GABA(B) receptors as potential targets for drugs able to prevent excessive excitatory amino acid transmission in the spinal cord

Pharmacological Profile of MP-101, a Novel Non-racemic Mixture of R- and S-dimiracetam with Increased Potency in Rat Models of Cognition, Depression and Neuropathic Pain

The racemic mixture dimiracetam negatively modulates NMDA-induced glutamate release in rat spinal cord synaptosomal preparations and is orally effective in models of neuropathic pain. In this study, we compared the effects of dimiracetam, its R- or S-enantiomers, and the R:S 3:1 non-racemic mixture (MP-101). In vitro, dimiracetam was more potent than its R- or S-enantiomers in reducing the NMDA-induced [³H]D-aspartate release in rat spinal cord synaptosomes. Similarly, acute oral administration of dimiracetam was more effective than a single enantiomer in the sodium monoiodoacetate (MIA) paradigm of painful osteoarthritis. Then, we compared the in vitro effects of a broad range of non-racemic enantiomeric mixtures on the NMDA-induced [³H]D-aspartate release. Dimiracetam was a more potent blocker than each isolated enantiomer but the R:S 3:1 non-racemic mixture (MP-101) was even more potent than dimiracetam, with an IC₅₀ in the picomolar range. In the chronic oxaliplatin-induced neuropathic pain model, MP-101 showed a significantly improved anti-neuropathic profile, and its effect continued one week after treatment suspension. MP-101 also performed better than dimiracetam in animal models of cognition and depression. Based on the benign safety and tolerability profile previously observed with racemic dimiracetam, MP-101 appears to be a novel, promising clinical candidate for the prevention and treatment of several neuropathic and neurological disorders.

Blockade of GABA(B) receptors facilitates evoked neurotransmitter release at spinal dorsal horn synapse

Metabotropic GABA type B (GABA(B)) receptors are abundantly expressed in the rat spinal dorsal horn. Activation of GABA(B) receptors by exogenous agonists inhibits synaptic transmission, which is believed to underlie the GABA(B) receptor-mediated analgesia. However, little effort has been made to test whether endogenous GABA might also mediate inhibition by acting on GABA(B) receptors. In this study, whole-cell recording techniques were employed to study the effect of endogenous GABA on GABA(B) receptors in substantia gelatinosa (SG) neurons in adult rat spinal cord slices. In current-clamp mode, blockade of GABA(B) receptors by their selective antagonist 3-[[[(3,4-dichlorophenyl)methyl]amino]propyl] (diethoxy-methyl) phosphinic acid (CGP 52432) facilitated presynaptic stimulation-induced action potential discharge and increased amplitude of postsynaptic potentials (PSPs), meaning a GABA(B) receptor-mediated inhibition of SG neuron excitability. In voltage-clamp mode, blockade of GABA(B) receptors increased the amplitude of evoked excitatory postsynaptic currents (eEPSCs) and decreased paired-pulse ratio, indicating a presynaptic CGP 52432 action. Primary afferent Aδ or C fiber-evoked EPSCs were also facilitated by CGP 52432 application. Amplitudes of evoked GABAergic and glycinergic inhibitory postsynaptic currents (eIPSCs) were enhanced by GABA(B) receptor blockade. The facilitation of amplitude persisted in the presence of a specific GABA transporter 1 (GAT-1) blocker, tiagabine, or GAT-2/3 blocker SNAP5114. However, blockade of GABA(B) receptors had no effect on action potential-independent miniature EPSCs (mEPSCs), miniature IPSCs (mIPSCs), or membrane conductance. Taken together, these results suggest that endogenous GABA modulates evoked synaptic transmission in SG neurons by acting on GABA(B) receptors. This GABA(B) receptor-mediated homeostatic regulation of neuronal excitability and neurotransmitter release might contribute to modulation of nociception in spinal dorsal horn.

Comparison of GAD65 and 67 immunoreactivity in the lumbar spinal cord between young adult and aged dogs

We investigated distribution and age-related changes in two isoforms of GABA synthesizing enzymes, glutamic acid decarboxylase (GAD) 65 and 67, in the lumbar levels (L(5)-L(6)) of the dog spinal cord. Male German shepherds were used at 1-2 years (young adult dogs) and 10-12 years (aged dogs) of age. GAD65 immunoreaction was observed in neuropil, not in cell bodies, in all laminae of the adult lumbar spinal cord: Many punctate GAD65-immunoreactive structures were shown in all laminae. The density of GAD65 immunoreactive structures was highest in laminae I-III, and lowest in lamina VII. In the aged dog, the distribution pattern of GAD65 immunoreactivity was similar to that in the adult dog; however the density of GAD65-immunoreactive structures and its protein levels were significantly increased in the aged lumbar spinal cord. GAD67 immunoreaction in the adult dog was also distributed in all laminae of the lumbar spinal cord like GAD65; however, we found that small GAD67-immunoreactive cell bodies were observed in laminae II, III and VIII. In the aged dogs, GAD67 immunoreactivity and its protein levels were also increased compared to those in the adult group. In conclusion, our results indicate that the distribution of GAD65-immunoreactive structures is different from GAD67-immunoreactive structures and that their immunoreactivity in the aged dogs is much higher than the adult dogs.

