Downregulation of spinal astrocytic connexin43 leads to upregulation of interleukin-6 and cyclooxygenase-2 and mechanical hypersensitivity in mice.

Abstract

Connexin43 (Cx43), involved in intercellular signaling, is expressed in spinal dorsal horn astrocytes and crucial in the maintenance of neuropathic pain. Downregulation of spinal astrocytic Cx43 in mice enhances glutamatergic neurotransmission by decreasing glutamate transporter GLT-1 expression, resulting in cutaneous hypersensitivity. Decreased expression of astrocytic Cx43 could lead to altered expression of other nociceptive molecules. Transfection of Cx43-targeting siRNA in cultured spinal astrocytes increased expression of the pronociceptive cytokine interleukin-6 (IL-6) and the prostaglandin synthesizing enzyme cyclooxygenase-2 (COX-2). Increased expression of IL-6 and COX-2 was due to decreased Cx43 expression rather than due to diminished Cx43 channel function. In mice, downregulation of spinal Cx43 expression by intrathecal treatment with Cx43-targeting siRNA increased IL-6 and COX-2 expression and induced hind paw mechanical hypersensitivity. Cx43 siRNA-induced mechanical hypersensitivity was attenuated by intrathecal treatment with anti-IL-6 neutralizing antibody and intraperitoneal treatment of selective COX-2 inhibitor celecoxib, demonstrating that these molecules play a role in nociceptive processing following Cx43 downregulation. Restoring spinal Cx43 by intrathecal injection of an adenovirus vector expressing Cx43 in mice with a partial sciatic nerve ligation reduced spinal IL-6 and COX-2 expression. Suppression of glycogen synthase kinase-3β (GSK-3β), a serine/threonine protein kinase, prevented upregulation of IL-6 and COX-2 expression induced by Cx43 downregulation in both cultured astrocytes and in mouse spinal dorsal horn. Inhibition of spinal GSK-3β also ameliorated Cx43 siRNA-induced mechanical hypersensitivity. The current findings indicate that downregulation of spinal astrocytic Cx43 leads to changes in spinal expression of pronociceptive molecules underlying the maintenance of pain following nerve injury.