Differential pattern of cAMP response element-binding protein activation in the rat brain after conditioned aversion as a function of the associative process engaged: taste versus context association

A neural mechanism for learning from delayed postingestive feedback

Animals learn the value of foods based on their postingestive effects and thereby develop aversions to foods that are toxic1-6 and preferences to those that are nutritious7-14. However, it remains unclear how the brain is able to assign credit to flavors experienced during a meal with postingestive feedback signals that can arise after a substantial delay. Here, we reveal an unexpected role for postingestive reactivation of neural flavor representations in this temporal credit assignment process. To begin, we leverage the fact that mice learn to associate novel15-18, but not familiar, flavors with delayed gastric malaise signals to investigate how the brain represents flavors that support aversive postingestive learning. Surveying cellular resolution brainwide activation patterns reveals that a network of amygdala regions is unique in being preferentially activated by novel flavors across every stage of the learning process: the initial meal, delayed malaise, and memory retrieval. By combining high-density recordings in the amygdala with optogenetic stimulation of genetically defined hindbrain malaise cells, we find that postingestive malaise signals potently and specifically reactivate amygdalar novel flavor representations from a recent meal. The degree of malaise-driven reactivation of individual neurons predicts strengthening of flavor responses upon memory retrieval, leading to stabilization of the population-level representation of the recently consumed flavor. In contrast, meals without postingestive consequences degrade neural flavor representations as flavors become familiar and safe. Thus, our findings demonstrate that interoceptive reactivation of amygdalar flavor representations provides a neural mechanism to resolve the temporal credit assignment problem inherent to postingestive learning.

Intraperitoneal injection of lithium chloride induces lateralized activation of the insular cortex in adult mice

Insular cortex is a critical brain region that participates in the interoceptive sensations. Here, we combined the iDISCO + method and Fos immunostaining to confirm that the middle part of the right-side, but not the left-side, insular cortex in adult male mice is activated by intraperitoneal injection of lithium chloride. Lateralized activation of the insular cortex is also observed in adult female mice, but not in young or aged male mice. Furthermore, asymmetrical activation of the insular cortex was completely blocked when both sides of the vagal nerve are transected, whereas intravenous injection of lithium chloride has no effect on the insular activation. Combined together, these results indicate that the insular cortex unilaterally responds to aversive visceral stimuli in an age-dependent way and this process depends on the vagal afferent pathways.

Differential changes in GAP-43 or synaptophysin during appetitive and aversive taste memory formation

Association between events in time and space is a major mechanism for all animals, including humans, which allows them to learn about the world and potentially change their behavior in the future to adapt to different environments. Conditioning taste aversion (CTA) is a single-trial learning paradigm where animals are trained to avoid a novel flavor which is associated with malaise. Many variables can be analyzed with this model and the circuits involved are well described. Thus, the amygdala and the gustatory cortex (GC) are some of the most relevant structures involved in CTA. In the present study we focused in plastic changes that occur during appetitive and/or aversive taste memory formation. Previous studies have demonstrated that memory consolidation, in hippocampal dependent paradigms, induces plastic changes like increase in the concentration of proteins considered as markers of neuronal plasticity, such as the growth associated protein 43 (GAP-43) and synaptophysin (SYN). In the present experiment in male rats we evaluated changes in GAP-43 and SYN expression, using immunofluorescence, induce by the formation of aversive and appetitive taste memory. We found that taste aversive memory formation can induce an increase in GAP-43 in the granular layer of the GC. Furthermore, we also found an increase in SYN expression in both layers of the GC, the basolateral amygdala (BLA) and the central amygdala (CeA). These results suggest that aversive memory representation induces a new circuitry (inferred from an increase in GAP 43). On the other hand, an appetitive taste learning increased SYN expression in the GC (both layers), the BLA and the CeA without any changes in GAP 43. Together these results indicate that aversive memory formation induces structural and synaptic changes, while appetitive memory formation induces synaptic changes; suggesting that aversive and appetitive memories require a different set of cortical and amygdala plastic changes.

