…

The nucleus accumbens shell occupies a distinctive position within the brain’s reward circuitry, receiving dense projections from the ventral tegmental area and sending outputs that interface with limbic structures governing emotion and motivation. This anatomical positioning helps explain why the shell appears especially sensitive to the hedonic qualities of consumed substances rather than to the more abstract decision variables handled by neighboring regions. When a rat tastes sucrose for the first time, the convergence of gustatory input from the brainstem and hypothalamic hunger and satiety signals creates a moment of maximal informational value, a situation in which the nervous system must decide whether this new flavor predicts benefit or harm. The neurochemical measurements obtained through microdialysis in this study capture that moment directly, sampling the extracellular fluid surrounding shell neurons at intervals while animals freely consume sugar solutions of varying familiarity.

Microdialysis itself deserves attention as a method, because it differs fundamentally from faster imaging techniques that dominate contemporary neuroscience reporting. A fine probe with a semipermeable membrane is lowered into the target region, and artificial cerebrospinal fluid perfuses slowly through it, collecting molecules that diffuse inward from the extracellular space. The collected samples are then quantified with high-performance liquid chromatography coupled to electrochemical detection, a technique well suited to small, electroactive molecules such as dopamine and, with appropriate enzymatic steps, acetylcholine. The temporal resolution is on the order of minutes rather than milliseconds, which means the measurements reflect sustained changes in tonic signaling rather than the brief phasic bursts that individual neurons fire. Nevertheless, these slower signals are physiologically meaningful, because tonic extracellular dopamine shapes the overall excitability of medium spiny neurons and sets the gain on how subsequent phasic events are interpreted. The choice to sample during actual consumption rather than during passive infusion preserves the natural coordination of taste, smell, swallowing, and postingestive feedback that accompanies real eating behavior.

The divergent patterns observed across the three familiarity groups illustrate a principle that reward researchers have increasingly emphasized: the response of the mesolimbic system to a stimulus is not fixed but depends on the animal’s history with that stimulus. Novel sucrose produced a robust and prolonged elevation of acetylcholine in the shell, persisting even after the sugar was removed, which is consistent with the idea that cholinergic interneurons flag salient or unexpected events for further processing. These interneurons, though numbering only a small fraction of the neurons in the striatum, exert outsized influence through their widespread axonal arbors and their capacity to modulate dopamine release via nicotinic receptors located on dopaminergic terminals. When acetylcholine surges in this way, it can promote the kind of attentional sharpening that helps an animal encode the features of a new food, including any aversive qualities that later guide avoidance. The finding that this cholinergic response is absent or even reversed after prolonged exposure suggests that the shell’s internal circuitry has been recalibrated by experience, so that a once-salient flavor no longer commands the same attentional resources.

The dopamine results add an important nuance to the familiar narrative that sugar simply drives dopamine upward. In the mildly familiarized group, dopamine rose during consumption, matching the classical picture of palatable food engaging mesolimbic transmission. Yet in the novel group, no such increase appeared, and in the highly familiarized group the response was likewise absent. This inverted pattern, with the strongest dopaminergic activation occurring at intermediate familiarity, echoes the reward prediction error framework in which dopamine neurons respond most vigorously when outcomes exceed expectations. A completely novel taste may be too ambiguous to generate a confident prediction, whereas a taste experienced once or twice now carries a positive expectation that is fulfilled upon consumption, producing a prediction error signal. After two weeks of continuous access, the sugar has presumably lost its capacity to generate such errors, because its value is fully anticipated, and the dopaminergic system accordingly falls silent. This interpretation aligns with broader findings that dopamine responses habituate with repeated exposure to the same reward, even as the animal continues to consume it eagerly.

The elevated baseline acetylcholine observed in the highly familiarized group is perhaps the most intriguing finding, because it suggests a lasting change in the resting state of the shell rather than merely an altered response to the sugar itself. Elevated tonic acetylcholine could reflect compensatory upregulation following chronic stimulation, or it could indicate a shift in the balance of cholinergic tone that alters how the shell processes all incoming information, not just sugar-related signals. During consumption, this elevated baseline dropped, meaning the relative change from baseline was negative even though absolute levels may have remained within normal ranges. Such a decrease during consumption of a highly familiar reward could represent a release from attentional engagement, a neurochemical signature of automaticity in which the animal consumes without the deliberative processing that novel flavors require. The negative correlation between familiarity and both dopaminergic and cholinergic responsiveness implies that these two transmitter systems, though often studied separately, are functionally coupled in the shell, with cholinergic interneurons gating when dopaminergic terminals release their contents.

