Simultaneous expression of excitatory postsynaptic potential/spike potentiation and excitatory postsynaptic potential/spike depression in the hippocampus

Epileptic pilocarpine-treated rats exhibit aberrant hippocampal EPSP-spike potentiation but retain long-term potentiation

Hippocampal neuron plasticity is strongly associated with learning, memory, and cognition. In addition to modification of synaptic function and connectivity, the capacity of hippocampal neurons to undergo plasticity involves the ability to change nonsynaptic excitability. This includes altering the probability that EPSPs will generate action potentials (E‐S plasticity). Epilepsy is a prevalent neurological disorder commonly associated with neuronal hyperexcitability and cognitive dysfunction. We examined E‐S plasticity in chronically epileptic Sprague–Dawley rats 3–10 weeks after pilocarpine‐induced status epilepticus. CA1 neurons in hippocampal slices were assayed by whole‐cell current clamp to measure EPSPs evoked by Schaffer collateral stimulation. Using a weak spike‐timing‐dependent protocol to induce plasticity, we found robust E‐S potentiation in conjunction with weak long‐term potentiation (LTP) in saline‐treated rats. In pilocarpine‐treated rats, a similar degree of LTP was found, but E‐S potentiation was reduced. Additionally, the degree of E‐S potentiation was not correlated with the degree of LTP for either group, suggesting that they independently contribute to neuronal plasticity. E‐S potentiation also differed from LTP in that E‐S plasticity could be induced solely from action potentials generated by postsynaptic current injection. The calcium chelating agent BAPTA in the intracellular solution blocked LTP and E‐S potentiation, revealing the calcium dependence of both processes. These findings suggest that LTP and E‐S potentiation have overlapping but nonidentical mechanisms of inducing neuronal plasticity that may independently contribute to cognitive disruptions observed in the chronic epileptic state.

More than synaptic plasticity: role of nonsynaptic plasticity in learning and memory

Decades of research on the cellular mechanisms of memory have led to the widely held view that memories are stored as modifications of synaptic strength. These changes involve presynaptic processes, such as direct modulation of the release machinery, or postsynaptic processes, such as modulation of receptor properties. Parallel studies have revealed that memories might also be stored by nonsynaptic processes, such as modulation of voltage-dependent membrane conductances, which are expressed as changes in neuronal excitability. Although in some cases nonsynaptic changes can function as part of the engram itself, they might also serve as mechanisms through which a neural circuit is set to a permissive state to facilitate synaptic modifications that are necessary for memory storage.

Changes in neuronal excitability and synaptic function in a chronic model of temporal lobe epilepsy

Long-term potentiation and depression of glutamatergic synaptic responses are accompanied by an increased firing probability of neurons in response to a given excitatory input. This property, named excitatory postsynaptic potential/spike potentiation, has also been described in epileptic tissue and has pro-epileptic consequences. In this study, we show that excitatory postsynaptic potential/spike potentiation can be reversed in the kainic acid lesioned rat hippocampus, a chronic model of temporal lobe epilepsy. Simultaneous in vitro extracellular recordings in stratum radiatum and stratum pyramidale were performed in the CA1 area of the kainic acid lesioned rat hippocampal slices. Fifteen minutes, application of the K(+) channel blocker tetraethylammonium resulted in excitatory postsynaptic potential/spike potentiation (measured 90min after the start of the washout period) which could be reversed by subsequent low-frequency or tetanic stimuli. Excitatory postsynaptic potential/spike potentiation and its subsequent reversal by an electrical conditioning stimulus were found to have a N-methyl-D-aspartate receptor-independent component. Tetraethylammonium treatment also resulted in excitatory postsynaptic potential/spike potentiation of pharmacologically isolated N-methyl-D-aspartate receptor-mediated responses which could be reversed by subsequent low-frequency or tetanic stimuli. We conclude that excitatory postsynaptic potential/spike potentiation can be reversed in epileptic tissue, even in the absence of synaptic plasticity. These results suggest the presence of endogenous regulatory mechanisms which are able to decrease cell excitability.

Interaction between adenosine A1 and A2 receptor-mediated responses in the rat hippocampus in vitro

