Forebrain contribution to the induction of a brainstem correlate of conditioned taste aversion. II. Insular (gustatory) cortex

Efferent projections of CGRP/Calca-expressing parabrachial neurons in mice

The parabrachial nucleus (PB) is composed of glutamatergic neurons at the midbrain-hindbrain junction. These neurons form many subpopulations, one of which expresses Calca, which encodes the neuropeptide calcitonin gene-related peptide (CGRP). This Calca-expressing subpopulation has been implicated in a variety of homeostatic functions, but the overall distribution of Calca-expressing neurons in this region remains unclear. Also, while previous studies in rats and mice have identified output projections from CGRP-immunoreactive or Calca-expressing neurons, we lack a comprehensive understanding of their efferent projections. We began by identifying neurons with Calca mRNA and CGRP immunoreactivity in and around the PB, including populations in the locus coeruleus and motor trigeminal nucleus. Calca-expressing neurons in the PB prominently express the mu opioid receptor (Oprm1) and are distinct from neighboring neurons that express Foxp2 and Pdyn. Next, we used Cre-dependent anterograde tracing with synaptophysin-mCherry to map the efferent projections of these neurons. Calca-expressing PB neurons heavily target subregions of the amygdala, bed nucleus of the stria terminalis, basal forebrain, thalamic intralaminar and ventral posterior parvicellular nuclei, and hindbrain, in different patterns depending on the injection site location within the PB region. Retrograde axonal tracing revealed that the previously unreported hindbrain projections arise from a rostral-ventral subset of CGRP/Calca neurons. Finally, we show that these efferent projections of Calca-expressing neurons are distinct from those of neighboring PB neurons that express Pdyn. This information provides a detailed neuroanatomical framework for interpreting experimental work involving CGRP/Calca-expressing neurons and opioid action in the PB region.

Cortico-amygdala interaction determines the insular cortical neurons involved in taste memory retrieval

The insular cortex (IC) is the primary gustatory cortex, and it is a critical structure for encoding and retrieving the conditioned taste aversion (CTA) memory. In the CTA, consumption of an appetitive tastant is associated with aversive experience such as visceral malaise, which results in avoidance of consuming a learned tastant. Previously, we showed that levels of the cyclic-AMP-response-element-binding protein (CREB) determine the insular cortical neurons that proceed to encode a conditioned taste memory. In the amygdala and hippocampus, it is shown that CREB and neuronal activity regulate memory allocation and the neuronal mechanism that determines the specific neurons in a neural network that will store a given memory. However, cellular mechanism of memory allocation in the insular cortex is not fully understood. In the current study, we manipulated the neuronal activity in a subset of insular cortical and/or basolateral amygdala (BLA) neurons in mice, at the time of learning; for this purpose, we used an hM3Dq designer receptor exclusively activated by a designer drug system (DREADD). Subsequently, we examined whether the neuronal population whose activity is increased during learning, is reactivated by memory retrieval, using the expression of immediate early gene c-fos. When an hM3Dq receptor was activated only in a subset of IC neurons, c-fos expression following memory retrieval was not significantly observed in hM3Dq-positive neurons. Interestingly, the probability of c-fos expression in hM3Dq-positive IC neurons after retrieval was significantly increased when the IC and BLA were co-activated during conditioning. Our findings suggest that functional interactions between the IC and BLA regulates CTA memory allocation in the insular cortex, which shed light on understanding the mechanism of memory allocation regulated by interaction between relevant brain areas.

Genetic Access to Gustatory Disgust-Associated Neurons in the Interstitial Nucleus of the Posterior Limb of the Anterior Commissure in Male Mice

Orofacial and somatic disgust reactions are observed in rats following intraoral infusion of not only bitter quinine (innate disgust) but also sweet saccharin previously paired with illness (learned disgust). It remains unclear, however, whether these innate and learned disgust reactions share a common neural basis and which brain regions, if any, host it. In addition, there is no established method to genetically access neurons whose firing is associated with disgust (disgust-associated neurons). Here, we examined the expression of cFos and Arc, two markers of neuronal activity, in the interstitial nucleus of the posterior limb of the anterior commissure (IPAC) of male mice that showed innate disgust and mice that showed learned disgust. Furthermore, we used a targeted recombination in active populations (TRAP) method to genetically label the disgust-associated neurons in the IPAC with YFP. We found a significant increase of both cFos-positive neurons and Arc-positive neurons in the IPAC of mice that showed innate disgust and mice that showed learned disgust. In addition, TRAP following quinine infusion (Quinine-TRAP) resulted in significantly more YFP-positive neurons in the IPAC, compared to TRAP following water infusion. A significant number of the YFP-positive neurons following Quinine-TRAP were co-labeled with Arc following the second quinine infusion, confirming that Quinine-TRAP preferentially labeled quinine-activated neurons in the IPAC. Our results suggest that the IPAC activity is associated with both innate and learned disgust and that disgust-associated neurons in the IPAC are genetically accessible by TRAP.

Sensory Cortical Activity Is Related to the Selection of a Rhythmic Motor Action Pattern

Rats produce robust, highly distinctive orofacial rhythms in response to taste stimuli-responses that aid in the consumption of palatable tastes and the ejection of aversive tastes, and that are sourced in a multifunctional brainstem central pattern generator. Several pieces of indirect evidence suggest that primary gustatory cortex (GC) may be a part of a distributed forebrain circuit involved in the selection of particular consumption-related rhythms, although not in the production of individual mouth movements per se. Here, we performed a series of tests of this hypothesis. We first examined the temporal relationship between GC activity and orofacial behaviors by performing paired single-neuron and electromyographic recordings in awake rats. Using a trial-by-trial analysis, we found that a subset of GC neurons shows a burst of activity beginning before the transition between nondistinct and taste-specific (i.e., consumption-related) orofacial rhythms. We further showed that shifting the latency of consumption-related behavior by selective cueing has an analogous impact on the timing of GC activity. Finally, we showed the complementary result, demonstrating that optogenetic perturbation of GC activity has a modest but significant impact on the probability that a specific rhythm will be produced in response to a strongly aversive taste. GC appears to be a part of a distributed circuit that governs the selection of taste-induced orofacial rhythms.

SIGNIFICANCE STATEMENT

In many well studied (typically invertebrate) sensorimotor systems, top-down modulation helps motor-control regions "select" movement patterns. Here, we provide evidence that gustatory cortex (GC) may be part of the forebrain circuit that performs this function in relation to oral behaviors ("gapes") whereby a substance in the mouth is rejected as unpalatable. We show that GC palatability coding is well timed to play this role, and that the latency of these codes changes as the latency of gaping shifts with learning. We go on to show that by silencing these neurons, we can change the likelihood of gaping. These data help to break down the sensory/motor divide by showing a role for sensory cortex in the selection of motor behavior.

