Selective effects of thiol reagents on the binding sites for imipramine and neurotransmitter amines in the rat brain

Effect of thiamazole on kainic acid-induced seizures in mice

Kainic acid (KA) induced epileptic seizures in mice is a commonly used experimental model of epilepsy. Previous studies have suggested the roles of various neurotransmitters and oxidative stress in KA-induced seizures. An important role of hypothyroidism has also been suggested in epilepsy. Thiamazole (TZ) is an anti-hyperthyroid drug with antioxidant property. This study reports the effect of TZ on KA-induced epileptic seizures in mice, produced by intraperitoneal (IP) injection of KA (18 mg/kg). Prior to KA injection, the animals were treated with TZ (12.5, 25 and 50 mg/kg IP). Our results showed that in KA alone group, about half of the animals developed seizures. Pre-treatment of mice with TZ significantly increased the frequency of seizures in dose-dependent manner. Administration of TZ significantly reduced the latency time and aggravated the severity of seizures. TZ also increased the mortality in KA-treated mice. Striatal dopamine and serotonin levels were markedly increased in KA alone treated mice, which were not significantly affected by TZ treatment. Among the indices of oxidative stress, we observed a significant reduction in cerebral vitamin E whereas the levels of cerebral malondialdehyde and conjugated dienes were significantly increased in animals with high severity of seizures. In conclusion, TZ potentiated the frequency and severity of experimental seizure in mice. There is a possibility of altered metabolism of KA in presence of TZ that might have potentiated the toxicity of KA. These findings suggest a caution while administering anti-hyperthyroid drugs in epileptic seizures.

External cysteine residues in the serotonin transporter

Hydropathy analysis predicts three cysteines (C109, C200, and C209) in extracellular loops of the rat serotonin transporter (SERT). We mutated these residues, singly and in combination, to either alanine or serine and expressed the mutant transporters in HeLa cells using the vaccinia-T7 transient expression system. Mutation of C109 to alanine had no effect on transport activity or surface expression of the transporter. In Na+-containing solutions, methanethiosulfonate (MTS) reagents had little effect on transport activity in the wild type or in the C109A mutant. When Na+ was replaced with Li+, inactivation of wild type by MTS reagents increased dramatically, but C109A was still resistant. The results suggest that C109 is exposed to the external medium in a manner dependent on cation binding. Replacing either C200 or C209 with serine resulted in either a partial (C200S) or almost total (C209S) loss of transport activity. MTS reagents rapidly inactivated transport activity in mutant C200S, suggesting increased accessibility of a previously unreactive cysteine residue. The double mutants C200S-C109A and C200S-C209S each retained partial activity. C200S-C109A was very sensitive to MTS reagents, but the C200S-C209S mutant was much less sensitive, similar to the wild type transporter. Replacement of C200 or C209 with serine dramatically decreased surface expression of the fully glycosylated transporter. Expression was normal, however, in the C200S-C209S double mutant. The Na+ dependence of transport and ligand binding was abnormal in both C200S and C200S-C209S mutants. Replacing C200 or C209 had similar effects on Na+ dependence and surface expression. Together with the increased MTS reactivity of C200S, these results support the possibility that C200 and C209 may be linked by a disulfide bond in the second external loop of SERT.

Effects of sodium metabisulphite on guinea pig contractile airway smooth muscle responses in vitro

BACKGROUND

Sodium metabisulphite (MBS) is known to induce bronchoconstriction in asthmatic patients. The effects of MBS on guinea pig airway smooth muscle and on neurally mediated contraction in vitro have been examined.

METHODS

Tracheal and bronchial airway segments were placed in oxygenated buffer solution and electrical field stimulation was performed in the presence of indomethacin (10(-5) M) and propranolol (10(-6) M) for the measurement of isometric tension. Atropine (10(-6) M) was added to bronchial tissues.

