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

Neurons, Receptors, and Channels

Here, I recount some adventures that I and my colleagues have had over some 60 years since 1957 studying the effects of drugs and neurotransmitters on neuronal excitability and ion channel function, largely, but not exclusively, using sympathetic neurons as test objects. Studies include effects of centrally active drugs on sympathetic transmission; neuronal action and neuroglial uptake of GABA in the ganglia and brain; the action of muscarinic agonists on sympathetic neurons; the action of bradykinin on neuroblastoma-derived cells; and the identification of M-current as a target for muscarinic action, including experiments to determine its distribution, molecular composition, neurotransmitter sensitivity, and intracellular regulation by phospholipids and their hydrolysis products. Techniques used include electrophysiological recording (extracellular, intracellular microelectrode, whole-cell, and single-channel patch-clamp), autoradiography, messenger RNA and complementary DNA expression, antibody injection, antisense knockdown, and membrane-targeted lipidated peptides. I finish with some recollections about my scientific career, funding, and changes in laboratory life and pharmacology research over the past 60 years.

Potentiation of GABA(A) receptor agonists by GABA uptake inhibitors in the rat ventral midbrain

Whole-cell patch recordings were made from dopamine-containing neurons in the ventral tegmental area (VTA) and substantia nigra zona compacta (SNC). Isoguvacine evoked an outward current (at -60 mV) in a concentration-dependent manner with an EC50 of 62+/-8 microM. The gamma-aminobutyric acid (GABA) uptake inhibitor 1-(2(((diphenylmethylene)imino)oxy)ethyl)-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride (NO 711) (3 microM) shifted the isoguvacine concentration-response curve to the left, with a new EC50 of 22+/-4 microM. L-Arginine (3 mM) also shifted the isoguvacine concentration-response curve to the left, with a new EC50 of 29+/-5 microM. L-Arginine (3 mM) increased the currents evoked by GABA (100 microM) and muscimol (1 microM) by 208% and 261%, respectively. The GABA uptake inhibitor 4,5,6,7,-tetrahydroisoxazolo[4,5-c]-pyridin-3-ol hydrobromide (THPO) (300 microM) not only mimicked but also occluded the ability of L-arginine (3 mM) to potentiate currents evoked by isoguvacine. Equimolar replacement of Na+ with choline increased GABA-evoked currents, suggesting that a low Na+ concentration has an inhibitory effect on GABA transport. Low Na+ concentration (25 mM) inhibited isoguvacine currents but still occluded the potentiating effects of L-arginine. We conclude that GABA uptake inhibitors potentiate the actions of the GABA(A) receptor agonists, isoguvacine and muscimol, probably because they are effective substrates for GABA transporters in the ventral midbrain.

Action of alpha-dendrotoxin on K+ currents in nerve terminal regions of axons in rat olfactory cortex

1. In the rat olfactory cortex, unmyelinated axons give rise to synapses en passant. This tissue was used to study the pharmacology of axonal K(+)-currents. Responses were measured from a group of these axons as unclamped field currents, with a polarizable suction electrode. 2. A single stimulus to the axons elicited a tetrodotoxin-sensitive Na(+)-dependent transient K(+)-currents were revealed by positive polarization of the suction electrode and were manifest as a negative current following the Na(+)-component. 3. In the presence of tetraethylammonium (TEA, 5 mM) and Cd2+ (100 microM), the K(+)-component was depressed by 3,4-diaminopyridine (3,4-DAP; 1 to 20 microM; IC50 2.0 +/- 0.4 microM). alpha-Dendrotoxin (DTX; 15-1500 nM) also attenuated the aminopyridine-sensitive component (IC50 93 +/- 4 nM). At the highest DTX concentration, depression of the K(+)-current was incomplete, the residual K+ current being reduced by 3,4-DAP (0.1 to 5 microM). 4. These results indicate the presence of two aminopyridine-sensitive K+ currents in this preparation distinguished by their susceptibility to DTX.