The Behavioral Relevance of Cortical Neural Ensemble Responses Emerges Suddenly

Whereas many laboratory-studied decisions involve a highly trained animal identifying an ambiguous stimulus, many naturalistic decisions do not. Consumption decisions, for instance, involve determining whether to eject or consume an already identified stimulus in the mouth and are decisions that can be made without training. By standard analyses, rodent cortical single-neuron taste responses come to predict such consumption decisions across the 500 ms preceding the consumption or rejection itself; decision-related firing emerges well after stimulus identification. Analyzing single-trial ensemble activity using hidden Markov models, we show these decision-related cortical responses to be part of a reliable sequence of states (each defined by the firing rates within the ensemble) separated by brief state-to-state transitions, the latencies of which vary widely between trials. When we aligned data to the onset of the (late-appearing) state that dominates during the time period in which single-neuron firing is correlated to taste palatability, the apparent ramp in stimulus-aligned choice-related firing was shown to be a much more precipitous coherent jump. This jump in choice-related firing resembled a step function more than it did the output of a standard (ramping) decision-making model, and provided a robust prediction of decision latency in single trials. Together, these results demonstrate that activity related to naturalistic consumption decisions emerges nearly instantaneously in cortical ensembles. Significance statement: This paper provides a description of how the brain makes evaluative decisions. The majority of work on the neurobiology of decision making deals with "what is it?" decisions; out of this work has emerged a model whereby neurons accumulate information about the stimulus in the form of slowly increasing firing rates and reach a decision when those firing rates reach a threshold. Here, we study a different kind of more naturalistic decision--a decision to evaluate "what shall I do with it?" after the identity of a taste in the mouth has been identified--and show that this decision is not made through the gradual increasing of stimulus-related firing, but rather that this decision appears to be made in a sudden moment of "insight."

Bilateral lesions in a specific subregion of posterior insular cortex impair conditioned taste aversion expression in rats

The gustatory cortex (GC) is widely regarded for its integral role in the acquisition and retention of conditioned taste aversions (CTAs) in rodents, but large lesions in this area do not always result in CTA impairment. Recently, using a new lesion mapping system, we found that severe CTA expression deficits were associated with damage to a critical zone that included the posterior half of GC in addition to the insular cortex (IC) that is just dorsal and caudal to this region (visceral cortex). Lesions in anterior GC were without effect. Here, neurotoxic bilateral lesions were placed in the anterior half of this critical damage zone, at the confluence of the posterior GC and the anterior visceral cortex (termed IC2 ), the posterior half of this critical damage zone that contains just VC (termed IC3), or both of these subregions (IC2 + IC3). Then, pre- and postsurgically acquired CTAs (to 0.1 M NaCl and 0.1 M sucrose, respectively) were assessed postsurgically in 15-minute one-bottle and 96-hour two-bottle tests. Li-injected rats with histologically confirmed bilateral lesions in IC2 exhibited the most severe CTA deficits, whereas those with bilateral lesions in IC3 were relatively normal, exhibiting transient disruptions in the one-bottle sessions. Groupwise lesion maps showed that CTA-impaired rats had more extensive damage to IC2 than did unimpaired rats. Some individual differences in CTA expression among rats with similar lesion profiles were observed, suggesting idiosyncrasies in the topographic representation of information in the IC. Nevertheless, this study implicates IC2 as the critical zone of the IC for normal CTA expression.

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.

CREB regulates memory allocation in the insular cortex

The molecular and cellular mechanisms of memory storage have attracted a great deal of attention. By comparison, little is known about memory allocation, the process that determines which specific neurons in a neural network will store a given memory. Previous studies demonstrated that memory allocation is not random in the amygdala; these studies showed that amygdala neurons with higher levels of the cyclic-AMP-response-element-binding protein (CREB) are more likely to be recruited into encoding and storing fear memory. To determine whether specific mechanisms also regulate memory allocation in other brain regions and whether CREB also has a role in this process, we studied insular cortical memory representations for conditioned taste aversion (CTA). In this task, an animal learns to associate a taste (conditioned stimulus [CS]) with the experience of malaise (such as that induced by LiCl; unconditioned stimulus [US]). The insular cortex is required for CTA memory formation and retrieval. CTA learning activates a subpopulation of neurons in this structure, and the insular cortex and the basolateral amygdala (BLA) interact during CTA formation. Here, we used a combination of approaches, including viral vector transfections of insular cortex, arc fluorescence in situ hybridization (FISH), and designer receptors exclusively activated by designer drugs (DREADD) system, to show that CREB levels determine which insular cortical neurons go on to encode a given conditioned taste memory.

Correlation Between Activation of the Prelimbic Cortex, Basolateral Amygdala, and Agranular Insular Cortex During Taste Memory Formation

Conditioned taste aversion (CTA) is a well-established learning paradigm, whereby animals associate tastes with subsequent visceral illness. The prelimbic cortex (PL) has been shown to be involved in the association of events separated by time. However, the nature of PL activity and its functional network in the whole brain during CTA learning remain unknown. Here, using awake functional magnetic resonance imaging and fiber tracking, we analyzed functional brain connectivity during the association of tastes and visceral illness. The blood oxygen level-dependent (BOLD) signal significantly increased in the PL after tastant and lithium chloride (LiCl) infusions. The BOLD signal in the PL significantly correlated with those in the amygdala and agranular insular cortex (IC), which we found were also structurally connected to the PL by fiber tracking. To precisely examine these data, we then performed double immunofluorescence with a neuronal activity marker (c-Fos) and an inhibitory neuron marker (GAD67) combined with a fluorescent retrograde tracer in the PL. During CTA learning, we found an increase in the activity of excitatory neurons in the basolateral amygdala (BLA) or agranular IC that project to the PL. Taken together, these findings clearly identify a role of synchronized PL, agranular IC, and BLA activity in CTA learning.

High-resolution lesion-mapping strategy links a hot spot in rat insular cortex with impaired expression of taste aversion learning

Gustatory cortex (GC), an assemblage of taste-responsive neurons in insular cortex, is widely regarded as integral to conditioned taste aversion (CTA) retention, a link that has been primarily established using lesion approaches in rats. In contrast to this prevailing view, we found that even the most complete bilateral damage to GC produced by ibotenic acid was insufficient to disrupt postsurgical expression of a presurgical CTA; nor were such lesions sufficient to disrupt postsurgical acquisition and initial expression of a second CTA. However, some rats with lesions were significantly impaired on these tests. Further examination of all conditioned rats with lesions, regardless of the lesion topography, revealed a significant positive association between damage in the posterior portion of GC and especially within adjacent posterior regions of insular cortex. Accordingly, we developed a high-resolution lesion-mapping program that permitted the overlay of the individual lesion maps from rats with CTA impairments to produce a groupwise aggregate lesion map. Comparison of this map with one derived from the unimpaired counterparts indicated a specific lesion "hot spot" associated with CTA deficits that included the most posterior end of GC and overlying granular layer and encompassed an area provisionally referred to in the literature as visceral cortex. Thus, the detailed mapping of the lesion in behaviorally defined subgroups of rats allowed us to exploit the variability in performance to uncloak an important potential component of the functional topography of insular cortex; such an approach could have general applicability to other brain structure-function endeavors as well.

