The molecular cascades of long-term potentiation underlie memory consolidation of one-trial avoidance in the CA1 region of the dorsal hippocampus, but not in the basolateral amygdala or the neocortex

PTPN2 dephosphorylates STAT3 to ameliorate anesthesia-induced cognitive decline in aged rats by altering the microglial phenotype and inhibiting inflammation

Perioperative neurocognitive disorders (PNDs) are common neurological complications after anesthesia in the elderly. Protein tyrosine phosphatase non-receptor type 2 (PTPN2) regulates signal transducer and activator of transcription protein 3 (STAT3) signaling to control inflammation in certain organs, but its role in PNDs remains unknown. Herein, we constructed a PND model in 18-month-old rats by treating them with sevoflurane. PND rats developed neuroinflammation, along with a significant decrease in PTPN2 expression and a rise in STAT3 phosphorylation in the hippocampus. Ptpn2 overexpression alleviated the behavioral disorders of PND rats, ameliorated neuronal injury, inhibited neuroinflammation, inflammasome activation, microglial activation, and microglial phenotype switching. Similar results were observed in sevoflurane-treated HMC3 microglia with PTPN2 overexpression, while PTPN2 silencing showed the opposite results. Additionally, PTPN2 seems to be a target of T-box transcription factor 2 (TBX2). These results contribute to the evidence supporting the idea that PTPN2 is a regulatory factor in PND progression.

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.

Using Postmeal Measures and Manipulations to Investigate Hippocampal Mnemonic Control of Eating Behavior

Episodic meal-related memories provide the brain with a powerful mechanism for tracking and controlling eating behavior because they contain a detailed record of recent energy intake that likely outlasts the physiological signals generated by feeding bouts. This review briefly summarizes evidence from human participants showing that episodic meal-related memory limits later eating behavior and then describes our research aimed at investigating whether hippocampal neurons mediate the inhibitory effects of meal-related memory on subsequent feeding. Our approach has been inspired by pioneering work conducted by Ivan Izquierdo and others who used posttraining manipulations to investigate memory consolidation. This review describes the rationale and value of posttraining manipulations, how Izquierdo used them to demonstrate that dorsal hippocampal (dHC) neurons are critical for memory consolidation, and how we have adapted this strategy to investigate whether dHC neurons are necessary for mnemonic control of energy intake. I describe our evidence showing that ingestion activates the molecular processes necessary for synaptic plasticity and memory during the early postprandial period, when the memory of the meal would be undergoing consolidation, and then summarize our findings showing that neural activity in dHC neurons is critical during the early postprandial period for limiting future intake. Collectively, our evidence supports the hypothesis that dHC neurons mediate the inhibitory effects of ingestion-related memory on future intake and demonstrates that post-experience memory modulation is not confined to artificial laboratory memory tasks.

Memory and eating: A bidirectional relationship implicated in obesity

This paper reviews evidence demonstrating a bidirectional relationship between memory and eating in humans and rodents. In humans, amnesia is associated with impaired processing of hunger and satiety cues, disrupted memory of recent meals, and overconsumption. In healthy participants, meal-related memory limits subsequent ingestive behavior and obesity is associated with impaired memory and disturbances in the hippocampus. Evidence from rodents suggests that dorsal hippocampal neural activity contributes to the ability of meal-related memory to control future intake, that endocrine and neuropeptide systems act in the ventral hippocampus to provide cues regarding energy status and regulate learned aspects of eating, and that consumption of hypercaloric diets and obesity disrupt these processes. Collectively, this evidence indicates that diet-induced obesity may be caused and/or maintained, at least in part, by a vicious cycle wherein excess intake disrupts hippocampal functioning, which further increases intake. This perspective may advance our understanding of how the brain controls eating, the neural mechanisms that contribute to eating-related disorders, and identify how to treat diet-induced obesity.

