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

The crucian carp retina was used to study the effects of the melatonin anta gonist DH97 (N-pentanoyl 2-benzpltrypramine) and the melatonin agonists [+] - and [-]AMMTC (N-acetyl-4-aminomethyl-6-methoxy-9-methyl-1,2,3,4-tetrahydr ocarbazole) on horizontal cell spinule formation, an indicator of the state of retinal adaptation. DH97 was capable of both counteracting dark-adaptiv e 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 D-1 dopamine receptor an tagonist, being more effective than the D-2 receptor antagonist sulpiride. DH97 induced a twofold increase in dopamine release. We conclude that melat onin acts as a dark signal within the teleost retina by inhibiting the dopa minergic system, In accordance with this, both enantiomers of AMMTC prevent ed light-induced spinule formation, and reduced dopamine release to below d ark-adaptive baseline levels. We suggest that the suppression of spinule fo rmation by AMMTC may be due to either a direct inhibitory interaction betwe en the melatonin agonist and horizontal cell dopamine D-1 receptors, or an inhibitory effect on the activity of the dopamine-releasing interplexiform cells.