Yuji Ikegaya

Mossy fiber Zn2+ spillover modulates heterosynaptic N-methyl-D-aspartate receptor activity in hippocampal CA3 circuits.

Although Zn2+ is contained in large amounts in the synaptic terminals of hippocampal mossy fibers (MFs), its physiological role in synaptic transmission is poorly understood. By using the newly developed high-sensitivity Zn2+ indicator ZnAF-2, the spatiotemporal dynamics of Zn2+ was monitored in rat hippocampal slices. When high-frequency stimulation was delivered to the MFs, the concentration of extracellular Zn2+ was immediately elevated in the stratum lucidum, followed by a mild increase in the stratum radiatum adjacent to the stratum lucidum, but not in the distal area of stratum radiatum. The Zn2+ increase was insensitive to a non–N-methyl-d-aspartate (NMDA) receptor antagonist but was efficiently attenuated by tetrodotoxin or Ca2+-free medium, suggesting that Zn2+ is released by MF synaptic terminals in an activity-dependent manner, and thereafter diffuses extracellularly into the neighboring stratum radiatum. Electrophysiological analyses revealed that NMDA receptor–mediated synaptic responses in CA3 proximal stratum radiatum were inhibited in the immediate aftermath of MF activation and that this inhibition was no longer observed in the presence of a Zn2+-chelating agent. Thus, Zn2+ serves as a spatiotemporal mediator in imprinting the history of MF activity in contiguous hippocampal networks. We predict herein a novel form of metaplasticity, i.e., an experience-dependent non-Hebbian modulation of synaptic plasticity.

BDNF attenuates hippocampal LTD via activation of phospholipase C: implications for a vertical shift in the frequency–response curve of synaptic plasticity

Recent evidence shows that neurotrophins are not only involved in neuronal survival and differentiation during development but also in modulating synaptic strength in the mature brain. To understand how neurotrophins alter this synaptic modification, we have investigated the effect of brain‐derived neurotrophic factor (BDNF) on long‐term depression (LTD) at Schaffer collateral–CA1 synapses in rat hippocampal slices. The slices treated with BDNF for 5u2003min showed significantly less LTD in response to a 1‐Hz tetanus compared with controls but displayed normal LTD when the afferents were tetanized at 10u2003Hz. Because BDNF enhanced long‐term potentiation (LTP) induced by a 30‐Hz tetanus, the synaptic modification threshold (θm) as defined in the ‘BCM’ theory of Bienenstock Cooper & Monroe [Bienenstock et al. (1982), J. Neurosci., 2, 32–48] was not shifted. BNDF is likely to alter the capability of the plastic changes in synaptic efficacy, i.e. to produce an upward shift in the BCM curve. The suppressive effect of BDNF on LTD was prevented by either the tyrosine kinase (Trk) receptor inhibitor K252a or the phospholipase C inhibitor U73122. Thus, TrkB activation may attenuate LTD through phospholipase C signalling pathway.

Cytoskeleton disruption causes apoptotic degeneration of dentate granule cells in hippocampal slice cultures

Colchicine, a potent microtubule-depolymerizing agent, is well known to selectively kill dentate granule cells in the hippocampal formation in vivo. Using organotypic cultures of rat entorhino-hippocampal slices, we confirmed that in vitro exposure to 1 microM and 10 microM of colchicine reproduced a specific degeneration of the granule cells after 24 h. Similar results were obtained with other types of microtubule-disrupting agents, i.e., nocodazole, vinblastine, and Taxol. Interestingly, the actin-depolymerizing agents cytochalasin D and latrunculin A also elicited selective neurotoxicity in the dentate gyrus without affecting survival of hippocampal pyramidal cells. The selective pattern of degeneration was observable 24 h after a brief treatment with the toxins as short as 5 min, but this delayed neuronal death was unlikely to be a result of excitotoxicity because it was virtually unaffected by glutamate receptor antagonists, tetrodotoxin, or extracellular Ca(2+)-free conditions. The damaged tissues contained a large number of TUNEL-positive neurons and exhibited an increased level in caspase-3-like activity, suggesting that cytoskeleton disruption triggers an apoptosis-like process in dentate granule cells. Thus, this study may provide a basis for understanding the distinctive mechanism that supports granule cell survival.

