Abstract

After maintenance of rat pineal organs in organ culture for 12 hr, brief exposure to choleragen caused a 4-fold stimulation of adenylate cyclase activity and induction of serotonin N-acetyltransferase activity. Choleragen also caused a sharp, transient increase in the intracellular content of adenosine cyclic 3',5'-monophosphate (cAMP) and a rapid reduction in guanosine cyclic 3',5'-monophosphate (cGMP). Cyclic nucleotide phosphodiesterase activity was also increased after choleragen exposure. Addition of 100 µM norepinephrine caused an increase in cAMP and a reduction in cGMP; however, phosphodiesterase activity was not affected. The stimulation of adenylate cyclase activity by norepinephrine did not occur in pineals that had previously been exposed to 10 µM norepinephrine for 1 hr. Incubation with choleragen (50 µg/ml) and subsequent culture in the presence of 10 µM norepinephrine resulted in a larger induction of serotonin N-acetyltransferase activity than that caused by either treatment alone. Neither the adenylate cyclase response to norepinephrine nor the cAMP content after norepinephrine administration was markedly increased by previous choleragen exposure; however, the increase in cAMP content was maintained over a longer time period. Addition of the phosphodiesterase inhibitor isobutylmethylxanthine (1 mM) caused induction of N-acetyltransferase and phosphodiesterase activity. This treatment led to a 2-3-fold increase in intracellular cAMP and a 5-6-fold increase in intracellular cGMP. The rise in N-acetyltransferase activity caused by choleragen was blocked by the addition of 1 µM ouabain on 80 mM K⁺ to the culture medium. The results indicate that adenylate cyclase activation is the critical signal for postsynaptic enzyme induction in the cultured rat pineal. Beta adrenoceptor sensitivity may be affected both by the sensitivity of adenylate cyclase to stimulation by norepinephrine and by regulation of the rate of cyclic nucleotide degradation through changes in phosphodiesterase activity.