Paired-pulse and frequency facilitation in the CA1 region of the in vitro rat hippocampus

A comparison of recurrent inhibition and of paired-pulse facilitation in hippocampal slices from normal and genetically epileptic mice

Tottering mice exhibit inherited generalized epilepsy of the 'absence' type. In hippocampal slices from these mutant mice studied in vitro, pairing an alvear antidromic stimulus to an orthodromic one revealed a strong recurrent inhibition (RI) of CA1 pyramidal neurons. RI was maximal at 10 ms inter-pulse interval (IPI 70% decrease of population spike, PS) gradually decreasing to 15% at 320 ms IPI. At 10 ms IPI it shifted the input/output curves to the right and decreased maximum PS. In the group of slices from epileptic mice the early part of RI (2.5-60 ms) was indistinguishable from that of normal mice, with respect to both its strength and its liability to activity-dependent decrement induced by a train of antidromic stimuli (8 s, 5 Hz). However, the delayed part (80-320 ms) was slightly stronger in the epileptic group. Also in this group only the train of antidromic pulses caused a significant and lasting decrease in the unconditioned orthodromic PS. Paired-pulse facilitation was equally strong in the 2 groups of slices. It is concluded that mechanisms underlying epileptogenic hyperexcitability in the tottering mutant may not include a failure of inhibition, at least in the CA1 area of the hippocampus. On the contrary some inhibitory mechanisms may be stronger.

Differential effects of mu- and delta-receptor selective opioid agonists on feedforward and feedback GABAergic inhibition in hippocampal brain slices

Previous studies have suggested that opioid receptor activation in the hippocampus increases pyramidal neuron excitability by reducing GABAergic inhibition. This hypothesis has received support with regard to mu-receptor agonists but has not been adequately tested with selective delta-receptor agonists. In the present investigation we compared the effects of the selective mu-opioid receptor agonist [Tyr-(D-Ala)-Gly-(N-Me-Phe)-Gly-ol]-enkephalin (DAGO) and the delta-receptor agonist [D-Pen2,D-Pen5]-enkephalin (DPDPE) to those of bicuculline methiodide (BMI) on extracellularly recorded feedforward (FFW) and recurrent (feedback; FB) inhibition. It was discovered that the control population spike response, evoked by Schaffer collateral/commissural axon stimulation, increased in response to DAGO, DPDPE, and BMI, while the secondary or test response increased only in the presence of DAGO and BMI. The resulting hypothesis that delta-opioid receptor activation facilitates synaptically evoked responses independently of a reduction of inhibition was investigated by examining the effect of DPDPE on the field EPSP response recorded in stratum radiatum of CA1, or postsynaptically on a burst response activated through antidromic stimulation of pyramidal neurons in low calcium medium. delta-Opioid receptor activation had no effect on either the field EPSP response or the burst response, suggesting that neither synaptic transmission nor postsynaptic excitability were augmented. Finally, the possibility that DPDPE acts to enhance pyramidal cell excitability independently of GABAergic transmission was further investigated by examining responses to both mu- and delta-opioid agonists following treatment with BMI (30 microM). Responses to DPDPE and DAGO were completely blocked by this treatment, supporting the involvement of a GABAergic circuit in the actions of these enkephalins. These results suggest that the delta-opioid receptor agonist DPDPE may mediate a reduction in GABAergic inhibition which is not detectable using paired stimulation techniques designed to examine FFW and FB inhibition in the hippocampal slice.

Long-term enhancement of synaptic responses in the songbird hippocampus

Electrophysiological recordings from the hippocampus were taken using brain slices from 1-year-old, female, song sparrows (Melospiza melodia). The evoked responses were smaller and less stable as compared with those obtained from the mammalian hippocampus. They consisted of two spikes. The first spike had low calcium dependency and represented mainly fiber potential. The second spike demonstrated a clear calcium dependency proving its synaptic origin. Paired-pulse facilitation showed inhibition of the second response at latencies below 20 ms, facilitation at 30 ms and no changes above 30 ms. Train stimulation with 20 Hz (each train consisted of 1 s stimulation with 10-s intervals repeated 3 times) evoked a stable increase in the size of the evoked response lasting approximately 2 h. These data indicate that the avian hippocampus possesses several neurophysiological properties that typify the mammalian hippocampus including the long-term enhancement of the synaptic response following certain patterns of stimulation.

GABAB receptors play a major role in paired-pulse facilitation in area CA1 of the rat hippocampus

Extracellular recordings of field potentials in area CA1 of the rat hippocampal slice have been used to investigate paired-pulse facilitation. Field potentials were evoked by maximal stimulation of the Schaffer collateral/commissural fibres. The height of the population spike (PS) in stratum pyramidale (str. pyr.) and the area under the field excitatory postsynaptic potential (EPSP) following the PS in the stratum radiatum (str. rad.) were quantified. These values were used to describe the time course of paired-pulse facilitation. Facilitation of the PS was maximal 50 ms after the conditioning pulse and was present over a period of about 500 ms. However, facilitation of the late area (LA) of the field EPSP was maximal afer 125 ms and had an overall duration of 1-2 s. The N-methyl-D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonovaleric acid (APV), had no effect on paired-pulse facilitation of either the LA or the PS. The gamma-aminobutyric acid-B (GABAB) agonist baclofen increased facilitation of the PS. This was mainly due to a reduction of the unconditioned response. Facilitation of the LA was reduced by both baclofen and the GABAB antagonist, 2-OH-saclofen. Baclofen increased the LA of the unconditioned response, while this was unaffected by 2-OH-saclofen. The LA of facilitated responses was decreased by 2-OH-saclofen while the effect of baclofen on these responses was more complex. Baclofen reduced the LA of maximally facilitated responses, while the LA of slightly facilitated responses was increased. The results show that different mechanisms are involved in the facilitation of the LA and the PS. Furthermore, activation of GABAB receptors makes a large contribution to paired-pulse facilitation of the field EPSP. It is also suggested that recording of extracellular fields in str. rad. in response to paired-pulse stimulation provides a simple electrophysiological model for testing the effect of agents which act at the GABAB receptor.

