CA1 Neurons Of Mice Research Articles

Persistent representation of the environment in the hippocampus

In the hippocampus, environmental changes elicit rearrangement of active neuronal ensembles or remapping of place cells. However, it remains elusive how the brain ensures a consistent representation of a certain environment itself despite salient events occurring there. Here, we longitudinally tracked calcium dynamics of dorsal hippocampal CA1 neurons in mice subjected to contextual fear conditioning and extinction training. Overall population activities were significantly changed by fear conditioning and were responsive to footshocks and freezing. However, a small subset of neurons, termed environment cells, were consistently active in a specific environment irrespective of experiences. A decoder modeling study showed that these cells, but not place cells, were able to predict the environment to which the mouse was exposed. Environment cells might underlie the constancy of cognition for distinct environments across time and events. Additionally, our study highlights the functional heterogeneity of cells in the hippocampus.

Synaptic plasticity in hippocampal CA1 neurons of mice lacking inositol-1,4,5-trisphosphate receptor-binding protein released with IP3 (IRBIT).

In hippocampal CA1 neurons of wild-type mice, a short tetanus (15 or 20 pulses at 100 Hz) or a standard tetanus (100 pulses at 100 Hz) to a naive input pathway induces long-term potentiation (LTP) of the responses. Low-frequency stimulation (LFS; 1000 pulses at 1 Hz) 60 min after the standard tetanus reverses LTP (depotentiation [DP]), while LFS applied 60 min prior to the standard tetanus suppresses LTP induction (LTP suppression). We investigated LTP, DP, and LTP suppression of both field excitatory postsynaptic potentials and population spikes in CA1 neurons of mice lacking the inositol 1,4,5-trisphosphate (IP3) receptor (IP3R)-binding protein released with IP3 (IRBIT). The mean magnitudes of LTP induced by short and standard tetanus were not different in mutant and wild-type mice. In contrast, DP and LTP suppression were attenuated in mutant mice, whereby the mean magnitude of responses after LFS or tetanus were significantly greater than in wild-type mice. These results suggest that, in hippocampal CA1 neurons, IRBIT is involved in DP and LTP suppression, but is not essential for LTP. The attenuation of DP and LTP suppression in mice lacking IRBIT indicates that this protein, released during or after priming stimulations, determines the direction of LTP expression after the delivery of subsequent stimulations.

Sodium sensitivity of KNa channels in mouse CA1 neurons.

Potassium channels play an important role regulating transmembrane electrical activity in essentially all cell types. We were especially interested in those that determine the intrinsic electrical properties of mammalian central neurons. Over 30 different potassium channels have been molecularly identified in brain neurons, but there often is not a clear distinction between molecular structure and the function of a particular channel in the cell. Using patch-clamp methods to search for single potassium channels in excised inside-out (ISO) somatic patches with symmetrical potassium, we found that nearly all patches contained non-voltage-inactivating channels with a single-channel conductance of 100-200 pS. This conductance range is consistent with the family of sodium-activated potassium channels (Slo2.1, Slo2.2, or collectively, KNa). The activity of these channels was positively correlated with a low cytoplasmic Na+ concentration (2-20 mM). Cell-attached recordings from intact neurons, however, showed little or no activity of this K+ channel. Attempts to increase channel activity by increasing intracellular sodium concentration ([Na+]i) with bursts of action potentials or direct perfusion of Na+ through a whole cell pipette had little effect on KNa channel activity. Furthermore, excised outside-out (OSO) patches across a range of intracellular [Na+] showed less channel activity than we had seen with excised ISO patches. Blocking the Na+/K+ pump with ouabain increased the activity of the KNa channels in excised OSO patches to levels comparable with ISO-excised patches. Our results suggest that despite their apparent high levels of expression, the activity of somatic KNa channels is tightly regulated by the activity of the Na+/K+ pump.NEW & NOTEWORTHY We studied KNa channels in mouse hippocampal CA1 neurons. Excised inside-out patches showed the channels to be prevalent and active in most patches in the presence of Na+. Cell-attached recordings from intact neurons, however, showed little channel activity. Increasing cytoplasmic sodium in intact cells showed a small effect on channel activity compared with that seen in inside-out excised patches. Blockade of the Na+/K+ pump with ouabain, however, restored the activity of the channels to that seen in inside-out patches.

