Tomi Taira

Long-Lasting GABA-Mediated Depolarization Evoked by High-Frequency Stimulation in Pyramidal Neurons of Rat Hippocampal Slice Is Attributable to a Network-Driven, Bicarbonate-Dependent K+ Transient

Biphasic GABAA-mediated postsynaptic responses can be readily evoked in CA1 pyramidal neurons of rat hippocampal slices by high-frequency stimulus (HFS) trains in the presence of ionotropic glutamate receptor antagonists. In the present experiments with sharp microelectrodes, whole-cell techniques, and K+-selective microelectrodes, an HFS train (40 pulses at 100 Hz) applied in stratum radiatum close to the recording site evoked a brief hyperpolarizing IPSP (hIPSP), which turned into a prolonged (2–3 sec) depolarization ( GABA-mediated depolarizing postsynaptic potential; GDPSP). The I–V relationships of the postsynaptic currents (hIPSC and GDPSC) had distinct characteristics: the hIPSC and the early GDPSC showed outward rectification, whereas the late GDPSC was reduced with positive voltage steps to zero or beyond (inward rectification), but often no clear reversal was seen. That two distinct currents contribute to the generation of the GDPSP was also evident from the finding that a second HFS train at peak or late GDPSP induced a prompt GABAA-mediated hyperpolarization. The GDPSP/C was dependent on the availability of bicarbonate, but not on interstitial or intrapyramidal carbonic anhydrase activity. The HFS train evoked a rapid GABAA-mediated bicarbonate-dependent increase in the extracellular K+ concentration ([K+]o), and the GDPSP followed the K+ transient in a sub-Nernstian manner. The spatial and pharmacological characteristics of the [K+]o shift indicated that it is generated by a local network of GABAergic interneurons. The brief ascending phase of the GDPSP is linked to a K+-dependent accumulation of intracellular Cl−. Thereafter, a nonsynaptic mechanism, a direct depolarizing effect of the [K+]oshift, is responsible for the most conspicuous characteristics of the GDPSP: its large amplitude and prolonged duration.

Transgenic mice overexpressing the full-length neurotrophin receptor trkB exhibit increased activation of the trkB–PLCγ pathway, reduced anxiety, and facilitated learning

We have investigated the biochemical, physiological, and behavioral properties of transgenic mice overexpressing the full-length neurotrophin receptor trkB (trkB.TK+). The highest trkB.TK+ mRNA overexpression was achieved in the cerebral cortex and hippocampal subfields, both areas also showing strongly increased trkB.TK+ receptor protein expression and phosphorylation. Furthermore, as a result of trkB.TK+ overexpression, partial activation of trkB downstream signaling was observed. Phosphorylation of phospholipaseCgamma-1 was increased but unexpectedly, the expression and phosphorylation levels of signaling molecules Shc and mitogen-activated protein kinase (MAPK) were unaltered. Behavioral studies revealed improved learning and memory in the water maze, contextual fear conditioning, and conditioned taste aversion tests, and reduced anxiety in the elevated plus maze (EPM) and light-dark exploration tests in trkB.TK+ transgenic mice. Electrophysiological studies revealed a reduced long-term potentiation (LTP) at the Schaffer collateral-CA1 synapse in trkB.TK+ mice. Altogether, overexpression of the trkB.TK+ receptor postnatally leads to selective activation of trkB signaling pathways and enhanced learning and memory.

Syndecan-3-Deficient Mice Exhibit Enhanced LTP and Impaired Hippocampus-Dependent Memory

Syndecan-3 (N-syndecan) is a transmembrane heparan sulfate proteoglycan expressed predominantly in the nervous system in a developmentally regulated manner. Syndecan-3 has been suggested to play a role in the development and plasticity of neuronal connections by linking extracellular signals to the regulation of the cytoskeleton. To study its physiological functions, we produced mice deficient in syndecan-3 by gene targeting. The mutant animals are healthy, are fertile, and have no apparent defects in the structure of the brain. We focused on characterizing the functions of the hippocampus, a brain area where expression of syndecan-3 is prominent in adults. Mice lacking syndecan-3 exhibited an enhanced level of long-term potentiation (LTP) in area CA1, while basal synaptic transmission and short-term plasticity were similar to those in wild-type animals. Further, the mutant mice were not responsive to the syndecan-3 ligand heparin-binding growth-associated molecule, which inhibits LTP in area CA1 in wild-type animals. Behavioral testing of the syndecan-3-deficient mice revealed impaired performance in tasks assessing hippocampal functioning. We suggest that syndecan-3 acts as an important modulator of synaptic plasticity that influences hippocampus-dependent memory.