GABAB receptors: physiological functions and mechanisms of diversity

GABA(B) receptors are the G-protein-coupled receptors (GPCRs) for gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the central nervous system. GABA(B) receptors are implicated in the etiology of a variety of psychiatric disorders and are considered attractive drug targets. With the cloning of GABA(B) receptor subunits 13 years ago, substantial progress was made in the understanding of the molecular structure, physiology, and pharmacology of these receptors. However, it remained puzzling that native studies demonstrated a heterogeneity of GABA(B) responses that contrasted with a very limited diversity of cloned GABA(B) receptor subunits. Until recently, the only firmly established molecular diversity consisted of two GABA(B1) subunit isoforms, GABA(B1a) and GABA(B1b), which assemble with GABA(B2) subunits to generate heterodimeric GABA(B(1a,2)) and GABA(B(1b,2)) receptors. Using genetic, ultrastructural, biochemical, and electrophysiological approaches, it has been possible to identify functional properties that segregate with these two receptors. Moreover, receptor modifications and factors that can alter the receptor response have been identified. Most importantly, recent data reveal the existence of a family of auxiliary GABA(B) receptor subunits that assemble as tetramers with the C-terminal domain of GABA(B2) subunits and drastically alter pharmacology and kinetics of the receptor response. The data are most consistent with native GABA(B) receptors minimally forming dimeric assemblies of units composed of GABA(B1), GABA(B2), and a tetramer of auxiliary subunits. This represents a substantial departure from current structural concepts for GPCRs.

Presynaptic metabotropic glutamate and GABAB receptors

Glutamate and GABA, the two most abundant neurotransmitters in the mammalian central nervous system, can act on metabotropic receptors that are structurally quite dissimilar from those targeted by most other neurotransmitters/modulators. Accordingly, metabotropic glutamate receptors (mGluRs) and GABA(B) receptors (GABA(B)Rs) are classified as members of family 3 (or family C) of G protein-coupled receptors. On the other hand, mGluRs and GABA(B)Rs exhibit pronounced and partly unresolved differences between each other. The most intriguing difference is that mGluRs exist as multiple pharmacologically as well as structurally distinct subtypes, whereas, in the case of GABA(B)Rs, molecular biologists have so far identified only one structurally distinct heterodimeric complex whose few variants seem unable to explain the pharmacological heterogeneity of GABA(B)Rs observed in many functional studies. Both mGluRs and GABA(B)Rs can be localized on axon terminals of different neuronal systems as presynaptic autoreceptors and heteroreceptors modulating the exocytosis of various transmitters.

Depression of glutamate and GABA release by presynaptic GABAB receptors in the entorhinal cortex in normal and chronically epileptic rats

Presynaptic GABA(B) receptors (GABA(B)R) control glutamate and GABA release at many synapses in the nervous system. In the present study we used whole-cell patch-clamp recordings of spontaneous excitatory and inhibitory synaptic currents in the presence of TTX to monitor glutamate and GABA release from synapses in layer II and V of the rat entorhinal cortex (EC)in vitro. In both layers the release of both transmitters was reduced by application of GABA(B)R agonists. Quantitatively, the depression of GABA release in layer II and layer V, and of glutamate release in layer V was similar, but glutamate release in layer II was depressed to a greater extent. The data suggest that the same GABA(B)R may be present on both GABA and glutamate terminals in the EC, but that the heteroreceptor may show a greater level of expression in layer II. Studies with GABA(B)R antagonists suggested that neither the auto- nor the heteroreceptor was consistently tonically activated by ambient GABA in the presence of TTX. Studies in EC slices from rats made chronically epileptic using a pilocarpine model of temporal lobe epilepsy revealed a reduced effectiveness of both auto- and heteroreceptor function in both layers. This could suggest that enhanced glutamate and GABA release in the EC may be associated with the development of the epileptic condition.

