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

Modulation of excitatory synaptic transmission by adenosine released from single hippocampal pyramidal neurons

Adenosine is a potent neuromodulator in the CNS, but the mechanisms that regulate adenosine concentrations in the extracellular space remain unclear. The present study demonstrates that increasing the intracellular concentration of adenosine in a single hippocampal CA1 pyramidal neuron selectively inhibits the excitatory postsynaptic potentials in that cell. Loading neurons with high concentrations of adenosine via the whole-cell patch-clamp technique did not affect the GABAA-mediated inhibitory postsynaptic potentials, the membrane resistance, or the holding current, whereas it significantly increased the adenosine receptor-mediated depression of excitatory postsynaptic currents. The effects of adenosine could not be mimicked by an agonist at the intracellular adenosine P-site, but the effects could be antagonized by a charged adenosine receptor antagonist and by adenosine deaminase, demonstrating that the effect was mediated via adenosine acting at extracellular adenosine receptors. The effect of adenosine loading was not blocked by BaCl2 and therefore was not caused by an adenosine-activated postsynaptic potassium conductance. Adenosine loading increased the paired-pulse facilitation ratio, demonstrating that the effect was mediated by presynaptic adenosine receptors. Finally, simultaneous extracellular field recordings demonstrated that the increase in extracellular adenosine was confined to excitatory synaptic inputs to the loaded cell. These data demonstrate that elevating the intracellular concentration of adenosine in a single CA1 pyramidal neuron induces the release of adenosine into the extracellular space in such a way that it selectively inhibits the excitatory inputs to that cell, and the data support the general conclusion that adenosine is a retrograde messenger used by pyramidal neurons to regulate their excitatory input.

Calcium-dependent paired-pulse facilitation of miniature EPSC frequency accompanies depression of EPSCs at hippocampal synapses in culture

Two forms of evoked neurotransmitter release at excitatory synapses between cultured hippocampal neurons have been described. After an action potential, it has been shown that transmitter initially is released synchronously, and this is followed by a period of "slow" asynchronous release. The "fast" synchronous component of release at these synapses has been found routinely to demonstrate paired-pulse and tetanic depression, whereas the short-term plasticity of asynchronous release has not been investigated. In the present experiments, we have used the whole-cell patch-clamp technique to record from pairs of neurons in a low-density hippocampal culture preparation to determine both the properties and underlying mechanisms of short-term plasticity of asynchronous release. It was found that an increase in miniature EPSC (mEPSC) frequency accompanied both single and multiple stimuli, and this mEPSC increase was facilitated during paired stimuli, even when the evoked synchronous release was depressed. In addition, both the activity-dependent depression of evoked EPSCs and facilitation of asynchronous mEPSC release were dependent on Ca accumulation in the nerve terminal. However, the Ca-dependent mechanisms underlying these two processes could be distinguished by the differential effects of two membrane-permeant calcium chelators, BAPTA-AM and EGTA-AM. Frequency-dependent depression of evoked EPSCs involves a rapid rise in intraterminal Ca, which likely triggers a process that proceeds in a Ca-independent manner, whereas the asynchronous release may be linked more directly to a sustained increase in intraterminal Ca.

Regulation of EPSPs by the synaptic activation of GABAB autoreceptors in rat hippocampus

1. Intracellular recording was used to study the influence of GABAB autoreceptor-mediated regulation of monosynaptic GABAA and GABAB receptor-mediated hyperpolarizing inhibitory postsynaptic potentials (IPSPAs and IPSPBs, respectively) on alpha-amino-3-hydroxy-5-methyl -4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic potentials (EPSPAs and EPSPNs, respectively) in the CA1 region of rat hippocampal slices. To achieve this, synaptic potential were evoked monosynaptically by near stimulation following blockade of either EPSPNs, by the NMDA receptor antagonist (R)-2-amino-5-phosphonopentanoate (AP5; 0.05 mM), or EPSPAs, by the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 0.01 mM). 2. Paired-pulse stimulation at 3-50 Hz caused an increase in the duration (paired-pulse widening) of EPSPAs, which paralleled the time course of paired-pulse depression of monosynaptic IPSCs, and a potentiation of the amplitude (paired-pulse potentiation) of EPSPAs, which did not. Paired-pulse stimulation also caused frequency-dependent changes in EPSPNs. At frequencies > 40 Hz it produced paired-pulse depression of EPSPNs, along with marked summation of IPSPS, and at frequencies < 40 Hz it caused paired-pulsed enlargement of EPSPNs, concomitant with a reduction in IPSPS. 3. Paired-pulse potentiation of EPSPAs at 50 Hz was enhanced by picrotoxin (0.1 mM) but was not significantly affected by 3-amino-propyl(diethoxymethyl)phosphinic acid (CGP 35348; 1 mM). Paired-pulse depression of EPSPNs at 50 Hz was converted to paired-pulse enlargement by picrotoxin but was unaffected by CGP 35348. These effects can be explained by block of IPSPAs by picrotoxin. 4. Paired-pulsed widening of EPSPAs at 5 Hz was occluded by picrotoxin and abolished by CGP 35348. Similarly, paired-pulsed enlargement of EPSPNs at 5 Hz was occluded, and in some cases converted to paired-pulse depression, by picrotoxin. The effects of CGP 35348 were more complex in that this antagonist reduced paired-pulse enlargement of EPSPNs in control medium whereas it eliminated paired-pulsed depression of EPSPNs in the presence of picrotoxin, effects consistent with its block of GABAB autoreceptors and IPSPBS, respectively. 5. 'Priming' using a 'priming stimulation protocol' (a single 'priming stimulus' followed at 1-50 Hz ('priming frequency') by a 'primed burst' of four shocks at 20-100 Hz ('burst frequency')) caused an increase in both 'primed' EPSPAs and EPSPNs compared with 'unprimed' EPSPAs and EPSPNs. This effect was optimal when the respective priming and burst frequencies were 5 and 100 Hz. 6. In the presence of either picrotoxin or CGP 35348 the primed EPSPAs and EPSPNs resembled unprimed EPSPAs and EPSPNs, respectively. This was because picrotoxin occluded whereas CGP 35348 blocked the effect of priming on EPSPS. 7. CGP 35348 had only modest effects on EPSPAs but enhanced EPSPNs evoked by a tetanus (20 stimuli at 100 Hz), in either the presence or absence of picrotoxin. In the absence of picrotoxin, CGP 35348 also promoted depolarization by enhancing a depolarizing GABAA receptor-mediated component (IPSPD). These effects can all be attributed to block of IPSPBS by CGP 35348. 8. CGP 35348 blocked the induction of long-term potentiation (LTP) of extracellularly recorded field EPSPs elicited by a priming stimulation protocol in control medium but was ineffective in the presence of picrotoxin. CGP 35348 was also ineffective at preventing tetanus-induced LTP (100 Hz, 1 s) in both the absence and presence of picrotoxin. 9. These data demonstrate the complex regulation of AMPA and NMDA receptor-mediated EPSPs during various patterns of synaptic activation caused by the dynamic changes in GABA-mediated synaptic inhibition, which are orchestrated by GABAA autoreceptors in a frequency-dependent

Presynaptic glutamatergic function in dentate gyrus in vivo is diminished by chronic exposure to inorganic lead

