Early-life and chronic exposure to high-fat diet alters noradrenergic and glutamatergic neurotransmission in the male rat amygdala and hippocampus under cognitive challenges.

Childhood obesity increases the risk of health and cognitive disorders in adulthood. Consuming high-fat diets (HFD) during critical neurodevelopmental periods, like childhood, impairs cognition and memory in humans and animals, affecting the function and connectivity of brain structures related to emotional memory. However, the underlying mechanisms of such phenomena need to be better understood. This study aimed to investigate the neurochemical profile of the amygdala and hippocampus, brain structures involved in emotional memory, during the acquisition of conditioned odor aversion in male rats that consumed a HFD from weaning to adulthood. The rats gained weight, experienced metabolic changes, and reduced insulin sensitivity and glucose tolerance. Rats showed enhanced odor aversion memory, contrary to the expected cognitive impairments. This memory enhancement was accompanied by increased noradrenergic and glutamatergic neurotransmission in the amygdala and hippocampus. Importantly, this upregulation was specific to stimuli exposure, as basal neurotransmitter levels remained unaltered by the HFD. Our results suggest that HFD modifies cognitive function by altering neurochemical signaling, in this case, upregulating neurotransmitter levels rendering a stronger memory trace, demonstrating that metabolic dysfunctions do not only trigger exclusively detrimental plasticity processes but also render enhanced plastic effects depending on the type of information.

Synergistic photoactivation of VTA-catecholaminergic and BLA-glutamatergic projections induces long-term potentiation in the insular cortex.

The presentation of novel stimuli induces a reliable dopamine release in the insular cortex (IC) from the ventral tegmental area (VTA). The novel stimuli could be associated with motivational and emotional signals induced by cortical glutamate release from the basolateral amygdala (BLA). Dopamine and glutamate are essential for acquiring and maintaining behavioral tasks, including visual and taste recognition memories. In this study, we hypothesize that the simultaneous activation of dopaminergic and glutamatergic projections to the neocortex can underlie synaptic plasticity. High-frequency stimulation of the BLA-IC circuit has demonstrated a reliable long-term potentiation (LTP), a widely acknowledged synaptic plasticity that underlies memory consolidation. Therefore, the concurrent optogenetic stimulation of the insula's glutamatergic and dopaminergic terminal fibers would induce reliable LTP. Our results confirmed that combined photostimulation of the VTA and BLA projections to the IC induces a slow-onset LTP. We also found that optogenetically-induced LTP in the IC relies on both glutamatergic NMDA receptors and dopaminergic D1/D5 receptors, suggesting that the combined effects of these neurotransmitters can trigger synaptic plasticity in the neocortex. Overall, our findings provide compelling evidence supporting the essential role of both dopaminergic and glutamatergic projections in modulating synaptic plasticity within the IC. Furthermore, our results suggest that the synergistic actions of these projections have a pivotal influence on the formation of motivational memories.

Transforming experiences: Neurobiology of memory updating/editing.

Long-term memory is achieved through a consolidation process where structural and molecular changes integrate information into a stable memory. However, environmental conditions constantly change, and organisms must adapt their behavior by updating their memories, providing dynamic flexibility for adaptive responses. Consequently, novel stimulation/experiences can be integrated during memory retrieval; where consolidated memories are updated by a dynamic process after the appearance of a prediction error or by the exposure to new information, generating edited memories. This review will discuss the neurobiological systems involved in memory updating including recognition memory and emotional memories. In this regard, we will review the salient and emotional experiences that promote the gradual shifting from displeasure to pleasure (or vice versa), leading to hedonic or aversive responses, throughout memory updating. Finally, we will discuss evidence regarding memory updating and its potential clinical implication in drug addiction, phobias, and post-traumatic stress disorder.

Dopamine activity on the perceptual salience for recognition memory.

