Transitional function of DG to CA3 in the hippocampus

Using single cell channel model, the transmission features of CA3-DG network in the hippocampus are investigated. The influence of the stimulation on discharge pattern of pyramidal neurons is analyzed, which shows that it starts with period spiking discharges, followed by period-doubling bifurcation to chaos, and period 3 discharge evolving into chaos, and ultimately a period of bursting discharges. By the synaptic model, the CA3-DG network model is constructed, which analyzes the summation of postsynaptic currents in the network, the influence of postsynaptic current on discharge rhythm as well as the mechanism of bursting discharges. The strong capacity of spatiotemporal encoding in the network indicates the features of CA3 network during the information transmission process in the hippocampus. The modeling result with time delay of the synaptic transmission is in accordance with the experimental phenomena of action potential in the hippocampus.     

Activation of NMDA receptors blocks GABAergic inhibition in an in vitro model of epilepsy

The application of tetanic electrical stimuli to the stratum radiatum fibre pathway in the hippocampus in vitro produces an NMDA (N-methyl-D-aspartate) receptor-dependent enhancement of synaptic efficacy. Repeated application of such stimuli produces a progressive enhancement of synaptic efficacy leading to the genesis of spontaneous and stimulation-evoked epileptiform discharges. We have used this in vitro approach to explore the cellular mechanisms which underlie the kindling model of epilepsy. Kindling of the stratum radiatum fibre pathway in vitro induced a progressive, long-lasting reduction of both spontaneous and stimulation-evoked GABAergic (gamma-aminobutyric acid-mediated) inhibitory postsynaptic potentials (i.p.s.ps). The reduction of i.p.s.ps by kindling was associated with a profound decrease in the sensitivity of CA1 pyramidal neurons to ionophoretically applied GABA and an increase in sensitivity to NMDA. The reduction of i.p.s.ps and GABA sensitivity was prevented by kindling in the presence of the NMDA receptor antagonist D-2-amino-5-phosphonovalerate (D-APV). These results demonstrate that kindling-like stimulus patterns produce a reduction of GABAergic inhibition in the hippocampus resulting from a stimulus-induced postsynaptic activation of NMDA receptors. The modulation of GABAergic inhibition by NMDA receptors may cause the synaptic plasticity which underlies the kindling model of epilepsy.     

Presynaptic enhancement shown by whole-cell recordings of long-term potentiation in hippocampal slices

Long-term potentiation (LTP) of synaptic transmission in the hippocampus is a widely studied model system for understanding the cellular mechanisms of memory. In region CA1, LTP is triggered postsynaptically by Ca2(+)-dependent activation of protein kinases, but the locus of persistent modification remains controversial. Statistical analysis of synaptic variability has been proposed as a means of settling this debate, although a major obstacle has been the poor signal-to-noise ratio of conventional intracellular recordings. We have applied the whole-cell voltage clamp technique to study synaptic transmission in conventional hippocampal slices (compare refs 28-30). Here we report that robust LTP can be recorded with much improved signal resolution and biochemical access to the postsynaptic cell. Prolonged dialysis of the postsynaptic cell blocks the triggering of LTP, with no effect on expression of LTP. The improved signal resolution unmasks a large trial-to-trial variability, reflecting the probabilistic nature of transmitter release. Changes in the synaptic variability, and a decrease in the proportion of synaptic failures during LTP, suggest that transmitter release is significantly enhanced.     

A physiological role for endogenous zinc in rat hippocampal synaptic neurotransmission.

The mammalian central nervous system (CNS) contains an abundance of the transition metal zinc, which is highly localized in the neuronal parenchyma. Zinc is actively taken up and stored in synaptic vesicles in nerve terminals, and stimulation of nerve fibre tracts that contain large amounts of zinc, such as the hippocampal mossy fibre system, can induce its release, suggesting that it may act as a neuromodulator. The known interaction of zinc with the major excitatory and inhibitory amino-acid neurotransmitter receptors in the CNS supports this notion. That zinc has a role in CNS synaptic transmission, however, has so far not been shown. Here we report a physiological role for zinc in the young rat hippocampus (postnatal, P3-P14 days). Our results indicate that naturally occurring spontaneous giant depolarizing synaptic potentials (GDPs) in young CA3 pyramidal neurones, mediated by the release of GABA (gamma-aminobutyric acid), are induced by endogenously released zinc. These synaptic potentials are inhibited by specific zinc-chelating agents. GDPs are apparently generated by an inhibitory action of zinc on both pre- and postsynaptic GABAB receptors in the hippocampus. Our study implies that zinc modulates synaptic transmission in the immature hippocampus, a finding that may have implications for understanding benign postnatal seizures in young children suffering with acute zinc deficiency.     

