Bicuculline- and Phaclofen-Resistant Hyperpolarizations Evoked by Glutamate Applications to Stratum Lacunosum-Moleculare in CA1 Pyramidal Cells of the Rat Hippocampus In Vitro

In the hippocampus, different types of interneurons may mediate distinct gamma-aminobutyric acid (GABA) responses, i.e. the early and late inhibitory postsynaptic potentials (IPSPs). To verify this hypothesis, intracellular recordings were obtained from CA1 pyramidal cells (n=63) in rat hippocampal slices. Glutamate (1 mM) was locally ejected in stratum lacunosum-moleculare to activate interneurons in this region. Glutamate-evoked hyperpolarizing responses were characterized in pyramidal cells and compared to the early IPSP and the late IPSP elicited by stratum radiatum electrical stimulation. Several characteristics were similar for the glutamate-evoked IPSPs and late IPSPs: their amplitude was small (-3.4 versus -4.9 mV, respectively), each was associated with a small conductance increase (5.0 versus 9.3 nS, respectively), their peak latency was slow (124.4 versus 129.8 ms, respectively) and in the majority of cells, each displayed little response reversal. However, the equilibrium potential of the glutamate IPSP (-76.5 mV) was similar to that of the early IPSP (-73.8 mV). Perfusion with a low Ca2+ (0.5 mM)/high Mg2+ (8 mM) medium or with tetrodotoxin (1 microM), which blocked synaptic transmission, also reduced the glutamate IPSP. Therefore the glutamate IPSP may be mediated indirectly by inhibitory interneurons. The GABAA antagonist bicuculline (10 microM), or picrotoxin (10-20 microM), blocked the early IPSP, but not the glutamate IPSP. The GABAB antagonist phaclofen (1 mM) attenuated the late IPSP, but did not affect the glutamate IPSP. The results of these experiments suggest that glutamate stimulation of interneurons in stratum lacunosum-moleculare evokes a slow IPSP different from the GABA-mediated early and late IPSPs in CA1 pyramidal cells of the hippocampus.     

Activity-dependent enhancement of hyperpolarizing and depolarizing gamma-aminobutyric acid (GABA) synaptic responses following inhibition of GABA uptake by tiagabine.

The effects of the 7-aminobutyric acid (GABA) uptake blocker tiagabine on isolated inhibitory postsynaptic potentials (IPSPs) were examined in CA1 pyramidal cells of the rat hippocampal slice preparation. The IPSPs were elicited by either single stimuli or by high frequency (100 Hz, 200 ms) stimulation (HFS) of inhibitory interneurons. Bath applied tiagabine (20 microM) produced little or no increase in the amplitude of IPSPs evoked by low (30-50 microA) or high (200-400 microA) intensity single stimuli. Only the duration of IPSPs evoked by high intensity stimuli was substantially prolonged by tiagabine, the time integral of the hyperpolarizing response being increased 3.2-fold. HFS elicited much larger fast and slow IPSPs than a single stimulus. In addition, with increments in the intensity (80-550 microA) of HFS, a GABA(A) receptor-mediated depolarizing response of progressively larger amplitude appeared between, and overlapped with, the fast and slow hyperpolarizing components of the IPSP. Tiagabine application markedly increased the GABA-mediated responses evoked by both low and high intensity HFS. Increasing the intensity of HFS enhanced the drug effect. Thus, measurements of the time integral of evoked responses showed that with weak (60 microA) HFS, tiagabine caused a 3.6-fold increase in the area of hyperpolarization while, in contrast, with strong (530 microA) HFS, tiagabine produced a 13.5-fold increase in the depolarizing actions of GABA. Our results suggest that tiagabine, a therapeutically effective anticonvulsant, may paradoxically increase, through a GABA(A) receptor-mediated mechanism, neuronal depolarization during the high frequency discharge of neurons involved in epileptiform activity.     

