Adam Gordon-Fennell

Summary: When an animal feels ill after consumption of a novel food, it rapidly learns to avoid this food in the future, a form of learning termed conditioned flavour avoidance (CFA). Postingestive malaise often occurs long after consumption, necessitating a neural mechanism that can facilitate plasticity between temporally distant events. Neuromodulators, acting through G-protein-coupled receptors (GPCRs) that can influence neuronal excitability on extended timescales, may facilitate this process. The projection of parabrachial (PB) Calca neurons to the central amygdala (CeA) promotes formation of CFA. Here, we demonstrate that these neurons overlap with a PB population that releases the neuromodulator acetylcholine (ACh) in the CeA during both consumption of a novel solution and subsequent visceral malaise, consistent with a role in CFA acquisition. Calcium imaging in brain slices reveals that ACh widely activates CeA neurons and enhances glutamatergic responsivity on a timescale consistent with CFA learning. CRISPR-Cas9-mediated genetic knockdown and optogenetics demonstrate that ACh from PB facilitates CFA acquisition. Loss of ACh signalling to CeA blocks key signatures of CFA-associated plasticity. Together, these data reveal that cholinergic input from PB to central amygdala is a critical neuromodulatory signal that links activity over long timespans to facilitate associative learning.

Cognitive flexibility refers to the adaptive neural processes that adjust learned behaviours as circumstances shift, supporting optimal decision-making and behavioural control. This includes the capacity to modify specific behaviours as the contingency between cues and rewards degrades. Across species^1,2,3,4, the medial prefrontal cortex (mPFC) has a well-established role in controlling contingency degradation⁵; however, the precise neural circuit mechanisms underlying this cognitive process remain unclear. To address this gap, we developed a quantitative model of cognitive flexibility that incorporates a meta-learning parameter into an established reward prediction error learning model^6,7. Our meta-reward prediction error model significantly improves accurate representation of mouse cue-evoked licking behaviour in response to degraded or enhanced cue–reward associations. Using longitudinal two-photon calcium imaging and single-cell holographic optogenetics, we found that a subset of neurons in the mPFC specifically encode the contingency degradation in a significant and causal manner. Recognizing that behavioural flexibility probably requires interactions between the mPFC and canonical reward learning circuitry, we then examined how mPFC neural signalling during contingency degradation interacts with the ventral tegmental area (VTA)—a critical hub for reward processing⁸. Our imaging and optogenetics data show that mPFC sends this signal to VTA, with most mPFC→VTA neurons reflecting this transmission, and that selective optogenetic stimulation of these ensembles accelerates contingency degradation. These findings reveal how prefrontal circuits facilitate flexibility, selectively halting learned behaviours through connections with subcortical reward networks.

The neurotransmitter histamine is involved in control of food intake, yet its dynamics during individual feeding episodes remain unexplored. Therefore, we used the novel genetically‐encoded histamine sensor, HisLightG, combined with fiber photometry to measure histamine release in two hypothalamic regions critical for the food‐suppressive effects of histamine, the paraventricular nucleus of the hypothalamus (PVH), and the ventromedial hypothalamus (VMH). Male mice were tested under different conditions to assess whether hunger, time of day, or the caloric content of the solution they were given affected histamine fluctuations. We found that histamine levels changed rapidly in response to eating. These histamine fluctuations were influenced by experimental conditions, with slightly smaller responses when the test solution was sucralose (both regions) or during the light cycle (PVH only). Notable regional differences were identified, such that in the PVH histamine rebounded to above baseline levels, whereas in the VMH histamine remained lower than baseline for at least 10 s after licking ceased. In a separate cohort of male and female mice, enhancing histamine tone via administration of a histamine precursor (L‐histidine) reduced the number of licks across multiple sucrose concentrations. Together, these findings indicate that histaminergic activity is modulated rapidly during ingestive episodes, and that understanding these release patterns will give insight into histamine's role in appetite suppression. image