Long-term potentiation is associated with increases in quantal content and quantal amplitude.

Long-term potentiation (LTP) of synaptic transmission in CA1 neurons of the hippocampus, elicited by the conjunction of presynaptic firing and postsynaptic depolarization, is an important model of plasticity, which may underlie memory storage. Although induction of LTP takes place in the postsynaptic cell, it is not clear whether it is expressed through an enhancement of transmitter release or through an increased postsynaptic response to the same amount of transmitter. Analysis of the trial-to-trial amplitude fluctuations of synaptic signals, that is quantal analysis, gives an important insight into the probabilistic mechanisms of transmission, although attempts to apply it to the mode of expression of LTP have so far yielded inconsistent results, at least in part because they have relied on models of transmitter release that have not been confirmed experimentally. Here we report clear evidence for quantal fluctuation in a subset of cells. Induction of LTP in these cells causes abrupt increases in either quantal content or quantal amplitude, or both. This shows that two different mechanisms can underlie the maintenance of LTP.     

Two components of long-term potentiation induced by different patterns of afferent activation

Long-term potentiation (LTP) of excitatory synaptic transmission could be a mechanism underlying memory. Induction of LTP requires Ca2+ influx into postsynaptic neurons through ion channels gated by NMDA (N-methyl-D-aspartate) receptors in hippocampus (area CA1 and dentate gyrus) and neocortex. Here we report that a component of LTP not requiring the activation of NMDA receptors can be induced in area CA1. The component is dependent on tetanus frequency, requires increases in postsynaptic intracellular Ca2+ concentrations, and is suppressed by an antagonist of voltage-dependent Ca2+ channels.     

Presynaptic release probability influences the locus of long-term potentiation.

The quantal hypothesis proposes that chemical synaptic transmission involves the probabilistic release of multimolecular packets of transmitter. Analysis of the resulting trial-to-trial fluctuations in postsynaptic response can provide estimates both of the number of quanta released and of the size of their postsynaptic effect. This in turn permits the quantification of the relative contributions of pre- and postsynaptic factors to the strength of a given synapse. Quantal analysis of excitatory synapses in the hippocampus has proved difficult and has led to contradictory conclusions when applied to long-term potentiation. Here we report the use of a combination of quantal analysis procedures to provide evidence that both pre- and postsynaptic changes can contribute substantially to the maintenance of long-term potentiation in the CA1 region of the hippocampus. The initial setting of the presynaptic release mechanism seems to determine their relative importance.     

Long-term potentiation of NMDA receptor-mediated synaptic transmission in the hippocampus.

Neurotransmission at most excitatory synapses in the brain operates through two types of glutamate receptor termed alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors; these mediate the fast and slow components of excitatory postsynaptic potentials respectively. Activation of NMDA receptors can also lead to a long-lasting modification in synaptic efficiency at glutamatergic synapses; this is exemplified in the CA1 region of the hippocampus, where NMDA receptors mediate the induction of long-term potentiation (LTP). It is believed that in this region LTP is maintained by a specific increase in the AMPA receptor-mediated component of synaptic transmission. We now report, however, that a pharmacologically isolated NMDA receptor-mediated synaptic response can undergo robust, synapse-specific LTP. This finding has implications for neuropathologies such as epilepsy and neurodegeneration, in which excessive NMDA receptor activation has been implicated. It adds fundamentally to theories of synaptic plasticity because NMDA receptor activation may, in addition to causing increased synaptic efficiency, directly alter the plasticity of synapses.     

