Serotonin2B receptor blockade in the rat dorsal raphe nucleus suppresses cocaine-induced hyperlocomotion through an opposite control of mesocortical and mesoaccumbens dopamine pathways.

Serotonin receptor (5-HTR) antagonists inhibit cocaine-induced hyperlocomotion independently of changes of accumbal dopamine (DA) release. Given the tight relationship between accumbal DA activity and locomotion, and the inhibitory role of medial prefrontal cortex (mPFC) DA on subcortical DA neurotransmission and DA-dependent behaviors, it has been suggested that the suppressive effect of 5-HTR antagonists on cocaine-induced hyperlocomotion may result from an activation of mPFC DA outflow which would subsequently inhibit accumbal DA neurotransmission. Here, we tested this hypothesis by means of the two selective 5-HTR antagonists, RS 127445 and LY 266097, using a combination of neurochemical, behavioral and cellular approaches in male rats. The intraperitoneal (i.p.) administration of RS 127445 (0.16 mg/kg) or LY 266097 (0.63 mg/kg) potentiated cocaine (10 mg/kg, i.p.)-induced mPFC DA outflow. The suppressant effect of RS 127445 on cocaine-induced hyperlocomotion was no longer observed in rats with local 6-OHDA lesions in the mPFC. Also, RS 127445 blocked cocaine-induced changes of accumbal glycogen synthase kinase (GSK) 3β phosphorylation, a postsynaptic cellular marker of DA neurotransmission. Finally, in keeping with the location of 5-HTRs on GABAergic interneurons in the dorsal raphe nucleus (DRN), the intra-DRN perfusion of the GABAR antagonist bicuculline (100 μM) prevented the effect of the systemic or local (1 μM, intra-DRN) administration of RS 127445 on cocaine-induced mPFC DA outflow. Likewise, intra-DRN bicuculline injection (0.1 μg/0.2 μl) prevented the effect of the systemic RS 127445 administration on cocaine-induced hyperlocomotion and GSK3β phosphorylation. These results show that DRN 5-HTR blockade suppresses cocaine-induced hyperlocomotion by potentiation of cocaine-induced DA outflow in the mPFC and the subsequent inhibition of accumbal DA neurotransmission.