Neurabin contributes to hippocampal long-term potentiation and contextual fear memory

Neurabin is a scaffolding protein that interacts with actin and protein phosphatase-1. Highly enriched in the dendritic spine, neurabin is important for spine morphogenesis and synaptic formation. However, less is known about the role of neurabin in hippocampal plasticity and its possible effect on behavioral functions. Using neurabin knockout (KO) mice, here we studied the function of neurabin in hippocampal synaptic transmission, plasticity and behavioral memory. We demonstrated that neurabin KO mice showed a deficit in contextual fear memory but not auditory fear memory. Whole-cell patch clamp recordings in the hippocampal CA1 neurons showed that long-term potentiation (LTP) was significantly reduced, whereas long-term depression (LTD) was unaltered in neurabin KO mice. Moreover, increased AMPA receptor but not NMDA receptor-mediated synaptic transmission was found in neurabin KO mice, and is accompanied by decreased phosphorylation of GluR1 at the PKA site (Ser845) but no change at the CaMKII/PKC site (Ser831). Pre-conditioning with LTD induction rescued the following LTP in neurabin KO mice, suggesting the loss of LTP may be due to the saturated synaptic transmission. Our results indicate that neurabin regulates contextual fear memory and LTP in hippocampal CA1 pyramidal neurons.     

Indomethacin enhances learning and memory potential by interacting with CaMKII

The present study examined the effect of indomethacin (IM), a cyclooxygenase inhibitor, on learning and memory functions. IM activated Ca(2+) /calmodulin-dependent protein kinase II (CaMKII) in cultured rat hippocampal neurons. IM (100 μM) significantly increased the rate of spontaneous AMPA receptor-mediated miniature excitatory postsynaptic currents elicited from CA1 pyramidal neurons of rat hippocampal slices, without affecting the amplitude, and enhanced extracellular high K(+) (20 mM)-induced glutamate release from rat hippocampal slices, indicating that IM stimulates presynaptic glutamate release. Those IM effects were clearly inhibited by the CaMKII inhibitor KN-93. IM persistently facilitated synaptic transmission monitored from the CA1 region of rat hippocampal slices in a concentration (1-100 μM)-dependent manner that was also abolished by KN-93. In the water maze test, IM (1 mg/kg, i.p.) enhanced spatial learning and memory ability for normal rats, and ameliorated scopolamine-induced spatial learning and memory impairment or age-related spatial learning and memory deterioration for senescence-accelerated mouse-prone 8 mice. In the test to learn 15 numbers consisting of three patterns of five digit number for healthy human subjects, oral intake with IM (25 mg/kg) significantly raised the scores of correct number arrangements that subjects memorized 5 min and 3 days after the test. The results of the present study indicate that IM could enhance learning and memory potential by facilitating hippocampal synaptic transmission as a result from stimulating presynaptic glutamate release under the control of CaMKII.     

SNAP-25 in hippocampal CA3 region is required for long-term memory formation

SNAP-25 is a synaptosomal protein of 25 kDa, a key component of synaptic vesicle-docking/fusion machinery, and plays a critical role in exocytosis and neurotransmitter release. We previously reported that SNAP-25 in the hippocampal CA1 region is involved in consolidation of contextual fear memory and water-maze spatial memory (Hou et al. European J Neuroscience, 20: 1593-1603, 2004). SNAP-25 is expressed not only in the CA1 region, but also in the CA3 region, and the SNAP-25 mRNA level in the CA3 region is higher than in the CA1 region. Here, we provide evidence that SNAP-25 in the CA3 region is also involved in learning/memory. Intra-CA3 infusion of SNAP-25 antisense oligonucleotide impaired both long-term contextual fear memory and water-maze spatial memory, with short-term memory intact. Furthermore, the SNAP-25 antisense oligonucleotide suppressed the long-term potentiation (LTP) of field excitatory post-synaptic potential (fEPSP) in the mossy-fiber pathway (DG-CA3 pathway), with no effect on paired-pulse facilitation of the fEPSP. These results are consistent with the notion that SNAP-25 in the hippocampal CA3 region is required for long-term memory formation.     

