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M A Dichter

Publications and source records attributed to M A Dichter.

At least 55 records · Page 3Linked to original sources
PubMed1992

Desensitization of GABA-activated currents and channels in cultured cortical neurons.

Application of GABA to rat neocortical neurons maintained in cell culture produced a response that declined over several seconds, even in the continued presence of agonist. The decrement could be attributed to both a redistribution of Cl- and a true decline in GABA-induced membrane conductance, or desensitization. The extent and rate of desensitization were dose dependent in a manner similar to the dose dependence of the GABA-induced current, but were not related to the absolute magnitude of the current or to the charge transfer. Bicuculline slowed desensitization while diazepam enhanced the rate of desensitization, consistent with a localization of desensitization to the agonist-receptor binding site. When measured in the whole-cell recording mode, desensitization was voltage dependent, becoming much slower as the membrane was depolarized. Changes in extracellular or intracellular [Ca2+] did not appear to grossly affect the desensitization process or its voltage dependence. GABA-activated single channels, recorded in the outside-out configuration, also desensitized in the continued presence of agonist. However, desensitization differed from that seen in the same neurons in the whole-cell mode. Desensitization was considerably more rapid and did not show any voltage sensitivity. Moreover, single-channel responses often failed to recover after only a few exposures to agonist. Desensitization of GABA responses may play a role in the regulation of cortical inhibition, especially under conditions of intense excitatory and inhibitory synaptic activation.

Animals↗
PubMed1991

Modulation of the NMDA receptor by polyamines.

Results of recent biochemical and electrophysiological studies have suggested that a recognition site for polyamines exists as part of the NMDA receptor complex. This site appears to be distinct from previously described binding sites for glutamate, glycine, Mg++,Zn++, and open-channel blockers such as MK-801. The endogenous polyamines spermine and spermidine increase the binding of open-channel blockers and increase NMDA-elicited currents in cultured neurons. These polyamines have been termed agonists at the polyamine recognition site. Studies of the effects of natural and synthetic polyamines on the binding of [3H]MK-801 and on NMDA-elicited currents in cultured neurons have led to the identification of compounds classified as partial agonists, antagonists, and inverse agonists at the polyamine recognition site. Polyamines have also been found to affect the binding of ligands to the recognition sites for glutamate and glycine. However, these effects may be mediated at a site distinct from that at which polyamines act to modulate the binding of open-channel blockers. Endogenous polyamines may modulate excitatory synaptic transmission by acting at the polyamine recognition site of the NMDA receptor. This site could represent a novel therapeutic target for the treatment of ischemia-induced neurotoxicity, epilepsy, and neurodegenerative diseases.

Animals↗
PubMed1991

Electrophysiological comparison of pyramidal and stellate nonpyramidal neurons in dissociated cell culture of rat hippocampus.

Differences in electrophysiological properties between pyramidal and nonpyramidal neurons have been previously demonstrated in the hippocampal slice preparation. However, it has also been shown that nonpyramidal neurons from several hippocampal regions have different morphologies as well as different active membrane characteristics. In this study, active and passive electrophysiological characteristics of pyramidal and a single morphological type of nonpyramidal neuron in rat dissociated hippocampal culture were examined with intracellular recording at room temperature and at 33-35 degrees C. No significant differences were noted between the two groups of neurons, at either temperature, with regard to action potential amplitude, rate of rise and fall, duration at half maximal amplitude, adaptation to steady state depolarization or resting membrane potential and input resistance. We conclude that these electrophysiological properties cannot be used to distinguish these two neuron types in dissociated hippocampal cell culture.

Animals↗
PubMed1990

Hippocampal commissural connections in the neonatal rat.

The presence and specific connections of the hippocampal commissure were investigated in neonatal Sprague-Dawley rats. Unilateral hippocampal injections into specific subfields with rhodamine-labelled microspheres (RLM) were performed between 3 and 5 days of postnatal life. Retrograde transport of the RLM was allowed to proceed for 48 h following the injection, at which time brains were removed and prepared for histology. Frozen sections were cut at 30-microns intervals in the coronal plane and examined with fluorescence microscopy. All 5 brains which had an injection restricted to CA1 demonstrated contralateral retrograde transport to the homotopic CA1 subfield only, and all 5 brains with focal CA3 injections had contralateral homotopic CA3 labelling. Eight brains had injections involving more than one subfield; in 5 of these brains, the results were consistent with the CA3 to CA3 commissural projection demonstrated by the more restricted injections. However, 3 brains in which the injection involved CA1 as well as CA3, contralateral CA1 labelling was not found, most likely due to 'missing' the projection site in CA1. We conclude: (1) the hippocampal commissural projection exists in the early postnatal rat, (2) a CA3 to CA3 homotopic commissural projection exists, as it does in the adult, and (3) a CA1 to CA1 projection is present in the neonatal rat.

