BACKGROUND:Reward-associated cues guide reward-seeking behaviors. These cues include conditioned stimuli (CSs), which occur following seeking actions and predict reward delivery, and discriminative stimuli (DSs), which occur response-independently and signal that a seeking action will produce reward. Metabotropic group II glutamate (mGlu2/3) receptors in the basolateral amygdala (BLA) modulate CS-guided reward seeking; however, their role in DS effects is unknown. METHODS:We developed a procedure to assess DS and CS effects on reward seeking in the same subjects within the same test session. Female and male rats self-administered sucrose where DSs signaled periods of sucrose availability (DS+) and unavailability (DS-). During DS+ trials, lever presses produced sucrose paired with a CS+. During DS- trials, lever presses produced a CS- and no sucrose. Across 14 sessions, rats learned to load up on sucrose during DS+ trials and inhibit responding during DS- trials. We then determined the effects of intra-BLA microinfusions of the mGlu2/3 receptor agonist LY379268 on cue-evoked sucrose seeking during a test where the DSs and CSs were presented response-independently, without sucrose. Before testing, rats received intra-BLA microinjections of artificial cerebrospinal fluid (aCSF) or LY379268. RESULTS:Under aCSF, only the DS+ and DS+CS+ combination triggered increases in reward-seeking behavior. The CS+ alone was ineffective. Intra-BLA LY379268 suppressed the increases in sucrose seeking triggered by the DS+ and DS+CS+ combination. CONCLUSIONS:Using a new procedure to test reward seeking induced by DSs and CSs, we show that BLA mGlu2/3 receptor activity mediates the incentive motivational effects of reward-predictive DSs.

Environmental cues guide animals towards resources vital for survival but can also drive maladaptive reward-seeking behaviours, as in gambling and eating disorders. While conditioned stimuli (CSs) are paired with reward delivery after reward-seeking actions, discriminative stimuli (DSs) signal reward availability independently of behaviour. We introduce a procedure to compare CS and DS effects on reward-seeking behaviour, in the same subjects within a single session. Female and male Sprague–Dawley rats learned to self-administer sucrose. During each session, DS+ trials signaled that lever pressing would produce sucrose paired with a CS+ , and DS- trials signaled no sucrose and a CS-. Next, in the absence of sucrose, we assessed the ability of the cues to i) reinforce lever pressing and ii) increase sucrose seeking when presented response-independently. We also assessed the effects of the mGlu2/3 receptor agonist LY379268 and d-amphetamine on cue-induced sucrose seeking. By the end of self-administration training, lever pressing peaked during DS+ trials and dropped during DS- trials. The DS+ was a conditioned reinforcer of sucrose seeking in both sexes, whereas the CS+ was more effective in males. Response-independent presentations of the DS+ invigorated sucrose seeking in both sexes, whereas the CS+ was effective only in males. LY379268 suppressed DS+ -triggered sucrose seeking in females, with no effect in males. D-amphetamine enhanced sucrose seeking non-specifically across cue conditions in males, with no effect in females. Our new trial-based procedure can be used to identify unique and similar mechanisms underlying DS and CS influences on appetitive behaviour.

The µ-opioid system is involved in the reinstatement of responding that is immediately evoked by alcohol-predictive cues. The extent of its involvement in reinstatement observed in a new model that evaluates the delayed effects of re-exposure to alcohol, however, is unclear. The current study investigated the role of µ-opioid receptors (MORs) in the delayed reinstatement of an extinguished, Pavlovian conditioned response that was evoked 24 h after alcohol re-exposure. Female and male Long-Evans rats received Pavlovian conditioning in which a conditioned stimulus (CS) was paired with the delivery of an appetitive unconditioned stimulus (US; Experiments 1, 2, 4: 15% v / v alcohol; Experiment 3: 10% w / v sucrose) that was delivered into a fluid port for oral intake. During subsequent extinction sessions, the CS was presented as before but without the US. Next, the US was delivered but without the CS. A reinstatement test was conducted 24 h later, during which the CS was presented in the absence of the US. Silencing MORs via systemic naltrexone (0.3 or 1.0 mg/kg) attenuated reinstatement of port entries elicited by an alcohol-CS, but not those elicited by a sucrose-CS. Finally, blocking MORs in the ventral hippocampus via bilateral microinfusion of D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; 2.5 or 5.0 µg/hemisphere) prevented reinstatement of port alcohol-CS port entries. These data show that MORs are involved in the delayed reinstatement of a Pavlovian conditioned response in an alcohol-specific manner. Importantly, these data illustrate, for the first time, that MORs in the ventral hippocampus are necessary for responding to an alcohol-predictive cue.

