Clozapine increases reward evaluation but not overall ingestive behaviour in rats licking for sucrose

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.

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.

Clozapine reliably increases the motivation for food: parsing the role of the 5-HT2c and H1 receptors

RATIONALE AND OBJECTIVES

Although clozapine is effective in treating schizophrenia, it is associated with adverse side effects including weight gain and metabolic syndrome. Despite this, the role of clozapine on feeding behaviour and food intake has not been thoroughly characterised. Clozapine has a broad pharmacological profile, with affinities for several neurotransmitter receptors, including serotonin (5-hydroxytriptamine, 5-HT) and histamine. Given that the serotonin 5-HT_2C receptor and histaminergic H₁ receptor are involved in aspects of feeding behaviour, the effect of clozapine on feeding may be linked to its action at these receptors.

METHODS

We assessed, in rats, the effect of acute and subchronic administration of clozapine on responding for food under a progressive ratio (PR) schedule under conditions of food restriction and satiety. We also examined the effect of antagonists of the serotonin 5-HT_2C and histaminergic H₁ receptors on the same schedule. Clozapine reliably increased responding for food, even when rats had ad libitum access to food. The effect of clozapine on responding for food was reproduced by combined (but not individual) antagonism of the serotonin 5-HT_2C and histaminergic H₁ receptors.

CONCLUSION

These findings show that clozapine enhances the motivation to work for food, that this effect is stable over repeated testing, and is independent of hunger state of the animal. This effect may relate to a combined action of clozapine at the serotonin 5-HT_2C and histaminergic H₁ receptors.

Antidepressant Drugs Correct the Imbalance Between proBDNF/p75NTR/Sortilin and Mature BDNF/TrkB in the Brain of Mice with Chronic Stress

Depression is a worldwide problem with a great social and economic burden in many countries. In our previous research, we found that the expression of proBDNF/p75NTR/sortilin is upregulated in patients with major depressive disorder. In addition, the treatment of proBDNF antibodies reversed both the depressive behaviors and the reduced BDNF mRNA detected in our rodent chronic stress models. Antidepressant drugs are usually only effective in a subpopulation of patients with major depression with a delayed time window of 2-4 weeks to exert their efficacy. The mechanism underlying such delayed response is not known. In this study, we hypothesize that antidepressant drugs exert their therapeutic effect by modulating proBDNF/p75NTR and mature BDNF/TrkB signaling pathways. To test the hypothesis, C57 mice were randomly divided into normal control, chronic unpredictable mild stress (CUMS), vehicle (VEH), fluoxetine (FLU), and clozapine (CLO) groups. Behavioral tests (sucrose preference, open field, and tail suspension tests) were performed before and after 4 weeks of CUMS. The gene and protein expression of proBDNF, the neurotrophin receptor (p75NTR), sortilin, and TrkB in the cortex and hippocampus were examined. At the protein level, CUMS induced a significant increase in proBDNF, p75NTR, and sortilin production while the TrkB protein level was found to be lower in the cortex and hippocampus compared with the control group. Consistently, at the mRNA level, p75NTR expression increased with reduced BDNF/TrkB mRNA in both cortex and hippocampus, while sortilin increased only in the hippocampus after CUMS. FLU and CLO treatments of CUMS mice reversed all protein and mRNA expression of the biomarkers in both cortex and hippocampus, except for sortilin mRNA in the cortex and proBDNF in the hippocampus, respectively. This study further confirms that the imbalance between proBDNF/p75NTR/sortilin and mBDNF/TrkB production is important in the pathogenesis of depression. It is likely that antidepressant FLU and antipsychotic CLO exert their antidepressant-like effect correcting the imbalance between proBDNF/p75NTR/sortilin and mBDNF/TrkB.

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

RATIONALE

Analysis of lick pattern for sucrose and NaCl and of the forced swimming response after dopamine antagonist administration led us to suggest that dopamine on D₁-like receptors is involved in behavioural activation, and the level of activation is "reboosted" on the basis of an evaluation process involving D₂-like receptors. Although some studies investigated licking microstructure for water after dopamine antagonists, the within-session time course of their effect was never investigated.

