Effects of (-)baclofen on inhibitory neurons in the guinea pig hippocampal slice

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Cellular and molecular mechanisms of epilepsy in the human brain

Animal models have provided invaluable data for identifying the pathogenesis of epileptic disorders. Clearly, the relevance of these experimental findings would be strengthened by the demonstration that similar fundamental mechanisms are at work in the human epileptic brain. Epilepsy surgery has indeed opened the possibility to directly study the functional properties of human brain tissue in vitro, and to analyze the mechanisms underlying seizures and epileptogenesis. Here, we summarize the findings obtained over the last 40 years from electrophysiological, histochemical and molecular experiments made with the human brain tissue. In particular, this review will focus on (i) the synaptic and non-synaptic properties of neocortical neurons along with their ability to produce synchronous activity; (ii) the anatomical and functional alterations that characterize limbic structures in patients presenting with mesial temporal lobe epilepsy; (iii) the issue of antiepileptic drug action and resistance; and (iv) the pathophysiology of seizure genesis in Taylor's type focal cortical dysplasia. Finally, we will address some of the problems that are inherent to this type of experimental approach, in particular the lack of proper controls and possible strategies to obviate this limitation.

GABA(B) receptor activation modulates GABA(A) receptor-mediated inhibition in chicken nucleus magnocellularis neurons

Neurons of nucleus magnocellularis (NM), a division of avian cochlear nucleus that performs precise temporal encoding, receive glutamatergic excitatory input solely from the eighth nerve and GABAergic inhibitory input primarily from the ipsilateral superior olivary nucleus. GABA activates both ligand-gated Cl- channels [GABA(A) receptors (GABA(A)Rs)] and G protein-coupled receptors (GABA(B) receptors). The net effect of GABA(A)R-mediated input to NM is inhibitory, although depolarizing. Several studies have shown that this shunting, inhibitory GABAergic input can evoke action potentials in postsynaptic NM neurons, which could interfere with their temporal encoding. While this GABA-mediated firing is limited by a low-voltage-activated K+ conductance, we have found evidence for a second mechanism. We investigated modulation of GABA(A)R-mediated responses by GABA(B)Rs using whole cell recording techniques. Bath-applied baclofen, a GABA(B)R agonist, produced dose-dependent suppression of evoked inhibitory postsynaptic currents (eIPSCs). This suppression was blocked by CGP52432, a potent and selective GABA(B)R antagonist. Baclofen reduced the frequency but not the amplitude of miniature IPSCs (mIPSCs) and did not affect postsynaptic currents elicited by puff application of a specific GABA(A)R agonist muscimol, suggesting a presynaptic mechanism for the GABA(B)R-mediated modulation. Firing of NM neurons by synaptic stimulation of GABAergic inputs to NM was eliminated by baclofen. However, endogenous GABA(B)R activity in the presynaptic inhibitory terminals was not observed. We propose that presynaptic GABA(B)Rs function as autoreceptors, regulating synaptic strength of GABA(A)R-mediated inhibition, and prevent NM neurons from generating firing during activation of the inhibitory inputs.

Mechanism of GABAB receptor-mediated inhibition of spontaneous GABA release onto cerebellar Purkinje cells

gamma-Aminobutyric acid (GABA)(B) receptor-mediated modulation of spontaneous GABA release onto Purkinje cells was investigated in cerebellar slices from 3- to 5-week-old mice. The GABA(B) receptor agonists baclofen and CGP 44533 each reduced the frequency of miniature inhibitory postsynaptic currents (mIPSCs), with no significant effect on mIPSC amplitude; together, consistent with a presynaptic site of action. The GABA(B) receptor antagonist CGP 55845 blocked baclofen-induced inhibition. The sulphydryl alkylating agent N-ethylmaleimide occluded baclofen effects, implicating G(i/o) subunits in mediating a GABA(B) G protein-coupled receptor pathway. Baclofen-induced inhibition persisted in the presence of Ba(2+), a blocker of K(+) channels, and Cd(2+), a blocker of Ca(2+) channel-mediated GABA release. Application of nominally Ca(2+)-free extracellular solutions reduced mIPSC frequency and amplitude; however, baclofen produced a significant inhibition in mIPSC frequency, further suggesting that this pathway was independent of Ca(2+) influx. Spontaneous GABA release was increased by the adenylate cyclase activator, forskolin, and the phorbol ester, phorbol 12,13-dibutyrate. However, baclofen-induced inhibition was not significantly changed in either condition. Baclofen action was also not affected by the adenylate cyclase inhibitor SQ 22536 or the protein kinase C inhibitor chelerythrine chloride. Baclofen still reduced mIPSC frequency in the presence of the polyvalent cation ruthenium red, which acts as a secretagogue here; however, baclofen-induced inhibition was reduced significantly. Furthermore, baclofen produced no clear inhibition during high-frequency mIPSCs bursts induced by the potent secretagogue alpha-Latrotoxin. Together, these results suggest that GABA(B) inhibition occurs downstream of Ca(2+) influx and may be mediated, in part, by an inhibition of the vesicular release mechanism.

GABAA receptor-dependent synchronization leads to ictogenesis in the human dysplastic cortex

Patients with Taylor's type focal cortical dysplasia (FCD) present with seizures that are often medically intractable. Here, we attempted to identify the cellular and pharmacological mechanisms responsible for this epileptogenic state by using field potential and K+-selective recordings in neocortical slices obtained from epileptic patients with FCD and, for purposes of comparison, with mesial temporal lobe epilepsy (MTLE), an epileptic disorder that, at least in the neocortex, is not characterized by any obvious structural aberration of neuronal networks. Spontaneous epileptiform activity was induced in vitro by applying 4-aminopyridine (4AP)-containing medium. Under these conditions, we could identify in FCD slices a close temporal relationship between ictal activity onset and the occurrence of slow interictal-like events that were mainly contributed by GABAA receptor activation. We also found that in FCD slices, pharmacological procedures capable of decreasing or increasing GABAA receptor function abolished or potentiated ictal discharges, respectively. In addition, the initiation of ictal events in FCD tissue coincided with the occurrence of GABAA receptor-dependent interictal events leading to [K+]o elevations that were larger than those seen during the interictal period. Finally, by testing the effects induced by baclofen on epileptiform events generated by FCD and MTLE slices, we discovered that the function of GABAB receptors (presumably located at presynaptic inhibitory terminals) was markedly decreased in FCD tissue. Thus, epileptiform synchronization leading to in vitro ictal activity in the human FCD tissue is initiated by a synchronizing mechanism that paradoxically relies on GABAA receptor activation causing sizeable increases in [K+]o. This mechanism may be facilitated by the decreased ability of GABAB receptors to control GABA release from interneuron terminals.

Ca2+-independent muscarinic excitation of rat medial entorhinal cortex layer V neurons

Cholinergic activation of entorhinal cortex (EC) layer V neurons plays a crucial role in the medial temporal lobe memory system and in the pathophysiology of temporal lobe epilepsy. Here, we demonstrate that muscarinic activation by focal application of carbachol depolarizes EC layer V neurons and induces epileptiform activity in rat brain slices. These seizure-like bursts are associated with a somatic [Ca2+]i increase of 293 +/- 82 nm and are blocked by the glutamate receptor antagonists CNQX and APV. Muscarinic activation did not directly evoke a [Ca2+]i increase, but subthreshold and suprathreshold depolarization did. Functional axon mapping revealed local axon branching as well as axon collaterals ascending to layers II and III. During blockade of ionotropic glutamatergic AMPA and NMDA receptors, carbachol depolarized layer V neurons by +7.5 +/- 3.4 mV. This direct muscarinic depolarization was associated with a conductance increase of 35 +/- 10.3% (+4.3 +/- 1.25 nS). Intracellular buffering of [Ca2+]i changes did not block this depolarization, but prolonged action potential duration and reduced adaptation of action potential firing. The muscarinic depolarization was neither blocked by combining intracellular Ca2+-buffering (EGTA or BAPTA) with non-specific Ca2+-channel inhibition by Ni+ (1 mm), nor by Ba2+ (1 mm) nor during inhibition of the h-current by 2 mm Cs+. In whole-cell patch-clamp recording, reversal of the muscarinic current occurred at about -45 mV and -5 mV with complete substitution of intrapipette K+ with Cs+. Thus, muscarinic depolarization of EC layer V neurons appears to be primarily mediated by Ca2+-independent activation of non-specific cation channels that conduct K+ about three times as well as Na+.

