Antagonism of the aconitine-induced inexcitability by the structurally related Aconitum alkaloids, lappaconitine and ajacine.

Aconitine, lappaconitine and ajacine are structurally related alkaloids occurring in several species of the Aconitum genus. While aconitine is known to activate the voltage-dependent sodium channel, lappaconitine has been reported to block this channel. To investigate a possible antagonism of the aconitine action on neuronal activity by lappaconitine and the closely related alkaloid ajacine, we have performed extracellular recordings of stimulus evoked population spikes and field excitatory postsynaptic potential (EPSP) in rat hippocampal slices. Aconitine (10-100 nM) diminished the amplitude of the orthodromic population spike in a concentration-dependent manner. When aconitine was applied in presence of 10 microM lappaconitine, the concentration-response curve was shifted to the right. Furthermore, the complete suppression of the population spike evoked by 100 nM aconitine was reversed by 10 microM lappaconitine. The action of lappaconitine was mimicked by ajacine, however, the latter alkaloid was less potent. Both lappaconitine and ajacine shifted the input-output relationship of the presynaptic fiber spike as function of the stimulation intensity and of the field EPSP as function of the presynaptic fiber spike to the right. After pharmacological isolation, the presynaptic fiber spike was decreased by both compounds in a frequency-dependent manner indicative for a use-dependent action. Thus, electrophysiologically these alkaloids seem to inhibit predominantly the excitability of the afferent fibres and, in consequence, neurotransmission between Schaffer collaterals and the CA1 neurons, thereby suppressing the firing of the latter. Ajacine and lappaconitine inhibited stimulus-triggered epileptiform population bursts in area CA1 elicited by omission of Mg(2+) as well as spontaneously occurring epileptiform discharges in area CA3 elicited by omission of Mg(2+) and elevation of K(+). It is concluded that the inhibitory and antiepileptiform effect of ajacine and lappaconitine is mediated by a frequency-dependent inhibition of the voltage-dependent sodium channel, thereby decreasing the excitability which might be important for filtering high frequency bursts of action potentials characteristic for epileptiform activity in the hippocampus. Moreover, these alkaloids are naturally occurring antagonists of the sodium channel activator aconitine.     

Inhibition of neuronal activity in rat hippocampal slices by Aconitum alkaloids

The structurally related Aconitum alkaloids aconitine, lappaconitine, and 6-benzoylheteratisine inhibited the orthodromic and antidromic population spike in hippocampal CA1 area in a frequency-dependent manner. Aconitine (1 μM) completely suppressed epileptiform activity induced by omission of Mg²⁺ as well as normal neuronal activity, whereas lappaconitine (10 μM) and 6-benzoylheteratisine (10 μM) diminished epileptiform activity by sparing normal neuronal activity.     

Structure-dependent differences in the effects of the Aconitum alkaloids lappaconitine, N-desacetyllappaconitine and lappaconidine in rat hippocampal slices

Lappaconitine, a C₁₉ diterpenoid alkaloid from Aconitum sinomontanum has been reported to possess analgesic and antiinflammatory properties in vivo and to inhibit neuronal activity in brain slices. In the present study the effect of lappaconitine has been compared with the effects of its main metabolite N-desacetyllappaconitine and the structurally related alkaloid lappaconidine. For comparison of drug effects population spikes and field excitatory postsynaptic potentials (EPSPs) evoked by stimulation of stratum radiatum or the alveus were studied in normal rat hippocampal slices and in slices treated with low Mg²⁺-medium. At concentrations of 3–100 μM, both lappaconitine and N-desacetyllappaconitine inhibited population spikes elicited by stratum radiatum and alvear stimulation as well as the field EPSP recorded in CA1 stratum radiatum. The drug-induced depression of field potential responses was increased with rising stimulus frequency, indicating an activity-dependent mode of action. The effect of N-desacetyllappaconitine on each parameter investigated was significantly stronger than the effect of lappaconitine. Despite the structural relationship, lappaconidine failed to affect neuronal excitability in concentration below 100 μM, and an increase in stimulus frequency did not potentiate its effect. Moreover, lappaconitine and N-desacetyllappaconitine suppressed epileptiform activity induced by bicuculline or by omission of Mg²⁺ from the bathing medium.     

