Regulation of rat pineal melatonin synthesis: effect of monoamine oxidase inhibition

Inhibition of pineal monoamine oxidase (MAO) activity either by harmine or pargyline in adult male Sprague-Dawley rats housed in a 12 : 12 LD cycle resulted in increase pineal N-acetyltransferase (NAT) activity. Pineal MAO inhibition also increased pineal melatonin content, presumably as a result of the increased NAT activity. Conjunct treatment with propranolol, a beta-adrenergic receptor antagonist, nullified these effects, regardless of the MAO inhibitor (harmine, pargyline or both) used or the inhibitor dose given. MAO inhibition during continuous light resulted in increased NAT activity greater than that observed following MAO inhibition during a 12 : 12 LD cycle. On the other hand, the increase in melatonin content following MAO inhibition during continuous light was not significantly different from that following MAO inhibition during a 12 : 12 LD cycle. Conjunct propranolol administration negated the effects of MAO inhibition on both the level of NAT activity and melatonin content, regardless of the lighting conditions. The level of pineal NAT activity is apparently regulated by the level of pineal beta-adrenergic receptor stimulation. While melatonin production appears to be dependent on increases in NAT activity, biosynthesis of this methoxyindole may also be regulated, in part, by other factors or processes in metabolic pathway.     

Type II Thyroxine 5''-Deiodinase Activity in the Rat Brown Adipose Tissue, Pineal Gland, Harderian Gland, and Cerebral Cortex: Effect of Acute Cold Exposure and Lack of Relationship to Pineal Melatonin Synthesis

The effect of acute cold exposure for 6 hours on nocturnal type II thyroxine 5'-deiodinase (5'-D) activity was studied in brown adipose tissue (BAT), Harderian gland, cerebral cortex, and pineal gland of the rat. Moreover, the effect of iopanoic acid (IOP), a potent inhibitor of 5'-D activity, on both pineal N-acetyltransferase (NAT) activity and melatonin content in rats maintained in a cold environment was also examined. Results show that acute cold exposure significantly increases 5'-D activity in BAT but not in either the pineal gland, Harderian gland, or cerebral cortex. In all tissues, the injection of IOP reduced dramatically 5'-D activity, while exposure of the animals to light at night reduced 5'-D activity in pineal gland but not in either the Harderian gland or BAT while light exposure at night increased cerebrocortical 5'-D activity. Cold exposure did not change either pineal NAT activity or the melatonin content of the gland. Finally, when pineal 5'-D activity was inhibited by IOP treatment, neither nocturnal pineal NAT activity nor melatonin content was affected.     

Responsiveness of Pineal N-Acetyltransferase and Melatonin in the Cotton Rat Exposed to Either Artificial or Natural Light at Night

In three separate experiments, the effect of acute exposure to either artificial or natural light during darkness of pineal N-acetyltransferase (NAT) activity and melatonin content was studied in the cotton rat (Sigmodon hispidus). The exposure of animals to an artificial-light irradiance of 160,000 microW/cm2 during darkness for either 1 s, 5 s, or 30 min was followed by a precipitous decline in pineal NAT activity and melatonin content when measured at either 15 or 30 min after light onset. When cotton rats were acutely exposed to light at night for 5 s, irradiances of either 3.2, 32, 320, and 3,200 did not suppress either pineal NAT or melatonin 30 min later; however, if the 5-s exposure had an irradiance of either 32,000 or 160,000 microW/cm2, the pineal enzyme activity and indole content were depressed. Moonlight, which had a maximal irradiance of 0.32 microW/cm2, was unable to suppress pineal NAT activity and melatonin content even when the animals were exposed to the moonlight for 30 min. The treatment of cotton rats with either norepinephrine or its agonist, isoproterenol, before their exposure to light at night retarded slightly the suppressive effect of light on the pineal constituents measured. Also, these drug treatments suppressed the pre-exposure levels of both NAT activity and melatonin content in the cotton rat pineal gland.     