Aging↗
PubMed1990

Electrophysiological effects of somatostatin-14 and somatostatin-28 on mammalian central nervous system neurons.

Somatostatin (SOM) exists in at least two active forms in the central nervous system (CNS): SOM-14 and SOM-28. These peptides have multiple actions on neurons in the CNS and these actions appear to be mediated by different receptors. Thus, SOM-14 can enhance voltage-dependent K currents, whereas SOM-28 inhibits these same currents, sometimes even in the same neurons. These effects are not mediated via cAMP, but do seem mediated by GTP-binding proteins. On the other hand, both forms of SOM inhibit a voltage-dependent Ca current, again via a GTP-binding protein. SOM can also interact with the GABA(A) receptor to modulate responses to this inhibitory transmitter. The physiological effects of SOM in an integrated circuit within the CNS will depend on the form of SOM released, the kinds and numbers of receptors present on the postsynaptic neurons, and the presence of other neurotransmitters.

Animals↗
PubMed1990

Characterization of polyamines having agonist, antagonist, and inverse agonist effects at the polyamine recognition site of the NMDA receptor.

The endogenous polyamines spermine and spermidine increase the binding of [3H]MK-801 to NMDA receptors. This effect is antagonized by diethylenetriamine (DET). We report here that spermine increases the rates of both association and dissociation of binding of [3H]MK-801, suggesting that it increases the accessibility of the binding site for MK-801 within the ion channel of the receptor complex. 1,10-Diaminodecane (DA10) inhibited the binding of [3H]MK-801. This effect was due to a decrease in the rate of association with no change in the rate of dissociation of [3H]MK-801. The effect of DA10 was not mediated by an action of DA10 at the binding sites for glutamate, glycine, Mg2+, or Zn2+, and was attenuated by DET. This suggests that DA10 acts at the polyamine recognition site. In hippocampal neurons the NMDA-elicited current was decreased by DA10, an effect opposite to that of spermine. The effects of spermine and DA10 were selectively blocked by DET. It is concluded that DA10 acts as a negative allosteric modulator or inverse agonist at the polyamine recognition site of the NMDA receptor.

Animals↗
PubMed1989

Calcium currents in cultured rat cortical neurons.

Rat neocortical neurons grown in dissociated cell culture for 4-12 weeks were studied with whole-cell patch-clamp techniques in order to characterize the calcium currents present in these cells. When voltage-dependent Na and K currents were inhibited, depolarizations from negative holding potentials induced inward currents which had 3 components: a low threshold activated, small, relatively persistent component, which was completely inactivated at holding potentials more positive then -60 mV; a higher threshold, relatively persistent component (which was not inactivated at VH = -50 mV); and a higher threshold, larger, transient component. All 3 components were reduced by removal of Ca, and blocked by Cd and Ni at appropriate concentrations. The components were differentially affected by low concentrations of Ni (500 microM), nifedipine (500 microM) and Ba (1.8 mM). Only the first two components were present in very young neurons.

Animals↗
PubMed1989

Cellular mechanisms of epilepsy and potential new treatment strategies.

Over the last 15 years, neurobiologists have begun to unravel the cellular mechanisms that underlie epileptiform activity. Such investigations have two main objectives: (1) to develop new methods for treating, "curing," or preventing epilepsy: and (2) to learn more about the normal functioning of the human brain, at the cellular/molecular and the neurological/psychological levels by analyzing abnormal brain functioning. The electroencephalogram (EEG) spike is a marker for the hyperexcitable cortex and arises in or near an area with a high epileptogenic potential. The depolarizing shift (DS) that underlies the interictal discharge (ID) appears to be generated by a combination of excitatory synaptic currents and intrinsic voltage-dependent membrane currents. The hyperpolarization that follows the DS (post-DS HP) limits ID duration, determines ID frequency, and prevents ID deterioration into seizures. The disappearance of the post-DS HP in some models is related to the onset of seizures and the spread of epileptiform activity. During the transition to seizures, the usually self-limited ID spreads in time and anatomical space. Several processes may intervene in the pathophysiological dysfunction. These include enhancing GABA-mediated inhibition, dampening NMDA-mediated excitability, interfering with specific Ca2+ currents in central neurons, and perhaps stimulating "gating" pathways.

