Melatonin Inhibits Cyclic Amp Production in Cultured Ovine Pars Tuberalis Cells

Abstract

Primary cultures of ovine pars tuberalis (PT) cells of the pituitary were established to investigate the mode of action of melatonin. The heterogeneous population of cells was shown to bind the radioligand 2-[125I]-melatonin over 72 h in culture, although there was a progressive decline in specific binding with time. In cells cultured for 24 h, forskolin (1 μmol/l) was found to stimulate a 12-fold increase in cyclic AMP accumulation. This response could be inhibited by melatonin in a dose-dependent manner, with an IC50 of approx. 6 pmol/l. However, melatonin did not inhibit basal levels of cyclic AMP. In homogenates of ovine PT, forskolin stimulated a dose-dependent increase in cyclic AMP, although the magnitude of this response was found to be lower than that observed in cells. This response was not inhibited by either 10 nmol or 1 μmol melatonin/l, and was also unaffected by GTP. These results provide the first evidence that the melatonin-binding site on ovine PT, recently characterized using the radioligand 2-[125I]-melatonin, functions as a physiological receptor.

Expression and functional characterization of a melatonin‐sensitive receptor in Xenopus oocytes

“…The MEL receptors are coupled to G proteins and their activation often leads to the inhibition of the adenylyl cyclase, resulting in a decrease in the cAMP level of the cells [2,7,10]. Recently it has been reported that MEL is also able to influence the phosphoinositide breakdown in chicken brain slices [13].…”

“…It has been reported that in the cells of the pars tuberalis MEL-sensitive receptors are functionally coupled to a G protein, which causes a decrease in the cyclic AMP level [2,7,10]. To test whether the expressed MEL-sensitive receptors are additionally able to influence the cyclic AMP level in oocytes, the effects of MEL on the action of forskolin, an unspecific activator of the adenylyl cyclase, was tested.…”

“…The MEL receptors are functionally coupled to G-proteins which inhibit the activity of the adenylyl cyclase [10]. The activation of MEL receptors causes a reduction of the forskolin-stimulated cAMP level in the cells [2,7].…”

Generation of the Melatonin Endocrine Message in Mammals: A Review of the Complex Regulation of Melatonin Synthesis by Norepinephrine, Peptides, and Other Pineal Transmitters

“…Their activation induces a decrease in forskolin-induced cAMP accumulation (Carlson et al, 1989;Morgan et al, 1989). This effect is generally mediated by a pertussis toxinsensitive G-protein (G i /G o ; Reppert et al, 1994).…”

Melatonin Inhibits the Activation of Cyclic AMP‐Dependent Protein Kinase in Cultured Pars Tuberalis Cells from Ovine Pituitary

“…These developments have added new impctus to attcmpts to understand the cellular and neural mechanisms of action of melatonin. In tissue explants of rat pituitary or hamster median eminence/pars tuberalis and in primary cells cultures of ovine Pars tuberalis, it has been demonstrated that melatonin is able to inhibit adenylate cyclase activity by acting through a receptor coupled to one or more inhibitory C-proteins (17)(18)(19)(20)(21)(22). In ovine pars tuberalis, melatonin does not acutely affect basal or stimulated levels of intracellular free calcium nor the turnover of inositol phosphates (23,24).…”