The ionic mechanism of postsynaptic inhibition in motoneurones of the frog spinal cord

Differences in the activation of inhibitory motoneuron receptors in the frog Rana ridibunda by GABA and glycine and their interaction

Intracellular recording of potentials was used in isolated spinal cord segments from the frog Rana ridibunda to compare the inhibitory effects of GABA and glycine on the motoneuron membrane. At equal concentrations, the response (a change in membrane potential) to application of glycine was 1.5-2 times greater than the response to GABA in terms of amplitude, and EC(50) values were 0.75 and 1.57 mM, respectively. The response to simultaneous application of GABA and glycine averaged 79.1 +/- 2.4% (n = 19) of the sum of the individual responses and 130.1 +/- 1.5% (n = 19) of the glycine response (partial occlusion). Preliminary application of glycine decreased the GABA response by 85.3 +/- 0.2% (n = 10), while preapplication of GABA decreased the glycine response by only 52.9 +/- 0.3% (n = 11). The glycine and GABA responses were specifically suppressed by strychnine and bicuculline. These results provide evidence that as in mammals, amphibian motoneurons have both glycine (predominantly) and GABA(A) receptors; they also show that asymmetrical cross inhibition can occur.

Dual effect of GABA on descending monosynaptic excitatory postsynaptic potential in frog lumbar motoneurons

Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked by stimulating ipsilateral ventrolateral column (VLC) in the thoracic section were recorded in lumbar motoneurons within the isolated spinal cord of the frog Rana ridibunda. Bath application of the selective GABAB receptor agonist (-)-baclofen (0.05 mM) caused a reduction in the peak amplitude of VLC EPSP. Baclofen did not cause any consistent change in the membrane potential or in the EPSP waveform within frog motoneurones. The selective GABA(B) receptor antagonist saclofen (0.1 mM) completely blocked the effect of (-)-baclofen on VLC EPSP. A decrease in VLC EPSP peak amplitude was also observed during GABA (0.5 mM) application. Unlike (-)-baclofen, inhibition of VLC EPSP induced by GABA was accompanied by a shortening of the EPSP time course and a reduction in membrane input resistance within lumbar motoneurons. The decrease in VLC EPSP peak amplitude induced by (-)-baclofen and GABA was accompanied by an increase in the paired-pulse facilitation. These data provide evidence for a dual pre- and postsynaptic GABAergic inhibition of the VLC monosynaptic EPSP in lumbar motoneurons within the frog spinal cord.

Comparison of changes evoked by GABA (γ-aminobutyric acid) and anoxia in [K+]o, [Cl-]o, and [Na+]o in stratum pyramidale and stratum radiatum of the guinea pig hippocampus

