GABA-induced chloride current in rat isolated Purkinje cells

Emerging roles for multifunctional ion channel auxiliary subunits in cancer

Several superfamilies of plasma membrane channels which regulate transmembrane ion flux have also been shown to regulate a multitude of cellular processes, including proliferation and migration. Ion channels are typically multimeric complexes consisting of conducting subunits and auxiliary, non-conducting subunits. Auxiliary subunits modulate the function of conducting subunits and have putative non-conducting roles, further expanding the repertoire of cellular processes governed by ion channel complexes to processes such as transcellular adhesion and gene transcription. Given this expansive influence of ion channels on cellular behaviour it is perhaps no surprise that aberrant ion channel expression is a common occurrence in cancer. This review will focus on the conducting and non-conducting roles of the auxiliary subunits of various Ca2+, K+, Na+ and Cl- channels and the burgeoning evidence linking such auxiliary subunits to cancer. Several subunits are upregulated (e.g. Cavβ, Cavγ) and downregulated (e.g. Kvβ) in cancer, while other subunits have been functionally implicated as oncogenes (e.g. Navβ1, Cavα2δ1) and tumour suppressor genes (e.g. CLCA2, KCNE2, BKγ1) based on in vivo studies. The strengthening link between ion channel auxiliary subunits and cancer has exposed these subunits as potential biomarkers and therapeutic targets. However further mechanistic understanding is required into how these subunits contribute to tumour progression before their therapeutic potential can be fully realised.

Toxin-related seizures

Toxin-related seizures result from an imbalance in the brain's equilibrium of excitation-inhibition. Fortunately, most toxin-related seizures respond to standard therapy using benzodiazepines. However, a few alterations in the standard approach are recommended to ensure optimal care and expedient termination of seizure activity. If 2 doses of a benzodiazepine do not terminate the seizure activity, a therapeutic dose of pyridoxine (5 g intravenously in an adult and 70 mg/kg intravenously in a child) should be considered. Phenytoin should be avoided because it is ineffective for many toxin-induced seizures and is potentially harmful when used to treat seizures induced by theophylline or cyclic antidepressants.

GABA-mediated component in the feedback response of turtle retinal cones

The negative feedback from horizontal cells to cone photoreceptors contributes to the formation of the receptive-field surround in cone photoreceptors. Recently, studies on the modulation of voltage-gated Ca(2+) currents in cone photoreceptors have led to great progress in our understanding of the mechanism of horizontal-cone feedback. Another highly probable hypothesis is that GABA mediates this feedback. This hypothesis is supported by the facts that cone photoreceptors respond to GABA and that horizontal cells release GABA. However, GABA-mediated synaptic inputs from horizontal cells to cone photoreceptors have not been demonstrated. In the present study, we examined whether cone photoreceptors receive GABAergic inputs from horizontal cells using a slice patch technique in the turtle retina. When 1 mM of GABA was applied to the cone photoreceptors, GABA-induced currents were activated. GABA-induced currents reversed their polarity at the equilibrium potential of Cl-. The application of 30 microM of SR95531, an antagonist of GABAA receptors, alone did not produce any change in the holding currents. When 200 microM of pentobarbital was introduced to potentiate the GABAergic inputs to the cone photoreceptors, however, the inhibitory action of SR95531 on GABAergic inputs became detectable. The amplitude of the GABAergic inputs, potentiated by pentobarbital, increased when the horizontal cells were depolarized by the application of 20 microM of kainate, while the amplitude decreased when the horizontal cells were hyperpolarized by the application of 10 microM of CNQX. When the cone photoreceptors were voltage clamped at a potential at which the voltage-gated Ca(2+) current was inactive, horizontal-cone feedback was not observed. However, the horizontal-cone feedback became detectable when the GABAergic inputs to the cone photoreceptors were potentiated by pentobarbital. We concluded that the contribution of GABAergic inputs from horizontal cells to cone pedicles in the formation of the receptive-field surround in cone photoreceptors is very limited but that the modulation of voltage-gated Ca(2+) currents in cone photoreceptors is a physiologically relevant mechanism for horizontal-cone feedback.

