I Introduction UNTIL 35 yr ago, most scientists did not take research on the pineal gland seriously The decade beginning in 1956, however, provided several discoveries that laid the foundation for what has become a very active area of investigation These important early observations included the findings that, 1), the physiological activity of the pineal is influenced by the photoperiodic environment (1–5); 2), the gland contains a substance, N-acetyl-5-methoxytryptamine or melatonin, which has obvious endocrine capabilities (6, 7); 3), the function of the reproductive system in photoperiodically dependent rodents is inextricably linked to the physiology of the pineal gland (5, 8, 9); 4), the sympathetic innervation to the pineal is required for the gland to maintain its biosynthetic and endocrine activities (10, 11); and 5), the pineal gland can be rapidly removed from rodents with minimal damage to adjacent neural structures using a specially designed trephine (12) Since the mid 1960s, research on t

Pineal Melatonin: Cell Biology of Its Synthesis and of Its Physiological Interactions*

Bright artificial light suppressed nocturnal secretion of melatonin in six normal human subjects. Room light of less intensity, which is sufficient to suppress melatonin secretion in other mammals, failed to do so in humans. In contrast to the results of previous experiments in which ordinary room light was used, these findings establish that the human response to light is qualitatively similar to that of other mammals.

https://science.sciencemag.org/content/sci/210/4475/1267.full.pdf

Light Suppresses Melatonin Secretion in Humans

https://www.cell.com/neuron/pdf/0896-6273(94)90055-8.pdf

Cloning and characterization of a mammalian melatonin receptor that mediates reproductive and circadian responses

A G protein-coupled receptor for the pineal hormone melatonin was recently cloned from mammals and designated the Mel1a melatonin receptor. We now report the cloning of a second G protein-coupled melatonin receptor from humans and designate it the Mel1b melatonin receptor. The Mel1b receptor cDNA encodes a protein of 362 amino acids that is 60% identical at the amino acid level to the human Mel1a receptor. Transient expression of the Mel1b receptor in COS-1 cells results in high-affinity 2-[125I]iodomelatonin binding (Kd = 160 +/- 30 pM). In addition, the rank order of inhibition of specific 2-[125I]iodomelatonin binding by eight ligands is similar to that exhibited by the Mel1a melatonin receptor. Functional studies of NIH 3T3 cells stably expressing the Mel1b melatonin receptor indicate that it is coupled to inhibition of adenylyl cyclase. Comparative reverse transcription PCR shows that the Mel1b melatonin receptor is expressed in retina and, to a lesser extent, brain. PCR analysis of human-rodent somatic cell hybrids maps the Mel1b receptor gene (MTNR1B) to human chromosome 11q21-22. The Mel1b melatonin receptor may mediate the reported actions of melatonin in retina and participate in some of the neurobiological effects of melatonin in mammals.

https://www.pnas.org/content/pnas/92/19/8734.full.pdf

Molecular characterization of a second melatonin receptor expressed in human retina and brain: the Mel1b melatonin receptor

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.

Generation of the Melatonin Endocrine Message in Mammals: A Review of the Complex Regulation of Melatonin Synthesis by Norepinephrine, Peptides, and Other Pineal Transmitters

The distribution and characterization of specific melatonin binding sites were studied using 125I-melatonin. Autoradiography revealed only three sites of specific melatonin binding in brain: the suprachiasmatic nuclei, the median eminence, and the small part of choroid plexus at the caudal end of the fourth ventricle. Two other sites were detected outside the CNS: the anterior pituitary and the retina. The specific binding of 125I-melatonin was saturable and reversible. The dissociation constant (KD) of the binding sites was 60 pM. The concentration of the binding sites (Bmax) in the median eminence was 26 fmol/mg protein, and in the pituitary 3 fmol/mg protein. Specificity of the binding sites was tested by displacement of 125I-melatonin. The order of potency--melatonin much less than N-acetyl-5-hydroxytryptamine less than 5-methoxytryptamine much less than 5-hydroxytryptamine = 3,4-dihydroxyphenylethylamine = noradrenaline--shows high specificity of the binding sites for melatonin.

Melatonin Binding Sites

THE ACTIVITY of serotonin N-acetyltransferase (acetyl CoA: arylamine N-acetyltransferase, EC 2.3.1.5), the first specific enzyme in the branch pathway for melatonin synthesis in the rat pineal gland (WEISSBACH et al., 1960), as well as the concentration of melatonin itself (LYNCH, 1971: OZAKI et a/., 1976), exhibit a marked diurnal rhythm with night values much higher than daytime ones (KLEIN & WELLER, 1970). The rhythm is generated by sympathetic nerve terminals innervating the pineal gland, presumably by diurnal release of the neurotransmitter noradrenaline (BROWNSTEIN & AXELROD, 1974) and is abolished either by exposing rats to constant light (KLEIN & WELLER, 1970) or by denervation of the gland through bilateral superior cervical ganglionectomy or by decentralisation of the ganglia (KLEIN et a!.; 1971). Exposure of rats to light at night causes a precipitous decline in pineal N-acetyltransferase activity (KLFIN & WELLER. 1972: DEGUCHI & AXELROD, 197%) and melatonin content (ILLNEROVA et al., 1978). The drop was observed in rats exposed to light for the whole time interval before killing. The enzyme activity (KLEIN & WELLER, 1972) as well as melatonin concentration (ILLNEROVA et a/., 1978) reached their lowest level in l S 1 5 m i n . The present study was undertaken to find out, whcther even a very brief exposure to light at night-time, such as for 1 min, after which rats would continue to be in darkness, could trigger the decrease in N-acetyltransferase activity and melatonin content to low levels. We now report that 1 min cxposure to light 5.5 h after evcning onset of darkness causes a rapid decrease of serotonin N-acetyl transferase activity and melatonin conlent in rat pineal with a halving time less than 5 min. Moreover neither enzyme activity nor melatonin concentration increase again to high night lcvels within 5 h in darkness following 1 min exposure to light.

J Vanĕcek

Papers

Melatonin Binding Sites

Effect of one minute exposure to light at night on rat pineal serotonin N-acetyltransferase and melatonin