The in vivo inactivation of GABA and other inhibitory amino acids in the cat nervous system

GABAB Receptors Tonically Inhibit Motoneurons and Neurotransmitter Release from Descending and Primary Afferent Fibers

Motoneurons receive thousands of excitatory and inhibitory synapses from descending tracts and primary afferent fibers. The excitability of these neurons must be precisely regulated to respond adequately to the requirements of the environment. In this context, GABAA and GABAB receptors regulate motoneuron synaptic strength. GABAA and GABAB receptors are expressed on primary afferent fibers and motoneurons, while in the descending afferent fibers, only the GABAB receptors are expressed. However, it remains to be known where the GABA that activates them comes from since the GABAergic interneurons that make axo-axonic contacts with primary afferents have yet to be identified in the descending afferent terminals. Thus, the main aim of the present report was to investigate how GABAB receptors functionally modulate synaptic strength between Ia afferent fibers, excitatory and inhibitory descending fibers of the dorsolateral funiculus, and spinal motoneurons. Using intracellular recordings from the spinal cord of the turtle, we provide evidence that the GABAB receptor antagonist, CGP55845, not only prevents baclofen-induced depression of EPSPs but also increases motoneuron excitability and enhances the synaptic strength between the afferent fibers and motoneurons. The last action of CGP55845 was similar in excitatory and inhibitory descending afferents. Interestingly, the action of baclofen was more intense in the Ia primary afferents than in the descending afferents. Even more, CGP55845 reversed the EPSP depression induced by the increased concentration of ambient GABA produced by interneuron activation and GABA transporter blockade. Immunofluorescence data corroborated the expression of GABAB receptors in the turtle's spinal cord. These findings suggest that GABAB receptors are extrasynaptic and tonically activated on descending afferent fibers and motoneurons by GABA released from astrocytes and GABAergic interneurons in the cellular microenvironment. Finally, our results also suggest that the antispastic action of baclofen may be due to reduced synaptic strength between descending fibers and motoneurons.

Structure, function, and plasticity of GABA transporters

GABA transporters belong to a large family of neurotransmitter:sodium symporters. They are widely expressed throughout the brain, with different levels of expression in different brain regions. GABA transporters are present in neurons and in astrocytes and their activity is crucial to regulate the extracellular concentration of GABA under basal conditions and during ongoing synaptic events. Numerous efforts have been devoted to determine the structural and functional properties of GABA transporters. There is also evidence that the expression of GABA transporters on the cell membrane and their lateral mobility can be modulated by different intracellular signaling cascades. The strength of individual synaptic contacts and the activity of entire neuronal networks may be finely tuned by altering the density, distribution and diffusion rate of GABA transporters within the cell membrane. These findings are intriguing because they suggest the existence of complex regulatory systems that control the plasticity of GABAergic transmission in the brain. Here we review the current knowledge on the structural and functional properties of GABA transporters and highlight the molecular mechanisms that alter the expression and mobility of GABA transporters at central synapses.

Effect of extracellular pH on GABA-activated current in rat recombinant receptors and thin hypothalamic slices

We studied the effects of extracellular pH (pHo) on gamma-aminobutyric acid (GABA)-mediated Cl- current in rat hypothalamic neurons and recombinant type-A GABA (GABA(A)) receptors stably expressed in human embryonic kidney cells (HEK 293), using whole cell and outside-out patch-clamp recordings. In alpha3beta2gamma2s receptors, acidic pH decreased, whereas alkaline pH increased the response to GABA in a reversible and concentration-dependent manner. Acidification shifted the GABA concentration-response curve to the right, significantly increasing the EC50 for GABA without appreciably changing the slope or maximal current induced by GABA. We obtained similar effects of pH in alpha1beta2gamma2 receptors and in GABA-activated currents recorded from thin hypothalamic brain slices. In outside-out patches recorded from alpha3beta2gamma2 recombinant receptors, membrane patches were exposed to 5 microM GABA at control (7.3), acidic (6.4), or alkaline (8.4) pH. GABA activated main and subconductance states of 24 and 16 pS, respectively, in alpha3beta2gamma2 receptors. Alkaline pH(o) increased channel opening frequency and decreased the duration of the long closed state, resulting in an increase in open probability (from 0.0801 +/- 0.015 in pH 7.3 to 0.138 +/- 0.02 in pH 8.4). Exposure of the channels to acidic pH(o) had the opposite effects on open probability (decreased to 0.006 +/- 0.0001). Taken together, our results indicate that the function of GABA(A) receptors is modulated by extracellular pH. The proton effect is similar in recombinant and native receptors and is dependent on GABA concentration. In addition, the effect appears to be independent of the alpha-subunit isoform, and is due to the ability of H+ to alter the frequency of channel opening. Our findings indicate that GABAergic signaling in the CNS may be significantly altered during conditions that increase or decrease pH.

Inhibitory nature of tiagabine-augmented GABAA receptor-mediated depolarizing responses in hippocampal pyramidal cells

Tiagabine is a potent GABA uptake inhibitor with demonstrated anticonvulsant activity. GABA uptake inhibitors are believed to produce their anticonvulsant effects by prolonging the postsynaptic actions of GABA, released during episodes of neuronal hyperexcitability. However, tiagabine has recently been reported to facilitate the depolarizing actions of GABA in the CNS of adult rats following the stimulation of inhibitory pathways at a frequency (100 Hz) intended to mimic interneuronal activation during epileptiform activity. In the present study, we performed extracellular and whole cell recordings from CA1 pyramidal neurons in rat hippocampal slices to examine the functional consequences of tiagabine-augmented GABA-mediated depolarizing responses. Orthodromic population spikes (PSs), elicited from the stratum radiatum, were inhibited following the activation of recurrent inhibitory pathways by antidromic conditioning stimulation of the alveus, which consisted of either a single stimulus or a train of stimuli delivered at high-frequency (100 Hz, 200 ms). The inhibition of orthodromic PSs produced by high-frequency conditioning stimulation (HFS), which was always of much greater strength and duration than that produced by a single conditioning stimulus, was greatly enhanced following the bath application of tiagabine (2-100 microM). Thus, in the presence of tiagabine (20 microM), orthodromic PSs, evoked 200 and 800 ms following HFS, were inhibited to 7.8 +/- 2.6% (mean +/- SE) and 34.4 +/- 18.5% of their unconditioned amplitudes compared with only 35.4 +/- 12.7% and 98.8 +/- 12.4% in control. Whole cell recordings revealed that the bath application of tiagabine (20 microM) either caused the appearance or greatly enhanced the amplitude of GABA-mediated depolarizing responses (DR). Excitatory postsynaptic potentials (EPSPs) evoked from stratum radiatum at time points that coincided with the DR were inhibited to below the threshold for action-potential firing. Independently of the stimulus intensity with which they were evoked, the charge transferred to the soma by excitatory postsynaptic currents (EPSCs), elicited in the presence of tiagabine (20 microM) during the large (1,428 +/- 331 pA) inward currents that underlie the DRs, was decreased on the average by 90.8 +/- 1.7%. Such inhibition occurred despite the presence of the GABAB receptor antagonist, CGP 52 432 (10 microM), indicating that GABAB heteroreceptors, located on glutamatergic terminals, do not mediate the observed reduction in the amplitude of excitatory postsynaptic responses. The present results suggest that despite facilitating the induction of GABA-mediated depolarizations, tiagabine application may nevertheless increase the effectiveness of synaptic inhibition during the synchronous high-frequency activation of inhibitory interneurons by enhanced shunting.