GABA(A) receptors in Müller glial cells of the human retina

The present study was aimed at characterizing the GABA(A) receptor-mediated currents in acutely isolated glial (Müller) cells of the human retina and investigating their subcellular localization across the Müller cell membrane. Extracellular application of GABA evoked two current responses in human Müller cells: a fast transient GABA(A) receptor-mediated current that inactivated within 10 s and that was independent of extracellular Na(+), and a sustained current that was dependent on extracellular Na(+) and that was mediated by high-affinity GABA transporters. The receptor current was half-maximally activated at a GABA concentration of 32 microM, while the transporter current showed an affinity constant of 7.9 microM GABA. The receptor currents were blocked by bicuculline and picrotoxin and were also activated by muscimol or by other amino acids. The receptor currents are Cl(-) currents, as indicated by the close relationship between the reversal potential of these currents and the Cl(-) equilibrium potential. Using perforated-patch recordings, a mean intracellular Cl(-) concentration of 37 +/- 12 mM was determined in human Müller cells. Using electrophysiological and fluorescence imaging methods, it was revealed that GABA(A) receptors are unevenly distributed across the Müller cell membrane, with higher densities at the endfoot, at the soma, and at the distal sclerad end of the cells. It is concluded that GABA(A) receptor expression may allow a sensing of retinal GABAergic neuronal signal transmission by Müller cells.

Dissociation of spikes, synaptic activity, and activity-dependent increments in rat cerebellar blood flow by tonic synaptic inhibition

Functional neuroimaging relies on the robust coupling between neuronal activity, metabolism and cerebral blood flow (CBF) to map the brain, but the physiological basis of the neuroimaging signals is still not well understood. Here we applied a pharmacological approach to separate spiking activity, synaptic activity, and the accompanying changes in CBF in rat cerebellar cortex. We report that tonic synaptic inhibition achieved by topical application of gamma-aminobutyric acid type A (GABAA) (muscimol) or GABAB (baclofen) receptor agonists abolished or reduced spontaneous Purkinje cell spiking activity without affecting basal CBF. The magnitude of CBF responses evoked by climbing fiber stimulation decreased gradually over time after exposure to muscimol, being more pronounced in the superficial than in the deep cortical layers. We provide direct evidence in favor of a laminar-specific regulation of CBF in deep cortical layers, independent of dilatation of surface vessels. With prolonged exposure to muscimol, activity-dependent CBF increments disappeared, despite preserved cerebrovascular reactivity to adenosine and preserved local field potentials (LFP). This dissociation of CBF and LFPs suggests that CBF responses are independent of extracellular synaptic currents that generate LFPs. Our work implies that neuronal and vascular signals evoked by glutamatergic pathways are sensitive to synaptic inhibition, and that local mechanisms independent of transmembrane synaptic currents adjust flow to synaptic activity in distinct cortical layers. Our results provide fundamental insights into the functional regulation of blood flow, showing important interference of GABAA receptors in translating excitatory input into blood flow responses.

Postnatal ethanol exposure blunts upregulation of GABAA receptor currents in Purkinje neurons

Recently, we found that early postnatal ethanol exposure inhibits the maturation of GABAA receptors (GABAARs) in developing medial septum/diagonal band (MS/DB) neurons, suggesting that these receptors may represent a target for ethanol related to fetal alcohol syndrome (FAS). To determine whether GABAARs on other neurons are also sensitive to a postnatal ethanol insult, postnatal day (PD) 4-9, rat pups were artificially reared and exposed to ethanol (4.5 g kg-1 day-1, 10.2% v/v). The pharmacological profile of acutely dissociated cerebellar Purkinje cell GABAARs from untreated, artificially reared controls and ethanol-treated animals was examined with conventional whole-cell patch clamp recordings during PD 12-16 (juveniles) and PD 25-35 (young adults). For untreated animals, GABA (0.3-100 microM) consistently induced inward Cl- currents in a concentration-dependent manner showing an age-related increase in maximum response without change in EC50 or slope value. Acute ethanol (100 mM) consistently inhibited 3 microM GABA currents (10-20%); positive modulators, pentobarbital (10 microM), midazolam (1 microM) and loreclezole (10 microM), consistently potentiated; the negative modulator, Zn2+ (30 microM), inhibited GABA currents across both juvenile and young adult groups. Loreclezole potentiation increased while Zn2+ inhibition decreased with age in untreated Purkinje neurons. Postnatal ethanol exposure (PD 4-9) decreased GABAAR maximum current density in young adult Purkinje cells but not in juvenile neurons. However, sensitivity to allosteric modulators did not change after ethanol. These data are consistent with the hypothesis that postnatal ethanol exposure during the brain growth spurt can disturb GABAAR development across the brain, although the mechanism(s) underlying this action remains to be determined.

