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)     

Circadian phototransduction: phase resetting and frequency of the circadian clock of Gonyaulax cells in red light

Constant red light (RR) influences the Gonyaulax clock in several ways: (1) Phase resetting by white or blue light pulses is stronger under background RR than in constant white light (WW); (2) frequency of the rhythm is less in RR than in WW; and (3) the amplitude of the spontaneous flashing rhythm is greater in RR than in WW. The phase response curve (PRC) to 4-hr white or blue light pulses is of high amplitude (Type 0) for cells in RR, but is of lower amplitude (Type 1) for cells in WW. In all cases, the PRC is highly asymmetrical: The magnitude of advance phase resetting is far higher than that of delay resetting. Consistent with this PRC, Gonyaulax cells in RR (free-running period greater than 24 hr) will entrain to T cycles of between 21 and 26.5 hr. The bioluminescence rhythms exhibit "masking" by blue light pulses while entrained to these T cycles. The fluence response of phase resetting to light-pulse intensity is not linear or logarithmic--rather, it is discontinuous. This feature is consistent with a limit cycle interpretation of Type 0 resetting of circadian clocks. Light pulses that cause large phase shifts also shorten the subsequent free-running period. The phase angle difference between the clock and the previous LD cycle is within 2 hr of the same phase between 16 degrees C and 25 degrees C, as determined from the light PRCs at various temperatures. Several drugs that inhibit mitochondria and/or electron transport will partially inhibit the phase shift by light.     

Circadian rhythm resetting in sparrows: early response to doublet light pulses

Circadian responses were studied using the perching activity of house sparrows (Passer domesticus). The sparrows were subjected to single or double 4-hr light pulses (the single pulses or the second pulses of the doublets scanned 24 hr) in the first cycle after previous entrainment to a light-dark cycle (LD 12:12). The differences in times at which the birds commenced perch-hopping in LD 12:12 before the pulses and in the five cycles immediately following the pulses were determined (phase shifts). A 24-hr time profile for phase shifts in response to single light pulses replicated our previous study: Early-night pulses delayed the rhythm (-1.7 hr), while late-night pulses advanced the rhythm (+3.8 hr). After pretreatment with a light pulse that advanced the birds +2.7 hr, the resetting curve was advanced. There were no delays; the range of average shifts was +0.1 hr to +6.2 hr. After pretreatment with a light pulse that delayed the birds -1.7 hr, the resetting curve was delayed. Average delays as much as -1.1 hr and advances up to +2.1 hr were measured. The data for double pulses were interpreted from predictions made from single-pulse data.     

Circadian regulation of pineal melatonin and reproduction in the Djungarian hamster

Gonadal state, pineal melatonin rhythms, and locomotor activity rhythms were examined in juvenile male Djungarian hamsters exposed to non-24-hr light cycles ("T-cycles") or to full photoperiods. At the end of 1 month, hamsters exposed to a 1-hr pulse of light every 24.33 hr (T 24.33) exhibited small testes, whereas those receiving the same amount of light every 24.78 hr (T 24.78) displayed stimulated gonads, ten-fold larger in size. Accompanying the nonstimulatory effect of the T 24.33 cycle were nocturnal peaks in both pineal melatonin content and serum melatonin concentration which were longer by approximately 4 hr than those observed on the photostimulatory T 24.78 cycle. Exposure to an intermediate-length T-cycle (T 24.53) resulted in a mixed gonadal response and in pineal and serum melatonin peaks of intermediate duration. Wheel-running activity was entrained to the T-cycles such that light was present only near the beginning of the subjective night, its phase (relative to activity onset) differing only slightly among T-cycle groups. Hence the durational differences observed in the melatonin peaks were apparently not due to the acute suppressive or phase-advancing effects of morning light on melatonin biosynthesis, but were rather the result of differences in the endogenous control of pineal activity by the circadian pacemaker system. While no strong correlation was detected between gonadal state and the phase of locomotor activity onset relative to the light pulse, a significant correlation was observed between gonadal state and the duration of daily locomotor activity (alpha). These data were compared to similar measures obtained from hamsters exposed to long-versus short-day full photoperiods (LD 16:8 vs. LD 10:14). In summary, the results of this study indicate involvement of the circadian pacemaker system of Djungarian hamsters in the control of pineal melatonin synthesis and secretion, and in photoperiodic time measurement. Furthermore, these data strengthen the hypothesis that it is the duration of nocturnal pineal melatonin secretion that is the critical feature of this neuroendocrine gland's photoperiodic signal.     

