Role of ventral hippocampus in acquisition, consolidation and retrieval of rat's passive avoidance response memory trace

Royal jelly improves learning and memory deficits in an amyloid β-induced model of Alzheimer's disease in male rats: Involvement of oxidative stress

Alzheimer's disease (AD) as the commonest type of dementia is associated with the cognitive function failure. Oxidative stress performs an essential role in the progression of AD. Royal jelly (RJ) is a natural product of bees with antioxidant and anti-inflammatory properties. The present research aimed to investigate the possible protective effect of RJ on learning and memory in a rat model of Aβ-induced AD. Forty male adult Wistar rats were equally distributed into five groups: control, sham-operated, Aβ (receiving intracerebroventricular (ICV) injection of amyloid beta (Aβ1-40)), Aβ + RJ 50 mg/kg, and Aβ + RJ 100 mg/kg. RJ was administered daily post-surgery by oral gavage for four weeks. Behavioral learning and memory were examined using the novel object recognition (NOR) and passive avoidance learning (PAL) tests. Also, oxidative stress markers, such as malondialdehyde (MDA), total oxidant status (TOS) and total antioxidant capacity (TAC), were assessed in the hippocampus. Aβ reduced step-through latency (STLr) and increased time spent in the dark compartment (TDC) in the PAL task and also decreased discrimination index in the NOR test. Administration of RJ ameliorated the Aβ-related memory impairment in both NOR and PAL tasks. Aβ decreased TAC and increased MDA and TOS levels in the hippocampus, whereas RJ administration reversed these Aβ-induced alterations. Our results indicated that RJ has the potential to ameliorate learning and memory impairment in the Aβ model of AD via attenuating oxidative stress.

RTP801/REDD1 Is Involved in Neuroinflammation and Modulates Cognitive Dysfunction in Huntington's Disease

RTP801/REDD1 is a stress-regulated protein whose levels are increased in several neurodegenerative diseases such as Parkinson's, Alzheimer's, and Huntington's diseases (HD). RTP801 downregulation ameliorates behavioral abnormalities in several mouse models of these disorders. In HD, RTP801 mediates mutant huntingtin (mhtt) toxicity in in vitro models and its levels are increased in human iPSCs, human postmortem putamen samples, and in striatal synaptosomes from mouse models of the disease. Here, we investigated the role of RTP801 in the hippocampal pathophysiology of HD. We found that RTP801 levels are increased in the hippocampus of HD patients in correlation with gliosis markers. Although RTP801 expression is not altered in the hippocampus of the R6/1 mouse model of HD, neuronal RTP801 silencing in the dorsal hippocampus with shRNA containing AAV particles ameliorates cognitive alterations. This recovery is associated with a partial rescue of synaptic markers and with a reduction in inflammatory events, especially microgliosis. Altogether, our results indicate that RTP801 could be a marker of hippocampal neuroinflammation in HD patients and a promising therapeutic target of the disease.

The nucleus reuniens, a thalamic relay for cortico-hippocampal interaction in recent and remote memory consolidation

The consolidation of declarative memories is believed to occur mostly during sleep and involves a dialogue between two brain regions, the hippocampus and the medial prefrontal cortex. The information encoded during experience by neuronal assemblies is replayed during sleep leading to the progressive strengthening and integration of the memory trace in the prefrontal cortex. The gradual transfer of information from the hippocampus to the medial prefrontal cortex for long-term storage requires the synchronization of cortico-hippocampal networks by different oscillations, like ripples, spindles, and slow oscillations. Recent studies suggest the involvement of a third partner, the nucleus reuniens, in memory consolidation. Its bidirectional connections with the hippocampus and medial prefrontal cortex place the reuniens in a key position to relay information between the two structures. Indeed, many topical works reveal the original role that the nucleus reuniens occupies in different recent and remote memories consolidation. This review aimed to examine these contributions, as well as its functional embedment in this complex memory network, and provide some insights on the possible mechanisms.

The ventral hippocampus CA3 is critical in regulating timing uncertainty in temporal decision-making

Timing uncertainty is a critical component of temporal decision-making, as it determines the decision strategies that maximize reward rate. However, little is known about the biological substrates of timing uncertainty. In this study, we report that the CA3 subregion of the ventral hippocampus (vCA3), a relatively unexplored area in timing, is critical in regulating timing uncertainty that informs temporal decision making. Using a variant of the differential reinforcement of low rates of responding (DRL) task that incorporates differential levels of approach-avoidance conflict, rats were trained to wait a minimum of 6 s to earn a reward that was paired with varying durations of foot shock. Post-training chemogenetic inhibition of the vCA3 reduced timing uncertainty without affecting mean wait times, irrespective of the level of conflict experienced. Simulations based on the information-processing variant of scalar expectancy theory (SET) revealed that the vCA3 may be important in modulating decision threshold or switch closure latency variability.

Protein Hydrolysate of Silkworm Pupa Prevents Memory Impairment Induced by Oxidative Stress in Scopolamine-Induced Mice via Modulating the Cholinergic Nervous System and Antioxidant Defense System

Silkworm pupae (Bombyx mori) is an edible insect that has been reported to contain high-quality proteins, lipids, minerals, and vitamins, and to possess high antioxidant activity. However, there have been no studies on the neuroprotective effects of silkworm pupae. Therefore, we investigated a water extract of silkworm pupae with protease (WSP) as a functional and therapeutic candidate for neurodegenerative disorders. First, we evaluated the effect of WSP on oxidative stress-induced mouse hippocampal neuronal cells (HT-22 cells). Cell viability diminished by addition of glutamate but was significantly recovered by WSP treatment. Furthermore, WSP significantly decreased the release of lactate dehydrogenase and generation of intracellular reactive oxygen species in oxidative stress-induced cells. In addition, in scopolamine-treated mice, WSP attenuated memory impairment, as demonstrated in the Morris water maze and passive avoidance tests, indicating protection of neuronal cells against oxidative damage. Moreover, WSP prevented scopolamine-induced increases in acetylcholinesterase activity and decreases in choline-acetyltransferase activity. Finally, treatment with WSP enhanced the antioxidant defense system by regulating the activities of antioxidant enzymes. Overall, this study showed that WSP exerted antioxidant and memory enhancing action against oxidative stress.

