Amygdala and cortex relationships during learning of a sensory discrimination task.

During learning of a sensory discrimination task, cortical and sub-cortical regions display complex spatiotemporal dynamics. During learning, both amygdala and cortex link stimulus information to its appropriate association, for example a reward. In addition, both structures are also related to non-sensory parameters such as body movements and licking during the reward period. However, the emergence of cortico-amygdala relationships during learning is largely unknown. To study this, we combined wide-field cortical imaging with fiber photometry to simultaneously record cortico-amygdala population dynamics as male mice learn a whisker-dependent go/no-go task. We were able to simultaneously record neuronal populations from the posterior cortex and either the basolateral amygdala (BLA) or central medial amygdala (CEM). Prior to learning, somatosensory and associative cortex responded during sensation, while amygdala areas did not show significant responses. As mice became experts, amygdala responses emerged early during the sensation period, increasing in CEM, while decreasing in BLA. Interestingly, amygdala and cortical responses were associated with task-related body movement, displaying significant responses ∼200 ms before movement initiation which led to licking for the reward. A correlation analysis between cortex and amygdala revealed negative and positive correlation with BLA and CEM respectively, only in the expert case. These results imply that learning induces an involvement of cortex and amygdala which may aid to link sensory stimuli with appropriate associations.Significance Statement Levitan and Gilad study neuronal dynamics in cortex and amygdala as mice learn a sensory discrimination task. They find that amygdala areas display opposing responses only after the mice learn the task. In contrast cortical areas display responses both before and after learning. These results imply that learning induces an involvement of amygdala and cortical areas in order to optimally link sensory stimuli with appropriate associations.

Single and population coding of taste in the gustatory cortex of awake mice.

Electrophysiological analysis has revealed much about the broad coding and neural ensemble dynamics that characterize gustatory cortical (GC) taste processing in awake rats and about how these dynamics relate to behavior. With regard to mice, however, data concerning cortical taste coding have largely been restricted to imaging, a technique that reveals average levels of neural responsiveness but that (currently) lacks the temporal sensitivity necessary for evaluation of fast response dynamics; furthermore, the few extant studies have thus far failed to provide consensus on basic features of coding. We have recorded the spiking activity of ensembles of GC neurons while presenting representatives of the basic taste modalities (sweet, salty, sour, and bitter) to awake mice. Our first central result is the identification of similarities between rat and mouse taste processing: most mouse GC neurons (~66%) responded distinctly to multiple (3-4) tastes; temporal coding analyses further reveal, for the first time, that single mouse GC neurons sequentially code taste identity and palatability, the latter responses emerging ~0.5 s after the former, with whole GC ensembles transitioning suddenly and coherently from coding taste identity to coding taste palatability. The second finding is that spatial location plays very little role in any aspect of taste responses: neither between- (anterior-posterior) nor within-mouse (dorsal-ventral) mapping revealed anatomic regions with narrow or temporally simple taste responses. These data confirm recent results showing that mouse cortical taste responses are not "gustotopic" but also go beyond these imaging results to show that mice process tastes through time.NEW & NOTEWORTHY Here, we analyzed taste-related spiking activity in awake mouse gustatory cortical (GC) neural ensembles, revealing deep similarities between mouse cortical taste processing and that repeatedly demonstrated in rat: mouse GC ensembles code multiple aspects of taste in a coarse-coded, time-varying manner that is essentially invariant across the spatial extent of GC. These data demonstrate that, contrary to some reports, cortical network processing is distributed, rather than being separated out into spatial subregion.

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.

Memory Retrieval Has a Dynamic Influence on the Maintenance Mechanisms That Are Sensitive to ζ-Inhibitory Peptide (ZIP).

In neuroscientists' attempts to understand the long-term storage of memory, topics of particular importance and interest are the cellular and system mechanisms of maintenance (e.g., those sensitive to ζ-inhibitory peptide, ZIP) and those induced by memory retrieval (i.e., reconsolidation). Much is known about each of these processes in isolation, but less is known concerning how they interact. It is known that ZIP sensitivity and memory retrieval share at least some molecular targets (e.g., recycling α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, AMPA, receptors to the plasma membrane); conversely, the fact that sensitivity to ZIP emerges only after consolidation ends suggests that consolidation (and by extension reconsolidation) and maintenance might be mutually exclusive processes, the onset of one canceling the other. Here, we use conditioned taste aversion (CTA) in rats, a cortically dependent learning paradigm, to test this hypothesis. First, we demonstrate that ZIP infusions into gustatory cortex begin interfering with CTA memory 43-45 h after memory acquisition-after consolidation ends. Next, we show that a retrieval trial administered after this time point interrupts the ability of ZIP to induce amnesia and that ZIP's ability to induce amnesia is reengaged only 45 h after retrieval. This pattern of results suggests that memory retrieval and ZIP-sensitive maintenance mechanisms are mutually exclusive and that the progression from one to the other are similar after acquisition and retrieval. They also reveal concrete differences between ZIP-sensitive mechanisms induced by acquisition and retrieval: the latency with which ZIP-sensitive mechanisms are expressed differ for the two processes. SIGNIFICANCE STATEMENT: Memory retrieval and the molecular mechanisms that are sensitive to ζ-inhibitory peptide (ZIP) are the few manipulations that have been shown to effect memory maintenance. Although much is known about their effect on maintenance separately, it is unknown how they interact. Here, we describe a model for the interaction between memory retrieval and ZIP-sensitive mechanisms, showing that retrieval trials briefly (i.e., for 45 h) interrupt these mechanisms. ZIP sensitivity emerges across a similar time window after memory acquisition and retrieval; the maintenance mechanisms that follow acquisition and retrieval differ, however, in the latency with which the impact of ZIP is expressed.

