Melatonin synthesis in the greenfrog retina in culture: II. Dopaminergic and adrenergic control

Serotonin N-acetyltransferase (NAT) activity and melatonin show a daily rhy thm with high levels at night. Although the rhythmic properties of NAT and melatonin are similar in pineal gland and retina, great differences in the light perception and transmission mechanisms exist. We have analyzed the ef fects of adrenergic and dopaminergic agents on greenfrog (Rana perezi) eyec up culture, in order to identify the receptors involved in the regulation o f retinal melatonin synthesis. A D-2-like receptor is directly involved in the regulation of NAT activity and melatonin release in R. perezi retina. Q uinpirole mimics the effect of light, reducing the darkness-stimulated NAT activity and melatonin release, while sulpiride antagonized these actions. Neither D-1-agonist (SKF 38393) nor D-1-antagonist (SCH 23390) had effect o n NAT activity. However, a significant inhibition of darkness-evoked melato nin release was produced by SKF 38393 after 6 hours of culture. The beta- a nd alpha(1)-agonists showed a clear inhibition. However, a direct effect of beta, alpha(1) and D-1-agonists on photoreceptors is unproven, being more probable that the adrenergic actions imply a non-photoreceptor retinal cell . In conclusion, eyecup culture of Rana perezi revealed a dopaminergic cont rol of melatonin synthesis and a possible modulation of dopaminergic tone b y adrenergic receptors. Melatonin release is a more sensitive parameter tha n NAT activity to the action of neuroactive agents, suggesting that melaton in synthesis can be regulated by more than one enzymatic step in Rana perez i.