Role of dopamine D1-like and D2-like receptors in the activation of ingestive behaviour in thirsty rats licking for water

Licking microstructure in response to novel rewards, reward devaluation and dopamine antagonists: Possible role of D1 and D2 medium spiny neurons in the nucleus accumbens

Evidence on the effect of dopamine D1 and D2-like antagonists and of manipulations of reward value on licking microstructure is reanalysed considering recent findings on the role of nucleus accumbens (NAc) medium spiny neurons (MSNs) in the control of sugar intake. The results of this analysis suggest that D1 MSN activation, which is involved in the emission of licking bursts, might play a crucial role in response to novel rewards. D2 MSN activation, which results in reduction of burst size and suppression of licking, might mediate the response to reward devaluation. Elucidating the neural mechanisms underlying the licking response might lead to a better definition of its microstructural measures in behaviourally and psychologically meaningful functional terms. This could further support its use as a behavioural substrate in the study of the neural mechanisms of ingestive behaviour and motivation, as well as in animal models of pathological conditions such as eating disorders and obesity.

Dopamine, activation of ingestion and evaluation of response efficacy: a focus on the within-session time-course of licking burst number

RATIONALE

Evidence on the effect of dopamine D1-like and D2-like receptor antagonists on licking microstructure and the forced swimming response led us to suggest that (i) dopamine on D1-like receptors plays a role in activating reward-directed responses and (ii) the level of response activation is reboosted based on a process of evaluation of response efficacy requiring dopamine on D2-like receptors. A main piece of evidence in support of this hypothesis is the observation that the dopamine D2-like receptor antagonist raclopride induces a within-session decrement of burst number occurring after the contact with the reward. The few published studies with a detailed analysis of the time-course of this measure were conducted in our laboratory.

OBJECTIVES

The aim of this review is to recapitulate and discuss the evidence in support of the analysis of the within-session burst number as a behavioural substrate for the study of the mechanisms governing ingestion, behavioural activation and the related evaluation processes, and its relevance in the analysis of drug effects on ingestion.

CONCLUSIONS

The evidence gathered so far suggests that the analysis of the within-session time-course of burst number provides an important behavioural substrate for the study of the mechanisms governing ingestion, behavioural activation and the related evaluation processes, and might provide decisive evidence in the analysis of the effects of drugs on ingestion. However, further evidence from independent sources is necessary to validate the use and the proposed interpretation of this measure.

Does partial blockade of dopamine D2 receptors with Amisulpride cause anhedonia? An experimental study in healthy volunteers

BACKGROUND

Anhedonia is a frequent cause of functional im^pairment in psychosis. Although it is plausible that medication-induced D2 receptor blockade could diminish hedonic responding, there is little experimental research testing this hypothesis in humans.

METHODS

To inspect possible effects of partial D2 blockade on hedonic experiences, we administered 300 mg of Amisulpride or placebo to 85 participants in a randomized, double-blind, placebo-controlled trial. Participants were then subjected to an emotional evocation task utilizing standardized pictorial pleasant, neutral, and unpleasant stimuli.

RESULTS

We observed lower positivity ratings in the Amisulpride group compared to placebo across all stimulus categories (p = .026, f = 0.25) and no group differences in negativity or arousal ratings. The Amisulpride group also showed lower electrodermal responses across all stimulus categories compared to placebo (p = .017, f = 0.27). The electrodermal response was especially diminished for pleasant stimuli.

CONCLUSION

We interpret our findings as evidence that D2 blockade via Amisulpride can reduce at-the-moment hedonic responsivity in healthy volunteers. If these results can be confirmed in drug-naïve clinical samples, this would indicate that antipsychotic medication contributes to clinical anhedonia, probably via antagonistic effects at the dopamine D2 receptor.

Dorsomedial prefrontal cortex activation disrupts Pavlovian incentive motivation

The dorsomedial prefrontal cortex (dmPFC) is known to make important contributions to flexible, reward-motivated behavior. However, it remains unclear if the dmPFC is involved in regulating the expression of Pavlovian incentive motivation, the process through which reward-paired cues promote instrumental reward-seeking behavior, which is modeled in rats using the Pavlovian-instrumental transfer (PIT) task. The current study examined this question using a bidirectional chemogenetic strategy in which inhibitory (hM4Di) or excitatory (hM3Dq) designer G-protein coupled receptors were virally expressed in dmPFC neurons, allowing us to later stimulate or inhibit this region by administering CNO prior to PIT testing. We found that dmPFC inhibition did not alter the tendency for a reward-paired cue to instigate instrumental reward-seeking behavior, whereas dmPFC stimulation disrupted the expression of this motivational influence. Neither treatment altered cue-elicited anticipatory activity at the reward-delivery port, indicating that dmPFC stimulation did not lead to more widespread motor suppression. A reporter-only control experiment indicated that our CNO treatment did not have non-specific behavioral effects. Thus, the dmPFC does not mediate the expression of Pavlovian incentive motivation but instead has the capacity to exert pronounced inhibitory control over this process, suggesting that it is involved in adaptively regulating cue-motivated behavior.

