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

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(-11) 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.