Locally evoked potentials in slices of the rat nucleus accumbens: NMDA and non-NMDA receptor mediated components and modulation by GABA

In a slice preparation of the rat nucleus accumbens (Acb), local electrical stimulation elicited a field potential composed of two negative peaks, followed by a positive wave. The early negative peak was identified as a non-synaptic compound action potential, the late negative peak as a monosynaptic population spike (PS) and the positive wave as a mixture of an excitatory and an inhibitory postsynaptic potential (PSP). Both the PS and the PSP exhibited a marked degree of paired-pulse facilitation. The quisqualate/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 2 microM) and the broadly acting glutamate receptor antagonist kynurenic acid (300 microM) reversibly abolished or reduced both the PS and PSP. In contrast, nicotinic, muscarinic and N-methyl-D-aspartate (NMDA) receptor antagonists had no suppressive action. Washout of Mg2+ from the superfusion medium reversibly enhanced and prolonged the PSP and this effect was blocked by the NMDA receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP-5). The gamma-aminobutyric acid antagonist picrotoxin (60 microM) enhanced the PS and induced secondary spikes which were superimposed on a prolonged PSP. Most of this prolongation was abolished by D-AP-5. It is concluded that locally evoked synaptic responses in the Acb are mediated by glutamate or aspartate, and that NMDA receptor mediated activity evoked by low frequency stimulation is substantial in Mg2(+)-free medium or during reduced GABAA receptor activity, but not under normal conditions.

Nicotine interferes with GABA-mediated inhibitory processes in mouse hippocampus

Previous studies indicated that the excitatory effects of nicotine may be mediated via interference with GABAergic transmission. Here, several variants of the paired-pulse paradigm were employed to ascertain whether nicotine interferes with endogenous inhibitory circuits in the hippocampus. Nicotine attenuated the inhibition evoked by antidromic (alvear) stimulation in the CA1 region in a concentration-dependent manner (EC50 = 60-75 microM). This same phenomenon was also observed for the GABAA receptor antagonist bicuculline (0.1 microM). Orthodromic-orthodromic paired-pulse paradigms were found to be unsuitable for investigating the effects of epileptogenic agents such as nicotine and bicuculline on endogenous inhibition.

Patterns of changes in field potentials in the isolated hippocampal slice on withdrawal from chronic ethanol treatment of mice in vivo

Extracellular recordings were made from isolated hippocampal slices, CA1 area, following withdrawal from chronic ethanol administration to mice of the C57 strain. The field potentials were followed for 7 h from preparation of the slices, in the absence of ethanol. Paired pulse potentiation was increased, and paired pulse inhibition decreased, in slices from ethanol-treated mice during the first four hours of the recording period. Orthodromic thresholds for elicitation of single and multiple population spikes were decreased by the ethanol treatment, in the later part of the recording period. The input/output curves for population spike area and population excitatory postsynaptic potential slope showed a shift to the left for the slices from ethanol-treated animals, but no change in the maximal response. Antidromic stimulation also demonstrated decreases in thresholds for single and multiple population spikes in tissues from ethanol-treated animals, during the later half of the recording period. The results indicate that there are several mechanisms by which neuronal excitability increases on withdrawal from chronic ethanol treatment. The changes follow different time courses and suggest multiple mechanisms underlying the behavioural signs seen during the ethanol withdrawal syndrome.

Elevated extracellular potassium concentration enhances synaptic activation of N-methyl-D-aspartate receptors in hippocampus

The biophysical properties of N-methyl-D-aspartate (NMDA) receptor-mediated conductances have been well described, but the means by which NMDA receptors are synaptically activated under physiological conditions remain unclear. Activation of NMDA receptors in the CA1 region of the rat hippocampus by paired and multiple stimuli occurred here with orthodromic stimulation at 10 Hz. Elevating extracellular potassium ion concentration [( K+]o) to 7.5 mM selectively enhanced the NMDA receptor-mediated component of the response to repetitive stimulation, and led to burst-firing of pyramidal cells. The effects of elevated [K+]o on NMDA receptor activation were accompanied by relatively small changes in resting potential, however, and may in part result from a decrease in the potassium equilibrium potential producing slowed repolarization. Supporting this hypothesis, low concentrations of the K(+)-channel blocker tetraethylammonium added to normal [K+]o solution slowed repolarization and reproduced the effects of elevated [K+]o on NMDA receptor activation. Significant changes in [K+]o occur with neuronal activity in the central nervous system, and thus may play a role in regulating NMDA receptor-mediated postsynaptic activity.

Acute effects of lithium on synaptic transmission in rat hippocampus studied in vitro

Acute effects of lithium on synaptic transmissions in the CA1 neurones of rat hippocampus were examined. Perfusion of 2-10 mM lithium chloride (LiCl) produced a dose-dependent increase in the amplitude of field EPSPs, whereas change in the population spikes was variable. The increasing ratio of second field EPSP, which was examined by paired-pulse stimulation, was reduced about 10% by 5 mM LiCl. Intracellularly recorded EPSPs and IPSPs were facilitated by 10 mM LiCl, and the soma membrane was depolarized about 3.2 mV. Intracellular calcium concentration was measured in single hippocampal neurones using fura-2. Although calcium concentration at rest was approximately 30 nM and was increased to an average of 220 nM by 10(-5) M glutamate, 10 mM LiCl had no influence on it. The effects of Li on calcium-dependent processes were not manifested in this study. Variable changes of population spikes may be dependent on the balance between the excitatory and inhibitory postsynaptic potentials during lithium application.

Developmental changes in synaptic properties in hippocampus of neonatal rats

The properties of synaptic responses in area CA1 of hippocampus were analyzed in slices prepared from 7-9 and 12-15 day old neonate rats. As expected from earlier work, only slices of two-week-old animals showed a consistent degree of long-term potentiation (LTP) in response to patterned high frequency stimulation. Several other synaptic properties were found to change during this developmental period. Inhibitory responses were absent in 7-9 day old but not in 12-15 day old neonates. Paired-pulse facilitation and the calcium sensitivity of postsynaptic responses were considerably reduced in 7-9 as compared to 12-15 day old rats. However, phorbol esters and 4-aminopyridine treatment still produced a strong facilitation of field potentials. The N-methyl-D-aspartate (NMDA) component of responses to single pulse stimulation in low magnesium medium was found to be larger in slices of 7-9 than 12-15 day old or adult animals. At the two time periods examined, trains of high frequency stimulation applied in the presence of regular magnesium elicited an NMDA dependent response. It is concluded that the differences in synaptic properties observed between 7-9 and 12-15 day old neonates may not account for the absence of LTP in the younger animals.

