Recruitment of hippocampal and thalamic pathways to the central amygdala in the control of feeding behavior under novelty.

It is adaptive to restrict eating under uncertainty, such as during habituation to novel foods and unfamiliar environments. However, sustained restrictive eating can become maladaptive. Currently, the neural substrates of restrictive eating are poorly understood. Using a model of feeding avoidance under novelty, our recent study identified forebrain activation patterns and found evidence that the central nucleus of the amygdala (CEA) is a core integrating node. The current study analyzed the activity of CEA inputs in male and female rats to determine if specific pathways are recruited during feeding under novelty. Recruitment of direct inputs from the paraventricular nucleus of the thalamus (PVT), the infralimbic cortex (ILA), the agranular insular cortex (AI), the hippocampal ventral field CA1, and the bed nucleus of the stria terminals (BST) was assessed with combined retrograde tract tracing and Fos induction analysis. The study found that during consumption of a novel food in a novel environment, larger number of neurons within the PVTp and the CA1 that send monosynaptic inputs to the CEA were recruited compared to controls that consumed familiar food in a familiar environment. The ILA, AI, and BST inputs to the CEA were similarly recruited across conditions. There were no sex differences in activation of any of the pathways analyzed. These results suggest that the PVTp-CEA and CA1-CEA pathways underlie feeding inhibition during novelty and could be potential sites of malfunction in excessive food avoidance.

Immune sensing of food allergens promotes avoidance behaviour.

In addition to its canonical function of protection from pathogens, the immune system can also alter behaviour1,2. The scope and mechanisms of behavioural modifications by the immune system are not yet well understood. Here, using mouse models of food allergy, we show that allergic sensitization drives antigen-specific avoidance behaviour. Allergen ingestion activates brain areas involved in the response to aversive stimuli, including the nucleus of tractus solitarius, parabrachial nucleus and central amygdala. Allergen avoidance requires immunoglobulin E (IgE) antibodies and mast cells but precedes the development of gut allergic inflammation. The ability of allergen-specific IgE and mast cells to promote avoidance requires cysteinyl leukotrienes and growth and differentiation factor 15. Finally, a comparison of C57BL/6 and BALB/c mouse strains revealed a strong effect of the genetic background on the avoidance behaviour. These findings thus point to antigen-specific behavioural modifications that probably evolved to promote niche selection to avoid unfavourable environments.

Sex differences in fear memory consolidation via Tac2 signaling in mice.

Memory formation is key for brain functioning. Uncovering the memory mechanisms is helping us to better understand neural processes in health and disease. Moreover, more specific treatments for fear-related disorders such as posttraumatic stress disorder and phobias may help to decrease their negative impact on mental health. In this line, the Tachykinin 2 (Tac2) pathway in the central amygdala (CeA) has been shown to be sufficient and necessary for the modulation of fear memory consolidation. CeA-Tac2 antagonism and its pharmacogenetic temporal inhibition impair fear memory in male mice. Surprisingly, we demonstrate here the opposite effect of Tac2 blockade on enhancing fear memory consolidation in females. Furthermore, we show that CeA-testosterone in males, CeA-estradiol in females and Akt/GSK3ß/ß-Catenin signaling both mediate the opposite-sex differential Tac2 pathway regulation of fear memory.

Neural correlates of safety learning.

Accurate discrimination between safe and dangerous stimuli is essential for survival. Prior research has begun to uncover the neural structures that are necessary for learning this discrimination, but exploration of brain regions involved in this learning process has been mostly limited to males. Recent findings show sex differences in discrimination learning, with reduced fear expression to safe cues in females compared to males. Here, we used male and female Sprague Dawley rats to explore neural activation, as measured by Fos expression, in fear and safety learning related brain regions. Neural activation after fear discrimination (Discrimination) was compared between males and females, as well as with fear conditioned (Fear Only) and stimulus presented (Control) conditions. Correlations of discrimination ability and neural activation were also calculated. We uncovered a correlation between central amygdala (CeA) activation and discrimination abilities in males and females. Anterior medial bed nucleus of the stria terminalis (BNST) was the only region where sex differences in Fos counts were observed in the Discrimination condition, and the only region where neural activation significantly differed between Fear Only and Discrimination conditions. Together, these findings indicate the importance of fear expression circuitry in mediating discrimination responses and generate important questions for future investigation.