GABA(B) receptors: structure and function

In the basal ganglia the effects of gamma-aminobutyrate (GABA) are mediated by both ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors. Although the existence and widespread distribution in the CNS of the GABA(B) receptor had been established in the 1980s the field of GABA(B) research was revolutionized with the discovery that two related G-protein-coupled receptors (GPCRs) needed to dimerize to form the functional GABA(B) receptor at the cell surface. This finding lead to a number of studies of oligomerization in GPCRs and detailed pharmacological studies of the cloned receptors and their splice variants. Particular interest has focused on the proteins interacting with the receptor which may be important in mediating the longer term signalling effects of the receptor and modifying its cellular localization or physiology. The cloning of the GABA(B) receptors also lead to the identification of the first compounds interacting in an allosteric fashion with the receptor some of which may have therapeutic value. Most recently "knockouts" of both the GABA(B) subunits have been produced where in general as expected there is a loss of the majority of the inhibitory effects of the GABA(B) receptor.

Inhibition of mechanosensitivity in visceral primary afferents by GABAB receptors involves calcium and potassium channels

GABA(B) receptors inhibit mechanosensitivity of visceral afferents. This is associated with reduced triggering of events that lead to gastro-esophageal reflux, with important therapeutic consequences. In other neuronal systems, GABA(B) receptor activation may be linked via G-proteins to reduced N-type Ca(2+) channel opening, increased inward rectifier K(+) channel opening, plus effects on a number of intracellular messengers. Here we aimed to determine the role of Ca(2+) and K(+) channels in the inhibition of vagal afferent mechanoreceptor function by the GABA(B) receptor agonist baclofen. The responses of three types of ferret gastro-esophageal vagal afferents (mucosal, tension and tension mucosal receptors) to graded mechanical stimuli were investigated in vitro. The effects of baclofen (200 microM) alone on these responses were quantified, and the effects of baclofen in the presence of the G-protein-coupled inward rectifier potassium channel blocker Rb(+) (4.7 mM) and/or the N-type calcium channel blocker omega-conotoxin GVIA (0.1 microM). Baclofen inhibition of mucosal receptor mechanosensitivity was abolished by both blockers. Its inhibitory effect on tension mucosal receptors was partly reduced by both. The inhibitory effect of baclofen on tension receptors was unaffected. The data indicate that the inhibitory action of GABA(B) receptors is mediated via different pathways in mucosal, tension and tension mucosal receptors via mechanisms involving both N-type Ca(2+) channels and inwardly rectifying K(+) channels and others.

Molecular structure and physiological functions of GABA(B) receptors

GABA(B) receptors are broadly expressed in the nervous system and have been implicated in a wide variety of neurological and psychiatric disorders. The cloning of the first GABA(B) receptor cDNAs in 1997 revived interest in these receptors and their potential as therapeutic targets. With the availability of molecular tools, rapid progress was made in our understanding of the GABA(B) system. This led to the surprising discovery that GABA(B) receptors need to assemble from distinct subunits to function and provided exciting new insights into the structure of G protein-coupled receptors (GPCRs) in general. As a consequence of this discovery, it is now widely accepted that GPCRs can exist as heterodimers. The cloning of GABA(B) receptors allowed some important questions in the field to be answered. It is now clear that molecular studies do not support the existence of pharmacologically distinct GABA(B) receptors, as predicted by work on native receptors. Advances were also made in clarifying the relationship between GABA(B) receptors and the receptors for gamma-hydroxybutyrate, an emerging drug of abuse. There are now the first indications linking GABA(B) receptor polymorphisms to epilepsy. Significantly, the cloning of GABA(B) receptors enabled identification of the first allosteric GABA(B) receptor compounds, which is expected to broaden the spectrum of therapeutic applications. Here we review current concepts on the molecular composition and function of GABA(B) receptors and discuss ongoing drug-discovery efforts.

Activaton of GABAB receptor inhibits the excitability of rat small diameter trigeminal root ganglion neurons

A selective GABA(B) receptor agonist, baclofen, is known to suppress neuropathic pain. In the present study, we investigated the effect of baclofen on the excitability of trigeminal root ganglion (TRG) neurons by using the whole cell and perforated patch-clamp recording techniques. Under voltage-clamp (V(h)=-60 mV), voltage-dependent K(+) currents were recorded in the small diameter TRG neurons (<30 microm) and isolated by blocking Na(+) and Ca(2+) currents with appropriate ion replacement. Separation of the K(+) current components was achieved by the response to variation in the conditioning voltage. Two distinct K(+) current components, a transient (I(A)) and a sustained (I(k)), were identified. Baclofen significantly increased I(A) by 74.8% (50 microM) and in a dose-dependent manner (1-50 microM). Similarly, I(K) was also enhanced by baclofen administration (41.8%: 50 microM). The relative amplitude of potentiation of I(A) was significantly higher than that of I(K) (P<0.05). Baclofen-sensitive I(A) and I(K) currents were antagonized by K(+) channel blockers, 4-aminopyridine (4-AP) and tetraethylammonium (TEA). The augmentation of K(+) currents was antagonized by 3-amino-2-(4-chlorophenyl)-2-hydroxypropylsulfonic acid (saclofen; GABA(B) antagonist). In the current clamp mode, the resting membrane potential was -62+/-1.6 mV (n=24). Hyperpolarization of the membrane potential was elicited by baclofen (10-50 microM), and the response was associated with a decrease in the input resistance. Baclofen induced-hyperpolarization was blocked by saclofen (100 microM). In the presence of both 4-AP and TEA, no significant changes in membrane potential induced by baclofen application were observed. In the presence of BaCl(2), baclofen-evoked hyperpolarization with decreased resistance was observed. During application of baclofen, the firing rate of the action potentials by depolarizing step pulses was decreased. Application of baclofen reduced action potential duration evoked by a depolarization current pulse.These results indicated that activation of GABA(B) receptors inhibits the excitability of rat small diameter TRG neurons and this inhibitory action is mediated by potentiation of voltage-dependent K(+) currents. We therefore concluded that modification of nociceptive transmission in the trigeminal system by activation of GABA(B) receptors occurs at the level of small TRG neuron cell bodies and/or their primary afferent terminals, which are potential targets of analgesia by baclofen.

