Human platelet imipramine recognition sites: biochemical and pharmacological characterization

Glycine Transporter 2: Mechanism and Allosteric Modulation

Neurotransmitter sodium symporters (NSS) are a subfamily of SLC6 transporters responsible for regulating neurotransmitter signalling. They are a major target for psychoactive substances including antidepressants and drugs of abuse, prompting substantial research into their modulation and structure-function dynamics. Recently, a series of allosteric transport inhibitors have been identified, which may reduce side effect profiles, compared to orthosteric inhibitors. Allosteric inhibitors are also likely to provide different clearance kinetics compared to competitive inhibitors and potentially better clinical outcomes. Crystal structures and homology models have identified several allosteric modulatory sites on NSS including the vestibule allosteric site (VAS), lipid allosteric site (LAS) and cholesterol binding site (CHOL1). Whilst the architecture of eukaryotic NSS is generally well conserved there are differences in regions that form the VAS, LAS, and CHOL1. Here, we describe ligand-protein interactions that stabilize binding in each allosteric site and explore how differences between transporters could be exploited to generate NSS specific compounds with an emphasis on GlyT2 modulation.

The mechanism of a high-affinity allosteric inhibitor of the serotonin transporter

The serotonin transporter (SERT) terminates serotonin signaling by rapid presynaptic reuptake. SERT activity is modulated by antidepressants, e.g., S-citalopram and imipramine, to alleviate symptoms of depression and anxiety. SERT crystal structures reveal two S-citalopram binding pockets in the central binding (S1) site and the extracellular vestibule (S2 site). In this study, our combined in vitro and in silico analysis indicates that the bound S-citalopram or imipramine in S1 is allosterically coupled to the ligand binding to S2 through altering protein conformations. Remarkably, SERT inhibitor Lu AF60097, the first high-affinity S2-ligand reported and characterized here, allosterically couples the ligand binding to S1 through a similar mechanism. The SERT inhibition by Lu AF60097 is demonstrated by the potentiated imipramine binding and increased hippocampal serotonin level in rats. Together, we reveal a S1-S2 coupling mechanism that will facilitate rational design of high-affinity SERT allosteric inhibitors.

Binding of tricyclic antidepressant drugs to trophozoites of giardia lamblia

1. Parameters affecting the binding of the tricyclic drugs imipramine and 3-chloroimipramine to Giardia lamblia trophozoites were studied. 2. Two to three times more chlorimipramine than imipramine was bound, consistent with a similar difference in suppressing parasite growth (Weinbach et al., 1985). 3. Kinetic analysis and the ease with which bound drugs can be washed out of the parasites indicate that noncovalent forces are involved in the drug-parasite interaction. 4. Evidence is presented that such drugs probably bind to parasite protein at common binding sites. 5. The data relate to our earlier observation that chlorimipramine is about ten times more effective than metronidazole (Crouch et al., 1986) in suppressing G. lamblia growth in vitro. Tricyclic drugs, therefore, merit serious consideration as novel therapeutic agents against giardiasis.

Subclasses of platelet 3H-imipramine binding sites

The possible presence of multiple high affinity 3H-imipramine (3H-IMI) binding sites on blood platelets was studied using trypsin digestion and cyanoimipramine (CNIMI), a pseudo-irreversible inhibitor of 3H-IMI binding and serotonin uptake. Increasing concentrations of CNIMI resulted in a discontinuous curve with a plateau at intermediate concentrations (0.05-0.35 nM). CNIMI sensitive (0.25 nM) sites accounted for approximately half of total high affinity 3H-IMI binding as defined by displacement with 100 microM desipramine. Similar results were obtained when platelet membranes were pretreated with trypsin (0.21-0.84 mg/ml), and no additional inhibition was evident with a combination of both treatments. The present results suggest that 3H-IMI may bind to two separate types of high affinity sites. One subclass is apparently proteinaceous and sensitive to low concentrations of CNIMI, whereas the other is apparently nonproteinaceous and is CNIMI resistant.