Optogenetic Stimulation of the Central Amygdala Using Channelrhodopsin.

Optogenetics allows for the targeted temporary inhibition or stimulation of specific brain regions in vivo with precise temporal resolution. Here, we describe the steps to perform intracranial optogenetic surgery in rodents as well as instructions to build an optogenetic headcap and set up an optogenetic testing environment to conduct experiments. Behavioral studies have implemented these methods to stimulate the central amygdala (CeA) to create an addictive-like preference for reward.

The Role of Dorsal Raphe Serotonin Neurons in the Balance between Reward and Aversion.

BACKGROUND: Reward processing is fundamental for animals to survive and reproduce. Many studies have shown the importance of dorsal raphe nucleus (DRN) serotonin (5-HT) neurons in this process, but the strongly correlative link between the activity of DRN 5-HT neurons and rewarding/aversive potency is under debate. Our primary objective was to reveal this link using two different strategies to transduce DRN 5-HT neurons. METHODS: For transduction of 5-HT neurons in wildtype mice, adeno-associated virus (AAV) bearing the mouse tryptophan hydroxylase 2 (TPH2) gene promoter was used. For transduction in Tph2-tTA transgenic mice, AAVs bearing the tTA-dependent TetO enhancer were used. To manipulate the activity of 5-HT neurons, optogenetic actuators (CheRiff, eArchT) were expressed by AAVs. For measurement of rewarding/aversive potency, we performed a nose-poke self-stimulation test and conditioned place preference (CPP) test. RESULTS: We found that stimulation of DRN 5-HT neurons and their projections to the ventral tegmental area (VTA) increased the number of nose-pokes in self-stimulation test and CPP scores in both targeting methods. Concomitantly, CPP scores were decreased by inhibition of DRN 5-HT neurons and their projections to VTA. CONCLUSION: Our findings indicate that the activity of DRN 5-HT neurons projecting to the VTA is a key modulator of balance between reward and aversion.

Basolateral amygdala is required for reconsolidation updating of heroin-associated memory after prolonged withdrawal.

Postretrieval extinction procedures are effective nonpharmacological interventions for disrupting drug-associated memories. Nonetheless, the conditioned stimulus (CS) memory retrieval-extinction procedure is ineffective in inhibiting drug craving and relapse after prolonged withdrawal, which significantly undermines its therapeutic potential. In the present study, we showed that, unlike the CS memory retrieval-extinction procedure, noncontingent heroin injections (unconditioned stimulus [UCS]) 1 hour before the extinction sessions decreased the heroin-priming-induced reinstatement, renewal, and spontaneous recovery of heroin seeking after 28 days of withdrawal (ie, remote heroin-associated memories) in rats. The UCS retrieval manipulation induced reactivation of the basolateral amygdala (BLA) after prolonged withdrawal, and this reactivation was absent with the CS retrieval manipulation. Chemogenetic inactivation of the BLA abolished the inhibitory effect of the UCS memory retrieval-extinction procedure on heroin-priming-induced reinstatement after prolonged withdrawal. Furthermore, the combination of chemogenetic reactivation of BLA and CS retrieval-extinction procedure resembled the inhibitory effect of UCS retrieval-extinction procedure on heroin seeking after prolonged withdrawal. We also observed that the inhibitory effect of the UCS retrieval-extinction procedure is mediated by regulation of AMPA receptor endocytosis in the BLA. Our results demonstrate critical engagement of the BLA in reconsolidation updating of heroin-associated memory after prolonged withdrawal, extending our knowledge of the boundary conditions of the reconsolidation of drug-associated memories.

A VTA GABAergic Neural Circuit Mediates Visually Evoked Innate Defensive Responses.

Innate defensive responses are essential for animal survival and are conserved across species. The ventral tegmental area (VTA) plays important roles in learned appetitive and aversive behaviors, but whether it plays a role in mediating or modulating innate defensive responses is currently unknown. We report that VTAGABA+ neurons respond to a looming stimulus. Inhibition of VTAGABA+ neurons reduced looming-evoked defensive flight behavior, and photoactivation of these neurons resulted in defense-like flight behavior. Using viral tracing and electrophysiological recordings, we show that VTAGABA+ neurons receive direct excitatory inputs from the superior colliculus (SC). Furthermore, we show that glutamatergic SC-VTA projections synapse onto VTAGABA+ neurons that project to the central nucleus of the amygdala (CeA) and that the CeA is involved in mediating the defensive behavior. Our findings demonstrate that aerial threat-related visual information is relayed to VTAGABA+ neurons mediating innate behavioral responses, suggesting a more general role of the VTA.

