Rat gustatory memory requires protein kinase C activity in the amygdala and cortical gustatory area

The α1-adrenergic receptors in the amygdala regulate the induction of learned despair through protein kinase C-beta signaling

Hyperactivity of amygdala is observed in patients with major depressive disorder. Although the role of α1-adrenoceptor in amygdala on fear memory has been well studied, the role of α1-adrenoceptor in amygdala on depression-like behaviors remains unclear. Therefore, we investigated the effect of α1A-adrenoreceptor in amygdala on despair behavior, evaluated by the immobility time during tail suspension test (TST), pharmacological intervention, and immunohistological methods. C57BL6/J mice given a bilateral intra-amygdala injection of artificial cerebrospinal fluid exhibited an increased duration of immobility in the latter half of both trials of TST with a 24-h interval, a phenomenon known as learned despair. Intra-amygdala injection of WB4101 (1.7 nmol/0.1 µl), an α1 adrenoreceptor antagonist, but not propranolol (250 pmol/0.1 µl), a β-adrenoreceptor antagonist, blocked the induction of learned despair during TST. Immunostaining experiments revealed that ~61-75% of α1A-adrenoreceptor-positive neurons were colocalized with GAD65/67 in amygdala, implying that the α1-adrenoceptors in amygdala may enormously regulate the GABA release. Protein kinase C-beta (PKCβ) was predominantly expressed in the α1A-adrenoreceptor-positive neurons in the BLA, whereas protein kinase C-epsilon (PKCε) was highly expressed with the α1A-adrenoreceptor in the Central nucleus of amygdala. Intra-amygdala injection of ruboxistaurin (10 pmol/0.1 µl), a PKCβ inhibitor, blocked the induction of learned despair during TST, whereas neither TAT-εV1-2 (500 ng/0.1 μl), a cell-permeant PKCε inhibitory peptide, nor HBDDE (50 pmol/0.1 µl), an inhibitor of PKCα and -γ, affected the duration of immobility during TST. These data suggest that the α1-adrenoreceptor in amygdala regulates the induction of learned despair via PKCβ.

Systemic mechanism of taste, flavour and palatability in brain

Taste is considered as one of the five traditional senses and has the ability to detect the flavour of food and certain minerals. Information of taste is transferred to the cortical gustatory area for identification and discrimination of taste quality. Animals have memory recognition power to maintain the familiar foods which are already encountered. Animal shows neophobic response when it encounters novel taste and shows no hesitation when the food is known to be safe. Palatability is the hedonic reward provided by foods and fluids. Palatability is closely related to neurochemicals, and this chemical influences the consumption of food and fluid. Even though, the food is palatable that can become aversive and avoided as a consequence of postingestional unpleasant experience such as malaise. This review presents the overall view on brain mechanisms of taste, flavour and palatability.

The integrated role of ACh, ERK and mTOR in the mechanisms of hippocampal inhibitory avoidance memory

The purpose of this review is to summarize the present knowledge on the interplay among the cholinergic system, Extracellular signal-Regulated Kinase (ERK) and Mammalian Target of Rapamycin (mTOR) pathways in the development of short and long term memories during the acquisition and recall of the step-down inhibitory avoidance in the hippocampus. The step-down inhibitory avoidance is a form of associative learning that is acquired in a relatively simple one-trial test through several sensorial inputs. Inhibitory avoidance depends on the integrated activity of hippocampal CA1 and other brain areas. Recall can be performed at different times after acquisition, thus allowing for the study of both short and long term memory. Among the many neurotransmitter systems involved, the cholinergic neurons that originate in the basal forebrain and project to the hippocampus are of crucial importance in inhibitory avoidance processes. Acetylcholine released from cholinergic fibers during acquisition and/or recall of behavioural tasks activates muscarinic and nicotinic acetylcholine receptors and brings about a long-lasting potentiation of the postsynaptic membrane followed by downstream activation of intracellular pathway (ERK, among others) that create conditions favourable for neuronal plasticity. ERK appears to be salient not only in long term memory, but also in the molecular mechanisms underlying short term memory formation in the hippocampus. Since ERK can function as a biochemical coincidence detector in response to extracellular signals in neurons, the activation of ERK-dependent downstream effectors is determined, in part, by the duration of ERK phosphorylation itself. Long term memories require protein synthesis, that in the synapto-dendritic compartment represents a direct mechanism that can produce rapid changes in protein content in response to synaptic activity. mTOR in the brain regulates protein translation in response to neuronal activity, thereby modulating synaptic plasticity and long term memory formation. Some studies demonstrate a complex interplay among the cholinergic system, ERK and mTOR. It has been shown that co-activation of muscarinic acetylcholine receptors and β-adrenergic receptors facilitates the conversion of short term to long term synaptic plasticity through an ERK- and mTOR-dependent mechanism which requires translation initiation. It seems therefore that the complex interplay among the cholinergic system, ERK and mTOR is crucial in the development of new inhibitory avoidance memories in the hippocampus.

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

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

The role of PKC signaling in CRF-induced modulation of startle

RATIONALE

Hypersignaling of corticotropin releasing factor (CRF) has been implicated in stress disorders; however, many of its downstream mechanisms of action remain unclear. In vitro, CRF1 receptor activation initiates multiple cell signaling cascades, including protein kinase A (PKA), protein kinase C (PKC), and mitogen-activated protein kinase kinase MEK1/2 signaling. It is unclear, however, which of these signaling cascades mediate CRF-induced behaviors during stress.

OBJECTIVES

We examined the role of PKA, PKC, and MEK1/2 signaling pathways in CRF-induced anxiety as measured by startle hyperreactivity.

