Additivity and independence of neuroprotective effects of GABAA and GABAB receptor agonists in complete global cerebral ischemia

Neuroprotective effects of GABA_B agonist baclofen and GABA_A agonists THIP and muscimol are completely additive. GABA_A and GABA_B receptor antagonists block the neuroprotective effects of the corresponding agonists without cross-blocking. In fact, the blockade of combined effect of two agonists with selective GABA_A and GABA_B antagonists separately and in combination is not below the estimated value. Therefore, GABA_A and GABA_B receptor mechanisms of the neuroprotective effects independently and separately contribute to the improvement of brain resistance to global ischemia. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 130, No. 8, pp. 187–189, August, 2000     

Effect of bilateral destruction of the subretrofacial area on receptor mechanisms of neuroprotective effect of GABAergic agents

Both GBA_A (muscimol, gaboxadol, and isonipecotate) and GABA_B (baclofen) receptor agonists produce marked neuroprotective effect during total brain ischemia. The antagonists of GABA_A receptors bicuculline and picrotoxin attenuate the effect of muscimol, and the GABA_B receptor antagonists hydroxysaclofen and aminovaleriate decrease the effect of baclofen. The GABAergic substances protect the brain via GABA receptors of both types. The effect of the GABA agonists is central in nature. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 125, No. 2, pp. 162–164, February, 1998.     

Balysum-2 inhibitis evoked activity of the pyramidal neurons in hippocampus

The effects of Balysum-2 on the responsiveness of field CA1 hippocampal pyramidal neurons was studied in experiments with cultured cerebral slices. Addition to the medium of 10⁻⁴M Balysum-2 or its active ingredient (comenic acid) led to a reversible decrease in the peak amplitude of focal response (pop-spike) recorded in the pyramidal layer CA1 during stimulation of the radial layer. Perfusion of hippocampal slices with a solution containing the noncompetitive GABA_A-antagonist picrotoxin prevented the effects of Balysum-2 and comenic acid. Inhibition of hippocampal pyramids by Balysum-2 and comenic acid is probably caused by an incease in the inhibitory effect mediated by GABA_A-receptors. Translated fromByulleten' Eksperimental'noi Biologii I Meditsiny, Vol. 125, No. 1, pp. 63–65, January, 1998     

Norbornan,a new irreversible ligand of the GABAA-receptor chloride channels

In vitro experiments demonstrate the high affinity of the toxic convulsant norbornan [2,2-di-(trifluoromethyl)-3,3-dicyano-5,6-dichloronorbornane] for the GABA_A-receptor chloride channels of membranes from rat brain. Repeated washing and centrifugation do not restore the ionophore density on norbornan-treated membranes, suggesting that norbornan may be counted among the agents acylating the chloride channels. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 121, No. 4, pp. 444–446, April, 1996     

Mechanisms of modulation of GABA-induced currents in isolated cerebellar cells by tacrine

Tacrine, when applied by the concentration clamp technique to isolated Purkinje cells from rat cerebellum, dose-dependently reduced the amplitude of GABA-activated chloride currents recorded by the patch clamp technique. Half-maximal inhibition (IC₅₀) of currents activated by 2 μM GABA was observed at a tacrine concentration of 107 μM. Tacrine produced a right shift of the dose-response curves for GABA-induced currents without affecting their peak amplitudes. It is suggested that suppression of GABA-induced currents caused by tacrine can not be attributed to its interaction with the benzodiazepine site of the GABA_A receptor. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 127, No. 5, pp. 539–542, May, 1999     

Involvement of the GABA-ergic and N-cholinergic systems in the establishment of terminal respiration

In experiments with Nembutal-anesthetized cats, in which lobeline, picrotoxin, tubocurarine, and naloxone were injected into the fourth brain ventricle, respiratory disturbances resulting from activation of the central N-cholinergic receptors by lobeline injected when central GABA_A receptors were blocked by picrotoxin led to respiration of the gasping type. After naloxone was administered additionally, the normal rhythmic pattern of breathing was restored. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 119, N ^o 1, pp. 21–24, January, 1995 Presented by Yu. A. Romanov, Member of the Russian Academy of Medical Sciences     

Activation of neostriatal dopaminergic system in rats prevents toxic effects of picrotoxin administered into globus pallidus

Multiple bilateral microinjections of 5 μg picrotoxin (antagonist of chloride channels coupled with GABA_A receptors) into the globus pallidus induced lethal seizures in some rats and impaired conditioned avoidance responses in survivors. Administration of 15 μg amphetamine into the rostral neostriatum prevented picrotoxin-induced lethal seizures and reduced its negative effects on active avoidance behavior. Activation of the neostriatal dopaminergic system in animals receiving no intrapallidal picrotoxin promoted recovery of conditioned responses after implantation procedure. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 130, No. 8, 144–146, August, 2000     

