Role for GABA and Glu plasma membrane transporters in the interplay of inhibitory and excitatory neurotransmission

Neurotransmitter plasma membrane transporters do have much more to perform than simply terminating synaptic transmission and replenishing neurotransmitter pools. Findings in the past decade have evidenced their function in maintaining physiological synaptic excitability, and their actions in critical or pathological conditions, also. Conclusively these findings indicated a previously unrecognized role for neurotransmitter plasma membrane transporters in both, synaptic and nonsynaptic signaling. Major inhibitory and excitatory neurotransmitters within the brain, GABA and Glu, have long been considered to operate through independent systems (GABAergic or Gluergic), each of them characterized by its own localization, function and dedicated GABAergic or Gluergic cell phenotypes. Recent advances, however, have challenged this long-standing paradigm. Localization of GABA in Gluergic terminals and Glu in GABAergic cells were reported. Specific plasma membrane transporters for GABA and Glu are also co-localized in different brain areas. Although, their role in regulating each other's signal is still far from being understood, emerging lines of evidence on interplaying GABAergic and Gluergic processes through plasma membrane transporters opens up a new avenue in the field of more specific therapeutic intervention.     

Extracellular level of GABA and Glu: in vivo microdialysis-HPLC measurements

In spite of several studies showing specific physiological functions of changes in the extracellular level of the major excitatory and inhibitory transmitters, Glu and GABA within the brain ([Glu](EXT), [GABA](EXT)) the exact origin (neuronal vs. astroglial, synaptic vs. extrasynaptic) of Glu and GABA present in dialysate samples is still a matter of debate. For better understanding the significance of in vivo microdialysis data, here we discuss methodological details and problems in addition to regulation of [Glu](EXT) and [GABA](EXT). Changes in [Glu](EXT) and [GABA](EXT) under pathological conditions such as ischemia and epilepsy are also reviewed. Based on recent in vivo microdialysis data we argue that ambient [Glu](EXT) and [GABA](EXT)may have a functional role. It is suggested that specific changes in concentrations of Glu and GABA in dialysate samples together with their alterations independent of neuronal activity indicate the involvement of Glu and GABA in the information processing of the brain as essential signaling molecules of nonsynaptic transmission as well. Since various drugs are able to interfere with extrasynaptic signals in vivo, studying the extracellular cell-to-cell communication of brain cells represents a new aspect to improve drugs modulating Gluergic as well as GABAergic neurotransmission.     

Co-existence of GABA and Glu transporters in the central nervous system

Co-localization of transporters able to recapture the released or endogenously synthesized transmitter (homotransporters) and of transporters that can selectively take up transmitters/modulators originating from neighbouring structures (heterotransporters) has been demonstrated to occur within the same axon terminal of several neuronal phenotypes. Activation of terminal heterotransporters invariably leads to the release of the transmitter specific to the terminal. Heterotransporters are also increasingly reported to exist on neuronal soma/dendrites and nerve terminals, on the basis of morphological experiments. The functions of somatodendritic heterotransporters has been investigated only in a very limited number of cases. Release-regulating GABA heterotransporters of the GAT-1 type exist on Glu nerve terminals in different rodent brain regions including spinal cord. Activation of GABA heterotransporters provokes release of Glu, which takes place by reversal of the Glu homotransporter and by anion channel opening. Interestingly, the release of Glu induced by GABA in spinal cord is dramatically enhanced in a transgenic mouse model of amyotrophic lateral sclerosis and this effect seems to represent the most precocious mechanism that increases extracellular Glu concentration, reported to occur in the pathomechanism.     

