Functional Status of the Serotonin 5-HT 2C Receptor (5-HT 2C R) Drives Interlocked Phenotypes that Precipitate Relapse-Like Behaviors in Cocaine Dependence

Relapse vulnerability in cocaine dependence is rooted in genetic and environmental determinants, and propelled by both impulsivity and the responsivity to cocaine-linked cues (‘cue reactivity’). The serotonin (5-hydroxytryptamine, 5-HT) 5-HT 2C receptor (5-HT 2C R) within the medial prefrontal cortex (mPFC) is uniquely poised to serve as a strategic nexus to mechanistically control these behaviors. The 5-HT 2C R functional capacity is regulated by a number of factors including availability of active membrane receptor pools, the composition of the 5-HT 2C R macromolecular protein complex, and editing of the 5-HT 2C R pre-mRNA. The one-choice serial reaction time (1-CSRT) task was used to identify impulsive action phenotypes in an outbred rat population before cocaine self-administration and assessment of cue reactivity in the form of lever presses reinforced by the cocaine-associated discrete cue complex during forced abstinence. The 1-CSRT task reliably and reproducibly identified high impulsive (HI) and low impulsive (LI) action phenotypes; HI action predicted high cue reactivity. Lower cortical 5-HT 2C R membrane protein levels concomitant with higher levels of 5-HT 2C R:postsynaptic density 95 complex distinguished HI rats from LI rats. The frequency of edited 5-HT 2C R mRNA variants was elevated with the prediction that the protein population in HI rats favors those isoforms linked to reduced signaling capacity. Genetic loss of the mPFC 5-HT 2C R induced aggregate impulsive action/cue reactivity, suggesting that depressed cortical 5-HT 2C R tone confers vulnerability to these interlocked behaviors. Thus, impulsive action and cue reactivity appear to neuromechanistically overlap in rodents, with the 5-HT 2C R functional status acting as a neural rheostat to regulate, in part, the intersection between these vulnerability behaviors.