Rationale Alcohol use is reliably preceded by discrete and contextual stimuli which, through diverse learning processes, acquire the capacity to promote alcohol use and relapse to alcohol use. Objective We review contemporary extinction, renewal, reinstatement, occasion setting, and sex differences research within a conditioning framework of relapse to alcohol use to inform the development of behavioural and pharmacological therapies. Key findings Diverse learning processes and corresponding neurobiological substrates contribute to relapse to alcohol use. Results from animal models indicate that cortical, thalamic, accumbal, hypothalamic, mesolimbic, glutamatergic, opioidergic, and dopaminergic circuitries contribute to alcohol relapse through separable learning processes. Behavioural therapies could be improved by increasing the endurance and generalizability of extinction learning and should incorporate whether discrete cues and contexts influence behaviour through direct excitatory conditioning or occasion setting mechanisms. The types of learning processes that most effectively influence responding for alcohol differ in female and male rats. Conclusion Sophisticated conditioning experiments suggest that diverse learning processes are mediated by distinct neural circuits and contribute to relapse to alcohol use. These experiments also suggest that gender-specific behavioural and pharmacological interventions are a way towards efficacious therapies to prevent relapse to alcohol use.

Re-exposure to an unconditioned stimulus (US) can reinstate extinguished conditioned responding elicited by a conditioned stimulus (CS). We tested the hypothesis that the reinstatement of responding to an appetitive CS is driven by an excitatory association formed between the US and the context that the US was ingested in during US re-exposure. Male, Long-Evans rats were acclimated to drinking alcohol (15%, v / v ) in the home-cage, then trained to associate an auditory CS with an alcohol-US that was delivered into a fluid port for oral intake. During subsequent extinction sessions, the CS was presented as before, but without alcohol. After extinction, rats were re-exposed to alcohol as in training, but without the CS (alcohol re-exposure). 24 h later at test, the CS was presented as in training, but without alcohol. First, we tested the effect of extinguishing the context-alcohol association, formed during alcohol re-exposure, on reinstatement. Conducting four context extinction sessions across four days (spaced extinction) after the alcohol re-exposure session did not impact reinstatement. However, four context extinction sessions conducted across two days (massed extinction) prevented reinstatement. Next, we conducted alcohol re-exposure in a context that either differed from, or was the same as, the test context. One alcohol re-exposure session in a different context did not affect reinstatement, however, three alcohol re-exposure sessions in a different context significantly reduced reinstatement during the first CS trial. These results partially support the view that a context-US association formed during US re-exposure drives the reinstatement of responding to an appetitive, alcohol-predictive CS.

Environmental contexts that are reliably associated with the use of pharmacologically active substances are hypothesized to contribute to substance use disorders. In this review, we provide an updated summary of parallel preclinical and human studies that support this hypothesis. Research conducted in rats shows that environmental contexts that are reliably paired with drug use can renew extinguished drug-seeking behavior and amplify responding elicited by discrete, drug-predictive cues. Akin to drug-associated contexts, interoceptive drug stimuli produced by the psychopharmacological effects of drugs can also influence learning and memory processes that play a role in substance use disorders. Findings from human laboratory studies show that drug-associated contexts, including social stimuli, can have profound effects on cue reactivity, drug use, and drug-related cognitive expectancies. This translationally relevant research supports the idea that treatments for substance use disorders could be improved by considering drug-associated contexts as a factor in treatment interventions. We conclude this review with ideas for how to integrate drug-associated contexts into treatment-oriented research based on 4 approaches: pharmacology, brain stimulation, mindfulness-based relapse prevention, and cognitive behavioral group therapy. Throughout, we focus on alcohol- and tobacco-related research, which are two of the most prevalent and commonly misused drugs worldwide for which there are known treatments.