Perfusion with lithium modifies neurophysiological responses in the CA1 region of the hippocampal slice preparation

The effects of acute lithium exposure on extracellular electrophysiological responses in the CA1 region of the in vitro hippocampus were investigated. Field potentials were assessed while perfusing slices with normal media or media in which LiCl was substituted for NaCl in 30, 20, 10 and 2 mM amounts. Lithium concentration in the slice following 20 min perfusion with 20 mM lithium was determined to be about 14 mM. At the higher concentrations, lithium exposure depressed the presynaptic fiber volley and antidromic population spike. On the other hand, the population EPSP and orthodromic population spike were enhanced. No significant changes were found at 2 mM. The findings are compatible with one action of lithium being on the excitability of axons and synaptic terminals. Comparisons were drawn between previous studies involving chronic lithium exposure and the present results. In this acute preparation lithium effects, as reflected in the population EPSP, were in opposition to those found with chronic lithium exposure. Changes demonstrated in this preparation in fiber volley and antidromic population spike paralleled those found with chronic lithium exposure.     

Heterosynaptic long-term depression is facilitated by blockade of inhibition in area CA1 of the hippocampus

Non-associative long-term depression (LTD) of the stratum radiatum input to area CA1 was studied in rat hippocampal slices. Tetanization of either the alveus or stratum oriens produced > 30min depression of the radiatum field EPSP and population spike, but generally only in the presence of picrotoxin. The spike depression was accounted for by the EPSP depression, and could be blocked by prior administration of anN-methyl-d-aspartate receptor antagonist. These data suggest that the induction of non-associative LTD is depolarization-dependent and involves theN-methyl-d-aspartate receptor/channel complex.     

Hippocampal excitability and changes in extracellular potassium

Extracellular recordings were made from dendritic and somatic sites in the CA1 region of guinea pig hippocampal slices maintained in vitro. We studied the effects of increasing extracellular K⁺ from 3.25 to 15.25 mm on potentials elicited by synaptic activation of stratum radiatum fibers. Increasing K⁺ from 3.25 to 12.25 mm had little or no detectable effect on the excitability of presynaptic fibers, increased the amplitude of the somatic response (the population spike), and increased the size of the dendritic field EPSP in the range of 3.25 to 9.25 mm. When epileptiform activity induced by penicillin was studied as a function of [K⁺]₀, it was found that elevating K⁺ increased the amplitude and duration of the abnormal population spike. However, significant epileptiform activity was present in 3.25 mm [K⁺]₀. All evoked activity was reversibly abolished by perfusion with 15.25 mm K⁺. These results indicate that [K⁺]₀ has an important role in regulating neuronal excitability and may effect changes both pre- and postsynaptically.     

Sensory modulation of hippocampal transmission. II. Evidence for a cholinergic locus of inhibition in the Schaffer-CA1 synapse

The present work studied the neurotransmitter mediating the depressive effect of sensory stimulation on the Schaffer-CA1 transmission. Field responses of the CA1 region evoked by ipsilateral CA3 stimuli were recorded in paralyzed, locally anesthetized rats following the same experimental paradigm as in the previous work. The tissue zone under recording was perfused in vivo by an implanted hollow fiber (brain dialysis device) with either Krebs-Ringer bicarbonate (KRB), or KRB with penicillin, atropine, acetylcholine or eserine. Results were the following: (1) atropine increased the field excitatory postsynaptic potential (EPSP) amplitude in a dose-dependent manner and totally abolished the modulatory action of sensory stimulation; (2) both the field EPSP and the modulatory action of sensory stimulation remained unaltered during the blockade of GABAergic activity by penicillin; (3) acetylcholine as well as eserine induced a great diminution of both field EPSP and population spike amplitudes, without altering the effect of sensory stimulation; (4) penicillin and atropine induced multiple population spikes, reversing the effect of sensory stimulation and increasing the cell excitability. These results demonstrate that the sensory modulation of information transfer through the Schaffer-CA1 synapse is mediated by a muscarinic cholinergic mechanism. The dose-dependent increase in the field EPSP by muscarinic blockade is evidence for the existence of a cholinergic presynaptic inhibition on the Schaffer collateral terminals.     

