Kinetic study of melatonin release from rat pineal glands using a perifusion technique

Connexin36 localization to pinealocytes in the pineal gland of mouse and rat

Several cell types in the pineal gland are known to establish intercellular gap junctions, but the connexin constituents of those junctions have not been fully characterized. Specifically, the expression of connexin36 (Cx36) protein and mRNA has been examined in the pineal, but the identity of cells that produce Cx36 and that form Cx36-containing gap junctions has not been determined. We used immunofluorescence and freeze fracture replica immunogold labelling (FRIL) of Cx36 to investigate the cellular and subcellular localization of Cx36 in the pineal gland of adult mouse and rat. Immunofluorescence labelling of Cx36 was visualized exclusively as puncta or short immunopositive strands that were distributed throughout the pineal, and which were absent in pineal sections from Cx36 null mice. By double immunofluorescence labelling, Cx36 was localized to tryptophan hydroxylase-positive and 5-hydroxytryptamine-positive pinealocyte cell bodies and their large initial processes, including at intersections of those processes and at sites displaying a confluence of processes. Labelling for the cell junction marker zonula occludens-1 (ZO-1) either overlapped or was closely associated with labelling for Cx36. Pinealocytes thus form Cx36-containing gap junctions that also incorporate the scaffolding protein ZO-1. FRIL revealed labelling of Cx36 at ultrastructurally defined gap junctions between pinealocytes, most of which was at gap junctions having reticular, ribbon or string configurations. The results suggest that the endocrine functions of pinealocytes and their secretion of melatonin is supported by their intercellular communication via Cx36-containing gap junctions, which may now be tested by the use of Cx36 null mice.

A noradrenergic sensitive endogenous clock is present in the rat pineal gland

The aim of this study was to examine the occurrence of endogenous oscillations of Per1, Per2, Bmal1 and Rev-erbα genes in rat pineal explants and to investigate their regulation by adrenergic ligands. Our results show a significant and sustained rhythm of Per2,Bmal1 and Rev-erbα gene expression for up to 48 h in cultured pineal gland with a pattern similar to that observed in vivo. By contrast, the rhythms of Per1 and Aa-nat, the rate-limiting enzyme for melatonin synthesis, were strongly attenuated after 24 h in culture. Addition of the exogenous adrenergic agonist isoproterenol on cultured pineal glands induced a short-term increase in mRNA levels of Per1 and Aa-nat, but not those of Per2,Bmal1 and Rev-erbα. This study demonstrates that the rat pineal gland hosts a circadian oscillator as evidenced by the sustained, noradrenergic-independent, endogenous oscillations of Per2, Bmal1 and Rev-erbα mRNA levels in cultured tissues. Only expression of Per1 was stimulated by adrenergic ligands suggesting that, in vivo, the adrenergic input could synchronize the pineal clock by acting selectively on Per1.

Daily rhythm and regulation of clock gene expression in the rat pineal gland

Rhythms in pineal melatonin synthesis are controlled by the biological clock located in the suprachiasmatic nuclei. The endogenous clock oscillations rely upon genetic mechanisms involving clock genes coding for transcription factors working in negative and positive feedback loops. Most of these clock genes are expressed rhythmically in other tissues. Because of the peculiar role of the pineal gland in the photoneuroendocrine axis regulating biological rhythms, we studied whether clock genes are expressed in the rat pineal gland and how their expression is regulated.Per1, Per3, Cry2 and Cry1 clock genes are expressed in the pineal gland and their transcription is increased during the night. Analysis of the regulation of these pineal clock genes indicates that they may be categorized into two groups. Expression of Per1 and Cry2 genes shows the following features: (1) the 24 h rhythm persists, although damped, in constant darkness; (2) the nocturnal increase is abolished following light exposure or injection with a beta-adrenergic antagonist; and (3) the expression during daytime is stimulated by an injection with a beta-adrenergic agonist. In contrast, Per3 and Cry1 day and night mRNA levels are not responsive to adrenergic ligands (as previously reported for Per2) and daily expression of Per3 and Cry1 appears strongly damped or abolished in constant darkness. These data show that the expression of Per1 and Cry2 in the rat pineal gland is regulated by the clock-driven changes in norepinephrine, in a similar manner to the melatonin rhythm-generating enzyme arylalkylamine N-acetyltransferase. The expression of Per3 and Cry1 displays a daily rhythm not regulated by norepinephrine, suggesting the involvement of another day/night regulated transmitter(s).