Nonsynaptic receptors for GABA and glutamate

The concept of nonsynaptic communication between neurons, once a heretic idea, has become a self-evident fact during the almost forty years since its original discovery. In this review we investigate whether the archetypical synaptic transmitters of the central nervous system, Glu and GABA, can operate via nonsynaptic transmission. While experimental data supporting the general concept of nonsynaptic transmission has been progressively accumulating during these years, most of the evidence regarding nonsynaptic transmission by Glu and GABA are results of the last decade. In this paper we collect evidence for different forms of nonsynaptic transmission by the Gluergic and GABAergic system. We investigate two theoretical predictions of the concept of nonsynaptic transmission in the light of recent progress in the field: i) since extrasynaptic receptors experience a lower concentration of agonist, they are likely to have higher affinity for the agonist ii) extrasynaptic receptors are expected to be more important pharmacological targets.     

5-HT2 presynaptic receptors mediate inhibition of glutamate release from cerebellar mossy fibre terminals.

'Giant' synaptosomes originating from mossy fibre terminals and having sedimentation properties different from those of standard synaptosomes were obtained from rat cerebellum. Exposure of superfused giant synaptosomes to 15 mM KCl caused the release of endogenous glutamate in a largely (about 80%) calcium-dependent manner. The K(+)-evoked overflow of glutamate was inhibited in a concentration-dependent manner by 5-hydroxytryptamine (5-HT) and by the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI), but not by the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). The effects of 5-HT and DOI were quite potent, already reaching significant inhibition (about 25%) at 10 nM. The 5-HT2 receptor antagonist ketanserin counteracted the inhibitory effect of 5-HT. In cerebellar slices, ketanserin increased on its own the calcium-dependent K(+)-evoked release of glutamate and this effect was not prevented by tetrodotoxin (TTX). The results support the idea that cerebellar mossy fibres use glutamate as a transmitter and show that the release of glutamate can be inhibited via presynaptic heteroreceptors of the 5-HT2 type probably localized on the mossy fibre terminals.     

GABA_B受体对牛磺酸调节突触体氨基酸释放的影响

目的　牛磺酸 (Tau)调节大鼠大脑皮层突触体Asp、Glu、GABA的释放的机制尚不清楚 ,本研究从GABA受体的角度进行探讨。方法　将 phaclofen、biculline、baclofen加入悬浮有突触体的KRB液中 ,氨基酸测定采用Dans Cl柱前衍生 ,高效液相测定。结果　Tau对GABA释放的抑制作用能有效被Phaclofen所拮抗 ,而它们对GluAsp的释放均无影响。结论　Tau抑制去极化引发GABA的释放是通过激发突触体前膜GABAB 受体而起作用的 ,同时还作用于Asp、Glu神经末梢的突触体前膜 ,从而抑制去极化引发的Asp、Glu的释放。     

Regulation of synaptic transmission in the mossy fibre-granule cell pathway of rat cerebellum by metabotropic glutamate receptors.

The role of metabotropic glutamate receptors (mGluRs) in the mossy fibre-granule cell pathway in rat cerebellum was studied using slice preparations and electrophysiological techniques. Application of the group I selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) evoked, in a concentration-dependent manner (EC50 = 33 microM), a depolarising/hyperpolarising complex response from granule cells which was preferentially inhibited by the group I selective antagonist (S)-4-carboxyphenylglycine (4CPG). The group III selective agonist L-amino-4-phosphonobutyrate (AP4) evoked a hyperpolarising response (EC50 = 10 microM) which was inhibited by the group II/III selective antagonist (S)-alpha-methyl-4-phosphonophenylglycine (MPPG). The group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxylcyclopropyl)glycine (DCG-IV) elicited no measurable voltage change. The amplitude of the synaptically-mediated mossy fibre response in granule cells was unaffected during application of AP4, was reduced by DHPG and was enhanced by DCG-IV (EC50 = 80 nM). These effects were inhibited by the group selective antagonists 4CPG and (2S,1'S,2'S,3'R)-2-(2'-carboxy-3'-phenylcyclopropyl)glycine (PCCG-4), respectively. Further investigation using patch-clamp recording revealed that DCG-IV potently inhibited spontaneous GABAergic currents. We conclude that group I and III (but not group II) mGluRs are functionally expressed by granule cells, whereas unexpectedly group II or III mGluRs do not appear to be present presynaptically on mossy fibre terminals. Group II mGluRs are located on Golgi cell terminals; when activated these receptors cause disinhibition, a function which may be important for gating information transfer from the mossy fibres to the granule cells.     

