GK-2 Reduces Death of Cultured Granule Neurons in Cerebellum Induced by the Toxic Effects of Zinc Ions

Peptide mimetic of nerve growth factor GK-2 in a dose of 1-2 mg/liter improves survival of cultured rat cerebellar granule neurons exposed to the cytotoxic effect of zinc ions, but has no protective effect against copper ion cytotoxicity. Experiments on cultured rat hippocampal slices demonstrated that GK-2 did not affect reactivity of pyramidal neurons and long-term potentiation in the hippocampal field CA1 and the probability of glutamate release from presynaptic terminals in the synapses of the CA3-CA1 fields. The results suggest that GK-2 does not affect the functional properties of synaptic transmission under normal conditions, but protects neurons from the toxic effects of zinc, which creates prerequisites for GK-12 use in the treatment of neurodegenerative diseases.

The mechanisms of potentiation and inhibition of GABAA receptors by non-steroidal anti-inflammatory drugs, mefenamic and niflumic acids

Fenamates mefanamic and niflumic acids (MFA and NFA) induced dual potentiating and inhibitory effects on GABA currents recorded in isolated cerebellar Purkinje cells using the whole-cell patch-clamp and fast-application techniques. Regardless of the concentration, both drugs induced a pronounced prolongation of the current response. We demonstrated that the same concentration of drugs can produce both potentiating and inhibitory effects, depending on the GABA concentration, which indicates that both processes take place simultaneously and the net effect depends on the concentrations of both the agonist and fenamate. We found that the NFA-induced block is strongly voltage-dependent. The Woodhull analysis of the block suggests that NFA has two binding sites in the pore - shallow and deep. We built a homology model of the open GABA_AR based on the cryo-EM structure of the open α1 GlyR and applied Monte-Carlo energy minimization to optimize the ligand-receptor complexes. A systematic search for MFA/NFA binding sites in the GABA_AR pore revealed the existence of two sites, the location of which coincides well with predictions of the Woodhull model. In silico docking suggests that two fenamate molecules are necessary to occlude the pore. We showed that MFA, acting as a PAM, competes with an intravenous anesthetic etomidate for a common binding site. We built structural models of MFA and NFA binding at the transmembrane β(+)/α(-) intersubunit interface. We suggested a hypothesis on the molecular mechanism underlying the prolongation of the receptor lifetime in open state after MFA/NFA binding and β subunit specificity of the fenamate potentiation.

Development of 1,3-thiazole analogues of imidazopyridines as potent positive allosteric modulators of GABAA receptors

Structure-activity relationship studies were conducted in the search for 1,3-thiazole isosteric analogs of imidazopyridine drugs (Zolpidem, Alpidem). Three series of novel γ-aminobutyric acid receptor (GABA_AR) ligands belonging to imidazo[2,1-b]thiazoles, imidazo[2,1-b][1,3,4]thiadiazoles, and benzo[d]imidazo[2,1-b]thiazoles were synthesized and characterized as active agents against GABA_AR benzodiazepine-binding site. In each of these series, potent compounds were discovered using a radioligand competition binding assay. The functional properties of highest-affinity compounds 28 and 37 as GABA_AR positive allosteric modulators (PAMs) were determined by electrophysiological measurements. In vivo studies on zebrafish demonstrated their potential for the further development of anxiolytics. Using the OECD "Fish, Acute Toxicity Test" active compounds were found safe and non-toxic. Structural bases for activity of benzo[d]imidazo[2,1-b]thiazoles were proposed using molecular docking studies. The isosteric replacement of the pyridine nuclei by 1,3-thiazole, 1,3,4-thiadiazole, or 1,3-benzothiazole in the ring-fused imidazole class of GABA_AR PAMs was shown to be promising for the development of novel hypnotics, anxiolytics, anticonvulsants, and sedatives drug-candidates.

