Amygdala stimulation evokes time-varying synaptic responses in the gustatory cortex of anesthetized rats

Perturbation of amygdala-cortical projections reduces ensemble coherence of palatability coding in gustatory cortex

Taste palatability is centrally involved in consumption decisions-we ingest foods that taste good and reject those that don't. Gustatory cortex (GC) and basolateral amygdala (BLA) almost certainly work together to mediate palatability-driven behavior, but the precise nature of their interplay during taste decision-making is still unknown. To probe this issue, we discretely perturbed (with optogenetics) activity in rats' BLA→GC axons during taste deliveries. This perturbation strongly altered GC taste responses, but while the perturbation itself was tonic (2.5 s), the alterations were not-changes preferentially aligned with the onset times of previously-described taste response epochs, and reduced evidence of palatability-related activity in the 'late-epoch' of the responses without reducing the amount of taste identity information available in the 'middle epoch.' Finally, BLA→GC perturbations changed behavior-linked taste response dynamics themselves, distinctively diminishing the abruptness of ensemble transitions into the late epoch. These results suggest that BLA 'organizes' behavior-related GC taste dynamics.

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.

Valence Encoding Signals in the Human Amygdala and the Willingness to Eat

One of the strongest drivers of food consumption is pleasure, and with a large variety of palatable food continuously available, there is rarely any necessity to eat something not tasty. The amygdala is involved in hedonic valuation, but its role in valence assignment during food choices is less understood. Given recent evidence for spatially segregated amygdala signatures encoding palatability, we applied a multivariate approach on fMRI data to extract valence-specific signal patterns during an explicit evaluation of food liking. These valence localizers were then used to identify hedonic valuation processes while the same healthy human participants (14 female, 16 male; in overnight fasted state on both scanning days) performed a willingness-to-eat task in a separate fMRI measurement. Valence-specific patterns of amygdala signaling predicted decisions on food consumption significantly. Findings could be validated using the same valence localizers to predict consumption decisions participants made on a separate set of food stimuli that had not been used for localizer identification. Control analyses revealed these findings to be restricted to a multivariate compared with a univariate approach, and to be specific for valence processing in the amygdala. Spatially distributed valuation signals of the amygdala thus appear to modulate appetitive consumption decisions, and may be useful to identify current hedonic valuation processes triggering food choices even when not explicitly instructed.SIGNIFICANCE STATEMENT The expectation of tastiness is a particularly strong driver in everyday decisions on food consumption. The amygdala is important for hedonic valuation processes and involved in valence-related behavior, but the relationship between both processes is less understood. Here, we show that hedonic values of food are represented in spatially distributed activation patterns in the amygdala. The engagement of these patterns during food choices modulates consumption decisions. Findings are stable in a separate stimulus set. These results suggest that valence-specific amygdala signals are integrated into the formation of food choices.

Synaptic Integration of Thalamic and Limbic Inputs in Rodent Gustatory Cortex

Neurons in the gustatory cortex (GC) process multiple aspects of a tasting experience, encoding not only the physiochemical identity of tastes, but also their anticipation and hedonic value. Information pertaining to these stimulus features is relayed to GC via the gustatory thalamus (VPMpc) and basolateral amygdala (BLA). It is not known whether these inputs drive separate groups of neurons, thus activating separate channels of information, or are integrated by neurons that receive both afferents. Here, we used anterograde labeling and in vivo intracellular recordings in anesthetized rats to assess the potential convergence of BLA and VPMpc inputs in GC, and to investigate the dynamics of integration of these inputs. We report substantial anatomic overlap of BLA and VPMpc axonal fields across GC, and identify a population of GC neurons receiving converging BLA and VPMpc inputs. Our data show that BLA modulates the gain of VPMpc-evoked responses in a time-dependent fashion and that this modulation is dependent on the recruitment of synaptic inhibition by both BLA and VPMpc. Our results suggest that BLA shapes cortical processing of thalamic inputs by dynamically gating the excitatory/inhibitory balance of the GC circuit.

