Characterization of adenylyl cyclase in goldfish brain

Further characterization of 5-HT1A receptors in the goldfish retina: role of cyclic AMP in the regulation of the in vitro outgrowth of retinal explants

The presence of serotonin 5-HT1A receptors and their physiological role were further characterized in the goldfish retina. The effects of the 5-HT6/7 receptor antagonists pimozide, fluphenazine and amoxapine, the 5-HT1A receptor antagonist WAY-100,135, and the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline, on the 5-HT1A receptor agonist [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding to retinal membranes, were evaluated. In addition, the effects of serotonin, 8-hydroxy-2-(di-n-propylamino)tetralin, WAY-100,135, the adenylate cyclase inhibitors SQ22536 and MDL12330A, and the cyclic AMP analog 8-bromoadenosine-3':5' cyclic monophosphate were also studied on neuritic outgrowth from retinal explants. WAY-100,135 but not 5-HT6/7 receptor antagonists inhibited [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding to retinal membranes N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline decreased [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding sites up to 70%, while receptor turnover was similar to that reported in other tissues. Serotonin and 8-hydroxy-2-(di-n-propylamino)tetralin stimulated cyclic AMP production, both ex vivo and in vitro, and these increases were related to inhibition of neuritic outgrowth. The inhibitory effect was reduced by SQ22536 and by WAY-100,135, and was mimicked by 8-bromoadenosine-3':5'cyclic monophosphate. This study supports previous findings about the role of serotonin as a regulator of axonal outgrowth during in vitro regeneration of the goldfish retina and demonstrates that this effect is mediated, at least in part, by 5-HT1A receptors through a mechanism which involves an increase of cyclic AMP levels.

5HT1A receptor agonist differentially increases cyclic AMP concentration in intact and lesioned goldfish retina. In vitro inhibition of outgrowth by forskolin

5HT1A receptors occur in the retina of various species and the administration of 5HT1A agonists results in the inhibition of outgrowth from postcrush goldfish retinal explants. The levels of cyclic AMP (cAMP) play a role in the modulation of the outgrowth of the nevous system. Moreover, the stimulation of central 5HT1A receptors with the agonist 8-hydroxy-2-(di-n-propylamino)tetralin has been reported to produce an increase or decrease in the activity of adenylate cyclase. In the present investigation we studied the effect of adenylate cyclase stimulation by forskolin, as well as the modulatory effects of 5HT1A receptor agonists and antagonists on the production of cAMP in the goldfish retina, and on the outgrowth of this tissue in vitro. 8-Hydroxy-2-(di-n-propylamino)tetralin produced a significant and dose-dependent increase in cAMP concentration. This effect was not additive to the stimulation produced by forskolin. By contrast, as previously described, the 5HT1A agonist decreased cAMP concentration in the hippocampus of the rat. Both effects were significantly impaired by the 5HT1A antagonist WAY-100,135. A significant effect of the antagonist alone was observed only in the goldfish retina. The increase in cAMP levels was greater in the intact than in the postcrush retina. In addition, forskolin decreased the outgrowth of postcrush retinal explants in a dose-dependent manner, suggesting the importance of critical levels of cAMP in this process. Taken together, 5HT1A receptors seem to be positively coupled to adenylate cyclase in the goldfish retina, where cAMP plays a role as a modulator of outgrowth and regeneration. The inhibitory effect of 5HT1A receptor agonists on retinal outgrowth might be mediated through the production of cAMP. The activation of other subtypes of 5HT receptors positively coupled to adenylate cyclase by the 5HT1A agonist, such as 5HT7, cannot be discarded.