Enhanced Hippocampal Long-Term Potentiation in Mice Lacking Heparin-Binding Growth-Associated Molecule

Heparin-binding growth-associated molecule (HB-GAM) (pleiotrophin) is a highly conserved extracellular matrix-associated protein implicated in a diverse range of developmental processes, including the formation and plasticity of neuronal connections. Using gene targeting, we have in the present study created HB-GAM-deficient mice that are viable and fertile and show no gross anatomical abnormalities. The hippocampal structure as well as basal excitatory synaptic transmission in the area CA1 appear normal in the mice lacking HB-GAM. However, hippocampal slices from HB-GAM-deficient mice display a lowered threshold for induction of long-term potentiation (LTP), which reverts back to the wild-type level by application of HB-GAM. HB-GAM expression in hippocampus is activity-dependent and upregulated in several neuropathological conditions. Thus, we suggest that HB-GAM acts as an inducible signal to inhibit LTP in hippocampus.

Functional maturation of CA1 synapses involves activity-dependent loss of tonic kainate receptor-mediated inhibition of glutamate release.

Early in development, excitatory synapses transmit with low efficacy, one mechanism for which is a low probability of transmitter release (Pr). However, little is known about the developmental mechanisms that control activity-dependent maturation of the presynaptic release. Here, we show that during early development, transmission at CA3-CA1 synapses is regulated by a high-affinity, G protein-dependent kainate receptor (KAR), which is endogenously activated by ambient glutamate. By tonically depressing glutamate release, this mechanism sets the dynamic properties of neonatal inputs to favor transmission during high frequency bursts of activity, typical for developing neuronal networks. In response to induction of LTP, the tonic activation of KAR is rapidly down regulated, causing an increase in Pr and profoundly changing the dynamic properties of transmission. Early development of the glutamatergic connectivity thus involves an activity-dependent loss of presynaptic KAR function producing maturation in the mode of excitatory transmission from CA3 to CA1.

Effects of C02 on excitatory transmission apparently caused by changes in intracellular pH in the rat hippocampal slice

Abstract It is generally known that hyperventilation produces an increase in neuronal excitability. However, the mechanism whereby a change in CO2 partial pressure (Pco2) leads to changes in neural excitability is not known. We have studied this phenomenon in rat hippocampal slices using double-barrelled microelectrodes for simultaneous recording of field excitatory postsynaptic potentials (EPSPs) and extracellular pH in stratum radiatum of area CAI. A drop in Pco2 from the control level, 36 mmHg to 7 mmHg, produced an increase in extracellular pH of 0.4–0.6 pH units and a transient increase in EPSP slope by about 20–30%. Despite the stable extracellular alkalosis, the EPSP reverted back to its original level within 10 min. Switching back to 36 mmHg Pco2 restored the original extracellular pH and caused a transient decrease in the EPSP slope. Pharmacological blockade of NMDA receptor and/or GABAA receptor had no influence on the effects of C02. An increase in PCO2 to 145 mmHg led to a stable fall in extracellular pH by 0.6 units and to a transient 30–50% decrease in EPSP slope. The above results indicate that the CO2-induced changes in neuronal excitability were not caused by changes in extracellular pH but they might have been mediated by changes in intracellular pH. Indeed, exposing the slices to the permeant weak base, trimethylamine (20 mM), which is known to produce a rise in intracellular pH, increased the EPSP slope by 50–70%. Application of 20 mM propionate (a permeant weak acid) decreased the EPSP slope by 40–60%. We conclude that the transient changes in the EPSP seen in response to changes in Pco2 are mediated by in intracellular pH.

pH transients due to monosynaptic activation of GABAA receptors in rat hippocampal slices.