Typifying presynaptic M-cholinergic receptors in different structures of rat brain

Pre- and postsynaptic M-cholinergic receptors are typified by radioligand analysis with selective cholinoblockers by comparing ligand binding in homogeneous and synaptosomal fractions of different structures of rat brain. It is found that presynaptic receptors of the hemispheres belong to M₃ subtype and those of the brain stem are probably of M₄ subtype. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 122, No. 7, pp. 75–77, July, 1996     

New insights on the role of gephyrin in regulating both phasic and tonic GABAergic inhibition in rat hippocampal neurons in culture

Gephyrin is a tubulin-binding protein that acts as a scaffold for clustering glycine and GABA_A receptors at postsynaptic sites. In this study, the role of gephyrin on GABA_A receptor function was assessed at the post-translational level, using gephyrin-specific single chain antibody fragments (scFv-gephyrin). When expressed in cultured rat hippocampal neurons as a fusion protein containing a nuclear localization signal, scFv-gephyrin were able to remove endogenous gephyrin from GABA_A receptor clusters. Immunocytochemical experiments revealed a significant reduction in the number of synaptic γ2-subunit containing GABA_A receptors and a significant decrease in the density of the GABAergic presynaptic marker vesicular GABA transporter (VGAT). These effects were associated with a slow down of the onset kinetics, a reduction in the amplitude and in the frequency of miniature inhibitory postsynaptic currents (mIPSCs). The quantitative analysis of current responses to ultrafast application of GABA suggested that changes in onset kinetics resulted from modifications in the microscopic gating of GABA_A receptors and in particular from a reduced entry into the desensitized state. In addition, hampering gephyrin function with scFv-gephyrin induced a significant reduction in GABA_A receptor-mediated tonic conductance. This effect was probably dependent on the decrease in GABAergic innervation and in GABA release from presynaptic nerve terminals. These results indicate that gephyrin is essential not only for maintaining synaptic GABA_A receptor clusters in the right position but also for regulating both phasic and tonic inhibition.     

Melatonin potentiates the GABAA receptor-mediated current in cultured chick spinal cord neurons

The effect of melatonin on the γ-aminobutyric acid_A (GABA_A) receptor-mediated response was studied in cultured chick spinal cord neurons using the whole-cell voltage-clamp recording technique. Melatonin rapidly and reversibly potentiated the GABA-induced current in a dose-dependent fashion, with an EC₅₀ of 766 μM and a maximal potentiation of 148%. Potentiation of the GABA response by melatonin was mediated by increasing the potency of GABA rather than the efficacy. Prolonged exposure to a saturating concentration of the disulfide-reducing agent dithiothreitol did not attentuate the effect of melatonin on the GABA response, indicating that melatonin does not act through the redox site. Furthermore, our results demonstrate that melatonin and 5α-pregnan-3α-ol-20-one (a positive steroid modulator of the GABA_A receptor) act through different sites.     

Suppression of LTP-like plasticity in human motor cortex by the GABAB receptor agonist baclofen

Previous experiments in slice preparations revealed that pharmacological activation of GABA_B receptors down- or up-regulates long-term potentiation (LTP), depending on whether increase of GABA_B receptor mediated inhibitory postsynaptic potentials or decrease of presynaptic auto-inhibition of GABA_A receptor mediated inhibition predominates. The effects of GABA_B receptor activation on LTP in humans in vivo are unknown. Here we show, by using transcranial magnetic stimulation, that the GABA_B receptor agonist baclofen decreases paired associative stimulation induced LTP-like plasticity in human motor cortex. This suggests that increased GABA_B mediated inhibitory postsynaptic potentials drive this effect, and that baclofen may have a negative impact on LTP-dependent behavioural processes such as motor learning.     

Subcellular localization of GABAA/benzodiazepine receptor-like immunoreactivity in the superficial gray layer of the rat superior colliculus

The monoclonal antibody bd-17, which recognizes the β₂ and β₃-subunits of GABA_A/benzodiazepine receptors, was used to determine the cellular and subcellular localization of receptor-like immunoreactivity in the superficial gray layer of the rat superior colliculus. In numerous dendrites, very strong immunostaining was present in the cytoplasm and on the postsynaptic dendritic membrane of synaptic junctions. The extrasynaptic portion of the dendritic membrane also very often showed [β₂ + β₃]-like immunoreactivity. However, due to methodological limitations, it could not be stated with certainty whether presynaptic β₂- and β₃-subunits of GABA_A/benzodiazepine receptors actually occur in this mesencephalic visual structure. In conclusion, these results strongly suggest that synaptic and non-synaptic GABA_A/benzodiazepine receptors are present in the superficial gray layer of the rat superior colliculus. These receptors may modulate neuronal cell activity in different ways, depending on their location.     