Nonphotic enhancement of adjustment to new light-dark cycles: masking interpretation discounted

The adjustment of hamsters to advanced light-dark (LD) cycles can be greatly accelerated by scheduling a single 3-hr bout of extra activity in a novel running wheel, starting about 7 hr before the time when the animals become active in the preceding LD cycle. The present experiments were designed to provide stronger evidence that this effect depends on a shift in the pacemaker rather than on masking. It was shown that when hamsters were put into continuous darkness (DD) 1 day after the exercise-accelerated phase shift, their free-running rhythms took off from a time nearer to the onset of darkness in the new LD cycle than in the preceding LD cycle. An incidental finding was that in DD the free-running period of the hamsters with the accelerated phase shifts was longer than that of the control animals. Further evidence that the 3-hr exercise pulse had produced a greater phase advance than that occurring in undisturbed control animals was obtained by giving a light pulse at the same clock time to all animals after they had been in DD for 8 days. The animals that had previously exercised for the additional 3-hr phase-advanced in response to the light pulse, while the undisturbed control animals phase-delayed.     

Locomotory and stridulatory circadian rhythms in the cricket, Teleogryllus commodus

Male crickets of the species Teleogryllus commodus express circadian rhythms in both their stridulatory and locomotory behaviours. Both forms of activity show the same free-running period (τ) in either DD (23·4 hr) or LL (25·1 hr). Although some overlap is seen between periods of locomotion and stridulation, the majority of each activity is found in different phases of the circadian cycle: locomotion occurs mainly in the subjective day and stridulation in the subjective night. Entraining LD cycles with photoperiods of 12 hr produce exogenous effects that can obscure endogenous components of the rhythms. Red light (λ>600 nm) causes the period to lengthen and RD cycles can entrain both rhythms. Single white light pulses of 2 or 6 hr did not produce significant phase shifts, but did cause τ to shorten when given in the subjective night. The significance of these observations is discussed. Given the results obtained to date, it is not likely that each rhythm is under the control of a separate circadian pacemaker.     

Synchronizing Effect of Photoperiod on the Dual Phasing of Demand-Feeding Rhythms in Sea Bass

Under natural environmental conditions, sea bass feeding rhythms are nocturnal in winter and diurnal the rest of the year. In this paper we describe the effect of contracting and expanding photoperiods and two skeleton photoperiods (SP) on the dual feeding rhythms of sea bass ( Dicentrarchus labrax L. ). To this end, twelve animals were held individually with access to self-feeders. First, the lights on and lights off were progressively delayed and advanced respectively by one hour in group 1 (G1), and conversely in group 2 (G2), so that the fish were exposed from a light/dark (LD) 12L:12D cycle to 2:22 LD (G1) and DL (G2) cycles and finally 0.25:23.75 LD (G1) and DL (G2). In the second experiment two SP's were used involving two light pulses separated by 12 hours, each pulse lasting 0.25 hours during the first two weeks and one hour during the succeeding two weeks. The results showed that diurnal and nocturnal sea bass tended to confine their feeding phase following the contraction of the LD cycle. Both SP's failed to simulate a complete photoperiod. In conclusion, the LD cycle appeared to drive the daily feeding rhythms but, the photoperiod length did not itself control the inversions of nocturnal and diurnal fish, so that other factors, in addition to photoperiod, may be involved in the control of the annual rhythms of phase inversions in sea bass.     

