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Biomedical subjects

I Mody

Publications and source records attributed to I Mody.

At least 55 records · Page 3Linked to original sources
PubMed1996

Zinc-induced collapse of augmented inhibition by GABA in a temporal lobe epilepsy model.

In the kindling model of temporal lobe epilepsy, several physiological indicators of inhibition by gamma-aminobutyric acid (GABA) in the hippocampal dentate gyrus are consistent with an augmented, rather than a diminished, inhibition. In brain slices obtained from epileptic (kindled) rats, the excitatory drive onto inhibitory interneurons was increased and was paralleled by a reduction in the presynaptic autoinhibition of GABA release. This augmented inhibition was sensitive to zinc most likely after a molecular reorganization of GABAA receptor subunits. Consequently, during seizures, inhibition by GABA may be diminished by the zinc released from aberrantly sprouted mossy fiber terminals of granule cells, which are found in many experimental models of epilepsy and in human temporal lobe epilepsy.

2-Amino-5-phosphonovalerate↗
PubMed1996

The effects of raising intracellular calcium on synaptic GABAA receptor-channels.

The effects of various calcium (Ca2+) loads imposed through whole-cell patch electrodes on dentate gyrus granule cells were investigated on synaptic GABAA receptor-channels. The kinetics of spontaneous inhibitory postsynaptic currents (sIPSCs) were similar when recorded without any exogenous Ca2+ buffers in the patch electrode or with up to 30 mM BAPTA in the pipette. Unbuffered Ca2+ concentrations of 20-100 microM in the patch pipettes induced a gradual prolongation of miniature IPSC (mIPSC) decays over the course of the recording (10-40 min) with no apparent change in their rise times, peak amplitudes, or frequency of occurrence. This effect was not mimicked by other divalent cations such as strontium. Infusion into the cells of free ionic Ca2+ concentrations buffered with various affinity chelators in the pipette had more pronounced effects on synaptic GABAA currents. Free ionic Ca2+ buffered in the range of 200-400 nM with BAPTA prolonged the decay time constant of mIPSCs. Introducing buffered Ca2+ into the neurons in excess of 1 microM, with a relatively low affinity buffer such as Br2BAPTA, resulted in a marked inhibition of mIPSCs. A similar effect was observed following release of Ca2+ from intracellular stores induced by caffeine (10 mM). We conclude that Ca2+ has a biphasic effect on synaptic GABAA receptor-channels. A high affinity potentiation, consistent with a prolongation of channel burst duration, and a low affinity depression of channel activity both contribute to a complex regulation of synaptic GABAA receptors by [Ca2+]i that has a profound bearing on cellular mechanisms of plasticity and pathological alterations in neuronal excitability.

Animals↗
PubMed1995

GABAergic inhibition of granule cells and hilar neuronal synchrony following ischemia-induced hilar neuronal loss.

In the dentate gyrus, granule cells are ischemia-resistant, but at least five types of predominantly spiny hilar neurons are extremely vulnerable to ischemia. Many of the ischemia-sensitive subtypes of hilar neurons appear to be involved in: (i) the regulation of GABAergic inhibition in the dentate gyrus, and (ii) the generation of hilar neuronal synchrony. The present study examined functional consequences of ischemia-induced hilar neuronal loss on GABAergic inhibition of granule cells and hilar neuronal synchrony. Transient (15 min) forebrain ischemia was induced by a modification of the four-vessel-occlusion method producing a substantial hilar neuronal loss as demonstrated by the Gallyas silver stain method. Three months later, we have examined spontaneous and stimulus-evoked inhibitory postsynaptic currents mediated by both GABA(A) and GABA(B) receptors, and inhibitory bursts induced by 4-aminopyridine (50 microM) using whole-cell recordings in coronal brain slices maintained at 34-36 degree C in the presence of excitatory amino acid receptor blockers. Spontaneous dentate spikes reflecting hilar neuronal synchrony and synaptic responses evoked by perforant path stimulation were also recorded in vivo to assess synchrony and inhibition in the dentate gyrus. In spite of significant damage to several types of hilar neurons, there were no marked differences in the conductance, kinetics, and 4-aminopyridine-induced burst frequencies of synaptic GABA(A) and GABA(B) responses in granule cells. Furthermore, both paired-pulse inhibition and dentate spikes appeared to be normal in vivo. We conclude that there appears to be little impairment of GABAergic inhibition of granule cells or of hilar neuronal synchrony three months following a massive ischemic damage to spiny hilar neurons.