Extensive lesions in the gustatory cortex in the rat do not disrupt the retention of a presurgically conditioned taste aversion and do not impair unconditioned concentration-dependent licking of sucrose and quinine

Although damage to gustatory cortex (GC) in the rat has been reported to severely impair, if not eliminate, retention of a presurgically conditioned taste aversion (CTA), it has equivocal effects on taste preference as measured by intake tests. Because intake tests can be influenced by nongustatory (e.g., postingestive) factors, we employed the brief-access taste test to assess the effects of ibotenic acid-induced lesions targeting the GC on unconditioned licking to a sucrose and then a quinine concentration series in a specialized lickometer. As a functional lesion assessment, a presurgical CTA to 0.1M NaCl was established in thirsty rats by following 15-min intake with intraperitoneal administration of either LiCl (or NaCl for control) on 2 occasions. Both conditioned sham-operated (SHAM) rats and rats with histologically confirmed extensive damage to the GC (GCX) avoided a NaCl concentration series relative to unconditioned controls in a postsurgical brief-access CTA test, with no difference between the surgical groups in their responses to NaCl or similar concentrations of KCl. GCX rats also did not differ from SHAM rats in the EC50 of concentration-response functions for sucrose or quinine. Clearly, the critical cortical area required for the retention of a presurgical CTA falls outside of the extensive area of damage, which was well centered within the conventionally defined gustatory zone of the insular cortex. The absence of an effect on unconditioned responsiveness to sucrose and quinine suggests that the damaged region is also unnecessary for the normal expression of affective licking responses to tastants.

Involvement of the insular cortex in the consolidation and expression of contextual fear conditioning

The insular cortex (IC) has been reported to be involved in the modulation of memory and autonomic and defensive responses. However, there is conflicting evidence about the role of the IC in fear conditioning. To explore the IC involvement in both behavioral and autonomic responses induced by contextual fear conditioning, we evaluated the effects of the reversible inhibition of the IC neurotransmission through bilateral microinjections of the non-selective synapse blocker CoCl2 (1 mm) 10 min before or immediately after the conditioning session or 10 min before re-exposure to the aversive context. In the conditioning session, rats were exposed to a footshock chamber (context) and footshocks were used as the unconditioned stimulus. Forty-eight hours later, the animals were re-exposed to the aversive context for 10 min, but no shock was given. Behavioral (freezing) as well as cardiovascular (arterial pressure and heart rate increases) responses induced by re-exposure to the aversive context were analysed. It was observed that the local IC neurotransmission inhibition attenuated freezing and the mean arterial pressure and heart rate increase of the groups that received the CoCl2 either immediately after conditioning or 10 min before re-exposure to the aversive context, but not when the CoCl2 was injected before the conditioning session. These findings suggest the involvement of the IC in the consolidation and expression of contextual aversive memory. However, the IC does not seem to be essential for the acquisition of memory associated with aversive context.

Context and the renewal of conditioned taste aversion: the role of rat dorsal hippocampus examined by electrolytic lesion

An extinguished conditioned response can sometimes be restored. Previous research has shown that this renewal effect depends on the context in which conditioning versus extinction takes place. Here we provide evidence that the dorsal hippocampus is critically involved in the representation of context that underscores the renewal effect. We performed electrolytic lesions in dorsal hippocampus, before or after extinction, in a conditioned taste aversion paradigm with rats. Rats that underwent all conditioning, extinction and testing procedures in the same experimental context showed no renewal during testing in the original context. In contrast, rats that underwent extinction procedures in a different experimental context than the one in which they had acquired the conditioned response, showed a reliable renewal effect during testing in the original context. When electrolytic lesion was performed prior to extinction, the context-dependent renewal effect was disrupted. When electrolytic lesion was undertaken after extinction, we observed a complex pattern of data including the blockage of the conventional renewal effect, and the appearance of an unconventional renewal effect. The implications of these results are discussed with respect to current views on the role of the dorsal hippocampus in processing context information.

Mitogen-activated protein kinase in the amygdala plays a critical role in lithium chloride-induced taste aversion learning

The intracellular mitogen-activated protein kinase (MAPK) pathway in the brain is necessary for the formation of a variety of memories including conditioned taste aversion (CTA) learning. However, the functional role of MAPK activation in the amygdala during lithium chloride (LiCl)-induced CTA learning has not been established. In the present study, we investigated if local microinjection of SL327, a MAPK kinase inhibitor, into the rat amygdala could alleviate LiCl-induced CTA learning. Our results revealed that acute administration of a high dose of LiCl (0.15M, 12 ml/kg, i.p.) rapidly increased the level of phosphorylated MAPK (pMAPK)-positive cells in the central nucleus of the amygdala (CeA) and nucleus of the solitary tract (NTS) of rats as measured by immunohistochemistry. Local microinjection of SL327 (1 μg/0.5 μl/hemisphere) into the CeA 10 min before LiCl administration decreased both the strength of LiCl-induced CTA paired with 0.125% saccharin and the level of LiCl-induced pMAPK-positive cells in the CeA, but not in the NTS. Our data suggest that the intracellular signaling cascade of the MAPK pathway in the CeA plays a critical role in the processing of visceral information induced by LiCl for CTA learning.

Intra-amygdalar okadaic acid enhances conditioned taste aversion learning and CREB phosphorylation in rats

Protein phosphatases (PPs) regulate many substrates implicated in learning and memory. Conditioned taste aversion (CTA) learning, in which animals associate a novel taste paired with a toxin and subsequently avoid the taste, is dependent on several serine/threonine phosphatase substrates and the PP1-binding protein spinophilin. In order to examine the effects of PP1/2A blockade on CTA acquisition and extinction, rats received bilateral infusions of okadaic acid (OA) (100nM, 1microl/hemisphere) or vehicle (0.15M NaCl) into the amygdala either 5min prior to, or 5min after, a single pairing of sodium saccharin (0.125%, 10-min access) and LiCl or NaCl (0.15M, 3ml/kg i.p.). Two-bottle, 24-h preference tests were conducted for 13days to measure CTA expression and extinction. Rats conditioned with saccharin and LiCl showed a decreased preference for saccharin, and OA administered before (but not after) the pairing of saccharin and LiCl resulted in a significantly stronger CTA that did not extinguish over 13days. The enhancement of the CTA was not due to aversive effects of OA, because rats given OA and a pairing of saccharin and NaCl did not acquire a CTA. Finally, OA administration increased levels of phosphorylated CREB immunoreactivity following a CTA trial. Together, these results suggest a critical role for PP1/2A during normal CTA learning. Because CTA learning was enhanced only when OA was given prior to conditioning, phosphatase activity may be a constraint on learning during the taste-toxin interval but not during acquisition and consolidation processes that occur after toxin administration.