Effects of chronic imipramine treatment on subclasses of platelet 3H-imipramine binding sites and plasma cortisol

A decreased density of platelet 3H-imipramine (3H-IMI) binding sites has been proposed as a putative trait marker of major depressive illness. However, subsequent studies have demonstrated that the number of such sites is increased so as to be more like normal controls upon chronic treatment with antidepressant drugs. In addition, there is some evidence to suggest that altered 3H-IMI binding may be secondary to elevated plasma cortisol levels which are common in depressed patients and which normalize with remission. The present study compares platelet 3H-imipramine binding, plasma cortisol levels, and clinical improvement of 10 endogenous depressed patients before and after 6 weeks of treatment with imipramine-HCl. Total high affinity 3H-IMI binding sites were further differentiated into two subclasses on the basis of their relative sensitivities to cyanoimipramine (CNIMI) inhibition. Treatment was associated with a significant increase (134%) in CNIMI resistant binding but a decrease (45.2%) in CNIMI sensitive binding. While the former was significantly correlated with posttreatment cortisol levels, no significant correlation was found between cortisol and CNIMI specific binding. Neither site appeared to be directly related to mood state. The significance of these findings to the evaluation of platelet binding as a trait dependent marker is discussed.

Seasonal changes in cyanoimipramine specific platelet 3H-imipramine binding in depression

Seasonal variations in cyanoimipramine (CNIMI) sensitive and CNIMI resistant subclasses of platelet 3H-imipramine (3H-IMI) binding sites were studied in depressed patients and normal volunteers. Sinusoidal rhythms of the binding of both subclasses were found in patients and controls with peak levels in mid-February. Patient values of CNIMI sensitive binding fluctuated about a yearly average that was 32% lower than the average of controls. Patient deviations from the normal pattern were apparently bimodally distributed, whereas those of controls were normally distributed. CNIMI resistant binding was also normally distributed in controls, but not in depressed patients, although patient mesor values were not lower than those of controls. Platelet binding was not correlated with the severity of illness as measured by the Hamilton Rating Scale for Depression, and individual symptoms failed to discriminate between patients with high and low Bmax values.

Biochemical characterization and purification of the neuronal sodium-dependent noradrenaline transporter

The protein properties of the neuronal sodium-dependent noradrenaline (NA) transporter of PC12 (rat pheochromocytoma) cells and of bovine adreno-medullary cells were studied by means of binding of 3H-desipramine (3H-DMI). 3H-DMI binding was decreased by proteases, phospholipase A2, by disulfide reducing agents and by the sulfhydryl-group alkylating agent N-ethylmaleimide. The NA transporter was partially purified by anion exchange and affinity chromatography. Tritiated desmethylxylamine (3H-DMX) bound irreversibly and in a DMI-sensitive manner to two PC12 membrane proteins (32kd and 53kd) which may represent components of the NA transporter.

Changes in platelet 3H-imipramine binding with chronic imipramine treatment are not state-dependent

One month of imipramine treatment increased both the Kd and Bmax of platelet 3H-imipramine binding in 11 endogenous unipolar depressed patients. Continued treatment (13 weeks) of 5 patients subsequently lowered the Bmax values of 2 patients who had initially shown the largest increases, so that binding was no longer significantly elevated after 13 weeks. The observed changes in Kd but not in Bmax, could be explained by the carryover of tightly bound drug to the binding assay, although neither of the measures were correlated with plasma imipramine levels. Posttreatment Bmax (4 weeks) values were inversely related to plasma cortisol levels, although a weak but positive correlation was found before treatment. No significant change was found in plasma cortisol with treatment. Clinical responses were not related to cortisol or Bmax changes, although optimal improvement was associated with extreme values (high and low) of pretreatment Bmax. The present results, obtained with imipramine, and similar results obtained after nortriptyline and electroconvulsive shock by others, suggest that at least some antidepressants may induce transient changes in the Bmax of platelet binding that are independent of affective state.

