Dual coupling of MT1 and MT2 melatonin receptors to cyclic AMP and phosphoinositide signal transduction cascades and their regulation following melatonin exposure

In this investigation, we wanted to determine whether MT1 or MT2 melatonin receptors are capable of coupling to the phosphoinositide (PI) signal transduction cascade. In addition, we wanted to assess the effects of chronic melatonin exposure on MT1 and MT2 melatonin receptor-mediated stimulation of PI hydrolysis. We also assessed the effects of chronic melatonin exposure on other parameters of the MT2 melatonin receptor function including total specific 2-[I-125]-iodomelatonin binding, the affinity of melatonin for the receptor, and melatonin (I nM)-mediated inhibition of cyclic 3',5'-adenosine monophosphate (cAMP) accumulation, Investigation of the PI signal transduction cascade activated by either the MT1 or MT2 melatonin receptor expressed in Chinese hamster ovary (CHO) cells showed that melatonin (I pM to I mM) was able to stimulate the formation of PIs to similar to40-60% over basal [EC50: MT1 = 29 nM (2-300 nM) and MT2 = 1.1 nM (0.32-3.5 nM), N = 5]. This response was mediated via receptors based upon the findings that melatonin did not stimulate the formation of PIs in CHO cells devoid of receptor and that antagonism of MT2 melatonin receptors by 4P-PDOT (AH 024; 4-phenyl-2-propionamidotetralin) attenuated melatonin-mediated stimulation of PI hydrolysis in CHO cells expressing the MT2 melatonin receptor. The consequence of chronic melatonin exposure on MT1 and MT2 receptor function was also examined. Pretreatment of either MT1- or MT2-CHO cells with melatonin (I M for 5 hr) resulted in: (a) a complete loss of melatonin-mediated stimulation of PI hydrolysis, and (b) an attenuation of melatonin (I nM)-mediated inhibition of forskolin-induced cAMP accumulation by similar to20-40%. The desensitization of the PI hydrolysis signal transduction cascades coupled to either MT1 or MT2 melatonin receptors following chronic melatonin exposure was not due to depleted phospholipid pools, to elevated basal levels, or to decreases in receptor affinity and density. This dual coupling of melatonin receptors to different signal transduction cascades may contribute to the diversity of melatonin receptor function in vivo. (C) 2002 Elsevier Science Inc. All rights reserved.