RESULTS

Concentrations of MBS up to 10(-3) M had no direct effect on airway smooth muscle contraction and did not alter either tracheal smooth muscle contraction induced by electrical field stimulation at all frequencies or acetylcholine-induced tracheal smooth muscle contraction. There was a similar response in the absence of epithelium, except for potentiation of the response induced by electrical field stimulation at 0.5 Hz (24 (10)% increase). However, MBS (10(-5), 10(-6) and 10(-7) M) augmented neurally-mediated non-adrenergic non-cholinergic contractile responses in the bronchi (13.3 (3.2)%, 23.8 (9.6)%, and 6.4 (1.6)%, respectively). MBS had no effect on the contractile response induced by substance P, but at higher concentrations (10(-3) M and 10(-4) M) it caused a time-dependent attenuation of responses induced by either electrical field stimulation or exogenously applied acetylcholine or substance P.

CONCLUSIONS

MBS had no direct contractile responses but enhanced bronchoconstriction induced by activation of non-cholinergic neural pathways in the bronchus, probably through increased release of neuropeptides. At high concentrations MBS inhibited contractile responses initiated by receptor or neural stimulation.

RC3/neurogranin structure and expression in the caprine brain in relation to congenital hypothyroidism

In view of the profound effects of thyroid hormone deficiency on the central nervous system (CNS), neuronal genes regulated by thyroid hormone could potentially be involved in the development of the CNS. Expression of the neuronal gene RC3/neurogranin was shown to be induced by thyroid hormone in the rat. No data are available on RC3 expression in mammals with prenatal brain development, like humans. To study RC3 mRNA expression in a genetic in vivo model of congenital hypothyroidism, which also resembles the human situation in the timing of brain development relative to birth, we used an inbred strain of congenitally hypothyroid goats. We isolated a cDNA for the caprine RC3 homolog. The deduced amino acid sequence had 99% similarity with the rat and bovine protein sequence. An analysis of the developmental expression of RC3 mRNA levels showed a 3-fold increase between E90 and P0. In situ hybridization analysis showed that in euthyroid goats, the RC3 expression pattern was region-specific and resembled that in rats. However, in contrast to rats, hypothyroid goats showed only a reduced RC3 mRNA expression in the striatum. Hypothyroidism had no effect on RC3 mRNA expression in all other brain regions. T4-treatment of the hypothyroid fetus increased RC3 mRNA expression in the striatum to euthyroid control levels. These data suggest that thyroid hormone is a regulator of RC3 gene expression in the caprine brain, and that the striatum is highly sensitive to thyroid hormone deficiency.

Sodium ion-dependent transporters for neurotransmitters: a review of recent developments

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Effect of oxidative stress on receptors and signal transmission

Reactive oxygen metabolites affect binding of ligands to membrane receptors and also coupling of receptors to G-proteins and effector enzymes. Peroxidation of membrane lipids may lead to a lowered receptor density and also will alter the viscosity of the plasma membrane, which affects receptor coupling. Reactive oxygen species may also interact with thiol/disulfide moieties on receptor proteins or on other factors in the receptor system, which is responsible for alterations in receptor binding or coupling. Moreover, lipid peroxidation is associated with the phospholipase A2 pathway, which might indirectly affect receptor function. Moreover, oxidative stress may lead to a disturbance in cellular Ca(2+)-homeostasis. This might be related to an effect on Ca(2+)-mobilizing receptors, but there is also evidence for a decreased Ca(2+)-sequestration by ATPases. In addition, peroxidation of membrane lipids increases membrane permeability to Ca2+. Finally, reactive oxygen species interfere with actions of nitric oxide, thus affecting another pharmacological messenger system.

