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

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.     

Interaction between the basolateral amygdala and dorsal hippocampus is critical for cocaine memory reconsolidation and subsequent drug context-induced cocaine-seeking behavior in rats

Contextual stimulus control over instrumental drug-seeking behavior relies on the reconsolidation of context-response-drug associative memories into long-term memory storage following retrieval-induced destabilization. According to previous studies, the basolateral amygdala (BLA) and dorsal hippocampus (DH) regulate cocaine-related memory reconsolidation; however, it is not known whether these brain regions interact or independently control this phenomenon. To investigate this question, rats were trained to lever press for cocaine reinforcement in a distinct environmental context followed by extinction training in a different context. Rats were then briefly re-exposed to the cocaine-paired context to destabilize cocaine-related memories, or they were exposed to an unpaired context. Immediately thereafter, the rats received unilateral microinfusions of anisomycin (ANI) into the BLA plus baclofen/muscimol (B/M) into the contralateral (BLA/DH disconnection) or ipsilateral DH, or they received contralateral or ipsilateral microinfusions of vehicle. They then remained in their home cages overnight or for 21 d, followed by additional extinction training and a test of cocaine-seeking behavior (nonreinforced active lever responding). BLA/DH disconnection following re-exposure to the cocaine-paired context, but not the unpaired context, impaired subsequent drug context-induced cocaine-seeking behavior relative to vehicle or ipsilateral ANI + B/M treatment. Prolonged home cage stay elicited a time-dependent increase, or incubation, of drug-context-induced cocaine-seeking behavior, and BLA/DH disconnection inhibited this incubation effect despite some recovery of cocaine-seeking behavior. Thus, the BLA and DH interact to regulate the reconsolidation of cocaine-related associative memories, thereby facilitating the ability of drug-paired contexts to trigger cocaine-seeking behavior and contributing to the incubation of cocaine-seeking behavior.     

Neuronal substrates of relapse to cocaine-seeking behavior: role of prefrontal cortex

The return to drug seeking, even after prolonged periods of abstinence, is a defining feature of cocaine addiction. The neural circuitry underlying relapse has been identified in neuropharmacological studies of experimental animals, typically rats, and supported in brain imaging studies of human addicts. Although the nucleus accumbens (NAcc), which has long been implicated in goal-directed behavior, plays a critical role in this circuit, the prefrontal cortex (PFC) appears to process the events that directly trigger relapse: exposure to acute stress, cues previously associated with the drug, and the drug itself. In this paper, we review animal models of relapse and what they have revealed about the mechanisms underlying the involvement of the NAcc and PFC in cocaine-seeking behavior. We also present electrophysiological data from PFC illustrating how the hedonic, motor, motivational, and reinforcing effects of cocaine can be analyzed at the neuronal level. Our preliminary findings suggest a role for PFC in processing information related to cocaine seeking but not the hedonic effects of the drug. Further use of this recording technology can help dissect the functions of PFC and other components of the neural circuitry underlying relapse.     

Role of matrix metalloproteinases in the acquisition and reconsolidation of cocaine-induced conditioned place preference

Persistent drug seeking/taking behavior involves the consolidation of memory. With each drug use, the memory may be reactivated and reconsolidated to maintain the original memory. During reactivation, the memory may become labile and susceptible to disruption; thus, molecules involved in plasticity should influence acquisition and/or reconsolidation. Recently, matrix metalloproteinases (MMPs) have been shown to influence neuronal plasticity, presumably by their regulation of extracellular matrix (ECM) molecules involved in synaptic reorganization during learning. We hypothesized that inhibition of MMP activity would impair the acquisition and/or reconsolidation of cocaine-conditioned place preference (CPP) in rats. Intracerebral ventricular (i.c.v.) microinjection of a broad spectrum MMP inhibitor, FN-439, prior to cocaine training suppressed acquisition of CPP and attenuated cocaine-primed reinstatement in extinguished animals. In a separate experiment, the cocaine memory was reactivated on two consecutive days with a cocaine priming injection. On these two days, artificial cerebral spinal fluid (aCSF) or FN-439 was administered either 30 min prior to or 1 min after cocaine-primed reinstatement sessions. Infusion of FN-439 partially impaired retrieval of the cocaine-associated context when given 30 min prior to cocaine. In both groups, however, FN-439 suppressed reinstatement compared with controls on the third consecutive test for cocaine-primed reinstatement, when no FN-439 was given. Control experiments demonstrated that two injections of FN-439 + cocaine given in the home cage, or of FN-439 + saline priming injections in the CPP chambers did not disrupt subsequent cocaine-primed reinstatement. These results show for the first time that (1) MMPs play a critical role in acquisition and reconsolidation of cocaine-induced CPP, and (2) rats demonstrate apparent disruption of reconsolidation by an MMP inhibitor after extinction and while they are under the influence of cocaine during reinstatement.     

