Nicotine Modifies Responding for Cocaine in a Concurrent Rodent Self-administration Model
Prevailing preclinical models of cocaine use have not resulted in an FDA-approved treatment for cocaine use disorder, potentially due to a focus on cocaine use in isolation, which may not translate well to polysubstance use in clinical populations. Clinically, nicotine has been shown to increase cocaines potency and reinforcing efficacy, but some preclinical studies suggest that non-contingent nicotine exposure is not sufficient to alter cocaine self-administration in rats; therefore, this experiment examined if the addition of nicotine to the cocaine solution would alter self-administration behavior. Male Sprague Dawley rats (N=7) were trained to self-administer cocaine (0.75mg/kg/inf), and tested on a long access, fixed ratio 1 schedule of reinforcement (6 hour sessions, unlimited inf, 5 days), for cocaine alone (0.75mg/kg/inf), followed by cocaine and nicotine (0.75mg/kg/inf cocaine+0.03mg/kg/inf nicotine). Finally, rats responded on a progressive ratio schedule for varied doses of cocaine with and without concurrent nicotine at a consistent dose (1.5, 0.75, 0.375, 0.19mg/kg/inf cocaine{+/-} 0.03mg/kg/inf nicotine). Unexpectedly, under long access conditions, rats self-administering cocaine and nicotine responded less than for cocaine alone, and did not escalate responding. However, under progressive ratio conditions, responding for cocaine and nicotine was greater than responding for cocaine alone across low and moderate cocaine doses, and decreased at high cocaine doses, indicating a leftward shift in the dose response curve. Together, these data highlight the importance of evaluating multiple outcome measures in nicotine + cocaine paradigms, and suggest that concurrent self-administration of cocaine and nicotine results in greater motivated responding than for cocaine alone.