Molecular mechanisms of taste-recognition memory

The Neural Basis of Salt Perception: A Focus on Potassium Chloride as a Sodium Alternative

Excessive dietary sodium intake is a major risk factor for hypertension, prompting interest in potassium chloride (KCl) as a sodium chloride (NaCl) alternative. While KCl preserves saltiness, its neural processing compared to NaCl remains underexplored. This study investigates the neural correlates of taste perception for NaCl, KCl, and their mixture using gustatory event-related potentials (ERPs) in a sample of 28 healthy young adults. Participants rated the intensity, saltiness, and pleasantness of the stimuli, which were matched for iso-intensity and iso-pleasantness. High-density EEG data revealed distinct microstate patterns associated with each condition, particularly in the later stages of processing, which align with the endogenous phases of taste perception. Source localization identified the insula and opercular regions as primary sites for gustatory processing, with specific differences in activation patterns between NaCl and KCl. These findings suggest that while KCl elicits comparable behavioral responses to NaCl, its neural representation involves unique processes that may reflect its distinct chemical properties. This study advances our understanding of the neural dynamics of salt taste perception, providing insights into the potential use of KCl as a potentially healthier alternative in dietary interventions.

Scopolamine infusion in the basolateral amygdala after saccharin intake induces conditioned taste avoidance in rats

RATIONALE

Muscarinic receptor activity in the basolateral amygdala (BLA) is known to be involved in plasticity mechanisms that underlie emotional learning. The BLA is involved in the Attenuation of Neophobia, an incidental taste learning task in which a novel taste becomes familiar and recognized as safe.

OBJECTIVE

Here we assessed the role of muscarinic receptor activity in the BLA in incidental taste learning.

METHODS

Young adult male Wistar rats were bilaterally implanted with cannulas aimed at BLA. After recovery, rats were randomly assigned to either vehicle or muscarinic antagonist group, for each experiment. We tested the effect of specific and non-specific muscarinic antagonists administered either 1) 20 min before novel taste presentation; 2) immediately after novel taste presentation; 3) immediately after retrieval (the second taste presentation on Day 5 -S2-) or immediately after the fifth taste presentation on Day 8 (S5).

RESULTS

Non-specific muscarinic receptor antagonist scopolamine infused prior to novel taste, while not affecting novel taste preference, abolished AN, i.e., the increased preference observed in control animals on the second presentation. When administered after taste consumption, intra-BLA scopolamine not only prevented AN but caused a steep decrease in the taste preference on the second presentation. This scopolamine-induced taste avoidance was not dependent on taste novelty, nor did it generalize to another novel taste. Targeting putative postsynaptic muscarinic receptors with specific M1 or M3 antagonists appeared to produce a partial taste avoidance, while M2 antagonism had no effect.

CONCLUSION

These data suggest that if a salient gustatory experience is followed by muscarinic receptors antagonism in the BLA, it will be strongly and persistently avoided in the future. The study also shows that scopolamine is not just an amnesic drug, and its cognitive effects may be highly dependent on the task and the structure involved.

Investigations on the Ability of the Insular Cortex to Process Peripheral Immunosuppression

The brain and immune system communicate through complex bidirectional pathways, but the specificity by which the brain perceives or even remembers alterations in immune homeostasis is still poorly understood. Recent data revealed that immune-related information under peripheral inflammatory conditions, termed as "immunengram", were represented in specific neuronal ensembles in the insular cortex (IC). Chemogenetic reactivation of these neuronal ensembles was sufficient to retrieve the inflammatory stages, indicating that the brain can store and retrieve specific immune responses. Against this background, the current approach was designed to investigate the ability of the IC to process states of immunosuppression pharmacologically induced by the mechanistic target of rapamycin (mTOR) inhibitor rapamycin. We here show that the IC perceives the initial state of immunosuppression, reflected by increased deep-brain electroencephalography (EEG) activity during acute immunosuppressive drug treatment. Following an experienced period of immunosuppression, though, diminished splenic cytokine production as formerly induced by rapamycin could not be reinstated by nonspecific chemogenetic activation or inhibition of the IC. These findings suggest that the information of a past, or experienced status of pharmacologically induced immunosuppression is not represented in the IC. Together, the present work extends the view of immune-to-brain communication during the states of peripheral immunosuppression and foster the prominent role of the IC for interoception.

Locus Ceruleus Dynamics Are Suppressed during Licking and Enhanced Postlicking Independent of Taste Novelty

Attending to salient sensory attributes of food, such as tastes that are new, displeasing, or unexpected, allows the procurement of nutrients without food poisoning. Exposure to new tastes is known to increase norepinephrine (NE) release in taste processing forebrain areas, yet the central source for this release is unknown. Locus ceruleus norepinephrine neurons (LC-NE) emerge as a candidate in signaling salient information about taste, as other salient sensory stimuli (e.g., visual, auditory, somatosensation) are known to activate LC neurons. To determine if LC neurons are sensitive to features of taste novelty, we used fiber photometry to record LC-NE activity in water-restricted mice that voluntarily licked either novel or familiar substances of differential palatability (saccharine, citric acid). We observed that LC-NE activity was suppressed during lick bursts and transiently activated upon the termination of licking and that these dynamics were independent of the familiarity of the substance consumed. We next recorded LC dynamics during brief and unexpected consumption of tastants and found no increase in LC-NE activity, despite their responsiveness to visual and auditory stimuli, revealing selectivity in LC's responses to salient sensory information. Our findings suggest that LC activity during licking is not influenced by taste novelty, implicating a possible role for non-LC noradrenergic nuclei in signaling critical information about taste.

The anterior insula and its projection to amygdala nuclei modulate the abstinence-exacerbated expression of conditioned place preference

RATIONALE

Relapse into substance use is often triggered by exposure to drug-related environmental cues. The magnitude of drug seeking depends on the duration of abstinence, a phenomenon known as the incubation of drug craving. Clinical and preclinical research shows that the insular cortex is involved in substance use disorders and cue-induced drug seeking. However, the role of the insula on memory retrieval and motivational integration for cue-elicited drug seeking remains to be determined.

