Stimulatory Effects of LH on Release of Melatonin and Activities of its Synthesizing Enzymes NAT and HIOMT in Organ-Cultured Pineal Glands of Female Rats

Abstract

The pineal hormone melatonin (N-acetyl-5-methoxytryptamine) exerts antigonadotropic effects in some mammalian species. To evaluate the effect of luteinizing hormone (LH) on melatonin release and its synthesizing enzyme activities in pineal glands, pineals of adult female rats undergoing diestrus were organ-cultured in a medium containing 10(-12), 10(-10) or 10(-8) M LH for 6 h. Melatonin release increased significantly in pineals cultured with 10(-12) and 10(-10) M LH, as compared to control values. Similarly, the activity of arylalkylamine N-acetyltransferase (NAT), the key regulatory enzyme in melatonin biosynthesis, was significantly higher in pineals cultured with 10(-12) and 10(-10) M LH for 6 h, while LH at 10(-8) M had no effect. Although LH at 10(-10) M increased pineal hydroxyindole-O-methyltransferase (HIOMT) activity, which catalyzes the final step of melatonin biosynthesis, LH at 10(-12) and 10(-8) M had no effect. These results demonstrate that at relatively low physiological levels, LH stimulates pineal melatonin synthesis and release, mainly by increasing NAT activity.

Expression of luteinizing hormone/chorionic gonadotropin receptor in the rat pineal gland

“…Similar changes in the estrous cycle have been reported in HIOMT activity [18] and AANAT activity [22]. We showed that LH at relatively low physiological levels stimulates melatonin secretion from the female pineal gland, whereas high LH levels, which are about equivalent to those in the circulating plasma during the preovulatory LH surge, have no effect [16]. Thus, LH may affect pineal melatonin secretion, particularly the nocturnal secretion, under physiological conditions.…”

“…Previous studies have reported that luteinizing hormone (LH) increases melatonin content and release in organ‐cultured pineal glands from adult male rats [13, 14]. We further showed that LH at relatively low physiological levels, which are equivalent to levels in the plasma at diestrus, stimulates melatonin secretion, mainly by increasing AANAT activity, in organ‐cultured pineal glands from adult female rats [16]. These studies suggest that pineal melatonin‐synthesizing cells, namely pinealocytes, are direct targets for LH.…”

“…As a result, pineal melatonin synthesis and release show a circadian rhythm. In addition, evidence suggests that circulating concentrations of reproductive hormones influence pineal melatonin synthesis and release [13][14][15][16][17]. Previous studies have reported that luteinizing hormone (LH) increases melatonin content and release in organcultured pineal glands from adult male rats [13,14].…”

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

“…Finally, it has been proposed that some pineal cells synthesize LHRH or a LHRH-like peptide (Pévet et al, 1980b). Until now, few studies have reported an effect of this peptide on pineal metabolism: it is proposed to stimulate HIOMT activity (Cardinali et al, 1976;Cardinali and Vacas, 1979), regulate the formation of granular vesicles and the process of protein and/or peptide secretion (Haldar-Misra and Pévet, 1983), and increase MEL secretion, although only moderately (Mess et al, 1991), mainly via an activation of AA-NAT (Hosaka et al, 2002). Seasonal variations of LHRH have been observed in the pineal gland of the rat (with a maximum in March/ Joseph, 1976) and sheep (King and Millar, 1981).…”

Melatonin and its precursor, L‐tryptophan: influence on pancreatic amylase secretion in vivo and in vitro

“…Melatonin is produced from l ‐tryptophan, through a four step reaction, controlled by specific enzymes, one of them is N‐acetyl‐serotonin transferase (NAT) [16]. Gene expression for this enzyme has been found in numerous tissues including brain, retina, and digestive system [17].…”