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.

Characterization of the 5-HT2 receptor antagonist MDL 100907 as a putative atypical antipsychotic: behavioral, electrophysiological and neurochemical studies

Progress toward understanding the role of the 5-hydroxytryptamine (5-HT)2 receptor in the therapy for schizophrenia has been hampered by the lack of highly selective antagonists. We now report on the effects of MDL 100,907 [R(+)-alpha-(2,3-dimethoxyphenyl)-1- [2-(4-fluorophenylethyl)]-4-piperidine-methanol], a highly selective and potent 5-HT2 receptor antagonist, in behavioral, electrophysiological and neurochemical models of antipsychotic activity and extrapyramidal side-effect liability. In mice, MDL 100,907 blocked amphetamine-stimulated locomotion at doses that did not significantly affect apomorphine-stimulated climbing behavior. Neither MDL 100,907 nor clozapine reduced apomorphine-induced stereotypies or produced catalepsy in rats. MDL 100,907 blocked the slowing of ventral tegmental area (A10) dopaminergic neurons by amphetamine but, like clozapine, produced only small increases in the number of active substantia nigra zona compacta (A9) and A10 dopamine neurons after acute administration. When administered chronically, MDL 100,907 and clozapine selectively reduced the number of spontaneously active A10 neurons, whereas haloperidol reduced activity in both the A9 and A10 regions. Consistent with their acute effect on A9 and A10 activity, neither MDL 100,907 nor clozapine increased dopamine metabolism in the striatum or nucleus accumbens, whereas acute haloperidol accelerated dopamine turnover in both regions. The administration of the dopamine uptake blocker amfonelic acid with haloperidol produced a massive increase in DA metabolism characteristic of typical antipsychotics. In contrast, MDL 100,907 and clozapine were without effect on dopamine turnover when given in the presence of amfonelic acid. These data indicate that MDL 100,907 has a clozapine-like profile of potential antipsychotic activity with low extrapyramidal sid-effect liability.(ABSTRACT TRUNCATED AT 250 WORDS)

5-HT2 receptors exert a state-dependent regulation of dopaminergic function: studies with MDL 100,907 and the amphetamine analogue, 3,4-methylenedioxymethamphetamine

The highly selective 5-HT2 receptor antagonist, MDL 100,907, was used to explore the role of serotonin in the stimulation of dopaminergic function produced by the amphetamine analogue 3,4-methylenedioxymethamphetamine (MDMA). MDL 100,907 blocked MDMA-stimulated dopamine synthesis in vivo without affecting basal synthesis. The long-term deficits in 5-HT concentrations believed to be a consequence of MDMA-induced dopamine release were also blocked by MDL 100,907 over the same dose range. In vivo microdialysis confirmed that 5-HT2 receptor blockade with MDL 100,907 attenuated MDMA-induced increases in extracellular concentrations of striatal dopamine. In contrast to its effect on MDMA-induced synthesis, MDL 100,907 did not alter dopamine synthesis stimulated by haloperidol or reserpine. In vivo dopamine release produced by haloperidol was also unaffected by MDL 100,907. The results suggest a permissive role for 5-HT2 receptors in the activation of the dopamine system which occurs during states of high serotonergic activity or during conditions of elevated dopamine efflux with high D2 receptor occupancy.

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.

Mechanisms of synaptic fatigue in a developing autonomic ganglion

Synaptic transmission in developing systems has often been noted to exhibit depression or failure at moderate frequencies of stimulation. While this is often presumed to be a transient, nonspecific inability of developing systems to meet the demands of synaptic transmission, this report demonstrates that such failure in the choroidal neurons of the embryonic ciliary ganglion is due to muscarinically mediated inhibition. Although the ganglion is composed of both choroid and ciliary neurons, only the choroid neurons exhibit the muscarinic depression, and only during embryonic development. The pharmacological properties of the relevant receptor are different from those of the muscarinic receptor involved in presynaptic inhibition in adult autonomic systems. Receptor-mediated, synaptic failure during development may serve to protect immature postsynaptic neurons from potentially toxic overstimulation.

The 5-HT2 receptor antagonist, MDL 28,133A, disrupts the serotonergic-dopaminergic interaction mediating the neurochemical effects of 3,4-methylenedioxymethamphetamine

The selective 5-HT2 receptor antagonist MDL 28,133A dose dependently-blocked the long-term deficits in rat brain 5-HT concentrations produced by the substituted amphetamine analogue 3,4-methylenedioxymethamphetamine (MDMA). This protective effect of MDL 28,133A could be abolished by coadministration of the dopamine precursor, L-dihydroxyphenylalanine (L-DOPA). Electrophysiological experiments demonstrated that the ability of MDL 28,133A to block the MDMA-induced slowing of A9 dopaminergic neurons was also sensitive to L-DOPA administration. Both sets of experiments suggest an interaction of MDL 28,133A at the level of dopamine synthesis. Consistent with this explanation, MDL 28,133A antagonized the MDMA-induced stimulation of dopamine synthesis in vivo. MDMA-induced 5-HT release did not reduce the firing rate of dopaminergic neurons as assessed by dopamine depletion following synthesis inhibition with alpha-methyl-p-tyrosine (alpha-MPT). This indicates that the effect of 5-HT2 receptor antagonists on MDMA-induced dopamine synthesis is not due simply to the removal of an inhibitory serotonergic input followed by an increase in dopamine cell firing and autoreceptor activation. MDL 28,133A was also shown to be without effect on the sensitivity of terminal dopamine autoreceptors. The results are consistent with the hypothesis that 5-HT2 receptors are permissive for the stimulation of dopamine synthesis necessary to support MDMA-induced transmitter efflux.

Specificity of G protein beta and gamma subunit interactions

Multiple heterotrimeric guanine nucleotide binding protein (G protein) subunits have evolved to couple a large variety of receptors to intracellular effectors. G protein beta gamma subunits are essential for efficient coupling of alpha subunits to receptors, and they are also important for modulation of effectors. Several different beta and gamma subunits exist, but it is not known whether all possible combinations of beta and gamma can form functional dimers. To answer this question, we have compared the ability of in vitro translated beta 1, beta 2, and beta 3 to form dimers with either gamma 1 or gamma 2. Dimerization was monitored by gel filtration, resistance to tryptic digestion, and chemical cross-linking. The results indicate that beta 1 binds both gamma subunits, beta 2 binds only gamma 2, and beta 3 will bind neither gamma 1 or gamma 2. Hence, the occurrence of beta gamma dimers may be partially regulated by the ability of the subunits to associate. Specificity of dimerization might allow cells to co-express multiple beta and gamma subunits while maintaining efficient and specific signal transduction.