Co-existence of GABA and Glu in the hippocampal granule cells: implications for epilepsy

The granule cells of the Dentate Gyrus are one of the most exciting and intriguing cells in the central nervous system. Besides containing and releasing Glu, they have been shown to contain and release peptides (somatostatin, neuropeptide Y, neurokinin B, cholecystokinin, dynorphin, enkephalin), Zn(++) ion, and brain-derived neurotrophic factor (BDNF). The recent addition of GABA to this list suggests that these cells can also function as inhibitory cells. Indeed, evidence has been presented of co-localization of all markers of the GABAergic phenotype in granule cells: GABA, the enzyme for its synthesis (Glu decarboxylase) and the membrane and vesicular transporters of GABA. These markers of the GABAergic phenotype are up-regulated after epileptic seizures. When this occurs, monosynaptic GABA receptor-mediated transmission emerges in the mossy fiber synapse thus restraining excitation and mediating antiepileptic and neuroprotective actions.     

Presynaptic inhibition by clonidine of neurotransmitter amino acid release in various brain regions.

The release of endogenous aspartic acid (Asp), glutamic acid (Glu) and gamma-aminobutyric acid (GABA) was investigated in synaptosomes prepared from various regions of the rat brain. The basal release of Asp, Glu and GABA from various regions was 12-35, 24-107 and 15-43 pmol/min per mg protein, respectively. Exposure to a depolarizing concentration of KCl (30 mM) resulted in 1.7 to 3.6-fold increases in Asp, Glu and GABA release. When clonidine (10(-4) M) was added to the perfusion medium, the K(+)-evoked overflow of both Asp and Glu was inhibited by 50-90% in the anterior cortex, thalamus and hypothalamus. Clonidine inhibited the K(+)-evoked Glu overflow by 30-40% in the posterior cortex and hippocampus. No significant effects were observed in the other brain regions (olfactory bulb, striatum, midbrain, cerebellum, pons, medulla oblongata). The inhibitory effects of clonidine were counteracted by an alpha 2-adrenoceptor antagonist, rauwolscine. The data suggest that the basal and K(+)-evoked release of Asp, Glu and GABA from nerve terminals is different in rat brain regions and that the presynaptic alpha 2-adrenoceptors which regulate the release of excitatory amino acids are mainly distributed in the anterior cerebral cortex, thalamus and hypothalamus of the rat brain.     

Bifemelane enhances high K(+)-evoked release of glutamate from mossy fiber synaptosomes of guinea-pig hippocampus.

We have shown that bifemelane augments long-term potentiation in the mossy fiber-CA3 system, but not in the Schaffer collateral-CA1 system. To elucidate the mechanism of action of bifemelane in relation to pathway-specific augmentation of long-term potentiation, we prepared a mossy fiber terminal-rich synaptosomal fraction (P3) from guinea-pig hippocampus and investigated the effect of bifemelane on the release of glutamate from these synaptosomes, using an in vitro superfusion technique. Bifemelane (0.01-1 microM) dose dependently increased the 30 mM K(+)-evoked release of glutamate from the P3 fraction, without affecting glutamate release from a conventional synaptosomal P2 fraction. This stimulatory effect of 1 microM bifemelane was abolished by 100 microM H-7, which also suppressed the increase in K(+)-evoked glutamate release by phorbol 12,13-dibutyrate (1 microM). Bifemelane (1 microM) induced the translocation of protein kinase C activity from cytosol to membrane in the P3 fraction (which contains large and irregular-shaped synaptosomes probably derived from mossy fiber terminals), but not in the P2 fraction. These findings suggest that bifemelane directly acts on mossy fiber terminals to potentiate depolarization-induced glutamate release, which may be at least partly mediated by the translocation (activation) of protein kinase C.     