Long term inhibition by estradiol or progesterone of melatonin secretion after administration of a mammary carcinogen, the dimethyl benz(a)anthracene, in Sprague-Dawley female rat; inhibitory effect of Melatonin on mammary carcinogenesis

A single intragastric administration of 7,12-dimethylbenz(a)anthracene (DMBA) has been shown to induce mammary tumors in young cycling female Sprague-Dawley rats. The appearance of the tumors is preceded by a series of neuroendocrine disturbances, including attenuation of the preovulatory Luteinizing Hormone surge and Gonadotropin-Releasing Hormone release and amplification of the preovulatory 17beta-Estradiol (E2) surge. In this study, we examined the hypothesis that a single administration of DMBA increases the E2 and Progesterone inhibition of the spontaneous and Isoproterenol-induced Melatonin (MT) secretion from the pineal gland, during the latency phase. Also, the incidence of mammary tumors, as well as the possible preventive effect of various doses of Melatonin, were recorded up to 6 months after daily administration. For all studies, Sprague-Dawley rats, 55-60 days of age, received, on the Estrous day of the Estrous cycle, a single dose of 15 mg DMBA delivered by intragastric intubation. For the study on ovarian steroids, they were ovariectomized 5 days later and then sacrificed by decapitation at 10 a.m., one month later. Pineal glands were removed and placed in perifusion chambers containing Hanks 199 medium. The medium was saturated with O2/CO2 (95%/5%) and its pH was 7.4. Ten independent chambers were immersed in a water bath at 37 degrees C. Each pineal gland received medium (flow rate: 0.16 ml/min) through a system of input lines. The fractions were collected every 10 min, and immediately frozen at -20 degrees C until Melatonin RIA. Experiments were repeated to obtain up to five experimental points for each treatment. E2 (10(-11)-10(-9) M) and Progesterone (10(-9)-10(-7) M) were applied during the entire perifusion period (7 h). Isoproterenol (10(-6) M) was applied for 20 min after 2.5 h in perifusion. Melatonin concentrations and Areas Under the Curves were compared using two-factor ANOVA as well as parametric or nonparametric two-sample methods after testing sample normality. For the study on the possible preventive effect of Melatonin, they were daily treated, by the intragastric route, with increasing doses of Melatonin for 6 months. The percentage of female rats having at least one mammary carcinoma were compared using the Fischer exact t-test. During the latency phase, in vehicle-treated rats, E2 and Progesterone treatments lead an almost significant inhibition of the Isoproterenol-induced stimulation of Melatonin secretion. In DMBA-treated rats, E2 treatment leads to a complete blunting of the Isoproterenol-induced stimulation of Melatonin and Progesterone treatment leads to a cyclic inhibition of the Isoproterenol-induced Melatonin secretion. During the promotion phase, there was a dose-dependent inhibitory effect (up to 65% inhibition) of the daily administration of Melatonin, on mammary tumors occurrence. In conclusion, the long term inhibition of DMBA upon Melatonin secretion from the pineal gland might accelerate the promotion of mammary tumors induced by the mammary carcinogen. Inversely, the daily administration of Melatonin for 6 months induces a long lasting protective effect against the formation of mammary tumors.

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Mechanisms regulating the marked seasonal variation in melatonin synthesis in the European hamster pineal gland

Like many wild species, the European hamster (Cricetus cricetus) adapts to the marked seasonal changes in its environment, namely by hibernation and inhibition of sexual activity in winter. These annual functions are driven by the variation in the environmental factors (light, temperature) that are transmitted to the body through large variations in the duration and amplitude of the nocturnal melatonin rhythm. Here we report that the seasonal variation in melatonin synthesis is mainly driven by arylalkylamine N-acetyltransferase gene transcription and enzyme activation. This, however, does not exclude participation of hydroxyindole-O-methyltransferase, which may relay environmental temperature information. The in vivo experiments show that norepinephrine stimulates melatonin synthesis, this effect being gated at night. The possibility that the variation in pineal metabolism depends on a seasonal change in the suprachiasmatic nuclei clock circadian activity that is transmitted by norepinephrine is discussed.