Activity of Insula to Basolateral Amygdala Projecting Neurons is Necessary and Sufficient for Taste Valence Representation

Conditioned taste aversion (CTA) is an associative learning paradigm, wherein consumption of an appetitive tastant (e.g., saccharin) is paired to the administration of a malaise-inducing agent, such as intraperitoneal injection of LiCl. Aversive taste learning and retrieval require neuronal activity within the anterior insula (aIC) and the basolateral amygdala (BLA). Here, we labeled neurons of the aIC projecting to the BLA in adult male mice using a retro-AAV construct and assessed their necessity in aversive and appetitive taste learning. By restricting the expression of chemogenetic receptors in aIC-to-BLA neurons, we demonstrate that activity within the aIC-to-BLA projection is necessary for both aversive taste memory acquisition and retrieval, but not for its maintenance, nor its extinction. Moreover, inhibition of the projection did not affect incidental taste learning per se, but effectively suppressed aversive taste memory retrieval when applied either during or before the encoding of the unconditioned stimulus for CTA (i.e., malaise). Remarkably, activation of the projection after novel taste consumption, without experiencing any internal discomfort, was sufficient to form an artificial aversive taste memory, resulting in strong aversive behavior upon retrieval. Our results indicate that aIC-to-BLA projecting neurons are an essential component in the ability of the brain to associate taste sensory stimuli with body states of negative valence and guide the expression of valence-specific behavior upon taste memory retrieval.SIGNIFICANCE STATEMENT In the present study we subjected mice to the conditioned taste aversion paradigm, where animals learn to associate novel taste with malaise (i.e., assign it negative valence). We show that activation of neurons in the anterior insular cortex (aIC) that project into the basolateral amygdala (BLA) in response to conditioned taste aversion is necessary to form a memory for a taste of negative valence. Moreover, artificial activation of this pathway (without any feeling of pain) after the sampling of a taste can also lead to such associative memory. Thus, activation of aIC-to-BLA projecting neurons is necessary and sufficient to form and retrieve aversive taste memory.

From Hiring to Firing: Activation of Inhibitory Neurons and Their Recruitment in Behavior

The investigation of GABAergic inhibitory circuits has substantially expanded over the past few years. The development of new tools and technology has allowed investigators to classify many diverse groups of inhibitory neurons by several delineating factors: these include their connectivity motifs, expression of specific molecular markers, receptor diversity, and ultimately their role in brain function. Despite this progress, however, there is still limited understanding of how GABAergic neurons are recruited by their input and how their activity is modulated by behavioral states. This limitation is primarily due to the fact that studies of GABAergic inhibition are mainly geared toward determining how, once activated, inhibitory circuits regulate the activity of excitatory neurons. In this review article, we will outline recent work investigating the anatomical and physiological properties of inputs that activate cortical GABAergic neurons, and discuss how these inhibitory cells are differentially recruited during behavior.

The Insula and Taste Learning

The sense of taste is a key component of the sensory machinery, enabling the evaluation of both the safety as well as forming associations regarding the nutritional value of ingestible substances. Indicative of the salience of the modality, taste conditioning can be achieved in rodents upon a single pairing of a tastant with a chemical stimulus inducing malaise. This robust associative learning paradigm has been heavily linked with activity within the insular cortex (IC), among other regions, such as the amygdala and medial prefrontal cortex. A number of studies have demonstrated taste memory formation to be dependent on protein synthesis at the IC and to correlate with the induction of signaling cascades involved in synaptic plasticity. Taste learning has been shown to require the differential involvement of dopaminergic GABAergic, glutamatergic, muscarinic neurotransmission across an extended taste learning circuit. The subsequent activation of downstream protein kinases (ERK, CaMKII), transcription factors (CREB, Elk-1) and immediate early genes (c-fos, Arc), has been implicated in the regulation of the different phases of taste learning. This review discusses the relevant neurotransmission, molecular signaling pathways and genetic markers involved in novel and aversive taste learning, with a particular focus on the IC. Imaging and other studies in humans have implicated the IC in the pathophysiology of a number of cognitive disorders. We conclude that the IC participates in circuit-wide computations that modulate the interception and encoding of sensory information, as well as the formation of subjective internal representations that control the expression of motivated behaviors.

