Paraoxon block of chloride conductance in cell R2 of Aplysia californica

In the presence of paraoxon, the amplitudes of chloride currents activated by acetylcholine (ACh) or gamma-aminobutyric acid (GABA) were reduced in cell R2 of Aplysia californica. IC 50 values were 12 and 9.7 μM for ACh and GABA responses, respectively. Paraoxon did not affect resting membrane potential, input resistance, or chloride reversal potential. Both the slopes and maxima of ACh and GABA concentration-response curves were reduced by paraoxon, suggesting that paraoxon antagonism of these responses is not competitive. The antagonism of ACh and GABA responses by paraoxon was not related to inhibition of acetylcholinesterase.