2-nitroimipramine: a selective irreversible inhibitor of [3H] serotonin uptake and [3H] imipramine binding in platelets

Tritiation of imipramine and pseudo irreversible analogues 3-Cyanoimipramine and 2-Nitroimipramine at high specific activity

Methods are presented to synthesize and characterize [N-methyl-3H] imipramine 3, [N-methyl-3H] 3-cyanoimipramine 6 and [N-methyl-3H] 2-nitroimipramine 9.

Seasonal variability in blood platelet 3H-imipramine binding in healthy controls: age and gender effects

Binding of 3H-imipramine to blood platelet membranes was determined four times (once each season) in 26 healthy volunteers (11 men and 15 women), over the course of 1 year to determine possible seasonal variations. Blood platelets were obtained in April-May, July-August, October-November, and January-February. Significant seasonal variations in the maximum number of binding sites were found in women but not in men, with circannual peak in summer and a nadir in spring. The pattern of seasonal variations was not the same in men and women. The present results highlight the importance of monitoring for gender and season in binding studies. We found no significant correlation between 3H-imipramine binding parameters and age.

Modulation of [3H]paroxetine binding to the 5-hydroxytryptamine uptake site in an animal model of depression

The effects of learned helplessness on the 5-hydroxytryptamine (5-HT) uptake site were studied in rats using [3H]paroxetine binding. This ligand was chosen because it was demonstrated to label directly the 5-HT uptake site whereas the [3H]imipramine binding site has been demonstrated to be heterogeneous in nature. Moreover, [3H]imipramine appears to bind to a presynaptic recognition site different from the uptake site. Exposure to uncontrollable shock training and testing resulted in an overall increase in [3H]paroxetine binding in all the groups studied [nonhelpless (NLH), learned helpless (LH), spontaneously helpless (SPLH)] as compared to naive controls (NC). However, the increase in [3H]paroxetine binding was significantly higher in the LH and SPLH groups. The maximum number of [3H]paroxetine binding sites in the rat hippocampus was increased significantly in learned helpless rats (LH and SPLH) at day 4 and day 30 after the shock escape test as compared to NC and NLH rats. By contrast, in the rat hypothalamus the maximum number of [3H]paroxetine binding sites was reduced significantly in the LH rats as compared to naive controls and NLH rats during the same time course. There was no change in [3H]paroxetine binding sites in any other brain regions examined in LH, NLH, and NC rats. The results suggest that a hippocampal hypothalamic connection might play a role in the serotonergic mediation of learned helpless behavior.

Characterization of a genetically reconstituted high-affinity system for serotonin transport

By transfecting mouse fibroblast L-M cells with human genomic DNA, we have established and identified several clonal cell lines that stably express a high-affinity serotonin (5-HT)-uptake mechanism absent in untransfected host cells. One such cell line, L-S1, possesses features of 5-[3H]HT uptake similar to those previously characterized in the central nervous system and blood platelets: (i) specificity for 5-HT; (ii) antagonism by imipramine, a known inhibitor of high-affinity 5-HT uptake; (iii) both Na+ and temperature dependences; (iv) kinetic saturability; and (v) high affinity for 5-HT (Km = 0.39 +/- 0.10 microM; Vmax = 2.14 +/- 0.55 pmol/min per mg of protein). This cell line can be used to compare the relative efficacies of known blockers of 5-HT uptake and thereby offers a rapid and reliable assay system for testing novel inhibitors of this system. Since L-S1 contains stably integrated human DNA in its genome, we postulate that the observed 5-HT-uptake system resulted from the expression of human gene(s) coding for the 5-HT transporter. Thus, cell lines such as L-S1 may represent novel means for screening and developing therapeutic agents specific for neurotransmitter-uptake systems as well as substrates for the cloning and elucidation of the genes encoding the various neurotransmitter transporters.