Effect of renal impairment on the pharmacokinetics and tolerability of tiagabine

PURPOSE

We wished to determine the effect of renal impairment on the pharmacokinetics and tolerability of the new antiepileptic drug tiagabine (TGB).

METHODS

We assessed TGB pharmacokinetics and tolerability in 25 subjects with various degrees of renal function (based on creatinine clearance, n = 4-6 per group) from healthy (group I) to requiring hemodialysis (group V) in a single and multiple dose (every 12 h), one-period (groups I-IV) or a single dose, two-period (group V) study (4-mg oral doses of TGB x HCl). Blood samples were collected after the first dose (both periods for group V) and after the last dose on day 5 (groups I-IV). TGB plasma concentrations and plasma protein binding were determined by high-performance liquid chromatography (HPLC) and ultrafiltration, respectively.

RESULTS

TGB was well tolerated by all study subjects. The pharmacokinetics of TGB were similar in all subjects; no pharmacokinetic parameter (based on either total or unbound concentrations) was statistically correlated with creatinine clearance. For total TGB in plasma, single-dose mean values of the maximum plasma concentration, clearance, and half-life (t(1/2)) ranged from 52 to 108 ng/ml, from 7.14 to 11.02 l/h, and from 6.4 to 8.4 h, respectively.

CONCLUSIONS

TGB pharmacokinetics and tolerability were independent of renal function; therefore, dosage adjustment is unnecessary for epilepsy patients with renal impairment.

Tiagabine: a novel drug with a GABAergic mechanism of action

The various possibilities for manipulating the gamma-aminobutyric acid (GABA) system to augment GABAergic inhibition have been surveyed with reference to the relevant antiepileptic compounds that have been successfully or unsuccessfully investigated in relation to these different mechanisms of action. The first clinical studies of tiagabine (TGB), a novel GABA-uptake inhibitor are now available. These studies utilized a novel design, the enrichment (Amery) design, which is put into perspective compared to classical clinical trial designs. Possible advantages and disadvantages of TGB, as seen at this stage in development, have been identified.

Postsynaptic action of endogenous GABA released by nipecotic acid in the hippocampus

Intracellular and whole-cell recording from CA1 pyramidal cells and dentate granule cells was used to study the release of endogenous GABA by nipecotic acid. Local application of nipecotic acid produced responses that could be entirely blocked by a combination of the GABAA receptor antagonist picrotoxin and the GABAB receptor antagonist CGP 35348. These responses were due to the heteroexchange release of endogenous GABA because they were blocked by low Na+ which blocks the GABA transporter and by SKF 89976 which is a competitive antagonist of the GABA transporter. Local application of nipecotic acid could, depending on the location, evoke pure GABAA or pure GABAB responses supporting proposals that GABAA and GABAB receptors can be segregated at separate inhibitory synapses.

Inward current caused by sodium-dependent uptake of GABA in the crayfish stretch receptor neurone

A two-microelectrode current-voltage clamp and Cl(-)-selective microelectrodes were used to examine the effects of gamma-aminobutyric acid (GABA) on membrane potential, current and intracellular Cl- activity (aiCl) in the crayfish stretch receptor neurone. All experimental solutions were CO2-HCO3- free. 2. GABA (500 microM) produced a mono- or biphasic depolarization (amplitude < or = 10 mV), often with a prominent initial depolarizing component followed by a transient shift to a more negative level. In some neurones, an additional depolarizing phase was seen upon washout of GABA. Receptor desensitization, being absent, played no role in the multiphasic actions of GABA. 3. The pronounced increase in membrane conductance evoked by GABA (500 microM) was associated with an increase in aiCl which indicates that the depolarizing action was not due to a current carried by Cl- ions. 4. The currents activated by GABA under voltage clamp conditions were inwardly directed when recorded at the level of the resting membrane potential, and they often revealed a biphasic character. The reversal potential of peak currents activated by pulses of 500 microM-GABA (EGABA) was 9-12 mV more positive than the reversal potential of the simultaneously measured net Cl- flux (ECl). ECl was 2-7 mV more negative than the resting membrane potential. 5. EGABA (measured using pulses of 500 microM-GABA) was about 10 mV more positive than the reversal potential of the current activated by 500 microM-muscimol, a GABA agonist that is a poor substrate of the Na(+)-dependent GABA uptake system. 6. In the absence of Na+, the depolarization and inward current caused by 500 microM-GABA were converted to a hyperpolarization and to an outward current. Muscimol produced an immediate outward current both in the presence and absence of Na+. 7. Following block of the inhibitory channels by picrotoxin (100-200 microM), the depolarizing effect of 500 microM-GABA was enhanced and the transient hyperpolarizing shifts were abolished. 8. In the presence of picrotoxin, GABA (> or = 2 microM) produced a concentration-dependent monophasic inward current which had a reversal potential of +30 to +60 mV. This current was inhibited in the absence of Na+ and by the GABA uptake blocker, nipecotic acid. Unlike the channel-mediated current, the picrotoxin-insensitive current was activated without delay also at low (2-10 microM) concentrations of GABA.(ABSTRACT TRUNCATED AT 400 WORDS)

Anticonvulsant activity of intracerebroventricularly administered glial GABA uptake inhibitors and other GABAmimetics in chemical seizure models

The antiseizure activities of glial or neuronal GABA uptake inhibitors and GABA agonists were compared following intracerebroventricular administration in 2 acute models of chemoconvulsion in rats. The glia-selective GABA uptake inhibitor, 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol (THPO), given at doses of 100-750 micrograms, i.c.v., protected against maximal pentylenetetrazol (PTZ) seizures and increased the latency to isonicotinic acid hydrazide (INH) seizures for at least 1 h following central administration. THPO failed to increase PTZ seizure thresholds. In contrast, the more potent partly glia-selective GABA uptake inhibitor, cis-4-hydroxynipecotic acid (30-300 micrograms), which is also a substrate for neuronal and glial transport systems, protected only 33% of rats against PTZ-induced tonic extension and had no effect on INH seizure latency. The neuron-selective uptake inhibitor L-2,4-diaminobutyric acid (DABA) at 1500 micrograms exhibited anti-PTZ activity initially and then, after a delay, produced proconvulsant behavior and spontaneous myoclonus in some animals. Intracerebroventricular injection of the GABA receptor agonist, muscimol, at toxic doses, gave rise to mixed anticonvulsant (INH seizures) and proconvulsant (PTZ seizure thresholds) effects. The results suggest that THPO, of the 4 compounds tested, possesses significant anticonvulsant activity. Its ability to suppress tonic but not generalized minor seizures suggests that it may block seizure spread.

