A role for MAPK and PI-3K signaling pathways in brain-derived neurotrophic factor modification of conditioned taste aversion retention

Neurotrophin-3 into the insular cortex strengthens conditioned taste aversion memory

Memory consolidation is an essential process of long-term memory formation. Neurotrophins have been suggested as key regulators of activity dependent changes in the synaptic efficacy and morphology, which are considered the downstream mechanisms of memory consolidation. The neurotrophin 3 (NT-3), a member of the neurotrophin family, and its high affinity receptor TrkC, are widely expressed in the insular cortex (IC), a region with a critical role in the consolidation of the conditioned taste aversion (CTA) paradigm, in which an animal associates a novel taste with nausea. Nevertheless, the role of this neurotrophin in the cognitive processes that the IC mediates remains unexamined. To answer whether NT-3 is involved in memory consolidation at the IC, adult male Wistar rats were administered with NT-3 or NT-3 in combination with the Trk receptors inhibitor K252a into the IC, immediately after CTA acquisition under two different conditions: a strong-CTA (0.2 M lithium chloride i.p.) or a weak-CTA (0.1 M lithium chloride i.p.). Our results show that NT-3 strengthens the memory trace of CTA, transforming a weak conditioning into a strong one, in a Trk-dependent manner. The present evidence suggests that NT-3 has a key role in the consolidation process of an aversive memory in a neocortical region.

Small Activating RNAs: Towards the Development of New Therapeutic Agents and Clinical Treatments

Small double-strand RNA (dsRNA) molecules can activate endogenous genes via an RNA-based promoter targeting mechanism. RNA activation (RNAa) is an evolutionarily conserved mechanism present in diverse eukaryotic organisms ranging from nematodes to humans. Small activating RNAs (saRNAs) involved in RNAa have been successfully used to activate gene expression in cultured cells, and thereby this emergent technique might allow us to develop various biotechnological applications, without the need to synthesize hazardous construct systems harboring exogenous DNA sequences. Accordingly, this thematic issue aims to provide insights into how RNAa cellular machinery can be harnessed to activate gene expression leading to a more effective clinical treatment of various diseases.

Another Example of Conditioned Taste Aversion: Case of Snails

Simple Summary

It is important to decide what to eat and what not to eat in the life. Children are likely to reject new foods. When eating a new food results in a negative experience, the child will avoid that specific food in the future. This phenomenon is called ‘conditioned taste aversion’ in mammals, and it is considered necessary for survival by preventing subsequent ingestion of sickening foods. Many researchers study the same kind of phenomenon in invertebrates, too. For example, the formation of conditioned taste aversion was found in the pond snail, Lymnaea stagnalis, with the selective associability between a sweet sucrose solution and a bitter KCl solution. A sweet food attracts many kinds of animals, resulting in the feeding response, whereas a KCl solution is an aversive stimulus, inducing a withdrawal response in snails. After repeated temporally-contingent presentations of these two stimuli, the sucrose solution no longer elicits a feeding response, and this phenomenon persists for a long term. In the present review, we first outline the mechanisms of conditioned taste aversion in mammals, then introduce the conditioned taste aversion in snails, and compare them. Furthermore, the molecular events in snails are discussed, suggesting the general mechanism in conditioned taste aversion.

Abstract

Conditioned taste aversion (CTA) in mammals has several specific characteristics: (1) emergence of a negative symptom in subjects due to selective association with a taste-related stimulus, (2) robust long-term memory that is resistant to extinction induced by repeated presentation of the conditioned stimulus (CS), (3) a very-long-delay presentation of the unconditioned stimulus (US), and (4) single-trial learning. The pond snail, Lymnaea stagnalis, can also form a CTA. Although the negative symptoms, like nausea, in humans cannot be easily observed in invertebrate animal models of CTA, all the other characteristics of CTA seem to be present in snails. Selective associability was confirmed using a sweet sucrose solution and a bitter KCl solution. Once snails form a CTA, repeated presentation of the CS does not extinguish the CTA. A long interstimulus interval between the CS and US, like in trace conditioning, still results in the formation of a CTA in snails. Lastly, even single-trial learning has been demonstrated with a certain probability. In the present review, we compare, in detail, CTA in mammals and snails, and discuss the possible molecular events in CTA.

