The effects of social contact on cocaine intake in female rats

Abstract

Background: Studies conducted in male rats report that social contact can either facilitate or inhibit drug intake depending on the behavior of social partners. The purpose of the present study was to: (1) examine the effects of social contact on cocaine intake in female rats, (2) examine the behavioral mechanisms by which social contact influences cocaine intake, and (3) examine whether the estrous cycle moderates the effects of social contact on cocaine intake.

Methods: Female rats were assigned to either isolated or pair-housed conditions in which a social partner either had access to cocaine (cocaine partner) or did not have access to cocaine (abstinent partner). Pair-housed rats were tested in custom-built operant conditioning chambers that allowed both rats to be tested simultaneously in the same chamber.

Results: Rats housed with a cocaine partner self-administered more cocaine than isolated rats and rats housed with an abstinent partner. A behavioral economic analysis indicated that these differences were driven by a greater intensity of cocaine demand (i.e., greater intake at lower unit prices) in rats housed with a cocaine partner. Multivariate modeling revealed that the estrous cycle did not moderate the effects of social contact on cocaine intake.

Conclusions: These findings indicate that: (1) social contact influences cocaine self-administration in females in a manner similar to that reported in males, (2) these effects are due to differences in the effects of social contact on the intensity of cocaine demand, and (3) these effects are consistent across all phases of the estrous cycle.

Keywords: Behavioral economics; Demand curve; Estrous; Social environment; Social learning; Substance use.

Figure 1. Social Contact Influences Cocaine Self-Administration

Left Panel: Responding maintained by cocaine on an FR1 schedule of reinforcement. Data are shown for socially housed rats with an abstinent partner [social: abstinent partner; n = 10], isolated rats [isolated; n = 13], and socially housed rats with a cocaine partner [social: cocaine partner = 10]. Vertical axis depicts number of infusions during 2-hr session. Horizontal axis depicts dose of cocaine in mg/kg/infusion. Points above 0.0 depict the effects of saline. Vertical lines indicate SEM; where not indicated, the SEM fell within the data point. Right Panel: Area under the curve (AUC) estimates for cocaine in rats responding on an FR1 schedule of reinforcement. Vertical lines indicate SEM. Asterisks (*) indicate significant difference from socially housed rats with a cocaine partner (p < .05).

Figure 2. Social Contact Influences Intensity of Cocaine Demand

Demand curves computed from the dose-response data of socially housed rats with an abstinent partner, isolated rats, and socially housed rats with a cocaine partner. Vertical axis depicts consumption (measured as intake in mg/kg in log units). Horizontal axis depicts unit price (depicted as responses/mg/kg in log units). An exponential demand equation was fit to the data and plotted as mean values. Vertical lines surrounding data points represent the SEM; where not indicated, the SEM fell within the data point.

Figure 3. Non-reinforced Responding of Abstinent Rats Mirrors Cocaine Intake of Social Partners

Left Panel: Cocaine-reinforced responses of socially housed rats with an abstinent partner (data are redrawn from Figure 1). Center Panel: Unreinforced responses of abstinent partners without access to cocaine. Right Panel: Cocaine intake (mg/kg) of socially housed rats with an abstinent partner. Horizontal axes reflect cocaine dose condition (mg/kg/infusion) of the partner with access to cocaine. Vertical lines indicate SEM; where not indicated, the SEM fell within the data point. Polynomial contrasts revealed that the non-reinforced responding of abstinent partners followed a linear (but not quadratic) trend. Cocaine-reinforced responding of their social partners followed a quadratic (but not linear) trend, but cocaine-intake of their social partners followed a linear (but not quadratic) trend.