Evidence that changes in presynaptic calcium currents are not responsible for long-term potentiation in hippocampus

We used two approaches to test the possibility that changes in presynaptic calcium currents might be responsible for the long-term potentiation (LTP) effect induced by high-frequency stimulation in area CA1 of hippocampal slices. In a first series of experiments, we compared the effect of LTP induction on paired-pulse facilitation with that produced by changes in extracellular calcium concentration, a procedure that modifies presynaptic calcium currents during depolarization by changing the ionic gradient for calcium. In hippocampus, as in peripheral synapses, increasing concentrations of extracellular calcium caused a marked reduction in the degree of facilitation obtained with paired-pulse stimulation; LTP, conversely, did not affect the facilitation ratio. The differential effect of changing calcium concentrations versus LTP induction on paired-pulse facilitation was observed with different interpulse intervals as well as in conditions in which the changes in response size produced by the two manipulations were comparable. In the second approach, we measured calcium dependency curves of synaptic responses before and after LTP induction or application of 4-aminopyridine, a blocker of potassium channels that increases presynaptic calcium currents by slowing spike repolarization. Procedures that increase calcium entry into terminals during transmission should shift to the left the sigmoidal function relating extracellular calcium to the slope of the extracellular response. This in turn should result in disproportionate effects of the procedure as a function of the calcium concentration. This prediction was realized with 4-aminopyridine but did not occur following LTP induction: control and potentiated responses were similarly affected by changes in calcium concentration. Although indirectly, these data strongly suggest that LTP is not accompanied by alterations in the presynaptic calcium dynamics associated with transmitter release.

Paradoxical sleep deprivation does not affect neuronal excitability in the rat hippocampus

Before, during and after paradoxical sleep deprivation, field potentials, evoked in the CA1 region and in the fascia dentata of the hippocampus by means of paired pulse stimulation, were measured. Paradoxical sleep deprivation was applied for 3 days, using the platform and the pendulum technique. No changes were observed on evoked field potential amplitude, population spike or paired pulse depression during and after the deprivation period. These results suggest that the neuronal excitability in the rat hippocampus, measured with the evoked potential technique, does not change as a result of paradoxical sleep deprivation.

The action of hydrogen peroxide on paired pulse and long-term potentiation in the hippocampus

The action of a reactive oxygen intermediate, that is, hydrogen peroxide (H2O2) on modulation of synaptic transmission was examined in the hippocampal brain slice preparation. Microinjection of H2O2 into the apical dendritic region of the CA1 pyramidal cells produced no change in either the pattern or amplitude of paired pulse facilitation compared to saline injection (control). Long term potentiation (LTP), induced by high frequency stimulation of homosynaptic inputs, however, was blocked by microinjection of H2O2 into the dendritic tree. LTP was seen in only 2 out of 10 slices investigated when treated with H2O2 while LTP was seen in 4 out of 5 slices when saline injected. The results suggest that a reactive oxygen intermediate can selectively modify synaptic mechanisms in the hippocampus.

Rate-limiting step for transmission at excitatory synapses in hippocampus

To examine mechanisms that might be responsible for limiting transmission at excitatory synapses in hippocampus, we analyzed the relationship between extracellular calcium concentrations (1-6 mM) and postsynaptic responses in field CA1 of hippocampal slices using low stimulation intensities and a paired-pulse paradigm. Three effects were observed: One, the relationship between calcium levels and the slope (or amplitude) of the postsynaptic response was described by a sigmoidal function with an asymptote at about 4 mM. Double reciprocal pilots relating calcium concentration to the initial slope of EPSPs provided evidence for the cooperativity expected between calcium ions and transmitter release. Two, both the rise time and half-decay time of the postsynaptic responses were reduced with increasing calcium concentrations. These effects of calcium were more pronounced on the first response elicited by paired-pulse stimulation and were considerably attenuated by 2 microM bicuculline, indicating that feed-forward inhibition was positively related to calcium concentration and differentially activated by repetitive stimulation. However, inhibition was not responsible for the asymptotic relationship observed between calcium and response size. Three, while increasing the calcium concentration beyond 4 mM did not further affect the initial slope of excitatory postsynaptic potentials (EPSPs), paired-pulse facilitation and 4-aminopyridine were still effective in increasing response size. These results suggest 1) that neither the number of postsynaptic receptors nor the number of transmitter quanta available for release were limiting transmission as a function of the calcium concentration; and 2) that calcium entry into presynaptic terminals was likely to represent the limiting step under the conditions used.

Facilitation of hippocampal long-term potentiation in slices perfused with high concentrations of calcium

The effect of increased extracellular calcium on long-term potentiation (LTP) of synaptic transmission has been examined in the CA1 region of guinea pig hippocampal slice preparation using extracellular recordings from the dendritic layer. The application of high calcium (4 mM) led to an increase in the initial slope of the field potential that reversed following return to control (2 mM calcium) solution. The magnitude of the field potential change was unaffected by prior induction of LTP, and inputs tetanized after return to control solution showed the same amount of LTP as those tetanized before the high calcium application. These results suggest that the calcium application by itself did not induce LTP. Inputs tetanized in the high calcium solution showed a greater amount of potentiation than in control solution, any given train producing about twice as much potentiation. However, using long trains (40 impulses) at high strength (2 x test strength) gave similar LTP values in the two solutions. The facilitatory effect of high calcium on LTP was completely blocked by raising extracellular magnesium from 2 to 4 mM. As in control solution. LTP evoked in the high calcium solution was blocked by 2-amino-5-phosphono-valerate. The results support the view that calcium influx through postsynaptic N-methyl-D-aspartate receptor channels is directly involved in the induction of LTP.