d-Serine facilitates the effects of extinction to reduce cocaine-primed reinstatement of drug-seeking behavior

Male Sprague Dawley rats were allowed to self-administer cocaine (0.5 mg/kg) during 90 min sessions for a period of 15 days. On day 16, rats were either held abstinent in their home cage environment or experienced an extinction session in which the active lever had no programmed consequences. Facilitating N-methyl-d-aspartate (NMDA) receptor activity with the coagonist d-serine (100 mg/kg i.p.) before or following the extinction session significantly reduced the subsequent cocaine-primed reinstatement of drug-seeking behavior tested on day 17. d-Serine significantly reduced drug-primed reinstatement only when combined with extinction, and its effectiveness when administered following the training session suggested that an enhancement of consolidation of extinction learning had occurred. In contrast, d-serine treatment did not reduce sucrose-primed reinstatement, indicating that the beneficial effects of this adjunct pharmacotherapy with extinction training were specific to an addictive substance (cocaine) and did not generalize to a natural reward (sucrose).     

Stimulus–reinforcer relations established during training determine resistance to extinction and relapse via reinstatement

The baseline rate of a reinforced target response decreases with the availability of response‐independent sources of alternative reinforcement; however, resistance to disruption and relapse increases. Because many behavioral treatments for problem behavior include response‐dependent reinforcement of alternative behavior, the present study assessed whether response‐dependent alternative reinforcement also decreases baseline response rates but increases resistance to extinction and relapse. We reinforced target responding at equal rates across two components of a multiple schedule with pigeons. We compared resistance to extinction and relapse via reinstatement of (1) a target response trained concurrently with a reinforced alternative response in one component with (2) a target response trained either concurrently or in separate components from the alternative response across conditions. Target response rates trained alone in baseline were higher but resistance to extinction and relapse via reinstatement tests were greater after training concurrently with the alternative response. In another assessment, training target and alternative responding together, but separating them during extinction and reinstatement tests, produced equal resistance to extinction and relapse. Together, these findings are consistent with behavioral momentum theory—operant response–reinforcer relations determined baseline response rates but Pavlovian stimulus–reinforcer relations established during training determined resistance to extinction and relapse. These findings imply that reinforcing alternative behavior to treat problem behavior could initially reduce rates but increase persistence.     

Hippocampal regulation of context-dependent neuronal activity in the lateral amygdala

Pavlovian fear conditioning is a robust and enduring form of emotional learning that provides an ideal model system for studying contextual regulation of memory retrieval. After extinction the expression of fear conditional responses (CRs) is context-specific: A conditional stimulus (CS) elicits greater conditional responding outside compared with inside the extinction context. Dorsal hippocampal inactivation with muscimol attenuates context-specific CR expression. We have previously shown that CS-elicited spike firing in the lateral nucleus of the amygdala is context-specific after extinction. The present study examines whether dorsal hippocampal inactivation with muscimol disrupts context-specific firing in the lateral amygdala. We conditioned rats to two separate auditory CSs and then extinguished each CS in separate and distinct contexts. Thereafter, single-unit activity and conditional freezing were tested to one CS in both extinction contexts after saline or muscimol infusion into the dorsal hippocampus. After saline infusion, rats froze more to the CS when it was presented outside of its extinction context, but froze equally in both contexts after muscimol infusion. In parallel with the behavior, lateral nucleus neurons exhibited context-dependent firing to extinguished CSs, and hippocampal inactivation disrupted this activity pattern. These data reveal a novel role for the hippocampus in regulating the context-specific firing of lateral amygdala neurons after fear memory extinction.     

Extinction training regulates neuroadaptive responses to withdrawal from chronic cocaine self-administration

Cocaine produces multiple neuroadaptations with chronic repeated use. Many of these neuroadaptations can be reversed or normalized by extinction training during withdrawal from chronic cocaine self-administration in rats. This article reviews our past and present studies on extinction-induced modulation of the neuroadaptive response to chronic cocaine in the mesolimbic dopamine system, and the role of this modulation in addictive behavior in rats. Extinction training normalizes tyrosine hydroxylase levels in the nucleus accumbens (NAc) shell, an effect that could help ameliorate dysphoria and depression associated with withdrawal from chronic cocaine use. Extinction training also increases levels of GluR1 and GluR2/3 AMPA receptor subunits, while normalizing deficits in NR1 NMDA receptor subunits, in a manner consistent with long-term potentiation of excitatory synapses in the NAc shell. Our results suggest that extinction-induced increases in AMPA and NMDA receptors may restore deficits in cortico-accumbal neurotransmission in the NAc shell and facilitate inhibitory control over cocaine-seeking behavior. Other changes identified by gene expression profiling, including up-regulation in the AMPA receptor aggregating protein Narp, suggest that extinction training induces extensive synaptic reorganization. These studies highlight potential benefits for extinction training procedures in the treatment of drug addiction.     

