Enhancement of in Vivo Dopamine Efflux in the Rat Nucleus Accumbens by Orexin Ox2 Receptor Blockade Is Reduced in Two Chronic Pain Models

Background Orexin-A and -B are neuropeptides implicated in food consumption, energy homeostasis and sleep-wake regulation. These neuropeptides act through two pharmacologically distinct orexin receptor (-R) subtypes: OX1 and OX2-Rs (Marcus et al., 2001). While orexin-A exhibits comparable affinities for both OX1 and OX2-Rs, orexin-B has a lower affinity for OX1-Rs than for OX2-Rs (Sakurai et al., 1998). Studies in experimental animals suggest that activation of orexin neurons originating in the lateral hypothalamus can reduce pain transmission (Fakhoury et al., 2020). Aims & Objectives We previously demonstrated, using isolated C-fibre-like neurons, that chronic pain may reduce orexinergic neural activity (Ishikawa et al., 2017; Yamaguchi et al., 2020). Furthermore, we showed that the nucleus accumbens (NAc), a major terminal area of the mesolimbic dopaminergic system, contains OX2-Rs that inhibit basal dopamine (DA) efflux (Kawashima et al., 2022). To elucidate how chronic pain affects orexin-R-mediated changes in accumbal dopaminergic neural activity, we analyzed the effects of intra-accumbal infusion of orexin-R ligands on accumbal DA efflux in rats using in vivo microdialysis. Method Male Sprague-Dawley rats were used. Chronic pain was induced experimentally using two models: inflammatory pain via intra-plantar injection of the proinflammatory compound carrageenan into the hind paws, and neuropathic pain via sciatic nerve ligation. DA levels in accumbal perfusates, collected every 5 minutes, were quantified by HPLC-ECD. Doses of compounds infused into NAc are expressed as total amounts (ng) over a 60-minute infusion period. Results Decreased paw withdrawal thresholds following carrageenan treatment or sciatic nerve ligation were inhibited by morphine (1.0 mg/kg, s.c.). However, meloxicam (5.8 mg/kg, i.p.) reversed these changes in carrageenan-treated rats but not in those with sciatic nerve ligation. Neither carrageenan injection nor sciatic nerve ligation altered basal accumbal DA efflux. The increase in accumbal dopamine (DA) efflux induced by the OX1- and OX2-R antagonist MK-4305 (50 ng) and the OX2-R antagonist EMPA (90 ng) observed in carrageenan-treated and sciatic nerve-ligated rats was reduced compared to their respective vehicle-treated or sham-operated controls. The OX2-R agonist orexin-B (5 ng), which did not alter basal DA levels on its own, counteracted the EMPA-induced increase in DA efflux in both pain models. Discussion & Conclusions The behavioral effects of meloxicam and morphine support our earlier findings that, in rats, carrageenan injection and sciatic nerve ligation induce inflammatory and neuropathic pain, respectively (Ishikawa et al., 2017; Yamaguchi et al., 2020). The effects of EMPA were counteracted by co-administration of orexin-B (Kawashima et al., 2022), suggesting that DA efflux in NAc is under inhibitory regulation by endogenous orexin agonists via OX2-Rs. Moreover, selective OX2-R antagonists enhanced accumbal DA efflux through disinhibition. These findings indicate that the reduced effects of EMPA on extracellular DA levels in this study reflect reductions in orexinergic input onto OX2-Rs during experimentally induced inflammatory and neuropathic pain, which inhibit DA release from accumbal nerve terminals.