Stereotyped and complex motor routines expressed during cocaine self-administration: results from a 24-h binge of unlimited cocaine access in rats

Rationale Cocaine binges represent the most severe form of cocaine taking due to high levels of cocaine consumed and a potential loss of self-control over cocaine taking. Experimentally, regulation of intravenous cocaine self-administration is observed during binges as revealed by nearly constant sequential inter-infusion intervals, suggesting that pharmacological and unconditioned behavioral effects of cumulative cocaine leave intact its reinforcing effect. Objective To elucidate factors that contribute to the patterning of cocaine self- administration behavior during binges, the current study quantifies the expression of specific motor routines that emerge from the beginning of a cocaine infusion until the next cocaine reinforcement is received, in six separate rats over 24 h of continuous access. Materials and methods During each 24-h cocaine binge, behavior was continuously monitored using a force plate actometer that provides reliable quantitation of rodent behaviors, including rotational behavior, locomotor activity, and focused stereotypy. Results Corroborating earlier results, each rat accumulated between 4 and 11mg/kg/h during periods of active cocaine self-administration. All rats displayed a similar narrow range of unconditioned behavioral responses to cocaine, yet each rat varied with regard to the intensity and specific predominant pattern of behavioral activation. Focused stereotyped behavior, in particular, was apparent in all rats and continued for as long as cocaine self-administration behavior remained active. Conclusions The current results support the hypothesis that individual differences in cocaine's pharmacodynamics may contribute to specific behaviors expressed during cocaine self-administration, but leave unresolved whether or not intense unconditioned behavior (e.g., focused stereotypy) conflicts with, or contributes to, ongoing self-administration behavior.