3‑N‑Butyphthalide improves learning and memory in rats with vascular cognitive impairment by activating the SIRT1/BDNF pathway

Vascular cognitive impairment (VCI) is a type of cerebral vascular disorder that leads to learning and memory decline. VCI models can be induced by chronic cerebral hypoperfusion via permanent bilateral common carotid artery occlusion. 3-N-Butylphthalide (NBP) is a neuroprotective drug used for the treatment of ischemic cerebrovascular diseases. Silent information regulator 1 (SIRT1) plays an important role in memory formation and cognitive performance, and its abnormal reduction is associated with cognitive dysfunction in neurodegenerative diseases. Brain-derived neurotrophic factor (BDNF) is a neurotrophic factor that plays critical roles in promoting neuronal growth and injury repair. The present study was performed to investigate the effects and the underlying mechanism of NBP on learning deficits in a rat model of VCI. Rats were divided into a control group, model group, low-NBP-dose group (30 mg/kg/day), high-NBP-dose group (60 mg/kg/day), NBP + SIRT1 inhibitor group and NBP + BDNF inhibitor group. Rats were then subjected to Morris water maze and T-maze tests, which identified that NBP treatment significantly attenuated memory impairments in VCI rats. Molecular examination indicated that SIRT1 and BDNF expression levels in the hippocampus were increased by NBP treatment. However, NBP failed to ameliorate cognitive function after inhibition of the SIRT1/BDNF signaling pathway. In addition, NBP in combination with a SIRT1 inhibitor suppressed BDNF protein expression, but inhibition of BDNF did not inhibit SIRT1 protein expression in rats with VCI. The present results suggested that the neuroprotective effects of NBP on learning deficits in a rat model of VCI may be via regulation of the SIRT1/BDNF signaling pathway, in which SIRT1 may be the upstream signaling molecule. Therefore, the SIRT1/BDNF pathway could be a potential therapeutic target for VCI.

Sonic hedgehog signaling in spinal cord contributes to morphine-induced hyperalgesia and tolerance through upregulating brain-derived neurotrophic factor expression

Purpose

Preventing opioid-induced hyperalgesia and tolerance continues to be a major clinical challenge, and the underlying mechanisms of hyperalgesia and tolerance remain elusive. Here, we investigated the role of sonic hedgehog (Shh) signaling in opioid-induced hyperalgesia and tolerance.

Methods

Shh signaling expression, behavioral changes, and neurochemical alterations induced by morphine were analyzed in male adult CD-1 mice with repeated administration of morphine. To investigate the contribution of Shh to morphine-induced hyperalgesia (MIH) and tolerance, Shh signaling inhibitor cyclopamine and Shh small interfering RNA (siRNA) were used. To explore the mechanisms of Shh signaling in MIH and tolerance, brain-derived neurotrophic factor (BDNF) inhibitor K252 and anti-BDNF antibody were used.

Results

Repeated administration of morphine produced obvious hyperalgesia and tolerance. The behavioral changes were correlated with the upregulation and activation of morphine treatment-induced Shh signaling. Pharmacologic and genetic inhibition of Shh signaling significantly delayed the generation of MIH and tolerance and associated neurochemical changes. Chronic morphine administration also induced upregulation of BDNF. Inhibiting BDNF effectively delayed the generation of MIH and tolerance. The upregulation of BDNF induced by morphine was significantly suppressed by inhibiting Shh signaling. In naïve mice, exogenous activation of Shh signaling caused a rapid increase of BDNF expression, as well as thermal hyperalgesia. Inhibiting BDNF significantly suppressed smoothened agonist-induced hyperalgesia.