Stimulation of α1-adrenoceptors reduces glutamatergic synaptic input from primary afferents through GABAA receptors and T-type Ca channels

Activation of the descending noradrenergic system inhibits nociceptive transmission in the spinal cord. Although both α₁- and α₂-adrenoceptors in the spinal cord are involved in the modulation of nociceptive transmission, it is not clear how α₁-adrenoceptors regulate excitatory and inhibitory synaptic transmission at the spinal level. In this study, inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs, respectively) were recorded from lamina II neurons in rat spinal cord slices. The specific α₁-adrenoceptor agonist phenylephrine significantly increased the frequency of GABAergic spontaneous IPSCs in a concentration dependent manner, and this effect was abolished by the α₁-adrenoceptor antagonist 2-(2,6-dimethoxyphenoxy)ethylaminomethyl-1,4-benzodioxane (WB4101). Phenylephrine also significantly reduced the amplitude of monosynaptic and polysynaptic EPSCs evoked from primary afferents. The inhibitory effect of phenylephrine on evoked monosynaptic glutamatergic EPSCs was largely blocked by the GABA_A receptor antagonist picrotoxin and, to a lesser extent, by the GABA_B receptor antagonist CGP55845. Furthermore, blocking T-type Ca²⁺ channels with amiloride or mibefradil diminished the inhibitory effect produced by phenylephrine or the GABA_A receptor agonist muscimol on monosynaptic EPSCs evoked from primary afferents. Collectively, these findings suggest that activation of α₁-adrenoceptors in the spinal cord increases synaptic GABA release, which attenuates glutamatergic input from primary afferents mainly through GABA_A receptors and T-type Ca²⁺ channels. This mechanism of presynaptic inhibition in the spinal cord may be involved in the regulation of nociception by the descending noradrenergic system.     

Additivity and independence of neuroprotective effects of GABAA and GABAB receptor agonists in complete global cerebral ischemia

Neuroprotective effects of GABA_B agonist baclofen and GABA_A agonists THIP and muscimol are completely additive. GABA_A and GABA_B receptor antagonists block the neuroprotective effects of the corresponding agonists without cross-blocking. In fact, the blockade of combined effect of two agonists with selective GABA_A and GABA_B antagonists separately and in combination is not below the estimated value. Therefore, GABA_A and GABA_B receptor mechanisms of the neuroprotective effects independently and separately contribute to the improvement of brain resistance to global ischemia. Translated fromByulleten’ Eksperimental’noi Biologii i Meditsiny, Vol. 130, No. 8, pp. 187–189, August, 2000     

GABAA receptor-mediated modulation of neuronal activity propagation upon tetanic stimulation in rat hippocampal slices

Tetanic stimulation (100 Hz), which can induce long-term potentiation in synaptic connections in the hippocampal CA1 region, causes γ-aminobutyric acid (GABA)_A receptor-mediated long-lasting depolarization of postsynaptic neurons. However, it is not clear how this stimulation modulates neuronal activity propagation. We studied tetanic burst-induced neuronal responses in the hippocampal CA1 region by using optical-recording methods employing a voltage-sensitive dye and focused on GABA_A receptor-mediated modulation. We observed that burst stimulation induced long-lasting depolarization and progressive decrease in individual excitatory postsynaptic potentials (EPSPs). Both these effects were suppressed by picrotoxin, a GABA_A receptor antagonist. Under whole-cell voltage-clamp conditions, we observed a long-lasting inhibitory current (IPSC) and a prominent progressive decrease in the amplitude of the excitatory postsynaptic current (EPSC). Further, picrotoxin inhibited the IPSC and the progressive decrease in EPSC. The optically recorded long-lasting depolarization and progressive decrease of EPSPs were strongly dependent on the distance between the recording electrode and the stimulation site. Optical recordings performed across a wide swatch of CA1 revealed that the decrease in activity propagation was followed by facilitation of propagation after recovery and that this facilitation also depended on GABA_A receptors. Intense activation of GABA_A receptors is a key factor shaping the spatiotemporal patterns of high-frequency stimulation-induced responses in the CA1 region.     

