Distracting stimuli evoke ventral tegmental area responses in rats during ongoing saccharin consumption.

Disruptions in attention, salience and increased distractibility are implicated in multiple psychiatric conditions. The ventral tegmental area (VTA) is a potential site for converging information about external stimuli and internal states to be integrated and guide adaptive behaviours. Given the dual role of dopamine signals in both driving ongoing behaviours (e.g., feeding) and monitoring salient environmental stimuli, understanding the interaction between these functions is crucial. Here, we investigate VTA neuronal activity during distraction from ongoing feeding. We developed a task to assess distraction exploiting self-paced licking in rats. Rats trained to lick for saccharin were given a distraction test, in which three consecutive licks within 1 s triggered a random distractor (e.g. light and tone stimulus). On each trial they were quantified as distracted or not based on the length of their pauses in licking behaviour. We expressed GCaMP6s in VTA neurons and used fibre photometry to record calcium fluctuations during this task as a proxy for neuronal activity. Distractor stimuli caused rats to interrupt their consumption of saccharin, a behavioural effect which quickly habituated with repeat testing. VTA neural activity showed consistent increases to distractor presentations and, furthermore, these responses were greater on distracted trials compared to non-distracted trials. Interestingly, neural responses show a slower habituation than behaviour with consistent VTA responses seen to distractors even after they are no longer distracting. These data highlight the complex role of the VTA in maintaining ongoing appetitive and consummatory behaviours while also monitoring the environment for salient stimuli.

Physiological state gates acquisition and expression of mesolimbic reward prediction signals.

Phasic dopamine signaling participates in associative learning by reinforcing associations between outcomes (unconditioned stimulus; US) and their predictors (conditioned stimulus; CS). However, prior work has always engendered these associations with innately rewarding stimuli. Thus, whether dopamine neurons can acquire prediction signals in the absence of appetitive experience and update them when the value of the outcome changes remains unknown. Here, we used sodium depletion to reversibly manipulate the appetitive value of a hypertonic sodium solution while measuring phasic dopamine signaling in rat nucleus accumbens. Dopamine responses to the NaCl US following sodium depletion updated independent of prior experience. In contrast, prediction signals were only acquired through extensive experience with a US that had positive affective value. Once learned, dopamine prediction signals were flexibly expressed in a state-dependent manner. Our results reveal striking differences with respect to how physiological state shapes dopamine signals evoked by outcomes and their predictors.

Ghrelin acts as an interface between physiological state and phasic dopamine signaling.

Brief, high-concentration (phasic) spikes in nucleus accumbens dopamine critically participate in aspects of food reward. Although physiological state (e.g., hunger, satiety) and associated hormones are known to affect dopamine tone in general, whether they modulate food-evoked, phasic dopamine specifically is unknown. Here, we used fast-scan cyclic voltammetry in awake, behaving rats to record dopamine spikes evoked by delivery of sugar pellets while pharmacologically manipulating central receptors for the gut "hunger" hormone ghrelin. Lateral ventricular (LV) ghrelin increased, while LV ghrelin receptor antagonism suppressed the magnitude of dopamine spikes evoked by food. Ghrelin was effective when infused directly into the lateral hypothalamus (LH), but not the ventral tegmental area (VTA). LH infusions were made in close proximity to orexin neurons, which are regulated by ghrelin and project to the VTA. Thus, we also investigated and found potentiation of food-evoked dopamine spikes by intra-VTA orexin-A. Importantly, intra-VTA blockade of orexin receptors attenuated food intake induced by LV ghrelin, thus establishing a behaviorally relevant connection between central ghrelin and VTA orexin. Further analysis revealed that food restriction increased the magnitude of dopamine spikes evoked by food independent of any pharmacological manipulations. The results support the regulation of food-evoked dopamine spikes by physiological state with endogenous fluctuations in ghrelin as a key contributor. Our data highlight a novel mechanism by which signals relating physiological state could influence food reinforcement and food-directed behavior.

Glucagon-like peptide-1 receptor activation in the nucleus accumbens core suppresses feeding by increasing glutamatergic AMPA/kainate signaling.

