Muscimol prevents long-lasting potentiation of dorsal horn field potentials in rats with chronic constriction injury exhibiting decreased levels of the GABA transporter GAT-1.

The inhibitory activity of gamma-aminobutyric acid (GABA) is considered critical in setting the conditions for synaptic plasticity, and many studies support an important role of GABA in the suppression of nociceptive transmission in the dorsal horn. Consequently, any injury-induced modification of the GABA action has the potential to critically modify spinal synaptic plasticity. We have previously reported that chronic constriction injury of the sciatic nerve was accompanied by long-lasting potentiation of superficial spinal dorsal horn field potentials following high-frequency tetanus. In this study we examined whether the GABA-A receptor agonist muscimol would modify post-tetanic responses in rats with chronic constriction injury. In animals exhibiting maximal thermal hyperalgesia as one sign of neuropathic pain 7 days after loose ligation of the sciatic nerve, spinal application of muscimol (5, 10 or 20 microg) before the high-frequency (50 Hz) tetanus produced a long-lasting depression (rather than potentiation) of spinal dorsal horn field potentials. In separate but related Western immunoblot experiments, we also established that the chronic constriction injury was accompanied by significant decreases in the content of the GABA transporter GAT-1. These data demonstrated that GABA-A receptor agonists may effectively influence the expression of long-lasting synaptic plasticity in the spinal dorsal horn, and that an injury-induced loss in GABA transporter content may have contributed to a depletion of GABA from its terminals within the spinal dorsal horn. These data lent further support to the notion that the loss of GABA inhibition may have important consequences for the development of neuropathic pain.