BOTH PERTUSSIS TOXIN-SENSITIVE AND INSENSITIVE G-PROTEINS LINK MELATONIN RECEPTOR TO INHIBITION OF ADENYLATE-CYCLASE IN THE OVINE PARS TUBERALIS

Bordetella pertussis toxin (islet activating protein, IAP) has been used to investigate the G-proteins involved in mediating the action of the melatonin receptor. Melatonin inhibits forskolin-stimulated cyclic AMP production in ovine pars tuberalis (PT) cells. In cells treated with IAP for 16 h this response is attenuated in a dose-dependent manner, but not abolished. IAP catalyses the incorporation of [P-32-ADP]ribose into a 41 kd protein present in PT membranes, but this labelling can be reduced if PT cells are preincubated with IAP for 16 h. Treatment of crude membrane preparations with IAP (20-mu-g/ml) suppresses the binding of 2-[I-125]iodomelatonin by 20%, whereas 1 mM GTP alone reduces binding by 40%, and in combination with IAP its effect is additive (60% inhibition). Therefore, these results indicate that the melatonin receptor acts via two G-proteins, one pertussis toxin-sensitive and the other pertussis toxin-insensitive.