OPIOID INHIBITION OF I-H VIA ADENYLYL-CYCLASE

Opioids are coupled through G proteins to both ion channels and adenylyl cyclase. This study describes opioid modulation of the voltage-dependent cation channel, I-h in cultured guinea pig nodose ganglion neurons. Forskolin, PGE(2), and cAMP analogs shifted the voltage dependence of activation of I-h to more depolarized potentials and increased the inward current at -60 mV. Opioids had no effect on I-h alone, but reversed the effect of forskolin on I-h. This action of opioids was blocked by naloxone. Opioids had no effect on lh in the presence of cAMP analogs, suggesting that modulation occurs at the level of adenylyl cyclase. The shift in the voltage dependence of I-h by agents that induce inflammation (i.e., PGE(2)) is one potential mechanism to mediate an increased excitability. Opioid inhibition of adenylyl cyclase and subsequent inhibition of I-h may be a mechanism by which opioids inhibit primary afferent excitability and relieve pain.