Frequency-dependent depression of inhibition in guinea-pig neocortex in vitro by GABAB receptor feed-back on GABA release

1. The mechanisms involved in the lability of inhibition at higher frequencies of stimulation were investigated in the guinea-pig in vitro neocortical slice preparation by intracellular recording techniques. We attempted to test the possibility of a feedback depression of GABA on subsequent release. 2. At resting membrane potential (Em, -75.8 +/- 5.2 mV) stimulation of either the pial surface or subcortical white matter evoked a sequence of depolarizing and hyperpolarizing synaptic components in most neurones. An early hyperpolarizing component (IPSPA) was usually only obvious as a pronounced termination of the EPSP, followed by a later hyperpolarizing event (IPSPB). Current-voltage relationships revealed two different conductances of about 200 and 20 nS and reversal potentials of -73.0 +/- 4.4 and -88.6 +/- 6.1 mV for the early and late component, respectively. 3. The conductances of IPSPA and IPSPB were fairly stable at a stimulus frequency of 0.1 Hz. At frequencies between 0.5 and 2 Hz both IPSPs were attenuated with the second stimulus and after about five stimuli a steady state was reached. Concomitantly IPSPs were shortened. The average decrease in synaptic conductance between 0.1 and 1 Hz was 80% for the IPSPA and 60% for the IPSPB. At these frequencies the reversal potentials decreased by 5 and 2 mV, respectively; Em and input resistance (Rin) were not consistently affected. 4. The amplitudes of field potentials, action potentials and EPSPs of pyramidal cells were attenuated less than 10% at stimulus frequencies up to 1 Hz, suggesting that alterations in local circuits between the stimulation site and excitatory input onto inhibitory interneurones may play only a minor role in the frequency-dependent decay of IPSPs. 5. Localized application of GABA produced multiphasic responses. With low concentrations and application near the soma an early hyperpolarization prevailed followed by a depolarizing late component. Brief application of GABA at low frequencies induced constant responses; at higher frequencies, the responses sometimes declined. The current-voltage relationships of the two GABA responses were similar to each other and to the early IPSP. An apparently fivefold higher conductance was estimated at lower Ems, suggesting that the GABA response had a voltage sensitivity. The slope conductance of IPSPs was decreased by up to 50% for tens of seconds after postsynaptically detectable effects of GABA had dissipated. 6. Application of the GABA uptake inhibitor nipecotic acid (50-500 microM) reduced the conductance of both components of orthodromically evoked inhibition and shortened the IPSP at low frequencies, but had no additional effects at higher stimulation rates.(ABSTRACT TRUNCATED AT 400 WORDS)

Experimental studies and controlled clinical testing of valproate and vigabatrin

Gamma-aminobutyric acid (GABA) is the most important inhibitory transmitter, quantitatively, in the CNS. Evidence exists that decreased GABAergic neurotransmission may play a role in some forms of epilepsy. Consequently, manipulating the GABA system may be a therapeutic possibility in the treatment of this disease. Inhibition of the major GABA degrading enzyme, GABA-transaminase (GABA-T), seems to be the most promising approach. Currently, 2 antiepileptic drugs, valproate (VPA) and vigabatrin, gamma-vinyl GABA (GVG), are available, which are supposed to inhibit the degradation of GABA. Both drugs cause an increase in the total concentration of GABA in the brain, but to a different extent. VPA produces a moderate elevation, which seems to be the result of a marked increase in the transmitter-related GABA pool, while the pronounced elevation in GABA concentration observed during treatment with GVG seems to be caused mainly by an increase in the non-transmitter-related (glial) GABA pool. In order to investigate this apparently differential influence of VPA and GVG on the GABA system, a number of studies were undertaken in selectively cultured astrocytes and neurons from mice. For both drugs neuronal GABA-T proved far more sensitive with regard to inhibition than glial GABA-T. In order to obtain a more direct measure of a potential GABAergic mechanism of action of VPA and GVG, synaptic release of endogenous GABA was determined after culturing neurons in the presence of clinically relevant concentrations of the drugs. GVG caused a significant increase in GABA release, even at concentrations as low as 25 microM. For VPA only the highest of the investigated concentrations (300 microM) augmented GABA release. It is concluded that the antiepileptic effect of GVG seems to be caused by a direct GABAergic mechanism of action. For VPA an influence on the GABA system may play a role in the antiepileptic effect of the drug. However, the lack of definite data on human brain levels of VPA after chronic treatment, combined with evidence that VPA exhibits a number of other effects that may be relevant for its antiepileptic properties, makes the interpretation of a GABAergic mechanism of action difficult. Controlled clinical trials have been increasingly applied within all areas of medicine. In 1982 a survey of the literature identified 29 studies of antiepileptic drugs, where the design involved randomization, the double-blind principle and a statistical analysis of the results.(ABSTRACT TRUNCATED AT 400 WORDS)

Uptake inhibitors potentiate gamma-aminobutyric acid-induced contractile responses in the isolated ileum of the guinea-pig

The gamma-aminobutyric acid (GABA)-induced contractile responses in the guinea-pig isolated ileum, maintained in Krebs-bicarbonate solution (pH 7.4, 37 degrees C), were significantly potentiated by inhibitors of GABA uptake, with a greater potentiation of the responses in the presence of (+/-)-cis-3-aminocyclohexane-carboxylic acid (ACHC) greater than L-2,4-diaminobutyric acid (DABA) greater than (+/-)-nipecotic acid greater than beta-alanine, whilst simultaneous addition of DABA with beta-alanine caused a greater potentiation of the GABA-induced responses than did nipecotic acid with beta-alanine, or any of the uptake blockers applied alone. The concentration-response curves for the GABA-induced ileal contraction were shifted to the left in the presence of the uptake inhibitors, this shift being more prominent over the lower concentration range of GABA (1-20 microM). By contrast, contractile responses to muscimol or 3-amino-1-propanesulphonic acid (3APS) were not potentiated by the uptake blockers, neither were their concentration-response curves altered. Bicuculline methochloride shifted the GABA concentration-response curve to the right, whilst picrotoxinin both shifted the concentration-response curve for GABA to the right and depressed the maximum response. In the presence of the uptake inhibitors, the rightward shift of the concentration-response curves for GABA induced by bicuculline was less than that induced by bicuculline alone. The rightward shift with picrotoxinin was similarly reduced in the presence of the uptake inhibitors, without altering the depression of the maximum by picrotoxinin. Bicuculline caused a rightward shift of the concentration-response curves for 3APS and muscimol, with the curve for 3APS most affected. Picrotoxinin similarly shifted the concentration-response curves for 3APS and muscimol but depressed the maximum, with the curve for 3APS again being most affected. None of the inhibitors of GABA uptake influenced the concentration-response curves for 3APS or muscimol in the presence of bicuculline or picrotoxinin. 5. In conclusion, a saturable GABA uptake system is present in the enteric nervous system of the guinea-pig intestine, where neuronal GABA uptake appears to predominate over glial uptake.