Effects of nucleus accumbens insulin inactivation on microstructure of licking for glucose and saccharin in male and female rats

Insulin of pancreatic origin enters the brain where several regions express a high density of insulin receptors. Functional studies of brain insulin signaling have focused predominantly on hypothalamic regulation of appetite and hippocampal regulation of learning. Recent studies point to involvement of nucleus accumbens (NAc) insulin signaling in a diet-sensitive response to glucose intake and reinforcement of flavor-nutrient learning. The present study used NAc shell microinjection of an insulin inactivating antibody (InsAb) to evaluate effects on the microstructure of licking for flavored 6.1% glucose. In both male and female rats, InsAb had no effect on the number of lick bursts emitted (a measure of motivation and/or satiety), but decreased the size of lick bursts (a measure of reward magnitude) in a series of five 30 min test sessions. This effect persisted beyond microinjection test sessions and was shown to depend on previous flavored glucose consumption under InsAb treatment rather than InsAb treatment alone. This suggests learning of diminished reward value and aligns with the previous finding that InsAb blocks flavor-nutrient learning. Specificity of the InsAb effect for nutrient reward was indicated by failure to affect any parameter of licking for flavored 0.25% saccharin solution. Finally, maintenance of rats on a 'Western' diet for twelve weeks produced a decrease in lick burst size for glucose in male rats, but an increase in lick burst size in females. Possible implications of these results for flavor-nutrient learning, maladaptive consequences of NAc insulin receptor subsensitivity, and the plausible involvement of distinct insulin-regulated mechanisms in NAc are discussed.

Predictive and motivational factors influencing anticipatory contrast: A comparison of contextual and gustatory predictors in food restricted and free-fed rats

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Using an anticipatory negative contrast (ANC) paradigm, food restricted animals can act selectively in their eating behavior.

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Contextual and gustatory predictors in a within-subject design are sufficient for anticipatory negative contrast development.

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Changes in reward palatability may underlie contextually-driven anticipatory negative contrast.

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An increase in premature port entries to the unavailable sipper – a second measure of ANC – in all groups reveals a direct influence of response competition on ANC development.

In anticipation of palatable food, rats can learn to restrict consumption of a less rewarding food type resulting in an increased consumption of the preferred food when it is made available. This construct is known as anticipatory negative contrast (ANC) and can help elucidate the processes that underlie binge-like behavior as well as self-control in rodent motivation models. In the current investigation we aimed to shed light on the ability of distinct predictors of a preferred food choice to generate contrast effects and the motivational processes that underlie this behavior. Using a novel set of rewarding solutions, we directly compared contextual and gustatory ANC predictors in both food restricted and free-fed Sprague-Dawley rats. Our results indicate that, despite being food restricted, rats are selective in their eating behavior and show strong contextually-driven ANC similar to free-fed animals. These differences mirrored changes in palatability for the less preferred solution across the different sessions as measured by lick microstructure analysis. In contrast to previous research, predictive cues in both food restricted and free-fed rats were sufficient for ANC to develop although flavor-driven ANC did not relate to a corresponding change in lick patterning. These differences in the lick microstructure between context- and flavor-driven ANC indicate that the motivational processes underlying ANC generated by the two predictor types are distinct. Moreover, an increase in premature port entries to the unavailable sipper – a second measure of ANC – in all groups reveals a direct influence of response competition on ANC development.

Further characterization of the effect of the prototypical antidepressant imipramine on the microstructure of licking for sucrose

We previously reported that treatment with the prototypical antidepressant imipramine induced a dose-dependent reduction of the ingestion of a 10% sucrose solution, due to reduction of the licking burst number, thus suggesting reduced motivation and/or increased satiation. Importantly, the experimental sessions were performed in an alternate order, either 1-h or 24-h after imipramine administration. The observation that imipramine effect was more pronounced in the “1-h after-treatment” sessions, i.e. at the time of the brain drug C_max, led us to suggest that it was likely related to brain drug levels at testing time. However, such an experimental design does not allow to rule out the alternative possibility that the observed effect might be due to post-session administration, as previously observed with memantine. To determine whether imipramine-induced decrease of sucrose ingestion could be observed even in absence of post-session administration, we examined the effect of a daily 22 day treatment with imipramine (5, 10 and 20 mg/kg). In the first half of the treatment period all behavioural tests were performed 1-h after administration. In the second half of the treatment period, tests were performed alternatively either 1-h or 24-h after imipramine administration. The results confirm that imipramine reduces sucrose ingestion due to a reduction of the licking burst number. Most importantly, these results demonstrate that this effect does not require imipramine post-session administration, since it was present before the beginning of post-session administrations. This supports the interpretation of the reduction of sucrose ingestion as a consequence of reduced motivation and/or increased satiation. Thus, these findings, taken together with the results of our previous study, might be relevant in explaining the effects of imipramine in models of drug-seeking and in body weight gain reduction in rats, but not in accounting for the antidepressant therapeutic effect. At variance with the results of our previous study, an increase in burst size was present in the first half of the treatment period, which might be interpreted as a prohedonic effect and/or as a compensatory effect.