Melatonin release from rat pineals in vitro is stimulated by both the alpha(2)-adrenoceptor agonist medetomidine and the antagonist atipamezole

This study was done to clarify the role of alpha(2)-adrenoceptors in the regulation of pineal melatonin synthesis. Rat pineal glands were incubated in oxygenated Krebs-Ringer solution in perifusion chambers, and perifused for 30 min with alpha(2)-adrenoceptor ligands. The melatonin concentrations were measured from the perifusate by radioimmunoassay. Both medetomidine and atipamezole (>/=10(-5) M) increased melatonin release. Yohimbine blocked the increase caused by medetomidine but not by atipamezole. The effects of medetomidine and atipamezole were also additive: the maximum response to atipamezole could be significantly increased by medetomidine. These results suggest that the two drugs stimulate the melatonin synthesis through different mechanisms: medetomidine through alpha(2)-adrenoceptors and atipamezole possibly through nonadrenergic mechanisms. The results differ from previous in vivo experiments suggesting that alpha(2)-adrenoceptor ligands affect melatonin synthesis both centrally and locally in the pineal gland. The local effects are most likely masked under the central regulatory systems in vivo.

Estimation of frequently sampled nocturnal melatonin production in humans by deconvolution analysis: evidence for episodic or ultradian secretion

In order to investigate nocturnal melatonin production and clearance rates, we applied deconvolution analysis to plasma melatonin concentration time series obtained every 20 min for 12 hr from 20:00 h to 08:00 h in two groups of healthy subjects at rest (group 1, 12 male subjects, 22-26 yr; group 2, ten female subjects, 31-42 yr). The estimated melatonin production rate from group 1 (0.55 +/- 0.21 microg/kg/night) was significantly higher than that of group 2 (0.26 +/- 0.19 microg/kg/night), as well as the mass of melatonin released per burst (275 +/- 110 vs. 145 +/- 130 pg/mL for groups 1 and 2, respectively), the amplitude of secretory bursts (7.8 +/- 3.2 vs. 4.7 +/- 3.5 pg/mL/min), and the pulsatile melatonin production rate (2.76 +/- 1.14 vs. 1.27 +/- 0.97 ng/mL/night). These differences could reflect alterations related to age and or gender. No differences were observed between the two groups in the secretory burst half-duration and frequency and the interburst interval. Melatonin production rates, as estimated by deconvolution analysis, are in agreement with other independent estimates, especially the isotopic method, and disclose an ultradian rhythmicity.

Inducible cyclic AMP early repressor protein in rat pinealocytes: a highly sensitive natural reporter for regulated gene transcription

Rhythmic activity of arylalkylamine N-acetyltransferase (AANAT) determines melatonin synthesis in rat pineal gland. The transcriptional regulation of AANAT involves the activating and inhibiting transcription factors of the cyclic AMP (cAMP)-signaling pathway, cAMP response element-binding protein and inducible cAMP early repressor (ICER), respectively. Activation of this pathway is centered around norepinephrine, stimulating beta(1)-adrenergic receptors, but various other transmitters can modulate melatonin biosynthesis. To compare the transcriptional impact of norepinephrine with that of other neurotransmitters on melatonin synthesis, we determined ICER protein levels in pinealocytes and, in parallel, hormone secretion. The dose-dependent inductions of ICER protein by norepinephrine, the beta(1)-adrenergic receptor agonist isoproterenol, vasoactive intestinal peptide, pituitary adenylate cyclase-activating polypeptide, and adenosine are correlated to regulatory dynamics in melatonin production. Importantly, ICER protein induction required lower ligand concentrations than the induction of melatonin biosynthesis. Although neuropeptide Y, glutamate, and vasopressin altered norepinephrine-stimulated hormone production without affecting ICER levels, the activation of voltage-gated cation channels increased ICER without affecting hormone synthesis. Sensitivity and versatility of ICER induction in pinealocytes make these neuroendocrine cells a valuable model system in which to study molecular interactions determining a regulated gene expression.