NMDA receptor-mediated depolarizing action of proline on CA1 pyramidal cells

This study investigated the actions of proline on CA1 hippocampal pyramidal cells with use of slice preparations. Bath-applied L-proline first induced these cells to fire multiple orthodromic population spikes in response to a single stimulus and then blocked their response to both orthodromic and antidromic stimulation. These effects could be explained by postsynaptic depolarization followed by depolarization block. Grease-gap studies confirmed that L-proline depolarizes CA1 pyramidal cells. D-Proline was inactive in these tests. Excitatory amino acid antagonists reduced depolarizing responses to proline and N-methyl-D-aspartate (NMDA) in parallel. Mn2+ failed to attenuate proline-evoked depolarizations at concentrations that substantially inhibited synaptic transmission, but at a higher concentration it reduced responses to both proline and NMDA. These results suggest that proline depolarized CA1 pyramidal cells mainly by activating postsynaptic NMDA receptors. The neuroexcitatory and neurotoxic actions of proline in the hippocampus may contribute to the seizures and mental retardation associated with hyperprolinemia.

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)

Differential effect of zinc on the vertebrate GABAA-receptor complex

1. gamma-Aminobutyric acid (GABA) responses were recorded from rat superior cervical ganglia (SCG) in culture using the whole cell recording technique. 2. Zinc (50-300 microM) reversibly antagonized the GABA response in embryonic and young post-natal neurones, while neurones cultured from adult animals were far less sensitive and occasionally resistant to zinc blockade. Cadmium (100-300 microM) also antagonised the GABA response, while barium (100 microM-2 mM) was ineffective. 3. The differential blocking effect of zinc on cultured neurones of different ages also occurred in intact SCG tissue. 4. The GABA log dose-response curve constructed with foetal or adult cultured neurones was reduced in a non-competitive manner by zinc. This inhibition was minimally affected by the membrane potential. 5. The GABA response recorded intracellularly from guinea-pig pyriform cortical slices was enhanced by zinc (300-500 microM), which occurred concurrently with a decrease in the input conductance of the cell. The enhancement was unaffected by prior blockade of the GABA uptake carrier by 1 mM nipecotic acid. This phenomenon could be reproduced by barium (300 microM) and cadmium (300 microM). 6. We conclude that the vertebrate neuronal GABAA-receptor becomes less sensitive to zinc with neural (GABAA-receptor?) development, and the enhanced GABA response recorded in the CNS is a consequence of the reduction in the input conductance and not due to a direct effect on the receptor complex.

The effect of captopril on the membrane properties of central neurons in-vitro

Captopril, a potent antihypertensive that acts via inhibition of angiotensin converting enzyme also has apparent central actions. Its effects on membrane properties on particular central neurons in-vitro has therefore been investigated. In the substantia nigra, where there is a high concentration of angiotensin converting enzyme, captopril caused a dose-dependent depolarization without any apparent change in conductance, but possibly requiring the integrity of the dendritic arbour. A similar effect occurred when captopril was applied to neurons in either the thalamus or hippocampus, where levels of angiotensin converting enzyme are relatively low. Further studies with homologues of captopril revealed that the -SH group on the molecule was a prerequisite of the effect observed. It is concluded that the -SH group on the captopril molecule has an electrogenic effect on diverse central neurons, independent of inhibition of angiotensin converting enzyme, but preferentially manifest at the level of the dendrite.

The effects of gamma-hydroxybutyrate on the membrane properties of guinea-pig pars compacta neurons in the substantia nigra in vitro

The effects of gamma-hydroxybutyrate have been studied on the membrane properties of pars compacta neurons within the guinea-pig substantia nigra maintained in vitro. The effects of gamma-hydroxybutyrate are to (i) lower input resistance, (ii) hyperpolarize the cell membrane in a dose-dependent manner and (iii) facilitate calcium conductances. These effects are resistant to the blockade of sodium channels with tetrodotoxin and blockade of potassium channels with tetraethylammonium or 4-aminopyridine. Furthermore, these effects are only partially blocked by high doses of the GABA receptor antagonist bicuculline: indeed the effects of the GABA receptor agonist muscimol can be differentiated from those of gamma-hydroxybutyrate in that the latter is sensitive to application of barium ions. The results suggest that gamma-hydroxybutyrate acts to increase utilization of calcium, which in turn leads to an initiation of calcium-dependent events. The functional consequences of these effects of gamma-hydroxybutyrate are discussed with regard to its possible endogenous modulatory actions.