Phenytoin does not block hippocampal long-term potentiation or frequency potentiation

Recently, a theoretical scheme offered mechanisms by which phenytoin exerts its antiepileptic effects. Two predictions arising from this proposal are that phenytoin would suppress alterations in the potency of excitatory neurotransmission engendered by repetitive neural activation and that this effect would be augmented by displacing the extracellular concentration of K+ ([K+]0) away from its normal resting level. In the present study, we tested these predictions by examining the effects of phenytoin on short- and long-term functional plasticity in vitro in the hippocampus. Extracellular field potentials were recorded in the CA1 region of the rat hippocampal slice in response to stimulation of the Schaeffer collaterals. Phenytoin (20 micrograms/ml) did not affect baseline excitatory neurotransmission as measured by input-output curves (population spike amplitude versus stimulus intensity) obtained at low stimulus rates. The drug also had no effect on either frequency potentiation (2.5 Hz) or long-term potentiation (50 Hz, 500 msec; or 400 Hz, 20 msec). When [K+]0 was raised to levels seen during seizures, the drug still did not alter frequency potentiation or long-term potentiation induced by either type of stimulus train. Phenytoin also had no effect on either stimulus-locked or spontaneous epileptiform bursts that appear in conjunction with elevated [K+]0 or on stimulus-locked bursts that appear in the presence of 0.75 mM extracellular calcium. These results, showing that certain forms of functional synaptic plasticity are not affected by phenytoin, suggest a means by which phenytoin could exert its antiepileptic action without interfering with normal brain function.

Phorbol ester-induced synaptic potentiation differs from long-term potentiation in the guinea pig hippocampus in vitro

The relationship between the synaptic potentiations evoked by the protein kinase C activator phorbol-12,13-diacetate and by afferent tetanization has been examined in the CA1 region of the hippocampal slice preparation using extracellular recording. It has been found that the potentiation of the field excitatory postsynaptic potential produced by 1 microM phorbol ester does not affect the amount of long-term potentiation (LTP) that can be evoked by afferent tetanization, and vice versa. A dissociation between phorbol ester-induced and tetanus-induced potentiation is also indicated by the fact that only the former was associated with changes in paired-pulse facilitation. On the other hand, as previously described, higher concentrations (10 microM) of phorbol ester blocked the tetanus-induced potentiation. Since the total potentiation given by 10 microM phorbol ester and tetanization depended on the order of presentation of the potentiation-inducing stimuli, it appears that the blockade of LTP is, at least partly, independent of the phorbol ester-induced potentiation.

Waveform and amplitude characteristics of evoked responses to dendritic stimulation of CA1 guinea-pig pyramidal cells

1. Dendritic synaptic responses were evoked in CA1 hippocampal pyramidal cells using a microstimulation protocol which included focal excitation of proximal and distal apical afferents. Ensembles of excitatory postsynaptic potentials (EPSPs) were analysed for magnitude, waveform parameters and fluctuation characteristics between responses. 2. The peak amplitude of the minimal detectable responses to dendritic stimulation ranged from 0.12 to 0.89 mV. Control experiments showed separation of the minimal EPSPs from extracellular field potentials and somatic inhibition. The EPSPs demonstrated no significant amplitude trends over ensembles of 150-300 responses, at a 2 Hz stimulation rate. 3. Both minimal (less than 1 mV) and large (1-5 mV) proximal and distal evoked EPSPs were clearly different in terms of all waveform parameters analysed. However, the large EPSPs exhibited substantially less proximal-distal separation than the minimal responses. 4. The separation of minimal proximal and distal responses was similar to that predicted by earlier dendritic model simulations, after revision of the model parameter determining shape of the synaptic input. The proximal and distal synapses are separated by approximately 0.6 dendritic length constants (lambda), within an average apical dendritic tree of 0.9 lambda. 5. The decrease in proximal-distal separation with increasing EPSP size may stem from recruitment of non-laminar dendritic afferents and the addition of feed-forward inhibitory potentials. These circuitry features of the CA1 region lead to an effective electrical shortening of the apical dendritic tree for large EPSPs and in particular to an enhanced efficacy of distal synapses.

Caffeine inhibits post-tetanic potentiation but does not alter long-term potentiation in the rat hippocampal slice

The effects of caffeine were investigated on the extracellular excitatory postsynaptic potentials (EPSPs) recorded in the stratum radiatum of CA1 of the rat hippocampal slice in response to stimulation of the Schaffer collaterals. Caffeine in concentrations from 500 microM to 10 mM caused an increase in the amplitude of the EPSP, which reached a peak after 5-10 min perfusion. This increase was antagonized by pretreatment with 20 microM trifluoperazine. Paired-pulse facilitation, augmentation and potentiation were strongly inhibited by caffeine. Augmentation was most sensitive to caffeine, being abolished by 500 microM caffeine. Long-term potentiation (LTP) induced by high-frequency stimulation was not significantly inhibited by caffeine either by 5-10 min or by 60 min perfusion with 10 mM caffeine. Moreover, the caffeine-induced increase in the low-frequency EPSP could be reversed by 30 min washout, demonstrating that it was not an LTP type increase.

Temporal requirements of associative short-term potentiation in CA1 neurons of rat hippocampus

Temporal separation and juxtaposition of stimulations for the induction of associative short-term potentiation in the rat hippocampus were examined. A series of stimulations of stratum radiatum during brief tetanic stimulations of stratum oriens resulted in a short-term potentiation (of about 3 min duration) of stratum radiatum-induced CA1 population excitatory postsynaptic potential. Potentiation was evident when the stratum radiatum stimulus preceded the onset of the conditioning tetanus by no more than 50 ms, or followed by no more than 80 ms. These parameters closely resemble those found for associative long-term potentiation, suggesting the possibility of shared mechanisms for the induction of these two forms of potentiation.