Neuroprotective Activity of Polyphenol-Rich Ribes diacanthum Pall against Oxidative Stress in Glutamate-Stimulated HT-22 Cells and a Scopolamine-Induced Amnesia Animal Model

Ribes diacanthum Pall, a native Mongolian medicinal plant, has been reported to show antioxidant activities due to its polyphenol and flavonoid content, and is especially rich in the ethyl acetate fraction from an 80% methanol extraction (RDP). We assessed the cytoprotective effect of RDP on glutamate-caused oxidative stress and apoptosis in mouse hippocampal neuronal cells (HT-22 cells). Cell viability was significantly recovered by RDP treatment. Also, RDP effectively decreased the glutamate-induced production of intracellular reactive oxygen species (ROS). In flow cytometric analysis, apoptotic cells and the mitochondrial membrane potential were suppressed by RDP. In the Western blotting analysis, we found that RDP not only decreased the release of apoptotic proteins but also recovered anti-apoptotic protein. Additionally, RDP enhanced the antioxidant defense system by regulating the expression of antioxidant enzymes. Furthermore, treatment with RDP activated the BDNF/TrkB pathway. In accordance with the in vitro results, RDP meliorated memory deficit by defending hippocampal neuronal cells against oxidative damage in scopolamine-injected mice. Taken together, our present study showed that RDP exerted antioxidant and neuroprotective actions against oxidative stress. Therefore, RDP might facilitate the development of candidates for functional health foods for neurodegenerative disorders.

The Antidepressant-Like Effects of Hesperidin in Streptozotocin-Induced Diabetic Rats by Activating Nrf2/ARE/Glyoxalase 1 Pathway

The co-occurrence of diabetes and depression is a challenging and underrecognized clinical problem. Alpha-carbonyl aldehydes and their detoxifying enzyme glyoxalase 1 (Glo-1) play vital roles in the pathogenesis of diabetic complications, including depression. Hesperidin, a naturally occurring flavanone glycoside, possesses numerous pharmacological properties, but neuroprotection by hesperidin in depression-like behaviors in diabetes was not observed. This study aimed to investigate the mechanisms and signaling pathways by which hesperidin regulates depression-like behaviors in diabetic rats and to identify potential targets of hesperidin. Rats with streptozotocin-induced diabetes were treated orally with hesperidin (50 and 150 mg/kg) or the nuclear factor erythroid 2-related factor 2 (Nrf2) inducer tert-butylhydroquinone (TBHQ, 25 mg/kg) for 10 weeks. After behavioral test, the brains were collected to evaluate the effects of hesperidin on Glo-1, Nrf2, protein glycation, and oxidative stress. Hesperidin showed antidepressant and anxiolytic effects in diabetic rats, as evidenced by the decreased immobility time in the forced swimming test, increased time spent in the center area of the open field test, and increased percentage of open-arm entries and time spent in the open arms in the elevated plus maze, as well as by the enhancement of Glo-1 and the inhibition of the AGEs/RAGE axis and oxidative stress in the brain. In addition, hesperidin caused significant increases in the Nrf2 levels and upregulated γ-glutamylcysteine synthetase, a well-known target gene of Nrf2/ARE signaling. In vitro, the effects of hesperidin on N2a cell injury caused by high glucose (HG) was assessed by MTT and LDH, and the effects on Nrf2 signaling were also assessed. We found that the Nrf2 inhibitor ML385 reversed the protective effects of hesperidin on the cell injury induced by HG. Hesperidin prevented the HG-induced reduction in the Nrf2 and Glo-1 levels, and ML385 reversed the effects of hesperidin on the expression of the proteins mentioned above, indicating that Nrf2 signaling is involved in the hesperidin-induced neuroprotective effects. Our findings indicate that the effects of hesperidin on ameliorating the depression- and anxiety-like behaviors of diabetic rats, which are mediated by the enhancement of Glo-1, may be due to the activation of the Nrf2/ARE pathway.

Enteromorpha prolifera Extract Improves Memory in Scopolamine-Treated Mice via Downregulating Amyloid-β Expression and Upregulating BDNF/TrkB Pathway

Enteromorpha prolifera, a green alga, has long been used in food diets as well as traditional remedies in East Asia. Our preliminary study demonstrated that an ethyl acetate extract of Enteromorpha prolifera (EAEP) exhibited the strongest antioxidant activity compared to ethanol or water extracts. Nonetheless, there has been no report on the effect of EAEP on memory impairment due to oxidative damage. This study investigated whether EAEP could attenuate memory deficits in an oxidative stress-induced mouse model. EAEP was orally administered (50 or 100 mg/kg body weight (b.w.)) to mice and then scopolamine was administered. The oral administration of EAEP at 100 mg/kg b.w. significantly restored memory impairments induced by scopolamine, as evaluated by the Morris water maze test, and the passive avoidance test. Further, EAEP upregulated the protein expression of BDNF, p-CREB, p-TrkB, and p-Akt. Moreover, EAEP downregulated the expression of amyloid-β, tau, and APP. The regulation of cholinergic marker enzyme activities and the protection of neuronal cells from oxidative stress-induced cell death in the brain of mice via the downregulation of amyloid-β and the upregulation of the BDNF/TrkB pathway by EAEP suggest its potential as a pharmaceutical candidate to prevent neurodegenerative diseases.

Activation of TRPV1 receptors affects memory function and hippocampal TRPV1 and CREB mRNA expression in a rat model of biliary cirrhosis

OBJECTIVE

Memory impairment induced by biliary cirrhosis is associated with abnormalities in the function of different neurotransmitter systems. However, the exact molecular mechanisms involved in the learning and memory dysfunctions following biliary cirrhosis is largely unknown. This study set out to determine whether activation of transient receptor potential vanilloid type 1 (TRPV1) in the CA1 area of the hippocampus in rats improve memory impairment induced by biliary cirrhosis.

METHODS

To assess learning and memory, passive avoidance task was carried out using a shuttle box. The mRNA expression of TRPV1 and cAMP response element binding (CREB) protein in the hippocampus were also evaluated by qT-PCR.

RESULTS

Our results indicated that activation of TRPV1 channels by capsaicin significantly decreased memory impairment and increased mRNA expression of the TRPV1 and CREB in the hippocampus of rats with biliary cirrhosis. Our findings also demonstrated that a positive correlation existed between mRNA expression of TRPV1 and CREB, and between memory function and TRPV1 expression.

DISCUSSION

Taken together, the results of this study support the view that TRPV1 receptor may play an important role in the regulation of learning and memory functions, and suggest that activation of TRPV1 channels seems to be a promising therapeutic target for learning and memory impairments following biliary cirrhosis.

Curcumin ameliorates scopolamine-induced mice memory retrieval deficit and restores hippocampal p-Akt and p-GSK-3β

The loss of cholinergic neurons has been a major issue in researches on Alzheimer's disease (AD) for about 40 years. Therefore, the scopolamine model of amnesia has been widely used in AD researches. Recently, it was reported that the early stage amnesia of AD is related to memory retrieval deficit. Curcumin, as the main ingredient of turmeric, has been suggested to decrease the prevalence of AD in human population. This study was conducted to assess if curcumin prevents retrieval deficit induced by scopolamine in passive avoidance task. Moreover, according to the proposed link between cholinergic system and Akt/GSK-3β (Glycogen synthase kinase 3 beta) signaling, the hippocampal contents of these proteins were determined. Male NMRI mice (20-25 g body weight) were treated with 50 or 100 mg/kg/po curcumin or its vehicle for 10 days. On day 10, the animals were trained in passive avoidance apparatus. The retention trial was performed 24 h later. Scopolamine (1 mg/kg/i.p.) or its vehicle was administered 30 min before retention test. At the completion of behavioral studies, the hippocampi were removed and western blot analysis was performed to determine hippocampal phosphorylated and total Akt and GSK-3β and beta actin contents. The results showed that curcumin treatment at 50 and 100 mg/kg doses prevented scopolamine-induced memory retrieval deficit and restored Akt and GSK dephosphorylation caused by scopolamine. Overall, these findings showed that pre-test scopolamine administration disrupts memory retrieval along with the diminished Akt and GSK-3β phosphorylation in hippocampus while curcumin administration prevented those changes.

