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Relapse to cotinine seeking in rats: Differential effects of sex

Relapse is a defining feature of smoking and a significant challenge in cessation management. Elucidation of novel mechanisms underlying relapse may inform future treatments. Cotinine, the major metabolite of nicotine, has been shown to support intravenous self-administration in rats, suggesting it as one potential mechanism contributing to nicotine reinforcement. However, it remains unknown whether cotinine would induce relapse-like behaviors. The current study investigated relapse to cotinine seeking in two relapse models, the reinstatement of drug seeking and incubation of drug craving models. In the reinstatement model, rats were trained to self-administer cotinine, extinguished cotinine-associated responses, and underwent cue-, drug-, or stress-induced reinstatement. Conditioned cues associated with cotinine self-administration, cotinine (1-2 mg/kg), or the pharmacological stressor yohimbine (1.25-2.5 mg/kg) reinstated cotinine seeking. Female rats displayed more pronounced cue-induced, but not drug- or stress-induced reinstatement than male rats. In addition, an overall analysis revealed that female rats exhibited greater cotinine self-administration, but less extinction than male rats. In the incubation model, rats were trained to self-administer cotinine, and underwent forced withdrawal in home cages. Rats were tested for cue-induced cotinine seeking on both withdrawal day 1 and withdrawal day 18. Rats exhibited greater cotinine-seeking on withdrawal day 18 compared to withdrawal day 1, with no difference between male and female rats. These findings indicate that cotinine induces sex-dependent relapse to cotinine seeking in rats, suggesting that cotinine may be a novel mechanism contributing to relapse. These rat models are valuable preclinical tools for interrogation of neurobiological underpinnings of relapse to cotinine seeking. Significance Statement Relapse is a defining feature of smoking and a significant challenge in cessation management. Elucidation of novel mechanisms underlying relapse may inform future treatment. Cotinine, the major metabolite of nicotine, has previously been shown as a potential mechanism underlying the development of nicotine reinforcement because it supports intravenous self-administration in rats. The current study found that conditioned cues, priming injection of cotinine, and acute exposure to the pharmacological stressor yohimbine induced robust cotinine-seeking behavior in rats with a history of cotinine self-administration. In addition, significant sex effects were revealed in that female rats exhibited greater cotinine self-administration, less extinction, and more robust cue-induced reinstatement of cotinine seeking than male rats. These findings suggest that cotinine may be a novel mechanism contributing to relapse to nicotine seeking. In addition, these rat models are valuable preclinical tools for interrogation of behavioral and neurobiological underpinnings to relapse to cotinine seeking, thus forming basis for developing effective therapeutic strategy to aid in smoking cessation.