Autoradiographic study of the distribution of [3H]gamma-aminobutyrate-accumulating neural elements in guinea-pig intestine: evidence for a transmitter function of gamma-aminobutyrate

High affinity uptake, and the distribution of 3H-radiolabelled gamma-aminobutyrate (GABA), cis-3-aminocyclohexanecarboxylic acid, beta-alanine, proline, and leucine have been examined autoradiographically in laminar preparations of the myenteric plexus from the guinea-pig intestine. Following labelling with [3H]proline and [3H]leucine, which are incorporated into neurons, silver grains were concentrated over recognisable perikarya in the ganglia and meshworks of the plexus, whilst [3H]GABA labelled a smaller proportion of neurons and their processes. Specificity of labelling in the sites of [3H]GABA-uptake was established using combinations of labelled and unlabelled GABA, beta-alanine, and cis-3-aminocyclohexanecarboxylic acid, substrates for glial or neuronal high affinity GABA uptake systems. Only myenteric neurons and their processes were labelled significantly by [3H]GABA and its analogue cis-3-[3H]aminocyclohexanecarboxylic acid. Using autoradiographs of laminar preparations and paraffin sections, [3H]GABA labelling was found over nerve fibre bundles that could be traced from their ganglionic origins through the interconnecting meshworks of the myenteric plexus into the innervation of the deep muscular plexus of the circular muscle layer where GABA is evidently concerned with prejunctional modulation of transmitter release. The extensive but selective distribution of [3H]GABA high affinity uptake sites in neural elements of the guinea-pig myenteric plexus is consistent with GABA being an enteric neurotransmitter.

Effect of central noradrenaline depletion on corticosterone levels and gastric ulcerations in rats

Effects of central noradrenergic depletion on the stress responses of rats were explored using the new selective neurotoxin (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4)). Noradrenergic depletion using DSP-4 was followed by a reduction in basal corticosterone levels after 7 days. Three weeks after DSP-4 treatment, animals exhibited less severe and fewer gastric ulcerations than control animals following 23 h immobilization stress, but stress levels of corticosterone were similar for the two groups. No differences could be found in the peripheral gastric levels of noradrenaline between experimental and control animals, while central noradrenaline was reduced to approximately 30% of control levels. The data support previous findings using other methods that central noradrenaline is an important factor in stress-induced gastric ulceration. The peripheral mechanisms for the protective effects of DSP-4 remain to be elucidated, and studies of these may cast light on the efferent pathways between the central nervous system and gastric mucosa which are involved in stress-induced gastric pathology.

Pipecolic acid enhancement of GABA response in single neurons of rat brain

Using unit recording and microelectrophoresis, influence of pipecolic acid (PA), a major metabolite of lysine in the brain, on GABA and glycine responses was studied in the cerebral cortical and hippocampal pyramidal neurons of rats. With small currents, PA had no effect on the single neuron activities but enhanced GABA response without affecting glycine response. The finding provides a new evidence that PA may have a connection with central GABA system.

Prolongation of gamma-aminobutyric acid-mediated inhibitory postsynaptic potentials by 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol (THPO)

The effects of 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol (THPO), an inhibitor of gamma-aminobutyric acid (GABA) uptake, were examined in the rat hippocampal slice and compared to the actions of the potent GABA uptake inhibitor, cis-4-OH-nipecotic acid. THPO reversibly prolonged GABA-mediated inhibitory postsynaptic potentials (IPSPs) but did not prolong conductance changes evoked by iontophoresed GABA. In contrast, cis-4-OH-nipecotic acid prolonged responses to iontophoresed GABA but had little effect on IPSPs. THPO reduced the intensity of spontaneous epileptiform bursts observed in elevated (8.5 mM) potassium; this effect was absent after addition of 100 microM bicuculline. These results suggest that drugs similar to THPO may be useful for enhancing GABA-mediated inhibition, but that THPO acts by a mechanism distinct from that of the GABA uptake inhibitor, cis-4-OH-nipecotic acid.

Tetanic stimulation-induced changes in [3H]glutamate binding and uptake in rat hippocampus

Tetanus-induced (400 Hz, 200 pulses) long-lasting potentiation of the stratum radiatum-evoked CA1 population spike in hippocampal slices is not accompanied by any change in Na+-independent [3H]glutamate binding sites. Homosynaptic depression that occurs subsequent to either a low frequency tetanus (20 Hz, 600 pulses) or a transient exposure to Cl(-)-free (containing NO3-) medium is associated with an elevation in the amino acid binding. [3H]Glutamate uptake into slices was decreased following a high frequency (400 Hz, 200 pulses) tetanus but in the majority of cases was increased following a low frequency (20 Hz, 600 pulses) tetanus to stratum radiatum. When the high frequency tetanus was given in the absence of extracellular Ca2+, there was a further reduction in [3H]glutamate uptake.

Role of uptake in gamma-aminobutyric acid (GABA)-mediated responses in guinea pig hippocampal neurons

Intracellular recordings were obtained from hippocampal pyramidal neurons maintained in vitro. Measurements were made of the conductance change induced by iontophoretically applied gamma-aminobutyric acid (GABA) and, using voltage-clamp techniques, of inhibitory postsynaptic currents resulting from activation of inhibitory pathways. Analysis of GABA iontophoretic charge-response curves indicated that there was considerable variation among neurons with respect to the slope of this relation. The placement of the GABA-containing pipette did not appear to be responsible for the observed variation, since vertical repositioning of the pipette did not alter the slope of the charge-response relationship. Steady iontophoresis of GABA from one barrel of a double-barreled pipette markedly affected the charge-response relation obtained when short pulses were applied to the other barrel. The curve was shifted to the left, and the slope was decreased. Concomitantly, the enhanced GABA-induced responses were prolonged. Similar alterations in GABA responsiveness were observed when the uptake blocker, nipecotic acid, was iontophoretically applied. Furthermore, bath application of saline containing a reduced sodium concentration (25% of control) also produced a prolongation of GABA-mediated responses. Under voltage clamp, inhibitory postsynaptic currents were observed to have biphasic decays. The initial, fast decay was prolonged by an average of 18% by nipecotic acid, whereas the later, slow phase was prolonged by 23%. The results of these studies support the hypothesis that a saturable GABA uptake system is responsible for the observed variation in the charge-response curves and, in turn, underlies the apparent sensitizing effect of excess GABA application. The results also suggest that a reduction of transmitter uptake affects the time course of inhibitory postsynaptic currents in the hippocampus.

