Effects of Environmental Enrichment on Visceral Sensitivity and Brain Connectivity in a Mouse Model of Chronic Overlapping Pain Conditions

Environmental Enrichment (EE) has been shown to reduce pain- and anxiety-like behaviors, enhance learning and memory, and induce neural plasticity. We have developed a mouse model of comorbid pain hypersensitivity (CPH: stress during preexisting orofacial pain, Complete Freund's Adjuvant (CFA) induced masseter muscle inflammation) and examined the effects of EE on behavioral and brain connectivity changes induced by this model. Eighteen female mice underwent resting-state fMRI scans and were divided into 3 groups: 6 CPH + EE, 6 CPH only, and 6 naive mice. Referred pain (visceral pain correlate) was measured as the increase in responsiveness to von Frey stimulation of the lower abdomen above baseline mechanosensitivity following inflammation and stress. CPH increased referred pain in female mice for at least 4 weeks. EE blocked the referred pain development. Mice placed in an enriched environment exhibited significantly lower insula functional connectivity to the following regions compared to CPH mice without EE: Claustrum, Caudoputamen, Central medial nucleus of the thalamus, Central amygdalar nucleus, medial part (Allen Brain Atlas nomenclature). Additionally, CPH mice with EE show a trend of increased connectivity between insula and periaqueductal gray, which is similarly seen in naive mice compared to CPH group. These results demonstrate that EE can reduce referred pain and functional connectivity of areas involved in pain and stress processing, and may be strengthening the impaired endogenous pain inhibitory system seen in comorbid pain conditions. Funding: We acknowledge the support of the Department of Neural and Pain Sciences, University of Maryland School of Dentistry.