Partial hippocampal kindling increases GABAB receptor-mediated postsynaptic currents in CA1 pyramidal cells

In previous studies, we showed that partial hippocampal kindling decreased the efficacy of the presynaptic GABAB receptors on both GABAergic and glutamatergic terminals of CA1 neurons in hippocampal slices in vitro. In this study, GABAB receptor-mediated inhibitory postsynaptic currents (GABAB-IPSCs) were assessed by whole-cell recordings in CA1 pyramidal neurons in hippocampal slices of male Long-Evans rats. The peak GABAB-IPSC evoked by a brief train of supramaximal stratum radiatum stimuli (20 pulses of 300 Hz) in the presence of picrotoxin (0.1 mM) and kynurenic acid (1 mM) was larger in neurons of kindled (65.9 +/- 5.2 pA, N=42 cells) than control (45.8 +/- 4.8 pA, N=32 cells) rats (P<0.01). Adding GABA uptake blocker nipecotic acid (1 mM) or GABAB receptor agonist baclofen (0.01 mM) in the perfusate induced outward currents that were blocked by GABAB receptor antagonist CGP 55845A (1 microM). The peak outward current induced by nipecotic acid was larger in neurons of the kindled (55.4 +/- 5.7 pA, N=30) than the control group (39.8 +/- 4.5 pA, N=28) (P<0.05). However, the magnitude of the baclofen-induced current was not different between kindled (90.8 +/- 6.9 pA, N=29) and control (87.2 +/- 5.9 pA, N=21) groups (P>0.05). We concluded that partial hippocampal kindling increased GABAB-IPSCs in hippocampal CA1 pyramidal cells via multiple presynaptic mechanisms.

GABAB receptor transduction mechanisms, and cross-talk between protein kinases A and C, in GABAergic terminals synapsing onto neurons of the rat nucleus basalis of Meynert

The transduction mechanisms underlying presynaptic GABAB receptor-mediated inhibition of transmitter release have been characterized for a variety of synapses in the central nervous system (CNS). These studies have suggested a range of transduction mechanisms, including a role for second messengers such as protein kinases A (PKA) and C (PKC). In the present study, we have examined the intracellular signalling pathways underlying baclofen-induced inhibition of GABA release from terminals synapsing onto rat basalis of Meynert neurons using patch-clamp recordings. Baclofen, a selective GABAB receptor agonist, reversibly decreased both evoked and spontaneous, miniature, GABAergic inhibitory postsynaptic currents (eIPSCs and mIPSCs, respectively). Such baclofen actions were completely abolished by CGP55845A, a selective GABAB receptor antagonist, and by staurosporine, a non-selective PKA and PKC inhibitor. The mIPSC frequency was still decreased by baclofen even in the presence of 4 AP, a K+ channel blocker, and Cd2+, a voltage-dependent calcium channel blocker. Pharmacological activation or inhibition of PKC activity affected basal GABA release and mildly affected the response to baclofen. Inhibition of the cAMP/PKA cascade also affected basal GABA release and, in a subset of neurons, occluded the effects of baclofen, suggesting that the GABAB receptor-mediated inhibitory action on GABA release was mediated via decreases in PKA activity. In addition, PKA inhibition occluded the effects of PKC modulation on both basal GABA release and on the response to baclofen. Our results characterize the transduction pathway of baclofen at these nucleus basalis of Maynert (nBM) synapses and show, for the first time, some cross-talk between the cAMP/PKA and PKC pathways in mammalian presynaptic nerve terminals.

Suppression of epileptiform activity by GABA(B) receptors in wild type and weaver hippocampus 'in vitro'

Inhibition by GABA(B) receptors comprises activation of K(+) conductance and inhibition of Ca(2+) conductance, thereby reducing action potential dependent transmitter release and silencing neuronal activity. We compared epileptiform activity and its inhibition by the activation of GABA(B) receptors in homozygous weaver (wv/wv) and wild type (+/+) CA3 neurons disinhibited by GABA(A) receptor blockade. In wv/wv mice GABA(B) receptors have lost their ability to activate K(+) conductance (J. Neurosci. 18 (1998) 4001). Spontaneous synchronous burst discharges in elevated [K(+)](o) displayed only subtle differences in +/+ and wv/wv slices, except that the GABA(B) receptor agonist R-baclofen in low concentration (0.1 microM) strongly reduced the frequency of synchronous bursts in +/+ CA3 neurons, but not in wv/wv CA3 neurons. A high affinity GABA(B) antagonist, CGP55845A (0.5 microM) promoted the incidence of bursts in low [K(+)](o). Concentration dependence of the reduction of evoked EPSCs was identical in wv/wv and +/+ neurons (IC(50)=0.3 microM). Amplitudes of evoked IPSCs were reduced by 0.01 microM R-baclofen in +/+, but not in wv/wv CA3 neurons. The effect of the low concentration was abolished by Ba(2+), which is known to block Kir conductance. The data suggest that activation of Kir conductance is important for the control of GABA release by GABA(B) autoreceptors in the CA3 network. We conclude that the loss of a contribution of Kir conductance to GABA(B) receptor-mediated autoinhibition reduces the inclination towards spontaneous bursts of wv/wv CA3 pyramidal neurons.

Masking synchronous GABA-mediated potentials controls limbic seizures

PURPOSE

We determined how CA3-driven interictal discharges block ictal activity generated in the entorhinal cortex during bath application of 4-aminopyridine (4AP, 50 microM).

METHODS

Field potential and [K+]o recordings were obtained from mouse combined hippocampus-entorhinal cortex slices maintained in vitro.

RESULTS

4AP induced N-methyl-d-aspartate (NMDA) receptor-dependent ictal discharges that originated in the entorhinal cortex, disappeared over time, but were reestablished by cutting the Schaffer collateral (n = 20) or by depressing CA3 network excitability with local application of glutamatergic receptor antagonists (n = 5). In addition, two types of interictal activity occurred throughout the experiment. The first type was CA3 driven and was abolished by a non-NMDA glutamatergic receptor antagonist. The second type was largely contributed by gamma-aminobutyric acid type A (GABAA) receptor-mediated conductances and persisted during blockade of glutamatergic transmission. The absence of CA3-driven interictal discharges in the entorhinal cortex after Schaffer collateral cut facilitated the GABA-mediated interictal potentials that corresponded to large [K+]o elevations and played a role in ictal discharge initiation. Accordingly, ictal discharges along with GABA-mediated interictal potentials disappeared during GABAA-receptor blockade (n = 7) or activation of mu-opioid receptors that inhibit GABA release (n = 4).

CONCLUSIONS

Our findings suggest that CA3-driven interictal events restrain ictal discharge generation in the entorhinal cortex by modulating the size of interictal GABA-mediated potentials that lead to large [K+]o elevations capable of initiating ictal discharges in this structure.

GABA(B) receptor-mediated inhibition of mitral/tufted cell activity in the rat olfactory bulb: a whole-cell patch-clamp study in vitro

GABA, the major inhibitory neurotransmitter involved in information processing in the olfactory bulb, is hypothesized to act through GABA(B) receptors by depressing primary neurotransmitter release at the level of olfactory nerve axon endings. The present study was designed to analyze GABA(B) receptor-mediated inhibition mechanisms by performing whole-cell patch-clamp recordings of mitral/tufted cell activity in the rat in vitro. To do so, GABA(B) receptor-mediated action was mimicked by baclofen and antagonized by saclofen. Our protocol led us to provide an original description of GABA(B) receptor-mediated inhibition exerted on mitral/tufted cells. First, their spontaneous activity was shown to be drastically abolished by baclofen. Second, their responses to olfactory nerve electrical stimulation were graded by GABA(B) receptor-mediated inhibition. Indeed, this inhibition may be described as inducing effects ranked from a slight increase in response latency to a complete response suppression.Altogether, our results corroborate the hypothesis of a presynaptic extrasynaptic GABA(B) receptor-mediated inhibition influencing mitral/tufted cell olfactory nerve responsivity. However, the involvement of postsynaptic receptors, with different properties or with different anatomical locations, cannot be ruled out, particularly in the control of spontaneous activity. In conclusion, we underline that, in the vertebrate olfactory bulb, GABA(B) receptor-mediated action appears to contribute to make mitral/tufted cell responses more salient by reducing their resting activity.

