Differential mechanisms of transmission at three types of mossy fiber synapse

Timing in the absence of supraspinal input I: variable, but not fixed, spaced stimulation of the sciatic nerve undermines spinally-mediated instrumental learning

Rats with complete spinal transections are capable of acquiring a simple instrumentally trained response. If rats receive shock to one hind limb when the limb is extended (controllable shock), the spinal cord will learn to hold the leg in a flexed position that minimizes shock exposure. If shock is delivered irrespective of leg position, subjects do not exhibit an increase in flexion duration and subsequently fail to learn when tested with controllable shock (learning deficit). Just 6 min of variable intermittent shock produces a learning deficit that lasts 24 h. Evidence suggests that the neural mechanisms underlying the learning deficit may be related to those involved in other instances of spinal plasticity (e.g. windup, long-term potentiation). The present paper begins to explore these relations by demonstrating that direct stimulation of the sciatic nerve also impairs instrumental learning. Six minutes of electrical stimulation (mono- or biphasic direct current [DC]) of the sciatic nerve in spinally transected rats produced a voltage-dependent learning deficit that persisted for 24 h (experiments 1-2) and was dependent on C-fiber activation (experiment 7). Exposure to continuous stimulation did not produce a deficit, but intermittent burst or single pulse (as short as 0.1 ms) stimulation (delivered at a frequency of 0.5 Hz) did, irrespective of the pattern (fixed or variable) of stimulus delivery (experiments 3-6, 8). When the duration of stimulation was extended from 6 to 30 min, a surprising result emerged; shocks applied in a random (variable) fashion impaired subsequent learning whereas shocks given in a regular pattern (fixed spacing) did not (experiments 9-10). The results imply that spinal neurons are sensitive to temporal relations and that stimulation at regular intervals can have a restorative effect.

Differential changes in mGlu2 and mGlu3 gene expression following pilocarpine-induced status epilepticus: a comparative real-time PCR analysis

Group II metabotropic glutamate (mGlu II) receptors subtype 2 and 3 (mGlu2 and mGlu3) are subtle regulators of neuronal excitability and synaptic plasticity in the hippocampus. In recent years, researchers have investigated the potential neuroprotective and anticonvulsant effects of compounds acting on mGlu II receptors. However, abnormal expression and function of mGlu2 and mGlu3 have been reported in temporal lobe epilepsy, a phenomena that may limit the therapeutic effectiveness of these potentially new antiepileptic drugs. Here, we investigated seizure-induced changes in mGlu2 and mGlu3 mRNA following pilocarpine-inducted status epilepticus (SE) and subsequent epileptogenesis. Relative changes in gene expression were assessed by comparative analysis of quantitative real-time PCR (qrtPCR) by the delta-delta CT method. Pilocarpine-treated and control rats were sacrificed at different periods (24 h, 10 days, one month and more than two months) following SE. Total RNA was isolated from microdissected dentate gyrus and processed for RT-PCR and qrtPCR using glyceraldehyde-3-phosphate dehydrogenase (GAPDH) as an endogenous control gene. Analysis of relative quantification (RQ) ratios of mGlu2 and mGlu3 mRNA expression revealed a significant down-regulation of both targets at 24 h after SE. Gene expression partially recovered at 10 days following SE reaching control levels at one month after SE. Two month after SE, mGlu2 mRNA expression was significantly down-regulated to approximately 41% of control expression whereas mGlu3 mRNA was comparable to control levels. Our data indicate that mGlu2 and mGlu3 expression is dynamically down-regulated or selectively enhanced during critical periods of epileptogenesis. Seizure-induced differential dysregulation of mGlu2 and mGlu3 receptors may affect the availability of these molecular targets for therapeutic compounds in epilepsy.

Coincidence detection of convergent perforant path and mossy fibre inputs by CA3 interneurons

We performed whole-cell recordings from CA3 s. radiatum (R) and s. lacunosum-moleculare (L-M) interneurons in hippocampal slices to examine the temporal aspects of summation of converging perforant path (PP) and mossy fibre (MF) inputs. PP EPSPs were evoked from the s. lacunosum-moleculare in area CA1. MF EPSPs were evoked from the medial extent of the suprapyramidal blade of the dentate gyrus. Summation was strongly supralinear when examining PP EPSP with MF EPSP in a heterosynaptic pair at the 10 ms ISI, and linear to sublinear at longer ISIs. This pattern of nonlinearities suggests that R and L-M interneurons act as coincidence detectors for input from PP and MF. Summation at all ISIs was linear in voltage clamp mode demonstrating that nonlinearities were generated by postsynaptic voltage-dependent conductances. Supralinearity was not detected when the first EPSP in the pair was replaced by a simulated EPSP injected into the soma, suggesting that the conductances underlying the EPSP boosting were located in distal dendrites. Supralinearity was selectively eliminated with either Ni2+ (30 microm), mibefradil (10 microm) or nimodipine (15 microm), but was unaffected by QX-314. This pharmacological profile indicates that supralinearity is due to recruitment of dendritic T-type Ca2+channels by the first subthreshold EPSP in the pair. Results with the hyperpolarization-activated (Ih) channel blocker ZD 7288 (50 microm) revealed that Ih restricted the time course of supralinearity for coincidently summed EPSPs, and promoted linear to sublinear summation for asynchronous EPSPs. We conclude that coincidence detection results from the counterbalanced activation of T-type Ca2+ channels and inactivation of Ih.

High threshold, proximal initiation, and slow conduction velocity of action potentials in dentate granule neuron mossy fibers

Dentate granule neurons give rise to some of the smallest unmyelinated fibers in the mammalian CNS, the hippocampal mossy fibers. These neurons are also key regulators of physiological and pathophysiological information flow through the hippocampus. We took a comparative approach to studying mossy fiber action potential initiation and propagation in hippocampal slices from juvenile rats. Dentate granule neurons exhibited axonal action potential initiation significantly more proximal than CA3 pyramidal neurons. This conclusion was suggested by phase plot analysis of somatic action potentials and by local tetrodotoxin application to the axon and somatodendritic compartments. This conclusion was also verified by immunostaining for voltage-gated sodium channel alpha subunits and by direct dual soma/axonal recordings. Dentate neurons exhibited a significantly higher action potential threshold and slower axonal conduction velocity than CA3 neurons. We conclude that while the electrotonically proximal axon location of action potential initiation allows granule neurons to sensitively detect and integrate synaptic inputs, the neurons are sluggish to initiate and propagate an action potential.

Phase Precession Through Synaptic Facilitation

Phase precession is a relational code that is thought to be important for episodic-like memory, for instance, the learning of a sequence of places. In the hippocampus, places are encoded through bursting activity of so-called place cells. The spikes in such a burst exhibit a precession of their firing phases relative to field potential theta oscillations (4–12 Hz); the theta phase of action potentials in successive theta cycles progressively decreases toward earlier phases. The mechanisms underlying the generation of phase precession are, however, unknown. In this letter, we show through mathematical analysis and numerical simulations that synaptic facilitation in combination with membrane potential oscillations of a neuron gives rise to phase precession. This biologically plausible model reproduces experimentally observed features of phase precession, such as (1) the progressive decrease of spike phases, (2) the nonlinear and often also bimodal relation between spike phases and the animal's place, (3) the range of phase precession being smaller than one theta cycle, and (4) the dependence of phase jitter on the animal's location within the place field. The model suggests that the peculiar features of the hippocampal mossy fiber synapse, such as its large efficacy, long-lasting and strong facilitation, and its phase-locked activation, are essential for phase precession in the CA3 region of the hippocampus.