OBJECTIVES

We investigated the role of the anterior insular cortex (aIC) and its glutamatergic projection to amygdala nuclei (aIC-AMY) on the expression of conditioned place preference (CPP) during early and late abstinence.

METHODS

Male adult C57BL/6J mice underwent amphetamine-induced CPP, and their preference was tested following 1 or 14 days of abstinence. aIC and aIC-AMY functional role in CPP expression was assessed at both abstinence periods by employing optogenetic silencing and behavioral pharmacology.

RESULTS

Compared to a single day, an exacerbated preference for the amphetamine-paired context was observed after 14 days of abstinence. Photoinhibition of either aIC or aIC-AMY projection reduced CPP expression following late but not early abstinence. Similarly, the antagonism of aIC NMDA receptors reduced CPP expression after 14 days of abstinence but not 1 day.

CONCLUSIONS

These results suggest that aIC and its glutamatergic output to amygdala nuclei constitute critical neurobiological substrates mediating enhanced motivational cue reactivity during the incubation of amphetamine craving rather than contextual memory recall. Moreover, cortical NMDA receptor signaling may become sensitized during abstinence, ultimately modulating disproportioned drug seeking.

A neural substrate for short-term taste memories

Real-time decisions on what foods to select for consumption, particularly in the wild, require a sensitive sense of taste and an effective system to maintain short-term taste memories, also defined as working memory in the scale of seconds. Here, we used a behavioral memory assay, combined with recordings of neural activity, to identify the brain substrate for short-term taste memories. We demonstrate that persistent activity in taste cortex functions as an essential memory trace of a recent taste experience. Next, we manipulated the decay of this persistent activity and showed that early termination of the memory trace abolished the memory. Notably, extending the memory trace by transiently disinhibiting taste cortical activity dramatically extended the retention of a short-term taste memory. Together, our results uncover taste cortex as a neural substrate for working memory and substantiate the role of sensory cortex in memory-guided actions while imposing meaning to a sensory stimulus.

Effects of food deprivation on conditioned orthonasal olfactory preferences with caloric and non-caloric reinforcers

Three experiments were conducted to investigate Conditioned Olfactory Preferences using orthonasal inhalation, which is a less explored perceptual pathway compared to retronasal inhalation. In these experiments, odors were impregnated onto plastic disks to prevent the subjects from consuming or tasting them. The reinforcers used were a sucrose solution (Caloric groups) and a saccharin solution (Non-Caloric groups). The influence of nutritional deprivation was analyzed, with unrestricted access to food throughout the procedure in Experiment 1, food restriction during the conditioning phase in Experiment 2, and limited access to food during the test phase in Experiment 3. The results revealed conditioned preferences using both sucrose and saccharin as reinforcers. Furthermore, dietary restriction reduced the conditioned preference induced by saccharin, but not the preference induced by sucrose. These findings are discussed in light of the potential differences between orthonasal and retronasal presentation of odors during conditioning.

Five-minute exposure to a novel appetitive food substance is sufficient time for a microRNA-dependent long-term memory to form

The Garcia effect is a unique form of conditioned taste aversion which requires that a novel food stimulus be followed sometime later by a sickness state associated with the novel food stimulus. The long-lasting associative memory resulting from the Garcia effect ensures that organisms avoid toxic foods in their environment. Considering its ecological relevance, we sought to investigate whether a brief encounter (5 min) with a novel, appetitive food stimulus can cause a persisting long-term memory (LTM) to form that would in turn block the Garcia effect in Lymnaea stagnalis. Furthermore, we wanted to explore whether that persisting LTM could be modified by the alteration of microRNAs via an injection of poly-L-lysine (PLL), an inhibitor of Dicer-mediated microRNA biogenesis. The Garcia effect procedure involved two observations of feeding behavior in carrot separated by a heat stress (30 °C for 1 h). Exposing snails to carrot for 5 min caused a LTM to form and persist for 1 week, effectively preventing the Garcia effect in snails. In contrast, PLL injection following the 5-min carrot exposure impaired LTM formation, allowing the Garcia effect to occur. These results provide more insight into LTM formation and the Garcia effect, an important survival mechanism.

Top-down circuitry from the anterior insular cortex to VTA dopamine neurons modulates reward-related memory

The insular cortex (IC) has been linked to the processing of interoceptive and exteroceptive signals associated with addictive behavior. However, whether the IC modulates the acquisition of drug-related affective states by direct top-down connectivity with ventral tegmental area (VTA) dopamine neurons is unknown. We found that photostimulation of VTA terminals of the anterior insular cortex (aIC) induces rewarding contextual memory, modulates VTA activity, and triggers dopamine release within the VTA. Employing neuronal recordings and neurochemical and transsynaptic tagging techniques, we disclose the functional top-down organization tagging the aIC pre-synaptic neuronal bodies and identifying VTA recipient neurons. Furthermore, systemic administration of amphetamine altered the VTA excitability of neurons modulated by the aIC projection, where photoactivation enhances, whereas photoinhibition impairs, a contextual rewarding behavior. Our study reveals a key circuit involved in developing and retaining drug reward-related contextual memory, providing insight into the neurobiological basis of addictive behavior and helping develop therapeutic addiction strategies.

Novel taste, sickness, and memory: Lipopolysaccharide to induce a Garcia-like effect in inbred and wild strains of Lymnaea stagnalis

Food is not only necessary for our survival but also elicits pleasure. However, when a novel food is followed sometime later by nausea or sickness animals form a long-lasting association to avoid that food. This phenomenon is called the 'Garcia effect'. We hypothesized that lipopolysaccharide (LPS) could be used as the sickness-inducing stimulus to produce a Garcia-like effect in inbred and wild populations of Lymnaea stagnalis. We first demonstrated that the injection of 25 μg (6.25 µg/mL) of Escherichia coli-derived LPS serotype O127:B8 did not by itself alter snails' feeding behavior. Then we showed that the presentation of a novel appetitive stimulus (i.e., carrot slurry) and LPS resulted in a taste-specific and long-lasting feeding suppression (i.e., the Garcia-like effect). We also found strain-specific variations in the duration of the long-term memory (LTM). That is, while the LTM for the Garcia-like effect in W-strain snails persisted for 24h, LTM persisted for 48h in freshly collected Margo snails and their F1 offspring. Finally, we demonstrated that the exposure to a non-steroidal anti-inflammatory drug, aspirin (acetylsalicylic acid) before the LPS injection prevented both the LPS-induced sickness state and the Garcia-like effect from occurring. The results of this study may pave the way for new research that aims at (1) uncovering the conserved molecular mechanisms underlying the Garcia-like effect, (2) understanding how cognitive traits vary within and between species, and (3) creating a holistic picture of the complex dialogue between the immune and central nervous systems.

