NMDA receptor activation during epileptiform responses in the dentate gyrus of epileptic patients.

We previously showed that a low frequency (1 Hz) train of perforant path stimulation evokes burst discharges in the dentate gyrus of hippocampal slices obtained from patients surgically treated for intractable temporal lobe epilepsy. We report here that multiple population spikes that characterize the burst discharge are blocked reversibly by the specific NMDA receptor antagonist, D-(-)-2-amino-5-phosphonovaleric acid (D-APV). The epileptiform discharge evoked in human dentate gyrus by stimulation trains of 1 Hz could be reproduced in the rat dentate gyrus in vitro by the same stimulation protocol but required the presence of low concentrations (0.2-0.6 mM) of extracellular magnesium. We suggest that low frequency orthodromic stimulation of dentate granule cells through the perforant path progressively evokes an increase in the activation of NMDA receptors resulting in burst discharges in tissue from epileptic patients.     

A hypoxic injury potential in the hippocampal slice

In rat hippocampal slices, neurons in the stratum pyramidale of the CA1 were stimulated orthodromically and antidromically while the resultant extracellular population spikes were monitored. Hypoxic conditions were then induced. After disappearance of the orthodromic population spike, a second orthodromic population spike appeared. We have titled this the hypoxic injury potential since it reflects the onset of permanent injury to neurons in area CA1 of the hippocampus.     

The functional relationship between antidromically evoked field responses of the dentate gyrus and mossy fiber reorganization in temporal lobe epileptic patients

Field recordings from the dentate granule cell layer of in vitro brain slices of temporal lobe epileptic patients were evoked by antidromic stimulation. Tissue from the same specimen was stained by the Timm-sulfide method to assess the pattern and degree of mossy fiber reorganization into the supragranular layer. A wide range of physiological responses and Timm staining patterns was present across patients. A significant correlation was observed between the abnormality of antidromic responses, reflected by multiple secondary population spikes, and the degree of Timm staining of the supragranular layer. This relationship lends support to the hypothesis that mossy fiber synapses located in the supragranular layer may have functional implications for granule cell excitability in human epileptic tissue.     

Differential paired-pulse effects of gabazine and bicuculline in rat hippocampal CA3 area

Field potentials were evoked in hippocampal area CA3 of anaesthetised rats by commissural stimulation, in order to study the effect of the prototypic gamma-aminobutyric acid (GABA)A antagonists gabazine (SR-95531; GBZ) and bicuculline methiodide (BMI) on paired-pulse interaction. Prominent paired-pulse inhibition of the orthodromic population spike (PS2) was observed when the interpulse interval (IPI) was < or = 40 ms, while facilitation occurred at IPIs >100 ms. Paired-pulse facilitation was lost at 500 ms. The antidromic population spike (PS1) presented paired-pulse facilitation at low-IPI, which decayed exponentially at increasing IPI. When the recording micropipettes contained millimolar concentrations of either GBZ, or BMI, single stimuli evoked repetitive (epileptiform) orthodromic PS2, of higher amplitude, while the antidromic PS1 was only weakly influenced. BMI reduced, but GBZ enhanced the low-IPI paired-pulse inhibition of the orthodromic PS2. Furthermore, BMI blunted paired-pulse facilitation of the antidromic PS1 at low-IPI, while GBZ caused strong paired-pulse inhibition of PS1 at IPI < or = 60 ms. The differential effects of GBZ and BMI on paired-pulse interaction might reflect different mechanisms of action of these compounds.     

Comparison of 5α-Pregnan-3α-ol-20-one and Phenobarbital on Cortical Synaptic Activation and Inhibition Studied In Vitro

Summary: The effects of 5Ø-pregnan-3Ø-ol-20-one (3Ø-OH-DHP) and phenobarbital (PB) on synaptic excitation and inhibition in rat hippocampal slices in vitro were compared. Stimulations were made orthodromically and antidromically while we recorded extracellularly from the dendritic and the somatic layer of the CA1 region. Perfusion with 5 μg/ml of 3Ø-OH-DHP for 30 min significantly increased the recurrent inhibition evoked by antidromic stimulation. The effect was most pronounced at short interstimulus intervals. The duration of the recurrent inhibition also was prolonged. There was no effect on the conditioned population spike after orthodromic paired-pulse stimulation. Furthermore, no effect was observed on the amplitude of the orthodromic fiber volley, the rate of increase in the field excitatory postsynaptic potential (EPSP) and the latency and amplitude of the CA1 population spike. Qualitative and quantitative similar findings were observed during perfusion with PB 0.1 mg/ml, (i.e., a concentration 20 times higher than that of 3Ø-OH-DHP). Higher concentrations of PB also affected synaptic excitation. The findings suggest a similar effect of 3Ø-OH-DHP and PB on recurrent GABA-ergic inhibition; however, 3Ø-OH-DHP appears to be much more potent.     