Efficient Ca2+ buffering in fast-spiking basket cells of rat hippocampus

Fast-spiking parvalbumin-expressing basket cells (BCs) represent a major type of inhibitory interneuron in the hippocampus. These cells inhibit principal cells in a temporally precise manner and are involved in the generation of network oscillations. Although BCs show a unique expression profile of Ca(2+)-permeable receptors, Ca(2+)-binding proteins and Ca(2+)-dependent signalling molecules, physiological Ca(2+) signalling in these interneurons has not been investigated. To study action potential (AP)-induced dendritic Ca(2+) influx and buffering, we combined whole-cell patch-clamp recordings with ratiometric Ca(2+) imaging from the proximal apical dendrites of rigorously identified BCs in acute slices, using the high-affinity Ca(2+) indicator fura-2 or the low-affinity dye fura-FF. Single APs evoked dendritic Ca(2+) transients with small amplitude. Bursts of APs evoked Ca(2+) transients with amplitudes that increased linearly with AP number. Analysis of Ca(2+) transients under steady-state conditions with different fura-2 concentrations and during loading with 200 microm fura-2 indicated that the endogenous Ca(2+)-binding ratio was approximately 200 (kappa(S) = 202 +/- 26 for the loading experiments). The peak amplitude of the Ca(2+) transients measured directly with 100 microm fura-FF was 39 nm AP(-1). At approximately 23 degrees C, the decay time constant of the Ca(2+) transients was 390 ms, corresponding to an extrusion rate of approximately 600 s(-1). At 34 degrees C, the decay time constant was 203 ms and the corresponding extrusion rate was approximately 1100 s(-1). At both temperatures, continuous theta-burst activity with three to five APs per theta cycle, as occurs in vivo during exploration, led to a moderate increase in the global Ca(2+) concentration that was proportional to AP number, whereas more intense stimulation was required to reach micromolar Ca(2+) concentrations and to shift Ca(2+) signalling into a non-linear regime. In conclusion, dentate gyrus BCs show a high endogenous Ca(2+)-binding ratio, a small AP-induced dendritic Ca(2+) influx, and a relatively slow Ca(2+) extrusion. These specific buffering properties of BCs will sharpen the time course of local Ca(2+) signals, while prolonging the decay of global Ca(2+) signals.

The two sides of hippocampal mossy fiber plasticity

Two studies in this issue of Neuron (Kwon and Castillo and Rebola et al.) show that the mossy fiber-CA3 pyramidal neuron synapse, a hippocampal synapse well known for its presynaptic plasticity, exhibits a novel form of long-term potentiation of NMDAR-mediated currents, which is induced and expressed postsynaptically.

Temporal compression mediated by short-term synaptic plasticity

Time scales of cortical neuronal dynamics range from few milliseconds to hundreds of milliseconds. In contrast, behavior occurs on the time scale of seconds or longer. How can behavioral time then be neuronally represented in cortical networks? Here, using electrophysiology and modeling, we offer a hypothesis on how to bridge the gap between behavioral and cellular time scales. The core idea is to use a long time constant of decay of synaptic facilitation to translate slow behaviorally induced temporal correlations into a distribution of synaptic response amplitudes. These amplitudes can then be transferred to a sequence of action potentials in a population of neurons. These sequences provide temporal correlations on a millisecond time scale that are able to induce persistent synaptic changes. As a proof of concept, we provide simulations of a neuron that learns to discriminate temporal patterns on a time scale of seconds by synaptic learning rules with a millisecond memory buffer. We find that the conversion from synaptic amplitudes to millisecond correlations can be strongly facilitated by subthreshold oscillations both in terms of information transmission and success of learning.

Long-term synaptic plasticity in hippocampal feedback inhibitory networks

Recent studies clearly indicate that long-term synaptic plasticity in hippocampal networks not only takes place at excitatory synapses of hippocampal granule and pyramidal cells, but also at excitatory synapses onto inhibitory interneurons. Various forms of long-term potentiation (LTP) and depression (LTD) have now been reported at glutamatergic synapses of interneurons in dentate gyrus (DG), CA3, and CA1 regions of the hippocampus. Importantly, the presence and type of these changes in synaptic efficacy appear to depend on the interneuron subtype, including its specific role within the hippocampal network. The data reviewed here suggest the existence of cell-type specific rules for synaptic plasticity in hippocampal feed-forward and feedback inhibitory networks. This specialized tuning of inhibition is likely important for global hippocampal function.

Transsynaptic modulation of the synaptic vesicle cycle by cell-adhesion molecules

Delicate control of the synaptic vesicle cycle is required to meet the demands imposed on synaptic transmission by the brain's complex information processing. In addition to intensively analyzed intrinsic regulation, extrinsic modulation of the vesicle cycle by the postsynaptic target neuron has become evident. Recent studies have demonstrated that several families of synaptic cell-adhesion molecules play a significant role in transsynaptic retrograde signaling. Different adhesion systems appear to specifically target distinct steps of the synaptic vesicle cycle. Signaling via classical cadherins regulates the recruitment of synaptic vesicles to the active zone. The neurexin/neuroligin system has been shown to modulate presynaptic release probability. In addition, reverse signaling via the EphB/ephrinB system plays an important role in the activity-dependent induction of long-term potentiation of presynaptic transmitter release. Moreover, the first hints of involvement of cell-adhesion molecules in vesicle endocytosis have been published. A general hypothesis is that specific adhesion systems might use different but parallel transsynaptic signaling pathways able to selectively modulate each step of the synaptic vesicle cycle in a tightly coordinated manner.

Differential mechanisms of transmission and plasticity at mossy fiber synapses

The last few decades have seen the hippocampal formation at front and center in the field of synaptic transmission. However, much of what we know about hippocampal short- and long-term plasticity has been obtained from research at one particular synapse; the Schaffer collateral input onto principal cells of the CA1 subfield. A number of recent studies, however, have demonstrated that there is much to be learned about target-specific mechanisms of synaptic transmission by study of the lesser known synapse made between the granule cells of the dentate gyrus; the so-called mossy fiber synapse, and its targets both within the hilar region and the CA3 hippocampus proper. Indeed investigation of this synapse has provided an embarrassment of riches concerning mechanisms of transmission associated with feedforward excitatory and inhibitory control of the CA3 hippocampus. Importantly, work from a number of labs has revealed that mossy fiber synapses possess unique properties at both the level of their anatomy and physiology, and serve as an outstanding example of a synapse designed for target-specific compartmentalization of synaptic transmission. The purpose of the present review is to highlight several aspects of this synapse as they pertain to a novel mechanism of bidirectional control of synaptic plasticity at mossy fiber synapses made onto hippocampal stratum lucidum interneurons. It is not my intention to pour over all that is known regarding the mossy fiber synapse since many have explored this topic exhaustively in the past and interested readers are directed to other fine reviews (Henze et al., 2000; Urban et al., 2001; Lawrence and McBain, 2003; Bischofberger et al., 2006; Nicoll and Schmitz, 2005).

Target-cell-dependent plasticity within the mossy fibre-CA3 circuit reveals compartmentalized regulation of presynaptic function at divergent release sites

Individual axons of central neurons innervate a large number of distinct postsynaptic targets belonging to divergent functional categories such as glutamatergic principal cells and inhibitory interneurons. While each bouton along a common axon should experience the same activity pattern in response to action potential firing within the parent presynaptic neuron, accumulating evidence suggests that neighbouring boutons contacting functionally distinct postsynaptic targets regulate their release properties independently, despite being separated by only a few microns. This target-cell-specific autonomy of presynaptic function can greatly expand the computational prowess of central axons to allow for precise coordination of large neuronal ensembles within a given circuit. An excellent example of target-cell-specific presynaptic mechanisms occurs in the CA3 hippocampus where mossy fibre (MF) axons of dentate gyrus granule cells target both principal cells and local circuit inhibitory interneurons via both anatomically and functionally specialized terminals. Of particular interest, mechanisms of both short- and long-term plasticity remain autonomous at these divergent release sites due to an anatomical and biochemical segregation of discrete molecular signalling cascades. Here we review roughly a decades worth of research on the MF-CA3 pathway to showcase the target-cell dependence of presynaptically expressed NMDA receptor-independent synaptic plasticity.

