title: 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 p697 (N-pentanoyl 2-benzyltryptamine) and the melatonin agonists [+]- and [-]-AMMTC (N-acetyl-4-aminomethyl-6-methoxy-9-methyl-1,2,3,4-tetrahydrocarbazol  e) 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.
authors.affiliation: Anatomisches Institut, Universität Tübingen, Germany.
authors.forename: U D
authors.initials: UD
authors.lastname: Behrens
authors.forename: R H
authors.initials: RH
authors.lastname: Douglas
authors.forename: D
authors.initials: D
authors.lastname: Sugden
authors.forename: D J
authors.initials: DJ
authors.lastname: Davies
authors.forename: H J
authors.initials: HJ
authors.lastname: Wagner
mesh_terms: D000221:Adaptation, Ocular / Q000502:physiology; D000818:Animals; D002227:Carbazoles / Q000494:pharmacology; D002851:Chromatography, High Pressure Liquid; D002940:Circadian Rhythm / Q000502:physiology; D003623:Dark Adaptation / Q000502:physiology; D004298:Dopamine / Q000032:analysis / Q000378:metabolism*; D006054:Goldfish; D008550:Melatonin / Q000819:agonists* / Q000037:antagonists & inhibitors*; D008854:Microscopy, Electron; D009473:Neuronal Plasticity / Q000502:physiology; D012160:Retina / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D014363:Tryptamines / Q000494:pharmacology*
chemical_list: D002227:Carbazoles; C102697:N-acetyl-4-aminomethyl-6-methoxy-9-methyl-1,2,3,4-tetrahydrocarbazole; C408803:N-pentanoyl-2-benzyltryptamine; D014363:Tryptamines; D008550:Melatonin; D004298:Dopamine