Effect of Endogenous Neuropeptide Cycloprolylglycine on GABAA Receptors in Cerebellar Purkinje Cells

Voltage clamp and concentration-jump methods were employed to examine the effects of endogenous neuropeptide cycloprolylglycine on GABA-activated ionic currents in isolated cerebellar Purkinje cells. In the concentration range of 0.1-10.0 μM, short-term (600 msec) external application of cycloprolylglycine against the background of GABA-evoked current produced no effect on its amplitude. In contrast, application of 1 μM cycloprolylglycine increased current up to 177±15% control level. The development of potentiating effect and return to the control level of ionic current were slow, which was indicative of possible implication of second messenger systems in these processes. Functional augmentation of GABA_A receptors under the action of cycloprolylglycine can underlie the established neuroprotective and anxiolytic effects of this endogenous dipeptide.

Modulation of GABA- and Glycine-Activated Ionic Currents with Semax in Isolated Cerebral Neurons

The concentration-clamp experiments with neurons isolated from the rat brain showed that nootropic and neuroprotective drug Semax added to perfusion solution at concentration of 1 μM augmented the amplitude of GABA-activated ionic currents in cerebellum Purkinje cells by 147±13%. In addition, Semax in perfusion solution (0.1 and 1 μM) diminished the amplitude of glycine-activated chloride currents in hippocampal pyramidal neurons down to 68 and 43% control level, respectively. Both potentiating and inhibitory effects developed slowly, and they were poorly reversible, which indicated a probable implication of second messengers in the observed phenomena. Semax accelerated the falling edge of glycine-activated current both after a short-term co-application with agonist and after addition of this peptide into perfusion solution.

Block of GABA(A) receptor ion channel by penicillin: electrophysiological and modeling insights toward the mechanism

GABA(A) receptors (GABA(A)R) mainly mediate fast inhibitory neurotransmission in the central nervous system. Different classes of modulators target GABA(A)R properties. Penicillin G (PNG) belongs to the class of noncompetitive antagonists blocking the open GABA(A)R and is a prototype of β-lactam antibiotics. In this study, we combined electrophysiological and modeling approaches to investigate the peculiarities of PNG blockade of GABA-activated currents recorded from isolated rat Purkinje cells and to predict the PNG binding site. Whole-cell patch-сlamp recording and fast application system was used in the electrophysiological experiments. PNG block developed after channel activation and increased with membrane depolarization suggesting that the ligand binds within the open channel pore. PNG blocked stationary component of GABA-activated currents in a concentration-dependent manner with IC50 value of 1.12mM at -70mV. The termination of GABA and PNG co-application was followed by a transient tail current. Protection of the tail current from bicuculline block and dependence of its kinetic parameters on agonist affinity suggest that PNG acts as a sequential open channel blocker that prevents agonist dissociation while the channel remains blocked. We built the GABA(A)R models based on nAChR and GLIC structures and performed an unbiased systematic search of the PNG binding site. Monte-Carlo energy minimization was used to find the lowest energy binding modes. We have shown that PNG binds close to the intracellular vestibule. In both models the maximum contribution to the energy of ligand-receptor interactions revealed residues located on the level of 2', 6' and 9' rings formed by a bundle of M2 transmembrane segments, indicating that these residues most likely participate in PNG binding. The predicted structural models support the described mechanism of PNG block.

Modulation of ATP-induced currents by zinc in acutely isolated hypothalamic neurons of the rat

1. Whole-cell patch-clamp and fast perfusion were used to study the effects of zinc on adenosine 5'-triphosphate (ATP)-induced responses of histaminergic neurons. 2. At 10-30 micro M ATP, Zn(2+) had biphasic effects on ATP responses. Zn(2+) at 3-100 micro M increased the ATP-induced currents, but inhibited them at higher concentrations. 3. At 300 micro M ATP, Zn(2+) predominantly but incompletely inhibited the currents. 4. At 5 and 50 micro M, Zn(2+) shifted to the left the concentration-response curve for ATP-induced currents, without changing the maximal response. At 1 mM, Zn(2+) inhibited ATP-induced currents in a noncompetitive way, reducing the maximal response by 58%. .Zn(2+) increased the decay time of ATP-evoked currents nine fold with an EC(50) of 63 micro M. Upon removal of high concentrations of Zn(2+), there was a rapid increase of the current followed by a slow decline towards the response amplitude seen with ATP alone. The appearance of a tail current is consistent with a Zn(2+)-induced increase of ATP affinity and an inhibition of its efficacy. 6. Thus, Zn(2+) acts as a bidirectional modulator of ATP receptor channels in tuberomamillary neurons, which possess functional P2X(2) receptors. The data are consistent with the existence of two distinct modulatory sites on the P2X receptor, which can be occupied by Zn(2+). 7. Our data suggest that zinc-induced potentiation of ATP-mediated currents is caused by the slowing of ATP dissociation from the receptor, while inhibition of ATP-induced currents is related to the suppression of ATP receptor gating.

