Role Of Infralimbic Cortex In Extinction And Reinstatement Of Heroin Seeking Behavior In Rats

ObjectiveTo investigate the role and possible mechanisms of infralimbic cortex in the extinction andreinstatement of heroin seeking behavior.MethodsExp.1: Adult male SD rats were trained to heroin self-administration under a FR1schedulefor consecutive14d. On d1-5of extinction, rats underwent15-min extinction sessions andreceived bilateral microinjection of GABA receptor agonists Baclofen+Muscimol (B+M,0.3+0.03nmol/side) or vehicle into IL immediately after each extinction session. On d6-12ofextinction, rats underwent2-h extinction sessions that were used to assess the retention ofextinction.Exp.2: Adult male SD rats were trained to heroin self-administration under a FR1schedulefor consecutive14d. Rats underwent6daily2-h extinction sessions and15min prior to eachextinction session rats received bilateral microinjection of AMPA receptor allosteric potentiatorPEPA (30ng/side) or vehicle into IL. The responses were recorded to access the effect of PEPAmicroinjection into IL on extction of heroin seeking.Exp.3: Adult male SD rats were trained to heroin self-administration under a FR1schedulefor consecutive14d. Followed by14daily2-h extinction training, cue-induced heroin seekingreinstatement was tested for2h. AMPA receptor allosteric potentiator PEPA (30ng/side) or vehiclewere microinjected bilaterally into IL15min prior to reinstatement test. Western blot was used todetect the expression of AMPA receptor GluR1subunit in IL and NAc immediately afterreinstatement test.Exp.4: Adult male SD rats were trained to heroin self-administration under a FR1schedulefor consecutive14d. Followed by14daily2-h extinction training, cue-induced heroin seekingreinstatement was tested for2h. β2-AR agonists Clenbuterol (8ng/side) or vehicle weremicroinjected bilaterally into IL15min prior to reinstatement test. Western blot was used to detectthe expression of β2-AR and p-CREB in PL, IL, NAcore and NAshell immediately afterreinstatement test.ResultsExp.1: For the2-h extinction sessions on days6-12, a two-way repeated-measures ANOVAfound a significant effect of time (F(6,78)=13.82, P <0.01) and a significant interaction (F(6,78)=4.365, P <0.05) but no effect of group (P>0.05). Post-hoc LSD test indicated that those rats that received B+M had significantly higher active pokes on day6of extinction compared with vehiclecontrols (P <0.05).Exp.2: A two-way repeated-measures ANOVA found a significant effect of time (F(5,100)=5.311, P <0.05), a significant effect of group (F(1,90)=4.646, P <0.05) and a significantinteraction (F(5,90)=4.556, P <0.01).Exp.3: One-way ANONA found that those rats that received PEPA had significantly loweractive pokes in cue-induced reinstatement (P <0.05). And Western blot showed that PEPAmicroinjection into IL could significantly up-regulate the GluR1subunit expression in IL and NAc(P <0.01).Exp.4: One-way ANONA found that those rats that received Clenbuterol had significantlylower active pokes in cue-induced reinstatement (P <0.05). And Western blot showed thatClenbuterol microinjection into IL could significantly down-regulate the β2-AR expression inNAshell (P <0.01) and the p-CREB expression in IL and NAshell (P <0.05, P <0.01) butsignificantly up-regulate the p-CREB expression in NAcore (P <0.01).ConclusionsThe present study provides the evidence that the reversible inactivation of IL impairs theretention of extinction. Meanwhile, microinjection of PEPA into IL facilitates the extinctionresponses of heroin seeking. Besides these, both PEPA and Clenbuterol microinjections into IL caninhibit the responses during cue-induced reinstatement. And the regulation of AMPA receptorGluR1subunit, β2-AR and p-CREB expression in IL and NAc may contribute to the behaviorreduction in heroin seeking reinstatement.