Gamma-aminobutyric acid uptake and the termination of inhibitory synaptic potentials in the rat hippocampal slice

Intracellular recordings were made from CA1 pyramidal cells in the rat hippocampal slice to study the processes that influence the time course of inhibitory post-synaptic potentials (i.p.s.p.s) mediated by gamma-aminobutyric acid (GABA), and conductance changes evoked by ionophoretically applied GABA. The GABA-uptake inhibitors, nipecotic acid and cis-4-OH-nipecotic acid (1 mM), greatly prolonged conductance increases associated with both hyperpolarizing and depolarizing responses to ionophoretically applied GABA. In contrast to their effects on GABA-evoked conductances, uptake inhibitors only slightly prolonged antidromically evoked i.p.s.p.s. Their primary effect occurred after the i.p.s.p. had decayed to 5-30% of its peak. 4-OH-isonipecotic acid, a nipecotic acid analogue that does not inhibit GABA uptake, did not prolong i.p.s.p.s or ionophoretically evoked conductance changes. Sodium pentobarbitone (100 microM), a drug that prolongs the open time of GABA-activated chloride channels, potentiated both i.p.s.p.s and responses to ionophoretically applied GABA. Whereas pentobarbitone also prolonged i.p.s.p.s, it did not prolong responses to ionophoretically applied GABA. The prolongation of i.p.s.p.s by pentobarbitone occurred equally in both the early and late phases of the i.p.s.p., in contrast to the effects of GABA-uptake inhibitors. I.p.s.p.s did not usually decay exponentially. The observation that uptake inhibitors prolonged the late but not the early decay phase of the i.p.s.p., together with the previous finding that the conductance change persists for the duration of the i.p.s.p., indicate that GABA is present in the synapse throughout much of the i.p.s.p. These data suggest that diffusion of GABA out of the synapse, a non-exponential process, is an important determinant of the i.p.s.p. decay time course. Increasing the extracellular potassium concentration from 3.5 to 8.5 mM resulted in spontaneously occurring, synchronous burst firing of pyramidal cells. Cis-4-OH-nipecotic acid significantly reduced the number and amplitude of extracellularly recorded population spikes within each burst. We conclude that diffusion, channel open time and GABA uptake all influence the time course of GABA-mediated i.p.s.p.s. The time course of a single, brief i.p.s.p. is determined predominantly by post-synaptic channel kinetics and diffusion of GABA out of the synapse, whereas the inhibition produced by prolonged synaptic bursts or relatively long application of exogenous GABA can be markedly influenced by GABA uptake.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological evidence that nipecotic acid can be used in vivo as a false transmitter

Dentate gyrus of the rat hippocampal formation was perfused with the potent inhibitor of GABA uptake, nipecotic acid, by means of an implanted dialytrode. Evoked population spikes in dentate gyrus were decreased in amplitude and often abolished during perfusion. However, multiple (2-4) population spikes developed shortly after nipecotic acid withdrawal. This excitability increase, which presented a pattern of repetitive discharge resembling that following blocking of GABAergic transmission was interpreted as electrophysiological evidence that nipecotic acid can act as a false transmitter 'in vivo', as previously postulated from uptake and release 'in vitro' studies.

Inhibition of GABA uptake potentiates the conductance increase produced by GABA-mimetic compounds on single neurones in isolated olfactory cortex slices of the guinea-pig

Membrane potential and input conductance were recorded in single neurones in slices of guinea-pig olfactory cortex in vitro. gamma-Aminobutyric acid (GABA) and GABA-mimetic compounds were applied by bath-perfusion. Potency was measured as the concentration required to double the input conductance. The potency of GABA was increased (i.e. the equi-effective concentrations were reduced) by 15.5 +/- 2.3 times (mean +/- s.e. mean) on reducing external [Na+] from 144 to 20 mmol l-1, by replacement with Mg2+. Corresponding potency changes for other agonists were + 10.8 +/- 2.5 for 3-aminopropanesulphonic acid (3-APS); 3.25 +/- 1.06 for isoguvacine and 2.43 +/- 0.69 for muscimol. Nipecotic acid (0.5 mM) produced the following increases in potency: GABA 2.68 +/- 0.02; 3-aminopropanesulphonic acid, 3.11 +/- 0.07; isoguvacine, 1.92 +/- 0.34; muscimol, 2.24 +/- 0.17. The concentration of GABA in the bathing fluid necessary to double input conductance increased with increasing depth of the recording site from the cut surface. The apparent potency fell 10 times for each 60 micron depth increment up to 150 micron. The recording depth also affected the apparent potency of muscimol and 3-APS but to a lesser extent. Reduction of external [Na+] reduced the depth-dependence of both GABA and 3-APS potency. No clear change in the duration of the recurrent inhibitory postsynaptic conductance could be detected in the presence of 0.5 mmol l-1 nipecotic acid. 6 It is suggested that agonist uptake by a Na+-dependent, nipecotic acid-sensitive mechanism severely attenuates the responses of olfactory neurones to exogenous GABA and to its analogues 3-APS, muscimol and isoguvacine, but has little immediate influence on the duration of the GABA-mediated inhibitory postsynaptic conductance.

Some functional consequences of GABA uptake by brain cells

Recent electrophysiological studies on the rat hippocampus (in vivo and in vitro) provide further evidence that neuronal and glial uptake of the inhibitory transmitter gamma-aminobutyric acid (GABA) limits the intensity and the duration of effects not only of locally applied exogenous GABA but also of GABAergic inhibitory synaptic potentials (IPSPs). There is good reason to believe that such uptake is at least partly responsible for the 'fading' of GABA action. Moreover, because it is probably driven by the transmembrane Na+ electrochemical gradient and is accompanied by Na+ influx, GABA uptake is potentially electrogenic and therefore may have a depolarizing effect on both neurons and glia.

The time course of GABA action on the crayfish stretch receptor: evidence for a saturable GABA uptake

The conductance increase induced by bath application of GABA has been measured in voltage-clamped stretch-receptor neurones of crayfish. A rapid conductance increase was obtained only at GABA concentrations above 3.3 X 10(-4) M. The response to lower GABA concentrations (between 10(-4) and 10(-6) M) developed slowly over 30-60 min. Repetitive application of intermediate GABA concentrations induced postsynaptic conductance changes which were progressively enhanced in their onset and magnitude. In the presence of nipecotic acid or in Na+-free Ringer solutions, the response to all GABA concentrations was rapid and constant for each concentration. The time course of inhibitory postsynaptic currents was unaffected by nipecotic acid. These results suggest the presence of a saturable GABA uptake system which limits the access of bath-applied GABA to postsynaptic receptors. This system has little if any effect on the termination of response to synaptically released GABA.