4-Aminopyridine↗
PubMed1995

Tonic inhibition originates from synapses close to the soma.

Central neurons are subject to a tonic barrage of randomly occurring spontaneous inhibitory events (mIP-SCs) resulting from the action potential-independent release of gamma-aminobutyric acid (GABA). Do the terminals making synapses onto somatic versus dendritic sites, which arise from specific populations of interneurons, differ in their ability to generate mIPSCs? We have combined the techniques of whole-cell patch-clamp recording and computational simulation to demonstrate that in granule cells of the dentate gyrus, most of the action potential-independent inhibition taking place as mIPSCs originates from proximal sites. Indeed, removal of the bulk (> 50%) of the dendritic tree did not change the characteristics of mIPSCs. These results are consistent with a functional segregation of GABAergic terminals synapsing at proximal versus distal portions of central neurons. Thus, proximal GABAergic terminals are responsible for tonic inhibition targeted at the soma.

Action Potentials↗
PubMed1995

NMDA receptor-dependent excitotoxicity: the role of intracellular Ca2+ release.

Massive activation of glutamate receptors can result in excessive rises in cytoplasmic Ca2+ that are thought to underlie the fundamental processes ultimately leading to neuronal death. Preventing such cellular Ca2+ rises in the brain may reduce considerably the neuronal damage produced by stroke, head trauma, or epilepsy. Activation of NMDA receptors is instrumental in this type of neurotoxicity. Recent findings, discussed here by Istvan Mody and John MacDonald, indicate that a large proportion of the neurotoxic Ca2+ that enters nerve cells following NMDA receptor activation originates from an intracellular Ca2+ pool. The release of Ca2+ from this pool is sensitive to the skeletal muscle relaxant dantrolene, and this may constitute a novel and alternative therapeutic approach against NMDA receptor-mediated excitotoxicity.

Animals↗
PubMed1995

Ca(2+)-dependent plasticity of miniature inhibitory postsynaptic currents after amputation of dendrites in central neurons.

1. The effects of cutting off the bulk (> 2/3) of the dendritic tree (dendrotomy) on GABAergic miniature inhibitory postsynaptic currents (mIPCSs) were studied in granule cells of the adult rat dentate gyrus in 400-microns-thick slices in vitro. 2. After dendrotomy carried out in warm (32 degrees C) control artificial cerebrospinal fluid (ACSF), only small antidromic population spikes could be evoked in the granule cell layer, and no viable whole cell recordings could be obtained. However, when dendrotomy was performed in cold (8-10 degrees C) control ACSF, the amplitude of the antidromic population spikes increased, and stable whole cell recordings became possible. 3. Whole cell recordings, with CsCl-filled pipettes, from granule cells dendrotomized in cold control ACSF, revealed significant alterations, lasting > 10 h, in the decay kinetics of mIPSACs. The change consisted of a calcium-dependent transformation of the normal, single exponential decay into a prolonged double exponential that effectively increased the charge transferred by the synaptic events (the total area of the currents) by 67%. When 30 mM 1,2 bis-(2-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) was included in the pipette, the changes in the mIPSCs decay kinetics could still be observed after dendrotomy, indicating that the maintenance phase of this plasticity did not depend on elevated intracellular calcium levels. 4. Viable whole cell recordings could also be obtained in dendrotomized granule cells when the amputation of dendrites was carried out at 32 degrees C after incubation for 2 h with the cell-permeant Ca2+ chelator, BAPTA-AM (50 microM), or the cutting process was done in an ACSF containing either a combination of excitatory amino acid receptor antagonists 2-amino-5-phosphonovaleric acid (APV; 25 microM) + 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM), a blocker of intracellular Ca2+ release dantrolene-Na (20 microM), or the voltage-gated Na+ channel blocker tetrodotoxin (TTX; 1 microM). 5. After dendrotomy in BAPTA-AM, APV + CNQX, APV + CNQX + TTX, and/or dantrolene, the changes in decay kinetics were prevented, indicating that a rise in intracellular Ca2+ concentration plays a pivotal role in this plasticity. 6. Computer simulations of mIPSCs suggested that changes in single channel kinetics alone can, in principle, account for the Ca(2+)-dependent changes in mIPSC decay kinetics. 7. These findings are consistent with a lasting Ca(2+)-dependent increase in gamma-aminobutyric acid-A (GABAA) receptor function in cells that survive physical injury to their dendrites.