Rapid plasticity of visually evoked responses in rat monocular visual cortex

Sensory cortical circuits are shaped by experience during sensitive periods in development. In the primary visual cortex (V1) altered visual experience results in changes in visual responsiveness of cortical neurons. The experience-dependent refinement of the circuit in V1 is thought to rely on competitive interactions between feedforward circuits driven by the two eyes. However, recent data have provided evidence for an additional role of cortico-cortical circuits in this process. Indeed, experience-dependent changes in intracortical circuits can be induced rapidly and may result in rapid-onset functional changes. Unilateral occlusion of vision rapidly alters visual responsiveness, synaptic strength and connectivity of local circuits in the binocular region of V1 (V1b), where the inputs from the two eyes converge. In the monocular region of rodent V1 (V1m), where feedforward inputs from the ipsilateral eye are virtually absent, visual deprivation induces rapid plasticity in local circuits; however, functional changes seem to occur only after long periods of deprivation. In V1m there is currently no evidence for functional changes occurring within a time window compatible with that of local circuit plasticity. Here, we probed the visual responsiveness of neurons in rat V1m and assessed the effect of one day unilateral eye lid suture on single neuron visual responses. We report a novel form of plasticity within V1m that occurs on a timescale consistent with the earliest known changes in synaptic strength. Our data provide new insights into how sensory experience can rapidly modulate neuronal responses, even in the absence of direct competition between feedforward thalamocortical inputs.

Amyloid β Peptide-Induced Changes in Prefrontal Cortex Activity and Its Response to Hippocampal Input

Alterations in prefrontal cortex (PFC) function and abnormalities in its interactions with other brain areas (i.e., the hippocampus) have been related to Alzheimer Disease (AD). Considering that these malfunctions correlate with the increase in the brain's amyloid beta (Aβ) peptide production, here we looked for a causal relationship between these pathognomonic signs of AD. Thus, we tested whether or not Aβ affects the activity of the PFC network and the activation of this cortex by hippocampal input stimulation in vitro. We found that Aβ application to brain slices inhibits PFC spontaneous network activity as well as PFC activation, both at the population and at the single-cell level, when the hippocampal input is stimulated. Our data suggest that Aβ can contribute to AD by disrupting PFC activity and its long-range interactions throughout the brain.

Fifty shades of inhibition

Inhibitory circuits are essential for brain function. Our understanding of their synaptic organization has advanced extensively with the identification and classification of an impressive variety of neuron groups, receptor types, and patterns of connectivity. However, the conceptual discussion regarding the role of in neural circuits still revolves around the idea that its primary role is to regulate circuit excitability. Here, I will focus on recent findings from cortical circuits and argue that inhibitory circuits are central to the integration of incoming inputs and can promote sophisticated fine-scale control of local circuits. I propose that inhibitory circuits should not be viewed so much as brakes on principal neurons activity, but as primary contributors to a variety of neural network functions.

Laminar- and Target-Specific Amygdalar Inputs in Rat Primary Gustatory Cortex

The primary gustatory cortex (GC) receives projections from the basolateral nucleus of the amygdala (BLA). Behavioral and electrophysiological studies demonstrated that this projection is involved in encoding the hedonic value of taste and is a source of anticipatory activity in GC. Anatomically, this projection is largest in the agranular portion of GC; however, its synaptic targets and synaptic properties are currently unknown. In vivo electrophysiological recordings report conflicting evidence about BLA afferents either selectively activating excitatory neurons or driving a compound response consistent with the activation of inhibitory circuits. Here we demonstrate that BLA afferents directly activate excitatory neurons and two distinct populations of inhibitory neurons in both superficial and deep layers of rat GC. BLA afferents recruit different proportions of excitatory and inhibitory neurons and show distinct patterns of circuit activation in the superficial and deep layers of GC. These results provide the first circuit-level analysis of BLA inputs to a sensory area. Laminar- and target-specific differences of BLA inputs likely explain the complexity of amygdalocortical interactions during sensory processing.

SIGNIFICANCE STATEMENT

Projections from the basolateral nucleus of the amygdala (BLA) to the cortex convey information about the emotional value and the expectation of a sensory stimulus. Although much work has been done to establish the behavioral role of BLA inputs to sensory cortices, very little is known about the circuit organization of BLA projections. Here we provide the first in-depth analysis of connectivity and synaptic properties of the BLA input to the gustatory cortex. We show that BLA afferents activate excitatory and inhibitory circuits in a layer-specific and pattern-specific manner. Our results provide important new information about how neural circuits establishing the hedonic value of sensory stimuli and driving anticipatory behaviors are organized at the synaptic level.