Dopaminylation in Psychostimulant Use Disorder Protects Against Psychostimulant Seeking Behavior by Normalizing Nucleus Accumbens (NAc) Dopamine Expression

BACKGROUND

Repeated cocaine administration changes histone acetylation and methylation on Lys residues and Deoxyribonucleic acid (DNA) within the nucleus accumbens (NAc). Recently Nestler's group explored histone Arg (R) methylation in reward processing models. Damez-Werno et al. (2016) reported that during human investigations and animal self-administration experiments, the histone mark protein-R-methyltransferase-6 (PRMT6) and asymmetric dimethylation of R2 on histone H3 (H3R2me2a) decreased in the rodent and cocaine-dependent human NAc. Overexpression of PRMT6 in D2-MSNs in all NAc neurons increased cocaine seeking, whereas PRMT6 overexpression in D1-MSNs protects against cocaine-seeking.

HYPOTHESIS

The hypothesis is that dopaminylation (H3R2me2a binding) occurs in psychostimulant use disorder (PSU), and the binding inhibitor Srcin1, like the major DRD2 A2 allelic polymorphism, protects against psychostimulant seeking behavior by normalizing nucleus accumbens (NAc) dopamine expression.

DISCUSSION

Numerous publications confirmed the association between the DRD2 Taq A1 allele (30-40 lower D2 receptor numbers) and severe cocaine dependence. Lepack et al. (2020) found that acute cocaine increases dopamine in NAc synapses, and results in histone H3 glutamine 5 dopaminylation (H3Q5dop) and consequent inhibition of D2 expression. The inhibition increases with chronic cocaine use and accompanies cocaine withdrawal. They also found that the Src kinase signaling inhibitor 1 (Srcin1 or p140CAP) during cocaine withdrawal reduced H3R2me2a binding. Consequently, this inhibited dopaminylation induced a "homeostatic brake."

CONCLUSION

The decrease in Src signaling in NAc D2-MSNs, (like the DRD2 Taq A2 allele, a well-known genetic mechanism protective against SUD) normalizes the NAc dopamine expression and decreases cocaine reward and motivation to self-administer cocaine. The Srcin1 may be an important therapeutic target.

Microstructure analysis of sucrose ingestion in the course of chronic treatment with imipramine

The analysis of licking microstructure provides measures which might be interpreted in terms of psychological constructs, such as pleasure and motivation, relevant for the interpretation of the effects of antidepressant drugs. The aim of this study was to characterise the effect of the prototypical antidepressant imipramine on the microstructure of licking for a 10% sucrose solution. In particular, ten 30-min sessions were performed in the course of a daily 21 day treatment with imipramine - 5, 10 and 20 mg/kg/die administered intraperitoneally. To interpret drug effects in relation to the presumed concentration of imipramine and its active metabolite desipramine, the experimental sessions were performed in an alternate order either 1-h or 24-h after imipramine administration. In the sessions performed 1-h after drug administration, the results showed a dose-dependent reduction of sucrose ingestion, accounted for by a reduction of the licking burst number. Moreover, reduced intra-burst lick rate and increased latency to lick were observed with the highest doses. Imipramine effect in the sessions performed 24-h after drug administration was similar but less pronounced. These results are consistent with the hypothesis that the reduction of sucrose ingestion might be due to reduced motivation and/or to a potentiation of satiety signals. These effects appear to be related, at least in part, to brain drug levels at testing time, and do not seem related to the mechanisms underlying the antidepressant therapeutic effect. However, these results might be relevant in explaining the effects of imipramine in models of drug-seeking and on body weight.

Investigating the Effect of Physiological Need States on Palatability and Motivation Using Microstructural Analysis of Licking

The study of consummatory responses during food intake represents a unique opportunity to investigate the physiological, psychological and neurobiological processes that control ingestive behavior. Recording the occurrence and temporal organization of individual licks across consumption, also called lickometry, yields a rich data set that can be analyzed to dissect consummatory responses into different licking patterns. These patterns, divided into trains of licks separated by pauses, have been used to deconstruct the many influences on consumption, such as palatability evaluation, incentive properties, and post-ingestive processes. In this review, we describe commonly used definitions of licking patterns and how various studies have defined and measured these. We then discuss how licking patterns can be used to investigate the impact of different physiological need states on processes governing ingestive behavior. We also present new data showing how licking patterns are changed in an animal model of protein appetite and how this may guide food choice in different protein-associated hedonic and homeostatic states. Thus, recording lick microstructure can be achieved relatively easily and represents a useful tool to provide insights, beyond the measurement of total intake, into the multiple factors influencing ingestive behavior.