Pharmacology of amino acid receptors on vertebrate primary afferent nerve fibres

Structure-activity of primary afferent depolarising action (PAD) mediated by gamma-aminobutyrate (GABA) analogues suggests a difference between subsynaptic receptors located at fibre terminations within the dorsal horn and axonal receptors which are distributed throughout non-synaptic regions. The interaction of the bicuculline-sensitive GABA receptor (GABA A) ionophore complex with barbiturates and benzodiazepines suggests that at least three binding sites are required to explain the independent GABA-mimetic, GABA-potentiating and picrotoxin-reversing effects of such agents. Difficulties with explanation of the depressant effects of baclofen on spinal transmission, in terms of the bicuculline-resistant GABA (GABA B) receptor hypothesis, are mentioned. Glutamate-induced PAD of low threshold afferents is mediated indirectly through release of potassium. However, such terminals possess receptors (possibly autoreceptors for L-glutamate), activated by (+)2-amino-4-phosphonobutyrate, which cause depression of transmitter release. Primary afferent C-fibres possess receptors which are selectively activated by kainate and which mediate picrotoxin-resistant PAD. Such receptors may be involved in the presynaptic conditioning of C-fibre transmitter release. The peripheral terminals of vestibular primary afferents, in amphibia, possess excitatory amino acid receptors which are probably activated by the transmitter released from hair cells.     

Presynaptic inhibition by kainate receptors converges mechanistically with presynaptic inhibition by adenosine and GABAB receptors

Kainate receptors are widely reported to regulate the release of neurotransmitter in the CNS, but the mechanisms involved remain controversial. Previous studies have found that the kainate receptor agonist ATPA, which selectively activates Glu(K5)-containing kainate receptors, depresses glutamate release at Schaffer-collateral synapses in the hippocampus. In the present study, we provide pharmacological evidence that this depressant effect is mediated by Glu(K5)-containing heteromers, but is distinct from a similar depressant effect engaged by the kainate receptor agonist domoate. The depressant effect of ATPA is insensitive to antagonists for GABA(A), GABA(B), and adenosine receptors, and is also unaffected by lowering the release probability by reducing extracellular calcium. However, the effect of ATPA is partly occluded by prior activation of GABA(B) receptors and completely occluded by prior activation of adenosine receptors, suggesting a mechanistic convergence of heteromeric Glu(K5) kainate receptor signaling with GABA(B) receptors and adenosine receptors. The effects of domoate are partially occluded by both adenosine and GABA(B) receptor agonists, indicating at least a partial convergence of Glu(K5)-lacking kainate receptor signaling with these other pathways. The depressant effect of ATPA is not blocked by inhibition of serine/threonine protein kinases. These results suggest that ATPA and domoate inhibit glutamate release through mechanisms that converge with those of classical metabotropic receptor agonists, although they do so through different receptors.     

Complex interaction between quisqualate and kainate receptors as revealed by measurement of GABA release from striatal neurons in primary culture

