Effects of pimozide on the hedonic properties of sucrose: analysis by the taste reactivity test

The Functional and Neurobiological Properties of Bad Taste

The gustatory system serves as a critical line of defense against ingesting harmful substances. Technological advances have fostered the characterization of peripheral receptors and have created opportunities for more selective manipulations of the nervous system, yet the neurobiological mechanisms underlying taste-based avoidance and aversion remain poorly understood. One conceptual obstacle stems from a lack of recognition that taste signals subserve several behavioral and physiological functions which likely engage partially segregated neural circuits. Moreover, although the gustatory system evolved to respond expediently to broad classes of biologically relevant chemicals, innate repertoires are often not in register with the actual consequences of a food. The mammalian brain exhibits tremendous flexibility; responses to taste can be modified in a specific manner according to bodily needs and the learned consequences of ingestion. Therefore, experimental strategies that distinguish between the functional properties of various taste-guided behaviors and link them to specific neural circuits need to be applied. Given the close relationship between the gustatory and visceroceptive systems, a full reckoning of the neural architecture of bad taste requires an understanding of how these respective sensory signals are integrated in the brain.

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.

Mu-opioid receptor activation in the medial shell of nucleus accumbens promotes alcohol consumption, self-administration and cue-induced reinstatement

Endogenous opioid signaling in ventral cortico-striatal-pallidal circuitry is implicated in elevated alcohol consumption and relapse to alcohol seeking. Mu-opioid receptor activation in the medial shell of the nucleus accumbens (NAc), a region implicated in multiple aspects of reward processing, elevates alcohol consumption while NAc opioid antagonists reduce it. However, the precise nature of the increases in alcohol consumption, and the effects of mu-opioid agonists on alcohol seeking and relapse are not clear. Here, we tested the effects of the mu-opioid agonist [D-Ala(2), N-MePhe(4), Gly-ol]-enkephalin (DAMGO) in rat NAc shell on lick microstructure in a free-drinking test, alcohol seeking during operant self-administration, extinction learning and expression, and cue-reinforced reinstatement of alcohol seeking. DAMGO enhanced the number, but not the size of drinking bouts. DAMGO also enhanced operant alcohol self-administration and cue-induced reinstatement, but did not affect extinction learning or elicit reinstatement in the absence of cues. Our results suggest that mu-opioid agonism in NAc shell elevates alcohol consumption, seeking and conditioned reinforcement primarily by enhancing the incentive motivational properties of alcohol and alcohol-paired cues, rather than by modulating palatability, satiety, or reinforcement.

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.

Differential dependence of Pavlovian incentive motivation and instrumental incentive learning processes on dopamine signaling

Here we attempted to clarify the role of dopamine signaling in reward seeking. In Experiment 1, we assessed the effects of the dopamine D(1)/D(2) receptor antagonist flupenthixol (0.5 mg/kg i.p.) on Pavlovian incentive motivation and found that flupenthixol blocked the ability of a conditioned stimulus to enhance both goal approach and instrumental performance (Pavlovian-to-instrumental transfer). In Experiment 2 we assessed the effects of flupenthixol on reward palatability during post-training noncontingent re-exposure to the sucrose reward in either a control 3-h or novel 23-h food-deprived state. Flupenthixol, although effective in blocking the Pavlovian goal approach, was without effect on palatability or the increase in reward palatability induced by the upshift in motivational state. This noncontingent re-exposure provided an opportunity for instrumental incentive learning, the process by which rats encode the value of a reward for use in updating reward-seeking actions. Flupenthixol administered prior to the instrumental incentive learning opportunity did not affect the increase in subsequent off-drug reward-seeking actions induced by that experience. These data suggest that although dopamine signaling is necessary for Pavlovian incentive motivation, it is not necessary for changes in reward experience, or for the instrumental incentive learning process that translates this experience into the incentive value used to drive reward-seeking actions, and provide further evidence that Pavlovian and instrumental incentive learning processes are dissociable.

Calculating utility: preclinical evidence for cost-benefit analysis by mesolimbic dopamine

RATIONALE

Throughout our lives we constantly assess the costs and benefits of the possible future outcomes of our actions and use this information to guide behavior. There is accumulating evidence that dopamine contributes to a fundamental component of this computation-how rewards are compared with the costs incurred when obtaining them.

OBJECTIVE

We review the evidence for dopamine's role in cost-benefit decision making and outline a simple mathematical framework in which to represent the interactions between rewards, costs, behavioral state and dopamine.