METHODS

Mice treated with intracerbroventricular (ICV) ovine CRF (oCRF) were pretreated with the PKA inhibitor Rp-cAMPS, PKC inhibitor bisindolylmaleimide (BIM), or MEK1/2 inhibitor PD98059 (ICV) and assessed for acoustic startle reactivity.

RESULTS

The PKC inhibitor BIM significantly attenuated CRF-induced increases in startle. BIM was also able to block startle increases induced by oCRF when both compounds were infused directly into the bed nucleus of stria terminalis (BNST). PKA and MEK1/2 inhibition had no significant effects on CRF-induced changes in startle at the dose ranges tested. CRF-induced disruption of prepulse inhibition was not significantly reversed by any of the three pretreatments at the dose ranges tested.

CONCLUSIONS

PKC signaling is required for CRF-induced increases in startle, and this effect is mediated at least in part at the BNST. These findings suggest that PKC signaling cascades (1) may be important for the acute effects of CRF to induce startle hyperreactivity and (2) support further research of the role of PKC signaling in startle abnormalities relevant to disorders such as posttraumatic stress disorder.

Memory of conditioned taste aversion is erased by inhibition of PI3K in the insular cortex

The conditioned taste aversion (CTA) paradigm, in which association between a novel taste and visceral malaise is formed, gives a unique experimental setting to examine the mechanisms underlying memory acquisition and extinction processes. AKT is a main kinase of the phosphoinositide 3-kinase cascade (PI3K) and has been implicated in long-term memory. We have recently reported that blockade of PI3K in the basolateral amygdala (BLA) before retrieval of fear memory was associated with long-term reduction in fear responses, suggesting a possible role of PI3K inhibition in fear erasure. In this study, we aimed to elucidate whether PI3K has a similar role in the insular cortex (IC), which has a crucial role in CTA acquisition, consolidation, maintenance, and extinction. To that end, we (1) monitored AKT phosphorylation in the IC following CTA acquisition and extinction and (2) inhibited PI3K by local microinjection of the PI3K inhibitor LY294002 at different stages of CTA acquisition and extinction. Our results show that while AKT phosphorylation is increased following CTA learning, it is decreased following CTA extinction. Inhibition of AKT phosphorylation in the IC before or after the first CTA retrieval test resulted in reduction in the aversion index. This reduction in aversion is due to the erasure of the original CTA trace memory, as re-application of the unconditioned stimulus (lithium chloride) did not induce the recovery of aversion in LY294002-treated animals. Our present data add new evidence to suggest that PI3K is engaged in consolidation of aversive memories, as its inhibition is associated with erasure of CTA memory.

Gene Network Analysis in Amygdala following Taste Aversion Learning in Rats

Conditioned taste aversion (CTA) is an adaptive behavior that benefits survival of animals including humans and also serves as a powerful model to study the neural mechanisms of learning. Memory formation is a necessary component of CTA learning and involves neural processing and regulation of gene expression in the amygdala. Many studies have been focused on the identification of intracellular signaling cascades involved in CTA, but not late responsive genes underlying the long-lasting behavioral plasticity. In this study, we explored in silico experiments to identify persistent changes in gene expression associated with CTA in rats. We used oligonucleotide microarrays to identify 248 genes in the amygdala regulated by CTA. Pathway Studio and IPA software analyses showed that the differentially expressed genes in the amygdala fall in diverse functional categories such as behavior, psychological disorders, nervous system development and function, and cell-to-cell signaling. Conditioned taste aversion is a complex behavioral trait which involves association of visceral and taste inputs, consolidation of taste and visceral information, memory formation, retrieval of stored information, and extinction phase. In silico analysis of differentially expressed genes is therefore necessary to manipulate specific phase/stage of CTA to understand the molecular insight.

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.

The role of protein phosphorylation in the gustatory cortex and amygdala during taste learning

Protein phosphorylation and dephosphorylation form a major post-translation mechanism that enables a given cell to respond to ever-changing internal and external environments. Neurons, similarly to any other cells, use protein phosphorylation/dephosphorylation to maintain an internal homeostasis, but they also use it for updating the state of synaptic and intrinsic properties, following activation by neurotransmitters and growth factors. In the present review we focus on the roles of several families of kinases, phosphatases, and other synaptic-plasticity-related proteins, which activate membrane receptors and various intracellular signals to promote transcription, translation and protein degradation, and to regulate the appropriate cellular proteomes required for taste memory acquisition, consolidation and maintenance. Attention is especially focused on the protein phosphorylation state in two forebrain areas that are necessary for taste-memory learning and retrieval: the insular cortex and the amygdala. The various temporal phases of taste learning require the activation of appropriate waves of biochemical signals. These include: extracellular signal regulated kinase I and II (ERKI/II) signal transduction pathways; Ca(2+)-dependent pathways; tyrosine kinase/phosphatase-dependent pathways; brain-derived neurotrophicfactor (BDNF)-dependent pathways; cAMP-responsive element bindingprotein (CREB); and translation-regulation factors, such as initiation and elongation factors, and the mammalian target of rapamycin (mTOR). Interestingly, coding of hedonic and aversive taste information in the forebrain requires activation of different signal transduction pathways.