Physiological Needs: Sensations and Predictions in the Insular Cortex

Physiological needs create powerful motivations (e.g., thirst and hunger). Studies in humans and animal models have implicated the insular cortex in the neural regulation of physiological needs and need-driven behavior. We review prominent mechanistic models of how the insular cortex might achieve this regulation and present a conceptual and analytical framework for testing these models in healthy and pathological conditions.

Local memory allocation recruits memory ensembles across brain regions

Memories are thought to be stored in ensembles of neurons across multiple brain regions. However, whether and how these ensembles are coordinated at the time of learning remains largely unknown. Here, we combined CREB-mediated memory allocation with transsynaptic retrograde tracing to demonstrate that the allocation of aversive memories to a group of neurons in one brain region directly affects the allocation of interconnected neurons in upstream brain regions in a behavioral- and brain region-specific manner in mice. Our analysis suggests that this cross-regional recruitment of presynaptic neurons is initiated by downstream memory neurons through a retrograde mechanism. Together with statistical modeling, our results indicate that in addition to the anterograde flow of information between brain regions, the establishment of interconnected, brain-wide memory traces relies on a retrograde mechanism that coordinates memory ensembles at the time of learning.

Transforming experiences: Neurobiology of memory updating/editing

Long-term memory is achieved through a consolidation process where structural and molecular changes integrate information into a stable memory. However, environmental conditions constantly change, and organisms must adapt their behavior by updating their memories, providing dynamic flexibility for adaptive responses. Consequently, novel stimulation/experiences can be integrated during memory retrieval; where consolidated memories are updated by a dynamic process after the appearance of a prediction error or by the exposure to new information, generating edited memories. This review will discuss the neurobiological systems involved in memory updating including recognition memory and emotional memories. In this regard, we will review the salient and emotional experiences that promote the gradual shifting from displeasure to pleasure (or vice versa), leading to hedonic or aversive responses, throughout memory updating. Finally, we will discuss evidence regarding memory updating and its potential clinical implication in drug addiction, phobias, and post-traumatic stress disorder.

Structural basis for strychnine activation of human bitter taste receptor TAS2R46

Taste sensing is a sophisticated chemosensory process, and bitter taste perception is mediated by type 2 taste receptors (TAS2Rs), or class T G protein-coupled receptors. Understanding the detailed molecular mechanisms behind taste sensation is hindered by a lack of experimental receptor structures. Here, we report the cryo-electron microscopy structures of human TAS2R46 complexed with chimeric mini-G protein gustducin, in both strychnine-bound and apo forms. Several features of TAS2R46 are disclosed, including distinct receptor structures that compare with known GPCRs, a new "toggle switch," activation-related motifs, and precoupling with mini-G protein gustducin. Furthermore, the dynamic extracellular and more-static intracellular parts of TAS2R46 suggest possible diverse ligand-recognition and activation processes. This study provides a basis for further exploration of other bitter taste receptors and their therapeutic applications.

Tag and capture: how salient experiences target and rescue nearby events in memory

The long-term fate of a memory is not exclusively determined by the events occurring at the moment of encoding. Research at the cellular, circuit, and behavioral levels is beginning to reveal how neurochemical activations in the moments surrounding an event can retroactively and proactively rescue weak memory for seemingly mundane experiences. We review emerging evidence showing enhancement of weakly formed memories encoded minutes to hours before or after a related motivationally relevant experience. We discuss proposed neurobiological mechanisms for strengthening weak memories formed in temporal proximity to a strong event, and how this knowledge could be leveraged to improve memory for information that is prone to forgetting.

The capacity and organization of gustatory working memory

Remembering a particular taste is crucial in food intake and associative learning. We investigated whether taste can be dynamically encoded, maintained, and retrieved on short time scales consistent with working memory (WM). We use novel single and multi-item taste recognition tasks to show that a single taste can be reliably recognized despite repeated oro-sensory interference suggesting active and resilient maintenance (Experiment 1, N = 21). When multiple tastes were presented (Experiment 2, N = 20), the resolution with which these were maintained depended on their serial position, and recognition was reliable for up to three tastes suggesting a limited capacity of gustatory WM. Lastly, stimulus similarity impaired recognition with increasing set size, which seemed to mask the awareness of capacity limitations. Together, the results advocate a hybrid model of gustatory WM with a limited number of slots where items are stored with varying precision.

Photostimulation of Ventral Tegmental Area-Insular Cortex Dopaminergic Inputs Enhances the Salience to Consolidate Aversive Taste Recognition Memory via D1-Like Receptors

Taste memory involves storing information through plasticity changes in the neural network of taste, including the insular cortex (IC) and ventral tegmental area (VTA), a critical provider of dopamine. Although a VTA-IC dopaminergic pathway has been demonstrated, its role to consolidate taste recognition memory remains poorly understood. We found that photostimulation of dopaminergic neurons in the VTA or VTA-IC dopaminergic terminals of TH-Cre mice improves the salience to consolidate a subthreshold novel taste stimulus regardless of its hedonic value, without altering their taste palatability. Importantly, the inhibition of the D1-like receptor into the IC impairs the salience to facilitate consolidation of an aversive taste recognition memory. Finally, our results showed that VTA photostimulation improves the salience to consolidate a conditioned taste aversion memory through the D1-like receptor into the IC. It is concluded that the dopamine activity from the VTA into IC is required to increase the salience enabling the consolidation of a taste recognition memory. Notably, the D1-like receptor activity into the IC is required to consolidate both innate and learned aversive taste memories but not appetitive taste memory.

