Blockade of spinal α5-GABAA receptors differentially reduces reserpine-induced fibromyalgia-type pain in female rats

Abstract

The role of spinal α₅ subunit-containing GABA_A (α₅-GABA_A) receptors in chronic pain is controversial. The purpose of this study was to investigate the participation of spinal α₅-GABA_A receptors in the reserpine-induced pain model. Reserpine administration induced tactile allodynia and muscle hyperalgesia in female and male rats. Intrathecal injection of L-655,708 and TB 21007 (7 days after the last reserpine injection) decreased tactile allodynia and, at a lesser extent, muscle hyperalgesia in female rats. The effects of these drugs produced a lower antiallodynic and antihyperalgesic effect in male than in female rats. Contrariwise, these drugs produced tactile allodynia and muscle hyperalgesia in naïve rats and these effects were lower in naïve male than female rats. Intrathecal L-838,417 prevented or reversed L-655,708-induced antiallodynia in reserpine-treated female rats. Repeated treatment with α₅-GABA_A receptor small interfering RNA (siRNA), but not scramble siRNA, reduced reserpine-induced allodynia in female rats. Accordingly, α₅-GABA_A receptor siRNA induced nociceptive hypersensitivity in naïve female rats. Reserpine enhanced α₅-GABA_A receptors expression in spinal cord and dorsal root ganglia (DRG), while it increased CD11b (OX-42) and glial fibrillary acidic protein (GFAP) fluorescence intensity in the lumbar spinal cord. In contrast, reserpine diminished K⁺-Cl^- co-transporter 2 (KCC2) protein in the lumbar spinal cord. Data suggest that spinal α₅-GABA_A receptors play a sex-dependent proallodynic effect in reserpine-treated rats. In contrast, these receptors have a sex-dependent antiallodynic role in naïve rats.

Keywords: Disinhibition; Fibromyalgia; Functional pain; α(5)-GABA(A) receptors.