Long-lasting Postnatal Sensory Deprivation Alters Dendritic Morphology of Pyramidal Neurons in the Rat Hippocampus: Behavioral Correlates

The role of normal sensory inputs in the development of sensory cortices is well known, however, their impacts on the hippocampus, an integrator of sensory modalities with important roles in cognitive functions, has received much less attention. Here, we applied a long-term sensory deprivation paradigm by trimming the rats' whiskers bilaterally, from postnatal day 3 to 59. Female sensory-deprived (SD) rats showed more on-wall rearing and visits to the center of the open-field box, shorter periods of grooming, less defecation and less anxiety-like behaviors in the elevated plus-maze compared to controls, who had their intact whiskers brushed. Passive avoidance memory retention was sex-dependently impaired in the female SD rats. In the radial arm maze, however, reference spatial memory was impaired only in the male SD rats. Nonetheless, working memory errors increased in both sexes of SD rats. Besides depletion of CA1 and CA3 pyramidal neurons in SD rats, Sholl analysis of Golgi-Cox stained neurons revealed that prolonged sensory deprivation has retracted the arborization of CA1 basal dendrites in SD group, while solely female SD rats had diminished CA1 apical dendrites. Sholl analysis of CA3 neurons in SD animals also disclosed significantly more branched apical dendrites in males and basal dendrites in females. Sensory deprivation also led to a considerable spine loss and variation of different spine types in a sex-dependent manner. Our findings suggest that experience-dependent structural plasticity is capable of spreading far beyond the manipulated sensory zones and the inevitable functional alterations can be expressed in a multifactorial sex-dependent manner.

Ghrelin restores memory impairment following olfactory bulbectomy in mice by activating hippocampal NMDA1 and MAPK1 gene expression

Ghrelin (Ghrl) is an orexigenic peptide with potential roles in the modulation of anxiety- and depressive-like symptoms induced by bilateral olfactory bulbectomy (OB) in rodents. In the present work, we assessed whether intrahippocampal Ghrl could reverse OB-induced depressive-like and amnesic effects by regulating molecular mechanisms related to neuroplasticity. Adult female albino Swiss mice were divided into sham and OB groups, and infused with saline (S) or Ghrl 0.03 nmol/μl, 0.3 nmol/μl, or 3 nmol/μl into the hippocampus before exposition to open-field test (OFT) and tail suspension test (TST) or immediately after training in the object recognition test (ORT). After test phase in ORT, animals were euthanized and their hippocampi were dissected to study the expression of genes related to memory. The OB-S animals presented hyperlocomotion in OFT, increased immobility in TST and memory impairment compared to sham-S (p < 0.05), but acute intrahippocampal infusion of Ghrl 0.3 nmol/μl produced an improvement on these parameters in OB animals (p < 0.05). In addition, this dose of Ghrl reversed OB-induced low expression of NMDA1 and MAPK1 iso1 and up-regulated the expression of CaMKIIa iso1 and iso2, and MAPK1 iso2 (p < 0.05). These results extend the existing literature regarding OB-induced behavioral and neurochemical changes, and provide mechanisms that could underlie the antidepressant effect of Ghrl in this model.

"Special K" Drug on Adolescent Rats: Oxidative Damage and Neurobehavioral Impairments

Ketamine is used in clinical practice as an anesthetic that pharmacologically modulates neurotransmission in postsynaptic receptors, such as NMDA receptors. However, widespread recreational use of ketamine in "party drug" worldwide since the 1990s quickly spread to the Asian orient region. Thus, this study aimed at investigating the behavioral and oxidative effects after immediate withdrawal of intermittent administration of ketamine in adolescent female rats. For this, twenty female Wistar rats were randomly divided into two groups: control and ketamine group (n = 10/group). Animals received ketamine (10 mg/kg/day) or saline intraperitoneally for three consecutive days. Three hours after the last administration, animals were submitted to open field, elevated plus-maze, forced swim tests, and inhibitory avoidance paradigm. Twenty-four hours after behavioral tests, the blood and hippocampus were collected for the biochemical analyses. Superoxide dismutase, catalase, nitrite, and lipid peroxidation (LPO) were measured in the blood samples. Nitrite and LPO were measured in the hippocampus. The present findings demonstrate that the early hours of ketamine withdrawal induced oxidative biochemistry unbalance in the blood samples, with elevated levels of nitrite and LPO. In addition, we showed for the first time that ketamine withdrawal induced depressive- and anxiety-like profile, as well as short-term memory impairment in adolescent rodents. The neurobehavioral deficits were accompanied by the hippocampal nitrite and LPO-elevated levels.