Animals↗
PubMed1994

Rapid turnover rate of the hippocampal synaptic NMDA-R1 receptor subunits.

The decrease in the number of synaptic NMDA receptors produced by prior i.c.v. injections of antisense oligonucleotides was examined in the CA1 stratum radiatum of hippocampal slices. Isolated NMDA and AMPA receptor-mediated components of extracellular synaptic responses were compared in slices obtained from control animals and from animals treated for 48 h with an 18-mer phosphorothioate antisense oligodeoxynucleotide directed against NMDA-R1. In slices ipsilateral to the injected hemisphere, the synaptic NMDA component was decreased by 29% compared with the contralateral side and by 34% compared with animals injected with vehicle alone. At the same time, synaptic AMPA receptor-mediated components were not affected. These results are consistent with a rapid turnover rate of synaptic NMDA-R1 and demonstrate the feasibility of antisense oligonucleotide techniques for the depletion of specific synaptic receptor channels.

Animals↗
PubMed1994

Lasting potentiation of inhibition is associated with an increased number of gamma-aminobutyric acid type A receptors activated during miniature inhibitory postsynaptic currents.

Whole-cell patch-clamp recordings unveiled a substantial increase in the amplitude, but no change in the frequency, of miniature inhibitory postsynaptic currents (mIPSCs) in dentate gyrus granule cells following chronic epilepsy induced by kindling. This novel and persistent enhancement of gamma-aminobutyric acid type A (GABAA) receptor-mediated inhibition lasted for at least 48 hr following its induction. Nearly a doubling of the number of activated functional postsynaptic GABAA receptor channels during mIPSCs without any change in single-channel conductance or kinetics could be demonstrated using nonstationary fluctuation analysis. As postsynaptic GABAA receptors are likely to be pharmacologically saturated by the transmitter concentration in the cleft, incrementing the number of functional receptor channels may be the most effective means to augment inhibition in the mammalian brain.

Animals↗
PubMed1994

Regulation of NMDA channel function by endogenous Ca(2+)-dependent phosphatase.

Protein kinases modulate the activity of several ligand-gated ion channels, including the NMDA (N-methyl-D-aspartate) subtype of glutamate receptor. Although phosphorylation and dephosphorylation of glutamate receptors may participate in several lasting physiological and pathological alterations of neuronal excitability, the physiological control of this cycle for NMDA channels has not yet been established. Using cell-attached recordings in acutely dissociated adult rat dentate gyrus granule cells, we now demonstrate that inhibitors of an endogenous serine/threonine phosphatase prolong the duration of single NMDA channel openings, bursts, clusters and superclusters. Okadaic acid, a non-selective phosphatase inhibitor, prolongs channel openings only at a concentration that inhibits the Ca2+/calmodulin-dependent phosphatase 2B (calcineurin), and is ineffective when Ca2+ entry through NMDA channels is prevented. Furthermore, FK506, an inhibitor of calcineurin, mimics the effects of okadaic acid. Thus in adult neurons, calcineurin, activated by calcium entry through native NMDA channels, shortens the duration of channel openings. Simulated synaptic currents were enhanced after phosphatase inhibition, which is consistent with the importance of phosphorylation of the NMDA-receptor complex in the short- and long-term control of neuronal excitability.

