Postsynaptic mechanisms underlying responsiveness of amygdaloid neurons to nociceptin/orphanin FQ

Effects of nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the opioid-like orphan receptor (ORL), were investigated in the rat lateral (AL) and central (ACe) amygdala in vitro. Approximately 98% of presumed projection neurons in the AL responded to N/OFQ with an increase in inwardly rectifying potassium conductance, resulting in an impairment in cell excitability. Half-maximal effects were obtained at 30.6 nM; the Hill coefficient was 0.63. In the ACe, 31% of the cells displayed responses similar to that in the AL, 44% were nonresponsive, and 25% responded with a small potassium current with a linear current-voltage relationship. Responses to N/OFQ were reduced by 100 microM Ba2+, were insensitive to 10 microM naloxone, and were blocked by a selective ORL antagonist, [Phe1psi(CH2-NH)Gly2]NC(1-13)NH2 (IC50 = 760 nM). Involvement of G-proteins was indicated by irreversible effects and blockade of action of N/OFQ during intracellular presence of GTP-gamma-S (100 microM) and GDP-beta-S (2 mM), respectively, and prevention of responses after incubation in pertussis toxin (500 ng/ml). These mechanisms may contribute to the role of N/OFQ in the reduction of fear responsiveness and stress that have recently been suggested on the basis of histochemical and behavioral studies.