A gustocentric perspective to understanding primary sensory cortices

Most of the general principles used to explain sensory cortical function have been inferred from experiments performed on neocortical, primary sensory areas. Attempts to apply a neocortical view to the study of the gustatory cortex (GC) have provided only a limited understanding of this area. Failures to conform GC to classical neocortical principles have been implicitly interpreted as a demonstration of GC's uniqueness. Here we propose to take the opposite perspective, dismissing GC's uniqueness and using principles extracted from its study as a lens for looking at neocortical sensory function. In this review, we describe three significant findings related to gustatory cortical function and advocate their relevance for understanding neocortical sensory areas.

Extensive lesions in rat insular cortex significantly disrupt taste sensitivity to NaCl and KCl and slow salt discrimination learning

While studies of the gustatory cortex (GC) mostly focus on its role in taste aversion learning and memory, the necessity of GC for other fundamental taste-guided behaviors remains largely untested. Here, rats with either excitotoxic lesions targeting GC (n = 26) or sham lesions (n = 14) were assessed for postsurgical retention of a presurgically LiCl-induced conditioned taste aversion (CTA) to 0.1M sucrose using a brief-access taste generalization test in a gustometer. The same animals were then trained in a two-response operant taste detection task and psychophysically tested for their salt (NaCl or KCl) sensitivity. Next, the rats were trained and tested in a NaCl vs. KCl taste discrimination task with concentrations varied. Rats meeting our histological inclusion criterion had large lesions (resulting in a group averaging 80% damage to GC and involving surrounding regions) and showed impaired postsurgical expression of the presurgical CTA (LiCl-injected, n = 9), demonstrated rightward shifts in the NaCl (0.54 log10 shift) and KCl (0.35 log10 shift) psychometric functions, and displayed retarded salt discrimination acquisition (n = 18), but eventually learned and performed the discrimination comparable to sham-operated animals. Interestingly, the degree of deficit between tasks correlated only modestly, if at all, suggesting that idiosyncratic differences in insular cortex lesion topography were the root of the individual differences in the behavioral effects demonstrated here. This latter finding hints at some degree of interanimal variation in the functional topography of insular cortex. Overall, GC appears to be necessary to maintain normal taste sensitivity to NaCl and KCl and for salt discrimination learning. However, higher salt concentrations can be detected and discriminated by rats with extensive damage to GC suggesting that the other resources of the gustatory system are sufficient to maintain partial competence in these tasks, supporting the view that such basic sensory-discriminative taste functions involve distributed processes among central gustatory structures.

Anatomical and electrophysiological mechanisms for asymmetrical excitatory propagation in the rat insular cortex: in vivo optical imaging and whole-cell patch-clamp studies

The insular cortex (IC) integrates limbic information from the amygdala and hypothalamic nucleus to multimodal sensory inputs, including visceral, gustatory, and somatosensory information. However, the functional framework of excitation in the IC is still unknown. We performed optical imaging and single pyramidal neuronal staining using a whole-cell patch-clamp technique in urethane-anesthetized rats to elucidate the precise anatomical and physiological features of IC pyramidal neurons, which regulate cortical information processing via their horizontal connections. Optical imaging revealed that electrical stimulation of the granular (GI) or dysgranular (DI) IC elicited characteristic excitatory propagations along the rostrocaudal axis parallel to the rhinal fissure, with a preference toward the rostral direction. Spatial patterns of the dendrites and axons of layer II/III pyramidal cells in the DI/GI support these functional features of excitation; for example, rostrocaudal axonal arbors tend to extend with a rostral directional preference. The mean length of the axons from the soma to the farthest site rostrally was ∼50% longer than that of the caudal length. Pyramidal cells in the DI/GI exhibited spontaneous membrane oscillation in the UP and DOWN states. Similarly to the evoked signals obtained by optical imaging, repetitive electrical stimulation of the caudal IC ∼1 mm away from the recorded cells (five pulses at 50 Hz) induced the summation of evoked excitatory postsynaptic potentials during the DOWN state and profound inhibitory postsynaptic potentials during the UP state. Clarification of the excitation feature with its cellular basis provides new clues about the functional mechanisms of the asymmetric propagation of neural activities in the IC.

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.