GABA receptors on the cell-body membrane of an identified insect motor neuron

The pharmacology of a gamma-aminobutyric acid (GABA) receptor on the cell body of an identified motor neuron of the cockroach (Periplaneta americana) was investigated by current-clamp and voltage-clamp methods. Iontophoretic application of GABA increased membrane conductance to chloride ions, and prolonged application resulted in desensitization. Hill coefficients, determined from dose-response data, indicated that binding of at least two GABA molecules was required to activate the chloride channel. Differences between vertebrate GABAA receptors and insect neuronal GABA receptors were detected. For the GABA receptor of motor neuron Df, the following rank order of potency was observed: isoguvacine greater than muscimol greater than or equal to GABA greater than 3-aminopropanesulphonic acid. The GABAB receptor agonist baclofen was inactive. Of the potent vertebrate GABA receptor antagonists (bicuculline, pitrazepin, RU5135 and picrotoxin), only picrotoxin (10(-7) M) produced a potent, reversible block of the response to GABA of motor neuron Df. Both picrotoxinin and picrotin also blocked GABA-induced currents. Bicuculline hydrochloride (10(-4) M) and bicuculline methiodide (10(-4) M) were both ineffective when applied at resting membrane potential (-65 mV), although at hyperpolarized levels partial block of GABA-induced current was sometimes observed. Pitrazepin (10(-4) M) caused a partial, voltage-independent block of GABA-induced current. The steroid derivative RU5135 was inactive at 10(-5) M. In contrast to the potent competitive blockade of vertebrate GABAA receptors by bicuculline, pitrazepin and RU5135, none of the weak antagonism caused by these drugs on the insect GABA receptor was competitive. Flunitrazepam (10(-6) M) potentiated GABA responses, providing evidence for a benzodiazepine site on an insect GABA-receptor-chloride-channel complex.

Sodium-dependent proline uptake in the rat hippocampal formation: association with ipsilateral-commissural projections of CA3 pyramidal cells

Na+-dependent uptake of L-[3H]proline was measured in a crude synaptosomal preparation from the entire rat hippocampal formation or from isolated hippocampal regions. Among hippocampal regions, Na+-dependent proline uptake was significantly greater in areas CA1 and CA2-CA3-CA4 than in the fascia dentata, whereas there was no marked regional difference in the distribution of Na+-dependent gamma-[14C]aminobutyric acid ([14C]GABA) uptake. A bilateral kainic acid lesion, which destroyed most of the CA3 hippocampal pyramidal cells, reduced Na+-dependent proline uptake by an average of 41% in area CA1 and 52% in area CA2-CA3-CA4, without affecting the Na+-dependent uptake of GABA. In the fascia dentata, neither proline nor GABA uptake was significantly altered. Kinetic studies suggested that hippocampal synaptosomes take up proline by both a high-affinity (KT = 6.7 microM) and a low-affinity (KT = 290 microM) Na+-dependent process, whereas L-[14C]glutamate is taken up predominantly by a high-affinity (KT = 6.1 microM) process. A bilateral kainic acid lesion reduced the Vmax of high-affinity proline uptake by an average of 72%, the Vmax of low-affinity proline uptake by 44%, and the Vmax of high affinity glutamate uptake by 43%, without significantly changing the affinity of the transport carriers for substrate. Ipsilateral-commissural projections of CA3 hippocampal pyramidal cells appear to possess nearly as great a capacity for taking up proline as for taking up glutamate, a probable transmitter of these pathways. Therefore proline may play an important role in transmission at synapses made by the CA3-derived Schaffer collateral, commissural, and ipsilateral associational fibers.

