Excitatory synaptic transmission and its modulation by PKC is unchanged in the hippocampus of GAP-43-deficient mice

We compared excitatory synaptic transmission between hippocampal pyramidal cells in dissociated hippocampal cell cultures and in area CA3 of hippocampal slice cultures derived from wild-type mice and mice with a genetic deletion of the presynaptic growth associated protein GAP-43. The basal frequency and amplitude of action potential-dependent and -independent spontaneous excitatory postsynaptic currents were similar in both groups. The probability that any two CA3 pyramidal cells in wild-type or GAP-43 knockout (-/-) slice cultures were synaptically connected was assessed with paired recordings and was not different. Furthermore, unitary synaptic responses were similar in the two genotypes. Bath application of phorbol 12,13-diacetate (0.6-3 microM) elicited a comparable increase in the frequency of miniature excitatory synaptic currents in wild-type and GAP-43 (-/-) cultures. This effect was blocked by the protein kinase C inhibitor, bisindolylmaleimide I (1.2 microM). Finally, 3 microM phorbol 12,13-diacetate potentiated the amplitude of unitary synaptic currents to a comparable extent in wild-type and GAP-43 (-/-) slice cultures. We conclude that GAP-43 is not required for normal excitatory synaptic transmission or the potentiation of presynaptic glutamate release mediated by activation of protein kinase C in the hippocampus.     

Effects of GABA and baclofen on pyramidal cells in the developing rabbit hippocampus: an 'in vitro' study

Using the in vitro hippocampal slice preparation, we have investigated the effects of gamma-aminobutyric acid (GABA) and its analogue beta-(p-chlorophenyl)-GABA (baclofen) on CA1 and CA3 pyramidal cells in the developing rabbit hippocampus. Somatic applications: both GABA and baclofen, when applied to CA1 pyramidal cells from immature tissue, led to cell depolarization from resting membrane potential; this baclofen depolarization may be indirectly mediated. In contrast, CA3 pyramidal cells at the same age were primarily hyperpolarized by both drugs. In mature tissue, both GABA and baclofen applied at the soma induce cell hyperpolarizations. Dendritic applications: immature CA1 cells responded to dendritic GABA and baclofen application with depolarizations associated with increased cell excitability; here, too, the baclofen depolarization may be due to indirect 'disinhibition'. Both depolarizing and hyperpolarizing responses were recorded in immature tissue when GABA was applied to CA3 pyramidal cell dendrites: baclofen produced only hyperpolarizations. In mature CA1 cells, dendritic GABA application produced membrane depolarization, but dendritic baclofen application produced hyperpolarizations. In mature CA3 cells, dendritic GABA and baclofen application produced predominant hyperpolarizations. Mature CA1 pyramidal cells appear to retain some of the GABA-induced depolarizations characteristic of immature tissue. In contrast, mature CA3 neurons show only hyperpolarizing responses to GABA and baclofen application. In all cases, responses to GABA and baclofen are associated with a decrease in cell input resistance. We conclude that the GABAergic receptor/channel complexes mature differently in the CA1 and CA3 regions of the hippocampus.     

Neuropeptide FF inhibition of morphine effects in the rat hippocampus

Opioids have an excitatory effect on CA1 pyramidal neurons in the hippocampus due to the inhibition of γ-aminobutyric acid (GABA) release from interneurons. Electrophysiologically, this pyramidal cell excitation is manifest as an increase in extracellularly recorded population spikes, while the reduction in synaptic GABA release is manifest as a decrease in the amplitude of intracellularly recorded inhibitory postsynaptic potentials (IPSPs). Recent studies suggest that some of the behavioral effects of opioids, such as antinociception, can be inhibited antiopioid peptides such as neuropeptide FF (NPFF). In the present study, we have used the hippocampal response to opioids to examine the potential interactions between morphine and NPFF in vitro. Morphine alone (20–200 μM) caused reversible concentration-dependent increases in population spikes and decreases in IPSPs. In extracellular experiments, NPFF (1 μM) alone had no effect on population spikes, but significantly and concentration-dependently inhibited the morphine-induced increases in these responses. Intracellular experiments indicated that while NPFF had no effect on IPSP amplitude, or other pyramidal neurons membrane properties (membrane potential, input resistance, afterhyperpolarization, action potential frequency), it significantly reduced the decrease in IPSP amplitude caused by morphine. These results demonstrate that NPFF can attenuate the effects of morphine on population spikes and IPSPs in the hippocampus, and suggest that this effect occurs at a presynaptic site, possibly involving GABAergic interneurons.     