Cellular activity in insular cortex across seconds to hours: Sensations and predictions of bodily states

Our wellness relies on continuous interactions between our brain and body: different organs relay their current state to the brain and are regulated, in turn, by descending visceromotor commands from our brain and by actions such as eating, drinking, thermotaxis, and predator escape. Human neuroimaging and theoretical studies suggest a key role for predictive processing by insular cortex in guiding these efforts to maintain bodily homeostasis. Here, we review recent studies recording and manipulating cellular activity in rodent insular cortex at timescales from seconds to hours. We argue that consideration of these findings in the context of predictive processing of future bodily states may reconcile several apparent discrepancies and offer a unifying, heuristic model for guiding future work.

Bidirectional role of dopamine in learning and memory-active forgetting

Dopaminergic neurons projecting from the Substantia Nigra to the Striatum play a critical role in motor functions while dopaminergic neurons originating in the Ventral Tegmental Area (VTA) and projecting to the Nucleus Accumbens, Hippocampus and other cortical structures regulate rewarding learning. While VTA mainly consists of dopaminergic neurons, excitatory (glutamate) and inhibitory (GABA) VTA-neurons have also been described: these neurons may also modulate and contribute to shape the final dopaminergic response, which is critical for memory formation. However, given the large amount of information that is handled daily by our brain, it is essential that irrelevant information be deleted. Recently, apart from the well-established role of dopamine (DA) in learning, it has been shown that DA plays a critical role in the intrinsic active forgetting mechanisms that control storage information, contributing to the deletion of a consolidated memory. These new insights may be instrumental to identify therapies for those disorders that involve memory alterations.

To eat or not to eat: a Garcia effect in pond snails (Lymnaea stagnalis)

Taste aversion learning is universal. In animals, a single presentation of a novel food substance followed hours later by visceral illness causes animals to avoid that taste. This is known as bait-shyness or the Garcia effect. Humans demonstrate this by avoiding a certain food following the development of nausea after ingesting that food ('Sauce Bearnaise effect'). Here, we show that the pond snail Lymnaea stagnalis is capable of the Garcia effect. A single 'pairing' of a novel taste, a carrot slurry followed hours later by a heat shock stressor (HS) is sufficient to suppress feeding response elicited by carrot for at least 24 h. Other food tastes are not suppressed. If snails had previously been exposed to carrot as their food source, the Garcia-like effect does not occur when carrot is 'paired' with the HS. The HS up-regulates two heat shock proteins (HSPs), HSP70 and HSP40. Blocking the up-regulation of the HSPs by a flavonoid, quercetin, before the heat shock, prevented the Garcia effect in the snails. Finally, we found that snails exhibit Garcia effect following a period of food deprivation but the long-term memory (LTM) phenotype can be observed only if the animals are tested in a food satiated state.

Cortical neurochemical signaling of gustatory stimuli and their visceral consequences during the acquisition and consolidation of taste aversion memory

The insular cortex (IC) has a crucial role in taste recognition memory, including conditioned taste aversion (CTA). CTA is a learning paradigm in which a novel taste stimulus (CS) is associated with gastric malaise (US), inducing aversion to the CS in future encounters. The role of the IC in CTA memory formation has been extensively studied. However, the functional significance of neurotransmitter release during the presentation of taste stimuli and gastric malaise-inducing agents remains unclear. Using microdialysis in free-moving animals, we evaluated simultaneous changes in glutamate, norepinephrine and dopamine release in response to the presentation of an innate appetitive or aversive gustatory novel stimulus, as well as after i.p. administration of isotonic or hypertonic gastric malaise-inducing solutions. Our results demonstrate that the presentation of novel stimuli, regardless of their innate valence, induces an elevation of norepinephrine and dopamine. Administration of a gastric malaise inducing agent (LiCl) promotes an elevation of glutamate regardless of its concentration. In comparison, norepinephrine release is related to the LiCl concentration and its equimolar NaCl control. Additionally, we evaluated their functional role on short and long-term taste aversion memory. Results indicate that the blockade of noradrenergic β1,2 receptors in the IC spares CTA acquisition and memory consolidation. In contrast, blockade of dopamine D1/D5 receptors impaired CTA consolidation, whereas the NMDA receptor blockade impedes both acquisition and consolidation of CTA. These results suggest that dopaminergic and noradrenergic release are related to the salience of conditioned taste stimuli. However, only cortical D1/D5 dopaminergic activity, but not the noradrenergic β1,2 activity, is involved in the acquisition and consolidation of taste memory formation. Additionally, glutamatergic activity signals visceral distress caused by LiCl administration and activates NMDA receptors necessary for the acquisition and consolidation of long-lasting taste aversion memory.

Inactivation of the ventral hippocampus facilitates the attenuation of odor neophobia in rats

Food neophobia is a behavior observed in rodents involving reduced consumption of a novel food or drink. In the absence of negative post-ingestive consequences, consumption increases with exposure (attenuation of neophobia), which is seen as an associative safe memory. Olfaction and gustation are sensory modalities essential for the development of a food preference. However, little is known about the neural mechanisms underlying neophobia to a food-related odor stimulus. In the present study, we examined the effect of pharmacological inactivation of the ventral hippocampus (vHPC) on neophobia to orally consumed solutions in rats using muscimol, a gamma aminobutyric acid type A receptor agonist. Two different types of solutions, almond odor (benzaldehyde) and sweet taste (saccharin), were prepared. In the results, microinjections of muscimol into the bilateral vHPC before the first odor and taste exposures did not alter the neophobic reactions of the rats to each stimulus. However, in the second odor, but not taste, exposure, the muscimol-injected rats showed higher consumption in comparison to that observed in the control rats, suggesting that the vHPC inactivation facilitates the attenuation of odor neophobia. On the other hand, intra-vHPC muscimol microinjections after the first odor and taste exposures did not facilitate consumption at the second exposures. These results indicate that neural activations within vHPC during orally consuming a novel odor, but not taste, solution play an inhibitory role in the subsequent attenuation of neophobia.