Conditioned taste aversion dependent regulation of amygdala gene expression

The present experiments investigated gene expression in the amygdala following contingent taste/LiCl treatment that supports development of conditioned taste aversion (CTA). The use of whole genome chips and stringent data set filtering led to the identification of 168 genes regulated by CTA compared to non-contingent LiCl treatment that does not support CTA learning. Seventy-six of these genes were eligible for network analysis. Such analysis identified "behavior" as the top biological function, which was represented by 15 of the 76 genes. These genes included several neuropeptides, G protein-coupled receptors, ion channels, kinases, and phosphatases. Subsequent qRT-PCR analyses confirmed changes in mRNA expression for 5 of 7 selected genes. We were able to demonstrate directionally consistent changes in protein level for 3 of these genes; insulin 1, oxytocin, and major histocompatibility complex class I-C. Behavioral analyses demonstrated that blockade of central insulin receptors produced a weaker CTA that was less resistant to extinction. Together, these results support the notion that we have identified downstream genes in the amygdala that contribute to CTA learning.

Activation of protein kinase C isozymes for the treatment of dementias

Memories are much more easily impaired than improved. Dementias, a lasting impairment of memory function, occur in a variety of cognitive disorders and become more clinically dominant as the population ages. Protein kinase C is one of the "cognitive kinases," and plays an essential role in both memory acquisition and maintenance. Deficits in protein kinase C (PKC) signal cascades in neurons represent one of the earliest changes in the brains of patients with Alzheimer's disease (AD) and other types of memory impairment, including those related to cerebral ischemia and ischemic stroke. Inhibition or impairment of PKC activity results in compromised learning and memory, whereas an appropriate activation of certain PKC isozymes leads to an enhancement of learning and memory and/or antidementic effects. In preclinical studies, PKC activators have been shown to increase the expression and activity of PKC isozymes, thereby restoring PKC signaling and downstream activity, including stimulation of neurotrophic activity, synaptic/structural remodeling, and synaptogenesis in the hippocampus and related cortical areas. PKC activators also reduce the accumulation of neurotoxic amyloid and tau protein hyperphosphorylation and support anti-apoptotic processes in the brain. These observations strongly suggest that PKC pharmacology may represent an attractive area for the development of effective cognition-enhancing therapeutics for the treatment of dementias.

Genetically induced cholinergic hyper-innervation enhances taste learning

Acute inhibition of acetylcholine (ACh) has been shown to impair many forms of simple learning, and notably conditioned taste aversion (CTA). The most adhered-to theory that has emerged as a result of this work – that ACh increases a taste’s perceived novelty, and thereby its associability – would be further strengthened by evidence showing that enhanced cholinergic function improves learning above normal levels. Experimental testing of this corollary hypothesis has been limited, however, by side-effects of pharmacological ACh agonism and by the absence of a model that achieves long-term increases in cholinergic signaling. Here, we present this further test of the ACh hypothesis, making use of mice lacking the p75 pan-neurotrophin receptor gene, which show a resultant over-abundance of cholinergic neurons in sub-regions of the basal forebrain (BF). We first demonstrate that the p75−/− abnormality directly affects portions of the CTA circuit, locating mouse gustatory cortex (GC) using a functional assay and then using immunohistochemisty to demonstrate cholinergic hyper-innervation of GC in the mutant mice – hyper-innervation that is unaccompanied by changes in cell numbers or compensatory changes in muscarinic receptor densities. We then demonstrate that both p75−/− and wild-type (WT) mice learn robust CTAs, which extinguish more slowly in the mutants. Further testing to distinguish effects on learning from alterations in memory retention demonstrate that p75−/− mice do in fact learn stronger CTAs than WT mice. These data provide novel evidence for the hypothesis linking ACh and taste learning.

Intra-amygdala ZIP injections impair the memory of learned active avoidance responses and attenuate conditioned taste-aversion acquisition in rats

We have investigated the effect of protein kinase Mzeta (PKMζ) inhibition in the basolateral amygdala (BLA) upon the retention of a nonspatial learned active avoidance response and conditioned taste-aversion (CTA) acquisition in rats. ZIP (10 nmol/μL) injected into the BLA 24 h after training impaired retention of a learned avoidance-jumping response assessed 7 d later when compared with control groups injected with scrambled-ZIP. Nevertheless, a retraining session applied 24 h later indicated no differences between the groups. Additionally, a similar ZIP injection into the BLA during the conditioned stimulus-unconditioned stimulus (CS-US) interval attenuated CTA acquisition. These findings support the BLA PKMζ role in various forms of memory.

Intracellular calcium chelation and pharmacological SERCA inhibition of Ca2+ pump in the insular cortex differentially affect taste aversive memory formation and retrieval

Variation in intracellular calcium concentration regulates the induction of long-term synaptic plasticity and is associated with a variety of memory/retrieval and learning paradigms. Accordingly, impaired calcium mobilization from internal deposits affects synaptic plasticity and cognition in the aged brain. During taste memory formation several proteins are modulated directly or indirectly by calcium, and recent evidence suggests the importance of calcium buffering and the role of intracellular calcium deposits during cognitive processes. Thus, the main goal of this research was to study the consequence of hampering changes in cytoplasmic calcium and inhibiting SERCA activity by BAPTA-AM and thapsigargin treatments, respectively, in the insular cortex during different stages of taste memory formation. Using conditioned taste aversion (CTA), we found differential effects of BAPTA-AM and thapsigargin infusions before and after gustatory stimulation, as well as during taste aversive memory consolidation; BAPTA-AM, but not thapsigargin, attenuates acquisition and/or consolidation of CTA, but neither compound affects taste aversive memory retrieval. These results point to the importance of intracellular calcium dynamics in the insular cortex during different stages of taste aversive memory formation.

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.

