Biomedical subjects

I Mody

Publications and source records attributed to I Mody.

At least 91 records · Page 5Linked to original sources

PubMed1990

Kindling-induced epilepsy alters calcium currents in granule cells of rat hippocampal slices.

Single electrode voltage-clamp recordings were obtained from dentate gyrus granule cells (GCs) in hippocampal slices of control and commissurally kindled rats. Two types of calcium currents, a transient and a sustained current, were studied in control and kindled neurons. The threshold of the transient calcium current was lowered in kindled GCs. The sustained calcium current was absent in kindled neurons but it could be restored by the intracellular administration of the calcium chelator EGTA. Our findings are consistent with the hypothesis that the loss of an intraneuronal calcium binding protein (Calbindin-D28K; CaBP) reduces the intraneuronal calcium buffering capacity in kindled neurons and results in the enhanced calcium-dependent inactivation of sustained calcium currents.

Animals↗

PubMed1990

Differential activation of glutamate receptors by spontaneously released transmitter in slices of neocortex.

Whole-cell recordings were made from neurons in neocortical brain slices in order to characterize excitatory synaptic currents mediated by glutamate receptors. Glutamate receptor antagonists, D-aminophosphonovalerate (D-APV) and CNQX, selectively attenuated distinct components in evoked synaptic currents, and were used to differentiate spontaneous synaptic currents mediated by N-methyl-D-aspartate (NMDA) and non-NMDA receptors. Spontaneous excitatory synaptic currents were independent of action potentials, varied linearly with voltage, and were blocked by the non-NMDA receptor antagonist CNQX. An NMDA receptor-mediated component was not apparent in these spontaneous synaptic currents, however, when magnesium was omitted from the recording medium, fluctuations in current and sustained inward current became apparent, and these were blocked by the NMDA receptor antagonist D-APV. Based on these findings, we conclude that NMDA and non-NMDA receptors are activated differentially by transmitter released independently of action potentials.

Animals↗

PubMed1990

The PCP site of the NMDA receptor complex.

Evidence from electropharmacological experimentation favors the hypothesis that the PCP site is intimately associated with the channel domain of the NMDA receptor. But it is too early to state that this site lies deep within the NMDA channel pore. Determining the molecular details of the PCP site will require a complete and detailed kinetic analysis of NMDA single channel behavior. Furthermore, it is likely that hydrophobic receptor site(s) are responsible for some aspects of the blockade by at least some members of the dissociative anaesthetic family.

Anesthetics, Dissociative↗

PubMed1989

A method for isolating and patch-clamping single mammalian taste receptor cells.

Individual taste receptor cells were isolated from the tongue of the mouse by enzymatic treatment followed by mechanical dissociation. The cells were morphologically identical with taste cells from amphibians. Whole-cell voltage-clamp recordings indicated that the murine taste cells possess a variety of voltage-dependent inward and outward currents. Delayed rectifier currents were blocked by denatonium benzoate, one of the most bitter compounds known. This preparation should permit a detailed electrophysiologcal investigation of taste transduction in mammals at the level of taste receptor cells.

Animals↗

PubMed1989

Dantrolene-Na (Dantrium) blocks induction of long-term potentiation in hippocampal slices.

Long-term potentiation (LTP) is characterized by a long lasting increase in the efficacy of neurotransmission which may consist of two phases. First an induction phase, with an absolute requirement for post-synaptic activation. Second, a maintenance phase, possibly involving pre-synaptic mechanisms. An essential function for calcium ions in the induction of LTP has been established and a particular emphasis has been placed on the role of N-methyl-D-aspartate (NMDA) receptor activation in gating a postsynaptic influx of calcium. We now report that pharmacological blockade of intraneuronal calcium release with 20 microM dantrolene-sodium (dantrium) completely blocks the induction of LTP in the CA1 region of the rat hippocampal slice. This drug inhibits calcium release from the sarcoplasmic reticulum and also diminishes the rise in intraneuronal calcium ion concentrations elicited by NMDA receptor activation in cultured CA1 pyramidal cells. Dantrolene does not block NMDA gated membrane currents or voltage activated Ca2+ currents in these cells. We suggest that release of intraneuronal calcium, rather than calcium influx may be the critical post-synaptic feature underlying LTP induction. We do not however exclude a pre-synaptic involvement in the specificity and/or maintenance of long-term potentiation.

