Melatonin In Denervated Melanophores

Denervation affects the pigment aggregation response mediated by noradrena-line to become more sensitive and displace the concentration-response curve to the left. Therefore, denervation might then also affect the response mediated by melatonin. Actually, melatonin lost its modulatory effect and became a potent as well as a full agonist (8). Since no functional nerve endings are present in this preparation, that means that the studied receptors have a postsynaptic location.

For decades, it has been known that yohimbine is a selective a2-adrenoceptor antagonist, and the compound has been used to characterize adrenoceptors. Yohimbine is an effective antagonist against both noradrenaline and B-HT 920, but with slightly different pharmacological profiles. We originally thought that yohimbine would be the negative control to exclude a2-adrenoceptor involvement in melatonin-induced pigment aggregation. The substance to be tested as antagonist against melatonin, was the ai-adrenoceptor antagonist prazosin, because it has been reported to have affinity for a second type of melatonin receptors, MT3. We found that prazosin in no

Melatonin Receptor

Figure 5. The effects of melatonin on pigment aggregation in innervated (•) and chemically denervated (guanethidin treated) (•) on isolated fish melanophores. Melatonin had only a slight effect on innervated melanophores but caused complete aggregation after chemical denervation. The aggregation could be counteracted by 1 mM yohimbine (■) but not by 1 mM prazosin (□), suggesting that a2-adrenoceptor feature has been retained. (With kind permission from Elsevier, ref 9.)

Figure 5. The effects of melatonin on pigment aggregation in innervated (•) and chemically denervated (guanethidin treated) (•) on isolated fish melanophores. Melatonin had only a slight effect on innervated melanophores but caused complete aggregation after chemical denervation. The aggregation could be counteracted by 1 mM yohimbine (■) but not by 1 mM prazosin (□), suggesting that a2-adrenoceptor feature has been retained. (With kind permission from Elsevier, ref 9.)

way interfered with melatonin, but yohimbine was a most effective antagonist against melatonin (Figure 5). More precisely, yohimbine almost totally inhibited the melatonin-induced pigment aggregation. This further suggests a melatonins-adrenoceptor interaction.

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