Glucagon-like peptide-1 receptor (GLP-1R) activation in the nucleus accumbens (NAc) core is pharmacologically and physiologically relevant for regulating palatable food intake. Here, we assess whether GLP-1R signaling in the NAc core of rats modulates GABAergic medium spiny neurons (MSNs) through presynaptic-glutamatergic and/or presynaptic-dopaminergic signaling to control feeding. First, ex vivo fast-scan cyclic voltammetry showed that the GLP-1R agonist exendin-4 (Ex-4) does not alter dopamine release in the NAc core. Instead, support for a glutamatergic mechanism was provided by ex vivo electrophysiological analyses showing that Ex-4 activates presynaptic GLP-1Rs in the NAc core to increase the activity of MSNs via a glutamatergic, AMPA/kainate receptor-mediated mechanism, indicated by increased mEPSC frequency and decreased paired pulse ratio in core MSNs. Only a small, direct excitatory effect on MSNs by Ex-4 was observed, suggesting that the contribution of postsynaptic GLP-1R to MSN activity is minimal. The behavioral relevance of the electrophysiological data was confirmed by the finding that intracore injection of the AMPA/kainate receptor antagonist CNQX attenuated the ability of NAc core GLP-1R activation by Ex-4 microinjection to suppress food intake and body weight gain; in contrast, intracore NMDA receptor blockade by AP-5 did not inhibit the energy balance effects of NAc core Ex-4. Together, these data provide evidence for a novel glutamatergic, but not dopaminergic, mechanism by which NAc core GLP-1Rs promote negative energy balance.

Adolescents are more vulnerable to cocaine addiction: behavioral and electrophysiological evidence.

In humans, adolescence is a period of heightened propensity to develop cocaine addiction. It is unknown whether this is attributable to greater access and exposure to cocaine at this age, or whether the adolescent brain is particularly vulnerable to the addictive properties of cocaine. Here, we subjected male adolescent (P42) and adult (∼P88) rats to a wide range of cocaine self-administration procedures. In addition, to determine whether behavioral differences are associated with developmental differences in dopaminergic activity, we examined and manipulated the activity of dopamine neurons. Relative to adults, adolescent rats took cocaine more readily, were more sensitive to lower doses, showed greater escalation of cocaine intake, and were less susceptible to increases in price (i.e., were more "inelastic"). In parallel, adolescents also showed elevated activity of ventral tegmental area dopamine neurons, a feature known to be associated with increased self-administration behavior. Pharmacological manipulation of dopamine D2 receptor function with quinpirole (agonist) or eticlopride (antagonist), to alter dopamine neuron activity, eliminated age differences in cocaine self-administration. These data suggest a causal relationship between behavioral and electrophysiological determinants of cocaine addiction liability. In conclusion, adolescents show behavioral and electrophysiological traits of heightened addiction liability.

Involvement of A13 dopaminergic neurons in prehensile movements but not reward in the rat.

Tyrosine hydroxylase (TH)-containing neurons of the dopamine (DA) cell group A13 are well positioned to impact known DA-related functions as their descending projections innervate target regions that regulate vigilance, sensory integration, and motor execution. Despite this connectivity, little is known regarding the functionality of A13-DA circuits. Using TH-specific loss-of-function methodology and techniques to monitor population activity in transgenic rats in vivo, we investigated the contribution of A13-DA neurons in reward and movement-related actions. Our work demonstrates a role for A13-DA neurons in grasping and handling of objects but not reward. A13-DA neurons responded strongly when animals grab and manipulate food items, whereas their inactivation or degeneration prevented animals from successfully doing so-a deficit partially attributed to a reduction in grip strength. By contrast, there was no relation between A13-DA activity and food-seeking behavior when animals were tested on a reward-based task that did not include a reaching/grasping response. Motivation for food was unaffected, as goal-directed behavior for food items was in general intact following A13 neuronal inactivation/degeneration. An anatomical investigation confirmed that A13-DA neurons project to the superior colliculus (SC) and also demonstrated a novel A13-DA projection to the reticular formation (RF). These results establish a functional role for A13-DA neurons in prehensile actions that are uncoupled from the motivational factors that contribute to the initiation of forelimb movements and help position A13-DA circuits into the functional framework regarding centrally located DA populations and their ability to coordinate movement.