Effects of bicuculline and baclofen on paired-pulse depression in the dentate gyrus of epileptic patients

Paired-pulse field responses were recorded from the granule cell layer of the dentate gyrus in brain slices from temporal lobe epileptic patients. Paired-pulse depression (PPD) was examined using perforant path stimulation of low to moderate intensity at an inter-stimulus interval (ISI) of 20 ms. The paired-pulse ratio (PS₂/PS₁) was expressed as the population spike amplitude of the second response (PS₂) relative to that of the first response (PS₁). Representative tissue responses from each patient biopsy were divided into two groups that were significantly different based on the magnitude of the highest paired-pulse ratio recorded for each biopsy specimen: the strong paired-pulse depression group (PS₂/PS₁ = 0.12 ± 0.03; n = 15) and the weak paired-pulse depression group (PS₂/PS₁ = 0.68 ± 0.06; n = 13). Paired-pulse ratios from the strong PPD group were relatively independent of stimulus intensity, whereas, PPD was dependent on stimulus intensity in the weak PPD group; i.e., PPD was greatest at the lowest intensity and reached a plateau at higher intensities. Bicuculline (20 μM) and low concentrations of baclofen (0.1–0.2 μM) reduced paired-pulse depression in the strong PPD group, but did not significantly change the paired-pulse ratio in the weak PPD group. Paired-pulse facilitation was observed in some cases after inhibition was blocked pharmacologically. The number of population spikes was increased in the presence of bicuculline but was unchanged by baclofen. In the strong PPD group, baclofen significantly altered the EPSP-population spike (E-S) relationship by increasing the slope of the relationship for the second response, without having an effect on the slope of the first response. Baclofen had no effect on the E-S relationship of either response in the weak PPD group. The data are consistent with (1) less inhibition in the weak PPD group compared to the strong PPD group, (2) reduction of feedback inhibition in the strong PPD group by bicuculline and by low concentrations of baclofen, and (3) the occurrence of paired-pulse facilitation when inhibition was pharmacologically reduced in the dentate gyrus of temporal lobe epileptic patients. The results are also consistent with the presence of GABA_B receptors on human inhibitory interneurons that, when activated by baclofen, result in disinhibition of granule cells through feedback circuits. Although inhibition may be compromised in some epileptic human biopsy specimens, the presence of strong inhibition in other patients' biopsy material suggests the re-evaluation of the role of inhibition in epilepsy.     

The effects of antidromic discharges on orthodromic firing of primary afferents in the cat

This study investigated the effects of antidromically conducted nerve impulses on the transmission of orthodromic volleys in primary afferents of the hindlimb in decerebrated paralyzed cats. Two protocols were used: (A) Single skin and muscle afferents (N=20) isolated from the distal part of cut dorsal rootlets (L7-S1) were recorded while stimulation was applied more caudally. The results showed that during the trains of three to 20 stimuli, the orthodromic firing frequency decreased or ceased, depending on the frequency of stimulation. Remarkably, subsequent to these trains, the occurrence of orthodromic spikes could be delayed for hundreds of ms (15/20 afferents) and sometimes stopped for several seconds (10/20 afferents). Longer stimulation trains, simulating antidromic bursts reported during locomotion, caused a progressive decrease, and a slow recovery of, orthodromic firing frequency (7/20 afferents), indicating a cumulative long-lasting depressing effect from successive bursts. (B) Identified stretch-sensitive muscle afferents were recorded intra-axonally and antidromic spikes were evoked by the injection of square pulses of current through the micropipette. In this case, one to three antidromic spikes were sufficient to delay the occurrence of the next orthodromic spike by more than one control inter-spike interval. If the control inter-spike interval was decreased by stretching the muscle, the delay evoked by antidromic spikes decreased proportionally. Overall, these findings suggest that antidromic activity could alter the mechanisms underlying spike generation in peripheral sensory receptors and modify the orthodromic discharges of afferents during locomotion.     

