Cingulate Cortex Neuroadaptations in a Female Rat Model of Combined Complex Regional Pain Syndrome and Alcohol Neuropathy

Abstract

Complex Regional Pain Syndrome (CRPS) is an amplified musculoskeletal pain condition that develops following limb injury or immobilization and can persist long after the injury has healed or immobilization has ceased. Although the exact mechanisms underlying the long-lasting nature of the syndrome are unknown, CRPS is associated with central nervous system dysregulation and peripheral hyperalgesia. These symptoms are also observed in alcoholic neuropathy, leading us to hypothesize that these conditions may be pathophysiologically accretive. Additionally, people assigned female at birth appear to be more sensitive to both CRPS and alcoholic neuropathy. To investigate the biobehavioral mechanisms underlying these two conditions, we utilized a model of combined CRPS and alcoholic neuropathy in adult female rats (n=4-5/group). In order to determine the effect of circulating ovarian hormones on these pathologies, half of the animals received an ovariectomy (OVX) at the start of the experiment. To model alcoholic neuropathy, animals were fed either a Leiber-DeCarli alcohol liquid diet or a control diet for 10 weeks. CRPS was modeled in all animals via unilateral hind limb immobilization for 7 days, and mechanical hyperalgesia was measured using the von Frey test 3 days following cast removal. We found that cast immobilization and chronic alcohol both separately and additively decreased mechanical nociceptive thresholds, indicative of hyperalgesia. Interestingly, OVX had no effect on mechanical sensitivity. We next used Western blot analysis to investigate phosphoprotein levels in the cingulate cortex, a limbic region that mediates the affective components of pain. Chronic alcohol increased phosphorylation of glutamatergic receptor subunits GluR1 and NR1 as well as extracellular signal-regulated kinase (ERK) and cAMP-response element binding protein (CREB). Again, we did not observe any OVX effects on protein phosphorylation. Collectively, these findings suggest that chronic alcohol in the context of CRPS facilitates hyperalgesia via hyperexcitability in the cingulate cortex, establishing a novel therapeutic strategy for these conditions.