Vasoactive intestinal peptide stimulates N-acetyltransferase and hydroxyindole-O-methyltransferase activities and melatonin production in cultured rat but not in Syrian hamster pineal glands

The purpose of this study was to compare the responses of the Syrian hamster and rat pineal glands in organ culture to vasoactive intestinal peptide (VIP). The endpoints in these studies were the activities of pineal N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT), as well as pineal and medium melatonin levels. When rat pineal glands were incubated with either VIP (1 microM) or isoproterenol (1 microM), a beta-adrenergic agonist, a significant increase in NAT and HIOMT activities and melatonin levels were observed within 3 hr. Conversely, during the day, VIP (1 microM) was ineffective in stimulating these parameters in hamster pineal gland after incubation times of either 2, 4, 6, or 8 hr. In another experiment, hamster pineal glands were collected from animals killed in the late dark period (after 30 min light exposure). In these glands, isoproterenol promoted NAT activity and melatonin production; however, VIP was ineffective in stimulating either NAT or HIOMT activities; likewise, VIP had no stimulatory effect on pineal melatonin levels at night. Finally, when hamster pineal glands at night were incubated with either 0, 10 nM, 100 nM, 10 microM, or 100 microM VIP, no changes in any parameter of melatonin synthesis were measured. The results indicate that the hamster pineal gland, unlike that of the rat, may not respond to VIP with an increased melatonin production.     

Effect of Acute Light Exposure Upon Melatonin Content, NAT Activity, and Nuclear Volume in the Gerbil Pineal Complex

This report describes the response of both the superficial and deep components of the gerbil pineal gland to a 30-min pulse of fluorescent light at the time of the nocturnal peak in pineal activity. When gerbils were maintained in a 14 h light:10 h dark photoperiod, mean melatonin concentrations in the superficial pineal gland ranged between 72 and 108 pg/gland during the day and first 4 h of darkness, rose to a peak of 399 pg/gland 7 h after lights out, then returned to basal levels at the expected time of light onset. Pineal melatonin contents were reduced to daytime values within 30 min after exposure of gerbils to fluorescent light at the time of the nocturnal peak. These changes in superficial pineal melatonin content reflected the circadian pattern in serotonin N-acetyltransferase (NAT) activity, as would be expected if NAT controls melatonin synthesis. In addition to the reductions in NAT and melatonin content induced by nocturnal photic stimulation, there was a 30% reduction in mean pinealocyte nuclear volume. A nocturnal elevation in melatonin content but not in NAT activity occurred in the deep pineal gland. Unlike the superficial pineal gland, the deep pineal gland did not demonstrate a response in melatonin content, NAT activity, or pinealocyte nuclear volumes upon exposure of gerbils to light at the time of the nocturnal peak in superficial pineal activity.     

Unusual responses of nocturnal pineal melatonin synthesis and secretion to swimming: Attempts to define mechanisms

Abstract: The effect of swimming at night on rat pineal melatonin synthesis was compared with that of light exposure at night. Rats were forced to swim at 0030 hr (lights out at 2000 hr) and sacrificed by decapitation 15 and 30 min later, immediately after swimming. Other groups of animals were exposed to white light (650μW/cm²) for 15 and 30 min at same time. Swimming caused a rapid and highly significant drop in the melatonin content in the pineal gland; however, the activity of N-acetyltransferase (NAT), the supposed rate limiting enzyme in the melatonin production, was not changed. Despite the drop in pineal melatonin levels, serum concentrations of the indole remained elevated in the rats that swam. In contrast, melatonin levels in the pineal and serum of light exposed rats fell precipitously, accompanied by a significant suppression of NAT activity. Since we anticipated that the strenuous exercise associated with swimming may induce release of artrial natriuretic peptide (ANP) from the heart, which in turn could cause the release of pineal melatonin, in a second study we injected physiological saline intravenously to stretch the cardiac muscle and release ANP. Three milliliters of normal saline was injected during the day into the jugular vein of anesthetized rats that were pretreated with isoproterenol to stimulate pineal melatonin production. Animals were killed 15 min after the saline injection, and pineal NAT activity and pineal melatonin levels were measured. The saline injections caused no alteration in the elevated levels of either NAT or melatonin. These data suggest that the disparity in pineal NAT activity (which was high) and pineal melatonin (which was low), in animals swum at night, may not be caused by ANP which is released during strenuous exercise such as swimming.     