The pharmacology of amino-acid responses in septal neurons

Septal cholinergic neurons are known to play an important role in cognitive processes including learning and memory through afferent innervation of the hippocampal formation and cerebral cortex. The septum contains not only cholinergic neurons but also various types of neurons including GABA (gamma-aminobutyric acid)-ergic neurons. Although synaptic transmission in the septum is mediated primarily by the activation of excitatory and inhibitory amino-acid receptors, it is possible that a distinct phenotype of neuron is endowed with a different type for each of the amino-acid receptors and thus they play different roles from each other, since it has been demonstrated within the septum that there is a regional distribution of various types of amino-acid receptor subunits, their expression as different combinations within a specific cell may produce receptor channels with disparate functional properties. As a first step towards knowing the various functions of septal cholinergic neurons, we characterized the functional properties of glutamate, GABA (type A; GABAA) and glycine receptor channels on cultured rat septal neurons which were histologically identified to be cholinergic. These were similar to those of receptor channels on other types of neurons, except for the actions of some neuromodulators. The septal N-methyl-D-aspartate receptor channel was distinct in being less sensitive to Mg2+ and in a voltage-dependent action of Zn2+. The septal GABAA receptor channel exhibited a lanthanide site whose activation resulted in a positive allosteric interaction with a binding site of pentobarbital. The septal glycine receptor channel was only positively modulated by Zn2+; this action of Zn2+ was not accompanied by an inhibitory effect. Our data suggest that the amino-acid receptors on septal cholinergic neurons may play a distinct role compared to other types of neurons; this difference depends on the actions of neuromodulators and metal cations. It would be interesting to compare these effects recorded in tissue culture to those observed with septal cholinergic neurons in slice preparations.

Differences in the expression of GABA(A) receptors between functionally innervated and non-innervated granule neurons in neonatal rat cerebellar cultures

We had earlier found that granule neurons in cultures of small explants of neonatal rat cerebellar cortex could be placed in two groups: cells in one group showed spontaneous synaptic activity and also had a large response to applications of 1 microM gamma-aminobutyric acid (GABA) while cells in the other lacked spontaneous activity and also showed much lower sensitivity to GABA [25]. For convenience, the more responsive cells will be termed A-type neurons, while the less responsive cells will be termed B-type. We have undertaken a comparison of the responses mediated by activation of GABA A receptors for the two types of neurons. A-type neurons have a larger maximal response to GABA (about 10 times that for B-type neurons), suggesting that they express more functional GABA(A) receptors. The concentration of GABA producing half-maximal activation of A-type neurons is somewhat less (12 mu M) than that for B-type neurons (41 microM), while the Hill coefficients are similar. Responses of both types of cell desensitize to prolonged applications of GABA. At a given concentration of GABA the responses of A-type neurons desensitize more rapidly than the responses of B-type neurons, indicating that the physiological properties of the receptors differ. Responses of A-type neurons are also potentiated to a significantly lesser extent by either chlordiazepoxide or alphaxalone than are the responses of B-type neurons, indicating that the pharmacological properties of the receptors differ. These data indicate that A-type and B-type granule neurons in our cultures express GABA(A) receptors which differ in number, physiological properties and pharmacological responsiveness. We have also confirmed the observation that almost all A-type neurons also show spontaneous synaptic currents, while almost no B-type neurons do.

Dual mechanisms of GABAA response inhibition by beta-lactam antibiotics in the pyramidal neurones of the rat cerebral cortex

1. The effects of beta-lactam antibiotics on the gamma-aminobutyric acid (GABA)-induced Cl- current were investigated in pyramidal neurones freshly dissociated from the rat frontal cortex by the use of a nystatin-perforated patch recording mode under voltage-clamp conditions. 2. The GABA-induced inward current increased in a concentration-dependent manner with an EC50 of 6.7 x 10(-6) M at a holding potential of -40 mV. The GABA response was accompanied by an increase in the membrane conductance and reversed at near the Cl- equilibrium potential. 3. All beta-lactams (penicillin, imipenem, aztreonam and cefotiam) inhibited the 10(-5) M GABA-induced response in a concentration-dependent manner with an IC50 and Hill coefficient of 1.3 x 10(-3) M and 0.64 for penicillin, 9.6 x 10(-4) M and 0.83 for imipenem, 2.5 x 10(-3) M and 9.99 for aztreonam, and 2.9 x 10(-4) M and 1.03 for cefotiam. 4. Imipenem inhibited the GABA-response competitively while penicillin inhibited the same response in a noncompetitive fashion. 5. The inhibitory action of imipenem showed no voltage-dependency, whereas the effect of penicillin was voltage-dependent. 6. It is thus proposed that some classes of beta-lactams, including imipenem may have a mechanism that is different from penicillin and competitively affects the GABAA receptor.