Conclusion

These findings suggest that Shh signaling may be a critical mediator for MIH and tolerance by regulating BDNF expression. Inhibiting Shh signaling, especially during the early phase, may effectively delay or suppress MIH and tolerance.

CaMKII Requirement for in Vivo Insular Cortex LTP Maintenance and CTA Memory Persistence

Calcium-calmodulin/dependent protein kinase II (CaMKII) plays an essential role in LTP induction, but since it has the capacity to remain persistently activated even after the decay of external stimuli it has been proposed that it can also be necessary for LTP maintenance and therefore for memory persistence. It has been shown that basolateral amygdaloid nucleus (Bla) stimulation induces long-term potentiation (LTP) in the insular cortex (IC), a neocortical region implicated in the acquisition and retention of conditioned taste aversion (CTA). Our previous studies have demonstrated that induction of LTP in the Bla-IC pathway before CTA training increased the retention of this task. Although it is known that IC-LTP induction and CTA consolidation share similar molecular mechanisms, little is known about the molecular actors that underlie their maintenance. The purpose of the present study was to evaluate the role of CaMKII in the maintenance of in vivo Bla-IC LTP as well as in the persistence of CTA long-term memory (LTM). Our results show that acute microinfusion of myr-CaMKIINtide, a selective inhibitor of CaMKII, in the IC of adult rats during the late-phase of in vivo Bla-IC LTP blocked its maintenance. Moreover, the intracortical inhibition of CaMKII 24 h after CTA acquisition impairs CTA-LTM persistence. Together these results indicate that CaMKII is a central key component for the maintenance of neocortical synaptic plasticity as well as for persistence of CTA-LTM.

Effects of lesions in different nuclei of the amygdala on conditioned taste aversion

Conditioned taste aversion (CTA) is an adaptive learning that depends on brain mechanisms not completely identified. The amygdala is one of the structures that make up these mechanisms, but the involvement of its nuclei in the acquisition of CTA is unclear. Lesion studies suggest that the basolateral complex of the amygdala, including the basolateral and lateral amygdala, could be involved in CTA. The central amygdala has also been considered as an important nucleus for the acquisition of CTA in some studies. However, to the best of our knowledge, the effect of lesions of the basolateral complex of the amygdala on the acquisition of CTA has not been directly compared with the effect of lesions of the central and medial nuclei of the amygdala. The aim of this study is to compare the effect of lesions of different nuclei of the amygdala (the central and medial amygdala and the basolateral complex) on the acquisition of taste aversion in male Wistar rats. The results indicate that lesions of the basolateral complex of the amygdala reduce the magnitude of the CTA when compared with lesions of the other nuclei and with animals without lesions. These findings suggest that the involvement of the amygdala in the acquisition of CTA seems to depend particularly on the integrity of the basolateral complex of the amygdala.

Predictable Chronic Mild Stress during Adolescence Promotes Fear Memory Extinction in Adulthood

Early-life stress in adolescence has a long-lasting influence on brain function in adulthood, and it is mostly recognized as a predisposing factor for mental illnesses, such as anxiety and posttraumatic stress disorder. Previous studies also indicated that adolescent predictable chronic mild stress (PCMS) in early life promotes resilience to depression- and anxiety-like behaviors in adulthood. However, the role of PCMS in associated memory process is still unclear. In the present study, we found that adolescent PCMS facilitated extinction and inhibited fear response in reinstatement and spontaneous recovery tests in adult rats, and this effect was still present 1 week later. PCMS in adolescence increased the activity of brain-derived neurotrophic factor (BDNF)-extracellular signal-regulated kinase 1/2 (ERK1/2) signaling in infralimbic cortex (IL) but not prelimbic cortex in adulthood. Intra-IL infusion of BDNF antibody and the ERK1/2 inhibitor U0126 reversed PCMS-induced enhancement of fear extinction. Moreover, we found that PCMS decreased DNA methylation of the Bdnf gene at exons IV and VI and elevated the mRNA levels of Bdnf in the IL. Our findings indicate that adolescent PCMS exposure promotes fear memory extinction in adulthood, which reevaluates the traditional notion of adolescent stress.