Contributions of Glycine and GABAA Receptors to the Generation of Inhibitory Postsynaptic Potentials in Frog Spinal Cord Motoneurons

Intracellular recording from lumbar motoneurons in isolated spinal cord preparations from the frog Rana ridibunda was used to study the contributions of glycine and GABA_A receptors to the generation of inhibitory postsynaptic potentials (IPSP) induced by microstimulation of fibers close to these motoneurons. IPSP were identified by blockade of excitatory synaptic transmission using kynurenate, CNQX, and AP-5 and by reversion of polarity on injection of a depolarizing current (1–10 nA) via the microelectrode. The selective glycine receptor antagonist strychnine at 1–5 μM decreased IPSP amplitude in all the motoneurons studied (by an average factor of 4.7), while the GABA_A receptor antagonist bicuculline at 50–70 mM decreased the amplitude (by an average factor of 1.6) in only some motoneurons, while no decrease occurred in others. Sequential application of strychnine and bicuculline completely blocked IPSP. These data support the view that postsynaptic inhibition in frog motoneurons is mediated mainly by glycine and to a lesser extent by GABA_A receptors. These latter are probably partially extrasynaptic.     

Possible differences between the time courses of presynaptic and postsynaptic GABA<Subscript>B</Subscript> mediated inhibition in the human motor cortex

Paired-pulse transcranial magnetic stimulation (TMS) can be used to non-invasively evaluate human motor cortical inhibitory circuits such as short interval intracortical inhibition (SICI) and long interval intracortical inhibition (LICI). Pharmacological studies suggested that SICI is mediated by GABA_A receptors while LICI is probably mediated by GABA_B receptors. A previous study also showed that SICI and LICI are mediated by separate neuronal populations and that LICI inhibits SICI, possibly through presynaptic GABA_B receptors. The aim of this study was to examine whether the time course of motor-evoked potentials (MEP) inhibition by LICI, likely mediated through postsynaptic GABA_B receptors, is different from SICI inhibition by LICI, likely mediated through presynaptic GABA_B receptors. Nine healthy volunteers were studied and MEP were recorded from the first dorsal interosseous muscle. A triple-stimulus TMS paradigm was used to evaluate the effect of LICI at ISIs of 100 and 150 ms on SICI. LICI at 100 and 150 ms caused a similar degree of MEP inhibition. LICI at 100 ms led to a significant reduction of SICI but LICI at 150 ms had no effect on SICI. Repeated measures ANOVA revealed a significant interaction between the LICI mediated inhibition of SICI and ISI (P = 0.0072). These findings suggest that the time courses of presynaptic and postsynaptic GABA_B receptors mediated inhibition are different in the human motor cortex.     

Subsynaptic units as a universal system-forming and regulating factor of brain synapses

Dense projections and postsynaptic density — specialized structures of the synaptic cytoskeleton — are regarded as a single functional system of subsynaptic units. From this viewpoint, interneuronal relations in health and after brain ischemia are analyzed. It is shown that hyperplasia and recombination of the system of subsynaptic units are the major mechanisms responsible for reorganization of functionally mature interneuronal junctions in the mammalian and human brain. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 124, No. 7, pp. 4–12, July, 1997     

Calcium currents and GABAB receptors in the dorsal sensory cells of the lamprey spinal cord

Patch-clamp studies were performed on the isolated dorsal sensory cells of the spinal cords of three species of lamprey,Ichthyomyzon unicuspis, Petromyzon marinus, andLampetra fluviatilis, to measure changes in the amplitudes of calcium current induced by GABA and its specific antagonists and agonists. The experiments showed that GABA (4 mM) reduced the peak amplitude of the calcium current by 28.5±4.9%, with subsequent recovery to 96.2±9.2% of control (n=45). The GABA_B agonist baclofen had similar effects. The GABA_A agonists glycine and taurine had no effect on the Ca²⁺ current. The inhibitory effect of GABA was blocked by 2-hydroxysaclofen (a GABA_B antagonist), but persisted in the presence of bicuculline (a GABA_A antagonist). These results are evidence that the membranes of dorsal sensory cells contain GABA_B receptors, which significantly increases our under-standing of the mechanisms of presynaptic inhibition in the spinal cords of the cyclostomata. Translated from Rossiiskii Fiziologischeskii Zhurnal imeni I. M. Sechenova, Vol. 83, No. 11-12, pp. 79–91, November–December, 1997.     

Adaptation to physical load increases the activity of prostaglandins E and I2 and reduces stress reaction

Adaptation to physical load protects against stress and other damage. It is suggested that this protection is associated with activation of prostaglandins E (PGE) and I₂ (PGI₂). Plasma contents of PGE₂, PGI₂, and thromboxane A₂ (TxA₂) and the severity of stress reaction are measured in male Wistar rats adapted to swimming. Training increases the concentrations of these prostaglandins and the prostaglandin/TxA₂ ratio, reduces almost 2-fold the severity of stress reaction as assessed by the plasma corticosterone concentration and corticosterone/insulin ratio. After stress, the PGI₂ and PGI₂/TxA₂ in adapted rats were, respectively, 33 and 31% higher than in unadapted. These findings suggest that prostaglandins are involved in the reduction of stress reaction. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 122, No. 12, pp. 622–624, December, 1996