Expression and function of P2X purinoceptors in rat histaminergic neurons

(1) The pharmacology of ATP responses and the expression pattern of seven known subunits of the P2X receptor were investigated in individual histaminergic neurons of the tuberomamillary nucleus (TM). (2) ATP (3-1000 micro M) evoked fast non-desensitizing inward currents in TM neurons. 2-methylthioATP (2MeSATP) displayed the same efficacy but a lower potency, EC(50)s 84 micro M versus 48 micro M, when compared with ATP. Adenosine-diphosphate (ADP), uridine-triphosphate (UTP) and alpha beta methylene-ATP (alphabeta-meATP) were inactive. (3) ATP-mediated whole cell currents were potentiated by acidification of the recording solution (pH 7.5 and 6.6 were compared). (4) Single-cell RT-PCR (scRT-PCR) analysis revealed that the P2X(2) receptor is expressed in all PCR-positive neurons. Each of the P2X(1), P2X(3), P2X(4), P2X(5) and P2X(6) mRNAs were detected in less than 35% of the cells. (5) Suramin antagonized ATP responses with an IC(50) of 4.2 micro M and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 1 micro M) reduced ATP responses to 43% of control, when antagonists were pre-applied 90s before the agonist. Cibacron blue (3 micro M) given together with ATP potentiated control responses by 67%, but inhibited it to 10% after pre-application. (6) 2',3'-O-(2,4,6-Trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP) antagonized ATP responses with an IC(50) of 7 micro M. (7) Pharmacological properties of ATP responses together with scRT-PCR data suggest that P2X(2) is the major purinoceptor on the soma of TM neurons, however the presence of heteromeric P2X(2/5) receptors in some neurons cannot be excluded.

Neuroprotective effects of the antifungal drug clotrimazole

Pretreatment with 10 microM of the antifungal drug clotrimazole potently reduced the death of cultured rat cerebellar granule cells induced by oxygen/glucose deprivation, and the excitotoxic effect of glutamate on cultured hippocampal neurons and cerebellar granule cells. In patch-clamped hippocampal pyramidal neurons, 10-50 microM clotrimazole caused a decrease in the amplitude of N-methyl-D-aspartate (NMDA) receptor-mediated currents. Glutamate induced intracellular Ca(2+) overload, as measured by Fluo-3 confocal fluorescence imaging, while clotrimazole reduced Ca(2+) overload and promoted the recovery of intracellular calcium homeostasis after glutamate treatment. Using tetramethylrhodamine ethyl ester fluorescence as a marker of mitochondrial membrane potential we found that clotrimazole prevented the glutamate-induced loss of mitochondrial membrane potential. Our data provide evidence that the protective effect of clotrimazole against oxygen/glucose deprivation and excitotoxicity is due to the ability of this drug to partially block NMDA receptor-gated channel, thus causing both reduced calcium overload and lower probability of the mitochondrial potential collapse.

GABA(A) receptor heterogeneity in histaminergic neurons

Histaminergic neurons of the tuberomamillary nucleus display pacemaker properties; their firing rate is regulated according to behavioural state by gabaergic inhibition. Whole-cell recordings and single-cell RT-PCR from acutely isolated rat tuberomamillary neurons were used to characterize GABA -evoked currents and to correlate them with the expression pattern of 12 GABAA receptor subunits. We report differences in sensitivity to GABA and zinc as well as in the modulation of IPSC-decay times by zolpidem in histaminergic neurons expressing gamma-subunits at different levels. Immunocytochemistry and pharmacological analysis of whole-cell GABA-currents in these neurons revealed that all carry the gamma2-subunit protein and that all receptors contain at least one gamma-subunit. Neurons with different expression levels of gamma-subunits displayed a difference in cooperativity of GABA and zolpidem binding which we explain by the presence of one vs. two gamma-subunits in one receptor. Thus, we describe here native GABAA receptor function in relation to its stoichiometry.