Commissural responses of rat retrohippocampal neurons

Synaptic responses of commissurally activated rat subicular and entorhinal neurons were studied intracellularly in vivo by stimulating the contralateral dentate gyrus. The most prominent synaptic responses in both subicular and entorhinal neurons were inhibitory postsynaptic potentials (IPSPs). IPSPs were generated in combination with antidromic spikes and/or excitatory postsynaptic potentials (EPSPs) and orthodromic spikes. No dependency between any two response types were found. Commissurally projecting subicular neurons (identified by the presence of antidromic spikes evoked by contralateral stimulation) were found, extending previous anatomical studies. Commissurally projecting entorhinal neurons were found in layer II, confirming previous anatomical studies. Positive correlations between antidromic spike latency and depth of recording sites supported the interpretation that axons projected along the fiber bundles of the hippocampal commissures and angular bundle to distribute to their targets. Possible circuits that could have mediated the excitatory and inhibitory responses of these retrohippocampal neurons are considered.     

Long-lasting potentiation and epileptiform activity produced by GABAB receptor activation in the dentate gyrus of rat hippocampal slice

Bath application of the GABAB receptor agonist baclofen produced a concentration-dependent long-lasting potentiation (LLP) of the evoked population spike in the dentate gyrus of rat hippocampal slices. A high concentration of baclofen (5 microM) also produced a loss of inhibition that was manifested as the appearance of epileptiform, multiple evoked population spikes and a decrease in paired-pulse inhibition. Both baclofen-induced potentiation and epileptiform activity could be blocked or significantly reduced in slices from pertussis toxin-treated animals (1 microgram, intradentate) or in slices pretreated with the NMDA receptor antagonist D-(-)-2-amino-5-phosphonovaleric acid (10 microM). At a concentration that had no significant effect on individual evoked responses (0.1 microM) but still produced a loss in paired-pulse inhibition, baclofen facilitated the induction of beta-adrenergic receptor-mediated LLP. LLP was induced in the dentate gyrus by bath application of 1 microM, but not 0.1 microM, isoproterenol. Coapplication of baclofen and isoproterenol, both at a concentration (0.1 microM) that individually had no effect on the population spike, produced a synergistic LLP of the population spike. We propose that baclofen produces a selective disinhibitory effect in the granule cell layer of the dentate gyrus by inhibiting the activity of GABAergic interneurons. At a low concentration, the subtle loss of inhibition can facilitate the induction of isoproterenol-induced LLP. At a high concentration, baclofen can produce an LLP that is probably induced by a loss of inhibition.     

Development of inhibitory and excitatory synaptic transmission in the rat dentate gyrus

We studied the ontogeny of inhibitory and excitatory processes in the rat dentate gyrus by examining paired-pulse plasticity in the hippocampal slice preparation. The mature dentate gyrus produces characteristic paired-pulse responses across a wide range of interpulse intervals (IPI). Paired-pulse effects on population excitatory postsynaptic potential (EPSP) slope and population spike (PS) amplitude were analyzed at postnatal day 6 (PN6), PN7/8, PN9/10, PN15/16, and PN > 60. The synaptic paired-pulse profile (10–5,000 ms IPI) matured by PN7/8. The triphasic pattern of short-latency depression, a relative facilitation at intermediate intervals, and long-latency depression was present at all ages tested. Paired-pulse effects on granule cell discharge indicated the presence of weak short-latency (20 ms IPI) inhibition at PN6, the earliest day that a population spike could be evoked. By PN7/8, short-latency inhibition was statistically equivalent to the mature dentate gyrus. Long-latency (500–2,000 ms IPI) PS inhibition was present, and equal to the mature dentate gyrus by PN6. The most consistent difference between the mature and developing dentate gyrus occurred at intermediate IPIs (40–120 ms) where spike facilitation was significantly depressed in the developmental groups. The studies indicate that short-term plasticity matures rapidly in the dentate gyrus and suggest that the inhibitory circuitry can function at a surprisingly early age. © 1994 Wiley-Liss, Inc.     

