placed in long days between 2 and 7 wk of age and in short days thereafter. Half (n = 8) received no further treatment. The other half (n = 8) were infused nightly with melatonin during the 8-h dark phase of the 5-wk, long-day photoperiod. This increased the amplitude of the natural nighttime melatonin rises 3- to 4-fold, well into the adult range.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dynamics of the adjustment of clock gene expression in the rat suprachiasmatic nucleus to an asymmetrical change from a long to a short photoperiod

The molecular clockwork of the rat suprachiasmatic nucleus, the site of the circadian clock, is affected by the photoperiod (Sumová et al., 2003). The aim of the present study was to partly elucidate the dynamics of the adjustment of the clockwork to a change from a long to a short photoperiod accomplished by an asymmetrical prolongation of the dark period into the morning hours. Rats maintained under a regime with 16 h of light and 8 h of darkness per day (LD 16:8) were transferred to LD 8:16, and after 2, 3, and 13 days, daily profiles of Per1, Per2, Bmal1, and Cry1 mRNA were assessed by in situ hybridization. The rhythms of Per1, Per2, and Bmal1 expression adjusted to the change from a long to a short photoperiod with larger phase delays of the morning Per mRNA rise and Bmal1 mRNA decline than of the evening and nighttime Per mRNA decline and Bmal1 mRNA rise. The rhythm of Cry1 expression adjusted to the change by parallel delays of the Cry1 mRNA rise and decline. Adjustment of the Cry1 mRNA rhythm to short days was almost accomplished within 13 days, whereas adjustment of the Per1 and Bmal1 mRNA rhythms took longer. Different dynamics of the adjustment of rhythms in clock gene expression to a change from a long to a short photoperiod suggests complex resetting effects of the photoperiod change.     

Ontogeny of the daily profile of plasma melatonin in European starlings raised under long or short photoperiods

Photoperiodic manipulation of young European starlings suggests that their reproductive physiology is incapable of responding to a short photoperiod until they are fully grown. This study aimed to determine whether the lack of response to a short photoperiod is reflected in the daily profile of plasma melatonin concentrations. Five-day-old starlings taken from nest boxes showed a significant (p < 0.0001) rhythm in plasma melatonin concentrations, with high values during night. In nestlings hand-reared from 5 days of age on a long photoperiod (LD 16:8), equivalent to natural photoperiod at the time, the amplitude of the daily rhythm in melatonin increased significantly (p < 0.01) with age until birds were fully grown (20 days old). In nestlings reared on a short photoperiod (LD 8:16), the daily melatonin profile remained almost identical to that of long photoperiod birds until they were fully grown. However, after 20 days old, the duration of elevated nighttime melatonin began to extend to encompass the entire period of darkness. In contrast, fully grown starlings transferred from a long to a short photoperiod had partially adapted to the short photoperiod after 5 days; by 10 days, the daily melatonin profile was identical to that of birds held chronically on a short photoperiod. Thus, consistent with responses of reproductive physiology, the pineal of young birds appears to be incapable of perceiving, or adapting to, a short photoperiod.     

Circadian rhythm of sex pheromone-releasing behaviour in females of the dermestid beetle,Trogoderma glabrum: Regulation by photoperiod

The effects on sex pheromone-releasing, or calling behaviour, of diel photoperiods of varying daylength, of light cycle phase shifts, and of continuous illumination were investigated in Trogoderma glabrum females. On light régimes with 8 to 20 hr daylengths, calling maxima tended to centre close to photophase midpoints. Although influencing the time of day at which calling occurred, daylength had little effect on the amount of activity or the length of the calling period. When 16 : 8 LD light cycles were advanced or delayed by 4 hr, the time of day at which calling peaks were observed shifted within 2 to 4 cycles so that a constant phase relationship with photoperiod was maintained. Daily calling peaks were evident in groups of females exposed to between 1 and 5 days of continuous illumination, but mean calling time occurred earlier in the day as light exposures were lengthened. It was concluded that a circadian rhythm of calling behaviour exists in T. glabrum females. and that the rhythm can be entrained to 24 hr periodicity by photoperiod.     

