ACTIVATIONAL EFFECTS OF SOCIAL STRESS ON IV COCAINE SELF-ADMINISTRATION IN RATS

The objective of the current studies was to investigate how social str ess modulates IV cocaine self-administration. Specifically, an experim ental protocol was developed in rats that assessed the impact of recur rent non-injurious social confrontations with an aggressive opponent o n subsequent rate of cocaine self-administration behavior, maintained across a range of doses. Initially, the cocaine dose-effect function f or intravenous self-administration was determined, reinforcing every f ifth lever press (fixed ratio or FR 5), with each dose (0.031, 0.063, 0.125, 0.25 mg/infusion and saline) being assessed for at least 3 days . Subsequently, the rats were assigned to two groups, one group being exposed as intruders to the threats of an aggressive resident rat for 60 min, while being protected by a screen, immediately before a sessio n for IV self-administration with maximally 15 cocaine infusions. A se cond group served as control, being determined for a second cocaine do se-effect function without any social stress exposure. Additional rats performed conditioned lever pressing that was reinforced by food at a rate and pattern closely similar to that characteristic for cocaine I V self-administration (joint FI 3 min FR 5). Recurrent episodes of mil d social stress increased the rate of responding on the cocaine-reinfo rced lever. This increase is seen after rats have been exposed to the threats of an aggressive opponent, but not after social or single hous ing. These behaviorally activating effects of social stress (1) are lo ng-lasting, (2) are not subject to habituation, (3) are selective to r esponding that is reinforced by IV cocaine, do not extend to non-reinf orced lever pressing, or to food-reinforced lever pressing, (4) are mo st prominent in the time-out period during and after the cocaine infus ion, (5) do not shorten the intervals between consecutive cocaine infu sions, and (6) are most evident at lower to intermediate cocaine doses . These results suggest that social stress effectively activates the m otor routines that are involved in cocaine-seeking rather than increas ing the drug's reinforcing efficacy.