A specific effect of morphine on evoked activity in the rat hippocampal slice

The effect of morphine (0.5-50 microM) was examined on CA1 field potentials in the tranverse hippocampal slice. Morphine consistently produced an augmentation of evoked activity manifest as (i) a decrease in the threshold for generation of a population spike and (ii) generation of an additional population spike(s) whose amplitude was proportional to the position of the sampled response on its input/output curve. Both of these opiate effects were stereospecific and naloxone-reversible. Additional population spikes occurred in opiate medium with either orthodromic or antidromic activation of the pyramidal cells, and the antidromic effect was abolished when synaptic transmission was blocked, suggesting that morphine did not act directly upon the pyramidal cells. Recordings of population EPSPs in the dendrites of the pyramidal cells showed no changes due to opiate exposure near threshold. Opiate effects were mimicked by the gamma-aminobutyric acid (GABA) antagonist picrotoxin, and were partially to fully reversed by GABA itself, suggesting that disinhibition of pyramidal cells might be involved as a mechanism in this opiate effect. The data are evidence for a specific primary effect of morphine within the hippocampus in spite of the low numbers of opiate receptors in this brain region.     

Norepinephrine decreases synaptic inhibition in the rat hippocampus

The effects of norepinephrine (NE) on inhibitory synaptic potentials were studied on CA1 pyramidal neurons in the hippocampal slice in vitro. Norepinephrine caused the appearance of multiple population spikes in the CA1 region of the hippocampal slice, reminiscent of the actions of gamma-aminobutyric acid (GABA) antagonists. Intracellular recording revealed that NE causes a marked and reversible reduction in inhibitory postsynaptic potentials (IPSPs) recorded in CA1 pyramidal cells. This reduced IPSP results in a larger intracellular excitatory postsynaptic potential (EPSP), which can cause the cell to fire more than one action potential. This disinhibitory effect of NE appears to be mediated by an alpha-receptor, and occurs at a site presynaptic to the pyramidal cell, since NE does not change the reversal potential of the IPSP nor does it affect the amplitude or the reversal potential of iontophoretic GABA responses. In addition to reducing evoked IPSPs, NE causes an increase in the frequency of spontaneous IPSPs, suggesting that inhibition of interneuronal firing may not account for this disinhibitory action of NE.     

Excitatory action of opioid peptides and opiates on cultured hippocampal pyramidal cells

Bath application of low concentrations of opioid peptides and higher concentrations of opiates increased the amplitude and duration of excitatory postsynaptic potentials of pyramidal cells and induced long-lasting depolarization shifts. These actions were reversible and blocked by the opiate antagonist naloxone. Synaptic isolation of the cells by exposure of the cultures to 8 mM Mg2+ not only abolished all spiking and synaptic activity, but also obliterated the peptide effects on pyramidal cells, although these cells were still excited by bath-applied glutamate. The opioid peptides had no detectable effect on resting membrane potential and on the input resistance of the penetrated cells. Experiments in which pyramidal cells were synaptically activated by field stimulation provided direct evidence for a disinhibitory action of the peptides.     

Augmentation of N-methyl-D-aspartate induced depolarizations by GABA in neocortical and archicortical neurons

The influence of the inhibitory transmitter gamma-aminobutyric acid (GABA) on depolarizations elicited by the excitatory amino acid N-methyl-D-aspartate (NMDA) was tested in neurons of organotypic neocortical tissue cultures (newborn rat) and in CA3 neurons of the hippocampal slice (guinea pig). Drugs were applied through a 3-barrelled micropipette by pressure ejection. Applications of GABA before the ejection of NMDA increased the amplitude of the depolarizations induced by the excitatory amino acid. It is suggested that the enhancement of NMDA responses by GABA may be mainly mediated by an intracellular common pathway.     

Reduction of GABAB inhibitory postsynaptic potentials by serotonin via pre- and postsynaptic mechanisms in CA3 pyramidal cells of rat hippocampus in vitro.

