Geographic differences for delay of sexual maturation in Peromyscus leucopus: effects of photoperiod, pinealectomy, and melatonin

Effects of short-day photoperiod, pinealectomy, and melatonin on sexual maturation were tested in Peromyscus leucopus from either Connecticut (CT) or Georgia (GA). Laboratory reared-stocks from CT and GA were exposed to short daylength (photoperiod) from birth or 25 days of age. At 12 wk of age, delay in sexual maturation was indicated in most CT mice by decreased testis length, combined testes weight, and seminal vesicle weight. Conversely, GA animals did not delay sexual maturation when exposed to short-day photoperiod from either birth or 25 days of age. These results indicate that responses to short daylengths differ for juvenile CT and GA populations. In a second experiment, pinealectomized or sham-operated CT males were exposed to short-day (9L:15D) or long-day (16L:8D) photoperiod from birth. Pinealectomy blocked the effect of short daylength on reproduction. Therefore, the pineal must be involved in the delay of sexual maturation observed for short-day CT mice. The effects of melatonin, a pineal gland hormone, were tested with chronic s.c. implants or daily injections. In CT mice given either melatonin implants or afternoon injections, sexual maturation was delayed. GA mice were insensitive to all melatonin treatments. Further, no differences in circadian organization (phase angle, duration of activity, period under constant dark) between GA and CT animals were apparent. Collectively, these studies indicate that melatonin is involved in the mechanism responsible for delay of sexual maturation in CT mice. Short-day insensitivity of GA Peromyscus leucopus probably results from a deficiency in the melatonin effector pathway and is not due to a disruption of circadian organization.     

Two daily melatonin injections differentially induce nonshivering thermogenesis and gonadal regression in the mouse (Peromyscus leucopus)

Female white-footed mice (Peromyscus leucopus) were injected twice daily with 5, 10, 50, 100 micrograms melatonin (MEL) or saline. Injections were given for 7 weeks at 2 and 12 hours after lights-on under a long day (LD 16:8) photoperiod. Afternoon administration of MEL induced gonadal regression, although a dose of 50 micrograms or more was necessary to obtain a maximal response. A 5 micrograms MEL injection in the afternoon resulted in intermediate reproductive tract weights. In white-footed mice a morning MEL injection did not abolish the reproductive regression induced by an afternoon injection. Mice receiving 10, 50 or 100 micrograms MEL daily exhibited increased nonshivering thermogenesis (NST), irrespective of the timing of the injection. Daily injections of 5 micrograms MEL had little effect on NST. These observations suggest that "up and down regulation" of MEL receptors may not be important in P. leucopus. Further, the mechanism by which MEL controls reproduction is different from that for NST.     

6-MBOA affects testis size, but not delayed-type hypersensitivity, in white-footed mice (Peromyscus leucopus)

Many rodents use day length to time reproduction to occur when resources are abundant, but some species also use supplementary environmental cues. One supplementary cue is the plant-derived compound, 6-methoxy-2-benzoxazolinone (6-MBOA). Most rodents grow their gonads in response to 6-MBOA in their diets, but it is presently unknown whether they also use 6-MBOA to adjust other aspects of physiology, specifically their immune systems. 6-MBOA is structurally similar to melatonin, and seasonal changes in rodent immune activities are often mediated by melatonin. We therefore predicted that white-footed mice (Peromyscus leucopus), which breed seasonally and are reproductively sensitive to melatonin, would adjust their immune systems when fed 6-MBOA. 6-MBOA treated mice in long day lengths regressed their testes to a greater extent than mice fed a standard diet, or mice kept in short day lengths and fed 6-MBOA or a standard diet. One type of immune activity (delayed-type hypersensitivity) was not affected by 6-MBOA, however, although responses were greater in short versus long day mice. In sum, P. leucopus responded reproductively to 6-MBOA, although differently than other species; immune activity was unaffected. Other aspects of the immune system, especially in herbivorous rodents, may be affected by 6-MBOA and thus warrant further study.     

6-MBOA affects testis size, but not delayed-type hypersensitivity, in white-footed mice (Peromyscus leucopus).