Estrogen receptors α and ß in the central amygdala and the ventromedial nucleus of the hypothalamus: Sociosexual behaviors, fear and arousal in female rats during emotionally challenging events.

Estrogens receptors (ER) are involved in several sociosexual behaviors and fear responses. In particular, the ERα is important for sexual behaviors, whereas ERß modulates anxiolytic responses. Using shRNA directed either against the ERα or the ERß RNAs (or containing luciferase control) encoded within an adeno-associated viral vector, we silenced these receptors in the ventromedial nucleus of the hypothalamus (VMN) and the central amygdala (CeA). We exposed ovariectomized female rats, sequentially treated with estradiol benzoate and progesterone, to five stimuli, previously reported to elicit positive and negative affect. The subjects were housed in groups of 4 females and 3 males in a seminatural environment for several days before hormone treatment. We analyzed the frequency of a large number of behavior patterns. In addition, we performed analyses of co-occurrence in order to detect changes in the structure of behavior after infusion of the vectors. Silencing the ERα in the VMN disrupted lordosis and showed some anxiolytic properties in aversive situations, whereas silencing of the ERß in this structure had no effect. This was also the case after silencing the ERα in the CeA. Silencing of the ERß in this structure increased risk assessment, an expression of anxiety, and increased olfactory exploration of the environment. We hypothesize that the ERß in the CeA has an important role in the well-established anxiolytic effects of estrogens, and that it may modulate arousal level. Furthermore, it seems that the ERα in the VMN is anxiogenic in aversive or threatening situations, in agreement with other studies.

Influence of behavioral traits in the inter-individual variability of nociceptive, emotional and cognitive manifestations of neuropathic pain.

Neuropathic pain is a complex disorder associated with emotional and cognitive deficits that may impair nociceptive manifestations. There is high inter-individual variability in the manifestations of human neuropathic pain, which largely depends on personality traits. We aim to identify the influence of different behavioral traits in the inter-individual vulnerability to neuropathic pain manifestations using behavioral, electrophysiological and genetic approaches. We first selected mice with extreme social and emotional traits and look for correlation with the spontaneous neuronal activity in the central amygdala. Neuropathic pain was induced to these mice to evaluate the influence of behavioral traits on nociceptive manifestations and gene expression profiles in the amygdala. Our results show an association of the spontaneous central amygdala neuronal activity with the sociability behavior. We demonstrate that low sociable, high anxious and low depressive phenotypes develop enhanced nociceptive hypersensitivity after nerve injury. However, greater emotional alterations and cognitive impairment are observed in high sociable, anxious-like and depressive-like mice, indicating that nociceptive, emotional and cognitive manifestations of neuropathic pain do not correlate with each other. Gene analyses identify high Pdyn and Il6 levels in the amygdala as indicative of enhanced nociceptive hypersensitivity and reveal an association between high Gadd45 expression and attenuated emotional and cognitive manifestations of neuropathic pain.

The purinergic mechanism of the central nucleus of amygdala is involved in the modulation of salt intake in sodium-depleted rats.

The central nucleus of the amygdala (CeA) is a critical region in regulating sodium intake, and interestingly, purinergic receptors reportedly related to fluid balance, are also expressed in CeA. In this study, we investigated whether the purinergic mechanisms of CeA were involved in regulating sodium intake. Male Sprague-Dawley rats had cannulas implanted bilaterally into the CeA and were sodium depleted with furosemide (FURO 20 mg/kg) plus 24 h-sodium deficient food fed. Bilateral injections of the P2X purinergic agonist, α,ß-methyleneadenosine 5'-triphosphate (α,ß-methylene ATP 1.0, 2.0, 4.0 nmol, respectively) into the CeA region induced dose-related reductions in sodium intake without affecting water intake. Injection of P2X purinergic antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS 4.0 nmol/0.5 µl) into the CeA region did not alter sodium and water intake, however, prior injection of PPADS into the CeA area abolished the inhibitory effects on sodium intake by α,ß-methylene ATP. Interestingly, prior injection of γ-aminobutyric acid type A (GABAA) receptor antagonist, bicuculline (4.0 nmol/0.5 µl) into the CeA region partially reversed the deficit of sodium intake induced by α,ß-methylene ATP. These results suggest that purinergic receptors in the CeA are involved in the control of sodium intake in the sodium-depleted rats and this negative modulation may be, at least partly, mediated by the GABAA receptor.