Gustatory neural circuitry in the hamster brain stem

The nucleus of the solitary tract (NST) and the parabrachial nuclei (PbN) are the first and second central relays for the taste pathway, respectively. Taste neurons in the NST project to the PbN, which further transmits taste information to the rostral taste centers. Nevertheless, details of the neural connections among the brain stem gustatory nuclei are obscure. Here, we investigated these relationships in the hamster brain stem. Three electrode assemblies were used to record the activity of taste neurons extracellularly and then to electrically stimulate these same areas in the order: left PbN, right PbN, and right NST. A fourth electrode, a glass micropipette, was used to record from gustatory cells in the left NST. Results showed extensive bilateral communication between brain stem nuclei at the same level: 1) 10% of 96 NST neurons projected to the contralateral NST and 58% received synaptic input from the contralateral NST; and 2) 12% of 43 PbN neurons projected to the contralateral PbN and 21% received synaptic input from the contralateral PbN. Results also showed extensive communication between levels: 1) as expected, the majority of 119 NST neurons, 82%, projected to the ipsilateral PbN, but 85% of the 20 NST neurons tested received synaptic input from the ipsilateral PbN, as did 59% of 22 NST neurons that did not project to the PbN; and 2) although few, 3%, of 119 NST cells projected to the contralateral PbN and 38% received synaptic input from the contralateral PbN. These results demonstrated that taste neurons in the NST not only project to, but also receive descending input from the bilateral PbN and that gustatory neurons in the NST and PbN also communicate with the corresponding nucleus on the contralateral side.

Modulation of solitary taste neurons by electrical stimulation of the ventroposteromedial nucleus of the thalamus in the hamster

Taste neurons in the nucleus of the solitary tract (NST) not only send axons to the parabrachial nuclei (PbN), but also receive descending projections from gustatory nuclei in the forebrain in rodents. The parvicellular portion of the ventroposteromedial nucleus of the thalamus (VPMpc) receives projections from the bilateral PbN and transmits taste information to the gustatory cortex. Here, we examined the influence of bilateral stimulation of the VPMpc on taste-responsive neurons in the NST. Extracellular single unit activity was recorded from the urethane-anesthetized hamster. Taste responses were confirmed by delivery of four basic tastants to the anterior tongue. After identifying a taste neuron in the NST, the VPMpc was stimulated bilaterally. Thirty seven out of 83 neurons were orthodromically activated following VPMpc stimulation: 30 were excited and seven were inhibited. Among these cells, seven were excited and one was inhibited bilaterally. In addition, four NST neurons were antidromically invaded from the ipsilateral VPMpc. The effect of VPMpc activation on taste-driven responses was tested on 8 of 30 cells that were excited, and all seven cells that were inhibited by the VPMpc stimulation. The VPMpc stimulation enhanced responses to the effective taste stimuli or suppressed the taste-evoked activities in all eight and seven cells tested, respectively, parallel to the type of the inputs which they received from the VPMpc. These results suggest that a subset of taste neurons in the NST is under the influence from the bilateral VPMpc and that the VPMpc activation modulates taste responses of these cells.

Learning-related plasticity of temporal coding in simultaneously recorded amygdala-cortical ensembles

Emotional learning requires the coordinated action of neural populations in limbic and cortical networks. Here, we performed simultaneous extracellular recordings from gustatory cortical (GC) and basolateral amygdalar (BLA) neural ensembles as awake, behaving rats learned to dislike the taste of saccharin [via conditioned taste aversion (CTA)]. Learning-related changes in single-neuron sensory responses were observed in both regions, but the nature of the changes was region specific. In GC, most changes were restricted to relatively late aspects of the response (starting approximately 1.0 s after stimulus administration), supporting our hypothesis that in this paradigm palatability-related information resides exclusively in later cortical responses. In contrast, and consistent with data suggesting the amygdala's primary role in judging stimulus palatability, CTA altered all components of BLA taste responses, including the earliest. Finally, learning caused dramatic increases in the functional connectivity (measured in terms of cross-correlation peak heights) between pairs of simultaneously recorded BLA and GC neurons, increases that were evident only during taste processing. Our simultaneous assays of the activity of single neurons in multiple relevant brain regions across learning suggest that the transmission of taste information through amygdala-cortical circuits plays a vital role in CTA memory formation.

Taste aversion learning induced c-fos expression in the nucleus of the solitary tract after spontaneous flavor intake: role of the inter-stimulus interval

Taste aversion learning (TAL) can be induced by associating a flavor intake with the immediate or delayed (30 min) intragastric administration of a noxious substance, e.g., hypertonic NaCl. The objective of this study was to analyze the induction of c-Fos immunoreactivity in the intermediate nucleus of the solitary nucleus (iNST) after acquisition of a contiguous or delayed TAL, offering the flavor for voluntary consumption in both cases. The behavioral results obtained indicate that, although the learning was established under both experimental conditions, an increase in c-Fos induction was only produced in the group that learned by means of a non-delayed TAL. Immunohistochemical analyses revealed the participation of different brain structures in these two TAL modalities. Thus, the nucleus of the solitary tract may be involved in the TAL procedure in which voluntary flavor intake and intragastric administration of the noxious visceral stimulus are contiguous but not in delayed TAL, which would depend on other anatomical circuits that do not include the iNST.

Does taste or odor activate the same brain networks after retrieval of taste potentiated odor aversion?

When simultaneous presentation of odor and taste cues precedes illness, rats acquire robust aversion to both conditioned stimuli. Such a phenomenon referred to as taste-potentiated odor aversion (TPOA) requires information processing from two sensory modalities. Whether similar or different brain networks are activated when TPOA memory is retrieved by either the odor or the taste presentation remains an unsolved question. By means of Fos mapping, we investigated the neuronal substrate underlying TPOA retrieval elicited by either the odor or the taste conditioned stimulus. Whatever the sensory modality used to reactivate TPOA memory, a significant change in Fos expression was observed in the hippocampus, the basolateral nucleus of amygdala and the medial and the orbito-frontal cortices. Moreover, only the odor presentation elicited a significantly higher Fos immunoreactivity in the piriform cortex, the entorhinal cortex and the insular cortex. Lastly, according to the stimulus tested to induce TPOA retrieval, the BLA was differentially activated and a higher Fos expression was induced by the odor than by the taste in this nucleus. The present study indicates that even if they share some brain regions, the cerebral patterns induced by either the odor or the taste are different. Data are discussed in view of the relevance of each conditioned stimulus to reactivate TPOA memory and of the involvement of the different labeled brain areas in information processing and TPOA retrieval.

