Pontine and thalamic influences on fluid rewards: III. Anticipatory contrast for sucrose and corn oil

Altered reward sensitivity to sucrose outcomes prior to drug exposure in alcohol preferring rats

Addiction involves key impairments in reward sensitivity (RS). The current study explored impaired RS to natural reward as a predisposing factor to addictive-like behavior. Alcohol preferring (P) rats are selectively bred based on significantly greater ethanol consumption and preference and offer the ability to inspect differences in subjects with a positive family history of addictive-like behavior. P rat's RS was compared to RS in the well-used Sprague-Dawley (SD) strain. To assess RS in a novel manner, instrumental incentive contrast, discrimination and consumption of sucrose solution were examined. Animals performed in a free operant situation for different sucrose concentration solutions using a block of 'mixed' trials with alternating outcome concentrations (e.g., 5 and 10 % sucrose) to change outcome value in a predictable manner. Animals also performed for reward in blocks of single outcome trials (5 or 10 or 20 or 40 % sucrose daily exposure) surrounding the mixed block. RS (e.g., reward discrimination and contrast effects between and within-sessions) was measured by changes in trials completed, instrumental response latency and consumption. P rats expressed an altered profile of RS with a greater tendency toward equivalent responding to different outcomes within the same session and an absence of incentive contrast from diverse reward comparisons. In contrast, SD animals expressed within-session reward discrimination and a subset of incentive contrast effects. These effects were moderated by food deprivation more consistently in SD compared to P rats. P rat alterations in processing natural rewards could predispose them to addictive-like behaviors including greater alcohol consumption and preference.

Satb2 neurons in the parabrachial nucleus mediate taste perception

The neural circuitry mediating taste has been mapped out from the periphery to the cortex, but genetic identity of taste-responsive neurons has remained elusive. Here, we describe a population of neurons in the gustatory region of the parabrachial nucleus that express the transcription factor Satb2 and project to taste-associated regions, including the gustatory thalamus and insular cortex. Using calcium imaging in awake, freely licking mice, we show that Satb2 neurons respond to the five basic taste modalities. Optogenetic activation of these neurons enhances taste preferences, whereas chronic inactivation decreases the magnitude of taste preferences in both brief- and long-access taste tests. Simultaneous inactivation of Satb2 and calcitonin gene-related peptide neurons in the PBN abolishes responses to aversive tastes. These data suggest that taste information in the parabrachial nucleus is conveyed by multiple populations of neurons, including both Satb2 and calcitonin gene-related peptide neurons.

Satb2 neurons in the parabrachial nucleus mediate taste perception

The neural circuitry mediating taste has been mapped out from the periphery to the cortex, but genetic identity of taste-responsive neurons has remained elusive. Here, we describe a population of neurons in the gustatory region of the parabrachial nucleus that express the transcription factor Satb2 and project to taste-associated regions, including the gustatory thalamus and insular cortex. Using calcium imaging in awake, freely licking mice, we show that Satb2 neurons respond to the five basic taste modalities. Optogenetic activation of these neurons enhances taste preferences, whereas chronic inactivation decreases the magnitude of taste preferences in both brief- and long-access taste tests. Simultaneous inactivation of Satb2 and calcitonin gene-related peptide neurons in the PBN abolishes responses to aversive tastes. These data suggest that taste information in the parabrachial nucleus is conveyed by multiple populations of neurons, including both Satb2 and calcitonin gene-related peptide neurons.

Striatal Activity and Reward Relativity: Neural Signals Encoding Dynamic Outcome Valuation

The striatum is a key brain region involved in reward processing. Striatal activity has been linked to encoding reward magnitude and integrating diverse reward outcome information. Recent work has supported the involvement of striatum in the valuation of outcomes. The present work extends this idea by examining striatal activity during dynamic shifts in value that include different levels and directions of magnitude disparity. A novel task was used to produce diverse relative reward effects on a chain of instrumental action. Rats (Rattus norvegicus) were trained to respond to cues associated with specific outcomes varying by food pellet magnitude. Animals were exposed to single-outcome sessions followed by mixed-outcome sessions, and neural activity was compared among identical outcome trials from the different behavioral contexts. Results recording striatal activity show that neural responses to different task elements reflect incentive contrast as well as other relative effects that involve generalization between outcomes or possible influences of outcome variety. The activity that was most prevalent was linked to food consumption and post-food consumption periods. Relative encoding was sensitive to magnitude disparity. A within-session analysis showed strong contrast effects that were dependent upon the outcome received in the immediately preceding trial. Significantly higher numbers of responses were found in ventral striatum linked to relative outcome effects. Our results support the idea that relative value can incorporate diverse relationships, including comparisons from specific individual outcomes to general behavioral contexts. The striatum contains these diverse relative processes, possibly enabling both a higher information yield concerning value shifts and a greater behavioral flexibility.