Whole-cell and single-channel currents activated by GABA and glycine in granule cells of the rat cerebellum

1. Patch-clamp methods have been used to characterize GABA-and glycine-activated channels and spontaneous synaptic currents in granule cells in thin cerebellar slices from 7- to 20-day-old rats. 2. All granule cells responded to 10 microM GABA, while approximately 60% responded to 100 microM glycine. With repeated against application, whole-cell responses to GABA, but not those to glycine, declined over a period of minutes unless the pipette solution contained Mg-ATP. 3. Whole-cell concentration-response curves gave EC50 values at 45.2 and 99.6 microM and Hill slopes of 0.94 and 2.6 for GABA and glycine, respectively. At saturating concentrations, currents evoked by GABA were fivefold larger than those evoked by glycine. 4. Whole-cell current-voltage (I-V) relationships of GABA- and glycine-activated currents reversed close to the predicted Cl- equilibrium potential. Partial replacement of intracellular Cl- with F- shifted the GABA reversal potential to a more negative value. 'Instantaneous' I-V relationships produced by ionophoretic application of GABA were linear, while 'steady-state' I-V relationships produced by ramp changes in potential showed outward rectification. For glycine, 'steady-state' I-V plots were linear. 5. Responses to GABA were blocked by the GABAA receptor antagonists bicuculline (15 microM), SR-95531 (10 microM) and picrotoxinin (100 microM) while responses to glycine were selectively blocked by strychnine (200 nM), indicating the presence of two separate receptor types. 6. In outside-out membrane patches, GABA opened channels with conductances of 16 and 28 pS. The proportion of openings to each of the conductances varied between patches, possibly indicating the activation of two distinct channel types. Glycine-activated single-channel currents had conductances of 32, 55 and 104 pS. Single-channel I-V relationships were linear. 7. Spontaneous synaptic currents with a rapid rise time and biexponential decay were present in more than half of the cells examined. These currents were eliminated by bicuculline (15 microM) or SR-95331 (10 microM) and were greatly reduced in frequency by tetrodotoxin (TTX; 300 nM), suggesting that they were mediated by GABA and arose from spontaneous activity in Golgi interneurones. In granule cells where this spontaneous synaptic activity was apparent, glycine and low concentrations of GABA increased the frequency of the synaptic currents.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparison of the actions of glycine and related amino acids on isolated third order neurons from the tiger salamander retina

Whole cell voltage and current clamp recordings were obtained from third order neurons isolated from the salamander retina. Using cross desensitization, the structure-function relationship of short chain amino acids on the glycine receptor were examined. L-Serine, L-alanine, beta-alanine and taurine all cross desensitized with glycine, but did not show significant cross desensitization with GABA. This indicates that these amino acids act at the glycine receptor. The order of potency was glycine >> beta-alanine > taurine >> L-alanine > L-serine. TAG, a reputed selective taurine antagonist, was equally effective in blocking taurine and glycine currents. There is no evidence for distinct receptors for taurine. Amino acids with larger moieties at the alpha carbon, such as threonine and valine, produced inactive ligands. Placing a methyl group on the amine of glycine or esterification of the carboxyl group also greatly reduced activity. Based on these modifications of the glycine molecule, it appears that selectivity at the glycine receptor results in part from steric restrictions at all three sites in the glycine chain. The steric interference is most critical at the carboxyl and amino ends, and less limiting at the alpha carbon. Doses of L-serine that had only slight effects in voltage clamp experiments, nevertheless produced large effects in current clamp experiments. This indicates that several endogenous amino acids can have significant effects on membrane voltage, even when their shunting activity may be small. High concentrations of agonists produced desensitization in the voltage clamp records, but there was little evidence of desensitization in the current clamp experiments. These results indicate that several endogenous amino acids can activate the glycine receptor, but there is no evidence for a discrete receptor for taurine, beta-alanine, L-alanine or L-serine. Since all these endogenous amino acids have similar amino and acid terminals, reduction in potency results from steric interference around the alpha carbon. This graded potency may have functional significance in mediating inhibition.

Stereoselective and non-stereoselective actions of isoflurane on the GABAA receptor

1. Acutely dissociated cerebellar Purkinje neurones from 8-14 day old rats were studied under voltage clamp in the whole-cell patch-clamp configuration. Cl- currents induced by bath application of gamma-aminobutyric acid (GABA) were measured (using symmetrical Cl- solutions) at both low (2 microM) non-desensitizing and high (300 microM) desensitizing concentrations of GABA. 2. At 2 microM GABA, the bicuculline-sensitive Cl- currents were potentiated by racemic isoflurane and both of its optical isomers. Isoflurane had no effect on membrane current in the absence of GABA. The dose-response data for potentiation by racemic isoflurane could be fitted with a Hill equation with an EC50 = 320 +/- 20 microM isoflurane and a Hill coefficient of h = 2.7 +/- 0.4 (means +/- s.e.mean). 3. The potentiations produced by the optical isomers of isoflurane at 2 microM GABA were stereoselective at moderate and high anaesthetic concentrations. The maximum stereoselectivity, about two fold, occurred at the EC50 concentration for general anaesthesia (310 microM isoflurane), with S(+)-isoflurane being more effective than R(-)-isoflurane. At sub-anaesthetic concentrations, the stereoselectivity was less marked and vanished at the lowest concentration used (77 microM isoflurane). 4. The sustained residual current remaining after exposure of neurons to a desensitizing concentration of GABA (300 microM) was inhibited non-stereoselectively, but only at high concentrations of isoflurane. The ratio of inhibitions by S(+)- and R(-)-isoflurane (mean +/- s.e.mean) was 1.14 +/- 0.21 at 770 microM isoflurane. At the EC50 concentration for general anaesthesia, however, the inhibition was barely significant. 5. The above results are discussed in relation to the possible role of the GABAA receptor channel in general anaesthesia.