Urethane anesthesia suppresses hippocampal subthreshold activity and neuronal synchronization

Urethane, an anesthetic utilized for animal experiments, induces neocortical slow oscillations in which a large number of neurons emit rhythmic synchronized activity. However, it remains unclear how urethane affects neuronal activity in the hippocampus. In this study, we obtained in vivo patch-clamp recordings from dorsal hippocampal CA1 neurons in mice and found a reduction in the fluctuation of subthreshold membrane potentials during urethane anesthesia, implying reduced synaptic activity in the hippocampus. We then performed spike unit recordings from dorsal hippocampal CA1 neuronal ensembles in rats and found prominent reductions in the spike rates of the majority of hippocampal units, especially spatially selective units, during urethane anesthesia, whereas a subset of nonspatial units exhibited increased spike rates. The overall reductions in neuronal spike rates induced by urethane led to prominent decreases in spike synchronization across neuronal units. Consistently, the magnitude of hippocampal sharp wave ripples was also reduced by urethane. The suppression of hippocampal neuronal synchronization by urethane may lead to the disruption of offline memory reactivation mechanisms.

Transcriptomopathies of pre- and post-symptomatic frontotemporal dementia-like mice with TDP-43 depletion in forebrain neurons

TAR DNA-binding protein (TDP-43) is a ubiquitously expressed nuclear protein, which participates in a number of cellular processes and has been identified as the major pathological factor in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Here we constructed a conditional TDP-43 mouse with depletion of TDP-43 in the mouse forebrain and find that the mice exhibit a whole spectrum of age-dependent frontotemporal dementia-like behaviour abnormalities including perturbation of social behaviour, development of dementia-like behaviour, changes of activities of daily living, and memory loss at a later stage of life. These variations are accompanied with inflammation, neurodegeneration, and abnormal synaptic plasticity of the mouse CA1 neurons. Importantly, analysis of the cortical RNA transcripts of the conditional knockout mice at the pre−/post-symptomatic stages and the corresponding wild type mice reveals age-dependent alterations in the expression levels and RNA processing patterns of a set of genes closely associated with inflammation, social behaviour, synaptic plasticity, and neuron survival. This study not only supports the scenario that loss-of-function of TDP-43 in mice may recapitulate key behaviour features of the FTLD diseases, but also provides a list of TDP-43 target genes/transcript isoforms useful for future therapeutic research.

Exercise Prevents Memory Consolidation Defects Via Enhancing Prolactin Responsiveness of CA1 Neurons in Mice Under Chronic Stress.

We investigated the effects of regular exercise on chronic stress-induced memory consolidation impairment and its underlying mechanism. We focused on prolactin (PRL)-modulated calcium-permeable (CP)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (AMPARs) in neurons in the CA1 stratum lacunosum-moleculare (SLM) area of the dorsal hippocampus. Regular exercise protected against memory retention defects and prevented dendritic retraction in apical distal segments of hippocampal CA1 neurons, as indicated by enhanced dendritic ramification, dendritic length, spine density, and synaptic protein levels following chronic stress. Regular exercise normalized synaptic CP-AMPAR assembly in the hippocampal CA1 SLM area, as evidenced by an enhanced ratio of GluR1 to GluR2 during chronic stress. This alteration in AMPARs was critical to memory retention, whereby memory retention was blunted by local blockage of CP-AMPARs in the SLM of naïve and exercised mice. Regular exercise improved PRL responsiveness in the hippocampal CA1 region during chronic stress, which led to increased binding of PRL to its receptor (PRLR) and PRL-dependent enhancement in phosphorylated signal transducer and activator of transcription 5 levels. The improvement in PRL responsiveness contributed to memory retention during chronic stress, as the protective action of exercise on memory persistence during stress was abolished by PRLR knockdown in the hippocampal CA1 area. Finally, in primary hippocampal cultures, repeated treatment with corticosterone led to decreased AMPAR-mediated Ca2+ influx, which was restored by PRL treatment. The above findings suggest a protective role for exercise against chronic stress-evoked defects in memory consolidation via PRL-modulated incorporation of CP-AMPARs into hippocampal CA1 synapses.