Extracellular pH transients were evoked in rat hippocampal brain slices by activation of a monosynaptic inhibitory pathway following pharmacological blockade of glutaminergic transmission. Repetitive stimulation in stratum radiatum near the recording site in stratum pyramidale evoked an immediate alkaline shift which was potentiated by pentobarbital and blocked by picrotoxin but not by 2-hydroxy-saclofen. Benzolamide, a poorly permeant inhibitor of carbonic anhydrase (CA), and prontosil-dextran 5000, a macromolecular CA inhibitor, abolished the alkaline transients evoked by stimulation and by exogenous GABA. Thus an extracellular CA is involved in regulating interstitial pH in brain, and the stimulation-induced alkaline transients are caused by net influx of CO2 into CA1 neurons in response to efflux of bicarbonate across postsynaptic GABAA receptor channels.

Endogenous Activation of Kainate Receptors Regulates Glutamate Release and Network Activity in the Developing Hippocampus

Kainate receptors (KARs) are highly expressed throughout the neonatal brain, but their function during development is unclear. Here, we show that the maturation of the hippocampus is associated with a switch in the functional role of presynaptic KARs. In a developmental period restricted to the first postnatal week, endogenous l-glutamate tonically activates KARs at CA3 glutamatergic synapses to regulate release in an action potential-independent manner. At synapses onto pyramidal cells, KARs inhibit glutamate release via a G-protein and PKC-dependent mechanism. In contrast, at glutamatergic terminals onto CA3 interneurons, presynaptic KARs can facilitate release in a G-protein-independent mechanism. In both cell types, however, KAR activation strongly upregulates inhibitory transmission. We show that, through the interplay of these novel diverse mechanisms, KARs strongly regulate the characteristic synchronous network activity observed in the neonatal hippocampus. By virtue of this, KARs are likely to play a central role in the development of hippocampal synaptic circuits.

Role of heparin-binding growth-associated molecule (HB-GAM) in hippocampal LTP and spatial learning revealed by studies on overexpressing and knockout mice

Heparin-binding growth-associated molecule (HB-GAM) is an extracellular matrix-associated protein with neurite outgrowth-promoting activity and which is suggested to be implicated in hippocampal synaptic plasticity. To study the functions of HB-GAM in adult brain we have produced HB-GAM overexpressing mice and compared phenotypic changes in the transgenic mice to those in the HB-GAM null mice. Both mutants were viable and displayed no gross morphological abnormalities. The basal synaptic transmission was normal in the area CA1 of hippocampal slices from the genetically modified mice. However, long-term potentiation (LTP) was attenuated in the mice overexpressing HB-GAM, whereas enhanced LTP was detected in the HB-GAM-deficient mice. Changes in LTP seen in vitro were paralleled by behavioral alterations in vivo. The animals overexpressing HB-GAM displayed faster learning in water maze and decreased anxiety in elevated plus-maze, while the HB-GAM knockouts demonstrated an opposite behavioral phenotype. These results show that HB-GAM suppresses LTP in hippocampus and plays a role in regulation of learning-related behavior.

Presynaptic mechanisms involved in the expression of STP and LTP at CA1 synapses in the hippocampus

The study of long-term potentiation (LTP) has for many years been the centre of a raging debate as to whether the process is expressed by presynaptic or postsynaptic mechanisms. Here we present evidence that two forms of synaptic plasticity at CA3-CA1 synapses in the hippocampus are expressed by presynaptic changes. One form is short-term potentiation (STP) and the other a neonatal form of early-LTP (E-LTP). We review recent experimental data that suggests that this latter form of LTP involves an increase in the probability of neurotransmitter release (Pr). We describe how this is caused by the rapid down-regulation of a high affinity kainate receptor, which otherwise responds to ambient levels of l-glutamate by depressing Pr.