CIRCADIAN GROWTH RHYTHM IN JUVENILE SPOROPHYTES OF LAMINARIALES (PHAEOPHYTA)1

A circadian rhythm in growth was detected by computer-aided image analysis in 3–4-cm-long, juvenile sporophytes of the kelp species Pterygophora California Rupr. and in seven Laminaria spp. In P. californica, the free-running rhythm occurred in continuous white fluorescent light, had a period of 26 h at 10°or 15°C, and persisted for at least 2 weeks in white or blue light. The rhythm became insignificant in continuous green or red light after 3 cycles. Synchronization by white light-dark regimes, e.g. by 16 h light per day, resulted in an entrained period of 24 h and in a shift of the circadian growth minimum into the middle of the light phase. A morning growth peak represented the decreasing portion of the circadian growth curve, and an evening peak the increasing portion. The circadian growth peak was not visible during the dark phase, because growth rate decreased immediately after the onset of darkness. At night, some growth still occurred at 16 or 12 h light per day, whereas growth stopped completely at 8 h light per day, as in continuous darkness. During 11 days of darkness, the thallus area became reduced by 3.5%, but growth rate recovered in subsequent light–dark cycles, and the circadian growth rhythm reappeared in subsequent continuous light.     

Different mechanisms of phase delays and phase advances of the circadian rhythm in rat pineal N-acetyltransferase activity

The circadian rhythm in rat pineal N-acetyltransferase (NAT) activity, which drives the rhythm in melatonin production, is controlled by a pacemaker located in the suprachiasmatic nucleus of the hypothalamus. As the NAT rhythm has two well-defined phase markers--namely, the time of the evening activity rise and of the morning decline--it is suitable for studies of the entrainment of the pacemaker by environmental light. Phase delays of the NAT rhythm proceed more rapidly than phase advances. One day after a brief light pulse applied before midnight, or after a delay in evening lights-off, or a delay of a light-dark (LD) cycle, phase delays of the evening NAT rise result in almost corresponding delays of the morning NAT decline. Consequently, the NAT rhythm is phase-shifted, but its pattern does not change. One day after a brief light pulse applied past midnight, or after bringing forward morning lights-on, or after an advance of an LD cycle, the morning NAT decline is phase-advanced, but the evening rise is not phase-advanced at all or may even by phase-delayed. Consequently, the phase relationship between the evening NAT activity onset and the morning offset may be compressed considerably, and it may take several transient cycles before phase advances of the morning NAT decline are followed by corresponding advances of the evening NAT rise. Due to the phase-delaying effect of evening light on the NAT rise and to the phase-advancing effect of morning light on the NAT decline, the phase relationship between the NAT rise and the decline is compressed on long days and decompressed on short days. Different phase shifts of the evening NAT rise and of the morning decline, even in opposite directions, are consistent with the hypothesis of a complex, two-component (evening-morning, or E-M) pacemaker controlling the NAT rhythm. As the E-M phase relationship determines duration of the high night melatonin production, and the duration of the nocturnal melatonin pulse may convey information on daylength, the data are consistent with the internal coincidence model for photoperiodic time measurement.     

The effects of pinealectomy and blinding on the circadian locomotor activity rhythm in the Japanese newt,Cynops pyrrhogaster

We examined the effects of pinealectomy and blinding (bilateral ocular enucleation) on the circadian locomotor activity rhythm in the Japanese newt, Cynops pyrrhogaster. The pinealectomized newts were entrained to a light-dark cycle of 12 h light and 12 h darkness. After transfer to constant darkness they showed residual rhythmicity for at least several days which was gradually disrupted in prolonged constant darkness. Blinded newts were also entrained to a 12 h light/12 h dark cycle. In subsequent constant darkness they showed free-running rhythms of locomotor activity. However, the freerunning periods noticeably increased compared with those observed in the previous period of constant darkness before blinding. In blinded newts entrained to the light/dark cycle the activity rhythms were gradually disrupted after pinealectomy even in the presence of the light/dark cycle. These results suggest that both the pineal and the eyes are involved in the newt's circadian system, and also suggest that the pineal of the newt acts as an extraretinal photoreceptor which mediates the entrainment of the locomotor activity rhythm.Abbreviations circadian period - DD constant darkness - LD cycle, light-dark cycle - LD 12:12 light-dark cycle of 12 h light and 12 h darkness     