Effect of low glucose concentration on synaptic transmission in the rat hippocampal slice

Severe hypoglycemia in vivo is known to slow down the EEG, then to produce complete electrical silence in the brain. To find out why low glucose concentrations reduce electrical activity, synaptic transmission from Schaffer collateral/commissural fibers to CA1 pyramidal cells in the submerged rat hippocampal slice was investigated using extracellular recording techniques. Superfusion for 30 min with 1 mM glucose reversibly reduced population spike amplitude, without affecting the size of the presynaptic volley and the slope of the field EPSP. Lower glucose concentrations also affected the EPSP, although to a lesser extent than the population spike. Antidromic population spikes were not decreased by low glucose. Depolarization with 8-10 mM K+ reduced both presynaptic volley amplitude and EPSP, but enhanced the population spike, an effect clearly different from that of low glucose. The slope of the input/output curve between presynaptic volley and EPSP remained unaltered in 1 mM glucose but the slope between EPSP and population spike was reduced by about 50%. Results suggest that low glucose concentrations interrupt synaptic transmission by reducing, but not abolishing, the excitability of pyramidal cells.     

Two-dimensional current source density analysis of propagation delays for components of epileptiform bursts in rat hippocampal slices

Recordings of epileptiform burst activity in the CA1 region of the transverse rat hippocampal slices were made with a 32-channel surface electrode array. The 200 μm interelectrode resolution, the simultaneity of the data, and the use of a two-dimensional current source density analysis allowed accurate measurement of population spike peak times. Differences were found in the apparent propagation delays among 3 burst components: the compound action potential (CAP) along the Schaffer collaterals, the first population spike directly driven by the CAP, and the second (and succeeding) population spikes representing the bursting, epileptiform component. Delay measurements were applied to epileptiform bursts recorded in slices treated with picrotoxin (PTX), pentylenetetrazol (PTZ), and 0-Mg²⁺ medium. In 0-Mg²⁺ medium all components propagate at nearly the same velocity. In the PTZ and PTX media the second population spike propagated more slowly than the CAP. The first population spike propagated at the same velocity as the CAP for orthodromic Schaffer collateral stimulation. The first population spike propagated at the same, slower velocity as the second spike for antidromic Schaffer collateral stimulation.     

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 atropin-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 GABA_A and GABA_B-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.     

Presynaptic inhibitory action of acetylcholine in area CA1 of the hippocampus

The effect of iontophoretically applied acetylcholin (ACh) was investigated in area CA1 of transverse hippocampal slices maintained in vitro. In synaptically activated regions of the dendritic field, ACh reduced the amplitude of the population spike recorded from the pyramidal layer. In dendritic regions which were not synaptically activated, ACh increased the amplitude of the population spike or it had no effect. The depressing effect of ACh was abolished in denervated dendritic regions. The intracellularly recorded excitatory postsynaptic potential (EPSP) decreased in amplitude when ACh was applied at the synaptic site. The resting membrane potential, the time course of the EPSP, and the membrane resistance were unaffected. ACh increased the excitability of afferent fibers and this was independent of synaptic transmission. We conclude that ACh in addition to its postsynaptic effects has a presynaptic site of action.     

Mechanisms of long-term potentiation: EPSP/spike dissociation, intradendritic recordings, and glutamate sensitivity

Synaptic efficacy is modified following a brief train of high-frequency stimulation (HFS) to a cell's afferent fibers (long-term potentiation; LTP). An alteration in the postsynaptic response to endogenous neurotransmitter, as a result of an increase in the number of postsynaptic receptors, has been proposed (Baudry and Lynch, 1980). We tested this hypothesis in the CA1 hippocampus by intracellularly recording the postsynaptic response to localized application of glutamate before and after induction of LTP. When LTP was produced, there was no corresponding change in neuronal sensitivity to glutamate application. These findings are not consistent with the hypothesis that HFS of fibers in CA1 stratum radiatum induces an increase in the number of postsynaptic glutamate receptors in CA1 pyramidal cells. Previous reports concerning LTP have indicated a dissociation between the degree of potentiation in the population EPSP and population spike. Simultaneous recordings of the CA 1 population EPSP and population spike in hippocampal slices confirmed that the degree of potentiation of the population spike was not predicted by the degree of potentiation in the population EPSP. Intradendritic impalements were obtained to more accurately assess changes in the intracellular EPSP following HFS. When the population EPSP was potentiated, there was also a potentiated intradendritic EPSP. When the population spike was potentiated following HFS, however, the intradendritic EPSP was often unchanged; in the same cell, there was an increased probability of action potential discharge to stimulation which was originally (i.e., pre-HFS) subthreshold for spike initiation. These results indicate that the EPSP (intracellular or extracellular) may be potentiated following HFS, but this potentiation is not a prerequisite for, or a correlation of, potentiation in the population spike. Furthermore, these findings suggest that LTP is composed of 2 independent components--a synaptic component and an EPSP-to-spike coupling component.     