Mechanisms of GABA(A) receptor blockade by millimolar concentrations of furosemide in isolated rat Purkinje cells

The action of diuretic furosemide on the GABA(A) receptor was studied in acutely isolated Purkinje cells using the whole-cell recording and fast application system. Furosemide blocked stationary component of GABA-activated currents in a concentration-dependent manner with IC(50) value > 5 mM at -70 mV. The inhibition was rapid in the onset, fully reversible and did not require drug pre-perfusion. The termination of GABA and furosemide co-application was followed by transient increase in the inward current 'tail' current, which was not observed when furosemide was continuously present in the solution. The degree of furosemide block did not depend on GABA concentration. Furosemide block increased with membrane depolarization. Five millimolar furosemide depressed GABA currents by 32.4+/-1.3% at -70 mV and by 76.7+/-5.0% at +70 mV. Analysis of the voltage dependence of the block suggests that furosemide binds at the site located within GABA(A) channel pore with a dissociation constant of 5.3+/-0.5 mM at 0 mV and electric distance of 0.27. Our results provide evidence that furosemide interacts with Purkinje cell GABA(A) receptors (most probably composed of alpha1beta2/3gamma2 subunits) through a low affinity site located in channel pore and suggest that furosemide acts as a sequential open channel blocker, which prevents the dissociation of agonist while the channel is blocked.

Differential modulation of AMPA receptors by cyclothiazide in two types of striatal neurons

The modulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazol-propionate (AMPA) receptor-mediated currents by cyclothiazide was investigated in acutely isolated cells from rat striatum with whole-cell patch-clamp recording. Single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) was used to identify medium spiny and giant aspiny neurons and to determine their AMPA receptor subunit composition mostly in separate experiments. After pretreatment with cyclothiazide, kainate-induced AMPA responses were more strongly potentiated in medium spiny than in giant aspiny neurons; cyclothiazide induced a ninefold leftward shift in the kainate concentration-response curve for medium spiny neurons (not giant aspiny neurons). The EC50s for the cyclothiazide potentiation did not differ substantially between medium spiny neurons and giant aspiny neurons. The recovery of kainate-activated currents from modulation by cyclothiazide was slower for medium spiny neurons than for giant aspiny neurons. Medium spiny neurons expressed GluR-A, GluR-B and GluR-C, but not GluR-D subunits in both flip and flop splice variants. All giant aspiny neurons expressed GluR-A and GluR-D, exclusively in the flop form, half of them also expressed GluR-B and GluR-C. This is in keeping with slow and fast desensitization kinetics in medium spiny neurons and giant aspiny neurons, respectively, and differences in cyclothiazide modulation. The rate of cyclothiazide dissociation from the AMPA receptor, activated by glutamate, was approximately 90 times slower in medium spiny neurons than in giant aspiny neurons. In giant aspiny neurons (not medium spiny neurons) this rate was strongly dependent on the presence of an agonist; 1 mM glutamate increased it 30-fold. Thus, two major cell groups in the striatum display distinct AMPA receptor compositions carrying specific properties of glutamate responses. Excitatory transmission will thus be differentially affected by cyclothiazide-type compounds.