Opposing effects of striatonigral feedback pathways on midbrain dopamine cell activity

The existence of a striatonigral GABAergic pathway has been well established both anatomically and biochemically. During intracellular recording from identified DA neurons in vivo, stimulation of the striatum (100 microA, 50 microseconds pulses) elicits an inhibitory postsynaptic potential (IPSP) and a rebound depolarization. The IPSP is a short latency (1.8-2.2 ms) conductance increase to chloride, since: the reversal potential is near the chloride reversal potential reported for other cells (-68 mV); intracellular chloride injection progressively reverses the IPSP into a depolarization with a similar time course; and the response of DA cells to systemic injection of the chloride channel blocker, picrotoxin, also exhibits a similar reversal potential. In contrast, during extracellular recording, stimulation of the striatum at low levels of intensity (e.g. 20 microA at 10 Hz) increases the firing rate of DA cells. Stimulation of the striatum will, in addition, elicit IPSPs in a subclass of substantia nigra zona reticulata neurons at the same latency as the IPSPs triggered in DA cells. These IPSPs also reverse with intracellular chloride injection. However, their amplitude is larger and their duration longer than observed in DA cells, and there is no depolarizing rebound. The late component of the IPSP in the zona reticulata neurons corresponds temporally to the rebound depolarization seen in DA cells in response to striatal stimulation. In addition, when recorded extracellularly, striatal stimulation will inhibit the firing of this class of zona reticulata interneurons at the same stimulation parameters that will excite DA cells. These data suggest that striatal cells may send branched fast-conducting GABAergic projections to zona reticulata cells and DA cells. Furthermore, low levels of striatal stimulation can excite DA cells by preferentially inhibiting interneurons in the zona reticulata which are more sensitive to the inhibitory effects of GABA than are DA neurons.     

Hippocampal inputs to identified neurons in an in vitro slice preparation of the rat nucleus accumbens: evidence for feed-forward inhibition.

The aim of the present study was to analyze responses of nucleus accumbens neurons to stimulation of the fornix. The recorded neurons were labeled with biocytin and identified as medium spiny neurons. A large majority of cells generated a depolarizing postsynaptic potential in response to stimulation of the fornix. Using intracellular current injection, this depolarizing response was dissociated into an EPSP reversing at -6 +/- 6 mV and an IPSP reversing at -71 +/- 4 mV. Both the EPSP and IPSP were abolished by 6-cyano-7-nitroquinoxaline-2,3-dione. In addition, the IPSP was blocked by bicuculline and picrotoxin. The onset latency of the EPSP was constant in spite of varying stimulus intensities. In contrast, the onset latency of the IPSP increased with decreasing stimulus intensity. Notably, the stimulus threshold for evoking IPSPs was generally lower than for EPSPs. At stimulus intensities well above threshold, the IPSP onset was only slightly delayed with respect to the EPSP onset. These results indicate that the EPSP can be characterized as a monosynaptic and glutamate-mediated synaptic response. The IPSP, however, appears to be mediated by a disynaptic feed-forward pathway involving both glutamate and GABAA receptors. Recurrent and lateral inhibitory interactions have previously been proposed to be predominant organizational principles in the caudate-putamen and nucleus accumbens. This study indicates that feed-forward inhibition is an additional principle governing the activities of striatal neural networks.     

GABA(B) receptors inhibit backpropagating dendritic spikes in hippocampal CA1 pyramidal cells in vivo

Spike backpropagation has been proposed to enhance dendritic depolarization and synaptic plasticity. However, relatively little is known about the inhibitory control of spike backpropagation in vivo. In this study, the backpropagation of the antidromic spike into the dendrites of CA1 pyramidal cells was studied by extracellular recording in urethane-anesthetized rats. The population antidromic spike (pAS) in CA1 following stimulation of the alveus was recorded simultaneously with a 16-channel silicon probe and analyzed as current source density (CSD). The pAS current sink was shown to sequentially invade the soma and then the apical and basal dendrites. When the pAS was preceded <400 ms by a conditioning orthodromic CA3 stimulus, the apical and basal dendritic spike sinks were reduced and delayed. Dendritic spike suppression was large after a high-intensity CA3 conditioning stimulus that evoked a population spike, small after a low-intensity CA3 conditioning stimulus, and weak after conditioning by another pAS. The late (150-400 ms latency) inhibition of the backpropagating pAS at the apical and basal dendrites was partially relieved by a GABA(B) receptor antagonist, CGP35348 or CGP56999A, given intracerebroventricularly (icv). CGP35348 icv also decreased the latency of the antidromic spike sinks at all depths. A compartment cable model of a CA1 pyramidal cell with excitable dendrites, combined with a model of extracellular potential generation, confirms that GABA(B) receptor activation delays a backpropagating spike and blocks distal dendritic spikes. GABA(B) receptor-mediated conductance increase and hyperpolarization, amplified by removing dendritic I(A) inactivation, contribute to conditioned dendritic spike suppression. In addition, the model shows that slow Na(+) channel inactivation also participates in conditioned spike suppression, which may partly explain the small dendritic spike suppression after conditioning with a weak orthodromic stimulus or another antidromic spike. Thus, both theory and experiment confirm an important role of the GABA(B) receptors in controlling dendritic spike backpropagation.     

