A β-ADRENERGIC receptor coupled to adenylate cyclase regulates the activity of rat pineal N-acetyltransferase, an enzyme involved in the biosynthesis of melatonin1. At the onset of darkness there is an increase in the release of the neurotransmitter noradrenaline from sympathetic nerves2,3 which produces 30- to 50-fold increases in N-acetyltransferase activity4. During periods of exposure to light, nerve activity and noradrenaline turnover are lower than during peribds of darkness2,3, and N-acetyltransferase activity remains at its lowest level4.

The lag period in the induction of rat pineal N-acetyltransferase (arylamine acetyltransferase or acetyl-CoA:arylamine N-acetyltransferase EC 2.3.1.5) by catecholamines via the beta-adrenergic receptor varies with the length of exposure of the rat to light or darkness. If rats have been exposed to light and reduced sympathetic nerve activity for more than 12 hr, this lag period is 1-2 hr long. Under these conditions, actinomycin D completely blocks the induction of N-acetyltransferase by isoproterenol and by dibutyryl adenosine 3':5'-cyclic monophosphate (cyclic AMP). In contrast, if enzyme activity is caused to fall by brief exposure to light at night when N-acetyltransferase activity is high, reinduction by catecholamines occurs almost immediately. In this case, actinomycin D does not block the reinduction of N-acetyltransferase by isoproterenol or by dibutyryl cyclic AMP. Cycloheximide blocks N-acetyltransferase induction under all conditions tested. Thus, new protein synthesis is always required for N-acetyltransferase induction; however, the requirement for RNA synthesis is variable, and contributes to the length of the lag period for induction. It is postulated that both beta-adrenergic stimulation and dibutyryl cyclic AMP act intracellularly at two separate sites in the induction of pineal N-acetyltransferase. One site is in the stimulation of transcription, and the other is in the stimulation of post-transcriptional events.

Stimulation by isoproterenol causes large increases in the activity of rat pineal N-acetyltransferase (arylamine acetyltransferase or acetyl-CoA:arylamine N-acetyltransferase, EC 2.3.1.5) after a variable lag period. Lengthening periods of exposure to light cause rapid increases in the sensitivity of pineal N-acetyltransferase to induction by isoproterenol. This supersensitivity, which develops gradually over a 12 hr period, is correlated with increasingly longer lag periods in the induction of the enzyme, and with progressively greater maximal response. Repeated administration of isoproterenol to supersensitive animals rapidly reverses the above changes and causes relative subsensitivity. The sensitivity of N-acetyltransferase to induction by dibutyryl-cyclic tamp added to pineals in organ culture was found to change in parallel with the sensitivity to isoproterenol. Stimulation of cyclic AMP levels in the pineal by isoproterenol was also greater in supersensitive rats than in the subsensitive animals, whether the supersensitivity had been caused by denervation or by exposure to light for 12 hr. The above experiments suggest that there are two sites for the regulation of the sensitivity of N-acetyltransferase to induction by catecholamines. The first site regulates the capacity of intracellular processes (i.e., induction of tn-acetyltransferase) to respond to cyclic AMP as a second messenger.

The responsiveness of the pineal β-adrenergic receptor that regulates serotonin-N-acetyltransferase activity is nearly ten times greater at the end of the light period (0600 to 1800 hours) than at the end of the dark period (1800 to 0600 hours). These changes in sensitivity of the postsynaptic β-adrenergic receptor are related to diurnal changes in the release of noradrenaline from sympathetic nerves innervating the pineal. Supersensitivity of the receptor appears to result from decreased release of the neurotransmitter during daytime, and subsensitivity from increased release at night.