The pars tuberalis (PT) of the anterior pituitary is characterized by the presence of a high concentration of melatonin receptors, and acute exposure of cells from this tissue to melatonin inhibits the accumulation of cyclic AMP (cAMP) stimulated by forskolin. Conversely, exposure of ovine PT (oPT) cells to melatonin for periods of up to 16 h causes a progressive increase in subsequent basal and forskolin-stimulated production of cAMP. These observations are consistent with the possibility that the PT is involved in the mediation of melatonin-dependent phenomena in mammals. If the chronic effects of exposure to melatonin are indeed functionally significant, then one would anticipate that those responses of oPT cells known to be dependent upon levels of cAMP would also show an enhanced response to stimulation following prolonged exposure to the hormone. In the present study, the activation of cAMP-dependent protein kinase and the synthesis of secretory protein by oPT cells were found to be sensitized by prolonged exposure to physiological concentrations of melatonin. In the case of the synthesis of secretory protein this effect of melatonin was confined to those proteins whose synthesis has been shown to be sensitive to melatonin in acute experiments. These observations support the hypothesis that melatonin-induced sensitization modulates the putative biosynthetic and secretory function of the PT.
The present study also examined the mechanism of sensitization of oPT cells by melatonin. The development of sensitization was not affected by simultaneous exposure of oPT cells to forskolin (1 μm) during pretreatment with melatonin. This observation suggests that melatonin-induced sensitization occurs independently of the established acute effects of the hormone on cAMP levels in oPT cells. Since no effects of melatonin upon any other signalling cascade have been observed in these cells, the most plausible explanation for this finding is that sensitization is a direct consequence of prolonged activation of melatonin receptors. Such a mechanism might be linked to the partial down-regulation of melatonin receptors known to occur in oPT cells in response to prolonged exposure to the hormone. In order to test this hypothesis further, the process of recovery from the sensitizing effects of melatonin was examined. The recovery of oPT cells from the sensitizing effects of exposure to melatonin (100 pm, 16 h) took place gradually and, even after an interval of 16 h, cells that had previously been exposed to melatonin for 16 h remained sensitized to approximately 20% of the extent seen immediately following pretreatment with melatonin for 16 h. In contrast to the previously reported insensitivity of the development of sensitization to the protein synthesis inhibitor, cycloheximide, the recovery of oPT cells from melatonin-induced sensitization was completely blocked by cycloheximide (10 μg/ml). Taken together, these observations are consistent with the hypothesis that melatonin-induced sensitization of oPT cells is the result of a reduction in levels of certain as yet unidentified protein(s), involved in the tonic inhibition of adenylate cyclase activity, occurring in parallel with the down-regulation of melatonin receptors, and that, conversely, the resynthesis of these factor(s) is a prerequisite for the return of oPT cells to the desensitized condition.
Journal of Endocrinology (1994) 142, 127–138