Localization of molecular correlates of memory consolidation to buccal ganglia mechanoafferent neurons after learning that food is inedible in Aplysia.

Training paradigms affecting Aplysia withdrawal reflexes cause changes in gene expression leading to long-term memory formation in primary mechanoafferents that initiate withdrawal. Similar mechanoafferents are also found in the buccal ganglia that control feeding behavior, raising the possibility that these mechanoafferents are a locus of memory formation after a training paradigm affecting feeding. Buccal ganglia mechanoafferent neurons expressed increases in mRNA expression for the transcription factor ApC/EBP, and for the growth factor sensorin-A, within the first 2 h after training with an inedible food. No increases in expression were detected in the rest of the buccal ganglia. Increased ApC/EBP expression was not elicited by food and feeding responses not causing long-term memory. Increased ApC/EBP expression was directly related to a measure of the efficacy of training in causing long-term memory, suggesting that ApC/EBP expression is necessary for the expression of aspects of long-term memory. In behaving animals, memory is expressed as a decrease in the likelihood to respond to food, and a decrease in the amplitude of protraction, the first phase of consummatory feeding behaviors. To determine how changes in the properties of mechanoafferents could cause learned changes in feeding behavior, synaptic contacts were mapped from the mechanoafferents to the B31/B32 neurons, which have a key role in initiating consummatory behaviors and also control protractions. Many mechanoafferents monosynaptically and polysynaptically connect with B31/B32. Monosynaptic connections were complex combinations of fast and slow excitation and/or inhibition. Changes in the response of B31/B32 to stimuli sensed by the mechanoafferent could underlie aspects of long-term memory expression.

Consolidation and translation regulation.

mRNA translation, or protein synthesis, is a major component of the transformation of the genetic code into any cellular activity. This complicated, multistep process is divided into three phases: initiation, elongation, and termination. Initiation is the step at which the ribosome is recruited to the mRNA, and is regarded as the major rate-limiting step in translation, while elongation consists of the elongation of the polypeptide chain; both steps are frequent targets for regulation, which is defined as a change in the rate of translation of an mRNA per unit time. In the normal brain, control of translation is a key mechanism for regulation of memory and synaptic plasticity consolidation, i.e., the off-line processing of acquired information. These regulation processes may differ between different brain structures or neuronal populations. Moreover, dysregulation of translation leads to pathological brain function such as memory impairment. Both normal and abnormal function of the translation machinery is believed to lead to translational up-regulation or down-regulation of a subset of mRNAs. However, the identification of these newly synthesized proteins and determination of the rates of protein synthesis or degradation taking place in different neuronal types and compartments at different time points in the brain demand new proteomic methods and system biology approaches. Here, we discuss in detail the relationship between translation regulation and memory or synaptic plasticity consolidation while focusing on a model of cortical-dependent taste learning task and hippocampal-dependent plasticity. In addition, we describe a novel systems biology perspective to better describe consolidation.

A brief retraining regulates the persistence and lability of a long-term memory.

An experience extending the persistence of a memory after training Aplysia californica with inedible food also allows a consolidated memory to become sensitive to consolidation blockers. Long-term (24 h) memory is initiated by 5 min of training and is dependent on protein synthesis during the first few hours after training. By contrast, a more persistent (48 h) memory is dependent on a longer training session and on a later round of protein synthesis. When presented 24 h after training, a 3-min training that produces no memory alone can cause a memory that would have persisted for only 24 h to persist for 48 h. After a 48 h memory has been consolidated, 3 min of training also makes the memory sensitive to a protein-synthesis inhibitor. These findings suggest that a function of allowing a consolidated memory to become sensitive to blockers of protein synthesis may be to allow the memory to become more persistent.

Deletion of Stk11 and Fos in mouse BLA projection neurons alters intrinsic excitability and impairs formation of long-term aversive memory.

Conditioned taste aversion (CTA) is a form of one-trial learning dependent on basolateral amygdala projection neurons (BLApn). Its underlying cellular and molecular mechanisms remain poorly understood. RNAseq from BLApn identified changes in multiple candidate learning-related transcripts including the expected immediate early gene Fos and Stk11, a master kinase of the AMP-related kinase pathway with important roles in growth, metabolism and development, but not previously implicated in learning. Deletion of Stk11 in BLApn blocked memory prior to training, but not following it and increased neuronal excitability. Conversely, BLApn had reduced excitability following CTA. BLApn knockout of a second learning-related gene, Fos, also increased excitability and impaired learning. Independently increasing BLApn excitability chemogenetically during CTA also impaired memory. STK11 and C-FOS activation were independent of one another. These data suggest key roles for Stk11 and Fos in CTA long-term memory formation, dependent at least partly through convergent action on BLApn intrinsic excitability.

