Gustatory Neocortex Research Articles

Periaqueductal gray c-Fos expression varies relative to the method of conditioned taste aversion extinction employed

A conditioned taste aversion (CTA) is acquired when an animal consumes a novel taste (CS) and then experiences the symptoms of poisoning (US). Following CTA training, animals will avoid the taste that was previously associated with malaise. This defensive reaction to a learned fear can be extinguished by repeated exposure to the CS alone (CS-only; CSO-EXT). However, following a latency period in which the CS is not presented, the CTA will spontaneously recover (SR). Through the use of an explicitly unpaired extinction procedure (EU-EXT) we have shown that we can speed up extinction and attenuate SR of the CTA. Here we compared and contrasted the ability of CSO and EU extinction procedures to affect c-Fos expression in the periaqueductal gray (PAG). Fluid-deprived Sprague–Dawley rats acquired a strong CTA [via 3 pairings of 0.3% oral saccharin (SAC; the CS) and 81 mg/kg i.p. lithium chloride (LiCl; the US)] followed by extinction trials consisting of multiple exposures to either, (a) the CS every-other day (CSO-EXT), or (b) CS and US on alternate days (EU-EXT). A different group of rats did not receive multiple CS exposures and served as a “no extinction” (NE) control. Both extinction procedures resulted in ≥ 90% reacceptance of SAC (achieving asymptotic extinction). Some of the animals were sacrificed for c-Fos immunohistochemical analysis following asymptotic extinction. Other rats entered a 30-day latency period where they drank water only. These remaining animals were then tested for SR with a final exposure to SAC before being sacrificed for c-Fos immunohistochemistry. As reported previously, rats in the CS-only group exhibited a significant SR of the CTA. However, animals in the EU extinction group reached asymptotic extinction more rapidly than did CSO rats and they did not show SR of the CTA. As compared to rats that retained their CTA, both groups of extinguished rats showed suppression in the number of c-Fos-labeled neurons in all 4 longitudinal columns of the PAG. The number of c-Fos-labeled cells in the PAG was generally low but there was a reliable increase in c-Fos expression in dorsolateral PAG (dlPAG) following the SR test in the brains of rats that went through the EU-EXT procedure as compared with those that either went through the more-traditional CSO extinction procedure or experienced no extinction at all. The number of c-Fos-labeled neurons in the dlPAG was significantly correlated with the amount of SAC consumed at the SR test. Surprisingly, the brains of EU-extinguished rats and CSO extinguished rats did not differ in the number of c-Fos-labeled neurons in gustatory neocortex, medial prefrontal cortex, basolateral amygdala, or the central nucleus of the amygdala. Thus, behavioral differences in SR between the EU and CSO extinction animals were not represented by corresponding changes in the neural activity of several brain nuclei classically associated with extinction learning. However a detailed analysis of PAG c-Fos expression provided hints about some of the physiological changes evoked by these 2 extinction paradigms that produce very different behavioral outcomes. The findings are clinically relevant as we seek the development of treatments for deficits in fear extinction (e.g. PTSD, phobias).

Spontaneous recovery of a conditioned taste aversion differentially alters extinction-induced changes in c-Fos protein expression in rat amygdala and neocortex