Drug-motivated behavior in rats with lesions of the thalamic orosensory area

Rats suppress intake of a palatable taste cue when paired with a rewarding or an aversive stimulus in appetitive or aversive conditioning, respectively. A similar phenomenon occurs with drugs of abuse, but the nature of this conditioning has been subject for debate. While relatively little is known about the underlying neural circuitry, we recently reported bilateral lesions of the thalamic trigeminal orosensory area isolate drug-induced suppression of intake of a taste cue. The lesion blocks avoidance of the taste cue when paired with experimenter delivered drugs of abuse, yet has no effect on avoidance of the same cue when paired with an aversive agent or when it predicts access to a highly palatable sucrose solution. We hypothesize the lesion may blunt the rewarding properties of the drug. To test this, we used a runway apparatus, as running speed has been shown to increase with increasing reward value. Our hypothesis was supported by failure of the lesioned rats to increase running speed for morphine. Interestingly, lesioned rats did avoid intake of the drug-paired cue when presented in the runway apparatus and displayed naloxone-precipitated withdrawal. Using a partial crossover design, the lesion prevented avoidance of a cocaine-paired cue when presented in the home cage. We conclude that the lesion disrupts avoidance of a taste cue in anticipation of the rewarding properties of a drug but, at least in the presence of contextual cues, allows for avoidance of a taste cue as it elicits the onset of an aversive conditioned state of withdrawal.

Relative reward effects on operant behavior: Incentive contrast, induction and variety effects

Comparing different rewards automatically produces dynamic relative outcome effects on behavior. Each new outcome exposure is to an updated version evaluated relative to alternatives. Relative reward effects include incentive contrast, positive induction and variety effects. The present study utilized a novel behavioral design to examine relative reward effects on a chain of operant behavior using auditory cues. Incentive contrast is the most often examined effect and focuses on increases or decreases in behavioral performance after value upshifts (positive) or downshifts (negative) relative to another outcome. We examined the impact of comparing two reward outcomes in a repeated measures design with three sessions: a single outcome and a mixed outcome and a final single outcome session. Relative reward effects should be apparent when comparing trials for the identical outcome between the single and mixed session types. An auditory cue triggered a series of operant responses (nosepoke-leverpress-food retrieval), and we measured possible contrast effects for different reward magnitude combinations. We found positive contrast for trials with the greatest magnitude differential but positive induction or variety effects in other combinations. This behavioral task could be useful for analyzing environmental or neurobiological factors involved in reward comparisons, decision-making and choice during instrumental, goal-directed action.

Conditioned Aversion for a Cocaine-Predictive Cue is Associated with Cocaine Seeking and Taking in Rats

Rats emit aversive taste reactivity (TR) behavior (i.e., gapes) following intraoral delivery of a cocaine-paired taste cue, and greater conditioned aversive TR in well-trained rats predicts greater drug-taking. Here, we used a between-groups design and tracked the development of this conditioned aversive TR behavior on a trial by trial basis in an effort to determine when the change in behavior occurs and at what point individual differences in cue reactivity become predictive of cocaine-seeking and cocaine-taking. The results demonstrate that conditioned aversive TR to a cocaine-predictive flavor cue appears very early in training (i.e., following as few as 1 to 2 taste-drug pairings), stabilizes quickly, and becomes predictive of terminal self-administration within 3 to 4 trials. Indeed, rats exhibiting high conditioned aversive TR to the cocaine-paired cue also exhibited greater goal-directed behavior, were faster to take drug, self-administered more cocaine, and exhibited greater seeking during periods of drug non-availability. High conditioned aversive TR, then, develops quickly and is associated with a greater motivation for drug.