Platelet serotonin uptake and 3H-imipramine binding in panic disorder

Platelet serotonin uptake and 3H-imipramine binding were measured in eight patients with panic disorders and nine controls. The Vmax of serotonin uptake was significantly elevated in patients compared to controls (77 +/- 14 vs. 50 +/- 4 pmol/10(8) platelets/min; P less than 0.05) while Km values were not different (1.46 +/- 0.41 vs. 1.24 +/- 0.20 microM). 3H-Imipramine binding to ruptured platelet membranes was not significantly different between patients and controls for either Bmax (395 +/- 71 vs. 412 +/- 107 fmol/mg protein) or Kd (0.90 +/- 0.18 vs. 1.09 +/- 0.30 nM). The implications for a serotonergic dysfunction in panic disorders are discussed.

Long-term blockade of the dopamine uptake complex by metaphit, an isothiocyanate derivative of phencyclidine

[3H]Mazindol was used to label the dopamine uptake complex in mouse striatum in vitro in the presence and absence of metaphit, an isothiocyanate derivative of phencyclidine. In some experiments, metaphit was present in the incubation fluid throughout the procedure; in other experiments it was eliminated by several washings and centrifugations. It was found that after removal of the metaphit by washing and centrifugation, the mazindol binding was not restored. Membranes that were pretreated with metaphit and washed had a lower density of mazindol binding sites than control membranes; the remaining mazindol sites had the same afinity for [3H]mazindol. These findings are in agreement with the previous studies on [3H]cocaine and [3H]methylphenidate binding. The following observations support that metaphit is irreversibly acting and not slowly dissociating from the mazindol recognition sites of the dopamine uptake carrier complex: 1) Metaphit did not change the off-rate of [3H]mazindol binding, arguing against an allosteric action at a distinct site. 2) The presence of cocaine protected the mazindol binding sites from the action of metaphit, supporting binding of metaphit and mazindol to the same site. 3) Nine hours after metaphit pretreatment and removal, the degree of inhibition of mazindol binding was the same as immediately after pretreatment, consonant with an irreversible effect of metaphit. 4) The potency of metaphit in inhibiting mazindol binding was greater under slightly alkaline conditions, consistent with acylation of the mazindol sites. Furthermore, it was found that intracerebroventricular application of metaphit did not result in a decrease in the binding of [3H]mazindol 5 hr after the administration.

Tricyclic antidepressants and calcium channel blockers: interactions at the (-)-desmethoxyverapamil binding site and the serotonin transporter

The activity of various calcium channel blockers at the serotonin transporter, as determined by their effects on imipramine binding and serotonin uptake, was investigated in rat brain and human platelets. In addition, the effect of tricyclic antidepressants on the binding of calcium channel blockers was evaluated. Verapamil was found to be a competitive inhibitor of both imipramine binding and serotonin uptake in brain and platelets. The inhibitory activity of verapamil was calcium-independent. Chronic verapamil treatment resulted in a significant decrease (28%) in [3H]imipramine binding in the rat hypothalamus but had no effect on [3H]imipramine binding to cerebral cortex membranes or on [3H]serotonin uptake in these two brain regions. Tricyclic antidepressants inhibited (-)-[3H]desmethoxyverapamil binding but did not affect [3H]nitrendipine binding to rat cerebral cortex membranes. Chronic imipramine treatment induced a non-significant increase (34%) in (-)-[3H]desmethoxyverapamil binding but did not alter [3H]nitrendipine binding in rat cerebral cortex. These interactions may be relevant to an understanding of the beneficial effects of verapamil in major affective disorders and may suggest an involvement of calcium channels in the pharmacological activity of tricyclic antidepressants.

