NMDA receptor blockade in the basolateral amygdala disrupts consolidation of stimulus-reward memory and extinction learning during reinstatement of cocaine-seeking in an animal model of relapse

Abstract

Previous research from our laboratory has implicated the basolateral amygdala (BLA) complex in the acquisition and consolidation of cue-cocaine associations, as well as extinction learning, which may regulate the long-lasting control of conditioned stimuli (CS) over drug-seeking behavior. Given the well established role of NMDA glutamate receptor activation in other forms of amygdalar-based learning, we predicted that BLA-mediated drug-cue associative learning would be NMDA receptor dependent. To test this hypothesis, male Sprague-Dawley rats self-administered i.v. cocaine (0.6 mg/kg/infusion) in the absence of explicit CS pairings (2-h sessions, 5 days), followed by a single 1-h classical conditioning (CC) session, during which they received passive infusions of cocaine discretely paired with a light+tone stimulus complex. Following additional cocaine self-administration sessions in the absence of the CS (2-h sessions, 5 days) and extinction training sessions (no cocaine or CS presentation, 2-h sessions, 7 days), the ability of the CS to reinstate cocaine-seeking on three test days was assessed. Rats received bilateral intra-BLA infusions (0.5 microl/hemisphere) of vehicle or the selective NMDA receptor antagonist, 2-amino-5-phosphonovalerate (AP-5), immediately prior to the CC session (acquisition), immediately following the CC session (consolidation), or immediately following reinstatement testing (consolidation of conditioned-cued extinction learning). AP-5 administered before or after CC attenuated subsequent CS-induced reinstatement, whereas AP-5 administered immediately following the first two reinstatement tests impaired the extinction of cocaine-seeking behavior. These results suggest that NMDA receptor-mediated mechanisms within the BLA play a crucial role in the consolidation of drug-CS associations into long-term memories that, in turn, drive cocaine-seeking during relapse.

Figure 1

Schematic representing the phases of cocaine self-administration training (SA I and II), the classical conditioning session (CC), extinction (EXT), and CS reinstatement testing (CS I, II, and III). Arrows indicate when the bilateral intra-BLA microinfusions occurred. Animals in the acquisition and consolidation groups received their microinfusions immediately prior to and following the CC session, respectively, and underwent a single CS test. Animals in the consolidation of extinction learning group all received vehicle treatment at the time of the CC session (either at acquisition or consolidation) and then received subsequent microinfusions (either AP-5 or vehicle) immediately after each of the first two reinstatement tests.

Figure 2

Coronal sections (adapted from Paxinos and Watson, 1986) with graphical representations of cannula tip placements in the BLA. Numbers to the left of the sections indicate A/P distance from bregma in mm.

Figure 3

Mean (±SEM) active (top panel) and inactive (bottom panel) lever responses for the last 2 days of self-administration (SA) phases I and II, extinction (EXT), and during the CS reinstatement test. Animals received bilateral intra-BLA infusions of vehicle or AP-5 immediately prior to the CC session (acquisition). Significant differences are indicated as compared to extinction levels (***P < 0.005; **P < 0.01) or vehicle (†P < 0.05).

Figure 4

Mean (±SEM) active (top panel) and inactive (bottom panel) lever responses for the last 2 days of self-administration (SA) phases I and II, extinction (EXT), and during the CS reinstatement test. Animals received bilateral intra-BLA infusions of vehicle or AP-5 immediately after the CC session (consolidation). Significant differences are indicated as compared to extinction levels (***P < 0.005; **P < 0.01) or vehicle (†P < 0.05).

Figure 5

LEFT: Mean (±SEM) active (top panel) and inactive (bottom panel) lever responses for the last 2 days of self-administration (SA) phases I and II and extinction (EXT). All animals received vehicle during the initial CC session. RIGHT: Mean (±SEM) active (top panel) and inactive (bottom panel) lever responses during the three CS reinstatement tests. Animals received bilateral intra-BLA infusions of vehicle or AP-5 immediately after the first and second CS tests. Significant differences are indicated as compared to CS I (*P < 0.05) or vehicle (†P < 0.05).