Kainic acid and penicillin: Differential effects on excitatory and inhibitory interactions in the CA1 region of the hippocampal slice

The effects of kainic acid (KA, 0.05-1.0 microM), and penicillin (PN, 3.4 mM) were studied in the CA1 region of rat hippocampal slices. Three components of the overall input/output function were taken: (1) the amplitude of the presynaptic compound action potential (prevolley) vs stimulation current applied to Schaffer collaterals, (2) the magnitude of the focally recorded synaptic potential (population EPSP) vs prevolley amplitude; and (3) the amplitude of the focally recorded population spike vs population EPSP magnitude. Recurrent inhibition was measured using the antidromic-orthodromic paired pulse method. KA caused a significant and reversible enhancement of all 3 component input/output functions while having no effect on paired pulse inhibition. PN caused a left shift in the EPSP-population spike relationship and decreased or abolished paired pulse inhibition; the other two measures of excitability were not changed. These results suggest that PN and KA differ fundamentally in the mechanisms by which they produce seizures: PN by removing inhibition while not affecting neuronal excitability per se; KA by exerting a generalized excitatory effect on neural membranes and on synaptic function while leaving recurrent inhibition unchanged.     

Inhibition of nitric oxide synthase protects against hypoxic neuronal injury.

We investigated the action of nitric oxide in hypoxic neuronal injury, using the hippocampal slice. Inhibition of nitric oxide synthase with the competitive inhibitor, NG-monomethyl-L-arginine, provided significant protection against hypoxia for population spike, EPSP and fiber volley responses, with an EC50 of 30 microM for protection of antidromic population spike amplitude. Confirming a stereo-specific site of action, this protection was reversed by the addition of L-arginine, but not D-arginine. These results indicate that nitric oxide generation may mediate acute CA1 neuronal injury during hypoxia, and that inhibition of nitric oxide synthase may be a useful neuroprotective strategy.     

NMDA receptor antagonists attenuate a portion of the penicillin-induced epileptiform burst

In the CA1 region of the rat hippocampal slice, epileptiform activity was induced by the GABAA antagonist penicillin (PEN, 3.4 mM). The competitive N-methyl-D-aspartate (NMDA) receptor antagonists D-2-amino-7-phosphonoheptanoate (D-AP7) and D-2-amino-5-phosphonovolerate (D-AP5) attenuated extracellularly recorded evoked burst duration, the number of population spikes per evoked bursts and the frequency of spontaneously occurring bursts, but did not affect the sum of the population spike amplitudes or the evoked burst coastline measures due to increases in amplitude of the remaining secondary population spikes. Intracellular recordings showed that many of the secondary action potentials in the PEN burst were decreased in amplitude and broadened in duration, perhaps due to spike inactivation. D-AP7 allowed these secondary action potentials to increase in amplitude, which could explain the increases in secondary population spike amplitude seen extracellularly. Decrements in stimulus strength can mimic the effect of D-AP7 on PEN bursts. These data suggest that there is a portion of the PEN-induced epileptiform burst which is sensitive to NMDA antagonists.     