Progressive postnatal increases in Fos immunoreactivity in the forebrain and brain stem of rats after viscerosensory stimulation with lithium chloride

Interoceptive signals have a powerful impact on the motivation and emotional learning of animals during stressful experiences. However, current insights into the organization of interoceptive pathways stem mainly from observation and manipulation of adults, and little is known regarding the functional development of viscerosensory signaling pathways. To address this, we have examined central neural activation patterns in rat pups after treatment with lithium chloride (LiCl), a malaise-inducing agent. Rat pups were injected intraperitoneally with 0.15 M LiCl or 0.15 M NaCl (2% body wt) on postnatal day (P)0, 7, 14, 21, or 28, perfused 60 to 90 min postinjection, and their brains assayed for Fos protein immunolabeling. Compared with saline treatment, LiCl increased Fos only slightly in the area postrema, nucleus of the solitary tract, and lateral parabrachial nucleus on P0. LiCl did not increase Fos above control levels in the central nucleus of the amygdala, bed nucleus of the stria terminalis (BNST), or paraventricular nucleus of the hypothalamus on P0 but did on P7 and later. Maximal Fos responses to LiCl were observed on P14 in all areas except the BNST, in which LiCl-induced Fos activation continued to increase through P28. These results indicate that central LiCl-sensitive interoceptive circuits in rats are not fully functional at birth, and show age-dependent increases in neural Fos responses to viscerosensory stimulation with LiCl.

Behavioral alterations in mice lacking the translation repressor 4E-BP2

The requirement for de novo protein synthesis during multiple forms of learning, memory and behavior is well-established; however, we are only beginning to uncover the regulatory mechanisms that govern this process. In order to determine how translation initiation is regulated during neuroplasticity we engineered mutant C57Bl/6J mice that lack the translation repressor eukaryotic initiation factor 4E-binding protein 2 (4E-BP2) and have previously demonstrated that 4E-BP2 plays a critical role in hippocampus-dependent synaptic plasticity and memory. Herein, we examined the 4E-BP2 knockout mice in a battery of paradigms to address motor activity and motor skill learning, anxiety and social dominance behaviors, working memory and conditioned taste aversion. We found that the 4E-BP2 knockout mice demonstrated altered activity in the rotating rod test, light/dark exploration test, spontaneous alternation T-maze and conditioned taste aversion test. The information gained from these studies builds a solid foundation for future studies on the specific role of 4E-BP2 in various types of behavior, and for a broader, more detailed examination of the mechanisms of translational control in the brain.

Acute suppression, but not chronic genetic deficiency, of c-fos gene expression impairs long-term memory in aversive taste learning

Several lines of evidence have indicated that the establishment of long-term memory requires protein synthesis, including the synthesis of immediate-early gene products. Although the anatomical expression patterns of the c-fos gene, a transcription factor-encoding immediate-early gene, in conditioned taste aversion (CTA) are well documented, the functional roles of c-fos gene expression and Fos-mediated transcription remain to be clarified. Using the antisense oligodeoxynucleotide (AS-ODN) method in rats and gene-targeting knockout techniques in mice (c-fos(-/-) mice), we examined the roles of c-fos gene expression in the acquisition, retrieval, and retention of CTA. Preconditioning microinfusion of AS-ODN directed against c-fos mRNA (c-fos AS-ODN) into the parabrachial nucleus (PBN) impaired the acquisition, whereas infusion of control ODNs consisting of a randomized or inverted base order had no effect. Microinfusion of c-fos AS-ODN into either the amygdala or insular cortex did not impair the acquisition, whereas it attenuated the retention. Retrieval and subsequent retention of an acquired CTA were not disrupted by c-fos AS-ODN infusion into the PBN or amygdala. Microinfusion of another AS-ODN directed against zif268 (egr-1, krox-24, NGFI-A) mRNA into the PBN or amygdala did not affect the acquisition and retention. The genetic deficiency in c-fos(-/-) mice caused normal acquisition and retention. The present results suggest that the Fos-mediated gene transcription in the PBN, amygdala, or insular cortex plays critical roles in the acquisition and/or consolidation, but not the retrieval, of long-term taste memory; nevertheless, some other factors could compensate CTA mechanism when Fos-mediated transcription is not available.

Hippocampal inactivation enhances taste learning

Learning tasks are typically thought to be either hippocampal-dependent (impaired by hippocampal lesions) or hippocampal-independent (indifferent to hippocampal lesions). Here, we show that conditioned taste aversion (CTA) learning fits into neither of these categories. Rats were trained to avoid two taste stimuli, one novel and one familiar. Muscimol infused through surgically implanted intracranial cannulae temporarily inactivated the dorsal hippocampus during familiarization, subsequent CTA training, or both. As shown previously, hippocampal inactivation during familiarization enhanced the effect of that familiarization on learning (i.e., hippocampal inactivation enhanced latent inhibition of CTA); more novel and surprising, however, was the finding that hippocampal inactivation during training sessions strongly enhanced CTA learning itself. These phenomena were not caused by specific aspects of our infusion technique--muscimol infusions into the hippocampus during familiarization sessions did not cause CTAs, muscimol infusions into gustatory cortex caused the expected attenuation of CTA, and hippocampal inactivation caused the expected attenuation of spatial learning. Thus, we suggest that hippocampal memory processes interfere with the specific learning mechanisms underlying CTA, and more generally that multiple memory systems do not operate independently.

Mapping Conditioned Taste Aversion Associations Using c-Fos Reveals a Dynamic Role for Insular Cortex

Novel tastes are more effective than familiar tastes as conditioned stimuli (CSs) in taste aversion learning. Parallel to this, a novel CS-unconditioned stimulus (US) pairing induced stronger Fos-like immunoreactivity (FLI) in insular cortex (IC), amygdala, and brainstem than familiar CS-US pairing, suggesting a large circuit is recruited for acquisition. To better define the role of IC, the authors combined immunostaining with lesion or reversible inactivation of IC. Lesions abolished FLI increases to novel taste pairing in amygdala, suggesting a role in novelty detection. Reversible inactivation during taste preexposure increased FLI to familiar taste pairing in amygdala and brainstem. The difference between temporary inactivation, which blocked establishment of "safe" taste memory, and lesions points to a dual role for IC in taste learning.

Involvement of the supramammillary nucleus in aversive conditioning

Mechanisms for the retention and retrieval of conditioned taste aversions (CTAs) have yet to be fully defined. The authors explored relevant subcortical forebrain regions by tracking the expression of immediate early genes, c-fos and zif268. The supramammillary nucleus (SuM) was activated following both viscerally based CTA and somatically based inhibitory avoidance (IA). Excitotoxic lesions of the SuM before conditioning caused no disruption of acquisition but accelerated the extinction of both the CTA and IA. In contrast, lesions after CTA conditioning did not impair retention or retrieval. The present study indicates that the SuM is activated by memory-elicited discomfort during retrieval, suggesting that it plays a role in resisting the extinction of a long-term aversive memory.