Estrogen modulates the nocturnal synthesis of melatonin in peripubertal female rats

Abstract: Our objective was to evaluate the effects of estrogen deficit and of estrogen stimulation on the synthesis of pineal melatonin in female rats during the peripubertal period. The levels of melatonin and N-acetylserotonin (NAS) and the activities of N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) were determined in homogenates of pineal glands obtained from peripubertal female Sprague- Dawley rats 4 to 12 weeks of age in the mid-dark during the daily light/dark cycle. Animals were ovariectomized at 4 weeks of age; daily administration of estradiol benzoate (E₂B, 1.0 μg/d, s.c.) was initiated at 4 weeks of age. A peak in the pineal levels of melatonin and NAS and in NAT activity was observed in untreated (control) rats with intact ovaries at 6 weeks. HIOMT activity increased from Week 4 to 6 and remained unchanged thereafter. Ovariectomy at Week 4 led to significant increases in the levels of melatonin and of NAS and NAT in activity at Week 8. NAT activity Week 10 resembled that of control animals, but levels of melatonin and NAS were slightly elevated. Ovariectomy did not affect HIOMT activity. Subcutaneous injection of E₂B significantly decreased the levels of melatonin and NAS and of NAT activity at Week 4, as compared with those in control rats. E₂B suppressed the ovariectomy-induced elevation of levels of melatonin and NAS and of NAT activity, similar to the effect in control animals. E₂B did not affect HIOMT activity. Our results suggest that estrogen modulates the nocturnal synthesis of melatonin in the pineal gland in peripubertal female rats. The effects of estrogen on melatonin synthesis appeared to be mediated by the modulation of NAT activity.     

Photoreceptor damage and eye pigmentation: influence on the sensitivity of rat pineal N-acetyltransferase activity and melatonin levels to light at night

The threshold of light irradiance capable of inhibiting nighttime pineal serotonin N-acetyltransferase (NAT) activity and melatonin content, and the importance of intact photoreceptors and eye pigmentation on these changes, were investigated in the rat. Groups of intact albino and black-eyed rats and albino animals with light-induced photoreceptor damage were studied in the dark period before, and after 15 and 30 min of exposure to either 0.0005, 0.175 or 3.33 microW/cm2 irradiance of light. In animals with photoreceptor damage, the sensitivity of the pineal gland to light decreased so that only the highest irradiance tested (3.33 microW/cm2) was capable of totally inhibiting pineal NAT activity and melatonin levels. In one study, pineal NAT and melatonin levels in intact albino rats were inhibited by all three irradiances studied. In a second experiment, albino and black-eyed animals behaved identically, only responding with a depression in pineal NAT and melatonin after exposure to light irradiances of either 0.175 or 3.33 microW/cm2. In conclusion, the lowest irradiance of cool white light capable of inhibiting pineal NAT and melatonin in albino rats is around 0.0005 microW/cm2. At the irradiances studied, photoreceptor damage influences the response of pineal NAT and melatonin to acute light exposure at night. On the other hand, eye pigmentation does not seem to have a major effect on the nighttime inhibition of the pineal by light.     

Tryptophan administration inhibits nocturnal N-acetyltransferase activity and melatonin content in the rat pineal gland. Evidence that serotonin modulates melatonin production via a receptor-mediated mechanism.

In the rat pineal gland, the activity of serotonin N-acetyltransferase (NAT) and the concentration of melatonin are normally high at night; conversely, the concentration of serotonin (5-HT), the precursor of melatonin, is low. Since tryptophan administration increases the concentration of pineal 5-HT at night, we examined its effect of melatonin production. Nighttime tryptophan loading led to substantial increases in pineal 5-hydroxytryptophan, 5-hydroxyindole acetic acid (5-HIAA), and 5-HT but a highly significant reduction in NAT activity in comparison to saline-injected controls. In contrast to other measured indoles, melatonin levels also were significantly diminished by tryptophan loading. Nocturnally high pineal norepinephrine levels were unaltered by tryptophan administration. The idea that high concentrations of 5-HT could lead to substrate inhibition of NAT activity was not supported by kinetic analysis of control NAT levels versus tryptophan-inhibited NAT activity under varied substrate concentrations. Hypotheses to explain these results include the possibility that tryptophan inhibition of melatonin synthesis is mediated by the release of 5-HT from the pinealocyte and its subsequent autocrine action on melatonin production.     

