Dopamine receptor activation in bovine pinealocyte via a cAMP-dependent transcription pathway

D1- and D2-dopamine receptors in the bovine pineal gland were previously identified and characterized. The data indicate that the density of D1-dopamine receptors far exceeded that of D2-dopamine receptors. In our previous study, the mRNA for both the D1- and D2-dopamine receptors which elucidated the status of dopamine and its possible involvement in the pineal function was identified. A selective D1-agonist enhanced N-acetyltransferase (NAT) activity and increased the melatonin level, whereas, a selective D2-agonist inhibited NAT activity and decreased the melatonin level. An attempt has been made in the present study to clarify the mechanism of dopamine in controlling melatonin production in bovine pineal. The level of intracellular cyclic 3',5'-adenosine monophosphate (cAMP) was determined after a 2-hr incubation of bovine pinealocytes with selected combinations of drugs. SKF 38393, a selective D1-agonist, enhanced intracellular level of cAMP, and its effect was blocked by SCH 23390, a D1-selective antagonist. In contrast quinpirole, a selective D2-agonist, inhibited forskolin-stimulated intracellular level of cAMP while its effect was blocked by a D2-selective antagonist, spiperone. In addition, the dopamine-dependent phosphorylation of the transcription factors, cAMP responsive element-binding protein (CREB) was investigated. Immunoblots showed that SKF 38393 enhanced CREB phosphorylation and the stimulatory effect was abolished by SCH 23390 whereas quinpirole inhibited forskolin-stimulated phosphorylated CREB production and the inhibitory effect was prevented by spiperone. Taken together with our previous data, the results indicate that activation of D1-dopamine receptor in bovine pinealocyte stimulates NAT activity and enhances melatonin level whereas activation of D2-dopamine receptor leads to an inhibitory effect and these stimulatory and inhibitory effects act, in part, via a cAMP-dependent transcription mechanism.