Conditioned taste aversions (CTAs) may be acquired when an animal consumes a novel taste (conditioned stimulus; CS) and then experiences the symptoms of poisoning (unconditioned stimulus; US). Animals will later avoid the taste that was previously associated with malaise. Extinction of a CTA is observed following repeated, non-reinforced exposures to the CS and represents itself as a resumption of eating/drinking the once-avoided tastant. Spontaneous recovery (SR) of a CTA (a revival of the taste avoidance) occurs when the CS is offered after a latency period in which the CS was not presented. An initial study explored the experimental parameters required to produce a reliable SR following acquisition and extinction of a robust CTA in rats. A CTA was formed through 3 pairings of 0.3% oral saccharin (SAC) and 81 mg/kg i.p. lithium chloride (LiCl) followed by extinction training resulting in 90% reacceptance of SAC. After extinction training, some of the animals were also tested for SR of the CTA upon exposure to SAC following a 15-, 30-, or 60-day latency period of water drinking. We report here that latencies of 15, 30, or 60 days produced small, but reliable, SRs of the CTA—with longer latencies producing progressively more suppression of SAC consumption. A second study investigated changes in the amygdala (AMY), gustatory neocortex (GNC), and medial prefrontal cortex (mPFC) functioning during SR of a CTA. Using immunohistochemical methods, brain c-Fos protein expression was analyzed in rats that extinguished the CTA as well as those that exhibited SR of the CTA after a 30-day latency. Our previous studies indicated that the numbers of c-Fos-labeled neurons in GNC and mPFC is low following CTA acquisition and increase dramatically as rats fully extinguished the aversion. Here we report that cortical c-Fos protein expression declines significantly following SR of the CTA. Expression of c-Fos in basolateral AMY decreased significantly from EXT to SR, but control animals with an intact CTA also decreased significantly from a short-term CTA test to a long-term CTA test. Low levels of c-Fos expression in the central nucleus of the amygdala (CE) were observed throughout EXT with little change in expression detectable following SR. These measurements reflect the dynamic nature of brain activity during acquisition and extinction of a CTA and highlight an important role for cortical neurons in the brain reorganization that occurs during SR of a CTA. The data also suggest that certain sub-nuclei of the AMY may play a relatively minor role in SR of this defensive reaction to a learned fear.

Dynamic processing of taste aversion extinction in the brain

While substantial advances have been made in discovering how the brain learns and remembers, less is known about how the brain discards information, reorganizes information, or both. These topics are not only relevant to normal brain functioning but also speak to pathologies in which painful memories do not wane but are evoked time and again (e.g., post-traumatic stress disorder; PTSD). Here, we measured brain activity (as indicated by the regional expression of c-Fos protein) in rats during acquisition and throughout extinction of a conditioned taste aversion (CTA). We compared that brain activity with animals that had intact CTA memories or those that experienced an explicitly unpaired (EU) conditioned stimulus (CS; saccharin, SAC) and unconditioned stimulus (US; lithium chloride, LiCl). The data show a dynamic and nonuniform pattern of c-Fos protein expression in brain nuclei known to mediate gustation and CTAs. In particular, brainstem nuclei (e.g., nucleus of the solitary tract; NTS) and the basolateral nucleus of the amygdala (BLA) are active early as CTAs are formed and as extinction of the learned response begins. Later in the extinction process, the BLA reduces c-Fos expression relative to nonextinguished controls. Finally, as almost full reacceptance of the taste is achieved, the gustatory neocortex (GNC) expresses enhanced levels of c-Fos protein. Thus, extinction of a CTA is not represented by a simple reversal of the c-Fos activity evoked by CTA conditioning. Rather, the data demonstrate that extinction of conditioned responses is a dynamic process in which the activity levels of particular nuclei along the brain's taste pathway change depending on the extent to which the conditioned response has been extinguished.

Ipsilateral connections between the gustatory cortex, amygdala and parabrachial nucleus are necessary for acquisition and retrieval of conditioned taste aversion in rats

Acquisition and retrieval of conditioned taste aversion (CTA) is implemented by the interaction of several brain structures. In order to clarify the role of 3 important gustatory and visceral relays, gustatory neocortex (GC), amygdala (AM) and parabrachial nucleus (PB), two experimental series were carried out. In Expt. 1 reversible unilateral lesions of the PB and AM or of AM and GC were induced by 10 ng of tetrodotoxin (TTX) either in the same hemisphere or in the opposite hemispheres during CTA acquisition. It was found that contralateral TTX injections into the above structures caused a complete blockade of CTA acquisition which was not affected by ipsilateral TTX administration. In Expt. 2, retrieval of CTA acquired during unilateral blockade of the PB or AM in one hemisphere has been examined during unilateral TTX blockade of GC alone, AM alone or during combined GC and AM inactivation either in the same or in the contralateral hemispheres. A clear CTA retrieval impairment was found only when structures in the opposite hemispheres were blocked. These results indicate a complete lateralization of CTA acquisition and retrieval processes and suggest that the long-term CTA engram can be formed in each hemisphere separately without participation of the contralateral hemisphere.