Entrainment of split circadian activity rhythms in hamsters

Hamsters that showed splitting of their circadian rhythms of wheel-running activity following long-term exposure to constant illumination (LL) were exposed to light-dark (LD) cycles with 2-hr dark segments, and with periods of 24.00, 24.23 or 24.72 hr. For comparison, hamsters showing nonsplit rhythms were also studied. In all cases of split rhythms, at least one of the two split components entrained to the LD cycles. In some animals, the second component continued to free-run until it merged with the entrained component, while in others, the second component also entrained to the LD cycle but maintained a stable phase angle of 6-14.5 hr relative to dark onset. These results were obtained in cases where the period of the LD cycle was shorter than that of the split rhythms and in cases where it was longer, implying that split components can be phase-advanced as well as phase-delayed by 2 hr of darkness. Three hamsters that showed stable entrainment of split rhythms were allowed to free-run in LL. The LD cycles were then reinstated, but instead of overlapping with the first component, as it did before, the dark segment was timed to overlap with the second. The entrainment patterns that ensued were similar to the ones obtained during the first LD exposure, indicating that the two split components respond to darkness in a qualitatively similar fashion. These results are further evidence that the pacemaker system underlying split circadian activity rhythms in hamsters is composed of two mutually coupled populations of oscillators that have similar properties, including a bidirectional phase response curve. Such a dual-oscillator organization may also underlie normal, or nonsplit, activity rhythms, as suggested by Pittendrigh and Daan (1976c), but the data are also compatible with the alternative view that the circadian pacemaker consists of a large number of coupled oscillators, which only dissociate into two separate populations in some animals under conditions of moderate LL intensity.     

Synchronizing Effect of Photoperiod on the Dual Phasing of Demand-Feeding Rhythms in Sea Bass

Under natural environmental conditions, sea bass feeding rhythms are nocturnal in winter and diurnal the rest of the year. In this paper we describe the effect of contracting and expanding photoperiods and two skeleton photoperiods (SP) on the dual feeding rhythms of sea bass (Dicentrarchus labrax L.). To this end, twelve animals were held individually with access to self-feeders. First, the lights on and lights off were progressively delayed and advanced respectively by one hour in group 1 (G1), and conversely in group 2 (G2), so that the fish were exposed from a light/dark (LD) 12L:12D cycle to 2:22 LD (G1) and DL (G2) cycles and finally 0.25:23.75 LD (G1) and DL (G2). In the second experiment two SP's were used involving two light pulses separated by 12 hours, each pulse lasting 0.25 hours during the first two weeks and one hour during the succeeding two weeks. The results showed that diurnal and nocturnal sea bass tended to confine their feeding phase following the contraction of the LD cycle. Both SP's failed to simulate a complete photoperiod. In conclusion, the LD cycle appeared to drive the daily feeding rhythms but, the photoperiod length did not itself control the inversions of nocturnal and diurnal fish, so that other factors, in addition to photoperiod, may be involved in the control of the annual rhythms of phase inversions in sea bass.     

Influence of light on feeding anticipatory activity in goldfish

Most animals when presented with food on a periodic basis develop feeding anticipatory activity (FAA). The objective of the present study was to investigate the synchronization of activity rhythms to light and feeding in single goldfish given a single daily meal and exposed to different light regimes. In the 1st experiment, the group of fish fed at the end of the day showed a longer FAA (228 +/- 27 min) than the group fed 1 h after lights-on (97 +/- 31 min). In the 2nd experiment, goldfish under conflicting zeitgebers, LD (T = 26) and feeding (T = 24) cycles, initially synchronized to the light cycle although feeding time gradually assumed greater importance as the experiment progressed. In the 3rd experiment, after altering the feeding cycles by advancing or delaying feeding time by 4 h in LD 0.25:23.75, most goldfish resynchronized to the feeding stimulus and developed FAA. In conclusion, the authors' results revealed that FAA in goldfish is driven by an endogenous timing system that is entrainable by periodic feeding and influenced by the light regime.     

Light-Dark Entrainment of Circadian Activity Rhythms of the Diurnal Indian Palm Squirrel (Funambulus pennanti)

