Assessment of the contribution of alpha 1-acid glycoprotein to the serum binding of basic drugs using serum treated with sulphosalicylic acid and DEAE-cellulose

Treatment of human serum with DEAE-cellulose in acid conditions almost completely removed alpha 1-acid glycoprotein (alpha 1-AG) with little change in the concentration of albumin and beta-lipoprotein, while treatment with sulphosalicylic acid removed almost all the proteins except alpha 1-AG. The binding of various drugs to serum treated as above was measured by equilibrium dialysis and the contribution of alpha 1-AG to drug binding by human serum was assessed. Sulphosalicylic acid-treated serum exhibited a saturable binding for propranolol, which was considered to be due to the binding to alpha 1-AG while DEAE-cellulose-treated serum mostly exhibited non-saturable binding, for which albumin and beta-lipoprotein may be responsible. With this treated serum, alpha 1-AG was estimated to contribute approximately 40% to the binding of therapeutic concentrations of propranolol, 15% to that of imipramine and 15-20% to that of desipramine, respectively, in serum samples pooled from healthy adults. However, no contribution of alpha 1-AG was observed in the binding of salicylic acid to the serum. Dissociation constants of the propranolol binding to the high affinity site in control serum, sulphosalicylic acid-treated serum and purified alpha 1-AG showed similar values (3.7-6.7 microM). These results suggest that treatment of serum with sulphosalicylic acid and DEAE cellulose is useful in assessing the contribution of alpha 1-AG to the serum binding of basic drugs.     

The effects of electroconvulsive shock or imipramine on subtypes of alpha 1-adrenoceptors in the frontal cortex of the rat.

The effects of repeated treatment (14 days) with electroconvulsive shock (ECS) or imipramine on binding sites on alpha 1-adrenoceptors in the rat were studied. The binding of [3H]prazosin studied with WB4101 and phentolamine, as binding inhibitors, showed the existence of two subtypes of alpha 1-adrenoceptor (alpha 1A and alpha 1B). Proportions of the alpha 1A and alpha 1B binding sites were about 3:7 in the frontal cortex and 9:1 in the hippocampus. Pretreatment of the membranes with chlorethylclonidine (CEC) almost abolished the alpha 1B binding sites. Inhibition of the binding of [3H]prazosin studied with antidepressants (imipramine, desipramine, maprotiline and mianserin) showed that these drugs bound to alpha 1-adrenoceptors with low affinity, in an apparent monophasic manner. The characteristics of the alpha 1A and alpha 1B binding sites were studied by the binding assay with [3H]prazosin, in the presence of a small concentration (2 nM) of WB4101 to mask the alpha 1A binding sites, as well as the assay without WB4101, for the total alpha 1-adrenoceptor (alpha 1A and alpha 1B) binding. Repeated treatment with electroconvulsive shock increased but that with imipramine decreased, the density of the alpha 1B binding sites in the frontal cortex, without change of the affinity. Neither treatment affected the alpha 1A binding sites in the frontal cortex. The alpha 1-adrenoceptors (alpha 1A and alpha 1B) in the hippocampus were not affected at all by these repeated treatments. The electroconvulsive shock-induced increase in the alpha 1B binding sites in the frontal cortex of the rat could contribute to differences in clinical effects between electroconvulsive shock and antidepressant drugs.     

Auramine O as a fluorescent probe for the binding of basic drugs to human alpha 1-acid glycoprotein (alpha 1-AG). The development of a simple fluorometric method for the determination of alpha 1-AG in human serum

A cationic fluorescent dye, auramine O (AO), exhibited an intense increase in fluorescence after binding to human alpha 1-acid glycoprotein (alpha 1-AG). The interaction between AO and the protein was studied by fluorescence spectroscopy and by equilibrium dialysis. AO binds to the protein via a single site with a dissociation constant of 24 microM. Various basic drugs such as chlorpromazine, imipramine, desipramine, quinidine, propranolol and lidocaine, which are known to bind to the protein, competitively inhibited the AO binding to the protein. The dissociation constants of these basic drugs obtained from such inhibitory experiments were comparable to those obtained with other methods (equilibrium dialysis, quenching of protein intrinsic fluorescence, and the difference spectrophotometric method) and from the literature. It is concluded that AO may be a useful fluorescent probe that binds to a single basic drug binding site on alpha 1-AG. In addition, a simple fluorometric method for the determination of alpha 1-AG in serum was developed using AO, and the validity of this method was confirmed by comparing it with the conventional radial immunodiffusion method.     