Action Potentials↗

PubMed1989

Down-regulation of norepinephrine sensitivity after induction of long-term neuronal plasticity (kindling) in the rat dentate gyrus.

Actions of norepinephrine (NE) in the dentate gyrus were examined before and after kindling-induced epilepsy, neuronal plasticity produced by daily high-frequency stimulation. NE, acting on beta 1-receptors, depolarized granule cells, increased input resistance, firing and influx of Ca2+ in response to repetitive stimulation, and elicited long-lasting potentiation of synaptic potentials. In addition, NE acting via alpha 1-receptors, attenuated Ca2+-dependent regenerative potentials. After kindling-induced plasticity, there were marked reductions in all these effects of NE on granule cells, changes likely to influence kindling-induced seizures, protecting against further enhancement of excitability once plasticity is in place.

Action Potentials↗

PubMed1989

Whole-cell voltage-clamp recordings in granule cells acutely isolated from hippocampal slices of adult or aged rats.

Whole-cell voltage-clamp recordings were undertaken in granule cells acutely dissociated by a simple enzymatic procedure from the dentate gyri of hippocampal slices obtained from adult or aged rats. This dissociation procedure also allowed the concomitant isolation of other neuron types from diverse hippocampal subfields for the study of ionic currents through voltage- and neurotransmitter-gated channels. For example, in isolated granule cells of the dentate gyrus both transient and sustained Ca2+ currents could be observed in the presence of extracellular tetrodotoxin (TTX) and intracellular Cs+. In addition, ionic currents mediated by activation of excitatory amino acid receptors of the N-methyl-D-aspartate (NMDA) type were present. A technique for the rapid dissociation of neurons from brain slices of adult or aged rats is described in detail.

Aging↗

PubMed1989

A role for N-methyl-D-aspartate receptors in norepinephrine-induced long-lasting potentiation in the dentate gyrus.

Mechanisms of action of norepinephrine (NE) on dentate gyrus granule cells were studied in rat hippocampal slices using extra- and intracellular recordings and measurements of stimulus and amino acid-induced changes in extracellular Ca2+ and K+ concentration. Bath application of NE (10-50 microM) induced long-lasting potentiation of perforant path evoked potentials, and markedly enhanced high-frequency stimulus-induced Ca2+ influx and K+ efflux, actions blocked by beta-receptor antagonists and mimicked by beta agonists. Enhanced Ca2+ influx was primarily postsynaptic, since presynaptic delta [Ca2+]o in the stratum moleculare synaptic field was not altered by NE. Interestingly, the potentiation of both ionic fluxes and evoked population potentials were antagonized by the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate (APV). Furthermore, NE selectively enhanced the delta [Ca2+]o delta [K+]o and extracellular slow negative field potentials elicited by iontophoretically applied NMDA, but not those induced by the excitatory amino acid quisqualate. These results suggest that granule cell influx of Ca2+ through NMDA ionophores is enhanced by NE via beta-receptor activation. In intracellular recordings, NE depolarized granule cells (4.8 +/- 1.1 mV), and increased input resistance (RN) by 34 +/- 6.5%. These actions were also blocked by either the beta-antagonist propranolol or specific beta 1-blocker metoprolol. Moreover, the depolarization and RN increase persisted for long periods (93 +/- 12 min) after NE washout. In contrast, while NE, in the presence of APV, still depolarized granule cells and increased RN, APV made these actions quickly reversible upon NE washout (16 +/- 9 min). This suggested that NE induction of long-term, but not short-term, plasticity in the dentate gyrus requires NMDA receptor activation. NE may be enhancing granule cell firing by some combination of blockade on the late Ca2+-activated K+ conductance and depolarization of granule cells, both actions that can bring granule cells into a voltage range where NMDA receptors are more easily activated. Furthermore, NE also elicited activity-independent long-lasting depolarization and RN increases, which required functional NMDA receptors to persist.

