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M Dichter

Publications and source records attributed to M Dichter.

At least 19 recordsLinked to original sources
PubMed2001

Epileptic seizures may begin hours in advance of clinical onset: a report of five patients.

Mechanisms underlying seizure generation are traditionally thought to act over seconds to minutes before clinical seizure onset. We analyzed continuous 3- to 14-day intracranial EEG recordings from five patients with mesial temporal lobe epilepsy obtained during evaluation for epilepsy surgery. We found localized quantitative EEG changes identifying prolonged bursts of complex epileptiform discharges that became more prevalent 7 hr before seizures and highly localized subclinical seizure-like activity that became more frequent 2 hr prior to seizure onset. Accumulated energy increased in the 50 min before seizure onset, compared to baseline. These observations, from a small number of patients, suggest that epileptic seizures may begin as a cascade of electrophysiological events that evolve over hours and that quantitative measures of preseizure electrical activity could possibly be used to predict seizures far in advance of clinical onset.

Action Potentials↗
PubMed1996

Somatostatin receptor subtypes SSTR2 and SSTR5 couple negatively to an L-type Ca2+ current in the pituitary cell line AtT-20.

The somatostatin receptor subtypes SSTR2 and SSTR5 mediate distinct endocrine and exocrine functions of somatostatin and may also be involved in mediating the neuromodulatory actions of somatostatin in the brain. To investigate whether these receptors couple to voltage-sensitive Ca2+ channels, SSTR2 and SSTR5 selective agonists were tested for their effects on AtT-20 cells using whole cell patch clamp techniques. The SSTR2 selective agonist MK 678 inhibited Ca2+ currents in AtT-20 cells. The effects of MK 678 were reversible and blocked by pertussis toxin pretreatment, suggesting that SSTR2 couples to the L-type Ca2+ channels via G proteins. Other SSTR2-selective agonists, including BIM 23027 and NC8-12, were able to inhibit the Ca2+ currents in these cells. The SSTR5 selective agonist BIM 23052 also inhibited the Ca2+ currents in these cells and this effect was reversible and blocked by pertussis toxin treatment. The ability of SSTR5 to mediate inhibition of the Ca2+ current was greatly attenuated by pretreatment with the SSTR5-selective agonist BIM 23052, whereas SSTR2-mediated inhibition of the Ca2+ current was not altered by pretreatment with the SSTR2-selective agonist MK 678. Thus, the SSTR2 and SSTR5 couplings to the Ca2+ current are differentially regulated. The peptide L362,855, which we previously have shown to have high affinity for the cloned SSTR5, had minimal effects on Ca2+ currents in AtT-20 cells at concentrations up to 100 nM and did not alter the ability of MK 678 to inhibit Ca2+ currents. However, it completely antagonized the effects of the SSTR5-selective agonist BIM 23052 on the Ca2+ currents. L362,855 is an antagonist/partial agonist at SSTR5 since it can reduce Ca2+ currents in these cells at concentrations above 100 nM. L362,855 is also an antagonist/partial agonist at the cloned rat SSTR5 expressed in CHO cells since it is able to block the inhibition of cAMP accumulation induced by somatostatin at concentrations below 100 nM but at higher concentrations can inhibit cAMP formation itself. Structural analysis of L362,855 reveals that only a single hydroxyl group at residue seven in the peptide is needed to convert the compound from an antagonist/partial agonist to a full agonist at SSTR5. These studies reveal that two different somatostatin receptor subtypes, SSTR2 and SSTR5, can mediate the inhibition of an L-type Ca2+ channel in AtT-20 cells by somatostatin. The receptor subtype responses can be distinguished by selective agonists and antagonists and are regulated differently by agonist pretreatment. The inhibition of Ca2+ influx into endocrine cells and neurons may be a major cellular mechanism by which somatostatin modulates hormone and neurotransmitter release. Our results reveal that at least two receptor subtypes can mediate this cellular response.

Animals↗
PubMed1995

Mechanisms of neuronal plasticity as analyzed at the single cell level.

This chapter has highlighted how correlates of neuronal plasticity such as electrophysiological responsiveness and changes in gene expression may be examined in defined CNS regions as well as in single cells. The ability to simultaneously measure the mRNA levels for hundreds of different genes, to clone novel genes, and to characterize the physiology and morphology of the cell promises to provide insight into molecular mechanisms of plasticity. The importance of understanding how one gene product changes relative to another (coordinated changes) as well as subcellular distribution of mRNAs cannot be overstated. It is only through an analysis of both the molecular and cellular processes associated with plasticity that a thorough understanding of the mechanisms of neuronal plasticity can be gained.