Analysis of the function of GABA(B) receptors on inhibitory afferent neurons of Purkinje cells in the cerebellar cortex of the rat

Purkinje cells, the output neurons of the cerebellar cortex, receive inhibitory input from basket, stellate and neighbouring Purkinje cells. The aim of the present study was to clarify the role of GABAB receptors on neurons giving inhibitory input to Purkinje cells. In sagittal slices prepared from the cerebellar vermis of the rat, the GABAB receptor agonist baclofen lowered the frequency and amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) recorded in Purkinje cells. These effects were prevented by the GABAB receptor antagonist CGP 55845. Two mechanisms were involved in the depression of the inhibitory input to Purkinje cells. The first mechanism was suppression of the firing of basket, stellate and Purkinje cells. The second mechanism was presynaptic inhibition of GABA release from terminals of the afferent axons. This was indicated by the finding that baclofen decreased the amplitude of IPSCs occurring in Purkinje cells synchronously with action potentials recorded in basket cells. A further support for the presynaptic inhibition is the observation that baclofen decreased the amplitude of autoreceptor currents which are due to activation of GABAA autoreceptors at axon terminals of basket cells by synaptically released GABA. The presynaptic inhibition was partly due to direct inhibition of the vesicular release mechanism, because baclofen lowered the frequency of miniature IPSCs recorded in Purkinje cells in the presence of cadmium and in the presence of tetrodotoxin plus ionomycin. The results show that activation of GABAB receptors decreased GABAA receptor-mediated synaptic input to cerebellar Purkinje cells both by lowering the firing rate of the inhibitory input neurons and by inhibiting GABA release from their axon terminals with a presynaptic mechanism.

GABA(B)-mediated action in the frog olfactory bulb makes odor responses more salient

In the olfactory bulb, GABA(B) receptors are selectively located in the glomerular layer. A current hypothesis is that GABAergic inhibition mediated through these receptors would be, at least partly, presynaptic and would exerted by decreasing the release of the olfactory receptor neuron excitatory neurotransmitter. Here, we assessed, in the frog, the in vivo action of baclofen, a GABA(B) agonist, on single-unit mitral cell activity in response to odors. Local application of baclofen in the glomerular region of the olfactory bulb was shown to drastically affect mitral cell spontaneous activity, since they became totally silent. Moreover, under baclofen, mitral cells still responded to odors and still specified odor concentration increases through their temporal response patterns. The pharmacological specificity of the GABA(B) agonist action was confirmed by showing that saclofen, a GABA(B) antagonist, partly prevented the inhibitory action of baclofen and restored the initial rate of mitral cell spontaneous activity. The results show that GABA(B)-mimicked inhibition suppressed mitral cell spontaneous activity while odor responses were maintained. This suggests that olfactory receptor neurons partly drive spontaneous mitral cell activity. Moreover, the effect of GABA(B)-mediated inhibition was seen to be very close to that described previously for dopamine D(2) receptor-mediated inhibition. In conclusion, we propose that these two inhibitory mechanisms would offer the possibility to reduce or suppress mitral cell spontaneous activity so as to make their responses to odor especially salient.

GABAB receptors mediate frequency-dependent depression of excitatory potentials in rat perirhinal cortex in vitro

Excitatory synaptic transmission in the perirhinal cortex exhibits marked homosynaptic paired pulse depression (PPD) at inter-pulse intervals between 100 and 1000 ms, being maximal at 200 ms. Additionally, there is greater PPD with stimulation of the pathway from the temporal cortex side than with stimulation of the pathway from the entorhinal cortex side. We establish that this frequency-dependent depression relies on the activation of GABAB (gamma-aminobutyric acid) receptors. PPD in both temporal and entorhinal pathways is abolished by either of the selective GABAB receptor antagonists, 3-N[1-(S)-(3, 4-dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl-p-benzyl-phosphinic acid (CGP55845A) or 3-amino-propyl(diethoxymethyl)phosphinic acid (CGP35348). Barium which blocks G-protein-coupled, inwardly rectifying potassium channels, does not block PPD. Heterosynaptic depression mediated by GABAB receptors was also observed. The depression of the entorhinal pathway by stimulation of the temporal pathway is greater than depression of the temporal pathway by stimulation of the entorhinal pathway. Moreover, PPD increases with stimulus strength and the depression is enhanced by short trains of stimuli, consistent with stronger stimulation resulting in more GABA reaching GABAB receptors on excitatory glutamatergic synapses. Synaptic activation of GABAB receptors may be important in regulating excitability in a frequency-dependent manner with maximal depression occurring at approximately 5 Hz, which approximates to the theta rhythm. That homosynaptic and heterosynaptic depression by stimulation of the temporal pathway is greater than by stimulation of the entorhinal pathway suggests that activation of temporal feedforward connections to the perirhinal cortex can dominate the GABAergic control of synaptic activity within the perirhinal cortex.

The GABA(B) receptor antagonist CGP 55845A reduces presynaptic GABA(B) actions in neocortical neurons of the rat in vitro

Use-dependent depression of inhibitory postsynaptic potentials was investigated with intracellular recordings and the paired-pulse paradigm in rat neocortical neurons in vitro. Pairs of stimuli invariably reduced the second inhibitory postsynaptic potential-A (GABA(A) receptor-mediated inhibitory postsynaptic potential) of a pair; at interstimulus intervals of 500 ms, the amplitude of the second inhibitory postsynaptic potential-A was considerably smaller than the first (36.2 +/- 6.2%, n= 17). Decreasing the interstimulus interval reduced the second inhibitory postsynaptic potential-A further and with interstimulus intervals shorter than 330 ms the compound excitatory postsynaptic potential-inhibitory postsynaptic potential response reversed from a hyperpolarizing to a depolarizing response. The depression of the inhibitory postsynaptic potential-A exhibited a maximum at interstimulus intervals near 150 ms and recovered with a time constant of 282 +/- 96.2 ms. Elimination of excitatory transmission by the application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D(-)-2-amino-5-phosphonovaleric acid yielded an essentially unaltered time-course of paired-pulse depression (maximum depression near 150 ms, time constant of recovery 232 +/- 98 ms). The polarity change of the compound excitatory postsynaptic potential response at shorter interstimulus intervals was abolished in the presence of CNQX and D(- )-2-amino-5-phosphonovaleric acid. CNQX and D(-)-2-amino-5-phosphonovaleric acid also reduced the apparent depolarizing shift of the reversal potential between the first and second inhibitory postsynaptic potential-A from about 6 mV to less than 2 mV. Application of the GABA(B) receptor antagonist CGP 55845A in the presence of CNQX and (-)-2-amino-5-phosphonovaleric acid abolished the inhibitory postsynaptic potential-B and paired-pulse depression. Under these conditions, the amplitude of the second inhibitory postsynaptic potential was, on average, about 90% of the first, i.e. reduced by about 10%. The second inhibitory postsynaptic potential-A was approximately constant at interstimulus intervals between 100 and 500 ms. It is concluded that paired-pulse depression of cortical inhibition is predominantly mediated by presynaptic GABA(B) receptors of GABAergic interneurons. The abolition of net inhibition at interstimulus intervals near 330 ms may facilitate spread of excitation and neuronal synchrony during repetitive cortical activation near 3 Hz. This use-dependent depression of inhibition may contribute to highly synchronized slow electroencephalogram activity during spike-and-wave or delta activity.

GABAB-Receptor-mediated currents in interneurons of the dentate-hilus border

GABA(B)-receptor-mediated inhibition was investigated in anatomically identified inhibitory interneurons located at the border between the dentate gyrus granule cell layer and hilus. Biocytin staining was used to visualize the morphology of recorded cells. A molecular layer stimulus evoked a pharmacologically isolated slow inhibitory postsynaptic current (IPSC), recorded with whole cell patch-clamp techniques, in 55 of 63 interneurons. Application of the GABA(B) receptor antagonists, CGP 35348 (400 microM) or CGP 55845 (1 microM) to a subset of 25 interneurons suppressed the slow IPSC by an amount ranging from 10 to 100%. In 56% of these cells, the slow IPSC was entirely GABA(B)-receptor-mediated. However, in the remaining interneurons, a component of the slow IPSC was resistant to GABA(B) antagonists. Subtraction of this antagonist resistant current from the slow IPSC isolated the GABA(B) component (IPSC(B)). This IPSC(B) had a similar onset and peak latency to that recorded from granule cells but a significantly shorter duration. The GABA(B) agonist, baclofen (10 microM), produced a CGP 55845-sensitive outward current in 19 of 27 interneurons. In the eight cells that lacked a baclofen current, strong or repetitive ML stimulation also failed to evoke an IPSC(B), indicating that these cells lacked functional GABA(B) receptor-activated potassium currents. In cells that expressed a baclofen current, the amplitude of this current was approximately 50% smaller in interneurons with axons that projected into the granule cell dendritic layer (22.2 +/- 5.3 pA; mean +/- SE) than in interneurons with axons that projected into or near the granule cell body layer (46.1 +/- 10.0 pA). Similarly, the IPSC(B) amplitude was smaller in interneurons projecting to dendritic (9.4 +/- 2.7 pA) than perisomatic regions (34.3 +/- 5.1 pA). These findings suggest that GABA(B) inhibition more strongly regulates interneurons with axons that project into perisomatic than dendritic regions. To determine the functional role of GABA(B) inhibition, we examined the effect of IPSP(B) on action potential firing and synaptic excitation of these interneurons. IPSP(B) and IPSP(A) both suppressed depolarization-induced neuronal firing. However, unlike IPSP(A), suppression of firing by IPSP(B) could be easily overcome with strong depolarization. IPSP(B) markedly suppressed N-methyl-D-aspartate but not AMPA EPSPs, suggesting that GABA(B) inhibition may play a role in regulating slow synaptic excitation of these interneurons. Heterogeneous expression of GABA(B) currents in hilar border interneurons therefore may provide a mechanism for the differential regulation of excitation of these cells and thereby exert an important role in shaping neuronal activity in the dentate gyrus.