2-Amino-5-phosphonovalerate↗

PubMed1989

The regulation of NMDA receptors in the central nervous system.

1. Excitatory amino acids are major transmitters within the CNS. 2. Postsynaptic receptors for these transmitters, particularly those of the NMDA subtype are important in a variety of physiological and pathological processes. Therefore psychoactive drugs and potentially therapeutic drugs may target these receptors. 3. NMDA receptors can be regulated by competitive antagonists or by dissociative anaesthetics which act as use dependent blockers of the NMDA channel. 4. Alternatively drugs could modify intracellular second messenger systems that control expression of NMDA receptors.

Allosteric Regulation↗

PubMed1989

Regulation of N-methyl-D-aspartate receptors revealed by intracellular dialysis of murine neurones in culture.

1. The whole-cell patch clamp recording technique was employed to investigate the intracellular regulation of N-methyl-D-aspartate (NMDA) receptors in cultured murine hippocampal neurones. Excitatory amino acids were repeatedly applied at regular intervals during intracellular dialysis with solutions of various composition. 2. Currents evoked by L-aspartate, an agonist of NMDA receptors, gradually 'washed out' to approximately 50% of their initial amplitude during dialysis with an intracellular solution containing CsCl and EGTA as a calcium buffer. In contrast, responses to kainate did not wash out. The wash-out of L-aspartate currents followed an exponential time course with a time constant of about 150 s. Wash-out did not appear to be related to desensitization of NMDA receptors. 3. Following wash-out, L-aspartate responses were blocked by Mg2+, ketamine or D-2-amino-5-phosphonovalerate indicating that these responses were still mediated by NMDA receptors. Furthermore, responses to NMDA itself showed wash-out to the same extent and with a time course similar to that for L-aspartate responses. 4. Neither the time course nor the extent of the wash-out of responses to L-aspartate was affected when the Ca2+ concentration of the dialysate was varied from zero to 1.5 x 10(-5) M. In addition, wash-out was unaffected by substitution of BAPTA for EGTA, indicating that wash-out was not a consequence of changes in intracellular pH related to the binding of Ca2+ to the buffer or to the kinetics of this binding. Therefore, the wash-out of NMDA currents could not be attributed to a gradual elevation of the concentration of intracellular Ca2+. 5. The extent of the wash-out of L-aspartate currents was similar for cells held at +40 versus -60 mV although the rate of wash-out was slower at the depolarized potential. In addition, the reversal potential of these currents was not altered, demonstrating that a change in driving force did not account for a component of the wash-out. 6. Inclusion of an ATP regeneration solution (Forscher & Oxford, 1985) in the dialysate prevented the wash-out of L-aspartate currents. ATP alone was less effective in preventing wash-out whereas phosphocreatine and creatine phosphokinase were ineffective by themselves. Wash-out also occurred when ATP was replaced with the non-hydrolysable analogue, beta, gamma-methyleneATP, or with GTP. In cells where wash-out of L-aspartate currents had been established, subsequent dialysis with the ATP regenerating solution partially reversed this wash-out.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine Triphosphate↗

PubMed1988

Requirement of NMDA receptor/channels for intracellular high-energy phosphates and the extent of intraneuronal calcium buffering in cultured mouse hippocampal neurons.