Calcium-permeable AMPA receptors are present in nucleus accumbens synapses after prolonged withdrawal from cocaine self-administration but not experimenter-administered cocaine.

Repeated noncontingent cocaine injections, which lead to behavioral sensitization, increase AMPA receptor (AMPAR) transmission in the rodent nucleus accumbens (NAc) in a withdrawal-dependent manner. On withdrawal days (WD) 10-21, this is attributable to upregulation of GluA1A2-containing AMPARs. However, synaptic incorporation of GluA2-lacking/Ca(2+)-permeable AMPARs (CP-AMPARs) was observed after longer withdrawal (WD35) from repeated noncontingent cocaine injections in young mice (Mameli et al., 2009). CP-AMPARs had previously been observed in NAc synapses only after prolonged (WD30-WD47) withdrawal from extended-access cocaine self-administration. Our goal was to determine whether rats receiving repeated noncontingent cocaine injections during adulthood similarly exhibit CP-AMPARs in the NAc after prolonged withdrawal. For comparison, we began by evaluating CP-AMPARs on WD35-WD49 after extended-access cocaine self-administration. Confirming our previous results, whole-cell recordings revealed inwardly rectifying AMPAR EPSCs, a hallmark of CP-AMPARs. This was observed in both core and shell. Next, we conducted the same analysis in adult rats treated with eight daily noncontingent cocaine injections and recorded on WD35-WD49. AMPAR EPSCs in core and shell did not show inward rectification and were insensitive to 1-naphthylacetylspermine (a selective antagonist of CP-AMPARs). Locomotor sensitization could still be demonstrated after this long withdrawal period, although the upregulation of GluA1A2-containing AMPARs observed at earlier withdrawal times was no longer detected. In conclusion, in adult rats, accumulation of synaptic CP-AMPARs in the NAc occurs after prolonged withdrawal from extended-access cocaine self-administration but not after prolonged withdrawal from noncontingent cocaine injections.

Group I mGluR activation reverses cocaine-induced accumulation of calcium-permeable AMPA receptors in nucleus accumbens synapses via a protein kinase C-dependent mechanism.

Following prolonged withdrawal from extended access cocaine self-administration in adult rats, high conductance Ca2+ -ermeable AMPA receptors (CP-AMPARs) accumulate in nucleus accumbens (NAc) synapses and mediate the expression of "incubated" cue-induced cocaine craving. Using patch-clamp recordings from NAc slices prepared after extended access cocaine self-administration and >45 d of withdrawal, we found that group I metabotropic glutamate receptor (mGluR) stimulation using 3,5-dihydroxyphenylglycine (DHPG; 50 µm) rapidly eliminates the postsynaptic CP-AMPAR contribution to NAc synaptic transmission. This is accompanied by facilitation of Ca2+ -impermeable AMPAR (CI-AMPAR)-mediated transmission, suggesting that DHPG may promote an exchange between CP-AMPARs and CI-AMPARs. In saline controls, DHPG also reduced excitatory transmission but this occurred through a CB1 receptor-dependent presynaptic mechanism rather than an effect on postsynaptic AMPARs. Blockade of CB1 receptors had no significant effect on the alterations in AMPAR transmission produced by DHPG in the cocaine group. Interestingly, the effect of DHPG in the cocaine group was mediated by mGluR1 whereas its effect in the saline group was mediated by mGluR5. These results indicate that regulation of synaptic transmission in the NAc is profoundly altered after extended access cocaine self-administration and prolonged withdrawal. Furthermore, they suggest that activation of mGluR1 may represent a potential strategy for reducing cue-induced cocaine craving in abstinent cocaine addicts.

Dopamine neurons in the ventral tegmental area fire faster in adolescent rats than in adults.

