Axonal expression of sodium channels in rat spinal neurones during postnatal development.

1. Postnatal expression of Na+ channels and development of somatic excitability were studied in dorsal horn neurones of rat using patch-clamp recordings from spinal cord slices in combination with the 'entire soma isolation' method. 2. The amplitude of Na+ current in the intact neurone in the slice increased with postnatal development (days 0-39) with a mean rate of 83 pA day-1. 3. The Na+ current in the neuronal soma did not increase with age and the soma, separated from the axon, was not able to fire spikes at any stage of development studied. 4. It is concluded that the postnatal development of the spinal dorsal horn neurone is accompanied by intensive expression of Na+ channels in the axonal but not somatic membrane. The estimated minimum density of Na+ channels in the axon initial segment is approximately 160 channels microm-2.