OBJECTIVES

The aims of this study were to further investigate the role of dopamine receptors in the mechanisms governing water ingestion, focussing on the within-session time course of the microstructure parameters, and to test the proposed hypothesis.

MATERIALS AND METHODS

The effects of the dopamine D₁-like receptor antagonist SCH 23390 (0.01-0.04 mg/kg) and of the dopamine D₂-like receptor antagonist raclopride (0.025-0.25 mg/kg) on licking microstructure for water were examined in 20-h water-deprived rats in 30-min sessions.

RESULTS

As previously observed with sucrose and NaCl, SCH 23390 reduced licking by reducing burst number, suggesting reduced behavioural activation. Moreover, it resulted in an increased burst size. Raclopride reduced the size of licking bursts, while their number was either increased or decreased depending on the dose.

CONCLUSION

The results support the suggestion that D₁ receptors are involved in behavioural activation and D₂ receptors are involved in a related evaluation process. Within the framework of the proposed hypothesis, the increased burst size after D₁-like receptor blockade might be interpreted as a pro-hedonic effect consequent to the increased cost of the activation of the licking response.

Affective biases and their interaction with other reward-related deficits in rodent models of psychiatric disorders

Major depressive disorder (MDD) is one of the leading global causes of disability. Symptoms of MDD can vary person to person, and current treatments often fail to alleviate the poor quality of life that patients experience. One of the two core diagnostic criteria for MDD is the loss of interest in previously pleasurable activities, which suggests a link between the disease aetiology and reward processing. Cognitive impairments are also common in patients with MDD, and more recently, emotional processing deficits known as affective biases have been recognised as a key feature of the disorder. Studies in animals have found similar affective biases related to reward. In this review we consider these affective biases in the context of other reward-related deficits and examine how affective biases associated with learning and memory may interact with the wider behavioural symptoms seen in MDD. We discuss recent developments in how analogues of affective biases and other aspects of reward processing can be assessed in rodents, as well as how these behaviours are influenced in models of MDD. We subsequently discuss evidence for the neurobiological mechanisms contributing to one or more reward-related deficits in preclinical models of MDD, identified using these behavioural assays. We consider how the relationships between these selective behavioural assays and the neurobiological mechanisms for affective bias and reward processing could be used to identify potential treatment strategies.

An Open Source Automated Two-Bottle Choice Test Apparatus for Rats

Two-bottle choice tests are a widely used paradigm in rodents to determine preference between two liquids, with utility for testing animal models of addiction, depression and anhedonia. The following paper describes a 3D-printed, Arduino controlled two-bottle choice test that automatically reads and records drinking behavior in rats to allow for detailed analysis of their drinking microstructure. While commercial products exist use lickometers to measure the microstructure of licking, this design uniquely incorporates hydrostatic depth sensors to allow for real-time volumetric measurements in addition to traditional beam break lick sensing, allowing for licking and drinking microstructure analysis. The goal of this design is to provide a user friendly, affordable apparatus that can study unique, complex behaviors without requiring the purchase of specialized scientific equipment or software. Its applications range from studying alcohol preference in animal models of addiction to sucrose preference in motivational deficits and reward evaluation. This design costs less than $180 CAD to build with decreased cost on each additional device. This design has been successfully tested for accuracy and validated using alcohol preference as an example. The apparatus showed consistency between drinking bouts and volume consumed and is shown to be accurate to ±0.086 ml of the actual volume. This design makes using the two-bottle choice paradigm more accurate, while also making its data more robust and informative while allowing for microstructure analysis of both licking behavior and volume consumed.

Characterizing ingestive behavior through licking microstructure: Underlying neurobiology and its use in the study of obesity in animal models

Ingestive behavior is controlled by multiple distinct peripheral and central physiological mechanisms that ultimately determine whether a particular food should be accepted or avoided. As rodents consume a fluid they display stereotyped rhythmic tongue movements, and by analyzing the temporal distribution of pauses of licking, it is possible through analyses of licking microstructure to uncover dissociable evaluative and motivational variables that contribute to ingestive behavior. The mean number of licks occurring within each burst of licking (burst and cluster size) reflects the palatability of the consumed solution, whereas the frequency of initiating novel bouts of licking behavior (burst and cluster number) is dependent upon the degree of gastrointestinal inhibition that accrues through continued fluid ingestion. This review describes the analysis of these measures within a context of the behavioral variables that come to influence the acceptance or avoidance of a fluid, and the neurobiological mechanisms that underlie alterations in the temporal distribution of pauses of licks. The application of these studies to models of obesity in animals is also described.