Somatic and dendritic actions of gamma-aminobutyric acid agonists and uptake blockers in the hippocampus in vivo

In rats under urethane anaesthesia gamma-aminobutyric acid agonists and uptake blockers were microiontophoretically applied in the pyramidal layer of CA1 and in the apical dendrites using a twin set of multibarrelled micropipettes. Thus, the somatic and dendritic field potentials elicited by commissural stimulation were recorded simultaneously and the effects of iontophoretic applications at either site studied. Somatic applications of gamma-aminobutyric acid, isoguvacine or muscimol produced an inhibition of the somatic population spike; this showed rapid fade and was followed by an "off" response i.e. an enhancement of the population spike discharge and the occurrence of a second (and occasionally third) spike. The order of potency with regard to the "off" response was muscimol greater than isoguvacine much greater than gamma-aminobutyric acid. In contrast, the inhibition of the population spike produced by 4,5,6,7-tetrahydroisoxazolo(5,4-C) pyridin 3-OL showed little fade and no prominent "off" response. The fade and "off" response were not associated with significant changes in the dendritic field excitatory postsynaptic potential concommittantly recorded and were exclusively restricted to the immediate vicinity of the pyramidal layer. Ejection of gamma-aminobutyric acid and its agonists in the stratum radiatum produced a reduction of the field excitatory postsynaptic potential and the somatic spike, this effect however showed no fade (even during prolonged applications of high doses) and no "off" response. Somatic applications of the uptake blockers nipecotic acid or guvacine consistently produced: an increase in the effectiveness of the inhibition produced by gamma-aminobutyric acid and its analogues: a decrease in the latency to peak of the inhibition and an increase in the time to recovery; a full blockade of the fade and the "off" response. All of these effects were rapid and fully reversible without significant changes in either the field excitatory postsynaptic potential or the (control) somatic spikes. The more specific glial uptake blocker, 4,5,6,7-tetrahydroisoxazolo(4,5-C) pyridin 3-OL occasionally blocked the "off" response, however it was less potent and also tended to reduce the spike amplitude. Dendritic applications of the uptake blockers reduced the excitatory postsynaptic potential and the somatic spike but failed to produce prominent changes in the action of gamma-aminobutyric acid and its analogues.(ABSTRACT TRUNCATED AT 400 WORDS)

Glycine release from Y79 retinoblastoma cells

Glycine release, induced by a high concentration of potassium chloride (K+), was investigated in cultured human Y79 retinoblastoma cells. The cells were labeled by incubation with [2-3H]glycine prior to K+ depolarization. Depolarization with 55 mM K+ caused an immediate, Ca2+-dependent release of approximately 20% of the cellular radiolabeled glycine content. Chemical analysis of the intracellular free glycine content also showed that approximately 20%, 2.4 nmol/mg protein, was released after K+ depolarization. Glycine release from labeled Y79 cells was not stimulated by incubation with 55 mM choline chloride. Based on measurements with an amino acid analyzer, it is concluded that of the free amino acids contained in the Y79 cell, only glycine is specifically released into the extracellular fluid by K+ depolarization. Although the intracellular content of serine and glutamate decreased, these amino acids were not released from the cells. Further studies with [U-14C]serine suggest that serine is converted into glycine in Y79 cells. Veratridine also caused an immediate release of [2-3H]glycine from the cells, and this was blocked by tetrodotoxin. This suggests that the Y79 cells possess voltage-dependent Na+ channels. These results indicate that K+- and veratridine-stimulated glycine release occurs in Y79 retinoblastoma cells, providing additional evidence that this continuously cultured line may be a useful model for certain human retinal and central nervous system functions.

Comparison of GABA analogues at the crayfish stretch receptor neurone

The conductance increase induced by GABA and structurally related compounds has been measured in voltage clamped stretch receptor neurones of crayfish. GABA induced only at 10(-3) M a rapid conductance increase. The response to lower concentrations between 10(-6) and 10(-4) M developed slowly (20-60 min). The postsynaptic conductance increase induced by repetitive application of the same GABA concentration was progressively enhanced in the speed and magnitude. In the presence of nipecotic acid or in Na+-free Ringer solutions, the response to all GABA concentrations was instantaneous and constant for each concentration. Muscimol between 10(-6) and 10(-3) M caused instantaneous dose-dependent conductance increases. These results suggest the presence of a saturable GABA uptake system limiting the access of bath applied GABA, but not of muscimol, to postsynaptic receptor sites.

The effects of temperature, pH and Cl-pump inhibitors on GABA responses recorded from cat dorsal root ganglia

GABA applied by iontophoresis produced GABA-induced currents (GCs) and GABA-induced depolarizations (GDs) which were recorded intracellularly from cat dorsal root ganglia (DRG). Lowering the temperature (37 to 27 degrees C) of the preparation depressed the amplitude of GCs while prolonging their rise-time and decay time. This depressant action was mainly due to a hyperpolarizing shift in the GABA equilibrium potential (EGABA). GABA responses could also be depressed by alkalinization of the superfusion solution or addition of putative chloride pump inhibitors, e.g. SITS, furosemide or bumetanide. However, the mechanism by which these latter procedures depressed GABA responses was not due to a shift in EGABA as occurred with lowered temperature. Instead we suggest that alkalinization or the putative chloride pump inhibitors affect the chloride channel or some other site associated with the GABA receptor complex and cause the depression we observed. GABA responses could be facilitated by lowering the pH of the superfusion solution or by injecting ammonium ion into a DRG. These results suggest that a temperature-sensitive, inwardly directed chloride pump that is resistant to SITS, furosemide or bumetanide, operates in cat DRG.

Anticonvulsant activity of GABA uptake inhibitors and their prodrugs following central or systemic administration

The activity of compounds inhibiting neuronal or glial GABA uptake has been assessed following intracerebroventricular (i.c.v.) or intraperitoneal (i.p.) administration in DBA/2 mice (sound-induced seizures) or Swiss S mice (pentylenetetrazol-induced seizures). Sound-induced seizures are suppressed by the i.c.v. injection of (+/-)-nipecotic acid, 3.2 mumol, or (+/-)-cis-4-hydroxynipecotic acid, 2 mumol, but not by i.p. injection of (+/-)-nipecotic acid, 3.2 mmol/kg or (+/-)-cis-4-hydroxynipecotic acid 4 mmol/kg. Pentylenetetrazol-induced seizures are not suppressed by i.c.v. injection of (+/-)-nipecotic acid 1-4 mumol, or (+/-)-cis-4-hydroxynipecotic acid, 2-4 mumol. THPO (4,5,6,7-tetrahydroisoxazolo[4.5-c]pyridin-3-ol), 1-5 mumol i.c.v. or 1-4 mmol/kg i.p., protects against sound-induced seizures. There is no protection against pentylenetetrazol seizures after i.c.v. THPO injection, but THPO, 2-8 mmol/kg i.p., is protective. Among prodrugs, (+/-)-nipecotic acid pivaloyloxymethyl ester protects against sound-induced seizures, when given i.c.v. (3.2 mumol) or i.p. (1.6-3.2 mmol/kg) and against pentylenetetrazol seizures when given i.p. (0.5-4 mmol/kg). (+/-)-cis-4-hydroxynipecotic acid methyl ester protects against sound-induced seizures when given i.p. (3.2 mmol/kg), but is only partially protective against pentylenetetrazol seizures, when given i.p. (4 mmol/kg). Some prodrugs induce myoclonus following either i.c.v. or i.p. administration.

Progabide: a controlled trial in partial epilepsy

Progabide (SL 76002) was studied in a randomized double-blind crossover trial using 20 outpatients suffering from partial complex seizures. Progabide was added to the concomitant antiepileptic treatment in a fixed dosage schedule. The design included an open therapy control unit. No significant difference was established between the number of partial seizures during treatment with progabide and placebo. A trend was observed for lower seizure frequency of secondary generalized seizures during treatment with progabide. Only mild and transient side effects were observed. There was no difference between the side effects of progabide and placebo.

