Differential mechanisms of transmission at three types of mossy fiber synapse

Activity-dependent induction of multitransmitter signaling onto pyramidal cells and interneurons of hippocampal area CA3

The granule cells of the dentate gyrus (DG) are considered to be glutamatergic, but they contain glutamic acid decarboxylase, gamma-amino butyric acid (GABA), and the vesicular GABA transporter mRNA. Their expression is regulated in an activity-dependent manner and coincides with the appearance of GABAergic transmission from the mossy fibers (MF) to pyramidal cells in area CA3. These data support the hypothesis that MF are able to release glutamate and GABA. Following the principle that a given neuron releases the same neurotransmitter(s) onto all its targets, we here demonstrate the emergence, after a generalized convulsive seizure, of MF GABAergic signaling sensitive to activation mGluR-III onto pyramidal cells and interneurons of CA3. Despite this, excitation overrides inhibition in interneurons, preventing disinhibition. Furthermore, on blockade of GABA and glutamate ionotropic receptors, an M1-cholinergic depolarizing signal is also revealed in both targets, which postsynaptically modulates the glutamatergic and GABAergic fast neurotransmission. The emergence of these nonglutamatergic signals depends on protein synthesis. In contrast to cholinergic responses evoked by associational/commissural fibers activation, cholinergic transmission evoked by DG stimulation is only observed after seizures and is strongly depressed by the activation of mGluR-II, whereas both are depressed by M2-AChR activation. With immunohistological experiments, we show that this cholinergic pathway runs parallel to the MF. Thus seizures compromise a delicate balance of excitation and inhibition, on which a complex interaction of different neurotransmitters emerges to counteract excitation at pre- and postsynaptic sites. Particularly, MF GABAergic inhibition emerges to exert an overall inhibitory action on CA3.

AMPA receptors regulate dynamic equilibrium of presynaptic terminals in mature hippocampal networks

The formation and disassembly of synapses in mature neuropil could provide a substrate to encode experience in the brain. Although there is evidence for postsynaptic spine dynamics in mature systems, contributions to circuit rearrangements by presynaptic terminals have remained unclear. We used hippocampal slice cultures from mice expressing spectral variants of green fluorescent protein (GFP) that are targeted to the membrane and/or synaptic vesicles in neuronal subsets to image identified presynaptic terminals. In mature tissues with no net change in synapse numbers, subpopulations of presynaptic terminals appeared and disappeared within 1-3 days. The three terminal types established by mossy fibers had distinct properties. High-frequency stimulation increased the fraction of dynamic terminals for 1-2 days, a process mediated by activation of AMPA receptors, protein kinase A (PKA) and protein synthesis. Thus, synaptic activity can make stable presynaptic terminals become dynamic, providing a candidate mechanism to convert experience into changes in network connectivity.

Ca2+ buffer saturation underlies paired pulse facilitation in calbindin-D28k-containing terminals

Ca2+ buffer saturation was proposed as a mechanism of paired pulse facilitation (PPF). However, whether it operates under native conditions remained unclear. Here we show that saturation of the endogenous fast Ca2+ buffer calbindin-D28k (CB) plays a major role in PPF at CB-containing synapses. Paired recordings from synaptically connected interneurons and pyramidal neurons in the mouse neocortex revealed that dialysis increased the amplitude of the first response and decreased PPF. Loading the presynaptic terminals with BAPTA or CB rescued the effect of the CB washout. We extended the study to the CB-positive facilitating excitatory mossy fiber-CA3 pyramidal cell synapse. The effects of different extracellular Ca2+ concentrations and of EGTA indicated that PPF in CB-containing terminals depended on Ca2+ influx rather than on the initial release probability. Experiments in CB knockout mice confirmed that buffer saturation is a novel basic presynaptic mechanism for activity-dependent control of synaptic gain.