Neuronal connections of the central amygdalar nucleus with refeeding-activated brain areas in rats.

Following fasting, satiety is accompanied by neuronal activation in brain areas including the central amygdalar nucleus (CEA). Since CEA is known to inhibit food intake, we hypothesized that CEA contributes to the termination of meal during refeeding. To better understand the organization of this satiety-related circuit, the interconnections of the CEA with refeeding-activated neuronal groups were elucidated using retrograde (cholera toxin-ß subunit, CTB) and anterograde (phaseolus vulgaris leucoagglutinin, PHA-L) tracers in male rats. C-Fos-immunoreactivity was used as marker of neuronal activation. The refeeding-activated input of the CEA primarily originated from the paraventricular thalamic, parasubthalamic and parabrachial nuclei. Few CTB-c-Fos double-labeled neurons were detected in the prefrontal cortex, lateral hypothalamic area, nucleus of the solitary tract (NTS) and the bed nuclei of the stria terminalis (BNST). Only few refeeding-activated proopiomelanocortin-producing neurons of the arcuate nucleus projected to the CEA. Anterograde tract tracing revealed a high density of PHAL-labeled axons contacted with refeeding-activated neurons in the BNST, lateral hypothalamic area, parasubthalamic, paraventricular thalamic and parabrachial nuclei and NTS; a low density of labeled axons was found in the paraventricular hypothalamic nucleus. Chemogenetic activation of the medial CEA (CEAm) inhibited food intake during the first hour of refeeding, while activation of lateral CEA had no effect. These data demonstrate the existence of reciprocal connections between the CEA and distinct refeeding-activated hypothalamic, thalamic and brainstem nuclei, suggesting the importance of short feedback loops in the regulation of satiety and importance of the CEAm in the regulation of food intake during refeeding.

Suckling induced activation pattern in the brain of rat pups.

OBJECTIVES: The aim of the study was to understand the effects of suckling on the brain of the pups by mapping their brain activation pattern in response to suckling. METHODS: The c-fos method was applied to identify activated neurons. Fasted rat pups were returned to their mothers for suckling and sacrificed 2 hours later for Fos immunohistochemistry. Double labeling was also performed to characterize some of the activated neurons. For comparison, another group of fasted pups were given dry food before Fos mapping. RESULTS: After suckling, we found an increase in the number of Fos-immunoreactive neurons in the insular and somatosensory cortices, central amygdaloid nucleus (CAm), paraventricular (PVN) and supraoptic hypothalamic nuclei, lateral parabrachial nucleus (LPB), nucleus of the solitary tract (NTS), and the area postrema. Double labeling experiments demonstrated the activation of calcitonin gene-related peptide-ir (CGRP-ir) neurons in the LPB, corticotropin-releasing hormone-ir (CRH-ir) but not oxytocin-ir neurons in the PVN, and noradrenergic neurons in the NTS. In the CAm, Fos-ir neurons did not contain CRH but were apposed to CGRP-ir fiber terminals. Refeeding with dry food-induced Fos activation in all brain areas activated by suckling. The degree of activation was higher following dry food consumption than suckling in the insular cortex, and lower in the supraoptic nucleus and the NTS. Furthermore, the accumbens, arcuate, and dorsomedial hypothalamic nuclei, and the lateral hypothalamic area, which were not activated by suckling, showed activation by dry food. DISCUSSION: Neurons in a number of brain areas are activated during suckling, and may participate in the signaling of satiety, taste perception, reward, food, and salt balance regulation.

Divergent functions of the left and right central amygdala in visceral nociception.