Changes in nocturnal pineal melatonin synthesis during the perimenopausal period: Relation to estrogen levels in female rats

To evaluate changes in melatonin synthesis during the perimenopausal period in the female rat and to determine the effects of estrogen on melatonin synthesis, pineal levels of tryptophan, melatonin and norepinephrine and activities of N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) were determined. Homogenates for assay were prepared from the pineal glands of female virgin Sprague-Dawley rats between 4 and 24 months of age in the middle of the dark period of a daily light/dark cycle. Serum 17 beta-estradiol (E2) concentrations were also determined. Pineal melatonin levels significantly decreased from month 4 12 and significantly increased from month 12 16, decreasing thereafter. Serum E2 concentrations significantly decreased from month 12-16, and remained low thereafter. No significant changes in tryptophan or norepinephrine were seen. NAT activities paralleled the time course of changes in melatonin. HIOMT activities decreased gradually from month 4 24. Subcutaneous implantation of an E2 capsule between months 12 and 16 resulted in significant decreases in levels of melatonin and NAT activity at month 16. Ovariectomy at month 4 or 12 led to significant increases in the levels of melatonin and NAT activity. These findings represent a temporal increase in pineal melatonin synthesis during the perimenopausal period, and suggest that the increase in melatonin synthesis activity at that time might result from decreasing levels of endogenous estrogen. The effect of estrogen on melatonin synthesis appeared to involve modulation of NAT activity.     

Swimming depresses nighttime melatonin content without changing N-acetyltransferase activity in the rat pineal gland

Recently, it was shown that a 1.5-ml subcutaneous saline injection depressed N-acetyltransferase (NAT) activity and melatonin content in the rat pineal gland at night. The present studies were undertaken to determine if another perturbation, swimming, could duplicate this response. Rats swam at 23.10 h (lights out at 20.00 h) for 10 min and were killed 15 and 30 min after the unset of swimming. Pineal NAT activity was found to be unaffected while melatonin content was depressed dramatically. Hydroxyindole-O-methyltransferase (HIOMT) activity as well as the content of serotonin (5HT), 5-hydroxytryptophan (5HTP) and 5-hydroxyindoleacetic acid (5HIAA) were not changed by this treatment. In a second study, pineal melatonin again was depressed without a concomitant drop in NAT activity. Mean serum melatonin at 15 min after onset of swimming was increased although the rise was not statistically significant. In the final study, it was found that NAT activity was slightly increased in intact rats and unchanged in adrenalectomized rats at 7 min after swimming onset. At 15 min both intact and adrenalectomized animals had NAT activity values similar to those of controls. Pineal melatonin content in intact and adrenalectomized rats plummeted to 50% of control values at 7 min and fell further to 25% at 15 min. While the rate of melatonin synthesis was not directly measured, lack of change in the activities of the enzymes involved in melatonin synthesis and the contents of two melatonin precursors suggests that swimming depresses pineal melatonin content by enhancing melatonin efflux from the gland.     

Effect of Various Cations on the Activity of Pineal Gland N-Acetyltransferase (EC 2.3.1.5) and Hydroxyindole-O-Methyl Transferase (EC2.1.1.4) In Vitro

The effect of various cations on activity of hydroxyindole-O-methyltransferase (HIOMT) and N-acetyltransferase (NAT) from pineal gland was studied. Copper inhibited both enzymes while calcium and magnesium inhibited HIOMT; the mechanisms of these inhibitions are unknown at this stage. Calcium, magnesium, potassium, and sodium initially stimulated NAT activity, and as the cation concentration increased inhibitory activity was observed. These differential effects on NAT activity may in some way be involved in pineal regulation of cation levels.     

Beta-adrenergic and peptide N-terminal histidine and C-terminal isoleucine stimulation of N-acetyl-transferase activity and melatonin production in the cultured rat pineal gland.

The purpose of this investigation was to compare the effect of peptide N-terminal histidine and C-terminal isoleucine (PHI) with that of the beta-adrenergic agonist isoproterenol (ISO) on N-acetyltransferase (NAT) activity and melatonin production in the cultured rat pineal gland. Pineal glands were removed and placed in organ culture containing PHI, ISO, or PHI plus ISO at different dosages, and subsequent changes in NAT activity and melatonin levels were measured. PHI stimulated these parameters in both a time- and dose-dependent manner. Combined treatment with PHI (10(-6) M) and high doses of ISO (either 10(-7) or 10(-8) M) did not potentiate the effect of the peptide in terms of either NAT activity or melatonin levels in the pineal gland. However, at a lower concentration, ISO (10(-9) M) had additive effects to those of PHI in both cultured pineals and medium. The results suggest that PHI modulates melatonin synthesis in the rat pineal gland. Furthermore, stimulation of the pineal with both PHI and ISO demonstrates an additive effect rather than a synergistic action of these compounds. It is presumed that ISO and PHI stimulate pineal melatonin production via separate receptors, but they probably use the same intracellular second messenger, cAMP, to do so. This is the first study showing an effect of the peptide PHI on pineal melatonin production in any vertebrate.     

