Circadian- and sex-dependent increases in intravenous cocaine self-administration in Npas2 mutant mice

Substance use disorder is associated with disruptions in circadian rhythms. The circadian transcription factor neuronal PAS domain protein 2 (NPAS2) is enriched in reward-related brain regions and regulates reward, but its role in substance use is unclear. To examine the role of NPAS2 in drug taking, we measured intravenous cocaine self-administration (acquisition, dose-response, progressive ratio, extinction, cue-induced reinstatement) in wild-type (WT) and Npas2 mutant mice at different times of day. In the light (inactive) phase, cocaine reinforcement was increased in all Npas2 mutants, while self-administration and motivation were affected sex-dependently. These sex differences were amplified during the dark (active) phase with Npas2 mutation increasing self-administration, reinforcement, motivation, extinction responding and reinstatement in females, but only reinforcement in males. To determine whether circulating hormones are driving these sex differences, we ovariectomized WT and Npas2 mutant females and confirmed that unlike sham controls, ovariectomized mutant mice showed no increase in self-administration. To identify whether striatal brain regions are activated in Npas2 mutant females, we measured cocaine-induced ΔFosB expression. Relative to WT, ΔFosB expression was increased in D1+ neurons in the nucleus accumbens core and dorsolateral striatum in Npas2 mutant females after dark phase self-administration. We also identified potential target genes that may underlie the behavioral responses to cocaine in Npas2 mutant females. These results suggest NPAS2 regulates reward and activity in specific striatal regions in a sex and time of day specific manner. Striatal activation could be augmented by circulating sex hormones, leading to an increased effect of Npas2 mutation in females.Significance StatementCircadian disruptions are a common symptom of substance use disorders and chronic exposure to drugs of abuse alters circadian rhythms, which may contribute to subsequent substance use. Diurnal rhythms are commonly found in behavioral responses to drugs of abuse with drug sensitivity and motivation peaking during the dark (active) phase in nocturnal rodents. Emerging evidence links disrupted circadian genes to substance use vulnerability and drug-induced alterations to these genes may augment drug-seeking. The circadian transcription factor NPAS2 is enriched in reward-related brain regions and regulates reward, but its role in substance use is unclear. To examine the role of NPAS2 in drug taking, we measured intravenous cocaine self-administration in wild-type and Npas2 mutant mice at different times of day.