Promotion of the GTP-liganded state of the Go alpha protein by deletion of the C terminus

G proteins are active as long as GTP is bound to the alpha subunit. Activation ends when GTP is cleaved to GDP that then stays bound to the active site. Agonist-liganded receptors allow formation of the active state by decreasing the affinity of alpha subunits for GDP allowing exchange of GDP for GTP. Since receptors interact with the C terminus of the alpha subunits, we tested whether deletion of the C terminus could mimic activation by receptors. Three deletions and one point mutation at the C terminus of alpha o were engineered in alpha o cDNA by the polymerase chain reaction, transcribed into RNA, and translated in a rabbit reticulocyte lysate. The ability of in vitro synthesized protein to bind guanine nucleotide was inferred from analysis of native tryptic cleavage patterns, while the ability of the proteins to associate with beta gamma was measured by sucrose density gradient centrifugation. Deletion of 14 amino acids, alpha oD[341], from the C terminus causes a large decrease in GDP affinity, with little or no change in guanosine 5'-3-O-(thio)triphosphate affinity. When GTP is present, alpha oD[341] remains in the activated conformation because exchange of GTP for GDP is rapid. Deletion of 10 amino acids, alpha oD[345], lowers GDP affinity, but less dramatically than in alpha oD[341]. Deletion of 5 amino acids, alpha oD[350], or mutation of Arg-349 to proline alpha oR[349P] has no detectable effects on GDP affinity. Deletion of up to 10 amino acids from the C terminus does not prevent formation of alpha beta gamma heterotrimers. We propose that the C terminus of the alpha subunit is a mobile region that blocks dissociation of GDP. Agonist-liganded receptors may move it aside to allow release of GDP, exchange for GTP, and activation of the alpha subunit.

Mutagenesis of the amino terminus of the alpha subunit of the G protein Go. In vitro characterization of alpha o beta gamma interactions

Heterotrimeric guanine nucleotide-binding proteins are composed of alpha and beta gamma subunits and couple a variety of cell-surface receptors to intracellular enzymes or ion channels. The heterotrimer dissociates into alpha and beta gamma subunits when the alpha subunit is activated by guanine nucleoside triphosphates. Several lines of evidence show that the amino terminus of the alpha subunit is important for the interaction with the beta gamma subunit (Neer, E. J., Pulsifer, L., and Wolf, L. G. (1988) J. Biol. Chem. 263, 8996-9000; Fung, B. K.-K., and Nash, C. R. (1983) J. Biol. Chem. 258, 10503-10510). We have mutagenized the amino terminus of alpha o to dissect the relative contributions of amino-terminal myristoylation and specific amino acid sequences to subunit interaction. Wild-type and mutant alpha o cDNAs were translated in vitro in a rabbit reticulocyte lysate. All proteins were able to bind guanosine 5'-(gamma-thio)triphosphate and to achieve the necessary conformation for protection from tryptic digestion. Two assays of alpha o beta gamma interactions were used: sucrose density gradients to look for stable heterotrimer formation and ADP-ribosylation by pertussis toxin to detect weak or transient alpha o beta gamma interactions. Our results indicate that myristoylation is essential for stable heterotrimer formation, but that nonmyristoylated proteins are also capable of interacting with the beta gamma subunit. Amino acids 7-10 have an important role in alpha o beta gamma interactions whether alpha o is myristoylated or not. Deletion of this region diminishes the ability of alpha o to interact with the beta gamma subunit, but substitutions at this position indicate that other amino acids can be tolerated without affecting subunit interaction.

5-HT2 receptor antagonists reverse amphetamine-induced slowing of dopaminergic neurons by interfering with stimulated dopamine synthesis

The role of serotonin in the effect of amphetamine on the firing rate of midbrain dopaminergic neurons was examined using unit recordings of identified A10 dopamine neurons in the chloral hydrate-anesthetized rat. Amphetamine (1 mg/kg, i.v.) reduced the firing rate of these neurons approximately 50 to 60%. This effect was blocked in animals pretreated with the selective serotonin-2 (5-HT2) receptor antagonists, MDL 28,133A (0.2 mg/kg, i.v.) or ritanserin (1 mg/kg, i.v.). Although pretreatment with L-dopa (100 mg/kg, i.v.) plus carbidopa (25 mg/kg, i.p.) alone had no effect on amphetamine-induced slowing of A10 dopamine neurons, when coadministered with the 5-HT2 antagonists, the dopamine precursor completely restored this amphetamine-induced slowing. To verify the role of serotonin in these findings, rats were pretreated with the tryptophan hydroxylase inhibitor, p-chlorophenylalanine (250 mg/kg/day for 2 days) to deplete cortical serotonin levels. Consistent with the results observed with the 5-HT2 receptor antagonists, amphetamine did not produce a significant reduction in the firing rate of A10 neurons in serotonin-depleted rats. These results suggest that, under some conditions, serotonergic input via the activation of 5-HT2 receptors may regulate the availability of the pool of dopamine, which is subject to amphetamine release.

Effects of the serotonin releasers 3,4-methylenedioxymethamphetamine (MDMA), 4-chloroamphetamine (PCA) and fenfluramine on acoustic and tactile startle reflexes in rats

The substituted amphetamines 4-chloroamphetamine (PCA), 3,4-methylenedioxymethamphetamine (MDMA) and fenfluramine (FEN) share the common neurochemical action of acutely releasing central serotonin (5-HT), and yet their behavioral effects are quite different. The present study evaluated the effects of these compounds on acoustic and tactile startle reflexes. PCA and MDMA were qualitatively similar in producing dose-related increases in acoustic and tactile startle reflexes that were slow in onset, but sustained throughout the 3.5-hr test session. Changes in motor activity did not account for the observed excitation of startle. In marked contrast to MDMA and PCA, FEN did not alter tactile startle and tended to depress acoustic startle. The excitatory effect of 20 mg/kg of MDMA was prevented by the 5-HT uptake blockers MDL 27,777A and fluoxetine. MDMA excitation was not affected by a dose of the dopamine antagonist haloperidol that attenuated the startle-enhancing effect of d-amphetamine. MDMA excitation was greatly attenuated by a general depletion of central 5-HT produced by prior intraventricular injection of the 5-HT neurotoxin 5,7-dihydroxytryptamine. PCA and MDMA excitations of startle were attenuated in rats specifically depleted of spinal 5-HT or in rats with radio frequency lesions of the dorsal raphe nucleus. Thus, PCA and MDMA have similar prolonged excitatory effects on startle reflexes that are mediated by ascending (dorsal raphe) and descending (spinal) pathways, whereas FEN differs in its lack of excitation of startle. Differences in the neurochemical properties of these compounds or their patterns of 5-HT release may underlie their different behavioral profiles.