Changes in serotonin levels, N-acetyltransferase activity, hydroxyindole-O-methyltransferase activity, and melatonin levels in the pineal gland of the Richardson's ground squirrel in relation to the light-dark cycle

Pineal serotonin and melatonin levels and the activities of hydroxyindole-O-methyltransferase (HIOMT) and N-acetyltransferase (NAT) were studied over a 24-hour period in the pineal gland of the diurnally active Richardson's ground squirrel (Spermophilus richardsonii). Under alternating light-dark conditions (light:dark hours 14:10), pineal serotonin and melatonin levels exhibited a rhythm with high values occurring either during the day (serotonin) or during the night (melatonin). NAT activity was also markedly increased during darkness. HIOMT activity exhibited no 24-hour variation. Exposure of squirrels to constant light for 7 days exaggerated the serotonin rhythm, but obliterated the cycles of NAT and melatonin. Under constant darkness (for 7 days), the rhythms in serotonin, melatonin and NAT persisted, each having a period of about 24 h. In the second study, ground squirrels were exposed to light-dark cycles of either 8:16, 10:14 or 14:10. Under each of these photoperiodic environments, rhythms in pineal NAT and melatonin were apparent. Increasing the daily dark period from 10 to 14 h caused a prolongation of the elevated NAT and melatonin levels. However, a further prolongation of the daily dark period (to 16 h) did not further increase the duration of the rise in NAT and melatonin. The results show that continual light exposure (irradiance of 200 microW/cm2) for 7 days suppresses the pineal rhythms in both NAT activity and melatonin level in the Richardson's ground squirrel. Conversely, light exposure, rather than depressing the serotonin rhythm, actually exaggerates it. Constant darkness for 7 days has little influence on the 24-hour rhythms of either NAT or melatonin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pharmacological Studies on the Regulation of N-Acetyltransferase Activity and Melatonin Content of the Pineal Gland of the Syrian Hamster

Thus far, all attempts to stimulate melatonin synthesis by beta-adrenergic receptor agonists in the Syrian hamster pineal gland have failed. Neither a wide range of dosages of isoproterenol (0.5 mg/kg to 24 mg/kg), nor prolonged treatment with norepinephrine, the natural neurotransmitter, increased N-acetyltransferase (NAT) activity or melatonin production. In the present study, the administration of isoproterenol at night was likewise ineffective in advancing or enhancing the normal nightly melatonin peak. Also, we did not find a delayed effect 7 or 8 h after the administration of the drug. Furthermore, we tested the idea of coneurotransmitters such as octopamine or dopamine being possibly necessary for stimulation, but could not find any effect of these substances on melatonin synthesis. In addition, a parasympatholytic agent, atropine, did not increase the responsiveness to sympathomimetic agents. Administration of a phosphodiesterase inhibitor was also ineffective in stimulating NAT activity. On the other hand, isoproterenol did retard the drop in NAT and melatonin after lights-on at night, indicating that beta-receptors are involved in maintaining elevated melatonin levels.     

Electrical stimulation of the hypothalamic paraventricular nuclei inhibits pineal melatonin synthesis in male rats

Recent findings have shown that lesions of the hypothalamic paraventricular nuclei (PVN) disrupt the synthesis of melatonin in the rat pineal gland. In order to further clarify the role of the PVN in the control of pineal function, the effects of electrical stimulation of these nuclei were investigated in acutely blinded adult male Sprague-Dawley rats. Following electrical stimulation, pineal serotonin-N-acetyltransferase (NAT) activity and pineal melatonin content were measured by means of radioenzymatic and radioimmunoassay methods, respectively. Stimulation had no significant effect on pineal melatonin synthesis throughout the early part of the dark phase, but caused a significant reduction in NAT activity during the light phase and the latter part of the dark phase. The pineal melatonin content appeared reduced, but due to large individual variations this reduction was not statistically significant. Stimulation duration experiments reveal that reduction of NAT activity is time dependent, with significant inhibition occurring after 30 min of stimulation. These observations further support the involvement of the PVN in the melatonin rhythm generating pathway and suggest that electrical activation of fibers in the PVN is similar to the effects of light on pineal melatonin synthesis.     

