α₅GABA_A receptors mediate primary afferent fiber tonic excitability in the turtle spinal cord

Abstract

γ-Amino butyric acid (GABA) plays a key role in the regulation of central nervous system by activating synaptic and extrasynaptic GABAA receptors. It is acknowledged that extrasynaptic GABAA 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 α5GABAA receptors mediate the tonic state of excitability of primary afferents independently of the phasic primary afferent depolarization mediated by synaptic GABAA receptors. Blockade of α5GABAA 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 α5GABAA 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, α5GABAA receptors may serve as potential pharmacological targets for the treatment of pain.

The α5 subunit containing GABAA receptors contribute to chronic pain

“…7A/7B). As reported in turtles [41], there was α 5 -GABA A receptors immunoreactivity in DRG (Figs. 7C/7D) and peripheral afferent fibers of the rat (Figs.…”

“…Consistent with this hypothesis, we previously demonstrated that α 5 -GABA A receptor blockade leads to antinociception in the rat formalin test [11]. Notably, α 5 -GABA A receptors have been localized at the dorsal horn in mice and rats [43,53] and are also found in DRG and peripheral nerve fibers in turtles [41], making them well placed to modulate the nociceptive process. However, the role of α 5 -GABA A receptors in chronic inflammatory or neuropathic pain is unclear.…”

“…# SAB2100878; dilution 1:200, Sigma-Aldrich, St. Louis, MO) [41]. In order to determine the specific cellular and subcellular distribution of the α 5 -GABA A receptors in the spinal cord and DRG, we performed double staining with the following markers: NeuN was used as a specific neuronal marker (chicken anti-NeuN, Cat.…”

“…The subunit composition of the GABA A receptors determines whether the receptors are more likely to participate in the generation of transient post-synaptic or pre-synaptic inhibition or a form of tonic inhibition [25]. Tonic inhibition is generated by extrasynaptic α 5 subunit-containing GABA A receptors (α 5 -GABA A receptors), among others, in the brain [7,25,32,62,63] and particularly in the spinal cord [14,21,41]. It has been demonstrated that tonically active α 5 -GABA A receptors modulate dorsal root reflexes by affecting excitability via tonic depolarization, suggesting that they might play an important role in pain [21,41,62].…”

“…Tonic inhibition is generated by extrasynaptic α 5 subunit-containing GABA A receptors (α 5 -GABA A receptors), among others, in the brain [7,25,32,62,63] and particularly in the spinal cord [14,21,41]. It has been demonstrated that tonically active α 5 -GABA A receptors modulate dorsal root reflexes by affecting excitability via tonic depolarization, suggesting that they might play an important role in pain [21,41,62]. Consistent with this hypothesis, we previously demonstrated that α 5 -GABA A receptor blockade leads to antinociception in the rat formalin test [11].…”