A recent focus of chronobiological studies has been to establish diurnal models as alternatives to the more frequently used nocturnal rodents. In the present study, light-dark (LD) entrainment characteristics were examined in one diurnal species, the Indian palm squirrel ( Funambulus pennanti ). Palm squirrels showed strongly diurnal locomotor activity rhythms (~ 88 percent) under light-dark (LD) cycles, with activity bimodally distributed during the L phase. In comparison to a dim LD cycle, exposure to a bright LD cycle caused a phase advance in the onset of activity, an increase in daily activity levels and an increase in the duration of activity. Percentage diurnality, however, did not vary between bright and dim LD cycles. Activity rhythms reentrained in significantly fewer days after an 8 hour phase delay of the LD cycle compared to an 8 hour phase advance. In both cases, the direction of reentrainment followed the direction of the LD shift. When exposed to single light pulses (1 hour) presented at the same time each day, 6/7 squirrels entrained. Under a skeletal photoperiod cycle (2 x 1 hour light pulses each day), 6/8 squirrels showed stable entrainment. The remaining squirrels exhibited rhythm splitting, with each component synchronising in an unstable manner with one of the light pulses. Under entrainment to single light pulses and to the skeletal photoperiod cycle, the phase angle of entrainment was negatively correlated with t. Finally, when exposed to a skeletal scotoperiod cycle (2 x 1-hour dark pulses each day), only 3/8 squirrels entrained, while the others free-ran. Two of the entrained squirrels showed spontaneous phase reversals during entrainment. As with other species, the activity rhythm of palm squirrels appears to be controlled by two separate self-sustaining oscillators. The strongly diurnal nature of palm squirrels make them a promising diurnal model for studies examining endogenous and exogenous influences on circadian functioning.     

Light-Dark Entrainment of Circadian Activity Rhythms of the Diurnal Indian Palm Squirrel (Funambulus pennanti)

A recent focus of chronobiological studies has been to establish diurnal models as alternatives to the more frequently used nocturnal rodents. In the present study, light-dark (LD) entrainment characteristics were examined in one diurnal species, the Indian palm squirrel (Funambulus pennanti). Palm squirrels showed strongly diurnal locomotor activity rhythms (? 88 percent) under light-dark (LD) cycles, with activity bimodally distributed during the L phase. In comparison to a dim LD cycle, exposure to a bright LD cycle caused a phase advance in the onset of activity, an increase in daily activity levels and an increase in the duration of activity. Percentage diurnality, however, did not vary between bright and dim LD cycles. Activity rhythms reentrained in significantly fewer days after an 8 hour phase delay of the LD cycle compared to an 8 hour phase advance. In both cases, the direction of reentrainment followed the direction of the LD shift. When exposed to single light pulses (1 hour) presented at the same time each day, 6/7 squirrels entrained. Under a skeletal photoperiod cycle (2 x 1 hour light pulses each day), 6/8 squirrels showed stable entrainment. The remaining squirrels exhibited rhythm splitting, with each component synchronising in an unstable manner with one of the light pulses. Under entrainment to single light pulses and to the skeletal photoperiod cycle, the phase angle of entrainment was negatively correlated with t. Finally, when exposed to a skeletal scotoperiod cycle (2 x 1-hour dark pulses each day), only 3/8 squirrels entrained, while the others free-ran. Two of the entrained squirrels showed spontaneous phase reversals during entrainment. As with other species, the activity rhythm of palm squirrels appears to be controlled by two separate self-sustaining oscillators. The strongly diurnal nature of palm squirrels make them a promising diurnal model for studies examining endogenous and exogenous influences on circadian functioning.     

Circadian components of semilunar and lunar timing mechanisms

The timing of semilunar as well as lunar reproductive rhythms has been analyzed in different geographic populations of the intertidal chironomid Clunio. In stocks of three populations differing in period and phase relationship with the lunar month, these long-term rhythms were synchronized in the laboratory by using artificial moonlight cycles of 30 days in otherwise 24-hr light-dark (LD) cycles (0.4 lux during 4 successive nights every 30 days in LD 12:12). In LD cycles of various periods, a strong synchronization was only possible in LD 12:12 and LD 11:11, whereas in LD 10:10 and LD 15:15 the synchronization by the 30-"day" moonlight cycle was weak or even absent. The study demonstrates a limited range of circadian periods for entrainment of the long-term rhythms. It is concluded that an LD cycle with a period near 24 hr is an essential zeitgeber condition for semilunar and lunar timing in this marine insect. Further, it is suggested that the underlying physiological timing mechanism of Clunio consists of a circadian function for the perception of the monthly moonlight zeitgeber cycles that entrain the endogenous, temperature-compensated oscillator of the circasemilunar (or circalunar) period. The long-term oscillator triggers the metamorphosis of the insect, and thereby determines the time of its eclosion and reproduction on the shorelines, in correlation with days of spring tides recurring about every 14-15 days.     

Photoperiodic control of diapause induction and termination in Ostrinia nubilalis: two different clocks?