Soft Drink Consumption in Young Mexican Adults Is Associated with Higher Total Body Fat Percentage in Men but Not in Women

A high consumption of soft drinks (SDs) has been linked with the development of anthropometric and metabolic alterations. We evaluate the association between SD consumption and some anthropometric and metabolic variables. This study is an observational study, using a sample of 394 university students, of which 158 were men (40.1%) and 238 women (59.9%), between 18 and 30 years. An SD intake questionnaire provided the consumption of different SDs. The participants' weight, height, and waist and hip circumferences were collected. Metabolic biomarkers were analyzed. The average intake of caloric SDs (CSDs) was 1193.6 ± 1534.8 mL/week and 84.5 ± 115.02 mL/week for non-caloric SDs (NCSDs). Sex differences were found in the amount of SD consumption and these statistical differences were driven by those men subjects with a high total body fat percentage (TBF%). In men, correlations were found between the intake of CSDs and the body mass index, waist and hip circumferences, TBF%, and visceral fat percentage. In woman, a correlation was found with glucose and triglycerides. The prediction model revealed that the intake of CSDs predicts TBF% and low-density lipoprotein only in men. A high amount of CSD consumption in men was associated with a high TBF%, and this may be predictive of future development of metabolic abnormalities.

Muscarinic receptor signaling in the amygdala is required for conditioned taste aversion

The sense of taste provides information regarding the nutrient content, safety or potential toxicity of an edible. This is accomplished via a combination of innate and learned taste preferences. In conditioned taste aversion (CTA), rats learn to avoid ingesting a taste that has previously been paired with gastric malaise. Recent evidence points to a role of cholinergic muscarinic signaling in the amygdala for the learning and storage of emotional memories. The present study tested the participation of muscarinic receptors in the amygdala during the formation of CTA by infusing the non-specific antagonist scopolamine into the basolateral or central subnuclei before or after conditioning, as well as before retrieval. Our data show that regardless of the site of infusion, pre-conditioning administration of scopolamine impaired CTA acquisition whereas post-conditioning infusion did not affect its storage. Also, infusions into the basolateral but not in the central amygdala before retrieval test partially reduced the expression of CTA. Our results indicate that muscarinic receptors activity is required for acquisition but not consolidation of CTA. In addition, our data add to recent evidence pointing to a role of cholinergic signaling in peri-hippocampal structures in the process of memory retrieval.

Chemospecific deficits in taste sensitivity following bilateral or right hemispheric gustatory cortex lesions in rats

Our prior studies showed bilateral gustatory cortex (GC) lesions significantly impair taste sensitivity to salts (NaCl and KCl) and quinine ("bitter") but not to sucrose ("sweet"). The range of qualitative tastants tested here has been extended in a theoretically relevant way to include the maltodextrin, Maltrin, a preferred stimulus by rats thought to represent a unique taste quality, and the "sour" stimulus citric acid; NaCl was also included as a positive control. Male rats (Sprague-Dawley) with histologically confirmed neurotoxin-induced bilateral (BGCX, n = 13), or right (RGCX, n = 13) or left (LGCX, n = 9) unilateral GC lesions and sham-operated controls (SHAM, n = 16) were trained to discriminate a tastant from water in an operant two-response detection task. A mapping system was used to determine placement, size, and symmetry (when bilateral) of the lesion. BGCX significantly impaired taste sensitivity to NaCl, as expected, but not to Maltrin or citric acid, emulating our prior results with sucrose. However, in the case of citric acid, there was some disruption in performance at higher concentrations. Interestingly, RGCX, but not LGCX, also significantly impaired taste sensitivity, but only to NaCl, suggesting some degree of lateralized function. Taken together with our prior findings, extensive bilateral lesions in GC do not disrupt basic taste signal detection to all taste stimuli uniformly. Moreover, GC lesions do not preclude the ability of rats to learn and perform the task, clearly demonstrating that, in its absence, other brain regions are able to maintain sensory-discriminative taste processing, albeit with attenuated sensitivity for select stimuli.

Enhanced Retrieval of Taste Associative Memory by Chemogenetic Activation of Locus Coeruleus Norepinephrine Neurons

The ability of animals to retrieve memories stored in response to the environment is essential for behavioral adaptation. Norepinephrine (NE)-containing neurons in the brain play a key role in the modulation of synaptic plasticity underlying various processes of memory formation. However, the role of the central NE system in memory retrieval remains unclear. Here, we developed a novel chemogenetic activation strategy exploiting insect olfactory ionotropic receptors (IRs), termed "IR-mediated neuronal activation," and used it for selective stimulation of NE neurons in the locus coeruleus (LC). Drosophila melanogaster IR84a and IR8a subunits were expressed in LC NE neurons in transgenic mice. Application of phenylacetic acid (a specific ligand for the IR84a/IR8a complex) at appropriate doses induced excitatory responses of NE neurons expressing the receptors in both slice preparations and in vivo electrophysiological conditions, resulting in a marked increase of NE release in the LC nerve terminal regions (male and female). Ligand-induced activation of LC NE neurons enhanced the retrieval process of conditioned taste aversion without affecting taste sensitivity, general arousal state, and locomotor activity. This enhancing effect on taste memory retrieval was mediated, in part, through α₁- and β-adrenergic receptors in the basolateral nucleus of the amygdala (BLA; male). Pharmacological inhibition of LC NE neurons confirmed the facilitative role of these neurons in memory retrieval via adrenergic receptors in the BLA (male). Our findings indicate that the LC NE system, through projections to the BLA, controls the retrieval process of taste associative memory.SIGNIFICANCE STATEMENT Norepinephrine (NE)-containing neurons in the brain play a key role in the modulation of synaptic plasticity underlying various processes of memory formation, but the role of the NE system in memory retrieval remains unclear. We developed a chemogenetic activation system based on insect olfactory ionotropic receptors and used it for selective stimulation of NE neurons in the locus coeruleus (LC) in transgenic mice. Ligand-induced activation of LC NE neurons enhanced the retrieval of conditioned taste aversion, which was mediated, in part, through adrenoceptors in the basolateral amygdala. Pharmacological blockade of LC activity confirmed the facilitative role of these neurons in memory retrieval. Our findings indicate that the LC-amygdala pathway plays an important role in the recall of taste associative memory.

