Electrophysiological properties of neurons in intact rat dorsal root ganglia classified by conduction velocity and action potential duration

Abstract

1. L4 and L5 dorsal root ganglia of rats aged 4-5 wk were isolated in vitro with their dorsal roots and sciatic nerves intact. With the use of intracellular microelectrodes, conduction velocity (CV) was determined along both peripheral and central axons and active and passive membrane properties were investigated with the use of a single-electrode switching clamp. 2. Neurons were classified into one of the three subgroups, A alpha/beta, A delta, and C, on the basis of a combination of axonal CV and action potential duration. Soma diameters overlapped between these groups. 3. Action potentials elicited by nerve stimulation in all cells and by a somatic current step in A alpha/beta-cells were always blocked by tetrodotoxin (TTX) 0.1-1 microM), whereas somatic action potentials in a proportion of A delta-cells and all C cells were TTX-resistant. 4. Passive electrical properties differed significantly between A alpha/beta-, A delta-, and C cells. The contribution of the additional membrane of the axons to the recorded electrical properties was analyzed with the use of a compartmental model of the neurons (see APPENDIX). 5. Most neurons discharged only a single action potential at the onset of a depolarizing current step, but 33% of A alpha/beta-cells fired repetitively throughout the step. This was associated with a lower threshold for action potential initiation by depolarizing current and a shorter afterhyperpolarization than in other A alpha/beta-cells. 6. Afterhyperpolarizations varied in size and duration between neurons and most were either not or only slightly affected by replacing Ca2+ in the bathing solution with Co2+ or Ba2+ or by adding tetraethylammonium (1 and 10 mM). Outward tail currents following an active response could be fitted with one fast exponential (time constant = 13 +/- 1 ms, mean +/- SE) and, in 65% of cells, one to three slower time course currents (to which exponentials with time constants of approximately 50, 300, or 1,500 ms could be fitted). A very slow late-onset current (detected in 33% of C cells) resembled a Ca(2+)-dependent K+ conductance described in several other neurons. 7. Voltage transients showed "sag" during maintained hyperpolarizing current steps in 90% of A alpha/beta-cells and 70% of A delta-cells but only 13% of C cells. Time-dependent inward currents were recorded when membrane potential was hyperpolarized. These currents had mean activation time constants of approximately 40 ms at -120 mV and were Cs+ sensitive and Ba2+ insensitive. 8. The proportion of neurons with a transient outward current, IA, increased as CV decreased (36% of A alpha/beta-cells, 56% of A delta-cells, 63% of C cells). Outward currents in cells of all subgroups had either one or two of three inactivation time constants (means approximately 22, 120, and 800 ms). 9. This study shows that many of the electrical characteristics of isolated dorsal root ganglion neurons can be demonstrated in intact ganglia in which the neurons can be better identified functionally. The currents underlying the afterhyperpolarization in these cells are diverse across all subgroups and require further investigation. The electrical effects of retaining the axonal projections of the cells and the use of microelectrodes filled with 0.5 M KC1 are discussed in relation to the differences from data recorded in dissociated neurons.