Antidepressant drugs potentiate the alpha 1-adrenoceptor effect in hippocampal slices

The effect of prolonged treatment with antidepressant drugs on the phenylephrine- and norepinephrine (NE)-evoked reaction in hippocampal slices was examined by extracellular recording of the spontaneous activity of CA1 layer neurons. The alpha 1-adrenoceptor agonists, phenylephrine and methoxamine, depressed the neuronal discharges of most of the units tested, while NE evoked both excitatory and inhibitory effects which were blocked by propranolol and phentolamine or prazosin, respectively. Imipramine, mianserin, (+)- and (-)-oxaprotiline administered subchronically (10 mg/kg p.o., twice daily for 14 days, withdrawal 48 h), potentiated the inhibitory reaction to phenylephrine. Mianserin was the only drug tested in the acute dose to effectively augment the reaction to alpha 1-adrenoceptor stimulation. Prolonged administration of mianserin and imipramine attenuated the excitatory effect to NE, which probably reflects beta-receptor down-regulation; however, only mianserin, but not imipramine, enhanced the NE-induced inhibition. The observed potentiation of the alpha 1-adrenoceptor-related inhibitory reaction to phenylephrine produced by antidepressant drugs may reflect the development of the alpha 1-adrenergic system supersensitivity in the hippocampus.     

Antidepressant drugs given repeatedly increase binding to alpha 1-adrenoceptors in the rat cortex

The effect of antidepressant drugs, administered repeatedly, on binding to alpha 1-adrenoceptors in the rat cerebral cortex was studied using [3H]prazosin as a ligand. Imipramine, amitriptyline, citalopram and mianserin increased the binding (Bmax) assessed at 8:00 h. At 20:00 h such an effect was produced by imipramine, citalopram and mianserin. [3H]Prazosin binding in control rats was higher at 20:00 h than at 8:00 h. An increase in the density of alpha 1-adrenoceptors may be associated with the therapeutic activity of antidepressant drugs.     

Interaction between antidepressants and alpha 1-adrenergic receptor antagonists on the binding to alpha 1-acid glycoprotein

alpha 1-Receptor antagonists and antidepressant agents are basic (cationic) drugs that are known to bind to alpha 1-acid glycoprotein (AAG). Since these drugs are frequently co-administered and since they bind to the same protein, this investigation was designed to evaluate the "in vitro" ability of antidepressants, alpha 1-receptor antagonists, and propranolol to displace [3H]imipramine and [3H]prazosin from the AAG binding site(s). Equilibrium dialysis was employed. Of the drugs studied, the following order of potency in displacing [3H]prazosin was found: trazodone greater than prazosin greater than doxazosin greater than propranolol greater than doxepin = amoxapine = trimazosin = amitriptyline greater than imipramine greater than nortriptyline = desipramine = nomifensine greater than bupropion = maprotiline. [3H]lmipramine binding from AAG was displaced with the following potency order: prazosin greater than imipramine greater than propranolol greater than doxazosin greater than nortriptyline greater than desipramine greater than trimazosin. Tricyclic antidepressants produced similar degrees of displacement of both [3H]imipramine and [3H]prazosin from AAG; whereas, alpha 1-receptor antagonists were more effective displacers of [3H]prazosin than of [3H]imipramine. Furthermore, the demethylated metabolites of imipramine and amitriptyline were less potent displacers than their parent compounds. These results suggest that more than a single binding site may be available for binding to AAG and that hydrophobic bonding is important in the binding of drugs to AAG.     

