Voluntary oxycodone self-administration produces analgesic tolerance and sex-dependent hyperalgesia across genetically diverse rats
Long-term opioid therapy is limited by analgesic tolerance and opioid-induced hyperalgesia, but the roles of genetic background, sex, and drug exposure remain unclear. We used 20 inbred strains from the Hybrid Rat Diversity Panel to examine thermal sensitivity, oxycodone analgesia, tolerance, and hyperalgesia-like changes following voluntary intravenous oxycodone or saline self-administration. Rats underwent tail-immersion testing before self-administration (Pre-SA) and after self-administration (Post-SA). Oxycodone analgesia was assessed using the percent maximum possible effect time course and the corresponding area under the curve. Pre-SA thermal sensitivity differed across strains and between sexes, and Pre-SA oxycodone analgesia also differed across strains. Oxycodone self-administration produced a sex-dependent increase in thermal sensitivity that was most evident in males. During Post-SA testing, oxycodone self-administering rats showed reduced analgesic responsiveness compared with saline controls, and the magnitude of this difference varied across strains. Within-strain Pre-SA-to-Post-SA comparisons identified tolerance-like reductions in several strains. Across strains and sexes, oxycodone self-administering rats showed a greater Pre-SA-to-Post-SA reduction in analgesic responsiveness than saline controls, consistent with analgesic tolerance. Total oxycodone intake was not associated with tolerance at either the strain-mean or individual-animal level. Heritability estimates were higher for thermal sensitivity and analgesia (H2 {approx} 0.28-0.40) than for changes in thermal sensitivity and tolerance (H2 {approx} 0.18-0.27). These findings demonstrate strain variation in thermal sensitivity and oxycodone analgesia, sex-dependent hyperalgesia-like effects, and reduced analgesic responsiveness following voluntary oxycodone intake.