Context and topography determine the role of basolateral amygdala metabotropic glutamate receptor 5 in appetitive Pavlovian responding

Abstract

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 contextdependent 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. HistologyStandard histological procedures [21] were used to visualize placements of the microinjectors within targeted brain regions (see supplementary materials and methods). Data analyses and availability of materialsStatistical analyses were performed using SPSS 24 (IBM, NY, USA), and included paired ttests, repeated measures ANOVA, mixed-design ANOVA, Bonferroni-corrected post-hoc comparisons, and Pearson correlations. For repeated measures ANOVA, Greenhouse-Geisser sphericity corrections were used when ε < 0.75. The non-parametric Friedman's Two-Way ANOVA was used when data violated assumptions of homogeneity of variance. The behavior we measured was entries into the fluid port during different intervals of the session. These intervals included 10 s before each CS/NS (Pre-CS/NS), the 10 s CS/NS, and the variable inter-trial interval (ITI). Conditioned responding is depicted as an elevation score, calculated by subtracting Pre-CS port entries from CS port entries [30, 31]. Each experiment was run as a single replicate. The underlying raw data and Med-PC code are available on Figshare [32]. Results CS port entries were selectively elevated in the sugar contextWe previously reported a reliable and selective elevation in port entries elicited by a CS that predicted alcohol in an alcohol context, relative to a neutral context [21]. The impact of context on port entries elicited by a CS that predicted sugar is unknown. To examine this question, rats (n=17) were t...

Comparing ABA, AAB, and ABC Renewal of Appetitive Pavlovian Conditioned Responding in Alcohol- and Sucrose-Trained Male Rats

“…In both experiments, CS+ port entries in extinction were significantly higher in context A (the conditioning context) than in context B, and this effect occurred during the initial and second extinction phases. These results replicate prior operant conditioning research (Wing and Shoaib, 2008;Todd, 2013) and concur with our published data using a different Pavlovian conditioning procedure that equated exposure to contexts A and B before test (Remedios et al, 2014;Sparks et al, 2014;Millan et al, 2015;Sciascia et al, 2015;Valyear et al, 2018;Khoo et al, 2019).…”

“…Our primary variable of interest for assessing renewal was a difference score ( CS port entries), which was calculated by subtracting pre-CS port entries from port entries made during the corresponding CS. This variable accounted for individual differences in port entry behavior across rats and has been used previously in appetitive Pavlovian conditioning experiments (Campese and Delamater, 2013;Sciascia et al, 2014;Panayi and Killcross, 2018;Khoo et al, 2019).…”

Sex differences in the amplification of responding to an alcohol-predictive cue by an alcohol-associated context

“…Male rats were trained to discriminate between a context in which a discrete auditory CS was paired with alcohol delivery and a second different context in which a different auditory stimulus was presented without alcohol delivery. We found that responding to a discrete alcohol-predictive CS was amplified in the alcohol-associated context, compared to the context in which alcohol was never consumed (Khoo et al, 2019a(Khoo et al, , 2019bMillan et al, 2015;Remedios et al, 2014;Sciascia et al, 2015;Valyear et al, 2020). This result indicates that conditioned responding to an alcohol-predictive CS is context-dependent in male rats, but the context-dependency of conditioned responding to an alcohol-predictive CS has not been assessed in females.…”

“…Next, CS-elicited port-entries were extinguished in both contexts, and then we examined the impact of context in an alcohol-primed induced reinstatement test. Based on prior results (Khoo et al, 2019a(Khoo et al, , 2019bMillan et al, 2015;Remedios et al, 2014;Sciascia et al, 2015;Valyear et al, 2020), we predicted that males would show invigoration of CS-elicited port-entries in the alcohol-associated context compared to the neutral context, during the test and alcohol-primed reinstatement test. Based on growing evidence described above that behaviour in females is not context-dependent, we predicted that responding would not differ between the alcohol-associated context and the neutral context in female rats.…”

Metabotropic glutamate receptor 5 signaling and appetitive Pavlovian behavior: implications for the treatment of addiction

“…In their recent Neuropsychopharmacology article, Khoo and colleagues [6] reported that mGluR5 signaling exerts distinct effects on context discrimination and cue-elicited behavior, depending on the brain site of action. The authors examined the impact of a negative allosteric modulator of mGluR5, 3-((2methyl-4-thiazolyl)ethynyl)pyridine (MTEP), on cue-elicited behavior after Pavlovian conditioning with an auditory cue that predicted delivery of a fructose-glucose solution.…”