Opposing effects of spinal nerve ligation on calcium-activated potassium currents in axotomized and adjacent mammalian primary afferent neurons

Calcium-activated potassium channels regulate AHP and excitability in neurons. Since we have previously shown that axotomy decreases I-Ca in DRG neurons, we investigated the association between I-Ca and K((Ca)) currents in control medium-sized (30-39 mu M) neurons, as well as axotomized L5 or adjacent L4 DRG neurons from hyperalgesic rats following L5 SNL. Currents in response to AP waveform voltage commands were recorded first in Tyrode's solution and sequentially after: 1) blocking Na+ current with NMDG and TTX; 2) addition of K-(Ca) blockers with a combination of apamin 1 mu M, iberiotoxin 200 nM, and clotrimazole 500 nM; 3) blocking remaining K+ current with the addition of 4-AP, TEA-Cl, and glibenclamide; and 4) blocking I-Ca with cadmium. In separate experiments, currents were evoked (HP -60 mV, 200 ms square command pulses from -100 to +50 mV) while ensuring high levels of activation Of I-K(Ca) by clamping cytosolic Ca2+ concentration with pipette solution in which Ca2+ was buffered to 1 mu M. This revealed I-K(Ca) with components sensitive to apamin, clotrimazole and iberiotoxin. SNL decreases total I-K(Ca) in axotomized (L5) neurons, but increases total I-K((Ca)) in adjacent (L4) DRG neurons. All I-K(Ca) subtypes are decreased by axotomy, but iberiotoxin-sensitive and clotrimazole-sensitive current densities are increased in adjacent L4 neurons after SNL. In an additional set of experiments we found that small-sized control DRG neurons also expressed iberiotoxin-sensitive currents, which are reduced in both axotomized (L5) and adjacent (L4) neurons. Conclusions: Axotomy decreases I-K(Ca) due to a direct effect on K-(Ca) channels. Axotomy- induced loss of I-Ca. may further potentiate current reduction. This reduction in I-K(Ca) may contribute to elevated excitability after axotomy. Adjacent neurons (L4 after SNL) exhibit increased I-K(Ca) current. (c) 2006 Elsevier B.V. All rights reserved.