Histochemical localization of neurons with zinc-permeable AMPA/kainate channels in rat brain slices

Zinc modulates neurotransmission and may trigger neurodegeneration following brain injuries. Therefore, it is important to understand zinc dynamics in an anatomical context. Using a histochemical procedure on stimulated slices, we visualized zinc influx into neurons 'in situ'. Hippocampal, neocortical and cerebellar slices were loaded with zinc and stimulated with compounds known to open zinc-permeable channels. Zinc was then visualized by histochemical precipitation methods. Kainate stimulation labelled CA1 hippocampal pyramidal neurons, as well as subpopulations of interneurons in the hilus, CA1 and CA3 fields. Interneurons in the neocortex and many cell types of the cerebellum were also labelled. However, neither NMDA nor 50 mM K(+) stimulation resulted in comparable zinc accumulation in neurons. Immunofluorescent colocalization of parvalbumin with kainate-induced zinc staining in the hippocampus and neocortex showed a subset of zinc-sensitive neurons, positive for parvalbumin. These results confirm that zinc permeation through calcium-permeable AMPA/kainate channels constitutes a predominant route of zinc entry into different cell types. Furthermore, this technique provides a useful and versatile histochemical approach to assess the cell subpopulations of the central nervous system particularly sensitive to zinc accumulation under normal or pathological conditions.

GABA B receptor modulation of excitatory and inhibitory synaptic transmission onto rat CA3 hippocampal interneurons

Hippocampal stratum radiatum inhibitory interneurons receive glutamatergic excitatory innervation via the recurrent collateral fibers of CA3 pyramidal neurons and GABAergic inhibition from other interneurons. We examined both presynaptic- and postsynaptic-GABA(B) receptor-mediated responses at both synapse types. Postsynaptic GABA(B) receptor-mediated responses were absent in recordings from young (P16-18) but present in recordings from older animals (> or =P30) suggesting developmental regulation. In young animals, the GABA(B) receptor agonist, baclofen, inhibited the amplitude of evoked EPSCs and IPSCs, an effect blocked by prior application of the selective antagonist CGP55845. Baclofen enhanced the paired-pulse ratio and coefficient of variation of evoked EPSCs and IPSCs, consistent with a presynaptic mechanism of regulation. In addition, baclofen reduced the frequency of miniature IPSCs but not mEPSCs. However, baclofen reduced the frequency of KCl-induced mEPSCs; an effect blocked by Cd(2+), implicating presynaptic voltage-gated Ca(2+) channels as a target for baclofen modulation. In contrast, although Cd(2+) prevented the KCl-induced increase in mIPSC frequency, it failed to block baclofen's reduction of mIPSC frequency. Whereas N- and P/Q-types of Ca(2+) channels contributed equally to GABA(B) receptor-mediated inhibition of EPSCs, more P/Q-type Ca(2+) channels were involved in GABA(B) receptor-mediated inhibition of IPSCs. Finally, baclofen blocked the frequency-dependent depression of EPSCs and IPSCs, but was less effective at blocking frequency-dependent facilitation of EPSCs. Our results demonstrate that presynaptic GABA(B) receptors are expressed on the terminals of both excitatory and inhibitory synapses onto CA3 interneurons and that their activation modulates essential components of the release process underlying transmission at these two synapse types.

TwoB or not twoB: differential transmission at glutamatergic mossy fiber-interneuron synapses in the hippocampus

Mossy fiber (MF) synapses are key stations for flow of information through the hippocampal formation. A major component of the output of the MF system is directed towards inhibitory interneurons. Recent studies have revealed that the functional properties of MF-interneuron synapses differ substantially from those of MF-CA3 pyramidal neuron synapses. Mossy-fiber-interneuron synapses in the stratum lucidum represent a continuum of functional subtypes, in which the subunit composition of postsynaptic AMPA receptors and NMDA receptors appears to be regulated in a coordinated manner.

Single granule cells reliably discharge targets in the hippocampal CA3 network in vivo

Processing of neuronal information depends on interactions between the anatomical connectivity and cellular properties of single cells. We examined how these computational building blocks work together in the intact rat hippocampus. Single spikes in dentate granule cells, controlled intracellularly, generally failed to discharge either interneurons or CA3 pyramidal cells. In contrast, trains of spikes effectively discharged both CA3 cell types. Increasing the discharge rate of the granule cell increased the discharge probability of its target neuron and decreased the delay between the onset of a granule cell train and evoked firing in postsynaptic targets. Thus, we conclude that the granule cell to CA3 synapses are 'conditional detonators,' dependent on granule cell firing pattern. In addition, we suggest that information in single granule cells is converted into a temporal delay code in target CA3 pyramidal cells and interneurons. These data demonstrate how a neural circuit of the CNS may process information.

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.