Effects of antidepressants on monoamine transporters

1. Using [3H]antidepressants, high affinity binding sites associated with the neuronal transporter for serotonin, noradrenaline, dopamine and adrenaline have been identified. 2. The association of high affinity [3H]imipramine binding with the serotonin transporter in brain and platelets is well established. Although the exact relationship between the [3H]imipramine recognition site and the serotonin transporter remains to be elucidated, it appears that the [3H]imipramine labelled component of the serotonin transporter represents a novel receptor that functions to modulate serotonin uptake. 3. Most data available to date support the hypothesis that [3H]imipramine binding to platelet represents a biological marker in depression. The majority of studies indicate that the Bmax of platelet [3H]imipramine binding is lower in depressed, untreated patients than in the control population and that this finding is relatively specific to depression. 4. Among the [3H]antidepressant binding sites associated with the other monoaminergic transporters, the recent identification of [3H]desipramine binding to the neuronal transporter for adrenaline offers novel perspectives. Thus, given the high affinity for [3H]desipramine binding to the adrenaline transporter in the frog heart for not only desipramine but also imipramine and the atypical antidepressants mianserin and iprindol, it is possible that an interaction with the adrenaline transporter is of significance to the clinical effects of antidepressant drugs.

The 5-HT1A receptor probe [3H]8-OH-DPAT labels the 5-HT transporter in human platelets

The present study characterizes a serotonin (5-HT) binding site on human platelet membranes, using [3H]8-OH-DPAT as the radioligand. [3H]8-OH-DPAT binds specifically and saturably to a site on human platelet membranes with an average KD of 43 nM and Bmax of 1078 fmol/mg protein. Determinations of IC50 values for various serotonergic characterizing agents in platelets for displacement of [3H]8-OH-DPAT were performed. For example, 8-OH-DPAT 5HT1A had an IC50 of 117 nM; TFMPP 5HT1B (2.3 microM0 and PAPP 1A + 5HT2 (9 microM); ipsapirone 5HT1A (21.1 microM) and buspirone 5HT1A (greater than 100 microM); ketanserin 5HT2 (greater than 100 microM); 5-HT uptake inhibitors: paroxetine (13 nM); chlorimipramine (73 nM) and fluoxetine (653 nM). The pharmacological inhibitory profile of the platelet 8-OH-DPAT site is not consistent with profiles reported for brain. 8-OH-DPAT does not inhibit [3H]imipramine binding, however, it does inhibit [3H]5-HT uptake in human platelets near 5-HT's Km value (IC50 = 2-4 microM). These results suggest that the human platelet site labeled by [3H]8-OH-DPAT is pharmacologically different from the neuronal site and probably is a component of the 5-HT transporter.

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%.

Allosteric interaction between the site labeled by [3H]imipramine and the serotonin transporter in human platelets

The nature of interaction between the site labeled by [3H]imipramine (IMI) and the 5-hydroxytryptamine (5-HT, serotonin) transporter in human platelets was examined. The sulfhydryl characterizing agent N-ethylmaleimide (NEM) differentially affected [3H]5-HT uptake and [3H]IMI binding in human platelet preparations. Concentrations of NEM that completely abolished [3H]5-HT uptake only minimally reduced [3H]IMI binding. Examining the effect of IMI on the kinetics of human platelet [3H]5-HT uptake revealed significant reductions in maximal velocity (Vmax) without altering affinity (Km). IC50 values for selected uptake blockers on [3H]IMI binding and [3H]5-HT uptake were determined. IC50 values of these compounds for uptake and binding revealed that agents such as IMI, chlorpromazine, amitriptyline, and nisoxetine were preferential inhibitors of [3H]IMI binding whereas fluoxetine, CL 216, 303, pyrilamine, and bicifadine were preferential [3H]5-HT uptake blockers. 5-HT was a weak displacer of [3H]IMI binding (IC25 = 3.0 microM) and exhibited a rather low Hill coefficient (nH app = 0.46). Results reported herein support the notion of an allosteric interaction between the [3H]IMI binding site and the 5-HT transporter complex in human platelets.