Differential changes in GAP-43 or synaptophysin during appetitive and aversive taste memory formation

Association between events in time and space is a major mechanism for all animals, including humans, which allows them to learn about the world and potentially change their behavior in the future to adapt to different environments. Conditioning taste aversion (CTA) is a single-trial learning paradigm where animals are trained to avoid a novel flavor which is associated with malaise. Many variables can be analyzed with this model and the circuits involved are well described. Thus, the amygdala and the gustatory cortex (GC) are some of the most relevant structures involved in CTA. In the present study we focused in plastic changes that occur during appetitive and/or aversive taste memory formation. Previous studies have demonstrated that memory consolidation, in hippocampal dependent paradigms, induces plastic changes like increase in the concentration of proteins considered as markers of neuronal plasticity, such as the growth associated protein 43 (GAP-43) and synaptophysin (SYN). In the present experiment in male rats we evaluated changes in GAP-43 and SYN expression, using immunofluorescence, induce by the formation of aversive and appetitive taste memory. We found that taste aversive memory formation can induce an increase in GAP-43 in the granular layer of the GC. Furthermore, we also found an increase in SYN expression in both layers of the GC, the basolateral amygdala (BLA) and the central amygdala (CeA). These results suggest that aversive memory representation induces a new circuitry (inferred from an increase in GAP 43). On the other hand, an appetitive taste learning increased SYN expression in the GC (both layers), the BLA and the CeA without any changes in GAP 43. Together these results indicate that aversive memory formation induces structural and synaptic changes, while appetitive memory formation induces synaptic changes; suggesting that aversive and appetitive memories require a different set of cortical and amygdala plastic changes.

Accumbens and amygdala in taste recognition memory: The role of d1 dopamine receptors

The attenuation of taste neophobia (AN) is a good model for studying the structural and neurochemical mechanisms of the emotional component of memory because taste recognition memory exhibits the unique feature of being necessarily linked to hedonic properties. Whilst novel tastes elicit cautious neophobic responses, taste exposures which are not followed by aversive consequences attenuate neophobia as the taste becomes safe and palatable. Given the involvement of the nucleus accumbens in reward and of the amygdala in emotional memories, we applied c-Fos immunohistochemistry as an index of neural activity in Wistar rats that were exposed to a vinegar solution for one, two or six days. An inverse pattern of accumbens nucleus vs amygdala activity was found on the second exposure day on which AN occurred. The number of c-Fos positive cells in the nucleus accumbens shell increased whilst the number of c-Fos positive cells in the basolateral amygdala decreased. Further analyses revealed a positive correlation between AN and the number of c-Fos positive cells in the accumbens shell but a negative correlation in the basolateral amygdala. Furthermore the accumbens-amygdala interplay relevant for AN seems to be mediated by dopamine D1 receptors (D1DR). The injection of SCH23390 (D1DR antagonist) in both the accumbens shell and the basolateral amygdala on the second taste exposure resulted in selectively impaired AN but had opposite long term effects. This finding supports the relevance of a dopaminergic network mediated by D1DRs in the nucleus accumbens shell and basolateral amygdala which is critical for adding the emotional component during the formation of taste memory.

Taste Processing: Insights from Animal Models

Taste processing is an adaptive mechanism involving complex physiological, motivational and cognitive processes. Animal models have provided relevant data about the neuroanatomical and neurobiological components of taste processing. From these models, two important domains of taste responses are described in this review. The first part focuses on the neuroanatomical and neurophysiological bases of olfactory and taste processing. The second part describes the biological and behavioral characteristics of taste learning, with an emphasis on conditioned taste aversion as a key process for the survival and health of many species, including humans.

CB1 cannabinoid receptor-mediated plasticity of GABAergic synapses in the mouse insular cortex

The insular cortex plays pivotal roles in taste learning. As cellular mechanisms of taste learning, long-term potentiation (LTP) at glutamatergic synapses is well studied. However, little is known about long-term changes of synaptic efficacy at GABAergic synapses in the insular cortex. Here, we examined the synaptic mechanisms of long-term plasticity at GABAergic synapses in layer V pyramidal neurons of the mouse insular cortex. In response to a prolonged high-frequency stimulation (HFS), GABAergic synapses displayed endocannabinod (eCB)-mediated long-term depression (LTD_GABA). When cannabinoid 1 receptors (CB1Rs) were blocked by a CB1R antagonist, the same stimuli caused LTP at GABAergic synapses (LTP_GABA) which was mediated by production of nitric oxide (NO) via activation of NMDA receptors. Intriguingly, NO signaling was necessary for the induction of LTD_GABA. In the presence of leptin which blocks CB1 signaling, the prolonged HFS caused LTP_GABA which was mediated by NO signaling. These results indicate that long-term plasticity at GABAergic synapses in the insular cortex can be modulated by combined effects of eCB and NO signaling. These forms of GABAergic synaptic plasticity in the insular cortex may be crucial synaptic mechanisms in taste learning.

Food Disgust Scale: Spanish Version

INTRODUCTION

The Food Disgust Scale (FDS) was recently developed and validated in Swiss adult population. This study aims to: (1) validate the FDS for the first time in a Spanish-speaking Mexican population, (2) correlate food disgust sensitivity with picky eating measures, and (3) explore the association between food disgust sensitivity and body mass index (BMI).

MATERIALS AND METHODS

A Spanish version of the FDS (FDS-Sp) and its short version (FDS-Sp short) were tested with confirmatory factor analysis (CFA) in order to test the original item/factor structure. Bivariate correlations were performed to determine the association between FDS-Sp/FDS-Sp short scores and picky eating. Lastly, hierarchical linear regression analysis was carried out to determine the relationship between food disgust sensitivity and BMI.