L-carnitine prevents memory impairment induced by chronic REM-sleep deprivation

Sleep deprivation (SD) negatively impacts memory, which was related to oxidative stress induced damage. L-carnitine is a naturally occurring compound, synthesized endogenously in mammalian species and known to possess antioxidant properties. In this study, the effect of L-carnitine on learning and memory impairment induced by rapid eye movement sleep (REM-sleep) deprivation was investigated. REM-sleep deprivation was induced using modified multiple platform model (8h/day, for 6 weeks). Simultaneously, L-carnitine was administered (300mg/kg/day) intraperitoneally for 6 weeks. Thereafter, the radial arm water maze (RAWM) was used to assess spatial learning and memory. Additionally, the hippocampus levels of antioxidant biomarkers/enzymes: reduced glutathione (GSH), oxidized glutathione (GSSG), GSH/GSSG ratio, glutathione peroxidase (GPx), catalase, and superoxide dismutase (SOD) and thiobarbituric acid reactive substance (TBARS) were assessed. The results showed that chronic REM-sleep deprivation impaired both short- and long-term memory (P<0.05), whereas L-carnitine treatment protected against this effect. Furthermore, L-carnitine normalized chronic REM-sleep deprivation induced reduction in the hippocampus ratio of GSH/GSSG, activity of catalase, GPx, and SOD. No change was observed in TBARS among tested groups (P>0.05). In conclusion, chronic REM-sleep deprivation induced memory impairment, and treatment with L-carnitine prevented this impairment through normalizing antioxidant mechanisms in the hippocampus.

Satb2 Ablation Impairs Hippocampus-Based Long-Term Spatial Memory and Short-Term Working Memory and Immediate Early Genes (IEGs)-Mediated Hippocampal Synaptic Plasticity

Special AT-rich sequence-binding protein 2 (Satb2) is a protein binding to the matrix attachment regions of DNA and important for gene regulation. Patients with SATB2 mutation usually suffer moderate to severe mental retardation. However, the mechanisms for the defects of intellectual activities in patients with SATB2 mutation are largely unclear. Here we established the heterozygous Satb2 mutant mice and Satb2 conditional knockout mice to mimic the patients with SATB2 mutation and figured out the role of Satb2 in mental activities. We found that the spatial memory and working memory were significantly damaged in the heterozygous Satb2 mutant mice, early postnatal Satb2-deficient mice (CaMKIIα-Cre+Satb2fl/fl mice), and adult Satb2 ablation mice (Satb2fl/fl mice injected with CaMKIIα-Cre virus). Functionally, late phase long-term potentiation (L-LTP) in these Satb2 mutant mice was greatly impaired. Morphologically, in CA1 neurons of CaMKIIα-Cre+Satb2fl/fl mice, we found decreased spine density of the basal dendrites and less branches of apical dendrites that extended into lacunar molecular layer. Mechanistically, expression levels of immediate early genes (IEGs) including Fos, FosB, and Egr1 were significantly decreased after Satb2 deletion. And, Satb2 could regulate expression of FosB by binding to the promoter of FosB directly. In general, our study uncovers that Satb2 plays an important role in spatial memory and working memory by regulating IEGs-mediated hippocampal synaptic plasticity.

Effects of standardized Ginkgo biloba extract on the acquisition, retrieval and extinction of conditioned suppression: Evidence that short-term memory and long-term memory are differentially modulated