Adolescence may be a period of vulnerability to drug addiction. In rats, elevated firing activity of ventral tegmental area (VTA) dopamine neurons predicts enhanced addiction liability. Our aim was to determine if dopamine neurons are more active in adolescents than in adults and to examine mechanisms underlying any age-related difference. VTA dopamine neurons fired faster in adolescents than in adults as measured with in vivo extracellular recordings. Dopamine neuron firing can be divided into nonbursting (single spikes) and bursting activity (clusters of high-frequency spikes). Nonbursting activity was higher in adolescents compared with adults. Frequency of burst events did not differ between ages, but bursts were longer in adolescents than in adults. Elevated dopamine neuron firing in adolescent rats was also observed in cell-attached recordings in ex vivo brain slices. Using whole cell recordings, we found that passive and active membrane properties were similar across ages. Hyperpolarization-activated cation currents and small-conductance calcium-activated potassium channel currents were also comparable across ages. We found no difference in dopamine D2-class autoreceptor function across ages, although the high baseline firing in adolescents resulted in autoreceptor activation being less effective at silencing neurons. Finally, AMPA receptor-mediated spontaneous excitatory postsynaptic currents occurred at lower frequency in adolescents; GABA(A) receptor-mediated spontaneous inhibitory postsynaptic currents occurred at both lower frequency and smaller amplitude in adolescents. In conclusion, VTA dopamine neurons fire faster in adolescence, potentially because GABA tone increases as rats reach adulthood. This elevation of firing rate during adolescence is consistent with it representing a vulnerable period for developing drug addiction.

Taste uncoupled from nutrition fails to sustain the reinforcing properties of food.

Recent findings suggest the reward system encodes metabolic value independent of taste, provoking speculation that the hedonic value of taste could be derived from nutritional value as a secondary appetitive property. We therefore dissociated and compared the impact of nutrition and taste on appetitive behavior in several paradigms. Though taste alone induces preference and increased consumption, in the absence of nutritional value its reinforcing properties are greatly diminished and it does not, like sucrose, induce increased responding over time. In agreement with behavioral data, saccharin-evoked (but not sucrose-evoked) dopamine release is greatly attenuated following pre-exposure, suggesting that nutritional value is critical for dopamine-mediated reward and reinforcement. Further supporting the primacy of nutrition over taste, genetically increased dopaminergic tone enhances incentive associated with nutritional value with minimal impact on taste-based, hedonic incentive. Overall, we suggest that the sensory-hedonic incentive value associated with taste functions as a conditioned stimulus that requires nutritional value to sustainably organize appetitive behavior.

Sucrose-predictive cues evoke greater phasic dopamine release than saccharin-predictive cues.

Cues that have been paired with food evoke dopamine in nucleus accumbens (NAc) and drive approach behavior. This cue-evoked dopamine signaling could contribute to overconsumption of food. One manner in which individuals try to restrict caloric intake is through the consumption of foods containing artificial (non-nutritive) sweeteners. We were interested in whether cues paired with a non-nutritive sweetener (saccharin) would evoke similar dopamine release as cues paired with a nutritive sweetener (sucrose). We trained food-restricted rats to associate distinct cues with sucrose or saccharin pellets. In the first group of rats, training sessions with each pellet took place on different days, maximizing the opportunity for rats to detect nutritional differences. After training, voltammetry recordings in NAc core revealed that sucrose cues evoked greater phasic dopamine release than saccharin cues. In a second group of rats, on each training day, sucrose and saccharin pellets were presented in pseudorandom order within the same session, to mask nutritional differences. In this condition, the difference in dopamine between sucrose and saccharin cues was attenuated, but not abolished. These results suggest that sucrose-paired cues will more powerfully motivate behavior than saccharin-paired cues. The differing responses to each cue seem to be driven by overall preference with both the nutritional value that the pellets predict as well as other factors, such as taste, contributing.

Restriction of dietary protein in rats increases progressive-ratio motivation for protein.

Low-protein diets can impact food intake and appetite, but it is not known if motivation for food is changed. In the present study, we used an operant behavioral task - the progressive ratio test - to assess whether motivation for different foods was affected when rats were maintained on a protein-restricted diet (REST, 5% protein diet) compared to non-restricted control rats (CON, 18% protein). Rats were tested either with nutritionally-balanced pellets (18.7% protein, Experiment 1) or protein-rich pellets (35% protein, Experiment 2) as reinforcers. Protein restriction increased breakpoint for protein-rich pellets, relative to CON rats, whereas no difference in breakpoint for nutritionally-balanced pellets was observed between groups. When given free access to either nutritionally-balanced pellets or protein-rich pellets, REST and CON rats did not differ in their intake. We also tested whether a previous history of protein restriction might affect present motivation for different types of food by assessing breakpoint of previously REST animals that were subsequently put on standard maintenance chow (protein-repleted rats, REPL, Experiment 2). REPL rats did not show increased breakpoint, relative to their initial encounter with protein-rich pellets while they were protein-restricted. This study demonstrates that restriction of dietary protein induces a selective increased motivation for protein-rich food, a behavior that disappears once rats are not in need of protein.