NMDA receptor antagonists block norepinephrine-induced long-lasting potentiation and long-term potentiation in rat dentate gyrus

In the in vitro rat dentate gyrus, norepinephrine-induced long-lasting potentiation (NELLP) and long-term potentiation (LTP) of responses to perforant path stimulation were blocked by the N-methyl-D-aspartate (NMDA) receptor antagonists, D(-)-2-amino-5-phosphonovaleric acid (D(-)APV) and 3-[(+/-)-2-carboxypiperazin-4-yl]propyl-1-phosphonic acid (CPP). CPP and D(-)APV, but not L(+)APV, also depressed the orthodromic population spike but not the antidromic spike, which suggests that these receptors may function in low-frequency evoked activity of granule cells. We conclude that NELLP, like LTP in the dentate gyrus, requires NMDA receptor activation.     

The site for initiation of action potential discharge over the somatodendritic axis of rat hippocampal CA1 pyramidal neurons

Early electrophysiological studies in the mammalian hippocampus reported that orthodromic depolarization of pyramidal cells evoked action potential discharge (presumed Na+ dependent) both at the axon hillock and at one or more sites in the dendritic arborization (Cragg and Hamlyn, 1955; Andersen, 1959, 1960; Spencer and Kandel, 1961; Andersen and Lomo, 1966). Although tetrodotoxin (TTX)-sensitive spikes have been recorded at the dendritic level (Wong et al., 1979; Benardo et al., 1982; Miyakawa and Kato, 1986; Turner et al., 1989), the site for initiation of these potentials has not yet been determined. In this study, we examine the site for initiation of Na+ spike discharge over the cell axis of rat hippocampal CA1 pyramidal neurons. Intrasomatic and intradendritic recordings were obtained from pyramidal neurons of hippocampal slices maintained in vitro. Spike discharge was evoked by alvear (antidromic) stimulation or orthodromically by stimulation of afferent inputs in stratum oriens (SO) or stratum radiatum (SR). Antidromic and orthodromic spikes were greatest in amplitude in somatic recordings and declined over the apical dendritic axis, while spike half-width was shortest at the cell body and increased with distance from stratum pyramidale. Measurements of orthodromic spike threshold revealed that the only location at which spikes discharged at a consistent membrane potential at threshold intensity (voltage threshold) was the cell body region. Finally, at threshold intensity, SR-evoked intradendritic spikes were blocked by local application of TTX in stratum pyramidale, while spike blockade at suprathreshold intensity required the diffusion of TTX into the apical dendritic region. These results indicate that, for threshold intensities of stimulation, antidromic and orthodromic spike discharge in CA1 pyramidal cells is initiated in the region of the cell body layer, subsequently conducting over the apical dendrites in a retrograde fashion. In contrast, SR-evoked orthodromic spike discharge exhibits an intensity-dependent shift in the site of origin up to 200 microns within the apical dendritic arborization.     

Feedforward and feedback inhibition of hippocampal principal cell activity evoked by perforant path stimulation: GABA-mediated mechanisms that regulate excitability in vivo.

Hippocampal field potentials evoked by paired-pulse perforant path stimulation were used to identify normal feedforward and feedback inhibitory influences on hippocampal principal cells. Three distinct aspects of inhibitory function were identified in the dentate gyrus. They are: (1) first spike amplitude-dependent inhibition of the second spike, which at low stimulus frequency is primarily feedback in nature; (2) frequency-dependent inhibition of a single spike or the first spike of a pair, which occurs as stimulus frequency is increased from 0.1 to 1.0 Hz and which is primarily a reflection of feedforward inhibition; and (3) frequency-dependent inhibition of the second spike that is relatively independent of first spike amplitude and probably due to a combination of feedforward and feedback mechanisms. The results indicate that granule cell recurrent inhibition alone, evoked at low stimulus frequency, is relatively brief and weak. At higher frequencies, probably more relevant to physiological activity, feedforward inhibitory activity is recruited. The combination of feedforward and feedback mechanisms results in strong, maximal duration, granule cell inhibition. Similar frequency dependence of inhibition was not seen in area CA1 in response to ipsilateral perforant path stimulation since low frequency stimulation did not evoke CA1 spikes. CA3 stimulation did evoke large contralateral CA1 population spikes, but paired-pulse inhibition was weaker than that evoked by ipsilateral perforant path stimulation in terms of the duration of inhibition and the ability to suppress the development of epileptiform behavior. The identification of simple tests that reflect distinct inhibitory processes in vivo permits similar studies to be conducted in vitro to determine how to preserve inhibitory processes for cellular studies of normal and human epileptic tissue in which the state of excitatory--inhibitory balance is the subject. These results also form the basis for the interpretation of the following study (Sloviter, 1991), which addresses the relationship between selective dentate interneuron loss and the pathophysiology that accompanies it.     