Within-session decrement of the emission of licking bursts following reward devaluation in rats licking for sucrose

We previously observed that dopamine D2-like receptor blockade in rats licking for sucrose produced a within-session decrement of the emission of licking bursts similar to the effect of either reward devaluation, or neuroleptics, on operant responding for different rewards, which, accordingly, we interpreted as an extinction-like effect. This implies that exposing animals to reward devaluation would result in a drop of burst number taking place only after the contact with the devalued reward. To test this prediction, we compared the difference in the within-session time course of burst number in response to high (10%) versus low (2%) concentration sucrose solutions, either in a condition of reward devaluation (exposure to 2% after daily 10%), or in a condition which does not involve changes in the reward value (two groups of subjects each repeatedly exposed to only one of the two concentrations). Reward devaluation resulted in a within-session decrement of the burst number, with the response rate dropping only after the contact with the devalued reward, as predicted. This response pattern was reliably observed only in subjects at their first devaluation experience. In contrast, exposure of separate groups of animals to the two different concentrations yielded lower levels of burst number in the low concentration group apparent since the beginning of the session, as previously observed with dopamine D1-like receptor blockade. These results show that the analysis of burst number, but not of burst size, reveals a specific activation pattern in response to reward devaluation, which differs from the pattern observed comparing the response to two different sucrose concentrations in separate groups of subjects, i.e. in a condition not involving reward devaluation. Finally, the characterisation of the experimental measures of the analysis of licking microstructure in behaviourally (and psychologically) meaningful functional terms, might be relevant for the investigation of the mechanisms underlying behavioural activation and the related evaluation processes.

Disruptions in effort-based decision-making and consummatory behavior following antagonism of the dopamine D2 receptor

Dopamine is known to influence motivational processes, however the precise role of this neurotransmitter remains a contentious issue. In the current study we sought to further characterize dopamine signaling in reward-based decision-making and consummatory behavior in mice, via lateral ventricle infusion of the dopamine D2 receptor antagonist eticlopride. In Experiment 1, we examined effort-based decision-making, in which mice had a choice between one lever, where a single response led to the delivery of a low value reward (2% sucrose); and a second lever, which led to a higher value reward (20% sucrose) that gradually required more effort to obtain. As the response schedule for the high value reward became more strict, low dose (4μg in 0.5μl) central infusions of eticlopride biased preference away from the high value reward, and toward the lever that led to the low value reward. Similarly, a higher dose of eticlopride (8μg in 0.5μl) also disrupted choice responding for the high value reward, however it did so by increasing omissions. In Experiment 2, we assessed the effects of eticlopride on consumption of 20% sucrose. The antagonist led to a dose-dependent reduction in intake, and through an analysis of licking microstructure, it was revealed that this in part reflected a reduction in the motivation to engage in consummatory behavior, rather than alterations in the evaluation of the reward. These results suggest that disruptions in D2 receptor signaling reduce the willingness to engage in effortful operant responding and consumption of a desirable outcome.

Atypical antipsychotics and effects on feeding: from mice to men

RATIONALE

So-called atypical antipsychotics (AAPs) are associated with varying levels of weight gain and associated metabolic disturbances, which in patients with serious mental illness (SMI) have been linked to non-compliance and poor functional outcomes. Mechanisms underlying AAP-induced metabolic abnormalities are only partially understood. Antipsychotic-induced weight gain may occur as a result of increases in food intake and/or changes in feeding.

OBJECTIVE

In this review, we examine the available human and preclinical literature addressing AAP-related changes in feeding behavior, to determine whether changes in appetite and perturbations in regulation of food intake could be contributing factors to antipsychotic-induced weight gain.