Effects of melatonin feeding on smoltification in masu salmon (Oncorhynchus masou)

Influences of photoperiod on plasma melatonin profiles and effects of melatonin administration on long-day-induced smoltification in masu salmon (Oncorhynchus masou) were investigated in order to reveal the roles of melatonin in the regulation of smoltification in salmonids. Under light-dark (LD) cycles, plasma melatonin levels exhibited daily variation, with higher values during the dark phase than during the light phase. The duration of nocturnal elevation under short photoperiod (LD 8:16) was longer than that under long photoperiod (LD 16:8). Melatonin feeding (0.01, 0.1 and 1 mg/kg body weight) elevated plasma levels of melatonin in a dose-dependent manner for at least 7 h but not for 24 h. When masu salmon reared under short photoperiod were exposed to long photoperiod (LD 16:8) and fed melatonin (1 mg/kg body weight) 7 hours before the onset of darkness, a significantly smaller proportion of smolts appeared in the melatonin-fed group after 32 days than in the control group. However, after 59 days of the treatment, there was no difference in the proportion of smolts between the control and melatonin-treated groups. Thus, melatonin feeding mimicked the effects of short photoperiod, which delays but does not completely suppress smoltification. These results indicate that the day length is transduced into changes in the duration of nocturnal elevation in plasma melatonin levels, and that artificial modification of the plasma melatonin pattern possibly delays the physiological processes of smoltification induced by long-day photoperiodic treatment.     

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.     

Photoperiodic regulation of circulating leukocytes in juvenile Siberian hamsters: mediation by melatonin and testosterone

The reproductive system of Siberian hamsters (Phodopus sungorus) undergoes rapid phenotypic responses to changes in day length that occur around the time of weaning. The present experiments tested whether the immune system of Siberian hamsters is similarly photoperiodic early in life and whether photoperiodic changes in melatonin or gonadal hormone secretions mediate any such responses to day length. Circulating blood leukocyte concentrations (WBC) were measured in juvenile male Siberian hamsters that were gestated in long-days (LD), transferred to short-days (SD) on the day of birth, and subsequently either remained in SD or were transferred from SD to LD at 18 days of age (day 18). WBC values were comparable between LD and SD hamsters on day 18. Between day 18 and day 32, SD hamsters exhibited a 3-fold increase in WBC, whereas LD hamsters failed to undergo a significant increase in WBC during this interval. WBC of LD hamsters was significantly lower than that of SD hamsters on day 25 and on day 32. In LD housed males, peripheral injections of melatonin delivered so as to extend the nocturnal duration of elevated endogenous melatonin secretion (i.e., provided in late afternoon) on days 18-31 increased WBC as measured on day 32. Peripubertal (day 17) gonadectomy abolished the immunosuppressive effect of LD exposure on WBC, and treatment with silastic implants containing testosterone suppressed WBC independent of photoperiod treatment. These data indicate that juvenile Siberian hamsters are immunologically responsive to photoperiod and that the leukocyte responses to day length are the result of melatonin-mediated effects of photoperiod on testicular hormone secretion.     

Photoperiod requirements for puberty differ from those for the onset of the adult breeding season in female sheep

Reproductive responses to photoperiod were directly compared in mature ewes and in their spring-born twin female lambs. All females were ovariectomized and treated with oestradiol implants before transfer into artificial photoperiod; serum LH concentrations and pulsatile LH patterns provided an index of neuroendocrine reproductive activity. Mothers were transferred from natural photoperiod to artificial long days (16 h light:8 h dark) at the summer solstice so that no decrease in photoperiod would be experienced. These ewes began reproductive activity synchronously at the expected time in the autumn. One of each pair of twin lambs was treated exactly as the mothers; to determine the normal timing of puberty the remaining twin was maintained in a photoperiod simulating the natural decrease in daylength. In all 6 control lambs experiencing the simulated natural photoperiod, reproductive activity occurred synchronously at 28 +/- 1 weeks of age (2 October +/- 7 days). However, in their twin sisters which did not experience a decrease in photoperiod, only 2 of 6 lambs had begun reproductive activity by the end of the experiment at 52 weeks of age (March), and these were both delayed relative to their twin control lambs exposed to decreasing daylength. Therefore, a decrease in photoperiod is necessary for the normal timing of puberty in the spring-born, female sheep, whereas seasonally anoestrous, mature sheep can enter the breeding season at a normal time in the absence of decreasing photoperiod. We suggest that the requirement for a decreasing photoperiod by the spring-born lamb reflects its limited photoperiodic history as compared to the adult.     