Attenuation of cerebellar tremor with implantation of an intrathecal baclofen pump: the role of gamma-aminobutyric acidergic pathways. Case report

The authors present the case of a 49-year-old woman with disabling bilateral upper-extremity cerebellar tremor that resolved unexpectedly after placement of an intrathecal baclofen pump for lower-extremity spasticity. The tremor amplitude decreased nearly linearly with increasing intrathecal baclofen dosage, and disappeared completely at a dose of 250 microg/day. In this report the authors demonstrate the role of the gamma-aminobutyric acidergic system in the pathogenesis of cerebellar tremor, and these findings may lead to a new treatment modality for patients disabled by this manifestation of their disease.

Inhibitory effects of spinal baclofen on spinal dorsal horn neurones in inflamed and neuropathic rats in vivo

gamma-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter, which modulates afferent transmission of nociceptive information at different levels of the central nervous system. Plasticity of spinal GABAergic systems may contribute to aberrant nociceptive responses associated with inflammatory and neuropathic pain states. Here potential changes in spinal GABA(B) receptor function in rats with peripheral inflammation and nerve injury, compared to control were investigated. Extracellular recordings of electrically evoked responses of spinal dorsal horn neurones were made in halothane anaesthetised rats. Effects of spinal administration of the GABA(B) receptor agonist baclofen (0.1-10 microg/50 microL) on evoked responses of spinal neurones in control, hindpaw carrageenan inflamed, spinal nerve ligated and sham-operated rats were studied. In all groups of rats, spinal baclofen significantly reduced Abeta-, Adelta- and C-fibre evoked responses of spinal dorsal horn neurones in a dose related manner. Spinal pre-administration of the GABA(B) receptor antagonist, CGP-35348 (30 microg/50 microL) significantly blocked the inhibitory effects of baclofen on evoked neuronal responses in control rats. Estimated ED(50) values for each fibre type within experimental groups were calculated, a significant (P<0.05) difference between the values for Abeta-fibre-evoked and C-fibre mediated post-discharge responses of spinal dorsal horn neurones in spinal nerve ligated rats is reported. This finding may reflect decreased sensitivity of Abeta-fibre-evoked responses to baclofen, as well as an increased sensitivity of post-discharge responses to baclofen in spinal nerve ligated rats. Overall, we report that GABA(B)-receptor control of A- and C-fibre evoked responses of spinal neurones is not profoundly altered in models of inflammatory and neuropathic pain.

The effects of sympathectomy on capsaicin-evoked fos expression of spinal dorsal horn GABAergic neurons

Electrophysiological studies have suggested that activity of spinal GABAergic interneurons can be enhanced following intradermal injection of capsaicin (CAP). This activity is proposed to be involved in the generation of dorsal root reflexes (DRRs) that contribute to neurogenic inflammation. We have recently reported that NMDA or non-NMDA antagonists by intrathecal pretreatment attenuate the increased Fos expression in spinal dorsal horn GABAergic neurons after intradermal injection of CAP in rats. Sympathetic efferents have been suggested to modulate inflammatory pain possibly by interactions with primary afferent terminals. In electrophysiological studies by our group, enhancement of the CAP-induced DRRs could be prevented by surgical sympathectomy and blocked by intraarterial pretreatment of the foot with alpha(1)- but not by alpha(2)-adrenoceptor antagonists. In order to determine morphologically if surgical sympathectomy changes the expression of Fos in GABAergic neurons in the lumbosacral spinal cord induced by CAP injection, further experiments were performed using immunofluorescence double-labeling staining at 30 min following CAP or vehicle injection into the glabrous skin of one hind paw of anesthetized rats both in sham-operated and sympathectomized animals. Our results showed that the proportion of Fos-positive GABAergic neuronal profiles was significantly increased following CAP injection (48.8+/-4.76%) compared to vehicle injection (23.8+/-5.1%) in laminae I-V on the ipsilateral side (P<0.05). However, when sympathetic efferents were removed surgically 7-10 days prior to the experiment (n=6), only 32.07+/-9.03% of GABA-immunoreactive neuronal profiles were stained for Fos following CAP injection, a significant reduction in the CAP-evoked Fos-staining of GABAergic neurons after surgical sympathectomy. These findings support our previous electrophysiological studies that GABAergic neurons take part in nociceptive processing within the spinal dorsal horn and suggest that sympathetic efferents may affect nociceptive transduction in the periphery.