Differential modulation of short-term synaptic dynamics by long-term potentiation at mouse hippocampal mossy fibre synapses

Synapses continuously experience short- and long-lasting activity-dependent changes in synaptic strength. Long-term plasticity refers to persistent alterations in synaptic efficacy, whereas short-term plasticity (STP) reflects the instantaneous and reversible modulation of synaptic strength in response to varying presynaptic stimuli. The hippocampal mossy fibre synapse onto CA3 pyramidal cells is known to exhibit both a presynaptic, NMDA receptor-independent form of long-term potentiation (LTP) and a pronounced form of STP. A detailed description of their exact interdependence is, however, lacking. Here, using electrophysiological and computational techniques, we have developed a descriptive model of transmission dynamics to quantify plasticity at the mossy fibre synapse. STP at this synapse is best described by two facilitatory processes acting on time-scales of a few hundred milliseconds and about 10 s. We find that these distinct types of facilitation are differentially influenced by LTP such that the impact of the fast process is weakened as compared to that of the slow process. This attenuation is reflected by a selective decrease of not only the amplitude but also the time constant of the fast facilitation. We henceforth argue that LTP, involving a modulation of parameters determining both amplitude and time course of STP, serves as a mechanism to adapt the mossy fibre synapse to its temporal input.

GluR7 is an essential subunit of presynaptic kainate autoreceptors at hippocampal mossy fiber synapses

Presynaptic ionotropic glutamate receptors are emerging as key players in the regulation of synaptic transmission. Here we identify GluR7, a kainate receptor (KAR) subunit with no known function in the brain, as an essential subunit of presynaptic autoreceptors that facilitate hippocampal mossy fiber synaptic transmission. GluR7(-/-) mice display markedly reduced short- and long-term synaptic potentiation. Our data suggest that presynaptic KARs are GluR6/GluR7 heteromers that coassemble and are localized within synapses. We show that recombinant GluR6/GluR7 KARs exhibit low sensitivity to glutamate, and we provide evidence that presynaptic KARs at mossy fiber synapses are likely activated by high concentrations of glutamate. Overall, from our data, we propose a model whereby presynaptic KARs are localized in the presynaptic active zone close to release sites, display low affinity for glutamate, are likely Ca(2+)-permeable, are activated by single release events, and operate within a short time window to facilitate the subsequent release of glutamate.

Regional differences in GABAergic modulation for TEA-induced synaptic plasticity in rat hippocampal CA1, CA3 and dentate gyrus

Tetraethylammonium (TEA), a K(+)-channel blocker, reportedly induces long-term potentiation (LTP) of hippocampal CA1 synaptic responses, but at CA3 and the dentate gyrus (DG), the characteristics of TEA-induced plasticity and modulation by inhibitory interneurons remain unclear. This study recorded field EPSPs from CA1, CA3 and DG to examine the involvement of GABAergic modulation in TEA-induced synaptic plasticity for each region. In Schaffer collateral-CA1 synapses and associational fiber (AF)-CA3 synapses, bath application of TEA-induced LTP in the presence and absence of picrotoxin (PTX), a GABA(A) receptor blocker, whereas TEA-induced LTP at mossy fiber (MF)-CA3 synapses was detected only in the absence of GABA(A) receptor blockers. MF-CA3 LTP showed sensitivity to Ni(2+), but not to nifedipine. In DG, synaptic plasticity was modulated by GABAergic inputs, but characteristics differed between the afferent lateral perforant path (LPP) and medial perforant path (MPP). LPP-DG synapses showed TEA-induced LTP during PTX application, whereas at MPP-DG synapses, TEA-induced long-term depression (LTD) was seen in the absence of PTX. This series of results demonstrates that TEA-induced DG and CA3 plasticity displays afferent specificity and is exposed to GABAergic modulation in an opposite manner.

Differential expression of posttetanic potentiation and retrograde signaling mediate target-dependent short-term synaptic plasticity

Short-term synaptic plasticity influences how presynaptic spike patterns control the firing of postsynaptic targets. Here we investigated whether specific mechanisms of short-term plasticity are regulated in a target-dependent manner by comparing synapses made by cerebellar granule cell parallel fibers onto Golgi cells (PF-->GC synapse) and Purkinje cells (PF-->PC synapse). Both synapses exhibited similar facilitation, suggesting that any differential short-term plasticity does not reflect differences in the initial release probability. PF-->PC synapses were highly sensitive to stimulus bursts, which could result in either depression of subsequent responses, mediated by endocannabinoid-dependent retrograde signaling, or enhancement of responses through posttetanic potentiation (PTP). In contrast, stimulus bursts had remarkably little effect on the strength of PF-->GC synapses. Unlike PCs, GCs were unable to regulate their PF synapses by releasing endocannabinoids. Moreover, PTP was reduced at the PF-->GC synapse compared to the PF-->PC synapse. Thus, the target-dependence of PF synapses arises from the differential expression of both retrograde signaling and PTP.

The Role of the GluR2 Subunit in AMPA Receptor Function and Synaptic Plasticity

The AMPA receptor (AMPAR) GluR2 subunit dictates the critical biophysical properties of the receptor, strongly influences receptor assembly and trafficking, and plays pivotal roles in a number of forms of long-term synaptic plasticity. Most neuronal AMPARs contain this critical subunit; however, in certain restricted neuronal populations and under certain physiological or pathological conditions, AMPARs that lack this subunit are expressed. There is a current surge of interest in such GluR2-lacking Ca2+-permeable AMPARs in how they affect the regulation of synaptic transmission. Here, we bring together recent data highlighting the novel and important roles of GluR2 in synaptic function and plasticity.

Presynaptic inhibition of excitatory afferents to hilar mossy cells

The hippocampus contains one very strong recurrent excitatory network formed by associational connections between CA3 pyramidal cells and another that depends largely on a disynaptic excitatory pathway between dentate granule cells. The recurrent excitatory network in CA3 has long been considered a possible location of autoassociative memory storage, whereas changes in the level and arrangement of recurrent excitation between granule cells are strongly implicated in epileptogenesis. Hilar mossy cells are likely to receive collateral input from CA3 pyramidal cells and they are key intermediaries (by mossy fiber inputs) in the recurrent excitatory network between granule cells. The current study uses minimal stimulation techniques in an in vitro preparation of the rat dentate gyrus to examine presynaptic modulation of both mossy fiber and non-mossy fiber inputs to hilar mossy cells. We report that both mossy fiber and non-mossy fiber inputs to hilar mossy cells express presynaptic gamma-aminobutyric acid type B (GABA(B)) receptors that are subject to tonic inhibition by ambient GABA. We further find that only non-mossy fiber inputs express presynaptic muscarinic acetylcholine receptors, but that bath application of cholinergic agonists produces action potential-dependent increases in ambient GABA that can indirectly inhibit mossy fiber inputs. Finally, we demonstrate that mossy cells express high-affinity postsynaptic GABA(A) receptors that are also capable of detecting changes in ambient GABA produced by cholinergic agonists. Our results are among the first to directly characterize these important collateral inputs to hilar mossy cells and may help facilitate informed comparison between primary and collateral projections in two major excitatory pathways.

Differential regulation at functionally divergent release sites along a common axon

Information transfer within neuronal networks requires the precise coordination of distinct neuronal populations within a given circuit. Evidence from a variety of central pathways indicates that such coordination is mediated in part by the ability of neurons to differentially regulate release properties at functionally divergent presynaptic elements along their individual axons according to the identity of the postsynaptic cell being innervated. Recent findings have revealed the cellular mechanisms by which central afferents modify release properties at individual presynaptic sites independent of neighboring terminals. Such autonomy of presynaptic regulation enables target-cell-dependent short-term and long-term synaptic plasticity and ensures that distinct features of afferent activity are relayed to divergent target-cell populations.