THIP: a single-blind controlled trial in patients with epilepsy

In an early phase-2 study, THIP (4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol) was investigated in a single-blind controlled trial comprising 9 outpatients suffering from epilepsy. THIP was added to the concomitant antiepileptic treatment with increasing doses (15 to 120 mg/day) based on therapeutic effect or side-effects. The blood levels of concomitant therapy were kept constant. No significant difference was established between the number of seizures during treatment with maximal doses of THIP and placebo. A trend was observed for lower seizure frequency during a period on submaximal dose of THIP.

Substrate specificity and developmental aspects of a presynaptic GABA receptor regulating glutamate release in the rat cerebellum

In order to better characterize the presynaptic GABA receptors regulating glutamate release in the cerebellum [Levi and Gallo, 1981], a number of GABA agonists and GABA transport inhibitors were tested for their ability to potentiate the depolarization-induced release of the glutamate analog D-[3H]aspartate from superfused cerebellar synaptosomes. Of all the compounds tested, only those which are known to interact specifically with GABA receptors were effective when tested on synaptosomal preparations. The order of effectiveness found was the following: muscimol congruent to 3-APS greater than or equal to P4S greater than isoguvacine greater than THIP. GABA uptake inhibitors were unable to enhance D-[3H]aspartate evoked release from synaptosomes, but were effective when tested in cerebellar slices; in the latter case, the activation of the GABA receptors may be achieved indirectly, through an increase of the extracellular GABA concentrations. The substrate specificity of the presynaptic GABA receptors regulating cerebellar acidic amino acid release appears to be similar to that reported for GABA receptors in radioligand binding studies and for GABA autoreceptors. Studies on synaptosomes from immature cerebella suggested that the presence of the potentiating effect on the acidic amino acid release by GABA agonists is correlated with the development of the parallel fiber terminals, which are believed to be the main site from which glutamate is released in the adult cerebellum.

Glycine uptake by cultured human Y79 retinoblastoma cells: effect of changes in phospholipid fatty acid unsaturation

Glycine uptake was investigated in cultured Y79 retinoblastoma cells containing different degrees of phospholipid fatty acid unsaturation. The modifications were produced by growing the retinoblastoma cells in medium supplemented with various unsaturated fatty acids. Glycine was taken up by the retinoblastoma cells through two kinetically distinguishable process. The high-affinity system is totally dependent upon extracellular Na+ and partially dependent upon Ca2+. Of the glycine taken up by retinoblastoma cells, 85-90% remains as free intracellular glycine and less than 30% is incorporated into cellular protein. When the cells are grown in a medium containing 10% fetal bovine serum as the only source of fatty acids, the phospholipids contained 23% polyunsaturated fatty acids. Under these conditions the high-affinity system has a K'm of 34.2 +/- 3.7 micrometers and a V'max of 91.2 +/- 16.2 pmol min-1 mg protein -1. The low-affinity system has a K'm of 2.7 +/- 0.4 mM and a V'max of 4.1 +/- 0.5 nmol min-1 mg protein-1. When the polyunsaturated fatty acid content of the phospholipids was increased by supplementing the medium with linolenic or docosahexaenoic acids (n-3 polyunsaturates) or linoleic or arachidonic acids (n-6 polyunsaturates), the K'm and V'max of the high-affinity glycine uptake system were increased three- to fourfold. By contrast, supplementing the medium with oleic acid, and n-9 monounsaturate, did not significantly alter the K'm or V'max for glycine uptake. The results with this model system suggest that one of the effects of the high polyunsaturated fatty acid content normally present in neural cell membranes may be a modulation of the high-affinity transport system so that it functions more efficiently in regulating glycine uptake.

Involvement of amino acids in periodic inhibitions of bulbar respiratory neurones

As previously demonstrated, spontaneously firing bulbar inspiratory neurones are periodically inhibited either at the beginning of, or throughout expiration, while bulbar expiratory neurones are inhibited during inspiration. The aim of the present study was to test the hypothesis that amino acids act as transmitters of these periodic inhibitions. The study was performed using iontophoretic applications of drugs on bulbar respiratory neurones. On these neurones GABA and glycine-sensitive sites were identified and differentiated on the basis of the actions of agonist (muscimol) or antagonists (bicuculline, picrotoxin and strychnine). Using competitive antagonists (nipecotic acid, beta-alanine) mechanisms responsible for GABA uptake were found in the close vicinity of respiratory-related neurones. Some but not all types of periodic inhibition were found to be reduced following application of GABA or glycine antagonists. Strychnine was found to reduce periodic inhibitions occurring at the beginning of expiration in inspiratory neurones. GABA antagonists had an effect on those periodic depressions which were prolonged throughout expiration. A different and complementary role of glycine-like and GABA-like systems in central respiratory mechanisms is proposed.

Dose-conductance relationships for GABA agonists and the effect of uptake inhibitors in crayfish stretch receptor neurons

The interaction of GABA (gamma-aminobutyric acid) and structurally-related compounds with postsynaptic GABA receptors was studied quantitatively by measuring receptor-mediated increases in membrane input conductance in isolated crayfish stretch receptor neurons (SRN). The following compounds, in order of decreasing potency, were effective agonists: muscimol greater than GABA greater than isoguvacine greater than (-)gamma-amino-beta-hydroxybutyric acid greater than beta-gu anidinopropionic acid greater than 3-aminopropanesulfonic acid greater than (+)gamma-amino-beta-hydroxybutyric acid greater than isonipecotic acid greater than THIP. A highly significant correlation was found between the log potencies for GABA agonists that were obtained in the SRN and those obtained in our laboratory using mammalian GABA receptor binding assays. Hill plot analyses of the log concentration-conductance data from the SRN indicated a Hill slope (nH) of approximately 2 for all agonists except GABA and guanidinopropionic acid (nH greater than 2), two compounds known to be actively accumulated by cellular GABA uptake processes. Nipecotic acid, guvacine, and L-alpha, beta-diaminopropionic acid, blockers of GABA uptake processes, had essentially no effect by themselves on the SRN membrane input conductance at concentrations up to 5 mM, however, they potentiated the effects of sub-maximal concentrations of GABA and decreased the steepness of the log concentration-conductance curve, and consequently nH, for GABA. The effects of muscimol, however, were not affected. When the influence of uptake processes was considered, it appeared that all agonists tested acted by the same cooperative mechanism which required at least two molecules of agonist to activate a receptor-ionophore unit.

The stereotyped behavior syndrome: a new model and proposed therapy

The stereotyped behavior syndrome was induced in rats through local impairment of inhibitory GABA-ergic mechanisms in both caudate nuclei by bilateral microinjection of tetanus toxin, penicillin, or picrotoxin into the rostral part of the caudate nucleus. Intraperitoneally injected haloperidol suppressed the syndrome; this effect was dose-dependent. The same effect on the tetanus toxin-induced stereotyped behavior was produced by GABA microinjected bilaterally into the rostral part of the caudate nucleus of unrestrained rats. It was found in this model of tetanus toxin-induced stereotyped behavior that lithium chloride and diazepam can suppress the syndrome. Combined application of lithium chloride, diazepam, and haloperidol in minimal effective doses resulted in a much more complete and longer-lasting suppression of the syndrome than the separate use of these drugs. This effect is attributed to the joint specific actions of the drugs on pathogenetically interrelated components of a hyperactive determinant structure that arises in the caudate nuclei after impairment of the GABA control and this is responsible for the stereotyped behavior syndrome.