Endothelium-Derived Semaphorin 3G Regulates Hippocampal Synaptic Structure and Plasticity via Neuropilin-2/PlexinA4

The proper interactions between blood vessels and neurons are critical for maintaining the strength of neural circuits and cognitive function. However, the precise molecular events underlying these interactions remain largely unknown. Here, we report that the selective knockout of semaphorin 3G (Sema3G) in endothelial cells impaired hippocampal-dependent memory and reduced dendritic spine density in CA1 neurons in mice; these effects were reversed after restoration of Sema3G levels in the hippocampus by AAV transfection. We further show that Sema3G increased excitatory synapse density via neuropilin-2/PlexinA4 signaling and through activation of Rac1. These results provide the first evidence that, in the central nervous system, endothelial Sema3G serves as a vascular-derived synaptic organizer that regulates synaptic plasticity and hippocampal-dependent memory. Our findings highlight the role of vascular endothelial cells in regulating cognitive function through intercellular communication with neurons in the hippocampus.

Effect of a hallucinogenic serotonin 5-HT2A receptor agonist on visually guided, hippocampal-dependent spatial cognition in C57BL/6J mice.

By acting on serotonin 5-HT2A receptors (5-HT2A Rs), serotonergic psychedelic drugs induce perceptual and visual hallucinations by increasing neuronal excitability and altering visual-evoked neuronal responses. The present study was designed to examine whether the perceptual alterations induced by a serotonergic psychedelic drug would affect the integrity of hippocampal-dependent, visually guided spatial cognition. phenylalkylamine hallucinogen TCB-2 is a selective agonist of 5-HT2A Rs. Mice received TCB-2 (1.0 mgkg-1 , i.p.), and spatial behaviors and hippocampal electrophysiological responses were measured with water maze tasks and in vivo single-unit recording, respectively. TCB-2 did not affect visual cue approach behavior in the visible platform water maze, but increased the latency of trained mice to initiate goal-directed swimming during a probe test in the hidden platform Morris water maze, which could be prevented by 5-HT2A R antagonist MDL 11,939. Interestingly, TCB-2 did not affect the efficiency of the swim path or the proper use of distal visual cues during the probe test. Hippocampal place cell activity is considered to represent spatial and context-specific episodic memory. Systemic TCB-2 did not affect previously established place fields of CA1 neurons in mice exploring a familiar environment, or the remapping of place cells when the mice explored a novel environment. However, TCB-2 impaired the long-term stability of place fields for the novel environment initially encoded under the influence of TCB-2, which could be prevented by 5-HT2A R antagonist MDL 11,939. Our data indicate that hallucinogenic 5-HT2A R agonist delays the initiation of spatial search behavior, but does not impair the use of visual cues to guide goal-directed spatial behavior. Moreover, activation of 5-HT2A Rs does not impair the coding and retrieval of spatial information, but impairs the long-term stability of new formed place fields of CA1 neurons. © 2017 Wiley Periodicals, Inc.

Brivaracetam differentially affects voltage-gated sodium currents without impairing sustained repetitive firing in neurons.

SummaryAimsBrivaracetam (BRV) is an antiepileptic drug in Phase III clinical development. BRV binds to synaptic vesicle 2A (SV2A) protein and is also suggested to inhibit voltage‐gated sodium channels (VGSCs). To evaluate whether the effect of BRV on VGSCs represents a relevant mechanism participating in its antiepileptic properties, we explored the pharmacology of BRV on VGSCs in different cell systems and tested its efficacy at reducing the sustained repetitive firing (SRF).MethodsBrivaracetam investigations on the voltage‐gated sodium current (IN a) were performed in N1E‐155 neuroblastoma cells, cultured rat cortical neurons, and adult mouse CA1 neurons. SRF was measured in cultured cortical neurons and in CA1 neurons. All BRV (100–300 μM) experiments were performed in comparison with 100 μM carbamazepine (CBZ).ResultsBrivaracetam and CBZ reduced IN a in N1E‐115 cells (30% and 40%, respectively) and primary cortical neurons (21% and 47%, respectively) by modulating the fast‐inactivated state of VGSCs. BRV, in contrast to CBZ, did not affect IN a in CA1 neurons and SRF in cortical and CA1 neurons. CBZ consistently inhibited neuronal SRF by 75–93%.ConclusionsThe lack of effect of BRV on SRF in neurons suggests that the reported inhibition of BRV on VGSC currents does not contribute to its antiepileptic properties.