Ablation of Ca2+ channel beta3 subunit leads to enhanced N-methyl-D-aspartate receptor-dependent long term potentiation and improved long term memory

The beta subunits of voltage-dependent Ca(2+) channels (VDCCs) have marked effects on the properties of the pore-forming alpha(1) subunits of VDCCs, including surface expression of channel complexes and modification of voltage-dependent kinetics. Among the four different beta subunits, the beta(3) subunit (Ca(v)beta3) is abundantly expressed in the hippocampus. However, the role of Ca(v)beta3 in hippocampal physiology and function in vivo has never been examined. Here, we investigated Ca(v)beta3-deficient mice for hippocampus-dependent learning and memory and synaptic plasticity at hippocampal CA3-CA1 synapses. Interestingly, the mutant mice exhibited enhanced performance in several hippocampus-dependent learning and memory tasks. However, electrophysiological studies revealed no alteration in the Ca(2+) current density, the frequency and amplitude of miniature excitatory postsynaptic currents, and the basal synaptic transmission in the mutant hippocampus. On the other hand, however, N-methyl-d-aspartate receptor (NMDAR)-mediated synaptic currents and NMDAR-dependent long term potentiation were significantly increased in the mutant. Protein blot analysis showed a slight increase in the level of NMDAR-2B in the mutant hippocampus. Our results suggest a possibility that, unrelated to VDCCs regulation, Ca(v)beta3 negatively regulates the NMDAR activity in the hippocampus and thus activity-dependent synaptic plasticity and cognitive behaviors in the mouse.     

Formation of temporal memory requires NMDA receptors within CA1 pyramidal neurons

In humans the hippocampus is required for episodic memory, which extends into the spatial and temporal domains. Work on the rodent hippocampus has shown that NMDA receptor (NMDAR) -mediated plasticity is essential for spatial memory. Here, we have examined whether hippocampal NMDARs are also needed for temporal memory. We applied trace fear conditioning to knockout mice lacking NMDARs only in hippocampal CA1 pyramidal cells. This paradigm requires temporal processing because the conditional and unconditional stimuli are separated by 30 s (trace). We found that knockout mice failed to memorize this association but were indistinguishable from normal animals when the trace was removed. Thus, NMDARs in CA1 are crucial for the formation of memories that associate events across time.     

Altered cocaine effects in mice lacking Ca(v)2.3 (alpha(1E)) calcium channel

Much evidence indicates that calcium channel plays a role in cocaine-induced behavioral responses. We assessed the contributions of Ca(v)2.3 (alpha(1E)) calcium channel to cocaine effects using Ca(v)2.3 knockout mice (Ca(v)2.3-/-). Acute administration of cocaine enhanced the locomotor activity in wild-type mice (Ca(v)2.3+/+), but failed to produce any response in Ca(v)2.3-/- mice. Repeated exposure to cocaine induced the behavioral sensitization and conditioned place preference in both genotypes. Pretreatment with a D1-receptor antagonist, SCH23390, blocked the cocaine-induced place preference in Ca(v)2.3+/+ mice; however, it had no significant effect in Ca(v)2.3-/- mice. Microdialysis and RT-PCR analysis revealed that the levels of extracellular dopamine and dopamine D1 and D2 receptor mRNAs were not altered in Ca(v)2.3-/- mice. These data indicate that Ca(v)2.3 channel contributes to the locomotor-stimulating effect of cocaine, and the deletion of Ca(v)2.3 channel reveals the presence of a novel pathway leading to cocaine rewarding which is insensitive to D1 receptor antagonist.     

Cav1.2 calcium channels modulate the spiking pattern of hippocampal pyramidal cells

Ca(v)1.2 L-type calcium channels support hippocampal synaptic plasticity, likely by facilitating dendritic Ca2+ influx evoked by action potentials (AP) back-propagated from the soma. Ca2+ influx into hippocampal neurons during somatic APs is sufficient to activate signalling pathways associated with late phase LTP. Thus, mechanisms controlling AP firing of hippocampal neurons are of major functional relevance. We examined the excitability of CA1 pyramidal cells using somatic current-clamp recordings in brain slices from control type mice and mice with the Ca(v)1.2 gene inactivated in principal hippocampal neurons. Lack of the Ca(v)1.2 protein did not affect either affect basic characteristics, such as resting membrane potential and input resistance, or parameters of single action potentials (AP) induced by 5 ms depolarising current pulses. However, CA1 hippocampal neurons from control and mutant mice differed in their patterns of AP firing during 500 ms depolarising current pulses: threshold voltage for repetitive firing was shifted significantly by about 5 mV to more depolarised potentials in the mutant mice (p<0.01), and the latency until firing of the first AP was prolonged (73.2+/-6.6 ms versus 48.1+/- 7.8 ms in control; p<0.05). CA1 pyramidal cells from the mutant mice also showed a lowered initial spiking frequency within an AP train. In control cells, isradipine had matching effects, while BayK 8644 facilitated spiking. Our data demonstrate that Ca(v)1.2 channels are involved in regulating the intrinsic excitability of CA1 pyramidal neurons. This cellular mechanism may contribute to the known function of Ca(v)1.2 channels in supporting synaptic plasticity and memory.     