Studies in our laboratory have characterized the putative neuromodulatory effects of a standardized extract of the green leaves of Ginkgo biloba (EGb), which comprises a formulation of 24% ginkgo-flavoglycosides and 6% ginkgo-terpenoid lactones, on conditioned suppression. This model comprises a suitable animal model for investigating the behavioral changes and pharmacological mechanisms that underlie fear memory and anxiety. The characterization of the effects on distinct stages of fear memory or fear extinction will help illustrate both the beneficial and harmful effects. Three hundred adult male Wistar rats were randomly assigned to 30 groups according to the treatment as follows: i-ii) control groups (CS-US and CSno-US); iii) vehicle group (12% Tween®80); and iv-vi) EGb groups (250, 500 and 1000mgkg(-1)); or experimental procedures designed to assess the effects of EGb treatment prior to the acquisition (n=20 per group) and retrieval of conditioned fear (n=10 per group) or prior to the extinction training (n=10 per group) and extinction retention test (n=10 per group). Furthermore, to better understand the effects of acute EGb treatment on fear memory, we conducted two additional analyses: the acquisition of within- and between-session extinction of fear memory (short- and long-term memory, respectively). No difference was identified between the control and treatment groups during the retention test (P>0.05), with the exception of the CSno-US group in relation to all groups (P<0.05). A between-session analysis indicated that EGb at 250mgkg(-1) facilitated the acquisition of extinction fear memory, which was verified by the suppression ration in the first trial of extinction training (SR=0.39) and the extinction retention test session (SR=0.53, P<0.05), without impairments in fear memory acquisition, which were evaluated during the retention test (SR=0.79). Moreover, EGb administered at 1000mgkg(-1) prior to conditioning did not enhance the long-term extinction memory, i.e., it did not prevent the return of extinguished fear memory in the extinction retention test, in which the spontaneous recovery of fear was demonstrated (SR=0.63, P<0.05); however, it significantly facilitated short-term memory as verified by data from the within-session extinction (1 to 8-10 trials) during the retention test (SR=0.73 to SR=0.59; P<0.05) and the extinction retention test (SR=0.63 to SR=0.41; P<0.05). Moreover, spontaneous recovery was identified in response to a higher dose of EGb when administered prior to extinction training (SR=0.75, P<0.05) and the extinction retention test (SR=0.70; P<0.05). At dose of 500mgkg(-1) EGb reduced the suppression ratio when administered prior to the retention test (SR=0.57) and extinction training (SR=0.55; P<0.05) without preventing the acquisition of fear memory, which suggests that EGb has anti-anxiety effects. Taken together, the current findings suggest that EGb differentially modulates short- and long-term memory, as well as anxiety-like behavior. The actions of EGb may provide information regarding the beneficial effects in the prevention and treatment of neurocognitive impairments and anxiety disorders. Additional analyses are necessary to facilitate an understanding of these effects; however, previous data from our group suggest that GABAergic, serotoninergic and glutamatergic receptors are potential targets of the effects of EGb on conditioned suppression.

Effects of vitamin E on lead-induced impairments in hippocampal synaptic plasticity

Lead (Pb) exposure during development is associated with impaired cognitive function and long-term potentiation (LTP). Vitamin E (VE) is an antioxidant that could have protective effects against Pb intoxication. In this study, we examined the protective effects of vitamin E against Pb-induced LTP impairments. Forty-six adult male Wistar rats were randomly divided into 6 treatment groups: (1) control; (2) Pb exposure; (3) VE; (4) Pb +VE; (5) Pb exposure followed by VE 2 months after exposure; (6) VE followed by Pb exposure 1 month after treatment. Rats were exposed to Pb through daily consumption of Pb-contaminated distilled water; VE was administered by daily gavage for 3 months. After this period, the population spike (PS) amplitudes and the slopes of excitatory postsynaptic potentials (EPSPs) were measured in the dentate gyrus (DG) area of the hippocampus in adult rats in response to electrical stimulation applied to the perforant pathway in vivo. Blood samples were also collected to evaluate malondialdehyde (MDA) levels, total antioxidant capacity (TAC), and total oxidant status (TOS). Biochemical analyses demonstrated significant increases in plasma MDA and TOS levels in the Pb-exposed group compared to the control group. VE-protected groups revealed significant increases in TAC levels. Our results demonstrate that Pb decreased EPSP slopes and PS amplitudes compared to the control group, whereas VE increased these parameters compared to the control group. Co-administration of VE with Pb exposure inhibited Pb-induced effects. These findings suggest that VE via its antioxidant activity reverses Pb-induced impairments of synaptic plasticity in the DG.

Long-Term Dietary Alpha-Linolenic Acid Supplement Alleviates Cognitive Impairment Correlate with Activating Hippocampal CREB Signaling in Natural Aging Rats

Alpha-linolenic acid (ALA) is a major precursor of the essential n-3 polyunsaturated fatty acid (PUFA), whose deficiency alters the structure and function of membranes and induces cerebral dysfunctions. The major purpose of this study was to investigate the protective effect of prolonged ALA intake on cognitive function during natural aging. Female Sprague-Dawley rats aged 6 months were chronically treated with ALA and/or lard per day for 12 months. Regular diet-treated rats, both young and old (4 and 18 months old, respectively) served as controls. Rats fed on regular diet during aging showed memory deficits in Morris water maze, which were further exacerbated by lard intake. However, supplementation with ALA for 12 months dose-dependently improved the performance in spatial working memory tasks. Memory performance correlated well with the activation of cAMP response element-binding protein (CREB) and increases in both levels of brain-derived neurotrophic factor (BDNF) and its specific receptor tyrosine kinase B (TrkB) phosphorylation in the hippocampus. Further study identified that hippocampal extracellular signal-related kinase (ERK) and Akt rather than calcium calmodulin kinase IV (CaMKIV) and protein kinase A (PKA), the upstream signalings of CREB, were also activated by ALA supplement. Moreover, memory improvement was accompanied with alterations of hippocampal synaptic structure and number, suggestive of enhancement in synaptic plasticity. Together, these results suggest that long-term dietary intake of ALA enhances CREB/BDNF/TrkB pathway through the activation of ERK and Akt signalings in hippocampus, which contributes to its ameliorative effects on cognitive deficits in natural aging.