CONCLUSIONS

Dopamine's effects on cost-benefit decision making can be modeled using simple utility-function curves. This approach provides a useful framework for modeling existing data and generating experimental hypotheses that can be objectively and quantitatively tested by observing choice behavior without the necessity to account for subjective psychological states such as pleasure or desire. We suggest that dopamine plays a key role in overcoming response costs and enabling high-effort behaviors. A particularly important anatomical site of this action is the core of the nucleus accumbens. Here, dopamine is able to modulate activity originating from the frontal cortical systems that also assess costs and rewards. Internal deprivation states (e.g., hunger and thirst) also help to energize goal-seeking behaviors, probably in part by their rich influence on dopamine, which can in turn modify decision making policies.

What is the role of dopamine in reward: hedonic impact, reward learning, or incentive salience?

What roles do mesolimbic and neostriatal dopamine systems play in reward? Do they mediate the hedonic impact of rewarding stimuli? Do they mediate hedonic reward learning and associative prediction? Our review of the literature, together with results of a new study of residual reward capacity after dopamine depletion, indicates the answer to both questions is 'no'. Rather, dopamine systems may mediate the incentive salience of rewards, modulating their motivational value in a manner separable from hedonia and reward learning. In a study of the consequences of dopamine loss, rats were depleted of dopamine in the nucleus accumbens and neostriatum by up to 99% using 6-hydroxydopamine. In a series of experiments, we applied the 'taste reactivity' measure of affective reactions (gapes, etc.) to assess the capacity of dopamine-depleted rats for: 1) normal affect (hedonic and aversive reactions), 2) modulation of hedonic affect by associative learning (taste aversion conditioning), and 3) hedonic enhancement of affect by non-dopaminergic pharmacological manipulation of palatability (benzodiazepine administration). We found normal hedonic reaction patterns to sucrose vs. quinine, normal learning of new hedonic stimulus values (a change in palatability based on predictive relations), and normal pharmacological hedonic enhancement of palatability. We discuss these results in the context of hypotheses and data concerning the role of dopamine in reward. We review neurochemical, electrophysiological, and other behavioral evidence. We conclude that dopamine systems are not needed either to mediate the hedonic pleasure of reinforcers or to mediate predictive associations involved in hedonic reward learning. We conclude instead that dopamine may be more important to incentive salience attributions to the neural representations of reward-related stimuli. Incentive salience, we suggest, is a distinct component of motivation and reward. In other words, dopamine systems are necessary for 'wanting' incentives, but not for 'liking' them or for learning new 'likes' and 'dislikes'.

The anhedonias: Clinical and neurobiological aspects

During the last 30 years there has been renewed clinical interest in the state of 'lack of pleasure' (anhedonia) seen in conditions such as schizophrenia and depression. In spite of some important work, confusion still remains about the term, behaviours and explanatory concepts pertaining to anhedonia. This paper reviews the clinical and basic scientific studies that throw light on this interesting clinical phenomenon and then presents a new model of anhedonia which can be tested empirically and should facilitate research in this field.

d-fenfluramine's effects on normal ingestion assessed with taste reactivity measures

The effects of d-fenfluramine on intake and on hedonic responses to taste stimuli in rats were investigated using a modified taste reactivity paradigm. Subjects (n = 15) were first trained to consume a 3% sucrose solution. They were then pretreated with d-fenfluramine (0.3-3.0 mg/kg, i.p.), and tested with access to either 3% sucrose, or a 0.01% quinine HCl solution. In the modified taste reactivity test, chronic oral cannulation was not used; instead, taste reactivity measures were scored during periods of noningestion in a voluntary intake test. d-Fenfluramine reliably reduced both sucrose and quinine consumption, and increased latency to drink at the highest dose. d-Fenfluramine also spared aversive responses to quinine, but reduced positive ingestive responses to sucrose. These results are consistent with an effect of d-fenfluramine to reduce taste palatability, which may, in turn, be an important factor in the effect of this drug on feeding motivation.

Morphine enhancement of sucrose palatability: analysis by the taste reactivity test

The ability of morphine to modify sucrose palatability was assessed by the taste reactivity test. In Experiment 1, rats were injected with morphine (0.0, 0.5, 2.0, and 10.0 mg/kg, subcutaneously), 30 min before receiving a 10-min intraoral infusion of 2% or 20% sucrose solution. A dose of 2.0 mg/kg morphine enhanced ingestive reactions elicited by both concentrations of sucrose solution. In Experiment 2, the interval between morphine pretreatment and the taste reactivity test was manipulated. Rats given 2.0 mg/kg morphine 30 or 120 min before testing displayed enhanced ingestive reactions elicited by 20% sucrose solution during the first 5 min of a 10-min test. The results support the hypothesis that morphine enhances the hedonic assessment of sucrose solution.

