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C J Schmidt

Publications and source records attributed to C J Schmidt.

At least 37 records · Page 2Linked to original sources
PubMed1996

Studies on the mechanism of p-chloroamphetamine neurotoxicity.

Studies were conducted to investigate the sensitivity of p-chloroamphetamine (PCA)-induced neurochemical changes to various pharmacological manipulations known to block the neurochemical effects of 3,4-methylenedioxymethamphetamine (MDMA). The monoamine oxidase-B (MAO-B) inhibitor L-deprenyl (2 mg/kg) given 4 hr before a nonneurotoxic dose of PCA (2 mg/kg) was shown not to alter the amount of [3H]paroxetine bound to serotonin (5-HT) uptake sites 7 days after treatment. L-Deprenyl 4 hr before a neurotoxic dose of PCA (10 mg/kg) did not change the acute hyperthermia. Further, neither L-deprenyl nor another selective MAO-B inhibitor, MDL-72,974 (1.25 mg/kg), given 30 min before or daily for 4 days before a single dose of PCA attenuated or potentiated the decrease in the number of [3H]paroxetine binding sites measured 7 days after PCA treatment. The combination of the MAO-A inhibitor clorgyline (2.5 mg/kg) or a nonspecific dose of L-deprenyl (10 mg/kg) with the selective 5-HT releasing agent 5,6-methylenedioxy-2-aminoindan did not lead to changes in the levels of 5-HT, 5-hydroxyindoleacetic acid or dopamine 7 days after treatment. Finally, the 5-HT2A receptor antagonist MDL-11,939 (5 mg/kg) did not protect against the neurotoxicity of PCA. By comparing the present work with previous studies of MDMA, these results can be interpreted to suggest that the mechanism of the neurotoxicity induced by PCA is not identical to that induced by MDMA. The relationship of these results to the neurotoxicity induced by MDMA is also discussed.

Animals↗
PubMed1996

5-HT modulation of auditory and visual sensorimotor gating: I. Effects of 5-HT releasers on sound and light prepulse inhibition in Wistar rats.

Increasing evidence suggests an important role for serotonin (5-HT) neurons in the etiology and treatment of schizophrenia. The prepulse inhibition paradigm is used as a model for sensorimotor gating processes that are disrupted in schizophrenia. The present study assessed the general role of 5-HT in modulating auditory and visual prepulse inhibition in Wistar rats. A general overactivation of central serotonerigic pathways was produced pharmacologically by four different agents which all shared the common property of releasing 5-HT, i.e., p-chloroamphetamine, 3,4-methylenedioxymethamphetamine, N-ethyl-3,4-methylenedioxymethamphetamine, or fenfluramine. Within each test session, both sound and light prepulses were used to obtain a cross-modal assessment of auditory and visual sensory gating processes. All four 5-HT releasing agents produced dose-related disruptions of auditory and visual prepulse inhibition, with p-chloroamphetamine being the most potent. The releasers depressed baseline to varying degrees. The alpha 2-adrenergic agonist clonidine decreased baseline startle without substantially disrupting prepulse inhibition, demonstrating that the two effects were dissociable. Using fenfluramine as the most selective 5-HT releaser, two approaches were used to demonstrate 5-HT mediation of its disruptive effect on prepulse inhibition. In the first approach, the selective 5-HT uptake blocker MDL 28,618A was used to prevent fenfluramine-induced 5-HT release. In the second approach, prior exposure to a neurotoxic dose of p-chloroamphetamine (10 mg/kg) was used to produce a substantial, sustained depletion of cortical 5-HT, presumably reflecting the loss of 5-HT terminals. Both approaches reduced the disruptive effect of fenfluramine on auditory and visual prepulse inhibition, thereby demonstrating 5-HT mediation of these effects. Neither manipulation significantly affected the depressant effect of fenfluramine on startle baseline, demonstrating that the baseline-reducing and prepulse inhibition-reducing effects of fenfluramine could be dissociated. MDL 28,618A alone did not affect prepulse inhibition or basal startle levels, demonstrating an important functional difference between pharmacologically induced 5-HT uptake blockade and 5-HT release. In summary, these data indicate that serotonergic overactivation can disrupt auditory and visual sensorimotor gating as measured using sound and light prepulse inhibition in rats. These data support a potential role of excessive 5-HT activity as a contributing factor to disrupted sensory gating processes seen in schizophrenia and possibly other neuropsychiatric disorders.