Animals↗
PubMed1995

Pharmacological characterization of heterodimeric NMDA receptors composed of NR 1a and 2B subunits: differences with receptors formed from NR 1a and 2A.

Pharmacological and molecular biological evidence indicates the existence of multiple types of NMDA receptors within the CNS. We have characterized pharmacological properties of receptors assembled from the combination of NR 1a and NR 2B subunits (NR 1a/2B) expressed in transfected cells using both 125I-MK-801 binding assays and electrophysiological measures. Binding of 125I-MK-801 to cells transfected with NR 1a/2B is saturable with a KD of 440 pM. The binding is potently inhibited by ketamine, dextromethorphan, phencyclidine, and MK-801 and is stimulated by low concentrations of magnesium. These properties resemble those of native receptors and receptors produced by NR 1a/2A. However, 125I-MK-801 binding to membranes from cells transfected with NR 1a/2B is inhibited with high affinity by ifenprodil and is stimulated by spermidine, unlike receptors assembled from NR 1a/2A. NMDA-induced currents measured in cells transfected with either NR 1a/2A or NR 1a/2B have pharmacological properties that correlate well with the binding studies. Currents in cells transfected with NR 1a/2B are potentiated by spermidine and blocked with high affinity by ifenprodil, whereas currents in cells transfected with NR 1a/2A are not enhanced by spermidine and are weakly inhibited by ifenprodil. These data suggest that pharmacological heterogeneity in native NMDA receptors may be explained by combinations of different subunits.

Animals↗
PubMed1994

On the nature and differential distribution of mRNAs in hippocampal neurites: implications for neuronal functioning.

Neurons are highly polarized cells with a mosaic of cytoplasmic and membrane proteins differentially distributed in axons, dendrites, and somata. In Drosophila and Xenopus, mRNA localization coupled with local translation is a powerful mechanism by which regionalized domains of surface or cytoplasmic proteins are generated. In neurons, there is substantial ultrastructural evidence positing the presence of protein synthetic machinery in neuronal processes, especially at or near postsynaptic sites. There are, however, remarkably few reports of mRNAs localized to these regions. We now present direct evidence that an unexpectedly large number of mRNAs, including members of the glutamate receptor family, second messenger system, and components of the translational control apparatus, are present in individual processes of hippocampal cells in culture.

Animals↗
PubMed1991

Subtypes of brain somatostatin receptors couple to multiple cellular effector systems.

To investigate whether somatostatin (SRIF) receptor subpopulations mediate different physiological actions of SRIF, we tested the effects of SRIF and the SRIF agonists MK 678 and CGP 23996 on different biological responses in rat neocortical neurons in culture. Neocortical cells in culture express SRIF receptors that can be labeled with 125I-MK 678 and 125I-CGP 23996. Pharmacological analysis of the binding sites indicates that the radioligands label SRIF receptor subtypes with distinct pharmacological characteristics. These receptor subpopulations are similar to those expressed in adult rat brain. SRIF, MK 678, and CGP 23996 are able to inhibit forskolin-stimulated adenylate cyclase activity in rat neocortical membranes by 25-30%. Furthermore, they inhibit a high voltage-activated Ca2+ current in rat neocortical neurons in culture by 25-35%. Both SRIF and MK 678 potentiate a delayed rectifier K+ current in rat neocortical neurons in culture by 25-30%. In contrast, high concentrations of CGP 23996 do not alter the K+ current. In cells that do not respond to CGP 23996, MK 678 increases the delayed rectifier K+ current. The findings of these studies indicate that rat neocortical neurons in culture express functionally distinct SRIF receptor subtypes that can be differentially activated by SRIF agonists.

Adenylyl Cyclases↗
PubMed1990

Somatostatin-14 and somatostatin-28 inhibit calcium currents in rat neocortical neurons.