Electrical stimulation of the insular region attenuates nicotine-taking and nicotine-seeking behaviors

Pharmacological inactivation of the granular insular cortex is able to block nicotine-taking and -seeking behaviors in rats. In this study, we explored the potential of modulating activity in the insular region using electrical stimulation. Animals were trained to self-administer nicotine (0.03 mg/kg per infusion) under a fixed ratio-5 (FR-5) schedule of reinforcement followed by a progressive ratio (PR) schedule. Evaluation of the effect of stimulation in the insular region was performed on nicotine self-administration under FR-5 and PR schedules, as well on reinstatement of nicotine-seeking behavior induced by nicotine-associated cues or nicotine-priming injections. The effect of stimulation was also examined in brain slices containing insular neurons. Stimulation significantly attenuated nicotine-taking, under both schedules of reinforcement, as well as nicotine-seeking behavior induced by cues and priming. These effects appear to be specific to nicotine-associated behaviors, as stimulation did not have any effect on food-taking behavior. They appear to be anatomically specific, as stimulation surrounding the insular region had no effect on behavior. Stimulation of brain slices containing the insular region was found to inactivate insular neurons. Our results suggest that deep brain stimulation to modulate insular activity should be further explored.

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.

Inactivation of basolateral amygdala specifically eliminates palatability-related information in cortical sensory responses

Evidence indirectly implicates the amygdala as the primary processor of emotional information used by cortex to drive appropriate behavioral responses to stimuli. Taste provides an ideal system with which to test this hypothesis directly, as neurons in both basolateral amygdala (BLA) and gustatory cortex (GC)-anatomically interconnected nodes of the gustatory system-code the emotional valence of taste stimuli (i.e., palatability), in firing rate responses that progress similarly through "epochs." The fact that palatability-related firing appears one epoch earlier in BLA than GC is broadly consistent with the hypothesis that such information may propagate from the former to the latter. Here, we provide evidence supporting this hypothesis, assaying taste responses in small GC single-neuron ensembles before, during, and after temporarily inactivating BLA in awake rats. BLA inactivation (BLAx) changed responses in 98% of taste-responsive GC neurons, altering the entirety of every taste response in many neurons. Most changes involved reductions in firing rate, but regardless of the direction of change, the effect of BLAx was epoch-specific: while firing rates were changed, the taste specificity of responses remained stable; information about taste palatability, however, which normally resides in the "Late" epoch, was reduced in magnitude across the entire GC sample and outright eliminated in most neurons. Only in the specific minority of neurons for which BLAx enhanced responses did palatability specificity survive undiminished. Our data therefore provide direct evidence that BLA is a necessary component of GC gustatory processing, and that cortical palatability processing in particular is, in part, a function of BLA activity.

Effects of cue-triggered expectation on cortical processing of taste

Animals are not passive spectators of the sensory world in which they live. In natural conditions they often sense objects on the bases of expectations initiated by predictive cues. Expectation profoundly modulates neural activity by altering the background state of cortical networks and modulating sensory processing. The link between these two effects is not known. Here, we studied how cue-triggered expectation of stimulus availability influences processing of sensory stimuli in the gustatory cortex (GC). We found that expected tastants were coded more rapidly than unexpected stimuli. The faster onset of sensory coding related to anticipatory priming of GC by associative auditory cues. Simultaneous recordings and pharmacological manipulations of GC and basolateral amygdala revealed the role of top-down inputs in mediating the effects of anticipatory cues. Altogether, these data provide a model for how cue-triggered expectation changes the state of sensory cortices to achieve rapid processing of natural stimuli.

Neural processing of gustatory information in insular circuits

The insular cortex is the primary cortical site devoted to taste processing. A large body of evidence is available for how insular neurons respond to gustatory stimulation in both anesthetized and behaving animals. Most of the reports describe broadly tuned neurons that are involved in processing the chemosensory, physiological and psychological aspects of gustatory experience. However little is known about how these neural responses map onto insular circuits. Particularly mysterious is the functional role of the three subdivisions of the insular cortex: the granular, the dysgranular and the agranular insular cortices. In this article we review data on the organization of the local and long-distance circuits in the three subdivisions. The functional significance of these results is discussed in light of the latest electrophysiological data. A view of the insular cortex as a functionally integrated system devoted to processing gustatory, multimodal, cognitive and affective information is proposed.