<prompt> Exposure to darkness leads to increased melatonin secretion. It is brought about by:Decreasing the activity of suprachiasmatic nucleiIncreasing the serotonin N-acetyl transferase.Decreasing the hydroxy-indole-o-methyl transferase, activityBlocking the release of norepinephrine from sympathetic nerve terminals  <response>: Ans. is b i.e. increasing the serotonin N-acetyl transferase Ref Ganong 21ste p 468, 20the p448First lets see the synthesis of melatonin from serotoninSerotonin N. Acetyl transferase Acetyl CoA N-Acetyl serotoninHydroxy indole - 0 methyl transferase S-Adenosylmethionine Melatonin N-acetyl-5-methoxytryptamineRegulation of Melatonin SecretionIn humans and all other species studied to date, melatonin synthesis and secretion are increased during the dark period of the day and maintained at a low level during the day-light hours.The remarkable diurnal variation in secretion is brought about by training of the hypothalamus to the light-dark cycle via retino-hypothalamic nervesThe hypothalamus in turn cause norepinephrine secretion by the postganglionic sympathetic nerves nervi conarii that innervate the pineal glandThe norepinephrine acts via b adrenergic receptors in the pineal gland to increase intracellular cAMP, and the cAMP in turn produced a marked increase in N-acetyltransferase activity. This results in increased melatonin synthesis and secretionExposure to darknessRetino hypothalamic fibres supra chiasmatic nuclei in hypothalamusPreganglionic sympathetic neurons in spinal cord postganglionic sympatheticnerves nervi conarii arising in superior cervical ganglion norepinephrine secretion in pineal gland intracellular cAMP N-Acetyltransferase activity Hydroxyindole - O methyl transferase activity Melatonin 