Antioxidant effect of salvianolic acid B on hippocampal CA1 neurons in mice with cerebral ischemia and reperfusion injury

To investigate the neuroprotective effects and underlying mechanisms of salvianolic acid B (Sal B) extracted from Salvia miltiorrhiza on hippocampal CA1 neurons in mice with cerebral ischemia reperfusion injury. Forty male National Institute of Health (NIH) mice were randomly divided into 4 groups with 10 animals each, including the sham group, the model group, the SalB group (SalB 22.5 mg/kg) and the nimodipine (Nim) group (Nim 1 mg/kg). A mouse model of cerebral ischemia and reperfusion injury was established by bilateral carotid artery occlusion for 30 min followed by 24-h reperfusion. The malondialdehyde (MDA) content, the nitric oxide synthase (NOS) activity, the superoxide dismutase (SOD) activity and total antioxidant capability (T-AOC) of the pallium were determined by biochemistry methods. The morphologic changes and Bcl-2 and Bax protein expression in hippocampal CA1 neurons were observed by using hematoxylineosin staining and immunohistochemistry staining, respectively. In the SalB group, the MDA content and the NOS activity of the pallium in cerebral ischemia-reperfusion mice significantly decreased and the SOD activity and the T-AOC significantly increased, as compared with the model group (P<0.05 or P<0.01). The SalB treatment also rescued neuronal loss (P<0.01) in the hippocampal CA1 region, strongly promoted Bcl-2 protein expression (P<0.01) and inhibited Bax protein expression (P<0.05). SalB increases the level of antioxidant substances and decreases free radicals production. Moreover, it also improves Bcl-2 expression and reduces Bax expression. SalB may exert the neuroprotective effect through mitochondria-dependent pathway on hippocampal CA1 neurons in mice with cerebral ischemia and reperfusion injury and suggested that SalB represents a promising candidate for the prevention and treatment of ischemic cerebrovascular disease.

Caspase-6 activity in the CA1 region of the hippocampus induces age-dependent memory impairment

Active Caspase-6 is abundant in the neuropil threads, neuritic plaques and neurofibrillary tangles of Alzheimer disease brains. However, its contribution to the pathophysiology of Alzheimer disease is unclear. Here, we show that higher levels of Caspase-6 activity in the CA1 region of aged human hippocampi correlate with lower cognitive performance. To determine whether Caspase-6 activity, in the absence of plaques and tangles, is sufficient to cause memory deficits, we generated a transgenic knock-in mouse that expresses a self-activated form of human Caspase-6 in the CA1. This Caspase-6 mouse develops age-dependent spatial and episodic memory impairment. Caspase-6 induces neuronal degeneration and inflammation. We conclude that Caspase-6 activation in mouse CA1 neurons is sufficient to induce neuronal degeneration and age-dependent memory impairment. These results indicate that Caspase-6 activity in CA1 could be responsible for the lower cognitive performance of aged humans. Consequently, preventing or inhibiting Caspase-6 activity in the aged may provide an efficient novel therapeutic approach against Alzheimer disease.

Intracellular calcium ion dynamics involved in long-term potentiation in hippocampal CA1 neurons in mice lacking the IP 3 type 1 receptor

In the present study, mice lacking the type 1 inositol-1,4,5-trisphosphate receptor (IP 3R) were used to study the role of type 1 IP 3Rs in the induction of long-term potentiation (LTP) in hippocampal CA1 neurons. The magnitude of the LTP induced by high frequency stimulation (HFS) consisting of 20 pulses at 30 Hz in mice lacking type 1 IP 3Rs was significantly larger than that in wild-type mice in terms of the field excitatory postsynaptic potential and population spike. By measuring changes in the intracellular Ca 2+ concentration ([Ca 2+] i) in CA1 pyramidal neurons using fluorometry, we found that the decay time of the transient increase in the [Ca 2+] i evoked by the HFS in mutant mice was significantly longer than that in wild-type mice, whereas the [Ca 2+] i at rest and the magnitude of the [Ca 2+] i increases caused by the HFS were no different from those in wild-type mice. In slices from the mutant mice, paired-pulse stimulation (PPS) delivered at an interval of 10 ms resulted in significantly weaker paired-pulse inhibition (PPI) than in wild-type mice, suggesting that lack of type 1 IP 3Rs reduces the PPI induced by PPS in the CA1 region. These results indicate that a lack of type 1 IP 3Rs causes a slower decay of the transient [Ca 2+] i in CA1 pyramidal neurons and attenuates the activity of inhibitory interneurons, resulting in enhancement of LTP induction.