Previous work has been carried out on the effects of adenosine on transmitter release and on the excitability of postsynaptic neurones, but little is known about the effects of adenosine on the coupling between the two. In this study, we examine the effects of specific adenosine receptor agonists and antagonists on the population excitatory postsynaptic potential (population EPSP) slope, the population spike amplitude, and the relationship between the two (E-S coupling) in the CA1 area of rat hippocampus. Activation of adenosine A1 receptors by adenosine or the selective agonist N6-cyclopentyladenosine resulted in a decrease of the population spike amplitude by a greater extent than could be accounted for by the decrease in population EPSP slope, resulting in a dissociation in the E-S relationship, reflected as a right-shift in the E-S curve. Activation of adenosine A2A receptors by the selective agonist 2-p-(2-carboxyethy)phenethylamino-5'-N-ethylcarboxamidoadeno sine (CGS 21680), or blockade by antagonists ZM 241385 and CP 66713 had no effect on evoked responses. However, when both adenosine A1 and A2A receptors were activated at the same time, a significant attenuation of the inhibitory effects of N6-cyclopentyladenosine on population spike amplitude was observed, resulting in a left-shift in the E-S curve. Intracellular recording indicated that N6-cyclopentyladenosine raised the threshold for spike induction by pulses of depolarising current, even at a concentration which did not produce hyperpolarisation of the neurone. At 30 nM, CGS 21680 prevented this effect of N6-cyclopentyladenosine, and this apparent antagonism was prevented by the A2A receptor antagonist ZM 241385. The results show that adenosine A1 receptors change the coupling between presynaptic transmitter release and postsynaptic cell firing, and that this effect is attenuated by A2A receptor activation.

Molecular mechanisms that underlie structural and functional changes at the postsynaptic membrane during synaptic plasticity

The synaptic plasticity that is addressed in this review follows neurodegeneration in the brain and thus has both structural as well as functional components. The model of neurodegeneration that has been selected is the kainic acid lesioned hippocampus. Degeneration of the CA3 pyramidal cells results in a loss of the Schaffer collateral afferents innervating the CA1 pyramidal cells. This is followed by a period of structural plasticity where new synapses are formed. These are associated with changes in the numbers and shapes of spines as well as changes in the morphometry of the dendrites. It is suggested that this synaptogenesis is responsible for an increase in the ratio of NMDA to AMPA receptors mediating excitatory synaptic transmission at these synapses. Changes in the temporal and spatial properties of these synapses resulted in an altered balance between LTP and LTD. These properties together with a reduction in the inhibitory drive increased the excitability of the surviving CA1 pyramidal cells which in turn triggered epileptiform bursting activity. In this review we discuss the insights that may be gained from studies of the underlying molecular machinery. Developments in one of the collections of the cogs in this machinery has been summarized through recent studies characterizing the roles of neural recognition molecules in synaptic plasticity in the adult nervous systems of vertebrates and invertebrates. Such investigations of neural cell adhesion molecules, cadherins and amyloid precursor protein have shown the involvement of these molecules on the morphogenetic level of synaptic changes, on the one hand, and signal transduction effects, on the other. Further complex cogs are found in the forms of the low-density lipoprotein receptor (LDL-R) family of genes and their ligands play pivotal roles in the brain development and in regulating the growth and remodelling of neurones. Evidence is discussed for their role in the maintenance of cognitive function as well as Alzheimer's. The molecular mechanisms responsible for the clustering and maintenance of transmitter receptors at postsynaptic sites are the final cogs in the machinery that we have reviewed. Postsynaptic densities (PSD) from excitatory synapses have yielded many cytoskeletal proteins including actin, spectrin, tubulin, microtubule-associated proteins and calcium/calmodulin-dependent protein kinase II. Isolated PSDs have also been shown to be enriched in AMPA, kainate and NMDA receptors. However, recently, a new family of proteins, the MAGUKs (for membrane-associated guanylate kinase) has emerged. The role of these proteins in clustering different NMDA receptor subunits is discussed. The MAGUK proteins are also thought to play a role in synaptic plasticity mediated by nitric oxide (NO). Both NMDA and non-NMDA receptors are highly clustered at excitatory postsynaptic sites in cortical and hippocampal neurones but have revealed differences in their choice of molecular components. Both GABAA and glycine (Gly) receptors mediate synaptic inhibition in the brain and spinal cord. Whilst little is known about how GABAA receptors are localized in the postsynaptic membrane, considerable progress has been made towards the elucidation of the molecular mechanisms underlying the formation of Gly receptors. It has been shown that the peripheral membrane protein gephyrin plays a pivotal role in the formation of Gly receptor clusters most likely by anchoring the receptor to the subsynaptic cytoskeleton. Evidence for the distribution as well as function of gephyrin and Gly receptors is discussed. Postsynaptic membrane specializations are complex molecular machinery subserving a multitude of functions in the proper communication between neurones. Despite the fact that only a few key players have been identified it will be a fascinating to watch the story as to how they contribute to structural and functional plasticity unfold.

Pro-epileptic changes in synaptic function can be accompanied by pro-epileptic changes in neuronal excitability

Repetitive sensory input, stroboscopic lights or repeated sounds can induce epileptic seizures in susceptible individuals. In order to understand the process we have to consider multiple factors. The output of a set of neurones is determined by the amount of excitatory synaptic input, the degree of positive feedback and their inherent electrical excitability, which can be modified by synaptic inhibition. Recent research has shown that it is possible to separate these phenomena, and that they do not always behave in unison.