Interaction between copper and zinc at GABA(A) receptors in acutely isolated cerebellar Purkinje cells of the rat

Nanomolar concentrations of Cu(2+) induce a slowly reversible block of GABA(A) receptor-mediated currents which can be removed by chelating substances. The possible interaction of Cu(2+) with the Zn(2+) binding site on the GABA(A) receptor complex was studied in acutely isolated Purkinje cells using whole-cell recording and a fast drug application system. When Zn(2+) was applied together with 2 microM GABA, the Zn(2+)-induced block of GABA-mediated currents was not additive to the Cu(2+)-induced block. In the presence of 0.1 microM Cu(2+) in the bath solution the degree of inhibition of GABA-mediated responses by Zn(2+) was strongly attenuated. Preapplication of 100 microM Zn(2+) during 10 s, terminated 1 s before exposure to 2 microM GABA did not affect the GABA current in Cu(2+)-free solution, but relieved its block by 0.1 microM Cu(2+). This effect of Zn(2+) was concentration-dependent with an EC(50) of 72 microM. When the Cu(2+)-induced block was removed by histidine, preapplication of Zn(2+) did not increase the GABA current, indicating that the relief of Cu(2+) block by Zn(2+) is the result of its ability to actively remove Cu(2+) from the GABA receptor complex. It is proposed that the inhibitory effects of Zn(2+) and Cu(2+) on GABA-induced currents result from an action of these metal ions at distinct, but conformationally linked sites on the GABA(A) receptor protein. Under physiological conditions Zn(2+) would liberate Cu(2+) from the GABA(A) receptor, thus facilitating Cu(2+) turnover and its binding by other endogenous chelating molecules.

The open channel blocking drug, IEM-1460, reveals functionally distinct alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors in rat brain neurons

The properties of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors were examined in various cell types isolated from young rat hippocampus, striatum and cerebellum using patch-clamp and fast application techniques. A dicationic adamantane derivative, IEM-1460, reversibly inhibited kainate-induced currents. In the presence of 100 microM IEM-1460, kainate currents in striatal giant cholinergic interneurons and hippocampal non-pyramidal neurons were inhibited by 95% and 81%, respectively, at Vh = - 70 mV. Striatal GABAergic principal cells, hippocampal pyramidal neurons and cerebellar Purkinje cells had low sensitivity to IEM-1460 (inhibition by 4-15%). Analysis of averaged data from the cell types studied revealed a highly significant positive correlation (r= 0.93, P < 0.01) between percentage inhibition by 100 microM IEM-1460 and relative calcium permeability of AMPA receptors, P(Ca)/P(Na). Also, within each brain structure, the sensitivity of IEM-1460 block was lower the stronger the outward rectification of kainate currents. Some hippocampal neurons exhibited intermediate sensitivity to IEM-1460. Kainate currents were suppressed by 40% in the presence of 100 microM IEM-1460. Meanwhile, AMPA receptors in this cell type had low calcium permeability (P(Ca)/P(Na) = 0.13) and demonstrated outwardly rectifying kainate currents. The interrelation of different properties of AMPA receptors considering their assembly is discussed. The data obtained suggest that IEM-1460 may be a convenient and promising marker of native AMPA receptor assembly: it selectively inhibits Ca(2+)-permeable, GluR2-lacking AMPA receptors.

Characterization of AMPA receptor populations in rat brain cells by the use of subunit-specific open channel blocking drug, IEM-1460

Dicationic adamantane derivative, IEM-1460, which selectively blocks GluR2-lacking, Ca2+-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors, was used to characterize the distribution of AMPA receptors among populations of rat brain cells. IEM-1460 inhibited kainate-induced inward currents (at -80 mV) in a dose-dependent manner. IEM-1460 concentrations producing 50% inhibition of kainate-induced current amplitude (IC50) varied greatly depending on the cell type studied. Striatal giant cholinergic interneurons and putative Bergmann glial cells isolated from the cerebellum were found to be highly sensitive to IEM-1460 block (IC50=2.6 microM), indicating the expression of GluR2-lacking AMPA receptor subtype. Among hippocampal and cortical non-pyramidal neurons, there were cell-to-cell differences in the pattern of AMPA receptor subtype expression. Some cells which are known to express AMPA receptors lacking GluR2 subunit exhibited high sensitivity of IEM-1460 block (IC50 about 1 microM) but in the others, the part of AMPA receptor population seemed to be represented by GluR2-having receptor subtype. The latter subtype was mainly expressed by pyramidal neurons isolated from hippocampus (IC50=1102 microM) and sensorimotor cortex (IC50=357 microM) which showed low affinity for IEM-1460 block. In conclusion, IEM-1460 can be utilized as an indicator of the distribution of AMPA receptor subtypes among populations of rat brain cells, and pharmacological detection of the absence of GluR2 subunit in AMPA receptor assembly can provide useful information for the interpretation of physiological events.