Generalization of Conditioned Auditory Fear is Regulated by Maternal Effects on Ventral Hippocampal Synaptic Plasticity

Maternal care shapes individual differences in fear-associated neural circuitry. In rats, maternal licking and grooming (LG) in early life regulates ventral hippocampal (VH) function and plasticity in adulthood, but its consequent effect on the regulation of fear memories remains unknown. We report an effect of maternal care on generalization of learned fear, such that offspring of high LG mothers express generalized fear responses when confronted with neutral stimuli following auditory fear conditioning. These animals simultaneously display a reduction in the magnitude of VH long-term potentiation (LTP) expressed and reduced input-output transformation in Schaffer collateral synapses. Inhibition of VH-LTP during learning specifically increases fear generalization in offspring of low LG mothers during recall, suggesting a role for VH synaptic plasticity in the specification of fear memories. These findings suggest that rearing by low LG dams enhances the efficacy of fear-related neural systems to support accurate encoding of fear memories through effects on the VH.

Detrimental Effects of Helium Ion Irradiation on Cognitive Performance and Cortical Levels of MAP-2 in B6D2F1 Mice

The space radiation environment includes helium (⁴He) ions that may impact brain function. As little is known about the effects of exposures to ⁴He ions on the brain, we assessed the behavioral and cognitive performance of C57BL/6J × DBA2/J F1 (B6D2F1) mice three months following irradiation with ⁴He ions (250 MeV/n; linear energy transfer (LET) = 1.6 keV/μm; 0, 21, 42 or 168 cGy). Sham-irradiated mice and mice irradiated with 21 or 168 cGy showed novel object recognition, but mice irradiated with 42 cGy did not. In the passive avoidance test, mice received a slight foot shock in a dark compartment, and latency to re-enter that compartment was assessed 24 h later. Sham-irradiated mice and mice irradiated with 21 or 42 cGy showed a higher latency on Day 2 than Day 1, but the latency to enter the dark compartment in mice irradiated with 168 cGy was comparable on both days. ⁴He ion irradiation, at 42 and 168 cGy, reduced the levels of the dendritic marker microtubule-associated protein-2 (MAP-2) in the cortex. There was an effect of radiation on apolipoprotein E (apoE) levels in the hippocampus and cortex, with higher apoE levels in mice irradiated at 42 cGy than 168 cGy and a trend towards higher apoE levels in mice irradiated at 21 than 168 cGy. In addition, in the hippocampus, there was a trend towards a negative correlation between MAP-2 and apoE levels. While reduced levels of MAP-2 in the cortex might have contributed to the altered performance in the passive avoidance test, it does not seem sufficient to do so. The higher hippocampal and cortical apoE levels in mice irradiated at 42 than 168 cGy might have served as a compensatory protective response preserving their passive avoidance memory. Thus, there were no alterations in behavioral performance in the open filed or depressive-like behavior in the forced swim test, while cognitive impairments were seen in the object recognition and passive avoidance tests, but not in the contextual or cued fear conditioning tests. Taken together, the results indicate that some aspects of cognitive performance are altered in male mice exposed to ⁴He ions, but that the response is task-dependent. Furthermore, the sensitive doses can vary within each task in a non-linear fashion. This highlights the importance of assessing the cognitive and behavioral effects of charged particle exposure with a variety of assays and at multiple doses, given the possibility that lower doses may be more damaging due to the absence of induced compensatory mechanisms at higher doses.

Two Behavioral Tests Allow a Better Correlation Between Cognitive Function and Expression of Synaptic Proteins

The molecular substrate of age-associated cognitive decline (AACD) is still elusive. Evidence indicates that AACD is related to synaptic impairment in hippocampus, but different hippocampal regions play different roles, with the dorsal hippocampus (DH) associated to spatial learning, and the ventral hippocampus (VH) crucial for emotionality. If changes in hippocampal function contributes to AACD, this contribution may be reflected in alterations of synaptic protein levels. A commonly used approach to investigate this issue is western blotting. When this technique is applied to the entire hippocampus and the cognitive impairment is evaluated by a single task, changes in expression of a protein might undergo a "dilution effect", as they may occur only in a given hippocampal region. We show that two behavioral tests yield more accurate results than one test in evaluating the function of the whole rat hippocampus by studying the expression of synaptotagmin 1 (SYT1), a vesicular protein whose expression in aged hippocampus is reportedly inconsistent. Analysis of SYT1 levels in the whole hippocampus of rats selected by the Morris water maze (MWM) test only failed to highlight a difference, whereas analysis of SYT1 levels in the whole hippocampus of rats categorized by both the MWM and the step-through passive avoidance (STPA) tests demonstrated a significant increase of SYT1 level in impaired rats. These findings, besides showing that SYT1 increases in impaired aged rats, suggest that using the whole hippocampus in blotting studies may prevent false negative results only if animals are categorized with tests exploring both DH and VH.

Scopolamine-induced passive avoidance memory retrieval deficit is accompanied with hippocampal MMP2, MMP-9 and MAPKs alteration

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive loss of memory and cognitive deficit. The observed amnesia in the early stages of AD is suggested to be a retrieval problem, rather than encoding and consolidation deficit. According to the cholinergic hypothesis of AD, scopolamine is used to induce an animal model of amnesia. Howbeit the effect of scopolamine on memory retrieval is contradictory. This study aimed to assess the effect of scopolamine on passive avoidance memory retrieval. Additionally according to the reported changes of MMP-2, MMP-9 and MAPKs (ERK, P38 and JNK) in AD pathology the hippocampal contents of these proteins were determined. Male NMRI mice weighing 20-25g were trained in passive avoidance apparatus. The drug or its vehicle was injected 24h after training (30min before retention test). The hippocampal tissue was isolated and western blot analysis was done for MMP-2, MMP-9 and MAPKs (ERK, P38 and JNK). The results indicated that scopolamine (1mg/kg) disrupts passive avoidance memory retrieval. This scopolamine treatment resulted in hippocampal MMP-2 and MMP-9 decline while increased MAPKs in the hippocampus. These results suggest that cholinergic system has an important role in learnt memory retrieval. It might also suggest the positive role of MMP-2 and MMP-9 in this phase of memory while propose that MAPKs affect negatively the reactivation of memory which is compatible with MAPKs activation in AD.