Single neurones can initiate synchronized population discharge in the hippocampus

The synchronized firing of neuronal populations is frequently observed in the mammalian central nervous system. The generation of motor activities such as locomotion and respiration requires the simultaneous activation of many neurones and synchronous firing also underlies the cortical alpha rhythm and the hippocampal theta rhythm. However the influence that single neurones may have on such neuronal population discharges is not clear. We have examined this question using small isolated segments of the CA3 region of the guinea pig hippocampus. We report here that in the presence of picrotoxin, a gamma-aminobutyric acid (GABA) antagonist, these segments spontaneously generate synchronized rhythmic bursts comparable with the interictal epileptiform discharges observed in the hippocampus and neocortex in the presence of penicillin. The activation of some individual neurones by intracellular current injection can partially entrain and reset the rhythm. The probability that a synchronized burst will follow stimulation of a single cell increases with time after a spontaneous synchronized discharge, suggesting that each population discharge is followed by a period of relative population refractoriness. A delay of 40-200 ms elapses between the activation of a single neurone and the synchronized discharge. We suggest that during this time activity elicited in one neurone spreads to other neurones through multisynaptic excitatory pathways and leads eventually to the participation of the whole population in a synchronous burst.     

GABA autoreceptors regulate the induction of LTP.

Understanding the mechanisms involved in long-term potentiation (LTP) should provide insights into the cellular and molecular basis of learning and memory in vertebrates. It has been established that in the CA1 region of the hippocampus the induction of LTP requires the transient activation of the N-methyl-D-aspartate (NMDA) receptor system. During low-frequency transmission, significant activation of this system is prevented by gamma-aminobutyric acid (GABA) mediated synaptic inhibition which hyperpolarizes neurons into a region where NMDA receptor-operated channels are substantially blocked by Mg2+ (refs. 5, 6). But during high-frequency transmission, mechanisms are evoked that provide sufficient depolarization of the postsynaptic membrane to reduce this block and thereby permit the induction of LTP. We now report that this critical depolarization is enabled because during high-frequency transmission GABA depresses its own release by an action on GABAB autoreceptors, which permits sufficient NMDA receptor activation for the induction of LTP. These findings demonstrate a role for GABAB receptors in synaptic plasticity.     

Dendritic spine changes associated with hippocampal long-term synaptic plasticity.

Long-term enhancement of synaptic efficacy in the hippocampus is an important model for studying the cellular mechanisms of neuronal plasticity, circuit reorganization, and even learning and memory. Although these long-lasting functional changes are easy to induce, it has been very difficult to demonstrate that they are accompanied or even caused by morphological changes on the subcellular level. Here we combined a local superfusion technique with two-photon imaging, which allowed us to scrutinize specific regions of the postsynaptic dendrite where we knew that the synaptic changes had to occur. We show that after induction of long-lasting (but not short-lasting) functional enhancement of synapses in area CA1, new spines appear on the postsynaptic dendrite, whereas in control regions on the same dendrite or in slices where long-term potentiation was blocked, no significant spine growth occurred.     

Associative long-term depression in the hippocampus induced by hebbian covariance

A brief, high-frequency activation of excitatory synapses in the hippocampus produces a long-lasting increase in synaptic strengths called long-term potentiation (LTP). A test input, which by itself does not have a long-lasting effect on synaptic strengths, can be potentiated through association when it is activated at the same time as a separate conditioning input. Neural network modelling studies have also predicted that synaptic strengths should be weakened when test and conditioning inputs are anti-correlated. Evidence for such heterosynaptic depression in the hippocampus has been found for inputs that are inactive or weakly active during the stimulation of a conditioning input, but this depression does not depend on any pattern of test input activity and does not seem to last as long as LTP. We report here an associative long-term depression (LTD) in field CA1 that is produced when a low-frequency test input is negatively correlated in time with a high-frequency conditioning input. LTD of synaptic strength is also produced by activating presynaptic terminals while a postsynaptic neuron is hyperpolarized. This confirms theoretical predictions that the mechanism for associative LTD is homosynaptic and follows a hebbian covariance rule.     