Multiphasic desensitization of the GABAA receptor in outside-out patches

GABAA receptor function was studied in outside-out patches from guinea pig hippocampal neurons using a drug application system with an exchange time of under 1.5 ms. Application of GABA to these patches induced a Cl- conductance that desensitized with prolonged exposure. Increasing GABA concentrations induced larger conductance increases that were associated with more complex patterns of desensitization. Smaller GABA responses desensitized with monophasic kinetics, whereas large responses displayed bi- and triphasic kinetics. Desensitization of the response to 1 mM GABA was triphasic in about 70% of the patches (tau = 15.4, 207, and 1370 ms) and biphasic in about 30% of the patches (tau = 44 and 725 ms). All phases of desensitization reversed at the Cl- equilibrium potential. Over the concentration range from 3 microM to 3 mM, both the rate and the extent of desensitization increased; however, complete desensitization was rarely observed. The increase in desensitization rate was due to an increase in the relative contribution of the faster phases with increasing GABA. The time constants of the three phases were independent of concentration. The different phases are not mediated by separate receptor populations, because double pulse experiments demonstrated interconversion among the fastest phase and the two slower phases. We demonstrate the plausibility of a model in which multiphasic desensitization is a consequence of the faster association rate at higher GABA concentrations.

Processing of information from different sources: spatial synaptic integration in the dendrites of vertebrate CNS neurons

Most synapses on a neuron are distributed along the dendrites. Inputs from different types of presynaptic neurons often distribute to different dendritic compartments. This provides an anatomical framework for spatial synaptic integration. At the same time, a plethora of time- and voltage-dependent responses are present, usually with a distinct distribution over the somato-dendritic membrane. These intrinsic conductances shape the local dendritic response to ligand-gated conductances, and provide the dendrites with a dynamic way of regulating the interaction between synapses. Recent results from neurons in the vertebrate CNS exemplify these mechanisms of dendritic integration.

Characterization of the GABA-induced current in frog pituitary melanotrophs

The molecular mechanisms regulating GABAA receptor activity in cultured frog melanotrophs were studied using the patch-clamp technique. In the whole-cell configuration, application of GABA evoked a dose-related increase of inward chloride currents. The ED50 value, estimated from the sigmoidal dose-response curve was 2 x 10(-6) M and the Hill coefficient was 1.55. The amplitude of the GABA-induced current decayed with time. Kinetics analysis of the desensitization revealed that the time-course of the current decrement was fitted by one exponential. Graded doses of GABA or association of GABA with the benzodiazepine receptor agonist flunitrazepam accelerated the desensitization process. In contrast, the time-course of the current did not significantly vary at different holding potentials. In the outside-out configuration, GABA was found to activate channels which displayed three unitary conductance levels (8, 15 and 30 pS). The channel openings of the more frequent conductance level (30 pS) exhibited short and long lasting open states (1.2 and 28.3 ms at -60 mV). Altogether these data reveal that frog melanotrophs possess a single population of GABAA receptors which interconvert into a higher affinity state in the presence of benzodiazepine receptor agonists. Two GABA molecules must bind to the receptor to trigger long lasting channel openings. In addition, the activity of the GABAA receptor appears to be independent of the accumulation of intracellular chloride ions.

How quickly can GABAA receptors open?

We have examined GABAA receptor activation by making rapid applications of GABA to outside-out patches excised from cultured postnatal rat cerebellar neurons. The rate of development of current increases with increasing GABA concentration from a low to a high concentration asymptote. The low concentration asymptote is about 10 s-1 for patches taken from granule cells and 4 s-1 for patches from Purkinje cells. The high concentration asymptote is about 6000 s-1 for patches taken from either granule cells or Purkinje cells. The high concentration asymptote gives an estimate of the fastest rate at which these channels can open and indicates that agonist binding steps are not rate limiting. The concentration dependence of the development of current indicates that more than one GABA molecule is bound to most receptors with open channels and that the final binding step is of low affinity (about 500 microM). A comparison with GABA-mediated postsynaptic currents suggests that the properties of the GABAA receptor play a major role in determining the shape of inhibitory synaptic responses and that the cleft concentration of GABA reaches at least 500 microM.