L-DOPA potentiation of the serotonergic deficits due to a single administration of 3,4-methylenedioxymethamphetamine, p-chloroamphetamine or methamphetamine to rats

The role of dopamine in the serotonergic neurotoxicity of 3,4-methylenedioxymethamphetamine, p-chloroamphetamine, methamphetamine, N-ethyl-3,4-methylenedioxyamphetamine and fenfluramine was assessed by determining the long-term effect of their coadministration with the dopamine precursor, L-DOPA (L-2,4-dihydroxyphenylalanine). L-DOPA administration potentiated the regional deficits in brain concentrations of serotonin measured one week after a single high dose of 3,4-methylenedioxymethamphetamine, p-chloroamphetamine or methamphetamine but did not alter the neurochemical response to N-ethyl-3,4-methylenedioxyamphetamine nor to fenfluramine. Consistent with this, in vitro release studies found the latter two agents to be the weakest of the five at increasing [3H]dopamine efflux from preloaded rat striatal slices. As an estimate of in vivo release, the effect of each agent on striatal dopamine concentrations was determined. Only those agents showing a synergism with L-DOPA in the long-term studies also produced changes in striatal dopamine consistent with an increase in transmitter release and synthesis. These results provide additional support for the hypothesis that dopamine release plays a role in the neurotoxicity of methylenedioxymethamphetamine, p-chloroamphetamine and methamphetamine. The lack of effect of L-DOPA on the neurotoxicity of fenfluramine as well as the modest effects of fenfluramine on dopamine release indicate this drug may produce its long-term effects on the serotonergic system through a unique mechanism not involving dopamine.

Suppression of interleukin-1 beta and LDL scavenger receptor expression in macrophages by a selective protein kinase C inhibitor

A human monocytic cell line, THP-1, stimulated with 40 nM phorbol myristate acetate (PMA), differentiated to macrophage-like cells, and exhibited increased expression and release of interleukin-1 beta and expression of acetylated low density lipoprotein (ac-LDL) receptors. A selective inhibitor, MDL 29,152 (4-propyl-5-(4-quinolinyl)-2(3H)-oxazolone) was used to show that this induction required activation of protein kinase C. MDL 29,152 acts in the catalytic domain of protein kinase C and is at least 200-fold selective for protein kinase C over cAMP-dependent protein kinase in THP-1 cells. MDL 29,152 (50 microM) reduced levels of interleukin-1 beta mRNA in PMA-stimulated cells by 76% and eliminated detectable interleukin-1 beta in the media. Flow cytometric analysis showed that 48 h after THP-1 activation, approximately 50% of the cells expressed ac-LDL receptors, while in the presence of 100 microM MDL 29,152, less than 5% of the cells expressed receptors. The relationship between THP-1 differentiation and protein kinase C activation was determined by following the expression of the cell surface antigen MO-1. Expression of MO-1 antigen increases as monocytes differentiate to macrophages. After 48 h of phorbol activation, 90% of the THP-1 population was MO-1-positive; less than 16% of the population was MO-1-positive when 100 microM MDL 29,152 was present. By dual analysis, it was found that within the differentiated, MO-1-positive population, only approximately 50% of the cells also expressed ac-LDL receptors. Based on these findings, we conclude that protein kinase C promotes processes important in THP-1 activation and differentiation to macrophage-like cells including interleukin-1 beta expression and secretion, ac-LDL receptor and MO-1 expression.

Identification of a retinal protein in Drosophila with antibody to the alpha subunit of bovine brain G(o) protein

An antibody directed against the alpha(o) subunit of bovine brain G(o) (R4) was used to identify a Drosophila retinal protein which may be the analogue of vertebrate transducin. The immunoreactivity appears predominantly in the retinal and occellar rhabdomeres. On a Western blot, the antibody recognizes a 41 kDa protein that is present in the heads of yellow white flies, but not in the heads of eyeless mutant flies, eyes absent. This protein is not recognized by an antibody raised against Drosophila alpha(o). Antibody R4 intensely stains rhabdomeres and, to a lesser extent, the neuropil of the central nervous system in tissue sections of adult flies. Antibody to Drosophila alpha(o) stains the neuropil of the central nervous system, but does not stain rhabdomeres. In developing flies, faint immunoreactivity appears in the retinal rhabdomeres at about 70% of the time through pupal development and increases to its apparent adult maximal level about 1 day after eclosion. Tissue sections from a phototransduction mutant, norp A, have retinal immunoreactivity at normal levels up to about 1 week after eclosion, but by 2 weeks, immunoreactivity has largely disappeared. This disappearance parallels the degeneration of the retina in norp A mutants. In Drosophila and other invertebrates, light activates a phospholipase C in the retina. The identification of a protein in Drosophila rhabdomeres with an antibody raised against a mammalian G protein alpha subunit thought to be involved in phospholipase C activation suggests that there may be common structural features between the putative Drosophila transducin and alpha(o). The identification of regions common to mammalian alpha(o) and Drosophila transducin may then provide clues to the structural requirements for PLC activation.

In vitro synthesis of G protein beta gamma dimers

The guanine nucleotide-binding proteins (G proteins), which play a central role in coupling membrane-bound receptors to intracellular effectors, are heterotrimers composed of alpha, beta, and gamma subunits. The beta and gamma subunits form a functional monomer that does not appear to separate under physiological conditions. This has made it difficult to differentiate the individual roles of beta and gamma subunits in signal transduction. To characterize the individual subunits, the 36-kDa beta subunit (beta 1), brain gamma (gamma 2), and transducin gamma (gamma t) were translated in vitro in a rabbit reticulocyte lysate system. Hydrodynamic studies and tryptic proteolysis were used to compare the physical properties of the in vitro translation products with those of beta gamma dimers purified from bovine brain. The hydrodynamic studies indicate that, without gamma subunits, the beta subunits are not stable but tend to aggregate into high molecular weight complexes. When beta and gamma subunits were co-translated, stable beta gamma dimers formed that bound alpha 0 in a guanine nucleotide-dependent manner. The beta gamma dimers were less hydrophobic than those purified from bovine brain. This may reflect a lack of post-translational modification in the reticulocyte lysate or other differences between the in vitro translation products and the purified beta gamma. When beta and gamma were translated separately and then mixed, beta gamma dimers also formed. Analysis of in vitro translated beta gamma subunits will provide ways to assess the function of these subunits and to determine the structural requirements for beta gamma formation.