Evidence for melatonin synthesis in rodent Harderian gland: a dynamic in vitro study

Melatonin content and release from Harderian glands (HGs) has been measured by an in vitro perifusion technique in three rodent species: Wistar rat, Syrian hamster, and Siberian hamster. Melatonin immunoreactive concentrations in HGs of animals killed at 10.00 hr were 0.31 +/- 0.031 pg/mg gland in male Wistar rat, 0.54 +/- 0.026 pg/mg gland in male Siberian hamster, 0.17 +/- 0.070 and 0.20 +/- 0.059 pg/mg gland in male and female Syrian hamster, respectively. In all species examined, isolated HGs perifused for 9-15 hr released melatonin but did not stabilize their melatonin release rate. No sex-related difference could be noted in the HG melatonin release rate. The total amount of melatonin released over a 15 hr long perifusion was about 0.075 +/- 0.004 ng/15 h/mg gland and 0.063 +/- 0.010 ng/15 hr/mg gland in male and female Wistar rat, respectively; 0.155 +/- 0.019 ng/15 hr/mg gland and 0.141 +/- 0.006 ng/15 hr/mg gland in male and female Siberian hamster, respectively; 0.035 +/- 0.003 ng/15 hr/mg gland and 0.045 +/- 0.004 ng/15 hr/mg gland in male and female Syrian hamster, respectively. This amount, which is higher than the tissue levels, demonstrates the de novo melatonin synthesis. This is confirmed by the fact that infusion of the indoleamine precursor, tryptophan (TRP), stimulated melatonin secretion from HGs. The melatonin release is increased by 2.5-fold in male and female Wistar rat, 1.5-fold in male and female Siberian hamster, and 2.0- and 3.0-fold in male and female Syrian hamster, respectively. Treatment with a TRP hydroxylase inhibitor, para-chlorophenylalanine, reduced basal melatonin release and inhibited the TRP-induced melatonin stimulation. Kinetics and amounts of melatonin released were not affected by pinealectomy, ruling out a possible plasmatic origin of the HG melatonin. Isoproterenol, a beta-adrenergic agonist, and dibutyryl cyclic AMP, a cyclic AMP analogue, failed to stimulate HG melatonin secretion. In conclusion, these results confirm the presence of melatonin in the HGs and demonstrate that melatonin is synthesized in and released from isolated rodent HGs.

Pharmacological, molecular and functional characterization of vasoactive intestinal polypeptide/pituitary adenylate cyclase-activating polypeptide receptors in the rat pineal gland

Melatonin secretion from the mammalian pineal gland is strongly stimulated by noradrenaline and also by vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP). Three types of receptors for VIP and PACAP have been characterized so far: VIP1/PACAP receptors and VIP2/PACAP receptors, which possess similar high affinities for VIP and PACAP, and PACAP1 receptors which exhibit a 100-1000-fold higher affinity for PACAP. The aim of the present study was to characterize the receptor subtype(s) mediating the stimulatory effects of VIP and PACAP on melatonin synthesis in the rat pineal gland. Autoradiographic studies showed that PACAP and VIP were equally potent in displacing binding of radioiodinated PACAP27 from pineal sections. Amplification of pineal complementary DNAs by polymerase chain reaction using specific primers for the different receptor subtypes revealed that all three receptor messenger RNAs are expressed and that VIP1/PACAP receptor messenger RNA was predominant over VIP2/PACAP receptor messenger RNA. In vitro, VIP and PACAP stimulated melatonin synthesis with similar high potency and the effect of the two peptides were not additive. The selective VIP1/PACAP receptor agonists [R16]chicken secretin (1-25) and [K15, R16, L27]VIP(1-7)/growth hormone releasing factor(8-27) were significantly more potent than the selective VIP2/PACAP receptor agonist RO 25-1553 in stimulating melatonin secretion. The stimulatory effects of VIP and PACAP were similarly inhibited by the VIP1/PACAP antagonist [acetyl-His1, D-Phe2, K15, R16, L27]VIP(3-7)/growth hormone releasing factor(8-27). These data strongly suggest that VIP and PACAP exert a stimulatory effect on melatonin synthesis mainly through activation of a pineal VIP1/PACAP receptor subtype.