Post-training reversible disconnection of the ventral hippocampal-basolateral amygdaloid circuits impairs consolidation of inhibitory avoidance memory in rats

The ventral hippocampus (VH) and the basolateral amygdala (BLA) are both crucial in inhibitory avoidance (IA) memory. However, the exact role of the VH–BLA circuit in IA memory consolidation is unclear. This study investigated the effect of post-training reversible disconnection of the VH–BLA circuit in IA memory consolidation. Male Wistar rats with implanted guide cannulae were trained with a one-trial IA task, then received immediate intracerebral injections of muscimol or saline, and were tested 24 h later. Muscimol injection into the bilateral BLA, or the unilateral VH and contralateral BLA, but not the unilateral VH and ipsilateral BLA, significantly decreased the retention latencies (versus saline treatment). The results suggest that the VH–BLA circuit could be an important circuit to modulate consolidation of IA memory in rats.

The protective effect of fermented Curcuma longa L. on memory dysfunction in oxidative stress-induced C6 gliomal cells, proinflammatory-activated BV2 microglial cells, and scopolamine-induced amnesia model in mice

BACKGROUND

Curcuma longa L. is a well-known medicinal plant that has been used for its anti-cancer, neuroprotective, and hepatoprotective effects. However, the neuroprotective effect of fermented C. longa (FCL) has not been reported. Therefore, in this study, the effectiveness of FCL for the regulation of memory dysfunction was investigated in two brain cell lines (rat glioma C6 and murine microglia BV2) and scopolamine-treated mice.

METHODS

C. longa powder was fermented by 5% Lactobacillus plantarum K154 containing 2% (w/v) yeast extract at 30 °C for 72 h followed by sterilization at 121 °C for 15 min. The protective effects of fermented C. longa (FCL) on oxidative stress induced cell death were analyzed by MTT assay in C6 cells. The anti-inflammatory effects of FCL were investigated by measuring the production of nitric oxide (NO) and prostaglandin E₂ (PGE₂) as well as the expression levels of inducible NO synthase (iNOS) and cyclooxygenase-2 (COX-2) in LPS-stimulated BV2 cells. The step-through passive avoidance test, Morris water maze test, acetylcholinesterase (AChE) activity, and expression of cAMP response element-binding protein (CREB) and brain-derived neurotropic factor (BDNF) were employed to determine the effects of FCL on scopolamine-induced memory deficit in mice. The contents of curcuminoids were analyzed through LC/MS.

RESULTS

Pretreatment with FCL effectively prevented the cell death induced by oxidative stress in C6 cells. Moreover, FCL inhibited the production NO and PGE₂ via the inhibition of iNOS and COX-2 expression in BV2 cells. FCL significantly attenuated scopolamine-induced memory impairment in mice and prevented scopolamine-induced AChE activity in the hippocampus. Additionally, FCL reversed the reduction of CREB and BDNF expression. The curcuminoids content in FCL was 1.44%.

CONCLUSION

FCL pretreatment could alleviate scopolamine-induced memory impairment in mice, as well as oxidative stress and inflammation in C6 and BV2 cells, respectively. Thus, FCL might be a useful material for preventing impairment of learning and memory.

Translational Approaches Targeting Reconsolidation

Maladaptive learned responses and memories contribute to psychiatric disorders that constitute a significant socio-economic burden. Primary treatment methods teach patients to inhibit maladaptive responses, but do not get rid of the memory itself, which explains why many patients experience a return of symptoms even after initially successful treatment. This highlights the need to discover more persistent and robust techniques to diminish maladaptive learned behaviours. One potentially promising approach is to alter the original memory, as opposed to inhibiting it, by targeting memory reconsolidation. Recent research shows that reactivating an old memory results in a period of memory flexibility and requires restorage, or reconsolidation, for the memory to persist. This reconsolidation period allows a window for modification of a specific old memory. Renewal of memory flexibility following reactivation holds great clinical potential as it enables targeting reconsolidation and changing of specific learned responses and memories that contribute to maladaptive mental states and behaviours. Here, we will review translational research on non-human animals, healthy human subjects, and clinical populations aimed at altering memories by targeting reconsolidation using biological treatments (electrical stimulation, noradrenergic antagonists) or behavioural interference (reactivation-extinction paradigm). Both approaches have been used successfully to modify aversive and appetitive memories, yet effectiveness in treating clinical populations has been limited. We will discuss that memory flexibility depends on the type of memory tested and the brain regions that underlie specific types of memory. Further, when and how we can most effectively reactivate a memory and induce flexibility is largely unclear. Finally, the development of drugs that can target reconsolidation and are safe for use in humans would optimize cross-species translations. Increasing the understanding of the mechanism and limitations of memory flexibility upon reactivation should help optimize efficacy of treatments for psychiatric patients.

Role of GABA(B) receptors in learning and memory and neurological disorders

Although it is evident from the literature that altered GABAB receptor function does affect behavior, these results often do not correspond well. These differences could be due to the task protocol, animal strain, ligand concentration, or timing of administration utilized. Because several clinical populations exhibit learning and memory deficits in addition to altered markers of GABA and the GABAB receptor, it is important to determine whether altered GABAB receptor function is capable of contributing to the deficits. The aim of this review is to examine the effect of altered GABAB receptor function on synaptic plasticity as demonstrated by in vitro data, as well as the effects on performance in learning and memory tasks. Finally, data regarding altered GABA and GABAB receptor markers within clinical populations will be reviewed. Together, the data agree that proper functioning of GABAB receptors is crucial for numerous learning and memory tasks and that targeting this system via pharmaceuticals may benefit several clinical populations.

Antidepressant-like activity of magnesium in the olfactory bulbectomy model is associated with the AMPA/BDNF pathway

RATIONALE

Numerous studies suggest agents that act on glutamatergic transmission as potential antidepressants. Preclinical and clinical evidence suggests that magnesium, an N-methyl-D-aspartate receptor blocker, may be useful in the treatment of depression.

OBJECTIVE

The aim of this study was to investigate the effects of magnesium on behavior; protein levels of GluN2A, GluN2B [N-methyl-D-aspartate receptor (NMDAR) subunits], GluA1 [α-amino-3-hydroxy-5 methyl-4-isoxazolepropionic acid (AMPA) subunit], phospho-Ser-831-GluA1 (P-S831), phospho-Ser-845-GluA1 (P-S845), and brain-derived neurotrophic factor (BDNF); and messenger RNA (mRNA) levels of GluN2A and GluN2B in different brain areas in the olfactory bulbectomy (OB) model of depression in rats.

METHODS

Magnesium was administered once daily for 14 days at three doses (10, 15, and 20 mg/kg, intraperitoneal) to sham and OB rats. Following treatment, open field and passive avoidance tests were performed in the sham and OB rats. After 24 h, the hippocampus, the prefrontal cortex (PFC), and the amygdala of rats treated with the most active dose (15 mg/kg) were harvested, and the protein and mRNA levels were determined.