The action of serotonin (5-HT) on GABAergic synaptic transmission was investigated with intracellular recordings in CA3 pyramidal cells of rat hippocampal slices. Local application of 5-HT (500 microM) hyperpolarized CA3 pyramidal cells, decreased cellular input resistance, and reduced slow afterhyperpolarizations. Serotonin attenuated the late (GABAB) component of polysynaptic inhibitory postsynaptic potentials (IPSPs; 47% of control) without affecting the early (GABAA) component. During bath application of the excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (20 microM) and 2-amino-5-phosphonovalerate (AP-5) (40 microM), 5-HT similarly decreased the amplitude of the late (GABAB) component (17% of control) of monosynaptic IPSPs but did not affect the early (GABAA) component. The mean reversal potentials of poly- and monosynaptic IPSPs were unaffected by 5-HT. The conductance increases associated with the late component of poly- and monosynaptic IPSPs were reduced by 5-HT. Hyperpolarizing responses evoked in CA3 pyramidal cells by somatic applications of gamma-aminobutyric acid (GABA) were unaffected by 5-HT. During bath application of bicuculline (20-50 microM), hyperpolarizing responses elicited by dendritic GABA application were reduced by 5-HT (71% of control). The effect of 5-HT on these direct GABAB hyperpolarizations (29% decrease in response) does not appear sufficient to fully account for the effect of 5-HT on late GABAB IPSPs (53-83% decrease in response). Therefore, 5-HT may reduce GABAB IPSPs in CA3 pyramidal cells 1) by a postsynaptic action on pyramidal cells and 2) by a selective presynaptic action on GABAergic interneurons mediating the GABAB IPSP. This presynaptic action of 5-HT does not appear to involve excitatory afferents onto inhibitory interneurons.     

Action of vasopressin on CA1 pyramidal neurons in rat hippocampal slices

The effects of arginine⁸-vasopressin (AVP) on the excitability of 47 pyramidal cells of the CA1 region of the hippocampus were determined by using intracellular recording techniques in a submerged slice preparation. Addition of 10⁻⁶ M AVP to the bathing medium evoked an increase in spike discharge which was slow in onset and only gradually reversible. The discharge was accompanied by an increase in excitatory postsynaptic potentials without significant change of the resting input resistance. AVP-induced excitation was found in 81% of ventral and 29% of dorsal hippocampal CA1 pyramidal cells. In low Ca²⁺, high Mg²⁺ solution this excitatory action by AVP was blocked. Microiontophoretic application of AVP onto apical or basal dendrites or the cell body did not result in excitation. These observations suggest that the action of AVP on CA1 pyramidal cells is transsynaptic and is more pronounced in ventral than dorsal CA1.     

Excitatory effects of opiates on the spontaneous EMG activity in pigeon oesophagus

The effects and the mechanism of action of morphine, methionine-enkephalin and leucine-enkephalin were examined in transverse muscular strips from pigeon oesophagus. All the opiates produced a concentration-dependent excitatory effect on the spontaneous EMG activity, characterized mainly by an increase in the spike burst frequency. The maximal excitatory response to morphine and opioid peptides was fully antagonized by naloxone and tetrodotoxin, significantly reduced by atropine and it was not affected by guanethidine pretreatment. Treatment of pigeons with reserpine abolished the excitatory effects induced by opiates. The above results suggest the existence of specific opioid receptors in pigeon oesophagus. Opiates have no direct action on smooth muscle cells, increasing the EMG activity via excitatory both cholinergic and non-adrenergic non-cholinergic neurons. The hypothesis of a possible involvement of serotonergic interneurons might be advanced.     

Δ-Tetrahydrocannabinol and the hippocampus: Effects on CA1 field potentials in rats

The effects of Δ⁹-tetiahydrocannabinol (THC) on ortho- and antidromically elicited CA1 field potentials were observed in locally anesthetized rats and in rats anesthetized with urethane. THC augmented amplitudes of population EPSP's as well as orthodromic and antidromic population spikes from pyramidal cells in locally anesthetized animals. Latencies to peak amplitude of these responses were increased. Conditioning-test shock experiments revealed that THC also depressed recurrent inhibition probably mediated by basket cells. In animals under urethane anesthesia THC enhanced test responses, but failed to augment population responses to the conditioning stimulus. It was concluded that THC enhanced postsynaptic excitatory processes but attenuated recurrent inhibition. Urethane anesthesia completely blocked the postsynaptic excitatory effect of THC but had little apparent influence on THC's disinhibitory action.     