High-affinity copper block of GABA(A) receptor-mediated currents in acutely isolated cerebellar Purkinje cells of the rat

The actions of Cu2+ ions on GABAA receptor-mediated currents in acutely isolated Purkinje cells from rat cerebellum were studied using the whole-cell patch-clamp technique and a rapid perfusion system. Bath application of Cu2+ reduced currents induced by 2 microM gamma-aminobutyric acid (GABA) in a concentration-dependent manner with an IC50 of 35 nM. The Cu2+-induced block of GABA responses was not voltage-dependent. Increasing the GABA concentration (from 2 to 50 microM) decreased the blocking effect of Cu2+. Dose-response analysis for activation of GABAA receptors revealed a twofold decrease in apparent affinity for GABA in the presence of 0.1 microM Cu2+. Recovery from the block required several minutes after removal of Cu2+ from the medium. The block was removed by histidine, which preferentially forms complexes with Cu2+, or by other chelating substances. Application of 10 microM histidine immediately before application of 2 microM GABA completely relieved the block of GABA responses produced by 0.1 microM Cu2+. The effect of histidine was concentration-dependent with an EC50 of 0.75 microM. The results demonstrate that Cu2+ is a potent inhibitor of GABA-evoked responses in rat Purkinje cells. Copper may be an endogenous synaptic modulating factor. Cu2+ toxicity, notably in Wilson's disease, could result to some extent from chronic GABAA receptor blockade.

A simple perfusion system for patch-clamp studies

Many systems for rapid perfusion of isolated cells or membrane patches operate with separate capillaries for each drug to be applied. We describe here a simple way by which the solutions in a single capillary can be quickly exchanged during recording. The device consists of a 100 microliters reservoir mounted 20 mm above the preparation and a capillary through which test perfusion media are driven by gravity. The application system is used for preparations attached to a recording pipette and includes two capillaries with solution exchange capabilities within a larger bath tube; it is driven by a capstan motor which is held in a neutral position by a spring. An unlimited number of solutions can be applied through each channel. The volume for washing and refilling a channel is typically 0.5 ml; it may be as low as 0.1 ml. The time interval necessary for sequential applications of different solutions through the same channel is less than 20 s. The solution flows through the capillaries and the bath tube at a speed of about 3 mm/s (30 and 600 nl/s respectively). The mechanical displacement for concentration jumps is adjustable from 0 to 0.4 mm in two directions and can be set to occur within 2-100 ms.

Abecarnil enhances GABA-induced currents in acutely isolated cerebellar Purkinje cells

The effect of abecarnil, a beta-carboline derivative acting at central gamma-aminobutyric acid (GABAA)/benzodiazepine receptors, on the response to GABA of isolated Purkinje cells acutely dissociated from rat cerebellar slices was studied. Using a rapid superfusion system to apply drugs and whole-cell voltage-clamp recording configuration, abecarnil was found to be of similar efficacy to diazepam (DZP) in enhancing GABA-mediated responses. Abecarnil potentiated GABA-induced chloride currents maximally by 241%, while DZP showed a maximal potentiation of 217%. However, abecarnil was more potent than DZP and exhibited different potentiation kinetics. While the response to DZP was fast and reversible, abecarnil after a 1-3 sec application initially produced only a very small enhancement of the GABA response. The effect then developed gradually even after cessation of abecarnil application, and depended on both abecarnil concentration and exposure time. It is suggested that abecarnil accumulates in the lipid membrane resulting in slow effect kinetics and prolonged presence at the benzodiazepine binding site. Abecarnil is a full agonist at the GABAA/benzodiazepine receptor on Purkinje cell somatic membranes.