RESULTS

Chronic administration of magnesium (15 and 20 mg/kg) reduced the number of trials required to learn passive avoidance and reduced the OB-induced hyperactivity. OB increased the P-S845 level in the hippocampus, which was reduced by magnesium treatment. Magnesium significantly increased the levels of BDNF, GluN2B, P-S831, and P-S845 protein (and mRNA) primarily in the PFC and the hippocampus in OB rats.

CONCLUSION

For the first time, the present results demonstrate the antidepressant-like activity of magnesium in the OB animal model of depression and indicate the potential involvement of the AMPA/BDNF pathway in this activity.

Neurotrophin receptor p75(NTR) mediates Huntington's disease-associated synaptic and memory dysfunction

Learning and memory deficits are early clinical manifestations of Huntington's disease (HD). These cognitive impairments have been mainly associated with frontostriatal HD pathology; however, compelling evidence provided by several HD murine models suggests that the hippocampus may contribute to synaptic deficits and memory dysfunction in HD. The neurotrophin receptor p75(NTR) negatively regulates spine density, which is associated with learning and memory; therefore, we explored whether disturbed p75(NTR) function in the hippocampus could contribute to synaptic dysfunction and memory deficits in HD. Here, we determined that levels of p75(NTR) are markedly increased in the hippocampus of 2 distinct mouse models of HD and in HD patients. Normalization of p75(NTR) levels in HD mutant mice heterozygous for p75(NTR) prevented memory and synaptic plasticity deficits and ameliorated dendritic spine abnormalities, likely through normalization of the activity of the GTPase RhoA. Moreover, viral-mediated overexpression of p75(NTR) in the hippocampus of WT mice reproduced HD learning and memory deficits, while knockdown of p75(NTR) in the hippocampus of HD mice prevented cognitive decline. Together, these findings provide evidence of hippocampus-associated memory deficits in HD and demonstrate that p75(NTR) mediates synaptic, learning, and memory dysfunction in HD.

Additive effects of maternal iron deficiency and prenatal immune activation on adult behaviors in rat offspring

Both iron deficiency (ID) and infection are common during pregnancy and studies have described altered brain development in offspring as a result of these individual maternal exposures. Given their high global incidence, these two insults may occur simultaneously during pregnancy. We recently described a rat model which pairs dietary ID during pregnancy and prenatal immune activation. Pregnant rats were placed on iron sufficient (IS) or ID diets from embryonic day 2 (E2) until postnatal day 7, and administered the bacterial endotoxin, lipopolysaccharide (LPS) or saline on E15/16. In this model, LPS administration on E15 caused greater induction of the pro-inflammatory cytokines, interleukin-6 and tumor necrosis factor-α, in ID dams compared to IS dams. This suggested that the combination of prenatal immune activation on a background of maternal ID might have more adverse neurodevelopmental consequences for the offspring than exposure to either insult alone. In this study we used this model to determine whether combined exposure to maternal ID and prenatal immune activation interact to affect juvenile and adult behaviors in the offspring. We assessed behaviors relevant to deficits in humans or animals that have been associated with exposure to either maternal ID or prenatal immune activation alone. Adult offspring from ID dams displayed significant deficits in pre-pulse inhibition of acoustic startle and in passive avoidance learning, together with increases in cytochrome oxidase immunohistochemistry, a marker of metabolic activity, in the ventral hippocampus immediately after passive avoidance testing. Offspring from LPS treated dams showed a significant increase in social behavior with unfamiliar rats, and subtle locomotor changes during exploration in an open field and in response to amphetamine. Surprisingly, there was no interaction between effects of the two insults on the behaviors assessed, and few observed alterations in juvenile behavior. Our findings show that long-term effects of maternal ID and prenatal LPS were additive, such that offspring exposed to both insults displayed more adult behavioral abnormalities than offspring exposed to one alone.

Chemogenetic inactivation of ventral hippocampal glutamatergic neurons disrupts consolidation of contextual fear memory

Synaptic consolidation is a process thought to consolidate memory in the brain. Although lesion studies have mainly implicated the hippocampus (HPC) in this process, it is unknown which cell type(s) or regions of the HPC might be essential for synaptic consolidation. To selectively and reversibly suppress hippocampal neuronal activity during this process, we developed a new Gi-DREADD (hM4Di) transgenic mouse for in vivo manipulation of neuronal activity in freely moving animals. We found that CA1 pyramidal neurons could be dose-dependently inactivated by clozapine-n-oxide (CNO). Inactivation of hippocampal neurons within 6 h immediately after conditioned fear training successfully impaired the consolidation of contextual memory, without disturbing cued memory. To anatomically define the brain subregion critical for the behavioral effects, hM4Di viral vectors were transduced and selectively expressed in the glutamatergic neurons in either the dorsal or ventral HPC. Significantly, we found that selective inactivation of ventral but not dorsal glutamatergic hippocampal neurons suppressed the synaptic consolidation of contextual memory.

Neonatal exposure to permethrin pesticide causes lifelong fear and spatial learning deficits and alters hippocampal morphology of synapses

Background

During the neurodevelopmental period, the brain is potentially more susceptible to environmental exposure to pollutants. The aim was to determine if neonatal exposure to permethrin (PERM) pesticide, at a low dosage that does not produce signs of obvious abnormalities, could represent a risk for the onset of diseases later in the life.

Methods

Neonatal rats (from postnatal day 6 to 21) were treated daily by gavage with a dose of PERM (34 mg/kg) close to the no-observed-adverse-effect level (NOAEL), and hippocampal morphology and function of synapses were investigated in adulthood. Fear conditioning, passive avoidance and Morris water maze tests were used to assess cognitive skills in rats, whereas electron microscopy analysis was used to investigate hippocampal morphological changes that occurred in adults.

Results

In both contextual and tone fear conditioning tests, PERM-treated rats showed a decreased freezing. In the passive avoidance test, the consolidation of the inhibitory avoidance was time-limited: the memory was not impaired for the first 24 h, whereas the information was not retained 72 h following training. The same trend was observed in the spatial reference memories acquired by Morris water maze. In PERM-treated rats, electron microscopy analysis revealed a decrease of synapses and surface densities in the stratum moleculare of CA1, in the inner molecular layer of the dentate gyrus and in the mossy fibers of the hippocampal areas together with a decrease of perforated synapses in the stratum moleculare of CA1 and in the inner molecular layer of the dentate gyrus.

Conclusions

Early-life permethrin exposure imparts long-lasting consequences on the hippocampus such as impairment of long-term memory storage and synaptic morphology.

GABA-mediated presynaptic inhibition is required for precision of long-term memory

Though much attention has been given to the neural structures that underlie the long-term consolidation of contextual memories, little is known about the mechanisms responsible for the maintenance of memory precision. Here, we demonstrate a rapid time-dependent decline in memory precision in GABAB(1a) receptor knockout mice. First, we show that GABAB(1a) receptors are required for the maintenance, but not encoding, of a precise fear memory. We then demonstrate that GABAB(1a) receptors are required for the maintenance, but not encoding, of spatial memories. Our findings suggest that GABA-mediated presynaptic inhibition regulates the maintenance of memory precision as a function of memory age.