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.     

Kindling induces transient fast inhibition in the dentate gyrus--CA3 projection

The granule cells of the dentate gyrus (DG) send a strong glutamatergic projection, the mossy fibre tract, toward the hippocampal CA3 field, where it excites pyramidal cells and neighbouring inhibitory interneurons. Despite their excitatory nature, granule cells contain small amounts of GAD (glutamate decarboxylase), the main synthetic enzyme for the inhibitory transmitter GABA. Chronic temporal lobe epilepsy results in transient upregulation of GAD and GABA in granule cells, giving rise to the speculation that following overexcitation, mossy fibres exert an inhibitory effect by release of GABA. We therefore stimulated the DG and recorded synaptic potentials from CA3 pyramidal cells in brain slices from kindled and control rats. In both preparations, DG stimulation caused excitatory postsynaptic potential (EPSP)/inhibitory postsynaptic potential (IPSP) sequences. These potentials could be completely blocked by glutamate receptor antagonists in control rats, while in the kindled rats, a bicuculline-sensitive fast IPSP remained, with an onset latency similar to that of the control EPSP. Interestingly, this IPSP disappeared 1 month after the last seizure. When synaptic responses were evoked by high-frequency stimulation, EPSPs in normal rats readily summate to evoke action potentials. In slices from kindled rats, a summation of IPSPs overrides that of the EPSPs and reduces the probability of evoking action potentials. Our data show for the first time that kindling induces functionally relevant activity-dependent expression of fast inhibition onto pyramidal cells, coming from the DG, that can limit CA3 excitation in a frequency-dependent manner.     

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

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

Properties of stereotyped series of postsynaptic potentials in hypoglossal motoneurons

A stereotyped series of postsynaptic potentials produced in cat hypoglossal motoneurons by stimulation of the cerebral cortex, the inferior alveolar nerve or the lingual nerve was studied. These include an excitatory postsynaptic potentials (EPSP) and subsequently 3 different types of inhibitory postsynaptic potential (IPSPs). The first is a short-lasting IPSP which was blocked by strychnine administration. The second is a γ-aminobutyric acid (GABA) IPSP which was blocked by picrotoxin administration. This IPSP was sensitive to membrane polarization and dependent on a conductance increase. The third is a long-duration hyperpolarizing potential which was enhanced by the injection of picrotoxin and insensitive to membrane polarization. Moreover, we have demonstrated that the amplitude of cortically induced EPSPs decreased greatly with depolarization.     

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.     

Distinct types of ionic modulation of GABA actions in pyramidal cells and interneurons during electrical induction of hippocampal seizure-like network activity

It has recently been shown that electrical stimulation in normal extracellular fluid induces seizure-like afterdischarge activity that is always preceded by GABA-dependent slow depolarization. These afterdischarge responses are synchronous among mature hippocampal neurons and driven by excitatory GABAergic input. However, the differences in the mechanisms whereby the GABAergic signals in pyramidal cells and interneurons are transiently converted from hyperpolarizing to depolarizing (and even excitatory) have remained unclear. To clarify the network mechanisms underlying this rapid GABA conversion that induces afterdischarges, we examined the temporal changes in GABAergic responses in pyramidal cells and/or interneurons of the rat hippocampal CA1 area in vitro. The extents of slow depolarization and GABA conversion were much larger in the pyramidal cell group than in any group of interneurons. Besides GABA(A) receptor activation, neuronal excitation by ionotropic glutamate receptors enhanced GABA conversion in the pyramidal cells and consequent induction of afterdischarge. The slow depolarization was confirmed to consist of two distinct phases; an early phase that depended primarily on GABA(A)-mediated postsynaptic Cl- accumulation, and a late phase that depended on extracellular K+ accumulation, both of which were enhanced by glutamatergic neuron excitation. Moreover, extracellular K+ accumulation augmented each oscillatory response of the afterdischarge, probably by further Cl- accumulation through K+-coupled Cl- transporters. Our findings suggest that the GABA reversal potential may be elevated above their spike threshold predominantly in the pyramidal cells by biphasic Cl- intrusion during the slow depolarization in GABA- and glutamate-dependent fashion, leading to the initiation of seizure-like epileptiform activity.     