Animals↗
PubMed1994

Bridging the cleft at GABA synapses in the brain.

A fragile balance between excitation and inhibition maintains the normal functioning of the CNS. The dominant inhibitory neurotransmitter of the mammalian brain is GABA, which acts mainly through GABAA and GABAB receptors. Small changes in GABA-mediated inhibition can alter neuronal excitability profoundly and, therefore, a wide range of compounds that clearly modify GABAA-receptor function are used clinically as anesthetics or for the treatment of various nervous system disorders. Recent findings have started to unravel the operation of central GABA synapses where inhibitory events appear to result from the synchronous opening of only tens of GABAA receptors activated by a saturating concentration of GABA. Such properties of GABA synapses impose certain constraints on the physiological and pharmacological modulation of inhibition in the brain.

Hippocampus↗
PubMed1994

Kindling increases N-methyl-D-aspartate potency at single N-methyl-D-aspartate channels in dentate gyrus granule cells.

Dose-response studies of N-methyl-D-aspartate channel openings were carried out using cell-attached patches in dentate gyrus granule cells acutely isolated from control and kindled rats. The tips of the patch electrodes were first filled with regular extracellular solution, followed by backfilling through the shank with the agonist containing solution. As the two solutions joined, the agonist (N-methyl-D-aspartate, 25 microM) steadily diffused to the cell membrane, and the concentration gradually built up resulting in the progressive increase in the opening probability of N-methyl-D-aspartate channels. The reliability of this cell-attached diffusional drug delivery method was tested by determining the concentration dependence of competitive antagonism of N-methyl-D-aspartate induced channel activity by D(-)-2-amino-5-phosphonopentanoic acid. The Ki for D(-)-2-amino-5-phosphonopentanoic acid in the presence of 25 microM N-methyl-D-aspartate was found to be 6.8 microM. Twenty-four hours following the last seizure, N-methyl-D-aspartate channels on kindled neurons were consistently activated by lower N-methyl-D-aspartate concentrations than channels on control granule cells, indicating a higher potency of agonist at epileptic N-methyl-D-aspartate channels. The higher potency of the agonist is most likely a reflection of the long-term alterations in the modulation of N-methyl-D-aspartate receptor function in epileptic neurons.

Animals↗
PubMed1994

Noise analysis of miniature IPSCs in adult rat brain slices: properties and modulation of synaptic GABAA receptor channels.

1. The properties of synaptic gamma-aminobutyric acid (GABA)A receptor channels were resolved by using tight-seal, whole-cell recordings from granule cells of the dentate gyrus in adult rat hippocampal slices and by applying the technique of nonstationary noise analysis to study miniature inhibitory postsynaptic currents (mIPSCs) recorded in the presence of tetrodotoxin (TTX), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and D-2-amino-5-phosphonovaleric acid (D-AP5). This technique allowed us to extract information about the conductance, the number, and the kinetics of ligand gated channels underlying elementary synaptic currents. 2. To ascertain the validity of the nonstationary noise analysis method we have first tested it on computer simulated mIPSCs with different channel activation, lifetime kinetics, and opening probabilities. Using intraburst mean open times, shorter than the time to the first opening following activation, caused a large variance at the peak due to the stochastic channel properties. This resulted in a skewed mean current-variance relationship, which precluded proper estimation of unit conductance and especially the number of channels open at the peak of mIPSCs. Regardless of the probability of channel opening, accurate estimates of the unit conductance and the number of channels underlying each simulated mIPSC were obtained when channels had mean open times longer than the time to first opening. 3. Once the validity of the nonstationary analysis had been ascertained, it was used on mIPSCs recorded at 35 degrees C. The unit conductance of the synaptic GABAA channels was 28 +/- 1 (SE) pS and the average number of channels underlying mIPSCs was 46 +/- 4. The mean current-variance relationship was not skewed at higher amplitudes, suggesting that the intrinsic variance at the peak of the GABAA mIPSCs is low and that the open time of the channels is longer than the time to first opening. The estimated unit conductance of the channels was constant over a wide range of holding potentials. 4. The amplitude distribution of mIPSCs with rapid 10-90% rise times (290 +/- 20 microseconds) was clearly skewed towards low values. This skew was not due to filtering of electrotonically distant currents. Current-variance analysis revealed that the skewness resulted from differences in the number of GABAA receptor channels and not from the heterogeneity of unitary conductances at various synapses. Selection of mIPSCs with slower rise times yielded smaller unit conductance estimates.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