Epileptiform activity but not synaptic plasticity is blocked by oxidation of NMDA receptors in a chronic model of temporal lobe epilepsy

Simultaneous extracellular recordings were performed in stratum radiatum and stratum pyramidale of hippocampal slices 7 days following unilateral intracerebroventricular injections of kainic acid. In this ex vivo experimental model of human temporal lobe epilepsy, stimulation of the surviving commissural fibres in stratum radiatum produced graded epileptiform activity in the CA1 area. The oxidizing reagent 5,5'-dithiobis (2-nitrobenzoic acid) (DTNB) acting at NMDA receptors redox sites decreases NMDA receptor-mediated responses by half and suppresses evoked epileptiform discharges. We have examined the effect of DTNB on NMDA-dependent bidirectional synaptic plasticity and EPSP/spike coupling. DTNB treatment did not prevent either long-term potentiation induced by tetanic stimulation or long-term depression induced by low frequency stimulation of field EPSPs. Application of DTNB alone did not induce EPSP/spike dissociation. However, both high and low frequency stimulations induced EPSP/spike potentiation indicating that neurons had a high probability to discharge in synchrony. These results suggest that oxidizing reagents may provide novel antiepileptic treatments since they decrease NMDA-dependent evoked epileptiform activity but do not interfere with either NMDA-dependent synaptic plasticity or the probability of synchronous discharge.

A role for synaptic and network plasticity in controlling epileptiform activity in CA1 in the kainic acid-lesioned rat hippocampus in vitro

1. Stimulation of the surviving afferents in the stratum radiatum of the CA1 area in kainic acid-lesioned hippocampal slices produced graded epileptiform activity, part of which (> 20%) involved the activation of N-methyl-D-aspartate (NMDA) receptors. There was also a failure of synaptic inhibition in this region. In this preparation, we have tested the effects of low-frequency stimulation (LFS; 1 Hz for 15 min) on synaptic responses and epileptiform activity. 2. LFS resulted in long-term depression (LTD) of excitatory synaptic potentials (EPSPs), long-term decrease of population spike amplitudes (PSAs) and EPSP-spike (E-S) potentiation. Evoked epileptiform activity was reduced but neurons had a higher probability of discharge. LTD could be reversed by subsequent tetanic stimulation whereas E-S dissociation remained unchanged. Synaptic and network responses could be saturated towards either potentiation or depression. However, E-S potentiation was maximal following the first conditioning stimulus. 3. NMDA receptor-mediated responses were pharmacologically isolated. LFS resulted in LTD of synaptic responses, long-term decrease of PSAs and E-S depression. These depressions could not be reversed by subsequent tetanic stimulation. alpha-Amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and NMDA receptor-mediated responses were then measured in isolation before and following conditioning stimuli. LFS was shown to simultaneously produce LTD of AMPA and NMDA receptor-mediated responses. E-S potentiation of the AMPA component and E-S depression of the NMDA component occurred coincidentally. 4. LTD of AMPA and NMDA receptor-mediated responses were shown to be NMDA dependent. In contrast, E-S potentiation and depression occurred even when NMDA receptors were pharmacologically blocked. 5. These findings indicate that synaptic responses could be modified bidirectionally in the CA1 area of kainic acid-lesioned rat hippocampus in an NMDA receptor-dependent manner. However, E-S dissociations were independent of the activation of NMDA receptors, hinting at mechanisms different from those of synaptic LTD. We suggest that changes in E-S coupling were caused by a modification of the firing threshold of the CA1 pyramidal neurons. Furthermore, the firing mechanisms controlling NMDA and AMPA receptor-mediated network activity appeared to be different. The possible use of LFS applied to the hippocampus as a clinical intervention to suppress epileptiform activity is discussed.

Plasticity of AMPA and NMDA receptor-mediated epileptiform activity in a chronic model of temporal lobe epilepsy

We have investigated the consequences of tetanic stimulation on epileptiform activity mediated by NMDA and AMPA receptors in an experimental model of human temporal lobe epilepsy. Recordings were performed in the CA1 area of the hippocampus one week following intracerebroventricular injection of kainic acid. Data presented here show that, after tetanic stimulation, there was a long-term increase in the amplitude of the population spikes associated with the epileptiform burst. This activity was triggered by the simultaneous activation of both NMDA and AMPA receptors. However, whilst the pharmacologically isolated AMPA component of this burst underwent long-term enhancement, the NMDA component underwent a long-term decrease in amplitude. These data suggest that in this chronic model of epileptiform activity, there is long-term potentiation of excitatory mediated events regulated primarily by AMPA receptors. Furthermore, the slow time course of the NMDA receptor-mediated synaptic conductances was responsible for prolonging the duration of the epileptiform bursts. However, the powerful depression of NMDA receptor-mediated events following tetanic stimulation suppressed the normally large potentiation of the overall response. Thus although it has been suggested that the NMDA receptor-mediated synaptic events contribute to the epileptogenic properties of the neocortex and hippocampus, this evoked depression may act as an intrinsic anticonvulsant mechanism.