Entrainment of the circadian rhythm in the rat pineal N-acetyltransferase activity by prolonged periods of light

Summary Entrainment of the circadian rhythm in the pineal N-acetyltranferase activity by prolonged periods of light was studied in rats synchronized with a light:dark regime of 1212 h by observing phase-shifts in rhythm after delays in switching off the light in the evening or after bringing forward of the morning onset of light. When rats were subjected to delays in switching off the light of up to 10 h and then were released into darkness, phase-delays of the evening N-acetyltransferase rise during the same night corresponded roughly to delays in the light switch off. However, phasedelays of the morning decline were much smaller. After a delay in the evening switch off of 11 h, no N-acetyltransferase rhythm was found in the subsequent darkness. The evening N-acetyltransferase rise was phase-delayed by 6.2 h at most 1 day after delays. Phase-delays of the morning Nacetyltransferase decline were shorter than phasedelays of the N-acetyltransferase rise by only 0.7 h to 0.9 h at most. Hence, 1 day after delays in the evening switch off, the period of the high night N-acetyltransferase activity may be shortened only slightly. The N-acetyltransferase rhythm was abolished only after a 12 h delay in switching off the light.Rats were subjected to a bringing forward of the morning light onset and then were released into darkness 4 h before the usual switch off of light. In the following night, the morning N-acetyltransferase decline, but not the evening rise, was phase advanced considerably. Moreover, when the onset of light was brought forward to before midnight, the N-acetyltransferase rise was even phase-delayed. Hence, 1 day after bringing forward the morning onset of light, the period of the high night N-acetyltransferase activity may be drastically reduced. When rats were subjected to a 4 h light pulse around midnight and then released into darkness, the N-acetyltransferase rhythm in the next night was abolished.The data are discussed in terms of a two-component pacemaker controlling the N-acetyltransferase rhythm. It is suggested that delays in the evening switch off of light may disturb the N-acetyltransferase rhythm the next day only a little, as the morning component may adjust to phasedelays of the evening component almost within one cycle. On the other hand, bringing forward the morning onset of light may disturb the N-acetyltransferase rhythm heavily the next day, as the evening component not only does not adjust to phase-advances of the morning component, but it may even be phase-delayed when the light onset occurs before midnight.Abbreviations NAT N-acetyltransferase - PRC phase response curve - E evening component of the N-acetyltransferase rhythm or of its pacemaker - M morning component of the N-acetyltransferase rhythm or of its pacemaker - LD xy light dark cycle comprising x h of light and y h of darkness     

Circadian Rhythm in Pineal N-Acetyltransferase Activity: Phase Shifting by Dark Pulses (III)

N-Acetyltransferase (NAT) is an enzyme whose rhythmic activity in the pineal gland and retina is thought to be responsible for melatonin circadian rhythms. The enzyme has circadian properties--its rhythm persists in constant conditions, and it is precisely controlled by light and dark. Experiments are reported in which 4-h light or dark pulses were imposed on chicks (Gallus domesticus) over a 24-h period. Pineal NAT profiles were measured during and subsequent to the pulses. The phase of the NAT cycle following pulses was plotted to obtain phase-response curves. Light pulses produced a maximum phase shift (advance of 5 h) 8 h after the expected time of lights-out; dark pulses produced a maximum phase shift (advance of 4 h) 3 h after the expected time of lights-out. Maximum phase delays (-2 h) occurred 1-2 h after the expected lights-out for light pulses and 8 h after expected lights-on for dark pulses.     

Use of benzodiazepines to manipulate the circadian clock regulating behavioral and endocrine rhythms

Extensive studies have now been carried out demonstrating that the systemic administration of the short-acting benzodiazepine, triazolam, can have pronounced effects on both behavioral and endocrine circadian rhythms. For example, three daily injections of triazolam can phase-advance the circadian rhythm of pituitary luteinizing hormone release and locomotor activity by about 2-3 h in female hamsters maintained in constant light. Triazolam has also been found to facilitate the rate of reentrainment of the activity rhythm following an 8-hour advance or delay in the light-dark cycle. Limited studies with other short-acting benzodiazepines indicate that the effects of triazolam on the circadian system of hamsters can be generalized to this class of drugs. Recent studies in humans indicate that treatment with triazolam can alter the time it takes for human endocrine rhythms to become reentrained following an 8-hour delay in the sleep-wake and light-dark cycle. Such findings raise the possibility that short-acting benzodiazepines may prove useful in reducing the symptoms associated with 'jet-lag' and rotating shift-work schedules as well as in the treatment of various physical and mental illnesses that have been associated with a disorder of biological timekeeping.     