Action Potentials↗
PubMed1989

Extracellular cAMP accumulation and degradation in rat cerebral cortex in dissociated cell culture.

Norepinephrine (NE) stimulated the accumulation of cAMP in embryonic rat cerebral cortex in dissociated cell culture. After exposure to NE for 10 min, the intracellular cAMP content of these cultures went from 22 +/- 12 to 202 +/- 75 pmol/mg protein. Using selective culturing techniques, evidence was obtained supporting the hypothesis that NE-stimulated production of cAMP is a property associated with the glial rather than the neuronal component of these cultures. Beta adrenergic agonist stimulation of cortical cultures also resulted in the efflux of cAMP into the medium. At the peak of extracellular accumulation of cAMP (following a 40-min exposure to isoproterenol), 180 pmol cAMP/mg protein had been transported into the extracellular medium. The fate of extracellular cAMP was investigated using thin-layer chromatography. Extracellular cAMP was degraded to AMP and adenosine; this degradation did not seem to be due to the presence of serum or serum components, suggesting the existence of an extracellular phosphodiesterase. In response to NE stimulation of glia, in particular astrocytes, cAMP or its metabolites may accumulate at high enough concentrations in the extracellular space in cerebral cortex to affect neuronal function, possibly via adenosine receptors.

Animals↗
PubMed1987

Cellular mechanisms of epilepsy: a status report.

The cellular phenomena underlying focal epilepsy are currently understood in the context of contemporary concepts of cellular and synaptic function. Interictal discharges appear to be due to a combination of synaptic events and intrinsic currents, the exact proportion of which in any given neuron may vary according to the anatomic and functional substrate involved in the epileptic discharge and the epileptogenic agent used in a given model. The transition to seizure appears to be due to simultaneous increments in excitatory influences and decrements in inhibitory processes--both related to frequency-dependent neuronal events. A variety of specific hypotheses have been proposed to account for the increased excitability that occurs during epileptiform activity. Although each of the proposed mechanisms is likely to contribute significantly to the epileptic process, no single hypothesis provides an exclusive unifying framework within which all kinds of focal epilepsy can be understood. The spread of epileptic activity throughout the brain, the development of primary generalized epilepsy, the existence of "gating" mechanisms in specific anatomic locations, and the extrapolation of hypotheses derived from simple models of focal epilepsy to explain more complex forms of human epilepsy, all are not yet fully understood.

Action Potentials↗
PubMed1987

Fibroblasts modulate expression of Thy-1 on the surface of skeletal myoblasts.

Thy-1 antigen is a well-characterized cell-surface glycoprotein known to be variably expressed in many different tissues, including lymphocytes, brain, and muscle. Its function remains unknown. In skeletal muscle, both in vivo and in vitro, the antigen has been reported on immature but not on adult tissue, and its disappearance corresponds roughly to the time of myoblast fusion. Using monoclonal H36 antibody to identify myoblasts unambiguously, we demonstrate here that Thy-1 is expressed only on a small (less than 1%) fraction of rat skeletal muscle myoblasts in heterogeneous primary cultures, but the number of myoblasts that express Thy-1 rises to a steady level of about 70% when fibroblasts are removed from secondary cultures. Restitution of fibroblasts or growth of purified myoblasts in medium conditioned by fibroblasts greatly suppresses this increase in myoblast Thy-1 expression. Thus an interaction between fibroblasts and myoblasts, mediated by a soluble nondialyzable molecule, modulates expression of Thy-1 on the myoblast outer membrane.

Animals↗
PubMed1987

Differential expression and modulation of Thy-1 on myoblast clones.

In the preceding paper [J. S. Schweitzer, M. A. Dichter, and S. J. Kaufman, 1987, Exp. Cell Res. 172, 1] we demonstrated that expression of Thy-1 antigen by rat skeletal muscle myoblasts in culture is modulated by fibroblasts. Growth of myoblasts with myofibroblasts, or in medium in which fibroblasts have been grown, causes a sharp reduction in the percentage of myoblasts that express Thy-1 antigen on their membrane. In this paper we demonstrate that there are two populations of myoblasts that can be distinguished by their capacity to respond to the Thy-1-modulating factor (TMF). The majority of Thy-1-positive or -negative myoblasts grown at clonal densities are responsive to TMF. A second myoblast clonal type is Thy-1 positive and fuses to form contractile myotubes, but is insensitive to TMF and has a more fibroblast-like morphology. These clonal phenotypes are stable upon passage in vitro and may represent distinct subsets of the myogenic lineage.