Electrophysiological evidence for the existence of GABAA receptors in cultured frog melanotrophs

The neurotransmitter GABA exerts a biphasic effect on alpha-melanocyte-stimulating hormone (alpha-MSH) secretion from pars intermedia cells: GABA induces a rapid and transient stimulation followed by a sustained inhibition of alpha-MSH release. In the present study, we have investigated the effect of GABA on the electrophysiological properties of frog melanotrophs in primary culture using the patch-clamp technique in the whole cell configuration. In all cells tested, GABA stimulated an inward current and induced depolarization. A transient period of intense firing was consistently observed at the onset of GABA administration. During the depolarization phase, the membrane potential reached a plateau corresponding to the Cl- equilibrium potential. When repeated hyperpolarizing pulses were applied, an increase of membrane conductance was observed throughout the response evoked by GABA. The effect of GABA was abolished by the chloride channel blocker picrotoxin, and by antagonists of GABAA receptors (bicuculline and SR 95531). The depolarizing action of GABA was mimicked by muscimol, an agonist of GABAA receptors. Taken together, our results indicate that the rapid and transient stimulation of alpha-MSH release induced by GABA can be accounted for by activation of a chloride conductance which causes membrane depolarization. These data support the notion that the transient stimulation of alpha-MSH secretion induced by GABA can be accounted for by membrane depolarization which provokes activation of voltage-operated calcium channels. Since no evidence was found for GABA-induced hyperpolarization, the intracellular mechanisms leading to the strong inhibitory effect of GABA on alpha-MSH secretion remain to be elucidated.     

Excitatory and inhibitory transmission from dorsal root afferents to neonate rat motoneurons in vitro

Intracellular recordings were made from antidromically identified motoneurons in neonate (12-22 days) rat transverse spinal cord slices and the transmitters and receptors probably involved in initiating the excitatory (EPSP) and inhibitory (IPSP) postsynaptic potentials were investigated. Stimulation of dorsal roots elicited in motoneurons an EPSP, an IPSP, or an EPSP followed by an IPSP. EPSPs in 70% of motoneurons had a short latency (less than or equal to 1 ms) and in the remaining cells a latency longer than 1 ms. The IPSPs had a long latency (greater than or equal to 1 ms). Short- and long-latency EPSPs were enhanced by the acidic amino acid uptake inhibitor L-aspartic acid-beta-hydroxamate (AAH) and depressed by the non-selective glutamate receptor antagonists gamma-D-glutamylglycine (DGG) and kynurenic acid. Short-latency EPSPs were suppressed by the quisqualate/kainate (QA/KA) receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) but not by the N-methyl-D-aspartate (NMDA) receptor antagonists D-(-)-2-amino-5-phosphonovaleric acid (APV) and ketamine. Long-latency EPSPs were reduced by DNQX as well as by APV and ketamine. Superfusion of the slices with a Mg-free solution increased the EPSPs and unmasked a late, APV-sensitive component. The IPSP was reduced by the glycine antagonist strychnine as well as by APV and ketamine but resistant to DNQX. The results indicate that stimulation of dorsal roots elicited in motoneurons a monosynaptic EPSP mediated by glutamate/aspartate acting predominantly on the QA/KA subtype of glutamate receptors; an NMDA component can be unveiled in Mg-free solution.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dissociation of the IPSP and response to GABA during spreading depression-like depolarizations in hippocampal slices