Coactivation of pre- and postsynaptic signaling mechanisms determines cell-specific spike-timing-dependent plasticity

Synapses may undergo long-term increases or decreases in synaptic strength dependent on critical differences in the timing between pre-and postsynaptic activity. Such spike-timing-dependent plasticity (STDP) follows rules that govern how patterns of neural activity induce changes in synaptic strength. Synaptic plasticity in the dorsal cochlear nucleus (DCN) follows Hebbian and anti-Hebbian patterns in a cell-specific manner. Here we show that these opposing responses to synaptic activity result from differential expression of two signaling pathways. Ca2+/calmodulin-dependent protein kinase II (CaMKII) signaling underlies Hebbian postsynaptic LTP in principal cells. By contrast, in interneurons, a temporally precise anti-Hebbian synaptic spike-timing rule results from the combined effects of postsynaptic CaMKII-dependent LTP and endocannabinoid-dependent presynaptic LTD. Cell specificity in the circuit arises from selective targeting of presynaptic CB1 receptors in different axonal terminals. Hence, pre- and postsynaptic sites of expression determine both the sign and timing requirements of long-term plasticity in interneurons.

Light triggers expression of philanthotoxin-insensitive Ca2+-permeable AMPA receptors in the developing rat retina

Ca2+-permeable AMPA receptors (AMPARs) are expressed throughout the adult CNS but yet their role in development is poorly understood. In the developing retina, most investigations have focused on Ca2+ influx through NMDARs in promoting synapse maturation and not on AMPARs. However, NMDARs are absent from many retinal cells suggesting that other Ca2+-permeable glutamate receptors may be important to consider. Here we show that inhibitory horizontal and AII amacrine cells lack NMDARs but express Ca2+-permeable AMPARs. Before eye-opening, AMPARs were fully blocked by philanthotoxin (PhTX), a selective antagonist of Ca2+-permeable AMPARs. After eye-opening, however, a subpopulation of Ca2+-permeable AMPARs were unexpectedly PhTX resistant. Furthermore, Joro spider toxin (JSTX) and IEM-1460 also failed to antagonize, demonstrating that this novel pharmacology is shared by several AMPAR channel blockers. Interestingly, PhTX-insensitive AMPARs failed to express in retinae from dark-reared animals demonstrating that light entering the eye triggers their expression. Eye-opening coincides with the consolidation of inhibitory cell connections suggesting that the developmental switch to a Ca2+-permeable AMPAR with novel pharmacology may be critical to synapse maturation in the mammalian retina.

Compartmentalized Ca(2+) channel regulation at divergent mossy-fiber release sites underlies target cell-dependent plasticity

Hippocampal mossy fibers (MFs) innervate CA3 targets via anatomically distinct presynaptic elements: MF boutons (MFBs) innervate pyramidal cells (PYRs), whereas filopodial extensions (Fils) of MFBs innervate st. lucidum interneurons (SLINs). Surprisingly, the same high-frequency stimulation (HFS) protocol induces presynaptically expressed LTP and LTD at PYR and SLIN inputs, respectively. This differential distribution of plasticity indicates that neighboring, functionally divergent presynaptic elements along the same axon serve as autonomous computational elements capable of modifying release independently. Indeed we report that HFS persistently depresses voltage-gated calcium channel (VGCC) function in Fil terminals, leaving MFB VGCCs unchanged despite similar contributions of N- and P/Q-type VGCCs to transmission at each terminal. Selective Fil VGCC depression results from HFS-induced mGluR7 activation leading to persistent P/Q-type VGCC inhibition. Thus, mGluR7 localization to MF-SLIN terminals and not MFBs allows for MF-SLIN LTD expression via depressed presynaptic VGCC function, whereas MF-PYR plasticity proceeds independently of VGCC alterations.

Mossy fiber synaptic transmission: communication from the dentate gyrus to area CA3

Communication between the dentate gyrus (DG) and area CA3 of the hippocampus proper is transmitted via axons of granule cells--the mossy fiber (MF) pathway. In this review we discuss and compare the properties of transmitter release from the MFs onto pyramidal neurons and interneurons. An examination of the anatomical connectivity from DG to CA3 reveals a surprising interplay between excitation and inhibition for this circuit. In this respect it is particularly relevant that the major targets of the MFs are interneurons and that the consequence of MF input into CA3 may be inhibitory or excitatory, conditionally dependent on the frequency of input and modulatory regulation. This is further complicated by the properties of transmitter release from the MFs where a large number of co-localized transmitters, including GABAergic inhibitory transmitter release, and the effects of presynaptic modulation finely tune transmitter release. A picture emerges that extends beyond the hypothesis that the MFs are simply "detonators" of CA3 pyramidal neurons; the properties of synaptic information flow from the DG have more subtle and complex influences on the CA3 network.

Models, structure, function: the transformation of cortical signals in the dentate gyrus

Our central question is why the hippocampal CA3 region is the only cortical area capable of forming interference-free representations of complex environmental events (episodes), given that apparently all cortical regions have recurrent excitatory circuits with modifiable synapses, the basic substrate for autoassociative memory networks. We review evidence for the radical (but classic) view that a unique transformation of incoming cortical signals by the dentate gyrus and the subsequent faithful transfer of the resulting code by the mossy fibers are absolutely critical for the appropriate association of memory items by CA3 and, in general, for hippocampal function. In particular, at the gate of the hippocampal formation, the dentate gyrus possesses a set of unusual properties, which selectively evolved for the task of code transformation between cortical afferents and the hippocampus. These evolutionarily conserved anatomical features enable the dentate gyrus to translate the noisy signal of the upstream cortical areas into the sparse and specific code of hippocampal formation, which is indispensable for the efficient storage and recall of multiple, multidimensional memory items. To achieve this goal the mossy fiber pathway maximally utilizes the opportunity to differentially regulate its postsynaptic partners. Selective innervation of CA3 pyramidal cells and interneurons by distinct terminal types creates a favorable condition to differentially regulate the short-term and long-term plasticity and the motility of various mossy terminal types. The utility of this highly dynamic system appears to be the frequency-dependent fine-tuning the excitation and inhibition evoked by the large and the small mossy terminals respectively. This will determine exactly which CA3 cell population is active and induces permanent modification in the autoassociational network of the CA3 region.

Timing and efficacy of transmitter release at mossy fiber synapses in the hippocampal network

It is widely accepted that the hippocampus plays a major role in learning and memory. The mossy fiber synapse between granule cells in the dentate gyrus and pyramidal neurons in the CA3 region is a key component of the hippocampal trisynaptic circuit. Recent work, partially based on direct presynaptic patch-clamp recordings from hippocampal mossy fiber boutons, sheds light on the mechanisms of synaptic transmission and plasticity at mossy fiber synapses. A high Na(+) channel density in mossy fiber boutons leads to a large amplitude of the presynaptic action potential. Together with the fast gating of presynaptic Ca(2+) channels, this generates a large and brief presynaptic Ca(2+) influx, which can trigger transmitter release with high efficiency and temporal precision. The large number of release sites, the large size of the releasable pool of vesicles, and the huge extent of presynaptic plasticity confer unique strength to this synapse, suggesting a large impact onto the CA3 pyramidal cell network under specific behavioral conditions. The characteristic properties of the hippocampal mossy fiber synapse may be important for pattern separation and information storage in the dentate gyrus-CA3 cell network.

Interneurons targeting similar layers receive synaptic inputs with similar kinetics

GABAergic interneurons play diverse and important roles in controlling neuronal network dynamics. They are characterized by an extreme heterogeneity morphologically, neurochemically, and physiologically, but a functionally relevant classification is still lacking. Present taxonomy is essentially based on their postsynaptic targets, but a physiological counterpart to this classification has not yet been determined. Using a quantitative analysis based on multidimensional clustering of morphological and physiological variables, we now demonstrate a strong correlation between the kinetics of glutamate and GABA miniature synaptic currents received by CA1 hippocampal interneurons and the laminar distribution of their axons: neurons that project to the same layer(s) receive synaptic inputs with similar kinetics distributions. In contrast, the kinetics distributions of GABAergic and glutamatergic synaptic events received by a given interneuron do not depend upon its somatic location or dendritic arborization. Although the mechanisms responsible for this unexpected observation are still unclear, our results suggest that interneurons may be programmed to receive synaptic currents with specific temporal dynamics depending on their targets and the local networks in which they operate.

