ROLE OF ADENOSINE 3′, 5′‐MONOPHOSPHATE IN THE REGULATION OF CIRCADIAN OSCILLATION OF SEROTONIN N‐ACETYLTRANSFERASE ACTIVITY IN CULTURED CHICKEN PINEAL GLAND

—When pineal glands of 10–12‐day‐old chicks were organ‐cultured in darkness, serotonin N‐acetyltransferase activity was low during the daytime, increased at midnight and then decreased to the daytime level the next morning. The pattern of increase and decrease of enzyme activity in cultured pineal glands was comparable to the circadian rhythm of N‐acetyltransferase activity in vivo . When pineal glands were kept at a low temperature for 5 h prior to culture, the phase of autonomous rhythm of enzyme activity was delayed. When chicken pineal glands were cultured during the daytime for 6 h, derivatives of adenosine 3′, 5′‐monophosphate (cyclic AMP), cholera toxin, a high concentration of KCl and phosphodiesterase inhibitors increased N‐acetyltransferase activity 3–7‐fold, indicating an involvement of cyclic AMP in the regulation of N‐acetyltransferase activity in chicken pineal gland as has been shown in rat pineal gland. When pineal glands were cultured at night in darkness, cholera toxin or a high KCl did not enhance the night‐time increase of the enzyme activity. Derivatives of cyclic AMP or phosphodiesterase inhibitors enhanced the autonomous night‐time increase of N‐acetyltransferase activity in an additive or more than additive manner in cultured pineal glands. These observations suggest that adenylate cyclase of pinealocytes is inactive during daytime, but is activated at night in darkness, which is transduced to the synthesis of N‐acetyltransferase molecules. Catecholamines suppressed the basal level and the nocturnal increase of N‐acetyltransferase activity via α‐adrenergic receptor. The nocturnal increase of enzyme activity was prevented by cycloheximide or actinomycin D. Cocaine, which stabilizes cell membrane potential or light exposure, blocked the nighttime increase of N‐acetyltransferase activity in cultured chicken pineal glands.