Protein kinase C activators as synaptogenic and memory therapeutics

The last decade has witnessed a rapid progress in understanding of the molecular cascades that may underlie memory and memory disorders. Among the critical players, activity of protein kinase C (PKC) isoforms is essential for many types of learning and memory and their dysfunction, and is critical in memory disorders. PKC inhibition and functional deficits lead to an impairment of various types of learning and memory, consistent with the observations that neurotoxic amyloid inhibits PKC activity and that transgenic animal models with PKCbeta deficit exhibit impaired capacity in cognition. In addition, PKC isozymes play a regulatory role in amyloid production and accumulation. Restoration of the impaired PKC signal pathway pharmacologically results in an enhanced memory capacity and synaptic remodeling / repair and synaptogenesis, and, therefore, represents a potentially important strategy for the treatment of memory disorders, including Alzheimer's dementia. The PKC activators, especially those that are isozyme-specific, are a new class of drug candidates that may be developed as future memory therapeutics.

Different effect of chronic stress on learning and memory in BALB/c and C57BL/6 inbred mice: Involvement of hippocampal NO production and PKC activity

Nitric oxide (NO) has been involved in many pathophysiological brain processes. Recently, we showed that neuronal nitric oxide synthase (nNOS)-mediated decrease in NO production is involved in memory impairment induced by chronic mild stress (CMS) in BALB/c mice. Two genetically different inbred murine strains, C57BL/6 and BALB/c, show distinct behavioral responses, neurodevelopmental and neurochemical parameters. Here, we perform a comparative study on CMS effects upon learning and memory in both strains, analyzing the role of NO production and its regulation by protein kinase C (PKC). Stressed BALB/c, but not C57Bl/6 mice, showed a poor learning performance in both the open field and passive avoidance inhibitory tasks. Also, CMS induced a diminished NO production by nNOS, associated with an increment in gamma and zeta PKC isoenzymes in BALB/c mice. In C57BL/6 mice, CMS had no effect on NO production, but increased delta and decreased betaI PKC isoforms. In vivo administration of a NOS inhibitor induced behavioral alterations in both strains. These results suggest a differential effect of stress, with BALB/c being more vulnerable to stress than C57BL/6 mice. This effect could be related to a differential regulation of NOS and PKC isoenzymes, pointing to an important role of NO in learning and memory.

PKC blockade differentially affects aversive but not appetitive gustatory memories

After consumption of a new taste, there are mainly two possible outcomes for the establishment of a taste memory, either it will be aversive or safe depending on the consequences of taste consumption. It has been proposed that both types of learning share a common initial taste memory trace, which will lead to two different memory traces, safe or aversive. To study the role of PKC activity in aversive or safe taste memory formation, we administered chelerythrine, a PKC inhibitor, into the insular cortex or parietal cortex 20 min before conditioned taste aversion or attenuation of neophobia training. The results suggest that PKC activity is needed in the insular cortex for the establishment of aversive taste memory, but not for safe taste memory.

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.

Different signal transduction cascades are activated simultaneously in the rat insular cortex and hippocampus following novel taste learning

Novel taste learning is a robust one-trial incidental learning process, dependent on functional activity of the insular (taste) cortex. In contrast to that of the cortex, the role of the hippocampus in taste learning is controversial. We set out to identify the time courses of the activation of mitogen-associated protein kinase (MAPK), transcription factor cAMP-response element-binding protein (CREB) and Akt/PKB (protein kinase B) in the insular cortex and hippocampus of rats subsequent to novel taste learning. Following taste learning, an early response (20 min) occurred at the same time in the insular cortex and the hippocampus. However, whereas MAPK was activated specifically in the insular cortex, CREB and Akt were phosphorylated in the hippocampus but not in the cortex. In addition, the immediate early gene, CCAAT/enhancer binding protein (C/EBPbeta) was induced in both the hippocampus and the insular cortex 18 h following taste learning. The results demonstrate, for the first time, correlative activation and gene expression in the hippocampus following novel taste learning. Moreover, the results suggest that different signal transduction cascades necessary for taste learning are activated in concert in different brain structures, to enable taste learning and consolidation.

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.

Memory-dependent c-Fos expression in the nucleus accumbens and extended amygdala following the expression of a conditioned taste aversive in the rat

Retrieving the memory of a conditioned taste aversion involves multiple forebrain areas. Although the amygdala clearly plays a role in the expression of a conditioned taste aversion, critical regions, downstream from the amygdala remain to be defined. To this end, Fos immunoreactivity was used in the rat to explore forebrain structures associated with retrieval that have an anatomical relationship with the amygdala. The results showed that expression of a conditioned taste aversion to 0.5 M sucrose elicited neuronal activation in the nucleus accumbens and in a complex of structures collectively referred to as the extended amygdala. The posterior hypothalamus and parasubthalamic nucleus, which receive inputs from the extended amygdala, were also activated upon re-exposure to the sucrose conditioned stimulus. Fos immunoreactivity did not increase in these regions in response to an innately aversive tastant, quinine hydrochloride (conditioned stimulus control), nor to LiCl-induced visceral stimulation in unconditioned animals (unconditioned stimulus control). In addition, these regions did not respond to the sucrose conditioned stimulus in sham-conditioned animals. These results suggest that conditioned and innately aversive tastes are differentially processed in the forebrain circuitry that includes the nucleus accumbens and extended amygdala.