Does glial uptake affect GABA responses? AN intracellular study on rat dorsal root ganglion neurones in vitro

1. Using single barrel pipettes, intracellular records were obtained from surface neurones of isolated rat dorsal root ganglia (DRG) impaled under microscopic vision.2. Responses to gamma-aminobutyric acid (GABA) were elicited either by ionophoresis or by placing drops of concentrated GABA solutions directly into the flow of superfusing Ringer. Using this latter method it was estimated that the GABA concentration eliciting threshold ( approximately 1 mV) responses was 3-20 muM.3. Short (</= 1 sec) ionophoretic or drop administrations of GABA elicited depolarizing responses associated with an increased membrane conductance. With longer applications the initial depolarization was not sustained but decayed to a lower plateau level (desensitization) associated with a minimal conductance change.4. Low chloride superfusions did not affect subsequent responses to GABA unless GABA was also administered during the low chloride superfusion, in which case responses declined markedly. This suggests that GABA caused appreciable chloride fluxes when it was administered regularly (e.g. for 1 sec every minute).5. Glial GABA uptake was inhibited by adding 1 mM-beta-alanine or 0.25 mM-chlorpromazine to the bicarbonate-Ringer superfusate or by substituting lithium for sodium in a Tris-Ringer superfusate. Uptake inhibition had no consistent effect on any of the parameters studied, namely membrane potential, input resistance, amplitude and time course of responses to GABA, and GABA desensitization.6. Muscimol and isoguvacine, which are probably not substrates for the glial GABA carrier, elicited responses with time course and desensitization characteristics indistinguishable from those of responses to GABA.7. GABA superfused at concentrations as low as 1 muM could reduce responses to ionophoretic GABA, i.e. cause a desensitization of GABA receptors.8. It is concluded firstly that in DRG, glial uptake does not affect the amplitude or time course of responses to GABA when the neurone under study is close to the source of GABA; and secondly that desensitization can occur independently of GABA uptake.9. The findings are discussed in relation to their possible relevance to GABA systems in the central nervous system.

Electrophoretic study of pipecolic acid, a biogenic imino acid, in the mammalian brain

Pipecolic acid (PA), one of the imino acids, is a normal constituent in the mammalian brain. It is said that PA is a major intermediate of lysine metabolism in the rat brain. Biochemical studies have suggested that PA may be involved in the regulation of synaptic mechanism in the CNS. Moreover, the pathophysiological significance of PA has been also suggested by some investigators. However, there has so far been no good evidence based on the comprehensive electrophysiological experiments. Using unit recording and microelectrophoretic technique, the action of PA on single neuron activities in the rat brain was examined. PA depressed the firing of 88 out of 115 cortical neurons tested. Only 2 were excited and 25 remained unaffected. All the identified hippocampal pyramidal neurons examined were uniformly inhibited. It has been reported that PA inhibits the uptake of GABA into the brain slices and enhances the release of GABA from the slices. Thus, it is likely that the inhibitory response due to PA may have some connections with GABAergic transmission. On the other hand, it remains to be clarified whether the specific PA sensitive receptors exist in the brain. Our findings provide a clue to the elucidation of the presumed synaptic involvement of PA in the CNS.

Inhibitors of the GABA uptake systems

This review describes a novel class of heterocyclic GABA uptake inhibitor with no affinity for the GABA receptors. The parent compound nipecotic acid is a potent inhibitor of neuronal and glial GABA uptake, and nipecotic acid is a substrate for the transport carriers concerned. The structurally related cyclic amino acids guvacine and cis-4-hydroxynipecotic acid are also potent inhibitors of both GABA transport systems. Even minor structural alterations of these compounds result in considerable or complete loss of activity. Whereas homonipecotic acid is a weak but selective inhibitor of glial GABA uptake, homoguvacine is virtually inactive. Similarly the lower homologues of nipecotic acid and guvacine, beta-proline and 3-pyrroline-3-carboxylic acid, respectively, show some selectivity with respect to inhibition of glial GABA uptake, but these compounds are much weaker than the parent compounds. The bicyclic compounds THPO and THAO, in which the carboxyl groups of nipecotic acid and homonipecotic acid have been replaced by 3-isoxazolol units are moderately potent and practically specific inhibitors of glial GABA uptake. cis-4-Mercaptonipecotic acid is considerably weaker than the closely related analogue cis-4-hydroxynipecotic acid, but the former compound may interact irreversibly with the GABA transport carriers. The results demonstrate a pronounced substrate specificity of the glial and in particular the neuronal GABA transport system. It is evident that the GABA molecule is transported in a conformation different from that, in which it activates its receptors. These findings are of importance for the development of drugs for selective pharmacological regulation of the functions of central GABA-mediated synapses in certain neurological diseases.

Influence of cellular transport on the interaction of amino acids with gamma-aminobutyric acid (GABA)-receptors in the isolated olfactory cortex of the guinea-pig

1 Freshly cut guniea-pig olfactory cortex slices contained 2.2 mmol gamma-aminobutyric acid (GABA)/kg tissue weight. This declined during in vitro incubation at 25 degrees C in the absence of exogenous GABA, but increased to 6.95 mmol/kg after 1.5 h incubation in 1 mM GABA. 2 Uptake of [3H]-GABA (1 microM) was inhibited by 1 mM (+/-)-nipecotic acid (-83%), beta-amino-n-butyric acid (BABA) (-59%), L-2,4-diaminobutyric acid (DABA) (-63%), (+/-)cis-3-aminocyclohexane carboxylic acid (ACHC) (-53%), and 3-aminopropanesulphonic acid (3-APS) (-26%), but was increased by beta-alanine (BALA) (+23%). 3 Autoradiographs showed steep concentration gradients of radioactivity across slices incubated for short periods in [3H]-GABA. 4 Efflux of [3H]-GABA from pre-loaded slices was accelerated strongly by nipecotic acid, BABA, DABA and ACHC but weakly or not all by BALA or 3-APS. 5 Nipecotic acid (1 mM) potentiated the surface-depolarization of the slice produced by GABA but not that produced by 3-APS. 6 The depolarizing actions of DABA, BABA, nipecotic acid and ACHC, but not that of 3-APS or BALA, were potentiated when the endogenous GABA content of slices was raised. 7 It is concluded that: (a) the depolarizing action of exogenous GABA is limited by cellular uptake; (b) surface-depolarizations produced by nipecotic acid, DABA, BABA and ACHC may be mediated by the release of GABA; and (c) neuronal, rather than glial, transport systems are responsible for these effects.