Many rodents use day length to time reproduction to occur when resources are abundant, but some species also use supplementary environmental cues. One supplementary cue is the plant-derived compound, 6-methoxy-2-benzoxazolinone (6-MBOA). Most rodents grow their gonads in response to 6-MBOA in their diets, but it is presently unknown whether they also use 6-MBOA to adjust other aspects of physiology, specifically their immune systems. 6-MBOA is structurally similar to melatonin, and seasonal changes in rodent immune activities are often mediated by melatonin. We therefore predicted that white-footed mice (Peromyscus leucopus), which breed seasonally and are reproductively sensitive to melatonin, would adjust their immune systems when fed 6-MBOA. 6-MBOA treated mice in long day lengths regressed their testes to a greater extent than mice fed a standard diet, or mice kept in short day lengths and fed 6-MBOA or a standard diet. One type of immune activity (delayed-type hypersensitivity) was not affected by 6-MBOA, however, although responses were greater in short versus long day mice. In sum, P. leucopus responded reproductively to 6-MBOA, although differently than other species; immune activity was unaffected. Other aspects of the immune system, especially in herbivorous rodents, may be affected by 6-MBOA and thus warrant further study.     

Population dynamics of a generalist rodent in relation to variability in pulsed food resources in a fragmented landscape

1. Pulsed food resources are often considered equivalent in their potential impact on the reproduction and population dynamics of consumers, but differences in the attributes of food pulses and their distribution in the landscape may cause differences in their effects. 2. We tested whether a perishable pulsed resource (periodical cicadas, Magicicada spp.) had similar effects on the population dynamics of a generalist forest rodent, Peromyscus leucopus, as have been reported for a cacheable pulsed resource (acorn mast). 3. Because the availability of periodical cicadas may vary between edge and interior habitat, we also tested whether habitat type (edge vs. interior) and fragment size affected the abundance of cicadas and P. leucopus. 4. Nearly 90% of the variation in the relative population densities of P. leucopus was explained by the variation in the relative densities of periodical cicadas, and fragments with more cicadas tended to have more reproductive female mice and litters. 5. We found more cicadas and more P. leucopus in edge than interior habitat, but no differences in the relative densities of either in relation to fragment size. 6. Data from a non-emergence year revealed no differences other than the presence of periodical cicadas that could explain the 50% higher relative densities of P. leucopus in the emergence year. 7. At the beginning of the emergence of periodical cicadas, the three fragments with the highest numbers of emergence holes had three times more mice than the fragments with the lowest numbers of emergence holes, suggesting that P. leucopus is able to anticipate the emergence of periodical cicadas and increase reproduction prior to the pulse. 8. Hence, despite differences in perishability, seasonal timing and nutritional quality of pulsed food resources in a fragmented landscape, they appear to have similar positive effects on the population dynamics of the generalist rodent, P. leucopus and, in fact, P. leucopus may be able to anticipate resource pulses.     

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.     

Melatonin and activity rhythm responses to light pulses in mice with the Clock mutation

Melatonin and wheel-running rhythmicity and the effects of acute and chronic light pulses on these rhythms were studied in Clock(Delta19) mutant mice selectively bred to synthesize melatonin. Homozygous melatonin-proficient Clock(Delta19) mutant mice (Clock(Delta19/Delta19)-MEL) produced melatonin rhythmically, with peak production 2 h later than the wild-type controls (i.e., just before lights on). By contrast, the time of onset of wheel-running activity occurred within a 20-min period around lights off, irrespective of the genotype. Melatonin production in the mutants spontaneously decreased within 1 h of the expected time of lights on. On placement of the mice in continuous darkness, the melatonin rhythm persisted, and the peak occurred 2 h later in each cycle over the first two cycles, consistent with the endogenous period of the mutant. This contrasted with the onset of wheel-running activity, which did not shift for several days in constant darkness. A light pulse around the time of expected lights on followed by constant darkness reduced the expected 2-h delay of the melatonin peak of the mutants to approximately 1 h and advanced the time of the melatonin peak in the wild-type mice. When the Clock(Delta19/Delta19)-MEL mice were maintained in a skeleton photoperiod of daily 15-min light pulses, a higher proportion entrained to the schedule (57%) than melatonin-deficient mutants (9%). These results provide compelling evidence that mice with the Clock(Delta19) mutation express essentially normal rhythmicity, albeit with an underlying endogenous period of 26-27 h, and they can be entrained by brief exposure to light. They also raise important questions about the role of Clock in rhythmicity and the usefulness of monitoring behavioral rhythms compared with hormonal rhythms.     