To survive, animals must recognize relevant stimuli and distinguish them from inconspicuous information. Usually, the properties of the stimuli, such as intensity, duration, frequency, and novelty, among others, determine the salience of the stimulus. However, previously learned experiences also facilitate the perception and processing of information to establish their salience. Here, we propose "perceptual salience" to define how memory mediates the integration of inconspicuous stimuli into a relevant memory trace without apparently altering the recognition of the physical attributes or valence, enabling the detection of stimuli changes in future encounters. The sense of familiarity is essential for successful recognition memory; in general, familiarization allows the transition of labeling a stimulus from the novel (salient) to the familiar (non-salient). The novel object recognition (NOR) and object location recognition (OLRM) memory paradigms represent experimental models of recognition memory that allow us to study the neurobiological mechanisms involved in episodic memory. The catecholaminergic system has been of vital interest due to its role in several aspects of recognition memory. This review will discuss the evidence that indicates changes in dopaminergic activity during exposure to novel objects or places, promoting the consolidation and persistence of memory. We will discuss the relationship between dopaminergic activity and perceptual salience of stimuli enabling learning and consolidation processes necessary for the novel-familiar transition. Finally, we will describe the effect of dopaminergic deregulation observed in some pathologies and its impact on recognition memory.

Voluntary physical activity improves spatial and recognition memory deficits induced by post-weaning chronic exposure to a high-fat diet.

Childhood and adolescent exposure to obesogenic environments has contributed to the development of several health disorders, including neurocognitive impairment. Adolescence is a critical neurodevelopmental window highly influenced by environmental factors that affect brain function until adulthood. Post-weaning chronic exposure to a high-fat diet (HFD) adversely affects memory performance; physical activity is one approach to coping with these dysfunctions. Previous studies indicate that voluntary exercise prevents HFD's detrimental effects on memory; however, it remains to evaluate whether it has a remedial/therapeutical effect when introduced after a long-term HFD exposure. This study was conducted on a diet-induced obesity mice model over six months. After three months of HFD exposure (without interrupting the diet) access to voluntary physical activity was provided. HFD produced weight gain, increased adiposity, and impaired glucose tolerance. Voluntary physical exercise ameliorated glucose tolerance and halted weight gain and fat accumulation. Additionally, physical activity mitigated HFD-induced spatial and recognition memory impairments. Our data indicate that voluntary physical exercise starting after several months of periadolescent HFD exposure reverses metabolic and cognitive alterations demonstrating that voluntary exercise, in addition to its known preventive effect, also has a restorative impact on metabolism and cognition dysfunctions associated with obesity.

Cognitive Impairment in Alzheimer's and Metabolic Diseases: A Catecholaminergic Hypothesis.

Catecholaminergic transmission plays an essential role in both physiological and pathological cognitive functions. Plastic changes subserving learning and memory processes are highly dependent on catecholaminergic activity, altering their function and impacting cognition. This review assesses changes in the dopaminergic and norepinephrine systems as part of the mechanisms underlying cognitive impairment in Alzheimer's disease as associated with metabolic dysfunctions such as type 2 diabetes, metabolic syndrome, and neuroinflammation and peripheral inflammation. Understanding the role of catecholaminergic systems in these conditions is relevant for identifying etiological factors that could advance diagnostic and therapeutic approaches for ameliorating cognitive alterations, disease onset, and progression.

Maintenance of conditioned place avoidance induced by gastric malaise requires NMDA activity within the ventral hippocampus.

It has been reported that during chemotherapy treatment, some patients can experience nausea before pharmacological administration, suggesting that contextual stimuli are associated with the nauseating effects. There are attempts to reproduce with animal models the conditions under which this phenomenon is observed to provide a useful paradigm for studying contextual aversion learning and the brain structures involved. This manuscript assessed the hippocampus involvement in acquiring and maintaining long-term conditioned place avoidance (CPA) induced by a gastric malaise-inducing agent, LiCl. Our results demonstrate that a reliable induction of CPA is possible after one acquisition trial. However, CPA establishment requires a 20-min confinement in the compartment associated with LiCl administration. Interestingly, both hippocampal regions seem to be necessary for CPA establishment; nonetheless, inactivation of the ventral hippocampus results in a reversion of avoidance and turns it into preference. Moreover, we demonstrate that activation of dorsal/ventral hippocampal NMDA receptors after CS-US association is required for long-term CPA memory maintenance.

Cortical neurochemical signaling of gustatory stimuli and their visceral consequences during the acquisition and consolidation of taste aversion memory.