Animals↗
PubMed1987

A small subset of cortical astrocytes in culture accumulates glycogen.

We are interested in identifying the target cells for norepinephrine in cerebral cortex and in characterizing the effects of norepinephrine on these target cells. Norepinephrine inhibits the incorporation of tritiated glucose into glycogen in rat cerebral cortex in dissociated cell culture. To identify which cells store glycogen in these cultures we combined glycogen cytochemistry with glial fibrillary acidic protein immunocytochemistry. Using this technique we show that cytochemically detectable glycogen is restricted to a small subset of astrocytes as well as an unidentified cell type which does not contain glial fibrillary acidic protein. These results demonstrate that only a minority of astrocytes in cortical cultures accumulate glycogen. Therefore cortical astrocytes are differentiated with respect to glycogen accumulation, an important metabolic function. We do not know if glycogen accumulation in astrocytes is a constitutive or facultative property. In either case the subset of astrocytes which accumulates glycogen might be one of the major cellular targets for norepinephrine in cerebral cortex.

Animals↗
PubMed1986

Immunoreactive thymopoietin in the mouse central nervous system.

A thymopoietin-immunoreactive substance (TP-IRS) has been detected in homogenates of mouse spinal cord and brain using a radioimmunoassay; levels were maximal at birth. TP-IRS was also detected in supernatants of mouse neuroblastoma (NIE-115) and primary spinal cord cultures but not human astrocytic and meningeal tumors or mouse primary astrocyte cultures. With affinity purified rabbit anti-TP globulin, immunofluorescent staining was seen in mouse spinal cord cultures in association with nuclear membranes of neurons and, to a lesser degree, flat background cells. From supernatants of NIE-115 cells grown in tritiated leucine and lysine, proteins of approximately 8000 and 4500 Da were isolated by TP affinity chromatography (compared with 5562 Da for thymic thymopoietin). When injected into mice, these neural proteins partially blocked neuromuscular transmission in a manner similar to thymic thymopoietin.

Animals↗
PubMed1986

Antiidiotypic antibody to reovirus binds to neurons and protects from viral infection.

A syngeneic monoclonal antiidiotype directed against the idiotype of an antireovirus type 3 hemagglutinin demonstrates several of the biological actions of the original viral hemagglutinin and binds to rat and murine cortical neurons grown in dissociated cell culture. Receptor-bearing neurons appear within 24 hours of plating in cultures from mouse or rat cortex taken on embryonic day 15; these neurons are demonstrable for the duration of the culture life span (4 to 8 weeks). When cortical cultures are incubated with antiidiotype before or during exposure to reovirus, the antiidiotype protects neurons from type 3 infection without inhibiting infection of nonneuronal cells with either type 3 or type 1. Thus an antibody directed against a viral receptor can prevent infection of receptor-bearing cells without directly neutralizing the virus.

Animals↗
PubMed1986

Effects of valproic acid in cultured mammalian neurons.

Valproic acid (VPA) has been postulated to exert its anticonvulsant effects by interaction with the postsynaptic GABA receptor. To test that hypothesis, we applied VPA in clinically appropriate concentrations to cortical neurons in dissociated cell culture. VPA did not enhance the postsynaptic effect of GABA, but did decrease the generation of sodium-dependent action potentials. VPA may exert anticonvulsant effects by inhibiting spike generation, independent of the GABAergic system.

Action Potentials↗
PubMed1985

Effects of norepinephrine on rat neocortical neurons in dissociated cell culture.

Intracellular recordings were made from neurons in dissociated cell culture of neocortex during application of norepinephrine (NE) or other adrenergic agonists. In the population of neurons generally studied, greater than 18 micron in diameter, adrenergic agonists from 1 nM to 50 microM produced no change in membrane potential or input resistance 120 cells). Adrenergic agonists increased synaptic activity impinging on the impaled cell in 25/120 neurons (21%). In neurons in cocultures of locus coeruleus and cerebral cortex, again the same synaptic response to perfusion with NE was noted in 13/93 neurons (14%). In addition, direct effects of NE were noted on 6/93 neurons recorded from in cocultures, all close to the explant. In these cells, NE hyperpolarized the membrane in association with a small decrease in input resistance (11%). These responsive cells may have originated within the explant. A paradigm was used for testing the possibility of a responsive element in the cultures distinct from the impaled soma. 'Hot spots' were found using concentrations of isoproterenol as low as 10 nM.

Age Factors↗