Inactivation of the ventral tegmental area abolished the general excitatory influence of Pavlovian cues on instrumental performance

Pavlovian stimuli can markedly elevate instrumental responding, an effect known as Pavlovian-instrumental transfer (PIT). As the role of the ventral tegmental area (VTA) in PIT is yet unknown, we examined the effects of transient VTA inactivation by direct microinjections of a mixture of the GABA(A) and GABA(B) receptor agonists, muscimol and baclofen. Results reveal that PIT, i.e., the increase in instrumental responding during presentation of a Pavlovian stimulus, was abolished by intra-VTA microinjections of muscimol/baclofen. These data provide the first evidence that the VTA mediates Pavlovian influences on instrumental behavior.     

Post-retrieval beta-adrenergic receptor blockade: effects on extinction and reconsolidation of cocaine-cue memories

Contexts and discrete cues associated with drug-taking are often responsible for relapse among addicts. Animal models have shown that interference with the reconsolidation of drug-cue memories can reduce seeking of drugs or drug-paired stimuli. One such model is conditioned place preference (CPP) in which an animal is trained to associate a particular environment with the rewarding effects of a drug. Previous work from this laboratory has shown that intra-nucleus accumbens core infusions of a MEK inhibitor can interfere with reconsolidation of these drug-cue memories. A question that remains is whether post-retrieval drug effects on subsequent memories represent an interference with reconsolidation processes or rather a facilitation of extinction. In this experiment, we explore the effect of post-retrieval injections of propranolol, a beta-adrenergic receptor antagonist, on reconsolidation and extinction of cocaine CPP. After acquisition of cocaine CPP, animals were given post-retrieval propranolol injections once or each day during a protocol of unreinforced preference tests, until the animals showed no preference for the previously cocaine-paired environment. Following a cocaine priming injection, the animals that received daily post-test propranolol injections did not reinstate their preference for the drug-paired side. In contrast, a single post-retrieval propranolol injection followed by multiple days of unreinforced preference tests failed to blunt subsequent cocaine reinstatement of the memory. These data suggest that daily post-retrieval systemic injections of propranolol decrease the conditioned preference by interfering with reconsolidation of the memory for the association between the drug-paired side and the reinforcing effects of the drug, rather than facilitating new extinction learning.     

RESISTANCE TO EXTINCTION AND RELAPSE IN COMBINED STIMULUS CONTEXTS

Reinforcing an alternative response in the same context as a target response reduces the rate of occurrence but increases the persistence of that target response. Applied researchers who use such techniques to decrease the rate of a target problem behavior risk inadvertently increasing the persistence of the same problem behavior. Behavioral momentum theory asserts that the increased persistence is a function of the alternative reinforcement enhancing the Pavlovian relation between the target stimulus context and reinforcement. A method showing promise for reducing the persistence‐enhancing effects of alternative reinforcement is to train the alternative response in a separate stimulus context before combining with the target stimulus in extinction. The present study replicated previous findings using pigeons by showing that combining an “alternative” richer VI schedule (96 reinforcers/ hr) with a “target” leaner VI schedule (24 reinforcers/hr) reduced resistance to extinction of target responding compared with concurrent training of the alternative and target responses (totaling 120 reinforcers/hr). We also found less relapse with a reinstatement procedure following extinction with separate‐context training, supporting previous findings that training conditions similarly influence both resistance to extinction and relapse. Finally, combining the alternative stimulus context was less disruptive to target responding previously trained in the concurrent schedule, relative to combining with the target response trained alone. Overall, the present findings suggest the technique of combining stimulus contexts associated with alternative responses with those associated with target responses disrupts target responding. Furthermore, the effectiveness of this disruption is a function of training context of reinforcement for target responding, consistent with assertions of behavioral momentum theory.     