Effects of acute cold exposure at night on pineal N-acetyltransferase activity and melatonin content in white-footed mice, Peromyscus leucopus

1. Pineal N-acetyltransferase (NAT) activity in long-day white-footed mice (Peromyscus leucopus) exposed to cold soon after onset of darkness was depressed relative to controls, whereas mice exposed to cold later at night had slightly elevated enzyme activity. 2. NAT activity in short-day mice exposed to cold soon after lights off did not differ from controls. Pineal melatonin in these mice, however, did not rise, as it did in controls. 3. These results suggest that acute cold exposure may modulate NAT activity, which is controlled primarily by the L:D cycle.     

Resetting of the hamster circadian system by dark pulses

Circadian rhythms of animals are reset by exposure to light as well as dark; however, although the parameters of photic entrainment are well characterized, the phase-shifting actions of dark pulses are poorly understood. Here, we determined the tonic and phasic effects of short (0.25 h), moderate (3 h), and long (6-9 h) duration dark pulses on the wheel-running rhythms of hamsters in constant light. Moderate- and long-duration dark pulses phase dependently reset behavioral rhythms, and the magnitude of these phase shifts increased as a function of the duration of the dark pulse. In contrast, the 0.25-h dark pulses failed to evoke consistent effects at any circadian phase tested. Interestingly, moderate- and long-dark pulses elevated locomotor activity (wheel-running) on the day of treatment. This induced wheel-running was highly correlated with phase shift magnitude when the pulse was given during the subjective day. This, together with the finding that animals pulsed during the subjective day are behaviorally active throughout the pulse, suggests that both locomotor activity and behavioral activation play an important role in the phase-resetting actions of dark pulses. We also found that the robustness of the wheel-running rhythm was weakened, and the amount of wheel-running decreased on the days after exposure to dark pulses; these effects were dependent on pulse duration. In summary, similarly to light, the resetting actions of dark pulses are dependent on both circadian phase and stimulus duration. However, dark pulses appear more complex stimuli, with both photic and nonphotic resetting properties.     

Response to enrichment, type and timing: small mammals vary in their response to a springtime cicada but not a carbohydrate pulse

1. Masting events in the autumn provide a carbohydrate-rich pulse of resources that can influence the dynamics of small mammals and their natural enemies. Similar patterns are observed with the periodical cicada emergence which provides a protein-rich pulse in the spring, but comparisons are confounded by timing and food type. 2. We compared the influence of a naturally occurring spring pulse of cicadas with an experimental spring pulse of carbohydrate-rich seeds. We used a replicated population level field experiment and capture-mark-recapture techniques to record the vital rates, demographics, and abundance of Peromyscus leucopus (the white-footed mouse), as well as other small mammals and their parasites. 3. The density of P. leucopus on grids where cicadas emerged was 55% higher than controls as a consequence of early breeding. This was followed by an increase in the prevalence of the nematode Pterygodermatities peromysci, reduced breeding and decreased recruitment rates. Other small mammals including Tamias striatus (eastern chipmunk) and Blarina brevicauda (short-tailed shrew), increased in density, but there was no affect on Sorex cinereus (masked shrew). 4. In contrast to the presence of cicadas, there was no influence of sunflower seed supplementation on small mammal density, vital rates, or reproduction with the exception of an increase in B. brevicauda density. The response of small mammals to seasonal pulses depends on timing, food type, and species.     

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.     