Transient incorporation of native GluR2-lacking AMPA receptors during hippocampal long-term potentiation

Postnatal glutamatergic principal neuron synapses are typically presumed to express only calcium-impermeable (CI), GluR2-containing AMPARs under physiological conditions. Here, however, we demonstrate that long-term potentiation (LTP) in CA1 hippocampal pyramidal neurons causes rapid incorporation of GluR2-lacking calcium-permeable (CP)-AMPARs: CP-AMPARs are present transiently, being replaced by GluR2-containing AMPARs approximately 25 min after LTP induction. Thus, CP-AMPARs are physiologically expressed at CA1 pyramidal cell synapses during LTP, and may be required for LTP consolidation.

Induction and expression mechanisms of postsynaptic NMDA receptor-independent homosynaptic long-term depression

The induction of long-term depression (LTD) can be divided into two main forms, one dependent upon activation of postsynaptic NMDAR, and another independent of postsynaptic NMDAR. Non-postsynaptic NMDAR-LTD (non-NMDAR-LTD) occurs in many regions of the brain, and encompasses a wide variety of induction and expression mechanisms. In this article, the induction and expression mechanisms of such LTD in over 10 brain regions are described, with a number of common mechanisms compared across a large range of types of LTD. The article describes the involvement of different presynaptic or postsynaptic receptors in the induction of non-NMDAR-LTD, especially metabotropic glutamate receptors, cannabinoid receptors and dopamine receptors. An increase in presynaptic or postsynaptic intracellular Ca concentration is a key event in induction, commonly followed by activation of certain kinases, especially PKC, p38 MAPK and ERK. Expression mechanisms are either presynaptic via a reduction in release probability, or postsynaptic involving a decrease in AMPAR via phosphorylation of a glutamate receptor subunit, especially GluR2, followed by clathrin-mediated endocytosis. Retrograde signalling from postsynaptic to presynaptic occurs when induction is postsynaptic and expression is presynaptic.

Synaptic plasticity at hippocampal mossy fibre synapses

The dentate gyrus provides the main input to the hippocampus. Information reaches the CA3 region through mossy fibre synapses made by dentate granule cell axons. Synaptic plasticity at the mossy fibre-pyramidal cell synapse is unusual for several reasons, including low basal release probability, pronounced frequency facilitation and a lack of N-methyl-D-aspartate receptor involvement in long-term potentiation. In the past few years, some of the mechanisms underlying the peculiar features of mossy fibre synapses have been elucidated. Here we describe recent work from several laboratories on the various forms of synaptic plasticity at hippocampal mossy fibre synapses. We conclude that these contacts have just begun to reveal their many secrets.

Long- and short-term plasticity at mossy fiber synapses on mossy cells in the rat dentate gyrus

Mossy cells give rise to the commissural and associational pathway of the dentate gyrus, and receive their major excitatory inputs from the mossy fibers of granule cells. Through these feed-back excitatory connections, mossy cells have been suggested to play important roles in both normal signal processing in learning and memory, as well as in seizure propagation. However, the nature of the activity-dependent modifications of the mossy fiber inputs to mossy hilar cells is not well understood. We studied the long- and short-term plasticity properties of the mossy fiber-mossy cell synapse, using the minimal stimulation technique in slices in whole cell recorded mossy cells retrogradely prelabeled with the fluorescent dye DiO from the contralateral dentate gyrus. Following tetanic stimulation, mossy fiber synapses showed significant NMDA receptor-independent long-term potentiation (LTP), associated with increased excitatory postsynaptic currents (EPSC) amplitude and decreased failure rates. Coefficient of variance and failure rate analyses suggested a presynaptic locus of LTP induction. Mossy fiber synapses on mossy cells also showed activity-dependent short-term modification properties, including both frequency-dependent facilitation (stimuli at higher frequencies evoked larger EPSCs with lower failure rates) and burst facilitation (each EPSC in a burst had a larger amplitude and higher probability of occurrence than the preceding EPSCs within the burst). The data show that mossy fiber-mossy cell synapses exhibit both long- and short-term plasticity phenomena that are generally similar to the mossy fiber synapses on CA3 pyramidal cells.

Studies of the synaptic plasticity of field CA3 of the hippocampus during tetanization of the perforant path

Studies on living slices of hippocampus-entorhinal cortex formation from adult rats were performed to investigate changes in responses in field CA3 to stimulation of mossy fibers in conditions of perforant path tetanization with different parameters. Tetanization of the perforant path at frequencies of 10 and 100 Hz induced depression of responses in CA3 on testing of this same path. Tetanization of the perforant path at a frequency of 10 Hz and an amplitude subthreshold for potentiating mossy fiber synapses in CA3 became threshold if preceded by tetanization of the perforant path at a frequency of 100 Hz. Tetanization of mossy fibers at 10 Hz resulted in potentiation of the input to CA3, while tetanization at 100 Hz induced depression. High-frequency tetanization of the perforant path (100 Hz) delivered in trains following at the frequency of the theta rhythm, led mainly to depression of field CA3 responses to stimulation of mossy fibers.

Mechanisms of target-cell specific short-term plasticity at Schaffer collateral synapses onto interneurones versus pyramidal cells in juvenile rats

Although it is presynaptic, short-term plasticity has been shown at some synapses to depend upon the postsynaptic cell type. Previous studies have reported conflicting results as to whether Schaffer collateral axons have target-cell specific short-term plasticity. Here we investigate in detail the short-term dynamics of Schaffer collateral excitatory synapses onto CA1 stratum radiatum interneurones versus pyramidal cells in acute hippocampal slices from juvenile rats. In response to three stimulus protocols that invoke different forms of short-term plasticity, we find differences in some but not all forms of presynaptic short-term plasticity, and heterogeneity in the short term plasticity of synapses onto interneurones. Excitatory synapses onto the majority of interneurones had less paired-pulse facilitation than synapses onto pyramidal cells across a range of interpulse intervals (20-200 ms). Unlike synapses onto pyramidal cells, synapses onto most interneurones had very little facilitation in response to short high-frequency trains of five pulses at 5, 10 and 20 Hz, and depressed during trains at 50 Hz. However, the amount of high-frequency depression was not different between synapses onto pyramidal cells versus the majority of interneurones at steady state during 2-10 Hz trains. In addition, a small subset of interneurones (approximately 15%) had paired-pulse depression rather than paired-pulse facilitation, showed only depression in response to the high-frequency five pulse trains, and had more steady-state high-frequency depression than synapses onto pyramidal cells or the majority of interneurones. To investigate possible mechanisms for these differences in short-term plasticity, we developed a mechanistic mathematical model of neurotransmitter release that explicitly explores the contributions to different forms of short-term plasticity of the readily releasable vesicle pool size, release probability per vesicle, calcium-dependent facilitation, synapse inactivation following release, and calcium-dependent recovery from inactivation. Our model fits the responses of each of the three cell groups to the three different stimulus protocols with only two parameters that differ with cell group. The model predicts that the differences in short-term plasticity between synapses onto CA1 pyramidal cells and stratum radiatum interneurones are due to a higher initial release probability per vesicle and larger readily releasable vesicle pool size at synapses onto interneurones, resulting in a higher initial release probability. By measuring the rate of block of NMDA receptors by the open channel blocker MK-801, we confirmed that the initial release probability is greater at synapses onto interneurones versus pyramidal cells. This provides a mechanism by which both the initial strength and the short-term dynamics of Schaffer collateral excitatory synapses are regulated by their postsynaptic target cell.