Repeated cocaine experience facilitates sucrose-reinforced operant responding in enriched and isolated rats

The purpose of the present experiment was to determine whether repeated cocaine exposure differentially affects sucrose-reinforced operant responding in rats raised in an enriched condition (EC) or an isolated condition (IC). Specifically, the performance of EC and IC rats pressing a lever for sucrose under a high fixed-ratio schedule (FR 30) prior to and after 10 days of exposure to cocaine (15 mg/kg, i.p.) or saline was compared. Regardless of rearing condition, rats repeatedly exposed to cocaine had shorter reacquisition latencies to complete a sucrose-reinforced FR 30 task than saline controls. The results suggest that cocaine exposure may have cross-sensitized both EC and IC rats to the reinforcing effects of sucrose or sucrose-associated cues, thus facilitating reacquisition of operant responding.     

Deepened extinction following compound stimulus presentation: noradrenergic modulation

Behavioral extinction is an active form of new learning involving the prediction of nonreward where reward has previously been present. The expression of extinction learning can be disrupted by the presentation of reward itself or reward-predictive stimuli (reinstatement) as well as the passage of time (spontaneous recovery) or contextual changes (renewal). The following experiments replicated the demonstration that presenting multiple previously rewarded stimuli in compound during extinction enhances extinction learning. To explore the pharmacological basis for this we next examined the effects of pharmacological treatments that either facilitated or blocked noradrenergic activity to test the hypothesis that increased noradrenergic activity at the time of extinction training would improve, whereas blockade of noradrenergic activity would impair the extinction of appetitive stimulus-reward memories. Different groups of rats were trained in a discriminative stimulus paradigm to lever-press for food reward. Once stable responding was achieved, responding was extinguished for 2 d. Prior to a third extinction session, rats received systemic administration of either saline, yohimbine (α2 antagonist), atomoxetine (norepinephrine reuptake inhibitor), or propranolol (β-receptor antagonist). Spontaneous recovery of responding to the stimuli was tested 4 wk later. Our results indicate that increasing noradrenergic activity during extinction augments extinction learning resulting in less recovery of responding at test. These results have important implications for models of relapse to drug seeking and the development of extinction-based therapies.     

Cocaine self-administration alters the relative effectiveness of multiple memory systems during extinction

