Evidence for a cholinergic influence on pineal hydroxyindole O-methyltransferase activity with changes in environmental lighting

Abstract

Hydroxyindole O-methyltransferase (HIOMT) activity of pineals of rats kept in constant light for one week was significantly lower than HIOMT in pineals of rats kept in constant darkness for one week. These inhibitory effects of light on pineal HIOMT have been shown by others to be mediated by the sympathetic nervous system. What factors mediate the facilitatory effects of the absence of light? In order to answer this question we studied the role of the parasympathetic system by giving female rats on constant light, constant dark or normal lighting schedules daily injections of atropine sulfate, atropine methyl bromide or the cholinomimetic, oxotremorine oxalate, with atropine methyl bromide. The peripherally acting parasympathetic effector blocker, atropine methyl bromide, inhibited the high levels of HIOMT in constant darkness, while the cholinomimetic restored HIOMT activity in darkness. Rats on a normal lighting schedule also had significantly higher levels of HIOMT after oxotremorine than after atropine sulfate. These results indicate that cholinergic mechanisms are involved peripherally, perhaps also centrally, in mediating the response of pineal HIOMT in darkness.