The left and right central amygdalae (CeA) are limbic regions involved in somatic and visceral pain processing. These 2 nuclei are asymmetrically involved in somatic pain modulation; pain-like responses on both sides of the body are preferentially driven by the right CeA, and in a reciprocal fashion, nociceptive somatic stimuli on both sides of the body predominantly alter molecular and physiological activities in the right CeA. Unknown, however, is whether this lateralization also exists in visceral pain processing and furthermore what function the left CeA has in modulating nociceptive information. Using urinary bladder distension (UBD) and excitatory optogenetics, a pronociceptive function of the right CeA was demonstrated in mice. Channelrhodopsin-2-mediated activation of the right CeA increased visceromotor responses (VMRs), while activation of the left CeA had no effect. Similarly, UBD-evoked VMRs increased after unilateral infusion of pituitary adenylate cyclase-activating polypeptide in the right CeA. To determine intrinsic left CeA involvement in bladder pain modulation, this region was optogenetically silenced during noxious UBD. Halorhodopsin (NpHR)-mediated inhibition of the left CeA increased VMRs, suggesting an ongoing antinociceptive function for this region. Finally, divergent left and right CeA functions were evaluated during abdominal mechanosensory testing. In naive animals, channelrhodopsin-2-mediated activation of the right CeA induced mechanical allodynia, and after cyclophosphamide-induced bladder sensitization, activation of the left CeA reversed referred bladder pain-like behaviors. Overall, these data provide evidence for functional brain lateralization in the absence of peripheral anatomical asymmetries.

Amygdala subnuclei resting-state functional connectivity sex and estrogen differences.

The amygdala is a hub in emotional processing, including that of negative affect. Healthy men and women have distinct differences in amygdala responses, potentially setting the stage for the observed sex differences in the prevalence of fear, anxiety, and pain disorders. Here, we examined how amygdala subnuclei resting-state functional connectivity is affected by sex, as well as explored how the functional connectivity is related to estrogen levels. Resting-state functional connectivity was measured using functional magnetic resonance imaging (fMRI) with seeds placed in the left and right laterobasal (LB) and centromedial (CM) amygdala. Sex differences were studied in 48 healthy men and 48 healthy women, matched for age, while the association with estrogen was analyzed in a subsample of 24 women, for whom hormone levels had been assessed. For the hormone analyses, the subsample was further divided into a lower and higher estrogen levels group based on a median split. We found distinct sex differences in the LB and CM amygdala resting-state functional connectivity, as well as preliminary evidence for an association between estrogen levels and connectivity patterns. These results are potentially valuable in explaining why women are more afflicted by conditions of negative affect than are men, and could imply a mechanistic role for estrogen in modulating emotion.

The influence of µ-opioid and noradrenaline reuptake inhibition in the modulation of pain responsive neurones in the central amygdala by tapentadol in rats with neuropathy.

Treatments for neuropathic pain are either not fully effective or have problematic side effects. Combinations of drugs are often used. Tapentadol is a newer molecule that produces analgesia in various pain models through two inhibitory mechanisms, namely central µ-opioid receptor (MOR) agonism and noradrenaline reuptake inhibition. These two components interact synergistically, resulting in levels of analgesia similar to opioid analgesics such as oxycodone and morphine, but with more tolerable side effects. The right central nucleus of the amygdala (CeA) is critical for the lateral spinal ascending pain pathway, regulates descending pain pathways and is key in the emotional-affective components of pain. Few studies have investigated the pharmacology of limbic brain areas in pain models. Here we determined the actions of systemic tapentadol on right CeA neurones of animals with neuropathy and which component of tapentadol contributes to its effect. Neuronal responses to multimodal peripheral stimulation of animals with spinal nerve ligation or sham surgery were recorded before and after two doses of tapentadol. After the higher dose of tapentadol either naloxone or yohimbine were administered. Systemic tapentadol resulted in dose-dependent decrease in right CeA neuronal activity only in neuropathy. Both naloxone and yohimbine reversed this effect to an extent that was modality selective. The interactions of the components of tapentadol are not limited to the synergy between the MOR and α2-adrenoceptors seen at spinal levels, but are seen at this supraspinal site where suppression of responses may relate to the ability of the drug to alter affective components of pain.

Amygdalar neuronal plasticity and the interactions of alcohol, sex, and stress.