Effects of thyroid status on the characteristics of alpha 1-, alpha 2-, beta, imipramine and GABA receptors in the rat brain

The effects of a chronic treatment with L-triiodothyronine (T3; 100 mg/rat/day s.c. for 7 days) or with propylthiouracil (PTU; 50 mg/rat/day for 35 days by stomach tube) on the characteristics of alpha 1, alpha 2, beta, imipramine and GABA binding sites in different brain areas of the adult rat have been studied. T3-treatment caused an increase in the number of [3H]dihydroalprenolol and a decrease in the number of [3H]muscimol binding sites in the cerebral cortex. PTU-treatment caused a decrease in the number of [3H]prazosin, [3H]yohimbine and [3H]dihydroalprenolol binding sites in the cerebral cortex, while the number of [3H]imipramine binding sites was reduced in the cerebral cortex and hypothalamus, and increased in the hippocampus. Affinity constants were never modified. Concurrent experiments showed that the "in vitro" addition of T3 and PTU did not influence the binding of any of the ligands employed to control rat brain membranes. The present data further support the view that neurotransmission in the CNS is influenced by the thyroid status.

Dopaminergic dysfunction in neonatal hypothyroidism: differential effects of GM1 ganglioside

The effects of daily treatment with GM1 ganglioside (30 mg/kg s.c.) from birth to day 30, on striatal pre- and postsynaptic markers of the dopaminergic system in euthyroid- and 32 day-old hypothyroid rats were studied. The purpose was to assess whether GM1 could prevent the extensive, hypothyroidism-provoked impairment of dopaminergic neurotransmission. Neonatal administration of GM1 well counteracted the hypothyroidism-related deficits in striatal synaptosomal uptake of [3H]dopamine and in membrane binding of [3H]tyramine, a putative marker for the vesicular carrier of dopamine. In the hypothyroid striatum, the decrease of concentrations of DOPAC and HVA, the loss of [3H]SCH-23,390-labelled D1-receptors and the decrease of basal- or dopamine-stimulated, D1-mediated activity of adenylate cyclase were not prevented by GM1. Although somatic and neurobehavioural aberrations of hypothyroids were not at all or only partially ameliorated, a slight improvement of the thyroid status was suggested by less decreased levels of serum thyroxine (T4) after treatment with GM1. The ganglioside-driven selective recovery of the transport and storage process of [3H]dopamine might result either from a chronically-exerted stimulation by GM1 on the NA/K- and Mg-ATPase activities, thus reflecting on the ATPase-dependent neuronal and vesicular transport processes of dopamine or from a GM1-promoted maturation of the otherwise retarded functionality of dopaminergic nerve endings in the neonatal hypothyroid striatum.

Differential scanning calorimetry study of the interaction of antidepressant drugs, noradrenaline, and 5-hydroxytryptamine with a membrane model

Differential scanning calorimetry was used to study the interaction of a membrane model with two neuromediators [noradrenaline and 5-hydroxytryptamine (Serotonin)] and four antidepressant drugs (imipramine, indalpine, citalopram, and milnacipran), known to be uptake inhibitors of these neuromediators. The study was carried out on dipalmitoyl phosphatidylcholine liposomes, a bilayer phospholipid system taken as a simplified model of biological membranes. Analysis of the thermograms led us to classify the molecules according to their lipophilic action, as in the conventional measurement of the water-octanol partition coefficient, and also enabled us to precisely determine their location along the phospholipid bilayer. A hypothesis based on this localization is put forward concerning the competitive, or otherwise, character of the blocking of uptake of the neuromediators. The extreme cases of interaction and localization of imipramine and milnacipran, a new antidepressant, relative to the bilayer are also analyzed in terms of side effects.

Chemical heterogeneity of [3H]imipramine binding sites on human platelet membranes

Functional groups essential for high- and low-affinity [3H]imipramine (IMI) binding were determined by the method of chemical modification. The high-affinity recognition sites contained cysteine and lysine amino acid residues, but not aspartic or glutamic acid residues. The low-affinity recognition sites contained only cysteine residues. Moreover, probably only part of these sites contained these residues. The arginine, tyrosine and histidine residues are not likely to be functionally important for the [3H]IMI binding process. Analysis of the structure-function interaction of drug molecules reveals that, for all substances with high displacement ability, there is a conformation in which they can react with high-affinity IMI recognition sites. Data obtained allowed us to construct a tentative structure model of the high-affinity recognition IMI binding site.