Training on an Appetitive Trace-Conditioning Task Increases Adult Hippocampal Neurogenesis and the Expression of Arc, Erk and CREB Proteins in the Dorsal Hippocampus

Adult hippocampal neurogenesis (AHN) plays an essential role in hippocampal-dependent memory consolidation. Increased neurogenesis enhances learning, whereas its ablation causes memory impairment. In contrast, few reports suggest that neurogenesis reduces after learning. Although the interest in exploring the role of adult neurogenesis in learning has been growing, the evidence is still limited. The role of the trace- and delay-appetitive-conditioning on AHN and its underlying mechanism are not known. The consolidation of trace-conditioned memory requires the hippocampus, but delay-conditioning does not. Moreover, the dorsal hippocampus (DH) and ventral hippocampus (VH) may have a differential role in these two conditioning paradigms. Here, we have investigated the changes in: (A) hippocampal cell proliferation and their progression towards neuronal lineage; and (B) expression of Arc, Erk1, Erk2, and CREB proteins in the DH and VH after trace- and delay-conditioning in the rat. The number of newly generated cells significantly increased in the trace-conditioned but did not change in the delay-conditioned animals compared to the control group. Similarly, the expression of Arc protein significantly increased in the DH but not in the VH after trace-conditioning. Nonetheless, it remains unaltered in the delay-conditioned group. The expression of pErk1, pErk2, and pCREB also increased in the DH after trace-conditioning. Whereas, the expression of only pErk1 pErk2 and pCREB proteins increased in the VH after delay-conditioning. Our results suggest that appetitive trace-conditioning enhances AHN. The increased DH neuronal activation and pErk1, pErk2, and pCREB in the DH may be playing an essential role in learning-induced cell-proliferation after appetitive trace-conditioning.

The role of the gustatory cortex in incidental experience-evoked enhancement of later taste learning

The strength of learned associations between pairs of stimuli is affected by multiple factors, the most extensively studied of which is prior experience with the stimuli themselves. In contrast, little data is available regarding how experience with "incidental" stimuli (independent of any conditioning situation) impacts later learning. This lack of research is striking given the importance of incidental experience to survival. We have recently begun to fill this void using conditioned taste aversion (CTA), wherein an animal learns to avoid a taste that has been associated with malaise. We previously demonstrated that incidental exposure to salty and sour tastes (taste preexposure-TPE) enhances aversions learned later to sucrose. Here, we investigate the neurobiology underlying this phenomenon. First, we use immediate early gene (c-Fos) expression to identify gustatory cortex (GC) as a site at which TPE specifically increases the neural activation caused by taste-malaise pairing (i.e., TPE did not change c-Fos induced by either stimulus in isolation). Next, we use site-specific infection with the optical silencer Archaerhodopsin-T to show that GC inactivation during TPE inhibits the expected enhancements of both learning and CTA-related c-Fos expression, a full day later. Thus, we conclude that GC is almost certainly a vital part of the circuit that integrates incidental experience into later associative learning.

The Insula and Taste Learning

The sense of taste is a key component of the sensory machinery, enabling the evaluation of both the safety as well as forming associations regarding the nutritional value of ingestible substances. Indicative of the salience of the modality, taste conditioning can be achieved in rodents upon a single pairing of a tastant with a chemical stimulus inducing malaise. This robust associative learning paradigm has been heavily linked with activity within the insular cortex (IC), among other regions, such as the amygdala and medial prefrontal cortex. A number of studies have demonstrated taste memory formation to be dependent on protein synthesis at the IC and to correlate with the induction of signaling cascades involved in synaptic plasticity. Taste learning has been shown to require the differential involvement of dopaminergic GABAergic, glutamatergic, muscarinic neurotransmission across an extended taste learning circuit. The subsequent activation of downstream protein kinases (ERK, CaMKII), transcription factors (CREB, Elk-1) and immediate early genes (c-fos, Arc), has been implicated in the regulation of the different phases of taste learning. This review discusses the relevant neurotransmission, molecular signaling pathways and genetic markers involved in novel and aversive taste learning, with a particular focus on the IC. Imaging and other studies in humans have implicated the IC in the pathophysiology of a number of cognitive disorders. We conclude that the IC participates in circuit-wide computations that modulate the interception and encoding of sensory information, as well as the formation of subjective internal representations that control the expression of motivated behaviors.

The entorhinal cortex is involved in conditioned odor and context aversions

In a natural environment, avoidance of a particular food source is mostly determined by a previous intake experience during which sensory stimuli such as food odor, become aversive through a simple associative conditioned learning. Conditioned odor aversion learning (COA) is a food conditioning paradigm that results from the association between a tasteless scented solution (conditioned stimulus, CS) and a gastric malaise (unconditioned stimulus, US) that followed its ingestion. In the present experimental conditions, acquisition of COA also led to acquisition of aversion toward the context in which the CS was presented (conditioned context aversion, CCA). Previous data have shown that the entorhinal cortex (EC) is involved in the memory processes underlying COA acquisition and context fear conditioning, but whether EC lesion modulates CCA acquisition has never be investigated. To this aim, male Long-Evans rats with bilateral EC lesion received CS-US pairings in a particular context with different interstimulus intervals (ISI). The results showed that the establishment of COA with long ISI obtained in EC-lesioned rats is associated with altered CCA learning. Since ISI has been suggested to be the determining factor in the odor- and context-US association, our results show that the EC is involved in the processes that control both associations relative to ISI duration.