Mossy fibre synaptic NMDA receptors trigger non-Hebbian long-term potentiation at entorhino-CA3 synapses in the rat

Hippocampal CA3 pyramidal cells receive two independent afferents from the enthorinal cortex, i.e. a direct input via the temporoammonic pathway (TA, perforant path) and an indirect input via the mossy fibres (MF) of dentate granule cells. In spite of past suggestions that the TA is assigned an important role in exciting the pyramidal cells, little is known about their physiological properties. By surgically making an incision through the sulcus hippocampi and a small part of the dentate molecular layer, we succeeded in isolating TA‐mediated monosynaptic responses in CA3 stratum lacunosum‐moleculare. The TA‐CA3 synaptic transmission was completely blocked by a combination of d,l‐2‐amino‐5‐phosphonopentanoic acid (AP5) and 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX), NMDA and non‐NMDA receptor antagonists, respectively, and displayed paired‐pulse facilitation and NMDA receptor‐dependent long‐term potentiation, which are all typical of glutamatergic synapses. We next addressed the heterosynaptic interaction between TA‐CA3 and MF‐CA3 synapses. The TA‐CA3 transmission was partially attenuated by single‐pulse MF pre‐stimulation at inter‐pulse intervals of up to 70 ms. However, surprisingly, burst stimulation of the MF alone induced long‐lasting facilitation of TA‐CA3 synaptic efficacy. This non‐Hebbian form of synaptic plasticity was efficiently prevented by local application of AP5 into the MF synapse‐rich area. Therefore, MF‐activated NMDA receptors are responsible for the heterosynaptic modification of TA‐CA3 transmission, and thereby, the history of MF activity may be etched into TA‐CA3 synaptic strength. Our findings predict a novel form of spatiotemporal information processing in the hippocampus, i.e. a use‐dependent intersynaptic memory transfer.

β-Amyloid Enhances Glial Glutamate Uptake Activity and Attenuates Synaptic Efficacy

Although amyloid β-protein (Aβ) has long been implicated in the pathogenesis of Alzheimers disease, little is known about the mechanism by which Aβ causes dementia. Aβ leads to neuronal cell death in vivo and in vitro, but recent evidence suggests that the property of the amnesic characteristic of Alzheimers disease can be explained by a malfunction of synapses rather than a loss of neurons. Here we show that prolonged treatment with Aβ augments the glutamate clearance ability of cultured astrocytes and induces a dramatic decrease in glutamatergic synaptic activity of neurons cocultured with the astrocytes. Biotinylation assay revealed that the enhancement of glutamate uptake activity was associated with an increase in cell-surface expression of GLAST, a subtype of glial glutamate transporters, without apparent changes in the total amount of GLAST. This phenomenon was blocked efficiently by actin-disrupting agents. Thus, Aβ-induced actin-dependent GLAST redistribution and relevant synaptic malfunction may be a cellular basis for the amnesia of Alzheimers disease.

Group II metabotropic glutamate receptor activation is required for normal hippocampal mossy fibre development in the rat

Glutamate is the main neurotransmitter at hippocampal mossy fibre (MF) terminals. Because neurotransmitters have been proposed as regulating factors of neural network formation and neurite morphogenesis in the developing CNS, we examined the possible contribution of glutamate to MF pathfinding. Entorhino‐hippocampal slices prepared from early postnatal rats were cultivated in the presence of glutamate receptor antagonists. Timm histochemical staining revealed that pharmacological blockade of metabotropic glutamate receptors (mGluR), but not of ionotropic glutamate receptors, induced abnormal outgrowth of the MFs. When slices were cultured in the presence of mGluR antagonists, DiI‐labelled MF axons displayed a great degree of defasciculation, and MF‐mediated EPSPs in the CA3 pyramidal cells were altered. Similar results were obtained for a selective antagonist of group II mGluR, but not of group I or III mGluR. Glutamate is, therefore, likely to regulate MF outgrowth via activation of group II mGluR. The present study may provide a novel role of glutamate in hippocampal development.