Afferent and efferent connections of the parvicellular subdivision of iNTS: defining a circuit involved in taste aversion learning

Conditioned taste aversion (CTA) expression is associated with strong increases in Fos-like immunoreactivity (FLI) in a region of the brainstem identified as the parvicellular subdivision of the intermediate nucleus of the solitary tract (iNTSpc). To identify the projections to and from cells in iNTSpc which display strong FLI in response to expression of a CTA, anterograde and retrograde tract tracing was used. When appropriate, tract tracing was combined with double labeling for FLI in animals which received CTA training as well as tracer injections and were re-exposed to the CS taste. With respect to afferent projections, iNTSpc receives a strong, direct, ipsilateral projection from amygdala and the distribution of the fiber terminals yields a striking match to that of cells expressing FLI after CTA expression. As for efferent projections, these cells in iNTSpc are characterized by a mixed, rather than homogeneous, projection pattern. Targets of these cells include pons and forebrain as well as local medullary sites, all of which are known to be involved in gastrointestinal function. Thus, activation of these cells may provide a circuit through which gastrointestinal/visceral responses are coordinated as a component of the conditioned aversion.

Insular cortex lesions alter conditioned taste avoidance in rats differentially when using two methods of sucrose delivery

The insular gustatory cortex may be essential for the evaluation of saliency and representation of the incentive values of tastes. Gustatory cortex lesions should interfere with conditioned taste avoidance according to these factors, which depend on the conditioned taste avoidance protocol used. The present study was aimed at investigating the effects of bilateral lesions of the gustatory cortex-focusing on electrolytic and excitotoxic lesions. Lesioned and sham-operated male Long-Evans rats were intoxicated using LiCl after drinking sucrose from a tube (SD) or having the same amount of sucrose fed directly into their mouths through a chronically implanted intra-oral (IO) cannula. Every aspect of the experiment was carefully counterbalanced between the experimental groups. In the control groups, the acquired avoidance towards sucrose was strongly preserved over eight extinction test days in SD rats but not in IO rats, in which a progressive decline was recorded. Electrolytic gustatory cortex lesions impaired but did not suppress conditioned taste avoidance in both protocols. Excitotoxic lesions tend to impair CTA also, but differentially according to the SD or IO protocols. Extinction of CTA was selectively impaired in the SD protocol by small lesions destroying the anterior insular cortex.

Centrifugal inputs modulate taste aversion learning associated parabrachial neuronal activities

Our previous studies have demonstrated that gustatory neurons in the parabrachial nucleus (PBN) show altered responses after the acquisition of conditioned taste aversion (CTA) to NaCl. The present study was conducted 1) to examine centrifugal influences on the altered gustatory activity of CTA-trained rats, and 2) to evaluate the role of amiloride-sensitive (ASN) and -insensitive NaCl (AIN) best units in coding the taste of NaCl. Animals were separated into 2 groups: a CTA group that had acquired taste aversion to 0.1 M NaCl and a control group that underwent pseudoconditioning before the recording experiment. Single-neuron activity, in 2 separate series of experiments, was extracellularly recorded in anesthetized rats. In the stimulation studies, the effects of electrical stimulation of the gustatory cortex (GC) or the central nucleus of amygdala (CeA) were examined on firing of PBN taste units. CeA stimulation produced excitatory effect in significantly more neurons in the CTA group (n = 8) than in the control group (n = 1). Furthermore, ASN-best units in the CTA group showed larger responses to NaCl than similar units in the control group. In the decerebration experiment, there was no statistical difference among the taste responses between the 2 groups in any best-stimulus category. These results suggest that CTA conditioning uses an effective central amygdaloid input to modulate activity of gustatory neurons in the PBN. Data also substantiate that amiloride-sensitive components of NaCl-best neurons play a critical role in the recognition of distinctive taste of NaCl.

Lateral parabrachial lesions impair lithium chloride-induced aversive responses but not saccharin-induced flavor preference

Behavioral taste-guided experiments, as well as molecular studies employing c-FLI expression in response to aversive/appetitive unconditioned stimulus, have strongly suggested a visceral role for the lateral parabrachial subnuclei (lPB). The main objective in the present study was to further evaluate the functional role of the lPB in lithium chloride-induced behavioral/physiological responses. We employed a lesion/behavioral experimental strategy combining a lithium chloride-induced place aversion procedure together with the simultaneous evaluation of behavioral ("Lying on Belly", "LOB") and physiological (body temperature) responses elicited by the toxin. Data showed that lPB-lesioned animals failed to avoid the chamber previously paired with lithium chloride. Moreover, "LOB", and not hypothermia, in response to lithium chloride was impaired in parabrachial lesioned animals. Finally, all the animals were tested in a free discriminative flavor-preference task induced by saccharin, a non-caloric reinforcer, which precludes visceral feedback as essential in acquiring the learned response. As expected, both control and lesioned animals developed a clear flavor-preference to the flavor previously paired with saccharin, which shows normal gustatory and associative processing in lPB-lesioned animals. This study extends previous results on the functional visceral role of lPB subnuclei by providing alternative behavioral evidence other than taste-guided behavior, that the lPB is pivotal in visceral processing. Present data are discussed in the context of the visceral hypothesis that holds that the lPB is critically involved in processing post-oral visceral feedback.

Increased cellular activity in rat insular cortex after water and salt ingestion induced by fluid depletion

Insular cortex (IC) receives inputs from multiple sensory systems, including taste, and from receptors that monitor body electrolyte and fluid balance and blood pressure. This work analyzed metabolic activity of IC cells after water and sodium ingestion induced by sodium depletion. Rats were injected with the diuretic furosemide (10 mg/kg body wt), followed 5 min later by injections of the angiotensin-converting enzyme inhibitor captopril (5 mg/kg body wt). After 90 min, some rats received water and 0.3 M NaCl to drink for 2 h while others did not. A third group had access to water and saline but was not depleted of fluids. All rats were killed for processing of brain tissue for Fos-immunoreactivity (Fos-ir). Nondepleted animals had weak-to-moderate levels of Fos-ir within subregions of IC. Fluid-depleted rats without fluid access had significantly increased Fos-ir in all areas of IC. Levels of Fos-ir were highest in fluid-depleted rats that drank water and sodium. Fos-ir levels were highest in anterior regions of IC and lowest in posterior regions of IC. These results implicate visceral, taste, and/or postingestional factors in the increased metabolic activity of cells in IC.