Reductions in membrane Ca2+ channel currents and depolarization-evoked neurotransmitter release have been repeatedly observed as a result of acute exposure to Pb2+. This study was performed to determine whether hippocampal glutamate and GABA release are impaired in intact animals chronically exposed to lead (Pb). As paired-pulse facilitation in the hippocampus is primarily mediated by an enhancement of glutamate release, this neurophysiological measure was also assessed in the dentate gyrus of Pb-exposed animals. Pregnant dams received 0.2% Pb acetate in the drinking water at parturition, and male offspring were weaned to the same solution as that given their dams. Control animals were maintained on distilled water. As adults, animals had intracerebral dialysis probes inserted through guide cannulae implanted 2-4 days previously and the hippocampal CAI-dentate area was perfused with modified Ringer's solution. Transmitter release was induced by perfusion with 150 mM K+ with half the animals in each group tested with Ca2+ present in the perfusate (total release) and the other half with Ca2+ absent (Ca(2+)-independent release). K(+)-stimulated total glutamate release was reduced in Pb-exposed animals relative to controls. No group differences were observed under Ca(2+)-free conditions, indicating that Ca(2+)-dependent glutamate release was decreased in exposed rats. In contrast no group differences in K(+)-stimulated total GABA release were evident, whereas an augmentation in GABA release under Ca(2+)-free conditions was revealed in Pb-exposed animals. The effects of exposure on the Ca(2+)-dependent components of release are consistent with in vitro evidence indicating an inhibitory action of Pb2+ at voltage-sensitive Ca2+ channels. A separate group of animals was prepared under urethane anesthesia with stimulating and recording electrodes placed in the perforant path and dentate gyrus, respectively. Pairs of stimulus pulses were delivered at interpulse intervals (IPI) of 10-250 ms. Pb exposure induced an increase in paired-pulse depression at the 20 ms 1PI and reduced paired-pulse facilitation at the 30 ms IPI. Decreases in paired-pulse facilitation could not be attributed to the reported effects of Pb2+ on N-methyl-D-aspartate (NMDA) receptors as MK-801 (1.0 mg/kg, s.c.) administration produced an opposing pattern of effects on paired-pulse measures. The Pb-induced suppression of paired-pulse facilitation is consistent with exposure-related decreases in total glutamate release. The impact of these effects of Pb exposure on hippocampal glutamatergic transmission may contribute to the reported effects of Pb on other forms of synaptic plasticity including long-term potentiation, a model of learning and memory.

Chronic developmental lead exposure increases the threshold for long-term potentiation in rat dentate gyrus in vivo

Chronic developmental lead (Pb) exposure has been long associated with cognitive dysfunction in children and animals. In an attempt to more directly relate the behavioral observations of impaired cognitive ability to Pb-induced effects on neuronal activity, we utilized the long-term potentiation (LTP) model of neural plasticity to assess synaptic function. Male rats were chronically exposed to 0.2% Pb(2+)-acetate through the drinking water of the pregnant dam, and directly through their own water supply at weaning. As adults, field potentials evoked by perforant path stimulation were recorded in the dentate gyrus under urethane anesthesia. LTP threshold was determined by applying a series of stimulus trains of increasing intensities. Baseline testing of dentate gyrus field potentials indicated that input/output functions, maximal response amplitudes, and threshold currents required to evoke a population spike (PS) did not differ for control and Pb-exposed animals. Despite similarities in baseline synaptic transmission, Pb-exposed animals required a higher train intensity to evoke LTP than controls. With maximal train stimulation, however, control and Pb animals exhibited comparable levels of potentiation. These findings suggest that the mechanisms of LTP induction are preferentially impaired by Pb exposure. Although baseline synaptic transmission was not altered in Pb-exposed animals, decreases in glutamate release following high K+ perfusion and reductions in paired pulse facilitation have been reported in the intact animal. Pb-induced reductions in calcium influx through voltage-sensitive or N-methyl-D-aspartate (NMDA) receptor-dependent channels may mediate increases in LTP threshold. It is possible that the threshold changes in the induction of LTP reported here contribute to cognitive impairments associated with Pb exposure.

Effects of pharmacologically facilitating glutamatergic transmission in the trisynaptic intrahippocampal circuit

The effects of a recently synthesized benzoyl-piperidine drug that enhances currents mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors were tested on monosynaptic and polysynaptic responses in hippocampal slices of the rat. Stimulation of perforant path inputs to the dentate gyrus evoked extracellular responses in field CA1 that had latencies and laminar profiles indicating that they were relayed through the trisynaptic intrahippocampal circuit. Under control conditions, trisynaptic field excitatory postsynaptic potentials did not show larger paired-pulse facilitation than monosynaptic responses and failed to exhibit frequency facilitation. Low concentrations of picrotoxin greatly enhanced trisynaptic responses and, under these conditions, frequency facilitation was obtained. Benzoyl-piperidine-12 (250 microM) had a three-fold greater effect on the amplitude of trisynaptic responses than on monosynaptic field excitatory postsynaptic potentials, indicating that the drug's effect is amplified across the successive stages of a polysynaptic circuit. The AMPA receptor modulator did not change the frequency characteristics of monosynaptic potentials and had only a modest influence on those of the trisynaptic response. The effect of benzoyl-piperidine-12 on trisynaptic responses was significantly greater when GABAergic inhibition was partially blocked with picrotoxin; the GABA blocker did not alter the effects of benzoyl-piperidine-12 on monosynaptic responses. These results indicate that centrally active AMPA receptor modulators are likely to have a greater influence on brain operations involving long chains of connections than on those mediated by simple reflex-like circuits, and will vary markedly in their effects depending upon the excitability of local interneurons.

Modulation of excitatory synaptic transmission by adenosine released from single hippocampal pyramidal neurons

Adenosine is a potent neuromodulator in the CNS, but the mechanisms that regulate adenosine concentrations in the extracellular space remain unclear. The present study demonstrates that increasing the intracellular concentration of adenosine in a single hippocampal CA1 pyramidal neuron selectively inhibits the excitatory postsynaptic potentials in that cell. Loading neurons with high concentrations of adenosine via the whole-cell patch-clamp technique did not affect the GABAA-mediated inhibitory postsynaptic potentials, the membrane resistance, or the holding current, whereas it significantly increased the adenosine receptor-mediated depression of excitatory postsynaptic currents. The effects of adenosine could not be mimicked by an agonist at the intracellular adenosine P-site, but the effects could be antagonized by a charged adenosine receptor antagonist and by adenosine deaminase, demonstrating that the effect was mediated via adenosine acting at extracellular adenosine receptors. The effect of adenosine loading was not blocked by BaCl2 and therefore was not caused by an adenosine-activated postsynaptic potassium conductance. Adenosine loading increased the paired-pulse facilitation ratio, demonstrating that the effect was mediated by presynaptic adenosine receptors. Finally, simultaneous extracellular field recordings demonstrated that the increase in extracellular adenosine was confined to excitatory synaptic inputs to the loaded cell. These data demonstrate that elevating the intracellular concentration of adenosine in a single CA1 pyramidal neuron induces the release of adenosine into the extracellular space in such a way that it selectively inhibits the excitatory inputs to that cell, and the data support the general conclusion that adenosine is a retrograde messenger used by pyramidal neurons to regulate their excitatory input.

Calcium-dependent paired-pulse facilitation of miniature EPSC frequency accompanies depression of EPSCs at hippocampal synapses in culture

Two forms of evoked neurotransmitter release at excitatory synapses between cultured hippocampal neurons have been described. After an action potential, it has been shown that transmitter initially is released synchronously, and this is followed by a period of "slow" asynchronous release. The "fast" synchronous component of release at these synapses has been found routinely to demonstrate paired-pulse and tetanic depression, whereas the short-term plasticity of asynchronous release has not been investigated. In the present experiments, we have used the whole-cell patch-clamp technique to record from pairs of neurons in a low-density hippocampal culture preparation to determine both the properties and underlying mechanisms of short-term plasticity of asynchronous release. It was found that an increase in miniature EPSC (mEPSC) frequency accompanied both single and multiple stimuli, and this mEPSC increase was facilitated during paired stimuli, even when the evoked synchronous release was depressed. In addition, both the activity-dependent depression of evoked EPSCs and facilitation of asynchronous mEPSC release were dependent on Ca accumulation in the nerve terminal. However, the Ca-dependent mechanisms underlying these two processes could be distinguished by the differential effects of two membrane-permeant calcium chelators, BAPTA-AM and EGTA-AM. Frequency-dependent depression of evoked EPSCs involves a rapid rise in intraterminal Ca, which likely triggers a process that proceeds in a Ca-independent manner, whereas the asynchronous release may be linked more directly to a sustained increase in intraterminal Ca.

