Involvement of the dorsal raphe serotonin system in basal and punished heroin self-administration and stress-induced reinstatement in male and female rats

RATIONALE: Opioid use disorders are marked by both drug use despite negative consequences and drug use to avoid negative physiological and psychological states. The dorsal raphe nucleus (DRN) serotonin (5-HT) system modulates reward- and stress-related responses, but its role in both male and female rats remains incompletely understood. OBJECTIVE: To investigate the role of the DRN 5-HT system in heroin-motivated behaviors in male and female rats, and, in parallel, to examine the impacts of stress-induced reinstatement of heroin seeking on DRN 5-HT cell electrophysiological profiles. METHODS: To probe DRN 5-HT contributions to basal, punished, and stress-driven heroin consumption, we performed chemogenetic manipulations in transgenic Tph2-iCre Sprague-Dawley rats. Twenty-two- and 50-kHz ultrasonic vocalizations (USVs), reflecting negative and positive affective state, respectively, were recorded during basal and punished heroin self-administration and a novel dual stressor-induced reinstatement model to delineate affective profiles. In a separate cohort of conventional Sprague-Dawley rats, ex vivo electrophysiology in DRN 5-HT cells examined the impact of stress-induced reinstatement of heroin seeking on spontaneous inhibitory postsynaptic currents (sIPSCs). RESULTS: Activation of the DRN 5-HT system increased heroin consumption under basal and punished conditions but did not alter stress-induced reinstatement or affective measures. Although females showed higher active lever pressing and consumption of heroin intake during acquisition of self-administration, responses to DRN 5-HT activation did not differ by sex. Ex vivo recordings revealed elevated sIPSCs in DRN 5-HT neurons from post-reinstatement females, but not males, indicating increased inhibitory drive. Notably, chemogenetic viral expression was densest in the dorsal DRN (DRD), associated with anxiety-related behavior, whereas electrophysiological recordings targeted ventrolateral/ventral DRN (DRVL/DRV) regions linked to anxiolysis. CONCLUSIONS: These findings demonstrate a role for the DRN 5-HT system in basal and punished heroin self-administration and a link between stress-induced heroin reinstatement and enhanced GABAergic inhibition of DRN 5-HT neurons in females. Serotonergic involvement in heroin consumption and sex differences in stress-driven heroin seeking patterns may reflect differential engagement of anatomically and functionally distinct DRN subdivisions.