GABA-mediated synaptic interaction between the visual and vestibular pathways of Hermissenda

The synaptic convergence of the eyes and the vestibular hair cells in the nudibranch mollusc Hermissenda has been shown previously to mediate the learning of simple visual-vestibular associations. The neurotransmitter mediating this interaction between the visual and vestibular organs was characterized. HPLC chromatography, confirmed by mass spectroscopic analysis, demonstrated endogenous GABA in the statocysts, in a concentration approximately 150 times greater than in the whole CNS. Additional confirmation was provided by immunocytochemical localization of GABA in hair cell axons and branches that converge with photoreceptor terminal branches. Depolarization of the hair cells in the caudal region of the statocyst in response to positive current injection or vibratory stimulation caused a hyperpolarization and a cessation of the type B photoreceptor impulse activity. The inhibition of the B cell was unaffected by addition to the artificial sea water bath of the adrenergic antagonist yohimbine (250 microM), the cholinergic antagonist atropine (250 microM), and the serotonergic antagonist imipramine (50 microM). In contrast, the GABAA antagonist bicuculline (250 microM) significantly reduced the inhibitory interaction. Moreover, the GABA reuptake inhibitor guvisine (250 microM) increased the hyperpolarization. Pressure microapplication of GABA (12.5 or 25 microM) onto the terminal branches of the B cell resulted in a concentration-dependent hyperpolarization and cessation of spikes in the B cell. Depolarization of the caudal hair cell, or direct GABA application, decreased input resistance across the B cell soma membrane. Moreover, removal of chloride from the extracellular solution reduced inhibition of the B cell induced by GABA application or hair cell stimulation. Furthermore, application of the GABAB agonist baclofen hyperpolarized the type B cell and reduced or eliminated spontaneous impulse activity at the resting membrane potential. The reversal potentials for inhibition induced in all three procedures ranged from -70 to -80 mV and were consistent with mixed Cl- and K+ conductances. These results implicate GABA as the endogenous neurotransmitter mediating visual-vestibular interactions in this animal, and suggest a possible role of GABA in visual-vestibular associative learning.

GABA responses and their partial occlusion by glycine in cultured rat medullary neurons

Whole-cell current responses to bath application of GABA and glycine were studied in medullary neurons cultured from embryonic rats. Two current components were seen in the responses to bath application of GABA, one component which desensitized and another which did not. These two current components have different dose-response characteristics for GABA, with the nondesensitizing component being activated more effectively and reaching its peak amplitude at lower agonist concentrations than the desensitizing one. The agonist concentrations producing half of the maximum responses are 2.8 +/- 0.3 (+/- S.E.M., n = 9) and 14.7 +/- 2.7 (n = 5) microM for the nondesensitizing and desensitizing components, respectively. The two current components for GABA are differentially affected by the antagonists, picrotoxin and bicuculline. The antagonist concentrations which block 50% of the control desensitizing and nondesensitizing responses to GABA are 33 and 320 microM for picrotoxin, and 3 and 50 microM for bicuculline, respectively. Thus, the characteristics of the GABA responses are analogous to those described previously for glycine in that there are two components which are differentially sensitive to agonist concentration [Lewis et al. (1991) J. Neurophysiol, 40, 1178-1187]. We now find there is occlusion between the responses to GABA and glycine, indicating that they share a population of receptors or channels. The occlusion was incomplete (< 80%) in half of the cells, suggesting that both agonists also activate unique receptors. Furthermore, the current responses to 35 microM GABA are blocked by the glycinergic antagonist, strychnine, with half-maximal blocking concentrations equal to 2 and 30 microM for the desensitizing and nondesensitizing components, respectively. This strychnine sensitivity is less than that for the glycine receptor. At the same time, the current responses to 100 microM glycine are sensitive to the GABAergic antagonists, picrotoxin and bicuculline. The half-maximal blocking concentrations are 36 and 120 microM picrotoxin, and 120 and 500 microM bicuculline, for the desensitizing and nondesensitizing components of the glycine response, respectively. Consequently, these results suggest that these cultured cells have at least three types of inhibitory receptors: glycine receptors, GABA receptors and GABA/glycine receptors, with all three receptors sensitive to block by strychnine, bicuculline and picrotoxin. The GABA/glycine receptor may be an immature form of the inhibitory receptor. Alternatively, some GABA and glycine receptors may have common ionophores.