NMDA or non-NMDA receptor antagonists attenuate increased Fos expression in spinal dorsal horn GABAergic neurons after intradermal injection of capsaicin in rats

GABAergic neurons play an important role in the generation of primary afferent depolarization, which results in presynaptic inhibition and, if large enough, triggers dorsal root reflexes. Recent electrophysiological studies by our group have suggested that increased excitation of spinal GABAergic neurons by activation of N-methyl-D-aspartate (NMDA) and non-NMDA receptors following intradermal injection of capsaicin results in the generation of DRRs that contribute to neurogenic inflammation. The present study was to determine if changes in the expression of Fos protein occur in GABAergic neurons in the lumbosacral spinal cord following injection of capsaicin into the glabrous skin of one hind paw of anesthetized rats and if pretreatment with an NMDA receptor antagonist, D-(-)-2-amino-7-phosphonoheptanoic acid (AP7) or a non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocks Fos expression in these neurons. The experiments used western blots and immunofluorescence double labeling staining following capsaicin or vehicle injection. Western blots showed that Fos protein was increased on the ipsilateral side in spinal cord tissue 0.5 h after capsaicin injection. Pretreatment with AP7 or CNQX caused a decrease in capsaicin-induced Fos expression. Immunofluorescence double labeling showed that the proportion of Fos-positive GABAergic neuronal profiles was significantly increased following capsaicin injection (48.8+/-4.8%) compared to the vehicle injection (23.8+/-5.1%) in superficial laminae on the ipsilateral side in lumbosacral spinal cord (P<0.05). However, when the spinal cord was pretreated with AP7 (5 microg) or CNQX (0.2 microg), only 9.1+/-0.6% or 7.1+/-0.8% of GABA-immunoreactive neuronal profiles were stained for Fos following capsaicin injection. The blockade of the capsaicin-evoked Fos staining was dose-dependent. These findings suggest that GABAergic neurons take part in dorsal horn circuits that modulate nociceptive information and that the function of GABAergic neurons following capsaicin injection is partially mediated by NMDA and non-NMDA receptors.

Baclofen attenuates harmaline induced tremors in rats

Recent experimental and clinical studies clearly suggest the role of gamma-aminobutyric acid (GABA) in the pathogenesis of tremors. The present study was undertaken to investigate the effect of baclofen, a GABA B receptor agonist on harmaline induced tremors. Four groups of female Wistar rats weighing 100+/-15 g were injected with harmaline (10 mg/kg, intraperitoneally) for inducing experimental tremors. The animals in groups 2, 3 and 4 were given baclofen by gavage at doses of 2.5, 5 and 10 mg/kg, respectively, half an hour before harmaline administration, whereas, the rats in group 1 served as control and received water. The latency of onset, intensity and duration of tremor and electromyographic (EMG) responses were recorded. Treatment with baclofen resulted in a dose dependent decrease in the intensity of tremor. Our EMG study also revealed a significant decrease in the amplitude of tremors in baclofen treated rats. A highly significant increase in latency of onset of tremor was observed in the rats treated with high dose (10 mg/kg) of baclofen only. This study clearly suggests beneficial effects of baclofen in harmaline induced tremors.

Ionotropic and metabotropic GABA and glutamate receptors in primate basal ganglia

The functions of glutamate and GABA in the CNS are mediated by ionotropic and metabotropic, G protein-coupled, receptors. Both receptor families are widely expressed in basal ganglia structures in primates and nonprimates. The recent development of highly specific antibodies and/or cDNA probes allowed the better characterization of the cellular localization of various GABA and glutamate receptor subtypes in the primate basal ganglia. Furthermore, the use of high resolution immunogold techniques at the electron microscopic level led to major breakthroughs in our understanding of the subsynaptic and subcellular localization of these receptors in primates. In this review, we will provide a detailed account of the current knowledge of the localization of these receptors in the basal ganglia of humans and monkeys.