RESULTS

In general, human studies point to disruption by AAPs of feeding behaviors and food consumption. In rodents, increases in cumulative food intake are mainly observed in females; however, changes in feeding microstructure or motivational aspects of food intake appear to occur independent of sex.

CONCLUSIONS

The findings from this review indicate that the varying levels of AAP-related weight gain reflect changes in both appetite and feeding behaviors, which differ by type of AAP. However, inconsistencies exist among the studies (both human and rodent) that may reflect considerable differences in study design and methodology. Future studies examining underlying mechanisms of antipsychotic-induced weight gain are recommended in order to develop strategies addressing the serious metabolic side effect of AAPs.

Involvement of Endogenous Enkephalins and β-Endorphin in Feeding and Diet-Induced Obesity

Studies implicate opioid transmission in hedonic and metabolic control of feeding, although roles for specific endogenous opioid peptides have barely been addressed. Here, we studied palatable liquid consumption in proenkephalin knockout (PENK KO) and β-endorphin-deficient (BEND KO) mice, and how the body weight of these mice changed during consumption of an energy-dense highly palatable 'cafeteria diet'. When given access to sucrose solution, PENK KOs exhibited fewer bouts of licking than wild types, even though the length of bouts was similar to that of wild types, a pattern that suggests diminished food motivation. Conversely, BEND KOs did not differ from wild types in the number of licking bouts, even though these bouts were shorter in length, suggesting that they experienced the sucrose as being less palatable. In addition, licking responses in BEND, but not PENK, KO mice were insensitive to shifts in sucrose concentration or hunger. PENK, but not BEND, KOs exhibited lower baseline body weights compared with wild types on chow diet and attenuated weight gain when fed cafeteria diet. Based on this and related findings, we suggest endogenous enkephalins primarily set a background motivational tone regulating feeding behavior, whereas β-endorphin underlies orosensory reward in high need states or when the stimulus is especially valuable. Overall, these studies emphasize complex interplays between endogenous opioid peptides targeting μ-receptors, such as enkephalins and endorphins, underlying the regulation of feeding and body weight that might explain the poor efficacy of drugs that generally target μ-opioid receptors in the long-term control of appetite and body weight.

Acute and chronic effects of NMDA receptor antagonists in rodents, relevance to negative symptoms of schizophrenia: a translational link to humans

Negative symptoms of schizophrenia remain an unmet clinical need as they are common, persistent, respond poorly to existing treatments and lead to disability. Blunted affect, alogia, asociality, anhedonia and avolition are regarded as key negative symptoms despite DSM-IV-TR specifying a more limited range. The key to development of improved therapies is improved animal models that mimic the human condition in terms of behaviour and pathology and that predict efficacy of novel treatments in patients. Accumulating evidence shows that NMDA receptor (NMDAR) antagonists mimic cognitive deficits of relevance to schizophrenia in animals, along with associated pathological changes. This review examines evidence for the ability of NMDAR antagonists to mimic anhedonia and asociality, two negative symptoms of schizophrenia, in animals. The use of various species, paradigms and treatment regimens are reviewed. We conclude that sub-chronic treatment with NMDAR antagonists, typically PCP, induces social withdrawal in animals but not anhedonia. NMDAR antagonists have further effects in paradigms such as motivational salience that may be useful for mimicking other aspects of negative symptoms but these require further development. Sub-chronic treatment regimens of NMDAR antagonists also have some neurobiological effects of relevance to negative symptoms. It is our view that a sub-chronic treatment regime with NMDAR antagonists, particularly PCP, with animals tested following a wash-out period and in a battery of tests to assess certain behaviours of relevance to negative symptoms and social withdrawal (the animal equivalent of asociality) is valuable. This will enhance our understanding of the psycho and neuropathology of specific negative symptom domains and allow early detection of novel pharmacological targets.