On the sites of presynaptic inhibition by neuropeptide y in rat hippocampus in vitro

Neuropeptide Y (NPY) reduces excitatory synaptic transmission between stratum radiatum and CA1 pyramidal cells in rat hippocampal slice in vitro by a presynaptic action. To understand NPY's role in the control of excitability in hippocampus, its actions on excitatory and inhibitory synaptic transmission were examined, using intracellular, sharp microelectrode, and tight-seal, whole cell recordings from principal neurons in areas CA1, CA3, and dentate. Bath application of 1 μM NPY reversibly inhibited excitatory postsynaptic potentials (EPSPs) evoked in CA1 pyramidal cells from either stratum radiatum or stratum oriens by about 50%. Neuropeptide Y also inhibited EPSPs at mossy fiber-CA3, stratum oriens-CA3, and CA3-CA3 synapses by between 45% and 55%. As in CA1, the action of NPY was presynaptic. By contrast, NPY did not inhibit EPSPs evoked in dentate granule cells from either perforant path or commissural inputs. Neuropeptide Y did not alter postsynaptic membrane properties in any cell type. Although NPY attenuated the orthodromically evoked (stratum radiatum) inhibitory postsynaptic potentials in CA1 pyramidal cells by about the same amount as it inhibited the EPSPs, it did not affect the IPSPs evoked in the same cells by antidromic stimulation from alveus. Inhibitory postsynaptic potentials evoked in pharmacological isolation in CA1, CA3, or dentate were also not significantly affected by NPY. The evidence supports the hypothesis that NPY acts at feedforward excitatory synapses to presynaptically reduce the amplitude of excitation as it travels through hippocampal circuits. By contrast, synaptically mediated inhibition is not directly affected by NPY. Neuropeptide Y is the only known endogenous substance that selectively reduces feedforward excitatory transmission without causing changes in other properties of the hippocampal circuitry.     

GluK1 inhibits calcium dependent and independent transmitter release at associational/commissural synapses in area CA3 of the hippocampus

CA3 pyramidal cells receive three main excitatory inputs: the first one is the mossy fiber input, synapsing mainly on the proximal apical dendrites. Second, entorhinal cortex cells form excitatory connections with CA3 pyramidal cells via the perforant path in the stratum lacunosum moleculare. The third input involves the ipsi‐and contralateral connections, termed the associational/commissural (A/C) pathway terminating in the stratum radiatum of CA3, thus forming a feedback loop within this region. Since this excitatory recurrent synapse makes the CA3 region extremely prone to seizure development, understanding the regulation of synaptic strength of this connection is of crucial interest. Several studies suggest that kainate receptors (KAR) play a role in the regulation of synaptic strength. Our aim was to characterize the influence of KAR on A/C synaptic transmission: application of ATPA, a selective agonist of the GluK1 KAR, depressed the amplitude fEPSP without affecting the size of the fiber volley. Blockade of GABA receptors had no influence on this effect, arguing against the influence of interneuronal KARs. Pharmacological and genetic deletion studies could show that this effect was selectively due to GluK1 receptor activation. Several lines of evidence, such as PPF changes, coefficient of variance–analysis and glutamate uncaging experiments strongly argue for a presynaptic locus of suppression. This is accompanied by an ATPA‐mediated reduction in Ca²⁺ influx at excitatory synaptic terminals, which is most likely mediated by a G‐Protein dependent mechanism, as suggested by application of pertussis toxin. Finally, analysis of miniature EPSCs in the presence and absence of extracellular Ca²⁺ suggest that presynaptic KAR can also reduce transmitter release downstream and therefore independent of Ca²⁺ influx. © 2010 Wiley Periodicals, Inc., Inc.     

Timing of the Developmental Switch in GABAA Mediated Signaling from Excitation to Inhibition in CA3 Rat Hippocampus Using Gramicidin Perforated Patch and Extracellular Recordings

Summary:  The timing of the developmental switch in the GABA_A mediated responses from excitatory to inhibitory was studied in Wistar rat CA3 hippocampal pyramidal cells using gramicidin perforated patch-clamp and extracellular recordings. Gramicidin perforated patch recordings revealed a gradual developmental shift in the reversal potential of synaptic and isoguvacine-induced GABA_A mediated responses from –55 ± 4 mV at postnatal days P0–2 to −74 ± 3 mV at P13–15 with a midpoint of disappearance of the excitatory effects of GABA at around P8. Extracellular recordings in CA3 pyramidal cell layer revealed that the effect of isoguvacine on multiple unit activity (MUA) switched from an increase to a decrease at around P10. The effect of synaptic GABA_A mediated responses on MUA switched from an increase to a decrease at around P8. It is concluded that the developmental switch in the action of GABA via GABA_A receptors from excitatory to inhibitory occurs in Wistar rat CA3 pyramidal cells at around P8–10, an age that coincides with the transition from immature to mature hippocampal rhythms. We propose that excitatory GABA contributes to enhanced excitability and ictogenesis in the neonatal rat hippocampus.     