The interaction of antidepressant drugs with enteric 5-HT7 receptors

In this study the functional interaction of the antidepressant drugs amitriptyline, mianserin, maprotiline, imipramine, fluoxetine and the putative antidepressant drug flibanserin has been studied on 5-HT7-mediated responses to 5-carboxamidotryptamine (5-CT) in the guinea-pig ileum. 5-CT induced a concentration-dependent inhibition of the contractile response to substance P (100 nM). Except for fluoxetine and flibanserin, all the antidepressants antagonized by different degrees the 5-CT inhibitory response with the following rank affinity order: mianserin > maprotiline > imipramine > amitriptyline. Mianserin was the only antidepressant to show a profile of competitive antagonism at 5-HT7 receptors in a tenfold range of concentrations (0.1-1 microM), with an affinity (pA2) value of 8.1 +/- 0.6. The antagonism of the other antidepressants was not concentration-dependent (amitriptyline) or was associated with slight or moderate reduction of the maximal 5-CT response (imipramine or maprotiline). The apparent affinity (pKB) values were: amitriptyline, 7.0 +/- 0.2; maprotiline, 7.3 +/- 0.6; imipramine, 7.2 +/- 0.4. Our results show that various antidepressant drugs belonging to different chemical classes behave as antagonists at enteric 5-HT7 receptors through competitive or allosteric mechanisms. This evidence extends our previous findings demonstrating the interaction of antidepressants with other 5-HT receptors, namely 5-HT3 and 5-HT4 receptors.     

Inhibition and possible induction of rat CYP2D after short- and long-term treatment with antidepressants

The aim of this study was to investigate the influence of tricyclic antidepressants (imipramine, amitriptyline, clomipramine, desipramine), selective serotonin reuptake inhibitors (SSRIs: fluoxetine, sertraline) and novel antidepressant drugs (mirtazapine, nefazodone) on the activity of CYP2D, measured as a rate of ethylmorphine O-deethylation. The reaction was studied in control liver microsomes in the presence of the antidepressants, as well as in microsomes of rats treated intraperitoneally for one day or two weeks (twice a day) with pharmacological doses of the drugs (imipramine, amitriptyline, clomipramine, nefazodone 10 mg kg(-1) i.p.; desipramine, fluoxetine, sertraline 5 mg kg(-1) i.p.; mirtazapine 3 mg kg(-1) i.p.), in the absence of the antidepressants in-vitro. Antidepressants decreased the activity of the rat CYP2D by competitive inhibition of the enzyme, the potency of their inhibitory effect being as follows: clomipramine (K(i) = 14 microM) > sertraline approximate, equals fluoxetine (K(i) = 17 and 16 microM, respectively) > imipramine approximate, equals amitriptyline (K(i) = 26 and 25 microM, respectively) > desipramine (K(i) = 44 microM) > nefazodone (K(i) = 55 microM) > mirtazapine (K(i) = 107 microM). A one-day treatment with antidepressants caused a significant decrease in the CYP2D activity after imipramine, fluoxetine and sertraline. After prolonged administration of antidepressants, the decreased CYP2D activity produced by imipramine, fluoxetine and sertraline was still maintained. Moreover, amitriptyline and nefazodone significantly decreased, while mirtazapine increased the activity of the enzyme. Desipramine and clomipramine did not produce any effect when administered in-vivo. The obtained results indicate three different mechanisms of the antidepressants-CYP2D interaction: firstly, competitive inhibition of CYP2D shown in-vitro, the inhibitory effects of tricyclic antidepressants and SSRIs being stronger than those of novel drugs; secondly, in-vivo inhibition of CYP2D produced by both one-day and chronic treatment with tricyclic antidepressants (except for desipramine and clomipramine) and SSRIs, which suggests inactivation of the enzyme apoprotein by reactive metabolites; and thirdly, in-vivo inhibition by nefazodone and induction by mirtazapine of CYP2D produced only by chronic treatment with the drugs, which suggests their influence on the enzyme regulation.     