Effects of conditioned food aversions on nutritional behavior in humans

Conditioned food aversion (CFA) and taste aversion (CTA) are widely occurring phenomena mediating rejection of solids or liquids, the ingestion of which has induced the onset of post-ingestional malaise. It is a powerful and durable imprint learning that may influence food choice and intake in all animals, including humans. For ethical reasons, CTA has been extensively investigated in a wide variety of laboratory animal's species but only incidentally in humans. Nevertheless, convincing evidence has been provided that CFA and CTA learning are possible in a wide range of human subjects. The results in humans may have some limitations in accuracy since data are sparse, sometimes indirect, and poorly controlled. There is only limited information on the extent of CFA in the elderly since most studies have employed questionnaire and/or interview methods on young people (i.e. college students). The present review evaluates the literature derived both from laboratory animals and humans. In the first instance, the salient features of food and taste aversion learning and the neural mechanisms involved in this learning behavior will be examined. Then, the problems encountered when trying to assess the role of learned food and taste aversions in the nutritional status of healthy as well as sick young or elderly people will be considered. In particular, the importance of CFA on the nutritional status of cancer patients and treatment of alcoholism will be examined. It is concluded that the data are compelling enough to warrant further research and, some indications and recommendations are suggested.

Induction of a brainstem correlate of conditioned taste aversion expression: role of the pontine parabrachial nucleus

Increases in Fos-like immunoreactivity (FLI) in the intermediate division of the nucleus of the solitary tract (iNTS) are seen following the expression of a conditioned taste aversion (CTA). In studies limited to behavioral assessment, the pontine parabrachial nucleus (PBN) has been demonstrated to play a critical role in the acquisition, but not the expression, of CTAs. To better define the role of the PBN in taste aversion learning, the present study examined the effects of PBN lesions on FLI in iNTS in animals with lesions placed either before or after CTA training. As is the case with behavioral expression of a CTA, timing of PBN lesions was found to be critical. Lesions placed prior to conditioning blocked evidence of conditioning, including both taste rejection and FLI in iNTS. Lesions placed after conditioning, but before testing, did not interfere with either taste rejection or FLI. These results support and extend prior claims that PBN is critical for CTA acquisition but not expression. They also demonstrate that input from PBN to iNTS is not necessary for the FLI seen there during CTA expression.

Enhanced intake of high-fat food following striatal mu-opioid stimulation: microinjection mapping and fos expression

Our previous studies have shown that stimulation of mu-opioid receptors within the nucleus accumbens preferentially enhances intake of palatable food containing sucrose and fat; thus, opioids in this brain area may mediate the rewarding characteristics of food by modulating taste and macronutrient preference. The present study was designed to further explore the nature of the involvement of striatal opioids in feeding behavior, such as the location of sensitive subregions of the ventral striatum and the brain neural circuits involved in opioid-mediated hyperphagia. In Experiment 1, we conducted a microinfusion mapping study of feeding behavior by microinfusion of the mu receptor agonist, D-Ala(2),NMe-Phe(4), Glyol(5)-enkephalin (0, 0.025 and 0.25 microg/0.5 microl per side; equivalent to 0, 0.04 and 0.40 nmol/0.5 microl per side), into several striatal subregions. In Experiment 2, detection of the expression of the immediate early gene, c-fos, was used to examine brain areas activated following intra-striatal microinfusion of D-Ala(2), NMe-Phe(4),Glyol(5)-enkephalin. The microinjection mapping study demonstrated a broad anatomical gradient within the striatum, with sensitivity highest in relatively more lateral and ventral regions of the striatum (ventrolateral striatum, lateral shell and core). The Fos mapping study demonstrated that circuitry including hypothalamic areas, the ventral tegmental area, the substantia nigra and the nucleus of the solitary tract was recruited by stimulation of mu receptors within the nucleus accumbens. A similar pattern was observed following stimulation of mu receptors in the dorsal striatum; however, the extent of activation was much smaller in magnitude. These results suggest that the role of mu receptors within the striatum in palatable feeding primarily involves ventral and lateral regions. Moreover, the pattern of activation in hypothalamic, midbrain and gustatory-visceral relay areas suggests that striatal mu receptors may participate in integrating motivational, metabolic and autonomic aspects of ingestive behavior.

The role of identified neurotransmitter systems in the response of insular cortex to unfamiliar taste: activation of ERK1-2 and formation of a memory trace

In the behaving rat, the consumption of an unfamiliar taste activates the extracellular signal-regulated kinase 1-2 (ERK1-2) in the insular cortex, which contains the taste cortex. In contrast, consumption of a familiar taste has no effect. Furthermore, activation of ERK1-2, culminating in modulation of gene expression, is obligatory for the encoding of long-term, but not short-term, memory of the new taste (Berman et al., 1998). Which neurotransmitter and neuromodulatory systems are involved in the activation of ERK1-2 by the unfamiliar taste and in the long-term encoding of the new taste information? Here we show, by the use of local microinjections of pharmacological agents to the insular cortex in the behaving rat, that multiple neurotransmitters and neuromodulators are required for encoding of taste memory in cortex. However, these systems vary in the specificity of their role in memory acquisition and in their contribution to the activation of ERK1-2. NMDA receptors, metabotropic glutamate receptors, muscarinic, and beta-adrenergic and dopaminergic receptors, all contribute to the acquisition of the new taste memory but not to its retrieval. Among these, only NMDA and muscarinic receptors specifically mediate taste-dependent activation of ERK1-2, whereas the beta-adrenergic function is independent of ERK1-2, and dopaminergic receptors regulate also the basal level of ERK1-2 activation. The data are discussed in the context of postulated novelty detection circuits in the central taste system.

Molecular neurobiology of ingestive behavior

The concepts and tools of molecular biology may be applied to almost any component of the animal involved in ingestion, but two categories of model system are particularly relevant for molecular analysis: homeostatic regulation of neuropeptide expression in the hypothalamus and neuronal plasticity underlying persistent changes in ingestive behavior. Molecular approaches to these models are reviewed, focusing on our strategy for analyzing conditioned taste aversion learning. Three questions must be answered: Where do the long-term changes occur within the distributed neural network that mediates feeding? This answer reveals the site of neuronal restructuring mediated by gene expression. When does the transition occur from short-term expression to long-term persistence of the change in behavior? This transition reveals the critical time of gene expression. What genes are expressed during the change in behavior? The expression of thousands of genes in discrete subpopulations of cells is likely to be required during critical periods of neuronal restructuring. The identification of these genes is a general challenge for molecular neurobiology. The analysis of ingestive behavior can profit from molecular tools, but ingestion also provides informative models that elucidate the principles of time- and neuron-specific gene expression mediating complex behaviors.