Alcohol abuse and alcoholism are major medical problems affecting both men and women. Previous animal studies reported a difference in c-Fos neuronal activation after chronic alcohol exposure; however, females remain an understudied population. To model chronic alcohol exposure match-pair fed adult male and female rats were administered 14 days of a liquid ethanol containing diet. Analysis focused on the central nucleus of the amygdala (CeA), a region integral to stress sensitivity and substance abuse. Immunocytochemical approaches identified cells containing ΔFosB, a marker of sustained neuronal activation, and activity patterns within the CeA were mapped by subdivision and rostral-caudal extent. Significant interactions were present between all groups, with gender differences noted among control groups, and ethanol exposed animals having the greatest number of ΔFosB immunoreactive cells indicating baseline dysregulation. Compared with c-Fos, a marker of recent neuronal activation, male ethanol treated animals had similar activity to controls, indicating a neuronal habituation not seen in females. Next, a cohort of animals were exposed to the forced swim test (FST), and c-Fos was examined in addition to FST behavior. Neuronal activity was increased in ethanol exposed animals compared to controls, and control females compared to males, indicating a potentiated stress response. Further, a population of activated neurons were shown to contain either corticotropin releasing factor or enkephalin. The present data suggest that dysregulation in the CeA neuronal activity may underlie some of the negative sequelae of alcohol abuse, and may, in part, underlie the distinctive response seen between genders to alcohol use.

Nf1 regulates alcohol dependence-associated excessive drinking and gamma-aminobutyric acid release in the central amygdala in mice and is associated with alcohol dependence in humans.

BACKGROUND: The neurofibromatosis type 1 (Nf1) gene encodes a GTPase activating protein that negatively regulates small GTPases of the Ras family. METHODS: We assessed alcohol-related behaviors including alcohol sensitivity, dependent and nondependent drinking, and basal and alcohol-induced gamma-aminobutyric acid (GABA) release in the central nucleus of the amygdala (CeA) in Nf1 heterozygous null mice (Nf1(+/-)). We also investigated the associations of NF1 polymorphisms with alcohol dependence risk and severity in humans. RESULTS: Nf1(+/-) mice do not differ from wild-type mice in nondependent drinking, such as 24-hour, 2-bottle choice drinking in the dark binge drinking or limited access 2-bottle choice. However, Nf1(+/-) mice failed to escalate alcohol drinking following chronic intermittent ethanol vapor exposure (CIE) to induce dependence. Alcohol acutely increases GABA release in the CeA and alcohol dependence is characterized by increased baseline GABA release in CeA. Interestingly, GABA release in Nf1(+/-) mice is greater at baseline than wild-type mice, is not elevated by induction of dependence by CIE, and failed to show alcohol-induced facilitation both before and after CIE. Additionally, we observed that multiple variants in the human NF1 gene are associated with a quantitative measure of alcohol dependence in both African Americans and European Americans. CONCLUSIONS: In this translational investigation, we found that Nf1 activity regulates excessive drinking and basal and ethanol-stimulated GABA release in the mouse central amygdala. We also found that genetic variation in NF1 may confer an inherent susceptibility to the transition from nondependent to dependent drinking in humans.

Restoration of quinine-stimulated Fos-immunoreactive neurons in the central nucleus of the amygdala and gustatory cortex following reinnervation or cross-reinnervation of the lingual taste nerves in rats.

Remarkably, when lingual gustatory nerves are surgically rerouted to inappropriate taste fields in the tongue, some taste functions recover. We previously demonstrated that quinine-stimulated oromotor rejection reflexes and neural activity (assessed by Fos immunoreactivity) in subregions of hindbrain gustatory nuclei were restored if the posterior tongue, which contains receptor cells that respond strongly to bitter compounds, was cross-reinnervated by the chorda tympani nerve. Such functional recovery was not seen if instead, the anterior tongue, where receptor cells are less responsive to bitter compounds, was cross-reinnervated by the glossopharyngeal nerve, even though this nerve typically responds robustly to bitter substances. Thus, recovery depended more on the taste field being reinnervated than on the nerve itself. Here, the distribution of quinine-stimulated Fos-immunoreactive neurons in two taste-associated forebrain areas was examined in these same rats. In the central nucleus of the amygdala (CeA), a rostrocaudal gradient characterized the normal quinine-stimulated Fos response, with the greatest number of labeled cells situated rostrally. Quinine-stimulated neurons were found throughout the gustatory cortex, but a "hot spot" was observed in its anterior-posterior center in subregions approximating the dysgranular/agranular layers. Fos neurons here and in the rostral CeA were highly correlated with quinine-elicited gapes. Denervation of the posterior tongue eliminated, and its reinnervation by either nerve restored, numbers of quinine-stimulated labeled cells in the rostralmost CeA and in the subregion approximating the dysgranular gustatory cortex. These results underscore the remarkable plasticity of the gustatory system and also help clarify the functional anatomy of neural circuits activated by bitter taste stimulation.