Interactions between associative synaptic potentials in the piriform cortex of the in vitro isolated guinea pig brain

The interaction between synaptic potentials generated by the activation of separate sets of associative fibres was investigated in the piriform cortex of an in vitro isolated guinea pig brain preparation. Restricted regions of the piriform cortex served by separate contingents of afferent fibres of the lateral olfactory tract were isolated surgically. The activity generated by these patches of cortex in response to afferent stimulation propagates to remote cortical regions along cortico-cortical associative fibres. Current source density (CSD) analysis of field potential laminar profiles evoked by lateral olfactory tract stimulation confirmed that the synaptic sinks induced by distinct associative fibre contingents converge on the apical dendrites of piriform cortex neurons in the superficial lb layer. Pairing between potentials evoked by activation of two separate sets of associative fibres resulted in an almost linear summation when the two responses coincided. For interstimulus intervals of <100 ms, heterosynaptic pairing of independent associative inputs induced a facilitation of the conditioned associative potential, which correlated with an increase in the associative sink located in layer lb, as demonstrated by CSD analysis. The evaluation of the pairing intervals suggests that the heterosynaptic facilitation of the conditioned associative potentials may be due to the summation of local and remote associative synaptic events. It is concluded that separate associative inputs converge on the apical dendrites of piriform cortex pyramidal neurons to generate synaptic potentials through the activation of spatially close but independent synapses. The role of associative synaptic integration in the functional organization of the olfactory cortex is discussed.

Comparative sensitivity to adenosine of paired-pulse inhibition and single field potentials in the rat hippocampus

If excitatory terminals onto inhibitory interneurones were more sensitive to adenosine than excitatory terminals onto pyramidal cells in the hippocampus it might explain the effect of adenosine to decrease paired-pulse inhibition and account for reported excitatory effects of low concentrations of adenosine. We have compared the concentration-response relationships for the effect of adenosine on single evoked field potentials and on paired-pulse inhibition in the CA1 area of the rat hippocampal slice in order to test this hypothesis. Adenosine caused a concentration-dependent decrease in both single evoked population spike size and in paired-pulse inhibition between potentials. The concentration-response relationships for both effects was very similar, ruling out the possibility that excitatory terminals onto inhibitory interneurones are more sensitive to adenosine than excitatory terminals onto pyramidal cells, and suggesting that the receptors located at the two sites may be indistinguishable.

Dentate gyrus field potentials evoked by stimulation of the basolateral amygdaloid nucleus in anesthetized rats

We previously found that long-term potentiation (LTP) in the dentate gyrus (DG) is attenuated by lesion of the basolateral amygdala (BLA), but it remained unclear whether or not there is neural connection between the BLA and the DG. In the present study, we tried to provide physiological evidence that the DG receives neural inputs from the BLA. Single-pulse electrical stimulation of the BLA evoked two distinct components of field potentials in the ipsilateral DG: the P1 component with 26-ms peak latency was elicited by lower intensity of BLA stimulation, whereas the P2 component with 14-ms peak latency was elicited at higher stimulus intensity. The PI response (1) was evoked when the stimulating electrode was positioned within the BLA, (2) showed the laminar profile similar to the perforant path (PP)-evoked response and (3) exhibited strong paired-pulse facilitation. On the other hand, the P2 wave (1) was evoked even with the stimulating electrode outside the BLA, (2) did not reverse its polarity at any location in the DG and (3) showed only slight facilitation by paired-pulse stimulation. These data indicate that the P1 component represents synaptic responses of DG granule cells to neural inputs from the BLA. Furthermore, the BLA-evoked DG field potentials were not affected by local injection of tetracaine into the PP and displayed LTP independently of the PP-evoked response, suggesting that neural inputs from the BLA are not mediated by the PP.

Paired pulse facilitation of GABAergic IPSCs in ventral horn neurons in neonatal rat spinal cord

Whole-cell patch-clamp recording of GABAergic inhibitory postsynaptic currents (IPSCs) were made in ventral horn neurons of neonatal rat lumbar spinal cord in slice. In contrast to the hippocampus where paired pulse depression is reported to be observed for GABAergic IPSCs, double pulse stimulation of GABAergic inputs resulted in enhancement in the amplitude of the second IPSC in the spinal ventral horn. The facilitation ratio was decreased during enhanced synaptic transmission by increasing Ca2+ concentration in the external recording solution. Baclofen and adenosine. which are reported to depress synaptic transmission by presynaptic mechanisms, depressed IPSCs and increased the facilitation ratio. A postsynaptic manipulation such as application of bicuculline or changing the driving force did not affect the facilitation ratio. These results suggest that paired pulse facilitation of GABAergic IPSCs observed in neonatal rat spinal ventral horn appears to be based upon a mechanism similar to that underlying frequency-dependent facilitation of excitatory synaptic transmission, and is sensitive to presynaptic changes in synaptic strength.

Paired-pulse facilitation and depression at unitary synapses in rat hippocampus: quantal fluctuation affects subsequent release

1. Excitatory synaptic transmission between pairs of monosynaptically coupled pyramidal cells was examined in rat hippocampal slice cultures. Action potentials were elicited in single CA3 pyramidal cells impaled with microelectrodes and unitary excitatory postsynaptic currents (EPSCs) were recorded in whole-cell voltage-clamped CA1 or CA3 cells. 2. The amplitude of successive unitary EPSCs in response to single action potentials varied. The amplitude of EPSCs was altered by adenosine or changes in the [Mg2+]/[CA2+] ratio. We conclude that single action potentials triggered the release of multiple quanta of glutamate. 3. When two action potentials were elicited in the presynaptic cell, the amplitude of the second EPSC was inversely related to the amplitude of the first. Paired-pulse facilitation (PPF) was observed when the first EPSC was small, i.e. the second EPSC was larger than the first, whereas paired-pulse depression (PPD) was observed when the first EPSC was large. 4. The number of trials displaying PPD was greater when release probability was increased, and smaller when release probability was decreased. 5. PPD was not postsynaptically mediated because it was unaffected by decreasing ionic flux with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or receptor desensitization with aniracetam. 6. PPF was maximal at an interstimulus interval of 70 ms and recovered within 500 ms. Recovery from PPD occurred within 5 s. 7. We propose that multiple release sites are formed by the axon of a CA3 pyramidal cell and a single postsynaptic CA1 or CA3 cell. PPF is observed if the first action potential fails to release transmitter at most release sites. PPD is observed if the first action potential successfully triggers release at most release sites. 8. Our observations of PPF are consistent with the residual calcium hypothesis. We conclude that PPD results from a decrease in quantal content, perhaps due to short-term depletion of readily releasable vesicles.

Differences in the refractory properties of two distinct inhibitory circuitries in field CA1 of the hippocampus

Extracellular reflections of IPSPs were examined in two distinct circuitries in field CA1 of the hippocampus. Stimulation in the stratum radiatum in the presence of AMPA receptor antagonists elicited positive potentials in the same stratum that were eliminated by picrotoxin, a blocker of GABAA receptors. Laminar profile analysis revealed that the response was maximal in the stratum radiatum at a point well distal to the pyramidal cell body layer and had a negative reflection in the stratum oriens. These field IPSPs presumably mediate the feedforward inhibition normally activated by the Schaffer-commissural projections to field CA1. Stimulation of the alveus produced an antidromic response followed by a much slower positive potential in recordings collected in the pyramidal cell layer. The latter response was suppressed by AMPA receptor antagonists or picrotoxin, as expected for disynaptic, recurrent (feedback) inhibition. The laminar profile for the feedback field IPSPs had its maximum near the pyramidal cell layer and its negative dipole in the stratum radiatum. Feedforward IPSPs were inhibited by about 50% if they were preceded within 200 ms by a priming pulse while feedback IPSPs were reduced by less than 20% under comparable conditions. The refractory effect was minimally dependent on stimulation intensity but was strongly affected by an antagonist of GABAB receptors. Attempts to modify IPSPs in the s. radiatum with long trains of low frequency stimulation or with theta-burst stimulation were not successful, suggesting that GABAergic synapses do not have the plasticities found in their glutamatergic counterparts. These results indicate that interneurons contacted by the extrinsic afferents of hippocampus form GABAergic synapses that differ in terms of spatial location and functional properties from the synapses generated by interneurons innervated by the recurrent collaterals of the pyramidal cells. The findings also suggest that repetitive afferent activity, while reducing the influence of dendritic IPSPs on excitatory input, will leave feedback suppression of cell spiking largely intact.

Interaction of the beta-carboline harmaline with a GABA-benzodiazepine mechanism: an electrophysiological investigation on rat hippocampal slices

An interaction of harmaline (HA), a beta-carboline, with benzodiazepine (Bzd) receptors, has been reported. HA perfusion induced a similar, although less potent, depressing effect as clonazepam (CLO) on the amplitude of the population spikes (PS) evoked by Schaffer collateral stimulation in the CA1 area of rat hippocampal slices. The suppressant effect of both CLO and HA on PS amplitude was reversed by simultaneous perfusion of the GABA antagonist picrotoxin. These results suggest that HA acts as a weak or partial agonist at Bzd receptors.

