Pain and hyperalgesia occured when L 5 and L 6 spinal nerves were ligated. To ev aluate the electrophysiological changes that contribute to this sensory patholog y, whole cell current clamp recording was performed in DRGs that was obtained from neuropathic pain and control rats. After nerve ligation, action potential threshold reduced (more negative) in b oth small and median sized DRG neurons (-18.98±0.69 mV vs -11.12±1.06 mV in c ontrol small sized neurons; -19.44 ±2.22 mV vs -14.55±1.81 mV in control m edi an sized neurons), but resting membrane potential action potential duration at half amplitude (APD 1/2 ) and action potential amplitude did not change sign ifica ntly. In addition, obvious membrane potential oscillations were observed in smal l sized DRG neurons from neuropathic pain rats (P<0.05 compared with control group). These results suggested that neuropathic pain increased the exci tability of nociceptors, which may be an important mechanism underlying peripher al hypersensitivity.
The electrophysiological characters of hyerpolarization activated current (Ih) and its distribution in the cell bodies of rat dorsal root ganglion (DRG) neuron s with different diameters were studied by whole cell voltage and current cla mp methods. DRG neurons were classified into three group: small (diameter<30 μm ), median (30 μm ≤ d ≤ 40 μm ) and large (>40 μm) types. Ih was found as a slowly activating inward current evoked by hyperpolarizing voltage steps from a holding potential of -60 mV, and reversed at -32.34±2.56 mV(n=4). Ih could be specifically blocked by 2.0 mM Cs 2+ but not 1.0 mM Ba 2+ , furthermore, 100 μM cAMP applied into pipette solution could shift the activa tion curve to positive di rection (-91.4 mV vs -100.6 mV in control). Ih was observed in 11 of 49 small, 17 of 22 median and 7 of 7 large diameter DRG neurons tested. The Density of Ih current in median and large cells was significantly higher than those in small o nes (11.88±1.59 pA/pF, 5.05±0.39 pA/pF and 2.59±0.20 pA/pF, respectively). Ac tion potential at half amplitude (APD 1/2 ) varied in different types of DR G neurons. It was shown that large diameter cells had the shortest APD 1/2 than the other two types. These data suggested that Ih was mainly distributed in large and median diameter DRG neurons. Because some of Ih expressed neurons a re nociceptors, so it suggests that Ih may play a role in chronic pain.
