Development and validation of a novel oral oxycodone self-administration protocol for female and male rats
The increased abuse of opioids - such as oxycodone - poses major challenges for health and socioeconomic systems. Human prescription opioid abuse is marked by continuous, voluntary, oral intake, and sex differences. Therefore the field would benefit from a preclinical in-depth characterization of sex differences in a chronic oral voluntary, free choice, and continuous access paradigm. Here we show in an oral oxycodone continuous access two-bottle choice paradigm sex-dependent voluntary drug intake, dependence, and motivation to take the drug. Adult female and male Long-Evans rats were given unlimited, continuous home cage access to two bottles containing water (Control) or one bottle of water and one bottle of oxycodone dissolved in water (Experimental). Most experimental rats voluntarily drank oxycodone ([~]10 mg/kg/day) and escalated their intake over 22 weeks. Females self-administered twice as much oxycodone as males, leading to greater blood levels of oxycodone, and engaged in more gnawing behavior. Precipitated withdrawal revealed high levels of dependence in both sexes. Reflecting motivation to drink oxycodone, ascending concentration of citric acid suppressed the intake of oxycodone (Experimental) and the intake of water (Control); however Experimental rats returned to pre-citric acid preference levels whereas Controls rats did not. Thus, female rats consumed and preferred oxycodone more than males in this chronic two-bottle oral choice paradigm. Both sexes displayed many features of human oxycodone abuse, and behavioral pre-screening predicted parameters of intake and withdrawal. This model provides an additional paradigm for understanding mechanisms that mediate long-term voluntary drug use and for exploring potential treatment options.\n\nHIGHLIGHTSAdult rats offered continuous choice of oral oxycodone vs. water preferred oxycodone\n\nRats self-titrated oxycodone, yet females preferred and escalated more than males\n\nBoth sexes were motivated to drink oxycodone, as shown by a citric acid aversion test\n\nBoth sexes became dependent on oxycodone, as shown by precipitated withdrawal\n\nBehavioral prescreening predicted later aspects of oxycodone intake and dependence