Ion-selective microelectrode recordings were made to assess a possible contribution of extracellular γ-aminobutyric acid (GABA) accumulation to early responses evoked in the brain by anoxia and ischemia. Changes evoked by GABA or N2 in [K+]o, [Cl-]o, [Na+]o, and [TMA+]o were recorded in the cell body and dendritic regions of the stratum pyramidale (SP) and stratum radiatum (SR), respectively, of pyramidal neurons in CA1 of guinea pig hippocampal slices. Bath application of GABA (1-10 mM) for approximately 5 min evoked changes in [K+]o and [Cl-]o with respective EC50 levels of 3.8 and 4.1 mM in SP, and 4.7 and 5.6 mM in SR. In SP 5 mM GABA reversibly increased [K+]o and [Cl-]o and decreased [Na+]o; replacement of 95% O2 -5% CO2 by 95% N2 -5% CO2 for a similar period of time evoked changes which were for each ion in the same direction as those with GABA. In SR both GABA and N2 caused increases in [K+]o and decreases in [Cl-]o and [Na+]o. The reduction of extracellular space, estimated from levels of [TMA+]o during exposures to GABA and N2, was 5-6% and insufficient to cause the observed changes in ion concentration. Ion changes induced by GABA and N2 were reversibly attenuated by the GABAA receptor antagonist bicuculline methiodide (BMI, 100 µM). GABA-evoked changes in [K+]o in SP and SR and [Cl-]o in SP were depressed by >=90%, and of [Cl-]o in SR by 50%; N2-evoked changes in [K+]o in SP and SR were decreased by 70% and those of [Cl-]o by 50%. BMI blocked Δ [Na+]o with both GABA and N2 by 20-30%. It is concluded that during early anoxia: (i) accumulation of GABA and activation of GABAA receptors may contribute to the ion changes and play a significant role, and (ii) responses in the dendritic (SR) regions are greater than and (or) differ from those in the somal (SP) layers. A large component of the [K+]o increase may involve a GABA-evoked Ca2+-activated gk, secondary to [Ca2+]i increase. A major part of [Cl-]o changes may arise from GABA-induced gCl and glial efflux, with strong stimulation of active outward transport and anion exchange at SP, and inward Na+/K+/2Cl- co-transport at SR. Na+ influx is attributable mainly to Na+-dependent transmitter uptake, with only a small amount related to GABAA receptor activation. Although the release and (or) accumulation of GABA during anoxia might be viewed as potentially protectant, the ultimate role may more likely be an important contribution to toxicity and delayed neuronal death.

Key words: brain slices, ion-selective microelectrodes, stratum pyramidale, stratum radiatum, bicuculline methiodide, extracellular space shrinkage.

Changes in [K+]o evoked by baclofen in guinea pig hippocampus

K+-sensitive microelectrodes were used to record changes evoked by baclofen in extracellular potassium concentration ([K+]o) and field potentials in the stratum pyramidale (SP) and stratum radiatum (SR) in the CA1b region of guinea pig hippocampal slices in vitro. Bath applications of ( ±)-baclofen (1 µM - 3 mM for approx 5 min) evoked changes in [K+]o, which were in most cases sustained throughout agonist application and reversed during washout. The maximal (Rmax) values for curves fitted to the concentration-response data were for SP and SR, respectively, 0.59 ± 0.03 and 0.65 ± 0.03 mM, and EC50 values were 39.7 and 39.4 µM, respectively. The evoked K+ and field potential changes were significantly correlated and could be blocked by 2-OH-saclofen (50 µM) and CGP 35348 (50 µM). In <= 10% of experiments baclofen (10-50 µM) induced either a decrease or a transient increase ( <= 1 min duration) in [K+]o; in some slices with concentrations >=20 µM an initial decrease preceded a progressive increase. Pressure ejection of baclofen (100 µM for 100-900 ms) evoked increases in [K+]o and field potentials, which were larger in SR than in SP. In <= 10% of slices brief and (or) sustained application of baclofen (by either bath perfusion or pressure ejection) also evoked synchronous, repetitive interictal and ictal discharges at frequencies approx 1/s and 1/12 s, respectively, an observation that affirms a proconvulsant capacity. It is concluded that (i) although increases in [K+]o evoked by baclofen in SR compared with SP are slightly larger, they are not significantly different, (ii) GABAB receptor subtype(s) in SR and SP appear similar, as they have identical affinities, and (iii) [K+]o accumulations evoked by GABA likely include a contribution from a GABAB receptor activated K+ conductance, especially in dendritic regions.Key words: brain slices, stratum pyramidale, stratum radiatum, GABAB receptors, ion-selective microelectrodes, epileptiform activity.