Primary food reward and reward-predictive stimuli evoke different patterns of phasic dopamine signaling throughout the striatum.

Phasic changes in dopamine activity play a critical role in learning and goal-directed behavior. Unpredicted reward and reward-predictive cues evoke phasic increases in the firing rate of the majority of midbrain dopamine neurons--results that predict uniformly broadcast increases in dopamine concentration throughout the striatum. However, measurement of dopamine concentration changes during reward has cast doubt on this prediction. We systematically measured phasic changes in dopamine in four striatal subregions [nucleus accumbens shell and core (Core), dorsomedial (DMS) and dorsolateral striatum] in response to stimuli known to activate a majority of dopamine neurons. We used fast-scan cyclic voltammetry in awake and behaving rats, which measures changes in dopamine on a similar timescale to the electrophysiological recordings that established a relationship between phasic dopamine activity and reward. Unlike the responses of midbrain dopamine neurons, unpredicted food reward and reward-predictive cues evoked a phasic increase in dopamine that was subregion specific. In rats with limited experience, unpredicted food reward evoked an increase exclusively in the Core. In rats trained on a discriminative stimulus paradigm, both unpredicted reward and reward-predictive cues evoked robust phasic dopamine in the Core and DMS. Thus, phasic dopamine release in select target structures is dynamic and dependent on context and experience. Because the four subregions assayed receive different inputs and have differential projection targets, the regional selectivity of phasic changes in dopamine has important implications for information flow through the striatum and plasticity that underlies learning and goal-directed behavior.

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

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.

Age-dependent effects of protein restriction on dopamine release.

Despite the essential role of protein intake for health and development, very little is known about the impact of protein restriction on neurobiological functions, especially at different stages of the lifespan. The dopamine system is a central actor in the integration of food-related processes and is influenced by physiological state and food-related signals. Moreover, it is highly sensitive to dietary effects during early life periods such as adolescence due to its late maturation. In the present study, we investigated the impact of protein restriction either during adolescence or adulthood on the function of the mesolimbic (nucleus accumbens) and nigrostriatal (dorsal striatum) dopamine pathways using fast-scan cyclic voltammetry in rat brain slices. In the nucleus accumbens, protein restriction in adults increased dopamine release in response to low and high frequency trains of stimulation (1-20 Hz). By contrast, protein restriction during adolescence decreased nucleus accumbens dopamine release. In the dorsal striatum, protein restriction at adulthood has no impact on dopamine release but the same diet during adolescence induced a frequency-dependent increase in stimulated dopamine release. Taken together, our results highlight the sensitivity of the different dopamine pathways to the effect of protein restriction, as well as their vulnerability to deleterious diet effects at different life stages.

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.

Mode of Sucrose Delivery Alters Reward-Related Phasic Dopamine Signals in Nucleus Accumbens.

In studies of appetitive Pavlovian conditioning, rewards are often delivered to subjects in a manner that confounds several processes. For example, delivery of a sugar pellet to a rodent requires movement to collect the pellet and is associated with sensory stimuli such as the sight and sound of the pellet arrival. Thus, any neurochemical events occurring in proximity to the reward may be related to multiple coincident phenomena. We used fast-scan cyclic voltammetry in rats to compare nucleus accumbens dopamine responses to two different modes of delivery: sucrose pellets, which require goal-directed action for their collection and are associated with sensory stimuli, and intraoral infusions of sucrose, which are passively received and not associated with external stimuli. We found that when rewards were unpredicted, both pellets and infusions evoked similar dopamine release. However, when rewards were predicted by distinct cues, greater dopamine release was evoked by pellet cues than infusion cues. Thus, dopamine responses to pellets, infusions as well as predictive cues suggest a nuanced role for dopamine in both reward seeking and reward evaluation.

No evidence that portion size influences food consumption in male Sprague Dawley rats.

