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M Dichter

Publications and source records attributed to M Dichter.

At least 19 recordsLinked to original sources
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

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↗