Activation of alpha-adrenoceptors indirectly facilitates sodium pumping in frog motoneurons

The effects of clonidine on Na+ pumping in motoneurons of the isolated frog spinal cord was investigated using sucrose gap recordings from ventral roots. Na+ pump activity, induced in motoneurons either by tetanizing the dorsal root or by rapidly exposing the cord to normal medium following 30 min in K(+)-free Ringer's solution (K(+)-activated hyperpolarizations), was increased by application of clonidine (100 microM). These actions of clonidine were blocked by the preferential alpha 2-adrenergic antagonist yohimbine, but not by alpha 1-adrenergic antagonist prazosin or the beta-blocker propranolol. Clonidine's effects on Na+ pumping appeared to be indirect (presumably via interneurons) because its effects on K(+)-activated hyperpolarizations were reduced by tetrodotoxin (TTX) or high concentrations of Mg2+. This indirect mechanism involved activation of non-NMDA excitatory amino acid receptors. Thus, in the presence of clonidine, CNQX, but not APH, limited the ability of clonidine to enhance K(+)-activated hyperpolarizations. The AMPA receptor may play a role in the process, K(+)-activated hyperpolarizations were augmented by the presence of AMPA; NMDA had no effect. The present results are consistent with the idea that activation of alpha 2-adrenoceptors produces the following: the release of excitatory amino acids from interneurons; the activation of non-NMDA receptors on motoneurons; increased Na+ influx and loading and increased Na+ pump activity.

Can Cl- ions be extruded from a gamma-aminobutyric (GABA)-acceptive nerve cell via GABAA receptors on the plasma membrane cytoplasmic side?

1. In this commentary we discuss results obtained by a micromethod for the study of Cl- permeability across single nerve membranes from rabbit Deiters' neurons. 2. These results showed the presence of GABAA receptors on the nerve cell membrane cytoplasmic side. 3. We could show that these receptor complexes have a higher affinity for GABA than their extracellularly facing counterparts. Moreover, they present a phenomenon of desensitization. Another distinct property is that upon activation by GABA, they expose positive charges at their cytoplasmic mouths. 4. We propose that these receptor complexes could function in situ as a device for extruding Cl- anions from the nerve cell interior. This phenomenon would create an electrochemical gradient for Cl- penetration into the cell upon the action of extracellular GABA, after its presynaptic release.

Changes in extracellular K+ evoked by GABA, THIP and baclofen in the guinea-pig hippocampal slice

Changes in [K+]0 evoked by the inhibitory amino acid transmitter, GABA (gamma-aminobutyric acid) and its agonists were recorded with ion-selective microelectrodes in the CA1 stratum pyramidale of guinea-pig hippocampal slices. Bath applications of GABA (0.1-10 mM) produced dose-dependent increases in [K+]0 (EC50 = 4 mM, Rmax = 1.6 mM), with a peak and decline during exposure, followed by undershoot during recovery. In contrast the selective GABAA agonist, THIP (4,5,6,7-tetrahydroisoxazolo-(5,4-c)-pyridin-3-ol) (0.01-1 mM) showed approximately ten-fold greater potency and evoked only increases in [K+]0 (EC50 = 0.5 mM, Rmax = 2 mM). Reduction of temperature from 34 degrees to 22 degrees C caused a more than two-fold augmentation of the K+0 accumulation evoked by GABA, but no change in that due to THIP. The GABAA antagonist, BMI (bicuculline methiodide) (100 microM) completely blocked responses to THIP and partially antagonized those to GABA. Responses to GABA were synergistically enhanced by pentobarbital (100 microM). Only small, delayed and inconsistent changes could be evoked by relatively high concentrations of the GABAB agonist, DL-baclofen (0.01-1 mM). The K+ changes evoked by GABA appear to be mediated by the activation of GABAA receptors with low affinity and to be related to their depolarizing action. Although the response includes an electrogenic component which suggests the involvement of Na-dependent transmitter uptake/transport, the increase in K+0 probably reflects an outward counter/co-transport of K+ with Cl/HCO3 anion shifts and/or activation of a voltage-dependent K+ conductance.