BDNF Boosts Spike Fidelity in Chaotic Neural Oscillations

Oscillatory activity and its nonlinear dynamics are of fundamental importance for information processing in the central nervous system. Here we show that in aperiodic oscillations, brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, enhances the accuracy of action potentials in terms of spike reliability and temporal precision. Cultured hippocampal neurons displayed irregular oscillations of membrane potential in response to sinusoidal 20-Hz somatic current injection, yielding wobbly orbits in the phase space, i.e., a strange attractor. Brief application of BDNF suppressed this unpredictable dynamics and stabilized membrane potential fluctuations, leading to rhythmical firing. Even in complex oscillations induced by external stimuli of 40 Hz (gamma) on a 5-Hz (theta) carrier, BDNF-treated neurons generated more precisely timed spikes, i.e., phase-locked firing, coupled with theta-phase precession. These phenomena were sensitive to K252a, an inhibitor of tyrosine receptor kinases and appeared attributable to BDNF-evoked Na(+) current. The data are the first indication of pharmacological control of endogenous chaos. BDNF diminishes the ambiguity of spike time jitter and thereby might assure neural encoding, such as spike timing-dependent synaptic plasticity.

REGIONALLY SELECTIVE NEUROTOXICITY OF NMDA AND COLCHICINE IS INDEPENDENT OF HIPPOCAMPAL NEURAL CIRCUITRY

The mechanisms by which cerebral ischemia and several neurotoxins cause regionally selective damages to the hippocampal formation are largely unknown. The CA1-selective toxicity of N-methyl--aspartate (NMDA), the CA3-selective toxicity of kainate, and the dentate gyrus (DG)-selective toxicity of colchicine were observed in organotypic entorhino-hippocampal cultures. The selective neurotoxicity of NMDA and colchicine but not kainate was present in isolated tissue cultures of each hippocampal subregion, suggesting that the regional vulnerability is irrespective of the hippocampal trisynaptic pathway. Dispersed cultures of neurons prepared from Ammons horn and the DG still exhibited a preference for susceptibility to NMDA and colchicine, respectively. Thus, the neurons per se appear to be inherently susceptible to specific toxins independently of their original loci, intrinsic neural circuits, vascular system, or other systemic factors.

Rapid regrowth of hippocampal mossy fibres and preceding maturation of NMDA receptor-mediated neurotransmission

Early in postnatal development, glutamatergic synapses contain primarily NMDA receptors and progressively acquire AMPA receptor function. To determine whether this transformation occurs in a process of regenerative synaptogenesis following axotomy, we investigated the recovery of AMPA and NMDA receptor‐mediated neurotransmission after the transection of mossy fibres (MF) in organotypic hippocampal cultures. An NMDA component could already be elicited 1u2003day after the lesion and reached a saturated level after 3u2003days. Thereafter, an AMPA component appeared and slowly matured after 10u2003days. The preceding establishment of NMDA receptor function implies that immature MF synapses are functionally silent at least for the first several days of recovery. The appearance of AMPA receptor‐mediated neurotransmission was unchanged in the presence of an NMDA‐receptor antagonist or tetrodotoxin, which suggests that the AMPA receptor maturation is virtually independent of neuronal activity. Thus, the conversion of silent to functional synapses is not unique to synaptic plasticity or developmental processes but also occurs in recovery after brain damage, but its mechanism is likely to differ from NMDA receptor‐dependent recruitment of AMPA receptors in synaptic plasticity.

Mossy Fiber Pathfinding in Multilayer Organotypic Cultures of Rat Hippocampal Slices

Abstract1. Using a novel technique of organotypic cultures, in which two hippocampal slices were cocultured in a bilayer style, we found that the mossy fibers arising from the dentate gyrus grafted onto another dentate tissue grew along the CA3 stratum lucidum of the host hippocampal slice. The same transplantation of a CA1 microslice failed to form a network with the host hippocampus.2. Thus, the type of grafted neurons is important to determine whether they can form an appropriate network after transplantation.