Temperature change-induced potentiation: a comparative study of facilitatory mechanisms in aged and young rat hippocampal slices

The effect of temperature changes in the medium on the evoked potentials of pyramidal neurons in response to the electrical stimulation of Schaffer collaterals was investigated in both young (one to two months) and aged (24-28 months) rat hippocampal slices. Temperature increase was shown to reduce, and subsequent temperature decrease to enhance, the population spike amplitude in both young and aged rats. Temperature decrease produced a long-lasting (> 30 min) and important potentiation (180%) of population spike amplitude in young but not in aged rats. Long-term post-tetanic potentiation was maintained in young but suppressed in aged rats by temperature changes. The impairment of temperature-induced potentiation in aged rats is suggested as a tool for studying promnesic drugs.

Synaptic transmission and paired-pulse behaviour of CA1 pyramidal cells in hippocampal slices from a hibernator at low temperature: importance of ionic environment

To investigate the effects of ionic changes possibly associated with hibernation, hippocampal slices prepared from golden hamsters were studied in artificial cerebrospinal fluid (ACSF) of variable composition (K+ 3-5 mM, Ca2+ 2-4 mM, Mg2+ 2-4 mM, pH 7.0-7.7) at temperatures of 15-20 degrees C, just above the temperature below which synaptic transmission is blocked. Population action potentials (population spikes, PSs) of CA1 pyramidal cells were evoked by stimulation of the Schaffer collaterals/commissural fibers with paired pulses (interpulse interval 50 ms, interval between pairs 30 s). The responses evoked at given temperatures were investigated as a function of extracellular ion concentrations. In ACSF containing 3 mM K+, 2 mM Ca2+ and 2 mM Mg2+, PSs could be evoked at temperatures of > approximately 16 degrees C whereas at lower temperatures synaptic transmission was blocked. The threshold temperature was slightly higher for the first (PS1) than for the second PS (PS2) evoked by paired-pulse stimulation. The slices displayed paired-pulse facilitation (PPF) at all temperatures. Elevation of [K+]o from 3 to 5 mM depressed the amplitudes of both PS1 and PS2, with a stronger effect on PS2. PPF was reduced and, at near-threshold temperatures, turned into paired-pulse depression (PPD). Elevation of [Ca2+]o from 2 to 4 mM increased the amplitude of PS1. The amplitude of PS2, in contrast, was reduced at near-threshold temperatures. PPF turned into PPD. Elevation of [Mg2+]o from 2 to 4 mM reduced the amplitudes of both PS1 and PS2, with a stronger effect on PS1. Accordingly, PPF was increased. Acidification by 0.3 pH units strongly depressed the amplitudes of PS1 as well as PS2 and increased PPF. Alkalization by 0.4 pH units had only weak effects in the opposite direction. Changes in the ionic composition comparable to those investigated in the present study presumably occur in the brain interstitium of hamsters during entrance into hibernation. According to our results, such changes depress synaptic transmission at low temperatures in the hamster hippocampus in vitro. This modulation may be important for the regulation of neuronal activity during entrance into hibernation.

Cannabinoids selectively decrease paired-pulse facilitation of perforant path synaptic potentials in the dentate gyrus in vitro

Perforant path synaptic potentials recorded from the outer molecular layer of the dentate gyrus were tested for paired-pulse potentiation and stimulus sensitivity in the presence and absence of the potent cannabinoid receptor ligand, WIN 55,212-2. Extracellular perforant path synaptic potential amplitudes were increased by 51% in 2 mM Ca2+ medium and 60% in 3 mM Ca2+ medium at a conditioning-test (C-T) interval of 10 ms, decreasing to 10-15% facilitation at an 80 ms C-T interval. Exposure to the potent cannabinoid receptor ligand WIN 55,212-2 produced a marked and dose-dependent reduction in the amplitude of the facilitated perforant path synaptic potentials. Maximum paired-pulse facilitation was reduced to 35% and 25% in 2.0 and 5.0 microM WIN 55,212-2 respectively. The effect was selective for potentials facilitated at C-T intervals of 10-60 ms. Input/output (I/O) curves of perforant path field potentials were shifted to the right in a dose-dependent (2.0 and 5.0 microM) manner by WIN 55,212-2. Significant differences in peak amplitudes of perforant path potentials were obtained at all suprathreshold stimulus intensities. A comparison of WIN 55,212-2 (5 microM) with the GABAB receptor agonist baclofen (200 microM) showed that when both drugs were administered independently each produced similar decreases in perforant path paired-pulse potentiation. However when administered together at these concentrations baclofen and WIN failed to potentiate each other, suggesting nonadditivity due to effects on a common process.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation by adenosine of a neuronal inhibitory interaction in the rat hippocampus

Adenosine is acknowledged to have a primarily inhibitory function in the central nervous system, but is believed to have little effect on inhibitory neurones themselves. It is, however, difficult to determine the effect of adenosine on inhibitory synaptic potentials since adenosine directly depresses evoked potentials and, in the presence of bicuculline to block GABAA-mediated inhibition, the bicuculline-resistant fraction of paired-pulse inhibition (ppi) is greater between pairs of small potentials than between pairs of larger potentials. Here, adenosine increased bicuculline-resistant ppi when stimulus strength was constant between adenosine and control but ppi of responses in adenosine was markedly less than ppi of control responses of the same size. Adenosine had less effect on the size of 'conditioned' potentials than on control potentials. It is concluded that adenosine can reduce the bicuculline-resistant fraction of paired-pulse inhibition in the hippocampus. Further quantitative comparison of the effects of adenosine on ppi and on single evoked potentials excluded a difference in the potency of adenosine at excitatory and inhibitory terminals as an explanation for this activity. The results suggest that adenosine may diminish bicuculline-resistant paired-pulse inhibition by enhancing a simultaneous facilitatory component of the neuronal responses.

Temporal information transformed into a spatial code by a neural network with realistic properties

Neurons exhibit a wide range of properties in addition to postsynaptic potential (PSP) summation and spike generation. Although other neuronal properties such as paired-pulse facilitation (PPF) and slow PSPs are well characterized, their role in information processing remains unclear. It is possible that these properties contribute to temporal processing in the range of hundreds of milliseconds, a range relevant to most complex sensory processing. A continuous-time neural network model based on integrate-and-fire elements that incorporate PPF and slow inhibitory postsynaptic potentials (IPSPs) was developed here. The time constants of the PPF and IPSPs were estimated from empirical data and were identical and constant for all elements in the circuit. When these elements were incorporated into a circuit inspired by neocortical connectivity, the network was able to discriminate different temporal patterns. Generalization emerged spontaneously. These results demonstrate that known time-dependent neuronal properties enable a network to transform temporal information into a spatial code in a self-organizing manner--that is, with no need to assume a spectrum of time delays or to custom-design the circuit.

Effect of adenosine on bicuculline-resistant paired-pulse inhibition in the rat hippocampal slice

This study extends previous investigations into the effect of adenosine on bicuculline-resistant paired-pulse inhibition between field potentials evoked 300 ms apart in the CA1 area of the rat hippocampal slice. A direct assessment of the effect of adenosine on paired-pulse inhibition is complicated by the facts that adenosine directly depresses evoked potentials and bicuculline-resistant paired-pulse inhibition is greater between pairs of small potentials than between pairs of larger potentials. Adenosine increased bicuculline-resistant paired-pulse inhibition when stimulus strength was constant between adenosine and control but paired-pulse inhibition of responses in adenosine was markedly less than paired-pulse inhibition of control responses of the same size. Furthermore, adenosine decreased the size of conditioned potentials to a significantly lesser extent than unpaired potentials of the same initial size. Taken together the results indicate that adenosine can decrease bicuculline-resistant paired-pulse inhibition in the hippocampus. A possible mechanism for this effect is that adenosine is suppressing transmission at excitatory terminals onto interneurones which would suggest that these receptors are more sensitive to adenosine than those on the Schaffer collateral/CA1 pyramidal cell synapses. In this case adenosine should reduce paired-pulse inhibition at lower concentrations than are required for depression of single evoked potentials. A comparison of the concentration-response relationships for the effects of adenosine on paired-pulse inhibition and on single evoked potentials ruled out greater sensitivity of adenosine receptors at excitatory terminals onto interneurones as an explanation for adenosine's action on bicuculline-resistant paired-pulse inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term potentiation: evidence against an increase in transmitter release probability in the CA1 region of the hippocampus

It is widely accepted that N-methyl-D-aspartate (NMDA)-receptor-dependent long-term potentiation (LTP) in the CA1 region of the hippocampus is triggered postsynaptically, but there is considerable debate as to the site at which the increase in synaptic strength is expressed. The irreversible open-channel blocking action of the NMDA receptor antagonist MK-801 has been used to test whether the probability of transmitter release (Pr) is increased during LTP. Although the rate of decline of the amplitude of the NMDA receptor-mediated excitatory postsynaptic current (EPSC) in the presence of MK-801 strongly depends on Pr, the rate of decline of the EPSC evoked at synapses expressing LTP is identical to that observed at synapses not expressing LTP. These findings are difficult to reconcile with models in which the expression of LTP is due to an increase in Pr.