gamma-Aminobutyric acid-induced ion movements in the guinea pig hippocampal slice

gamma-Aminobutyric acid (GABA)-induced regional changes of extracellular Cl, K and Na concentration ([Cl]o, [K]o, [Na]o), as well as of the extracellular space were measured with ion-sensitive microelectrodes in guinea pig hippocampal slices. Microdrop application of GABA to the pyramidal cell layer of CA3 or CA1 induced a decrease of [Cl]o, while application to the dendritic layer of CA3 or CA1 induced an increase of [Cl]o in addition. All changes of [Cl]o persisted in the presence of TTX and were blocked by bath-applied bicuculline. The GABA-induced decrease of [Cl]o was reduced by bicuculline application to the pyramidal cell layer. The increase of [Cl]o was blocked by bicuculline application to the dendritic layer. Additionally, GABA induced an increase of [K]o and decreases/increases of [Na]o. Changes of [Cl]o, [K]o and [Na]o together were approximately electroneutral. [Cl]o increases were exaggerated and [Cl]o decreases partly masked by shrinkage of the extracellular space after GABA application. Changing [K] in the superfusate transiently changed GABA-induced [Cl]o movements in a way predicted from a change in driving force due to the effect of [K] on membrane potential. Then a partial recovery followed towards the original [Cl]o change. We conclude that inward and outward Cl transports maintain [Cl]i below equilibrium in CA3 and CA1 pyramidal somata and above equilibrium in CA3 and CA1 dendrites. The significance of this Cl-distribution for hippocampal inhibition is discussed.

Inhibitory postsynaptic potentials in neonatal rat sympathetic preganglionic neurones in vitro

1. Intracellular recordings were made from antidromically identified sympathetic preganglionic neurones (SPNs) in transverse sections of thoraco-lumbar spinal cord from neonatal (12-22 day) rats. 2. Two types of hyperpolarizing (inhibitory) postsynaptic potentials (IPSPs) were recorded in the SPNs. The first type, which we have termed unitary IPSPs, were small, discrete IPSPs that occurred spontaneously and also following chemical or electrical stimulation applied to the spinal cord slices. The second type IPSP was a hyperpolarizing response evoked by either dorsal or ventral root stimulation. 3. Spontaneously occurring unitary IPSPs had an amplitude of 1 to 5 mV, and reversal potential of -60 to -75 mV; they were reversibly abolished by low Ca2+, tetrodotoxin (TTX) or strychnine but not by bicuculline and picrotoxin. 4. Pressure application of N-methyl-D-aspartate (NMDA), an excitatory amino SPNs; these were abolished by either strychnine or by the NMDA receptor antagonist D-2-amino-5-phosphonovalerate. Furthermore, electrical stimulation of dorsal rootlets elicited in several SPNs the discharge of strychnine-sensitive unitary IPSPs. 5. Electrical stimulation applied to dorsal or ventral rootlets elicited in nineteen and eight SPNs, respectively, an IPSP of larger amplitude (5 to 15 mV). The IPSP exhibited a reversal potential of -60 to 75 mV; it was changed to a depolarizing response in a low [Cl-]o solution, but was not significantly affected in a low [K+]o. Strychnine but not bicuculline or picrotoxin reversibly blocked the IPSPs in nearly all the SPNs. Additionally, hexamethonium and d-tubocurarine antagonized the IPSPs evoked by ventral but not by dorsal root stimulations. 6. Our results suggest that unitary and evoked IPSPs recorded in SPNs are due primarily to an increase of Cl- conductance by glycine or a glycine-like substance, released from interneurones, that can be activated by NMDA. Furthermore, IPSPs evoked by ventral root stimulation appear to represent a disynaptic event whereby nicotinic activation of a glycine-releasing interneurone results in a release of the inhibitory transmitter; this is then analogous to the Renshaw cell circuitry of the spinal motoneurones.