The insular cortex (IC) has a crucial role in taste recognition memory, including conditioned taste aversion (CTA). CTA is a learning paradigm in which a novel taste stimulus (CS) is associated with gastric malaise (US), inducing aversion to the CS in future encounters. The role of the IC in CTA memory formation has been extensively studied. However, the functional significance of neurotransmitter release during the presentation of taste stimuli and gastric malaise-inducing agents remains unclear. Using microdialysis in free-moving animals, we evaluated simultaneous changes in glutamate, norepinephrine and dopamine release in response to the presentation of an innate appetitive or aversive gustatory novel stimulus, as well as after i.p. administration of isotonic or hypertonic gastric malaise-inducing solutions. Our results demonstrate that the presentation of novel stimuli, regardless of their innate valence, induces an elevation of norepinephrine and dopamine. Administration of a gastric malaise inducing agent (LiCl) promotes an elevation of glutamate regardless of its concentration. In comparison, norepinephrine release is related to the LiCl concentration and its equimolar NaCl control. Additionally, we evaluated their functional role on short and long-term taste aversion memory. Results indicate that the blockade of noradrenergic ß1,2 receptors in the IC spares CTA acquisition and memory consolidation. In contrast, blockade of dopamine D1/D5 receptors impaired CTA consolidation, whereas the NMDA receptor blockade impedes both acquisition and consolidation of CTA. These results suggest that dopaminergic and noradrenergic release are related to the salience of conditioned taste stimuli. However, only cortical D1/D5 dopaminergic activity, but not the noradrenergic ß1,2 activity, is involved in the acquisition and consolidation of taste memory formation. Additionally, glutamatergic activity signals visceral distress caused by LiCl administration and activates NMDA receptors necessary for the acquisition and consolidation of long-lasting taste aversion memory.

Catecholaminergic stimulation restores high-sucrose diet-induced hippocampal dysfunction.

Increasing evidence suggests that long-term consumption of high-caloric diets increases the risk of developing cognitive dysfunctions. In the present study, we assessed the catecholaminergic activity in the hippocampus as a modulatory mechanism that is altered in rats exposed to six months of a high-sucrose diet (HSD). Male Wistar rats fed with this diet developed a metabolic disorder and showed impaired spatial memory in both water maze and object location memory (OLM) tasks. Intrahippocampal free-movement microdialysis showed a diminished dopaminergic and noradrenergic response to object exploration during OLM acquisition compared to rats fed with normal diet. In addition, electrophysiological results revealed an impaired long-term potentiation (LTP) of the perforant to dentate gyrus pathway in rats exposed to a HSD. Local administration of nomifensine, a catecholaminergic reuptake inhibitor, prior to OLM acquisition or LTP induction, improved long-term memory and electrophysiological responses, respectively. These results suggest that chronic exposure to HSD induces a hippocampal deterioration which impacts on cognitive and neural plasticity events negatively; these impairments can be ameliorated by increasing or restituting the affected catecholaminergic activity.

Artificial taste avoidance memory induced by coactivation of NMDA and ß-adrenergic receptors in the amygdala.

The association between a taste and gastric malaise allows animals to avoid the ingestion of potentially toxic food. This association has been termed conditioned taste aversion (CTA) and relies on the activity of key brain structures such as the amygdala and the insular cortex. The establishment of this gustatory-avoidance memory is related to glutamatergic and noradrenergic activity within the amygdala during two crucial events: gastric malaise (unconditioned stimulus, US) and the post-acquisition spontaneous activity related to the association of both stimuli. To understand the functional implications of these neurochemical changes on avoidance memory formation, we assessed the effects of pharmacological stimulation of ß-adrenergic and glutamatergic NMDA receptors through the administration of a mixture of L-homocysteic acid and isoproterenol into the amygdala after saccharin exposure on specific times to emulate the US and post-acquisition local signals that would be occurring naturally under CTA training. Our results show that activation of NMDA and ß-adrenergic receptors generated a long-term avoidance response to saccharin, like a naturally induced rejection with LiCl. Moreover, the behavioral outcome was accompanied by changes in glutamate, norepinephrine and dopamine levels within the insular cortex, analogous to those displayed during memory retrieval of taste aversion memory. Therefore, we suggest that taste avoidance memory can be induced artificially through the emulation of specific amygdalar neurochemical signals, promoting changes in the amygdala-insular cortex circuit enabling memory establishment.

Early memory consolidation window enables drug induced state-dependent memory.