Importance of duration of nocturnal melatonin secretion in determining the reproductive response to inductive photoperiod in the ewe

The pineal gland, through its nocturnal melatonin secretion, mediates the effects of inhibitory (long) and stimulatory (short) photoperiod on reproduction in female sheep. Earlier studies revealed that duration of the nighttime melatonin rise is important in determining the inhibitory effect of day length on reproduction in the ewe. The present study tested whether the duration is also important in mediating the inductive response to short days. Pinealectomized ewes, housed under long days, received a short-day melatonin infusion (16-h duration) for 90 days. Reproductive status was monitored from the response to estradiol negative feedback of luteinizing hormone (LH) secretion. This short-day melatonin pattern led to unambiguous reproductive induction, despite the exposure to inhibitory long days. The increase in serum LH was comparable, in terms of latency and magnitude, to that in pinealectomized controls receiving the same short-day melatonin pattern under short days, and in pineal-intact controls transferred from long to short days. Since the reproductive status conformed to the length of time that melatonin was elevated each day rather than to photoperiod, these results support the conclusion that duration of the nighttime melatonin rise mediates the reproductive response of the ewe to an inductive photoperiod. In all, the melatonin rhythm is considered an integral component of the physiologic mechanism measuring day length; through duration of its nocturnal secretion, melatonin encodes both inhibitory and stimulatory photoperiods.     

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)     

Effect of naloxone on gonadotrophin secretion at various stages of development in the ewe lamb

Spring-born crossbred ewe lambs were raised in a natural photoperiod and saline (N = 6) or naloxone (1 mg/kg) in saline (N = 6) was injected (i.m.) every 2 h for 6 h at 5, 10 and 15 weeks of age and for 8 h at 20, 25 and 30 weeks of age. Blood samples were taken every 12 min during treatment periods. Naloxone had no effect on time to first oestrus (controls 235 +/- 6 days, naloxone 242 +/- 7 days). Mean serum LH concentrations and LH pulse frequency were elevated by naloxone in ewe lambs at 20, 25, and 30 weeks of age (P less than 0.05). The only FSH response to naloxone was a depression of mean serum concentrations at 30 weeks of age (P less than 0.05). LH pulse amplitude was elevated at 5 weeks of age in all ewe lambs and declined thereafter to a nadir at 30 weeks of age in control, but not in naloxone-treated animals (P less than 0.05). LH pulse frequency was elevated at 10 weeks of age in control ewe lambs and in all animals at 30 weeks of age (P less than 0.05). FSH pulse frequency declined from 5 weeks of age in control ewe lambs (P less than 0.05), with very few pulses noted in 25- and 30-week-old animals. We conclude that (1) opioidergic suppression of LH, but not FSH, secretion developed at 20 weeks of age in the growing ewe lambs used in the present study, with no obvious change in suppression before the onset of first oestrus: (2) pulsatile FSH secretion occurred in the young ewe lamb but was lost as the lamb matured: (3) attainment of sexual maturity was preceded by an elevation in LH pulse frequency.     

Changes in melatonin binding sites under artificial light-dark, constant light and constant dark conditions in the masu salmon brain

To test whether the affinity (Kd) and total binding capacity (Bmax) of melatonin receptors exhibit daily and circadian changes in teleost fish whose melatonin secretion is not regulated by intra-pineal clocks, we examined the changes in melatonin binding sites in the brains of underyearling masu salmon Oncorhynchus masou under artificial light-dark (LD), constant light (LL) and constant dark (DD) conditions. In Experiment 1, fish were reared under a long (LD 16:8) or short (LD 8:16) photoperiod for 69 days. Blood and brains were sampled eight times at 3 h intervals. Plasma melatonin levels were high during the dark phase and low during the light phase in both photoperiodic groups. The Bmax exhibited no daily variations. Although the Kd slightly, but significantly, changed under LD 8:16, this may be of little physiological significance. In Experiment 2, fish reared under LD 12:12 for 27 days were exposed to LL or DD from the onset of the dark phase under LD 12:12. Blood and brains were sampled 13 times at 4 h intervals for two complete 24 h cycles. Plasma melatonin levels were constantly high in the DD group and low in the LL group. No significant differences were observed in the Kd and the Bmax between the two groups, and the Kd and the Bmax exhibited no circadian variation either in the LL or DD groups. These results indicate that light conditions have little effect on melatonin binding sites in the masu salmon brain.     