The reuniens and rhomboid nuclei: neuroanatomy, electrophysiological characteristics and behavioral implications

The reuniens and rhomboid nuclei, located in the ventral midline of the thalamus, have long been regarded as having non-specific effects on the cortex, while other evidence suggests that they influence behavior related to the photoperiod, hunger, stress or anxiety. We summarise the recent anatomical, electrophysiological and behavioral evidence that these nuclei also influence cognitive processes. The first part of this review describes the reciprocal connections of the reuniens and rhomboid nuclei with the medial prefrontal cortex and the hippocampus. The connectivity pattern among these structures is consistent with the idea that these ventral midline nuclei represent a nodal hub to influence prefrontal-hippocampal interactions. The second part describes the effects of a stimulation or blockade of the ventral midline thalamus on cortical and hippocampal electrophysiological activity. The final part summarizes recent literature supporting the emerging view that the reuniens and rhomboid nuclei may contribute to learning, memory consolidation and behavioral flexibility, in addition to general behavior and aspects of metabolism.

Administration of Phytoceramide Enhances Memory and Upregulates the Expression of pCREB and BDNF in Hippocampus of Mice

This study was aimed at investigating the possible effects of phytoceramide (Pcer) on learning and memory and their underlying mechanisms. Phytoceramide was orally administered to ICR mice for 7 days. Memory performances were assessed using the passive avoidance test and Y-maze task. The expressions of phosphorylated cAMP response element binding protein (pCREB), brain-derived neurotrophic factor (BDNF) were measured with immunoblot. The incorporation of 5-bromo-2-deoxyuridine (BrdU) in hippocampal regions was investigated by using immunohistochemical methods. Treatment of Pcer enhanced cognitive performances in the passive avoidance test and Y-maze task. Immunoblotting studies revealed that the phosphorylated CREB and BDNF were significantly increased on hippocampus in the Pcer-treated mice. Immunohistochemical studies showed that the number of immunopositive cells to BrdU was significantly increased in the hippocampal dentate gyrus regions after Pcer-treatment for 7 days. These results suggest that Pcer contribute to enhancing memory and BDNF expression and it could be secondary to the elevation of neurogenesis.

Effects of CA1 glutamatergic systems upon memory impairments in cholestatic rats

BACKGROUND

Bile duct ligation (BDL) is shown to induce cholestasis-related liver function impairments as well as consequent cognitive dysfunctions (i.e. impaired learning and memory formation). Glutamatergic neurotransmission plays an important role in hippocampal modulation of learning and memory function. The present study aimed to investigate the possible involvement of dorsal hippocampal (CA1) glutamatergic systems upon cholestasis-induced amnesia.

METHOD

Cholestasis was induced in male Wistar rats through double-ligation of the main bile duct (at two points) and transection of the interposed segment. Step-through passive avoidance test was employed to examine rats' learning and memory function. All drugs were injected into CA1 region of the hippocampus.

RESULTS

our results indicated a decrease in memory retrieval following cholestasis (11, 17 and 24 days post BDL). Only subthreshold doses of N-methyl-d-aspartate (NMDA; 0.125 and 0.25 μg/μl) but not its effective dose (0.5 μg/μl), restored the cholestasis-induced amnesia in step-through passive avoidance test, 11, 17 and 24 days post BDL. This effect was blocked by the subthreshold dose of D-[1]-2-amino-7-phosphonoheptanoic acid (D-AP7, NMDA receptor antagonist; 0.0625 μg/μl, intra-CA1) at 0.125 μg/μl and 0.25 μg/μl doses of NMDA. Moreover, our data revealed that only effective doses of D-AP7 (0.125 and 0.25 μg/μl, intra-CA1) potentiate memory impairments in 11 days after BDL. It was noted that none of applied drugs/doses exerted an effect on memory acquisition and locomotors activity, 10 and 12 days post laparotomy, respectively.

CONCLUSION

Our findings suggest the potential involvement of CA1 glutamatergic system(s) in cholestasis-induced memory deficits.

Regulation of hippocampal cGMP levels as a candidate to treat cognitive deficits in Huntington's disease

Huntington’s disease (HD) patients and mouse models show learning and memory impairment associated with hippocampal dysfunction. The neuronal nitric oxide synthase/3',5'-cyclic guanosine monophosphate (nNOS/cGMP) pathway is implicated in synaptic plasticity, and in learning and memory processes. Here, we examined the nNOS/cGMP pathway in the hippocampus of HD mice to determine whether it can be a good therapeutic target for cognitive improvement in HD. We analyzed hippocampal nNOS and phosphodiesterase (PDE) 5 and 9 levels in R6/1 mice, and cGMP levels in the hippocampus of R6/1, R6/2 and Hdh^Q7/Q111 mice, and of HD patients. We also investigated whether sildenafil, a PDE5 inhibitor, could improve cognitive deficits in R6/1 mice. We found that hippocampal cGMP levels were 3-fold lower in 12-week-old R6/1 mice, when they show deficits in object recognition memory and in passive avoidance learning. Consistent with hippocampal cGMP levels, nNOS levels were down-regulated, while there were no changes in the levels of PDE5 and PDE9 in R6/1 mice. A single intraperitoneal injection of sildenafil (3 mg/Kg) immediately after training increased cGMP levels, and improved memory in R6/1 mice, as assessed by using the novel object recognition and the passive avoidance test. Importantly, cGMP levels were also reduced in R6/2 mouse and human HD hippocampus. Therefore, the regulation of hippocampal cGMP levels can be a suitable treatment for cognitive impairment in HD.

Changes in mouse cognition and hippocampal gene expression observed in a mild physical- and blast-traumatic brain injury

Warfare has long been associated with traumatic brain injury (TBI) in militarized zones. Common forms of TBI can be caused by a physical insult to the head-brain or by the effects of a high velocity blast shock wave generated by the detonation of an explosive device. While both forms of trauma are distinctly different regarding the mechanism of trauma induction, there are striking similarities in the cognitive and emotional status of survivors. Presently, proven effective therapeutics for the treatment of either form of TBI are unavailable. To be able to develop efficacious therapies, studies involving animal models of physical- and blast-TBI are required to identify possible novel or existing medicines that may be of value in the management of clinical events. We examined indices of cognition and anxiety-like behavior and the hippocampal gene transcriptome of mice subjected to both forms of TBI. We identified common behavioral deficits and gene expression regulations, in addition to unique injury-specific forms of gene regulation. Molecular pathways presented a pattern similar to that seen in gene expression. Interestingly, pathways connected to Alzheimer's disease displayed a markedly different form of regulation depending on the type of TBI. While these data highlight similarities in behavioral outcomes after trauma, the divergence in hippocampal transcriptome observed between models suggests that, at the molecular level, the TBIs are quite different. These models may provide tools to help define therapeutic approaches for the treatment of physical- and blast-TBIs. Based upon observations of increasing numbers of personnel displaying TBI related emotional and behavioral changes in militarized zones, the development of efficacious therapies will become a national if not a global priority.