We have compared the sensitivity of CA1 and CA3 hippocampal pyramidal cells, in mature and immature tissue, to spreading depression-like depolarization episodes. Using hippocampal slices from rabbit, we have found that mature and immature tissue, and CA1 and CA3 neurons, were differentially prone to depolarization episodes, depending on the method used to produce the depolarization. CA1 region was generally more sensitive than CA3. Spontaneous and stimulus-evoked depolarizations were seen more frequently in immature tissue than in mature slices, but anoxia-induced depolarizations were much more likely to occur in mature tissue. Synaptic transmission and responses to somatic gamma-aminobutyric acid (GABA) ejection were compared during anoxia-induced depolarizations in mature slices. The early component of the inhibitory postsynaptic potential (IPSP) normally had the same reversal potential as the GABA response. During anoxia-induced depolarization, both the drug response and the PSPs were lost. Synaptic transmission generally disappeared before the response to exogenous GABA application; the GABA response reappeared before synaptic function was restored. During the recovery of resting potential (RMP) following depolarization, the reversal potential of the early IPSP differed significantly from that of the GABA response; when the cell had recovered to RMP, the IPSP was depolarizing, whereas GABA application produced a 'normal' cell hyperpolarization. IPSPs and GABA-mediated responses attained their pre-depolarization form within a few minutes of RMP recovery. These observations suggest that, at least under special circumstances, the early component of the IPSP and GABA-mediated hyperpolarizations can be dissociated. Therefore, the early IPSP may be mediated by more complex mechanisms than a simple alteration in chloride conductance due to GABA-receptor interactions.     

Conductance changes and inhibitory actions of hippocampal recurrent IPSPs

Intracellular recordings were obtained from CA1 pyramidal neurons in obliquely cut in vitro hippocampal slices. Recurrent IPSPs were elicited by antidromic stimulation of alvear fibers. The mechanisms by which IPSPs depress pyramidal cell excitability were investigated.Recurrent IPSPs could be reversed in sign by small hyperpolarizing currents applied through the recording electrode, indicating an increased membrane conductance. By using an AC bridge circuit it was found that the maximum impedance decrease usually occurred slightly before the peak of the IPSP. Otherwise the time course of the impedance change matched that of the IPSP itself.Inhibitory actions of the conductance increase were studied by adjusting the membrane potential to the IPSP equilibrium potential, thus allowing only the IPSP conductance to play an inhibitory role. Under these conditions non-linear summation of recurrent IPSPs with EPSPs originating in the apical dendrites could be demonstrated only during the initial 15–25 msec of the IPSP, which is the period of maximum conductance increase.The inhibition afforded by the hyperpolarization of the recurrent IPSP far outlasts the period of effective EPSP shunting by the inhibitory synaptic currents. The mechanisms of recurrent inhibition in the hippocampus thus appear similar to those operating in spinal motoneuron IPSPs.     

Intracellular correlates of morphine excitation in the hippocampal slice preparation

The influences of morphine and opioid peptides on hippocampal CA1 pyramidal cells were investigated using intracellular recordings from the in vitro slice preparation. Morphine applied to somal and basal dendritic areas of CA1 cells via a pressure ejection system confirmed a number of excitatory actions of opiates and opioid peptides in this brain region. These included an increase in the amplitude and duration of orthodromically (radiatum) elicited EPSPs and a decrease in amplitude of the following IPSP. The increase in EPSP amplitude was accompanied by a reduction in stimulus intensity necessary for eliciting the action potential. Morphine delivered to the slice in this manner induced synaptically elicited and spontaneous multiple spike burst discharges. In slice maintained in 10⁻⁴ M pentobarbital¹, morphine reversed the presumably GABA mediated long-duration depolarization following orthodromic stimulation. Finally, depending on the specific site of application (apical or basal dendritic region) of the opiate, morphine produced two different effects on the resting membrane potential and input resistance of CA1 pyramidal cells. These findings are discussed as to whether opiates act directly excitatory influences in the hippocampus, or via blockade of GABA mediated inhibitory mechanisms.     

In vitro studies of the role of gamma-aminobutyric acid in inhibition in the lateral septum of the rat

Focal stimulation, stimulation of the fimbria, and stimulation of the medial septal area result in an inhibitory postsynaptic potential (IPSP) in lateral septal neurons. Increased stimulus intensity results in the appearance of a late hyperpolarizing potential (LHP). Treatment of the slice with bicuculline methiodide or picrotoxin results in blockade of the IPSP. When present, LHPs are enhanced in the presence of bicuculline or picrotoxin. Spontaneous and evoked IPSPs reverse near -70 mV, and LHPs reverse near -90 mV. Iontophoretic application of gamma-amino-butyric acid (GABA) results in hyperpolarizing, depolarizing, or biphasic potentials. Treatment with bicuculline or picrotoxin results in depression of biphasic GABA responses that appears selective for the depolarizing portion of the potential. At high concentrations of bicuculline, a portion of the hyperpolarizing GABA potential persists. The reversal potential of the depolarizing GABA potential is near -30 mV, and the reversal potential of monophasic hyperpolarizing GABA potential is near -70 mV. The bicuculline-resistant hyperpolarizing GABA response has a reversal potential near -90 mV. GABA activates three separate conductances on septal neurons, which are similar to those reported on hippocampal neurons. The resistance of the hyperpolarizing GABA potential to bicuculline appears to be due to the presence of a GABA-activated potassium conductance, which is similar to that activated by baclofen.     

