α5GABAA receptors mediate primary afferent fiber tonic excitability in the turtle spinal cord

gamma-Amino butyric acid (GABA) plays a key role in the regulation of central nervous system by activating synaptic and extrasynaptic GABA(A) receptors. It is acknowledged that extrasynaptic GABA(A) receptors located in the soma, dendrites, and axons may be activated tonically by low extracellular GABA concentrations. The activation of these receptors produces a persistent conductance that can hyperpolarize or depolarize nerve cells depending on the Cl -equilibrium potential. In an in vitro preparation of the turtle spinal cord we show that extrasynaptic alpha(5)GABA(A) receptors mediate the tonic state of excitability of primary afferents independently of the phasic primary afferent depolarization mediated by synaptic GABA A receptors. Blockade of alpha(5)GABA(A) receptors with the inverse agonist L-655,708 depressed the dorsal root reflex ( DRR) without affecting the phasic increase in excitability of primary afferents. Using RT-PCR and Western blotting, we corroborated the presence of the mRNA and the alpha(5)GABA(A) protein in the dorsal root ganglia of the turtle spinal cord. The receptors were localized in primary afferents in dorsal root, dorsal root ganglia, and peripheral nerve terminals using immunoconfocal microscopy. Considering the implications of the DRR in neurogenic inflammation, alpha(5)GABA(A) receptors may serve as potential pharmacological targets for the treatment of pain.