3,4-Methylenedioxyamphetamine↗
PubMed1996

Effects of the selective 5-HT2A receptor antagonist MDL 100,907 on MDMA-induced locomotor stimulation in rats.

(+/-)3,4-Methylenedioxymethamphetamine (MDMA) releases dopamine and serotonin in vivo and stimulates locomotor activity. Previous work demonstrated that MDMA-stimulated dopamine release could be reduced by the selective 5-HT2A receptor antagonist [R-(+)-a- (2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidinem ethanol] (MDL 100,907). In the present study MDL 100,907 significantly reduced MDMA-stimulated locomotion without affecting basal levels of locomotion. Other agents with 5-HT2A antagonist activity (ritanserin, clozapine, MDL 28,133A, or methiothepin), as well as agents that block 5-HT1A-(propranolol), D2-(haloperidol), or D1 receptors (SCH 23390) also reduced MDMA-stimulated locomotion. Intraventricularly administered 5,7-dihydroxytryptamine decreased regional 5-HT levels and attenuated MDMA-stimulated locomotion. These data support the conclusion that serotonin released onto 5-HT2A receptors contributes to MDMA-stimulated locomotion and suggest that MDMA-stimulated locomotion may be useful as an in vivo behavioral measure of 5-HT2A antagonism. The data also support previous reports of contributions of 5-HT1A, D1 and D2 receptors to MDMA-stimulated locomotion. A preliminary time-course analysis indicating time-dependent contributions of different receptors to MDMA-stimulated locomotion suggests the potential utility of this model for characterizing potential atypical antipsychotic compounds.

5,7-Dihydroxytryptamine↗
PubMed1996

Regional effects of MK-801 on dopamine release: effects of competitive NMDA or 5-HT2A receptor blockade.

The open channel N-methyl-D-aspartate (NMDA) receptor antagonists dizocilpine (MK-801) and phencyclidine (PCP) increase the firing rate of both A9 and A10 dopaminergic neurons in the rat. In the A10 nucleus, this effect of MK-801 is reportedly prevented by either competitive NMDA antagonists or serotonin2 (5-HT2) antagonists. The present study examined the neurochemical correlates of these effects using the technique of in vivo microdialysis in conscious rats. In contrast to its reported electrophysiological effects at the cell body level, MK-801 (2 mg/kg, i.p.) has divergent effects on dopamine release in the terminal fields of the A9 and A10 systems. MK-801 stimulated dopamine release in both the medial prefrontal cortex and the nucleus accumbens but tended to decrease release in the striatum. Stimulated dopamine release in the nucleus accumbens was selectively blocked by either the competitive NMDA receptor antagonist, MDL 100,453 or the 5-HT2A receptor antagonist, MDL 100,907. Neither MDL 100,453 nor MDL 100,907 affected MK-801-induced release in the medial prefrontal cortex. The results illustrate the complex regulation of the forebrain dopaminergic systems by glutamate and indicate that the serotonergic system, via the 5-HT2A receptor, may play an important role in this regulation.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1995

The G protein gamma subunit. Requirements for dimerization with beta subunits.

Guanine nucleotide-binding protein beta and gamma subunits form a tightly bound complex that can only be separated by denaturation. Assembly of beta and gamma subunits is a complicated process. The beta 1 and gamma 2 subunits can be synthesized in vitro in rabbit reticulocyte lysate and then assembled into dimers, but beta 1 cannot form beta gamma dimers when synthesized in a wheat germ extract. In contrast, gamma 2 translated in either system can dimerize with beta 1, suggesting that dimerization-competent gamma 2 can be synthesized without the aid of specific chaperonins or other cofactors. Dimerization-competent gamma 2 in solution forms an asymmetric particle with a Stokes radius of about 21 +/- 0.4 A (n = 4), s20,w of 0.9 S (range 0.8-1.0 S, n = 2), and frictional ratio of 1.57 (assuming no hydration). To define the part of gamma 2 that is needed for native beta gamma dimer formation, a series of N- and C-terminal truncations were generated, synthesized in vitro, and incubated with beta 1. Dimerization was assessed by stabilization of beta 1 to tryptic proteolysis. Truncation of up to 13 amino acids at the C terminus did not affect dimerization with beta 1, whereas removal of 27 amino acids prevented it. Therefore, a region between residues 45 and 59 of gamma 2 is important for dimerization. Truncation of 15 amino acids from the N terminus greatly diminished the formation of beta gamma dimers, while removal of 25 amino acids entirely blocked it. Thus, another region important for forming native beta gamma is near the N terminus. Extension of the N terminus by 12 amino acids that include the influenza virus hemagglutinin epitope did not prevent beta gamma dimerization. Furthermore, in intact 35S-labeled COS cells, epitope-tagged gamma 2 coimmunoprecipitates with beta and alpha subunits. The N-terminal epitope tag must lie at the surface of the heterotrimer since it prevents neither heterotrimer formation nor access of the antibody.

