Repeated lofexidine treatment attenuates stress-induced, but not drug cues-induced reinstatement of a heroin-cocaine mixture (speedball) seeking in rats.

Alpha-2 adrenoceptor agonists (lofexidine, clonidine) are used to alleviate short-term opioid withdrawal in humans. In rats, acute injections of these agents attenuate stress-induced reinstatement of heroin and cocaine seeking at time points that are beyond the acute drug withdrawal phase. Here, we studied whether exposure to lofexidine would attenuate reinstatement of a heroin-cocaine mixture (speedball) seeking induced by exposure to stress or to drug-associated cues. Rats were trained to lever press for speedball for 10 days, and the drug-reinforced behavior was then extinguished for 11 days in the presence (Experiment 1) or the absence (Experiment 2) of the drug cues. Subsequently, rats were tested for reinstatement of drug seeking after exposure to intermittent footshock stress (5-15 min; Experiment 1) or the drug cues (Experiment 2). Starting on day 7 of training, rats received daily injections of saline or lofexidine (0.1 or 0.2 mg/kg). Repeated lofexidine treatment significantly attenuated footshock-induced reinstatement, but did not alter drug cues-induced reinstatement of speedball seeking. In addition, lofexidine did not have a consistent effect on speedball self-administration and extinction behavior. Results extend previous reports with acute drug injections, indicating that lofexidine maintains its effect on stress-induced reinstatement after repeated treatment. The present data also suggest that the neurochemical events underlying stress- and drug cues-induced relapse are not identical.