Postsynaptic hyperpolarization during conditioning reversibly blocks induction of long-term potentiation

Activity-induced changes in the efficacy of synaptic transmission between neurones are central to several prominent theories of learning. In both in vivo and in vitro preparations of the hippocampus, a conditioning high-frequency stimulus delivered to afferent fibres results in a long-term potentiation of synaptic transmission at those inputs. Evidence has been provided supporting both presynaptic and postsynaptic sites as loci where critical events occur in the development of potentiation. In this study we report that long-term potentiation is reversibly blocked by intracellular injection of hyperpolarizing current in the postsynaptic cell during the conditioning high-frequency stimulus, suggesting the involvement of a voltage-dependent postsynaptic mechanism.     

An essential role for postsynaptic calmodulin and protein kinase activity in long-term potentiation

The phenomenon of long-term potentiation (LTP), a long lasting increase in the strength of synaptic transmission which is due to brief, repetitive activation of excitatory afferent fibres, is one of the most striking examples of synaptic plasticity in the mammalian brain. In the CA1 region of the hippocampus, the induction of LTP requires activation of NMDA (N-methyl-D-aspartate) receptors by synaptically released glutamate with concomitant postsynaptic membrane depolarization. This relieves the voltage-dependent magnesium block of the NMDA-receptor ion channel, allowing calcium to flow into the dendritic spine. Although calcium has been shown to be a necessary trigger for LTP (refs 11, 12), little is known about the immediate biochemical processes that are activated by calcium and are responsible for LTP. The most attractive candidates have been calcium/calmodulin-dependent protein kinase II (CaM-KII) (refs 13-16), protein kinase C (refs 17-19), and the calcium-dependent protease, calpain. Extracellular application of protein kinase inhibitors to the hippocampal slice preparation blocks the induction of LTP (refs 21-23) but it is unclear whether this is due to a pre- and/or postsynaptic action. We have found that intracellular injection into CA1 pyramidal cells of the protein kinase inhibitor H-7, or of the calmodulin antagonist calmidazolium, blocks LTP. Furthermore, LTP is blocked by the injection of synthetic peptides that are potent calmodulin antagonists and inhibit CaM-KII auto- and substrate phosphorylation. These findings demonstrate that in the postsynaptic cell both activation of calmodulin and kinase activity are required for the generation of LTP, and focus further attention on the potential role of CaM-KII in LTP.     

Different voltage-dependent thresholds for inducing long-term depression and long-term potentiation in slices of rat visual cortex

In the hippocampus and neocortex, high-frequency (tetanic) stimulation of an afferent pathway leads to long-term potentiation (LTP) of synaptic transmission. In the hippocampus it has recently been shown that long-term depression (LTD) of excitatory transmission can also be induced by certain combinations of synaptic activation. In most hippocampal and all neocortical pathways studied so far, the induction of LTP requires the activation of N-methyl-D-aspartate (NMDA) receptor-gated conductances. Here we report that LTD can occur in neurons of slices of the rat visual cortex and that the same tetanic stimulation can induce either LTP or LTD depending on the level of depolarization of the postsynaptic neuron. By applying intracellular current injections or pharmacological disinhibition to modify the depolarizing response of the postsynaptic neuron to tetanic stimulation, we show that the mechanisms of induction of LTD and LTP are both postsynaptic. LTD is obtained if postsynaptic depolarization exceeds a critical level but remains below a threshold related to NMDA receptor-gated conductances, whereas LTP is induced if this second threshold is reached.     