Purification and properties of a human plasma endogenous modulator for the platelet tricyclic binding/serotonin transport complex

An endogenous modulator for the site labeled by [3H]imipramine which is putatively coupled to the serotonin transporter in human platelets was isolated and purified from plasma. Procedures included sequential chromatography on Cibacron blue-Sepharose 4B, concanavalin A-Sepharose 4B, Mono Q HR 10/10 anion exchange, DuPont GF-250 gel permeation and Mono S HR 5/5 cation exchange columns. The purified modulator is a protein of Mr 45,000 with a very acidic pK (less than 3) and sensitive to various proteinases but heat- and acid-stable. This protein inhibited [3H]imipramine binding to platelet membranes competitively (IC50 approximately 6 microM) and enhanced serotonin uptake in fresh human platelets (EC50 approximately 7 microM). Various physicochemical properties, including chromatographic, electrophoretic and immunological as well as amino acid composition analysis revealed that the isolated protein is most probably the human alpha 1-acid glycoprotein.

Platelet 3H-imipramine binding in bipolar patients

Platelet 3H-imipramine binding was investigated in 31 control subjects and 19 hospitalized bipolar patients, either in the hypomanic or the depressed phase of illness. The mean Bmax value in the bipolar depressed patients did not differ significantly from that in the control subjects or the hypomanic patients. Differences in timing of the assay after blood collection, membrane preparation, protein content used in the assay, or binding of radioactive ligand to the equipment do not appear to explain the discrepancy between these results and previous findings.

Lower 3H-imipramine binding in platelets from untreated depressed patients compared to healthy controls

3H-Imipramine binding in platelets was measured in 63 severely depressed hospitalized patients, who had been drug free (with the exception of moderate doses of benzodiazepines) for at least 1 month, and in 53 healthy control subjects of comparable age and sex distribution. Bmax of 3H-imipramine binding was significantly lower in the depressed subjects (1012 +/- SD 295 vs. 1123 +/- SD 178 fmole/mg protein). Depressed patients who had attempted suicide by violent means tended to have higher Bmax than nonviolent attempters.

5-Hydroxytryptamine uptake and imipramine binding sites in neurotumor NCB-20 cells

NCB-20 cells (neuroblastoma X fetal Chinese hamster brain hybrids) are equipped with a [3H]5-hydroxytryptamine [( 3H]5-HT) uptake system and [3H]imipramine recognition sites. Approximately 80% of the radioactivity taken up by cells incubated with [3H]5-HT was identified with 5-HT. [3H]5-HT uptake was temperature-dependent, partially sodium-dependent, saturable (Km = 7.3 +/- 0.6 microM; Vmax = 2.0 +/- 0.6 pmol/min/mg), and inhibited by clomipramine, imipramine, fluoxetine, and desipramine, but not by iprindole, mianserin, or opipramol. Lineweaver-Burk plots showed a competitive type of inhibition by imipramine and fluoxetine. [3H]5-HT uptake was not inhibited by nisoxetine or benztropine. [3H]Imipramine binding sites had a KD of 12 +/- 2 nM and a Bmax of 22 +/- 7 pmol/mg protein. The binding was sodium-sensitive although to a lesser extent than that found with brain membranes. Imipramine binding was displaced by tricyclic antidepressants with the following order of potency: clomipramine greater than imipramine greater than fluoxetine greater than desipramine much greater than iprindole = mianserin greater than opipramol. These results suggest that imipramine binding sites are present together with the 5-HT uptake sites in NCB-20 cells and that these sites interact functionally but are different biochemically.