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.

Increasing CREB function in the CA1 region of dorsal hippocampus rescues the spatial memory deficits in a mouse model of Alzheimer's disease

The principal defining feature of Alzheimer's disease (AD) is memory impairment. As the transcription factor CREB (cAMP/Ca(2+) responsive element-binding protein) is critical for memory formation across species, we investigated the role of CREB in a mouse model of AD. We found that TgCRND8 mice exhibit a profound impairment in the ability to form a spatial memory, a process that critically relies on the dorsal hippocampus. Perhaps contributing to this memory deficit, we observed additional deficits in the dorsal hippocampus of TgCRND8 mice in terms of (1) biochemistry (decreased CREB activation in the CA1 region), (2) neuronal structure (decreased spine density and dendritic complexity of CA1 pyramidal neurons), and (3) neuronal network activity (decreased arc mRNA levels following behavioral training). Locally and acutely increasing CREB function in the CA1 region of dorsal hippocampus of TgCRND8 mice was sufficient to restore function in each of these key domains (biochemistry, neuronal structure, network activity, and most importantly, memory formation). The rescue produced by increasing CREB was specific both anatomically and behaviorally and independent of plaque load or Aβ levels. Interestingly, humans with AD show poor spatial memory/navigation and AD brains have disrupted (1) CREB activation, and (2) spine density and dendritic complexity in hippocampal CA1 pyramidal neurons. These parallel findings not only confirm that TgCRND8 mice accurately model key aspects of human AD, but furthermore, suggest the intriguing possibility that targeting CREB may be a useful therapeutic strategy in treating humans with AD.

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.

Increasing CREB in the auditory thalamus enhances memory and generalization of auditory conditioned fear

Although the lateral nucleus of the amygdala (LA) is essential for conditioned auditory fear memory, an emerging theme is that plasticity in multiple brain regions contributes to fear memory formation. The LA receives direct projections from the auditory thalamus, specifically the medial division of the medial geniculate nucleus (MGm) and adjacent posterior intralaminar nucleus (PIN). While traditionally viewed as a simple relay structure, mounting evidence implicates the thalamus in diverse cognitive processes. We investigated the role of plasticity in the MGm/PIN in auditory fear memory. First we found that auditory fear conditioning (but not control manipulations) increased the levels of activated CREB in both the MGm and PIN. Next, using viral vectors, we showed that exogenously increasing CREB in this region specifically enhanced formation of an auditory conditioned fear memory without affecting expression of an auditory fear memory, formation of a contextual fear memory, or basic auditory processing. Interestingly, mice with increased CREB levels in the MGm/PIN also showed broad auditory fear generalization (in contrast to control mice, they exhibited fear responses to tones of other frequencies). Together, these results implicate CREB-mediated plasticity in the MGm/PIN in both the formation and generalization of conditioned auditory fear memory. Not only do these findings refine our knowledge of the circuitry underlying fear memory but they also provide novel insights into the neural substrates that govern the degree to which acquired fear of a tone generalizes to other tones.

The brain, appetite, and obesity

Food intake and energy expenditure are controlled by complex, redundant, and distributed neural systems that reflect the fundamental biological importance of adequate nutrient supply and energy balance. Much progress has been made in identifying the various hormonal and neural mechanisms by which the brain informs itself about availability of ingested and stored nutrients and, in turn, generates behavioral, autonomic, and endocrine output. While hypothalamus and caudal brainstem play crucial roles in this homeostatic function, areas in the cortex and limbic system are important for processing information regarding prior experience with food, reward, and emotion, as well as social and environmental context. Most vertebrates can store a considerable amount of energy as fat for later use, and this ability has now become one of the major health risks for many human populations. The predisposition to develop obesity can theoretically result from any pathological malfunction or lack of adaptation to changing environments of this highly complex system.