Interferon-γ Prevents Death of Bystander Neurons during CD8 T Cell Responses in the Brain

T cells restricted to neurotropic viruses are potentially harmful as their activity may result in the destruction of neurons. In the Borna disease virus (BDV) model, antiviral CD8 T cells entering the brain of infected mice cause neurological disease but no substantial loss of neurons unless the animals lack interferon-gamma (IFN-gamma). We show here that glutamate receptor antagonists failed to prevent BDV-induced neuronal loss in IFN-gamma-deficient mice, suggesting that excitotoxicity resulting from glutamate receptor overstimulation is an unlikely explanation for the neuronal damage. Experiments with IFN-gamma-deficient mice lacking eosinophils indicated that these cells, which specifically accumulate in the infected brains of IFN-gamma-deficient mice, are not responsible for CA1 neuronal death. Interestingly, BDV-induced damage of CA1 neurons was reduced significantly in IFN-gamma-deficient mice lacking perforin, suggesting a key role for CD8 T cells in this pathological process. Specific death of hippocampal CA1 neurons could be triggered by adoptive transfer of BDV-specific CD8 T cells from IFN-gamma-deficient mice into uninfected mice that express transgene-encoded BDV antigen at high level in astrocytes. These results indicate that attack by CD8 T cells that cause the death of CA1 neurons might be directed toward regional astrocytes and that IFN-gamma protects vulnerable CA1 neurons from collateral damage resulting from exposure to potentially toxic substances generated as a result of CD8 T cell-mediated impairment of astrocyte function.

Anion channel blockers attenuate delayed neuronal cell death induced by transient forebrain ischemia

Chloride efflux is known to be involved in the progression of apoptosis in various cell types. We have recently shown that the volume-sensitive outwardly rectifying (VSOR) anion channel serves as the pathway for apoptotic chloride efflux in some cells. In the present study, we tested the neuroprotective effects of drugs that can block the VSOR anion channel, on delayed neuronal death (DND) induced by transient forebrain ischemia. The functional expression of the VSOR anion channel was first examined in hippocampal neurons in both primary culture and hippocampal slice preparations, by the whole-cell patch-clamp technique. We then tested the channel's sensitivity to an anion channel blocker, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), and a tyrosine kinase blocker, genistein. By histological examinations and cytochrome c release assessments, the protective effects of these drugs on the DND of hippocampal CA1 neurons in mice subjected to transient ischemia were examined. Drugs were administered via the jugular vein prior to ischemic treatment and into the peritoneal cavity after reperfusion. Hippocampal neurons were found to express the volume-sensitive Cl(-) channel, which exhibits outward rectification and is sensitive to DIDS and genistein. Administration of DIDS or genistein reduced cytochrome c release and the number of damaged neurons in the CA1 region after transient forebrain ischemia. This fact suggests that the DND induction mechanism involves the activity of the VSOR anion channel and that this channel may provide a therapeutic target for the treatment of stroke.

Inhibitory Gating Modulation of Small Conductance Ca2+-Activated K+Channels by the Synthetic Compound (R)-N-(Benzimidazol-2-yl)-1,2,3,4-tetrahydro-1-naphtylamine (NS8593) Reduces Afterhyperpolarizing Current in Hippocampal CA1 Neurons