5-HT2 antagonists stereoselectively prevent the neurotoxicity of 3,4-methylenedioxymethamphetamine by blocking the acute stimulation of dopamine synthesis: reversal by L-dopa

The active and inactive stereoisomers of the serotonin (5-HT2) antagonist, MDL 11,939, were used to examine the relationship between the acute effects of 3,4-methylenedioxymethamphetamine (MDMA) on the dopaminergic system and its long-term effects on the serotonergic system. Only the R-(+) stereoisomer of MDL 11,939 both reversed the acute stimulation of striatal dopamine synthesis by MDMA and prevented the deficit in forebrain 5-HT concentrations measured one week later. This acute activation of striatal dopamine synthesis by MDMA is a compensatory response to the carrier-mediated efflux of transmitter as shown by its sensitivity to the dopamine uptake inhibitor, nomifensine. It is suggested that in the absence of this enhanced synthesis, the dopaminergic neuron cannot sustain the carrier-mediated dopamine release which is a prerequisite for the development of MDMA-induced neurotoxicity. This hypothesis is supported by the observation that the administration of the dopamine precursor, L-dopa, with MDMA reverses the protective effects of 5-HT2 receptor antagonists.

MDL 27,032 [4-propyl-5-(4-pyridinyl)-2(3H)-oxazolone], an active site-directed inhibitor of protein kinase C and cyclic AMP-dependent protein kinase that relaxes vascular smooth muscle

MDL 27,032 [4-propyl-5-(4-pyridinyl)-2(3H)-oxazolone] is a novel vasodilator whose mechanism of action has not been elucidated. We investigated whether smooth muscle relaxation by MDL 27,032, in vitro, may involve an alteration in the activity of protein kinase C, cyclic AMP (cAMP)-dependent protein kinase or myosin light chain kinase by investigating the effects of MDL 27,032 on cyclic nucleotide phosphodiesterases (PDEs) and protein kinase activities. Strips of dog femoral artery or saphenous vein contracted with phorbol 12-myristate 13-acetate (PMA) were relaxed by 100 microM concentrations of MDL 27,032, as well as by other known inhibitors of PDEs [3-isobutyl-1-methylxanthine and papaverine], myosin light chain kinase (W-7) and protein kinase C (H-7 and polymyxin B). In contrast to 3-isobutyl-1-methylxanthine and papaverine, MDL 27,032 was either inactive or weak as an inhibitor of purified PDE types I, II, IVa and IVb. Similarly, it was a weak inhibitor of myosin light chain kinase. However, MDL 27,032 was a significantly more potent inhibitor of protein kinase C and cAMP-dependent protein kinase in cytosolic extracts of dog vein. Kinetic experiments utilizing purified rat brain protein kinase C revealed that inhibition with MDL 27,032 was competitive with Mg(++)-ATP (Ki 24 microM) and noncompetitive with phospholipid, diacylglycerol, PMA, calcium or substrate proteins. Inhibition of the catalytic subunit of cAMP-dependent protein kinase was also competitive with Mg(++)-ATP (Ki 14.3 microM). Similar results were obtained with MDL 27,032 and H-7 on both enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)

Chloral hydrate anesthesia antagonizes the neurotoxicity of 3,4-methylenedioxymethamphetamine

We report that maintaining rats under chloral hydrate anesthesia for the first 3 h following the administration of 3,4-methylenedioxymethamphetamine (MDMA) blocks the decrease in forebrain concentrations of 5-hydroxytryptamine (5-HT) measured 1 week later. In contrast, the acute effect of MDMA (3 h) on forebrain 5-HT was not altered by the anesthetic. This protective effect of chloral hydrate was not due to an anesthetic-induced hypothermia but may be related to the hypothesized role of dopamine in the neurotoxic effects of MDMA.

Selective 5-hydroxytryptamine2 receptor antagonists protect against the neurotoxicity of methylenedioxymethamphetamine in rats

The serotonergic deficits resulting from methylenedioxymethamphetamine (MDMA)-induced neurotoxicity were prevented by the simultaneous administration of 5-hydroxytryptamine2 (5-HT2) receptor antagonists such as MDL 11,939 or ritanserin. This effect was not region specific as protection was observed in the cortex, hippocampus and striatum 1 week after the administration of a single dose of MDMA. MDL 11,939 also showed some efficacy at reducing the deficits in 5-HT concentrations and tryptophan hydroxylase activity produced by multiple administrations of MDMA. Protection against the neurotoxicity required the administration of MDL 11,939 within 1 hr of MDMA indicating 5-HT2 receptor activation was an early event in the process leading to terminal damage. Examination of the effect of the 5-HT2 receptor blockade on the early neurochemical alterations induced by MDMA revealed an inhibitory effect on MDMA-stimulated dopamine synthesis. Analysis of these data and the associated changes in dopamine metabolites indicates that 5-HT2 receptor antagonists block MDMA-induced neurotoxicity by interfering with the ability of the dopamine neuron to maintain its cytoplasmic pool of transmitter and thereby sustain carrier-mediated dopamine release.

Activity of 5,7-dichlorokynurenic acid, a potent antagonist at the N-methyl-D-aspartate receptor-associated glycine binding site

5,7-Dichlorokynurenic acid (5,7-DCKA), one of the most potent excitatory amino acid receptor antagonists yet described, binds to a strychnine-insensitive glycine binding site located on the N-methyl-D-aspartate (NMDA) receptor complex (Ki = 79 nM versus [3H]glycine). 5,7-DCKA (10 microM) antagonized the ability of NMDA to stimulate the binding of the radiolabeled ion channel blocker N-[3H][1-(2-thienyl)cyclohexyl]-piperidine ([3]TCP). Glycine was able to overcome this effect and in the presence of 5,7-DCKA enhanced [3H]TCP binding to antagonist-free levels. 5,7-DCKA completely and noncompetitively antagonized several NMDA receptor-mediated biochemical and electrophysiological responses. Thus, micromolar concentrations of 5,7-DCKA inhibited NMDA-stimulated elevation of cytosolic calcium in cultured hippocampal neurons, cGMP accumulation in cerebellar slices, and norepinephrine release from hippocampal slices. The glycine antagonist could also block the action of synaptically released agonist, as shown by its ability to inhibit the increase in the magnitude of the population spike that follows tetanic stimulation of the hippocampus in vitro (long term potentiation). Inclusion of glycine or D-serine prevented all these effects of the antagonist. 5,7-DCKA was a potent anticonvulsant when administered intracerebroventricularly to mice. As in the in vitro experiments, the dose-response curve for the antagonist was shifted rightward in a parallel fashion when D-serine was coinjected. This spectrum of activity displayed by a compound acting at the glycine binding site suggests that the therapeutic utility of glycine antagonists will be similar to those proposed for other types of glutamate receptor antagonists.