Cell-Cell Connection Enhances Proliferation and Neuronal Differentiation of Rat Embryonic Neural Stem/Progenitor Cells

Cell-cell interaction as one of the niche signals plays an important role in the balance of stem cell quiescence and proliferation or differentiation. In order to address the effect and the possible mechanisms of cell-cell connection on neural stem/progenitor cells (NSCs/NPCs) proliferation and differentiation, upon passaging, NSCs/NPCs were either dissociated into single cell as usual (named Group I) or mechanically triturated into a mixture of single cell and small cell clusters containing direct cell-cell connections (named Group II). Then the biological behaviors including proliferation and differentiation of NSCs/NPCs were observed. Moreover, the expression of gap junction channel, neurotrophic factors and the phosphorylation status of MAPK signals were compared to investigate the possible mechanisms. Our results showed that, in comparison to the counterparts in Group I, NSCs/NPCs in Group II survived well with preferable neuronal differentiation. In coincidence with this, the expression of connexin 45 (Cx45), as well as brain derived neurotrophic factor (BDNF) and neurotrophin 3 (NT-3) in Group II were significantly higher than those in Group I. Phosphorylation of ERK1/2 and JNK2 were significantly upregulated in Group II too, while no change was found about p38. Furthermore, the differences of NSCs/NPCs biological behaviors between Group I and II completely disappeared when ERK and JNK phosphorylation were inhibited. These results indicated that cell-cell connection in Group II enhanced NSCs/NPCs survival, proliferation and neuronal differentiation through upregulating the expression of gap junction and neurotrophic factors. MAPK signals- ERK and JNK might contribute to the enhancement. Efforts for maintaining the direct cell-cell connection are worth making to provide more favorable niches for NSCs/NPCs survival, proliferation and neuronal differentiation.

Brain-derived neurotrophic factor into adult neocortex strengthens a taste aversion memory

Nowadays, it is known that brain derived neurotrophic-factor (BDNF) is a protein critically involved in regulating long-term memory related mechanisms. Previous studies from our group in the insular cortex (IC), a brain structure of the temporal lobe implicated in acquisition, consolidation and retention of conditioned taste aversion (CTA), demonstrated that BDNF is essential for CTA consolidation. Recent studies show that BDNF-TrkB signaling is able to mediate the enhancement of memory. However, whether BDNF into neocortex is able to enhance aversive memories remains unexplored. In the present work, we administrated BDNF in a concentration capable of inducing in vivo neocortical LTP, into the IC immediately after CTA acquisition in two different conditions: a "strong-CTA" induced by 0.2M lithium chloride i.p. as unconditioned stimulus, and a "weak-CTA" induced by 0.1M lithium chloride i.p. Our results show that infusion of BDNF into the IC converts a weak CTA into a strong one, in a TrkB receptor-dependent manner. The present data suggest that BDNF into the adult insular cortex is sufficient to increase an aversive memory-trace.

Effect of preexposure on methylphenidate-induced taste avoidance and related BDNF/TrkB activity in the insular cortex of the rat

RATIONALE

Exogenous brain-derived neurotrophic factor (BDNF) in the insular cortex (IC) is known to influence conditioned taste avoidance (CTA) learning, but little is known of its endogenous role in the phenomenon. Preexposure to many abusable compounds attenuates their ability to induce CTA, thus providing a possible platform from which to further elucidate the endogenous role of IC BDNF in CTA.

OBJECTIVES

The role of IC BDNF in CTA learning was examined by assessing the effect of preexposure to methylphenidate (MPH) on MPH-induced CTA, followed by expression between preexposure groups of BDNF in the IC, central nucleus of the amygdala (CeA), and the nucleus accumbens (NAc).

