Variation within the serotonin (5-HT) 5-HT₂C receptor system aligns with vulnerability to cocaine cue reactivity

Abstract

Cocaine dependence remains a challenging public health problem with relapse cited as a major determinant in its chronicity and severity. Environmental contexts and stimuli become reliably associated with its use leading to durable conditioned responses ('cue reactivity') that can predict relapse as well as treatment success. Individual variation in the magnitude and influence of cue reactivity over behavior in humans and animals suggest that cue-reactive individuals may be at greater risk for the progression to addiction and/or relapse. In the present translational study, we investigated the contribution of variation in the serotonin (5-HT) 5-HT2C receptor (5-HT2CR) system in individual differences in cocaine cue reactivity in humans and rodents. We found that cocaine-dependent subjects carrying a single nucleotide polymorphism (SNP) in the HTR2C gene that encodes for the conversion of cysteine to serine at codon 23 (Ser23 variant) exhibited significantly higher attentional bias to cocaine cues in the cocaine-word Stroop task than those carrying the Cys23 variant. In a model of individual differences in cocaine cue reactivity in rats, we identified that high cocaine cue reactivity measured as appetitive approach behavior (lever presses reinforced by the discrete cue complex) correlated with lower 5-HT2CR protein expression in the medial prefrontal cortex and blunted sensitivity to the suppressive effects of the selective 5-HT2CR agonist WAY163909. Our translational findings suggest that the functional status of the 5-HT2CR system is a mechanistic factor in the generation of vulnerability to cocaine-associated cues, an observation that opens new avenues for future development of biomarker and therapeutic approaches to suppress relapse in cocaine dependence.

Figure 1

Cocaine-dependent subjects exhibit attentional bias toward cocaine-related words on the cocaine-word Stroop task. The difference in mean reaction times between trials with cocaine-related words and those with neutral words was used as a measure of attentional bias toward cocaine-related words. (a) Cocaine-dependent subjects displayed longer reaction times (msec; mean±s.e.m.) to indicate the word color in trials with cocaine-related words vs trials with neutral words (P<0.01 vs neutral words). (b) Accuracy (ratio of correct responses to total trials on either neutral or cocaine-related word trials) did not differ between cocaine-related and neutral word trials in cocaine-dependent subjects.

Figure 2

The highest attentional bias is seen in African-American cocaine-dependent subjects with the Ser23 protein variant. Mean reaction times (msec±s.e.m.) for African-American subjects with the C/CC genotype which encodes for the Ser23 protein variant were significantly greater than that for African-American subjects with the G/GG genotype (*P<0.05 vs G/GG genotype).

Figure 3

Individual differences in appetitive approach behavior in rats are driven by discrete cocaine-associated stimuli. The levels of operant behavior within the cocaine-associated context (left) or reinforced by the discrete cue complex (right) at 24 h of withdrawal were assessed in rats stratified as high (HCR) vs low cue reactive (LCR). Previously-active lever presses did not differ between HCR (n=12) and LCR (n=12) rats upon exposure to the context in the absence of the discrete cue complex (first 10 min of session; NS). HCR rats displayed significantly higher cue-reinforced lever presses (second 60 min of session) vs LCR rats (*P<0.05 vs LCR rats).

Figure 4

High cue reactive (HCR) rats exhibit lower 5-HT_2CR protein expression in medial prefrontal cortex (mPFC) relative to low cue reactive (LCR) rats. (a) Mean total lever presses (±s.e.m.) on the previously-active and inactive levers are presented for the cue reactivity test session. Each previously-active lever press resulted in the presentation of the discrete cue complex in the absence of cocaine delivery on an FR1. Rats identified as HCR (n=5) displayed significantly higher lever presses for cocaine-associated cues vs LCR rats (n=6; *P<0.01). Inactive lever presses did not differ between HCR and LCR rats. (b) Qualitative and (c) quantitative data demonstrate phenotypic differences in mPFC 5-HT_2CR synaptosomal protein expression. HCR rats displayed lower cortical synaptosomal 5-HT_2CR protein levels relative to LCR rats (*P<0.05). (d) An inverse correlation was observed between mPFC 5-HT_2CR synaptosomal protein and responses on the previously-active lever for cocaine-associated cues in individual rats (r=0.815; P<0.01). The differential protein expression observed in HCR (0.051±0.002 arbitrary units) and LCR (0.076±0.008 arbitrary units) rats was not related to the cue reactivity test itself as comparable 5-HT_2CR mPFC protein levels were observed in HCR (n=6; 0.049±0.01 arbitrary units) and LCR rats (n=6; 0.087±0.02 arbitrary units) that remained in their home cage until sacrifice.

Figure 5

High cue reactive (HCR) rats exhibit lower sensitivity to the suppressive effects of the selective 5-HT_2CR agonist WAY163909 relative to low cue reactive (LCR) rats. Mean active lever presses (±s.e.m.) are presented for the cue reactivity test session. Each previously active lever press resulted in the presentation of the discrete cue complex in the absence of cocaine delivery on an FR1. Rats identified as HCR (n=16 rats per treatment) and LCR rats (n=16 rats per treatment) were injected with saline (15 min; 1 ml kg⁻¹, intraperitoneal) or WAY163909 (15 min; 0.5 mg kg⁻¹; intraperitoneal) prior to a cocaine cue reactivity test session on FA day 1. WAY163909 significantly attenuated lever presses for cocaine-associated cues in LCR, but not HCR, rats (*P<0.05 vs saline-treated LCR rats). FA, forced abstinence.