Pimozide enhances the aversiveness of quinine solution

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.

Palatability, food intake and the behavioural satiety sequence in male rats

Food intake is influenced not only by nutritional status but also by diverse environmental factors. Indeed, a unique quality of food reward is its strong modulation by palatability cues, such as taste, with animals generally preferring diets that are sweet and avoiding those that are either bitter or sour. As appetite suppressants (including those currently in development) could alter food intake by modifying taste sensitivity and/or palatability, the aim of the present study was to characterise the influence of taste adulteration on the normal structure of feeding behaviour, i.e., the behavioural satiety sequence (BSS). Adult male rats were initially habituated both to the basic test diet (mash) and the test arena. Following stabilisation of basal intake, a continuous monitoring technique was used to profile behaviour in weekly 1-h sessions during which the animals were presented, in counterbalanced order, with the basic diet (control) or one of four taste-adulterated variants (0.015% quinine, 0.04% quinine, 0.2% saccharin, 0.3% saccharin). Food intake was strongly suppressed by the higher quinine concentration but was not significantly altered by any of the other additives. Behavioural analysis revealed that this anorectic-like response to 0.04% quinine-adulterated food was associated with a significant reduction in the peak feeding response, highly atypical intermittent food sampling/digging and the virtual absence of resting behaviour. Importantly, this pattern of behavioural change is readily distinguishable from those seen in response to other manipulations that reduce intake, including selective anorectics, sedatives and psychostimulants. Despite the lack of significant effect on food intake or the duration of feeding behaviour, dietary adulteration with 0.015% quinine (and, to a lesser degree, 0.3% saccharin) produced some effects on behavioural structure/time course consistent with a mild aversive response, i.e., bouts of midsession food sampling and a delay in the transition from eating to resting. Data are discussed in relation to the specific behavioural signature to quinine-induced anorexia and its potential utility in identifying appetite suppressants that may modify intake via changes in taste sensitivity and/or palatability.

Slow cortical DC-potential responses to sweet and bitter tastes in humans

Processing of hedonic stimulus quality is assumed to be accompanied by a tuning of cortical arousal and excitability. In this pilot study in 11 healthy humans scalp-recorded DC potentials were assessed during application of a sweet (sucrose) and bitter (quinine hydrochloride) taste, i.e., primary reinforcers of positive and negative quality. Muscular, ocular, and skin potential activity were controlled. Application of sucrose induced a widespread positive DC-potential shift with an amplitude of 40-50 microV and persisting for more than 120-s post-stimulus onset. Following administration of quinine hydrochloride, this positive shift was reduced, most distinctly between 48- and 88-s post-stimulus onset. The reduction appeared to be most consistent at anterior midline recording sites (Fz, Cz). It is assumed that the higher DC-potential positivity during sweetness than during bitterness points to a differential tuning of cortical excitability by a widespread decrease in depolarization of apical dendrites.

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'.

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.

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.

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

The ability of various doses (0, 1.25, 2.5, and 5.0 mg/kg) of fenfluramine to modify the palatability of sucrose and quinine solutions was assessed by means of the taste reactivity test. Although fenfluramine did not modify the positive hedonic ingestive reactions elicited by sucrose solution, it consistently enhanced the negatively hedonic aversive reactions elicited by unpalatable 0.05% quinine solution and moderately palatable 2% sucrose solution. The results suggest that fenfluramine enhances the aversive properties of tastants without suppressing the positive hedonic properties of tastants. The results support a two-dimensional model of palatability.

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.

Effects of morphine injection into the parabrachial area on saccharin preference: modulation by lateral hypothalamic neurons

The aim of the present study was to analyze the effects of morphine injected into the second relay station of the gustatory input pathways, the parabrachial area, on preference for saccharin over water. This study was carried out using both rats whose lateral hypothalamic neurons had been lesioned by ibotenic acid and sham-lesioned rats. As already shown, an 0.3 mM solution of the sweetener, which was clearly preferred over water by the sham-lesioned animals, was neutral for the lesioned rats. The injection of 50 ng of morphine into each parabrachial area transformed this neutral response of the lesioned rats to a clear preference for the sweetener, whereas the preference of sham-lesioned rats for the same solution was converted to an aversive response. Likewise, with a more palatable solution of saccharin (2.5 mM), the injection of 50 ng of morphine decreased the preference of the nonlesioned rats but increased the preference of the lesioned animals. Using the 2.5 mM solution of saccharin, the intraparabrachial injection of higher doses of morphine (100 and 500 ng) did not greatly modify the preference for the sweetener but induced a significant decrease in total fluid intake that was still observed 11 h after the injection of the opiate. These results are discussed: the morphine-induced aversion observed in the nonlesioned rats could be explained either by a specific influence on certain opioid receptors in the parabrachial area or, more probably, by the stimulation of pathways involved in taste or visceral aversive processes and relaying in the parabrachial area.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Saccharin's aversive taste in rats: evidence and implications

In most biobehavioral research using rats as subjects, saccharin is viewed as a "sugar substitute"--sweet, palatable, and conveniently lacking in calories. This characterization has merit: Saccharin seems to share some sensory, affective, and motivating properties with sugars. This paper focuses on saccharin's lesser known, aversive properties. Evidence of saccharin's distinctive taste in rats is reviewed, followed by discussion of several modulating variables. Procedures that influence rats' responses to saccharin and their measurement are summarized, and the argument is advanced that saccharin can be used to study the relationship of taste and ingestion to learning, stress, and emotional processes in rats and humans.

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

The ability of the neuroleptic agent, pimozide, to modify sucrose palatability was assessed using three 10-min taste reactivity test sessions. Pimozide was found to suppress the ingestive response of tongue protrusions, but enhance the mildly ingestive/neutral response of mouth movements elicited by an intraoral infusion of sucrose solution. Since the pattern of taste reactivity responding shifted from highly ingestive to mildly ingestive/neutral, our results suggest that pimozide pretreatment reduces the palatability of sucrose solution. The temporal pattern of the modification of these taste reactivity responses was predicted by the Anhedonia Hypothesis.