GABA(B) receptor activation promotes seizure activity in the juvenile rat hippocampus

We analyzed how the GABA(B) receptor agonist baclofen (10-50 microM) influences the activity induced by 4-aminopyridine (4-AP, 50 microM) in the CA3 area of hippocampal slices obtained from 12- to 25-day-old rats. Interictal and ictal discharges along with synchronous GABA-mediated potentials occurred spontaneously in the presence of 4-AP. Baclofen abolished interictal activity (n = 29 slices) and either disclosed (n = 21/29) or prolonged ictal discharges (n = 8/29), whereas GABA-mediated potentials occurred at a decreased rate. The N-methyl-D-aspartate (NMDA) receptor antagonist 3,3-(2-carboxypiperazine-4-yl)-propyl-1-phosphate (CPP, 10 microM, n = 8) did not modify the GABA-mediated potentials or the ictal events recorded in 4-AP + baclofen. In contrast ictal, activity, but not GABA-mediated potentials, was blocked by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM, n = 5). Most baclofen effects were reversed by the GABA(B) receptor antagonist CGP 35348 (1 mM; n = 4). Baseline and transient increases in [K(+)](o) associated with the 4-AP-induced synchronous activity were unaffected by baclofen. Baclofen hyperpolarized CA3 pyramids (n = 8) recorded with K-acetate-filled electrodes by 4.8 +/- 1.3 mV and made spontaneous, asynchronous hyperpolarizing and depolarizing potentials disappear along with interictal depolarizations. GABA-mediated synchronous long-lasting depolarizations (LLDs) and asynchronous depolarizations were also studied with KCl-filled electrodes in 4-AP + CPP + CNQX (n = 6); under these conditions baclofen did not reduce LLD amplitude but abolished the asynchronous events. Dentate hilus stimulation at 0. 2-0.8 Hz suppressed the ictal activity recorded in 4-AP + baclofen (n = 8). Our data indicate that GABA(B) receptor activation by baclofen decreases transmitter release leading to disappearance of interictal activity along with asynchronous excitatory and inhibitory potentials. By contrast, GABA-mediated LLDs and ictal events, which reflect intense action potential firing invading presynaptic inhibitory and excitatory terminals respectively, are not abolished. We propose that the proconvulsant action of baclofen results from 1) block of asynchronous GABA-mediated potentials causing disinhibition and 2) activity-dependent changes in hippocampal network excitability.

GABAB-receptor-mediated control of GABAergic inhibition in rat histaminergic neurons in vitro

The onset of slow wave sleep may require an inhibition of histaminergic neurons by GABAergic afferents from the ventrolateral preoptic area. We have utilized electrophysiological methods in an in vitro brain slice preparation to examine the role of GABAB receptor activation in GABAergic synaptic inhibition in histaminergic neurons of the tuberomammillary nucleus. Tetrodotoxin blocked evoked GABAergic IPSPs but not miniature IPSPs or IPSCs. Evoked IPSPs varied in amplitude and exhibited failures of transmission. Baclofen reduced the amplitude of evoked IPSPs in all experiments and often caused an increase in failures of transmission. Responses elicited by application of exogenous GABA were insensitive to baclofen treatment. The action of baclofen was blocked by CGP-35348 (100 microm), a GABAB receptor antagonist, which also enhanced the amplitude of evoked IPSPs. The frequency of spontaneous and miniature IPSPs and IPSCs was reduced by baclofen. However, the amplitude distribution of mIPSCs was not altered. We conclude that GABA release onto TM neurons is under presynaptic control via GABAB receptors. This presynaptic control of transmission to tuberomammillary neurons may reduce inhibition, increasing histamine release and enhancing wakefulness.

GABAB receptor-mediated modulation of presynaptic currents and excitatory transmission at a fast central synapse

Large nerve terminals (calyces of Held) in the medial nucleus of the trapezoid body (MNTB) offer a unique opportunity to explore the modulation of presynaptic channels at a mammalian central synapse. In this study I examined gamma-aminobutyric acid-B (GABAB)-mediated presynaptic inhibition at the calyx of Held in slices of the rat auditory brain stem. The selective GABAB agonist baclofen caused a potent inhibition of synaptic transmission and presynaptic Ca2+ current. The inhibition of presynaptic Ca2+ channels was associated with a slowing of the activation kinetics of the underlying current, and the inhibition was relieved by strong depolarization. The inhibition of both synaptic transmission and presynaptic Ca2+ current was abolished by N-ethylmaleimide, a sulfhydryl alkylating agent that uncouples the G(o)/Gi class of G proteins from receptors. Baclofen does not activate a potassium conductance in the presynaptic terminal. Taken together, these results suggest that GABAB receptors inhibit synaptic transmission via G protein-mediated modulation of presynaptic Ca2+ channels at this large central synapse. Furthermore, these findings demonstrate that basic mechanisms of G protein-mediated inhibition of Ca2+ channels, proposed from recordings of neuron cell bodies, are well conserved at nerve endings in the mammalian brain.

Selective loss of early suppression in the dentate gyrus precedes kainic acid induced electrographic seizures

The role of inhibitory and facilitatory processes in the induction of seizures was studied in a kainic acid (KA) model of epilepsy. The dentate gyrus (DG) response to paired-pulse stimulation of the perforant path (PP) was monitored prior to and immediately following the initial KA induced afterdischarge (AD) in rats chronically prepared with stimulation recording electrodes. The subjects received a 1-h program of stimulation consisting of repeated sequences of pulse pairs at a short (20-30 ms), intermediate (45-90 ms), and long (200-300 ms) interpulse interval (IPIs). The stimulation program was administered both under control conditions and immediately following systemic injection of KA. During the control condition, stable suppression of population spike measures was obtained at the short (early phase) and long (late phase) IPIs, while facilitation was observed at the intermediate IPI. Administration of KA resulted in a progressive loss of suppression prior to the initial AD at the short IPI; neither facilitation nor the late phase of suppression were significantly affected. The early phase decreased further following the initial discharge. Since the early phase most likely reflects recurrent inhibition, these results provide evidence that inhibitory loss precedes the occurrence of KA induced AD, and that this inhibitory loss is increased further following the initial evoked AD. A use-dependent disinhibition is one possible explanation for the change in responsiveness that precedes the AD. This disinhibition could result from a depressed response at GABA-A receptors, an increased responsiveness at GABA-B receptors or possibly both.

Activation of presynaptic GABAB receptors inhibits evoked IPSCs in rat magnocellular neurons in vitro

1508-1517, 1998. Whole cell recordings (nystatin-perforated patch) were carried out on magnocellular neurons of the rat supraoptic nucleus (SON) to study the modulation of inhibitory postsynaptic currents (IPSCs) by gamma-aminobutyric acid-B (GABAB) receptors. Field stimulation adjacent to the SON in the presence of kynurenic acid, evoked monosynaptic GABAergic IPSCs. Baclofen reversibly reduced the amplitude of the IPSCs in a dose-dependent manner (EC50: 0.68 microM) without apparent effect on the holding current (Vh = -80 mV) or input resistance and altered neither the kinetic properties, nor the reversal potential of IPSCs. Concomittant to IPSC depression, baclofen enhanced the paired-pulse ratio for two consecutive IPSCs [interstimulus interval (ISI): 50 ms], an effect consistent with a presynaptic locus of action. Both actions of baclofen were abolished by CGP35348 (500 microM), a GABAB receptor antagonist. In testing for involvement of synaptically activated presynaptic GABAB receptors, we only recorded paired-pulse facilitation at most ISIs tested (50-500 ms), suggesting that the classical GABAB autoreceptors may not normally be activated in our conditions. However, enhancement of local GABA concentration by perfusion of a GABA uptake inhibitor (NO-711) revealed an action of endogenous GABA at these presynaptic GABAB receptors. The nonselective K+ channel blocker Ba2+ abolished baclofen's effect and pertussis toxin (PTX) pretreatment (200-500 ng/ml for 18-24 h) was ineffective in blocking the baclofen-induced inhibition, making an involvement of PTX-sensitive G protein unlikely. The present results show that presynaptic GABAB receptors that are coupled to PTX-insensitive G-proteins may be activated by endogenous GABA under conditions of reduced GABA uptake, thus regulating the inhibitory synaptic input to SON.