A comparison of paired-pulsed facilitation of AMPA and NMDA receptor-mediated excitatory postsynaptic currents in the hippocampus

Paired-pulse facilitation of excitatory synaptic transmission was investigated in the CA1 region of rat hippocampal slices using whole-cell patch-clamp recording. To optimise the measurement of excitatory synaptic transmission, gamma-amino-butyric acid (GABA)-mediated synaptic inhibition was eliminated using both GABAA and GABAB antagonists. Pure alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic currents (EPSCs) were then isolated pharmacologically. Paired-pulse facilitation of either AMPA or NMDA receptor-mediated EPSCs (EPSCA and EPSCN, respectively) was investigated using two stimuli of identical strength delivered at intervals of between 25 and 1000 ms. The paired-pulse facilitation profiles of both EPSCA and EPSCN were similar. Paired-pulse facilitation of EPSCA was independent of holding potential. In contrast paired-pulse facilitation of EPSCN was markedly voltage-dependent; maximum facilitation was recorded at hyperpolarised membrane potentials. At positive membrane potentials there was little or no paired-pulse facilitation and, in most neurones, paired-pulse depression was observed. Voltage-dependence of paired-pulse facilitation of EPSCN was similar in the presence of nominal absence of Mg2+ in the bathing medium, and was unaffected by extensive dialysis of neurones with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). These data are consistent with a presynaptic locus for paired-pulse facilitation of EPSCA. However, paired-pulse facilitation of EPSCN involves postsynaptic factors.

Factors affecting paired-pulse facilitation in hippocampal CA1 neurons in vitro

Factors underlying paired-pulse facilitation (PPF) were studied by intracellular and field recordings of CA1 neurons in the hippocampal slice in vitro, following stimulation of the Schaffer collaterals apical dendritic afferents. Similar magnitudes of PPF were found using the slopes or peaks of the excitatory postsynaptic potentials (EPSPs) recorded intracellularly or extracellularly at the soma or dendrites. The paired-pulse EPSP facilitation index (EPI), defined as the ratio of EPSP slope evoked by the second pulse (E2) to that evoked by the first pulse (E1), had a broad peak at 30-60 ms interpulse interval (IPI). EPI was largest at small E1 and decreased with an E1 increase. Spiking excitability was enhanced after the second as compared to the first pulse as evidenced by (1) a decreased latency to fire and (2) an increased tendency to fire double or multiple spikes. The PPF of spiking resulted partly from an increased E2 and partly from a diminished inhibition evoked by the second pulse. Whether the first pulse elicited a spike or not had no significant effect on the EPI. Multiple spiking evoked by the second pulse was partly blocked by the GABAB antagonist CGP35348 (1 mM). The PPF of the EPSP slopes, however, was not significantly affected by GABAB antagonists, GABAA antagonist bicuculline or NMDA antagonist 2-aminophosphonovalerate. In conclusion, PPF may serve as a means of amplification of synaptic transmission such that reliable spike output may result from a given set of synapses.

Modulation of long-term potentiation in rat hippocampal pyramidal neurons by zinc

The phenomenon of long-term potentiation is frequently promulgated as an example of learning and memory mechanisms at the synaptic level in the mammalian central nervous system. In the CA3 region of the hippocampus there is an abundance of zinc, which is located in presynaptic mossy fibre nerve terminals. Stimulation of these fibres can cause the release of zinc, which interacts with excitatory amino acid receptors and may therefore modulate long-term potentiation. We now demonstrate in CA1 and CA3 neurons that zinc (100-300 microM) enhances non-N-methyl-D-aspartate-receptor-mediated responses whilst reducing excitatory synaptic transmission and inhibiting long-term potentiation. However, by using zinc-chelating agents, endogenously released zinc following high-frequency stimulation in the stratum lucidum does not appear to have any modulatory role in excitatory synaptic transmission and long-term potentiation. These results indicate that an increase in the level of extracellular zinc can limit excitatory synaptic transmission in the CA1 or CA3 region and further suggests that pathologies that can be related to excessive levels of endogenous zinc may have implications for synaptic plasticity in CA3 neurons.

Modulation of GABA-mediated synaptic transmission by endogenous zinc in the immature rat hippocampus in vitro

1. Intracellular recordings from postnatal 2- to 12-day-old (P2-12) rat hippocampal CA3 pyramidal neurones exhibited spontaneous synaptic potentials mediated by GABAA receptors. These potentials can be separated on the basis of amplitude into two classes which are referred to as small and large. 2. The large depolarizing potentials were reversibly inhibited by the Zn2+ chelator 1,2-diethyl-3-hydroxypyridin-4-one (CP94). The small inhibitory postsynaptic potentials. (IPSPs) were apparently unaffected. 3. Stimulation of the mossy fibre pathway evoked composite excitatory postsynaptic potentials (EPSPs) and IPSPs. Threshold stimulus-evoked synaptic potentials were mediated by GABAA receptors and were reversibly blocked by CP94. The responses evoked by suprathreshold stimulation and persisting in the presence of bicuculline or CP94 were partially inhibited by 2-amino-5-phosphonopropionic acid (AP5) and were completely blocked with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 4. L-Histidine, which preferentially forms complexes with Cu2+ > Zn2+ > Fe2+ > Mn2+, inhibited both naturally occurring spontaneous and evoked GABAA-mediated large synaptic potentials without affecting the neuronal resting membrane properties. Exogenously applied Zn2+ induced large spontaneous synaptic potentials and prolonged the duration of the evoked potentials. These effects were reversibly blocked by histidine. 5. The metal chelating agent diethyldithiocarbamate had little effect on the large amplitude synaptic potentials. 6. The transition metal divalent cations Fe2+ and Mn2+ did not initiate large synaptic potentials in CA3 neurones; however, Cu2+ depolarized the membrane and enhanced both excitatory and inhibitory synaptic transmission, resulting in a transient increase in the frequency of the large amplitude events. In comparison, zinc increased the frequency of the large potentials and also induced such events in neurons (P4-21) where innate potentials were absent. The postsynaptic response to ionophoretically applied GABA was either unaffected or slightly enhanced by Zn2+. 7. Under conditions favouring the activation of non-NMDA receptors, excitatory synaptic transmission was unaffected by CP94 but was depressed by Zn2+. Responses to ionophoretically applied glutamate were not inhibited by Zn2+, indicating that Zn2+ affects excitatory synaptic transmission via a presynaptic mechanism. 8. We conclude that the naturally occurring large synaptic potentials in young CA3 neurones are apparently induced by endogenous Zn2+ which can promote or synchronize the release of GABA in the immature hippocampus.