Amino Acid Sequence↗
PubMed1995

The selective 5-HT2A receptor antagonist, MDL 100,907, increases dopamine efflux in the prefrontal cortex of the rat.

Diminished function within the mesocortical dopamine system has been to hypothesized to contribute directly to the negative and indirectly to the positive symptoms of schizophrenia. Based on the proposed role of 5-HT2 receptor blockade in the antipsychotic profile of clozapine and its preferential augmentation of prefrontal dopamine release, we have examined the effects of the selective 5-HT2A receptor antagonist, R-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-piperidi ne- methanol (MDL 100,907), on dopamine release in the rat medial prefrontal cortex using in vivo microdialysis. The results indicate that local 5-HT2A receptors exert a tonic inhibitory influence on dopamine efflux in the medial prefrontal cortex. These observations are consistent with the hypothesis that 5-HT2A receptor blockade contributes to the unique antipsychotic profile of clozapine and that MDL 100,907 may have antipsychotic activity.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1995

The role of 5-HT2A receptors in antipsychotic activity.

The correlation between the clinical activity of antipsychotic agents and their affinity for the D2 dopamine receptor has been the mainstay of the hypothesis that schizophrenia is due to excessive dopaminergic function. More recently, the unique clinical profile of the atypical antipsychotic clozapine has been proposed to involve actions on additional receptor systems. In particular, the high affinity of clozapine for the 5HT2A receptor subtype has been suggested to contribute to its reduced side-effect liability, greater efficacy and its activity in therapy-resistant schizophrenia. We have used the highly selective 5-HT2A antagonist MDL 100,907 to explore the contribution of 5-HT2A receptor blockade to antipsychotic activity. Biochemical, electrophysiological and behavioral studies reveal that selective 5HT2A receptor antagonists have the preclinical profile of an atypical antipsychotic. The limited clinical evidence available also suggests that compounds producing 5-HT2A receptor blockade are effective, in particular, against the negative symptoms of schizophrenia.

Animals↗
PubMed1995

Mixed D2/5-HT2A antagonism of amphetamine-induced facilitation of brain stimulation reward.

Recent experiments have demonstrated that 5-HT2A antagonists can modify electrophysiological, neurochemical, and behavioral responses to psychostimulants. These findings led to an interest in using 5-HT2A antagonists to block the effects of psychostimulants on brain reward mechanisms. The present experiments assessed the ability of mixed D2/5-HT2A antagonists to reverse amphetamine-induced facilitation of self-stimulation. The D2/5-HT2A antagonists MDL 28,133A and risperidone attenuated the effects of cocaine and amphetamine, but only at antagonist doses that elevated baseline self-stimulation thresholds. A comparison of the effects of the mixed antagonists to those of haloperidol and eticlopride revealed that all four antagonists produced similar anti-stimulant effects when the influence of the drugs on baseline responding was considered. The D2 activity of the antagonists appears to account for their ability to reduce the effects of psychostimulants on self-stimulation. 5-HT2A antagonism makes a negligible contribution to the anti-amphetamine effects.

Amphetamine↗
PubMed1995

Analysis of (R)-4-oxo-5-phosphononorvaline (MDL 100,453) in rat plasma and brain dialysate using liquid chromatography after derivatization with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate.

A liquid chromatographic (LC) method with precolumn derivatization and fluorescence detection has been developed for the quantitation of (R)-4-oxo-5-phosphononorvaline (MDL 100,453), which is a selective antagonist of N-methyl-D-aspartate receptor, in rat plasma and brain dialysate. The plasma samples were deproteinized with acetonitrile and then derivatized with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC). The brain dialysis samples were dried in vacuum, reconstituted with borate buffer, and derivatized with AQC. The derivatized MDL 100,453 was analyzed by LC with a Nova-Pak C18 column at 32 degrees C using a gradient mobile phase. Detection was accomplished by fluorescence with excitation at 250 nm and emission at 395 nm. This analytical method was used to follow the time course of drug concentrations in rat plasma and brain dialysate after intravenous (i.v.) bolus injection of MDL 100,453 or a combination of i.v. bolus injection and i.v. infusion.

