Kinetic characterization of the voltage-gated currents possessed by Xenopus embryo spinal neurons.

1. Using the whole-cell patch clamp technique, the voltage-gated currents of neurons acutely isolated from the Xenopus embryo spinal cord were studied. 2. The spinal neurons possessed a very fast Na+ current, which activated with time constants that ranged from 0.1 to 0.25 ms. It was also subject to rapid inactivation with time constants ranging from 0.3 to 8 ms. This current could only be fitted with Hodgkin-Huxley equations once the rapid inactivation that occurs by the time of the peak current had been taken into account. 3. Xenopus embryo neurons also possessed a mixture of kinetically similar Ca2+ currents, which activated with time constants that ranged from 0.3 to 0.8 ms. Sometimes the Ca2+ currents showed very slow inactivation at more positive voltages (> 20 mV). The Ca2+ current was modelled as a single non-inactivating current. 4. As might be expected, the embryonic neurons possessed a mixture of outward currents that were hard to separate either pharmacologically or through differences in voltage dependence. The delayed rectifier seemed to consist of varying proportions of two currents: a fast-activating K+ current (with time constants of activation ranging from 0.6 to 2 ms) and a slow K+ current (with time constants of activation ranging from 5 to 25 ms). The slow current was occasionally seen in isolation. 5. For the Ca2+, fast K+ and slow K+ currents the rate of deactivation was faster than would be predicted from the kinetics of activation. This was modelled by allowing the closing rate constant of the channels to be described by one of two different functions of voltage that between them covered the whole range of transmembrane voltage. Although this was done for empirical reasons, it could be interpreted to suggest that the channels have more than one open state and predominantly close from a state that is distinct from the one to which they originally opened.