Development of a behavioral task measuring reward "wanting" and "liking" in rats

Ventromedial prefrontal cortex is involved in preference and hedonic evaluation of tastes

The ventromedial prefrontal cortex (vmPFC) of rats has reciprocal connections with the gustatory and the hedonic impact coding structures. The main goal of the present study was to investigate the involvement of local neurons of vmPFC and their catecholaminergic innervations in taste preference and taste reactivity test. Therefore, kainate or 6-hydroxydopamine (6-OHDA) lesions were performed in the vmPFC by iontophoretic method. In the first experiment, taste preference was tested to 250 mM and 500 mM glucose solutions over water in two-bottle choice test. In the second experiment, taste reactivity was examined to 4 concentrations of glucose solutions (250 mM, 500 mM, 750 mM and 1000 mM) and 4 concentrations of quinine solutions (0.125 mM, 0.25 mM, 1.25 mM and 2.5 mM). Our results showed, that kainate microlesion of vmPFC did not modify the preference of 250 mM and 500 mM glucose solutions in two-bottle choice test. In contrast, 6-OHDA microlesion of vmPFC resulted in increased preference to the higher concentration of glucose (500 mM) solution over water. Results of taste reactivity test showed that kainate lesion resulted in more ingestive and less rejective responses to 750 mM glucose solution and elevated rejectivity to the higher concentrations (1.25 mM and 2.5 mM) of quinine solutions. 6-OHDA lesion of vmPFC increased the number of ingestive responses to highly concentrated (500 mM, 750 mM and 1000 mM) glucose solutions and decreased the number of ingestive responses to the lower concentration (0.125 mM) of quinine solution. The present data provide evidence for the important role of vmPFC neurons and catecholaminergic innervation of the vmPFC in the regulation of hedonic evaluation of tastes and in the hedonic consummatory behavior.

Nicotine-Induced Conditional Place Preference Is Affected by Head Injury: Correlation with Dopamine Release in the Nucleus Accumbens Shell

BACKGROUND

Traumatic brain injury is known to impact dopamine-mediated reward pathways, but the underlying mechanisms have not been fully established.

METHODS

Nicotine-induced conditional place preference was used to study rats exposed to a 6-psi fluid percussion injury with and without prior exposure to nicotine. Preference was quantified as a score defined as (C1 - C2) / (C1 + C2), where C1 is time in the nicotine-paired compartment and C2 is time in the saline-paired compartment. Subsequent fast-scan cyclic voltammetry was used to analyze the impact of nicotine infusion on dopamine release in the shell portion of the nucleus accumbens. To further determine the influence of brain injury on nicotine withdrawal, nicotine infusion was administered to the rats after fluid percussion injury. The effects of fluid percussion injury on conditional place preference after prior exposure to nicotine and abstinence or withdrawal from nicotine were also assessed.

RESULTS

After traumatic brain injury, dopamine release was reduced in the nucleus accumbens shell, and nicotine-induced conditional place preference preference was significantly impaired. Preference scores of control, sham-injured, and fluid percussion injury groups were 0.1627±0.04204, 0.1515±0.03806, and -0.001300±0.04286, respectively. Nicotine-induced conditional place preference was also seen in animals after nicotine pretreatment, with a conditional place preference score of 0.07805±0.02838. Nicotine preexposure substantially increased tonic dopamine release in sham-injured animals, but it did not change phasic release; nicotine exposure after fluid percussion injury enhanced phasic release, though not to the same levels seen in sham-injured rats. Conditioned preference was related not only to phasic dopamine release (r=0.8110) but also to the difference between tonic and phasic dopamine levels (r=0.9521).

CONCLUSIONS

Traumatic brain injury suppresses dopamine release from the shell portion of the nucleus accumbens, which in turn significantly alters reward-seeking behavior. These results have important implications for tobacco and drug use after traumatic brain injury.

Dopamine in the Brain: Hypothesizing Surfeit or Deficit Links to Reward and Addiction

Recently there has been debate concerning the role of brain dopamine in reward and addiction. David Nutt and associates eloquently proposed that dopamine (DA) may be central to psycho stimulant dependence and some what important for alcohol, but not important for opiates, nicotine or even cannabis. Others have also argued that surfeit theories can explain for example cocaine seeking behavior as well as non-substance-related addictive behaviors. It seems prudent to distinguish between what constitutes “surfeit” compared to” deficit” in terms of short-term (acute) and long-term (chronic) brain reward circuitry responsivity. In an attempt to resolve controversy regarding the contributions of mesolimbic DA systems to reward, we review the three main competing explanatory categories: “liking”, “learning”, and “wanting”. They are (a) the hedonic impact -liking reward, (b) the ability to predict rewarding effects-learning and (c) the incentive salience of reward-related stimuli -wanting. In terms of acute effects, most of the evidence seems to favor the “surfeit theory”. Due to preferential dopamine release at mesolimbic-VTA-caudate-accumbens loci most drugs of abuse and Reward Deficiency Syndrome (RDS) behaviors have been linked to heightened feelings of well-being and hyperdopaminergic states.The “dopamine hypotheses” originally thought to be simple, is now believed to be quite complex and involves encoding the set point of hedonic tone, encoding attention, reward expectancy, and incentive motivation. Importantly, Willuhn et al. shows that in a self-administration paradigm, (chronic) excessive use of cocaine is caused by decreased phasic dopamine signaling in the striatum. In terms of chronic addictions, others have shown a blunted responsivity at brain reward sites with food, nicotine, and even gambling behavior. Finally, we are cognizant of the differences in dopaminergic function as addiction progresses and argue that relapse may be tied to dopamine deficiency. Vulnerability to addiction and relapse may be the result of the cumulative effects of dopaminergic and other neurotransmitter genetic variants and elevated stress levels. We therefore propose that dopamine homeostasis may be a preferred goal to combat relapse.

"Liking" and "wanting" linked to Reward Deficiency Syndrome (RDS): hypothesizing differential responsivity in brain reward circuitry

In an attempt to resolve controversy regarding the causal contributions of mesolimbic dopamine (DA) systems to reward, we evaluate the three main competing explanatory categories: "liking,""learning," and "wanting" [1]. That is, DA may mediate (a) the hedonic impact of reward (liking), (b) learned predictions about rewarding effects (learning), or (c) the pursuit of rewards by attributing incentive salience to reward-related stimuli (wanting). We evaluate these hypotheses, especially as they relate to the Reward Deficiency Syndrome (RDS), and we find that the incentive salience or "wanting" hypothesis of DA function is supported by a majority of the evidence. Neuroimaging studies have shown that drugs of abuse, palatable foods, and anticipated behaviors such as sex and gaming affect brain regions involving reward circuitry, and may not be unidirectional. Drugs of abuse enhance DA signaling and sensitize mesolimbic mechanisms that evolved to attribute incentive salience to rewards. Addictive drugs have in common that they are voluntarily selfadministered, they enhance (directly or indirectly) dopaminergic synaptic function in the nucleus accumbens (NAC), and they stimulate the functioning of brain reward circuitry (producing the "high" that drug users seek). Although originally believed simply to encode the set point of hedonic tone, these circuits now are believed to be functionally more complex, also encoding attention, reward expectancy, disconfirmation of reward expectancy, and incentive motivation. Elevated stress levels, together with polymorphisms of dopaminergic genes and other neurotransmitter genetic variants, may have a cumulative effect on vulnerability to addiction. The RDS model of etiology holds very well for a variety of chemical and behavioral addictions.