Kainate receptors and mossy fiber LTP

There is considerable interest in understanding long-term potentiation (LTP) of glutamatergic synaptic transmission because the molecular mechanisms involved in its induction and expression are believed to be critical for learning and memory. There are two distinct forms of LTP. One type is triggered by synaptic activation of NMDA receptors and the other is NMDA receptor-independent. The latter type of LTP has been mostly studied at mossy fiber/CA3 synapses. Here we summarise some of our recent studies concerning the mechanisms of the induction of the NMDA receptor-independent form of LTP at these CA3 synapses. This form of LTP is triggered by the synaptic activation of kainate receptors. We also address the importance of Ca2+ availability in the extracellular environment and the release of Ca2+ from intracellular stores for this form of LTP.

mGluR7 is a metaplastic switch controlling bidirectional plasticity of feedforward inhibition

Plasticity of feedforward inhibition in the hippocampal mossy fiber (MF) pathway can dramatically influence dentate gyrus-CA3 dialog. Interestingly, MF inputs to CA3 stratum lucidum interneurons (SLINs) undergo long-term depression (LTD) following high-frequency stimulation (HFS), in contrast to MF-pyramid (PYR) synapses, where long-term potentiation (LTP) occurs. Furthermore, activity-induced potentiation of MF-SLIN transmission has not previously been observed. Here we report that metabotropic glutamate receptor subtype 7 (mGluR7) is a metaplastic switch at MF-SLIN synapses, whose activation and surface expression governs the direction of plasticity. In naive slices, mGluR7 activation during HFS generates MF-SLIN LTD, depressing presynaptic release through a PKC-dependent mechanism. Following agonist exposure, mGluR7 undergoes internalization, unmasking the ability of MF-SLIN synapses to undergo presynaptic potentiation in response to the same HFS that induces LTD in naive slices. Thus, selective mGluR7 targeting to MF terminals contacting SLINs and not PYRs provides cell target-specific plasticity and bidirectional control of feedforward inhibition.

Stress suppresses and learning induces plasticity in CA3 of rat hippocampus: a three-dimensional ultrastructural study of thorny excrescences and their postsynaptic densities

Chronic stress and spatial training have been proposed to affect hippocampal structure and function in opposite ways. Previous morphological studies that addressed structural changes after chronic restraint stress and spatial training were based on two-dimensional morphometry which does not allow a complete morphometric characterisation of synaptic features. Here, for the first time in such studies, we examined these issues by using three-dimensional (3-D) reconstructions of electron microscope images taken from thorny excrescences of hippocampal CA3 pyramidal cells. Ultrastructural alterations in postsynaptic densities (PSDs) of thorny excrescences receiving input from mossy fibre boutons were also determined, as were changes in numbers of multivesicular bodies (endosome-like structures) within thorny excrescences and dendrites. Quantitative 3-D data demonstrated retraction of thorny excrescences after chronic restraint stress which was reversed after water maze training, whilst water maze training alone increased thorny excrescence volume and number of thorns per thorny excrescence. PSD surface area was unaffected by restraint stress but water maze training increased both number and area of PSDs per thorny excrescence. In restrained rats that were water maze trained PSD volume and surface area increased significantly. The proportion of perforated PSDs almost doubled after water maze training and restraint stress. Numbers of endosome-like structures in thorny excrescences decreased after restraint stress and increased after water maze training. These findings demonstrate that circuits involving contacts between mossy fibre terminals and CA3 pyramidal cells at stratum lucidum level are affected conversely by water maze training and chronic stress, confirming the remarkable plasticity of CA3 dendrites. They provide a clear illustration of the structural modifications that occur after life experiences noted for their different impact on hippocampal function.

Chronic BDNF deficiency permanently modifies excitatory synapses in the piriform cortex

Brain-derived neurotrophic factor (BDNF), aside from its classic neurotrophic role in development and survival of neurons, has been shown to be involved in modification and plasticity of central synapses. In mice with BDNF gene deletion (BDNF+/-), deficits in synaptic transmission are often observed but are reversed readily by administration of BDNF, suggesting its acute effect. In support, blockade of BDNF signaling in wild-type hippocampal slices by TrkB-IgG closely reproduces synaptic alterations observed in BDNF+/- mice. We demonstrate that in BDNF+/- mice, lateral olfactory tract (LOT) synapses exhibit decreased release probability of glutamate, suggested by increased paired-pulse facilitation (PPF) of field excitatory postsynaptic potentials (fEPSPs), as well as by slower blocking rate of N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic currents (EPSCs) by MK-801 in the pyramidal neurons of the piriform cortex. The changes in PPF were not mimicked in wild-type mice by acute blockade of BDNF signaling by TkrB-IgG. These data imply that BDNF deficit during development might lead to chronic changes of excitatory transmission in LOT synapses. Modification of the LOT synapses in BDNF+/- mice was associated with altered inhibitory drive onto the mitral cells from the granule and glomerular neurons, which in turn exhibited decreased renewal rate compared to that in wild-type mice. Taken together, these data suggest that BDNF deficiency can have both acute and more permanent effects on synaptic function, particularly when BDNF signaling is compromised during the early stages of brain development. In the latter case, altered synaptic properties in BDNF+/- mice could be secondary to other complex changes in the brain, e.g., cell survival/proliferation.

Neurogranin is expressed by principal cells but not interneurons in the rodent and monkey neocortex and hippocampus

As a substrate of protein kinase C (PKC), neurogranin (NG) is involved in the regulation of calcium signaling and activity-dependent plasticity. Recently, we have shown that, in the rodent cerebellum, NG is exclusively expressed by gamma-aminobutyric acidergic Golgi cells, whereas, in the monkey cerebellum, brush cells were the only neuronal population expressing NG (Singec et al. [2003] J. Comp. Neurol. 459:278-289). In the present study, we analyzed the neocortical and hippocampal expression patterns of NG in adult mouse (C57Bl/6), rat (Wistar), and monkey (Cercopithecus aetiops). By using immunocytochemistry and nonradioactive in situ hybridization, we demonstrate strong NG expression by principal cells in different neocortical layers and in the hippocampus by granule cells of the dentate gyrus and pyramidal neurons of CA1-CA3. In contrast, double-labeling experiments in rodents revealed that neocortical and hippocampal interneurons expressing glutamate decarboxylase 67 (GAD67) were consistently devoid of NG. In addition, by using antibodies against parvalbumin, calbindin, and calretinin, we could demonstrate the absence of NG in interneurons of monkey frontal cortex and hippocampus. Together these findings corroborate the idea of different calcium signaling pathways in excitatory and inhibitory cells that may contribute to different modes of synaptic plasticity in these neurons.

Differential involvement of oriens/pyramidale interneurones in hippocampal network oscillations in vitro

Using whole-cell patch-clamp recordings in conjunction with post hoc anatomy we investigated the physiological properties of hippocampal stratum oriens and stratum pyramidale inhibitory interneurones, before and following the induction of pharmacologically evoked gamma frequency network oscillations. Prior to kainate-induced transient epochs of gamma activity, two distinct classes of oriens interneurones, oriens lacunosum-moleculare (O-LM) and trilaminar cells, showed prominent differences in their membrane and firing properties, as well as in the amplitude and kinetics of their excitatory postsynaptic events. In the active network both types of neurone received a phasic barrage of gamma frequency excitatory inputs but, due to their differential functional integration, showed clear differences in their output patterns. While O-LM cells fired intermittently at theta frequency, trilaminar interneurones discharged on every gamma cycle and showed a propensity to fire spike doublets. Two other classes of fast spiking interneurones, perisomatic targeting basket and bistratified cells, in the active network discharged predominantly single action potentials on every gamma cycle. Thus, within a locally excited network, O-LM cells are likely to provide a theta-frequency patterned output to distal dendritic segments, whereas basket and bistratified cells are involved in the generation of locally synchronous gamma band oscillations. The anatomy and output profile of trilaminar cells suggest they are involved in the projection of locally generated gamma rhythms to distal sites. Therefore a division of labour appears to exist whereby different frequencies and spatiotemporal properties of hippocampal rhythms are mediated by different interneurone subtypes.