Δ9-Tetrahydrocannabinol and the hippocampus: Effects on CA1 field potentials in rats

The effects of Δ⁹-tetiahydrocannabinol (THC) on ortho- and antidromically elicited CA1 field potentials were observed in locally anesthetized rats and in rats anesthetized with urethane. THC augmented amplitudes of population EPSP's as well as orthodromic and antidromic population spikes from pyramidal cells in locally anesthetized animals. Latencies to peak amplitude of these responses were increased. Conditioning-test shock experiments revealed that THC also depressed recurrent inhibition probably mediated by basket cells. In animals under urethane anesthesia THC enhanced test responses, but failed to augment population responses to the conditioning stimulus. It was concluded that THC enhanced postsynaptic excitatory processes but attenuated recurrent inhibition. Urethane anesthesia completely blocked the postsynaptic excitatory effect of THC but had little apparent influence on THC's disinhibitory action.     

5-Hydroxytryptamine increases excitability of CA1 hippocampal pyramidal cells.

In the presence of spiperone to block the 5-HT1A-mediated inhibition of pyramidal cell activity, 5-hydroxytryptamine (serotonin, 5-HT) produces a rapid transient increase in amplitude of the extracellularly recorded population spike from area CA1 of the hippocampus. Intracellular recording techniques in area CA1 of rat hippocampal slices were used to identify the ionic mechanism and to characterize the 5-HT receptor mediating this excitatory response to 5-HT. Most of the experiments were conducted in the presence of spiperone to block the 5HT1A hyperpolarization. Since spiperone also has high affinity for 5-HT2 receptors, any response mediated by 5-HT2 receptors would also be blocked. Bath perfusion of the slice with 5-HT increased the rectification of pyramidal cells in the subthreshold region, increased the resistance, and increased the amplitude of subthreshold excitatory postsynaptic potentials (EPSPs) to initiate spike firing. The 5-HT2,1C-selective agonist DOI mimicked this effect of 5-HT, and the 5-HT2,1C antagonist ketanserin (1 microM) blocked the effect of DOI. There was no change in the amplitude of the slow afterhyperpolarization (sAHP) or the amplitude of evoked inhibitory postsynaptic potentials (IPSPs). The increase in rectification and EPSP amplitude by 5-HT occurred even in the presence of the 5-HT4-selective antagonist BRL 24924 to prevent the decrease in amplitude of the sAHP by 5-HT. We conclude that 5-HT produces a fast excitatory response by increasing subthreshold conductance in CA1 hippocampal pyramidal cells. The identity of the receptor mediating this response was not conclusively identified, but resembled the 5-HT1C receptor.     

Effects of ethanol on CA1 and CA3 pyramidal cells in the hippocampal slice preparation: an intracellular study

Superfusion of ethanol (10-350 mM) sometimes caused weak hyperpolarization, but more often elicited weak depolarization or biphasic depolarizing, hyperpolarizing responses in CA1 and CA3 pyramidal neurons of the hippocampal slice. The occasional polarizations were sometimes accompanied by, but not always correlated with, small increases or decreases in input resistance. However, many cells in both areas showed no detectable change in membrane potential (36% of cells) or input resistance (57% of cells), even at very high ethanol concentrations (86-200 mM). Spontaneous spiking, when present, was occasionally accelerated or decelerated, although in CA3 a biphasic speeding-slowing sequence was often seen. The afterhyperpolarizations following bursts of action potentials evoked by current (CA1) or occurring spontaneously (CA3) were most often either slightly reduced in amplitude (CA3) or not affected (CA1) by ethanol superfusion. In contrast, synaptic potentials evoked by stimulation of the hilar mossy fiber pathway (for CA3) or the stratum radiatum (for CA1) were more sensitive to ethanol: excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) were most often reduced in amplitude in both CA1 and CA2, even at low ethanol concentrations (10-50 mM). The action on IPSPs may be exerted presynaptically, because responses to locally applied GABA were little affected. These results suggest that hippocampal evoked synaptic activity may be more sensitive than postsynaptic membrane properties to physiologically relevant ethanol concentrations.     