GABA-induced responses in Purkinje cell dendrites of the rabbit cerebellar slice

Pressure applications of GABA localized to Purkinje cell somas in a rabbit cerebellar slice produced uniphasic hyperpolarizing responses, whereas applications of GABA that were directed at the Purkinje cell dendrites produced complex, triphasic responses with hyperpolarizing and depolarizing components. Both somatic and dendritic application of GABA elicited fast hyperpolarization (GABAhf), but dendritic application also elicited a slower depolarization (GABAd) and a later, long-lasting hyperpolarization (GABAhl). All three types of responses were accompanied by increased conductance. Use of either GABA antagonist, bicuculline or picrotoxin, eliminated the GABAhf and GABAd responses but left the GABAhl response intact. Pressure delivery of the GABA agonist, baclofen, to the dendrites but not the soma elicited a GABAhl response. Application of baclofen paired with membrane depolarization sufficient to elicit local, calcium-dependent dendritic spiking produced a persistent reduction in the GABAhl response, whereas alternating presentations of baclofen and membrane depolarization or presentations of baclofen alone could not. The fact that GABA and baclofen inhibited Purkinje cell activity in the rabbit cerebellar slice and that picrotoxin and bicuculline eliminated some, but not all of the components of the GABA response suggests the presence of both GABAA and GABAB receptors. The ability of baclofen to inhibit Purkinje cells if it was applied to the dendrites but not if applied to the soma suggests that GABAB receptors are located predominantly on Purkinje cell dendrites. The pairing-specific change in the baclofen response suggests the existence of GABAB-mediated modifiability of Purkinje cell dendrites.(ABSTRACT TRUNCATED AT 250 WORDS)

Stellate cell inhibition of Purkinje cells in the turtle cerebellum in vitro

1. The stellate cell-mediated inhibition of Purkinje cells was studied by intracellular recordings in an in vitro slice preparation of the turtle cerebellar cortex. A graded inhibitory postsynaptic potential (IPSP) was recorded in Purkinje cells upon stimulation of the parallel fibre-stellate cell pathway. 2. The IPSP was abolished by bicuculline, and had a reversal potential around -75 mV, consistent with a GABAA receptor-operated Cl- conductance dominating the response investigated here. 3. Paired recordings from synaptically coupled stellate cells and Purkinje cells demonstrated that the inhibitory input from a single stellate cell is sufficient to reduce the firing in a Purkinje cell. 4. The extracellular-evoked IPSP interacted with the active postsynaptic membrane properties in the Purkinje cell. Interaction with both the Na+ plateau and the IA prolonged the responses to an IPSP, making the net effect of the inhibitory response dependent on the membrane potential in each postsynaptic neurone. 5. A precisely timed IPSP was particularly efficient in reducing dendritic Ca2+ influx. 6. The voltage-dependent Ca2+ component of a climbing fibre response (CFR) as well as of a parallel fibre (PF) input was reduced by the IPSP. 7. It is suggested that Ca2+ spike-mediated reduction in Purkinje cell excitability may be prevented by the stellate cell IPSP-mediated reduction in Ca2+ influx.

GABAergic synaptic current in dissociated nucleus basalis of Meynert neurons of the rat

gamma-Aminobutyric acid (GABA)-mediated spontaneous inhibitory postsynaptic currents (IPSCs) were recorded from dissociated rat nucleus basalis of Meynert neurons which still had their synaptic boutons attached. The membrane currents were recorded by the whole-cell patch-clamp technique. Elevated extracellular K+ concentration and the addition of the calcium ionophore, A23187, enhanced the amplitude and frequency of spontaneous IPSCs. Ryanodine and Ca(2+)-free external solution containing EGTA or BAPTA markedly decreased the spontaneous IPSC activities. Spontaneous IPSC activities were reversibly reduced by baclofen and increased by phaclofen, indicating that the GABAB receptor regulates the release of GABA from nerve terminals and acts as a negative autoreceptor.

Delayed potassium current and non-specific outward current in pyramidal neurones acutely dissociated from rat hippocampus

The electrical property of delayed K+ currents (IKD) was studied in pyramidal neurones freshly isolated from the rat hippocampal CA1 region. The IKD was separated pharmacologically from other membrane currents. Activation and inactivation processes of the IKD were highly voltage-dependent in the potential range between -30 and +20 mV. The steady-state inactivation of IKD was observed at -100 mV or more positive potentials. The potential for half steady-state inactivation was -65 mV. The IKD was fully inactivated around -20 mV. Reactivation of IKD consisted of two exponential components. After pharmacological suppression of IKD, the small amount of residual voltage-dependent outward current (one-fifteenth to one-twentieth of IKD amplitude) was observed. The current kinetics was similar to that of IKD and greatly reduced by substitution of internal K+ with N-methyl-D-glucamine+. It was concluded that the properties of IKD was basically similar to those of IKD in other excitable tissues and that the residual current might be non-specific outward current.