Both diapause induction and diapause termination are under photoperiodic control in the lepidopteran, Ostrinia nubilalis. In the present study, induction of diapause was maximal in light-dark (LD) cycles that contained 12 hr of light alternating with 12 hr of darkness (LD 12:12). Termination of diapause was maximal in LD 16:8. Diapause termination also occurred rapidly in non-24-hr LD cycles that possessed an 8-hr dark phase. In each of these cases, the period of the LD cycle was not important. Diapause termination did not, however, occur rapidly in non-24-hr LD cycles that lacked an 8-hr dark phase. Thus, the clock mechanism underlying the termination response resembles an hourglass in its behavior. This is in contrast with what is known about induction of diapause. Here it has been demonstrated that the circadian system is somehow involved. It is thus possible that two different physiological clocks underlie these responses.     

Circadian photoentrainment: parameters of phase delaying

Experiments were carried out using simulated den cages to delineate specific characteristics of phase delaying in circadian photoentrainment of a nocturnal rodent, the flying squirrel. The principal experiments entailed presentation of one to five consecutive 15-min white-light pulses per activity cycle at activity onset to animals free-running in darkness, in order to determine the immediate and final phase-shifting effect. Auxiliary experiments recorded entrainment patterns on light-dark (LD) schedules in the den cages. Phase response curves (PRCs) based on 15-min white-light pulses in standard wheel cages were also constructed for these animals as background information for interpreting the phase-delaying experiments. Exposure of a den animal to light by light sampling at the time of initial arousal from the rest state at circadian time (CT) 12, either by an LD schedule or by a 15-min light pulse, resulted in a return to the nest box for a short rest period. The phase delay occurring after a single light exposure at activity onset was equal to the induced rest, thus suggesting an immediate phase shift. The maximum delay was about 1 1/2 hr/cycle, with the amount of delay related to the number of light exposures. During the photoentrained state on an LD schedule, the activity rhythm of a den-housed animal was essentially free-running on the days following a phase delay. The data are used to expand current models for photoentrainment of circadian activity rhythms in nocturnal rodents.     

Masking of circadian activity rhythms in canaries by light and dark

Canaries (Serinus canaria) were kept singly in cages placed in an artificially illuminated, soundproof cabinet. Perch-hopping activity was recorded by means of a computer system. In three series of experiments, the activity rhythms of the birds were entrained to 24 hr by light-dark (LD) cycles with 4, 12, or 20 hr of light (L), respectively. The intensity of illumination was 10 lux in L and 0.25 lux in darkness (D). Under LD 4:20 and 12:12, the intensity of D was increased daily at the same zeitgeber time to 1 lux for 1 hr (L pulse) during about 8 consecutive days. This sequence was followed by 8 days without L pulses before giving another series of L pulses at a different zeitgeber time. Under LD 20:4, the intensity of L was decreased to 1 lux for 1 hr (D pulse). The activity of all birds was more or less increased by the L pulses (positive masking) and decreased by the D pulses (negative masking). The level of masking activity during the L and D pulses depended on the circadian phase at which the pulses were administered. Positive masking by L pulses was minimal about 5 hr after the beginning of D, and increased steadily thereafter. Negative masking by D pulses was maximal at the beginning and the end of L, and minimal during the middle.     

Circadian Rhythms and time Measurement in Locomotor Activity of the Scorpion Leiurus Quinquestriatus (Buthidae)

The circadian rhythms of locomotor activity of the scorpion Leiurus quinqueslriatus were examined under different light-dark cycles and in free-running conditions. The circadian rhythm is bimodal in LD 12:12 with alternating cycles of temperature (35°-25°C) with high intensity (1300 lux) or in LD 12: 12 with constant temperature 35° C with 300 lux. In LD 12:12 (1300 lux), in long or in short light spans with constant temperature, the bimodal pattern is slightly changed with the appearance of a third minor peak of activity. In free-running conditions, the bimodal rhythm of locomotor activity persists in DD with T about 24 hr, but in LL the rhythm becomes unimodal with T about 24 hr. Cosinor and power spectrum analysis showed the presence of more than one periodic component. It seems that there is a correlation between the range of light regimens, temperature, light intensity and the coincidence of these components. These components are independently entrained by the environmental light cycle. The mechanism of entrainment of components is discussed.     