Rats were trained to run a straight-alley maze for an oral cocaine or sucrose vehicle solution reward, followed by either response or latent extinction training procedures that engage neuroanatomically dissociable “habit” and “cognitive” memory systems, respectively. In the response extinction condition, rats performed a runway approach response to an empty fluid well. In the latent extinction condition, rats were placed at the empty fluid well without performing a runway approach response. Rats trained with the sucrose solution displayed normal extinction behavior in both conditions. In contrast, rats trained with the cocaine solution showed normal response extinction but impaired latent extinction. The selective impairment of latent extinction indicates that oral cocaine self-administration alters the relative effectiveness of multiple memory systems during subsequent extinction training.Understanding the psychological and neural mechanisms underlying the acquisition and extinction of drug-seeking behavior has important implications for therapies targeting drug addiction. A better understanding of the neurobiology of extinction can potentially allow for the development of treatments to produce more effective and persistent extinction learning. Dissociable hippocampus-dependent “cognitive” and dorsal striatal-dependent “habit” memory systems are engaged during the initial acquisition of learned behavior (for reviews, see Packard and Knowlton 2002; White and McDonald 2002; Squire 2004). Interestingly, recent evidence indicates that multiple memory systems can also be engaged during the new learning that occurs during behavioral extinction (Gabriele and Packard 2006). For example, the behavior of a rat trained to traverse a straight-alley runway for a food reward can be extinguished using either habit/response or cognitive/latent extinction training procedures. During response extinction, rats are allowed to perform the runway approach response to an empty food cup. In contrast, during latent extinction, rats are placed at the empty food cup without performing the runway approach response. Consistent with evidence indicating a selective role for the hippocampus in cognitive memory, neural inactivation of this brain structure impairs latent extinction and spares response extinction (Gabriele and Packard 2006). Moreover, consistent with evidence that the dorsal striatum selectively mediates habit memory (for review, see Packard and Knowlton 2002), neural inactivation of this brain region impairs response extinction and spares latent extinction (A. Gabriele and M.G. Packard, unpubl.).The transition from initial drug use to eventual addiction may involve, at least in part, the development of compulsive drug-seeking and drug-taking behaviors that are increasingly guided by dorsal striatal-dependent habit learning mechanisms (for reviews, see White 1996; Everitt et al. 2001; Everitt and Robbins 2005; Belin et al. 2008). This hypothesis raises the possibility that once “habit-like” drug-seeking behaviors are firmly acquired, the extinction of such behaviors may be differentially influenced by engaging habit and cognitive memory systems. In the present study, we examined this idea by comparing the relative effectiveness of response and latent extinction training procedures in rats trained to run a straight-alley maze for an oral cocaine reward. Consistent with criteria considered important for demonstrating drug dependence, oral cocaine self-administration produces withdrawal following forced abstinence (Barros and Miczek 1996) and additionally is resistant to reinforcer devaluation (Miles et al. 2003), indicating that this behavior becomes divorced from its consequences in a manner similar to the dorsal striatum-mediated compulsive drug-seeking behavior that may characterize addiction (for reviews, see White 1996; Everitt et al. 2001; Everitt and Robbins 2005; Belin et al. 2008).The apparatus was an elevated (86.4 cm) straight-alley maze with a black Plexiglas floor and clear Plexiglas sides (117.8 cm long, 11.4 cm wide, and 20.3 cm tall). A fluid cup (2.5-cm diameter) was located at the goal end of the maze. The maze was located in a room containing several extra-maze cues.Subjects were 32 adult male Long-Evans rats (275–300 g). Rats were individually housed on a 12:12-h light–dark cycle, with lights on from 8:00 a.m.–8:00 p.m. All animals received food ad libitum.During all behavioral procedures, water bottles were removed from home cages 24 h prior to training, and animals received 15 min/day access to water following each day''s procedures. Training began with 3 d of habituation to the solution to be used during training (cocaine–sucrose [0.1% cocaine HCl/20% sucrose in ddH₂0] or sucrose [20% in ddH₂0] alone). Each habituation day involved presentations of 0.5 mL of the solution in a novel environment consisting of a half-white, half-black box (41.9 cm long, 31.8 cm wide, 35.6 tall) with the fluid cup located in the center of the black side. The number of presentations increased with each habituation day (1, 2, and 4). Each individual presentation had a maximum time of 20 min, and rats were removed when the solution was consumed. Volume consumed and amount of time to consume the solution were recorded for each rat. Each sucrose rat was matched to a cocaine rat to ensure that there were no differences between groups in terms of volume of solution consumed prior to training. For each matched pair, the volume consumed by the rat receiving the cocaine solution during each presentation was measured, and an identical amount was made available to the matched sucrose animal. If, during any given presentation, the cocaine animal did not consume any solution, then the matched sucrose animal received 20 min in the habituation environment with no solution present.Behavioral procedures were similar to those of our previous study using food reward (Gabriele and Packard 2006). During maze training, animals received either the cocaine–sucrose solution or sucrose vehicle solution reward. On days 1–10 of solution-rewarded maze training (six trials per day), rats were placed in the start end and allowed to traverse the maze and drink the available reward solution (0.5 mL). Upon consuming the solution, rats were removed from the maze and placed in an opaque holding box adjacent to the maze for a 30-sec intertrial interval. On each trial, the latency (in seconds) to reach the fluid cup was recorded and used as the measure of task acquisition. If a rat failed to reach the fluid cup within 60 sec, it was removed for the intertrial interval and a latency of 60 sec was recorded.Twenty-four hours following the completion of training (i.e., day 11), rats were assigned to one of two extinction conditions; latent extinction (n = 18, 10 cocaine and eight sucrose) or response extinction (n = 14, seven cocaine and seven sucrose). For both the latent and response conditions, extinction training was administered over 3 d (six trials per day, 30-sec intertrial interval) with no reward solution present. In the latent extinction condition, rats were placed facing the empty fluid cup in the goal end of the maze and were confined for 60 sec by placement of a clear Plexiglas barrier 20 cm from the rear wall of the goal end of the maze. Following confinement, rats were removed from the maze and placed in the holding box for the 30-sec intertrial interval. In the response extinction condition, rats were placed into the start end of the maze as during training and allowed to run to an empty fluid cup at the goal end of the maze. Upon reaching the empty fluid cup and being allowed to discover its emptiness (or after 60 sec if the rat did not reach the reward cup), rats were removed from the maze and placed in the holding box for the 30-sec intertrial interval. Latency to reach the fluid cup was recorded and used as the measure of extinction behavior. On day 3 of extinction, 90 min following the sixth daily extinction trial, all rats were given an additional four extinction “probe” trials in which they were placed in the start end of the maze and latency to reach the empty fluid cup was recorded. These four trials allowed for an assessment of the effectiveness of each extinction procedure.Data from the runway acquisition sessions are presented in Figure 1. A two-way one-repeated-measure ANOVA (Group [cocaine vs. sucrose] × Session) comparing the latencies to reach the fluid cup during acquisition in rats that subsequently received latent extinction revealed a significant effect of Session (F_(9,16) = 61.03, P < 0.001), indicating that latency to reach the fluid cup during acquisition decreased across sessions. However, the absence of a main effect of Group (F_(1,16) = 1.94, n.s.) or interaction between Group and Session (F_(9,16) = 0.53, n.s.) indicates that rats trained to run for