The prosomatostatin-derived peptides, somatostatin-14 and somatostatin-28, are believed to function as neurotransmitters or neuromodulators in the cerebral cortex. To investigate the molecular mechanisms by which these peptides induce their physiological effects in the cerebral cortex, we have examined the effects of somatostatin-14 and somatostatin-28 on voltage-dependent Ca2+ currents in rat neocortical neurons in culture. Ca2+ currents were recorded using whole-cell patch-clamp techniques under conditions in which K+ and Na+ currents were blocked. Ca2+ currents were induced by depolarization from the holding potential of -80 mV. Somatostatin-14 (100 nM) and somatostatin-28 (100 nM) did not significantly affect low-voltage activated Ca2+ currents, but blocked high-voltage activated Ca2+ currents and slowed the activation of this current. The effects of both peptides were concentration-dependent and reversible. Furthermore, the effects of somatostatin-14 and somatostatin-28 on the high-voltage activated Ca2+ currents were not additive, suggesting that both peptides regulate this ionic current through similar cellular mechanisms. When patch pipettes used to record the Ca2+ currents contained 100 microM cAMP and 0.5 mM isobutylmethylxanthine, a phosphodiesterase inhibitor, somatostatin-14 and somatostatin-28 still inhibited Ca2+ currents, indicating that the effects of these peptides on the Ca2+ currents were cAMP-independent. Inclusion of the non-hydrolysable guanine triphosphate analogue, guanine triphos-somatostatin-14 or somatostatin-28, suggesting the involvement of guanine nucleotide binding proteins in the actions of the peptides on the Ca2+ currents.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Modulation of the N-methyl-D-aspartate channel by extracellular H+.

The influence of external [H+] on whole-cell and single-channel currents activated by glutamate agonists was studied in rat hippocampal neurons. In the pH range between 6.6 and 8.0, changes in external [H+] had negligible influence on the amplitude and kinetics of the monovalent ion-carrying currents activated by the agonists quisqualate and kainate. The divalent ion-carrying N-methyl-D-aspartate (NMDA)-activated current, on the other hand, was strongly modulated by extracellular [H+]. Increased external [H+] suppressed, whereas decreased external [H+] enhanced, the NMDA-activated current. Changes in internal [H+] had little or no effect on the NMDA-activated current. Modulation of the NMDA-activated current resulted primarily from changes in the number of channel openings. Neither the unitary conductance nor the individual open dwell-times were significantly affected. These results suggest that the protonation site is on the external aspect of the channel and is far removed from the channel permeation pathway. Because interactions between H+, NMDA, and glycine in activating the current were predominantly noncompetitive, our results suggest that the modulatory effect of H+ was not associated with changes in receptor-agonist affinities. These results suggest that modulation of the NMDA-receptor channel by [H+] may be an intrinsic protective mechanism by which calcium influx into neurons is regulated, particularly in hypoxic/ischemic conditions.

Animals↗
PubMed1990

Lack of cross-desensitization of somatostatin-14 and somatostatin-28 receptors coupled to potassium channels in rat neocortical neurons.

The effects of somatostatin-14 (SOM-14) and somatostatin-28 (SOM-28) on the delayed rectifier K+ current (IK) in rat neocortical neurons in culture were measured by using whole-cell patch clamp techniques. SOM-14 stimulated IK in a reversible manner. Continuous application of SOM-14 to the neocortical neurons led to a gradual desensitization of the SOM-14 response. Many cells became completely densensitized to SOM-14. SOM-28 also modulated IK in neocortical cells. However, SOM-28 reduced IK. This response was also reversible. Continuous application of SOM-28 to neocortical neurons led to a desensitization of the SOM-28 inhibition of IK. Many of the neurons that responded to SOM-28 became completely refractory to the peptide following prolonged SOM-28 pretreatment. While most neocortical neurons responded either to SOM-14 or to SOM-28, a population of neurons responded to both peptides. Chronic application of SOM-14 to these neurons completely desensitized the SOM-14 stimulation of IK but did not affect SOM-28 inhibition of this potassium current. Similarly, complete desensitization of SOM-28 responses in these cells was not associated with a modification of SOM-14 stimulation of IK. The lack of cross-desensitization between SOM-14 and SOM-28 induced responses suggests that these peptides act through different receptors to regulate IK.

Animals↗
PubMed1989

Quisqualate activates a rapidly inactivating high conductance ionic channel in hippocampal neurons.