Circadian response reduction in light and response restoration in darkness: a "skeleton" light pulse PRC study in mice (Mus musculus)

Entrainment may involve responses to dawn, to dusk, and to the light in between these transitions. Previous studies showed that the circadian system responds to only 2 light pulses, one at the beginning and one at the end of the day, in a similar way as to a full photoperiod, as long as the photoperiod is less than approximately 1/2 tau. The authors used a double 1-h light pulse protocol with different intervals of darkness in between (1, 2, 4, 7, 10, and 16 h) to study the phase responses of mice. The phase response curves obtained were compared to full light pulse PRCs of corresponding durations. Up to 6 hours, phase responses induced by double light pulses are virtually the same as by a corresponding full light pulse. The authors made a simple phase-only model to estimate the response reduction due to light exposure and response restoration due to dark exposure of the system. In this model, they assumed a 100% contribution of the first 1-h light pulse and fitted the reduction factor for the second light pulse to yield the best fit to the observations. The results suggest that after 1 h of light followed by less than 4 h of darkness, there is a considerable reduction in response to the second light pulse. Full response restoration requires more than 10 h of darkness. To investigate the influence of the duration of light on the response saturation, the authors performed a second series of experiments where the duration of the 2 light pulses was varied from 4 to 60 min each with a fixed duration of the stimulus (4 h). The response to 2 light pulses saturates when they are between 30 and 60 min long. In conclusion, double pulses replace single full light pulses of a corresponding duration of up to 6 h due to a response reduction during light, combined with response restoration during darkness. By the combined response reduction and response restoration, mice can maintain stable entrainment to the external LD cycle without being continuously exposed to it.     

Pheromone-induced reproductive inhibition in young female Peromyscus leucopus

Soiled bedding and urine from adult female white-footed mice (Peromyscus leucopus) were tested for their capacity to inhibit reproduction of young females. Test animals were given either physical or airborne contact with soiled bedding from adult females, adult female urine, clean bedding, or water from 21 to 150 days of age. Results indicate that reproductive inhibition is due to an airborne pheromone emitted by the adult females as a component of their urine. In the second experiment, young female mice were exposed to an adult female for 0, 1, 3, 6, 12, 18, or 24 h/day from 21 to 150 days of age. Results from this experiment show that exposure to adult females of as little as 3 h/day was sufficient to cause reproductive inhibition to occur. This phenomenon has important implications in terms of both female-female reproductive competition and socially mediated population regulation.     

Effect of Melatonin Treatment on 24-hour Variations in Hypothalamic Serotonin and Dopamine Turnover During the Preclinical Phase of Freund's Adjuvant Arthritis in Rats

The effect of melatonin treatment on time-of-day variations in hypothalamic serotonin (5-HT) and dopamine (DA) turnover was studied in rats treated with Freund's complete adjuvant (FCA). Animals received s.c. injections of 30 æg of melatonin or vehicle 1 h before lights off for 11 days. On day 10 of treatment, FCA or its vehicle was s.c. injected, and 2 days later, the rats were killed at 6 different time intervals throughout a 24-hour cycle. Hypothalamic 5-HT, 5-hydroxyindole-3-acetic acid (5-HIAA), DA and 3,4-dihydroxyphenylacetic acid (DOPAC) levels were measured by HPLC. 5-HT and DA turnover were estimated from the 5-HIAA/5-HT and DOPAC/DA ratios, respectively. In the anterior hypothalamus, time-of-day variation in 5-HT turnover was suppressed by FCA, an effect counteracted by melatonin treatment. Melatonin also prevented FCA effect on medial hypothalamic 5-HT turnover, while in the posterior hypothalamus, similar daily variations of 5-HT turnover were found in all experimental groups. As far as DA turnover, FCA or melatonin administration suppressed its daily variations in the anterior hypothalamus. Time-of-day variations in medial hypothalamic DA turnover were similar in all groups while only rats treated with melatonin and FCA or its vehicle exhibited significant daily changes of DA turnover in the posterior hypothalamus. Results indicate that melatonin treatment affects partly the 24-hour pattern of variation of hypothalamic 5-HT and DA turnover at an early phase of FCA arthritis in rats.     

Melatonin and the seasonal control of reproduction.