It is well established that newly acquired information is stabilized over time by processes underlying memory consolidation, these events can be impaired by many drug treatments administered shortly after learning. The consolidation hypothesis has been challenged by a memory integration hypothesis, which suggests that the processes underlying new memories are vulnerable to incorporation of the neurobiological alterations induced by amnesic drugs generating a state-dependent memory. The present experiments investigated the effects of amnesic drugs infused into the insular cortex of male Wistar rats on memory for object recognition training. The findings provide evidence that infusions of several amnesic agents including a protein synthesis inhibitor, an RNA synthesis inhibitor, or an NMDA receptor antagonist administered both after a specific period of time and before retrieval induce state-dependent recognition memory. Additionally, when amnesic drugs were infused outside the early consolidation window, there was amnesia, but the amnesia was not state-dependent. Data suggest that amnesic agents can induce state-dependent memory when administered during the early consolidation window and only if the duration of the drug effect is long enough to become integrated to the memory trace. In consequence, there are boundary conditions in order to induce state-dependent memory.

Differential requirement of de novo Arc protein synthesis in the insular cortex and the amygdala for safe and aversive taste long-term memory formation.

Several immediate early genes products are known to be involved in the facilitation of structural and functional modifications at distinct synapses activated through experience. The IEG-encoded protein Arc (activity regulated cytoskeletal-associated protein) has been widely implicated in long-term memory formation and stabilization. In this study, we sought to evaluate a possible role for de novo Arc protein synthesis in the insular cortex (IC) and in the amygdala (AMY) during long-term taste memory formation. We found that acute inhibition of Arc protein synthesis through the infusion of antisense oligonucleotides administered in the IC before a novel taste presentation, affected consolidation of a safe taste memory trace (ST) but spared consolidation of conditioned taste aversion (CTA). Conversely, blocking Arc synthesis within the AMY impaired CTA consolidation but had no effect on ST long-term memory formation. Our results suggest that Arc-dependent plasticity during taste learning is required within distinct structures of the medial temporal lobe, depending on the emotional valence of the memory trace.

Memory trace reactivation and behavioral response during retrieval are differentially modulated by amygdalar glutamate receptors activity: interaction between amygdala and insular cortex.

The insular cortex (IC) is required for conditioned taste aversion (CTA) retrieval. However, it remains unknown which cortical neurotransmitters levels are modified upon CTA retrieval. Using in vivo microdialysis, we observed that there were clear elevations in extracellular glutamate, norepinephrine, and dopamine in and around the center of the gustatory zone of the IC during CTA retrieval. Additionally, it has been reported that the amygdala-IC interaction is highly involved in CTA memory establishment. Therefore, we evaluated the effects of infusions of an AMPA receptor antagonist (CNQX) and a NMDA receptor antagonist (APV) into the amygdala on CTA retrieval and IC neurotransmitter levels. Infusion of APV into the amygdala impaired glutamate augmentation within the IC, whereas dopamine and norepinephrine levels augmentation persisted and a reliable CTA expression was observed. Conversely, CNQX infusion into the amygdala impaired the aversion response, as well as norepinephrine and dopamine augmentations in the IC. Interestingly, CNQX infusion did not affect glutamate elevation in the IC. To evaluate the functional meaning of neurotransmitters elevations within the IC on CTA response, we infused specific antagonists for the AMPA, NMDA, D1, and ß-adrenergic receptor before retrieval. Results showed that activation of AMPA, D1, and ß-adrenergic receptors is necessary for CTA expression, whereas NMDA receptors are not involved in the aversion response.

Neural ablation of the PARK10 candidate Plpp3 leads to dopaminergic transmission deficits without neurodegeneration.

