GABA(A)-RECEPTOR-MEDIATED CONDUCTANCE AND ACTION-POTENTIAL WAVE-FORM IN CUTANEOUS AND MUSCLE AFFERENT NEURONS OF THE ADULT-RAT - DIFFERENTIAL EXPRESSION AND RESPONSE TO NERVE INJURY

1. Whole cell patch-clamp recordings were obtained from identified cut aneous and muscle afferent neurons (33-60 mu m diam) in dissociated L( 4) and L(5) dorsal root ganglia (DRGs) from normal rats and from rats 2-3 wk after sciatic nerve ligation or crush injury. gamma-Aminobutyri c acid (GABA)-induced conductance was compared in normal and injured n eurons from both functional classes of sensory neurons. 2. Control cut aneous afferent neurons had a peak GABA-mediated conductance of 287 +/ - 27 (SE) nS compared with 457 +/- 42 nS for control muscle afferent n eurons. 3. An inflection on the downslope of the action potential was observed in 47% of cutaneous afferent neurons compared with 20% of mus cle afferent neurons. 4. After ligation and transection of the sciatic nerve there was no change in the GABA-mediated conductance of muscle afferent neurons or in the action potential waveform (23% inflected). However, the cutaneous afferent neurons displayed a greater than twofo ld increase in their GABA-mediated conductance and displayed a promine nt reduction in the number of neurons with inflected action potentials (13% inflected). Input resistance was similar in cutaneous and muscle afferent neurons and decreased after ligation in cutaneous but not mu scle afferents. Resting potential averaged from -50 to -56 mV in norma l and ligated groups for both cutaneous and muscle afferent neurons. 5 . After crush injury in cutaneous afferent neurons where the transecte d axons were allowed to regenerate into the distal nerve stump, GABA(A )-receptor-mediated conductance was elevated compared with controls. H owever, action potential waveform was not altered by crush injury, sug gesting a differential regulation of these two properties in cutaneous afferent neurons. 6. These data indicate that injury-induced plastici ty of GABA(A)-receptor-mediated conductance and action potential wavef orm occurs in cutaneous but not muscle afferent DRG neurons. It appear s that peripherally derived influences are critical in maintaining the electrophysiological phenotype of cutaneous afferent neurons but not muscle afferent neurons.