Inhibition of binedaline binding to human alpha 1-acid glycoprotein and other serum proteins by chlorpromazine, imipramine, and propranolol

Binedaline (1-[[2-(dimethylamino)ethyl]methylamino]- 3-phenylindole) binding to human alpha 1-acid glycoprotein and other serum proteins was studied in the presence of three basic compounds: chlorpromazine, propranolol, and imipramine. In serum, at therapeutic concentrations, binedaline binding was not modified by the presence of these three compounds, nor did binedaline inhibit the binding of these compounds. With isolated alpha 1-acid glycoprotein, the four drugs exhibited competitive inhibition indicating that they share a common binding site on this protein.     

Tail suspension test does not detect antidepressant-like properties of atypical antipsychotics

The purpose of this study was to investigate the potency of atypical antipsychotics (clozapine, olanzapine, amisulpride, quetiapine, aripiprazole, risperidone) to reduce immobility time and to increase the fighting power, and the number of fights in an automated version of the tail suspension test in C57BL/6J mice. Antidepressant drugs, citalopram and imipramine were tested for comparison. Olanzapine (0.125-5 mg/kg), amisulpride (0.5-2 mg/kg), quetiapine (0.25-2 mg/kg), aripiprazole (0.25-1 mg/kg), and risperidone (0.005-0.05 mg/kg) did not produce any antidepressant-like effect. Only clozapine (0.156-2.5 mg/kg), administered at a dose of 0.312 mg/kg, significantly increased the number of fight episodes. As expected, citalopram (20-40 mg/kg) and imipramine (10-30 mg/kg) dose dependently produced antidepressant-like activity in the same procedure. The effect of antipsychotics on spontaneous locomotor activity and MK-801-induced or d-amphetamine-induced hyperactivity, were also tested in CD-1 mice to confirm the active doses of these drugs in tests commonly used to predict antipsychotic-like activity. Careful screening of potential antipsychotics for their antidepressant effects is considered to be an important part of modern drug development. Our data suggest that the tail suspension test in mice may be relatively insensitive to antidepressant-like activity of atypical antipsychotic drugs with antidepressant properties confirmed by clinical trials.     

Down-regulation of central serotonin S2 receptors after repeated treatment with quinupramine in rats

The present studies were undertaken to determine whether the repeated administration of quinupramine caused down- or up-regulation of beta-, alpha 2-adrenergic, serotonin S2, imipramine and muscarinic cholinergic receptors, as had been demonstrated for tricyclic and atypical antidepressant drugs. Quinupramine administered at 10 mg/kg (p.o.) twice daily for 10 days caused a down-regulation of serotonin S2 receptors in the frontal cortex of the rat as determined by [3H]ketanserin binding. However, quinupramine did not alter the binding populations of beta-adrenergic, muscarinic cholinergic and alpha 2-adrenergic receptors in the rat brain as determined by the Scatchard analysis of the [3H]ligand binding data. Differing from quinupramine, imipramine caused down-regulation of beta-adrenergic and serotonin S2 receptor bindings, and it caused slight but significant up-regulation of muscarinic cholinergic receptor bindings. These results show that the antidepressant activity of quinupramine is associated with the central serotonin system, but not with the beta-adrenergic system. Accordingly, quinupramine, chemically one of the typical tricyclic antidepressant drugs, seems to be pharmacologically one of the atypical antidepressant drugs, and it was suggested that the central serotonin system plays an important role in the antidepressant activity of quinupramine.     

本院精神专科门诊抗抑郁药物使用情况调查分析

目的分析本院精神专科门诊抗抑郁药处方情况。方法对2010年9月与2011年9月本院精神专科门诊抗抑郁药处方,按照年龄分布、诊断及联合用药等进行统计分析。结果抗抑郁药使用人群以18～60岁为主;青少年使用比例,2011年9月显著高于2010年9月(P<0.05)。2年度中的9月,抗抑郁药处方的诊断均以抑郁症为主,其次为焦虑症。相比2010年9月,2011年9月焦虑症的诊断比例显著增高,精神分裂症则显著减低(P<0.05)。5-羟色胺再摄取抑制剂是目前临床应用较多的抗抑郁药。合并非典型抗精神病药物比例明显增加,而合并三环类药物比例呈明显下降趋势(P<0.05)。结论本院精神科门诊抗抑郁药物处方使用是合理和安全。     

