Cyclic AMP‐Dependent Induction of Serotonin N‐Acetyltransferase Activity in Photoreceptor‐Enriched Chick Retinal Cell Cultures: Characterization and Inhibition by Dopamine

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Abstract
The activity of serotonin N‐acetyltransferase (NAT), a key regulatory enzyme in the melatonin biosynthetic pathway, was examined in low‐density monolayer cultures of chick embryo retinal cells prepared with three levels of photoreceptor enrichment. In cultures prepared from embryonic day 8 retinas (E8), photoreceptors represented approximately 30% of the total cell population, whereas in those prepared from embryonic day 6 retinas (E6), approximately 70% of the cells were photoreceptors. In E8 retinas treated with kainic acid to destroy neurons (E8K), the relative content of photoreceptors was increased to approximately 50%. NAT activity was detectable in the cultures under all conditions studied, and was markedly increased by drugs that increase intracellular cyclic AMP levels and cyclic AMP‐dependent protein kinase activity: 8‐bromocyclic AMP, forskolin, and 3‐isobutyl‐1‐methylxanthine (IBMX). Consistent with the hypothesis that NAT is localized in photoreceptors, the effects of the stimulatory treatments were significantly greater in E6 and E8K cultures than in E8 cultures. The stimulation of NAT activity in E6 cultures was inhibited by actinomycin D and cycloheximide, suggesting the involvement of RNA and protein synthesis. Dopamine inhibited the induction of NAT activity by forskolin and IBMX, but not that elicited by 8‐bromocyclic AMP. The dopamine‐mediated suppression of activity was significantly inhibited by pertussis toxin and by spiperone and sulpiride, both D2‐dopamine receptor antagonists, but not by SCH 23390, a D1‐dopamine receptor blocker, or antagonists of α‐adrenergic, β‐adrenergic, or serotonergic receptors. Because the inhibitory effect of dopamine on E6 and E8K cultures was at least as great as that on E8 cultures, the results suggest that dopamine acts on D2‐like receptors on photoreceptors. The receptors appear to be coupled to adenylate cyclase through an inhibitory GTP‐binding protein and to mediate inhibition of cyclic AMP synthesis and consequent induction of NAT activity.