Mice lacking Ca(v)2.3 (alpha1E) calcium channel exhibit hyperglycemia.

To investigate the functional role of Ca(v)2.3 channel in glucose homeostasis, we performed in vivo glucose tolerance and insulin tolerance tests together with stress-induced glucose release tests using mice deficient in Ca(v)2.3 channel (Ca(v)2.3-/-). The Ca(v)2.3-/- mice were significantly heavier than wild-type mice. In glucose tolerance and insulin tolerance tests, Ca(v)2.3-/- mice showed a significantly higher blood glucose level compared to wild-type mice. However, stress-induced blood glucose changes in Ca(v)2.3-/- mice were similar to those in wild-type mice. These results suggest that Ca(v)2.3 channel plays a role in glucose homeostasis by reducing insulin sensitivity and that Ca(v)2.3-/- mice exhibit symptoms resembling non-insulin-dependent diabetes mellitus.     

Effect of Cardiac Arrest on Cognitive Impairment and Hippocampal Plasticity in Middle-Aged Rats

Cardiopulmonary arrest is a leading cause of death and disability in the United States that usually occurs in the aged population. Cardiac arrest (CA) induces global ischemia, disrupting global cerebral circulation that results in ischemic brain injury and leads to cognitive impairments in survivors. Ischemia-induced neuronal damage in the hippocampus following CA can result in the impairment of cognitive function including spatial memory. In the present study, we used a model of asphyxial CA (ACA) in nine month old male Fischer 344 rats to investigate cognitive and synaptic deficits following mild global cerebral ischemia. These experiments were performed with the goals of 1) establishing a model of CA in nine month old middle-aged rats; and 2) to test the hypothesis that learning and memory deficits develop following mild global cerebral ischemia in middle-aged rats. To test this hypothesis, spatial memory assays (Barnes circular platform maze and contextual fear conditioning) and field recordings (long-term potentiation and paired-pulse facilitation) were performed. We show that following ACA in nine month old middle-aged rats, there is significant impairment in spatial memory formation, paired-pulse facilitation n dysfunction, and a reduction in the number of non-compromised hippocampal Cornu Ammonis 1 and subiculum neurons. In conclusion, nine month old animals undergoing cardiac arrest have impaired survival, deficits in spatial memory formation, and synaptic dysfunction.     

Are beta-adrenergic receptors in the hippocampal CA1 region required for retrieval of contextual fear memory?

It is well established that β-adrenoceptors (β-ARs) in the hippocampal CA1 region are involved in regulating synaptic plasticity and are essential for acquisition and consolidation of spatial memory and contextual fear memory. Previous studies reported that β-ARs in the CA1 region are also involved in memory retrieval. The present study re-examined the role of hippocampal β-ARs in retrieval of conditioned contextual fear. We bilaterally infused a high dose of the β-AR antagonist propranolol (15 μg in 1 μl saline) into the CA1 region 30 min before retention test and found that propranolol produced no deficit in retrieval of either 1-day or 7-day contextual fear. We then examined if β-AR stimulation would produce a beneficial effect. The β-AR agonist isoproterenol (10 μg in 1 μl saline) was infused into the CA1 region 30 min before retention test. Surprisingly, isoproterenol did not enhance but severely disrupted retrieval of 7-day contextual fear memory, with no impact on retrieval of 1-day contextual fear memory. The present study argues against the previous conclusion that β-ARs in the CA1 region play a role in memory retrieval. β-ARs in the CA1 region may be dispensable for retrieval of conditioned contextual fear.     

Impairment of hippocampal long-term depression and defective spatial learning and memory in p35 mice

Cdk5 (cyclin-dependent kinase 5) activity is dependent upon association with one of two neuron-specific activators, p35 or p39. Genetic deletion of Cdk5 causes perinatal lethality with severe defects in corticogenesis and neuronal positioning. p35(-/-) mice are viable with milder histological abnormalities. Although substantial evidence implicates Cdk5 in synaptic plasticity, its role in learning and memory has not been evaluated using mutant mouse models. We report here that p35(-/-) mice have deficiencies in spatial learning and memory. Close examination of hippocampal circuitry revealed subtle histological defects in CA1 pyramidal cells. Furthermore, p35(-/-) mice exhibit impaired long-term depression and depotentiation of long-term potentiation in the Schaeffer collateral CA1 pathway. Moreover, the Cdk5-dependent phosphorylation state of protein phosphatase inhibitor-1 was increased in 4-week-old mice due to increased levels of p39, which co-localized with inhibitor-1 and Cdk5 in the cytoplasm. These results demonstrate that p35-dependent Cdk5 activity is important to learning and synaptic plasticity. Deletion of p35 may shift the substrate specificity of Cdk5 due to compensatory expression of p39.     