NMDA application potentiates synaptic transmission in the hippocampus

The NMDA (N-methyl-D-aspartate) class of glutamate receptor plays a critical role in a variety of forms of synaptic plasticity in the vertebrate central nervous system. One extensively studied example of plasticity is long-term potentiation (LTP), a remarkably long-lasting enhancement of synaptic efficiency induced in the hippocampus by brief, high-frequency stimulation of excitatory synapses. LTP is a strong candidate for a cellular mechanism of learning and memory. The site of LTP induction appears to be the postsynaptic cell and induction requires both activation of NMDA receptors by synaptically released glutamate and depolarization of the postsynaptic membrane. It is proposed that this depolarization relieves a voltage-dependent Mg2+ block of the NMDA receptor channel, resulting in increased calcium influx which is the trigger for the induction of LTP. This model predicts that application of a large depolarizing dose of NMDA should be sufficient to evoke LTP. In agreement with a previous study, we have found that NMDA or glutamate application does potentiate synaptic transmission in the hippocampus. This agonist-induced potentiation is, however, decremental and short-lived, unlike LTP. It is occluded shortly after the induction of LTP and a similar short-term potentiation can be evoked by synaptically released glutamate. We thus propose that LTP has two components, a short-term, decremental component which can be mimicked by NMDA receptor activation, and a long-lasting, non-decremental component which, in addition to requiring activation of NMDA receptors, requires stimulation of presynaptic afferents.     

Optical imaging of calcium accumulation in hippocampal pyramidal cells during synaptic activation

The dynamic response of nerve cells to synaptic activation and the spatial distribution of biochemical processes regulated by ion concentration are critically dependent on the cell-surface distribution of ion channels. In the hippocampus, intracellular calcium-ion concentration is thought to influence the biochemical events associated with kindling, excitotoxicity, and long-term potentiation. Computer models of hippocampal pyramidal cells also indicate that calcium-channel location influences dynamic characteristics such as bursting. Here, we have used in situ microfluorometric imaging in brain slices to directly measure the spatial distribution of calcium accumulation in guinea-pig CA1 pyramidal cells during trains of orthodromic synaptic stimulation. Calcium accumulation is substantial throughout the entire proximal section of the apical and basal dendrites. Most of this accumulation results from influx through non-NMDA (N-methyl-D-aspartate) voltage-gated calcium channels, and in the apical dendrite it drops steeply as the dendrite enters stratum moleculare, the termination zone of perforant path afferents. These results demonstrate a marked segregation of calcium-channel activity and directly show a spatial distribution of calcium accumulation during orthodromic synaptic activation.     

A frequency-dependent switch from inhibition to excitation in a hippocampal unitary circuit

The hippocampus, a brain structure essential for memory and cognition, is classically represented as a trisynaptic excitatory circuit. Recent findings challenge this view, particularly with regard to the mossy fibre input to CA3, the second synapse in the trisynaptic pathway. Thus, the powerful mossy fibre input to CA3 pyramidal cells might mediate both synaptic excitation and inhibition. Here we show, by recording from connected cell pairs in rat entorhinal-hippocampal slice cultures, that single action potentials in a dentate granule cell evoke a net inhibitory signal in a pyramidal cell. The hyperpolarization is due to disynaptic feedforward inhibition, which overwhelms monosynaptic excitation. Interestingly, this net inhibitory synaptic response changes to an excitatory signal when the frequency of presynaptic action potentials increases. The process responsible for this switch involves the facilitation of monosynaptic excitatory transmission coupled with rapid depression of inhibitory circuits. This ability to immediately switch the polarity of synaptic responses constitutes a novel synaptic mechanism, which might be crucial to the state-dependent processing of information in associative hippocampal networks.     

Dendritic spikes as a mechanism for cooperative long-term potentiation

Strengthening of synaptic connections following coincident pre- and postsynaptic activity was proposed by Hebb as a cellular mechanism for learning. Contemporary models assume that multiple synapses must act cooperatively to induce the postsynaptic activity required for hebbian synaptic plasticity. One mechanism for the implementation of this cooperation is action potential firing, which begins in the axon, but which can influence synaptic potentiation following active backpropagation into dendrites. Backpropagation is limited, however, and action potentials often fail to invade the most distal dendrites. Here we show that long-term potentiation of synapses on the distal dendrites of hippocampal CA1 pyramidal neurons does require cooperative synaptic inputs, but does not require axonal action potential firing and backpropagation. Rather, locally generated and spatially restricted regenerative potentials (dendritic spikes) contribute to the postsynaptic depolarization and calcium entry necessary to trigger potentiation of distal synapses. We find that this mechanism can also function at proximal synapses, suggesting that dendritic spikes participate generally in a form of synaptic potentiation that does not require postsynaptic action potential firing in the axon.