Effects of D₁- and D₂-dopamine receptor activation on melatonin synthesis in bovine pinealocytes

Previous studies have identified and characterized D₁- and D₂-dopamine receptors in bovine pineal glands. The data indicate that the density of D₁-dopamine receptors (974 fmol/mg protein) far exceed that of D₂-dopamine receptors (37 fmol/mg protein). The objective of this study was to identify the mRNAs for both D₁- and D₂-dopamine receptors and to elucidate the status of dopamine and its possible involvement in the pineal function, particularly on melatonin synthesis. The expression of these dopamine receptor subtypes were determined by using a reverse transcriptase-polymerase chain reaction technique with specific pairs of primers to amplify D₁- and D₂-dopamine receptor mRNAs. Amplification of RNAs from bovine striatum (positive control) and bovine pineal gland resulted in products of the predicted lengths of 231 bp for D₁- and 333 bp for D₂-dopamine receptors. The results indicate that both D₁- and D₂-dopamine receptor mRNAs are present in the bovine pineal gland. The role of dopamine receptors was investigated by studying the effects of selective D₁- and D ₂-dopamine agonists and antagonists on the N-acetyltransferase (NAT) activity of cultured bovine pinealocytes. The data showed that SKF-38393, a selective D₁-agonist, enhanced NAT activity, and increased melatonin level, and the stimulatory effect was blocked by SCH-23390, a D ₁-selective antagonist, whereas quinpirole, a selective D ₂-agonist, inhibited NAT basal activity and decreased the melatonin basal level. Furthermore the inhibitory effect was blocked by D ₂-selective antagonists, spiperone, haloperidol, and domperidone. The present results indicate that the pineal dopamine receptors have a distinct effect on pineal function. The precise mechanism whereby activation of dopamine receptors altered the NAT activity and melatonin level needs to be further delineated.