Adaptation of the GABAA-receptor complex in rat brain during chronic elevation of GABA by ethanolamine O-sulphate

Slice preparations of rat cuneate nucleus were used for studies on the gamma-aminobutyric acid GABAA-receptor complex following chronic and acute pretreatment with GABA-alpha-ketoglutarate aminotransferase (GABA-T) inhibitors. The whole brain GABA concentration was significantly increased 2.9 fold and 2.6 fold following treatment with ethanolamine O-sulphate (EOS, orally) for 15-30 days and 56-64 days, respectively. One hour after a single injection of gamma-acetylenic GABA (GAG) i.p., there was a significant 2.1 fold increase in whole brain GABA. Superfusion of a slice with muscimol or the GABA uptake inhibitor nipecotic acid depolarized the afferent nerve fibres. These effects were potentiated by flurazepam (1 microM) and pentobarbitone (10 microM) and antagonized by picrotoxin (3 microM, 30 microM). Following 15-30 days of EOS-treatment, the depolarization response to muscimol was decreased and that to nipecotic acid increased. These changes were no longer significant by 56-64 days of pretreatment. The acute dose of GAG did not affect the depolarization response to muscimol but increased that to nipecotic acid. The potentiations of muscimol by flurazepam (1 microM) and pentobarbitone (10 microM) were enhanced following chronic EOS treatment (15-64 days). The enhancement of flurazepam was less after 56-64 days than after 15-30 days pretreatment whereas the enhancement of pentobarbitone was similar at both times. Acute GAG treatment had no effect. The potency of picrotoxin as an antagonist of muscimol was reduced following chronic EOS treatment; the enhancement was less after 56-64 days than after 15-30 days pretreatment. Acute GAG treatment caused only a very small reduction in picrotoxin potency. Possible adaptations in the GABAA-receptor complex and its modulation during chronic elevation of brain GABA are discussed.

Quantitative evaluation of the potencies of GABA-receptor agonists and antagonists using the rat hippocampal slice preparation

CA1 population spikes recorded in the rat hippocampal slice were used to assess quantitatively the potencies of GABA-receptor agonists and antagonists on mammalian CNS neurones. Apart from GABA itself, GABA A-receptor agonists inhibited the CA1 population spikes with potencies that correlated closely (r = 0.96) with their ability to displace [3H]-GABA from GABAA-binding sites. The low potency of GABA in this preparation was attributed to the action of uptake processes as the GABA uptake inhibitor, cis-4-hydroxynipecotic acid (2 X 10(-4) M), produced an approximate 6 fold increase in the potency of GABA whilst having no effect on the potency of 4,5,6,7-tetrahydroisoxazolo [5,4-c] pyridin-3-ol (THIP), a GABAA-receptor agonist which is not a substrate for the GABA uptake system. The inhibitory effects of the selective GABAA-receptor agonists isoguvacine and muscimol were antagonized by bicuculline methochloride, which shifted the dose-response curves to the right in a parallel manner. The Schild plots for bicuculline methochloride against isoguvacine and muscimol had slopes of 1 and gave pA2 values of 6.24 and 6.10, respectively. Picrotoxin also antagonized the inhibitory effects of isoguvacine and produced parallel shifts to the right of the dose-response curve. However, the Schild plot for picrotoxin had a slope significantly less than unity (0.82) and gave a pA2 value of 6.89. The novel GABAA-receptor antagonist, pitrazepin, antagonized the inhibitory effects of isoguvacine in an apparently competitive manner. The Schild plot had a slope of 1 and gave a pA2 of 6.69. 6 The inhibitory effects of baclofen, GABA and kojic amine were not antagonized by GABAAreceptor antagonists and were presumed to be mediated by actions at GABA5-receptors. 7 The inhibitory effects of THIP and isoguvacine were antagonized with the same potency by bicuculline methobromide. These results do not support the suggestion that THIP acts preferentially at a 'synaptic' bicuculline-sensitive, GABA receptor. 8 It is concluded that the CAI population spike in the rat hippocampal slice is a useful test system for the quantitative analysis of both GABAA- and GABAB-receptor agonists and antagonists.