METHODS

Following preexposure to MPH (18 mg/kg), CTAs induced by MPH (0, 10, 18, and 32 mg/kg) were assessed in adult male Sprague-Dawley rats (n = 64). In separate groups (n = 31), differences in BDNF and tropomyosin-related kinase receptor-B (TrkB) were assessed using Western blots following similar preexposure and conditioning procedures.

RESULTS

Preexposure to MPH significantly blunted MPH-CTA compared to preexposure to vehicle. Unexpectedly, there were no significant effects of MPH on BDNF activity following CTA, but animals preexposed to MPH exhibited decreased activity in the CeA and enhanced activity in the IC and NAc.

CONCLUSIONS

Preexposure to MPH attenuates its aversive effects on subsequent presentations, and BDNF's impact on CTA learning may be dependent upon its temporal relation to other CTA-related intracellular cascades.

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.

Acute infusion of brain-derived neurotrophic factor in the insular cortex promotes conditioned taste aversion extinction

Brain-derived neurotrophic factor (BDNF) has emerged as one of the most potent molecular mediators not only for synaptic plasticity, but also for the behavioral organism-environment interactions. Our previous studies in the insular cortex (IC), a neocortical region that has been related with acquisition and retention of conditioned taste aversion (CTA), have demonstrated that intracortical microinfusion of BDNF induces a lasting potentiation of synaptic efficacy in the basolateral amygdaloid nucleus (Bla)-IC projection and enhances the retention of CTA memory of adult rats in vivo. The aim of the present study was to analyze whether acute BDNF-infusion in the IC modifies the extinction of CTA. Accordingly, animals were trained in the CTA task and received bilateral IC microinfusions of BDNF before extinction training. Our results showed that taste aversion was significantly reduced in BDNF rats from the first extinction trial. Additionally, we found that the effect of BDNF on taste aversion did not require extinction training. Finally we showed that the BDNF effect does not degrade the original taste aversion memory trace. These results emphasize that BDNF activity underlies memory extinction in neocortical areas and support the idea that BDNF is a key regulator and mediator of long-term synaptic modifications.

Effect of age on methylphenidate-induced conditioned taste avoidance and related BDNF/TrkB signaling in the insular cortex of the rat

RATIONALE

Drug use and abuse is thought to be a function of the balance between its rewarding and aversive effects, such that the rewarding effects increase the likelihood of use while the drug's dissociable aversive effects limit it. Adolescents exhibit a shift in this balance toward reward, which may ultimately lead to increased use. Importantly, recent work shows that adolescents are also protected from the aversive effects of many abusable drugs as measured by conditioned taste avoidance (CTA). However, such effects of methylphenidate (MPH, widely prescribed to adolescents with ADHD) have not been characterized.

OBJECTIVES

The effect of age on MPH-induced CTA was assessed. In addition, MPH-induced changes in brain-derived neurotrophic factor (BDNF) activity in the insular cortex (IC) and central nucleus of the amygdala (CeA), known to be important to CTA, were examined and related to CTAs in adolescents and adults.

METHODS

CTAs induced by MPH (0, 10, 18, and 32 mg/kg) were assessed in adolescent (n = 34) and adult (n = 33) male Sprague Dawley rats. Following MPH CTA, IC and CeA tissue was probed for differences in BDNF and tropomyosin-related kinase receptor-B (TrkB) using Western blots.

RESULTS

Blunted expression of MPH CTA was observed in the adolescents versus adults, which correlated with generally attenuated adolescent BDNF/TrkB activity in the IC, but the drug effects ran contrary to the expression of CTA.

CONCLUSIONS

Adolescents are protected from the aversive effects of MPH versus adults, but further work is needed to characterize the possible involvement of BDNF/TrkB.