GABA-induced potentiation of neuronal excitability occurs during contiguous pairings with intracellular calcium elevation

The temporal convergence of neuronal signals is commonly considered as a likely prerequisite for enhanced neuronal excitability underlying the induction of associative memories. Here we report that transmitter application on presynaptic terminals of the Hermissenda Type B photoreceptors, when paired with depolarization, results in a potentiation of the excitability of the B-cell which derives from an increase in input resistance across the B-cell soma membrane. Pressure microapplication of gamma-aminobutyric acid (GABA) (12.5 microM) on the terminal branches of the Hermissenda Type B photoreceptors results in the fast (less than 1 s) activation of an inward Cl- conductance, characterized by a decrease in neuronal membrane resistance and an accompanying hyperpolarization (3-6 mV) of the B-cell. A slower effect of GABA, characterized by a slight depolarization (2-4 mV) and increase in resistance was observed approximately 2 min after GABA application. Following bath application of the Cl- channel blocker picrotoxin (100 microM), this increase in resistance was observed within 20 s of GABA application, suggesting that it was normally masked by the faster Cl- conductance. The magnitude of the resistance increase in response to GABA was enhanced when the B-cell was held at depolarized membrane potentials (-40 to -20 mV), but was eliminated if Ca2+ was removed from the extracellular bath, or if the non-specific protein kinase inhibitor H7 (100 microM) was added to the extracellular bath. In a final experiment, GABA application was paired with a transient (10 s) depolarization of the B-cell (to -20 mV).(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium entry increases the sensitivity of cerebellar Purkinje cells to applied GABA and decreases inhibitory synaptic currents

The sensitivity to GABA of Purkinje cells in thin cerebellar slices was examined by recording either spontaneous inhibitory synaptic currents or ionic currents elicited by local GABA applications. The effects of Ca2+ entry induced by depolarizing voltage pulses were opposite for the two types of currents. Currents due to exogenous GABA applications were increased by a train of voltage pulses. This potentiation was transient with an average half recovery period of 3.7 min. Spontaneous synaptic currents were reduced by depolarizing voltage pulses, with a half recovery time of about 20 s. The inhibition was largely explained by a decrease of the frequency of synaptic events, suggesting that the primary location of the effect was presynaptic. Thus, a Ca2+ rise increases the sensitivity of Purkinje cells to GABA and induces a retrograde inhibition of presynaptic terminals. The latter effect may be due to a diffusible Ca2(+)-dependent messenger.

Strychnine-induced potassium current in isolated dorsal root ganglion cells of the rat

1. Effects of strychnine (Str) on the dissociated dorsal root ganglion (DRG) cells of the rat have been investigated in whole-cells configuration by a conventional patch-clamp technique. 2. gamma-Aminobutyric acid (GABA)-induced Cl- current (ICl) increased sigmoidally with increasing concentration. The half-maximal response (Ka) was 3 x 10(-5) M and the Hill coefficient was 1.5. Both Str and bicuculline inhibited the GABA-induced ICl in a concentration-dependent manner. 3. Str itself could elicit the current at concentrations over 10(-5) M, at which concentrations the GABA response was completely suppressed. The concentration-response curve for the Str-induced current was bell-shaped, and a nearly maximum response occurred at 3 x 10(-4) M. A transient 'hump' current appeared immediately after the wash-out of external solution containing high concentrations of Str over 3 x 10(-4) M. 4. The Str-induced outward current and a transient 'hump' current were augmented by the removal of extracellular K+ and were suppressed by the substitution of intracellular K+ for Cs+. But the current was not sensitive to extracellular Na+, Ca2+ and Cl-. 5. The reversal potential of Str-induced current (EStr) was -75 mV, which was close to the K+ equilibrium potential (EK = -76.3 mV). The change of EStr for a ten fold change in extracellular K+ concentration was 58 mV, indicating that the membrane behaves like a K+ electrode in the presence of Str. The reversal potential of the 'hump' current was also close to EK. 6. The Str-induced outward current was antagonized by K+ channel blockers such as Ba2+, tetraethylammonium (TEA)-chloride, and 4-aminopyridine (4-AP) in a concentration-dependent manner. 7. The Str-induced K+ current was not affected by internal perfusion of bis(gamma-aminophenoxy)ethane-N, N,N',N-tetraacetic acid (BAPTA), indicating that the Str response does not result in the activation of K+ conductance by the intracellular Ca2+.

GABA inhibits the rise in cytosolic free calcium concentration in depolarized immature cerebellar Purkinje cells

gamma-Aminobutyric acid (GABA) reduced the peak rise or slowed the rate of rise in cytosolic free calcium concentration ([Ca]in) induced by quisqualate, kainate, or high KCl in immature cerebellar Purkinje cell bodies. The sustained rise or repeated transient of [Ca]in induced by tetraethylammonium, veratridine, or Bay-K-8644 was lowered to the basal level by adding GABA, although the inhibition by GABA of Bay-K-8644-induced rise in [Ca]in was only slight and transient in some cells. These findings indicate that GABA inhibits the rise in [Ca]in by hyperpolarizing the membrane potentials at Purkinje cell bodies.