Lateral parabrachial lesions impair intraperitoneal but not intraventricular methylscopolamine-induced taste aversion learning

The role of the lateral parabrachial area (lPB) in the acquisition of a delayed taste aversion learning task (TAL) was examined by delivering the peripherally acting aversive compound, methylscopolamine (MSP), through two different routes, intraperitoneal and intraventricular. Consistent with previous anatomical, behavioral and molecular work, electrolytic lesions centered at the lPB did impair TAL when the MSP was injected intraperitoneally. However, lPB-lesioned animals exhibited intact learning capacities when MSP was administered intraventricularly. These results are interpreted in terms of the lPB as a critical anatomical relay involved in bottom-up visceral processing of aversive stimuli and also in relation to the relevance of forebrain structures in TAL.

Comparison of c-fos-like immunoreactivity in the brainstem following intraoral and intragastric infusions of chemical solutions in rats

To examine whether the activation of brainstem neurons during ingestion is due to orosensory afferents or post-ingestive factors, neuronal activation in response to intraoral and intragastric infusions of taste stimuli was compared in the area postrema (AP), nucleus tractus solitarius (NTS) and parabrachial nucleus (PBN) by the c-fos immunohistochemical method. An aliquot (7.5 ml) of 0.5 M sucrose, 5 mM sodium saccharin, 1 mM quinine hydrochloride and distilled water was delivered into the oral cavity or the stomach in each rat, which had been deprived of water and food overnight. Water induced little c-Fos-like immunoreactivity (c-FLI), but both intraoral and intragastric infusions of sucrose, but not non-caloric saccharin, induced strong c-FLI in the AP, caudal NTS and the external lateral subnucleus of the rostral PBN, suggesting that these areas receive general visceral inputs. Other areas in the NTS and PBN may receive gustatory inputs since more dominant c-FLI was detected by intraoral rather than intragastric infusions of the stimuli. Functional segregation of neurons reflecting qualitative and hedonic aspects of sweeteners (sucrose and saccharin) and bitter-tasting substance (quinine) was suggested in the PBN, but less evident in the NTS. These results indicate that c-fos induction in brainstem neurons during ingestion reflects gustatory inputs and postingestional factors depending on the kind of food ingested.

Conditioned c-Fos in mouse NTS during expression of a learned taste aversion depends on contextual cues

c-Fos expression in the nucleus tractus solitarius (NTS) of the rat has been found to follow administration of a variety of pharmacologically diverse unconditioned stimuli (US), and it has been proposed that NTS is a critical structure in transduction of the US during taste aversion learning. Before conditioning, the conditioned stimulus (CS) taste does not induce c-Fos in NTS, but following pairing of the CS and US, subsequent CS presentation induces c-Fos in NTS. Although it has been suggested that the shift in the c-Fos response following conditioning represents a molecular correlate of taste aversion learning, i.e. the formerly neutral CS now predicts the toxicity associated with the US, the data presented here suggest a more cautious interpretation of c-Fos expression in NTS. In mice, post-conditioning c-Fos expression to the CS depends on contextual cues: when conditioning and testing occur in a novel environment, CS saccharin causes an increase in c-Fos expression, and when conditioning and testing occur in the home cage, CS saccharin produces a decrease in c-Fos expression relative to controls. Furthermore, we show that merely placing an animal into a novel environment is sufficient to drive c-Fos expression in NTS. These data suggest that c-Fos expression in NTS can be driven by a number of different stimuli and conditions, and that these responses may depend on context-dependent activation of forebrain structures shown to drive conditioned c-Fos expression in NTS.

c-Fos induction in response to saccharin after taste aversion learning depends on conditioning method

Increases in c-Fos-like immunoreactivity (FLI) in the intermediate nucleus of the solitary tract (iNTS) have been seen consistently as a correlate of the expression of a conditioned taste aversion (CTA) when conditioning occurs using taste delivery through intraoral (I/O) infusions. The present study examined whether a similar FLI response would occur when conditioning was accomplished by presenting the taste solution in a bottle. I/O and bottle methods generated aversions that were comparable, when judged by the behavioral response of solution rejection. However, elevations in FLI were seen only in animals conditioned with the I/O method. This finding adds to evidence that the neural pathways underlying CTA learning differ as a function of the type of conditioning method used.

Insular cortex lesions and taste aversion learning: effects of conditioning method and timing of lesion

The specific role of insular cortex in acquisition and expression of a conditioned taste aversion was assessed using two different conditioning methods, which vary mode of taste delivery. Involvement of insular cortex in the induction of c-Fos-immunoreactivity in the nucleus of the solitary tract, a cellular correlate of the behavioral expression of a conditioned taste aversion, was also assessed. Electrolytic lesions of insular cortex blocked behavioral expression of a conditioned taste aversion and this was evident not only when lesions were placed prior to conditioning, but also when they were made after conditioning but before testing. In contrast to the effects on behavior, lesions did not completely block the c-Fos-immunoreactivity which accompanies re-exposure to the aversive taste. In addition, the blocking of behavioral evidence of aversion conditioning by cortical lesions was seen both in animals trained under an intraoral acquisition procedure and those trained with bottle-conditioning. This contrasts with previous work with amygdala lesions which showed that amygdala was absolutely necessary for taste aversions conditioned with the intraoral method but not for those conditioned using bottle presentation of the taste. Overall, these findings imply that the details of the neural circuitry involved in taste aversion learning, including its anatomical distribution, complexity and degree of redundancy, vary with the type of conditioning method employed.

Specific and differential activation of mitogen-activated protein kinase cascades by unfamiliar taste in the insular cortex of the behaving rat

Rats were given to drink an unfamiliar taste solution under conditions that result in long-term memory of that taste. The insular cortex, which contains the taste cortex, was then removed and assayed for activation of mitogen-activated protein kinase (MAPK) cascades by using antibodies to the activated forms of various MAPKs. Extracellular responsive kinase 1-2 (ERK1-2) in the cortical homogenate was significantly activated within <30 min of drinking the taste solution, without alteration in the total level of the ERK1-2 proteins. The activity subsided to basal levels within <60 min. In contrast, ERK1-2 was not activated when the taste was made familiar. The effect of the unfamiliar taste was specific to the insular cortex. Jun N-terminal kinase 1-2 (JNK1-2) was activated by drinking the taste but with a delayed time course, whereas the activity of Akt kinase and p38MAPK remained unchanged. Elk-1, a member of the ternary complex factor and an ERK/JNK downstream substrate, was activated with a time course similar to that of ERK1-2. Microinjection of a reversible inhibitor of MAPK/ERK kinase into the insular cortex shortly before exposure to the novel taste in a conditioned taste aversion training paradigm attenuated long-term taste aversion memory without significantly affecting short-term memory or the sensory, motor, and motivational faculties required to express long-term taste aversion memory. It was concluded that ERK and JNK are specifically and differentially activated in the insular cortex after exposure to a novel taste, and that this activation is required for consolidation of long-term taste memory.