The role of the intracellular and extracellular serotonin in the regulation of melatonin production in rat pinealocytes

This study investigated whether the activation of pinealocyte beta-adrenergic receptors is involved in the regulation of serotonin (5-HT) synthesis and release, as it is for melatonin production. In addition, the role of the intra- and extra-cellular 5-HT in modulating the synthesis of melatonin induced by the beta-adrenergic agonist isoproterenol (ISO) was also studied. The incubation of dissociated pinealocytes with 0.1-10 microM ISO resulted in a concentration-dependent increase of melatonin synthesis. 5-HT release and intracellular 5-HT content were increased by 0.1 and 1 microM ISO but they were reduced after ISO 10 microM. Moreover, when incubated with the tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA), the secretion of 5-HT as well as the intracellular 5-HT levels were markedly reduced in both ISO-stimulated and unstimulated conditions. Melatonin release was also inhibited by PCPA, although it responded in the expected manner to increasing concentrations of ISO. These data indicate that the release of 5-HT from pinealocytes depends on the availability of cytoplasmic 5-HT, which in turn is highly dependent on the tryptophan hydroxylase activity. In cells stimulated with moderate ISO concentrations, 5-HT release may be an important regulatory process of pineal 5-HT. After a large stimulation of N-acetyltransferase (NAT) activity by ISO, the synthesis of melatonin prevails on 5-HT release, whose decrease is associated to a deficit of intracellular 5-HT. On the other hand, the present study shows that the incubation of pineal cells with high concentrations of 5-HT or with a selective 5-HT2 receptor agonist, alpha-methyl-5-hydroxytryptamine, reverses partially the inhibitory effect of PCPA on the ISO-stimulated melatonin synthesis. In contrast the 5-HT2 antagonist, ketanserin, results in an inhibiton of the release of melatonin following ISO stimulation. These results suggest that released 5-HT may have a role in the full expression of the beta-adrenergically induced NAT activity and, thus, may contribute to the optimal melatonin synthesis at night.

Vasopressin potentiation of the melatonin synthetic pathway via specific V1a receptors in the rat pineal gland

The pineal gland releases the "time-keeping' hormone melatonin following a rhythmic sympathetic input which translates light information. The aim of this work was to study the role and mechanism of action of the central vasopressinergic input on pineal cAMP-dependent melatonin synthesis in the rat. The pineal was found to display vasopressin receptors of the V1a subtype, as the V1a antagonist [125I]HO-LVA bound in a saturable manner to pineal membranes with a high affinity (kd = 10 pM) and a maximal binding capacity (B(max)) of 13 fmol/mg protein. Vasopressin was able to displace [125I]HO-LVA binding in a dose-dependent manner (k(i) = 1.9 nM). Vasopressin had no effect on the basal cAMP level and melatonin secretion in cultured rat pinealocytes. However, it clearly and dose-dependently (EC50 = 7 nM) potentiated by 2-3 times cAMP accumulation and by 1.5-2.5 times melatonin secretion induced by moderate noradrenergic stimulation. On strongly stimulated pinealocytes, however, vasopressin could potentiate cAMP accumulation, but not melatonin secretion. The potentiatory effect of vasopressin was inhibited in the presence of the V1a antagonist. These results indicate that vasopressin is a potent modulator of rat pineal synthetic activity.

Evidence for a regulatory role of melatonin on serotonin release and uptake in the pineal gland

Melatonin has been proposed to exert some regulatory actions within the pineal gland itself. The present study examined the effect of melatonin on the release of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) from rat pineal glands by using an in vitro perifusion system. Melatonin induced a concentration-dependent stimulatory effect on 5-HT secretion from 10(-6) M to 10(-3) M. Maximal effects were obtained with melatonin 10(-3) M and concentrations lower than 10(-6) M were without effect. The secretion of 5-HIAA was inhibited by melatonin 10(-3) and 10(-4) M, but it was increased when pineals were incubated with 10(-5) and 10(-6) M of melatonin. The indoleamine secretion was also studied on peripherally denervated rat pineal glands. Basal output of 5-HT from these glands was increased when compared with those from control rats. In contrast, the secretion of 5-HIAA was strongly reduced after removal of the sympathetic input to the pineal gland. Melatonin 10(-3) M failed to stimulate 5-HT release from denervated pineal glands, although it inhibited 5-HIAA secretion. In contrast, melatonin 10(-5) M enhanced 5-HT release without altering 5-HIAA output. Fluoxetine, a 5-HT uptake inhibitor, produced similar effects than mM concentrations of melatonin on the indoleamine secretion from control pineal glands, but it had no effect on glands taken from peripherally denervated rats. These data suggest that mM concentrations of the pineal hormone are able to stimulate 5-HT release from the pinealocyte, while mM concentrations of melatonin increase extracellular 5-HT by inhibiting its reuptake in the adrenergic nerve endings. These findings are discussed in relation to the possible role of melatonin regulating the intra- and extracellular availability of 5-HT in the pineal gland and its significance as an autocrine factor.