Corticofugal influence on taste responses in the nucleus of the solitary tract in the rat.

1. Previous work has revealed a pervasive influence of the gustatory neocortex (GN) on the electrophysiological responses to taste in the parabrachial nucleus of the pons (PbN), the second synapse in the central pathway for gustation. Subsequent experiments have further suggested that direct projections from the GN to the PbN are not sufficiently dense to account for the widespread effects of cortical input. Because the main source of input to the PbN, i.e., the nucleus of the solitary tract (NTS), also receives input from the GN, the present experiment was conducted to test the hypothesis that changes in taste responses in the PbN after temporary elimination of GN input may be a normal reaction to altered input originating in the NTS. 2. Fourty-three taste-responsive neurons in the NTS were isolated initially in urethan-anesthetized rats. Single units were then classified as "relay" (n = 12) or "nonrelay" (n = 13) on the basis of their electrophysiological response to electrical shocks delivered to the taste-responsive portion of the PbN. After histological analyses, 18 units were classified as "unknown" because the PbN stimulating electrode was found to be outside the anatomically defined taste area in the pons. 3. Electrophysiological responses to sapid solutions of the NaCl (0.1 M), HCl (0.01 M), quinineHCl (0.01 M), sucrose (0.5 M), and Na-saccharin (0.004 M) were then recorded before and after recovery from infusions of procaineHCl into the GN. Both the ipsilateral and contralateral sides of the GN, in that order, received procaine infusions separated by a recovery period of at least 45 min. 4. Analysis of across-unit patterns of response was accomplished with the use of a vector space analysis. With this approach, the response of a given neuron to a given tastant is considered as a coordinate in n-dimensional space, where n is the number of neurons tested. The responses to each stimulus generate vectors whose length relates to the overall magnitude of response across the sample and whose relative directionality indicates similarity to other across-unit patterns. Measures derived from this type of analysis were used as input in a multidimensional scaling (MDS) analysis designed to summarize the organization of the across-unit patterns of response generated by the taste stimuli. This type of analysis creates a "taste space" in which similar across-unit patterns of response are placed close together and dissimilar patterns are placed far apart.(ABSTRACT TRUNCATED AT 400 WORDS)

Corticofugal input to taste-responsive units in the parabrachial pons

Taste responses in units in the parabrachial nucleus of the pons (PbN) that receive direct projections from the gustatory neocortex (GN) were examined in the rat. Electrical stimulation was applied to the GN with bipolar stimulating electrodes. Time-locked responses to GN stimulation were observed in 19 of 62 (31%) taste-responsive units identified in the PbN. Of these, 17 units produced excitatory responses following GN stimulation and 2 units showed evidence of inhibition. No evidence of antidromic activation of PbN units was apparent. These results are consistent with descriptions of the anatomical projections from the GN to the PbN and demonstrate that some portion of taste-responsive neural elements within the PbN receive direct cortical input. PbN units that followed GN stimulation could not be distinguished from those that did not follow either by their spontaneous rates or their response profiles. These results suggest that axons from the GN do not select targets within the PbN based on the particular response properties.

Clonidine in the prepyriform cortex blocked anorectic response to amino acid imbalance

Ingestion of imbalanced amino acid diets (IMB) has been associated with a decrease in norepinephrine (NE) concentration in the prepyriform cortex (PPC), an area essential for the anorectic response to IMB. Decreased NE could result from activity-induced release (and subsequent metabolism) of the transmitter. If activity of the NE system is important in the rat's anorectic response to IMB, reduced NE activity should result in increased IMB intake. Therefore, the feeding response to IMB was measured after injecting clonidine (Clon) into the PPC to inhibit NE release. At 3 and 6 h after Clon (1.0 and 1.5 micrograms/rat) injections, IMB intake was increased from 69 (the usual response to IMB in untreated animals) to > 100% of control intake. Effective injection sites did not include the gustatory neocortex, an area important for conditioned taste aversions. Thus activation of the NE system in the PPC may be associated with the initial reduced intake of IMB, suggesting that NE activity in the PPC has a role in the neural mechanisms that subserve recognition of amino acid deficiency.