SUMMARY: The lateral hypothalamic area (LHA) contains GABAergic and glutamatergic neurons that converge on the midbrain dopamine system and exert opposing influences on consummatory feeding behavior. However, the activity dynamics of these populations during consumption and their impact on striatal dopamine release remain poorly understood. Here, we show that LHA GABAergic and glutamatergic neurons independently scale their activity during the consumption of rewarding and aversive solutions while cooperatively regulating dopamine release along the anterior-posterior axis of the striatum in mice. Dopamine release exhibited widespread modulation during consumption, with anterior striatal regions showing stronger representation of solution value and its recent history—an effect dependent on LHA activity. These findings suggest that the LHA acts as a central regulator of striatal dopamine release during consumption, providing a mechanism through which hypothalamic circuits influence motivational and consummatory behaviors.

The neurotransmitter histamine is involved in control of food intake, yet its dynamics during individual feeding episodes remain unexplored. Therefore, we used the novel genetically-encoded histamine sensor, HisLightG, combined with fiber photometry to measure histamine release in two hypothalamic regions critical for the food-suppressive effects of histamine, the paraventricular nucleus of the hypothalamus (PVH), and the ventromedial hypothalamus (VMH). Male mice were tested under different conditions to assess whether hunger, time of day, or the caloric content of the solution they were given affected histamine fluctuations. We found that histamine levels changed rapidly in response to eating. These histamine fluctuations were influenced by experimental conditions, with slightly smaller responses when the test solution was sucralose (both regions) or during the light cycle (PVH only). Notable regional differences were identified, such that in the PVH histamine rebounded to baseline levels, whereas in the VMH histamine remained lower than baseline for at least 10 seconds after licking ceased. In a separate cohort of male and female mice, enhancing histamine tone via administration of a histamine precursor (L-histidine) reduced the number of licks across multiple sucrose concentrations. Together, these findings indicate that histaminergic activity is modulated rapidly during ingestive episodes, and that understanding these release patterns will give insight into histamine’s role in appetite suppression.

Understanding how neural ensemble dynamics evolve over time is crucial for grasping the mechanisms of neural and behavioral processes. Longitudinal identification of single cells profiled during these processes using activity alone is challenging when neural activity is anticipated to change over time. To address this, optical imaging approaches, such as those capturing calcium-induced fluorescence changes, align cell morphology and field position instead of activity to consistently track cell activity over days to weeks. A prominent optical method that both visualizes and records cellular activity is fluorescence microscopy of genetically encoded voltage, neuromodulator, or calcium indicators, such as GCaMP.¹ Because somatic calcium influx occurs coincident with neural action potentials,² the rise in GCaMP fluorescence can serve as an indicator for neural activity. In addition, this fluorescence can be averaged throughout a session to capture a spatial “footprint” of the cells within the field of view (FOV).¹

Head-fixed behavioral experiments in rodents permit unparalleled experimental control, precise measurement of behavior, and concurrent modulation and measurement of neural activity. Here, we present OHRBETS (Open-Source Head-fixed Rodent Behavioral Experimental Training System; pronounced ‘Orbitz’), a low-cost, open-source platform of hardware and software to flexibly pursue the neural basis of a variety of motivated behaviors. Head-fixed mice tested with OHRBETS displayed operant conditioning for caloric reward that replicates core behavioral phenotypes observed during freely moving conditions. OHRBETS also permits optogenetic intracranial self-stimulation under positive or negative operant conditioning procedures and real-time place preference behavior, like that observed in freely moving assays. In a multi-spout brief-access consumption task, mice displayed licking as a function of concentration of sucrose, quinine, and sodium chloride, with licking modulated by homeostatic or circadian influences. Finally, to highlight the functionality of OHRBETS, we measured mesolimbic dopamine signals during the multi-spout brief-access task that display strong correlations with relative solution value and magnitude of consumption. All designs, programs, and instructions are provided freely online. This customizable platform enables replicable operant and consummatory behaviors and can be incorporated with methods to perturb and record neural dynamics in vivo.