Subcortical modulation of synaptic plasticity in the hippocampus

A two-stage model of memory trace formation in the hippocampus considers exploratory behavior associated with theta activity and consumatory behaviors or immobility associated with the occurrence of synchronous pyramidal cell discharges. Threshold, frequency and extent of CA3 bursts differ between differently performing strains of mice and are subject to subcortical modulation. Mechanisms for such actions are described: interference with potassium and other cation channels through second messengers and a direct action of histamine on the NMDA receptor channel. Furthermore long-lasting potentiations of synaptic transmission and excitability following brief exposures of hippocampal slices to histamine, noradrenaline, and serotonin are mediated by cyclic AMP and presumably protein kinase A.

Tetrahydroaminoacridine blocks and prolongs NMDA receptor-mediated responses in a voltage-dependent manner

N-Methyl-D-aspartate (NMDA) receptor-mediated currents were recorded from acutely isolated rat hippocampal neurones using patch-clamp and fast perfusion techniques. Tetrahydroaminoacridine blocked NMDA receptor currents in a concentration-dependent fashion with IC50 25 +/- 6 microM and slope factor 2 +/- 0.2 at a membrane potential -80 mV. The block was voltage-dependent being greater at a hyperpolarized potential. The NMDA responses blocked by tetrahydroaminoacridine at concentrations greater than 25 microM were followed by a transient inward current hump with a decay time constant of about 200 ms at -90 mV. The tetrahydroaminoacridine-induced NMDA tail current was voltage-dependent, blocked by magnesium and tetrahydroaminoacridine itself and was not affected by NMDA and glycine recognition site antagonists. Magnesium suppressed the tail current amplitude without changing its time course whereas the tetrahydroaminoacridine block was accompanied by a dramatic prolongation. It is suggested that tetrahydroaminoacridine prevents the closing of the blocked NMDA channels thus keeping them in the activated state after the removal of agonist. The observed properties of the tetrahydroaminoacridine block could be explained in terms of a sequential model of an open channel block.

Histamine potentiates N-methyl-D-aspartate responses in acutely isolated hippocampal neurons

N-methyl-D-aspartate (NMDA)-evoked currents were recorded from acutely isolated rat hippocampal neurons, using the whole-cell patch-clamp technique and a rapid perfusion system. Histamine, at concentrations from 0.5 to 100 microM, reversibly enhanced NMDA currents by up to 50%. The effect cannot be ascribed to activation of the known histamine receptors (H1, H2, H3) but is occluded by spermine. These results suggest an interaction of histamine with the polyamine-binding site on the NMDA receptor complex. This modulatory action could allow the histaminergic system to determine time and loci of NMDA receptor-mediated events, such as memory formation according to behavioral state.

[Effects of gamma-aminobutyric acid antagonist, 4-ethyl bicyclo-phosphate, on total and synaptic evoked responses in neurons of hippocampal slices]

Influence of 4-E-BPE on the amplitude of population spices (PS) evoked in CA1 area by Shaffer collateral stimulation in hippocampal slices were analysed. Bath application of 4-E-BPE (10(-6)-10(-5) M) led to a pronounced increase in the amplitude of the PS, the appearance of secondary PS and then introduction of GABA led to restoring original state. The 4-E-BPE was more potent than picrotoxin. These findings suggest that 4-E-BPE suppress inhibitory synaptic transmission in the CA1 region of hippocampus.

[Tetrodotoxin--a sensitive component of the depolarizing response to GABA administration to the pyramidal neuron dendrites of the CA1 field of the hippocampus]

The intracellular recording of CA1 neurons in mouse hippocampal slice preparation was used to study the properties of depolarizing responses to iontophoretically applied GABA to their apical dendrites. Reversal potential of depolarizing responses was dependent on parameters of injecting current. It was about -60 mV and - (45-55) mV when iontophoretic currents 40-60 nA and 8-20 nA were used respectively. Application of tetrodotoxin (0.1-0.5 microM) resulted in decrease in amplitude of depolarizing responses evoked by weak currents, increase in slope of plot, reflecting relationship between response amplitude and membrane potential, and hyperpolarizing shift of reversal potential. Blocking++ of synaptic transmission with low calcium solution did not produce such changes. These results suggest that GABA depolarizing responses have a potential-sensitive component due to activation of sodium channels.

