Nonselective Ionic Channels in Aplysia Neurones

Single-channel recordings from outside-out patches of Aplysia neurones in K-free solutions revealed the presence in most membrane patches of ionic channels showing surprising selectivity properties, as deduced from reversal potential measurements. After complete substitution of external NaCl by mannitol (in the presence of internal CsCl), these channels are more permeable to Cl than to Cs, but are also slightly permeable to Cs: PCl/PCs = 4. Furthermore, in the presence of external NaCl, their ability to discriminate cations from anions seems lower than in external mannitol. Substitutions of external Cl by various anions showed that the channels are more permeable to NO3 than to Cl, and that they are appreciably permeable to isethionate, SO4 and methanesulfonate. Their elementary conductance is about 100 pS in 600 mM symmetrical Cl. However, different conductance states (usually 2 or 3) can often be detected in the same membrane patch. By using voltage ramps, we established the I-V curves corresponding to each of these states and found small but significant differences between the reversal potentials of each state.