Possible presynaptic actions of 2-amino-4-phosphonobutyrate in rat olfactory cortex

1 The effect of 2-amino-4-phosphonobutyrate (APB) on facilitation at the lateral olfactory tract (LOT)-superficial pyramidal cell synapse of the olfactory cortex has been studied by recording the relative changes in amplitude of the N-waves evoked on stimulation of the LOT by pairs of stimuli. 2 Although APB (0.01 to 5 mM) reduced the amplitude of the conditioning response there was an overall increase in facilitation over conditioning intervals of up to 1700 ms which was concentration-dependent and inversely related to the concentration of extracellular calcium (1.25 to 5 mM). 3 The L-(+)-isomer of APB was more potent than the D-(-)-form in increasing synaptic facilitation. 4 The potassium channel blockers 4-aminopyridine (0.25 mM), 3,4-diaminopyridine (0.1 mM), tetraethylammonium (10 mM) and catechol (1 mM) all reduced facilitation but failed to antagonize the increase in facilitation produced by APB (1 mM). In contrast, all 4 drugs antagonized APB-induced reductions in the amplitude of the conditioning response. 5 APB (1 mM) significantly reduced the K+-evoked release of endogenous aspartate and glutamate but not of gamma-aminobutyric acid from slices of olfactory cortex. 6 It is suggested that APB reduces the amplitude of the conditioning response and increases synaptic facilitation by reducing transmitter release from the LOT terminals. The mechanism is unlikely to involve activation of terminal potassium currents.

Calcium and the aging nervous system

Many aspects of calcium homeostasis change with aging. Numerous calcium compartments complicate studies of altered calcium regulation. However, age-related decreases in calcium permeation across membranes and mobilization from organelles may be a common fundamental change. Deficits in ion movements appear to lead to altered coupling of calcium-dependent biochemical and neurophysiological processes and may lead to pathological and behavioral changes. The calcium-associated changes during aging probably do not occur with equal intensity in all cell types or in different parts of the same cell. Thus, cells or compartments with a high proportion of calcium activated processes would be more sensitive to diminished calcium availability. These age-related changes may predispose the brain to the development of age-related neurological disorders. The effects of decreased ion movement may be further aggravated by an age-related decline in other calcium-dependent processes. Depression of some of these calcium-dependent functions appears physiologically significant, since increasing calcium availability ameliorates age-related deficits in neurotransmission and behavior. A better understanding of the interactions between calcium homeostasis and calcium-dependent processes during aging will likely help in the design of more effective therapeutic strategies.

Facilitated induction of hippocampal long-term potentiation in slices perfused with low concentrations of magnesium

The generation of long-term potentiation of synaptic transmission in area CA1 of hippocampal slices of the guinea-pig has been examined in solutions containing low concentrations of magnesium ions. It was found that the induction of long-term potentiation is greatly facilitated in slices perfused with 0.1 mM magnesium but much less so with 0.5 mM magnesium solution. The long-term potentiation evoked by brief tetanization in 0.1 mM magnesium was prevented following application of the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovalerate. Moreover, the response to tetanization, recorded in the dendritic layer, contained a much greater than normal component blocked by 2-amino-5-phosphonovalerate. The latter represents current through postsynaptic N-methyl-D-aspartate receptor channels, suggesting that the facilitation of long-term potentiation is related to a facilitated opening of these channels. The results support the notion that the generation of long-term potentiation is related to current through N-methyl-D-aspartate receptor channels which is under the control of extracellular magnesium ions.

Characteristics of CA1 activation through the hippocampal trisynaptic pathway in the unanaesthetized rat

The hippocampal CA1 field is activated by the entorhinal cortex mainly through the hippocampal excitatory trisynaptic circuit. Field responses of the CA1 region were evoked by ipsilateral CA3 or perforant path volley (mono- or trisynaptic activation, respectively) in paralyzed, locally anaesthetized rats and studied as a function of the stimulus patterns presented. The relationship of these responses with the concomitant EEG was also explored. Results showed that mono- and especially trisynaptically evoked responses were progressively enhanced by increasing the stimulus frequency from 0.1 to 1.0 Hz. At specific intensities the trisynaptically evoked population spike (PS) was present only with a rather fixed frequency of stimulation (approximately 0.5 Hz). PS was produced in 100% of the responses using 0.7 Hz, indicating the existence of a threshold-like level for this stimulus parameter. The frequency of presented paired pulses differentially affected pair-pulse facilitation of mono- and trisynaptically evoked excitatory postsynaptic potentials (EPSP): higher frequency decreased the former and increased the latter. All evoked responses studied (i.e. EPSP and PS) showed steep increments and decrements in amplitude, clearly developing several clusters. Moreover, the amplitude distribution of trisynaptic PS often varied spontaneously from maximal to negligible values, showing an all-or-none distribution. Clustering was interpreted as evidence of the existence in the hippocampus of functional neuronal aggregates. All-or-none distribution of trisynaptic PS was found to be associated with the EEG pattern, PS amplitude being maximal during irregular EEG activity and minimal during theta rhythm. Present results suggest that (1) the entorhinal cortex may exert modulatory actions on CA1 by a mechanism widely based on the frequency of the input; (2) information transfer from the entorhinal cortex to other brain areas throughout the hippocampus is biased by hippocampal EEG; and (3) electronic coupling may be functionally predominant in the hippocampus.

Abolition of the orthodromically evoked IPSP of CA1 pyramidal cells before the EPSP during washout of calcium from hippocampal slices

The complex EPSP-IPSP response of CA1 pyramidal cells to orthodromic activation of Schaffer collaterals-commissural afferent fibres was monitored during washout of Ca from hippocampal slices. The IPSP was clearly abolished before the EPSP and this occurred when the extracellular Ca([Ca2+]o) had fallen to between 1.03 and 0.7 mM. The loss of the IPSP was usually associated with a temporary increase in size of the EPSP and sometimes a membrane depolarization and the appearance of spontaneous activity. As [Ca2+]o fell further these effects were reversed and eventually the EPSP was abolished when [Ca2+]o had reached between 0.78 and 0.26 mM.