Basolateral amygdala glutamatergic activation enhances taste aversion through NMDA receptor activation in the insular cortex

In conditioned taste aversion (CTA), a subject learns to associate a novel taste with visceral malaise. Brainstem, limbic and neocortical structures have been implicated in CTA memory formation. Nevertheless, the role of interactions between forebrain structures during these processes is still unknown. The present experiment was aimed at investigating the possible interaction between the basolateral nucleus of the amygdala (BLA) and the insular cortex (IC) during CTA memory formation. Injection of a low dose of lithium chloride (30 mg/kg, i.p.) 30 min after novel taste consumption (saccharin 0.1%) induces a weak CTA. Unilateral BLA injection of glutamate (2 microg in 0.5 microL) just before low lithium induces a stronger CTA. Unilateral injection of an N-methyl-d-aspartate (NMDA) receptor antagonist (AP5, 5 microg in 0.5 microL) in IC has no effect. However, AP5 treatment in IC at the same time or 1 h after the ipsilateral BLA injection reverses the glutamate-induced CTA enhancement. Injection of AP5 in IC 3 h after BLA injection does not interfere with the glutamate effect. Moreover, the CTA-enhancing effect of glutamate was also blocked by contralateral IC injection of AP5 at the same time. These results provide strong evidence that NMDA receptor activation in the IC is essential to enable CTA enhancement induced by glutamate infusion in the BLA during a limited time period that extends to 1 but not to 3 hours. These findings indicate that BLA-IC interactions regulate the strength of CTA. The bilateral nature of these amygdalo-cortical interactions is discussed.

Effects of midazolam on the expression of conditioned taste aversion in rats

In conditioned taste aversion (CTA), the animals learn to avoid a taste substance (conditioned stimulus, CS) which was previously associated with visceral distress (unconditioned stimulus, US). The present study examined the effects of administration of midazolam (MDZ), a benzodiazepine agonist, after the acquisition of CTA on the expression of CTA. After ingestion of 0.5 M sucrose (CS) was paired with an intraperitoneal (i.p.) injection of 0.15 M LiCl (US), control rats showed strong CTA to the CS. However, a systemic injection of MDZ (1.5 mg/kg, i.p.) before the retention test prevented conditioned animals from rejecting the CS, but in the subsequent retention tests where the drug was not administrated, those animals again showed strong aversions to the CS. Aversive taste reactivity patterns to the intraorally infused sucrose and 0.3 M dl-alanine in the conditioned animals were also diminished by the similar injection of MDZ, but not by a serotonergic anxiolytic agent, buspirone (2.5 or 5.0 mg/kg, i.p.). General taste sensory deficit might not be induced by MDZ because the drug injection did not impair conditioned aversions to 0.2 M NaCl and 0.01 M HCl. Infusion of MDZ into the basolateral nucleus of the amygdala (BLA) also attenuated conditioned aversions to sucrose. These results suggest that systemic or intra-BLA administrations of MDZ impair the expression of CTA selectively to sweet-tasting substances, implying that a transient MDZ-induced CTA expression deficit is due to the enhancement of palatability of CSs with preferable tastes rather than general anxiolytic or amnesic effects of MDZ.

Perirhinal cortex muscarinic receptor blockade impairs taste recognition memory formation

The relevance of perirhinal cortical cholinergic and glutamatergic neurotransmission for taste recognition memory and learned taste aversion was assessed by microinfusions of muscarinic (scopolamine), NMDA (AP-5), and AMPA (NBQX) receptor antagonists. Infusions of scopolamine, but not AP5 or NBQX, prevented the consolidation of taste recognition memory using attenuation of neophobia as an index. In addition, learned taste aversion in both short- and long-term memory tests was exclusively impaired by scopolamine. These data provide neurochemical support for the theory that cholinergic activity of the perirhinal cortex participates in the formation of the taste memory trace and that it is independent of the NMDA and AMPA receptor activity. These results support the idea that cholinergic neurotransmission in the perirhinal cortex is also essential for acquisition and consolidation of taste recognition memory.

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.

New life in an old idea: the synaptic plasticity and memory hypothesis revisited

The notion that changes in synaptic efficacy underlie learning and memory processes is now widely accepted, although definitive proof of the synaptic plasticity and memory hypothesis is still lacking. This article reviews recent evidence relevant to the hypothesis, with particular emphasis on studies of experience-dependent plasticity in the neocortex and hippocampus. In our view, there is now compelling evidence that changes in synaptic strength occur as a consequence of certain forms of learning. A major challenge will be to determine whether such changes constitute the memory trace itself or play a less specific supporting role in the information processing that accompanies memory formation.

Novel Tastes Elevate c-fos Expression in the Central Amygdala and Insular Cortex: Implication for Taste Aversion Learning

Taste novelty strongly modulates the speed and strength of taste aversion conditioning. To identify molecular signals responsive to novel tastes, immunostaining for c-fos protein (Fos-like immunoreactivity [FLI]) was used to mark neurons that responded differentially to taste novelty. Novel saccharin induced larger increases in FLI than familiar saccharin. This pattern was seen in central amygdala and insular cortex, but not in basolateral amygdala, parabrachial nucleus, or nucleus of the solitary tract. Other parameters known to influence aversion learning were tested for effects on FLI. Manipulations known to reduce the strength of learning blunted the FLI response, supporting the idea that FLI marks neural pathways critical to taste processing during acquisition, and that c-fos expression is a key transcriptional event underlying this plasticity.

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.

