Rationale and objectives: Footshock stress reliably reinstates heroin seeking in rats, but the time course of the development of this effect following drug withdrawal is not known. Here we studied the effect of intermittent footshock stress on reinstatement of heroin seeking following different withdrawal periods (1-66 days). We also studied whether changes in corticotropin-releasing factor (CRF) mRNA in the central nucleus of the amygdala (CeA) and the bed nucleus of the stria terminalis (BNST) are correlated with this reinstatement after 1 day and 6 days of heroin withdrawal.
Methods: Rats were trained to self-administer heroin (9 h/day; 0.1 mg/kg per infusion) for 10 days. Tests for extinction behavior and footshock-induced reinstatement of heroin seeking were then conducted after 1, 6, 12, 25, or 66 days of heroin withdrawal. On the test day, rats were given five to ten 60-min extinction sessions until they reached the extinction criterion of less than 15 responses per 60 min on the lever previously associated with heroin. Rats were then exposed to intermittent foot-shock (0.8 mA; 10 min), and lever-pressing behavior was recorded for 120 min.
Results: Reinstatement of lever-pressing behavior by footshock followed an inverted U-shaped curve with maximal responding after 6 days and 12 days of heroin withdrawal. Surprisingly, foot-shock did not reinstate lever-pressing behavior on day 1 of withdrawal. Lever pressing during extinction, prior to exposure to footshock, also followed an inverted U-shaped curve, with higher responding after 6, 12, and 25 days of heroin withdrawal. Finally, compared with control groups not exposed to shock, CRF mRNA levels in response to footshock were increased in the CeA (day 1 of withdrawal) and the dorsal BNST (day 1 and day 6), but not in the ventral BNST.
Conclusions: The duration of the heroin withdrawal period is an important factor in the manifestation of (1) footshock stress-induced reinstatement of heroin seeking and (2) extinction of the heroin-reinforced behavior. Finally, the time-dependent changes in footshock stress-induced reinstatement following withdrawal from heroin were not correlated with alterations in CRF mRNA in the CeA and BNST.