Temperature dependence of drug interaction with the platelet 5-hydroxytryptamine transporter: a clue to the imipramine selectivity paradox

Although [3H]imipramine is a selective radioligand for the 5-hydroxytryptamine (5-HT) transporter in human platelets, its affinity for binding to the 5-HT transporter complex at 0 degrees C (0.6 nM) is significantly higher than its potency for inhibition of [3H]5-HT uptake at the physiological temperature of 37 degrees C (Ki = 29 nM). As this apparent discrepancy could be related to the assay temperature, we studied the thermodynamics of drug interaction with the 5-HT transporter at assay temperatures between 0 degrees C and 37 degrees C, using as radioligands [3H]imipramine (0 degrees C and 20 degrees C) and [3H]paroxetine (20 degrees C and 37 degrees C), a newly available probe for the 5-HT transporter. At 20 degrees C, Ki values of 14 tricyclic and nontricyclic drugs for inhibition of [3H]imipramine and [3H]paroxetine binding to human platelet membranes were highly significantly correlated (r = 0.98, p less than 0.001), validating the use of these two radioligands to study the 5-HT transporter over a temperature range larger than was previously possible with [3H]imipramine alone. The affinity of imipramine for the 5-HT transporter is progressively enhanced with decreasing incubation temperature, thus favoring the selectivity of [3H]imipramine for the 5-HT transporter at 0 degrees C. At 37 degrees C, the Ki of imipramine for inhibition of [3H]paroxetine binding is 32 nM, and equals its Ki value for inhibition of 5-HT uptake into human platelets. With the exception of chlorimipramine, other tricyclic 5-HT uptake inhibitors showed a temperature sensitivity in their interaction with the 5-HT transporter similar to that of imipramine.(ABSTRACT TRUNCATED AT 250 WORDS)

Platelet imipramine binding in patients with panic disorder and major familial depression

Platelet 3H-imipramine binding was investigated in 15 normal subjects, 17 patients with major depressive disorder and 43 patients with panic disorder, to further study the relationship between depressive and anxiety disorders. Whereas patients with major depression had a significantly lower mean Bmax value than healthy volunteers, mean Bmax values in patients with panic disorder did not differ significantly from normal controls. Furthermore, apparently normal Bmax values were observed even in those panic disorder patients who had concurrent major depression or a past history of depression. Thus, despite previous findings of an overlap between panic and depressive disorders, the present results suggest that the two syndromes may have distinct neurochemical substrates.

Modification of ligand binding to membranes by a soluble acceptor. Alpha-1-acid glycoprotein attenuates 3H-imipramine binding to cerebral membranes

The effect of individual plasma proteins on the binding of 3H-imipramine (3H-IMI) was investigated, using rat cerebral membranes as the binding site source. Addition of alpha-1-acid glycoprotein (aAGLP) to an incubation medium containing 4 nm 3H-IMI resulted in a concentration-dependent inhibition of 3H-IMI binding, with an IC50 value of 0.53 microM. Albumin (0.7 mM) and gamma globulin (3 microM) had no apparent effect. Scatchard analyses of 3H-IMI binding in the presence and absence of 0.5 microM aAGLP revealed that the inhibition of 3H-IMI binding was associated with an increased Kd, with no appreciable change in Bmax. The inhibitory action of platelet-free plasma (PFP) on 3H-IMI binding to cerebral membranes was eliminated after treatment of PFP with specific antibodies to aAGLP. It is suggested that the inhibition of 3H-IMI binding to cerebral membranes by PFP is due, at least partially, to the presence of aAGLP in the plasma. The observed inhibitory effect is consistent with a competition for the ligand between the membrane binding site and the soluble protein acceptor. These findings may explain the serum effect confounding radioreceptor assays of various drugs.

Inhibition of 3H-imipramine binding by plasma from depressed and normal subjects

The inhibition by human plasma of 3H-imipramine (3H-IMI) specific binding to rat cerebral membranes was investigated using platelet-free plasma (PFP) from normal and depressed subjects. The specific binding of 3H-IMI at a ligand concentration of 4 nM decreased with increasing PFP volumes, reaching 50% inhibition following the addition of 10-13 microliters PFP to a final incubation volume of 250 microliters. The extent of this inhibition was the same using PFP from controls or depressed patients. The inhibitory activity was associated with plasma proteins. Scatchard analysis of 3H-IMI binding in the presence and absence of PFP indicated that the inhibitory effect was associated with an increased Kd without an appreciable change in Bmax. It is suggested that an endogenous acceptor in the PFP, such as alpha 1-acid-glycoprotein may be responsible for the observed inhibition of 3H-IMI binding to the cerebral membranes.