Localization of BDNF expression in the developing brain of zebrafish

The brain-derived neurotrophic factor (BDNF) gene is expressed in differentiating and post-mitotic neurons of the zebrafish embryo, where it has been implicated in Huntington's disease. Little is known, however, about the full complement of neuronal cell types that express BDNF in this important vertebrate model. Here, we further explored the transcriptional profiles during the first week of development using real-time quantitative polymerase chain reaction (RT-qPCR) and whole-mount in situ hybridization (WISH). RT-qPCR results revealed a high level of maternal contribution followed by a steady increase of zygotic transcription, consistent with the notion of a prominent role of BDNF in neuronal maturation and maintenance. Based on WISH, we demonstrate for the first time that BDNF expression in the developing brain of zebrafish is structure specific. Anatomical criteria and co-staining with genetic markers (shh, pax2a, emx1, krox20, lhx2b and lhx9) visualized major topological domains of BDNF-positive cells in the pallium, hypothalamus, posterior tuberculum and optic tectum. Moreover, the relative timing of BDNF transcription in the eye and tectum may illustrate a mechanism for coordinated development of the retinotectal system. Taken together, our results are compatible with a local delivery and early role of BDNF in the developing brain of zebrafish, adding basic knowledge to the study of neurotrophin functions in neural development and disease.

BDNF and memory processing

Brain-derived neurotrophic factor has emerged as one of the most important molecules involved in memory. Its wide role in different types of memories that depend on different structures as well as its involvement in distinct memory stages points at BDNF as one likely target to treat cognitive impairments and anxiety-related memory disorders. However, regulation of BDNF expression is very complex as well as its modes of action. Here we describe the latest research carried out on the function of BDNF in memory to illustrate such complexity. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Neurotrophins and synaptic plasticity

It has been suggested that long-term modifications of synaptic transmission constitute the foundation of the processes by which information is stored in the central nervous system. A group of proteins called neurotrophins are considered powerful molecular mediators in central synaptic plasticity. Among these, brain-derived neurotrophic factor (BDNF) as well as neurotrophin-3 (NT-3) have emerged as having key roles in the neurobiological mechanisms related to learning and memory. In this chapter, we review the studies that have represented a significant step forward in understanding the role played by BDNF and NT-3 in long-term synaptic plasticity. The effects of BDNF and NT-3 on synaptic plasticity can be of a permissive nature, establishing the conditions under which plastic changes can take place, or it may be instructive, directly modifying the communication and morphology of synapses. The actions carried out by BDNF include its capacity to contribute to the stabilization and maturation of already-existing synapses, as well as to generate new synaptic contacts. One important finding that highlights the participation of these neurotrophins in synaptic plasticity is the observation that adding BDNF or NT-3 gives rise to drastic long-term increases in synaptic transmission, similar to the long-term potentiation in the hippocampus and neocortex of mammals. Because neurotrophins modulate both the electrical properties and the structural organization of the synapse, these proteins have been considered important biological markers of learning and memory processes.

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.

Inhibition of natural antisense transcripts in vivo results in gene-specific transcriptional upregulation

Here we demonstrate that natural antisense transcripts (NATs), which are abundant in mammalian genomes, can function as repressors of specific genomic loci and that their removal or inhibition by AntagoNAT oligonucleotides leads to transient and reversible upregulation of sense gene expression. As one example, we show that Brain-Derived Neurotrophic Factor (BDNF) is under the control of a conserved noncoding antisense RNA transcript, BDNF-AS, both in vitro and in vivo. BDNF-AS tonically represses BDNF sense RNA transcription by altering chromatin structure at the BDNF locus, which in turn reduces endogenous BDNF protein and function. By providing additional and analogous examples of endogenous mRNA upregulation, we suggest that antisense RNA mediated transcriptional suppression is a common phenomenon. In sum, we demonstrate a novel pharmacological strategy by which endogenous gene expression can be upregulated in a locus-specific manner.

