The melatonin generating system in the rat retina and pineal gland: effect of single and repeated electroconvulsive shock (ECS).

N-Acetyltransferase (NAT), a rate-limiting enzyme in the melatonin synthesis which converts serotonin to N-acetylserotonin, shows a distinct circadian rhythm in the rat pineal gland and retina, with low activities during the light phase and peak activities during the dark phase. Hydroxyindole-O-methyltransferase (HIOMT), an enzyme which methylates N-acetylserotonin to melatonin, did not show any significant diurnal variations in both analyzed tissues. Isoproterenol, a selective beta-adrenoceptor agonist, when administered during morning hours of the light phase, markedly increased NAT activity in the pineal gland, but not in the retina. Electroconvulsive shock (ECS), especially when applied repeatedly (ECS x 10, once daily) significantly increased NAT activity in the retina and tended to decrease the enzyme activity in the pineal gland in isoproterenol-treated rats. ECS x 10 slightly increased and decreased the nocturnally-stimulated NAT activity in the rat retina and pineal gland, respectively.