Three dimensional culture of pineal cell aggregates: a model of cell-cell co-operation

Three dimensional (3-D) cultures of pineal cell aggregates were obtained by constant gyratory shaking the heterogenous cell populations, obtained from the rat pineals, in the DMEM (Dulbecco's modified Eagle's medium). Within 4 days, the pineal cells became organized into a tissue like configuration appearing as a compact ball, evidenced by the scanning electron microscopy. The 3-D aggregates seemed to be mainly composed of pinealocytes (round-oval cells), glial (elongated cells) and other unknown cells. The heterogenous cells were separated by intercellular spaces. The ultrastructural characteristics revealed by transmission electron microscopy exhibited the presence of granular lysosomes, typical of pinealocytes actively involved in the secretion. These pineal cell aggregates secreted melatonin and other indole amines i.e. 5-methoxytryptamine (5-MT), indole acetic acid (IAA), 5-methoxy-3-indole acetic acid (5-MIAA), tryptophol (TOL) and 5-methoxytryptophol (5-MTL) in the culture medium, indicating the functional aspect of pinealocytes. The 3-D aggregates cultures had advantages over the pineal monolayer cultures as, after 4 days of culture, the amounts of indole amines secreted by 3-D aggregates were higher than those secreted by monolayer cultures. Besides, the 3-D aggregates remained functional till 24 days in the gyratory culture conditions. In the continuous perifusion system, the 3-D aggregates secreted melatonin while challanged with isoproterenol. This 3-D model of pineal cell aggregates might be useful, in future, to perform other kinetic studies of the release of indole amines in perifusion experiments as this system allows the maintenance of pineal cells for a long period of time.

Distinct modes of melatonin secretion in normal men

As for many hormones, melatonin levels in the blood suggest that it is discharged from the pineal gland in a pulsatile manner. Recently, the existence of short-term episodes, superimposed on the circadian pattern of circulating melatonin, has been questioned. Because plasma melatonin levels reflect not only the secretory process, but also the effects of distribution and degradation, secretory rates were estimated from peripheral levels, using a deconvolution procedure. Fourteen healthy volunteers were studied during the night, while sleeping in the dark (2300-0700), and seven of them subsequently were used in a replicate study. Plasma melatonin levels were measured at 10-min intervals by a direct, specific radioimmunoassay. Pulse analysis was performed using the computer program ULTRA. Approximately 30% more pulses were detected on the overall secretory were often superimposed on tonic basal secretion. Their number, amplitude, and distribution over time were variable depending on subjects. Also the mean melatonin secretory rate varied more than threefold across individuals. Despite the large interindividual variability, the subjects, who were used in replicate experiment, displayed a rather similar secretory profile. We conclude that in normal adult men, melatonin secretion undergoes two distinct secretory modes, in which episodic secretion is superimposed on tonic secretion in subject-dependent variable proportions.

Rat pineal cell aggregates: ultrastructural and functional characteristics

The aggregates were obtained by constant gyratory shaking of suspension cells freshly isolated from adult rat pineal glands. Their sizes ranged from 60 to 120 microns. Within 4-5 days, the aggregates formed by pinealocytes, astrocytes, and other unidentified cells became organized in a tissue-like configuration. There was no proliferation of the fibroblast cells. Ultrastructural characteristics of the aggregates were revealed by the presence of granular lysosomes, which are typical of pinealocytes, and are actively involved in the secretion. Functional characteristics were studied in static incubation. The aggregates secreted melatonin and other indole amines in culture medium. Basal melatonin release was detected until Day 24 of culture. This secretion was stimulated 230% with Isoproterenol (beta-adrenergic agonist), 725% with Epinephrine (alpha- and beta-adrenergic agonists), and 140% with Vasoactive Intestinal Peptide after 5 days in culture, then > 1200% with Forskolin 9 days later (14-day-old aggregates). The results indicate that three-dimensional aggregates obtained from isolated pineal gland cells were the functional multicellular structures with in vivo characteristics.