Subcellular site of action of imipramine in rodent brain

To determine the site of action of imipramine, the subcellular distribution of [3H]imipramine in rodents was followed after both in vivo administration and in vitro incubation with tissue slices under "physiological" conditions. Total [3H]imipramine (10-1,000 nM) binding was associated with all primary fractions, but in particular with the nuclear (P1) and mitochondrial (P2) pellets and the synaptosomal (P2B) and myelin (P2A) fractions. Using an excess of imipramine to define any nonspecific interactions, a specific association was observed mainly in those fractions containing isolated nerve terminals and to a lesser extent with the purified myelin fraction. Preparation of subsynaptosomal fractions by osmotic lysis indicated that [3H]imipramine was associated with the synaptic vesicle and microsomal fractions and also with synaptosomal membranes. The degree of binding to the vesicular and microsomal fractions was increased with the length of preparation time, whereas there was an inverse relationship between the length of preparation and the amount bound to the synaptosomal membrane fraction. There was no evidence of an intrasynaptosomal accumulation of [3H]imipramine at concentrations up to 1,000 nM. [3H]2-Nitroimipramine, a slowly dissociating imipramine derivative, was exclusively located in synaptic membrane fractions. Prior treatment of rats with a combination of 5,7-dihydroxytryptamine and desipramine reduced 5-hydroxytryptamine levels and the levels of [3H]imipramine associated with the synaptosomal fractions to the same extent. It is concluded that imipramine is associated with a binding site localised on 5-hydroxytryptaminergic nerve terminals and that there is a redistribution to other sites (vesicular and microsomal) during the isolation procedure.

Tritiated imipramine binding to platelets is decreased in patients with agoraphobia

Controversy exists regarding the relationship between anxiety states and major depression. We studied the binding of tritiated imipramine to platelet membranes in order to determine if patients with agoraphobia and panic attacks differed from depressed subjects or healthy volunteers on this biological parameter. Mean (+/- SD) Bmax and Kd values were significantly lower in patients with agoraphobia and panic attacks (787 +/- 276 fmole/mg protein and 0.35 +/- 0.14 nM, respectively) than in healthy volunteers (1237 +/- 201 fmole/mg protein and 0.71 +/- 0.37 nM, respectively). In addition, patients with agoraphobia and panic attacks had binding parameters that were similar to those of patients with bipolar or familial pure depressive disorder, but significantly lower than those of patients with depressive spectrum or sporadic depressive disorder. These findings have implications for both the nosology and pathophysiology of anxiety disorders.

Imipramine treatment differentially affects platelet 3H-imipramine binding and serotonin uptake in depressed patients

Uptake of serotonin and 3H-imipramine binding in platelets of depressed patients were investigated simultaneously with changes in clinical state. Both Vmax for serotonin uptake and Bmax for 3H-imipramine binding were significantly lower in unmedicated depressed patients with respect to normal subjects. Successful treatment with imipramine led to a significant increase in Bmax for 3H-imipramine binding, without significant change in Vmax for serotonin uptake. Bmax values increased to the normal range following complete, rather than partial clinical improvement. These data indicate that successful antidepressant treatment may increase the density of 3H-imipramine binding sites on platelets by a process which is independent of the uptake of serotonin.