In studies of eating behavior that have been conducted in humans, the tendency to consume more when given larger portions of food, known as the portion size effect (PSE), is one of the most robust and widely replicated findings. Despite this, the mechanisms that underpin it are still unknown. In particular, it is unclear whether the PSE arises from higher-order social and cognitive processes that are unique to humans or, instead, reflects more fundamental processes that drive feeding, such as conditioned food-seeking. Importantly, studies in rodents and other animals have yet to show convincing evidence of a PSE. In this series of studies, we used several methods to test for a PSE in adult male Sprague Dawley rats. Our approaches included using visually identifiable portions of a palatable food; training on a plate cleaning procedure; providing portion sizes of food pellets that were signaled by auditory and visual food-predictive cues; providing food with amorphous shape properties; and providing standard chow diet portions in home cages. In none of these manipulations did larger portions increase food intake. In summary, our data provide no evidence that a PSE is present in male Sprague Dawley rats, and if it is, it is more nuanced, dependent on experimental procedure, and/or smaller in size than it is in humans. In turn, these findings suggest that the widely-replicated PSE in humans may be more likely to reflect higher-order cognitive and social processes than fundamental conditioned behaviors.

Parallels and Overlap: The Integration of Homeostatic Signals by Mesolimbic Dopamine Neurons.

Motivated behaviors are often initiated in response to perturbations of homeostasis. Indeed, animals and humans have fundamental drives to procure (appetitive behaviors) and eventually ingest (consummatory behaviors) substances based on deficits in body fluid (e.g., thirst) and energy balance (e.g., hunger). Consumption, in turn, reinforces motivated behavior and is therefore considered rewarding. Over the years, the constructs of homeostatic (within the purview of the hypothalamus) and reward (within the purview of mesolimbic circuitry) have been used to describe need-based vs. need-free consumption. However, many experiments have demonstrated that mesolimbic circuits and "higher-order" brain regions are also profoundly influenced by changes to physiological state, which in turn generate behaviors that are poised to maintain homeostasis. Mesolimbic pathways, particularly dopamine neurons of the ventral tegmental area (VTA) and their projections to nucleus accumbens (NAc), can be robustly modulated by a variety of energy balance signals, including post-ingestive feedback relaying nutrient content and hormonal signals reflecting hunger and satiety. Moreover, physiological states can also impact VTA-NAc responses to non-nutritive rewards, such as drugs of abuse. Coupled with recent evidence showing hypothalamic structures are modulated in anticipation of replenished need, classic boundaries between circuits that convey perturbations in homeostasis and those that drive motivated behavior are being questioned. In the current review, we examine data that have revealed the importance of mesolimbic dopamine neurons and their downstream pathways as a dynamic neurobiological mechanism that provides an interface between physiological state, perturbations to homeostasis, and reward-seeking behaviors.

Restriction of dietary protein leads to conditioned protein preference and elevated palatability of protein-containing food in rats.

The mechanisms by which intake of dietary protein is regulated are poorly understood despite their potential involvement in determining food choice and appetite. In particular, it is unclear whether protein deficiency results in a specific appetite for protein and whether influences on diet are immediate or develop over time. To determine the effects of protein restriction on consumption, preference, and palatability for protein we assessed patterns of intake for casein (protein) and maltodextrin (carbohydrate) solutions in adult rats. To induce a state of protein restriction, rats were maintained on a low protein diet (5% casein) and compared to control rats on non-restricted diet (20% casein). Under these dietary conditions, relative to control rats, protein-restricted rats exhibited hyperphagia without weight gain. After two weeks, on alternate conditioning days, rats were given access to either isocaloric casein or maltodextrin solutions that were saccharin-sweetened and distinctly flavored whilst consumption and licking patterns were recorded. This allowed rats to learn about the post-ingestive nutritional consequences of the two different solutions. Subsequently, during a preference test when rats had access to both solutions, we found that protein-restricted rats exhibited a preference for casein over carbohydrate whereas non-restricted rats did not. Analysis of lick microstructure revealed that this preference was associated with an increase in cluster size and number, reflective of an increase in palatability. In conclusion, protein-restriction induced a conditioned preference for protein, relative to carbohydrate, and this was associated with increased palatability.