Brain phenotype of transgenic mice overexpressing cystathionine β-synthase

BACKGROUND

The cystathionine β-synthase (CBS) gene, located on human chromosome 21q22.3, is a good candidate for playing a role in the Down Syndrome (DS) cognitive profile: it is overexpressed in the brain of individuals with DS, and it encodes a key enzyme of sulfur-containing amino acid (SAA) metabolism, a pathway important for several brain physiological processes.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we have studied the neural consequences of CBS overexpression in a transgenic mouse line (60.4P102D1) expressing the human CBS gene under the control of its endogenous regulatory regions. These mice displayed a ∼2-fold increase in total CBS proteins in different brain areas and a ∼1.3-fold increase in CBS activity in the cerebellum and the hippocampus. No major disturbance of SAA metabolism was observed, and the transgenic mice showed normal behavior in the rotarod and passive avoidance tests. However, we found that hippocampal synaptic plasticity is facilitated in the 60.4P102D1 line.

CONCLUSION/SIGNIFICANCE

We demonstrate that CBS overexpression has functional consequences on hippocampal neuronal networks. These results shed new light on the function of the CBS gene, and raise the interesting possibility that CBS overexpression might have an advantageous effect on some cognitive functions in DS.

Aβ oligomers inhibit synapse remodelling necessary for memory consolidation

Extensive research has implicated the amyloid-β protein (Aβ) in the aetiology of Alzheimer's disease (AD). This protein has been shown to produce memory deficits when injected into rodent brain and in mouse models of AD Aβ production is associated with impaired learning and/or recall. Here we examined the effects of cell-derived SDS-stable 7PA2-derived soluble Aβ oligomers on consolidation of avoidance learning. At 0, 3, 6, 9 or 12h after training, animals received an intracerebroventricular injection of Aβ-containing or control media and recall was tested at 24 and 48 h. Immediately after 48 h recall animals were transcardially perfused and the brain removed for sectioning and EM analysis. Rats receiving injections of Aβ at 6 or 9h post-training showed a significant impairment in memory consolidation at 48 h. Importantly, impaired animals injected at 9h had significantly fewer synapses in the dentate gyrus. These data suggest that Aβ low-n oligomers target specific temporal facets of consolidation-associated synaptic remodelling whereby loss of functional synapses results in impaired consolidation.

Disrupted reinforcement signaling in the orbitofrontal cortex and caudate in youths with conduct disorder or oppositional defiant disorder and a high level of psychopathic traits

OBJECTIVE

Dysfunction in the amygdala and orbitofrontal cortex has been reported in youths and adults with psychopathic traits. The specific nature of the functional irregularities within these structures remains poorly understood. The authors used a passive avoidance task to examine the responsiveness of these systems to early stimulus-reinforcement exposure, when prediction errors are greatest and learning maximized, and to reward in youths with psychopathic traits and comparison youths.

METHOD

While performing the passive avoidance learning task, 15 youths with conduct disorder or oppositional defiant disorder plus a high level of psychopathic traits and 15 healthy subjects completed a 3.0-T fMRI scan.

RESULTS

Relative to the comparison youths, the youths with a disruptive behavior disorder plus psychopathic traits showed less orbitofrontal responsiveness both to early stimulus-reinforcement exposure and to rewards, as well as less caudate response to early stimulus-reinforcement exposure. There were no group differences in amygdala responsiveness to these two task measures, but amygdala responsiveness throughout the task was lower in the youths with psychopathic traits.

CONCLUSIONS

Compromised sensitivity to early reinforcement information in the orbitofrontal cortex and caudate and to reward outcome information in the orbitofrontal cortex of youths with conduct disorder or oppositional defiant disorder plus psychopathic traits suggests that the integrated functioning of the amygdala, caudate, and orbitofrontal cortex may be disrupted. This provides a functional neural basis for why such youths are more likely to repeat disadvantageous decisions. New treatment possibilities are raised, as pharmacologic modulations of serotonin and dopamine can affect this form of learning.

The n-butanolic extract of Opuntia ficus-indica var. saboten enhances long-term memory in the passive avoidance task in mice

Opuntia ficus-indica var. saboten Makino (Cactaceae) is used to treat burns, edema, dyspepsia, and asthma in traditional medicine. The present study investigated the beneficial effects of the n-butanolic extract of O. ficus-indica var. saboten (BOF) on memory performance in mice and attempts to uncover the mechanisms underlying its action. Memory performance was assessed with the passive avoidance task, and western blotting and immunohistochemistry were used to measure changes in protein expression and cell survival. After the oral administration of BOF for 7 days, the latency time in the passive avoidance task was significantly increased relative to vehicle-treated controls (P<0.05). Western blotting revealed that the expression levels of brain-derived neurotrophic factor (BDNF), phosphorylated cAMP response element binding-protein (pCREB), and phosphorylated extracellular signal-regulated kinase (pERK) 1/2 were significantly increased in hippocampal tissue after 7 days of BOF administration (P<0.05). Doublecortin and 5-bromo-2-deoxyuridine immunostaining also revealed that BOF significantly enhanced the survival of immature neurons, but did not affect neuronal cell proliferation in the subgranular zone of the hippocampal dentate gyrus. These results suggest that the subchronic administration of BOF enhances long-term memory, and that this effect is partially mediated by ERK-CREB-BDNF signaling and the survival of immature neurons.

Involvement of cyclin-dependent kinase-like 2 in cognitive function required for contextual and spatial learning in mice

Cyclin-dependent kinase-like 2 (Cdkl2) is a cdc2-related serine/threonine protein kinase that is postnatally expressed in various brain regions, including the cerebral cortex, entorhinal cortex, hippocampus, amygdala, and dorsal thalamus. The extremely high Cdkl2 expression in these regions suggests that it has a role in cognition and emotion. Recent genetic studies indicate that mutations of Cdkl family kinases are associated with neurodevelopmental and neuropsychiatric disorders in humans. To elucidate the physiologic role of Cdkl2, we behaviorally analyzed Cdkl2(LacZ/LacZ) mice lacking Cdkl2. Cdkl2(LacZ/LacZ) mice had reduced latencies to enter the dark compartment after electric footshock in an inhibitory avoidance task and attenuated contextual fear responses when exposed to mild training conditions. Hippocampal spatial learning in the Morris water maze was slightly anomalous with mice exhibiting an abnormal swimming pattern. The aversive response in a two-way avoidance task was slightly, but not significantly, enhanced. On the other hand, Cdkl2(LacZ/LacZ) mice did not exhibit altered sensitivity to aversive stimuli, such as electric footshock and heat, or deficits in the elevated plus maze or rotating rod test. These findings suggest that Cdkl2 is involved in cognitive function and provide in vivo evidence for the function of Cdkl family kinases expressed in terminally differentiated neurons in mice.