Mapping and Deciphering Neural Codes of NMDA Receptor-Dependent Fear Memory Engrams in the Hippocampus

Mapping and decoding brain activity patterns underlying learning and memory represents both great interest and immense challenge. At present, very little is known regarding many of the very basic questions regarding the neural codes of memory: are fear memories retrieved during the freezing state or non-freezing state of the animals? How do individual memory traces give arise to a holistic, real-time associative memory engram? How are memory codes regulated by synaptic plasticity? Here, by applying high-density electrode arrays and dimensionality-reduction decoding algorithms, we investigate hippocampal CA1 activity patterns of trace fear conditioning memory code in inducible NMDA receptor knockout mice and their control littermates. Our analyses showed that the conditioned tone (CS) and unconditioned foot-shock (US) can evoke hippocampal ensemble responses in control and mutant mice. Yet, temporal formats and contents of CA1 fear memory engrams differ significantly between the genotypes. The mutant mice with disabled NMDA receptor plasticity failed to generate CS-to-US or US-to-CS associative memory traces. Moreover, the mutant CA1 region lacked memory traces for “what at when” information that predicts the timing relationship between the conditioned tone and the foot shock. The degraded associative fear memory engram is further manifested in its lack of intertwined and alternating temporal association between CS and US memory traces that are characteristic to the holistic memory recall in the wild-type animals. Therefore, our study has decoded real-time memory contents, timing relationship between CS and US, and temporal organizing patterns of fear memory engrams and demonstrated how hippocampal memory codes are regulated by NMDA receptor synaptic plasticity.     

Leptin facilitates learning and memory performance and enhances hippocampal CA1 long-term potentiation and CaMK II phosphorylation in rats

Leptin, an adipocytokine encoded by an obesity gene and expressed in adipose tissue, affects feeding behavior, thermogenesis, and neuroendocrine status via leptin receptors distributed in the brain, especially in the hypothalamus. Leptin may also modulate the synaptic plasticity and behavioral performance related to learning and memory since: leptin receptors are found in the hippocampus, and both leptin and its receptor share structural and functional similarities with the interleukin-6 family of cytokines that modulate long-term potentiation (LTP) in the hippocampus. We therefore examined the effect of leptin on (1) behavioral performance in emotional and spatial learning tasks, (2) LTP at Schaffer collateral-CA1 synapses, (3) presynaptic and postsynaptic activities in hippocampal CA1 neurons, (4) the intracellular Ca(2+) concentration ([Ca(2+)](i)) in CA1 neurons, and (5) the activity of Ca(2+)/calmodulin protein kinase II (CaMK II) in the hippocampal CA1 tissue that exhibits LTP. Intravenous injection of 5 and/or 50mug/kg, but not of 500mug/kg leptin, facilitated behavioral performance in passive avoidance and Morris water-maze tasks. Bath application of 10(-12)M leptin in slice experiments enhanced LTP and increased the presynaptic transmitter release, whereas 10(-10)M leptin suppressed LTP and reduced the postsynaptic receptor sensitivity to N-methyl-d-aspartic acid. The increase in the [Ca(2+)](i) induced by 10(-10)M leptin was two times greater than that induced by 10(-12)M leptin. In addition, the facilitation (10(-12)M) and suppression (10(-10)M) of LTP by leptin was closely associated with an increase and decrease in Ca(2+)-independent activity of CaMK II. Our results show that leptin not only affects hypothalamic functions (such as feeding, thermogenesis, and neuroendocrine status), but also modulates higher nervous functions, such as the behavioral performance related to learning and memory and hippocampal synaptic plasticity.     

