Repeated stress impairs pain-induced GABA release, leading to increased glutamate release and NMDA-receptor-mediated overactivation on spinal nociceptive neurons, which enhances inflammatory hyperalgesia. Therapeutic strategies aimed at reducing a GABA/glutamate imbalance might be instrumental in management of chronic pain syndromes associated with high levels of anxiety and/or stress.
GABA and glutamate are both affected by stress and are involved in nociception. Thus, we determined whether stress-induced enhancement of inflammatory hyperalgesia is mediated by an imbalance between glutamate and GABA neurotransmission. Male rats were subjected daily to 10 to 20 minutes per day of either forced swimming (FS) or sham swimming for 3 consecutive days; nonconditioned rats served as controls. Some rats were treated i.p. with ketamine (5 mg/kg), diazepam (2 mg/kg), flumazenil (0.1 mg/kg), or vehicle (0.9% NaCl), 30 to 60 minutes before each conditioning session or nociception assessment. Pain behavior, spinal nociceptive neuronal activation and GABA and glutamate release were respectively evaluated by the formalin test, the expression of c-Fos and in vivo microdialysis of superficial laminae of the lumbar spinal cord, 48 hours after the last conditioning session. Nitric oxide metabolites (NOx) were determined as markers of post-synaptic NMDA receptor activation. FS stress enhanced formalin-induced hyperalgesia, increased pain-elicited c-Fos expression, decreased basal and delayed pain-induced GABA release, and increased basal and induced glutamate release. Hyperalgesia and c-Fos overexpression were blocked only by prestress treatment with diazepam and post-stress treatment with ketamine, whereas changes in GABA and glutamate release were reversed by prestress treatment with diazepam. Diazepam effects were blocked by flumazenil. NOx increased in lumbar spinal cord of FS rats by a mechanism antagonized by ketamine. Thus, stress-induced hyperalgesia is initiated by a decreased and delayed GABA release and GABA-A receptor activation, whereas it is maintained by increased glutamate release and NMDA glutamate receptor activation at the spinal level.