Advanced Age and Sex Modulate Multiple Mu Opioid Receptor Signaling Mechanisms in the Rat Midbrain Periaqueductal Gray: Implications for Analgesia

Chronic pain is exceedingly prevalent in individuals over 65 years of age. Still, it is under-managed due in large part to a dearth of knowledge regarding the suitable dosing of opioids for chronic pain management. We have shown that advanced age and sex alter morphine modulation of persistent inflammatory pain (induced by intraplantar Complete Freund's adjuvant (CFA)). Specifically, morphine potency was highest in adult male rats (2mos), with a 2-fold rightward shift in the dose-response curve for aged males (18mos) and females regardless of age. In this study, we tested the hypothesis that advanced age attenuates μ-opioid receptor (MOR) signaling in the ventrolateral periaqueductal gray (vlPAG), a critical region for the modulation of pain. We find that aged males exhibit reduced vlPAG MOR expression (Bmax) compared to adult males, and both aged males and aged females exhibit reduced vlPAG MOR affinity (Kd) compared to adult males. Further, aged males and females, regardless of age, show reduced vlPAG MOR binding potential and reduced G-protein activation efficiency compared to adult males. Western blots of phosphorylated MOR at serine 375 revealed no age or sex differences in MOR phosphorylation. DAMGO-induced cAMP inhibition was 2-fold higher in adult males than aged males or females. The regulation of opioid-induced G-protein signaling was further assessed using RNAscope to analyze mRNA expression of Regulator of G-Protein Signaling (RGS) family members RGS4 and RGS9-2. We found increased expression of RGS4 and RGS9-2 in the vlPAG of aged animals compared to adults, indicating that MOR signaling is subjected to greater negative regulation in the aged vlPAG. The observed age-related reductions in vlPAG MOR expression, MOR binding potential, G-protein activation, and cAMP inhibition, along with the observed increase in vlPAG RGS4 and RGS9-2, have significant implications in pain management in the aged population. Grant support from NIH R01DA041529 (AZM), NIH R01DA052340 (JS) NIH P50MH100023 (LJY).