Molecular mechanisms underlying memory consolidation of taste information in the cortex

The senses of taste and odor are both chemical senses. However, whereas an organism can detect an odor at a relatively long distance from its source, taste serves as the ultimate proximate gatekeeper of food intake: it helps in avoiding poisons and consuming beneficial substances. The automatic reaction to a given taste has been developed during evolution and is well adapted to conditions that may occur with high probability during the lifetime of an organism. However, in addition to this automatic reaction, animals can learn and remember tastes, together with their positive or negative values, with high precision and in light of minimal experience. This ability of mammalians to learn and remember tastes has been studied extensively in rodents through application of reasonably simple and well defined behavioral paradigms. The learning process follows a temporal continuum similar to those of other memories: acquisition, consolidation, retrieval, relearning, and reconsolidation. Moreover, inhibiting protein synthesis in the gustatory cortex (GC) specifically affects the consolidation phase of taste memory, i.e., the transformation of short- to long-term memory, in keeping with the general biochemical definition of memory consolidation. This review aims to present a general background of taste learning, and to focus on recent findings regarding the molecular mechanisms underlying taste–memory consolidation in the GC. Specifically, the roles of neurotransmitters, neuromodulators, immediate early genes, and translation regulation are addressed.

Late Protein Synthesis-Dependent Phases in CTA Long-Term Memory: BDNF Requirement

It has been proposed that long-term memory (LTM) persistence requires a late protein synthesis-dependent phase, even many hours after memory acquisition. Brain-derived neurotrophic factor (BDNF) is an essential protein synthesis product that has emerged as one of the most potent molecular mediators for long-term synaptic plasticity. Studies in the rat hippocampus have been shown that BDNF is capable to rescue the late-phase of long-term potentiation as well as the hippocampus-related LTM when protein synthesis was inhibited. Our previous studies on the insular cortex (IC), a region of the temporal cortex implicated in the acquisition and storage of conditioned taste aversion (CTA), have demonstrated that intracortical delivery of BDNF reverses the deficit in CTA memory caused by the inhibition of IC protein synthesis due to anisomycin administration during early acquisition. In this work, we first analyze whether CTA memory storage is protein synthesis-dependent in different time windows. We observed that CTA memory become sensible to protein synthesis inhibition 5 and 7 h after acquisition. Then, we explore the effect of BDNF delivery (2 μg/2 μl per side) in the IC during those late protein synthesis-dependent phases. Our results show that BDNF reverses the CTA memory deficit produced by protein synthesis inhibition in both phases. These findings support the notion that recurrent rounds of consolidation-like events take place in the neocortex for maintenance of CTA memory trace and that BDNF is an essential component of these processes.

Region-specific involvement of BDNF secretion and synthesis in conditioned taste aversion memory formation

Brain-derived neurotrophic factor (BDNF) and its receptor, tropomyosin-related kinase receptor B (TrkB), play a critical role in activity-dependent plasticity processes such as long-term potentiation, learning, and memory. It has been shown that BDNF exerts different or even opposite effects on behavior depending on the neural circuit. However, the detailed role of BDNF in memory process on the basis of its location has not been fully understood. Here, we aim to investigate the regional specific involvement of BDNF/TrkB in hippocampal-independent conditioned taste aversion (CTA) memory processes. We found region-specific changes in BDNF expression during CTA learning. CTA conditioning induced increased BDNF levels in the central nuclei of amygdala (CeA) and insular cortex, but not in the basolateral amygdala (BLA) and ventromedial prefrontal cortex. Interestingly, we found that the enhanced TrkB phosphorylation occurred at the time point before the increased BDNF expression, suggesting rapid induction of activity-dependent BDNF secretion by CTA learning. Moreover, targeted infusion of BDNF antibodies or BDNF antisense oligonucleotides revealed that activity-dependent BDNF secretion and synthesis in the CeA, but not the BLA, was respectively involved in the short- and long-term memory formation of CTA. Finally, we found that infusion of exogenous BDNF into the CeA could enhance CTA learning. These data suggest that region-specific BDNF release and synthesis temporally regulate different CTA memory phases through activation of TrkB receptors.