Conditioned taste aversion as a learning and memory paradigm

Conditioned taste aversion (CTA) is a well established learning and memory paradigm in rats and mice that is considered to be a special form of classical conditioning. Rodents--as well as many other species including man--learn to associate a novel taste (CS) with nausea (US), and as a consequence avoid drinking fluid with this specific taste. In contrast to other types of classical conditioning, even CS-US intervals lasting several hours lead to an aversion to the gustatory CS. With increasing CS-US delay duration, however, the aversion against the CS gradually decreases. Mice differ from rats in their reaction to the CS as well as the US. They tolerate a much higher concentration of saccharin and they do not show any clear signs of nausea when injected with the US. Advantages of this task are its relative independence of motor behavior, well described pathways for the CS and partly the US, and the wealth of available anatomical and pharmacological data implying several brain structures (e.g. parabrachial nucleus, amygdala, insular cortex), neurotransmitters and their receptors (e.g. cholinergic system, NMDA-receptors), and cellular processes (e.g. expression of immediate early genes, Ras-MAP kinase signaling pathway, CREB phosphorilation, protein tyrosine phosphorilation, protein synthesis) in CTA. The CTA paradigm has also been successfully used to phenotype mouse mutants.

Conditioned taste aversion and Ca/calmodulin-dependent kinase II in the parabrachial nucleus of rats

Bielavska and colleagues (Bielavska, Sacchetti, Baldi, & Tassoni, 1999) have recently shown that KN-62, an inhibitor of calcium/calmodulin-dependent kinase II (CaCMK), induces conditioned taste aversion (CTA) when introduced into the parabrachial nucleus (PBN) of rats. The aim of the present report was to assess whether activity of CaCMK in the PBN is changed during CTA. We induced CTA in one group of rats by pairing saccharin consumption with an ip injection of lithium chloride. Another group of rats received lithium alone (without being paired with saccharin consumption) to test whether lithium has an effect on CaCMK in the PBN, independent of those effects due to training. In animals receiving CTA training, CaCMK activity in extracts of PBN was reduced by approximately 30% at the postacquisition intervals of 12, 24, and 48 h, compared to control animals receiving saccharin with saline injection. By 120 h after CTA training, no effect on CaCMK was present. At those postacquisition intervals showing CaCMK activity effects due to CTA, there were no effects attributable to lithium alone. Lithium alone produced only a short-lasting reduction in CaCMK activity (at 20 min a 30% decrease, at 60 min a 23% decrease; and at 6, 12, and 24 h no decrease). The time course of lithium-induced effects differed markedly from that of CTA training. All changes were Ca2+/- -dependent; we did not observe any changes in Ca-independent activity. CTA effects on CaCMK were selective for PBN, insofar as we did not observe any CTA effects on CaCMK in the visual cortex, a brain region unrelated to taste pathways. Since CTA produces a relatively long-lasting reduction in CaCMK activity (lasting 2 days or more) specifically in the PBN, which is critical a relay for taste information, the reduction of CaCMK activity may enable the consolidation of taste memory in an aversive situation.

CAMKII inhibition in the parabrachial nuclei elicits conditioned taste aversion in rats

The conditioned taste aversion (CTA) paradigm was used to assess the role of Ca(2+)/calmodulin-dependent protein kinase (CAMKII) in associative learning. KN62, a specific inhibitor of CAMKII, was injected into the parabrachial nuclei (PBN) either immediately after saccharin drinking (CS) or after saccharin drinking and i.p. injection of LiCl (US). Injection of KN62 into the PBN after saccharin drinking elicited clear CTA (Exp. 1). This effect was dosage-dependent and site-specific (Exp. 2). The results are discussed in relation with an earlier report showing that CTA acquisition is disrupted by injection of Ca(2+)/phospholipid-dependent protein kinase (PKC) inhibitor chelerythrine into the PBN during CS-US interval. It is suggested that the principal serine/threonine kinases play different roles in CTA learning: whereas PKC activity is necessary for the gustatory short-term memory formation, CAMKII acts similarly to the US itself-an unexpected role of CAMKII in associative learning.

Bombesin-like peptides: studies on food intake and social behaviour with receptor knock-out mice

Bombesin (BN)-like peptides and receptors for these peptides are widely distributed in mammalian peripheral tissues and the central nervous system. The physiological and behavioural functions of these peptides have been clarified by both in vivo and in vitro studies. In spite of intensive investigations, the functions of endogenous BN-like peptides remain unclear. In order to specify these functions, our group and another laboratory generated by gene targeting mutant mice that lack one of the three BN-like peptide receptors found in mammals, ie neuromedin B receptor (NMB-R; BB1), gastrin-releasing peptide receptor (GRP-R; BB2), or bombesin receptor subtype-3 (BRS-3; BB3). Using these mutant mouse, we have found unexpected phenotypes, such as hyperphagia and obesity in the BRS-3-deficient mouse, and abnormal social behaviour in the GRP-R-deficient mouse. In the present study, we present our most recent findings in addition to previous studies and discuss the functions of BN-like peptides related to feeding and social behaviour from the point of view of knock-out mice studies.

Synaptic plasticity and memory: an evaluation of the hypothesis

Changing the strength of connections between neurons is widely assumed to be the mechanism by which memory traces are encoded and stored in the central nervous system. In its most general form, the synaptic plasticity and memory hypothesis states that "activity-dependent synaptic plasticity is induced at appropriate synapses during memory formation and is both necessary and sufficient for the information storage underlying the type of memory mediated by the brain area in which that plasticity is observed." We outline a set of criteria by which this hypothesis can be judged and describe a range of experimental strategies used to investigate it. We review both classical and newly discovered properties of synaptic plasticity and stress the importance of the neural architecture and synaptic learning rules of the network in which it is embedded. The greater part of the article focuses on types of memory mediated by the hippocampus, amygdala, and cortex. We conclude that a wealth of data supports the notion that synaptic plasticity is necessary for learning and memory, but that little data currently supports the notion of sufficiency.