Complex circadian regulation of pineal melatonin and wheel-running in Syrian hamsters

Circadian regulation of pineal melatonin content was studied in Syrian hamsters (Mesocricetus auratus), especially melatonin peak width and the temporal correlation to wheel-running activity. Melatonin was measured by radioimmunoassay in glands removed at different circadian times with respect to activity onset (= CT 12). Pineal melatonin peak width (h; for mean 125 pg/gland) and activity duration () were both 4–5 h longer after 12 or 27 weeks than after 5 or 6 days in continuous darkness (DD). Increased peak width was associated with a delay in the morning decline (M) of melatonin to baseline, correlated with a similar delay in wheel-running offset. In contrast, the evening rise (E) in melatonin occurred at approximately the same circadian phase regardless of the length of DD. Fifteen min light pulses produced similar phase-shifts in melatonin and activity. In a phase advance shift, M advanced at once, while E advanced only after several days of adjustment. Independent timing of shifts in the E and M components of the melatonin rhythm suggest that these events are controlled separately by at least two circadian oscillators whose mutual phase relationship determines melatonin peak width. This two-oscillator control of melatonin peak width is integral to the circadian mechanism of hamster photoperiodic time measurement.Abbreviations CT circadian time - DD continuous dark - L: D light: dark cycle - PMEL pineal melatonin - PRC phase response curve - RIA radioimmunoassay; , duration (h) of the active phase of the circadian wheel-running rhythm; , free-running period     

Circadian Rhythm in Pineal N-Acetyltransferase Activity: Rapid Phase Reversal and Response to Shorter than 24-Hour Cycles (IV)

N-Acetyltransferase (NAT) is an enzyme whose rhythmic activity in the pineal gland and retina is responsible for circadian rhythms in melatonin. The NAT activity rhythm has circadian properties such as persistence in constant conditions and precise control by light and dark. Experiments are reported in which chicks (Gallus domesticus), raised for 3 weeks in 12 h of light alternating with 12 h of dark (LD12:12), were exposed to 1-3 days of light-dark treatments during which NAT activity was measured in their pineal glands. (a) In LD12:12, NAT activity rose from less than 4.5 nmol/pineal gland/h during the light-time to 25-50 nmol/pineal gland/h in the dark-time. Constant light (LL) attenuated the amplitude of the NAT activity rhythm to 26-45% of the NAT activity cycle in LD12:12 during the first 24 h. (b) The timing of the increase in NAT activity was reset by the first full LD12:12 cycle following a 12-h phase shift of the LD12:12 cycle (a procedure that reversed the times of light and dark by imposition of either 24 h of light or dark). This result satisfies one of the criteria for NAT to be considered part of a circadian driving oscillator. (c) In less than 24-h cycles [2 h of light in alternation with 2 h of dark (LD2:2), 4 h of light in alternation with 4 h of dark (LD4:4), and 6 h of light in alternation with 6 h of dark (LD6:6)], NAT activity rose in the dark during the chicks' previously scheduled dark-time but not the previously scheduled light-time of LD12:12. In a cycle where 8 h of light alternated with 8 h of dark (LD8:8), NAT activity rose in both 8-h dark periods, even though the second one fell in the light-time of the prior LD12:12 schedule.(ABSTRACT TRUNCATED AT 250 WORDS)     

Entrainment of the circadian activity rhythm to the light-dark cycle can be altered by a short-acting benzodiazepine, triazolam

The circadian rhythm of locomotor activity in hamsters maintained in either constant darkness or constant light can be phase-shifted by a single injection of the short-acting benzodiazepine, triazolam. These results suggest that treatment with triazolam may also alter the entrainment pattern of circadian rhythms in animals that are synchronized to a light-dark (LD) cycle. To test this hypothesis, hamsters maintained on an LD 6:18 light cycle received daily injections of triazolam (or vehicle) for 10-12 days, and any subsequent effects on the phase relationship between the onset of activity and the LD cycle were determined. Daily injections of triazolam (but not vehicle) induced pronounced advances or delays in the phase relationship between the entrained activity rhythm and the LD cycle; the direction of the shift was dependent on the time of the injection. Taken together with data from previous studies, these results suggest that triazolam, and perhaps other short-acting benzodiazepines, can be used to manipulate the mammalian circadian clock under a variety of experimental conditions.     