The action of valproate on spontaneous epileptiform activity in the absence of synaptic transmission and on evoked changes in [Ca2+]o and [K+]o in the hippocampal slice

The effects of valproate (VPA) on neuronal excitability and on changes in extracellular potassium ([K+]o) and calcium ([Ca2+]o) were investigated with ion selective-reference electrode pairs in area CA1 of rat hippocampal slices. Field potential responses to single ortho- and antidromic stimuli were unaltered by VPA (1-5 mM). The afferent volley evoked in the Schaffer-commissural fibers was also unaffected. In contrast, VPA (1 mM) depressed frequency potentiation and paired pulse facilitation markedly. Decreases in [Ca2+]o induced either by repetitive stimulation or by application of the excitatory amino acids N-methyl-D-aspartate and quisqualate were reduced, and the latter results suggest that VPA interferes with postsynaptic Ca2+ entry. When synaptic transmission was blocked by lowering [Ca2+]o (0.2 mM) and elevating [Mg2+]o (7 mM), prolonged afterdischarges elicited by antidromic stimulation were blocked by VPA. VPA also suppressed the spontaneous epileptiform activity seen when [Ca2+]o was lowered to 0.2 mM, without elevating [Mg2+]o. The amplitudes of the rises in [K+]o induced by repetitive orthodromic stimulation were only slightly depressed and those elicited by antidromic stimulation were generally unaltered by VPA, as were laminar profiles of stimulus-evoked [K+]o signals. These results indicate that VPA has membrane actions in addition to known effects on excitatory and inhibitory transmitter pools.

Zinc and paired-pulse potentiation in the hippocampus

Zinc was added to the perfusate of rat hippocampal slices to determine the effect on the potentiation of evoked responses in subfield CA3 following paired-pulse stimulation of mossy fibres. Paired-pulse potentiation across a range of interpulse intervals and currents was significantly depressed by 50 and 500 microM zinc chloride. Additionally but only at the higher concentration, the amplitude of the first evoked potential was significantly increased. These results provide further evidence of a role for zinc in hippocampal neurotransmission.

Adenosine increases synaptic facilitation in the in vitro rat hippocampus: evidence for a presynaptic site of action

The effect of adenosine on paired synaptic responses was characterized in the CA1 region of the rat hippocampus in vitro. Adenosine increased the degree of synaptic facilitation at a 40 ms conditioning-testing interval under all conditions tested. Even when the stimulation intensity was increased so as to counteract the direct depressant effect of adenosine on synaptic transmission, its effect on facilitation was maintained. The ability of adenosine to increase synaptic facilitation was a complex function of several variables. The effect was enhanced by increasing the calcium concentration of the medium, and was most pronounced at short conditioning-testing intervals and at low response amplitudes. Adenosine was particularly efficacious in blocking the depression of synaptic responses observed in high-calcium medium at short conditioning-testing intervals. Because this depression most probably reflects depletion of the available store of releasable transmitter, one mechanism by which adenosine could reverse this effect would be by blocking the depletion of transmitter. These results suggest that adenosine diminishes transmitter release via an action at the presynaptic terminal. The reduction in the release of neurotransmitter, particularly at excitatory synapses, may be responsible for the depressant effects of adenosine upon the central nervous system.

Facilitation of hippocampal long-lasting potentiation by GABA antagonists

Long-lasting potentiation (LLP) of synaptic transmission in the CAI region of the hippocampal slice preparation has been examined. The effects of reduced postsynaptic inhibition given by application of gamma-aminobutyric acid (GABA) antagonists (mainly picrotoxin) on the generation of LLP were investigated. It was first demonstrated that picrotoxin had little effect on excitatory synaptic transmission itself as judged by the rising phase of the field EPSP. Moreover, there were largely no actions on short-lasting synaptic effects such as paired pulse facilitation and frequency potentiation. On the other hand, following drug application, much fewer afferent volleys were needed to generate a given amount of LLP. Long-lasting potentiation could be produced by trains containing as few as 2-5 impulses, trains that normally give rise to only short-lasting effects. There was no apparent difference in the maximal amount of LLP that could be produced for a given input, suggesting that the GABA antagonists do not operate by enhancing the capacity for LLP production but by facilitating its induction. As in normal solution, the LLP in the presence of the drugs was confined to the tetanized pathway. Tetanization in the treated slices was associated with enhanced somatic firing as well as an increase of the negative extracellular potential recorded in the dendritic layer. It is proposed that part of this increased negativity represents current through synaptically opened N-methyl-D-aspartate (NMDA) receptor channels. Furthermore, it is suggested that the facilitated induction of LLP in the presence of GABA antagonists is related to a facilitated activation of these NMDA receptor channels which is secondary to the higher levels of dendritic depolarization attained during tetanization under conditions of reduced postsynaptic inhibition.

Responses of opossum and rat hippocampal CA1 cells to paired stimulus volleys

Short-term synaptic plasticity was studied in the in vitro hippocampus of the North American opossum (Didelphis virginiana) and rat (Rattus norvegicus). Conditioning and test stimulus pulses were delivered to fibers in stratum radiatum, and intracellular and extracellular recordings were obtained from area CA1 pyramidal cells. In rat, the amplitude of the population spike in response to the second (test) of two stimulus impulses is suppressed at short inter-pulse-intervals (IPI's). In opossum, the amplitude of the test population spike is facilitated at comparable IPI's. Facilitation of the test population spike in rat occurs only when the test stimulus is separated from the first stimulus (conditioning) by a longer IPI. Peak values of facilitation do not significantly differ between species. Intracellular responses, elicited by stimulus pulses that were subthreshold for spike production, indicate that the amplitude of test EPSP's recorded from opossum pyramidal cells are facilitated at IPI's that result in suppression of test EPSP's in rat pyramidal cells.

Perforant pathway-evoked long-term potentiation of CA1 neurons in the hippocampal slice preparation

Previously, we have presented electrophysiological evidence reaffirming the existence of a controversial hippocampal pathway. These fibers are part of the perforant pathway and terminate directly on the CA1 cells. We now report that, in the hippocampal slice preparation, tetanic stimulation of the perforant pathway produces long-term potentiation (LTP) of CA1 cell responses. LTP of population spikes varied from 150% to 500%. The results were of interest because these axons synapse at distal sites on the apical dendrite. This location is usually thought to be a difficult site to evoke action potentials.

