Dopamine stimulates alpha 2-adrenoceptors on the Anolis melanophore

The inhibitory effect of dopamine on the action of MSH on the Anolis melanophore is blocked by the alpha 2-adrenoceptor antagonists, yohimbine and compound RX 781094, suggesting that the action of dopamine is mediated by alpha 2-adrenoceptors. Apomorphine and sulpiride had no effect on MSH action nor the inhibitory effect of dopamine on MSH action confirming that the Anolis melanophore does not possess dopamine receptors and that the action of dopamine is not mediated by dopamine receptors. Since the kinetics of competition between dopamine and MSH is that described for a single receptor it is concluded that interpretation of classical kinetics of competition in terms of action at a single receptor site requires modification to include an unrelated action at separate receptors, competition occurring at a post-receptor level.