Somatostatin depresses excitatory but not inhibitory neurotransmission in rat CA1 hippocampus

In rat CA1 hippocampal pyramidal neurons (HPNs), somatostatin (SST) has inhibitory postsynaptic actions, including hyperpolarization of the membrane at rest and augmentation of the K+ M-current. However, the effects of SST on synaptic transmission in this brain region have not been well-characterized. Therefore we used intracellular voltage-clamp recordings in rat hippocampal slices to assess the effects of SST on pharmacologically isolated synaptic currents in HPNs. SST depressed both (R, S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate and N-methyl--aspartate (NMDA) receptor-mediated excitatory postsynaptic currents (EPSCs) in a reversible manner, with an apparent IC50 of 22 nM and a maximal effect at 100 nM. In contrast, SST at concentrations up to 5 microM had no direct effects on either gamma-aminobutyric acid-A (GABAA) or GABAB receptor-mediated inhibitory postsynaptic currents (IPSCs). The depression of EPSCs by SST was especially robust during hyperexcited states when polysynaptic EPSCs were present, suggesting that this peptide could play a compensatory role during seizurelike activity. SST effects were greatly attenuated by the alkylating agent N-ethylmaleimide, thus implicating a transduction mechanism involving the Gi/Go family of G-proteins. Use of 2 M Cs+ in the recording electrode blocked the postsynaptic modulation of K+ currents by SST, but did not alter the effects of SST on EPSCs, indicating that postsynaptic K+ currents are not involved in this action of SST. However, 2 mM external Ba2+ blocked the effect of SST on EPSCs, suggesting that presynaptic K+ channels or other presynaptic mechanisms may be involved. These findings and previous results from our laboratory show that SST has multiple inhibitory effects in hippocampus.

G protein-coupled inwardly rectifying K+ channels (GIRKs) mediate postsynaptic but not presynaptic transmitter actions in hippocampal neurons

To study the role of G protein-coupled, inwardly rectifying K+ (GIRK) channels in mediating neurotransmitter actions in hippocampal neurons, we have examined slices from transgenic mice lacking the GIRK2 gene. The outward currents evoked by agonists for GABA(B) receptors, 5HT1A receptors, and adenosine A1 receptors were essentially absent in mutant mice, while the inward current evoked by muscarinic receptor activation was unaltered. In contrast, the presynaptic inhibitory action of a number of presynaptic receptors on excitatory and inhibitory terminals was unaltered in mutant mice. These included GABA(B), adenosine, muscarinic, metabotropic glutamate, and NPY receptors on excitatory synapses and GABA(B) and opioid receptors on inhibitory synapses. These findings suggest that a number of G protein-coupled receptors activate the same class of postsynaptic K+ channel, which contains GIRK2. In addition, the GIRK2 channels play no role in the inhibition mediated by presynaptic G protein-coupled receptors, suggesting that the same receptor can couple to different effector systems according to its subcellular location in the neuron.

cAMP modulates an S-type K+ channel coupled to GABAB receptors in mammalian respiratory neurons

Premotor respiratory neurons from neonatal rats express a Ba(2+)-insensitive outward rectifying K+ channel (KOR) which is activated by gamma-aminobutyric acid acting at its beta receptor. The biophysical properties of KOR are similar to those described for the S-channel (KS) which underlies simple forms of non-associative learning in the marine mollusc Aplysia. We show here that KOR, like the S-channel, is inhibited by cAMP. In addition, we demonstrate that this inhibition is due to a change in closed time kinetics. Our data suggests that the ionic and biochemical substrates underlying synaptic plasticity in Aplysia have been phylogenetically conserved in mammalian motor circuits such as that controlling rhythmic breathing movements.

GABAB receptor-mediated inhibition of spontaneous inhibitory synaptic currents in rat midbrain culture

1. Tight-seal, whole-cell recording was used to study GABAB receptor-mediated inhibition of spontaneous inhibitory synaptic currents in cultured rat midbrain neurones. 2. Spontaneous miniature inhibitory postsynaptic currents (mIPSCs) were recorded in tetrodotoxin (TTX), Cd2+ and Ba2+. (R)-(-)-baclofen reduced the frequency of mIPSCs through a presynaptic mechanism. The EC50 for this effect was 7 microM. It was antagonized by the GABAB receptor antagonist CGP55845A (0.5 microM). 3. In pertussis toxin (PTX)-treated cultures, some GABAB receptor-mediated reduction of the frequency of mIPSCs persisted. In contrast, PTX treatment totally abolished inhibition of miniature excitatory postsynaptic currents (mEPSCs). 4. In PTX-treated cultures, a saturating concentration of (R)-(-)-baclofen inhibited action potential-generated IPSCs but no EPSCs. 5. PTX treatment abolished the (R)-(-)-baclofen-mediated inhibition of high voltage-activated somatic Ca2+ currents and of spontaneous IPSCs depending on presynaptic Ca2+ entry. 6. We conclude that cellular mechanisms underlying GABAB receptor-mediated inhibition of mIPSCs contribute to auto-inhibition of GABA release.

Kainic acid-induced seizures enhance dentate gyrus inhibition by downregulation of GABA(B) receptors

Seizures cause a persistent enhancement in dentate synaptic inhibition concurrent with, and possibly compensatory for, seizure-induced hippocampal hyperexcitability. To study this phenomenon, we evoked status epilepticus in rats with systemic kainic acid (KA), and 2 weeks later assessed granule cell inhibition with paired-pulse stimulation of the perforant path (PP) in vitro. Controls demonstrated three components of paired-pulse inhibition: early inhibition (10-30 msec), intermediate facilitation (30-120 msec), and late inhibition (120 msec to 120 sec). After seizures, inhibition in all components was enhanced significantly. The GABA(A) antagonist bicuculline blocked only early enhanced inhibition, demonstrating that both GABA(A) and GABA(B) postsynaptic receptors contribute to seizure-induced enhanced inhibition. In controls, the GABA(B) antagonist CGP 35348 increased both GABA(A) and GABA(B) responses in granule cells, suggesting that CGP 35348 acts presynaptically, blocking receptors that suppress GABA release. In contrast, slices from KA-treated rats were markedly less sensitive to CGP 35348. To test the hypothesis that GABA(B) receptors regulating GABA release are downregulated after seizures, we measured paired-pulse suppression of recurrent IPSPs, or disinhibition, using mossy fiber stimuli. Early disinhibition (< 200 msec) was reduced after seizures, whereas late disinhibition remained intact. CGP 35348 blocked the early component of disinhibition in controls and, to a lesser extent, reduced disinhibition in KA slices. However, paired monosynaptic IPSPs recorded intracellularly showed no difference in disinhibition between groups. Our findings indicate that seizure-induced enhancement in dentate inhibition is caused, at least in part, by reduced GABA(B) function in the polysynaptic recurrent inhibitory circuit, resulting in reduced disinhibition and heightened GABA release.

GABAB receptor-activated inwardly rectifying potassium current in dissociated hippocampal CA3 neurons

GABA and the GABAB receptor agonist baclofen activated a potassium conductance in acutely dissociated hippocampal CA3 neurons. Baclofen-activated current required internal GTP, was purely potassium selective, and showed strong inward rectification. As with acetylcholine-activated current in atrial myocytes, external Cs+ blocked inward but not outward current in a highly voltage-dependent manner, whereas Ba2+ blocked with no voltage dependence. Unlike the cardiac current, however, the baclofen-activated current showed no intrinsic voltage-dependent relaxation. With fast solution exchange, current was activated by baclofen or GABA with a lag of approximately 50 msec followed by an exponential phase (time constant approximately 225 msec at saturating agonist concentrations); deactivation was preceded by a lag of approximately 150 msec and occurred with a time constant of approximately 1 sec. GABA activated the potassium conductance with a half maximally effective concentration (EC50) of 1.6 microM, much lower than that for activation of GABAA receptor-activated chloride current in the same cells (EC50 approximately 25 microM). At low GABA concentrations, activation of the GABAB current had a Hill coefficient of 1.4-2.1, suggesting cooperativity in the receptor-to-channel pathway. Although the maximal conductance activated by GABAB receptors is much smaller than that activated by GABAA receptors, its higher sensitivity to GABA and slower time course make it well suited to respond to low concentrations of extra-synaptic GABA.