Preclinical data have shown that the excitatory metabotropic Gαq-coupled glutamate receptor, mGluR5, has a role in substance abuse and relapse. However, little is known about the contribution of mGluR5 to the expression of conditioned responding elicited by appetitive Pavlovian cues. We investigated this question in rats that were trained to associate a discrete, auditory conditioned stimulus (CS) with a fructose-glucose solution (5.5% fructose/4.5% glucose; “sugar”). In subsequent tests for the expression of conditioned responding without sugar delivery, CS-elicited fluid port entries were elevated in a context associated with sugar, relative to an equally familiar, neutral context. Inhibiting mGluR5 via systemic injections of a negative allosteric modulator (MTEP; 5 mg/kg) reduced CS port entries in both the sugar context and neutral context. Targeting MTEP microinjections (3 µg/side; 0.3 µl/min) to the nucleus accumbens (Acb) core had no effect on CS port entries at test, whereas the same manipulation in the basolateral amygdala (BLA) produced effects that were topographically dependent. Specifically, microinjecting MTEP in the posterior BLA had no effect on behavior, whereas inhibiting mGluR5 in the anterior BLA enhanced the contextual discrimination of CS port entries. These data are the first to show a role of mGluR5 in the context-dependent expression of appetitive Pavlovian conditioned responding, with a topographically defined arrangement of mGluR5 in the BLA being particularly important for context-based responding to a discrete, appetitive cue.

BACKGROUND:Animal models are critical for studying causal explanations of relapse. Using a Pavlovian conditioning procedure with alcohol, we examined relapse after extinction triggered by either re-exposure to alcohol (reinstatement) or a delay between extinction and test (spontaneous recovery).METHODS:Male, Long-Evans rats were acclimated to 15% alcohol in the home-cage using an intermittent-access 2-bottle choice procedure. Next, they received Pavlovian conditioning sessions in which an auditory-conditioned stimulus (CS; 20 second white noise; 8 trials/session; variable time 240 seconds) was paired with 15% alcohol (0.3 ml/CS; 2.4 ml/session) that was delivered into a fluid port for oral ingestion. In subsequent extinction and test sessions, CS presentations occurred as before, but without alcohol.RESULTS:In experiment 1, exposure to either alcohol or water in the fluid port following extinction reinstated CS-elicited port entries at test 24 hours later. In a follow-up study using the same procedure (experiment 2), reinstatement was more robustly stimulated by alcohol, compared to a familiar lemon-flavored liquid. In experiment 3, systemic alcohol injections (0, 0.5, or 1.0 g/kg, intraperitoneal) administered either 24 hours or 15 minutes before test did not reinstate CS-elicited alcohol-seeking. Importantly, enzymatic assays in experiment 4 revealed detectable levels of alcohol in the blood following oral alcohol intake or intraperitoneal injection, suggesting that a pharmacological effect was likely with either route of administration. Last, in experiment 5, a 23-day delay between extinction and test resulted in a robust spontaneous recovery of CS-elicited alcohol-seeking.CONCLUSIONS:The reinstatement and spontaneous recovery effects revealed herein provide evidence of viable new behavioral paradigms for testing interventions against relapse.

Wheel running, unlike typical operant behavior, generates its own automatic reinforcement that alters the control exerted by extrinsic reinforcement on wheel running. The current study investigated the implications of the automatic reinforcement of wheel running by arranging different sucrose concentrations as extrinsic reinforcement for operant wheel running in ad-lib fed and food-deprived rats. Eleven female Long Evans rats ran on fixed revolution 30 schedules that delivered a drop of sucrose solution as reinforcement. Sucrose concentration varied across values of 0%, 2.5%, 5%, 10%, and 15% sucrose (w/v). Results showed that under ad-lib feeding, only the highest concentrations increased operant wheel-running rate. By contrast, under deprivation, all concentrations of sucrose increased the rate of wheel running. Despite the differences in sucrose-reinforced operant wheel-running rates by deprivation level (ad lib vs. deprived), wheel-running rates did not differ at the highest concentrations. Prior research on operant lever pressing, a response generating low (or no) automatic reinforcement, has shown considerably higher lever-pressing rates as a function of increasing amounts of sucrose reinforcement when rats are food deprived. Together, these previous observations and the current study suggest that automatic reinforcement generated by an operant decreases the control exerted by extrinsic reinforcement. Additionally, the regulation by extrinsic reinforcement on automatically reinforcing behavior depends on the organism’s motivation or deprivation level (ad lib vs. deprived).