Dopamine on D2-like receptors "reboosts" dopamine D1-like receptor-mediated behavioural activation in rats licking for a isotonic NaCl solution

RATIONALE

We recently suggested that dopamine on D1-like receptors is involved in the activation of goal-directed responses and the level of response activation is "reboosted" on the basis of an evaluation process involving D2-like receptors assessing "response efficacy". A main piece of evidence in support of this hypothesis was the observation of an "extinction mimicry" effect in the time course of licking bursts after dopamine D2-like receptor blockade in rats licking for sucrose.

OBJECTIVES

The aim of this study was to determine whether the pattern of licking observed with sucrose as a reward could be reproduced in rats licking for a different reward (0.9% NaCl).

MATERIALS AND METHODS

We investigated the effects of the dopamine D1-like receptor antagonist SCH 23390 (0.01-0.04 mg/kg) and of the dopamine D2-like receptor antagonist raclopride (0.025-0.25 mg/kg) on the microstructure of licking for a 0.9% NaCl solution in 12-h water-deprived rats in 30-min sessions.

RESULTS

As previously observed with sucrose as a reward, raclopride reduced the size of licking bursts and produced on the burst number time course an "extinction mimicry" effect, while SCH 23390 reduced licking exclusively by reducing burst number.

CONCLUSIONS

These results are consistent with the proposed hypothesis and provide support to the use of the study of licking microstructure as a valid model not only for the investigation of the mechanisms governing ingestive behaviour but also for the investigation of the mechanisms underlying behavioural activation and the related evaluation processes.

Licking and Liking: The Assessment of Hedonic Responses in Rodents

Affective processes are a key determinant of behaviour: At its simplest, liked stimuli are approached while disliked stimuli are avoided. Although assessing hedonic responses in nonverbal animals can be difficult, one relatively tractable approach relies on detailed analyses of rodents' consummatory behaviour. Rodents typically produce rhythmic sets of licks that can be grouped into clusters on the basis of the intervals between licks. The mean number of licks in a cluster (cluster size) is directly related to the concentration of palatable and unpalatable solutions. These relationships suggest that lick cluster size might be a useful index of an animal's hedonic reaction to the solution being consumed. I begin by reviewing studies of conditioned flavour preference and aversion that support the idea that lick cluster size can provide useful information about rats' hedonic reactions. I then describe how this methodology has been used to address previously intractable issues in the investigation of contrast effects as well as revealing an analogue of effort justification effects that, in humans, are commonly explained in terms of cognitive dissonance reduction. Finally, I consider how lick analysis might provide information about hedonic responses in animal models of human psychiatric disorders. In all these cases, how an animal did something was particularly informative about why it was doing it.

Effect of the dopamine D1-like receptor antagonist SCH 23390 on the microstructure of ingestive behaviour in water-deprived rats licking for water and NaCl solutions

The analysis of licking microstructure provides measures, size and number of licking bouts, which might reveal, respectively, reward evaluation and behavioural activation. Based on the different effects of the dopamine D1-like and D2-like receptor antagonists SCH 23390 and raclopride on licking for sucrose, in particular the failure of the former to reduce bout size and the ability of the latter to induce a within-session decrement of bout number resembling either reward devaluation or neuroleptics on instrumental responding, we suggested that activation of reward-associated responses depends on dopamine D1-like receptor stimulation, and its level is updated on the basis of a dopamine D2-like receptor-mediated reward evaluation. Consistent results were obtained in a study examining the effect of dopamine D2-like receptor antagonism in rats licking for NaCl solutions and water. In this study, we examined the effects of the dopamine D1-like receptor antagonist SCH 23390 (0, 10, 20 and 40 μg/kg) on the microstructure of licking for water and sodium chloride solutions (0.075 M, 0.15 M, 0.3 M) in 12 h water deprived rats. Rats were exposed to each solution for 60 s either after the first lick or after 3 min that the animals were placed in the chambers. Bout size, but not bout number, was decreased at the highest NaCl concentration. SCH 23390 produced a decrease of bout number and of lick number mainly due to the decreased number of subjects engaging in licking behaviour, and failed to reduce bout size for Na Cl and water at a dose which increased the latency to the 1st lick but did not affect the intra-bout lick rate. In agreement with previous observations, these results suggest that dopamine D1-like receptors play an important role in the activation of reward-oriented responses.