Activation of kinetically distinct synaptic conductances on inhibitory interneurons by electrotonically overlapping afferents

Mossy fiber (MF) and CA3 collateral (CL) axons activate common interneurons via synapses comprised of different AMPA receptors to provide feedforward and feedback inhibitory control of the CA3 hippocampal network. Because synapses potentially occur over variable electrotonic distances that distort somatically recorded synaptic currents, it is not known whether the underlying afferent-specific synaptic conductances are associated with different time courses. Using a somatic voltage jump technique to alter the driving force at the site of the synapse, we demonstrate that MF and CL synapses overlap in electrotonic location yet differ in conductance time course. Thus, afferent-specific conductance time courses allow single interneurons to differentially integrate feedforward and feedback information without the need to segregate distinct AMPA receptor subunits to different electrotonic domains.

Distinct NMDA receptors provide differential modes of transmission at mossy fiber-interneuron synapses

Dentate gyrus granule cells innervate inhibitory interneurons via a continuum of synapses comprised of either Ca(2+)-impermeable (CI) or Ca(2+)-permeable (CP) AMPA receptors. Synapses at the extreme ends of this continuum engage distinct postsynaptic responses, with activity at CI synapses being strongly influenced by NMDA receptor activation. NMDARs at CI synapses have a lower NR2B subunit composition and a higher open probability, which generate larger amplitude and more rapid EPSCs than their CP counterparts. A novel form of NMDAR-dependent long-term depression (iLTD) is associated with CI-mossy fiber synapses, whereas iLTD at CP synapses is dependent on Ca(2+)-permeable AMPA receptor activation. Induction of both forms of iLTD required elevation of postsynaptic calcium. Thus mossy fibers engage CA3 interneurons via multiple synapse types that will act to expand the computational repertoire of the mossy fiber-CA3 network.

Mediation of hippocampal mossy fiber long-term potentiation by presynaptic Ih channels

Hippocampal mossy fiber long-term potentiation (LTP) is expressed presynaptically, but the exact mechanisms remain unknown. Here, we demonstrate the involvement of the hyperpolarization-activated cation channel (Ih) in the expression of mossy fiber LTP. Established LTP was blocked and reversed by Ih channel antagonists. Whole-cell recording from granule cells revealed that repetitive stimulation causes a calcium- and Ih-dependent long-lasting depolarization mediated by protein kinase A. Depolarization at the terminals would be expected to enhance transmitter release, whereas somatic depolarization would enhance the responsiveness of granule cells to afferent input. Thus, Ih channels play an important role in the long-lasting control of transmitter release and neuronal excitability.

PTP and LTP at a hippocampal mossy fiber-interneuron synapse

The mossy fiber-CA3 pyramidal neuron synapse is a main component of the hippocampal trisynaptic circuitry. Recent studies, however, suggested that inhibitory interneurons are the major targets of the mossy fiber system. To study the regulation of mossy fiber-interneuron excitation, we examined unitary and compound excitatory postsynaptic currents in dentate gyrus basket cells, evoked by paired recording between granule and basket cells or extracellular stimulation of mossy fiber collaterals. The application of an associative high-frequency stimulation paradigm induced posttetanic potentiation (PTP) followed by homosynaptic long-term potentiation (LTP). Analysis of numbers of failures, coefficient of variation, and paired-pulse modulation indicated that both PTP and LTP were expressed presynaptically. The Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) did not affect PTP or LTP at a concentration of 10 mM but attenuated LTP at a concentration of 30 mM. Both forskolin, an adenylyl cyclase activator, and phorbolester diacetate, a protein kinase C stimulator, lead to a long-lasting increase in excitatory postsynaptic current amplitude. H-89, a protein kinase A inhibitor, and bisindolylmaleimide, a protein kinase C antagonist, reduced PTP, whereas only bisindolylmaleimide reduced LTP. These results may suggest a differential contribution of protein kinase A and C pathways to mossy fiber-interneuron plasticity. Interneuron PTP and LTP may provide mechanisms to maintain the balance between synaptic excitation of interneurons and that of principal neurons in the dentate gyrus-CA3 network.