Timely administration of melatonin accelerates reentrainment to phase-shifted light-dark cycles in the field mouse Mus booduga

The effect of melatonin on the rate of reentrainment after a 6h phase delay and a 6h phase advance in the light-dark (LD) cycle was assayed in the nocturnal field mouse Mus booduga. After a phase delay of 6h in the LD cycle, a single dose of melatonin (1 mg/kg) was administered for three consecutive days at about CT4 (circadian time 4). After a phase advance of 6h in the LD cycle, melatonin was administered for three consecutive days at about CT22. Melatonin was found to accelerate reentrainment in both cases. Melatonin-treated animals took significantly fewer cycles to reentrain compared to vehicle-treated (50% dimethylsulfoxide [DMSO]) and nontreated control animals.     

Influence of Photoperiod in Accelerating the Reentrainment in Drosophila

Efficacy of the short photoperiod (Spp) and the long photoperiod (Lpp) in accelerating the reentrainment was assessed in Drosophila biarmipes. The Spp accelerated the reentrainment after the phase advance of light-dark (LD) cycles, which was associated with the early activity onset (Ψo) and the short period of free-running rhythm (τ). The Lpp accelerated the reentrainment after the phase delay of LD cycles, which was associated with the late Ψo and the long τ. This study indicates that the photoperiodic modulation of the circadian waveform of the underlying pacemaker that controls activity rhythm influenced the rate of reentrainment in D. biarmipes. (Author correspondence: drdsjoshi08@gmail.com)     

Circadian rhythm of tryptophan hydroxylase activity in chicken retina

1. Retinal tryptophan hydroxylase activity in chickens (1-4 weeks old and embryos) was estimated by determination of levels of 5-hydroxytryptophan (5HTP) in retinas at defined intervals after inhibition of aromatic L-amino acid decarboxylase with m-hydroxybenzylhydrazine (NSD1015). 2. The relationship of tryptophan hydroxylase activity to photoperiod was explored. In chickens maintained on a 12-hr light: 12-hr dark cycle, a diurnal cycle in tryptophan hydroxylase activity was observed. Activity during middark phase was 4.4 times that seen in midlight phase. Cyclic changes in tryptophan hydroxylase activity persisted in constant darkness with a period of approximately 1 day, indicating regulation of the enzyme by a circadian oscillator. The phase of the tryptophan hydroxylase rhythm was found to be determined by the phase of the light/dark cycle. The relationship of the tryptophan hydroxylase rhythm to the light/dark cycle mirrors previously described rhythms of melatonin synthesis and serotonin N-acetyltransferase (NAT) activity in the retina. 3. Light exposure for 1 hr during dark phase suppressed NAT activity by 82%, while tryptophan hydroxylase activity was suppressed by only 30%. 4. Based on the differential responses of retinal NAT activity and tryptophan hydroxylase activity to acute light exposure during dark phase, it was predicted that exposure to light during dark phase would divert serotonin in the retina from melatonin biosynthesis to oxidation by MAO. In support of this, levels of 5-hydroxyindole acetic acid (5HIAA) in retina were found to be elevated approximately two-fold in chickens exposed to 30 min of light during dark phase. In pargyline-treated chickens, 2 hr of light exposure during dark phase was found to increase retinal serotonin levels by 64% over pargyline-treated controls. 5. Cyclic changes in tryptophan hydroxylase activity and NAT activity persisted for 2-3 days in constant light. Tryptophan hydroxylase activity at mid-night gradually decreased on successive days in constant light; on the first day of constant light, tryptophan hydroxylase activity at mid-night was 70% of activity seen during middark phase of the normal light/dark cycle and decreased further on subsequent days. In contrast, on each of 3 days of constant light, NAT activity at mid-night was approximately 15% of normal middark phase activity. 6. Cycloheximide completely inhibited the nocturnal increase in tryptophan hydroxylase activity when given immediately before light offset. The nocturnal increase in NAT activity was inhibited in a similar fashion. 7. Like the development of the NAT rhythm, cyclic changes of tryptophan hydroxylase activity in the retinas of chickens began on or immediately before the day of hatching. hatching.(ABSTRACT TRUNCATED AT 400 WORDS)     