Time-dependent recovery of taste aversion learning by fetal brain transplants in gustatory neocortex-lesioned rats

We recently showed that fetal brain transplants produced a significant recovery in the ability of gustatory neocortex-lesioned rats to learn a conditioned taste aversion. In this report we assessed the capability of gustatory neocortex fetal brain transplants to produce behavioral recovery at different times. Four groups of male Wistar rats showing disrupted taste aversions due to gustatory neocortex lesions were studied. The lesioned animals received fetal cortical grafts, obtained from 16-day-old fetuses, and were retrained in the behavioral procedure after 15, 30, 45, or 60 days postgraft. Behavioral results showed a very good functional recuperation at 60 days, slight recovery at 45 and 30 days, and a poor recovery at 15 days postgraft. Results with HRP histochemistry revealed that at 30, 45, and 60 days postgrafting there were increased connections with the ventromedial nucleus of the thalamus and with the amygdala. At 15 days postgrafting there was an absence of HRP-labeled cells. In addition, behavioral recovery was correlated with increased acetylcholinesterase activity, detected histochemically, and with morphological neuronal maturation, revealed by Golgi staining. These results suggest that morphological maturity and reconnectivity between grafts and host tissue are important for behavioral recovery in gustatory neocortex-lesioned rats.

Odor cue mediation of alcohol aversion learning in rats lacking gustatory neocortex.

Odor cue mediation of alcohol aversion learning in rats lacking gustatory neocortex.

Odor cue mediation of alcohol aversion learning in rats lacking gustatory neocortex.

Normal rats presented with a 5% alcohol solution followed by lithium chloride-induced illness quickly learned to avoid drinking alcohol. After training, the rats also avoided drinking water in the presence of the alcohol odor alone, whether tested immediately or 1 month later. In Experiment 1, rats with gustatory neocortex (GN) ablations also developed strong alcohol aversions when the alcohol solution was paired with illness. They also showed normal avoidance of drinking in the presence of the alcohol odor alone when tested soon after training. In Experiment 2, when normal rats were trained to avoid alcohol, given GN ablations, and then tested for retention 1 month later, avoidance of drinking water in the presence of the odor alone was significant but attenuated somewhat in relation to trained control rats. These data support the hypothesis that rats lacking GN partially acquire alcohol aversions by using odor cues and confirm that associative learning is intact in these rats despite the fact that GN rats display significant deficits in aversion learning when only tastes are paired with illness.

Corticofugal influence on taste responses in the parabrachial pons of the rat

Although the anatomy of centrifugal input to gustatory neural structures has been described, little is known of the physiological mechanisms that convey this influence or of their functional significance. As a first step in the investigation of these issues, the effect of a reversible lesion in the gustatory neocortex (GN) on the neural code for taste in the parabrachial nucleus of the pons (PbN) was studied in rats. Electrophysiological responses to taste stimuli bathed over the tongue were recorded from single units in the PbN before, after and following recovery from an infusion of procaine-HCl into the GN. Test stimuli consisted of sapid solutions of NaCl (0.1 M), HCl (0.01 M), sucrose (0.5 M), Na-saccharin (0.004 M) and quinine-HCl (0.01 M). Infusions of procaine into the GN were correlated with both specific and nonspecific effects on the responsitivity to gustatory stimuli in the PbN. Specific effects included: (1) changes in the magnitude of response to some tastants, but not others, in a given PbN unit, (2) changes in the across unit patterns produced by sweet stimuli and (3) the appearance of OFF responses in a subset of PbN units. Nonspecific effects were evidenced by changes in the spontaneous rates of activity and by enhancement or suppression of responses across all the tastantts tested in a subset of PbN units. Comparison of these results with reports on the effects of decerebration suggests that some of these effects may be accounted for by interruption of the descending input from the GN to the PbN. In addition, the stimulus-specific effects that were noted following procaine infusion into the GN provide support for the suggestion that the GN specifically modifies the electrophysiological patterns that are evoked by salient taste stimuli.