Comparative activities of the enantiomeric GABA(B) receptor agonists CGP 44532 and 44533 in central and peripheral tissues

In neocortical slices maintained in Mg(2+)-free Krebs medium, the gamma-aminobutyric acid (GABA(B)) receptor agonists baclofen, (3-amino-2(S)-hydroxypropyl)methylphosphinic acid (CGP 44532), and its (R)-enantiomer CGP 44533 depressed the frequency of spontaneous discharges in a concentration-dependent manner (EC(50)=10, 6.5, and 50 microM, respectively). These effects were reversibly antagonised by the GABA(B) receptor antagonist (+)-(S)-5,5 dimethylmorpholinyl-2-acetic acid (Sch 50911) (3, 10, and 30 microM) (average pA(2) value=6.0+/-0.2). In neocortical wedges, baclofen, CGP 44532 and CGP 44533 elicited concentration-dependent hyperpolarisations (the EC(50)s were 14, 7.5 and 16 microM, respectively) sensitive to Sch 50911 (1, 5, 10 microM) (average pA(2) value=6.0+/-0.1), whilst they also depressed ileal electrically elicited cholinergic twitch contractions (EC(50)=11, 7, and 50 microM) that were antagonised by Sch 50911 (average pA(2) value=6.0+/-0.1). In electrically stimulated brain slices preloaded with [3H]GABA, baclofen, CGP 44532 and CGP 44533 decreased [3H]GABA release (IC(50)=5, 0.45, and 10 microM); this effect was reversed by Sch 50911 (50 microM). It is concluded that CGP 44532 is a far more potent agonist at GABA(B) autoreceptors than at central or peripheral heteroreceptors.

K(+)-Evoked [(3)H]D-aspartate release in rat spinal cord synaptosomes: modulation by neuropeptide Y and calcium channel antagonists

This study was conducted to investigate mechanisms regulating the release of [(3)H]D-aspartate (or endogenous glutamate) in the rat spinal cord. Presynaptic modulation of glutamate release was studied in superfused synaptosomes depolarized with 20 mM KCl. Calcium-channel antagonists, omega-conotoxin GVIA (omega-CgTx GVIA; N-type), nifedipine (L-type), and omega-conotoxin MVIIC (omega-CmTx MVIIC; P/Q type), were used to characterize the voltage-operated Ca(2+) channels (VOCCs) involved in this release. Nifedipine had no significant effect on the K(+)-evoked release of [(3)H]D-aspartate, but the omega-conotoxins GVIA and MVIIC produced dose-dependent inhibitory effects that were additive. The most substantial reduction (54.30% +/- 4.40%) was seen with omega-CgTx GVIA, indicating that N-type channels play a major role in the release of glutamate in this tissue. We investigated the effects of neuropeptide Y (NPY), NPY(13-36), and [Leu(31)][Pro(34)]NPY on Ca(2+)-dependent, K(+)-evoked [(3)H]D-aspartate release. NPY and NPY(13-36) equipotently inhibited the release of glutamate in a concentration-dependent manner. The half-maximal response was observed at about 12 nM; maximal inhibition of 44.22% +/- 4.60% was achieved with 0.3 microM. The selective GABA(B) agonist (-)baclofen inhibited K(+)-evoked [(3)H]D-aspartate release from superfused spinal cord synaptosomes by 50.00% +/- 4.80% at 10 microM. When NPY(13-36) and (-)baclofen were used together at maximal doses, their release-inhibiting effects were not additive. In addition, neither of the agonists was able to enhance the inhibition produced by pretreating the synaptosomes with the selective inhibitor of N-type VOCCs omega-CgTx GVIA. These results are consistent with the hypothesis that presynaptic Y(2)-like and GABA(B) receptors regulate glutamate release by blocking Ca(2+) currents through N-type VOCCs. Characterization of the receptors that can inhibit the release of glutamate may provide useful information for treatment of conditions characterized by excessive glutamatergic transmission in the spinal cord.

GABA(B) receptors are the first target of released GABA at lamina I inhibitory synapses in the adult rat spinal cord

We have previously provided functional evidence that glycine and GABA are contained in the same synaptic vesicles and coreleased at the same synapses in lamina I of the rat spinal dorsal horn. However, whereas both glycine receptors (GlyRs) and GABA(A) receptors (GABA(A)Rs) are expressed on the postsynaptic target, under certain conditions inhibitory events appeared to be mediated by GlyRs only. We therefore wanted to test whether GABA(B) receptors could be activated in conditions where GABA released was insufficient to activate GABA(A)Rs. Focal stimulation in the vicinity of visually identified lamina I neurons elicited monosynaptic IPSCs in the presence of ionotropic glutamate receptor antagonists. Pairs of stimuli were given at different interstimulus intervals (ISI), ranging from 25 ms to 1 s to study the depression of the second of evoked IPSCs (paired pulse depression; PPD). Maximal PPD of IPSCs was 60 +/- 14% (SE) (of the conditioning pulse amplitude), at ISI between 150 and 200 ms. PPD was observed with IPSCs evoked at stimulus intensities where they had no GABA(A)R component. PPD of small evoked IPSCs was not affected by the GABA(A)R antagonist bicuculline but significantly attenuated by 10-30 microM CGP52432, a specific GABA(B) receptor antagonist. These data indicate that, under conditions where GABA released is insufficient to affect postsynaptic GABA(A)Rs at lamina I inhibitory synapses, significant activation of presynaptic GABA(B) receptors can occur.