Many mammalian species from temperate latitudes exhibit seasonal variations in breeding activity which are controlled by the annual photoperiodic cycle. Photoperiodic information is conveyed through several neural relays from the retina to the pineal gland where the light signal is translated into a daily cycle of melatonin secretion: high at night, low in the day. The length of the nocturnal secretion of melatonin reflects the duration of the night and it regulates the pulsatile secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus. Changes in GnRH release induce corresponding changes in luteinising hormone secretion which are responsible for the alternating presence or absence of ovulation in the female, and varying sperm production in the male. It is not yet known where and how this pineal indoleamine acts to exert this effect. Although melatonin binding sites are preferentially localised in the pars tuberalis (PT) of the adenohypophysis, the hypothalamus contains the physiological target sites of melatonin for its action on reproduction. Melatonin does not seem to act directly on GnRH neurons; rather it appears to involve a complex neural circuit of interneurons that includes at least dopaminergic, serotoninergic and excitatory aminoacidergic neurons.     

Photoperiodic variations of the polysialylated form of neural cell adhesion molecule within the hypothalamus and related reproductive output in the ewe

Cellular mechanisms induced by melatonin to synchronise seasonal reproduction in several species, including sheep, remain unclear. We sought to evaluate the scale and physiological significance of neural plasticity in order to explain the delay between the change of duration of melatonin secretion and the change of reproductive status following a transition from long days (LD, 16 h light/24 h) to short days (SD, 8 h light/24 h) and from SD to LD. Using Western blots in ovariectomised oestradiol-replaced ewes, we evaluated the content of the polysialylated form of neural cell adhesion molecule (PSA-NCAM), a plasticity marker, in the hypothalamus. From day 15 following a transition to SD, most hypothalamic areas showed a decrease of PSA-NCAM level that was particularly significant in the preoptic area (POA). Following a transition to LD, PSA-NCAM content increased at day 15 in most regions except in the premammillary hypothalamic area (PMH) in which a significant decrease was noted. The functional importance of PSA-NCAM variations for seasonal reproduction was assessed for the PMH and POA. PSA-NCAM was degraded by stereotaxic injections of endoneuraminidase N and luteinising hormone (LH) secretion was recorded in treated and control ewes. Degradation of PSA-NCAM in the PMH in SD-treated ewes failed to produce a significant effect on LH secretion, whereas a similar treatment in the POA before a transition to SD delayed activation of the gonadotroph axis in two-thirds of the ewes. Our results suggest that the photoperiod controls variations of the hypothalamic content of a plasticity marker and that these might be important for the regulation of seasonal reproduction, particularly in the POA.     

Effect of Single Low Intensity Light Pulse Exposure and Melatonin Treatment on the Circadian Variation of Melatonin in Indian Palm Squirrel Funambulus pennanti

The endogenous circadian rhythm of melatonin in mammals provides information regarding the resetting response of the mammalian circadian timing system in response to the changes in light dark cycle. Photoperiodic changes are reported to have acute and chronic effect on melatonin rhythm. Our aim in present experiment was to study the effect of single light pulse of low intensity on the circadian variation of melatonin in Indian palm squirrel. A short pulse of 5min was given to the animals at 22:55 h on day 16th in natural photoperiodic condition of long day length (LD ~ 13.55:10.05) and melatonin levels were estimated at every 4-h interval on ZT scale on day 17th (DD). Observations suggest that the light pulse given on day 16th suppressed the melatonin level on day 17th (DD). Besides this, it was also found that there was phase delay in the peak value of melatonin. Further, we tested the ability of single melatonin injection on the light pulse induced phase shift of acrophase of melatonin in this species F. pennanti. We injected the single physiological dose of melatonin (25 microgram/100 g body wt.) just 5 min prior to the commencement of light pulse (22:50 h) on day 16 and melatonin levels were estimated on day 17th as above. Injection of melatonin prior to light pulse altered the suppressing and phase shifting effect of light in terms of peak concentration of melatonin in squirrels. Above data may lead us to conclude that the biological clock mechanism controlling circadian rhythm of melatonin in this rodent is in response to the phase shifting effect of light and acute melatonin treatment. Further, we may suggest that single melatonin injection has the capability to entrain melatonin rhythm but a dose dependent study is required to facilitate the suggestion.     