Endogenous circannual cycles of ovarian activity and changes in prolactin and melatonin secretion in wild and domestic female sheep maintained under a long-day photoperiod

The present study examines the ovulatory activity of wild and domesticated ewes subjected to either a constant photoperiod of long days (16L:8D) or natural changes in daily photoperiod for 16 mo. The aim was to determine whether an endogenous reproductive rhythm controls seasonal reproductive activity in these sheep, and how the photoperiod might affect this. The effects of long-day photoperiods on long-term changes in prolactin and melatonin secretion were also evaluated. The two species showed changes in reproductive activity under the constant photoperiod conditions, suggesting the existence of an endogenous rhythm of reproduction. This rhythm was differently expressed in the two types of ewe (P < 0.05), with the domestic animals exhibiting much greater sensitivity to the effects of long days. A circannual rhythm of plasma prolactin concentration was also seen in both species and under both photoperiod conditions, although in both species the amplitude was always lower in the long-day animals (P < 0.01). The duration of the nocturnal melatonin plasma concentrations reflected the duration of darkness in both species and treatments. The peak melatonin concentration did not differ between seasons either under natural or long-day photoperiods.     

Response in peripheral plasma melatonin to photoperiod change and the effects of exogenous melatonin on seasonal quiescence in the tammar, Macropus eugenii

During the light phase of each of 3 photoperiods tested, plasma melatonin concentrations were less than 16 to 62 pg/ml and during the dark phase they were 31 to 169 pg/ml. When the photoperiod to which the tammars were exposed was altered from 15 h light:9 h dark to 12L:12D the onset of the nocturnal rise in melatonin was advanced from the first day, thereby extending its duration, and the females gave birth 32 +/- 0.4 (mean +/- s.e.m.) days later. To test whether melatonin mediated this effect of photo-period change, tammars in a second group were injected s.c. with melatonin (400 ng/kg, N = 6) or the arachis oil vehicle (N = 6), 2.5 to 2.25 h before dark during 15L:9D for 15 days before exposure to 12.5L:11.5D. The melatonin injections mimicked the endogenous melatonin profile of 12L:12D and the melatonin-injected tammars gave birth 32 +/- 0.8 days after the start of injections, which was the same as the interval from photoperiod change in Exp. 1 but was significantly different (P less than 0.005) from the interval in the control group (46.0 +/- 1.1 days). These results show that exogenous melatonin given 2.5 to 2.25 h in advance of the endogenous rise fully mimics the response of the tammar to photoperiod change.     

Diurnal Sensitivity of the Neuroendocrine-Reproductive Axis to the Antigonadotrophic Influence of Melatonin in Male Syrian Hamsters with Experimentally Altered Cortisol Rhythms

Two different experimental models were used to test if a temporal relationship exists between the rhythm of adrenal steroid secretion and the vulnerability of the hamster reproductive system to short photoperiod exposure or to the daily afternoon injection of melatonin. In the first experiment adrenalectomized hamsters were implanted with a Cortisol pellet to provide a sustained, rather than rhythmic, level of the hormone. The animals were either placed in short photoperiod or given a daily afternoon melatonin injection. In both cases the gonads underwent atrophy. In the second experiment adrenalectomized hamsters were given a Cortisol injection either in the morning (approx. 8 hr before the subsequent afternoon injection of melatonin) or in the afternoon (approx. 1 hr before the subsequent melatonin injection). Measurements of testicular and accessory organ weights 7 weeks later indicated regression of the reproductive system in both the groups when compared with their appropriate controls. Depressed levels of plasma LH. PRL, testosterone and thyroxine (T4) in these animals confirmed the melatonin induced gonadal collapse. The results suggest that apparently there is no temporal correlation between the rhythm of secretion of the adrenal steroids and the responsiveness of the reproductive system to late afternoon injection of melatonin. Interestingly, all the adrenalectomized Cortisol injected control animals (not receiving melatonin) had depressed plasma LH and PRL while the testicular weights and plasma testosterone titers remain unaffected.     

