Anxiogenic-like effects limit rewarding effects of cocaine in balb/cbyj mice.

Previous studies have reported intravenous cocaine self-administration behavior in several strains of mice with the exception of BALB/cByJ, a strain considered a mouse model of high emotional reactivity. The present experiments further investigated acquisition of self-administration in BALB/cByJ mice using a low dose and a habituation session. Following evidence of an initial drug-seeking behavior, we observed a progressive decline of intravenous self-administration. Pretreatment with diazepam (0.5 mg/kg, IP), reinstated cocaine-maintained responding. To test the hypothesis that injections directly into a reward-relevant brain region might support consistent cocaine-seeking behavior, BALB/cByJ mice implanted in the nucleus accumbens (NAc) or the caudate-putamen nucleus (CPu) were trained to discriminate between the arm enabling a microinjection of cocaine (30 pmol/50 nl or 150 pmol/50 nl) and the neutral arm of a Y-maze. Only NAc subjects exhibited a spatial discrimination toward the cocaine-reinforced arm and the D2 antagonist, sulpiride (50 mg/kg, IP) eliminated intra-NAc cocaine self-administration. However, after several days of cocaine self-injection, animals developed an approach/avoidance-like behavior between the start box and the reinforced arm. This behavior was suppressed by systemic diazepam (1 mg/kg, IP) pretreatment. We conclude that: (1) medio-ventral NAc is involved both in the rewarding (via a D2 dopaminergic mechanism) and aversive effects of cocaine in mice; and (2) anxiolytic pretreatment (diazepam) indirectly enhanced the reinforcing properties of cocaine in BALB/cByJ, suggesting that emotionality can act as a protective mechanism against stimulant abuse.