Neonatal hypothyroidism induces striatal dopaminergic dysfunction

Oral administration of the antithyroid drug methimazole (50 mg/kg per day) to rats during the last six days of pregnancy, and subsequent daily s.c. injection of methimazole (20-30 mg/kg) to their pups from birth to postnatal day 30 provoked hormonal and somatic alterations resembling (with all caution to any association between rodent and human data) those of congenital hypothyroidism. The steady-state concentrations of striatal dopamine were similar in hypothyroid and euthyroid, 32-day-old rats, while the levels of the dopamine metabolites 3,4-dihydroxyphenylacetic and homovanillic acids were markedly decreased in hypothyroidism. The results of this and our earlier study [Vaccari A. and Gessa G. L. (1989) Neurochem. Res. 14, 949-955] show that the maximal synaptosomal uptake of [3H]dopamine, an index for the density of nigrostriatal dopaminergic terminals, and the maximum number of membrane [3H]tyramine binding sites, reflecting the concentration of the vesicular transporter for dopamine, were decreased in the hypothyroid striatum. There was also a loss of those D1-type dopaminergic receptors claimed to be located on neurons intrinsic to the striatum, and, consequently, dopamine-stimulated, D1-regulated adenylate cyclase activity was depressed. It is suggested that individual dopaminergic nerve endings in the neonatal hypothyroid striatum must contain more dopamine, owing to some loss of pertinent innervation and, therefore, to the presence of less vesicular transport sites for dopamine. Hypothyroidism-related decreases in the maximum number of striatal D1- and, reportedly, D2-receptors, plus the impairment of D1-coupled second messenger activity, may play a role in the derangement of those neurobehavioural patterns where a dopaminergic regulation is putatively implied.

A common binding site for tricyclic and nontricyclic 5-hydroxytryptamine uptake inhibitors at the substrate recognition site of the neuronal sodium-dependent 5-hydroxytryptamine transporter

An investigation of the site of interaction of a variety of tricyclic and nontricyclic 5-HT uptake inhibitors with the neuronal sodium-dependent 5-HT transporter was undertaken. The dissociation of [3H]paroxetine binding induced by indalpine (10 microM), SL 81.0385 (10 microM), fluoxetine (10 microM), citalopram (10 microM), paroxetine (0.15 microM), imipramine (10 microM) and 5-HT (50 microM) produced monophasic dissociation curves and gave t1/2 values of dissociation similar to that induced by dilution alone. In inhibition studies of [3H]paroxetine binding with citalopram, imipramine and 5-HT, increases in the concentration of [3H]radioligand used led to parallel rightward shifts of the inhibition curves with no diminution of the maximum degree of inhibition (Imax). "Schild-type" analyses of the data obtained from the inhibition curves with these 3 compounds gave slopes close to unity. In chemical modification studies, treatment of membrane fractions with N-ethylmaleimide led to a pronounded reduction in specific [3H]paroxetine binding. Preincubation of these membranes with SL 81.0385, fluoxetine, imipramine, tryptamine and 5-HT provided significant protection against this NEM-induced inactivation. The above findings are interpreted to provide evidence for a common or at least overlapping binding site for the tricyclic and nontricyclic 5-HT uptake inhibitors with the substrate recognition site of the neuronal sodium-dependent 5-HT transporter.