The effects of non-NMDA receptor agonists were tested on endogenous GABA and [3H]GABA release from highly purified striatal neurons differentiated in primary culture. Kainate (KA), glutamate (Glu) and quisqualate (QA) stimulated [3H]GABA release with EC50S = 85 +/- 20 (n = 6), 6.21 +/- 1.42 (n = 3) and 0.135 +/- 0.035 (n = 3) microM, respectively. KA was the most potent (in term of efficacy) agonist (maximal response at 10 mM: 935 +/- 51% (n = 6) increase over basal release) followed by Glu (at 100 microM: 404 +/- 34% (n = 5) increase) and QA (at 10 microM: 91 +/- 6% (n = 6) increase). Phencyclidine (PCP), which was without effect on QA- and KA-evoked GABA release, inhibited the Glu response by about 50%. QA totally inhibited KA (50 microM)-evoked GABA release with an IC50 = 0.39 +/- 0.11 (n = 4) in a competitive manner (Ki = 0.39 +/- 0.07 microM (n = 3]. Competitive inhibition of the KA response was also observed with the other agonists of the quisqualate receptor, Glu and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), suggesting that Glu, QA and AMPA act as partial agonists at the KA receptor. gamma-D-Glutamylaminomethylsulfonic acid (GAMS) also inhibited (IC50 = 2.1 mM) the KA response competitively. However the inhibition by GAMS and QA was not additive. The response to QA was rapidly inactivated (no response after 3 min stimulation) in contrast to the KA-evoked GABA release which remained maximal for at least 3 min. When neurons were first exposed to concanavalin A (con A), a lectin known to inhibit Glu receptor desensitisation on insect muscles, the QA response remained maximal for at least 6 min. Con A greatly enhanced the maximal responses to QA and AMPA and decreased their apparent affinities. The KA-evoked GABA release (but not the veratridine and NMDA effects) was also augmented (no change in the EC50 value) by con A. It is proposed that QA, AMPA and KA act at the same receptor-channel complex (termed G2 receptor) which is desensitised more rapidly when stimulated by QA or AMPA than when stimulated by KA.     

Characterization of pre- and postsynaptic actions of (-)-baclofen in the guinea-pig hippocampus in vitro.

The effects of (-)-baclofen on evoked potentials in the hippocampus were examined through intracellular recordings from guinea-pig brain slices. The evoked responses were recorded in two fibre connections within the hippocampus: the Schaffer collateral/commissural-CA1 pyramidal cell, and the mossy fibre-CA3 pyramidal cell. The Schaffer collateral/commissural-CA1 response was suppressed by (-)-baclofen in concentrations over 2 X 10(-5)M, whereas (+)-baclofen, an inactive isomer, in a concentration of 10(-4)M had no effect on the response. A compound action potential of Schaffer collateral/commissural axons was unaffected by (-)-baclofen even at 10(-4)M, a concentration that almost completely depressed the evoked response in the CA1 pyramidal cell. The mossy fibre-CA3 response was not inhibited by (-)-baclofen (10(-4)M). The depressant action of (-)-baclofen on the Schaffer collateral/commissural-CA1 response was unaffected by bicuculline (10(-4)M), whereas the direct membrane effects of (-)-baclofen were antagonized by bicuculline (10(-5)M). It is suggested that (-)-baclofen may modulate neuronal transmission through presynaptic recognition sites possibly by decreasing transmitter release from nerve terminals and also may directly regulate the endogenous neuronal excitability through an activation of the postsynaptic recognition sites.     

Sensitivity of hippocampal neurones to kainic acid, and antagonism by kynurenate.

1. The sensitivity to kainic acid of neurones in the CA1 and CA3 regions of rat hippocampal slices has been examined by microiontophoresis and by superfusion methods. 2. When the iontophoretic currents needed to produce comparable plateaux of firing were compared, neurones in the pyramidal cell layer of the CA3 region were approximately 5 times more sensitive than cells in the CA1 region. No difference was noted in sensitivity to N-methyl-D-aspartate (NMDA) or quisqualate. 3. When kainate was superfused at known concentrations, the threshold for eliciting excitation in CA1 was 2.1 microM. The threshold concentration in CA3 was 0.24 microM. 4. Two weeks after the stereotaxic intrahippocampal injection of colchicine, the granule cells of the dentate gyrus and thus the mossy fibre projections to CA3 were destroyed. In slices prepared from animals thus treated the threshold concentration of kainate for eliciting excitation had risen to 1.64 microM. 5. Kainate was less effective in promoting the development of epileptiform bursts of neuronal firing in colchicine-treated slices than in controls. 6. Kynurenic acid antagonized the excitation of CA1 neurones elicited by kainate, NMDA or quisqualate. In the CA3 region kynurenate antagonized selectively responses to microiontophoretic NMDA, with little effect on responses to kainate or quisqualate. 7. In slices taken from colchicine-treated rats kynurenate was able to block responses to kainate in the CA3 area in parallel with responses to NMDA. 8. Taken together the results suggest that the excitatory responses to kainate in the CA3 region may be partly due to a presynaptic action on mossy fibre terminals to release endogenous amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)     