Evidence for modulation of GABAergic neurotransmission by nicotine

Bath-application of nicotine (800 microM) to mouse hippocampal slices resulted in an increase in the amplitude of the population spike and the appearance of multiple population spikes in the CA1 pyramidal cell layer. Similar effects were observed after perfusion of the GABAA antagonist bicuculline methiodide (2 microM) and the glutamate decarboxylase inhibitor L-C-allylglycine (4 mM). These apparently excitatory effects of nicotine (800 microM) could be reversed by bath-application of gamma-aminobutyric acid (GABA; 400 microM), as well as by the GABA uptake inhibitor nipecotic acid (5 mM) and the benzodiazepine flurazepam (4 microM). Nicotine did not alter binding of [3H]GABA or [3H]flunitrazepam to whole brain plasma membranes. The results are consistent with the hypothesis that the electrophysiological effects of nicotine on CA1 pyramidal cell excitability is mediated by disruption of GABAergic transmission.     

Differential distribution of KCC2 along the axo-somato-dendritic axis of hippocampal principal cells

The neuron-specific potassium-chloride cotransporter 2 (KCC2) plays a crucial role in adjusting intracellular Cl(-) concentrations. The lack of KCC2 in the plasma membrane of the axon initial segment (AIS) of pyramidal cells contributes to variable reversal potentials for perisomatic γ-aminobutyric acid (GABA)(A) receptor-mediated postsynaptic potentials, but the distribution of KCC2 in pyramidal dendrites remains to be established. We applied high-resolution pre-embedding immunolocalization to quantify KCC2 concentrations along dendritic, somatic and axonal regions of rat hippocampal principal cells. Confirming our results on neocortical pyramidal cells, membranes of AIS of CA1 pyramidal cells and dentate granule cells contained 6.4 ± 11.9% and 6.6 ± 14.1% of somatic KCC2 concentrations, respectively. Concentrations of KCC2 in basal dendritic shafts of stratum (str.) oriens were similar to somatic levels (109.2 ± 48.8%). Along apical dendritic shafts of CA1 pyramidal cells, the concentration of KCC2 showed a complex profile: normalized to somatic levels, the density of KCC2 was 124.5 ± 15.7%, 79 ± 12.4% and 98.2 ± 33.5% in the proximal and distal part of str. radiatum and in str. lacunosum moleculare, respectively. Dendritic spines of CA1 receiving excitatory inputs contained 39.9 ± 8.5% of KCC2 concentration measured in shafts of the same dendritic segments targeted by GABAergic inputs. Dendrites of dentate granule cells showed higher KCC2 concentration compared with the soma (148.9 ± 54%), but no concentration gradient was detected between proximal and distal dendrites. In conclusion, the density of KCC2 in hippocampal principal cells increases along the axo-somato-dendritic axis with cell type-specific distribution profiles within the dendritic tree.     

Effects of GABA on CA3 pyramidal cell dendrites in rabbit hippocampal slices

Using the in vitro rabbit hippocampal slice preparation, we have investigated the effects of gamma-aminobutyric acid (GABA) iontophoresis on CA3 pyramidal cell dendrites. The predominant response (70% of the cells tested) was a hyperpolarization associated with a 30% decrease in cell input resistance (Rm). These hyperpolarizations displayed a very pronounced voltage dependency: they were decreased by cell depolarization and flattened by hyperpolarization. Bicuculline methiodide (BMI, 50 microM) did not abolish this response, nor did intracellular iontophoresis of chloride ions. In 5% of the cells, an additional hyperpolarization was obtained with longer ejection times; it reversed close to the reversal potential of the early component of the IPSP. In 25% of the cells, dendritic GABA application produced a depolarization. This response was reversed with cell membrane depolarization and was associated with a large (80%) decrease in Rm. The depolarizations were abolished by BMI (50 microM) and greatly increased by increasing the intracellular chloride concentration. None of the responses to GABA were affected by blockade of synaptic transmission. We conclude that the predominant response of CA3 pyramidal cell dendrites to GABA application is a hyperpolarization mediated by GABAB receptors and probably carried by potassium ions. The depolarizing responses are mediated via GABAA receptors and depend on an increase in chloride permeability.