Hyperexcitability of CA3 pyramidal cells in mice lacking the potassium channel subunit Kv1.1

PURPOSE: To investigate further the membrane properties and postsynaptic potentials of the CA3 pyramidal cells in mice that display spontaneous seizures because of a targeted deletion of the Kcna1 potassium channel gene (encoding the Kv1.1 protein subunit). METHODS: Intracellular recordings were obtained from CA3 pyramidal cells in hippocampal slices prepared from Kcna1-null and control littermates. CA3 pyramidal cells were activated: orthodromically, by stimulating mossy fibers; antidromically, by activating Schaffer collaterals; and by injecting intracellular pulses of current. Responses evoked under these conditions were compared in both genotypes in normal extracellular medium (containing 3 mM potassium) and in medium containing 6 mM potassium. RESULTS: Recordings from CA3 pyramidal cells in Kcna1-null and littermate control slices showed similar membrane and action-potential properties. However, in 33% of cells studied in Kcna1-null slices bathed in normal extracellular medium, orthodromic stimulation evoked synaptically driven bursts of action potentials that followed a short-latency excitatory postsynaptic potential (EPSP)-inhibitory PSP (IPSP) sequence. Such bursts were not seen in cells from control slices. The short-latency gamma-aminobutyric acid (GABA)A-mediated IPSP event appeared similar in null and control slices. When extracellular potassium was elevated and excitatory synaptic transmission was blocked, antidromic activation or short pulses of intracellular depolarizing current evoked voltage-dependent bursts of action potentials in the majority of cells recorded in Kcna1 null slices, but only single spikes in control slices. CONCLUSIONS: Lack of Kv1.1 potassium channel subunits in CA3 pyramidal cells leads to synaptic hyperexcitability, as reflected in the propensity of these cells to generate multiple action potentials. The action-potential burst did not appear to result from loss of GABAA receptor-mediated inhibition. This property of CA3 neurons, seen particularly when tissue conditions become abnormal (e.g., elevated extracellular potassium), helps to explain the high seizure susceptibility of Kcna1-null mice.     

gamma-Aminobutyric acid (GABA) causes consistent depolarization of neurons in the guinea pig supraoptic nucleus due to an absence of GABAB recognition sites

The action of gamma-aminobutyric acid (GABA) in the supraoptic nucleus was investigated using guinea pig brain slices. GABA produced a membrane depolarization accompanied by a decrease in the input resistance. The action of GABA was concentration-dependent throughout a wide range of concentrations (10(-7)-10(-3) M). In none of the cells examined, a membrane hyperpolarization was observed. The reversal potential for the depolarization induced by GABA was about 25 mV positive to the resting membrane potential. The amplitude of the GABA-induced depolarization was increased to 1.5 X the control by reducing the external Cl- from 134.2 mM to 10.2 mM. The action of GABA was readily antagonized by relatively low concentrations of bicuculline (10(-5) M). The action of GABA in the hippocampus or in the anterior hypothalamus was markedly different from that in the supraoptic nucleus, i.e. GABA produced both depolarizing and hyperpolarizing responses in the hippocampus and consistently a hyperpolarization in the anterior hypothalamus. The depolarizing but not the hyperpolarizing response in the hippocampus was selectively blocked by picrotoxin (2 X 10(-5) M) or by bicuculline (10(-5) M). The depolarizing component was dependent on the external Cl- concentration and had a reversal potential similar to that of the depolarization induced by GABA in the supraoptic nucleus. The hyperpolarizing component was resistant to bicuculline and had a reversal potential about 30 mV negative to the resting membrane potential.(ABSTRACT TRUNCATED AT 250 WORDS)