Baclofen inhibits more effectively C-afferent than Adelta-afferent glutamatergic transmission in substantia gelatinosa neurons of adult rat spinal cord slices

Although intrathecal administration of baclofen, a selective GABA(B)-receptor agonist, is known to have an antinociceptive effect on various pain models, the role of presynaptic GABA(B) receptors in antinociception is not well characterized. In the present study, the action of baclofen on primary afferent-evoked glutamatergic excitatory transmission was examined in substantia gelatinosa (SG) neurons of an adult rat spinal cord slice with an attached dorsal root, prepared from the lumbar segment, by use of the blind whole-cell patch-clamp technique. Under the condition where a postsynaptic action of baclofen was inhibited, baclofen (1 microM) reduced the amplitudes of excitatory postsynaptic currents (EPSCs; V(H)=-70 mV) which were monosynaptically evoked by stimulating primary-afferent C- and/or Adelta-fibers and which were remarkably depressed by CNQX (10 microM). The identification of the C-fiber or Adelta-fiber EPSC was based on antidromic action potentials recorded from neurons of isolated dorsal root ganglia. The C-fiber EPSC was depressed in peak amplitude by baclofen (1 microM) to a larger extent than the Adelta-fiber EPSC (20 and 45% of control, respectively). Each of the baclofen actions was suppressed by a selective GABA(B)-receptor antagonist, CGP 35348 (50 microM). Baclofen (1 microM) did not affect a response of SG neurons to bath-applied AMPA (10 microM). These results indicate that baclofen inhibits the release of L-glutamate from Adelta and C primary-afferent terminals in the SG through the activation of GABA(B) receptor; this action is more effective to C-fiber than Adelta-fiber transmission. Considering that the SG is the main part of termination of Adelta- and C-fibers transmitting nociceptive information, the present finding would account for at least a part of the inhibitory action of baclofen on pain transmission.

GABA(B) and group I metabotropic glutamate receptors in the striatopallidal complex in primates

Glutamate and GABA neurotransmission is mediated through various types of ionotropic and metabotropic receptors. In this review, we summarise some of our recent findings on the subcellular and subsynaptic localisation of GABA(B) and group I metabotropic glutamate receptors in the striatopallidal complex of monkeys. Polyclonal antibodies that specifically recognise GABA(B)R1, mGluR1a and mGluR5 receptor subtypes were used for immunoperoxidase and pre-embedding immunogold techniques at the light and electron microscope levels. Both subtypes of group I mGluRs were expressed postsynaptically in striatal projection neurons and interneurons where they aggregate perisynaptically at asymmetric glutamatergic synapses and symmetric dopaminergic synaptic junctions. Moreover, they are also strongly expressed in the main body of symmetric synapses established by putative intrastriatal GABAergic terminals. In the globus pallidus, both receptor subtypes are found postsynaptically in the core of striatopallidal GABAergic synapses and perisynaptically at subthalamopallidal glutamatergic synapses. Finally, extrasynaptic labelling was commonly seen in the globus pallidus and the striatum. Moderate to intense GABA(B)R1 immunoreactivity was observed in the striatopallidal complex. At the electron microscope level, GABA(B)R1 immunostaining was commonly found in neuronal cell bodies and dendrites. Many striatal dendritic spines also displayed GABA(B)R1 immunoreactivity. Moreover, GABA(B)R1-immunoreactive axons and axon terminals were frequently encountered. In the striatum, GABA(B)R1-immunoreactive boutons resembled terminals of cortical origin, while in the globus pallidus, subthalamic-like terminals were labelled. Pre-embedding immunogold data showed that postsynaptic GABA(B)R1 receptors are concentrated at extrasynaptic sites on dendrites, spines and somata in the striatopallidal complex, perisynaptically at asymmetric synapses and in the main body of symmetric striatopallidal synapses in the GPe and GPi. Consistent with the immunoperoxidase data, immunoparticles were found in the presynaptic grid of asymmetric synapses established by cortical- and subthalamic-like glutamatergic terminals. These findings indicate that both GABA and glutamate metabotropic receptors are located to subserve various modulatory functions of the synaptic transmission in the primate striatopallidal complex. Furthermore, their pattern of localisation raises issues about their roles and mechanisms of activation in normal and pathological conditions. Because of their 'modulatory' functions, these receptors are ideal targets for chronic drug therapies in neurodegenerative diseases such as Parkinson's disease.