The effect of repeated administration of imipramine, citalopram and mianserin on responsiveness of central serotonergic, alpha 2-adrenergic and cholinergic system in mice

The effect of antidepressant drugs: imipramine, citalopram and mianserin given either in a single dose or twice a day for 14 days in a dose of 10 mg/kg was investigated in mice in tests for the responsiveness of central serotonergic (L-5 hydroxytryptophan-induced head twitches), alpha 2-adrenergic (donidine hypoactivity) and cholinergic (oxotremorine syndrome) systems. The effect of L-5 hydroxytryptophan was inhibited by repeated administration of citalopram and mianserin but unchanged by administration of imipramine. After a single administration only mianserin inhibited the L-5 hydroxytryptophan effect. Given repeatedly, the investigated antidepressant drugs did not affect the effect of clonidine; only mianserin potentiated the hypoactivity when given in a single dose. Repeatedly administered antidepressant drugs did not affect the action of oxotremorine, although imipramine (but not citalopram or mianserin) antagonized it after a single administration. The results indicate that under the present conditions repeatedly given mianserin and citalopram, but not imipramine, antagonize behavioral effects of L-5 hydroxytryptophan. No one of the investigated antidepressant drugs given repeatedly changed the responsiveness of alpha 2 -adrenergic and cholinergic systems to their agonists. It might be concluded that the changes in alpha 1-adrenergic and dopaminergic systems, observed previously after repeated administration of antidepressant drugs, are selective.     

Differential effects of antidepressants on GABAB and beta-adrenergic receptors in rat cerebral cortex.

The effects of chronic administration of antidepressant drugs on beta-adrenergic and gamma-amino-butyric acid (GABA)B receptors have been assessed with radioligand binding. Tricyclics [imipramine (IMI), 30 mg/kg/day, and desmethylimipramine (DMI), 10 mg/kg day] or monoamine oxidase inhibitors [(+/-)-tranylcypromine (TCP), 1 mg/kg/day, and phenelzine (PLZ), 10 mg/kg/day] were administered to male Sprague-Dawley rats by constant infusion via Alzet 2ML4 osmotic minipumps for 28 days. Pumps were implanted s.c. in the interscapular region. On day 28 the animals were killed and their brains removed; [3H]GABA binding to GABAB receptors was measured in frontal cortex and the remaining cortical tissue was used to measure [3H]dihydroalprenolol ([3H]DHA) binding to beta-adrenoceptors. All drugs tested induced a significant decrease in density (Bmax) of [3H]DHA binding, although no significant changes in affinity (Kd) were observed. [3H]GABA binding was not altered significantly by chronic antidepressant treatment. TCP-treated animals showed a tendency towards increased [3H]-GABA binding, but the differences did not reach statistical significance. No effects on Kd were observed. These data do not support the proposal that an increase in the total population of cortical GABAB receptors is a common effect of chronic antidepressant treatment.     

Differentiation of two components of specific [3H]imipramine binding in rat brain

Specific binding of [3H]imipramine to membrane preparation from rat cerebral cortex can be resolved in two distinct components: the high-affinity binding with a KD in nanomolar range (6.9 +/- 0.4 nM) and a maximum number of binding sites (Bmax) 285 +/- 19 fmol/mg protein and the low-affinity component with a KD of 292 +/- 45 nM and Bmax of 2459 +/- 428 fmol/mg protein. Tricyclic antidepressants, a non-tricyclic serotonin uptake inhibitor fluoxetine and a tetracyclic antidepressant maprotiline show a markedly different pattern in displacing [3H]imipramine binding at the high- and the low-affinity site. When the high-affinity sites were protected, the differences in potency of tested drugs in displacing specific [3H]imipramine binding, and the correlation with their ability to inhibit serotonin reuptake, were not observed. The Hill coefficients of competing drugs at low-affinity sites were markedly lower (0.45-0.69) and the shape of displacement curves indicated that more than one site may be involved in low affinity binding of [3H]imipramine.     