Timing of cognitive deficits following neonatal seizures: relationship to histological changes in the hippocampus

Neonatal seizures are frequently associated with cognitive impairment and reduced seizure threshold. Previous studies in our laboratory have demonstrated that rats with recurrent neonatal seizures have impaired learning, lower seizure thresholds, and sprouting of mossy fibers in CA3 and the supragranular region of the dentate gyrus in the hippocampus when studied as adults. The goal of this study was to determine the age of onset of cognitive dysfunction and alterations in seizure susceptibility in rats subjected to recurrent neonatal seizures and the relation of this cognitive impairment to mossy fiber sprouting and expression of glutamate receptors. Starting at postnatal day (P) 0, rats were exposed to 45 flurothyl-induced seizures over a 9-day period of time. Visual-spatial learning in the water maze and seizure susceptibility were assessed in subsets of the rats at P20 or P35. Brains were evaluated for cell loss, mossy fiber distribution, and AMPA (GluR1) and NMDA (NMDAR1) subreceptor expression at these same time points. Rats with neonatal seizures showed significant impairment in the performance of the water maze and increased seizure susceptibility at both P20 and P35. Sprouting of mossy fibers into the CA3 and supragranular region of the dentate gyrus was seen at both P20 and P35. GluR1 expression was increased in CA3 at P20 and NMDAR1 was increased in expression in CA3 and the supragranular region of the dentate gyrus at P35. Our findings indicate that altered seizure susceptibility and cognitive impairment occurs prior to weaning following a series of neonatal seizures. Furthermore, these alterations in cognition and seizure susceptibility are paralleled by sprouting of mossy fibers and increased expression of glutamate receptors. To be effective, our results suggest that strategies to alter the adverse outcome following neonatal seizures will have to be initiated during, or shortly following, the seizures.

A critical role of a facilitatory presynaptic kainate receptor in mossy fiber LTP

The mechanisms involved in mossy fiber LTP in the hippocampus are not well established. In the present study, we show that the kainate receptor antagonist LY382884 (10 microM) is selective for presynaptic kainate receptors in the CA3 region of the hippocampus. At a concentration at which it blocks mossy fiber LTP, LY382884 selectively blocks the synaptic activation of a presynaptic kainate receptor that facilitates AMPA receptor-mediated synaptic transmission. Following the induction of mossy fiber LTP, there is a complete loss of the presynaptic kainate receptor-mediated facilitation of synaptic transmission. These results identify a central role for the presynaptic kainate receptor in the induction of mossy fiber LTP. In addition, these results suggest that the pathway by which kainate receptors facilitate glutamate release is utilized for the expression of mossy fiber LTP.

Induction of mossy fiber --> Ca3 long-term potentiation requires translocation of synaptically released Zn2+

The mammalian CNS contains an abundance of chelatable Zn(2+) sequestered in the vesicles of glutamatergic terminals. These vesicles are particularly numerous in hippocampal mossy fiber synapses of the hilar and CA3 regions. Our recent observation of frequency-dependent Zn(2+) release from mossy fiber synaptic terminals and subsequent entry into postsynaptic neurons has prompted us to investigate the role of synaptically released Zn(2+) in the induction of long-term potentiation (LTP) in field CA3 of the hippocampus. The rapid removal of synaptically released Zn(2+) with the membrane-impermeable Zn(2+) chelator CaEDTA (10 mm) blocked induction of NMDA receptor-independent mossy fiber LTP by high-frequency electrical stimulation (HFS) in rat hippocampal slices. Mimicking Zn(2+) release by bath application of Zn(2+) (50-100 microm) without HFS induced a long-lasting potentiation of synaptic transmission that lasted more than 3 hr. Moreover, our experiments indicate the effects of Zn(2+) were not attributable to its interaction with extracellular membrane proteins but required its entry into presynaptic or postsynaptic neurons. Co-released glutamate is also essential for induction of LTP under physiological conditions, in part because it allows Zn(2+) entry into postsynaptic neurons. These results indicate that synaptically released Zn(2+), acting as a second messenger, is necessary for the induction of LTP at mossy fiber-->CA3 synapses of hippocampus.