Effects of adenosine uptake blockers and adenosine on evoked potentials of guinea-pig olfactory cortex

The olfactory cortex slice preparation from guinea-pig has been used to test compounds which inhibit the cellular uptake of adenosine. The uptake inhibitors dipyridamole (0.1-10 mumol/l), dilazep (1-10 mumol/l) nitrobenzylthioguanosine (1-10 mumol/l), nitrobenzylthioinosine (0.1-5 mumol/l), and hexobendine (1-100 mumol/l) increased the potency of adenosine (0.1-30 mumol/l) by up to 5-fold but did not potentiate cyclohexyladenosine (0.01-10 mumol/l). The benzodiazepine, diazepam (1 mumol/l) slightly increased the potency of adenosine (by 1.7-fold) whereas flurazepam (3 mumol/l) had no effect, suggesting that inhibition of adenosine uptake is probably not the major therapeutic action of these compounds. The uptake inhibitors depressed the amplitude of the monosynaptic epsp when added alone, an effect reversed by adenosine deaminase (1 unit/ml) whereas the adenosine deaminase inhibitor, erythro-9-(2-hydroxy-3-nonyl)adenine (10 mumol/l) had no effect on adenosine action. These results show that in this preparation (a) adenosine action is attenuated by an uptake mechanism and (b) endogenous adenosine release normally has no apparent effects on synaptic transmission at low stimulus rates. Nitrobenzylthioinosine and nitrobenzylthioguanosine are probably the best uptake blockers.

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)

Cellular uptake disguises action of L-glutamate on N-methyl-D-aspartate receptors. With an appendix: diffusion of transported amino acids into brain slices

Pharmacological properties of the guanosine 3'5'-cyclic monophosphate (cyclic GMP) responses to excitatory amino acids and their analogues were compared in slices and dissociated cells from the developing rat cerebellum maintained in vitro. The intention was to determine the extent to which cellular uptake might influence the apparent properties of receptor-mediated actions of these compounds. In slices, the potencies of the weakly (or non-) transported analogues, N-methyl-D-aspartate (NMDA) and kainate (KA) (EC50 = 40 microM each) were higher than those of the transported amino acids, D- and L-aspartate (EC50 = 250 microM and 300 microM) and D- and L-glutamate (EC50 = 540 microM and 480 microM). Quisqualate (up to 300 microM) failed to increase cyclic GMP levels significantly. The sensitivity of agonist responses to the NMDA receptor antagonist, DL-2-amino-5-phosphonovalerate (APV), was in the order NMDA greater than L-aspartate greater than L-glutamate, KA. In dissociated cells, L-glutamate was 280 fold more potent (calculated EC50 = 1.7 microM). L- and D-aspartate (calculated EC50 = 13 microM) and D-glutamate (EC50 = 130 microM) were also more effective than in slices. The potencies of NMDA and KA were essentially unchanged. Responses to NMDA, L-glutamate and L-aspartate under these conditions were equally sensitive to inhibition by APV but the response to KA remained relatively resistant to this antagonist. The implications of these results are that, in slices, cellular uptake is responsible for (i) the dose-response curves to L-glutamate, L- and D-aspartate bearing little or no relationship to the true (or relative) potencies of these amino acids; (ii) the potency of APV towards the actions of transported agonists acting at NMDA receptors being reduced and (iii) a differential sensitivity to APV of responses to L-glutamate and L-aspartate being created, the consequence being that a potent action of L-glutamate on NMDA receptors is disguised. These conclusions are supported by theoretical considerations relating to the diffusion of transported amino acids into brain slices, as elaborated in the Appendix.