The uptake and metabolism of 5-hydroxytryptamine by tissue slices of the cestode Hymenolepis diminuta

The in vitro uptake of [3H]5HT was investigated in tissue slices of the cestode Hymenolepis diminuta. A concentrative, sodium sensitive, high affinity uptake mechanism (Km 1.43 X 10(-6) M; Vmax 222 fmoles/mg wet wt/min), together with a sodium insensitive component (linear up to 5 X 10(-6) M) were present. In the presence of 2-nitroimipramine the sodium sensitive component was significantly suppressed (Vmax 33 fmoles/mg/wet wt/min) although the Km (1.37 X 10(-6) M) was not affected. Nitroimipramine showed an IC50 of approximately 2 X 10(-6) M. The sodium insensitive component was not affected by nitroimipramine. Biogenic amines and related indoleamines were weak inhibitors of the sodium sensitive and sodium insensitive components of 5HT uptake. The tricyclic antidepressants and fluoxetine were effective inhibitors of the sodium sensitive component of 5-HT uptake; receptor ligands were weak inhibitors or without effect. The metabolism of [3H]5HT in tissue slices of H. diminuta was examined by HPLC. The role of the sodium sensitive uptake and metabolism of 5HT in terms of inactivation and recycling of neurally released 5HT and the possible importance of exogenous recruitment of 5HT are discussed.

High-affinity 3H-imipramine binding in platelets of children and adolescents with major affective disorders

High-affinity 3H-imipramine binding to platelet membranes was evaluated in 12 untreated children and adolescents aged 11-17 years who met the DSM-III criteria for major affective disorders. The affective patients were compared to 13 nonaffective subjects and 15 normal controls of similar ages. No significant difference in the maximal binding of 3H-imipramine (Bmax) and Kd values could be found among the three groups. 3H-Imipramine binding values failed to discriminate between patients with major depression and those with bipolar disorder, depressed type. No association between a positive family history of major affective disorders and 3H-imipramine binding values was detected.

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.

Learned helplessness decreases [3H]imipramine binding in rat cortex

Specific binding of [3H]imipramine decreased in frontal neocortex from rats demonstrating learned helplessness, an animal model of depression. The decrease was in maximal binding but not in affinity for the receptor site. No change in [3H]imipramine binding was found in septum or hippocampus. The receptor changes found in frontal neocortex parallel behavioral and neurochemical changes produced by learned helplessness in this region. These changes are also similar to those found in the frontal neocortex from suicides and in platelets of patients with depression.

High affinity binding of [3H]paroxetine and [3H]imipramine to human platelet membranes

Paroxetine, one of the most potent and specific serotonin uptake inhibitors, was tritiated and used for binding studies with human platelet membranes. Specific, high affinity binding was demonstrated. The binding was compared with [3H]imipramine binding; it was found that the maximal binding (Bmax) was the same for [3H]paroxetine and [3H]imipramine, whereas the affinity was much higher for [3H]paroxetine (KD 0.08 nM and 0.56 nM for paroxetine and imipramine binding, respectively). IC50 was calculated for the inhibition of [3H]paroxetine and [3H]imipramine binding by a number of antidepressants; the corresponding Hill coefficients were also calculated.

Kinetic properties of [3H]2-nitroimipramine binding to human platelets

2-Nitroimipramine has previously been reported to be a slowly dissociating ligand at [3H]imipramine binding sites in rat brain. This binding site has been tentatively identified as the recognition site for the serotonin transport mechanism. Since we have previously solubilized imipramine binding sites from platelets, the slowly dissociating nature of this compound was of interest in the continuance of our molecular characterisation studies. Association of [3H]2-nitroimipramine to human platelets was complete within 2 h and saturation analyses implied that the affinity of this ligand was similar to that of [3H]imipramine. Moreover, the ligand binding was inhibited by non-radioactive compounds in a manner consistent with reversible, competitive kinetics. However the dissociation rate, after only 20 min association, was slow (t1/2 = 8.3 h) as compared with [3H]imipramine. After prolonged incubation with membranes (16-20 h) the dissociation rate for [3H]imipramine was essentially unaltered whereas that for [3H]2-nitroimipramine decreased approximately three fold. I conclude that [3H]2-nitroimipramine gradually becomes more slowly reversible upon prolonged incubation, but following short incubation periods it behaves essentially like [3H]imipramine.