Spread of low Mg2+ induced epileptiform activity from the rat entorhinal cortex to the hippocampus after kindling studied in vitro

Extracellular recordings were performed in in vitro combined hippocampal-entorhinal cortex (HC-EC) slices obtained from control and amygdala kindled rats to investigate the spread of epileptiform activity from the entorhinal cortex (EC) to the hippocampus (HC). Epileptiform activity was induced by lowering extracellular Mg2+ concentration. In control slices epileptiform activity was in most slices characterized by intericatal discharges and short recurrent discharges in areas CA1 and CA3 and by early seizure like events and late recurrent discharges in the EC and the subiculum. In spite of well preserved anatomical pathways in the combined HC-EC slice in which most of the fibre connectivity between the EC and the dentate gyrus (DG) is intact, seizure like events and late recurrent discharges generated in the EC had only moderate effects on the epileptiform activity in areas CA3 and CA1. In contrast in HC-EC slices obtained from kindled rats epileptiform activity generated in the EC spread to the DG and the areas CA3 and CA1. Kindling facilitates the propagation of seizure like events and late recurrent discharges through the HC-EC slice and appears to alter the filtering function of the DG.

Enhancement of paired-pulse depression in the dentate gyrus in vivo by the NMDA antagonist, MK-801, and electrical kindling

We have previously demonstrated that late paired-pulse depression of dentate granule cell field potentials decreases when stimulus intensity is increased from moderate to high levels. Voltage-dependent N-methyl-D-aspartate (NMDA) currents are increasingly activated within this stimulus range, and are enhanced following the development of kindled seizures. The NMDA antagonist, MK-801 (0.25 and 1.0 mg/kg, i.p.), was used in the present experiments to evaluate the contribution of NMDA currents to the loss of late paired-pulse depression at high stimulus intensities in naive and kindled rats. Paired-pulse stimulus intensity functions were obtained from animals prepared with chronic electrodes in the perforant path and dentate gyrus. MK-801 administration had no effect on the stimulus intensity function for early paired-pulse depression (20-30 ms interpulse intervals, IPI) in either preparation. Late paired-pulse depression (150-500 ms IPI) was significantly enhanced in naive rats by MK-801. In contrast, MK-801 had no effect on the potentiation of late paired-pulse depression recorded from kindled animals. These findings suggest that the ability of NMDA currents to reduce the strength of late paired-pulse depression in naive animals is altered following the development of kindled seizures. A decrease in late paired-pulse depression was observed at high stimulus intensities under all experimental conditions. The latter findings indicate that the processes responsible for the reduction in late paired-pulse depression at high stimulus intensities are unaffected by either NMDA or kindling-induced modulation of late paired-pulse depression.

GABAB receptor-mediated responses in GABAergic projection neurones of rat nucleus reticularis thalami in vitro

1. Whole-cell voltage-clamp recordings were obtained from GABAergic neurones of rat nucleus reticularis thalami (NRT) in vitro to assess pre- and postsynaptic GABAB receptor-mediated responses. Presynaptic inhibition of GABA release was studied at terminals on local axon collaterals within NRT as well as on projection fibres in the somatosensory relay nuclei. 2. The GABAB receptor agonist (R)-baclofen (10 microM) reduced monosynaptically evoked GABAA-mediated inhibitory postsynaptic currents (IPSCs) in NRT and somatosensory relay cells to 11 and 12% of control, respectively. 3. Action potential-independent miniature IPSCs (mIPSCs) were observed in both cell types. Mean mIPSC amplitude was 20 pA in both NRT and relay cells at a holding potential of 0 mV. The mean mIPSC frequencies were 0.83 and 2.2 Hz in NRT and relay cells, respectively. Baclofen decreased mIPSP frequency by about half in each cell type without affecting amplitude. 4. Paired-burst inhibition of evoked IPSCs was studied in relay and NRT cells by applying pairs of 100 Hz stimulus bursts separated by 600 ms. The mean ratio of second to first peak IPSC amplitudes was 0.77. 5. In NRT cells baclofen induced a linear postsynaptic conductance increase of 0.82 nS with an associated reversal potential of -121 mV. A small (0.14 nS) GABAB component of the evoked IPSC was detected in only a minority of NRT cells (3 of 18). 6. All pre- and postsynaptic effects of baclofen, as well as PBI, were largely reversed by the specific GABAB receptor antagonist CGP 35348 (0.5 mM). 7. We conclude that activation of GABAB receptors in NRT leads to presynaptic autoinhibition of IPSCs in both NRT and relay cells, and to direct activation of a small linear K+ conductance. In addition our experiments suggest that reciprocal connectivity within NRT can be partially mediated by a small GABAB inhibitory event.

Electrophysiological actions of GABAB agonists and antagonists in rat dorso-lateral septal neurones in vitro

1. The actions of GABAB-receptor agonists and antagonists on rat dorso-lateral septal neurones in vitro were recorded with intracellular microelectrodes. 2. In the presence of 1 microM tetrodotoxin to prevent indirect neuronal effects caused by action potential-dependent neurotransmitter release, bath application of baclofen (0.1-30 microM) or SK&F 97541 (0.01-3 microM) evoked concentration-dependent hyperpolarizations which reversed close to the potassium equilibrium potential; the EC50S were 0.55 and 0.05 microM, respectively. No significant desensitization was observed during prolonged agonist exposure (< or = 10 min). 3. Hyperpolarizations induced by baclofen were antagonized in a competitive manner by the following GABAB-receptors antagonists (calculated pA2 values in parentheses): CGP 36742 (4.0), 2-OH saclofen (4.2), CGP 35348 (4.5), CGP 52432 (6.7) and CGP 55845A (8.3). Responses to SK&F 97541 were also antagonized by CGP 55845A (pA2 = 8.4). 4. The amplitude of the late, GABAB receptor-mediated inhibitory postsynaptic potential (i.p.s.p.) was reduced by the GABAB antagonists as follows (means +/- s.e.mean): CGP 55845A (1 microM) 91 +/- 5%, CGP 52432 (1 microM) 64 +/- 5%, CGP 35348 (100 microM) 82 +/- 5%, CGP 36742 (100 microM) 76 +/- 8%, and 2-OH saclofen (100 microM) 68 +/- 3%. 5. It is concluded that neurones in the rat dorso-lateral septal nucleus express conventional GABAB receptors, which are involved in the generation of slow inhibitory postsynaptic potentials. CGP 55845A is the most potent GABAB receptor antagonist described in this brain area.

Prolonged field potentials evoked by 1 Hz stimulation in the dentate gyrus of temporal lobe epileptic human brain slices

An abnormal electrophysiological response in brain slices of the dentate gyrus from biopsy material from patients surgically treated for intractable epilepsy (46/57), exhibited characteristics similar to the physiological hallmark of epilepsy, the paroxysmal discharge, a prolonged (30-600 ms) and often large amplitude field potential. The most striking feature of the prolonged response to a single perforant path stimulus was a predominantly biphasic field potential (23/46 cases). The biphasic response was characterized by a negative field potential of substantial duration exceeding 180 ms which followed an initial shorter duration positive field potential. Multiple population spikes occurred during both phases of the response. During a 1 Hz stimulus train applied to the perforant path, the magnitude and duration of the negative component of the field response was significantly increased. Approximately half of the cases (Group 1; 30/57) exhibited potentiation of the biphasic response, while the remaining cases (Group 2; 27/57) exhibited no negative field component during 1 Hz stimulation trains. This repetitive stimulation, in general, increased the area of the field response in a large majority of cases (44/57) regardless of the sign of the field potential. The number of population spikes following 1 Hz stimulation increased significantly for cases in both groups, although the increase was greater for those in Group 1 than in Group 2. Paired pulse depression (20 ms ISI) was reduced in cases that exhibited potentiated biphasic responses during 1 Hz stimulation (Group 1) in comparison to cases that exhibited no negative field potentials (Group 2). Paired pulse depression at a 200 ms ISI was not significantly different between the groups. During a single stimulus, bicuculline disinhibition (20 microM) resulted in either a prolonged positive or biphasic field potential. Intracellularly recorded responses to single perforant path stimuli also exhibited prolonged and large depolarizations that were comparable in time course to the duration of field potentials recorded in the same area whether generated in the absence or presence of bicuculline. The prolonged field potential after bicuculline was reduced by APV (20 microM). We suggest that the prolonged field response, whether biphasic or monophasic when generated by either 1 Hz stimulation or bicuculline disinhibition, may be due directly or indirectly to an increase in membrane depolarization mediated by activation of the NMDA receptor.

