Hippocampal memory system function and the regulation of cocaine self-administration behavior in rats

There is considerable interest in elucidating neurocognitive mechanisms of cocaine addiction. This report focuses on the hippocampal memory system. Using food reward, two cognitive tasks were examined after lidocaine inactivation of the dorsal (dSUB) or ventral (vSUB) subiculum, the primary hippocampal output regions in rats. These tasks were conducted to first identify functionally relevant stereotaxic coordinates within the hippocampal memory system, in order to then examine its role in regulating drug-seeking and drug-taking behavior studied under a contextually discriminable FI 5-min(FR5:S) second-order schedule of cocaine and brief stimulus delivery. Inactivation of the dSUB and vSUB with 10microg lidocaine impaired hippocampal-dependent win-shift performance. Amygdalar-dependant conditioned cue preference, used as a test for behavioral specificity of lidocaine, was not affected following inactivation of either site. Inactivation of the dSUB with 100 microg lidocaine significantly reduced drug-seeking and drug-taking behavior studied during the cocaine self-administration maintenance phase. Following extinction, inactivation of neither the dSUB nor vSUB influenced reinstatement of drug-seeking behavior induced by drug-paired cues presented alone or with a cocaine priming injection. The impairments in win-shift performance are consistent with the spatial processing functions of the dSUB and vSUB, and the reduction in drug-taking behavior is consistent with the non-spatial temporal processing functions of the dSUB. The lack of an effect of dSUB and vSUB inactivation on reinstatement of drug-seeking behavior may relate to the fact that the contextual associations with cocaine were well-practiced at the time of cue reinstatement testing, and therefore, drug-seeking behavior was likely regulated by nonhippocampal-dependent mechanisms.