Glutamate receptor involvement in dentate granule cell epileptiform activity evoked by mossy fiber stimulation

In many persons with temporal lobe epilepsy, dentate granule cells form an interconnected synaptic network. This recurrent mossy fiber circuit mediates reverberating excitation that may facilitate seizure propagation by synchronizing granule cell discharge. The involvement of specific glutamate receptors in granule cell epileptiform activity evoked by stimulating the mossy fibers was investigated with use of rat hippocampal slices superfused with bicuculline, with or without increasing [K+](o) to 6 mM. The occurrence of short-latency mossy fiber-evoked granule cell epileptiform activity in slices from pilocarpine-treated rats correlated with the presence and extent of recurrent mossy fiber growth. Blockade of AMPA receptors nearly abolished the orthodromic component of the response; subsequent antagonism of kainate receptors as well appeared to have no further action. Antagonism of NMDA receptors reduced the duration of epileptiform discharge, but increased the amplitude of population spikes within the evoked burst. Thus AMPA and NMDA, but perhaps not kainate, receptors play an important role in this type of epileptiform activity. Activation of type II metabotropic glutamate receptors, which inhibits the release of glutamate from mossy fiber boutons, reduced the magnitude of epileptiform discharge. This action was reversed by a partial agonist of these receptors. However, neither an agonist nor an antagonist of type III metabotropic glutamate receptors significantly altered the response. Considering the importance of synchronous granule cell discharge for seizure propagation from the entorhinal cortex to the hippocampus, agonists of type II metabotropic glutamate receptors may be useful in suppressing such discharge both experimentally and clinically.

Long-term plasticity in interneurons of the dentate gyrus

Single interneurons influence thousands of postsynaptic principal cells, and the control of interneuronal excitability is an important regulator of the computational properties of the hippocampus. However, the mechanisms underlying long-term alterations in the input-output functions of interneurons are not fully understood. We report a mechanism of interneuronal plasticity that leads to the functional enhancement of the gain of glutamatergic inputs in the absence of long-term potentiation of the excitatory synaptic currents. Interneurons in the dentate gyrus exhibit a characteristic, limited (approximately 8 mV) depolarization of their resting membrane potential after high-frequency stimulation of the perforant path. The depolarization can be observed with either whole-cell or perforated patch electrodes, and it lasts in excess of 3 h. The long-term depolarization is specific to interneurons, because granule cells do not show it. The depolarization requires the activation of Ca(2+)-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and the rise of intracellular Ca(2+), but not N-methyl-d-aspartate (NMDA) receptor activation. Data on the maintenance of the depolarization point to a major role for a long-term change in the rate of electrogenic Na(+)/K(+)-ATPase pump function in interneurons. As a result of the depolarization, interneurons after the tetanus respond with action potential discharges to previously subthreshold excitatory postsynaptic potentials (EPSPs), even though the EPSPs are not potentiated. These results demonstrate that the plastic nature of the interneuronal resting membrane potential underlies a unique form of long-term regulation of the gain of excitatory inputs to gamma-aminobutyric acid (GABA)ergic neurons.

Monosynaptic GABAergic signaling from dentate to CA3 with a pharmacological and physiological profile typical of mossy fiber synapses

Mossy fibers are the sole excitatory projection from dentate gyrus granule cells to the hippocampus, where they release glutamate, dynorphin, and zinc. In addition, mossy fiber terminals show intense immunoreactivity for the inhibitory neurotransmitter GABA. Fast inhibitory transmission at mossy fiber synapses, however, has not previously been reported. Here, we show that electrical or chemical stimuli that recruit dentate granule cells elicit monosynaptic GABA(A) receptor-mediated synaptic signals in CA3 pyramidal neurons. These inhibitory signals satisfy the criteria that distinguish mossy fiber-CA3 synapses: high sensitivity to metabotropic glutamate receptor agonists, facilitation during repetitive stimulation, and NMDA receptor-independent long-term potentiation. GABAergic transmission from the dentate gyrus to CA3 has major implications not only for information flow into the hippocampus but also for developmental and pathological processes involving the hippocampus.

Interneurons unbound

Local-circuit, gamma-aminobutyric acid-releasing inhibitory interneurons of the hippocampus and cortex have traditionally been considered as the regulators of principal neuron activity--the yin to the excitatory yang. Recent evidence indicates that, in addition to that role, their network connectivity and the properties of their intrinsic voltage-gated currents are finely tuned to permit inhibitory interneurons to generate and control the rhythmic output of large populations of both principal cells and other populations of inhibitory interneurons. This review brings together recently described properties and emerging principles of interneuron function that indicate a much more complex role for these cells than just providers of inhibition.