Presynaptic inhibition by baclofen of miniature EPSCs and IPSCs in substantia gelatinosa neurons of the adult rat spinal dorsal horn

Intrathecal administration of baclofen, a GABA(B)-receptor agonist, affects pain behavior induced by formalin in a biphasic manner; baclofen at low doses enhances pain while producing antinociception at high doses. This may be due to the fact that baclofen modulates each of excitatory and inhibitory transmission in the dorsal horn of the spinal cord with a distinct sensitivity, resulting in a biphasic action on pain transmission. To address this issue, we examined the actions of baclofen on miniature excitatory (glutamatergic) and inhibitory (GABAergic) postsynaptic currents (mEPSCs and mIPSCs, respectively) in substantia gelatinosa (SG) neurons of adult rat spinal cord slices by using the whole-cell voltage-clamp technique. Baclofen reduced the frequency of both mEPSC and mIPSC without a change in their amplitudes. These actions were dose-dependent in a concentration range of 0.1-100 microM; the effective concentrations for the half-inhibition of mEPSC and mIPSC frequency were 4.44+/-0. 60 microM (n=7) and 4.31+/-0.77 microM (n=6), respectively. These results indicate that each of glutamatergic and GABAergic nerve terminals in the SG is endowed with the GABA(B) receptor, the activation of which depresses the release of neurotransmitter from the terminal; this provides a cellular basis for the modulation of pain by baclofen. It is suggested from a similar affinity for baclofen of the GABA(B) receptors on both terminals that the baclofen-induced biphasic action on pain behaviors cannot be accounted for by only its presynaptic actions in the SG and that other actions such as an inhibitory action of baclofen on postsynaptic neurons also have to be taken into consideration.

GABA(B) receptor antagonists elevate both mRNA and protein levels of the neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) but not neurotrophin-3 (NT-3) in brain and spinal cord of rats

In this study we show that single, physiologically-active and non-convulsive doses of the three GABA(B) receptor antagonists CGP 36742, CGP 56433A and CGP 56999A increase NGF and BDNF mRNA levels by 200-400% and protein levels by 200-250% in rat neocortex, hippocampus as well as spinal cord. In all areas examined the increase in NGF protein preceded that of BDNF. Peak levels of both neurotrophins are transient and occur between 24 and 72 h, depending on the region. In contrast, NT-3 protein concentrations in the neocortex and hippocampus were decreased significantly to 50% of control values within 48-96 h. The decrease in the spinal cord was less than 30% and did not reach significant levels. These data clearly demonstrate that GABA(B) receptor antagonists induce a specific neurotrophin expression in the central nervous system at physiologically relevant doses, as opposed to the extreme conditions of seizure paradigms. The results are in line with the concept that neuronal neurotrophin synthesis and release in brain are controlled by afferent nerve activity. GABA(B) receptor antagonists could therefore be a valuable new approach to selectively increase endogenous neurotrophin levels in the central nervous system.

Activation of the GABA(B) receptor inhibits transient lower esophageal sphincter relaxations in dogs

BACKGROUND & AIMS

Transient lower esophageal sphincter relaxation (TLESR) appears to be the most frequent motor event responsible for gastroesophageal reflux. Because TLESRs are considered to be triggered by activation of gastric mechanoreceptors, and because the gamma-aminobutyric acid type B (GABA(B))-receptor agonist baclofen is known to inhibit transmitter release from mechanosensitive afferents, the effects of baclofen on TLESRs in the dog were assessed.

METHODS

A total of 183 recordings of the pharyngeal, esophageal, lower esophageal sphincter, and gastric pressures as well as measurement of esophageal pH were performed in 15 awake dogs. Racemic baclofen, its enantiomers, and the GABA(B)-receptor antagonist CGP36742 were administered before stimulation of TLESRs by a liquid meal and air insufflation. The pharmacodynamics of baclofen were compared with its pharmacokinetics.

RESULTS

Baclofen dose-dependently inhibited TLESRs, with a 50% effective dose (ED(50)) of 1.0 micromol/kg after intravenous administration. The maximal inhibition amounted to approximately 80%. Intragastric baclofen was almost equally effective (ED(50), 1.8 micromol/kg), compatible with the complete oral availability of the drug (100%). The inhibitory effect of baclofen resided in the pharmacologically active R enantiomer, and CGP36742 reduced some of the effects of baclofen.

CONCLUSIONS

Baclofen is a potent and efficacious inhibitor of TLESRs and reflux in the dog. Activation of the GABA(B) receptor may be a new approach to the treatment of reflux disease.