RESULTS

The factor structure of the FDS was replicated and acceptable internal consistency values were observed for FDS-Sp subscales (α varied between 0.781 and 0.955). Moreover, FDS-Sp subscales and FDS-Sp short were correlated with picky eating. Higher score in VEGI subscale of the FDS-Sp was a significant predictor for higher BMI, explaining 4% of the variance.

CONCLUSION

FDS-Sp is a useful, reliable and robust psychometric instrument to measure the sensitivity to unpleasant food situations in a Mexican adult Spanish-speaking population. A relationship between food disgust sensitivity and picky eating, selective eating behaviors and neophobia in Mexicans was confirmed. BMI is multifactorial and only one subscale of FDS-Sp is a significant predictor for BMI status. These results are helpful to continue exploring food disgust in diverse populations.

Pavlovian Conditioning of Immunological and Neuroendocrine Functions

The phenomenon of behaviorally conditioned immunological and neuroendocrine functions has been investigated for the past 100 yr. The observation that associative learning processes can modify peripheral immune functions was first reported and investigated by Ivan Petrovic Pavlov and his co-workers. Their work later fell into oblivion, also because so little was known about the immune system’s function and even less about the underlying mechanisms of how learning, a central nervous system activity, could affect peripheral immune responses. With the employment of a taste-avoidance paradigm in rats, this phenomenon was rediscovered 45 yr ago as one of the most fascinating examples of the reciprocal functional interaction between behavior, the brain, and peripheral immune functions, and it established psychoneuroimmunology as a new research field. Relying on growing knowledge about efferent and afferent communication pathways between the brain, neuroendocrine system, primary and secondary immune organs, and immunocompetent cells, experimental animal studies demonstrate that cellular and humoral immune and neuroendocrine functions can be modulated via associative learning protocols. These (from the classical perspective) learned immune responses are clinically relevant, since they affect the development and progression of immune-related diseases and, more importantly, are also inducible in humans. The increased knowledge about the neuropsychological machinery steering learning and memory processes together with recent insight into the mechanisms mediating placebo responses provide fascinating perspectives to exploit these learned immune and neuroendocrine responses as supportive therapies, the aim being to reduce the amount of medication required, diminishing unwanted drug side effects while maximizing the therapeutic effect for the patient’s benefit.

Role of ASIC1a in Normal and Pathological Synaptic Plasticity

Acid-sensing ion channels (ASICs), members of the degenerin/epithelial Na+ channel superfamily, are broadly distributed in the mammalian nervous system where they play important roles in a variety of physiological processes, including neurotransmission and memory-related behaviors. In the last few years, we and others have investigated the role of ASIC1a in different forms of synaptic plasticity especially in the CA1 area of the hippocampus. This review summarizes the latest research linking ASIC1a to synaptic function either in physiological or pathological conditions. A better understanding of how these channels are regulated in brain circuitries relevant to synaptic plasticity and memory may offer novel targets for pharmacological intervention in neuropsychiatric and neurological disorders.

Extra-Hippocampal Learning Deficits in Young Apolipoprotein E4 Mice and Their Synaptic Underpinning

The E4 allele of apolipoprotein (apoE4) is the primary genetic risk factor for late onset Alzheimer's disease (AD), yet the exact manner in which apoE4 leads to the development of AD is undetermined. Human and animal studies report that apoE4-related memory deficits appear earlier than the AD clinical manifestation, thus suggesting the existence of early, pre-pathological, apoE4 impairments that may later lead to AD onset. While current research regards the hippocampus as the initial and primary effected locus by apoE4, we presently investigate the possibility that apoE4 innately impairs any brain area that requires synaptic plasticity. To test this hypothesis, we trained young (3-4-month-old) target-replacement apoE3 and apoE4 mice in conditioned taste aversion (CTA) acquisition and extinction learnings- hippocampus-independent learnings that are easily performed at a young age. Synaptic vesicular markers analysis was conducted in the gustatory cortex (GC), basolateral amygdala (BLA), medial prefrontal cortex (mPFC), and hippocampal CA3 to reveal underlying apoE4-related impairments. We have found that young apoE4 mice are severely impaired in CTA acquisition and extinction learning. CTA acquisition impairments were correlated with reduced vGat and vGlut levels in the BLA and GC, but not in the CA3. CTA extinction was correlated with lower synaptophysin and vGlut levels in the mPFC, a central region in CTA extinction. Our results support apoE4-related early-life plasticity impairments that precede the AD clinical manifestations and affect any brain area that depends on extensive plasticity; early impairments that may promote the development of AD pathologies later in life.

Neurolocalization of taste disorders

Neurolocalization of taste disorders requires a knowledge of the functional anatomy involved in mediating taste information from the peripheral mucosal surfaces through numerous peripheral cranial nerves to complex subcortical and cortical brain regions. Our understanding of this functional anatomy has advanced in recent years. Taste is an experience that is both innate and learned, and the "taste" experience involves the integration of information from other sensory modalities, such as olfaction and somatosensation. Normal taste perception is influenced by different neurophysiologic states, which involve endocrine function, emotions, and even attitudes and expectations toward eating. At its core, the normal effective ability to taste is a reflection of the proper function of many organ systems within the body and may be considered a marker for good health. Clinical taste disorders, on the other hand, involve the dysfunction of the normal neural taste pathways and/or aberrant influences on multisensory integration and cortical taste processing. The number of disease processes, which can adversely affect taste, are numerous and quite varied in their presentation. There may be contributory involvement of other organ systems within the body, and the appropriate management of taste disorders often requires a multidisciplinary approach to fully understand the disorder. Depending on the underlying cause, taste disorders can be effectively managed when identified. Treatments may include correcting underlying metabolic disturbances, eliminating infections, changing offending medications, replenishing nutritional deficiencies, operating on structural impairments, calming autoimmune processes, and even stabilizing electrochemical interactions.