The memory-enhancing effects of Euphoria longan fruit extract in mice

AIM OF THE STUDY

The fruit of Euphoria longan (Lour.) Steud. (Sapindaceae) is sweet and edible. Dried Euphoria longan fruit is prescribed as a tonic and for the treatment of forgetfulness, insomnia, or palpitations caused by fright in traditional Chinese medicine. The effects of aqueous extract of Euphoria longan fruit (ELE) on learning and memory and their underlying mechanisms were investigated.

MATERIALS AND METHODS

Aqueous extract of Euphoria longan fruit (ELE) was administered to ICR mice for 14 days. Piracetam was used as a positive control for its known memory-enhancing effects. Memory performances were assessed using the passive avoidance task. The expressions of phosphorylated extracellular signal-regulated kinase (pERK) 1/2, phosphorylated cAMP response element binding protein (pCREB), brain-derived neurotrophic factor (BDNF), doublecortin (DCX) and the incorporation of 5-bromo-2-deoxyuridine (BrdU) in hippocampal dentate gyrus and CA1 regions were investigated using immunohistochemical methods.

RESULTS

The step-through latency in the ELE-treated group was significantly increased compared with that in the vehicle-treated controls (P<0.05) in the passive avoidance task. Piracetam-treated group also showed enhanced cognitive performaces in the passive avoidance task. Immunohistochemical studies revealed that the number of cells immunopositive for BDNF, pCREB, or pERK 1/2 was significantly increased in the hippocampal dentate gyrus and CA1 regions after ELE treatment for 14 days (P<0.05). DCX and BrdU immunostaining also revealed that ELE significantly enhanced immature neuronal survival, but not neuronal cell proliferation in the subgranular zone of the dentate gyrus.

CONCLUSIONS

The present results suggest that subchronic administration of aqueous extract of Euphoria longan fruit enhances learning and memory, and that its beneficial effects are mediated, in part, by BDNF expression and immature neuronal survival.

Tanshinone I enhances learning and memory, and ameliorates memory impairment in mice via the extracellular signal-regulated kinase signalling pathway

BACKGROUND AND PURPOSE

The intracellular signalling kinase, extracellular signal-regulated kinase 1/2 (ERK1/2) is required for new memory formation, suggesting that control of ERK signalling might be a target for the treatment of cognitive dysfunction. Previously, we reported that tanshinone congeners have ameliorating effects on drug-induced memory impairment in mice. Here, we have investigated possible modes of action of tanshinone I on learning and memory, associated with ERK phosphorylation.

EXPERIMENTAL APPROACH

Using immunohistochemical, Western blot techniques, and behavioural testing, we studied the effect of tanshinone I on memory impairment induced by diazepam or dizocilpine (MK-801) in mice.

KEY RESULTS

Tanshinone I (2 or 4 mg.kg(-1), p.o.) increased latency times versus vehicle-treated control group in the passive avoidance task. Western blot analysis and immunohistochemical data showed that tanshinone I (4 mg.kg(-1)) increased levels of phosphorylated cAMP response element binding protein (pCREB) and phosphorylated ERK (pERK) in the hippocampus. These increases in pCREB and pERK were blocked by U0126 (inhibitor of ERK1/2), which also prevented the increase in passive avoidance task latency time after tanshinone I. In models of learning and memory impairment induced by diazepam and MK-801, tanshinone I (4 mg.kg(-1)) reversed learning and memory impairments detected by the passive avoidance test. Western blot analysis showed that tanshinone I reversed the diazepam- and MK-801-induced inhibitions of ERK and CREB activation in hippocampal tissues. These effects were also blocked by U0126.

CONCLUSIONS AND IMPLICATIONS

Tanshinone I ameliorates the learning and memory impairments induced by diazepam and MK-801 through activation of ERK signalling.

Bilateral and unilateral hippocampal inactivation did not differ in their effect on consolidation processes in the Morris water maze

Consolidation processes were studied in the rat by using functional inactivation techniques. Previous results showed that unilateral hippocampal inactivation alters consolidation. It is not clear if bilateral treatments increase the impairment. Wistar rats were trained in the Morris water maze during 4 consecutive days. Subjects received saline or tetrodotoxin in the dorsal hippocampus 1 min after training. Results showed that bilateral as well as unilateral treatments impair consolidation to the same degree, as shown by the mean latency to reach the platform. In both cases, the impairment is only visible in the first trial of the session following the blockade.

Aberrant hippocampal activity underlies the dopamine dysregulation in an animal model of schizophrenia

Evidence supports a dysregulation of subcortical dopamine (DA) system function as a common etiology of psychosis; however, the factors responsible for this aberrant DA system responsivity have not been delineated. Here, we demonstrate in an animal model of schizophrenia that a pathologically enhanced drive from the ventral hippocampus (vHipp) can result in aberrant dopamine neuron signaling. Adult rats in which development was disrupted by prenatal methylazoxymethanol acetate (MAM) administration display a significantly greater number of spontaneously firing ventral tegmental DA neurons. This appears to be a consequence of excessive hippocampal activity because, in MAM-treated rats, vHipp inactivation completely reversed the elevated DA neuron population activity and also normalized the augmented amphetamine-induced locomotor behavior. These data provide a direct link between hippocampal dysfunction and the hyper-responsivity of the DA system that is believed to underlie the augmented response to amphetamine in animal models and psychosis in schizophrenia patients.

Notch signalling becomes transiently attenuated during long-term memory consolidation in adult Wistar rats

Recent evidence has suggested a role for Notch in memory consolidation but the means by which this evolutionarily conserved mechanism serves these plasticity-related processes remains to be established. We have examined a role for this signalling pathway in the hippocampal dentate gyrus of Wistar rats at increasing times following passive avoidance conditioning. Our principal finding is that a transient attenuation of Notch signalling occurs at the 10-12h post-training time. In this period, extracellular Notch-1 protein fragment exhibited a significant 2- to 3-fold increase but, by contrast, Notch-1 mRNA levels were significantly reduced. Moreover, transient inactivation of Notch-1 signalling was further suggested by concomitant reductions in the Notch ligand Jagged-1 and Notch-1 target protein Hes-1 mRNA levels. The C-terminal fragment of PS-1, necessary for gamma-secretase activity, was also significantly reduced at the 12h post-training time. These events were commensurate with the increase of a Notch immunoreactive fragment of 66 kDa in the nuclear fraction of the dentate gyrus. This fragment, identified with two different Notch-1 antisera, was not the expected NICD polypeptide of approximately 110 kDa and its accumulation was found to correlate with a significantly reduced expression of the Hes-1 transcriptional repressor. During the period of reduced Notch activity, a transient increase in soluble beta-catenin and GSK-3beta phosphorylation was observed, indicating a reciprocal activation of the Wnt signalling pathway. As down-regulation of Notch signalling promotes differentiation and neurite outgrowth in post-mitotic neurons, it is proposed that this pathway regulates the integration of synapses transiently produced during memory consolidation.