Strychnine-sensitive, glycine-induced release of [3H]norepinephrine from rat hippocampal slices

The amino acid glycine is the primary inhibitory neurotransmitter of the mammalian spinal cord. Glycine has also been shown to facilitate the excitatory actions of glutamate at the N-methyl-D-aspartic acid receptor subtype. In this article, glycine is shown to increase the Ca2(+)-dependent release of [3H]norepinephrine from preloaded slices of the rat hippocampus. This effect was inhibited noncompetitively by nanomolar concentrations of strychnine, which differentiates it from the glycine site associated with the N-methyl-D-aspartate receptor. Glycine also released [3H]acetylcholine, but was without effect on the efflux of [3H]serotonin or gamma-[3H]aminobutyric acid from the same tissue preparation. The release of [3H]norepinephrine was reversibly blocked by tetrodotoxin, indicating the effect is not initiated at the noradrenergic terminals, but requires propagation of an action potential. The results suggest that a glycine site that is pharmacologically similar to that found in the spinal cord exists in the rat hippocampus. We suggest that this site may participate in modulating the release of specific neurotransmitters in the brain.

Reversal of the acute effects of 3,4-methylenedioxymethamphetamine by 5-HT uptake inhibitors

Recent evidence suggests that the acute 3,4-methylenedioxymethamphetamine (MDMA)-induced loss of tryptophan hydroxylase activity (TPH) may be due to the oxidation of critical sulf-hydryl groups on the molecule. To determine if TPH activity could be regenerated in vivo we administered a 5-HT uptake inhibitor at various times immediately after MDMA. Although enzyme activity began to decline immediately following MDMA administration, rats receiving the uptake inhibitor 1 h post MDMA showed a rapid recovery of TPH activity. Administration of an uptake inhibitor 3 h post MDMA was without effect on the time course of TPH inactivation. The results suggest that systems exist within the serotonergic neuron for the reductive regeneration of active TPH. Furthermore, these systems are acutely compromised following the administration of MDMA.

Antagonism of the neurotoxicity due to a single administration of methylenedioxymethamphetamine

The role of transmitter release in the serotonergic neurotoxicity of methylenedioxymethamphetamine (MDMA) was examined using treatments altering MDMA-induced release or its consequences. The long-term decrease in 5-HT concentrations and tryptophan hydroxylase activity produced by MDMA was antagonized by depletion of vesicular monoamines with reserpine or interruption of monoamine synthesis with the decarboxylase inhibitor, monofluoromethyl DOPA (dihydroxyphenylalanine). Similar results were achieved by selectively inhibiting dopamine synthesis with alpha-methyl-p-tyrosine or through bilateral lesions of the substantia nigra with 6-hydroxydopamine. The dopamine receptor antagonist haloperidol was also effective in this regard. Although these results strongly implicate dopamine release in the long-term neurochemical effects of MDMA, protection was also provided by selective 5-HT2 antagonists indicating that the neurotoxicity is dependent upon the release of both dopamine and 5-HT.

Neuronal expression of a newly identified Drosophila melanogaster G protein alpha 0 subunit

Guanine nucleotide-binding proteins (G proteins) mediate signals between activated cell-surface receptors and cellular effectors. A bovine G-protein alpha-subunit cDNA has been used to isolate similar sequences from Drosophila genomic and cDNA libraries. One class, which we call DG alpha 0, hybridized to position 47A on the second chromosome of Drosophila. The nucleotide sequence of the protein coding region of one cDNA has been determined, revealing an alpha subunit that is 81% identical with rat alpha 0. The cDNA hybridizes strongly to a 3.8 kb mRNA and weakly with a 5.3 kb message. Antibodies raised against a trp-E-DG alpha 0 fusion protein recognized a 39,000 Da protein in Drosophila extracts. In situ hybridization to adult Drosophila sections combined with immunohistochemical studies revealed expression throughout the optic lobes and central brain and in the thoracic and abdominal ganglia. DG alpha 0 message and protein were also detected in the antennae, oocytes, and ovarian nurse cells. The neuronal expression of this gene is similar to mammalian alpha 0, which is most abundantly expressed in the brain.

Myosin I heavy-chain genes of Acanthamoeba castellanii: cloning of a second gene and evidence for the existence of a third isoform

We have determined the complete sequence and structure of a second myosin I heavy-chain gene from Acanthamoeba castellanii. This gene, which we have named MIL, spans approx. 6kb, is split by 17 introns, encodes a 1147-aa polypeptide, and is transcribed in log-phase cells. The positions of six of the introns are conserved relative to a vertebrate muscle myosin gene. Similar to the previously characterized MIB heavy-chain gene, the deduced MIL heavy-chain aa sequence reveals a 125-kDa protein composed of a myosin globular head domain joined to a novel, approx. 50-kDa C-terminal domain that is rich in glycine, proline and alanine residues. There are differences, however, between MIL and MIB in the sequence organization of their unconventional C-terminal domains. We conclude from this and other data that Acanthamoeba express at least three myosin I heavy-chain isoforms: MIL, plus MIA and MIB, whose purifications have been published previously. Amoeba genomic DNA blots probed with a short, highly conserved sequence whose position is transposed between MIB and MIL indicate that the Acanthamoeba myosin I heavy-chain gene family may actually contain as many as six genes. Finally, we compared the myosin I sequences with those of two related proteins, Drosophila NinaC and the bovine myosin I-like protein, and found that a portion of the unconventional C-terminal domains of the amoeba myosins I and the bovine protein appear to be related.