Electrophysiological characterization of associational pathway terminating on dentate gyrus granule cells in the rat.

The functional topography and parameters of excitation and inhibition were determined in the in situ associational pathway of the rat dentate gyrus. The functional topography was found to be consistent with previous anatomical studies. The greatest amplitude population spikes and the strongest paired-pulse inhibition were generated with the stimulating electrode placed in the hilus at least 1.5 mm caudal to the ipsilateral dentate gyrus recording electrode. With this standard electrode configuration, neither long-term potentiation of the population spike nor of the population excitatory postsynaptic potential occurred. Hilar associational pathway activation of dentate gyrus granule cells elicited paired-pulse responses similar to those produced in granule cells by perforant path stimulation. Thus, the associational pathway provides another way to assess dentate granule cell function electrophysiologically.     

Epileptiform discharges evoked in hippocampal brain slices from epileptic patients

We have recorded from diseased hippocampal tissue which was surgically removed from epileptic patients for therapeutic purposes. When the perforant path was stimulated at a low frequency (1 Hz), the number of population spikes evoked in the dentate gyrus increased by a factor of as great as 8 during a 15 s train. This effect was transient. A similar epileptiform discharge could be generated in normal rat hippocampal brain slices by the same stimulus paradigm, but only in the presence of a low concentration (0.2 microM) of bicuculline. These results suggest that this frequency-dependent epileptiform discharge, evoked in the dentate gyrus of epileptic patients, may be due to a small reduction in GABAA-mediated inhibition and may involve factors that lead to the initiation of seizures.     

The effects of traumatic brain injury on inhibition in the hippocampus and dentate gyrus

Changes in inhibitory neuronal functioning may contribute to morbidity following traumatic brain injury (TBI). Evoked responses to orthodromic paired-pulse stimulation were examined in the hippocampus and dentate gyrus at 2 and 15 days following lateral fluid percussion TBI in adult rats. The relative strength of inhibition was estimated by measuring evoked paired pulses in three afferent systems: the CA3 commissural input to the CA1 region of the hippocampus; the entorhinal cortical input to the ipsilateral CA1 area (temporoammonic system); and the entorhinal input to the ipsilateral dentate gyrus (perforant path). In addition to quantitative electrophysiological estimates of inhibitory efficacy, levels of γ-aminobutyric acid (GABA) were qualitatively examined with immunohistochemical techniques. Effects of TBI on paired-pulse responses were pathway-specific, and dependent on time postinjury. At 2 days following TBI, inhibition of population spikes was significantly reduced in the CA3 commissural input to CA1, which contrasted with injury-induced increases in inhibition in the dentate gyrus seen at both 2 and 15 days postinjury. Low-level stimulation, subthreshold for population spikes, also revealed changes in paired-pulse facilitation of field extracellular postsynaptic potentials (fEPSPs), which depended on fiber pathway and time postinjury. Significant injury-induced electrophysiological changes were almost entirely confined to the hemisphere ipsilateral to injury. Intensity of GABA immunobinding exhibited a regional association with electrophysiological indices of inhibition, with the most pronounced increases in GABA levels and inhibition found in the dentate gyrus. TBI-induced effects showed a regional pattern within the hippocampus which corresponds closely to inhibitory changes reported to follow ischemia and kindling. This degree of similarity in outcome following dissimilar injuries may indicate common mechanisms in the nervous system response to injury.     

Brevetoxin depresses synaptic transmission in guinea pig hippocampal slices

Extracellular recordings were obtained from area CA1 of guinea pig hippocampal slices. PbTx-3, a brevetoxin fraction isolated from the red tide dinoflagellate Ptychodiscus brevis. was applied by bath perfusion. The toxin produced a concentration-dependent depression of the orthodromically evoked population spike with an EC50 of 37.5 nM. Brevetoxin concentrations below 10 nM were without effect, and concentrations above 100 nM led to total inhibition of evoked responses. PbTx-3 did not produce spontaneous synchronous discharges but did induce afterdischarges following evoked responses in about 50% of the slices tested, particularly at concentrations between 10 nM and 100 nM. Orthodromically evoked responses were more sensitive to PbTx-3 than were those elicited by antidromic stimulation. High-Ca²⁺ solution, 4-aminopyridine, and tetraethylammonium failed to antagonize either orthodromic or antidromic effects of the toxin. Although the precise mechanism by which PbTx-3 depresses evoked responses is not certain, depolarization of the presynaptic nerve terminals leading to failure of transmitter release could explain the toxin's actions. This is the first report of the effects of brevetoxin applied directly to central nervous system tissue.     