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

The action of the antithyroid drugs methimazole (MMI) and propylthiouracil (PTU) on the binding of [3H]-imipramine, [3H]-5-hydroxytryptamine [3H]-5-HT) (to 5-HT1-receptors) and [3H]-spiperone (to 5-HT2-, D2-receptors) of rat brain membranes has been examined. The synaptosomal uptake of [3H]-5-HT was also studied. Micromolar concentrations of the disulphide bond reducing agents MMI, PTU, dithiothreitol (DTT) and mercaptoethanol increased both the binding of [3H]-imipramine and the uptake of [3H]-5-HT. In contrast, they decreased the number of 5-HT1-receptors, and did not affect 5-HT2-and D2-sites. Reaction with membrane-bound sulphydryl (SH) groups by micromolar concentrations of N-ethylmaleimide (NEM), hydroxymercuribenzoic acid (PCMB), or Ellman's reagent (DTNB) decreased the binding of [3H]-imipramine, the number of 5-HT1-receptors, and the uptake of [3H]-5-HT. Millimolar concentrations of NEM were necessary in order to decrease partially 5-HT2- and D2-receptors. The effects of NEM on imipramine recognition sites and on the uptake of 5-HT could be prevented by DTT; protection was not obtained in other receptor systems. Three groups of receptors have been, thus, postulated, based upon their different sensitivity towards alterations in membrane [disulphide bridges in equilibrium SH] equilibrium: Group I, including imipramine recognition sites and the uptake system for 5-HT; Group II, including 5-HT1-receptors; Group III, including 5-HT2-and D2-receptors.

Serotonin uptake inhibitors differentially modulate high affinity imipramine dissociation in human platelet membranes

The influence of selective serotonin uptake inhibitors on the dissociation rate of 3H-imipramine from its high affinity binding site in human platelet membranes was studied. Of the uptake inhibitors tested, one group of compounds attenuated dissociation, another group accelerated it, while a third group had little effect on the dissociation process. Drugs unrelated to the serotonin uptake system were ineffective. Removal of sodium ions markedly increased imipramine dissociation. Dose response curves of the active compounds indicated that micromolar concentrations were required to exert an effect on imipramine dissociation. These results can be adequately explained by an allosteric model which includes effector binding sites and distinct conformational states of the high affinity imipramine binding site.

Effect of chronic desmethylimipramine or electroconvulsive shock on selected brain and platelet neurotransmitter recognition sites

Rats were treated with electroconvulsive shock (ECS), desmethylimipramine (DMI), ECS plus DMI, or diazepam. In vitro analyses showed that chronic ECS produced an elevated density of recognition sites for [3H]imipramine (IMI) in platelet membranes, but had no effect on membrane preparations derived from cortical tissue. A similar elevation in receptor binding was seen exclusively in platelets after chronic ECS plus DMI, whereas no effect was observed with DMI alone. Equilibrium dissociation constant (KD) values for [3H]IMI were also increased in platelet membranes from rats given chronic ECS or ECS plus DMI treatment. Chronic ECS or DMI administration produced a decreased density of beta-adrenergic recognition sites in frontal cortex and cerebellum as assessed by [3H]dihydroalprenolol (DHA) binding. The combination of ECS plus DMI produced a similar decrease. In addition, chronic diazepam administration produced a down-regulation of the beta-adrenergic receptor only in the cerebellum. These data provide evidence for the differential regulation of brain and peripheral neurotransmitter recognition sites.

Neuroleptics and beta-carbolines displace (3H)Imipramine from its binding sites in human and rat tissues

Most investigations dealing with the pharmacological characterization of (3H)imipramine binding sites focus on tricyclic antidepressants (TCA). This approach seemed to be justified since imipramine belongs to that chemical group. Langer and coworkers, however, introduced a tetrahydro-beta-carboline (TH beta C) as a possible endogenous ligand. Thus, the high affinity of imipramine towards the binding sites might not be due to its special chemical structure but due to its tricyclic nature. In the present paper the structure-activity-relationships of neuroleptics and beta-carbolines were investigated and compared with that of tricyclic antidepressants. Among the tricyclic neuroleptics those with an electron attracting substituent (-Cl) exerted highest affinity. The effect was attenuated by a long, cyclic side chain. The affinity of tricyclic neuroleptics was only slightly weaker than that of 6-Meo-TH beta C the suggested endogenous ligand. The experiments with other TH beta Cs supported the observation that an electron attracting substituent increases the affinity of a compound to the (3H)imipramine binding sites. Comparison of the binding characteristics of (3H)imipramine to membranes of human brain and thrombocytes as well as those of rat brain and thrombocytes revealed no differences among both species. Furthermore, the displacing potencies of neuroleptics were very similar with only slightly more activity in human tissue. As a methodological aspect the applicability of the "Lowry" method to determine the protein concentration is discussed.