Parkinson's disease (PD) is a multifactorial neurodegenerative disorder, characterised by the progressive loss of midbrain dopaminergic neurons and a variety of motor symptoms. The gene coding for the phospholipid phosphatase 3, PLPP3 (formerly PPAP2B or LPP3), maps within the PARK10 locus, a region that has been linked with increased risk to late-onset PD. PLPP3 modulates the levels of a range of bioactive lipids controlling fundamental cellular processes within the central nervous system. Here we show that PLPP3 is enriched in astroglial cells of the adult murine ventral midbrain. Conditional inactivation of Plpp3 using a Nestin::Cre driver results in reduced mesencephalic levels of sphingosine-1-phosphate receptor 1 (S1P1), a well-known mediator of pro-survival responses. Yet, adult PLPP3-deficient mice exhibited no alterations in the number of dopaminergic neurons or in the basal levels of striatal extracellular dopamine (DA). Potassium-evoked DA overflow in the striatum, however, was significantly decreased in mutant mice. Locomotor evaluation revealed that, although PLPP3-deficient mice exhibit motor impairment, this is not progressive or responsive to acute L-DOPA therapy. These findings suggest that disruption of Plpp3 during early neural development leads to dopaminergic transmission deficits in the absence of nigrostriatal degeneration, and without causing an age-related locomotor decline consistent with PD.

Differential involvement of glutamatergic and catecholaminergic activity within the amygdala during taste aversion retrieval on memory expression and updating.

During memory retrieval, consolidated memories are expressed and destabilized in order to maintain or update information through a memory reconsolidation process. Despite the key role of the amygdala during memory acquistion and consolidation, the participation of neurotransmitter signals in memory retrieval is poorly understood. Hence, we used conditioned taste aversion and in vivo microdialysis to evaluate changes in glutamate, norepinephrine and dopamine concentrations within the amygdala during memory retrieval. We observed that exposure to an aversive-conditioned stimulus induced an augmentation in glutamate, norepinephrine and dopamine levels within the amygdala, while exposure to a familiar and safe stimulus did not induce changes in these neurotransmitters levels. Also, we evaluated the amygdalar blockade of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), N-methyl-d-aspartate (NMDA), ß-adrenergic and dopamine D1 receptors in memory retrieval and updating. Results showed that during retrieval, behavioural expression was impaired by intra-amygdalar blockade of AMPA and ß-adrenergic receptors, whereas NMDA, D1 and ß-adrenergic receptors blockade hindered memory updating. In summary, during conditioned taste aversion retrieval there was an increase in the extracellular levels of glutamate, norepinephrine and dopamine within the amygdala, and their receptors activity were differentially involved in the behavioural expression and memory updating during retrieval.

Post-acquisition release of glutamate and norepinephrine in the amygdala is involved in taste-aversion memory consolidation.

Amygdala activity mediates the acquisition and consolidation of emotional experiences; we have recently shown that post-acquisition reactivation of this structure is necessary for the long-term storage of conditioned taste aversion (CTA). However, the specific neurotransmitters involved in such reactivation are not known. The aim of the present study was to investigate extracellular changes of glutamate, norepinephrine, and dopamine within the rat amygdala using in vivo microdialysis during the acquisition and 1-h post-acquisition of CTA paradigm. Microdialysis monitoring showed a significant norepinephrine increase related to novel taste exposure and a glutamate increase after gastric malaise induction by i.p. LiCl administration. Interestingly, we found a spontaneous concomitant increase of glutamate and norepinephrine, but not dopamine, 45 min after conditioning, suggesting the presence of aversive learning-dependent post-acquisition signals in the amygdala. These signals seem to be involved in CTA consolidation process, since post-trial blockade of N-methyl-D-aspartate or ß-adrenergic receptors impaired long- but not short-term memory. These data suggest that CTA long-term storage involves post-acquisition release of glutamate and norepinephrine in the amygdala.

Off-line concomitant release of dopamine and glutamate involvement in taste memory consolidation.

It has been postulated that memory consolidation process requires post-learning molecular changes that will support long-term experiences. In the present study, we assessed with in vivo microdialysis and capillary electrophoresis whether such changes involve the release of neurotransmitters at post-acquisition stages. Using conditioned taste aversion paradigm we observed spontaneous off-line (i.e. in absence of stimulation) dopamine and glutamate reactivation within the insular cortex about 45 min after the stimuli association. These increments did not appear in control groups that were unable to acquire the task, and it seems to be dependent on amygdala activity since its reversible inactivation by tetrodotoxin impaired cortical off-line release of both neurotransmitters and memory consolidation. In addition, blockade of dopaminergic D1 and/or NMDA receptors before the off-line activity impaired long- but not short-term memory. These results suggest that off-line extracellular increments of glutamate and dopamine have a significant functional role in consolidation of taste memory.