Attenuated Dopamine Receptor Signaling in Nucleus Accumbens Core in a Rat Model of Chemically-Induced Neuropathy

Abstract

ABSTRACTNeuropathy is major source of chronic pain that can be caused by mechanically or chemically induced nerve injury. Previous work in a rat model of neuropathic pain demonstrated that bilateral formalin injection into the hind paws produced mechanical hypersensitivity (allodynia) and depressed responding for intracranial self-stimulation (ICSS). To determine whether neuropathy alters dopamine receptor responsiveness in mesolimbic brain regions, we examined dopamine D1-like and D2-like receptor (D1/2R) signaling and expression in male rats 14 days after bilateral intraplantar formalin injections into both rear paws. D2R-mediated G-protein activation and expression of the D2R long, but not short, isoform were reduced in nucleus accumbens (NAc) core, but not in NAc shell, caudate-putamen (CPu) or ventral tegmental area (VTA) of formalin-compared to saline-treated rats. In addition, D1R-stimulated adenylyl cyclase (AC) activity was also reduced in NAc core, but not in NAc shell or prefrontal cortex, of formalin-treated rats, whereas D1R expression was unaffected. Expression of other proteins involved in dopamine neurotransmission, including dopamine uptake transporter (DAT) and tyrosine hydroxylase (TH), were unaffected by formalin treatment. In behavioral tests, the effects of D2R agonists on ICSS were attenuated in formalin-treated rats, whereas the effects of D1R agonists were unchanged. These results indicate that intraplantar formalin as a model of chemically induced neuropathy produces attenuation of highly specific DA receptor signaling processes in NAc core of male rats.