SK channels are small conductance Ca(2+)-activated K(+) channels important for the control of neuronal excitability, the fine tuning of firing patterns, and the regulation of synaptic mechanisms. The classic SK channel pharmacology has largely focused on the peptide apamin, which acts extracellularly by a pore-blocking mechanism. 1-Ethyl-2-benzimidazolinone (1-EBIO) and 6,7-dichloro-1H-indole-2,3-dione 3-oxime (NS309) have been identified as positive gating modulators that increase the apparent Ca(2+) sensitivity of SK channels. In the present study, we describe inhibitory gating modulation as a novel principle for selective inhibition of SK channels. In whole-cell patch-clamp experiments, the compound (R)-N-(benzimidazol-2-yl)-1,2,3,4-tetrahydro-1-naphtylamine (NS8593) reversibly inhibited recombinant SK3-mediated currents (human SK3 and rat SK3) with potencies around 100 nM. However, in contrast to known pore blockers, NS8593 did not inhibit (125)I-apamin binding. Using excised patches, it was demonstrated that NS8593 decreased the Ca(2+) sensitivity by shifting the activation curve for Ca(2+) to the right, only slightly affecting the maximal Ca(2+)-activated SK current. NS8593 inhibited all the SK1-3 subtypes Ca(2+)-dependently (K(d) = 0.42, 0.60, and 0.73 microM, respectively, at 0.5 microM Ca(2+)), whereas the compound did not affect the Ca(2+)-activated K(+) channels of intermediate and large conductance (hIK and hBK channels, respectively). The site of action was accessible from both sides of the membrane, and the NS8593-mediated inhibition was prevented in the presence of a high concentration of the positive modulator NS309. NS8593 was further tested on mouse CA1 neurons in hippocampal slices and shown to inhibit the apaminand tubocurarine-sensitive SK-mediated afterhyperpolarizing current, at a concentration of 3 microM.

A Spontaneous Mutation InvolvingKcnq2(Kv7.2) Reduces M-Current Density and Spike Frequency Adaptation in Mouse CA1 Neurons

The M-type K+ current [IK(M)] activates in response to membrane depolarization and regulates neuronal excitability. Mutations in two subunits (KCNQ2 and KCNQ3; Kv7.2 and Kv7.3) that underlie the M-channel cause the human seizure disorder benign familial neonatal convulsions (BFNC), presumably by reducing IK(M) function. In mice, the Szt1 mutation, which deletes the genomic DNA encoding the KCNQ2 C terminus and all of CHRNA4 (nicotinic acetylcholine receptor alpha4 subunit) and ARFGAP-1 (GTPase-activating protein that inactivates ADP-ribosylation factor 1), reduces seizure threshold, and alters M-channel pharmacosensitivity. Genomic deletions affecting the C terminus of KCNQ2 have been identified in human families with BFNC, and truncation of the C terminus prevents proper KCNQ2/KCNQ3 channel assembly in Xenopus oocytes. We showed previously that Szt1 mice have a reduced baseline seizure threshold and altered sensitivity to drugs that act at the M-channel. Specifically, the proconvulsant M-channel blocker linopirdine and anticonvulsant enhancer retigabine display increased and decreased potency, respectively, in Szt1 mice. To investigate the effects of the Szt1 mutation on IK(M) function explicitly, perforated-patch electrophysiology was performed in CA1 pyramidal neurons of the hippocampus in brain slices prepared from C57BL/6J-Szt1/+ and control C57BL/6J+/+ mice. Our results show that Szt1 reduces both IK(M) amplitude and current density, inhibits spike frequency adaptation, and alters many aspects of M-channel pharmacology. This is the first evidence that a naturally occurring Kcnq2 mutation diminishes the amplitude and function of the native neuronal IK(M), resulting in significantly increased neuronal excitability. Finally, the changes in single-cell biophysical properties likely underlie the altered seizure threshold and pharmacosensitivity reported previously in Szt1 mice.

Long-term potentiation and long-term depression in hippocampal CA1 neurons of mice lacking the IP 3 type 1 receptor

To investigate the role in synaptic plasticity of Ca 2+ released from intracellular Ca 2+ stores, mice lacking the inositol 1,4,5-trisphosphate type 1 receptor were developed and the physiological properties, long-term potentiation, and long-term depression of their hippocampal CA1 neurons were examined. There were no significant differences in basic synaptic functions, such as membrane properties and the input/output relationship, between homozygote mutant and wild-type mice. Enhanced paired-pulse facilitation at interpulse intervals of less than 60 ms and enhanced post-tetanic potentiation were observed in the mutant mice, suggesting that the presynaptic mechanism was altered by the absence of the inositol 1,4,5-trisphosphate type 1 receptor. Long-term potentiation in the field-excitatory postsynaptic potentials induced by tetanus (100 Hz, 1 s) and the excitatory postsynaptic currents induced by paired stimulation in hippocampal CA1 pyramidal neurons under whole-cell clamp conditions were significantly greater in mutant mice than in wild-type mice. Homosynaptic long-term depression of CA1 synaptic responses induced by low-frequency stimulation (1 Hz, 500 pulses) was not significantly different, but heterosynaptic depression of the non-associated pathway induced by tetanus was blocked in the mutant mice. Both long-term potentiation and long-term depression in mutant mice were completely dependent on N-methyl- d-aspartate receptor activity. To rule out the possibility of an effect compensating for the lack of the inositol 1,4,5-trisphosphate type 1 receptor occurring during development, an anti-inositol 1,4,5-trisphosphate type 1 receptor monoclonal antibody that blocks receptor function was diffused into the wild-type cell through a patch pipette, and the effect of acute block of inositol 1,4,5-trisphosphate type 1 receptor on long-term potentiation was examined. Significant enhancement of long-term potentiation was observed compared with after control immunoglobulin G injection, suggesting that developmental redundancy was not responsible for the increase in long-term potentiation amplitude observed in the mutant mouse. The properties of channels that could be involved in long-term potentiation induction were examined using whole-cell recording. N-methyl- d-aspartate currents were significantly larger in mutant mice than in wild-type mice only between holding potentials of −60 and −80 mV. We conclude that inositol 1,4,5-trisphosphate type 1 receptor activity is not essential for the induction of synaptic plasticity in hippocampal CA1 neurons, but appears to negatively regulate long-term potentiation induction by mild modulation of channel activities.

