Abstract
RationaleStudies in human and non-human primates demonstrate that social status is an important determinant of cocaine reinforcement. However, it is unclear whether social rank is associated with other traits that also predispose to addiction and whether social status similarly predicts cocaine self-administration in rats.ObjectivesThe objective of this study is to investigate whether social ranking assessed using a resource competition task affects (i) the acquisition, maintenance and reinstatement of cocaine self-administration; (ii) the dopaminergic markers in the striatum; and (iii) the expression of ancillary traits for addiction.MethodsSocial ranking was determined in group-housed rats based upon drinking times during competition for a highly palatable liquid. Rats were then evaluated for cocaine self-administration and cue-induced drug reinstatement or individual levels of impulsivity, anxiety and novelty-induced locomotor activity. Finally, dopamine content, dopamine transporter (DAT) and dopamine D2/D3 (D2/3) receptor binding were measured postmortem in the dorsal and ventral striatum.ResultsRats deemed socially dominant showed enhanced novelty reactivity but were neither more impulsive nor anxious compared with subordinate rats. Dominant rats additionally maintained higher rates of cocaine self-administration but showed no differences in the acquisition, extinction and reinstatement of this behaviour. D2/3 binding was elevated in the nucleus accumbens shell and dorsal striatum of dominant rats when compared to subordinate rats, and was accompanied by elevated DAT and reduced dopamine content in the nucleus accumbens shell.ConclusionsThese findings show that social hierarchy influences the rate of self-administered cocaine but not anxiety or impulsivity in rats. Similar to non-human primates, these effects may be mediated by striatal dopaminergic systems.Electronic supplementary materialThe online version of this article (doi:10.1007/s00213-015-4122-8) contains supplementary material, which is available to authorized users.
Highlights
Social dominance reflects the tendency of an individual to consistently strive for and achieve desirable outcomes within social encounters (Drews 1993)
D2/3 binding was elevated in the nucleus accumbens shell and dorsal striatum of dominant rats when compared to subordinate rats, and was accompanied by elevated dopamine transporter (DAT) and reduced dopamine content in the nucleus accumbens shell
These findings show that social hierarchy influences the rate of self-administered cocaine but not anxiety or impulsivity in rats
Summary
Social dominance reflects the tendency of an individual to consistently strive for and achieve desirable outcomes within social encounters (Drews 1993). Social defeat stress in rats enhances the acquisition of cocaine self-administration and increases the motivation to seek and take this drug under a progressive ratio schedule of reinforcement whilst having no effect on ‘dominant’ resident animals (Covington and Miczek 2001, 2005) Consistent with these findings, socially recessive cynomolgus monkeys readily acquire intravenous cocaine self-administration, distinct from dominant monkeys (Morgan et al 2002). Subordinate, psychostimulant vulnerable monkeys show an increased behavioural reaction to novel objects (Czoty et al 2010; Riddick et al 2009), an observation that accords with evidence in rats that high ambulatory activity in a novel environment (the ‘high-responder’ or HR phenotype) predicts an increased propensity to selfadminister psychostimulant drugs (Davis et al 2008; Piazza et al 1989) It is unclear whether social ranking is linked in any way to enhanced novelty-related behaviour in rats. This is a relevant question to ask as cocaine selfadministration in HR rats is modulated by social stress (Kabbaj et al 2001)
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