[Prolonged posttetanic potentiation in the neurons of cultured hippocampal tissue]

Long-lasting post-tetanic potentiation (LLPTP) of field potentials that lasted 1/2--4 hours was seen in vitro in hippocampal explants of 9--14-day-old mice on days 4--7 within CA1 and CA3 fields after tetanization of Schaffer's collaterals and mossy fibers. It is assumed that LLPTP is an intrinsic functional property of the hippocamp and might occur whatever afferent effects on the part of other brain structures.

["Inhibition of inhibition" in neurons of the cerebral cortex]

Light flashes and single electrical stimuli applied to the nucleus ventro-posterolateralis and to the LGB evoked protracted IPSPs in visual and sensorimotor cortical neurons respectively. High-frequency stimulation of the midbrain RF suppressed these IPSPs which was manifested in a shortening of the inhibitory pause and in a decrease in the amplitude of the IPSPs up to their complete elimination (disinhibition). Activation of noradrenergic pathways by stimuli applied to the locus coeruleus or to the lateral hypothalamus led to the same result. The suppression of cortical IPSPs followed by facilitation of cortical unit firing in response to thalamic or peripheral stimulation seems to be an imprortant mechanism underlying arousal.

[Antidromal action potentials as a manifestation of trace excitation]

The possibility of occurance of action potentials (AP) in neuronal terminals after preliminary excitation of a cortical area was studied in unanaesthetized, non-immobilized rabbits. It was found that a low-frequency subconvulsive stimulation of the sensorimotor cortex may lead to a spontaneous appearance of AP groups that spread antidromically toward the soma of callosal neurones in the opposite hemisphere. Groups of spontaneous antidromic AP were recorded in the cortex after the disappearance of convulsive activity caused by application of penicillin. It is assumed that the observed phenomena reflect a prolonged retention of increased excitability in the terminals of cortical neurones after the end of electrical stimulation or penicillin action.

[Development of action potentials spreading antidromally from an acute epileptogenic focus through the corpus callosum]

The activity of callosal neurones located in the sensorimotor cortical area was studied in unanaesthetized rabbits. An epileptogenic focus was produced by application of penicillin or strychnine in the homotopic region of the contralateral hemisphere. During hyperpolarization of the membrane, corresponding to the appearance of paraxysmal discharges in the homotopic area, some of the neurones exhibited a group of antidromic action potentials (APs) in their soma with a 70-80 msec latency. Analysis of the APs form and of interspike intervals between the tested APs and antidromic APs evoked by stimulation, allowed to identify spontaneous antidromic APs in the extracellular records of callosal neurones as well. The data obtained show that under the influence of epileptogenic agents in the axon terminals located in the focus of the epileptic activity APs may appear, spreading antidromally through the corpus callosum to the soma of the callosal neurones.

[Identification and properties of callosal neurons of the sensomotor region of the rabbit cerebral cortex]

Antidromic unit responses to the stimulation of the homotopic area of the contralateral hemisphere were specified in the cortical sensorimotor area of the unanaesthetized rabbit. The response latencies and their variability, the maximal reproduced stimulation frequencies, stability of the response, presence of certain contesting relationships between the background activity and the tested action potentials are as a whole reliable criteria for identifying extracellularly recorded callosal neurons. The criteria of certain relationships between the background and evoked action potentials allowed to disclose callosal neurons with slow conduction.

Reticular suppression of flash-evoked IPSPs in visual cortex neurons

In a majority of visual cortex (VC) neurons recorded intracellularly in chronically implanted rabbits, light flashes evoked responses consisting of protracted IPSPs (duration 160-200 msec) followed in some cases by a rebound. Inhibition of spontaneous activity through high frequency reticular stimulation (MRF) or through presentations of non-visual arousing stimuli, was not associated with strong hyperpolarization of the cell membrane. Reticular stimulation evoking both activation and inhibition of spontaneous neuronal activity elicited attenuation or complete elimination elimination (disinhibition) of flash-evoked protracted IPSPs and of the postinhibitory rebounds. Rhythmic neuronal discharges elicited by stimulation of the visual pathway was also reduced during reticular activation, due to attenuation of the IPSP-rebound sequence.