Age-induced changes in Mu Opioid Receptor signaling in the midbrain Periaqueductal Gray of male and female rats

The analgesic effects of opioids are attenuated in aged rats. Opioids such as morphine have decreased analgesic potency (but not efficacy) in aged rodents compared to adults; however, the neural mechanisms underlying this attenuated response are not yet known. The present study investigated the impact of advanced age and biological sex on opioid signaling in the ventrolateral periaqueductal gray (vlPAG) in the presence of chronic inflammatory pain. Assays measuring mu-opioid receptor (MOR) radioligand binding, GTP{gamma}S binding, receptor phosphorylation, cAMP inhibition, and regulator of G-protein signaling (RGS) protein expression were performed on vlPAG tissue from adult (2-3mos) and aged (16-18mos) male and female rats. Persistent inflammatory pain was induced by intraplantar injection of Complete Freunds Adjuvant (CFA). Adult males exhibited the highest MOR binding potential and the highest G-protein activation (activation efficiency ratio) in comparison to aged males and females (adult and aged). No impact of advanced age or sex on MOR phosphorylation state was observed. DAMGO-induced cAMP inhibition was highest in the vlPAG of adult males compared to aged males and females (adult and aged). vlPAG levels of RGS4 and RGS9-2, critical for terminating G-protein signaling, were assessed using RNAscope. Adult rats (both males and females) exhibited lower levels of vlPAG RGS4 and RGS9-2 mRNA expression compared to aged males and females. The observed age-related reductions in vlPAG MOR binding potential, G-protein activation efficiency, and cAMP inhibition, along with the observed age-related increases in RGS4 and RGS9-2 vlPAG expression, provide potential mechanisms whereby the potency of opioids is decreased in the aged population. These results have significant implications for pain management in this population. HighlightsO_LIAged males and females (adult and aged) exhibit reduced vlPAG -opioid receptor binding potential compared to adult males. C_LIO_LIAged males and females (adult and aged) exhibit reduced opioid-induced vlPAG G-protein activation compared to adult males. C_LIO_LIAged males and females (adult and aged) exhibit reduced vlPAG MOR mediated cAMP inhibition compared to adult males. C_LIO_LIAged rats (males and females) exhibit increased vlPAG mRNA expression of Regulator of G-Protein Signaling (RGS) proteins RGS4 and RGS9-2 compared to adult rats (males and females), which may explain the reduced receptor signaling observed in aged animals. C_LIO_LIThese coordinate decreases in opioid receptor signaling may explain the previously reported reduced potency of opioids to produce pain relief in females and aged rats. C_LI