Taste receptor function

This chapter summarizes the available data about taste receptor functions and their role in perception of food with emphasis on the human system. In addition we illuminate the widespread presence of these receptors throughout the body and discuss some of their extraoral functions. Finally, we describe clinical aspects where taste receptor signaling could be relevant.

Dorsal hippocampal damage disrupts the auditory context-dependent attenuation of taste neophobia in mice

Rodents exhibit neophobia for novel tastes, demonstrated by an initial reluctance to drink novel-tasting, potentially-aversive solutions. Taste neophobia attenuates across days if the solution is not aversive, demonstrated by increased consumption as the solution becomes familiar. This attenuation of taste neophobia is context dependent, which has been demonstrated by maintained reluctance to drink the novel tasting solution if the subject has to drink it after being brought to a novel environment. This spatial context-dependent attenuation of taste neophobia has been described and likely depends on the integrity of the dorsal hippocampus because this brain area is crucial for representing space and spatial context associations, but is unnecessary for processing taste memories per se. Whether changing the non-spatial auditory context causes a similar effect on attenuation of taste neophobia and the potential role of the dorsal hippocampus in processing this decidedly non-spatial information has not been determined. Here we demonstrate that changing the non-spatial auditory context affects the attenuation of taste neophobia in mice, and investigate the consequence of hippocampal lesion. The results demonstrate that the non-spatial auditory context-dependent attenuation of taste neophobia in mice is lost following NMDA excitotoxic lesions of the CA1 region of the dorsal hippocampus. These findings demonstrate that the dorsal hippocampus is crucial for the modulation non-associative taste learning by auditory context, neither of which provide information about space.

Flavor recognition memory related activity of the posterior piriform cortex in adult and aged rats

The relationship between the piriform cortex and flavor recognition memory was investigated in adult and aged rats. By using c-Fos immunohistochemistry, we assessed the piriform cortex activity induced by flavor familiarity. The results indicated increased activity in the rostral region of the posterior piriform cortex elicited by the most familiar cider vinegar solution after six exposures. Aged rats exhibited overall increased activity in the posterior, but not the anterior piriform cortex, which was not related to flavor familiarity. This suggests that the posterior piriform cortex is related to flavor recognition memory and that aging modifies its activity pattern which might underlie their slower attenuation of flavor neophobia.

Sucrose intensity coding and decision-making in rat gustatory cortices

Sucrose's sweet intensity is one attribute contributing to the overconsumption of high-energy palatable foods. However, it is not known how sucrose intensity is encoded and used to make perceptual decisions by neurons in taste-sensitive cortices. We trained rats in a sucrose intensity discrimination task and found that sucrose evoked a widespread response in neurons recorded in posterior-Insula (pIC), anterior-Insula (aIC), and Orbitofrontal cortex (OFC). Remarkably, only a few Intensity-selective neurons conveyed the most information about sucrose's intensity, indicating that for sweetness the gustatory system uses a compact and distributed code. Sucrose intensity was encoded in both firing-rates and spike-timing. The pIC, aIC, and OFC neurons tracked movement direction, with OFC neurons yielding the most robust response. aIC and OFC neurons encoded the subject's choices, whereas all three regions tracked reward omission. Overall, these multimodal areas provide a neural representation of perceived sucrose intensity, and of task-related information underlying perceptual decision-making.

The Placebo Effect in Pain Therapies

Pharmacological strategies for pain management have primarily focused on dampening ascending neurotransmission and on opioid receptor-mediated therapies. Little is known about the contribution of endogenous descending modulatory systems to clinical pain outcomes and why some patients are mildly affected while others suffer debilitating pain-induced dysfunctions. Placebo effects that arise from patients' positive expectancies and the underlying endogenous modulatory mechanisms may in part account for the variability in pain experience and severity, adherence to treatment, distinct coping strategies, and chronicity. Expectancy-induced analgesia and placebo effects in general have emerged as useful models to assess individual endogenous pain modulatory systems. Different systems and mechanisms trigger placebo effects that highly impact pain processing, clinical outcomes, and sense of well-being. This review illustrates critical elements of placebo mechanisms that inform the methodology of clinical trials, the discovery of new therapeutic targets, and the advancement of personalized pain management.

An Insula-Central Amygdala Circuit for Guiding Tastant-Reinforced Choice Behavior

For animals to survive, they must reliably predict during foraging which substances are suitable for consumption. Despite extensive study, the neural circuit mechanisms underlying such adaptive behavior remain poorly understood. Here, using a tastant (sucrose/quinine)-reinforced "go/no-go" task in male and female mice, we examined the anatomical and functional connectivity of the circuit linking the insular cortex (IC) to the central amygdala (CeA) and the role of this circuit in the establishment of appropriate behavioral responses. Using anatomic tracing approaches combined with optogenetics-assisted circuit mapping, we found that the gustatory region of the IC sends direct excitatory projections to the lateral division of the CeA (CeL), making monosynaptic excitatory connections with distinct populations of CeL neurons. Specific inhibition of neurotransmitter release from the CeL-projecting IC neurons prevented mice from acquiring the "no-go" response, and impaired the "go" responses in the go/no-go task. Furthermore, selective activation of the IC-CeL pathway with optogenetics drove unconditioned lick suppression in thirsty animals, induced aversive responses, and was sufficient to instruct conditioned action suppression in response to a cue predicting the optogenetic activation. These results indicate that activities in the IC-CeL circuit are critical for establishing taste-reinforced behavioral responses, including avoidance responses to an aversive tastant, and are sufficient to drive learning of anticipatory avoidance. Our findings suggest that the IC-CeL circuit plays an important role in guiding appropriate choices during foraging.SIGNIFICANCE STATEMENT An animal's ability to predict which substances are suitable for consumption and then produce an appropriate action to those substances is critical for survival. Here we found that activity in the circuit that links the insular cortex (IC) to the central amygdala (CeA) is necessary for establishing appropriate behavioral responses to taste-predicting cues. This neural circuit seems to be particularly tuned to avoid an unpleasant tastant, and is also sufficient to drive learning of such avoidance responses. These results suggest that the IC-CeA circuit is critical for generating appropriate behavioral responses during foraging when facing different choices.