The Persistence of Hippocampal-Based Memory Requires Protein Synthesis Mediated by the Prion-like Protein CPEB3

Consolidation of long-term memories depends on de novo protein synthesis. Several translational regulators have been identified, and their contribution to the formation of memory has been assessed in the mouse hippocampus. None of them, however, has been implicated in the persistence of memory. Although persistence is a key feature of long-term memory, how this occurs, despite the rapid turnover of its molecular substrates, is poorly understood. Here we find that both memory storage and its underlying synaptic plasticity are mediated by the increase in level and in the aggregation of the prion-like translational regulator CPEB3 (cytoplasmic polyadenylation element-binding protein). Genetic ablation of CPEB3 impairs the maintenance of both hippocampal long-term potentiation and hippocampus-dependent spatial memory. We propose a model whereby persistence of long-term memory results from the assembly of CPEB3 into aggregates. These aggregates serve as functional prions and regulate local protein synthesis necessary for the maintenance of long-term memory.

Chronic ethanol exposure during adolescence through early adulthood in female rats induces emotional and memory deficits associated with morphological and molecular alterations in hippocampus

There is increasing evidence that heavy ethanol exposure in early life may produce long-lasting neurobehavioral consequences, since brain structural maturation continues until adolescence. It is well established that females are more susceptible to alcohol-induced neurotoxicity and that ethanol consumption is increasing among women, especially during adolescence. In the present study, we investigated whether chronic ethanol exposure during adolescence through early adulthood in female rats may induce hippocampal histological damage and neurobehavioral impairments. Female rats were treated with distilled water or ethanol (6.5 g/kg/day, 22.5% w/v) by gavage from the 35(th)-90(th) day of life. Ethanol-exposed animals displayed reduced exploration of the central area and increased number of fecal boluses in the open field test indicative of anxiogenic responses. Moreover, chronic high ethanol exposure during adolescence induced marked impairments on short-term memory of female rats addressed on social recognition and step-down inhibitory avoidance tasks. These neurobehavioral deficits induced by ethanol exposure during adolescence through early adulthood were accompanied by the reduction of hippocampal formation volume as well as the loss of neurons, astrocytes and microglia cells in the hippocampus. These results indicate that chronic high ethanol exposure during adolescence through early adulthood in female rats induces long-lasting emotional and memory deficits associated with morphological and molecular alterations in the hippocampus.

Evaluation of the effect of pentoxifylline on sleep-deprivation induced memory impairment

In this study, we examined the ability of Pentoxifylline (PTX) to prevent sleep deprivation induced memory impairment probably through decreasing oxidative stress. Sleep deprivation was chronically induced 8 h/day for 6 weeks in rats using modified multiple platform model. Concurrently, PTX (100 mg/kg) was administered to animals on daily basis. After 6 weeks of treatment, behavioral studies were conducted to test the spatial learning and memory using the Radial Arm Water Maze. Additionally, the hippocampus was dissected; and levels/activities of antioxidant defense biomarkers glutathione reduced (GSH), glutathione oxidized (GSSG), GSH/GSSG ratio, glutathione peroxidase (GPx), catalase, and superoxide dismutase (SOD), were assessed. The results show that chronic sleep deprivation impaired short- and long-term memories, which was prevented by chronic treatment with PTX. Additionally, PTX normalized sleep deprivation-induced reduction in the hippocampus GSH/GSSG ratio (P < 0.05), and activities of GPx, catalase, and SOD (P < 0.05). In conclusion, chronic sleep deprivation induces memory impairment, and treatment with PTX prevented this impairment probably through normalizing antioxidant mechanisms in the hippocampus.