Food reward: brain substrates of wanting and liking

What are the neural substrates of food reward? Are reward and pleasure identical? Can taste pleasure be assessed in animals? Is reward necessarily conscious? These questions have re-emerged in recent years, and there is now sufficient evidence to prompt re-examination of many preconceptions concerning reward and its relation to brain systems. This paper reviews evidence from many sources regarding both the psychological structure of food reward and the neural systems that mediate it. Special attention is paid to recent evidence from "tasty reactivity" studies of affective reactions to food. I argue that this evidence suggests the following surprising possibilities regarding the functional components and brain substrates of food reward. (1) Reward contains distinguishable psychological or functional components--"liking" (pleasure/palatability) and "wanting" (appetite/incentive motivation). These can be manipulated and measured separately. (2) Liking and wanting have separable neural substrates. Mediation of liking related to food reward involves neurotransmitter systems such as opioid and GABA/benzodiazepine systems, and anatomical structures such as ventral pallidum and brainstem primary gustatory relays. Mediation of wanting related to food reward involves mesotelencephalic dopamine systems, and divisions of nucleus accumbens and amygdala. Both liking and wanting arise from vastly distributed neural systems, but the two systems are separable. (3) Neural processing of food reward is not confined to the limbic forebrain. Aspects of food reward begin to be processed in the brainstem. A neural manipulation can enhance reward or produce aversion but no single lesion or transection is likely abolish all properties of food reward. (4) Both wanting and liking can exist without subjective awareness. Conscious experience can distort or blur the underlying reward process that gave rise to it. Subjective reports may contain false assessments of underlying processes, or even fail at all to register important reward processes. The core processes of liking and wanting that constitute reward are distinct from the subjective report or conscious awareness of those processes.

The effects of nicotine and nicotine withdrawal on taste reactivity

The effect of nicotine pretreatment on the palatability of flavored solutions was assessed using the taste reactivity test. In Experiment 1, low doses of nicotine [0.2-0.4 mg/kg, subcutaneously (s.c.)] suppressed the aversive taste properties of quinine and quinine-sucrose mixture and enhanced the hedonic taste properties of sucrose (0.4 mg/kg, s.c.) in rats that were nicotine naive. In Experiment 2, rats were chronically preexposed to nicotine or saline over a period of 21 pretreatment days. Tolerance developed to the ability of nicotine to enhance the palatability of sucrose. Furthermore, rats that were chronically preexposed to nicotine displayed enhanced hedonic evaluation of sucrose 24 h after nicotine was withdrawn. These results confirm human self-reports that withdrawal from nicotine dependency enhances the palatability of sweet-tasting foods.

The role of dopamine in drug abuse viewed from the perspective of its role in motivation

Drugs of abuse share with conventional reinforcers the activation of specific neural pathways in the CNS that are the substrate of their motivational properties. Dopamine is recognized as the transmitter of one such neural pathway, being involved in at least three major aspects of motivation: modulation of motivational state, acquisition (incentive learning) and expression of incentive properties by motivational stimuli. Drugs of abuse of different pharmacological classes stimulate in the low dose range dopamine transmission particularly in the ventral striatum. Apart from psychostimulants, the evidence that stimulation of dopamine transmission by drugs of abuse provides the primary motivational stimulus for drug self-administration is either unconvincing or negative. However, stimulation of dopamine transmission is essential for the activational properties of drugs of abuse and might be instrumental for the acquisition of responding to drug-related incentive stimuli (incentive learning). Dopamine is involved in the induction and in the expression of behavioural sensitization by repeated exposure to various drugs of abuse. Sensitization to the dopamine-stimulant properties of specific drug classes leading to facilitation of incentive learning of drug-related stimuli might account for the strong control over behaviour exerted by these stimuli in the addiction state. Withdrawal from drugs of abuse results in a reduction in basal dopamine transmission in vivo and in reduced responding for conventional reinforcers. Although these changes are likely to be the expression of a state of dependence of the dopamine system their contribution to the motivational state of drug addiction is unclear.

Benzodiazepines, appetite, and taste palatability

Benzodiazepine agonists stimulate feeding in animals. This paper reviews evidence which indicates that benzodiazepine-induced feeding is due to a specific enhancement of the perceived palatability of food and fluids, and is not a mere secondary consequence of anxiety reduction. In studies of the effect of benzodiazepines on affective reactions that are naturally elicited from rats by tastes, we have shown that (a) benzodiazepines enhance hedonic taste palatability in a receptor-specific fashion; (b) the relevant receptors and the minimal neural circuitry required to mediate benzodiazepine-induced palatability enhancement both exist complete in the decerebrate brain stem; and (c) even in normal brains, receptors in the brain stem, not forebrain, are the primary substrate for the benzodiazepine-induced enhancement of taste palatability. We conclude that a 'benzodiazepine-GABA' neural system in the brain stem constitutes an important component of the neural hierarchy responsible for taste pleasure. The reason why benzodiazepine tranquilizers have not been reported to enhance palatability for humans may be that the appropriate studies have not yet been done, that human doses are low, and that the brain stem palatability system is less responsive to commonly prescribed agonists that are anxiety/arousal benzodiazepine systems. Finally, in keeping with the purpose of the symposium in which this paper was originally presented, we discuss a number of issues regarding the measurement and interpretation of taste reactivity data.