cocaine and sucrose acquired the task at similar rates (Fig. 1A). Similar results were observed in rats that subsequently received response extinction (Fig. 1B) in that there was a main effect of Session (F_(9,12) = 13.11, P < 0.001) but no main effect (F_(1,12) = 0.44, n.s.) or interaction (F_(9,12) = 1.50, n.s.) involving drug Group.Open in a separate windowFigure 1.Acquisition of maze runway behavior. (A) Acquisition of maze runway behavior by rats that subsequently received latent extinction. (B) Acquisition of maze runway behavior by rats that subsequently received response extinction. Mean ± SEM of latency (in seconds) to reach the solution cup over training days. For both extinction conditions, there were no group differences in the initial acquisition of runway behavior.The effects of oral cocaine self-administration on latent and response extinction are shown in Figure 2. A two-way ANOVA (Group × Extinction condition) comparing mean runway latencies (collapsed across the four probe trials) for each group revealed a significant main effect of Extinction condition (F_(1,28) = 32.440, P < 0.001), indicating that the response extinction procedures produced greater extinction of the runway response, and a significant interaction effect between Extinction condition and Group (F_(1,28) = 4.813, P < 0.05) but no effect of Group (F_(1,28) = 0.96, n.s.). Simple effects tests showed a significant effect of Group within the latent extinction condition (F_(1,16) = 5.688, P < 0.05) but not the response extinction condition (F_(1,12) = 0.663, n.s.), indicating that oral cocaine self-administration selectively impaired latent but not response extinction. Additionally, a two-way one-repeated-measure ANOVA (Group × Trial) computed on the latencies to reach the fluid cup during response extinction training revealed a main effect of Trial (F_(2,12) = 16.44, P < 0.001), but no significant main effect (F_(1,12) = 2.27, n.s.) or interaction (F_(2,12) = 0.88, n.s.) involving Group, further indicating that oral cocaine did not impair response extinction.Open in a separate windowFigure 2.Effects of oral cocaine self-administration on extinction. The effect of oral cocaine self-administration on runway latent and response extinction. Mean ± SEM latency (in seconds) to reach the fluid cup is shown over the four extinction probe trials. Oral cocaine self-administration impaired latent extinction, but did not impair response extinction.The present experiments investigated the effect of oral cocaine self-administration on response and latent extinction in a straight-alley maze. Following training, rats in the response extinction condition performed the approach response to an empty goal box, whereas rats in the latent extinction condition were placed in the goal box with no reward present. Consistent with previous studies using food reward (e.g., Seward and Levy 1949; Gabriele and Packard 2006), rats rewarded with a sucrose solution were able to extinguish the approach response following both response and latent extinction procedures. In contrast, rats rewarded with a cocaine solution displayed normal response extinction (see also Schoenbaum and Setlow 2005) but impaired latent extinction. The selective impairing effect of oral cocaine self-administration on latent extinction indicates that the drug does not impair processes that contribute to general maze behavior (e.g., motivational, motor, or sensory processes), as any such influence would also likely produce a deficit in response extinction.Previous findings indicate that latent extinction of runway behavior is hippocampus dependent, whereas response extinction is dorsal striatal dependent (Gabriele and Packard 2006; A. Gabriele and M.G. Packard, unpubl.). In view of evidence that the hippocampus and dorsal striatum mediate cognitive and habit learning mechanisms, respectively (for reviews, see Packard and Knowlton 2002; White and McDonald 2002; Squire 2004), the findings suggest that oral cocaine self-administration can affect the relative use of multiple memory systems during extinction learning. The medial prefrontal cortex and basolateral amygdala have been implicated in extinction of several forms of learned behavior, and prior cocaine exposure can impair some forms of extinction learning (Burke et al. 2006; Peters et al. 2008; Quirk and Mueller 2008). However, neural inactivation of medial prefrontal cortex or basolateral amygdala does not affect latent extinction of maze runway behavior (A. Gabriele and M.G. Packard, unpubl.), suggesting that cocaine-induced dysfunction of these structures does not account for the results observed here.One explanation of the cocaine-induced impairment of latent extinction is that the approach response acquired during task acquisition is guided by a supra-normal stimulus-response habit, thereby rendering cognitive learning mechanisms ineffectual during latent extinction training. Consistent with this possibility, drug-seeking behaviors underlying addiction may involve, at least in part, a transition from goal-directed behaviors to habitual behaviors that characterize the function of the dorsal striatal memory system (e.g., Tiffany 1990; White 1996; Packard 1999; Everitt et al. 2001; Porrino et al. 2004; Everitt and Robbins 2005; Belin et al. 2008). Indeed, recent evidence implicates the dorsal striatum in habitual drug-seeking behaviors. For example, intradorsal striatum administration of dopamine antagonists impairs cocaine seeking (Vanderschuren et al. 2005), and inactivation of the dorsal striatum attenuates drug seeking, following both abstinence and extinction (Fuchs et al. 2006; See et al. 2007). Interestingly, disconnection between the ventral and dorsolateral striatum impairs cocaine-seeking behavior (Belin and Everitt 2008), and extended cocaine use enhanced cue-selective firing in the dorsal striatum and reduced cue-selective firing in the ventral striatum in go/no go discrimination learning, indicating an accelerated shift to dorsolateral striatal control (Takahashi et al. 2007). In addition, dopamine release increases in the dorsal striatum of rats following presentation of a response-contingent cue associated with cocaine (Ito et al. 2002). Similar results from fMRI and PET studies of human cocaine addicts showed increased activation in the dorsal striatum (Garavan et al. 2000) and an increase in dopamine release within the dorsal striatum (Volkow et al. 2006) following cue-induced cravings.A second explanation of the cocaine-induced impairment in latent extinction is that drug intake during task acquisition may have affected hippocampal physiology in a manner that negatively impacted the hippocampus-dependent learning that subsequently mediates latent extinction. Consistent with this possibility, chronic cocaine exposure impairs subsequent performance of hippocampus-dependent tasks such as the Morris water maze and the win-shift radial arm maze task (Melnick et al. 2001; Quirk et al. 2001; Mendez et al. 2008). However, it should be noted that the impairments observed in the latter studies were observed following exposure to cocaine doses considerably higher than those used in the present oral self-administration study. Since the current experiments do not explicitly examine the potential neurobiological progression underlying the acquisition of runway responding, further research is necessary to determine whether the cocaine-induced impairment of latent extinction involves the interfering effect of a supra-normal response habit, or a direct impairing effect on hippocampal physiology. It should also be noted that both oral cocaine self-administration and a passive cocaine administration regimen produce results analogous to those presented here, in that they impair “cognitive” representations of rewards (Miles et al. 2003; Schoenbaum and Setlow 2005). However, the relationship between this type of cognitive reward representation (mediated by interactions between basolateral amygdala and orbitofrontal cortex) (Pickens et al. 2003) and cognitive representations in latent extinction mediated by the hippocampus (Gabriele and Packard 2006) is currently unclear.Finally, the selective impairing effect of cocaine self-administration on latent extinction may have implications for understanding the persistent ability of drug-predictive cues and contexts to compel drug-seeking behavior and relapse. Specifically, if the ability to use cognitive learning mechanisms to extinguish drug-seeking behaviors is impaired following the transition from initial to habitual and compulsive drug use, then contextual/relational cues might be expected to maintain greater control over behavior following extinction training. This in turn might suggest that incorporation of response extinction procedures into treatment strategies might provide greater therapeutic efficacy.     

