Modulation of Spinal Mu-Opioid Receptor Expression by Selective Adenosine A1 and A3 Receptor Agonists and Allopurinol in a Rat Model of Neuropathic Pain

Background and Objectives: Neuropathic pain presents a significant therapeutic challenge, often due to resistance to opioids. This study examined how selective adenosine receptor agonists and allopurinol modulate spinal mu-opioid receptor (MOR) mRNA expression in a rat model of neuropathic pain, aiming to develop a novel strategy for restoring opioid homeostasis. Materials and Methods: Male Sprague-Dawley rats were subjected to L5 spinal nerve ligation (SNL). The animals received treatment with selective A1AR (CCPA, 1 mg/kg) or A3AR (IB-MECA, 1 mg/kg) agonists, or allopurinol (10 or 50 mg/kg) for three days. Spinal MOR mRNA expression was measured on day 7 post-SNL using qRT-PCR. Results: SNL resulted in a non-significant upward trend in MOR transcription. Selective agonists (CCPA and IB-MECA) appeared to enhance or maintain MOR levels compared to the vehicle-treated group, but the most pronounced trend toward recovery of MOR mRNA expression was observed with high-dose allopurinol (50 mg/kg). Effect size analysis revealed large Cohen’s d values for high-dose allopurinol (d = 1.05) and CCPA (d = 0.90) versus sham, supporting the biological relevance of these preliminary trends. Conclusions: These preliminary findings suggest that augmenting endogenous adenosine bioavailability with allopurinol may be more effective than targeting individual receptor subtypes in modulating the spinal opioid system. As this is an exploratory pilot investigation, these results should be interpreted as hypothesis-generating rather than confirmatory. Since allopurinol is already a clinically established drug, these findings offer a mechanistic rationale warranting further investigation of its potential as an adjuvant for opioid resistance in chronic pain management.