Whole-cell patch-clamp recordings were undertaken in cultured mouse hippocampal neurons in order to investigate time-dependent changes in: (i) currents evoked by L-aspartic acid (Asp) and kainic acid (KAI), two excitatory amino acids active at N-methyl-D-aspartic acid (NMDA) and KAI receptor sites respectively, and (ii) tetrodotoxin (TTX) resistant voltage-dependent inward currents carried by Ca2+. Consistent with previous observations, Ca2+ currents gradually run down unless a support system containing Mg-ATP, phosphocreatine and creatinine phosphokinase is added to the intracellular medium. Here we report that, in addition to suppressing the rundown of currents through voltage-gated Ca2+ channels, such a support system is also necessary to prevent rundown of ionic currents through excitatory amino acid-gated channels of the NMDA type. When this support system was omitted from the recording pipette, currents induced by Asp, but not KAI, progressively declined over a period of 20 min and stabilized at values of about 50% of the initial. This progressive decline occurred regardless of the extent of intraneuronal Ca2+ buffering, indicating that it was not due to accumulation of cytosolic Ca2+. After the rundown, reversal potentials of ASP-induced currents were the same whether recorded with or without the intracellular support system and the Asp induced currents could be blocked by the specific NMDA channel blocker ketamine. We conclude that ionic currents through NMDA gated channels have two components: one requires high-energy phosphates and will run down if these are not supplied; the other requires no such supply and remains steady.

Adenosine Triphosphate↗

PubMed1988

Activation of N-methyl-D-aspartate receptors parallels changes in cellular and synaptic properties of dentate gyrus granule cells after kindling.

1. The cellular and synaptic properties of rat dentate gyrus granule cells (GCs) were examined using intra-/extracellular and Ca2+-sensitive microelectrode recordings following epilepsy induced by kindling of the hippocampal commissures or amygdala. 2. The recordings were made in hippocampal slices prepared from sham-stimulated controls and animals that have received daily stimuli to reach stage IV-V of kindling. The average number of stimulation trials (60 Hz/1 s, 100-150 microA) required to reach full motor seizures (stage V) was 23 +/- 2 for commissural kindling and 14 +/- 1 for amygdala kindling. 3. The resting membrane potential of GCs following kindling (RMP; -72 +/- 3 mV) was not significantly different from the RMP of control GCs (-70 +/- 2 mV). Similarly, action potential height and threshold were unaffected by kindling. However, kindling altered other cellular properties of GCs regardless of the site of stimulation (hippocampal commissures or amygdala), the stage of kindling reached (IV or V), or the time elapsed between the last kindling stimulus and preparation of the hippocampal slices (24 h-6 wk). The input resistance of kindled GCs (55 +/- 4 M omega) was significantly higher than that of controls (40 +/- 3 M omega). In contrast to most control GCs, the slope conductance (GS) of kindled neurons, measured with constant-amplitude current injections at various membrane potentials, generally increased at membrane potentials more negative than rest. Furthermore, other voltage-dependent ionic conductances (see below), that were not normally encountered in control GCs, were present in kindled neurons. 4. The intracellularly recorded monosynaptic excitatory postsynaptic potentials (EPSPs) of kindled GCs, evoked through the stimulation of the lateral perforant pathway, differed significantly from the EPSPs of control GCs. The amplitudes of control EPSPs increased upon hyperpolarizations and decreased following depolarizations of the membrane, as expected for conventional EPSPs without contribution from voltage-dependent conductances. In contrast, the EPSPs of kindled GCs invariably increased in amplitude and duration at membrane potentials 5-20 mV depolarized from rest, indicating the presence of a characteristic voltage-dependent component. Frequently, following the synaptically triggered action potentials, kindled GCs displayed depolarizing afterpotentials. 5. Perfusion of the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV; 30 microM) had no effect on the EPSPs of control GCs, but consistently reduced the amplitude and duration of EPSPs in kindled GCs.(ABSTRACT TRUNCATED AT 400 WORDS)

Amygdala↗

PubMed1987

Distribution of Calbindin-D28K 1 (CaBP) in the cerebral cortex and hippocampus of the epileptic (El) mouse.

The distribution of Calbindin-D28K (CaBP), a calcium-binding protein that binds Ca2+ with high affinity, was measured by radioimmunoassay in various cortical regions of the epileptic strain of mice El. The El strain, in which seizures are induced by repeated vestibular stimulation, had significantly lower levels of CaBP in the hippocampus and dorsal occipital cortical areas than the control CF-1 strain. Following induction of seizures in the El strain, a further decrease in CaBP levels was observed in the hippocampal formation and ventral temporal cortical regions, areas where paroxysmal activity is generated in this strain. Considering the role of CaBP as an intraneuronal calcium buffer, the present findings indicate that neuronal calcium regulation is genetically altered in the El strain and is further disturbed during the events that lead to induction of seizures.