Aminoquinolines↗
PubMed1994

The ancient regulatory-protein family of WD-repeat proteins.

WD proteins are made up of highly conserved repeating units usually ending with Trp-Asp (WD). They are found in all eukaryotes but not in prokaryotes. They regulate cellular functions, such as cell division, cell-fate determination, gene transcription, transmembrane signalling, mRNA modification and vesicle fusion. Here we define the common features of the repeating units, and criteria for grouping such proteins into functional subfamilies.

Amino Acid Sequence↗
PubMed1994

G(O), a guanine nucleotide binding protein, is expressed during neurite extension in the embryonic mouse.

The developmental pattern of expression of the G protein alpha o subunit and GAP43 were compared by immunohistochemical staining of mouse embryos. Staining for alpha o and GAP43 was identical and detected throughout the developing nervous system, and the antigens first appeared in neurons at the beginning of neuronal differentiation. GAP43 and alpha o were not detected in regions containing only neuroblasts. These observations suggest that alpha o and GAP43 may not be required for the decision to pass from neuroblast to differentiated neuron, but may play a role in signal transduction during early neuronal development.

Animals↗
PubMed1994

Blockade of striatal 5-hydroxytryptamine2 receptors reduces the increase in extracellular concentrations of dopamine produced by the amphetamine analogue 3,4-methylenedioxymethamphetamine.

5-Hydroxytryptamine2 (5-HT2) receptor antagonists have been shown to interfere with the stimulation of striatal dopamine synthesis and release produced by the amphetamine analogue 3,4-methylenedioxymethamphetamine (MDMA). To localize the receptors responsible for the attenuation of MDMA-induced release, 5-HT2 receptor antagonists were infused via the microdialysis probe directly into the brains of awake, freely moving rats before the systemic administration of MDMA. Intrastriatal infusions of the selective 5-HT2 antagonist MDL 100,907 produced a concentration-dependent inhibition of MDMA-induced dopamine release. Similar results were observed with intrastriatal infusions of the 5-HT2 antagonist amperozide. In contrast, infusion of MDL 100,907 into the mid-brain region near the dopaminergic cell bodies was without effect on the MDMA-induced elevation of extracellular dopamine in the ipsilateral striatum. Neither antagonist attenuated basal transmitter efflux nor the MDMA-stimulated release of [3H]dopamine from striatal slices in vitro indicating that the in vivo effect of the antagonists was not due to inhibition of the dopamine uptake carrier. Intrastriatal infusion of tetrodotoxin reduced both basal and MDMA-stimulated dopamine efflux and eliminated the effect of intrastriatal MDL 100,907. The results indicate that 5-HT2 receptors located in the striatum augment the release of dopamine produced by high doses of MDMA. Furthermore, these 5-HT2 receptors appear to be located on nondopaminergic elements of the striatum.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1993

A rat homolog of the Drosophila enhancer of split (groucho) locus lacking WD-40 repeats.

In Drosophila, neurogenic loci function in defining cellular fate by interpreting the identity of other cells in the immediate environment. To begin studies of mammalian homologs of these genes, we have isolated two rat homologs of the neurogenic locus Enhancer of split. The protein encoded by the Drosophila Enhancer of split locus is complex and contains five distinct regions based on amino acid composition. One region contains six WD-40 repeats, which were first described in the beta subunit of the heterotrimeric guanine nucleotide-binding protein. One of the rat cDNAs we isolated, R-esp1, encodes a novel form that lacks the WD-40 repeating units. Data are presented demonstrating that the R-esp1 cDNA is a full-length clone encoding an expressed 24-kDa protein. Antibodies raised against this protein stain the nucleus of both PC-12 and GH3 cells. The second clone, R-esp2, encodes a full-length homolog containing WD-40 repeats. The hydrodynamic properties of in vitro translated R-esp1 and R-esp2 proteins indicate that they do not stably self-associate or form heterodimers. A model is presented for the possible role of the R-esp1 protein in the negative regulation of Enhancer of split proteins containing WD-40 repeats.

Amino Acid Sequence↗
PubMed1993

G-protein alpha o subunit: mutation of conserved cysteines identifies a subunit contact surface and alters GDP affinity.