Serotonin transporter mutations associated with obsessive-compulsive disorder and phosphorylation alter binding affinity for inhibitors

Mutants of serotonin transporter that are altered in their regulation by cGMP were tested for the ability of cocaine and the antidepressant drugs imipramine, sertraline, citalopram and fluoxetine to inhibit serotonin transport. Mutation at Ile-425 to valine, found in some patients with obsessive-compulsive disorder, altered the response of SERT to cGMP (Kilic, F., Murphy, D.L., Rudnick, G., 2003. A human serotonin transporter mutation causes constitutive activation of transport activity. Mol. Pharmacol. 64, 440-446). This mutation selectively decreased the potency of sertraline for inhibiting serotonin transport. The potencies of imipramine, citalopram, fluoxetine and cocaine for inhibiting transport were not affected by this mutation. In binding measurements with the cocaine analog 2beta-carbomethoxy-3beta-(4-[(125)I]-iodophenyl)-tropane (beta-CIT), sertraline potency was reduced by the I425V mutation but citalopram potency was unchanged. Mutation at the site of cGMP-dependent phosphorylation, Thr-276, decreased the potency of each of the drugs tested. This effect was also observed in studies with beta-CIT where both citalopram and sertraline were less potent at displacing this high-affinity ligand. These results support an influence of Thr-276 on the conformation of inhibitor binding sites of serotonin transporter, and also suggest that the sertraline binding site contains unique determinants that are not shared with the other tested inhibitors.     

The inhibition of monoamine oxidase activity by various antidepressants: differences found in various mammalian species

The effects of the antidepressant drugs zimeldine, imipramine, maprotiline or nomifensine on mitochondrial monoamine oxidase (MAO) activity in mouse, rat, dog and monkey brains were compared in vitro. Mouse, rat, dog and monkey brain MAO-B activities were inhibited by zimeldine more potently than MAO-A activity. Imipramine inhibited MAO-B more potently than MAO-A activity in mouse and rat brains. When dog and monkey brains were investigated, MAO-A activity was inhibited more potently than MAO-B activity at high concentrations of imipramine, while at low concentrations, MAO-B activity was more potently inhibited. Maprotiline and nomifensine inhibited mouse and rat brain MAO-B activity more potently than MAO-A activity, while the inverse was true for dog and monkey brains. All four drugs are competitive inhibitors of MAO-A, but noncompetitive inhibitors of MAO-B in all animal brains. The respective Ki values of these reagents for monkey brain MAO-A and MAO-B were low compared to those of mouse, rat and dog. These results indicate that monkey brain MAOs are more sensitive to antidepressant drugs than those in rodent brain.     

Postdecapitation convulsions in the rat measured with an Animex motility meter: relation to central alpha-adrenoceptors

Postdecapitation convulsions (PDC) were quantified by recording the number of movements of the trunk of decapitated rats with an Animex motility meter. PDC were inhibited by alpha 1-adrenoceptor blocking agents, phenoxybenzamine and prazosin, but unaffected by alpha 2-adrenoceptor antagonists, yohimbine and piperoxan, or a beta-adrenoceptor blocker, propranolol. Chronic administration of antidepressant drugs: amitriptyline, imipramine and zimelidine augmented the PDC. Preliminary experiments with [3H]prazosin binding suggest that the increased PDC response is related to an increase in alpha 1-adrenoceptor binding, most likely owing to an increase in receptor density.     