Routing of spike series by dynamic circuits in the hippocampus

Recurrent inhibitory loops are simple neuronal circuits found in the central nervous system, yet little is known about the physiological rules governing their activity. Here we use simultaneous somatic and dendritic recordings in rat hippocampal slices to show that during a series of action potentials in pyramidal cells recurrent inhibition rapidly shifts from their soma to the apical dendrites. Two distinct inhibitory circuits are sequentially recruited to produce this shift: one, time-locked with submillisecond precision to the onset of the action potential series, transiently inhibits the somatic and perisomatic regions of pyramidal cells; the other, activated in proportion to the rate of action potentials in the series, durably inhibits the distal apical dendrites. These two operating modes result from the synergy between pre- and postsynaptic properties of excitatory synapses onto recurrent inhibitory neurons with distinct projection patterns. Thus, the onset of a series of action potentials and the rate of action potentials in the series are selectively captured and transformed into different spatial patterns of recurrent inhibition.

Quantal transmission at mossy fibre targets in the CA3 region of the rat hippocampus

Recent anatomical evidence that inhibitory interneurones receive approximately 10 times more synapses from mossy fibres than do principal neurones (Acsády et al. 1998) has led to the re-examination of the extent to which interneurones are involved in CA3 network excitability. Although many of the anatomical and physiological properties of mossy fibre-CA3 interneurone synapses have been previously described (Acsády et al. 1998; Tóth et al. 2000), an investigation into the quantal nature of transmission at this synapse has not yet been conducted. Here, we employed variance-mean (VM) analysis to compare the release probability, quantal size (q) and number of release sites (n) at mossy fibre target neurones in CA3. At six of seven interneurone synapses in which a high concentration of Ca2+ was experimentally imposed, the variance-mean relationship could be approximated by a parabola. Estimates of n were 1-2, and the weighted release probability in normal Ca2+ conditions ranged from 0.34 to 0.51. At pyramidal cell synapses, the variance-mean relationship approximated a linear relationship, suggesting that release probability was significantly lower. The weighted quantal amplitude was similar at interneurone synapses and pyramidal cell synapses, although the variability in quantal amplitude was larger at interneurone synapses. Mossy fibre transmission at CA3 interneurone synapses can be explained by a lower number of release sites, a broader range of release probabilities, and larger range of quantal amplitudes than at CA3 pyramidal synapses. Finally, quantal events on to interneurones elicited spike transmission, owing in part to the more depolarized membrane potential than pyramidal cells. These results suggest that although mossy fibre synapses on to pyramidal cells are associated with a larger number of release sites per synapse, the higher connectivity, higher initial release probability, and larger relative impact per quantum on to CA3 interneurones generate strong feedforward inhibition at physiological firing frequencies of dentate granule cells. Given the central role of CA3 interneurones in mossy fibre synaptic transmission, these details of mossy fibre synaptic transmission should provide insight into CA3 network dynamics under both physiological and pathophysiological circumstances.

Synaptic Dynamics on Different Time Scales in a Parallel Fiber Feedback Pathway of the Weakly Electric Fish

Synaptic dynamics comprise a variety of interacting processes acting on a wide range of time scales. This enables a synapse to perform a large array of computations, from temporal and spatial filtering to associative learning. In this study, we describe how changing synaptic gain via long-term plasticity can act to shape the temporal filtering of a synapse through modulation of short-term plasticity. In the weakly electric fish, parallel fibers from cerebellar granule cells provide massive feedback inputs to the pyramidal neurons of the electrosensory lateral line lobe. We demonstrate a long-term synaptic enhancement (LTE) of these synapses that is biochemically similar to the presynaptic long-term potentiation expressed by parallel fibers in the mammalian cerebellum. Using a novel stimulation protocol and a simple modeling paradigm, we then quantify the changes in short-term plasticity during the induction of LTE and show that these changes can be explained by gradual changes in only one model parameter, that which is associated with the baseline probability of transmitter release. These changes lead to a shift in the spike frequency preference of the synapse, suggesting that long-term plasticity is not only involved in controlling the gain of the parallel fiber synapse, but also provides a means of controlling synaptic filtering over multiple time scales.

Pharmacological dissection of calcium channel subtype-related components of strontium inflow in large mossy fiber boutons of mouse hippocampus

Several subtypes of voltage-dependent calcium channels (VDCCs) are present in the presynaptic terminals. In the mammalian hippocampus, P/Q-, N-, and R- but not L-type VDCCs are involved in the fast transmitter release from large mossy fiber (MF) boutons, which are associated with CA3 pyramidal cell dendrites. We investigated whether L-type VDCCs are indeed absent in these large MF boutons. With the use of Sr2+ as the Ca2+ substitute, the stimulus-evoked Sr2+ increment (delta[Sr2+]pre) was evaluated fluorometrically. Delta[Sr2+]pre appeared to be proportional to Sr2+ inflow through VDCCs and was specifically attenuated by conventional VDCC subtype-selective antagonists. The P/Q-type selective omega-agatoxin IVA (AgTx(IVA)) blocked delta[Sr2+]pre with an IC50 of 28 nM and by 30-35% at its maximum effective concentration of 0.5 microM. The N-type selective omega-conotoxin GVIA (CgTx(GVIA)) blocked delta[Sr2+]pre with an IC50 of 15 nM and by 20-25% at its maximum effective concentration of 1 microM. The R-type selective SNX-482 blocked delta[Sr2+]pre with an IC50 of 79 nM and by 20-25% at its maximum effective concentration of 1 microM. The effects of these toxins did not overlap at their maximum effective concentrations and about 70-80% of delta[Sr +]pre was blocked by the simultaneous exposure to these toxins. delta[Sr2+]pre component that is resistant to AgTx(IVA), CgTx(IVA), and SNX-482 was significantly potentiated by an L-type agonist, (S)-(-)-Bay K8644, and attenuated by an L-type antagonist, nimodipine, suggesting that L-type VDCCs are present in large MF terminals. The L-type agonist, (+/-)-Bay K8644, also potentiated Sr2+ inflow into individual boutons identified as large MF boutons under confocal microscopy. Almost similar results were observed for Ca2+ inflow-dependent fluorescence increments. L-type VDCCs appear to be present in large MF boutons and mediate a substantial Ca2+ inflow into presynaptic terminals during action potentials.

Interneuron diversity series: containing the detonation--feedforward inhibition in the CA3 hippocampus

Feedforward inhibitory circuits are involved both in the suppression of excitability and timing of action potential generation in principal cells. In the CA3 hippocampus, a single mossy fiber from a dentate gyrus granule cell forms giant boutons with multiple release sites, which are capable of detonating CA3 principal cells. By contrast, mossy fiber terminals form a larger number of Lilliputian-sized synapses with few release sites onto local circuit interneurons, with distinct presynaptic and postsynaptic properties. This dichotomy between the two synapse types endows the circuit with exquisite control over pyramidal cell discharge. Under pathological conditions where feedforward inhibition is compromised, focal excitation is no longer contained, rendering the circuit susceptible to hyperexcitability.

Synapse-specific mGluR1-dependent long-term potentiation in interneurones regulates mouse hippocampal inhibition

Hippocampal CA1 inhibitory interneurones control the excitability and synchronization of pyramidal cells, and participate in hippocampal synaptic plasticity. Pairing theta-burst stimulation (TBS) with postsynaptic depolarization, we induced long-term potentiation (LTP) of putative single-fibre excitatory postsynaptic currents (EPSCs) in stratum oriens/alveus (O/A) interneurones of mouse hippocampal slices. LTP induction was absent in metabotropic glutamate receptor 1 (mGluR1) knockout mice, was correlated with the postsynaptic presence of mGluR1a, and required a postsynaptic Ca2+ rise. Changes in paired-pulse facilitation and coefficient of variation indicated that LTP expression involved presynaptic mechanisms. LTP was synapse specific, occurring selectively at synapses modulated by presynaptic group II, but not group III, mGluRs. Furthermore, the TBS protocol applied in O/A induced a long-term increase of polysynaptic inhibitory responses in CA1 pyramidal cells, that was absent in mGluR1 knockout mice. These results uncover the mechanisms of a novel form of interneurone synaptic plasticity that can adaptively regulate inhibition of hippocampal pyramidal cells.

