Nitric oxide induces an increased Na+ conductance in identified neurons of Aplysia

The ionic mechanism of the effects of micropressure ejections of hydroxylamine (HOA) and sodium nitroprusside (SNP), nitric oxide (NO) generators, on the membrane of identified neurons (R9-R12) of Aplysia kurodai was investigated with conventional voltage-clamp, micropressure ejection, and ion-substitution techniques. Micropressure ejection of HOA and SNP onto the neurons caused a marked depolarization in the unclamped neurons. Clamping the same neurons at their resting potential level (-60 mV) and reejecting HOA and SNP with the same dose produced a slow inward current (Ii(HOA) and Ii(SNP), 3-7 nA in amplitude, 15-60 s in duration) associated with an increase in input membrane conductance. Bath-applied hemoglobin (50 microM), a nitric oxide scavenger, almost completely blocked Ii(HOA) and Ii(SNP), and 3-isobutyl-1-methylxanthine (IBMX, 50 microM) prolonged and enhanced both Ii(HOA) and Ii(SNP). An intracellular injection of cyclic guanosine 3',5'-monophosphate (cGMP) into the same neurons produced a slow inward current (Ii(cGMP)) which resembled the responses to HOA and SNP, and this current was enhanced in IBMX. Bath-applied methylene blue (10 microM), an inhibitor of guanylate cyclase, significantly reduced Ii(HOA) and Ii(SNP). The inward currents induced by HOA, SNP and cGMP were sensitive to changes in the external Na+ concentration. These results suggest that extracellular NO can induce a slow inward current associated with an increase in Na+ conductance, mediated by an increase in intracellular cGMP.