Reinstatement of extinguished conditioned responses and negative stimulus valence as a pathway to return of fear in humans

The present study investigated reinstatement of conditioned responses in humans by using a differential Pavlovian conditioning procedure. Evidence for reinstatement was established in a direct (fear rating) and in an indirect measure (secondary reaction time task) of conditioning. Moreover, the amount of reinstatement in the secondary reaction time task was significantly correlated with the difference in valence between the conditioned stimulus (CS)+ and the CS-after extinction. These data provide clear evidence for reinstatement and for the role of negative stimulus valence in the return of conditioned responding after extinction.     

Extinction-induced neuroplasticity attenuates stress-induced cocaine seeking: a state-dependent learning hypothesis

Chronic drug use weakens excitatory neocortical input to the nucleus accumbens (NAc). We previously reported that extinction training, a form of inhibitory learning that progressively reduces cocaine-seeking behaviour when reward is withheld, reverses this deficit by up-regulating GluR1 and GluR2/3 subunits of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) glutamate receptors in the NAc. The level of GluR1 up-regulation is positively associated with a reduction in cocaine seeking, suggesting that extinction-induced up-regulation in AMPA receptors in the NAc opposes motivational influences that maintain cocaine seeking. This hypothesis is supported by the finding that over-expression of GluR1 and GluR2 in the NAc facilitates extinction of cocaine self-administration. Furthermore, a single extinction training session conducted during GluR1 and GluR2 over-expression strongly and selectively attenuates the ability of an environmental stressor to trigger relapse to cocaine seeking long after GluR1 and GluR2 over-expression declines. These results could suggest that excitatory input to the NAc promotes extinction learning, but only when memory is recalled under stressful situations. Recent studies indicate that both environmental stress and the frustrative stress of withholding reward during extinction of drug self-administration induce similar neurochemical events in the NAc. These neurochemical events could impose a "state-dependency" on extinction learning such that subsequent exposure to stress acts as a cue to enhance retrieval of extinction memory. Our results suggest that extinction-induced up-regulation in NAc AMPA receptors acts reciprocally to facilitate state-dependent extinction learning, as stressful situations evoke extinction memories that exert powerful inhibitory control over drug-seeking behaviour. These results may have important therapeutic implications for behaviour-based approaches aimed at treating drug addiction.     