Differential effects of fluoxetine and venlafaxine on memory recognition: possible mechanisms of action

Serotonin-specific reuptake inhibitors (SSRI) and serotonin-norepinephrine reuptake inhibitors (SNRI) are antidepressant drugs commonly used to treat a wide spectrum of mood disorders (Wong and Licinio, 2001). Although they have been clinically used for more than 50 years, the molecular and cellular basis for the action of SSRIs and SNRIs is not clear. Considering that the changes in gene expression involved in the action of antidepressant drugs on memory have not been identified, in this study we investigated the impact of chronic treatment with a SSRI (fluoxetine) and a SNRI (venlafaxine) on the mRNA expression of genes related to memory cascade in the mouse hippocampus, namely, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), nitric oxide synthase 1 (NOS1), neurotrophic tyrosine kinase receptor type 2 (TrKB), mitogen-activated protein kinases (MAPK/ERK) and serotonin transporter (SERT). Animals treated with fluoxetine 10 mg/Kg/day for 28 days showed a significant decrease in the percentage of time spent in the novel object recognition test (p≤0.005) and induced MAPK1/ERK2 down-regulation (p=0.005). Our results suggest that the effect on cognition could probably be explained by fluoxetine interference in the MAPK/ERK memory pathway. In contrast, chronic treatment with venlafaxine did not reduce MAPK1/ERK2 expression, suggesting that MAPK1/ERK2 down-regulation is not a common effect of all antidepressant drugs. Further studies are needed to examine the effect of chronic fluoxetine treatment on the ERK-CREB system, and to determine whether there is a causal relationship between the disruption of the ERK-CREB system and the effect of this antidepressant on memory performance.

GABA(A) receptor blockade enhances memory consolidation by increasing hippocampal BDNF levels

Memory consolidation is the process by which acquired information is converted to something concrete to be retrieved later. Here we examined a potential role for brain-derived neurotrophic factor (BDNF) in mediating the enhanced memory consolidation induced by the GABA(A) receptor antagonist, bicuculline methiodide. With the administration of an acquisition trial in naïve mice using a passive avoidance task, mature BDNF (mBDNF) levels were temporally changed in the hippocampal CA1 region, and the lowest levels were observed 9 h after the acquisition trial. In the passive avoidance task, bicuculline methiodide administration within 1 h of training but not after 3 h significantly increased latency time in the retention trial 24 h after the acquisition trial. Concomitantly, 1 h post-training administration of bicuculline methiodide, which enhanced memory consolidation, significantly increased mBDNF levels 9 h after training compared to those of the vehicle-treated control group. In addition, exogenous human recombinant BDNF (hrBDNF) administration 9 h after training into the hippocampal CA1 region facilitated memory consolidation confirming that the increase in mBDNF at around 9 h after training plays a key role in the enhancement of memory consolidation. Moreover, the increases in latency time and immediate early gene expressions by bicuculline methiodide or hrBDNF were significantly blocked by anisomycin, a protein synthesis inhibitor, K252a, a tyrosine receptor kinase (Trk) inhibitor, or anti-TrkB IgG. These findings suggest that the increase in the level of mBDNF and its function during a restricted time window after training are required for the enhancement of memory consolidation by GABA(A) receptor blockade.

Involvement of the ventral tegmental area in the inhibitory avoidance memory in rats

The ventral tegmental area (VTA) is a neural structure that sends strong efferent projections to the hippocampus. Output from the VTA can affect hippocampal-dependent neural processes that are critical for learning and memory, including long term potentiation and theta activity. However, no study to date has elucidated what role the VTA plays in mediating the different stages of learning and memory. Therefore, the current study was designed to assess how reversible inactivation of the VTA may affect the acquisition, consolidation and retrieval of memory in rats using an inhibitory avoidance (IA) task. In this experiment, rats with chronically implanted cannulae aimed at the VTA were trained on an inhibitory avoidance task. They received intra-VTA infusions of lidocaine or saline immediately before training, after training or before a memory retention test. The results indicate that inactivation of the VTA prior to the first acquisition session increased the number of trials rats required to reach the acquisition criterion. Similarly, inactivation of the VTA after acquisition training decreased the step-through latency and increased the amount of time spent in the dark compartment relative to the saline-treated group. However, inactivation of the VTA immediately prior to the memory retention test failed to alter either step-through latency or the amount of time spent in the dark compartment. Overall, these results suggest the VTA facilitates the acquisition and consolidation of IA learning and memory.