Novel secoergoline derivatives inhibit both GABA and glutamate uptake in rat brain homogenates: synthesis, in vitro pharmacology, and modeling

Three of twelve secoergoline derivatives (Z ethyl 4-[(ethoxycarbonylmethyl)methylamino]-2-methyl-3-phenylpent-2-enoate, 8; ethyl 1,6-dimethyl-3-oxo-5-phenyl-1,2,3,6-tetrahydropyridine-2-carboxylate, 9; Z methyl 4-[(methoxycarbonylmethyl)methylamino)-2-methyl-3-phenylpent-2-enoate, 11), containing bioisosteric sequences of GABA and Glu, inhibited both GABA and Glu transport through cerebrocortical membranes specifically. Compounds 8, 9, and 11 appeared to be equipotent inhibitors of GABA and Glu transport with IC50 values between 270 and 1100 microM, whereas derivatives 1-7, 10, and 12 were without effects. In the presence of GABA and Glu transport-specific nontransportable inhibitors, inhibition of GABA and Glu transport by 8, 9, and 11 proceeded in two phases. The two phases of inhibition were characterized by IC50 values between 4 and 180 nM and 360-1020 microM and different selectivity sequences. These findings may indicate the existence of some mechanism possibly mediated by a previously unrecognized GABA-Glu transporter. Derivatives with the cis, but not the trans configuration of bulky ester groups (8 vs 7 and 11 vs 12) showed significant inhibitory effect (IC50 values of 270 microM vs >1000 microM and 1100 microM vs >1000 microM on GABA transport, respectively). The cis-trans selectivity can be explained by docking these secoergolines in a three-dimensional model of the second and third transmembrane helices of GABA transporter type 1.     

Dissociation between drug-induced increases in nerve terminal and non-nerve terminal pools of GABA in vivo.

To examine whether increased GABA levels produced by n-dipropylacetate (DPA) and amino-oxyacetic acid (AOAA) are associated with nerve terminals, we compared the effect of these drugs on the GABA content of substantia nigra (SN) in rats in which the GABAergic afferent projections to SN had been unilaterally destroyed. In the SN largely devoid of GABAergic nerve terminals, AOAA (30 mg/kg) produced a 2-fold increase in GABA, whereas DPA (300 mg/kg) was without effect. Since DPA and AOAA both increased GABA to a similar extent in the intact SN, it appears that the GABA increase produced by DPA is associated with GABAergic nerve terminals, while AOAA primarily elevates GABA in non-nerve terminal components (neural perikarya and glial cells) which are not destroyed by our lesions.     

Coexistence of carriers for dopamine and GABA uptake on a same nerve terminal in the rat brain.

The ability of gamma-aminobutyric acid (GABA) to affect the release of [3H]-dopamine in rat brain synaptosomes prepared from corpus striatum, frontal cortex and hypothalamus and prelabelled with the radioactive catecholamine in the presence of desipramine was examined. GABA (10-300 microM) increased in a concentration-dependent way the basal release of [3H]-dopamine from striatum and cortical synaptosomes; however, its effect was much less pronounced in hypothalamic nerve terminals. 2,4-Diaminobutyric acid (DABA) mimicked GABA although less potently. Neutral amino acids such as leucine, valine or alpha-aminoisobutyric acid (100-300 microM) did not affect or increased minimally the release of [3H]-dopamine. The GABA-induced [3H]-dopamine release was not prevented by the GABAA-receptor antagonists, bicuculline or picrotoxin. The GABAA-receptor agonist, muscimol (10-300 microM), displayed only a very weak, not significant, enhancing effect on [3H]-dopamine release. The GABAB-receptor agonist (-)-baclofen (100 or 300 microM) had no effect. Three novel and selective inhibitors of GABA uptake, N-(4,4-diphenyl-3-butenyl)-nipecotic acid (SK&F 89976A), N-(4,4-diphenyl-3-butenyl)-guvacine (SK&F 100330A) and N-(4,4-diphenyl-3-butenyl)-homo-beta-proline (SK&F 100561) potently counteracted the enhancing effect of GABA on [3H]-dopamine release. Nipecotic acid also reduced the effect of GABA. It is concluded that carriers for the uptake of dopamine and GABA may coexist on the same nerve terminal in the rat brain.     