Glutamate activates a number of different receptor-channel complexes, each of which may contribute to generation of excitatory postsynaptic potentials in the mammalian central nervous system. The rapid application of the selective glutamate agonist, quisqualate, activates a large rapidly inactivating current (3 to 8 milliseconds), which is mediated by a neuronal ionic channel with high unitary conductance (35 picosiemens). The current through this channel shows pharmacologic characteristics similar to those observed for the fast excitatory postsynaptic current (EPSC); it reverses near 0 millivolts and shows no significant voltage dependence. The amplitude of the current through this channel is many times larger than that through the other non-NMDA (N-methyl-D-aspartate) channels. These results suggest that this high-conductance quisqualate-activated channel may mediate the fast EPSC in the mammalian central nervous system.

Animals↗
PubMed1989

Somatostatin-14 and somatostatin-28 induce opposite effects on potassium currents in rat neocortical neurons.

The prosomatostatin-derived peptides somatostatin-14 (Som-14) and somatostatin-28 (Som-28) are believed to act as neurotransmitters in the central nervous system. To examine possible mechanisms by which these peptides induce their physiological actions in brain, the effects of Som-14 and Som-28 on voltage-dependent K+ currents in rat cerebral cortical neurons in culture were examined by using whole-cell patch-clamp techniques. Som-14 increased a delayed rectifier K+ current (IK) in the cortical neurons, while Som-28 reduced IK in the neurons, both in a concentration-dependent manner. Som-14 and Som-28 could induce opposite changes in IK in the same neurons. Elevating intracellular cAMP in the cortical neurons did not modify the effects of Som-14 or Som-28 on IK, indicating that the peptides can regulate this ionic current through cAMP-independent mechanisms. Pretreatment of the neocortical cells with pertussis toxin, which inactivates inhibitory GTP-binding proteins, abolished both Som-14 and Som-28 modulation of IK, indicating that Som-14 and Som-28 receptors are coupled to IK via GTP-binding proteins. These studies show that Som-14 and Som-28 can induce opposite biological effects, suggesting that Som-14 and Som-28, acting through distinct receptors, may function as different neurotransmitters or neuromodulators.

Animals↗
PubMed1988

Delayed and fast transient potassium currents in rat neocortical neurons in cell culture.

Cultured rat neocortical neurons were subjected to the whole-cell mode of voltage clamping to study outward K+ currents. Tetrodotoxin and Cd2+ were applied extracellularly to block Na+ and Ca2+ currents. Depolarizing voltage commands from a holding potential of -90 mV evoked an outward current which peaked early and decayed over 20-50 ms to attain a steady level. At holding potentials more positive than -50 mV, the fast, transient component of the outward current was largely inactivated while the late, steady one remained present. The two components of the outward current displayed different pharmacological sensitivities: the fast, transient one was blocked by 4-aminopyridine, while the late, persistent one was reduced by tetraethylammonium. These currents were present in neocortical cells as early as the third day in culture. Our experiments indicate that as in sympathetic, hippocampal and spinal cord neurons, neocortical cells possess both a fast, transient, and a delayed K+ current. These currents might play an important role in controlling neocortical excitability.

4-Aminopyridine↗
PubMed1981

Concurrent acetylcholinesterase staining and gamma-aminobutyric acid uptake of cortical neurons in culture.

Gamma-aminobutyric acid (GABA) uptake and acetylcholinesterase (AChE) content were demonstrated concurrently in cortical neurons grown in tissue culture. Positive reactions either for GABA uptake or for AChE content were encountered in pyramidal and stellate, as well as spindle-shaped neurons. Neither reaction was confined to a specific morphological subtype. Nearly half the neurons were negative for either reaction. Most of the remaining neurons were positive only for GABA or only for AChE. However, a subpopulation of neurons showed not only a high AChE content, but also an avid GABA uptake. Thus, four types of neurons could be identified on the basis of these two reactions. The high AChE content in some of the cortical neurons that also showed GABA uptake indicates that there are at least two distinct types of GABAergic neurons.

Acetylcholinesterase↗
PubMed1980

Biochemical correlates of GABA function in rat cortical neurons in culture.