Role of Na(+)/H(+) exchanger during O(2) deprivation in mouse CA1 neurons.

To determine the role of membrane transporters in intracellular pH (pH(i)) regulation under conditions of low microenvironmental O(2), we monitored pH(i) in isolated single CA1 neurons using the fluorescent indicator carboxyseminaphthorhodafluor-1 and confocal microscopy. After total O(2) deprivation or anoxia (PO(2) approximately equal to 0 Torr), a large increase in pH(i) was seen in CA1 neurons in HEPES buffer, but a drop in pH(i), albeit small, was observed in the presence of HCO(3)(-). Ionic substitution and pharmacological experiments showed that the large anoxia-induced pH(i) increase in HEPES buffer was totally Na(+) dependent and was blocked by HOE-694, strongly suggesting the activation of the Na(+)/H(+) exchanger (NHE). Also, this pH(i) increase in HEPES buffer was significantly smaller in Na(+)/H(+) exchanger isoform 1 (NHE1) null mutant CA1 neurons than in wild-type neurons, demonstrating that NHE1 is responsible for part of the pH(i) increase following anoxia. Both chelerythrine and H-89 partly blocked, and H-7 totally eliminated, this anoxia-induced pH(i) increase in the absence of HCO. We conclude that 1) O(2) deprivation activates Na(+)/H(+) exchange by enhancing protein kinase activity and 2) membrane proteins, such as NHE, actively participate in regulating pH(i) during low-O(2) states in neurons.

Synaptic plasticity in hippocampal CA1 neurons of mice lacking type 1 inositol-1,4,5-trisphosphate receptors.

In hippocampal CA1 neurons of wild-type mice, delivery of a standard tetanus (100 pulses at 100 Hz) or a train of low-frequency stimuli (LFS; 1000 pulses at 1 Hz) to a naive input pathway induces, respectively, long-term potentiation (LTP) or long-term depression (LTD) of responses, and delivery of LFS 60 min after tetanus results in reversal of LTP (depotentiation, DP), while LFS applied 60 min before tetanus suppresses LTP induction (LTP suppression). To evaluate the role of the type 1 inositol-1,4,5-trisphosphate receptor (IP3R1) in hippocampal synaptic plasticity, we studied LTP, LTD, DP, and LTP suppression of the field excitatory postsynaptic potentials (EPSPs) in the CA1 neurons of mice lacking the IP3R1. No differences were seen between mutant and wild-type mice in terms of the mean magnitude of the LTP or LTD induced by a standard tetanus or LFS. However, the mean magnitude of the LTP induced by a short tetanus (10 pulses at 100 Hz) was significantly greater in mutant mice than in wild-type mice. In addition, DP or LTP suppression was attenuated in the mutant mice, the mean magnitude of the responses after delivery of LFS or tetanus being significantly greater than in wild-type mice. These results suggest that, in hippocampal CA1 neurons, the IP3R1 is involved in LTP, DP, and LTP suppression but is not essential for LTD. The facilitation of LTP induction and attenuation of DP and LTP suppression seen in mice lacking the IP3R1 indicates that this receptor plays an important role in blocking synaptic potentiation in hippocampal CA1 neurons.