A role for the beta isoform of protein kinase C in fear conditioning

The protein kinase C family of enzymes has been implicated in synaptic plasticity and memory in a wide range of animal species, but to date little information has been available concerning specific roles for individual isoforms of this category of kinases. To investigate the role of the beta isoform of PKC in mammalian learning, we characterized mice deficient in the PKC beta gene using anatomical, biochemical, physiological, and behavioral approaches. In our studies we observed that PKC beta was predominantly expressed in the neocortex, in area CA1 of the hippocampus, and in the basolateral nucleus of the amygdala. Mice deficient in PKC beta showed normal brain anatomy and normal hippocampal synaptic transmission, paired pulse facilitation, and long-term potentiation and normal sensory and motor responses. The PKC beta knock-out animals exhibited a loss of learning, however; they suffered deficits in both cued and contextual fear conditioning. The PKC expression pattern and behavioral phenotype in the PKC beta knock-out animals indicate a critical role for the beta isoform of PKC in learning-related signal transduction mechanisms, potentially in the basolateral nucleus of the amygdala.

Different disruptive effects on the acquisition and expression of conditioned taste aversion by blockades of amygdalar ionotropic and metabotropic glutamatergic receptor subtypes in rats

Conditioned taste aversion (CTA) is based on the gustatory long-term memory established after association of the taste of food (conditioned stimulus, CS) with visceral signals of poisoning (unconditioned stimulus, US). After the acquisition of CTA, hedonics of the taste CS changes from positive to negative as indicated by reduced ingestive and increased aversive taste reactivities in response to re-exposures to the CS. We examined the effects of reversible and selective blockades of the amygdalar glutamate receptor subtypes, AMPA, NMDA and metabotropic glutamate receptors, on the formation of CTA. Blockades of each of the three receptor subtypes between ingestion of saccharin (CS) and malaise-inducing LiCl (US) disrupted the acquisition of CTA. After the acquisition of CTA, however, blockades of only AMPA receptors, but not NMDA or metabotropic receptors, impaired the expression of CTA. This effect was seen only during the period when the antagonistic action to AMPA receptors lasted. These results indicate that both ionotropic and metabotropic glutamate receptor subtypes in the amygdala are indispensable for the acquisition of CTA, but that the expression of acquired CTA is mediated only by AMPA receptors. The present results also suggest that the amygdalar glutamatergic neural transmission is involved in the formation and storage of long-term gustatory memory associated with the altered hedonics from positive to negative.

Redundant basal forebrain modulation in taste aversion memory formation

Mnemonic deficits resulting from excitotoxic lesion of the basal forebrain have been classically attributed to the resulting depletion of cortical acetylcholine activity. It has been demonstrated that in spite of the strong cholinergic depletion after injections into the basal forebrain of the immunotoxin 192IgG-saporin, no detectable deficit can be found in the acquisition of several learning tasks, including conditioned taste aversion. Conversely, NMDA-induced lesions of the basal forebrain strongly impair taste aversion learning. In this study we show that 192IgG-saporin produces an efficient and selective cholinergic deafferentation of the rat neocortex but not the amygdala. Furthermore, a stronger relationship between severity of memory impairment after NMDA lesions and basoamygdaloid cholinergic deafferentation was found. Therefore, in a second experiment, we show that combining NMDA-induced lesions into the basolateral amygdala with 192IgG-saporin injections into the basal forebrain results in a strong disruption of taste aversion learning, whereas none of these treatments were by themselves capable of producing any detectable impairment in this learning task. The double lesion effect was only paralleled by simple NMDA lesions into the basal forebrain, suggesting that the learning deficits associated to excitotoxic lesions of the basal forebrain are the result of the simultaneous destruction of the corticopetal and basoamygdaloid interaction. A model is proposed, according to which the modulation of learning processes exerted by the basal forebrain can be redundantly performed by both the basocortical and basoamygdaloid pathway.

Hyperresponsiveness to palatable and aversive taste stimuli in genetically obese (bombesin receptor subtype-3-deficient) mice

Taste preference in obese mice was examined using genetically obese (bombesin receptor subtype-3: BRS-3 deficient) animals. Preference for either sodium saccharin (0.2%). sodium chloride (0.9%), citric acid (0.1%), or quinine sulfate (0.002%) solution was examined using a two-bottle test situation, and BRS-3 deficient mice not only showed a stronger preference for saccharin solution, but also a stronger aversive response to quinine solution, relative to wild-type littermates. Furthermore, a conditioned taste-aversion test measured the consumption of sodium saccharin (0.2%) and sodium chloride (0.9%) solutions after intraperitoneal injection of LiCl (0.3 M, 1 mg/kg), and BRS-3-deficient mice exhibited stronger aversion to both solutions than did control animals. In situ hybridization demonstrated that the BRS-3 gene is expressed in the parabrachial nucleus, the medial and central nuclei of the amygdala, and the hypothalamic nuclei such as paraventricular nucleus, all of which are known to be involved in taste perception. These results suggest that expression of the BRS-3 gene in these nuclei is important for the modulation of taste preference, as well as the development of obesity.

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.

The MAPK cascade is required for mammalian associative learning

Mitogen-activated protein kinase (MAPK) is an integral component of cellular signaling during mitogenesis and differentiation of mitotic cells. Recently MAPK activation in post-mitotic cells has been implicated in hippocampal long-term potentiation (LTP), a potential cellular mechanism of learning and memory. Here we investigate the involvement of MAPK in learning and memory in behaving animals. MAPK activation increased in the rat hippocampus after an associative learning task, contextual fear conditioning. Two other protein kinases known to be activated during hippocampal LTP, protein kinase C and alpha-calcium/calmodulin protein kinase II, also were activated in the hippocampus after learning. Inhibition of the specific upstream activator of MAPK, MAPK kinase (MEK), blocked fear conditioning. Thus, classical conditioning in mammals activates MAPK, which is necessary for consolidation of the resultant learning.