Muscarinic modulation of synaptic transmission in slices of the rat ventral striatum is dependent on the frequency of afferent stimulation

Extracellular, intracellular and tight-seal patch-clamp recordings in ventral striatal slices were used to investigate whether the effectiveness of muscarinic neuromodulation of fast synaptic transmission may be dependent on the frequency of afferent stimulation. In all neurons tested, EPSPs were reversibly attenuated by muscarine or carbachol. This action was completely antagonized by atropine or pirenzepine. Several observations indicated a presynaptic site of action. In extracellular recordings, carbachol reduced the monosynaptic population spike but not the non-synaptic compound action potential. The acetylcholinesterase inhibitors eserine and pyridostigmine also induced an atropine-sensitive reduction of the EPSP. When the rate of afferent stimulation was increased, control EPSPs or EPSCs exhibited a decline in peak amplitude until reaching a steady-state value. Muscarinic modulation of steady-state EPSPs/EPSCs was significantly stronger in the range of lower frequencies (0.25-4 Hz) than at higher frequencies (8 and 12 Hz). The GABAA and GABAB-receptor/channel antagonists picrotoxin and 2-hydroxy-saclofen, the opiate receptor antagonist naloxone and atropine failed to alter the shape of the frequency-response curve. These results show that both exogenous and endogenous muscarinic receptor agonists are capable of activating a presynaptic mechanism by which fast excitatory inputs to the ventral striatum are depressed. The depressive effect is clearly stronger at lower rates of afferent stimulation than at high rates. This frequency-dependent attenuation of excitatory synaptic inputs exemplifies a new type of activity-dependent neuromodulation in central neural circuits.

Paired tone facilitation in dorsal cochlear nucleus neurons: a short-term potentiation model testable in vivo

It has been suggested that the dorsal cochlear nucleus (DCN) is involved in coding stimulus history or prior auditory activity [Manis (1989) J. Neurophys., 61, 149-161; Manis (1990) J. Neurosci., 10, 2338-2351]. The major output neurons of the DCN are the fusiform (pyramidal) cells which are thought to receive excitatory inputs from the descending branch of the acoustic nerve onto their basal dendrites and significant inhibitory glycinergic and GABAergic inputs to the soma and dendrites. The apical dendrites of these neurons lie within the molecular layer of the DCN and encounter parallel fibers which are thought to utilize the excitatory amino acid neurotransmitter glutamate. In this study of anesthetized chinchillas, we found that, in contrast to the responses of acoustic nerve fibers and most cochlear nucleus neurons which are masked by an appropriate preceding signal, many DCN principal cells are facilitated during the second of two identical stimuli. Facilitated DCN responses often have a reduced interspike interval and a more chopper-like temporal response pattern to the second characteristic frequency tone. This paired tone facilitation in the chinchilla DCN provides as in vivo model of short-term potentiation elicited by sensory stimulation similar to the paired-pulse facilitation observed with electrical stimulation in other models.

Roles of N-type and Q-type Ca2+ channels in supporting hippocampal synaptic transmission

Several types of calcium channels found in the central nervous system are possible participants in triggering neurotransmitter release. Synaptic transmission between hippocampal CA3 and CA1 neurons was mediated by N-type calcium channels, together with calcium channels whose pharmacology differs from that of L- and P-type channels but resembles that of the Q-type channel encoded by the alpha 1A subunit gene. Blockade of either population of channels strongly increased enhancement of synaptic transmission with repetitive stimuli. Even after complete blockade of N-type channels, transmission was strongly modulated by stimulation of neurotransmitter receptors or protein kinase C. These findings suggest a role for alpha 1A subunits in synaptic transmission and support the idea that neurotransmitter release may depend on multiple types of calcium channels under physiological conditions.

The pharmacology and function of central GABAB receptors

In conclusion, GABAB receptors enable GABA to modulate neuronal function in a manner not possible through GABAA receptors alone. These receptors are present at both pre- and postsynaptic sites and can exert both inhibitory and disinhibitory effects. In particular, GABAB receptors are important in regulating NMDA receptor-mediated responses, including the induction of LTP. They also can regulate the filtering properties of neural networks, allowing peak transmission in the frequency range of theta rhythm. Finally, GABAB receptors are G protein-coupled to a variety of intracellular effector systems, and thereby have the potential to produce long-term changes in the state of neuronal activity, through actions such as protein phosphorylation. Although the majority of the effects of GABAB receptors have been reported in vitro, recent studies have also demonstrated that GABAB receptors exert electrophysiological actions in vivo. For example, GABAB receptor antagonists reduce the late IPSP in vivo and consequently can decrease inhibition of spontaneous neuronal firing following a stimulus (Lingenhöhl and Olpe, 1993). In addition, blockade of GABAB receptors can increase spontaneous activity of central neurons, suggesting the presence of GABAB receptor-mediated tonic inhibition (Andre et al., 1992; Lingenhöhl and Olpe, 1993). Despite these electrophysiological effects, antagonism of GABAB receptors has generally been reported to produce few behavioral actions. This lack of overt behavioral effects most likely reflects the modulatory nature of the receptor action. Nevertheless, two separate behavioral studies have recently reported an enhancement of cognitive performance in several different animal species following blockade of GABAB receptors (Mondadori et al., 1992; Carletti et al., 1993). Because of their small number of side effects, GABAB receptor antagonists may represent effective therapeutic tools for modulation of cognition. Alternatively, the lack of overt behavioral effects of GABAB receptors may indicate that these receptors are more important in pathologic rather than normal physiological states (Wojcik et al., 1989). For example, a change in receptor affinity or receptor number brought on by the pathology could enhance the effectiveness of GABAB receptors. Of significance, CGP 35348 has been shown to block absence seizures in genetically seizure prone animals, while inducing no seizures in control animals (Hosford et al., 1992; Liu et al., 1992). Thus, GABAB receptors may represent effective sites for pharmacological regulation of absence seizures. Perhaps further behavioral effects of these receptors will become apparent only after additional studies have been performed using the highly potent antagonists that have been recently introduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of NMDA receptors in hippocampal area CA1 by low and high frequency orthodromic stimulation and their contribution to induction of long-term potentiation

N-methyl-D-aspartate (NMDA) receptors are important in many instances of synaptic plasticity. In hippocampal area CA1, long-term potentiation (LTP) can be induced by both NMDA receptor-dependent and -independent mechanisms. Using intracellular recordings and single-electrode voltage clamp, we isolated and characterized NMDA receptor-mediated synaptic responses. NMDA receptor-mediated responses evoked by low frequency orthodromic stimulation were inhibited in a dose-dependent manner by the competitive antagonist D,L-2-amino-5-phosphonovaleric acid (APV). High frequency (tetanic) stimulation, which facilitates synaptic release of glutamate, failed to overcome the blockade of NMDA receptors by APV. Using extracellular recordings of field potentials, we studied the contribution of NMDA receptors to LTP induced by different patterns of tetanic stimulation. LTP was inhibited in a dose-dependent manner by APV, but was more sensitive to APV than were NMDA receptor-mediated synaptic responses. This most likely reflects a threshold for NMDA receptor activation in LTP induction. A component of LTP that resisted blockade by APV was induced by high (200 Hz), but not low (25 Hz), frequency tetanization. This NMDA receptor-independent component of LTP persisted for > 4 hours and accounted for approximately half the potentiation induced by 200 Hz tetanization. Procedures necessary to induce LTP at the Schaffer collateral/commissural synapses in area CA1 by both NMDA receptor-dependent and -independent mechanisms are now well characterized. Using the same neuronal population, it will be possible to ask if processes involved in the maintenance of LTP are shared even when LTP is induced through two different mechanisms.

Evidence for all-or-none regulation of neurotransmitter release: implications for long-term potentiation

1. We have used the whole-cell patch-clamp recording technique to examine the modulation of dual-component excitatory postsynaptic currents (EPSCs) in CA1 pyramidal cells in guinea-pig hippocampal slices. 2. The dramatic difference in the reported sensitivities of the N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors to glutamate suggests that changes in transmitter concentration in the synaptic cleft would result in differential modulation of the two components of the EPSC. 3. To test whether presynaptic manipulations change transmitter concentration in the synaptic cleft, pharmacological modulation of transmitter release by the GABAB agonist baclofen or by the adenosine antagonist theophylline was used. These manipulations resulted in parallel changes of NMDA and non-NMDA receptor-mediated components of EPSCs over a sixteen-fold range. 4. Stimuli that induce long-term potentiation (LTP) did not cause a sustained enhancement of isolated NMDA receptor-mediated EPSCs evoked in the presence of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 5. To compare directly the effect of LTP on the components of the EPSC, dual-component EPSCs were elicited while the postsynaptic membrane potential was held at +30 mV. Induction of long-term potentiation by delivering low-frequency synaptic stimulation in conjunction with such depolarization led to differential enhancement of the non-NMDA receptor-mediated component of the EPSC. 6. These data support the notion that synaptic transmission at individual boutons occurs in an all-or-none fashion, without changing peak transmitter concentration in the synaptic cleft. Long-term potentiation could occur through a postsynaptic modification of receptors or through a presynaptic change involving increased transmitter concentration in the synaptic cleft, but is difficult to explain by a generalized increase in release probability.