[3H]tyramine binding: a comparison with neuronal [3H]dopamine uptake and [3H]mazindol binding processes

[3H]Spiperone [( 3H]SPI) binding sites in rat or bovine striata have been solubilized using CHAPS or digitonin detergents. Solubilized sites retained the binding characteristics of those in native membrane preparations. The same solubilized material, however, did not bind [3H]tyramine [( 3H]PTA), thus indicating that [3H]PTA binding sites and DA receptors are different chemico-physical entities. In membrane preparations or crude synaptosomes obtained from the c.striatum of neonatally-rendered hypothyroid rats, when central DA-pathways are impaired, both [3H]PTA binding and [3H]DA uptake processes were markedly decreased, with no effect on [3H]mazindol [( 3H]MAZ) binding, compared to euthyroids. Reserpine, a well-known inhibitor of DA-uptake into a variety of secretory vesicles, and a potent in vivo and in vitro inhibitor of [3H]PTA binding, did not affect the [3H]MAZ binding process. This further supported the suggestion that while [3H]PTA binding sites are almost totally associated with the vesicular transporter for DA, [3H]MAZ does label a site involved in the DA-translocation across the neuronal membrane. The latter process seems to be rather insensitive to thyroid hypofunction, when however the intracellular storage of DA might be consistently impaired. In conclusion, PTA might be well exploited as a marker of the DA vesicular transporter through its molecular characterization, whenever possible.

Evidence for the existence of at least two different binding sites for 5HT-reuptake inhibitors within the 5HT-reuptake system from human platelets

Chemical modification procedures have been used to study the interaction of tricyclic and non-tricyclic 5HT-reuptake inhibitors with the [3H]imipramine binding site (IBS). N-Ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) induced a pronounced loss in [3H]imipramine binding due to a reduction in Bmax. Preincubation with reuptake inhibitors and subsequent inactivation by EEDQ revealed that imipramine and 5HT prevented the EEDQ-induced inhibition, but citalopram and fluoxetine did not. Thiol modification studies demonstrated that reduction by dithiothreitol (DTT) enhanced the binding of [3H]imipramine by increasing the Bmax. The thioselective reagents 1,1-diazobis- (N,N-dimethylformamide) (diamide), phenyl-arsineoxide (PAO) and N-ethylmaleimide (NEM) attenuated the binding capacity by lowering the Bmax. PAO, a reversible thiol reagent, prevented NEM alkylation indicating that dithiols are involved in the NEM-induced inactivation. Binding of tricyclics or non-tricyclics prior to PAO inactivation revealed that tricyclics provide complete protection against thiol modification, while the non-tricyclics do not. The results support the hypothesis that the 5HT-reuptake system of human platelets possesses at least two distinguishable binding sites.

High- and low-affinity [3H]imipramine binding sites on human platelets: separate determination and involvement of sulphur-containing bonds

We have confirmed the presence of two different classes of [3H]imipramine ([3H]IMI) binding sites on human platelets: high-affinity (Kd = 0.52 nM, Bmax = 1670 fmol/mg protein) and low-affinity (Kd = 101 nM, Bmax = 8,000 fmol/mg protein) binding sites. The high-affinity component of [3H]IMI binding can also be obtained separately as the difference between specific [3H]IMI binding in Na-containing and Li-containing incubation buffer. The low-affinity component can be obtained as the difference between [3H]IMI binding in 50 mM Tris-HCl, 5 mM KCl, 120 mM LiCl, (pH 7.5) in the absence and presence of 0.1 mM IMI. The chemical modification of SH groups was performed with Ellman's reagent (10 mM, 40 min at 23 degrees C). The high-affinity component of the binding was totally inhibited while the low-affinity component only decreased by 39%. No decrease in [3H]IMI specific binding was observed when the modification of SH groups was carried out in the presence of 1 microM IMI. The inhibition of high- and low-affinity [3H]IMI binding was reversible since it was completely restored by incubation of modified membranes with 1,4-dithioerythritol (DTE). The reduction of SS groups by DTE (10 mM, 1 h at 23 degrees C) in the intact membrane preparation produced an increase in total number of binding sites of the high-affinity component of [3H]IMI binding by 50%.