Effect of melatonin agonists and antagonists on horizontal cell spinule formation and dopamine release in a fish retina

Abstract.

The crucian carp retina was used to study the effects of the melatonin antagonist DH97 (N-pentanoyl 2-benzyltryptamine) and the melatonin agonists [+]- and [–]-AMMTC (N-acetyl-4-aminomethyl-6-methoxy-9-methyl-1,2,3,4-tetrahydrocarbazole) on horizontal cell spinule formation, an indicator of the state of retinal adaptation. DH97 was capable of both counteracting dark-adaptive spinule degradation and inducing light-adaptive spinule formation at the beginning of the dark phase. Addition of dopamine receptor blockers opposed the action of DH97 on spinules, with SCH 23930, a D₁ dopamine receptor antagonist, being more effective than the D₂ receptor antagonist sulpiride. DH97 induced a twofold increase in dopamine release. We conclude that melatonin acts as a dark signal within the teleost retina by inhibiting the dopaminergic system. In accordance with this, both enantiomers of AMMTC prevented light-induced spinule formation, and reduced dopamine release to below dark-adaptive baseline levels. We suggest that the suppression of spinule formation by AMMTC may be due to either a direct inhibitory interaction between the melatonin agonist and horizontal cell dopamine D₁ receptors, or an inhibitory effect on the activity of the dopamine-releasing interplexiform cells.