Neurophysiologic effects of folate compounds in hippocampus, in vitro

Pteroylglutamic acid (PGA, folic acid) and related compounds were studied for their electropysiologic effects on guinea pig hippocampal slices in vitro. Folates are found to have marked neuroexcitatory actions in CA3 and lesser but substantial excitatory effects in CA1. The dentate gyrus is minimally activated by folates. Extracellular recordings in CA3 reveal augmentation in single unit activity, evoked responses, and at 50–100 μM PGA concentrations, spontaneous epileptiform discharges are generated. Progressive increase in PGA concentrations to at least 2 mM do not result in loss of this activity. At concentrations of 200 μM, epileptiform bursts in CA3, precede those in CA1 which are lost by sectioning the Schaeffer collateral pathway. However, with 500 μM PGA, spontaneous bursts occur in CA1 isolated from CA3. The change induced by PGA primarily affects the population spike resulting in lower stimulus threshold and higher amplitude response.Measurement of threshold concentration necessary to produce spontaneous epileptiform activity of folate related compounds reveals the following order of potency: pteroylglutamic acid (PGA) ≥ formyl tetrahydrofolic acid (folinic acid) > > methyltetrahydrofolate (MTHF) ≥ methotrexate (MTX) ≥ glutamic acid. Pteroic acid, pterin,N(p-aminobenzoyl)L-glutamic acid are inactive. Methotrexate does not diminish the response to PGA. Thus, the entire folate molecule is needed for full activity. Folates represent naturally occurring, highly epileptogenic compounds whose mechanism of action is not dependent on metabolic products. Possibly they act at a central receptor as a neuromodulator.     

Low magnesium epileptogenesis in the rat hippocampal slice: electrophysiological and pharmacological features

Extra- and intracellular recording techniques were used to study the epileptiform activity generated by rat hippocampal slices perfused with Mg2(+)-free artificial cerebrospinal fluid (ACSF). This procedure induced in both CA1 and CA3 subfields the appearance of synchronous, spontaneously occurring epileptiform discharges which consisted of extracellularly recorded 100-800 ms long, positive shifts with superimposed negative going population spikes. Simultaneous, extracellular recordings from CA1 and CA3 subfields revealed that the epileptiform discharges in CA3 preceded those occurring in CA1 by 5-25 ms. Surgical separation of the two areas led to the disappearance of spontaneous events in the CA1 but not in the CA3 subfield. In this type of experiment CA1 pyramidal cells still generated epileptiform discharges following orthodromic stimuli. The intracellular counterpart of both spontaneous and stimulus-induced epileptiform discharges in CA1 and CA3 pyramidal cells was a large amplitude depolarization with high frequency discharge of action potentials which closely resembled the paroxysmal depolarizing shift recorded in the experimental epileptogenic focus. A hyperpolarizing potential triggered by alvear stimuli was recorded in CA1 cells perfused with Mg2(+)-free ACSF. This hyperpolarization was blocked by bicuculline methiodide (BMI) indicating that it represented a GABAergic inhibitory postsynaptic potential (IPSP). BMI also caused a prolongation of both spontaneous and stimulus-induced Mg(+)-free epileptiform discharges. Perfusion of the slices with the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphono-valerate (APV) reduced and eventually abolished the Mg(+)-free epileptiform discharges. These effects were more pronounced in the CA1 than in the CA3 subfield. APV also reduced the amplitude and the duration of the alveus-induced IPSP. These data demonstrate that Mg(+)-free epileptiform activity is present in the hippocampal slice at a time when inhibitory GABAergic potentials are operant as well as that in the CA1 subfield this type of epileptiform activity is dependent upon NMDA-activated conductances. Our experiments also indicate that NMDA receptors might be involved in the neuronal circuit responsible for the hyperpolarizing IPSP generated by CA1 pyramidal neurons.     

Effects of pilocarpine on paired pulse inhibition in the CA3 region of the rat hippocampus