A study of spontaneous and evoked activity in the rat hippocampus under helium-oxygen high pressure

High pressures affect the physiology of the central nervous system. For a better understanding of this effect, we examined the hippocampal activity in the rat under high pressures (91 bars) of helium-oxygen. Effects of high pressure on hippocampal physiology are: an abnormally sustained 5-8 Hz pattern of spontaneous activity, followed, in some cases, by seizures; a marked decrease in the responses of CA1 pyramidal cells to stimulation of their commissural afferents; and a 50% decrease in the afterdischarge threshold. On the basis of the relatively well understood hippocampal physiology in normobaric conditions, our observations suggest that high pressures induce hypoexcitability of afferents and/or target cells.

Chronically elevating plasma Mg2+ improves hippocampal frequency potentiation and reversal learning in aged and young rats

One prediction of the hypothesis that the capacity for hippocampal frequency potentiation is relevant to behavioral plasticity is tested in this study, by chronically elevating magnesium in intact aged and young rats. Elevated extracellular Mg2+ specifically improves frequency potentiation in hippocampal slices, and chronic alterations in plasma Mg2+ can increase brain Mg2+ in intact animals. Aged and young rats on a diet that elevated plasma Mg2+ exhibited stronger frequency potentiation under urethane anesthesia, and showed improved maze reversal learning.

Modulation by dopamine of population spikes in area CA1 hippocampal neurons elicited by paired stimulus pulses

Extracellular recording techniques were used to study the effects of dopamine on postactivation excitability of rat area CA1 hippocampal neurons maintained in vitro. Population spikes were elicited by delivery of conditioning and test stimulus pulses to afferent fibers. The interval between the conditioning and test volley was set to separate delivery of stimuli by 10 to 80 msec. The effect of superfusion or microtopical application of dopamine (DA) on population responses to test stimulus pulses was studied. When paired stimulus volleys, separated by brief intervals (up to 40 msec), were delivered to afferent fibers, paired-pulse suppression (PPS) was indicated by the amplitude of the population spike elicited by the test volley being smaller than that elicited by the conditioning volley. When paired volleys were separated by longer intervals (40 to 80 msec), the response elicited by the test volley was larger in amplitude than that elicited by the conditioning volley, indicating paired-pulse facilitation (PPF). Following exposure to DA, the amplitude of the population response elicited by the conditioning volley was larger than the amplitude before exposure to DA. This effect was long-lasting, enduring for tens of minutes. However, when the amplitude of the conditioning population response was held constant, the PPS was decreased, indicating disinhibition. It is suggested that dopamine produces a long-lasting attenuation of an intervening inhibitory influence onto CA1 pyramidal neurons.

Hippocampal field potential: a microcomputer aided comparison of amplitude and integral

A microcomputer based system is described for the analysis of various aspects of hippocampal field potential response recorded in vitro, either in the dendritic or somatic region. A comparison is made between the average amplitude and the time integral of synaptically evoked population spike at the cell body layer, following various intensities of stimuli delivered to the stratum radiatum. There was a good correlation between the two measures at low and intermediate intensities of stimulation. With stimulus intensities beyond those eliciting the population spike of maximal integral, the amplitude of the population spike further increased while its duration decreased. It is concluded that the integral provides a more accurate estimate of the extent of neuronal firing than the amplitude.

Ephaptic interactions contribute to paired pulse and frequency potentiation of hippocampal field potentials

The contribution of ephaptic interactions to potentiation of the hippocampal CA1 extracellular population spike during paired pulse or frequency stimulation of stratum radiatum (SR) inputs was investigated using the in vitro hippocampal slice preparation. Records of the transmembrane potential revealed a depolarizing wave with an amplitude and latency that varied directly with that of the extracellular population spike. Paired pulse or repetitive stimulation of SR resulted in a potentiation of the population spike amplitude and a corresponding increase in the amplitude of the TMP depolarizing wave. Action potentials generated during the stimulus train consistently arose from the peak of the depolarizing wave. It is proposed that ephaptic interactions contribute to potentiation of the extracellular population spike through recruitment of subthreshold neurons within the population during repetitive afferent stimulation.

Chronic failure of inhibition of the CA1 area of the hippocampus following kainic acid lesions of the CA3/4 area

The chronic effects of the lesioning agent, kainic acid, on paired pulse inhibition in the CA1 area were investigated in the hippocampus both in vivo and in vitro. Pretreatment of animals with a unilateral intracerebroventricular (i.c.v.) injection of kainic acid resulted in a lesion of the CA3/4 area of the hippocampus ipsilateral to the injection site. On activating the surviving Schaffer collateral afferents in the contralateral hippocampus, normal paired-pulse inhibition of the extracellularly recorded population spike in CA1 was observed. On activating the surviving commissural afferents to the CA1 area ipsilateral to the lesion, no such inhibition could be observed. However, paired-pulse inhibition was recorded in the dentate gyrus ipsilateral to the lesion in response to stimulation of the perforant path. The chronic failure of inhibition following the unilateral i.c.v. injection of kainic acid further supports the use of this method to provide a chronic model in the rat for the study of epileptogenesis in the hippocampus.

Presynaptic involvement in frequency facilitation in the hippocampal slice

Orthodromic stimulation with frequencies from 1 to 30 Hz causes facilitation of extracellularly recorded population spikes and a decrease of extracellular Ca2+ concentration ( [Ca2+]o ) monitored with ion selective microelectrodes at the cell body layer of area CA1 of the hippocampal slice. Reducing the Ca2+ content of the perfusion medium impairs synaptic transmission. However, even under these conditions, weak stimulation evokes a significant decrease of [Ca2+]o, which is ascribed to a presynaptic Ca2+ entry. Stimulation with high intensities and/or frequencies initially induces an accelerated decrease in [Ca2+]o followed by the re-establishment of synaptic transmission, indicating a contribution of the presynaptic site to frequency facilitation.

Amnesia-producing drugs affect hippocampal frequency potentiation

The effects of intravenous injections of lorazepam, scopolamine and propranolol upon hippocampal potentiation produced by commissural stimulation have been investigated in rats anaesthetized with urethane. Administration of 250 micrograms/kg or 500 micrograms/kg lorazepam significantly delayed the onset of secondary potentiation (frequency potentiation) of the population spikes recorded in subfields CA1 and CA3 of the dorsal hippocampus. Scopolamine also delayed the onset of frequency potentiation in CA1, but only at high dose (10 mg/kg). No other measured parameters of frequency potentiation, paired-pulse potentiation or post-tetanic potentiation were affected by any of the drugs. Lorazepam (greater than or equal to 250 micrograms/kg) and propranolol (3 mg/kg) reduced the severity of hippocampal after-discharge. Rhythmic entrainment of after-discharges was occasionally observed. The results are discussed in relation to the possible link between hippocampal potentiation and memory processes.

