MELATONIN AGONISTS INDUCE PHOSPHOINOSITIDE HYDROLYSIS IN XENOPUS-LAEVIS MELANOPHORES

Melatonin, the principal hormone of the vertebrate pineal gland, has b een implicated in a variety of neurobiological processes such as circa dian rhythmicity and reproductive function. One of the earliest descri bed actions of melatonin was its ability to cause pigment translocatio n in the dermal melanophores of amphibians. Melatonin binding sites ha ve been identified in the brain of many species and in pigmented tumou r cell lines; however, the dermal melanophores of the frog Xenopus Lae vis possess the highest known density of melatonin binding sites. Thes e cells are the source from which a melatonin receptor has been cloned and provide an excellent model to study melatonin mediated signal tra nsduction in an isolated cell system. In Xenopus melanophores, melaton in induces a rapid perinuclear aggregation of intracellular pigment wh ich is associated with a pertussis toxin sensitive inhibition of cAMP. We have previously demonstrated that a sub type of melatonin binding sites found in selected regions of the pigeon brain and in Syrian Hams ter RPMI 1846 melatonin cells are functionally coupled to phosphoinosi tide hydrolysis as a second messenger. Here we now present evidence to suggest that Xenopus Laevis melanophores also possess melatonin bindi ng sites which are functionally linked to phosphoinositide hydrolysis. Melatonin agonists induced phosphoinositide hydrolysis in melanophore s in a concentration-dependent manner with a rank order of potency of 2-iodomelatonin > 6-chloromelatonin > N-acetylserotonin > melatonin. S timulatory response of 2-iodomelatonin was blocked by the melatonin an tagonist N-acetyltryptamine and the alpha adrenergic antagonist prazos in, which has been shown to have high affinity for melatonin binding s ites. Phosphoinositide hydrolysis induced by melatonin ago nists was n ot blocked by the serotonin antagonist ketanserin or by phentolamine, an alpha-adrenergic antagonist, indicating that the response observed was not due to stimulation of 5-HT2a/2c receptors or alpha-adrenergic receptors. Furthermore, incubation of melanophores with the non-hydrol yzable G-protein source GTP gamma-S attenuated the phosphoinositide do se response induced by 2-iodomelatonin, and pre incubation of the cell s with pertussis toxin had no effect on 2-iodomelatonin-induced phosph oinositide hydrolysis. The present data suggest that Xenopus Laevis Me lanophores possess G-protein linked pertussis toxin-insensitive melato nin binding sites which are functionally coupled to phosphoinositide h ydrolysis as a signal transduction mechanism. (C) 1997 Elsevier Scienc e Inc.