Fentanyl demand and seeking in female rats: Role of the orexin system and estrous cycle

The orexin system plays a major role in drug reward. Orexin-1 receptor (OxR1) blockade reduces fentanyl demand in males. However, there are a number of sex differences in the orexin system, and it is unclear how OxR1 antagonism would decrease fentanyl demand in females. Furthermore, the relationships between the estrous cycle and fentanyl intake are yet to be delineated. Here, we conducted a behavioral economics (BE) procedure in female rats to assess the effects of the OxR1 antagonist SB-334867 on fentanyl demand before and after short-(ShA), long- (LgA) or intermittent-access (IntA) self-administration of fentanyl; we also tested the effect of SB-334867 on cued reinstatement of fentanyl seeking. Finally, we measured the impact of the estrous cycle on fentanyl demand, intake and seeking. Results showed that in females SB-334867 did not consistently modulate demand for fentanyl before or after chronic access periods. However, SB-334867 at 30mg/kg reduced the number of active lever presses during cued-reinstatement of fentanyl seeking. We also found that fentanyl self-administration disrupted estrous cyclicity, in particular proestrus epochs, an effect that depended on short versus chronic access. Furthermore, extended access to fentanyl shifted the role of progesterone from facilitation of fentanyl demand during short periods, to attenuation of fentanyl demand after long term exposure. These results indicate that an orexin-based therapy in women for treating opioid use disorder must consider prior drug history as well as cycle phase.