FUNCTIONAL DISTRIBUTION OF 3 TYPES OF NA-CORD( CHANNEL ON SOMA AND PROCESSES OF DORSAL HORN NEURONS OF RAT SPINAL)

1. Voltage-gated Na+ channels and their distribution were studied by t he patch-clamp technique in intact dorsal horn neurones in slices of n ewborn rat spinal cord and in neurones isolated from the slice by slow withdrawal of the recording pipette. This new method of neurone isola tion was further used to study the roles of soma and axon in generatio n of action potentials. 2. Tetrodotoxin (TTX)-sensitive Na+ currents i n intact neurones consisted of three components. A fast component with an inactivation time constant (tau(f)) of 0.6-2.0 ms formed the major part (80-90%) of the total Na+ current. The remaining parts consisted of a slowly inactivating component (tau(8) of 5-20 ms) and a steady-s tate component. 3. Single fast and slow inactivating Na+ channels with conductances of 11.6 and 15.5 pS, respectively were identified in the soma of intact neurones in the slice. Steady-state Na+ channels were not found in the soma, suggesting an axonal or dendritic localization of these channels. 4. In the whole-cell recording mode, the entire som a of a dorsal horn neurone could be isolated from the slice by slow wi thdrawal of the recording pipette, leaving all or nearly all of its pr ocesses in the slice. The isolated structure was classified as: (1) 's oma' if it lost all of its processes, (2) 'soma+axon' complex if it pr eserved one process and at least 85% of its original peak Na+ current or (3) 'soma+dendrite' complex if it preserved one process but the rem aining Na+ current did not exceed those observed in the isolated 'soma ta'. 5. The spatial distribution of Na+ channels in the neurone was st udied by comparing Na+ currents recorded before and after isolation. T he isolated 'soma' contained 13.8 +/- 1.3% of inactivating Na+ current but no steady-state Na+ current. 'Soma+axon' complexes contained 93.6 +/- 1.4% of inactivating and 46% of steady-state Na+ current. 6. In c urrent-clamp experiments, the intact neurones and isolated 'soma+axon' complexes responded with 'all-or-nothing' action potentials to curren t injections. In contrast, isolated 'somata' showed only passive or lo cal responses and were unable to generate action potentials. 7. It is concluded that dorsal horn neurones of the spinal cord possess three t ypes of TTX-sensitive voltage-gated Na+ channels. The method of entire soma isolation described here shows that the majority of inactivating Na+ channels are localized in the axon hillock and only a small propo rtion (ca 1/7) are distributed in the soma. Steady-state Na+ channels are most probably expressed in the axonal and dendritic membranes. The soma itself is not able to generate action potentials. The axon or it s initial segment plays a crucial role in the generation of action pot entials.