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

1. Postnatal expression of Na+ channels and development of somatic excitabi lity were studied in dorsal horn neurones of rat using patch-clamp recordin gs 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 increase d 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 s oma, separated from the axon, was not able to fire spikes at any stage of d evelopment 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 axon al but not somatic membrane. The estimated minimum density of Na+ channels in the axon initial segment is similar to 160 channels mu m(-2).