The effect of prenatal photoperiodic history on the postnatal endocrine status of female lambs

Postnatal photoperiodic experience plays a pivotal role in determining the timing of ovarian activity in female lambs. This study examines whether a photoperiodic history gained while in utero is able to influence this timing.Pregnant Soay ewes were maintained in either long days (n = 7, 18 h light : 6 h dark; group PLD) or short days (n = 12, y h light : 18 h dark; group PSD) from 25 days of gestation. At birth, female lambs (n = 8 per group) were transferred to long days for 10 weeks, and then placed under short days until the end of the experiment at 38 weeks of age. Blood samples were collected from lambs on the day of birth and three times weekly for the duration of the study and the resulting plasma assayed for progesterone and prolactin.Although both gestational photoperiods produced, at best, abbreviated periods of ovarian activity, lambs born to ewes which experienced long days during gestation (group PLD) exhibited elevated plasma progesterone concentrations significantly earlier (P < 0.05) than lambs born to ewes exposed to short days during gestation (group PSD) (mean ± SEM, 193 ± 17 versus 244 ± 14 days for PLD and PSD groups, respectively. Plasma prolactin concentrations in newborn lambs born between late December and early April were not affected by the ambient photoperiod, but reflected the artificial daylength experienced by their mothers during gestation. Lambs born to ewes maintained under long days during gestation (group PLD) had significantly higher prolactin concentrations on the day of birth than lambs born to ewes maintained under short days during gestation (group PSD) (45 ± 5.4 ng/ml versus 7 ± 3.7 ng/ml respectively, P < 0.001). The mean birth weight, rate of live weight gain and live body weight of lambs at the end of the experiment did not vary significantly between treatment groups. These results suggest that the ovine foetus is sensitive to photoperiodic information prior to birth, and develops a photoperiodic history which, under the present experimental conditions, modulates the subsequent endocrine status of the neonatal lamb.     

Spontaneous c-Fos rhythm in the rat suprachiasmatic nucleus: location and effect of photoperiod

A recently reported circadian rhythm in the spontaneous c-Fos immunoreactivity in the rat suprachiasmatic nucleus (SCN) is expressed mostly in the dorsomedial (dm) SCN, where vasopressinergic cells are located. The aim of the present study is to find out whether day length, i.e., photoperiod, affects the dm-SCN rhythm and, if so, how the rhythm adjusts to a change from a long to a short photoperiod. In addition, a question of whether the spontaneous c-Fos production is localized in vasopressin- producing cells or in other cells is also studied to characterize further the dm-SCN rhythmicity. Combined immunostaining for c-Fos and arginine vasopressin (AVP) revealed that most of c-Fos immunopositive cells were devoid of AVP; the results suggest that c-Fos-producing cells in the dm-SCN are mostly not identical with those producing AVP. In rats maintained under a long photoperiod with 16:8-h light-dark cycle (LD 16:8) daily and then released into darkness, the time of the afternoon and evening decline of the spontaneous c-Fos immunoreactivity in the dm-SCN differed just slightly from the time in rats maintained originally under a short LD 8:16 photoperiod; however, the morning c-Fos rise occurred about 4 h earlier under the long than under the short photoperiod. After a change from a long to a short photoperiod, a rough but not yet a fine adjustment of the morning c-Fos rise to the change was accomplished within 3-6 days. The results show that similar to the recently reported ventrolateral SCN rhythmicity, the intrinsic dm-SCN rhythmicity is also affected by the photoperiod and suggest that the whole SCN state is photoperiod dependent.     

Effect of 12 hr temporal relation of serotonin and dopamine precursor drugs (5-HTP and L-DOPA) on photosexual responses of immature Japanese quail

To study the interaction of photoperiod and circadian neurotransmitter activity, L-DOPA L-dihydroxyphenylalanine, dopamine precursor) was administered daily, 12 hr after 5-HTP (5-hydroxy tryptophan, serotonin precursor) in sexually immature Japanese quail, raised under short photoperiod (LD8:16) since hatching. The 12 hr treatment of 5-HTP and L-DOPA was given under continuous condition of light for 11 days. After treatment the quail were transferred to intermediate day length (LD 13.5:10.5). The cloacal gland size of drug treated group increased significantly in comparison to control. The quail were then transferred to short photoperiod (LD 8:16). The cloacal gland size of both the groups started decreasing gradually but the rate was significantly low in drug treated quail in comparison to control. The results indicate that the endogenous mechanism controlling seasonality may be reset by drugs that influence serotonergic and dopaminergic activity. The 12 hr relation between the two drugs is stimulatory for gonadal growth under intermediate day length and retards the rate of regression when transferred to short days.