Guanine nucleotides are competitive inhibitors of N-methyl-D-aspartate at its receptor site both in vitro and in vivo

Guanine nucleotides were shown to alter N-methyl-d-aspartate (NMDA) receptor-effector coupling by competitive antagonism at the glutamate binding site, rather than via interaction with an intracellularly located GTP-binding protein. Thus, in contrast to known G-protein linked receptors, micromolar concentrations of guanine nucleotides and their analogs decreased both agonist [( 3H]glutamate) and antagonist [( 3H]-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid binding to the NMDA receptor complex. The most potent compound, the GDP analog guanosine-5'-O-(2-thiodiphosphate) (GDP beta S), was studied in detail. GDP beta S exhibited almost 200-fold selectivity for the glutamate recognition site vs. the strychnine-insensitive glycine binding site. IC50 values were 2.7 +/- 1.4 and 484 +/- 97 microM, respectively. GDP beta S also inhibited N-[1-(2-thienyl)cyclohexyl-3H]piperidine binding (IC50 was 28.0 +/- 3.7 microM) in an NMDA-reversible fashion. [3H]-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid saturation binding studies revealed an increase in Kd from 263 +/- 49 (control) to 552 +/- 134 nM (8 microM GDP beta S) without any change in maximum binding (4.94 +/- 0.34 and 5.19 +/- 0.58 pmol/mg of protein, respectively). GDP beta S was also a competitive inhibitor of the following NMDA-stimulated responses: elevation of cyclic GMP in neonatal rat cerebellar slices, release of preloaded [3H]norepinephrine from superfused rat hippocampal slices and elevation of cytosolic calcium concentration in fura-2-loaded cultured rat forebrain neurons. IC50 values were 78.4, 53.4 and 1.6 microM, respectively. Finally, GDP beta S resembled known NMDA receptor antagonists in its ability to block NMDA receptor-induced seizures after i.c.v. administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct central effects of acute methylenedioxymethamphetamine on serotonergic neurons

Acute peripheral administration of either the (+) or (-) stereoisomer of methylenedioxymethamphetamine (MDMA) to rats results in a rapid loss of tryptophan hydroxylase (TPH) activity in several brain regions. This decline in enzyme activity precedes a decrease in serotonin (5-HT) concentrations in the same areas. An initial rise in the concentration of 5-hydroxyindole acetic acid after drug administration suggests that an increase in the turnover of 5-HT is an early event in the development of these changes. Unsuccessful attempts to reproduce the in vivo effects of MDMA on TPH activity using in vitro preparations such as cortical slices or the mouse mastocytoma cell line, P-815, suggested a requirement for an intact neuronal system or metabolism of the drug. Injection of MDMA directly into several brain regions also had no effect on TPH activity or 5-HT concentrations. However, when brain concentrations of MDMA were maintained using a constant i.c.v. infusion, TPH activity declined as observed following peripheral administration. The results, therefore, indicate that the acute effect of MDMA on 5-HT synthesis is a direct central effect of the drug which may be triggered by a sustained increase in transmitter turnover.

Release of [3H]norepinephrine from rat hippocampal slices by N-methyl-D-aspartate: comparison of the inhibitory effects of Mg2+ and MK-801

The excitatory amino acid receptor subtype activated by N-methyl-D-aspartic acid (NMDA) was studied using superfused slices from the rat hippocampus preloaded with [3H]norepinephrine. NMDA-induced release was inhibited by the direct receptor antagonist CPP, and was sensitive to physiological concentrations of Mg2+. NMDA-induced transmitter release in the presence of Mg2+ was demonstrable if the slices were first depolarized by exposure to elevated K+ or kainic acid to relieve the voltage-dependent Mg2+ blockade. Transmitter release was also inhibited by the indirectly acting antagonists MK-801 and phencyclidine. This effect of MK-801 showed use dependence, while inhibition of release by Mg2+ remained at a constant level with repeated agonist application. Kinetic analysis indicated the mechanism of MK-801 inhibition was uncompetitive in that agonist was required for the association of the inhibitor with the receptor-channel complex. In contrast, Mg2+ inhibited NMDA-induced transmitter release through a noncompetitive process. The two antagonists also differed in terms of reversibility with inhibition by Mg2+ being evident only in the presence of the cation. The effect of MK-801, however, was still apparent for several stimuli after removal of the drug. These results demonstrate the utility in this in vitro release system for studying the unique characteristics of the NMDA receptor complex.

Effects of methamphetamine on central monoaminergic systems in normal and ascorbic acid-deficient guinea pigs

Repeated injections (s.c.) of methamphetamine (METH) were administered to normal and ascorbic acid-deficient (scorbutic) guinea pigs to assess a potential role for ascorbic acid in the METH-induced effects in central monoaminergic systems. The ascorbic acid-deficient condition differentially influenced the METH-induced responses of dopaminergic and serotonergic variables in the striatum: drug-induced changes in dopaminergic variables were identical in normal and scorbutic animals; METH-induced decreases in serotonergic variables [tryptophan hydroxylase activity, serotonin (5-HT) and 5-hydroxyindoleacetic acid concentrations], however, were prevented in scorbutic animals. The scorbutic condition did not alter significantly the distribution of METH in the brain, nor were striatal concentrations of dopamine (DA) or 5-HT affected. In vitro, ascorbic acid increased significantly DA-mediated [3H]5-HT release from striatal slices, thus suggesting a potential role for ascorbate in DA-mediated actions of METH on serotonergic systems. Although supplemental ascorbate failed to restore the METH-induced serotonergic effects in scorbutic guinea pigs, these data suggest that, in a normal animal, the effects of multiple injections of METH, on serotonergic systems, involve ascorbic acid.

Depression of rat brain tryptophan hydroxylase activity following the acute administration of methylenedioxymethamphetamine

The psychotomimetic agent, methylenedioxymethamphetamine, produced a rapid, persistent and dose-dependent reduction in cortical tryptophan hydroxylase activity when administered acutely to rats. This effect did not occur in vitro and did not require N-demethylase activity in the whole animal. Kinetic analysis revealed the loss of enzyme activity to be due to an alteration in Vmax with no change in the affinity of the enzyme for either its cofactor or substrate. Coadministration of the serotonin (5-HT) uptake inhibitor, citalopram, only partially antagonized the loss of tryptophan hydroxylase activity 3 hr after methylenedioxymethamphetamine, but completely prevented the loss of cortical 5-HT. Recovery of enzyme activity did occur by 1 week if the neurotoxic effect of methylenedioxymethamphetamine was blocked by fluoxetine. The effect of methylenedioxymethamphetamine on 5-HT synthesis was not affected by pretreatment with alpha-methyl-p-tyrosine, reserpine or yohimbine. Ketanserine and methiothepin, 5-HT receptor antagonists, did partially block the methylenedioxymethamphetamine-induced loss of tryptophan hydroxylase activity, suggesting a possible role for neurotransmitter release in the acute effects of the drug on enzyme activity.