Carriers for GABA and noradrenaline uptake coexist on the same nerve terminal in rat hippocampus

gamma-Aminobutyric acid (GABA; 1-300 microM) increased the basal release of [3H]noradrenaline ([3H]NA) from rat hippocampal synaptosomes. The effect of GABA at low concentrations (below 10 microM) was largely bicuculline-sensitive while the sensitivity to bicuculline decreased at higher concentrations. Muscimol mimicked GABA but only below 10 microM; bicuculline antagonized the effect of muscimol. Up to 300 microM (-)baclofen did not modify [3H]NA release. The effect of GABA was potently counteracted by SK & F 89976A, SK & F 100330A and SK & F 100561, three novel inhibitors of neuronal GABA uptake. These compounds could not entirely prevent the effect of GABA, being least effective at the lowest GABA concentrations (below 10 microM) and becoming progressively more effective when the concentrations of GABA were increased. The effect of muscimol was insensitive to SK & F 89976A. The effect of 100 microM GABA was totally prevented when bicuculline and uptake inhibitor were added together to the superfusion medium. The results suggest that the basal release of [3H]NA can be enhanced by GABA through two mechanisms: GABAA receptor activation and penetration into NA terminals by a GABA uptake process. Thus a carrier for the uptake of NA and a carrier for the uptake of GABA appear to coexist on the same nerve terminal in rat hippocampus.     

Effects of baclofen on synaptically-induced cell firing in the rat hippocampal slice

The effects of baclofen on the synaptically-induced firing of pyramidal and granule cell populations were tested in the rat hippocampal slice. Population spikes were evoked by stimulating excitatory pathways in the presence and absence of bath-applied drug. (+/-)-Baclofen (20 microM) completely blocked the firing of CA1 or CA3 hippocampal pyramidal cells subsequent to stimulation of projections that originate in area CA3. In contrast, the firing of dentate granule cells evoked by stimulation of the perforant path fibres was depressed by only 46% and baclofen did not affect the monosynaptic firing of CA3 pyramidal cells evoked by mossy fibre stimulation. These results are consistent with the effects of baclofen on the corresponding extracellularly-recorded excitatory postsynaptic potentials (e.p.s.ps). The Schaffer collateral-commissural population spike in area CA1 was depressed by (-)-baclofen (EC50 = 2.8 microM), GABA (EC50 = 2.2 mM) and 3-aminopropanesulphonic acid (3-APS) (EC50 = 0.34 mM). (-)-Baclofen was 180 times as potent as (+)-baclofen. Bicuculline methiodide (100 microM) did not reverse the depressant action of (-)-baclofen. GABA-induced depressions were antagonized to only a small degree, whilst the effect of 3-APS was readily reversed. Raising the concentration of bicuculline from 100 microM to 500 microM did not further reverse the action of GABA. The effects of (-)-baclofen and 3-APS on the relationship between extracellular e.p.s.p. and population spike were tested by stimulation of the Schaffer collateral-commissural fibres in area CA1. (-)-Baclofen shifted the 'input/output' curve to the right at a concentration of 1 microM, but less or not at all at 3 microM. In contrast, increasing the concentration of 3-APS shifted this curve farther to the right.