Effects of Short-Day Photoperiods and of N-(2,4-Dinitrophenyl)-5-Methoxytryptamine, a Putative Melatonin Antagonist, on Melatonin Synthesis in the Harderian Gland of the Syrian Hamster, Mesocricetus auratus

The Harderian glands of Syrian hamsters contain melatonin and the enzymes N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) which synthesize melatonin from serotonin. Because the Harderian glands share this metabolic pathway with the pineal gland, we examined the effects of short-day photoperiods, which stimulate pineal-mediated gonadal regression, and N-(2,4-dinitrophenyl)-5-methoxytryptamine (ML-23), which has been described as a melatonin antagonist, on melatonin synthesis in the Harderian glands of the hamster. Harderian glands of male hamsters kept in short days had reduced NAT activity and melatonin concentration, but HIOMT activity was unchanged from that of long-day controls. In males kept in short days, ML-23 restored melatonin concentrations to levels seen in long days but did not affect the short-day induced reduction in NAT activity. ML-23 had no effect upon NAT or HIOMT activity or melatonin concentration in male hamsters kept on long days. Harderian glands of female hamsters kept on short days had reduced melatonin concentrations, but NAT and HIOMT activities similar to those of long-day controls. ML-23 had no effect on Harderian NAT or HIOMT activities or melatonin concentration in females kept in short days. However, in females kept in long days, ML-23 treatment led to increased NAT activity and decreased melatonin concentrations. We conclude from these results that short-day photoperiods alter some aspects of melatonin synthesis in hamster Harderian glands and that these effects differ in males and females. ML-23 does not usually prevent the effects of short days on Harderian melatonin synthesis, suggesting that it is not a melatonin antagonist in the Syrian hamster.     

Stimulatory effect of isoproterenol but not of dibutyryl cyclic AMP on N-acetyltransferase activity and melatonin content of Syrian hamster pineal gland in organ culture

The purpose of this study was to compare the response of Syrian hamster pineal glands in organ culture either to isoproterenol, a beta-adrenergic agonist, or to dibutyryl cyclic AMP. When pineal glands were collected at night, hamsters were exposed to light for 30 min to depress pineal N-acetyltransferase (NAT) activity and melatonin values to low levels. Pineal glands were removed and placed in organ culture containing either isoproterenol or dibutyryl cyclic AMP and subsequent changes in NAT activity and melatonin levels were measured. At night, isoproterenol (10(-7) or 10(-6) M) induced an increase in the NAT activity and melatonin levels in both pineals and culture media. However, dibutyryl cyclic AMP was either ineffective or minimally effective in stimulating these parameters at either different incubation times (2, 4, and 6 h) or drug concentrations (0.1, 0.5, and 1.0 mM). Conversely, when rat pineal glands were incubated with either isoproterenol (10(-7)) or dibutyryl cyclic AMP (0.5 mM) dramatic rises in NAT activity and melatonin levels were observed. In another experiment, hamster pineal glands were collected from animals killed either late in the light period (19.00 h) or in the latter half of the dark period. Isoproterenol promoted NAT activity and melatonin production only in glands collected in the latter half of the dark phase.     

Regulation of melatonin production by catecholamines and adenosine in a photoreceptive pineal organ. An in vitro study in the pike and the trout

The pineal organ of fish contains photoreceptor cells with structural and functional analogies to retinal photoreceptors. In these cells, the light/dark (LD) cycle influences the production of melatonin by controlling the activity of one of its synthetizing enzymes, serotonin N-acetyltransferase (NAT). The daily rhythm in NAT activity is generated endogenously in the pike but not in the trout pineal. We report here that in addition to the LD information, chemical factors are also involved in the control of melatonin production. Adenosine and two of its analogs stimulated or inhibited NAT activity and melatonin release in cultured pike and trout pineals, depending on the experimental conditions. It is believed that the nucleoside, produced locally, exerts a modulatory role on the neurohormonal output via still enigmatic mechanisms, involving a transmembranous carrier. Nocturnal melatonin production in cultured pike pineals was inhibited by alpha-adrenergic agonists and stimulated by a beta-adrenergic agonist. No effect could be induced in trout pineals cultured under similar conditions. Because melatonin production by pineal photoreceptors is apparently regulated by both light and chemical inputs, we propose they might be multieffector cells.