Acute administration of methylenedioxymethamphetamine: comparison with the neurochemical effects of its N-desmethyl and N-ethyl analogs

The acute and long-term neurochemical effects of three methylenedioxyamphetamine analogs were examined in the serotonergic system of the rat brain. Methylenedioxymethamphetamine as well as its N-desmethyl and N-ethyl derivatives depleted cortical serotonin (5HT) concentrations to less than 30% of control 3 h after drug administration. All three compounds were also very similar in their effects on [3H]5HT release from superfused rat striatal slices. Increasing the size of the N-alkyl substituent did appear to reduce the potency of the agent for inducing [3H]dopamine release. One week following drug administration cortical 5HT concentrations had returned to control levels in animals treated with the N-ethyl derivative while the other two analogs produced persistent depletions in transmitter concentrations. The effects of the latter two drugs were correlated with a significant decrease in whole brain synaptosomal [3H]5HT uptake indicating serotonergic nerve terminal damage. N-ethyl-methylenedioxyamphetamine had no effect on synaptosomal 5HT uptake at one week. The (+) stereoisomer of methylenedioxyamphetamine was slightly more potent than the (-) enantiomer at producing the long-term 5HT depletion as previously shown for its N-methyl derivative suggesting similar mechanisms may be responsible for their neurotoxic effects.

In vitro and in vivo neurochemical effects of methylenedioxymethamphetamine on striatal monoaminergic systems in the rat brain

A single high dose of methylenedioxymethamphetamine, a psychedelic agent, produced a rapid and persistent depletion of striatal indoles similar to that observed following administration of the serotonergic neurotoxin p-chloroamphetamine. The drug had little effect on dopaminergic variables. Like p-chloroamphetamine, methylenedioxymethamphetamine was found to be a relatively selective agent for inducing [3H]serotonin release in vitro. The serotonin uptake inhibitor, citalopram, blocked both [3H]serotonin release in vitro and striatal serotonin depletion in vivo, indicating that both processes were carrier dependent. In vivo comparisons of the stereoisomers of methylenedioxymethamphetamine indicated two phases of serotonin depletion similar to those reported for p-chloroamphetamine. Although both the (+)- and (-)-stereoisomers produced an acute (3 hr) decrease in striatal indoles, the long-term effects of the drug showed stereoselectivity in that the (+)-enantiomer produced the most dramatic serotonin depletion. Comparison of the effects of the stereoisomers of methylenedioxymethamphetamine and its n-desmethyl analog, methylenedioxyamphetamine, on [3H]serotonin and [3H]dopamine release in vitro showed the (+)-enantiomer of both drugs to be the more potent releasing agent. In spite of its reported lack of hallucinogenic activity, (+)methylenedioxyamphetamine was found to be of a potency similar to that of (+)methylenedioxymethamphetamine in inducing [3H]serotonin release in vitro. The results are discussed in terms of the neurochemical similarities between methylenedioxymethamphetamine and p-chloroamphetamine as well as the proposed role of serotonin release in the behavioral effects of methylenedioxymethamphetamine.

Ascorbic acid-deficient condition alters central effects of methamphetamine

Effects of multiple doses of methamphetamine (METH) were examined in normal and ascorbic acid-deficient (scorbutic) guinea pigs. METH-induced decreases of striatal serotonin concentrations were completely antagonized in scorbutic animals. Elevations of nigral substance P-like immunoreactivity also differed significantly in METH-treated scorbutic compared to METH-treated normal animals. Various lines of evidence indicate that dopamine is an essential mediator of METH-induced effects in both serotonergic and substance P systems in the brain areas examined; however, results from the present study indicate that, along with dopamine, ascorbic acid also plays a role in mediating the effects of METH in the central nervous system.

Neurotoxicity of the psychedelic amphetamine, methylenedioxymethamphetamine

The neurochemical effects of the unique psychedelic agent, methylenedioxymethamphetamine (MDMA), indicate it may be a serotonergic neurotoxin related to agents such as p-chloroamphetamine. MDMA had a biphasic effect on cortical serotonin concentrations beginning with an acute depletion of the transmitter which reached a maximum between 3 and 6 hr after drug administration. This early phase of depletion was reversible because cortical serotonin concentrations had recovered to control levels by 24 hr. However, transmitter concentrations were reduced significantly 1 week later, indicating a second phase of depletion. The latter phase of depletion was associated with a decrease in synaptosomal [3H]serotonin uptake due to a loss in the number of uptake sites with no change in the affinity of the carrier for serotonin. This neurotoxic effect of MDMA was found to be a property of the (+)-stereoisomer of the drug as only this enantiomer produced the depletion of cortical serotonin and the decrease in synaptosomal serotonin uptake at 1 week. In contrast to this, both stereoisomers of the drug could produce the acute depletion of cortical serotonin measured 3 hr after drug administration. Coadministration of the selective serotonin uptake inhibitor, fluoxetine, completely blocked the reduction in cortical serotonin concentrations 1 week after MDMA. Administration of fluoxetine at various times after MDMA revealed that the long-term effects of the drug developed independently of the acute depletion of serotonin and could be partially blocked by the uptake inhibitor as long as 6 hr after drug administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of K+-stimulated [3H]dopamine and [14C]acetylcholine release by the putative dopamine autoreceptor agonist, B-HT 920

The inhibition of K+-stimulated [3H]dopamine and [14C]acetylcholine release from preloaded rat striatal slices was used to examine the presynaptic selectivity of the putative dopamine autoreceptor agonist, B-HT 920. In the micromolar range, B-HT 920 caused a concentration-dependent inhibition of the release of both labeled neurotransmitters as evoked by 20 mM K+. The effect of B-HT 920 on both [3H]dopamine and [14C]acetylcholine release was completely blocked by (+) butaclamol but not by (-) butaclamol. Sulpiride, a selective D2 antagonist, similarly blocked the inhibitory effect of B-HT 920 on the release of both labeled neurotransmitters indicating both responses were mediated by D2 receptors. (+) Butaclamol alone elevated stimulated [3H]dopamine release suggesting a significant amount of autoreceptor occupancy by endogenously released dopamine. Experiments with tolazoline and the alpha 2 agonist, B-HT 933, did not suggest any involvement of alpha-adrenoceptor activity in the inhibitory effects of B-HT 920 on the release of either transmitter. Inhibition of release was a selective effect of B-HT 920 as the drug was without effect on the K+-stimulated release of [3H]serotonin. The results indicate that in vitro B-HT 920 is active of both pre- and postsynaptic dopamine receptors in contrast to the pattern of effects observed after its in vivo administration.