The reversible association of alpha and beta gamma subunits of GTP-binding proteins is important for signal transmission from a variety of cell-surface receptors to intracellular effectors. Previous work showed that 1,6-bis(maleimido)hexane, which crosslinks cysteine residues, crosslinks alpha o and alpha i-1 to beta gamma. These crosslinks are likely to form through a conserved cysteine because 1,6-bis(maleimido)hexane can also crosslink alpha i-2, alpha 1, alpha s and Drosophila alpha 1 to give products of the same apparent molecular weight as crosslinked alpha o beta gamma and alpha i-1 beta gamma. These proteins have only two cysteines in common. Therefore, we mutated each of the two conserved cysteines of alpha o to alanines. Mutation of Cys215 prevents crosslinking to beta gamma, but does not affect binding of guanosine 5'-[gamma-thio]triphosphate or the ability of the mutated alpha subunit to bind beta gamma. In models of the alpha subunit based on the crystal structure of p21ras, Cys215 is located on the face opposite to the GTP-binding site and near an area that changes conformation depending on the nucleotide bound. This surface on the alpha subunit overlaps a putative effector binding region, raising important questions about the spatial organization of the proteins as they form ternary complexes. Mutation of Cys325 has no effect on crosslinking but, surprisingly, decreases by a factor of 10 the affinity of the mutated protein for GDP, relative to wild type, without changing the affinity for guanosine 5'-[gamma-thio]triphosphate. This mutation falls within a region thought to contact receptors and may represent a site through which receptors enhance the release of GDP.

Amino Acid Sequence↗
PubMed1993

Dose-related inhibition by dietary phenethyl isothiocyanate of esophageal tumorigenesis and DNA methylation induced by N-nitrosomethylbenzylamine in rats.

The purpose of this investigation was to establish a dose response for the effects of dietary phenethyl isothiocyanate (PEITC) on N-nitrosomethylbenzylamine (NMBA)-induced esophageal tumorigenesis and DNA methylation. Groups of 13-27 rats were randomly assigned to AIN-76A diets containing 0, 0.325, 0.75, 1.5 or 3.0 mumol PEITC/g. Two weeks later, rats were administered NMBA subcutaneously at a dose of 0.5 mg/kg once a week for 15 weeks. Animals were maintained on control or experimental diets for an additional 8 weeks and were terminated at week 25 of the experiment. No significant effects on weight gain or food intake were noted for any of the experimental diets when compared with control values. Animals receiving only NMBA developed 9.3 +/- 0.9 tumors/rat, with an incidence of 100%. Dietary PEITC at concentrations of 0.75, 1.5 and 3.0 mumol/g inhibited NMBA-induced esophageal tumor multiplicity by 39%, 90% and 100%, respectively. Esophageal tumor incidence in these groups was reduced by 0%, 40% and 100%, respectively. The 0.325 mumol/g PEITC diet did not significantly affect NMBA-induced esophageal tumorigenesis. These results indicate that the minimum inhibitory dietary concentration of PEITC is between 0.325 and 0.75 mumol/g. Groups of 20 rats were assigned to diets containing 0-3.0 mumol PEITC/g for two weeks as described above, and then sacrificed 24 hours after administration of [3H-methyl]NMBA. The esophageal DNA was isolated, purified, hydrolyzed, and analyzed by HPLC. PEITC inhibited DNA methylation in a dose-dependent manner, as was found in the tumor bioassay. The inhibition of tumor incidence was highly correlated with the percentage inhibition of either 7-methylguanine or O6-methylguanine. These latter results suggest that the inhibitory activity of PEITC in this model is manifested, at least in part, during the functional equivalent of tumor initiation.

Animals↗
PubMed1993

Electrophysiological, biochemical and behavioral evidence for 5-HT2 and 5-HT3 mediated control of dopaminergic function.

Several lines of evidence have suggested a link between serotonergic and dopaminergic systems in the brain. The interpretation of much of these early data needs careful reevaluation in light of the recent understanding of the plethora of serotonin receptor subtypes, their distribution in the brain and the new findings with more selective serotonin antagonists. Electrophysiological, biochemical and behavioral evidence obtained using highly selective antagonists of the 5-HT2 or 5-HT3 receptor subtypes, MDL 100,907 or MDL 73,147EF, respectively, supports the thesis that serotonin modulates the dopaminergic system. This modulation is most evident when the dopaminergic system has been activated.

Animals↗