Voltage-Gated Calcium Currents in Axotomized Adult Rat Cutaneous Afferent Neurons

Abstract

The effect of sciatic nerve injury on the somatic expression of voltage-gated calcium currents in adult rat cutaneous afferent dorsal root ganglion (DRG) neurons identified via retrograde Fluoro-gold labeling was studied using whole cell patch-clamp techniques. Two weeks after a unilateral ligation and transection of the sciatic nerve, the L₄–L₅DRG were dissociated and barium currents were recorded from cells 3–10 h later. Cutaneous afferents (35–50 μm diam) were classified as type 1 (possessing only high-voltage–activated currents; HVA) or type 2 (having both high- and low-voltage–activated currents). Axotomy did not change the percentage of neurons exhibiting a type 2 phenotype or the properties of low-threshold T-type current found in type 2 neurons. However, in type 1 neurons the peak density of HVA current available at a holding potential of −60 mV was reduced in axotomized neurons (83.9 ± 5.6 pA/pF, n = 53) as compared with control cells (108.7 ± 6.9 pA/pF, n = 58, P < 0.01, unpaired t-test). A similar reduction was observed at more negative holding potentials, suggesting differences in steady-state inactivation are not responsible for the effect. Separation of the type 1 cells into different size classes indicates that the reduction in voltage-gated barium current occurs selectively in the larger (capacitance >80 pF) cutaneous afferents (control: 112.4 ± 10.6 pA/pF, n = 30; ligated: 72.6 ± 5.0 pA/pF, n = 36; P < 0.001); no change was observed in cells with capacitances of 45–80 pF. Isolation of the N- and P\Q-type components of the HVA current in the large neurons using ω-conotoxin GVIA and ω-agatoxin TK suggests a selective reduction in N-type barium current after nerve injury, as the density of ω-CgTx GVIA-sensitive current decreased from 56.9 ± 6.6 pA/pF in control cells ( n = 13) to 31.3 ± 4.6 pA/pF in the ligated group ( n = 12; P < 0.005). The HVA barium current of large cutaneous afferents also demonstrates a depolarizing shift in the voltage dependence of inactivation after axotomy. Injured type 1 cells exhibited faster inactivation kinetics than control neurons, although the rate of recovery from inactivation was similar in the two groups. The present results indicate that nerve injury leads to a reorganization of the HVA calcium current properties in a subset of cutaneous afferent neurons.