Dissociated roles for the lateral and medial septum in elemental and contextual fear conditioning

Extensive evidence indicates that the septum plays a predominant role in fear learning, yet the direction of this control is still a matter of debate. Increasing data suggest that the medial (MS) and lateral septum (LS) would be differentially required in fear conditioning depending on whether a discrete conditional stimulus (CS) predicts, or not, the occurrence of an aversive unconditional stimulus (US). Here, using a tone CS-US pairing (predictive discrete CS, context in background) or unpairing (context in foreground) conditioning procedure, we show, in mice, that pretraining inactivation of the LS totally disrupted tone fear conditioning, which, otherwise, was spared by inactivation of the MS. Inactivating the LS also reduced foreground contextual fear conditioning, while sparing the higher level of conditioned freezing to the foreground (CS-US unpairing) than to the background context (CS-US pairing). In contrast, inactivation of the MS totally abolished this training-dependent level of contextual freezing. Interestingly, inactivation of the MS enhanced background contextual conditioning under the pairing condition, whereas it reduced foreground contextual conditioning under the unpairing condition. Hence, the present findings reveal a functional dissociation between the LS and the MS in Pavlovian fear conditioning depending on the predictive value of the discrete CS. While the requirement of the LS is crucial for the appropriate processing of the tone CS-US association, the MS is crucial for an appropriate processing of contextual cues as foreground or background information.

Lack of neprilysin suffices to generate murine amyloid-like deposits in the brain and behavioral deficit in vivo

Accumulation of the beta-amyloid peptide (Abeta) in the brain is a major pathological hallmark of Alzheimer's disease (AD), leading to synaptic dysfunction, neuronal death, and memory impairment. The levels of neprilysin, a major Abeta-degrading enzyme, are decreased in AD brains and during aging. Because neprilysin cleaves Abeta in vivo, its down-regulation may contribute to the pathophysiology of AD. The aim of this study was to assess the consequences of neprilysin deficiency on accumulation of murine Abeta in brains and associated pathologies in vivo by investigating neprilysin-deficient mice on biochemical, morphological, and behavioral levels. Aged neprilysin-deficient mice expressed physiological amyloid precursor protein (APP) levels and exhibited elevated brain Abeta concentrations and amyloid-like deposits in addition to signs of neuronal degeneration in their brains. Behaviorally, neprilysin-deficient mice acquired a significantly weaker conditioned taste aversion that extinguished faster than the aversion of age-matched controls. Our data establish that, under physiological APP expression levels, neprilysin deficiency is associated with increased Abeta accumulation in the brain and leads to deposition of amyloid-like structures in vivo as well as with signs of AD-like pathology and with behavioral deficits.

Forebrain structures specifically activated by conditioned taste aversion

This study investigates which forebrain structures show Fos protein expression during conditioned taste aversion (CTA) acquisition and whether Fos expression depends on the aversion strength. A novel taste paired with an intraperitoneal injection of a low dose of the malaise-inducing agent lithium chloride (LiCl) induced a weak CTA, whereas associating this novel taste with a high dose of LiCl induced a strong CTA. Increasing the strength of the gastric malaise alone enhanced Fos expression in central, basal, and lateral amygdala nuclei and decreased Fos expression in the nucleus accumbens core. Taste-malaise association induced specific Fos activation in the insular cortex (with both the low and the high doses of LiCl) and the nucleus accumbens shell (with the high LiCl dose only). No significant variation of Fos expression was measured in the perirhinal cortex. Several forebrain areas may be sites of taste-malaise convergence during CTA acquisition depending on the strength of the aversion.

Developmental regulation of cognitive abilities: modified composition of a molecular switch turns on associative learning

N-methyl-D-aspartate receptors (NMDARs) act as molecular coincidence detectors and allow for association or dissociation between pre- and postsynaptic neurons. NMDA receptors are central to remodeling of synaptic connections during postnatal development and associative learning abilities in adults. The ability to remodel neural networks is altered during postnatal development, possibly due to a change in the composition of NMDARs. That is, as forebrain systems (and cerebellum) develop, synaptic NR2B-containing NMDARs (NR2B-NMDARs) are replaced by NR2A-containing NMDARs (NR2A-NMDARs) and NR2B-NMDARs move to extrasynaptic sites. During the initial phase of the switch, synapses contain both NR2A- and NR2B-NMDARs and both long-term potentiation and long-term depression are enhanced. As NMDAR subunit expression decreases and NR2A-NMDARs come to predominate in the synapse, channel function and synaptic plasticity are reduced, and remodeling ability dissipates. The end result is a balance of plasticity and stability that is optimal for information processing and storage. Associative learning abilities involving different sensory modalities emerge sequentially, in accordance with synaptic maturation in related cortical and underlying brain structures. Thus, developmental alterations in NMDAR composition that occur at different ages in various brain structures may explain the protracted nature of the maturation of various associative learning abilities.