Training with inedible food in Aplysia causes expression of C/EBP in the buccal but not cerebral ganglion.

Training with inedible food in Aplysia increased expression of the transcription factor C/EBP in the buccal ganglia, which primarily have a motor function, but not in the cerebral or pleural ganglia. C/EBP mRNA increased immediately after training, as well as 1-2 h later. The increased expression of C/EBP protein lagged the increase in mRNA. Stimulating the lips and inducing feeding responses do not lead to long-term memory and did not cause increased C/EBP expression. Blocking polyADP-ribosylation, a process necessary for long-term memory after training, did not affect the increased C/EBP mRNA expression in the buccal ganglia.

New learning while consolidating memory during sleep is actively blocked by a protein synthesis dependent process.

Brief experiences while a memory is consolidated may capture the consolidation, perhaps producing a maladaptive memory, or may interrupt the consolidation. Since consolidation occurs during sleep, even fleeting experiences when animals are awakened may produce maladaptive long-term memory, or may interrupt consolidation. In a learning paradigm affecting Aplysia feeding, when animals were trained after being awakened from sleep, interactions between new experiences and consolidation were prevented by blocking long-term memory arising from the new experiences. Inhibiting protein synthesis eliminated the block and allowed even a brief, generally ineffective training to produce long-term memory. Memory formation depended on consolidative proteins already expressed before training. After effective training, long term memory required subsequent transcription and translation. Memory formation during the sleep phase was correlated with increased CREB1 transcription, but not CREB2 transcription. Increased C/EBP transcription was a correlate of both effective and ineffective training and of treatments not producing memory.

The differential role of cortical protein synthesis in taste memory formation and persistence.

The current dogma suggests that the formation of long-term memory (LTM) is dependent on protein synthesis but persistence of the memory trace is not. However, many of the studies examining the effect of protein synthesis inhibitors (PSIs) on LTM persistence were performed in the hippocampus, which is known to have a time-dependent role in memory storage, rather than the cortex, which is considered to be the main structure to store long-term memories. Here we studied the effect of PSIs on LTM formation and persistence in male Wistar Hola (n ≥ 5) rats by infusing the protein synthesis inhibitor, anisomycin (100 µg, 1 µl), into the gustatory cortex (GC) during LTM formation and persistence in conditioned taste aversion (CTA). We found that local anisomycin infusion to the GC before memory acquisition impaired LTM formation (P = 8.9E - 5), but had no effect on LTM persistence when infused 3 days post acquisition (P = 0.94). However, when we extended the time interval between treatment with anisomycin and testing from 3 days to 14 days, LTM persistence was enhanced (P = 0.01). The enhancement was on the background of stable and non-declining memory, and was not recapitulated by another amnesic agent, APV (10 µg, 1 µl), an N-methyl-d-aspartate receptor antagonist (P = 0.54). In conclusion, CTA LTM remains sensitive to the action of PSIs in the GC even 3 days following memory acquisition. This sensitivity is differentially expressed between the formation and persistence of LTM, suggesting that increased cortical protein synthesis promotes LTM formation, whereas decreased protein synthesis promotes LTM persistence.

Surgical management of gastric varices and morbid obesity: a novel approach.

BACKGROUND: Morbid obesity is a growing pandemic. The greater prevalence of chronic conditions such as diabetes, hypertension, and heart and liver disease has made management of obesity challenging. Many surgical techniques are in practice, each with some elements of restrictive or malabsorptive components. Nonalcoholic steatohepatitis can lead to portal hypertension, which can further manifest as upper gastrointestinal bleeding. METHODS: We performed sleeve gastrectomy at a nonuniversity tertiary care center, as a novel approach for the management of isolated gastric varices, in a morbidly obese cirrhotic patient. RESULTS: The operating time was 142 minutes. The estimated blood loss was 150 mL. The patient did not receive intraoperative or postoperative transfusions. The length of stay was prolonged to 10 days because of an ischemic cardiac event that was managed by coronary angioplasty on postoperative day 7. The patient did not develop any other complications. During the next couple of months, the patient lost significant weight and had no complaints. CONCLUSION: Sleeve gastrectomy with devascularization is a durable approach that will address the problems of both portal hypertension and morbid obesity, with the desired effect of weight reduction and treatment of gastric varices using a single surgical approach.

Long-term memory requires polyADP-ribosylation.

PolyADP-ribose-polymerase 1 is activated in neurons that mediate several forms of long-term memory in Aplysia. Because polyADP-ribosylation of nuclear proteins is a response to DNA damage in virtually all eukaryotic cells, it is surprising that activation of the polymerase occurs during learning and is required for long-term memory. We suggest that fast and transient decondensation of chromatin structure by polyADP-ribosylation enables the transcription needed to form long-term memory without strand breaks in DNA.