Effects of acidic amino acid antagonists on paired-pulse potentiation at the lateral perforant path

The glutamate analogue 2-amino-4-phosphonobutyric acid (APB) has been shown to selectively reduce synaptic transmission along the lateral portion of the perforant path input to the dentate gyrus. APB is studied here with respect to effects on paired-pulse potentiation (PPP) along the perforant path. Application of APB causes a reduction in lateral perforant path responses, but also an increase in the %PPP of that response. The effect does not result simply from reducing response size, because the amount of potentiation of matched first responses increases, and also because APB reduces the potentiated response proportionately less than a comparable first response. A similar effect is seen by decreasing extracellular calcium. Reducing lateral perforant path responses with kynurenic acid, which apparently acts on postsynaptic sites, does not have a similar effect on PPP. These results may indicate a presynaptic action of APB, possibly mediated via an effect on presynaptic calcium availability.

Evidence for late development of inhibition in area CA1 of the rat hippocampus

Development of recurrent inhibition in area CA1 of the rat hippocampus was monitored by a paired-pulse stimulation test. Inhibition of the second response was never observed before postnatal day six. In addition, a type of epileptiform activity, referred to as spontaneous unison firing, was observed frequently, only on postnatal days 4 and 5. Together these observations suggest that the inhibitory interneuronal network of area CA1 becomes effective on about postnatal day 6.

The action of piracetam on the electrical activity of the hippocampal slice preparation: a field potential analysis

The action of various doses of piracetam on the electrical responses of in vitro hippocampal slices from the rat was investigated. Piracetam increased dose dependently the amplitude of the population spike response of pyramidal neurons evoked by stimulation of the stratum radiatum. Piracetam began to affect the population spike at a dose of 100 microM and consistently increased it at a dose of 1 mM. The drug had a rapid onset of action and recovery was seen within a few minutes following its removal. The response of the pyramidal neurons to antidromic stimulation was not affected by piracetam at concentrations up to 50 mM. Also, neither the amplitude nor the slope of the dendritic response was changed by the drug.l Piracetam also did not affect either posttetanic or long-term potentiation of synaptic potentials. These findings suggest that the drug does not act through depolarization of the pyramidal cells or potentiation of the synaptic processes located on their dendrites. The action of piracetam on the feedforward- and feedback-mediated inhibition of pyramidal cells by basket cells was investigated in several series of experiments. At concentrations of 1 and 10 mM, piracetam did not change either of the two types of inhibitory mechanisms. Several hypothetical sites of action of piracetam are discussed.

Enhancement of synaptic transmission by 4-aminopyridine in hippocampal slices of the rat

1. The effects of 4-aminopyridine (4-AP) on neurotransmission in hippocampal slices of the rat were studied in vitro. 2. Extracellular recordings of field potentials and intracellular recordings of excitatory and inhibitory post-synaptic potentials, action potentials and TTX-resistant spikes were obtained from the somatic and dendritic fields of CA 1 hippocampal neurones. 3. Constant perfusion with, or bolus injection of 4-AP in micromolar concentrations resulted in substantial and persistent enhancement of the amplitude of synaptic field potentials and population spikes with no alteration of the afferent input. Somatic positive waves (p-waves) were increased with local micro-application of 4-AP. 4. Intracellular recordings revealed an increase in the amplitude of e.p.s.p.s. and i.p.s.p.s. I.p.s.p.s were prolonged by 4-AP. 5. The pyramidal cells were, after an initial hyperpolarization, depolarized by up to 10 mV during exposure to 4-AP; the membrane conductance was not consistently changed. 6. Spontaneous shifts of the membrane potential were described as giant e.p.s.p.s and i.p.s.p.s. Spontaneous i.p.s.p.s were increased in frequency and amplitude. 7. Paired-pulse facilitation of e.p.s.p.s was reduced by 4-AP suggesting a presynaptic modulation of transmitter output. 8. It is concluded that 4-AP enhances transmitter release at both excitatory and inhibitory synapses in the hippocampus.

Long-term potentiation of excitatory synaptic transmission in the rat hippocampus: the role of inhibitory processes

1. The possibility that changes in inhibitory processes are responsible for long-term potentiation (l.t.p.) was examined using the rat hippocampal slice preparation.2. Inhibitory pathways were characterized using both extra- and intracellular recordings from the CA1 pyramidal cell layer. Stimulating electrodes were placed in either stratum radiatum or the alveus to allow orthodromic or antidromic activation of the pyramidal cells.3. Using extracellular recordings, inhibition was studied by applying paired pulses at interstimulus intervals of 20-500 msec through either the same or different stimulating electrodes, and quantifying the reduction in the population spike. An antidromic conditioning pulse was least effective in influencing the test response, while paired stimuli delivered through separate stimulators in stratum radiatum revealed the longest duration effects. Inhibition was either reduced or enhanced, depending upon the stimulation paradigm, with increasing stimulus intensity.4. With l.t.p., alterations in paired-pulse inhibition were observed corresponding to the changes in conditioning pulse amplitude. Reducing stimulus intensity to restore the initial conditioning pulse amplitude eliminated these effects.5. Using intracellular recordings, the effects of l.t.p. on inhibition were studied by examining changes in e.p.s.p.-i.p.s.p. sequences, i.p.s.p.s evoked by antidromic stimulation, and spontaneous depolarizing i.p.s.p.s observed with KCl-filled electrodes.6. Following l.t.p. enhanced e.p.s.p.s and slightly reduced, but prolonged, i.p.s.p.s were observed in response to orthodromic stimulation. Antidromically evoked, as well as spontaneous, i.p.s.p.s were unaffected.7 It is concluded that alterations in inhibitory processes are not responsible for l.t.p. in hippocampal subfield CA1. However, changes in the strength of inhibitory synapses as a consequence of long-term potentiation may modify the functional character of the hippocampal connexions.