Effect of Single Low Intensity Light Pulse Exposure and Melatonin Treatment on the Circadian Variation of Melatonin in Indian Palm Squirrel Funambulus pennanti

The endogenous circadian rhythm of melatonin in mammals provides information regarding the resetting response of the mammalian circadian timing system in response to the changes in light dark cycle. Photoperiodic changes are reported to have acute and chronic effect on melatonin rhythm. Our aim in present experiment was to study the effect of single light pulse of low intensity on the circadian variation of melatonin in Indian palm squirrel. A short pulse of 5min was given to the animals at 22:55 h on day 16th in natural photoperiodic condition of long day length (LD ~ 13.55:10.05) and melatonin levels were estimated at every 4-h interval on ZT scale on day 17th (DD). Observations suggest that the light pulse given on day 16th suppressed the melatonin level on day 17th (DD). Besides this, it was also found that there was phase delay in the peak value of melatonin. Further, we tested the ability of single melatonin injection on the light pulse induced phase shift of acrophase of melatonin in this species F. pennanti . We injected the single physiological dose of melatonin (25 microgram/100 g body wt.) just 5 min prior to the commencement of light pulse (22:50 h) on day 16 and melatonin levels were estimated on day 17th as above. Injection of melatonin prior to light pulse altered the suppressing and phase shifting effect of light in terms of peak concentration of melatonin in squirrels. Above data may lead us to conclude that the biological clock mechanism controlling circadian rhythm of melatonin in this rodent is in response to the phase shifting effect of light and acute melatonin treatment. Further, we may suggest that single melatonin injection has the capability to entrain melatonin rhythm but a dose dependent study is required to facilitate the suggestion.     

Circadian Rhythm in Pineal N-Acetyltransferase Activity: Phase Shifting by Light Pulses (II)

Abstract: N-Acetyltransferase (NAT) is an enzyme whose rhythmic activity in the pineal gland and retina is thought responsible for melatonin circadian rhythms. The enzyme has properties of a circadian biological clock—its rhythm persists in constant conditions and it is precisely controlled by light and dark. Experiments are reported in which light pulses of 1 to 10 h duration were imposed on chicks during their dark-time. The effect of these pulses upon the NAT was measured and the effect of the pulses on subsequent NAT was also determined. The experiments support the conclusion that the amount and/or duration of dark-time NAT is limited. This finding is interpreted as supporting the idea that a fixed amount of some substance, an initiator, is synthesized during the subjective day.     

Differential reproductive response to short photoperiod in deer mice: role of melatonin

Summary Inhibitory photoperiod differentially effects reproduction in deer mice (Peromyscus maniculatus nebrascensis). Pituitary-testicular function is arrested in about one-third of short-day exposed males (reproductively responsive mice), while an equal number remain fertile (reproductively nonresponsive mice). Both phenotypes are found in natural populations and their disparate reproductive responses have a genetic basis. To assess whether this difference is attributable to a prepineal/pineal or post-pineal mechanism, we compared spermatogenic responses of known and unknown phenotype to exogenous melatonin. Melatonin significantly reduced mean sperm number in long-day housed mice of unknown phenotype. But, individual responses ranged from azoospermia to normal spermatogenesis, and this range was not significantly different from that previously recorded for short-day exposed mice. Reproductively nonresponsive males were unaffected by melatonin administration when housed under long or short daylength. In contrast, melatonin significantly suppressed sperm production in reproductively responsive males housed under long photoperiod, but had no additional suppressive effect in short-day housed mice with regressed testes. These data demonstrate that melatonin is only effective in eliciting testicular regression in reproductively responsive males. Taken together, these results suggest that differential testicular response to photoperiod are caused by a post-pineal mechanism.Abbreviations LD long day - SD short day - 16L:8D 16 h light, 8 h dark - 8L:16D 8 h light, 16 h dark