Raclopride reduces sucrose preference in rats

Dopaminergic D1 and D2 antagonists decrease the intake of sweet solutions during sham feeding. Because the decreased intake of 10% sucrose produced by the D1 and D2 antagonists has been demonstrated to occur in the absence of significant deficits in the initiation of ingestion, or of its motor performance, we investigated the hypothesis that raclopride decreases intake by lowering the reinforcing potency of the orosensory stimulation provided by sucrose during sham feeding. Rats were adapted to ingest two differently flavored 10% sucrose solutions for 5 min in one-bottle tests. The flavored solution that rats preferred was paired with pretreatment with a dose of raclopride (400 micrograms/kg, IP, 15 min) that produced a mean decrease of intake of 55%. The other flavored 10% sucrose solution was paired with vehicle (0.15 M NaCl) injections. After three or six pairings with raclopride or vehicle injection, two two-bottle preference tests were given without raclopride pretreatment. Preference for the flavored 10% sucrose solution previously paired with raclopride decreased significantly in both tests. We interpret this decreased preference as evidence that raclopride decreased the reinforcing potency of flavored 10% sucrose during one-bottle tests. This is consistent with our hypothesis and with the more general hypothesis of Wise that central dopaminergic mechanisms mediate the reinforcing effect of food.

Taste reactivity to ethanol in rats: influence of adrenalectomy or ipsapirone

The affective mimetic responses of male Wistar rats with prior access to 6% ethanol in their home cages were observed during intraoral infusions of an equivalent alcohol solution. Ethanol preference in the home cage appeared unrelated to measures of aversion and ingestion in the taste reactivity tests in normal rats. Adrenalectomy, which significantly reduced home cage ethanol preference, failed to influence the taste reactions elicited by ethanol or water. On the other hand, treatment of intact rats with the 5-HT1A receptor agonist ipsapirone (2.5 mg/kg), a drug that also decreases ethanol drinking in two-bottle intake tests, did increase the duration of aversive groomings, whereas measures of ingestion remained unaffected. These results suggest that ipsapirone, but not adrenalectomy, may alter the palatability of ethanol; this perceptual change may partly underlie the ability of ipsapirone to reduce home cage alcohol drinking in the rat.

Amphetamine-induced modification of quinine palatability: analysis by the taste reactivity test

The effects of low doses of d-amphetamine (0.25-0.5 mg/kg, IP) on taste reactions elicited by quinine solutions in a 5-10-min taste reactivity test were assessed in a series of three experiments. Amphetamine consistently suppressed aversive reactions elicited by quinine solutions. The results suggest that amphetamine, like morphine, attenuates the aversiveness of the taste of quinine solution.

Attenuation of drinking sweetened water following calcium channel blockade

Recent reports cite results that both cocaine-induced conditioned place preference and activity stimulation are attenuated by pretreatment with the calcium channel blocker isradipine (ISR) in rats. By blocking voltage-dependent L-type calcium channels, ISR may regulate neural dopamine release that, in turn, decreases the putative rewarding effects mediated by dopaminergic mechanisms. It is known that nonfluid deprived rats avidly consume sweetened fluids; this suggests that the sweet taste is rewarding. Three experiments were conducted to determine the effects of ISR on drinking sweetened and nonflavored water. Experiment 1 was designed to test whether ISR would attenuate the intake of a palatable solution in a dose-dependent manner. To this end, ISR was administered both peripherally (3.0-30 mg/kg) and centrally (0.3-30 micrograms/rat) prior to a solution of saccharin and d-glucose (S + G) being made available to rats (15 min/day) and intake was recorded. ISR produced dose-dependent decreases (38%-81%) in S + G intake dependent on the route of administration. In Experiment 2, water intake was measured in 18 h water-deprived rats following ISR (10 mg/kg) administration as well as comparing S + G drinking. The effect of two ISR vehicles, dimethyl sulfoxide and Tween 80, upon fluid intake was also determined. ISR injection did not attenuate water intake in 18 h water-deprived rats and the choice of vehicle did not affect the ISR-induced attenuation of S + G drinking. In Experiment 3, a single dose (30 micrograms) of ICV administered ISR, that attenuated S + G intake by approximately 44%, did not attenuate water intake in 18 h water-deprived rats.(ABSTRACT TRUNCATED AT 250 WORDS)