Patch-clamp recordings reveal powerful GABAergic inhibition in dentate hilar neurons.

Whole-cell and cell-attached patch-clamp recordings were used to investigate the nature of GABAA receptor-mediated inhibition in the adult rat dentate gyrus in standard 400-microns-thick horizontal slices. In the presence of the glutamate receptor antagonists D-2-amino-5-phosphonovaleric acid and 6-cyano-7-nitroquinoxaline-2,3-dione, whole-cell voltage-clamp experiments with chloride-filled electrodes ([Cl-]in = [Cl-]out) revealed a high degree of spontaneous activity (10-60 Hz) in all hilar neurons (HNs) recorded with access resistances lower than 20 M omega. The events were inward at negative holding potentials, reversed at around the Cl- equilibrium potential, and were completely abolished by the specific antagonists of the GABAA receptor channel picrotoxin and SR-95531 in a reversible manner, indicating that they were spontaneous inhibitory postsynaptic currents (sIPSCs) mediated by GABAA receptors. The majority of the sIPSCs were TTX-insensitive miniature currents resulting from the action potential-independent release of GABA. The 10-90% rise times and the monoexponential decay time constants of the sIPSCs were significantly longer in HNs than those found in neighboring granule cells (GCs). Furthermore, the decay time constant of the hilar sIPSCs was not voltage dependent, contrary to the voltage dependency of the decay time constant of the sIPSCs recorded from GCs. As HNs have longer electrotonic length than GCs do, dendritic filtering may contribute to the kinetic differences. Non-stationary fluctuation analysis showed that whereas the number of channels open at the peak of individual sIPSCs was similar, the single-channel conductances significantly differed between the two cell groups. The 21% smaller single-channel conductance and the existence of electrotonically close GABAergic synapses on HNs indicate that dendritic filtering alone cannot explain the differences between HNs and GCs. The distinct subunit composition of the GABAA receptor channels in HNs and GCs may also be responsible for the altered kinetics of IPSCs in HNs. However, the subunit specific benzodiazepine agonist zolpidem (3 microns) prolonged the monoexponential decay time constants in both HNs and GCs. Thus, differences between the GABAA receptors of the two cell types are not due to a simple all-or-none presence/absence of the alpha 5 subunit.(ABSTRACT TRUNCATED AT 400 WORDS)

2-Amino-5-phosphonovalerate↗
PubMed1993

The molecular basis of kindling.

Kindling is an experimental model of epilepsy that involves activity-dependent changes in neuronal structure and function. During kindling, pathological changes may occur at several organizational levels of the nervous system, from alterations in gene-expression in individual neurons to the loss of specific neuronal populations and rearrangement of synaptic connectivity resulting from sustained stimulation of major excitatory pathways. This review summarizes recent developments in alterations at single neuronal and molecular levels that may be responsible for kindling epileptogenesis.

Animals↗
PubMed1993

Characterization of synaptically elicited GABAB responses using patch-clamp recordings in rat hippocampal slices.

