Exposure to darkness leads to increased melatonin secretion. It is brought about by:

Ans. is 'b' i.e. increasing the serotonin N-acetyl transferase Ref Ganong 21st/e p 468, 20th/e p448First let's see the synthesis of melatonin from serotoninSerotonin| N. Acetyl transferase + Acetyl CoA N-Acetyl serotonin|Hydroxy indole - 0 methyl transferase + S-Adenosylmethionine Melatonin (N-acetyl-5-methoxytryptamine)Regulation 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 darkness|Retino hypothalamic fibres| supra chiasmatic nuclei in hypothalamus|Preganglionic 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