Conditioning Method Dramatically Alters the Role of Amygdala in Taste Aversion Learning

Although an important role for the amygdala in taste aversion learning has been suggested by work in a number of laboratories, results have been inconsistent and interpretations varied. The present series of studies reevaluated the role of the amygdala in taste aversion learning by examining the extent to which conditioning methods, testing methods and lesioning methods, influence whether amygdala lesions dramatically affect conditioned taste aversion (CTA) learning. Results indicated that when animals are conditioned with an intraoral (I/O) taste presentation, lesions of amygdala eliminate evidence of conditioning whether animals are tested intraorally or with a two-bottle solution presentation. Dramatic effects of amygdala lesions on CTA learning were seen whether lesions were made electrolytically or using an excitotoxin. In contrast, when animals were conditioned using bottle presentation of the taste, electrolytic lesions attenuated CTAs but did not eliminate them, and excitotoxic lesions had no effect. These results are consistent with the hypothesis that neural structures critical for CTA learning may differ depending on the extent to which the method of conditioned stimulus delivery incorporates a response component.

Chelerythrine, a specific PKC inhibitor, blocks acquisition but not consolidation and retrieval of conditioned taste aversion in rat

Association of the short-term memory of the gustatory conditioned stimulus (CS) with visceral malaise (unconditioned stimulus, US) in conditioned taste aversion (CTA) paradigm takes place in the parabrachial nuclei (PBN) of brainstem. In order to ascertain the role of protein-kinase C (PKC) during different phases of CTA acquisition and retrieval, four experimental series were carried out. In Experiment 1, 1 microl of 10 mM of PKC inhibitor chelerythrine prevented CTA acquisition when applied into PBN in the CS-US interval. In Experiment 2, the necessity of PKC activity in different phases of CTA acquisition was tested by prolonging the time interval between PBN administration of chelerythrine and i.p. LiCl. CTA acquisition was prevented when chelerythrine-induced blockade of PKC coincided with GSTM persistence but not with CTA consolidation. In Experiment 3, the interval between saccharin drinking and LiCl injection was prolonged to 120 min. Again, chelerythrine blockade of PKC activity prevented CTA formation when it interfered with GSTM persistence. In Experiment 4, the possibility that PKC activity is necessary also for CTA retrieval was tested by chelerythrine application into PBN 5 min before retrieval testing. In this case, the chelerythrine-induced PKC blockade did not impair CTA retrieval. It is concluded that PKC is important for GSTM formation and persistence but not for CTA consolidation or retrieval.

Short-term and long-term excitability changes of the insular cortical neurons after the acquisition of taste aversion learning in behaving rats

Conditioned taste aversion, a long-lasting type of learning established after a single pairing of a novel taste and subsequent internal malaise, is an adaptive behavior to prevent animals from repeated intakes of poisonous substances. The present study was designed to identify the time-dependent excitability changes of cortical neurons to gustatory stimuli after the acquisition of conditioned taste aversion in freely behaving rats. Conditioned taste aversion to saccharin was established by an intraperitoneal injection of lithium chloride, a sickness-inducing agent, soon after an intraoral infusion of saccharin. Twenty minutes after the pairing, 25 (29%) of 86 rats showed aversive taste reactivities to saccharin, and 30 min after the pairing, all of the rats showed aversive behaviors to saccharin; these behavioral changes lasted throughout the test session (over 360 min). When unit activities were recorded from the insular cortex simultaneously with the behavioral test, 14 (11%) of 122 neurons showed a significant enhancement of excitability in response to saccharin, but not to other taste stimuli, after the acquisition of taste aversion. Eight of these 14 neurons showed a short-term enhancement: significant effects were detected only 30 min after the pairing. The remaining six neurons exhibited a long-term enhancement: the effects lasted over 360 min after the pairing. The existence of such short-term and long-term excitability changes suggests that the gustatory insular cortex is involved in different aspects of taste aversion learning.

Roles of chemical mediators in the taste system

Recent advances in neural mechanisms of taste are reviewed with special reference to neuroactive substances. In the first section, taste transduction mechanisms of basic tastes are explained in two groups, whether taste stimuli directly activate ion channels in the taste cell membrane or they bind to cell surface receptors coupled to intracellular signaling pathways. In the second section, putative transmitters and modulators from taste cells to afferent nerves are summarized. The candidates include acetylcholine, catecholamines, serotonin, amino acids and peptides. Studies favor serotonin as a possible neuromodulator in the taste bud. In the third section, the role of neuroactive substances in the central gustatory pathways is introduced. Excitatory and inhibitory amino acids (e.g., glutamate and GABA) and peptides (substance P and calcitonin gene-related peptide) are proved to play roles in transmission of taste information in both the brainstem relay and cortical gustatory area. In the fourth section, conditioned taste aversion is introduced as a model to study gustatory learning and memory. Pharmacobehavioral studies to examine the effects of glutamate receptor antagonists and protein kinase C inhibitors on the formation of conditioned taste aversion show that both glutamate and protein kinase C in the amygdala and cortical gustatory area play essential roles in taste aversion learning. Recent molecular and genetic approaches to disclose biological mechanisms of gustatory learning are also introduced. In the last section, behavioral and pharmacological approaches to elucidate palatability, taste pleasure, are described. Dopamine, benzodiazepine derivatives and opioid substances may play some roles in evaluation of palatability and motivation to ingest palatable edibles.