Differential paired-pulse responses between the CA1 region and the dentate gyrus are related to altered CLC-2 immunoreactivity in the pilocarpine-induced rat epilepsy model

The epileptic hippocampus shows differential paired-pulse responses between the dentate gyrus and the CA1 region. However, little data are available to explain this phenomenon. In the present study, we identified the relationship between regional differences of paired-pulse response and voltage gated Cl(-) channel 2 (CLC-2)/vesicular GABA transport (VGAT) expression in a pilocarpine-induced rat model. During epileptogenic periods, paired-pulse inhibitions in the dentate gyrus and the CA1 region were markedly reduced. After recurrent seizure onset, paired-pulse inhibition in the dentate gyrus was markedly enhanced, while that in the CA1 region more reduced. Unlike VGAT, CLC-2 immunoreactivity was markedly reduced in the hippocampus during epileptogenic periods and was re-enhanced only in the dentate gyrus after recurrent seizure onset. Linear regression analysis showed an inverse proportional relationship between alterations in CLC-2 immunoreactivity and changes in normalized population spike amplitude ratio within the CA1 region and the dentate gyrus. Therefore, our findings suggest that the regionally specific alterations in CLC-2 immunoreactivity after SE may determine the properties of paired-pulse responses in the hippocampus of the pilocarpine-induced rat epilepsy model.     

Long-lasting reduction of dentate paired-pulse depression following LTP-inducing tetanic stimulations of perforant path

Effects of high-frequency stimulations of the perforant path on the dentate paired-pulse depression were examined in urethane-anesthetized rats. The tetanic stimulations produced a long-term potentiation (LTP) of the excitatory synaptic transmission at the perforant path-dentate granule cell synapses in almost all animals examined. The strength of the early paired-pulse depression at an inter-pulse interval (IPI) of 20 ms decreased significantly for at least 60 min after the tetanic stimulations, whereas the late paired-pulse depression at an IPI of 2 s remained almost unchanged. The reduction of the early paired-pulse depression was stepwise augmented by each of successive tetanic stimulations given at an interval of 10 min. A preceding antidromic stimulation of the mossy fibers depressed the population spike amplitude of perforant path response at an interval of 5-9 ms. The strength of the antidromic depression of population spike also decreased following the perforant path tetanic stimulations. These results suggest that tetanic stimulations of the perforant path produce a long-lasting reduction of the GABAergic recurrent inhibition in the dentate area associated with LTP. The possible mechanisms of the decrease in GABAergic inhibition produced by tetanic stimulations and its possible effects on the development of LTP with succeeding tetanic stimulations were discussed.     

Postsynaptic action potentials do not alter short-term potentiation in the dentate gyrus

The dendrites of neocortical neurons have been shown to support active action potentials which back-propagate from the soma after an output spike has been initiated. This observation has led to speculation that dendritic action potentials may participate in various forms of synaptic plasticity. The contribution of dendritic spikes to paired-pulse facilitation (PPF), a form of short-term plasticity, was investigated in the dentate gyrus of hippocampal slices. Paired orthodromic stimulation of the perforant path produced an average facilitation of the test population spike (PS) amplitude of 167% (n = 16, conditioning RESPONSE = 100%). There was also a small but significant increase in slope of the field EPSP (fEPSP) of 108%. To determine whether increased presynaptic drive could account for this facilitation, the relationship between fEPSP slope and spike amplitude (I-O) was determined for a range of stimulus intensities. An increase in fEPSP slope of 171% was associated with an increase in PS amplitude equal to the facilitation produced by paired-pulse stimulation (167%), suggesting a postsynaptic component in PPF. Electric field effects were then used as a tool to alter the excitability of granule cells during the conditioning response without changing synaptic drive. Any change in the test response associated with manipulation of the conditioning population spike amplitude would suggest that dendritic spikes may contribute to the postsynaptic component of PPF. Surprisingly, altering the number of neurons responding to the conditioning stimulus with an action potential had no effect on the test response, suggesting that dendritic action potentials do not participate in this form of short-term synaptic plasticity.