Continuous electrophysiological measurements of changes in cell volume of motoneurons in the isolated frog spinal cord

One type of ion-sensitive micro-electrode (K+ ligand Corning 477317) is sensitive to large quaternary ammonium ions such as choline or tetramethylammonium (TMA+). We have now used such electrodes for continuous electrophysiological measurements of changes in cell volume of motoneurons in the isolated frog spinal cord. The electrodes were double-barrelled with tip diameters of 1 micron. The reference barrel was filled with 100 mM choline or 100 mM TMA+ in 1 M Mg2+-acetate, the sensitive barrel contained the Corning K+ ligand. After the impalement of a motoneuron, choline or TMA+ diffused into the cell and about 1 h later, a steady-state concentration of these ions in the range of 10-20 mM was reached. Following this period, the motoneurons were activated by repetitive electrical stimulation or by application of amino acids via the bathing solution. All these stimuli led to a transient rise of the intracellular concentrations of choline or TMA+ (indicating a cell shrinkage of 3-10% difference to control volume).

"Intracellular" GABA affects the equilibrium distribution of Cl- across the plasma membrane of a GABA acceptive neuron

The permeability of Cl- ions through single microdissected plasma membrane from Deiters' neurons was studied by a microtechnique. In particular, the time course of the passage of 36Cl- ions from a microchamber, M1, to another one, M2, across the membrane was followed. This study was performed with or without gamma-amino-butyric acid (GABA) in the two microchambers. The results suggest that in basal conditions the high intracellular concentration normally present in these neurons, 3.3 mM (1), causes a higher permeability of Cl- in the direction inside----outside in the respect of the plasma membrane. "Extracellular" GABA, 0.1 mM, is able to abolish this imbalance in Cl- permeability in the two opposite directions. This event appears to be the basis for GABA induced hyperpolarization of these neurons.

GABAA receptor complexes are present on both sides of a GABA-acceptive neuronal membrane

A micromethod allowing the study of the characteristics of single GABA-acceptive membranes microdissected from Deiters' neurones was used in order to assess the effects of both "extra"- and "intra"-cellular GABA on Cl- permeability. The results indicate that GABA can activate Cl- permeability in the in----out direction when it is present on the cytoplasmic side of the membrane. Moreover, as already described, it can activate Cl- permeability in the opposite direction when present on the "extracellular" side of the membrane. Both these phenomena are blocked by GABAA receptor inhibitors, bicuculline and picrotoxin. The presence of GABAA receptors on both sides of the membrane is discussed as the possible basis for synaptically released GABA hyperpolarising action on these neurones.

gamma-Aminobutyric acid stimulates chloride permeability across microdissected Deiters' neuronal membrane

Fluxes of 36Cl- across freshly prepared Deiters' neuronal membranes have been studied in a two-compartment microchamber simulating the extra- and the intracellular space. The rate of 36Cl- influx was enhanced by gamma-aminobutyric acid (GABA) (10(-4) M), the effect being reversed by picrotoxin (10(-4) M) and by bicuculline (10(-5) M). Diazepam (10(-8)-10(-7) M) did not potentiate the response to GABA and rather depressed it. However, a barbiturate site is most probably present in the GABA receptor complexes since pentobarbitone (10(-4) M) was able to stimulate 36Cl- permeability to the same extent as GABA itself.

The role of chloride transport in postsynaptic inhibition of hippocampal neurons

Hippocampal inhibitory postsynaptic potentials are depolarizing in granule cells but hyperpolarizing in CA3 neurons because the reversal potentials and membrane potentials of these cells differ. Here the hippocampal slice preparation was used to investigate the role of chloride transport in these inhibitory responses. In both cell types, increasing the intracellular chloride concentration by injection shifted the reversal potential of these responses in a positive direction, and blocking the outward transport of chloride with furosemide slowed their recovery from the injection. In addition, hyperpolarizing and depolarizing inhibitory responses and the hyperpolarizing and depolarizing responses to the inhibitory neurotransmitter gamma-aminobutyric acid decreased in the presence of furosemide. These effects of furosemide suggest that the internal chloride activity of an individual hippocampal neuron is regulated by two transport processes, one that accumulates chloride and one that extrudes chloride.