A role for tyrosine phosphorylation in the regulation and sensitization ofadenylate cyclase by melatonin

Mimicking short photoperiod melatonin signals (16 h exposure) on primary ce ll cultures of melatonin target cells of the ovine pars tuberalis (PT) resu lts in an enhanced cAMP response to forskolin stimulation relative to untre ated cells, a phenomenon termed sensitization. The sensitized response of P T cells may be an important aspect of the interpretation of the melatonin s ignal to initiate appropriate seasonal physiological responses. The aim of this study is to add to our understanding of the molecular mechanisms invol ved in the sensitization of PT cells by melatonin, We demonstrate that sens itization of PT cells by melatonin is mediated via a G(i)-coupled melatonin receptor. The sensitized cAMP response is not only obtained with the pharm acological tool forskolin, but also with cholera toxin, an activator of G(s alpha). Changes in the level of G(s alpha) or G(i alpha) G-protein subunit s are ruled out as part of the sensitization mechanism. However, changes in tyrosine phosphorylation may be involved as tyrosine kinase inhibitors sen sitize ovine PT cells and tyrosine phosphatase inhibitors significantly blu nt adenylate cyclase activity, including the sensitized response to melaton in. The adenylate cyclase isoforms mediating the sensitized response may be broad as 7 of the 9 isoforms of adenylate cyclase are expressed in the PT.