Changes in phase-angle under light–dark cycles influenced by nonphotic stimulation

ABSTRACT

Most work looking at nonphotic effects on circadian rhythms is conducted when animals are held under freerunning conditions, usually constant darkness. However, for nonphotic effects to be functionally significant, they should be demonstrable under conditions in which most animals live, i.e., a 24-hr light–dark cycle (LD). Syrian hamsters held in LD 6:18 were administered nonphotic stimulation in the form of a 3-hr confinement to a novel wheel starting about 6 hr before the start of their normal nightly activity bout. This resulted in a 2.5-hr advance of their activity rhythm on the next day that gradually receded to about 1.5 hr over the next 10 days. When hamsters held in LD 6:18 were given five novel wheel confinements over 13 days their nightly activity onset advanced 3 hr and remained at that phase for at least 2 weeks. Home cage wheel deprivation experiments indicated that high levels of home cage activity are necessary to maintain the advanced phase. These results show that nonphotic stimulation can have large, long-lasting effects on daily rhythms in LD and suggest a possible mechanism whereby nocturnal rodents might achieve phase flexibility in response to seasonal changes.     

Effect of melatonin infusion duration and frequency on gonad, lipid, and body mass in pinealectomized male Siberian hamsters

The goal of this study was to discriminate between two hypotheses regarding how the circadian rhythm of pineal melatonin (MEL) production transmits photoperiodic information: (1) A circadian rhythm of sensitivity to MEL regulates the hormone's effect; (2) the duration of the MEL signal, rather than its circadian timing, is the critical parameter of the MEL rhythm. The experiment examined the response of pinealectomized (PINX) male Siberian hamsters to 10-hr (short-day-type) versus 6-hr (long-day-type) duration MEL infusions (10 ng/infusion) in cycles with period lengths (T) of 18, 24, 36, and 48 hr. After cannula implantation, animals were moved from LD 16:8 to LD 10:14 (lights-on from 0500 to 1500 hr, EST), where the timed infusions began. Additional T 24 cycles included as controls employed 18-hr MEL, 18-hr saline (SAL), and 10-hr SAL infusions: Body weight and food intake were measured weekly. After 6 weeks, animals were killed; blood samples were taken for radioimmunoassay (RIA) of serum follicle-stimulating hormone (FSH) and prolactin (PRL); and terminal body, epididymal white adipose tissue (EPIWAT), and paired testis weights were recorded. Six-hour MEL infusions failed to induce short-day-type effects, regardless of the period (T) of the infusion cycle. In contrast, compared to SAL and 6-hr MEL infusions, 10-hr MEL resulted in decreases in body, EPIWAT, and testis weights in T 24, but not in T 36 or T 48. In T 18, testis, body, and EPIWAT mass were decreased, but not to the same extent as in T 24. Similarly, daily 18-hr MEL infusions (T24) were less effective as a short-day stimulus than were 10-hr MEL infusions. The effectiveness of 10-hr, but not 6-hr, MEL infusions in T 18 and T 24 is consistent with the duration hypothesis and argues against the circadian hypothesis. Neither hypothesis could have predicted that all infusion cycles of T greater than or equal to 36 hr, regardless of the infusion durations, would fail to elicit short-day-type responses. This outcome suggests a need for relatively frequent (T less than 36 hr) MEL stimulation in addition to the requirement for adequate duration of each MEL infusion.     

Photoperiodic time measurement in the male deer mouse, Peromyscus maniculatus

Weanling male deer mice, Peromyscus maniculatus, were exposed for three weeks either to light-dark (LD) cycles with periods (T=L+D) ranging from T=23 (1L:22D) to T=25.16 (1L:24.16D) or to 24-h LD cycles with photoperiods ranging from 1 (1L:23D) to 19 (19L:5D) h. Both the circadian locomotor activity rhythms and the response of the reproductive system to these LD cycles were assessed. The results demonstrate that the photoperiodic effectiveness of light depends on the phase of the light relative to the animal's circadian system, as marked by the circadian activity rhythm. Light falling during the animal's subjective night, from activity onset to at least 11.8 h after activity onset, stimulates growth and maturation of the reproductive system, whereas light falling during the rest of the circadian cycle is nonstimulatory.