Lack of association between [3H]imipramine binding sites and uptake of serotonin in control, depressed and schizophrenic patients

Uptake of serotonin and [3H]imipramine binding were studied in parallel in the platelets of control, depressed and schizophrenic patients. In the depressed patient group, uptake of serotonin was consistently reduced while [3H]imipramine binding was only decreased in a number of these patients. In the schizophrenic group, uptake of serotonin was reduced to 62% of control with no changes in [3H]imipramine binding being observed. These data demonstrate a clear dissociation between uptake of serotonin and [3H]imipramine binding sites in human platelets. The possible functional role of these [3H]imipramine binding sites remains to be determined.

Noradrenergic and serotonergic input necessary for imipramine-induced changes in beta but not S2 receptor densities

Chronic imipramine administration decreases the density of serotonin S2 and beta-adrenergic receptors. The mode of action is not clear but might be assumed to be a result of imipramine interacting with its high affinity binding sites on presynaptic serotonin neurons. To investigate this possibility and the role of the serotonergic and noradrenergic systems on imipramine-induced receptor down-regulations, the effects of raphe lesions and 6-hydroxydopamine lesions were examined. The theory that imipramine acts through the serotonin system to down-regulate beta-adrenergic receptors was supported by the observation that raphe lesions attenuated the imipramine-induced beta receptor denumeration. However, the effect of imipramine on the S2 site may not be a result of serotonin uptake blockade. Raphe lesions which diminish 3H-imipramine high affinity binding did not alter the imipramine-induced decrease in S2 receptor numbers. 6-Hydroxydopamine lesions prevented the reduction in beta but not S2 binding. Imipramine may exert its effect on S2 binding through a site other than the high affinity, serotonin uptake site.

Quantitative autoradiography of [3H]nitroimipramine binding sites in rat brain

We have used the recent tritium-sensitive film method of quantitative autoradiography to localize in rat brain high-affinity binding sites for nitroimipramine (NI), a long-lasting inhibitor of serotonin (5-HT) transport in platelets. The distribution of NI binding sites in rat brain closely parallels the location of 5-HT terminals. There are high concentrations of binding sites in the dorsal and medial raphe nuclei, the basal portion of the mammilary nuclei, the medial forebrain bundle, the olfactory tubercule and the posterior basal nucleus of the amygdala. The association of [3H]NI binding sites with regions having a high content of 5-HT supports the notion that the high-affinity binding site for [3H]NI corresponds to some aspect of the presynaptic uptake site for 5-HT.

Subcellular distribution of high affinity [3H]imipramine binding and [3H]serotonin uptake in rat brain

The binding of [3H]imipramine to various subcellular fractions of rat brain was investigated. Using sodium free buffer or 0.5 microM chlorimipramine to define nonspecific binding, the specific binding was found to be associated with the synaptosomal fraction, as was the uptake of [3H]serotonin. A significant quantity of 'displaceable' binding was observed in the myelin and nuclear fractions when 'specific' binding was defined using only one concentration of [3H]imipramine and a large molar excess of chlorimipramine. This 'pseudospecific' binding was of low affinity and was not affected by the presence or absence of sodium.

[3H]2-Nitroimipramine: a selective "slowly-dissociating" probe of the imipramine binding site ("serotonin transporter") in platelets and brain

Previous studies have demonstrated a close functional and structural relationship between the "high affinity" binding site for [3H]imipramine and the presynaptic and platelet uptake site(s) for serotonin. Recently we have synthesized several nitro derivatives of imipramine which have a very high affinity for the imipramine binding site and which dissociate very slowly when incubations are performed at 0-4 degrees C. In this report, we describe the characteristics of [3H]2-nitroimipramine binding to platelet and brain membranes. Our results support the relative utility of this ligand for studying the impramine binding site (serotonin transporter) since this analogue has both a higher affinity and specific activity than [3H]imipramine. [3H]2-Nitroimipramine by virtue of its extremely slow dissociation rate should be a valuable tool in subsequent characterization and purification of the serotonin uptake or transport site.