Photoperiod modifies daily maps of light and dark sensitivity for N-acetyltransferase activity in pineal glands of 3-week oldGallus domesticus

Summary N-acetyltransferase (NAT) activity in pineal glands exhibits a circadian rhythm with peak activity occurring in the dark-time. We previously showed that inGallus domesticus chicks pretreated with LD12:12, NAT activity was increased by dark exposure (peak dark sensitivity occurred during the expected dark-time) or decreased by light at night (peak light sensitivity occurred early in the night during the time of dark sensitivity). In this study we mapped dark sensitivity vs time (for NAT activity increase in response to 2 h dark pulses), and light sensitivity vs time (for NAT activity decrease in response to 10 min or 30 min light pulses) over a cycle for 3-week old chicks,Gallus domesticus, pretreated with long (LD16:8) or short photoperiod (LD8:16). Sensitivity to light was increased in the second 8 h after L/D by LD8:16. Sensitivity to dark was increased in the first 8 h after L/D by LD16:8.Abbreviations LD16:8 a light-dark cycle consisting of 16 h of light alternating with 8 h of dark - LD8:16 a light-dark cycle consisting of 8 h of light alternating with 16 h of dark - DD constant dark - LL constant light - L/D lights-off - D/L lights-on - NAT pineal serotonin N-acetyltransferase - NAT activity is given in nmoles/pineal gland/h - chick used here to denote a young bird of either sex of the speciesGallus domesticus from hatching to three weeks of age     

Pineal-retinal relationships: rhythmic biosynthesis and immunocytochemical localization of melatonin in the retina of the pike (Esox lucius)

Summary The levels of melatonin and the activities of two enzymes of the melatonin biosynthetic pathway, serotonin N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT), were measured throughout the light-dark cycle in the retina of a teleost fish, the pike. HIOMT activity did not display significant variations, whereas NAT activity and melatonin content showed a daily rhythm, high levels occurring during the night. The profiles of the latter two rhythms did not closely match one another and differed from those previously described in the pineal organ of the same species. These results are discussed with respect to a possible paracrine role of retinal melatonin. Melatonin-like immunoreactivity was found in the photoreceptor cell layer and in the Müller cells of the inner nuclear layer. The intensity of the melatonin-like immunoreactivity varied throughout the 24 h light-dark cycle, in good correlation with the variations in the melatonin level as measured by radioimmunoassay.This article is dedicated to the memory of Dr. Klaus Hoffmann     

Daily and circadian variations of the pineal and ocular melatonin contents and their contributions to the circulating melatonin in the Japanese newt, Cynops pyrrhogaster

Daily and circadian variations of melatonin contents in the diencephalic region containing the pineal organ, the lateral eyes, and plasma were studied in a urodele amphibian, the Japanese newt (Cynops pyrrhogaster), to investigate the possible roles of melatonin in the circadian system. Melatonin levels in the pineal region and the lateral eyes exhibited daily variations with higher levels during the dark phase than during the light phase under a light-dark cycle of 12 h light and 12 h darkness (LD12:12). These rhythms persisted even under constant darkness but the phase of the rhythm was different from each other. Melatonin levels in the plasma also exhibited significant day-night changes with higher values at mid-dark than at mid-light under LD 12:12. The day-night changes in plasma melatonin levels were abolished in the pinealectomized (Px), ophthalmectomized (Ex), and Px+Ex newts but not in the sham-operated newts. These results indicate that in the Japanese newts, melatonin production in the pineal organ and the lateral eyes were regulated by both environmental light-dark cycles and endogenous circadian clocks, probably located in the pineal organ and the retina, respectively, and that both the pineal organ and the lateral eyes are required to maintain the daily variations of circulating melatonin levels.     