Serial biochemical studies of a rat cortical tissue culture system in which synapses regularly form showed that gamma-aminobutyric acid (GABA) is present in the cultures and increases with their maturation. The tissue GABA concentration in mature cultures is similar to that of adult rat cortex in vivo. The synthetic enzyme, glutamate decarboxylase, also increases with age as does high affinity GABA uptake. GABA uptake was blocked by L-2,4-diaminobutyrate (DABA) and had the properties of neuronal GABA uptake. Specific release by depolarizing media of both exogenous [3H]GABA and GABA synthesized from D-[U-14C]glucose was demonstrated. The GABA released by high potassium media had higher specific activity and a greater contribution from glucose (as compared to acetate) than GABA found in the medium in the absence of depolarization. Calcium dependency of evoked GABA release could be shown only after pretreatment of cultures with ethyleneglycol-bis-(beta-aminoethyl ether)-N,N'-tetraacetic acid or EGTA. Synaptosomes may exhibit greater calcium dependence of evoked transmitter release than intact cells in culture because their intracellular calcium stores are depleted during preparation. Glycine uptake by the cultures was much less in amount than was GABA uptake, and specific release of glycine could not be demonstrated. Specific binding of both a GABA agonist ([3H]muscimol) and an antagonist ([3H]bicuculline) was shown by membranes prepared from the cultures. By contrast, when [3H]muscimol binding to intact cells was studied, essentially all binding was sodium dependent and had the properties of GABA uptake binding. We conclude that the use of [3H]muscimol for receptor studies is valid only after the elimination of GABA uptake systems. Biochemical data from these studies support the concept that GABA is the transmitter for many cortical synapses. Glycine and taurine are not likely to be transmitters in these cortical cultures. When considered together with physiological data from the preceding paper, we have satisfied Werman's criteria (see ref. 36) for accepting GABA as the major inhibitory transmitter in the cortical culture system.

4-Aminobutyrate Transaminase↗
PubMed1980

Identification of GABA neurons in rat cortical cultures by GABA uptake autoradiography.

Autoradiographic studies of rat cortical cultures were conducted with tritiated transmitters and related drugs. Autoradiographs prepared from cultures incubated in [3H]GABA showed selective labeling: dense accumulations of silver grains over the somas and all processes of approximately 30-50% of the neuronal population, few grains over the non-neuronal cells. This labeling was blocked by diaminobutyric acid (DABA) and sodium-free media but not by beta-alanine and thus has the characteristics of GABA uptake in other neuronal systems. There were no obvious differences in the size, shape, number of processes or distribution in the culture between neurons which accumulated GABA and those which did not. Similar cultures incubated in either [3H]glycine or [3H]glutamate and processed by autoradiography resulted in a much different distribution of silver grains than that seen for [3H]GABA. Following incubation in [3H]glycine, silver grains were distributed uniformly over all cells in the culture, both neuronal and non-neuronal. This distribution suggests a metabolic and not a neurotransmitter role for glycine in the cultures, as would be expected of neuronal cells derived from cerebral cortex. Glutamate incubations resulted in the appearance of silver grains over only the non-neuronal cells with very few over the neuronal population. Autoradiograms were also prepared following incubation in the potent GABA receptor agonist [3H]muscimol. These autoradiograms were indistinguishable from those obtained following [3H]GABA incubation. Thus, a finite population of neurons was densely labeled, the labeling was blocked by the GABA uptake inhibitors DABA, nipecotic acid, guvacine and Na+-free media, while substances which interact with the GABA receptor, bicuculline methiodide, THIP, isoguvacine and the noncompetitive antagonist, picrotoxin, were without effect. These results demonstrate that the affinity of muscimol for the GABA uptake site far outweighs its affinity for the GABA receptor site in autoradiographic experiments where intact cells are employed, presumably because its binding to receptors is fleeting. Therefore, muscimol autoradiography may not be informative about GABA receptor localization. These autoradiographic studies suggest that nearly half the neurons in our culture system are GABA neurons but disclosed no morphological handle for GABA neurons.

Animals↗
PubMed1976

Electrical excitability of cultured adrenal chromaffin cells.

1. Adult human and gerbil adrenal medullary cells were maintained in dissociated cell culture and studied by micro-electrode penetration. 2. In the best recordings, chromaffin cell transmembrane potentials exceeded -50mV. 3. Chromaffin cells were capable of generating all-or-nothing over-shooting action potentials, similar to those generated by sympathetic neurones. 4. The action potentials were blocked by tetrodotoxin (TTX, 10(-6)g/ml.) but were not blocked by removal of Ca or by CoCl2 (10 mM). We conclude that the action potentials are probably generated by a Na mechanism. 5. Chromaffin cells are depolarized by the iontophoretic application of acetylcholine (ACh). This depolarization was accompanied by an increased membrane conductance and could trigger action potentials. 6. Action potentials were also found in cells in fresh slices of gerbil adrenal medullae.

Acetylcholine↗