Blood platelets in psychopharmacological research

Blood platelets are a very good model for nerve endings, for study of serotonin uptake and imipramine binding. The efficiency of various drugs, mainly antidepressants, can be evaluated using blood platelets instead of synaptosomes. A few neuronal active compounds are effective in inducing platelet secretion and shape change reaction. Platelet MAO and c-AMP levels are not yet established as useful in psychopharmacological studies.

5-hydroxytryptamine and depression: a model for the interaction of normal variance with pathology

1 Theories linking 5-hydroxytryptamine (5-HT) with depression are briefly reviewed. The various experimental strategies adopted to investigate this relationship, examination of autopsy data, CSF metabolite data, 5-HT re-uptake patterns in human blood platelets and imipramine binding studies in human platelets, are discussed. 2 Recent studies of 5-hydroxyindole acetic acid (5-HIAA) levels in cerebrospinal fluid have revealed a linkage between low 5-HIAA levels and suicide, aggression and impulsivity. Decreases in the number of imipramine binding sites have also been found in brains of suicide victims. 3 The available data lead to the conclusion that decreased 5-hydroxytryptaminergic function may be associated with an increased risk of depression, suicide, and some types of aggression.

Nitroimipramines - synthesis and pharmacological effects of potent long-acting inhibitors of [3H] serotonin uptake and [3H] imipramine binding

A series of nitro derivatives of imipramine have been prepared by nitration of imipramine. Several of the products, especially 2-nitroimipramine (2) and 2,8-dinitro-imipramine (4) were found to be very potent inhibitors of [3H] serotonin uptake and high affinity [3H]imipramine binding in human platelets. In contrast to the parent antidepressant, imipramine, the inhibition of platelet [3H] serotonin uptake and [3H] imipramine binding by the nitro derivatives of imipramine was long-acting and essentially irreversible at low temperatures. These compounds should prove to be valuable tools for subsequent studies on the purification and characterization of the transport protein(s) involved in serotonin uptake and may have novel behavioral and clinical effects.

Tritiated imipramine binding sites are decreased in the frontal cortex of suicides

Binding characteristics of tritiated imipramine were determined in the frontal cortex of suicides and well-matched controls. Maximal binding was significantly lower in brains from the suicides. This finding is consistent with reports of decreased tritiated imipramine binding in the platelets of patients diagnosed as having a major affective disorder.

High-affinity binding of [3H]desipramine to rat brain: a presynaptic marker for noradrenergic uptake sites

High-affinity binding sites (apparent KD = 1.5 nM) for [3H]desipramine have been demonstrated and characterized in membranes prepared from rat brain. The binding of [3H]desipramine was found to be saturable, reversible, heat-sensitive, sodium-dependent, and regionally distributed among various regions of the brain. High concentrations of [3H]desipramine binding sites were found in the septum, cerebral cortex, and hypothalamus, whereas lower concentrations were found in the medulla, cerebellum, and corpus striatum. A very good correlation (r = 0.81, P less than 0.001) was observed between the potencies of a series of drugs in inhibiting high-affinity [3H]desipramine binding and their capacity to block norepinephrine uptake into synaptosomes. In 6-hydroxydopamine-lesioned rats there was a marked decrease in [3H]norepinephrine uptake and [3H]desipramine binding with no significant alterations in either [3H]serotonin uptake or [3H]imipramine binding. These results suggest that the high-affinity binding of [3H]desipramine to rat brain membranes is pharmacologically and biochemically distinct from the high-affinity binding of [3H]imipramine, and that there is a close relationship between the high-affinity binding site for [3H]desipramine and the uptake site for norepinephrine.