Pineal melatonin rhythms in the lizardAnolis carolinensis: effects of light and temperature cycles

Summary Pineal and ocular melatonin was assessed, over 24 h periods, in male lizards (Anolis carolinensis) entrained to 24 h light-dark (LD) cycles and a constant 32 C, and in lizards entrained to both 24 h LD cycles and 24 h temperature cycles (32 C/20 C). At a constant temperature, the duration of the photoperiod has a profound effect on the duration, amplitude, and phase of the pineal melatonin rhythm (Fig. 1). The pineal melatonin rhythm under cyclic temperature peaks during the cool (20 C) phase of the cycle regardless of whether or not the cool phase occurs during the light or dark phase of a LD 1212 cycle (Fig. 3). Under a temperature cycle and constant dim illumination, a pineal melatonin rhythm is observed which peaks during the cool phase of the temperature cycle, but the amplitude of the rhythm is depressed relative to that observed under LD (Fig. 2). Illumination up to 2 h in duration does not suppress the nocturnal melatonin peak in theAnolis pineal (Fig. 4). No melatonin rhythm was observed in the eyes ofAnolis under either 24 h LD cycles and a constant temperature (Fig. 1), or under simultaneous light and temperature cycles (Fig. 3). Ocular melatonin content was, in all cases, either very low or non-detectable.Abbreviations HIOMT hydroxyindole-O-methyltransferase - NAT N-acetyltransferase     

Restricted daytime feeding attenuates reentrainment of the circadian melatonin rhythm after an 8-h phase advance of the light-dark cycle

It is well established that in the absence of photic cues, the circadian rhythms of rodents can be readily phase-shifted and entrained by various nonphotic stimuli that induce increased levels of locomotor activity (i.e., benzodiazepines, a new running wheel, and limited food access). In the presence of an entraining light-dark (LD) cycle, however, the entraining effects of nonphotic stimuli on (parts of) the circadian oscillator are far less clear. Yet, an interesting finding is that appropriately timed exercise after a phase shift can accelerate the entrainment of circadian rhythms to the new LD cycle in both rodents and humans. The present study investigated whether restricted daytime feeding (RF) (1) induces a phase shift of the melatonin rhythm under entrained LD conditions and (2) accelerates resynchronization of circadian rhythms after an 8-h phase advance. Animals were adapted to RF with 2-h food access at the projected time of the new dark onset. Before and at several time points after the 8-h phase advance, nocturnal melatonin profiles were measured in RF animals and animals on ad libitum feeding (AL). In LD-entrained conditions, RF did not cause any significant changes in the nocturnal melatonin profile as compared to AL. Unexpectedly, after the 8-h phase advance, RF animals resynchronized more slowly to the new LD cycle than AL animals. These results indicate that prior entrainment to a nonphotic stimulus such as RF may "phase lock" the circadian oscillator and in that way hinder resynchronization after a phase shift.     

Light-dark cues entrain oviposition time in the chicken hen

Single Comb White Leghorn hens, entrained to a 24-h light-dark cycle (LDC), were maintained under 16L8D and 20L4D LDC for studying LD cues affecting oviposition time. A shift in the onset of darkness (74% response) had two times the effect on timing the oviposition in a LDC compared to a shift in the onset of light (35–38% response). An effect of over 94% response shift in time of oviposition was found when both LD cues were shifted together. A shift in oviposition time followed the same direction of a displacement of the dark period in a LDC. The longer dark period in a LDC delayed the time of oviposition when counted from the onset of darkness (9.32–9.58 h for 20L4D and 10.50–10.98 h for 16L8D cycles). Data indicated that the onset of light and of darkness, the daily position and length of the dark period all determined the time of oviposition during a LDC.Contribution from the Missouri Agricultural Experiment Station. Journal Series Number10042.     

Regulation of Melatonin Production by Pineal Photoreceptor Cells: Role of Cyclic Nucleotides in the Trout (Oncorhynchus mykiss)

Abstract: The light/dark cycle influences the rhythmic production of melatonin by the trout pineal organ through a modulation of the serotonin N -acetyltransferase (NAT) activity. In static organ culture, cyclic AMP (cAMP) levels (in darkness) and NAT activity (in darkness or light) were stimulated in the presence of forskolin, isobutylmethylxanthine, or theophylline. Analogues of cAMP, but not of cyclic GMP, induced an increase in NAT activity. Light, applied after dark adaptation, inhibited NAT activity. This inhibitory effect was partially prevented in the presence of drugs stimulating cAMP accumulation. In addition, cAMP accumulation and NAT activity increase, induced by forskolin, were temperature dependent. Finally, melatonin release, determined in superfused organs under normal conditions of illumination, was stimulated during the light period of a light/dark cycle by adding an analogue of cAMP or a phosphodiesterase inhibitor. However, no further increase in melatonin release was observed during the dark phase of this cycle in the presence of the drugs. This report shows for the first time that cAMP is a candidate as intracellular second messenger participating in the control of NAT activity and melatonin production by light and temperature.