Release of acetylcholine, γ-aminobutyrate, dopamine and glutamate, and activity of some related enzymes, in rat gustatory neocortex

The gustatory neocortex (GN), final relay along the gustatory pathway, is a region of the brain involved in the neural integration of feeding behavior. Since information on the neurotransmitters in this nucleus is scarce, the aim of the present work was to establish whether acetylcholine (ACh), γ-aminobutyric acid (GABA), dopamine and glutamate may act as transmitters within this structure. It was found that GN slices are able to release labeled GABA, ACh and glutamate but not dopamine. Additionally, it was possible to detect significant glutamic acid decar☐ylase, choline acetyltransferase and acetylcholinesterase activities in GN homogenates. The activity of the two enzymes involved in acetylcholine metabolism was higher than that observed in other cortical regions. These findings suggest that GABA, ACh and glutamate probably are neurotransmitters in the GN, whereas dopamine is not.

Correlation between acetylcholine release and recovery of conditioned taste aversion induced by fetal neocortex grafts

Rats with lesions of the gustatory neocortex (GN) show deficits in the acquisition of taste aversion. Fetal GN grafts to a lesioned animal restore taste aversion learning and establish connections with the host brain. In this work, we examined whether the grafts are biochemically functional and whether this fact can be related to behavioral recovery. Gustatory or occipital cortices from rat fetuses were transplanted to GN-lesioned rats. Two months later, taste aversion recovery was tested and the release of labeled γ-aminobutyric acid (GABA), acetylcholine (ACh), dopamine and glutamate from the grafted tissue was assayed. Fetal GN grafts promoted recovery of learning and released GABA, ACh and glutamate in response to K + depolarization. Occipital cortex grafts did not induce behavioral recovery, although they were capable of releasing GABA. In contrast, these grafts did not release ACh. Moreover, GN-grafted rats in which behavioral recovery was not seen also failed to release ACh. These results are in agreement with previous findings that cholinergic transmission is important in the GN and suggest that ACh may play a role in the graft-mediated behavioral recovery observed in this model.

Fetal brain grafts induce recovery of learning deficits and connectivity in rats with gustatory neocortex lesion

Three groups of rats showing disrupted taste aversion due to gustatory neocortex lesions, were studied. One group received a transplant of homotopic cortical tissue, another of heterotopic tectal tissue, obtained from 17-day-old fetuses. The third group remained without transplant as a lesioned control group. Comparisons of the taste aversion scores before and after graft, revealed that cortical grafted animals significantly improved the taste aversion, whereas those which received tectal grafts, and the cortical-lesioned controls did not. Moreover, results with horseradish peroxidase (HRP) histochemistry revealed that the homotopic, but not the heterotopic, brain transplants were able to re-establish connections with amygdala and with the ventromedial nucleus of the thalamus areas who normally kept connectivity with the gustatory neocortex. These results support the hypothesis that fetal brain transplants can re-establish cognitive functions, as well as connectivity with its host tissue.

Alcohol aversion generalization in rats: Specific disruption of taste and odor cues with gustatory neocortex or olfactory bulb ablations.

Alcohol aversion generalization in rats: Specific disruption of taste and odor cues with gustatory neocortex or olfactory bulb ablations.

Partial recovery of gustatory function after neurol tissue transplantation to the lesioned gustatory neocortex

The ability of homotypic cortical tissue grafts to induce recovery of function after a gustatory neocortex (GN) lesion was studied using the conditioned taste aversion (CTA) paradigm. On acquisition day, 26 GN-lesioned and 8 sham-lesioned rats were presented with a saccharin solution, followed by an injection of the illness-inducing agent lithium chloride (LiCl). On the test day, 2 days later, saccharin was presented again. The GN-lesioned rats showed significantly less aversion to saccharin on the test day, indicating that the lesion impaired their ability to form taste-illness association. Nine of the lesioned rats were then bilaterally transplanted with fetal GN tissue. Nine weeks after the transplantation, the rats were presented with a LiCl solution, which served as both a tastant and an illness-inducing agent. An NaCl solution, which tasted very similar to the LiCl solution, was used to test the CTA to salt 3 days later. The nontransplanted rats consumed significantly more LiCl than the transplanted and sham-operated rats on the acquisition day, but both transplanted and nontransplanted rats consumed more NaCl than sham-operated rats on the test day. Nissl and Golgi stainings showed numerous somata and extensive arborization of neurons within the grafts. The results indicate that fetal GN grafts can restore the ability to integrate gustatory and visceral inputs but not to form long-lasting taste-illness associations.