The behavioral consequences of these neuroadaptations may extend beyond simple habituation. Prior work from the same laboratory indicated that rats with long-term sugar experience were impaired in learning new negative consequences associated with a familiar sweet taste, a form of appetitive relearning deficit that could contribute to escalated intake in environments where sugar availability fluctuates. If the shell’s cholinergic and dopaminergic signals normally support the formation of aversive associations by marking unexpected or devalued outcomes, then the blunted signaling observed after chronic consumption would plausibly weaken that protective learning. In other words, an animal whose shell has adapted to sugar may continue to treat the familiar flavor as safe and desirable even when new information indicates otherwise, a mechanism with obvious relevance to overconsumption in humans who encounter sugar in countless familiar contexts.

Insulin signaling provides a metabolic bridge between dietary history and these neurochemical changes. Cholinergic interneurons in the striatum express insulin receptors, and insulin excites these cells while simultaneously facilitating dopamine release at dopaminergic terminals. Chronic high-calorie consumption alters circulating insulin and the sensitivity of peripheral and central tissues to it, raising the possibility that the neuroadaptations observed here are partly mediated by metabolic hormones acting directly on the shell circuitry. Evidence that insulin manipulation affects the hedonic response to glucose but not to non-nutritive sweeteners suggests a selective coupling between metabolic state and the processing of nutritive sugars, a coupling that could explain why repeated sugar exposure, with its accompanying postingestive nutrient feedback, produces stronger adaptations than mere taste familiarity alone would predict.

Several limitations frame the interpretation of these findings. The study examined adult male rats, leaving open questions about sex differences, which are well documented in both dopaminergic and cholinergic signaling and in the metabolic consequences of sugar consumption. The sugar exposure consisted of a ten percent sucrose solution available ad libitum for fourteen days, a regimen that models voluntary chronic intake but differs from the intermittent access protocols used in some addiction-modeling studies, which can produce more pronounced binge-like patterns. Microdialysis sampling in freely moving animals necessarily involves habituation to the testing chambers, and although the investigators took care to habituate animals before sampling, the chamber environment itself differs from the home cage in ways that could influence baseline transmitter levels. Finally, extracellular measurements cannot distinguish among the multiple cellular sources contributing to the sampled signal, including dopaminergic terminals from the ventral tegmental area, cholinergic interneurons within the shell, and modulatory inputs from the prefrontal cortex, amygdala, and hippocampus that converge on this region.

Looking forward, these results suggest several productive avenues. Directly manipulating cholinergic receptors in the shell during sugar consumption could test whether the cholinergic decline after chronic exposure is causally involved in impaired aversive learning or merely a correlate. Comparing the shell with the core in the same animals would clarify how these neighboring subregions divide the labor of reward processing across familiarity levels. Extending the approach to females and to different sugar exposure patterns would establish the generality of the findings. And because the shell participates in memory formation for hedonic stimuli, examining how chronic sugar alters other forms of learning that depend on this region, including context-based associations and outcome devaluation, would reveal whether the neuroadaptations observed here represent a narrow tuning to sugar or a broader change in how the shell encodes value. The demonstration that prolonged consumption reshapes the neurochemical landscape of a key reward region in ways that negatively correlate with stimulus familiarity provides a concrete neurobiological foundation for understanding why dietary habits become entrenched and why familiar palatable foods can be so difficult to moderate once they have become a routine part of daily life.

Subject of Research: …

Article Title: Long-term sugar consumption alters extracellular dopamine and acetylcholine levels in the nucleus accumbens shell

Article References: Vera-Rivera, G., Olvera, M. J., & Miranda, M. I. (2026). Long-term sugar consumption alters extracellular dopamine and acetylcholine levels in the nucleus accumbens shell. Frontiers in Behavioral Neuroscience, 20, Article 1938066. https://doi.org/10.3389/fnbeh.2026.1938066

Image Credits: AI Generated

DOI: 10.3389/fnbeh.2026.1938066

Keywords: Long-term, sugar, consumption, alters, extracellular, dopamine, acetylcholine, levels, nucleus, accumbens, shell, scientific research