Thalamic contribution to cortical processing of taste and expectation

Taste-related information reaches the gustatory cortex (GC) through two routes: a thalamic and a limbic pathway. While evidence is accumulating on limbic-cortical interactions in taste, very little information is available on the function of the gustatory thalamus in shaping GC activity. Here we rely on behavioral electrophysiological techniques to study taste-evoked activity in GC before and after inactivation of the parvicellular portion of the ventroposteromedial nucleus of thalamus (VPMpc; i.e., the gustatory thalamus). Gustatory stimuli were presented to rats either alone or preceded by an anticipatory cue. The reliance on two different behavioral contexts allowed us to investigate how the VPMpc mediates GC responses to uncued tastants, cued tastants, and anticipatory cues. Inactivation of the thalamus resulted in a dramatic reduction of taste processing in GC. However, responses to anticipatory cues were unaffected by this manipulation. The use of a cue-taste association paradigm also allowed for the identification of two subpopulations of taste-specific neurons: those that responded to gustatory stimulation and to the cue (i.e., cue-and-taste) and those that responded to tastants only (i.e., taste-only). Analyses of these two populations revealed differences in response dynamics and connectivity with the VPMpc. The results provide novel evidence for the role of VPMpc in shaping GC activity and demonstrate a previously unknown association between responsiveness to behavioral events, temporal dynamics, and thalamic connectivity in GC.

Thalamic contribution to cortical processing of taste and expectation

Taste-related information reaches the gustatory cortex (GC) through two routes: a thalamic and a limbic pathway. While evidence is accumulating on limbic-cortical interactions in taste, very little information is available on the function of the gustatory thalamus in shaping GC activity. Here we rely on behavioral electrophysiological techniques to study taste-evoked activity in GC before and after inactivation of the parvicellular portion of the ventroposteromedial nucleus of thalamus (VPMpc; i.e., the gustatory thalamus). Gustatory stimuli were presented to rats either alone or preceded by an anticipatory cue. The reliance on two different behavioral contexts allowed us to investigate how the VPMpc mediates GC responses to uncued tastants, cued tastants, and anticipatory cues. Inactivation of the thalamus resulted in a dramatic reduction of taste processing in GC. However, responses to anticipatory cues were unaffected by this manipulation. The use of a cue-taste association paradigm also allowed for the identification of two subpopulations of taste-specific neurons: those that responded to gustatory stimulation and to the cue (i.e., cue-and-taste) and those that responded to tastants only (i.e., taste-only). Analyses of these two populations revealed differences in response dynamics and connectivity with the VPMpc. The results provide novel evidence for the role of VPMpc in shaping GC activity and demonstrate a previously unknown association between responsiveness to behavioral events, temporal dynamics, and thalamic connectivity in GC.

Bilateral lesions of the thalamic trigeminal orosensory area dissociate natural from drug reward in contrast paradigms

Substance abuse and addiction are associated with an apparent devaluation of, and inattention to, natural rewards. This consequence of addiction can be modeled using a reward comparison paradigm where rats avoid intake of a palatable taste cue that comes to predict access to a drug of abuse. Evidence suggests rats avoid intake following such pairings, at least in part, because the taste cue pales in comparison to the highly rewarding drug expected in the near future. In accordance, lesions of the gustatory thalamus or cortex eliminate avoidance of a taste cue when paired with either a drug of abuse or a rewarding sucrose solution, but not when paired with the aversive agent, LiCl. The present study used bilateral ibotenic acid lesions to evaluate the role of a neighboring thalamic structure, the trigeminal orosensory area (TOA), in avoidance of a gustatory cue when paired with sucrose (experiment 1), morphine (experiment 2), cocaine (experiment 3), or LiCl (experiment 4). The results show that the TOA lesion disrupts, but does not eliminate avoidance of a taste cue that predicts access to a preferred sucrose solution and leaves intact the development of a LiCl-induced conditioned taste aversion. The lesion does, however, eliminate the suppression of intake of a taste cue when paired with experimenter-administered morphine or cocaine using our standard parameters. As such, this is the first manipulation found to dissociate avoidance of a taste cue when mediated by a sweet or by a drug of abuse.