Alcohol aversion generalization in rats: specific disruption of taste and odor cues with gustatory neocortex or olfactory bulb ablations.

Rats with ablations of the gustatory neocortex (Experiment 1) and rats with olfactory bulb ablations (Experiment 2) were compared with normal rats for aversion generalization to both single taste solutions (sucrose, sodium chloride, quinine hydrochloride, hydrochloric acid) and compound taste solutions (pairs of the four single tastants) following alcohol aversion training. All rats acquired equal and strong alcohol aversions. Control rats showed consistent aversion generalization to both the sucrose + quinine and the sucrose + hydrochloric acid solutions; no significant generalization occurred to the single tastants except a weak generalization to sucrose in Experiment 2. Rats with gustatory neocortical ablations failed to show aversion generalization to any of the taste solutions. Rats with olfactory bulbectomies displayed the same aversion generalization functions as control rats but exhibited significantly faster extinction of the alcohol aversion than did the trained control rats. Results from the present experiments suggest that during alcohol aversion learning, rats lacking gustatory neocortex use odor cues (no taste generalization), whereas rats lacking olfactory bulbs utilize taste cues (normal taste generalization).

Fetal brain transplants induce recuperation of taste aversion learning

Rats showing disrupted taste aversion due to gustatory neocortex or amygdala lesions were transplanted into the lesioned area with homologous brain tissue obtained from 17-day-old fetuses. Comparisons of taste aversions scores before and after the graft, revealed that the grafted animals significantly recuperated taste aversions, whereaa cortical lesioned animals without grafts did not. Surprisingly, however, amygdala-lesioned animals without graft presented spontaneous recovery. These results not only support the hypothesis that fetal brain transplants can restore cognitive functions, but also that there are some fundamental functional differences between the gustatory neocortex and the amygdala in the regulation of the processes involved in the acquisition and retention of taste aversion.

Alcohol preference in rats lacking gustatory neocortex

Ingestion of alcohol solutions (in concentrations from 0.5 to 12% v/v) was examined in three experiments where rats lacking gustatory neocortex (GN) were compared with intact and control lesion rats. Two experiments tested alcohol consumption during a restricted schedule of fluid access with one or two bottle tests. The last experiment involved testing with continuous access to water and alcohol solutions in two bottle tests. Subgroups of rats in each experiment were presented with either an ascending or descending order of concentrations. In all experiments, GN rats consumed less total fluid during testing (relative to control rats). In general, GN rats exhibited similar patterns of alcohol consumption as that found in control rats. Where differences between GN rats and control rats were found, GN rats consumed more alcohol than control rats during tests with restricted fluid access. During continuous fluid access, however, no significant differences between GN rats and control rats were found.

Learned alcohol aversions in rats: Gustatory and olfactory components

The gustatory and oflactory basis of learned alcohol aversions was examined by testing rats with either gustatory neocortex ablations, olfactory bulb ablations, or a combination of both ablations. In the first experiment operated rats were compared with control rats in the acquisition of a learned alcohol aversion. In the second experiment, the effect of ablations on preoperatively-learned alcohol aversions was examined. Rats lacking gustatory neocortex learned and retained alcohol aversions normally although these rats extinguished the aversions faster than normal rats. Olfactory bulb ablation alone failed to disrupt normal aversion learning but completely eliminated retention of a previously acquired aversion. Combination ablations produced severe deficits both in acquisition and retention of learned alcohol aversions. The results indicate that, besides having gustatory qualities, the odor quality of alcohol is important in determining the associative and memorial characteristics of alcohol.