Effects of GABAB receptor antagonists on two models of focal epileptogenesis

The acute effects of two GABAB receptor antagonists (phaclofen and CGP-35348) were studied in two types of epileptogenic activity: that produced by intracortical injections of baclofen and that appearing after withdrawal of chronic intracerebral GABA infusion (the GABA-withdrawal syndrome, GWS). Intracortical baclofen induced two types of electrographic paroxysmal discharges: one consisting of single spike-and-wave (pattern I) and another of polyspike-and-wave patterns (pattern II). Both patterns showed similar latencies and temporal evolution of spike frequency discharges. Phaclofen, applied directly into the baclofen-induced epileptogenic focus, suppressed pattern II but was ineffective in modifying both pattern I and the GWS. CGP-35348, administered systemically, inhibited both patterns I and II. Intracortical microinjection of baclofen or phaclofen in rats showing a GWS had no effect, nor the systematically given CGP 35348. These results indicate a differential participation of GABAB receptors in GABA-related epileptic syndromes of cortical origin.

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

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

Adrenergic modulation of hilar neuron activity and granule cell inhibition in the guinea-pig hippocampal slice

To study the effects of norepinephrine on synaptic inhibition in the dentate gyrus, intracellular recordings were made from hilar neurons in the guinea-pig hippocampal slice. The effects of norepinephrine on hilar neurons were compared with changes in the frequency of spontaneous inhibitory postsynaptic potentials recorded from granule cells. Hilar neurons comprised two electrophysiologically distinct groups: type I hilar neurons displayed a pronounced single spike afterhyperpolarization and little spike frequency accommodation, type II hilar neurons had small afterhyperpolarizations and pronounced spike frequency accommodation. The majority of recordings were from type I hilar neurons which are presumably inhibitory to granule cells. In most instances, effects of norepinephrine (2-10 microM) on hilar neurons could be mimicked by the beta-adrenergic agonist isoproterenol (0.1-1 microM). Isoproterenol induced a slight depolarization, blocked a slow afterhyperpolarization and, in type II neurons, reduced spike frequency accommodation. These effects were associated with an increase in the spontaneous discharge rate and an enhancement of spontaneous excitatory and inhibitory postsynaptic potentials. In accordance, isoproterenol and norepinephrine increased the frequency of inhibitory postsynaptic potentials in granule cells. In the presence of the non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione and the N-methyl-D-aspartate receptor antagonist CGP 37849, isoproterenol and norepinephrine also increased the frequency of Cl- -dependent inhibitory postsynaptic potentials in granule cells. Under this experimental condition, however, norepinephrine reduced the discharge rate of type I hilar neurons through an effect on alpha-receptors. In the presence of GABAA receptor blockers, norepinephrine increased the frequency of spontaneously occurring K(+)-dependent inhibitory postsynaptic potentials in granule cells. Accordingly, the frequency of burst discharges in type I hilar neurons was increased. We suggest that the discrepancy in the effect of norepinephrine on the discharge rate of presumed inhibitory hilar neurons and the frequency of Cl- -dependent inhibitory postsynaptic potentials in granule cells results from a direct effect of norepinephrine on GABAergic terminals because norepinephrine also enhanced the frequency of tetrodotoxin-resistant inhibitory postsynaptic potentials in granule cells. Thus, the net effect of synaptically released norepinephrine on synaptic inhibition in the dentate gyrus will be determined by opposing actions of alpha- versus beta-receptor stimulation at the synapse on hilar neurons.

A physiological role for GABAB receptors and the effects of baclofen in the mammalian central nervous system

The inhibitory neurotransmitter GABA acts in the mammalian brain through two different receptor classes: GABAA and GABAB receptors. GABAB receptors differ fundamentally from GABAA receptors in that they require a G-protein. GABAB receptors are located pre- and/or post-synaptically, and are coupled to various K+ and Ca2+ channels presumably through both a membrane delimited pathway and a pathway involving second messengers. Baclofen, a selective GABAB receptor agonist, as well as GABA itself have pre- and post-synaptic effects. Pre-synaptic effects comprise the reduction of the release of excitatory and inhibitory transmitters. GABAergic receptors on GABAergic terminals may regulate GABA release, however, in most instances spontaneous inhibitory synaptic activity is not modulated by endogenous GABA. Post-synaptic GABAB receptor-mediated inhibition is likely to occur through a membrane delimited pathway activating K+ channels, while baclofen, in some neurons, may activate K+ channels through a second messenger pathway involving arachidonic acid. Some, but not all GABAB receptor-gated K+ channels have the typical properties of those G-protein-activated K+ channels which are also gated by other endogenous ligands of the brain. New, high affinity GABAB antagonists are now available, and some pharmacological evidence points to a receptor heterogeneity. The pharmacological distinction of receptor subtypes, however, has to await final support from a characterization of the molecular structure. The function importance of post-synaptic GABAB receptors is highlighted by a segregation of GABAA and GABAB synapses in the mammalian brain.

Stimulus parameters affecting paired-pulse depression of dentate granule cell field potentials. II. Low-frequency stimulation

Low frequency (1 Hz) stimulation of the perforant path produces a depression in the population spike (PS) of dentate granule cell field potentials and also may affect the strength of paired pulse depression. The effects of 1 Hz stimulation (30 s train) on paired pulse depression (20 and 200 ms interpulse intervals, IPI) were evaluated in the unanesthetized rat under two conditions: (i) when the stimulus intensity of both pulses was increased simultaneously (5-100%); and (ii) when the stimulus intensity of the first (conditioning) pulse was increased (5-100%), while the stimulus intensity of the second (test) pulse was held constant (50%). The test PS amplitude was predicted based upon either the conditioning PS amplitude at the end of the 1 Hz train or upon the additive effects of paired pulse depression and 1 Hz stimulation. These predicted values then were assessed for the best fit to observed values following 1 Hz trains. Under both stimulus conditions, the 1 Hz depression in the conditioning PS amplitude exhibited characteristics that were identical to late paired pulse depression recorded before the train. A decrease in the test PS amplitude also was observed following 1 Hz stimulation at the 20 and 200 ms IPIs. The best fit to observed values of the test PS at the end of 1 Hz trains was provided by estimates based upon the additive effects of 1 Hz stimulation and paired pulse depression. These results indicate that the strength of paired pulse depression in the unanesthetized rat is unchanged following 1 Hz stimulation, and further, that the 1 Hz depression in dentate granule cell field potentials most likely reflects the cumulative influence of late paired pulse depression.

Muscarinic amplification of fast excitation in hilar neurones and inhibition in granule cells in the guinea-pig hippocampus

1. Effects of the cholinergic agonist, carbachol (CCh), or the acetylcholinesterase inhibitor, eserine, on presumed inhibitory hilar neurones and on inhibition in granule cells were studied by intracellular recording in guinea-pig hippocampal slices. 2. CCh (1-5 microM) strongly enhanced the discharge activity of hilar neurones and spontaneous and evoked IPSPs in granule cells. 3. Eserine, in an atropine-sensitive manner, increased the excitability of hilar neurones through effects on membrane properties and on excitatory synaptic barrage. EPSPs readily triggered long-duration burst discharges. In granule cells, the amplitudes of evoked GABAA and GABAB receptor-mediated IPSPs were enhanced. 4. In the presence of eserine and antagonists for glutamatergic and GABAergic synaptic transmission, train stimulation evoked atropine-sensitive slow EPSPs. In contrast to those in granule cells, slow EPSPs in hilar neurones were invariably preceded by a strong burst-after-hyperpolarization. 5. We suggest that acetylcholine, released from septo-hippocampal fibres, amplifies fast synaptic excitation of inhibitory hilar neurones and inhibition of granule cells. In the dentate area, muscarinic receptor-mediated effects are faster than anticipated from the time course of the slow EPSP.