Presynaptic mechanism for heterosynaptic, posttetanic depression in area CA1 of rat hippocampus

Conditioning stimulation applied to afferent fibers in stratum radiatum or stratum oriens of hippocampal area CA1 produced heterosynaptic, posttetanic depression (PTD) of excitatory postsynaptic potentials (EPSPs). PTD amounted to a 60-80% reduction of EPSPs and recovered over a 5 min period. Conditioning stimulation also induced a posttetanic hyperpolarization (PTH) averaging 4 mV and decaying over a 1-1.5 min period. PTH was accompanied by a large reduction in input resistance. We sought to determine the pre- or postsynaptic locus of heterosynaptic PTD. Our results suggest that PTD reflects a presynaptic mechanism: (1) PTD was observed for both N-methyl-D-aspartate (NMDA) and non-NMDA receptor mediated EPSPs; (2) Direct depolarization of pyramidal cells, substituted for the synaptic depolarization induced by conditioning stimulation, did not elicit PTD; (3) PTD and PTH were differentially affected by pharmacological and postsynaptic manipulations; (4) Conditioning stimulation depressed responses to pressure applied glutamate, but the magnitude and duration were too small to account for PTD. Since afferent fiber volleys were not depressed following conditioning stimulation, while field EPSPs were, we conclude that conditioning stimulation suppresses synaptic release of glutamate.

Bicuculline-resistant paired-pulse inhibition in the rat hippocampal slice

1. An initial observation that paired-pulse inhibition in hippocampal slices was increased rather than decreased by bicuculline prompted the present study to explore the mechanism underlying bicuculline-resistant inhibition. 2. In the presence of bicuculline, paired-pulse interactions were dependent on the interpulse interval (i.p.i.) but a medium-latency inhibition was consistently observed at an i.p.i. of 300 to 500 ms. 3. The medium-latency (300 ms) bicuculline-resistant inhibition produced by paired orthodromic stimuli was substantially reduced by 2-hydroxysaclofen and was probably largely mediated by GABAB-receptor activation. Paired-pulse inhibition produced by an orthodromic/antidromic stimulation sequence was not affected by 2-hydroxysaclofen suggesting the possibility that the GABAB-receptors involved in orthodromic inhibition may be located presynaptically on the Schaffer collateral terminals rather than on the postsynaptic surface. The medium latency inhibition was also reduced by baclofen and under some conditions, by adenosine. 4. In addition to the GABAB-component, a hydroxysaclofen-resistant depression of postsynaptic excitability contributed to bicuculline-resistant paired-pulse inhibition at the 300 ms latency.

Age-related alterations in potentiation in the CA1 region in F344 rats

F344 rats of various ages (2-3 months, 15-16 months, and 24-25 months) were tested on a spatial memory task. The 15- and 24-month-old rat groups showed impaired acquisition and retention of the memory task, compared to the young animals. Extracellular field potential recordings in the CA1 region were subsequently performed in vitro, using hippocampal slices from both these tested rats and similar but untested F344 young and aged rats. Findings included: a) a positive correlation between baseline dendritic EPSP slope values and retention scores across age groups; b) a more rapid decay of both somatic and dendritic short-term potentiation in aged slices; c) decreased somatic but not dendritic long-term potentiation overall in aged slices, regardless of bath Mg2+ level; and d) decreased paired-pulse facilitation in slices from aged rats bathed in 4.0 mM Mg2+ media compared to young controls. These findings suggest an age-related alteration in both presynaptic and postsynaptic potentiation mechanisms, which may relate to the poor spatial memory acquisition and retention in the aged rats. These age-related differences point to substantial changes in neuronal signal processing capabilities and local circuit function in the hippocampus as a function of aging.

Current source density profiles of optical recording maps: a new approach to the analysis of spatio-temporal neural activity patterns

Spatio-temporal spreading of activity in the CA1 region of the rat hippocampal slice was studied by two experimental approaches. At identical locations in the tissue we measured both the extracellular field potential distribution with microelectrode recordings and the intracellular potential distribution by optical recording, using voltage-sensitive fluorescent dyes. Current source density analysis (CSD) was applied to the extracellular field potential distributions (eCSD) to enhance the spatial resolution. In order to obtain an analogous improvement for the optical recordings, we developed a new CSD transformation, which calculates the locations of the transmembrane current generators from the intracellular potential distributions (iCSD). Compared to the underlying fluorescence maps, the new iCSD profiles exhibit a considerable improvement in spatial resolution. Results can be directly interpreted in terms of physiological membrane processes, such as postsynaptic potentials and action potentials. The iCSD profiles show a surprisingly good correspondence with the classical eCSD profiles both qualitatively and quantitatively, the only difference being that cell body activity is reduced in amplitude. Thus, this new optical CSD analysis paves the way for a quantitative interpretation, rather than the hitherto predominantly qualitative interpretation of spatio-temporal activity profiles from optical recording measurements.

The opioid peptide dynorphin mediates heterosynaptic depression of hippocampal mossy fibre synapses and modulates long-term potentiation

The mossy fibre pathway in the hippocampus uses glutamate as a neurotransmitter, but also contains the opioid peptide dynorphin. Synaptic release of dynorphin causes a presynaptic inhibition of neighbouring mossy fibres and inhibits the induction and expression of mossy fibre long-term potentiation. These findings demonstrate a physiological role for a neuropeptide in the central nervous system, provide a functional basis for the coexistence of a neuropeptide with classic neurotransmitters and demonstrate the very different roles played by these two classes of signalling molecules.

NMDA receptors have a dominant role in population spike-paired pulse facilitation in the dentate gyrus of urethane-anesthetized rats

Paired-pulse facilitation was studied at the perforant path-granule cell synapses in the dentate gyrus of urethane-anesthetized rats. Extracellular field potentials comprising excitatory postsynaptic potentials (EPSPs) and population spikes (PSs) were used to compare facilitation of both responses at interpulse intervals between 10 and 1000 ms. In this model system EPSPs, produced at stimulus intensities well below the PS threshold, exhibited paired-pulse facilitation (PPF) at intervals less than 40 ms. Between 40 and 100 ms both responses were of equal size and from 200 to 1000 ms the second response exhibited paired-pulse depression (PPD). With higher stimulus intensities, PSs exhibited early (10-30 ms) and late (400-1000 ms) PPD and intermediate interval (40-200 ms) PPF. The enhanced PS amplitude during facilitation was accompanied by a modest decrease in EPSP slope and increase in PS latencies to onset and to peak. If PS amplitude was increased to the same extent by simply increasing stimulus intensity, an increase in EPSP slope and decrease in PS latencies to onset and peak were observed. Current source-density analysis revealed that the current sink generated by synaptic activation of granule cell dendrites was indistinguishable between control and facilitated responses up to the onset of the PS. However, the facilitated response exhibited a marked increase in the duration and amplitude of current flowing into the dendrites during the late phase of the EPSP. PPF of the PS was attenuated or blocked by the administration of the known NMDA receptor-ion channel blockers, MK-801, dextromethorphan and ketamine. The depressant effects of these pharmacological agents on facilitation shared the same time course as facilitation itself, peaking at 60 ms and lasting approximately 200 ms. These data suggest that facilitation in this situation is due to postsynaptic rather than presynaptic modulations, and is based upon an increase in the NMDA-mediated component of the evoked response. No increase in transmitter release, per se, could be demonstrated.