Presynaptic and postsynaptic effects of neuropeptide Y in the rat pineal gland

Neuropeptide Y is colocalized with noradrenaline in sympathetic fibers innervating the rat pineal gland. In this article we present a study of the effects and mechanisms of action of neuropeptide Y on the pineal noradrenergic transmission, the main input leading to the rhythmic secretion of melatonin. At the presynaptic level, neuropeptide Y inhibits by 45%, with an EC50 of 50 nM, the potassium-evoked noradrenaline release from pineal nerve endings. This neuropeptide Y inhibition occurs via the activation of pertussis toxin-sensitive G protein-coupled neuropeptide Y-Y2 receptors and is independent from, but additive to, the alpha 2-adrenergic inhibition of noradrenaline release. At the postsynaptic level, neuropeptide Y decreases by a maximum of 35%, with an EC50 of 5 nM, the beta-adrenergic induction of cyclic AMP elevation via the activation of neuropeptide Y-Y1 receptors. This moderate neuropeptide Y-induced inhibition of cyclic AMP accumulation, however, has no effect on the melatonin secretion induced by a beta-adrenergic stimulation. On the contrary, in the presence of 1 mM ascorbic acid, neuropeptide Y potentiates (up to threefold) the melatonin secretion. In conclusion, this study has demonstrated that neuropeptide Y modulates the noradrenergic transmission in the rat pineal gland at both presynaptic and postsynaptic levels, using different receptor subtypes and transduction pathways.

Glutamate inhibition of the adrenergic-stimulated production of melatonin in rat pineal gland in vitro

The effect of L-glutamate on the adrenergic-stimulated release of melatonin in the rat pineal gland was examined using an in vitro perfusion system. L-Glutamate by itself had no effect on melatonin secretion whereas L-glutamate administered prior to (-)-isoproterenol (beta-adrenergic agonist) and L-phenylephrine (alpha-adrenergic agonist) inhibited melatonin production by 42%. L-Glutamate did not inhibit melatonin secretion when glands were stimulated with (-)-isoproterenol alone. D-Glutamate, as well as the L-glutamate agonists kainate, N-methyl-D-aspartate, quisqualate, and trans-1-aminocyclopentane-1,3-dicarboxylic acid, had no effect on the (-)-isoproterenol- and L-phenylephrine-stimulated secretion of melatonin, which suggests that the inhibitory effects of glutamate are not mediated via any of the known glutamate receptor subtypes. The possibility that L-glutamate may be converted to another neuroactive compound (GABA) prior to the addition of (-)-isoproterenol and L-phenylephrine is suggested by the observation that simultaneous administration of L-glutamate with (-)-isoproterenol and L-phenylephrine did not inhibit melatonin production.

Changes in tyrosine hydroxylase activity and melatonin synthesis in rat pineal glands throughout 72 hours of incubation

Time-dependent modifications of N-acetyltransferase (NAT) activity, melatonin content, and tyrosine hydroxylase (TH) activity within the isolated rat pineal over a 72 h of incubation period were studied. Both pineal NAT activity and melatonin content displayed similar decreases after 12 h of incubation, and reached a maximal reduction of 82% or 93%, respectively, after 48 h. However, a maximal decrease of pineal TH activity was only 47% after 72 h of incubation. Neither the nuclear area of pinealocytes, nor lactate dehydrogenase (LDH) activity in the incubation medium showed any significant variation dependent on the incubation time. Treatment with isoproterenol caused an expected elevation of NAT activity and melatonin content but was unable to modify TH activity. Since a significant degree of pineal TH activity remained following 3 days of incubation, a non-sympathetic source of TH within the rat pineal gland is suggested.

Adrenergic signals direct rhythmic expression of transcriptional repressor CREM in the pineal gland

Transcription factor CREM appears to play a key physiological and developmental role within the hypothalamic-pituitary-gonadal axis. This axis is modulated by the pineal hormone melatonin, whose production is in turn driven by the endogenous clock. There is striking circadian fluctuation of a novel CREM isoform, ICER, which is expressed at high levels during the night. ICER is generated from an alternative, intronic promoter and functions as a powerful repressor of cyclic AMP-induced transcription. Rhythmic adrenergic signals originated by the clock direct ICER expression by stimulation of the cAMP signal transduction pathway.

