Infusion of the substance P analogue, DiMe-C7, into the ventral tegmental area induces reinstatement of cocaine-seeking behaviour in rats

Rationale: The mesocorticolimbic dopamine (DA) system is critically involved in mediating reinstatement of drug-seeking behaviour. Substance P (SP) is a neuropeptide that significantly interacts with the mesocorticolimbic system, therefore suggesting a possible role for the SP system in the mediation of relapse. Objectives: This study examined the effects of injections of the SP analogue, DiMe-C7, into the ventral tegmental area (VTA) on reinstatement of cocaine-seeking behaviour, as well as on locomotor activity in rats. Additionally, this study examined whether these effects are DA-dependent. Methods: Rats were trained to self-administer cocaine for 15 days followed by 15 days of extinction. Reinstatement of cocaine-seeking behaviour was then measured in response to bilateral intra-VTA microinjections of DiMe-C7 (0, 0.1, 0.5 and 2.5 mug). In a separate group of rats, locomotor activity was measured in response to intra-VTA injections of DiMe-C7 ( 0, 0.5, 1.5 and 3 mug). The effects of pre-treatment with DA receptor antagonists on DiMe-C7-induced reinstatement and locomotor activity were also examined. Animals were pre-treated with the D-1 and D-2 receptor antagonists, SCH23390 and haloperidol 0, 0.01 and 0.03 mg/kg, IP), respectively, prior to receiving intra-VTA injections of DiMe-C7 ( 0 and 2.5 mug). Results: Infusion of DiMe-C7 into the VTA increased locomotor activity and induced reinstatement of cocaine-seeking behaviour. Both SCH23390 and haloperidol blocked intra-VTA DiMe-C7-induced locomotor activation. In addition, SCH23390 attenuated DiMe-C7-induced reinstatement of cocaine-seeking behaviour, while haloperidol had no effect. Conclusions: These results suggest that interactions between SP and the mesocorticolimbic DA system may play a role in mediating reinstatement of cocaine-seeking behaviour and that the involvement of these interactions in reinstatement are dependent upon D-1 receptor mechanisms.