Animals↗

PubMed1987

Epileptiform activity induced by lowering extracellular [Mg2+] in combined hippocampal-entorhinal cortex slices: modulation by receptors for norepinephrine and N-methyl-D-aspartate.

Reduction of extracellular Mg2+ concentration induced spontaneous and evoked epileptiform activity in the entorhinal cortex (EC) and dentate gyrus (DG) of combined hippocampus (HC)-EC slices. Extracellular field potentials, as well as changes in extracellular Ca2+ and K+ concentrations, were measured in EC and DG with ion-selective/reference electrodes during both repetitive and single stimuli. In the EC, lowering extracellular [Mg2+] induces both spontaneous and single stimulus evoked ictal events consisting of extracellular negative potential shifts (up to 5 mV, 30 sec), decreases in [Ca2+]0 and increases in [K+]0. In the DG, spontaneous events were much shorter, but similar changes in [Ca2+]0, [K+]0 and field potentials (FPs) could be evoked by brief high-frequency stimulation. In both areas, the N-methyl-D-aspartate (NMDA) receptor antagonist 2-aminophosphonovalerate (2-APV) completely blocked spontaneous as well as stimulus evoked epileptiform events. The neurotransmitter norepinephrine (NE), which has previously been shown to modulate long-term potentiation in the DG, was found to exhibit differential modulation of epileptiform activity in the EC and DG. In the EC, NE, acting via alpha 1-receptors, completely blocked low Mg2+-induced epileptiform activity. In contrast, in the DG, NE exhibited a beta-receptor mediated prolongation of the low Mg2+-induced ictal events, and enhanced the stimulus-induced ionic and field potential changes. From these results, we conclude that lowering extracellular [Mg2+], acting in large part through the removal of the Mg2+ voltage-dependent blockade of NMDA receptors, leads to induction of epileptiform activity in both the EC and DG.(ABSTRACT TRUNCATED AT 250 WORDS)

2-Amino-5-phosphonovalerate↗

PubMed1987

Low extracellular magnesium induces epileptiform activity and spreading depression in rat hippocampal slices.

The effect of low extracellular Mg2+ concentration ([Mg2+]o) on neuronal activity was studied in rat hippocampal slices. After 20-40 min of perfusion with Mg2+-free medium, when [Mg2+]o declined to approximately 0.1-0.4 mM, spontaneous field potentials developed in the CA1 and CA3 regions, but not in the dentate gyrus. In the CA3 pyramidal cell layer, these potentials consisted of repetitive (0.3-0.5 Hz), 40- to 120-ms-long positive deflections (2-5 mV) with superimposed population spikes. In the stratum (str.) pyramidale of the CA1 region, positive-negative deflections (less than 3 mV) lasting for 30-80 ms were observed, which occurred with a frequency of 0.3-0.5 Hz. In some cases, longer lasting and rapidly recurring events were also observed. In CA3 pyramidal cells, the intracellular correlates of the field potential transients were 20- to 30-mV paroxysmal depolarization shifts (PDS) with superimposed bursts of action potentials, followed by large (greater than 10 mV), 500- to 1,200-ms-long afterhyperpolarizations (AHP). In contrast, pyramidal neurons of the CA1 area did not show PDSs; instead, sequences of excitatory postsynaptic potentials (EPSPs)/inhibitory postsynaptic potentials (IPSPs) accompanied the transient field potential changes. Occasionally, spontaneous EPSPs/IPSPs, occurring with high frequencies, could also be observed in CA1 without any field potential transients. In both hippocampal regions, the epileptiform activity evolved without significant alterations in the resting membrane potential (RMP) and input resistance (RN) of the neurons, although a 2- to 5-mV reduction in action potential threshold was noted. The spontaneous activity in Mg2+-free medium was readily suppressed by raising the extracellular Ca2+ concentration ([Ca2+]o) from 1.6 to 3.6 mM. The perfusion of 10-30 microns DL-2-amino-5-phosphonovaleric acid (2-APV), an antagonist for the glutamate receptors of the N-methyl-D-aspartate (NMDA) type, also attenuated or reversibly blocked the spontaneous activity. Surgical isolation of area CA1 from CA3 ceased the occurrence of the transients in CA1 but not in CA3. The synaptic input/output curves were shifted to the left in the absence of [Mg2+]o. Threshold intensity for eliciting population spikes was 50-75% of that in normal medium. Paired-pulse facilitation was still present near threshold, but was reduced at higher stimulus intensities. Decreases in [Ca2+]o, produced by repetitive stimulation (20-Hz/5-10 s) of the Schaffer collateral/commissural pathway and monitored with ion-selective microelectrodes in the CA1 region, were enhanced in Mg2+-free medium.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1986