The GABAergic phenotype of the “glutamatergic” granule cells of the dentate gyrus

The granule cells of the dentate gyrus (DG), origin of the mossy fibers (MFs), have been considered to be glutamatergic. However, data obtained with different experimental approaches in recent years may be calling for a redefinition of their phenotype. Although they indeed release glutamate for fast neurotransmission, immunohistological and molecular biology evidence has revealed that these glutamatergic cells also express GABAergic markers. The granule cell expression of a GABAergic phenotype is developmentally regulated. Electrophysiological studies reveal that during the first 3 weeks of age, mossy fiber stimulation provokes monosynaptic fast inhibitory transmission mediated by GABA, besides the monosynaptic excitatory glutamatergic transmission, onto their targets in CA3. After this age, mossy fiber GABAergic transmission abruptly disappears and the GABAergic markers are undetected. In the adult, the GABAergic markers are upregulated and GABA-mediated transmission emerges after induction of hyperexcitability. The simultaneous glutamate- and GABA-mediated signals share the same plastic and pharmacological characteristics that correspond to neurotransmission of mossy fiber origin. This intriguing evidence gives rise to two fundamental points of discussion. The first is the plausible fact that glutamate and GABA, two neurotransmitters of opposing actions, are coreleased from the mossy fibers. The second relates to its functional implications that can be immediately inferred, as the dentate gyrus can exert direct GABA-mediated excitatory actions early in life and inhibitory actions in young and adult hippocampus. This evidence poses the need to reevaluate and reinterpret some aspects of the physiology of the mossy fiber pathway under normal and pathological conditions. This work reviews the recent evidence that supports the assumption that glutamate and GABA can be coreleased from a single pathway, the mossy fibers, and makes some considerations about its functional implications.

Developmental decrease in synaptic facilitation at the mouse hippocampal mossy fibre synapse

Transmission at the hippocampal mossy fibre (MF)-CA3 pyramidal cell synapse is characterized by prominent activity-dependent facilitation, which is thought to provide a wide dynamic range in hippocampal informational flow. At this synapse in mice the magnitude of paired-pulse facilitation and frequency-dependent facilitation markedly decreased with postnatal development from 3 weeks (3W) to 9 weeks (9W). Throughout this period the mean amplitude and variance of unitary EPSCs stayed constant. By altering extracellular Ca2+/Mg2+ concentrations the paired-pulse ratio could be changed to a similar extent as observed during development. However, this was accompanied by an over 30-fold change in EPSC amplitude, suggesting that the developmental change in facilitation ratio cannot simply be explained by a change in release probability. With paired-pulse stimulation the Ca2+ transients at MF terminals, monitored using mag-fura-5, showed a small facilitation, but its magnitude remained similar between 3W and 9W mice. Pharmacological tests using CNQX, adenosine, LY341495, H-7 or KN-62 suggested that neither presynaptic receptors (kainate, adenosine and metabotropic glutamate) nor protein kinases are responsible for the developmental change in facilitation. Nevertheless, loading the membrane-permeable form of BAPTA attenuated the paired-pulse facilitation in 3W mice to a much greater extent than in 9W mice, resulting in a marked reduction in age difference. These results suggest that the developmental decrease in the MF synaptic facilitation arises from a change associated with residual Ca2+, a decrease in residual Ca2+ itself or a change in Ca2+-binding sites involved in the facilitation. A developmental decline in facilitation ratio reduces the dynamic range of MF transmission, possibly contributing to the stabilization of hippocampal circuitry.

Depolarization-induced long-term depression at hippocampal mossy fiber-CA3 pyramidal neuron synapses

Hippocampal CA3 pyramidal neurons receive two types of excitatory afferent innervation: mossy fibers (MFs) from granule cells of the dentate gyrus and recurrent collateral fibers (CFs) from other CA3 pyramidal neurons. At CF-CA3 pyramidal neuron synapses, membrane depolarization paired with low (0.33 Hz) presynaptic stimulation generated a heterogeneous response that ranged from long-term potentiation (LTP), long-term depression (LTD), to no alteration of synaptic strength. However, the same induction paradigm applied at MF-CA3 pyramidal neuron synapses consistently induced LTD. This novel form of LTD was independent of NMDARs, mGluRs, cannabinoid receptors, opioid receptors, or coincident synaptic activity, but was dependent on postsynaptic Ca2+ elevation through L-type Ca2+ channels and release from inositol 1,4,5-trisphosphate receptor-sensitive intracellular stores. Ca2+ imaging of both proximal and distal CA3 pyramidal neuron dendrites demonstrated that the depolarizing induction paradigm differentially elevated intracellular Ca2+ levels. L-type Ca2+ channel activation was observed only at the most proximal locations where mossy fibers make synapses. Depolarization-induced LTD did not occlude the conventional 1 Hz-induced LTD or vice versa, suggesting independent mechanisms underlie each form of plasticity. The paired-pulse ratio and coefficient of variation of synaptic transmission were unchanged after LTD induction, suggesting that the expression locus of LTD is postsynaptic. Moreover, peak-scaled nonstationary variance analysis indicated that depolarization-induced LTD correlated with a reduction in postsynaptic AMPA receptor numbers without a change in AMPA receptor conductance. Our results suggest that this novel form of LTD is selectively expressed at proximal dendritic locations closely associated with L-type Ca2+ channels.

A role for Ca2+ stores in kainate receptor-dependent synaptic facilitation and LTP at mossy fiber synapses in the hippocampus

Compared with NMDA receptor-dependent LTP, much less is known about the mechanism of induction of NMDA receptor-independent LTP; the most extensively studied form of which is mossy fiber LTP in the hippocampus. In the present study we show that Ca2+-induced Ca2+ release from intracellular stores is involved in the induction of mossy fiber LTP. This release also contributes to the kainate receptor-dependent component of the pronounced synaptic facilitation that occurs during high-frequency stimulation. We also present evidence that the trigger for this Ca2+ release is Ca2+ permeation through kainate receptors. However, these novel synaptic mechanisms can be bypassed when the Ca2+ concentration is raised (from 2 to 4 mM), via a compensatory involvement of L-type Ca2+ channels. These findings suggest that presynaptic kainate receptors at mossy fiber synapses can initiate a cascade involving Ca2+ release from intracellular stores that is important in both short-term and long-term plasticity.

Short-term frequency-dependent plasticity at recurrent mossy fiber synapses of the epileptic brain

The recurrent mossy fiber pathway of the dentate gyrus expands dramatically in human temporal lobe epilepsy and in animal models of this disorder, creating monosynaptic connections among granule cells. This novel granule cell network can support reverberating excitation but is difficult to activate with low-frequency stimulation. This study used hippocampal slices from pilocarpine-treated rats to explore the dependence of synaptic transmission in this pathway on stimulus frequency. Minimal electrically evoked EPSCs exhibited a high failure rate ( approximately 60%). Stimulus trains delivered at a frequency of <1 Hz depressed synaptic transmission, as evidenced by an increase in response failures. Conversely, stimulus trains delivered at higher frequencies reduced the percentage of response failures and increased the amplitude of compound EPSCs, including pharmacologically isolated NMDA receptor-mediated EPSCs. Short-term frequency-dependent facilitation was of modest size compared with mossy fiber synapses on other neuronal types. Facilitation depended on the activation of kainate receptors by released glutamate and was inhibited by feedback activation of type II metabotropic glutamate receptors. These results suggest that the recurrent mossy fiber pathway may be functionally silent during baseline asynchronous granule cell activity in vivo attributable, in part, to progressive transmission failure. The pathway may synchronize granule cell firing and may promote seizure propagation most effectively during the brief periods of high-frequency granule cell firing that occur during normal behavior, during the periods of hypersynchronous fast activity characteristic of epileptic brain and, most importantly, during the period of increasing granule cell activity that precedes a spontaneous seizure.