Changes in intrinsic inhibition in isolated hippocampal slices during ethanol withdrawal; lack of correlation with withdrawal hyperexcitability

1. Intracellular recordings were made from pyramidal cells in area CA1 in mouse isolated hippocampal slices, after chronic ethanol treatment in vivo. 2. Fast i.p.s.ps were isolated by injection of the impaled neurones with QX314 (to block fast sodium currents and the slow i.p.s.p.) and stimulating the interneurones in the presence of the glutamatergic blockers, CNQX and APV. 3. The isolated fast-inhibitory postsynaptic potential (f.-i.p.s.p.) was measured at intervals during the 7 h withdrawal period. The reversal potential and sensitivity to bicuculline suggested that the isolated f.-i.p.s.p. was mediated by activation of the GABAA receptor-chloride ionophore complex. 4. Measurement of stimulus-response relationships for the f.-i.p.s.ps revealed an initial increase in the maximum size of the i.p.s.p., evoked from a membrane potential of -50 mV, seen at 2 h into ethanol withdrawal. This was attributed to a negative shift in the reversal potential, Ei.p.s.p., with no observed change in conductance, Gi.p.s.p. 5. No differences in f.-i.p.s.ps evoked during ethanol withdrawal or in control slices were seen at 4 h or 6 h. At these times, epileptiform activity was seen in previous field potential recordings. 6. Paired pulse depression of the f.-i.p.s.p. was significantly increased at 2 h into withdrawal, when a 150 ms pulse interval was used. No differences were seen at later times in the ethanol withdrawal period. 7. The results suggest that ethanol withdrawal hyperexcitability in isolated hippocampal slices is not caused by primary decreases in inhibition mediated by the GABAA receptor-chloride ionophore complex.4. Measurement of stimulus-response relationships for the f.-i.p.s.ps revealed an initial increase in the maximum size of the i.p.s.p., evoked from a membrane potential of - 50 mV, seen at 2 h into ethanol withdrawal. This was attributed to a negative shift in the reversal potential, Ejp.sp with no observed change in conductance, Gj ps p.5. No differences in f.-i.p.s.ps evoked during ethanol withdrawal or in control slices were seen at 4 h or 6 h. At these times, epileptiform activity was seen in previous field potential recordings.6. Paired pulse depression of the f.-i.p.s.p. was significantly increased at 2 h into withdrawal, when a 150 ms pulse interval was used. No differences were seen at later times in the ethanol withdrawal period.7. The results suggest that ethanol withdrawal hyperexcitability in isolated hippocampal slices is not caused by primary decreases in inhibition mediated by the GABAA receptor-chloride ionophore complex.The increase in the f.-i.p.s.p. during the initial stages of the withdrawal might prevent the overt expression of epileptiform activity at this time.

Chronic ethanol treatment reduces inhibition in CA1 of the rat hippocampus

The effect of chronic ethanol exposure on inhibition in the rat hippocampal slice was investigated using paired-pulse stimulation techniques with stimulation in stratum radiatum or stratum oriens of CA1. Experimental animals were fed ethanol in a liquid diet for 20 weeks and were withdrawn for at least 8 weeks prior to electrophysiological recording. Prior ethanol treatment had no effect on basic input-output relationships for the extracellular population spike. Ethanol treatment significantly reduced the recurrent inhibition produced by antidromic stimulation in a manner dependent upon stimulus intensity. In addition, with orthodromic paired-pulse stimulation of either stratum radiatum or oriens, a trend toward an augmentation of the facilitation of population spike amplitude was observed, suggesting that feedforward inhibition may also be reduced. These results are similar to those found with treatments that reduce inhibition. Therefore, we conclude that chronic ethanol exposure produces an enduring disruption of inhibitory neuronal function in the rat hippocampus.

Observations of synaptic efficacy and paired-pulse facilitation in area CA1 of hippocampal slices from coriaria lactone-kindled rats

The changes of population spike(PS)/population excitatory postsynaptic potential (EPSP) slope relationship and paired-pulse facilitation (PPF) were primarily investigated with extracellular recording in stratum pyramidale to stimulation of Schaffer collaterals in CA1 region of hippocampal slices from coriaria lactone (CL)-kindled and control rats. The results were as follows: (1) neither spontaneous nor evoked epileptiform bursts were found in all hippocampal slices from CL-kindled and control rats; (2) the synaptic efficacy, expressed by the ratio of PS/EPSP slope, at low stimulation intensity ranging from 10-30% of its maximum was significantly increased on CL-kindled rats (P less than 0.05); and (3) although PPF was found in all slices, the PPF strength only at stimulation intensity of 10 and 20% of maximum was augmented remarkably in CL-kindled rats (P less than 0.002 and 0.024, respectively). According to the results from our previous work, we suggested that the increment of PS/EPSP slope ratio and PPF strength at low stimulation intensity may result from the potentiation of excitatory synaptic activity or/and the change of intrinsic excitability of pyramidal neurons.

Behavior-dependent paired-pulse responses in the hippocampal CA1 region

Paired-pulses of 20-100 ms interpulse interval (IPI) were delivered to the Schaffer collaterals/commissural fibers in order to excite the apical dendrites of the hippocampal CA1 region in freely behaving rats. Significant differences were observed for the paired-pulse responses during different behavioral states. The responses recorded during awake immobility (IMM), and slow-wave sleep (SWS) were similar, but as a group, were different from those during walking (WLK) and rapid-eye-movement sleep (REM). During WLK and REM, the population spike evoked by the second pulse (P2) at IPI of 30 and 50 ms, was greatly facilitated as compared to the population spike evoked by the first pulse (P1), i.e. P2 greater than P1. During IMM and SWS and using moderate stimulus intensities, P2 was generally smaller than P1 (paired-pulse suppression) at IPI of 30 and 50 ms. The P2/P1 relation with behavior was not caused by the slight variations of P1 with behavior. In addition, paired-pulse facilitation of the population excitatory postsynaptic potentials (EPSP) was relatively small and not significantly dependent on behavior. Behavioral dependence of the paired-pulse responses was not generally found for IPI of 20 or 100 ms. It is concluded that paired-pulse facilitation at 30-50 ms IPI can best be explained by EPSP facilitation combined with a behaviorally dependent disinhibition.

Potentiation of inhibition with perforant path kindling: an NMDA-receptor dependent process

Kindling produces a long-lasting enhancement of excitatory and inhibitory neurotransmission. Both long-term potentiation and kindling-induced potentiation of hippocampal excitatory neurotransmission are suppressed by N-methyl-D-aspartate (NMDA) receptor antagonists. These antagonists also greatly retard the development of electrical kindling. We have previously reported prolonged afterdischarges (AD) in animals stimulated in the perforant path and treated with the NMDA antagonist, dizocilpine maleate (MK-801), despite a retardation in the development of kindling. In the present study the potentiation of excitation and inhibition was assessed during perforant path kindling when NMDA channels were blocked with MK-801. Paired pulse inhibition at 8 interpulse intervals (IPI 20-1000 ms) was monitored before and during kindling development. MK-801 (1 mg/kg, i.p.) delivered 30 min prior to perforant path stimulation increased AD thresholds and delayed kindling development. Potentiation of the excitatory postsynaptic potential (EPSP) and of paired pulse inhibition measured 20-24 h after each drug administration/stimulation were suppressed in MK-801-treated animals. Paradoxically, AD durations were prolonged by MK-801. Longer AD durations could be accounted for by a higher incidence of secondary AD bouts in MK-801 relative to control animals. Development of potentiation of the early phase of paired pulse inhibition (IPI 20-30 ms) was delayed and the potentiation of the late phase of inhibition (IPIs of 200-1000 ms) was completely blocked by MK-801. Thus, some of the enhancement of inhibition seen with kindling is dependent upon NMDA neurotransmission. Suppression of this potentiated inhibition may account for prolonged focal ADs in the perforant path and dentate gyrus.

Chronic benzodiazepine treatment of rats induces reduction of paired-pulse inhibition in CA1 region of in vitro hippocampus

Paired-pulse inhibition was studied extracellularly in in vitro hippocampal slices from rats sacrificed 48 h or 7 days after 1 week flurazepam (FZP) treatment. Population spikes and field excitatory postsynaptic potentials (EPSPs) were recorded with NaCl-containing glass micropipettes in the stratum pyramidale and stratum radiatum, respectively, of the CA1 region. Conditioning pulses were delivered by stimulating Shaffer collaterals (orthodromic) or the alveus (antidromic). Orthodromic test pulses were delivered with interpulse intervals of 10-200 ms. There was a significant reduction in paired-pulse inhibition in slices from treated vs control rats in both the orthodromic-orthodromic and antidromic-orthodromic paradigms. Reduced inhibition was evident 48 h, but not 7 days, after the end of FZP treatment. Furthermore, there was a significant prolongation of the half decay time of the field EPSP, without a significant change in the initial slope or maximum amplitude. The results may suggest an impairment of endogenous gamma-aminobutyric acid function in the hippocampus after chronic benzodiazepine (BZ) treatment and may provide a basis for a mechanism of BZ tolerance.