Laminar profiles of the changes in extracellular calcium concentration induced by repetitive stimulation and excitatory amino acids in the rat dentate gyrus.

Changes in extracellular free calcium concentration ([Ca2+]o) were measured with ion-selective microelectrodes in the dentate gyrus of the rat hippocampal slice preparation. Repetitive stimulation (20 Hz/10 s) of the perforant path or mossy fibers, or alternatively, iontophoresis of the excitatory amino acids N-methyl-D-aspartate (NMDA) or quisqualate (Quis) elicited decreases in [Ca2+]o (delta Ca2+) which were unequally distributed along the axis of the granule cells. Laminar profiles of the [Ca2+]o changes revealed marked differences between stimulus- and amino acid-induced responses. The delta Ca2+ induced by either anti- or orthodromic stimulation were relatively small (less than 0.15 mM) and were found to be maximal at the cell body layer (stratum granulosum). In contrast, the excitatory amino acids NMDA and Quis evoked large delta Ca2+ (greater than 1.2 mM) which were maximal at dendritic sites, 100 microns away from the cell body layer in the inner stratum moleculare. The effect of NMDA was reversibly blocked by 2-amino-5-phosphonovaleric acid without antagonism of the synaptic responses or the stimulus-induced changes in [Ca2+]o. Therefore, under normal conditions, NMDA receptors appear not to participate in synaptically induced delta Ca2+ in the dentate gyrus.

Amino Acids↗

PubMed1985

Levels of hippocampal calcium and zinc following kindling-induced epilepsy.

Hippocampal calcium and zinc content was determined using atomic absorption spectrophotometry in control and commissural-kindled rats. In animals exhibiting 5-10 consecutive motor seizures hippocampal calcium was slightly elevated (356.7 parts per million (ppm), dry weight) but not significantly different from controls (329.8 ppm), whereas the amount of zinc was significantly higher (101.6 ppm) than in nonstimulated animals (88.3 ppm). These results are indicative of certain pathophysiological changes in kindled hippocampi, most likely localized to the granule cells of the dentate gyrus where the bulk of hippocampal zinc is confined.

Animals↗

PubMed1984

Blockade of tetanic- and calcium-induced long-term potentiation in the hippocampal slice preparation by neuroleptics.

The effect of neuroleptics which block calmodulin was studied on two forms of long-term potentiation of field responses evoked by stratum radiatum stimulation of the CA1 region in the in vitro preparation of hippocampal slices. Tetanic stimulation or brief exposure to 4 mM Ca2+ produced a long-lasting augmentation of the extracellular excitatory postsynaptic potentials EPSP) and of the responses of the population spikes. Both forms of potentiation were inhibited by perfusion of 10 microM trifluoperazine (TFP) or pimozide, an effect which is unlikely to involve interactions with dopamine or norepinephrine receptors, but rather a potent blockade of calmodulin-mediated events. The results suggest that induction of the "permanency" of both tetanic- and calcium-induced long term potentiation requires activation of calmodulin and involves some calmodulin-mediated mechanism(s).

Animals↗