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

Abstract

Mimicking short photoperiod melatonin signals (16 h exposure) on primary cell cultures of melatonin target cells of the ovine pars tuberalis (PT) results in an enhanced cAMP response to forskolin stimulation relative to untreated cells, a phenomenon termed sensitization. The sensitized response of PT cells may be an important aspect of the interpretation of the melatonin signal to initiate appropriate seasonal physiological responses. The aim of this study is to add to our understanding of the molecular mechanisms involved in the sensitization of PT cells by melatonin. We demonstrate that sensitization of PT cells by melatonin is mediated via a G(i)-coupled melatonin receptor. The sensitized cAMP response is not only obtained with the pharmacological tool forskolin, but also with cholera toxin, an activator of G(salpha). Changes in the level of G(salpha) or G(ialpha) G-protein subunits are ruled out as part of the sensitization mechanism. However, changes in tyrosine phosphorylation may be involved as tyrosine kinase inhibitors sensitize ovine PT cells and tyrosine phosphatase inhibitors significantly blunt adenylate cyclase activity, including the sensitized response to melatonin. 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.

The Human MT1 Melatonin Receptor Stimulates cAMP Production in the Human Neuroblastoma Cell Line SH‐SY5Y Cells Via a Calcium‐Calmodulin Signal Transduction Pathway

“…It is interesting to note that the response to melatonin did not desensitise during the time course of melatonin treatment. This is consistent with the lack of desensitisation in pars tuberalis cells where melatonin receptors can inhibit forskolin-stimulated cAMP accumulation despite an overnight pretreatment with melatonin (17,34).…”

Photoperiod regulates multiple gene expression in the suprachiasmatic nuclei and pars tuberalis of the Siberian hamster (Phodopus sungorus)

“…Melatonin regulates cAMP signalling pathways in the PT by distinct mechanisms. Acute exposure of PT cells to melatonin inhibits stimulated cAMP activity , whereas prolonged exposure to melatonin sensitizes PT cells to subsequent stimulation of cAMP (Hazlerigg et al, 1993;Barrett et al, 2000;von Gall et al, 2002). It is thought that these actions of melatonin on cAMP signalling regulate the timing of Per1 expression via its CRE elements .…”

Photoperiodic regulation of prolactin secretion: changes in intra-pituitary signalling and lactotroph heterogeneity

“…However, the mechanisms by which melatonin signal duration drives PT physiology are poorly understood. Prolonged (6-24 h) incubation of PT cell cultures with melatonin sensitises subsequent stimulation of cAMP (Hazlerigg et al 1993, von Gall et al 2002, probably resulting from changes in tyrosine phosphorylation of adenylyl cyclase (Barrett et al 2000). This had fuelled speculation that melatonin signal duration may modulate the induction of cAMP by stimulatory factor(s), such as pituitary adenylyl cyclase-activating polypeptide (PACAP) or adenosine (Barrett et al 2002, von Gall et al 2002.…”