FAST AND SLOW ACTIVATION KINETICS OF VOLTAGE-GATED SODIUM-CHANNELS INMOLLUSCAN NEURONS

Whole cell patch-clamp recordings of Na current (I-Na,) were made unde r identical experimental conditions from isolated neurons from cephalo pod (Loligo, Octopus) and gastropod (Aplysia, Pleurobranchaea, Doriops illa) species to compare properties of activation gating. Voltage depe ndence of peak Na conductance (g(Na)) is very similar in all cases, bu t activation kinetics in the gastropod neurons studied are markedly sl ower. Kinetic differences are very pronounced only over the voltage ra nge spanned by the g(Na)-voltage relation. At positive and negative ex tremes of voltage, activation and deactivation kinetics of I-Na are pr actically indistinguishable in all species studied. Voltage-dependent rate constants underlying activation of the slow type of Na channel fo und in gastropods thus appear to be much more voltage dependent than a re the equivalent rates in the universally fast type of channel that p redominates in cephalopods. Voltage dependence of inactivation kinetic s shows a similar pattern and is representative of activation kinetics for the two types of Na channels. Neurons with fast Na channels can t hus make much more rapid adjustments in the number of open Na channels at physiologically relevant voltages than would be possible with only slow Na channels. This capability appears to be an adaptation that is highly evolved in cephalopods, which are well known for their high-sp eed swimming behaviors. Similarities in slow and fast Na channel subty pes in molluscan and mammalian neurons are discussed.