Alpha 2-adrenoreceptors on human platelets: selective labelling by [3H]clonidine and [3H]yohimbine and competitive inhibition by antidepressant drugs

[3H]Clonidine and [3H]yohimbine have been used to characterize alpha 2-adrenoreceptors on human platelets. At 25 degrees C binding was rapid (t 1/2 of association, 1.8 and 2.7 min) and reversible (t 1/2 of dissociation, 0.5 and 8.2 min). The binding sites for [3H]clonidine and ]3H]yohimbine showed the specificity required for an alph 2-adrenoreceptor. The rank order of potency of inhibitors of [3h[clonidine binding was clonidine greater than yohimbine greater than phenylephrine greater than prazosin and of [3H]yohimbine binding was yohimbine greater than clonidine greater than phenylephrine greater than prazosin. Scatchard analysis of [3H]yohimbine binding indicated the existence of a single population of noninteracting sites (KD = 3.0 nM; Bmax = 188 fmol/mg protein). The high-affinity binding of [3H-clonidine had a lower affinity and a lower number of sites (KD = 5.0 nM; Bmax = 35 fmol/mg protein). [3H]Clonidine binding also showed evidence of a second site of much lower affinity and greater number (KD = 18.6 nM; Bmax = 77 fmol/mg protein) in 40% of the normal population. In vitro, antidepressant drugs competed with [3H]clonidine and [3H]yohimbine for the platelet alpha 2-adrenoreceptor. The rank order of potency of inhibitors of [3H]clonidine binding was mianserin greater than amitriptyline greater than iprindole greater than desipramine and of [3H]yohimbine binding was mianserin greater than amitriptyline greater than desipramine greater than iprindole. The inhibition constants (Ki) of adrenergic drugs and of various antidepressant drugs in competing with [3H]clonidine were correlated with the inhibition constants of these drugs in competing with [3H]yohimbine (r = 0.970; P less than 0.001) which suggests that both radioligands labelled the same alpha 2-adrenoreceptor on the human platelet. The inhibition of binding induced by all antidepressant drugs was competitive. In contrast, long-term administration of tricyclic antidepressant drugs to patients was recently found to be associated with a decrease in the number of binding sites for [3H]clonidine on platelet membranes. The present results indicate that both [3H]clonidine and [3H]yohimbine are useful tools for the quantification of alpha 2-adrenoreceptors on blood platelets and suggests that the specific binding of radiolabelled alpha 2-adrenoreceptor ligands to human platelet membranes might be used to monitor changes in alpha 2-adrenoreceptors during tricyclic antidepressant drug treatment.     

Regulation of high- and low-affinity [3H]imipramine recognition sites in rat brain by chronic treatment with antidepressants

Specific binding of [3H]imipramine to its recognition sites in frontal cortex and levels of serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and norepinephrine (NE) as well as uptake of serotonin by crude synaptosomal (P2) fraction were determined in a group of rats chronically (for 21 days) treated with different types of antidepressant drugs: nortriptyline, fluoxetine, iprindole, phenelzine (10 mg/kg per day), maprotiline (20 mg/kg per day) and vehicle only (controls). Quantitative analysis of imipramine competition curves confirmed the existence of two classes of [3H]imipramine sites: high-affinity with IC50 and 11.2 nM and low-affinity with IC50 of 630 nM for the competing ligand. The proportion of high- and low-affinity sites was 73 +/- 4 and 26 +/- 4%, respectively. Chronic treatment with all antidepressant drugs except iprindole significantly decreased the affinity but not the proportion of high-affinity sites for imipramine. IC50 of imipramine at low-affinity sites was even more markedly increased at low-affinity sites by all treatments except for phenelzine. Fluoxetine was by far the most effective in altering the affinity of both high- and low-affinity [3H]imipramine recognition sites. Both NE and 5-HT levels were significantly enhanced only by phenelzine treatment, whereas 5-HT and 5-HIAA levels were found to be lower after fluoxetine. Kinetics of 5-HT uptake were altered significantly only in rats treated with fluoxetine: rate of 5-HT uptake (Vmax) was decreased by 43% and Km value increased from 104 to 184 nM. Changes in affinity of imipramine for its binding sites were not found to be associated with the effect of tested drugs on 5-HT levels or uptake. They may be due to adaptive alterations in physico-chemical properties of binding proteins although the presence of residual drug interfering with the binding assay cannot be excluded. The observed changes are likely to represent the condition during chronic administration of these drugs in clinical therapy of depression.