1. Tight-seal, whole-cell voltage clamp recording techniques were used to characterize monosynaptically evoked GABAB currents in adult rat brain slices maintained at 34-35 degrees C. Responses were recorded from granule cells of the dentate gyrus following the blockade of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-, D-2-amino-5-phosphonovaleric acid (D-AP5)- and picrotoxin-sensitive fast synaptic transmission, so that the remaining synaptic currents could be studied in isolation. 2. Under these conditions, stimulation in the molecular layer elicited a slow outward current which was blocked by the selective GABAB antagonist CGP 35348 in a concentration-dependent manner (200-800 microM). This current was absent in recordings made with pipettes containing 10-15 mM of the lidocaine derivative QX-314 or when caesium was substituted for K+. 3. Increasing the [K+]o e-fold (from 2.5 to 6.8 mM) shifted the reversal potential of the GABAB current from -97.9 to -73.2 mV, as predicted by the Nernst equation. Peak conductance was constant, but in 6.8 mM [K+]o at voltages hyperpolarized to EK (equilibrium potential for potassium), a small outward rectification was evident. 4. The time course of the current could be described by fourth-power exponential activation kinetics with double exponential inactivation. At 34-35 degrees C, the average activation time constant (tau m) was 45.2 ms, while the two inactivation time constants (tau h1 and tau h2) were 110.2 and 516.2 ms, with corresponding weighting factors (wh1 and wh2) of 0.84 and 0.16, respectively. The Q10 (temperature coefficient) values for these time constants were between 1.82 and 2.31. Neither tau m, nor tau h1 and tau h2 were voltage dependent in the range from -45 to -95 mV. 5. Paired-pulse depression of the GABAB current was studied by giving identical conditioning and test stimuli over a wide range (50-5000 ms) of interstimulus intervals (ISIs). The maximal depression (48%) occurred at 200 ms ISI, and the depression lasted for over 5 s. The magnitude of paired-pulse depression was not dependent on the postsynaptic membrane potential. 6. Application of the competitive antagonist CGP 35348 such that the peak current was diminished by approximately 50% had no effect on the activation or inactivation kinetics of the current. Similarly, during paired-pulse depression the kinetics of test currents were identical to those of conditioning currents. These findings support the hypothesis that the mechanism responsible for paired-pulse depression involves a reduction in neurotransmitter release without postsynaptic alterations in K+ channel activation/inactivation kinetics.(ABSTRACT TRUNCATED AT 400 WORDS)

Algorithms↗
PubMed1993

Differential ontogenesis of presynaptic and postsynaptic GABAB inhibition in rat somatosensory cortex.

1. The postnatal maturation of gamma-aminobutyric acid (GABA)B receptor-mediated presynaptic inhibition was studied in brain slices of rat somatosensory cortex maintained in vitro. Patchclamp techniques were used to record whole-cell inhibitory post-synaptic currents from layer II-III neurons in animals from postnatal days (P) 7-24. Monosynaptic inhibitory postsynaptic currents (IPSCs) were evoked after N-methyl-D-aspartate (NMDA) and non-NMDA type glutamate receptors had been blocked by D-amino-phosphonovaleric acid (D-AP5, 20 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM), respectively. These IPSCs were solely mediated by postsynaptic GABAA receptors because they were abolished by bicuculline (10 microM), reversed polarity near the chloride equilibrium potential, and were recorded with electrodes that contained Cs+ to block postsynaptic GABAB responses. 2. When pairs of stimuli separated by intervals of 0.1-10 s were used to evoke IPSCs, the second response was depressed, an effect that was maximal at 300 ms. Evoked IPSCs were also depressed by baclofen (10 microM). The paired pulse depression (PPD) of monosynaptic IPSCs was decreased or eliminated by 2-OH-saclofen (200 microM). These findings indicate that PPD of monosynaptic IPSCs was due to presynaptic GABAB receptor-mediated inhibition of GABA release. 3. There were no significant differences in the amounts of PPD in neurons from different age groups (P7-10, P12-17, P22-24) at any interstimulus interval tested (0.1-10 s).(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