Differences in the Cs block of baclofen and 4-aminopyridine induced potassium currents of guinea pig CA3 neurons in vitro

Single-electrode current- and voltage-clamp techniques were employed to study responses elicited by (-)baclofen or gamma-aminobutyric acid (GABA) and 4-aminopyridine (4-AP) induced inhibitory postsynaptic potentials in CA3 pyramidal neurons in guinea pig hippocampal slices. All drugs were applied by the bath to submerged slices in which fast synaptic transmission was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM), bicuculline (50 microM), and picrotoxin (50 microM). (-)Baclofen (0.5 microM) and GABA (1 mM) induced equivalent-sized hyperpolarizations and input resistance decreases. The agonist induced hyperpolarization or current and 4-AP induced hyperpolarizations or currents (4-AP induced K-IPSPs or IPSCs) reversed in sign near the K-equilibrium potential (EK). The GABAB receptor antagonists, OH-saclofen (500 microM) and CGP 35348 (100 microM), reduced (-)baclofen responses, and 4-AP induced K-IPSPs, suggesting that they were mediated by GABAB receptors. Intracellular tetraethylammonium-, and extracellular barium-ions (1 mM) diminished the (-)baclofen induced current and 4-AP induced K-IPSCs. Intracellular Cs-ions blocked the (-)baclofen induced outward current at resting membrane potential but did not grossly affect the inward current recorded at membrane potentials negative to EK. 4-AP induced inwardly or outwardly directed K-IPSCs were not blocked by intracellular Cs-ions. Extracellular Cs-ions (5 mM) blocked the (-)baclofen induced inward K-current, but did not block 4-AP induced inwardly directed K-IPSCs. In conclusion, we found differences in the Cs block of activated by (-)baclofen or the endogenous transmitter GABA.(ABSTRACT TRUNCATED AT 250 WORDS)

The development of GABAB-mediated activity in the rat dentate gyrus

We examined the effects of GABAB receptor activation in the dentate gyrus of hippocampal slices prepared from 6-8-day-old rat pup. Baclofen (0.25-1.0 microM), a GABAB agonist, rapidly and potently disinhibited the developing dentate, similar to its effect in the mature organism. CGP 35348, a GABAB antagonist, quickly reversed the baclofen-induced disinhibition. However, GABAB antagonists did not reverse long-latency (500-1000 ms IPI) paired-pulse depression, suggesting that it is not caused by a late GABAB-mediated IPSP. GABAB-mediated disinhibition of the dentate gyrus can occur by postnatal day 6, providing a powerful mechanism for altering excitability in the developing hippocampus.

Divergence of hippocampal mossy fibers

By connecting the fascia dentata with the hippocampus proper, the axons of the granule cells, the mossy fibers, represent an important element of the main excitatory, trisynaptic pathway of the hippocampal formation. In this review the various synaptic connections of the mossy fibers are discussed. It turns out that the mossy fibers do not only establish synapses with the pyramidal neurons of regio inferior as traditionally assumed, but a variety of local circuit neurons as well as projection cells are also contacted by the mossy fibers. Thus there is an underestimated divergence of the impulse flow within the "trisynaptic" pathway at the level of the mossy fibers. Similarly, the pattern of afferent input to the granule cells, especially that of GABAergic neurons, is more complex than previously assumed. In this respect the concept of a unidirectional "trisynaptic" pathway certainly is an oversimplification. In particular, the hilus of the fascia dentata, that the mossy fibers traverse on their way to regio inferior, is often neglected in this concept. The hilar region comprises a large variety of morphologically and functionally distinct neuronal types that, to a large extent, are targets of hilar mossy fiber collaterals. By focusing on the mossy fiber system, an attempt is made in this review to summarize new data on hippocampal circuitries that have been accumulated since the original description of the trisynaptic pathway. This concept, which originally comprised the synapses of the perforant path fibers on dentate granule cells, the mossy fiber synapses on CA3 pyramidal neurons, and the synapses of the Schaffer collaterals on CA1 pyramidal cells, has been of great heuristic value but needs to be modified in view of recent morphological and physiological data.

Effects of serotonin on hilar neurons and granule cell inhibition in the guinea pig hippocampal slice

Intracellular recordings in guinea pig hippocampal slices were used to study the effects of serotonin (5-HT) on presumed inhibitory hilar neurons and on postsynaptic inhibition of granule cells. 5-HT applied by the bath hyperpolarized only 50% of the hilar neurons tested but all CA3 neurons and granule cells, presumably by activating a K-conductance. The bath application of 4-aminopyridine (4-AP, 50 microM) induced burst discharge activity in hilar neurons and giant inhibitory postsynaptic potentials (IPSPs) in granule cells consisting of a Cl- and K-component. 5-HT (5-10 microM) reversibly blocked the K-component of giant IPSPs in granule cells, but not their Cl-component. In the majority of hilar neurons 5-HT increased the frequency of 4-AP induced burst discharges even when hilar neurons were hyperpolarized. Only in a few hilar neurons 5-HT blocked 4-AP induced burst discharges. We conclude that the changes in burst discharge pattern of hilar neurons correspond with the differential effect of 5-HT on Cl- and K-mediated inhibition of granule cells.

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)

Role of glutamatergic synaptic transmission in synchronized discharges of hilar neurons in guinea pig hippocampal slices

The role of glutamatergic excitatory synaptic transmission in the synchronization of burst discharges in hilar neurons was studied using paired intracellular recording from hilar neurons and granule cells of guinea pig hippocampal slices. The convulsant 4-aminopyridine (4-AP, 50-100 microM) induced synchronous burst discharges in hilar neurons time locked to giant inhibitory postsynaptic potentials (IPSPs) in granule cells. The non-N-methyl-D-aspartic acid (non-NMDA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5-10 microM) disrupted this synchrony. The inhibitory effect of gamma-aminobutyric acid (GABA)B receptor stimulation on the frequency of synchronous activity was smaller in the presence of CNQX than in its absence. We conclude that glutamatergic synapses operating through non-NMDA receptors are required for the synchronization but not for the generation of burst discharges which are induced by 4-AP in hilar neurons.

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.

Characteristics of spontaneous and evoked EPSPs recorded from dentate spiny hilar cells in rat hippocampal slices

1. Excitation of the spiny subtype of hilar neurons in the fascia dentata was characterized by intracellular recording from hilar cells in hippocampal slices. Stimulation of the outer molecular layer was used to activate the perforant path. Evoked responses were examined, as well as the large spontaneous excitatory potentials that are a distinctive characteristic of spiny hilar cells. 2. Excitatory potentials that occurred spontaneously, as well as those that occurred in response to outer molecular layer stimulation, were similar among the cells that were sampled, regardless of morphological variations such as the presence or absence of thorny excrescences. Spontaneous and evoked excitatory postsynaptic potentials (EPSPs) were complex depolarizations that often had several discrete peaks. Spontaneous EPSPs increased in amplitude slightly with hyperpolarization, and evoked EPSPs clearly increased with hyperpolarization. 3. Applications of selective antagonists of excitatory amino acid receptors were used to determine which excitatory amino acid receptor mediates EPSPs of these cells. 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) was used to block the receptor subtype selective for the agonists alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainic acid (the "AMPA/kainate" receptor). 2-amino-5-phosphonovaleric acid (APV) was used to block receptors specific for the agonist N-methyl-D-aspartate (NMDA; the "NMDA" receptor). Perfusion with CNQX (5-25 microM) completely blocked all spontaneous and evoked excitation, even when activity was examined at relatively depolarized membrane potentials and a low concentration of extracellular magnesium (0.5 mM) was used. Under these conditions, APV (25-50 microM) had no detectable effect on spontaneous activity but did increase the stimulus strength required to elicit responses to outer molecular layer stimulation. 4. When extracellular magnesium was lowered to 0 mM (nominally), there was strong evidence for a contribution of NMDA receptors to spontaneous and evoked EPSPs. Thus, when cells were perfused with 0 mM extracellular magnesium and 5 microM CNQX, spontaneous depolarizations were present and EPSPs could be triggered by stimulation of the outer molecular layer. Both the spontaneous and evoked EPSPs were blocked by 25 microM APV. 5. Because gamma-aminobutyric acid (GABA)A receptors can cause depolarizations in hippocampal neurons, the GABAA receptor antagonist bicuculline was used to determine whether some of the EPSPs were mediated by GABAergic neurons that are normally activated by spontaneous release of excitatory amino acids. Bicuculline (5-25 microM) had no effect on spontaneous depolarizations, and led to an enhancement of evoked depolarizations. Therefore it does not appear that GABAA receptor-mediated depolarizations contribute to hilar cell depolarizations.(ABSTRACT TRUNCATED AT 400 WORDS)