Impaired long-term memory and long-term potentiation in N-type Ca2+ channel-deficient mice

Voltage-dependent N-type Ca(2+) channels, along with the P/Q-type, have a crucial role in controlling the release of neurotransmitters or neuromodulators at presynaptic terminals. However, their role in hippocampus-dependent learning and memory has never been examined. Here, we investigated hippocampus-dependent learning and memory and synaptic plasticity at hippocampal CA3-CA1 synapses in mice deficient for the alpha(1B) subunit of N-type Ca(2+) channels. The mutant mice exhibited impaired learning and memory in the Morris water maze and the social transmission of food preference tasks. In particular, long-term memory was impaired in the mutant mice. Interestingly, among activity-dependent long-lasting synaptic changes, theta burst- or 200-Hz-stimulation-induced long-term potentiation (LTP) was decreased in the mutant, compared with the wild-type mice. This type of LTP is known to require brain-derived neurotrophic factor (BDNF). It was found that both BDNF-induced potentiation of field excitatory postsynaptic potentials and facilitation of the frequency of miniature excitatory postsynaptic currents (mEPSCs) were reduced in the mutant. Taken together, these results demonstrate that N-type Ca(2+) channels are required for hippocampus-dependent learning and memory, and certain forms of LTP.     

Enhanced learning and memory in mice lacking Na+/Ca2+ exchanger 2

The plasma membrane Na(+)/Ca(2+) exchanger (NCX) plays a role in regulation of intracellular Ca(2+) concentration via the forward mode (Ca(2+) efflux) or the reverse mode (Ca(2+) influx). To define the physiological function of the exchanger in vivo, we generated mice deficient for NCX2, the major isoform in the brain. Mutant hippocampal neurons exhibited a significantly delayed clearance of elevated Ca(2+) following depolarization. The frequency threshold for LTP and LTD in the hippocampal CA1 region was shifted to a lowered frequency in the mutant mice, thereby favoring LTP. Behaviorally, the mutant mice exhibited enhanced performance in several hippocampus-dependent learning and memory tasks. These results demonstrate that NCX2 can be a temporal regulator of Ca(2+) homeostasis and as such is essential for the control of synaptic plasticity and cognition.     

Enhanced learning and memory in GAT1 heterozygous mice

γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the central nervous system. The termination of GABA transmission is through the action of a family of membrane proteins, called GABA transporters (GAT1-4). It is well established that GABA system is involved in the modulation of memory. Our previous study showed that homozygous GAT1(-/-) mice exhibited impaired hippocampus-dependent learning and memory. To evaluate the impact of endogenous reduced GABA reuptake on mice cognitive behaviors, the ability of learning and memory of heterozygous GAT1(+/-) mice was detected by the passive avoidance paradigm and Morris water maze. The hole board paradigm was also used to measure changes in anxiety-related behavior or exploratory behavior in such mice. As one form of synaptic plasticity, long-term potentiation was recorded in the mouse hippocampal CA1 area. We found that GAT1(+/-) mice displayed increased learning and memory, decreased anxiety-like behaviors, and highest synaptic plasticity compared with wild-type and homozygous GAT1(-/-) mice. Our results suggest that a moderate reduction in GAT1 activity causes the enhancement of learning and memory in mice.     

TRPV1 activation alleviates cognitive and synaptic plasticity impairments through inhibiting AMPAR endocytosis in APP23/PS45 mouse model of Alzheimer’s disease

Alzheimer's disease (AD) is one of the most common causes of neurodegenerative diseases in the elderly. The accumulation of amyloid‐β (Aβ) peptides is one of the pathological hallmarks of AD and leads to the impairments of synaptic plasticity and cognitive function. The transient receptor potential vanilloid 1 (TRPV1), a nonselective cation channel, is involved in synaptic plasticity and memory. However, the role of TRPV1 in AD pathogenesis remains largely elusive. Here, we reported that the expression of TRPV1 was decreased in the brain of APP23/PS45 double transgenic AD model mice. Genetic upregulation of TRPV1 by adeno‐associated virus (AAV) inhibited the APP processing and Aβ deposition in AD model mice. Meanwhile, upregulation of TRPV1 ameliorated the deficits of hippocampal CA1 long‐term potentiation (LTP) and spatial learning and memory through inhibiting GluA2‐containing α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid receptor (AMPAR) endocytosis. Furthermore, pharmacological activation of TRPV1 by capsaicin (1 mg/kg, i.p.), an agonist of TRPV1, dramatically reversed the impairments of hippocampal CA1 LTP and spatial learning and memory in AD model mice. Taken together, these results indicate that TRPV1 activation effectively ameliorates cognitive and synaptic functions through inhibiting AMPAR endocytosis in AD model mice and could be a novel molecule for AD treatment.