Elevated expression of serotonin 5-HT_2A receptors in the rat ventral tegmental area enhances vulnerability to the behavioral effects of cocaine

The dopamine mesocorticoaccumbens pathway which originates in the ventral tegmental area (VTA) and projects to the nucleus accumbens and prefrontal cortex is a circuit important in mediating the actions of psychostimulants.The function of this circuit is modulated by the actions of serotonin (5-HT) at 5-HT_2A receptors (5-HT_2AR) localized to the VTA. In the present study, we tested the hypothesis that virally mediated overexpression of 5-HT_2AR in the VTA would increase cocaine-evoked locomotor activity in the absence of alterations in basal locomotor activity. A plasmid containing the gene for the 5-HT_2AR linked to a synthetic marker peptide (Flag) was created and the construct was packaged in an adeno-associated virus vector (rAAV-5-HT_2AR-Flag). This viral vector (2 μl; 10^9-10 transducing units/ml) was unilaterally infused into the VTA of male rats, while control animals received an intra-VTA infusion of Ringer's solution.Virus-pretreated rats exhibited normal spontaneous locomotor activity measured in a modified open-field apparatus at 7, 14, and 21 days following infusion. After an injection of cocaine (15 mg/kg, ip), both horizontal hyperactivity and rearing were significantly enhanced in virus-treated rats (p < 0.05). Immunohistochemical analysis confirmed expression of Flag and overexpression of the 5-HT_2AR protein. These data indicate that the vulnerability of adult male rats to hyperactivity induced by cocaine is enhanced following increased levels of expression of the 5-HT_2AR in the VTA and suggest that the 5-HT_2AR receptor in the VTA plays a role in regulation of responsiveness to cocaine.