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates melatonin synthesis from rat pineal gland

The effect of pituitary adenylate cyclase polypeptide (PACAP) on rat pineal was examined. PACAP stimulated melatonin release from cultured dissociated pinealocytes with a 10(4) higher potency than isoproterenol (EC50 were 30 pM and 250 nM, respectively). The 10(-9) M PACAP stimulation was not inhibited by 5 x 10(-6) M VIP antagonist whereas that of 10(-9) M VIP was reduced by 54%. Kinetic analysis of melatonin release indicated that PACAP acts postsynaptically via receptor activation.

Long-Term Dynamic in vitro System for Investigating Rat Pineal Melatonin Secretion

Abstract Several details regarding the control of melatonin (MT) secretion from the pinealocytes are still to be clarified. To obtain more data on the mechanism and kinetics of MT secretion and on the interactions between bioactive materials affecting MT production, a perifusion system has been developed in our laboratory. In this dynamic in vitro system the surviving pinealocytes maintain their full responsiveness for at least 5 days. In order to determine the MT contents of large numbers of samples collected in the perifusion system, a sensitive MT radioimmunoassay was set up utilizing our specific MT antibody. In our perifusion system the basal MT release does not change significantly during the 5-day experiments. Norepinephrine (NE) was used at 1 muM concentration for 30 min as a marker of the responsiveness of pinealocytes, given at the beginning and at the end of the same experiments. No significant difference was found in the MT responses to NE stimulation over 5 days. The kinetics of MT response and the dose-response relationship were investigated after NE exposure at various concentrations (100 nM to 10 mM). NE at 100 nM was found to be ineffective. Between 1 muM and 1 mM concentrations NE increased the MT release in a dose-dependent manner. No significant difference was found between the responses above 1 mM concentration. NE seems to be a specific stimulator of pineal MT production. The MT production reached the maximum value after a relatively long lag (2 to 3 h) when NE application had been stopped, and returned to basal values after 5 to 6 h. This prolonged time-course of MT secretion, in contrast with the fast responses of pituitary cells to releasing hormones, suggests that NE stimulated the synthesis of MT rather than the release of stored hormone. The modulatory effects of light-dark cycle on basal and stimulated MT release of perifused pineals was also investigated: Neither basal nor NE-stimulated (100 nM to 10muM) MT release was influenced significantly by light-dark conditions showing that the light-dark cycle does not have a direct modulatory effect on MT secretion under in vitro circumstances. Based on our observations, this perifusion system should be a useful tool for investigating: 1) hormone interactions on the regulation of pineal MT release, and 2) accurate kinetics of MT response.

Vasopressin and oxytocin modulation of melatonin secretion from rat pineal glands

The rat pineal gland is known to release melatonin in response to noradrenergic stimulation. Since vasopressin (VP)- and oxytocin (OT)-containing fibers innervate the pineal gland, the effects of VP and OT on melatonin release from perifused rat pineal glands were investigated. VP (10(-7) M) and OT (10(-6) M) decreased the basal melatonin secretion. No dose-dependent effect was observed. At high concentrations (10(-5)) these peptides potentiated the isoproterenol-induced increase of melatonin secretion. Below 10(-5) M no potentiation was observed. Fragments of VP ([pGlu4,Cys6]VP(4-9] and OT (pGlu4,Cys6]OT(4-9] did not display any effect on the isoproterenol-induced melatonin secretion.

Vasoactive intestinal peptide stimulates melatonin release from perifused pineal glands of rats

The rat pineal gland is known to release melatonin in response to noradrenergic stimulation. The effect of vasoactive intestinal peptide (VIP), one of the neuropeptides present in the pineal, was examined on perifused rat pineal glands. VIP stimulated melatonin release with a dose-dependent effect above 10(-7) M. In regard of kinetic characteristics, the pattern of melatonin release after VIP stimulation was similar to that after isoproterenol stimulation. 10(-6) M VIP-stimulated melatonin release was not altered when the pineal glands were treated with 10(-5) M propranolol (a beta-adrenergic antagonist) or 10(-5) M prazosin (an alpha 1-adrenergic antagonist). Thus VIP has a noradrenergic-independent effect on melatonin secretion. Conversely, this VIP effect is greatly inhibited by the specific action of a VIPergic antagonist. This suggests that VIP acts on melatonin synthesis through its own binding sites. This study demonstrates that melatonin secretion from rat pineal glands may be elicited through a VIPergic system which is independent of the well-known noradrenergic system.