The effects of selective ibotenate lesions of the hippocampus on conditioned inhibition and extinction

Previously, Solomon (1977) reported that aspiration lesions of the dorsal hippocampus in rabbits had no effect either on the acquisition of Pavlovian conditioned inhibition or on performance during a subsequent retardation test. The present experiment confirmed and extended these findings by showing that rats with ibotenate lesions of the complete hippocampus (the dorsal and ventral hippocampus and the dentate gyrus) were also unimpaired on the same types of tasks. Additional tests with the same rats showed that removing the hippocampus significantly impaired extinction of responding to a stimulus that had been previously trained with an appetitive unconditioned stimulus. The performance of the lesioned rats on a summation test was also marginally, but not significantly, different from that of controls. The data are discussed with reference to the idea that the hippocampus is involved with the formation of some, but not all, types of inhibitory associations.     

Extinction of drug cue reactivity in methamphetamine-dependent individuals

Conditioned responses to drug-related environmental cues (such as craving) play a critical role in relapse to drug use. Animal models demonstrate that repeated exposure to drug-associated cues in the absence of drug administration leads to the extinction of conditioned responses, but the few existing clinical trials focused on extinction of conditioned responses to drug-related cues in drug-dependent individuals show equivocal results. The current study examined drug-related cue reactivity and response extinction in a laboratory setting in methamphetamine-dependent individuals. Methamphetamine cue-elicited craving was extinguished during two sessions of repeated (3) within-session exposures to multi-modal (picture, video, and in-vivo) cues, with no evidence of spontaneous recovery between sessions. A trend was noted for a greater attenuation of response in participants with longer (4-7 day) inter-session intervals. These results indicate that extinction of drug cue conditioned responding occurs in methamphetamine-dependent individuals, offering promise for the development of extinction- based treatment strategies.     

Acute and chronic effects of cocaine on extinction-induced aggression.

Pigeons worked individually in a chamber containing a response key and a mirror. Pecking on the key was controlled by a multiple schedule in which a brief period of continuous food reinforcement alternated with a 5-minute period of extinction. Under baseline conditions, aggressive behavior (responding on the mirror) occurred at the onset of each extinction period. In Experiment I (acute drug administration), the aggressive behavior was decreased by doses of cocaine that had little or no effect on key pecking. Such food-reinforced responding was disrupted, however, by higher doses of cocaine. An attempt to mimic the disruptive drugs effects by a prefeeding manipulation was unsuccessful. In Experiment II (chronic drug administration), some tolerance developed to the disruptive effects of cocaine on the food-reinforced responding, except at the highest dose tested. There was no clear-cut indication of tolerance to the initial effect of cocaine on the aggressive behavior at any dose.     

Assessing the combined effects of resurgence and reinstatement in children diagnosed with autism spectrum disorder

Resurgence and reinstatement are laboratory models of relapse following treatments for problem behavior that arrange alternative sources of reinforcement, such as differential reinforcement of alternative behavior and noncontingent reinforcement. Resurgence models the elimination or reduction of reinforcers during treatment and reinstatement models the re‐presentation of reinforcers previously maintaining problem behavior. The present study examined individual and combined effects of resurgence and reinstatement in a translational model of treatment relapse with three children diagnosed with Autism Spectrum Disorder. We first reinforced and then extinguished an arbitrary response while providing access to a preferred toy to model a version of noncontingent reinforcement with extinction. In the following phases, we examined resurgence by removing the toy, reinstatement by presenting the training reinforcer response‐independently, and a combination of resurgence and reinstatement. Overall, relapse of target responding reliably exceeded functionally similar responses never reinforced in the experimental situation. Most importantly, relapse tended to be greater when combining resurgence and reinstatement than when assessing either alone. These findings support previous studies showing that combinations of operations can increase treatment relapse. This translational model arranging simulated problem behavior with arbitrary tasks provides a platform from which to thoroughly and systematically assess methods for understanding and improving behavioral treatments.