Effects of neonatal antithyroid treatment on brain [3H]-imipramine binding sites

The action of the antithyroid, sulphydryl reagent methimazole (MMI) on the specific binding of [3H]-imipramine in the cerebral cortex and corpus striatum of immature and mature rats has been examined. Chronic administration of MMI through the first 30 days of life decreased the number of imipramine binding sites in cortical but not striatal membranes, as assessed 48 h after the last injection of goitrogen. A similar treatment did not affect the binding profile of [3H]-imipramine in mature rats. Acute administration of MMI to 30 day-old rats increased the number of imipramine binding sites shortly after the injection, an effect no longer evident 48 h later. MMI in vitro increased the binding of [3H]-imipramine. It is concluded that maturational impairment of the hypothyroid cortex, rather than any alteration of membrane bound thiol groups, was a major cause for the diminished binding of [3H]-imipramine in MMI-treated, immature rats.

Temperature-sensitive conformational changes in [3H]imipramine binding sites and the involvement of sulphur-containing bonds

Recently, a reversible, temperature-sensitive conformational change has been observed which alters [3H]imipramine binding parameters. I now report that the conformational change probably occurs within the binding site protein and is not due to allosteric interactions or altered ligand kinetics. Evidence comes from the identification of sulphur-containing bonds in the binding site molecule which are differentially exposed at different incubation temperatures. Binding of [3H]imipramine to outdated human platelets is maximal at 4 degrees C; when the incubation temperature is raised to 23 degrees C, a loss of affinity is observed. N-Ethylmaleimide, an alkylator of sulphydryl groups, could completely inhibit the binding at both temperatures but did so much faster at 23 degrees C than at 4 degrees C. Dithioerythritol (DTE) and beta-mercaptoethanol reduce disulphide bonds and were found to enhance [3H]imipramine binding. Again, these reagents were more potent at 23 degrees C than at 4 degrees C. None of these reagents caused these effects by interacting with the ligand. Simultaneous preincubation with excess fluoxetine could prevent the action of DTE on specific [3H]imipramine binding. It is concluded that sulphur-containing bonds are close to, or located at the [3H]imipramine binding site, and their conformation is sensitive to changing temperatures.

Platelet imipramine binding in autistic subjects

Previous reports of elevated platelet serotonin (5-HT) concentrations in autistic subjects suggest that platelet 5-HT uptake might be altered in autism. Parameters of 3H-imipramine (IMI) binding were measured in 11 drug-free autistic subjects and 10 normal volunteers. Similar means (+/- SD) for Bmax (autistics, 1350 +/- fmole/mg protein; normals, 1590 +/- 206 fmole/mg protein) and Kd (autistics, 0.98 +/- 0.10 nM; normals, 0.94 +/- 0.13 nM) were found in the two groups. The normal number (Bmax) and affinity (Kd) of the IMI binding site in autistic subjects suggest that the regulation of 5-HT uptake is not different in autism.

Effect of tricyclic antidepressant drugs on lymphocyte membrane structure

Tricyclic antidepressant-induced perturbations of murine splenic lymphocyte membranes and cell surface concanavalin A receptor mobility have been investigated using the fluorescent probes diphenylhexatriene and fluorescein-conjugated concanavalin A. Results of these studies illustrate the possible relationship between tricyclic antidepressant-induced membrane perturbations and tricyclic antidepressant-induced suppression of the normal murine lymphocyte mitogen response. Tricyclic antidepressant effects on murine splenic lymphocyte membranes are dose-, time- and temperature- dependent. Murine lymphocyte concanavalin A cell surface receptor mobility is not apparently altered by the tricyclic antidepressants.