Response of mesolimbic substance P systems to methamphetamine treatment

Administration of methamphetamine (METH) has been shown to increase substance P (SP) concentrations in striatonigral structures. Data described herein demonstrate that SP pathways associated with the mesolimbic systems also are influenced by METH treatment. Specifically, multiple doses of METH significantly decreased SP concentrations in the ventral tegmental area and the medial habenular nucleus; whereas, a single METH injection increased the levels of this neuropeptide in the ventral tegmental area. The possible significance of these observations is discussed.

Methylenedioxymethamphetamine: a potentially neurotoxic amphetamine analogue

The amphetamine analogue, methylenedioxymethamphetamine (MDMA) has received considerable attention recently as a novel and increasingly popular psychoactive agent. When administered acutely to rats in high doses, MDMA caused a selective and dramatic decrease in brain concentrations of serotonin and its metabolite, 5-hydroxyindoleacetic acid. The depletion of serotonin and its metabolite persisted for up to at least one week after a single injection of MDMA at approximately four to five times the acute dose reported for humans. These results are discussed in terms of the possible neurotoxic effects of MDMA.

Cell specificity and an effect of ras on human metallothionein gene expression

The expression of three human metallothionein (hMT) genes has been compared in various established cell lines and primary liver. The single gene for hMT isoform II is ubiquitously expressed in all cell types in response to cadmium. In contrast, two genes encoding hMT-I isoforms are expressed in a highly specific, reciprocal fashion that correlates with the embryonic germ layer origin of the cells. In one cell line that failed to express detectable amounts of hMT-IE, treatment with the demethylating agent 5-azacytidine led to cadmium-inducible expression of this subtype. The genes for both MT-I isoforms are coordinately inducible by heavy metals but differ in their response to glucocorticoids. Surprisingly, cells transformed with the Ha-ras oncogene contain elevated basal levels of both MT-I and MT-II RNA. The implications of these results for growth-related and developmental functions of MT are discussed.

Effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on striatal tyrosine hydroxylase and tryptophan hydroxylase in rat

The results reported here indicate that treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) caused significant changes in the dopamine-synthesizing enzyme, tyrosine hydroxylase. The authors examined the effects of two doses of MPTP on the activities of tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH) in the striatum, and also the time-course of these effects. Rats received an intraperitoneal loading dose, followed by a 24-hr infusion of MPTP (total doses of 21 or 42 mg) from subcutaneously-implanted osmotic pumps. Seven days after treatment, the activity of tyrosine hydroxylase was decreased by MPTP (42 mg); however, the activity of tryptophan hydroxylase was not affected. In time-course experiments, the activity of tyrosine hydroxylase was maximally reduced at 3 and 7 days after treatment with MPTP (42 mg). The activity of tryptophan hydroxylase did not significantly change at any time-point. Concurrent administration of haloperidol (HALO; 2 mg/kg, 4 doses) with MPTP significantly enhanced the depression of the activity of tyrosine hydroxylase in the striatum caused by MPTP, while treatment with haloperidol alone had no such effect. Concentrations of dopamine in the striatum were maximally decreased to approx. 50% of control in animals treated with haloperidol and MPTP (42 mg), whereas treatment with MPTP alone decreased concentrations of dopamine to approx. 70% of control.

Studies on the mechanism of tolerance to methamphetamine

We have reported that the ability of high doses of methamphetamine to impair dopamine and serotonin synthesis in the rat brain is attenuated when animals are pretreated with gradually increasing doses of methamphetamine. To examine the mechanism of this tolerance phenomenon, the effect of methamphetamine on several neurochemical parameters was determined in naive and methamphetamine-pretreated rats. The elevation of nigral substance P concentrations by methamphetamine was attenuated in pretreated compared to naive rats. The methamphetamine-induced reduction in [3H]sulpiride binding in the rat neostriatum and nucleus accumbens was similarly attenuated in animals pretreated with methamphetamine. Determination of brain concentrations of methamphetamine and amphetamine revealed significantly lower concentrations of both compounds in the brains of pretreated compared to naive animals. The results indicate a reduction in the ability of methamphetamine to increase dopamine transmission in the brains of methamphetamine-pretreated rats. Furthermore, this effect appears to be due, at least in part, to a change in the disposition of methamphetamine in pretreated animals.

Structure and expression of two human metallothionein-I isoform genes and a related pseudogene

Three members of the human metallothionein-I gene family have been cloned and characterized. Two of the genes encode closely related but distinct metallothionein-I subtypes. Both of these genes are functional as shown by their transcription in cultured hepatoblastoma cells and by their ability to render transfected cells resistant to cadmium toxicity. The cotranscription of these nonallelic genes shows that the previously observed microheterogeneity of metallothionein-I protein preparations is due to the expression of distinct gene products. The third clone is incapable of encoding a typical metallothionein due to an early termination codon and two nonconservative amino acid replacements. This nonfunctional pseudogene retains introns. Evolutionary comparisons reveal conserved DNA sequences in both the coding and regulatory regions of these genes.

Role of dopamine in the neurotoxic effects of methamphetamine

Multiple administrations of high doses of methamphetamine to rats cause long-term depression of both dopamine and serotonin synthesis. Coadministration of the catecholamine synthesis inhibitor, alpha-methyl-p-tyrosine, antagonizes this effect of methamphetamine on both neurotransmitter systems. However, when catecholamine synthesis was maintained by the administration of L-dopa and the peripheral decarboxylase inhibitor R04-4602, alpha-methyl-p-tyrosine no longer prevented the effects of methamphetamine on either dopamine or serotonin synthesis. In addition, the administration of the specific dopamine uptake blocker, amfonelic acid, significantly attenuated the changes in the serotonin synthesizing enzyme, tryptophan hydroxylase, resulting from multiple high doses of methamphetamine. The ability of a single administration of methamphetamine to depress tryptophan hydroxylase was also dependent on catecholamine synthesis. These results suggest that dopamine plays an important role in the changes mediated by the administration of methamphetamine in both the dopaminergic and serotonergic systems.