Pilocarpine (PILO), a muscarinic agonist, produces status epilepticus when administered to rats in vivo and induces interictal or ictal patterns of epileptiform activity in rat hippocampal slices. We investigated the effects of PILO (10 μM) on paired pulse inhibition (PPI) in the CA3 region of rat hippocampal slices. PPI was assessed by stimulating either the alveus or str. radiatum and recording the extracellular response from str. pyramidale of CA3. The evoked population spike following the second stimulus was compared to the first, PILO was bath applied for 1 h and then washed out to assess acute and long lasting effects. PILO decreased the amplitude of evoked population spikes measured in CA3. PPI following alveus stimulation was not affected by PILO; however, a significant loss of PPI at 15 and 30 ms interpulse intervals occurred following str. radiatum stimulation in the presence of PILO and 5 mM [K⁺]_o artificial cerebrospinal fluid (ACSF). The decrease in PPI at the 15 ms interval persisted following wash-out of PILO. PILO in 7.5 mM [K⁺]_o ACSF produced epileptiform activity and a resultant long lasting loss of PPI that followed str. radiatum stimulation. This effect was not observed following epileptiform activity produced by 7.5 mM [K⁺]_o alone, suggesting that the loss of PPI was due to PILO. Because str. radiatum-evoked PPI was selectively impaired, PILO appears to preferentially decreased feed-forward inhibition. The more dramatic loss of PPI following exposure to PILO and high [K⁺]_o may present the first steps in the development of chronic seizures that results from PILO-induced status epilepticus in rats.     

Action potential broadening induced by lithium may cause a presynaptic enhancement of excitatory synaptic transmission in neonatal rat hippocampus

Lithium enhances excitatory synaptic transmission in CA1 pyramidal cells, but the mechanisms remain unclear. The present study demonstrates that lithium enhances the N-methyl-d -aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-isoxazole propionic acid (AMPA) receptor-mediated components of the excitatory postsynaptic current (EPSC). Lithium decreased the magnitude of paired-pulse facilitation and presented an inverse correlation between the lithium-induced enhancement of synaptic transmission and initial paired-pulse facilitation, which is consistent with a presynaptic mode of action. The enhancement of synaptic strength is likely to act, at least in part, by increasing the amplitude of the presynaptic Ca²⁺ transient. One mechanism which could account for this change of the presynaptic Ca²⁺ transient is an increase in the duration of the action potential. We investigated action potential in hippocampal pyramidal neurons and found that lithium (0.5–6 mm ) increased the half-amplitude duration and reduced the rate of repolarization, whereas the rate of depolarization remained similar. To find out whether the lithium synaptic effects might be explained by spike broadening, we investigated the field recording of the excitatory postsynaptic potential (EPSP) in hippocampal slices and found three lines of evidence. First, the prolongation of the presynaptic action potential with 4-aminopyridine and tetraethylammonium blocked or reduced the synaptic effects of lithium. Second, the lithium-induced synaptic enhancement was modulated when presynaptic Ca²⁺ influx was varied by changing the external Ca²⁺ concentration. Finally, both effects, the synaptic transmission increment and the action potential broadening, were independent of inositol depletion. These results suggest that lithium enhances synaptic transmission in the hippocampus via a presynaptic site of action: the mechanism underlying the potentiating effect may be attributable to an increased Ca²⁺ influx consequent to the broadening effect of lithium on the action potential.     

Pregnanolone (3α-hydroxy-5α-pregnane-20-one), a progesterone metabolite, facilitates inhibition of synaptic transmission in the Schäffer collateral pathway of the guinea pig hippocampus in vitro

Pregnanolone (3α-hydroxy-5α-pregnane-20-one, a metabolite of progesterone) caused a significant depression of the amplitude of the population spike evoked in stratum pyramidale in CA1 of the guinea pig hippocampus in vitro. Local application of pregnanolone on the surface of the slice in stratum oriens depressed the population spike without effects on the presynaptic spike and the population excitatory postsynaptic potential simultaneously recorded in stratum radiatum. The depression was dose-dependent and was observed with a minimum latency of 10 s after application of a 0.5-nl droplet of 3.1 μM pregnanolone. The concentration at the recording site was computed to be 0.2 μM. The duration of the depression was 20–30 min. The depression was significantly reduced during perfusion of the slice bath with 100 μM picrotoxin in artificial cerebrospinal fluid. When pregnanolone was applied locally in stratum radiatum, the amplitudes of the presynaptic spike, the population excitatory postsynaptic potential and the population spike were depressed. The effects on the presynaptic spike and the population excitatory postsynaptic potential vanished with different time courses. It is concluded that the depression of the population spike was caused by GABA_A-mediated inhibition of the pyramidal neurones. The role of pregnanolone as a positive modulator of the GABA_A receptor and the effect of this modulation on the complex mechanisms underlying catamenial epilepsy are discussed.