Temperature-sensitive reversible loss of [3H]imipramine binding sites: evidence suggesting different conformational states

Previous studies have indicated that the tertiary tricyclic antidepressant imipramine binds with high affinity to a site related to the serotonin uptake mechanism. We have further characterized this site with regards to the effects of temperature, pH and calcium ions on both rat cerebral cortex homogenates and outdated human platelets. The binding of [3H]imipramine to these sites is maximal at 4 degrees C. When the assay temperature is increased to 23 degrees C, there is approximately a 50% decrease in the maximal number of binding sites (Bmax) with no apparent change in the affinity for the ligand. On further raising the temperature to 37 degrees C there is no further decrease in Bmax but there is an increase in the dissociation constant (KD). These temperature related changes are reversed when the assay temperature is again lowered to 4 degrees C. Decreasing the pH of the incubation mixture from 7.5 to 7 (which is the pH change also observed upon changing the temperature of the incubation mixture from 4 to 37 degrees C) decreased [3H]imipramine binding by 10-20%, an amount which does not account for the changes observed. Similarly, neither calcium nor EDTA altered these reversible changes. Irreversible loss of binding sites was observed on prolonged incubation at 23 or 37 degrees C. After 20 h preincubation at 23 or 37 degrees C, 15 or 30% of the binding respectively could not be restored by lowering the temperature to 4 degrees C. This permanent loss was enhanced by calcium and inhibited by EDTA. These results suggest that reversible conformational changes in the [3H]imipramine binding site can be observed.

3H-imipramine binding in platelets: influence of varying proportions of intact platelets in membrane preparations on binding

Human platelets possess high-affinity 3H-imipramine binding sites. A study in ten healthy volunteers showed that platelet preparations produced by mechanical disruption contained varying proportions of intact platelets, as measured by the cytoplasmic marker enzyme LDH. This may invalidate the protein reference. Higher proportions of membranes in the preparations lead to higher Bmax values of imipramine binding (fmol/mg protein). The use of intact platelets in imipramine binding studies is therefore preferable to membrane preparations, particularly in studies where the interindividual variation of binding parameters is of interest.

Serotonin modulates the dissociation of [3H]imipramine from human platelet recognition sites

The dissociation of [3H]imipramine from human platelet membrane recognition sites was studied. Simple exponential dissociation kinetics were observed indicating an apparent homogeneous class of sites which dissociate by a first order rate process. Inclusion of serotonin in the medium dramatically decreased the dissociation rate. At 0 degree C half-times of dissociation were 61.7 and 170 min in the absence and presence of serotonin, respectively. Other biogenic amines were ineffective in altering the dissociation kinetics. It is suggested that serotonin controls [3H]imipramine dissociation from an allosterically coupled site residing on a larger macromolecular complex.

Human brain receptor alterations in suicide victims

A comparison was made of human postmortem muscarinic-cholinergic, beta-adrenergic and serotonergic (presynaptic) recognition sites in cortical tissues derived from suicide and homicide (control) victims. An elevation of 47% and 35% in the suicide group compared to controls was observed in receptor ligand binding for 3H-quinuclidinyl benzilate (QNB, muscarinic antagonist) and 3H-imipramine (IMI, a presynaptic serotonin marker), respectively. In contrast, no appreciable differences in 3H-dihydroalprenolol (DHA, beta-adrenergic antagonist) binding were observed between the two groups. Additionally, tissues from both groups of subjects were analyzed for tricyclic antidepressive agent (TAD) content. High performance liquid chromatographic (HPLC) tissue analysis revealed no detectable levels of tricyclic agents with an assay sensitivity of 50 picograms/mg tissue. The results presented herein demonstrate neurotransmitter-receptor alterations in suicide subjects compared to homicide (control) victims. The attendant roles of serotonergic and muscarinic-cholinergic processes in the psychobiology of suicide and depression are addressed.