Demonstration of endogenous "imipramine like" material in rat brain

Isolation and characterization of an endogenous peptide from rat brain interacting specifically with the serotonergic 1B receptor subtypes

The existence of endogenous compounds interacting with the serotonergic system was previously postulated. In the present work, rat brain tissues were extracted by acidic and organic procedures. The resulting extract was tested for its capacity to interact with the binding of [3H]5-hydroxytryptamine ([3H]5-HT) to 5-HT1 receptors. Compounds responsible for the observed inhibitory activities were isolated and purified by high pressure liquid chromatography. A tetrapeptide corresponding to a novel amino acid sequence Leu-Ser-Ala-Leu (LSAL) was identified. It reduces the binding of [3H]5-HT to 5-HT1 receptors at low concentration (IC50 = 10(-10) M). This effect corresponds to a specific interaction at 5-HT1B receptors since LSAL does not significantly affect other neurotransmitter bindings. LSAL appears heterogeneously distributed throughout the brain (hippocampus > cerebellum > striatum > brain stem) and in peripheral tissues (kidney > lung > stomach > blood > liver > spleen). Two other peptides, Leu-Ser (LS) and Ala-Leu (AL), were also purified. They hardly affected [3H]5-HT binding compared with LSAL. They presumably represent degradation products of the functional peptide LSAL. The fact that LSAL interacts specifically with 5-HT1B receptors that inhibit the release of neurotransmitters and particularly that of 5-HT itself suggests that this peptide may be involved in mechanisms controlling 5-HT neurotransmission and, accordingly, may play an important role in pathophysiological functions related to 5-HT activity.

Further evidence for the existence of endogenous serotonin uptake inhibitors and their purification by calmodulin affinity chromatography

Experimental evidence suggests the occurrence of endogenous antidepressant-like compounds in the brain, blood plasma, and urine. After extensive purification by calmodulin-sepharose affinity chromatography, and further purification of the urine-derived sample by exclusion chromatography, at least three distinctive fractions have been identified. These fractions effectively inhibited serotonin uptake, imipramine, and/or paroxetine binding, and they apparently contained some compounds that were recognized by the anti-imipramine or anti-paroxetine antibodies. Their identification may have significant implications for studies of affective illness.

Further studies on the endogenous serotonin-uptake-inhibitor-like substances in the human cerebrospinal fluid

The properties of endogenous substances that inhibit 5HT uptake in human cerebrospinal fluid (CSF) were investigated. Human CSF was loaded onto a Sephadex G-25 column, and each fraction was tested for its ability to inhibit [3H]paroxetine binding in monkey brain preparations. We found four different inhibitory peaks with respective molecular weights (M.W.) of > 12400, 2000 and two of < 1350. The third and fourth peaks (F-3, F-4: < 1350 M.W.) of inhibitory activity were determined to consist of some monoamines (5HT, etc.) or their metabolites (5HIAA, etc.) and other unidentified compounds by using an HPLC-electrochemical detector. The second peak (F-2, M.W. about 2000) displaced [3H]paroxetine binding noncompetitively (decreased Bmax and did not change Kd) and inhibited [3H]5HT uptake noncompetitively (decreased Vmax and did not change Km), but had no effect on either [3H]norepinephrine uptake or [3H]dopamine uptake. These results suggest that the endogenous substances that selectively inhibit 5HT uptake are present in human CSF as low molecular weight compounds.

Serotonin uptake and imipramine binding in the blood platelets of obsessive-compulsive disorder patients

14C-Serotonin (5-HT) uptake and 3H-imipramine binding (IB) were studied in the blood platelets of 20 obsessive-compulsive disorder (OCD) patients, 53 normal controls (5-HT uptake) and 32 normal controls (IB binding). The maximum number of binding sites (Bmax) was significantly decreased in OCD patients compared to normal controls, but there was no difference in the affinity for 3H-imipramine (Kd). The affinity for 5-HT uptake (Km) was also decreased in the OCD patients but the maximum velocity of 5-HT uptake sites (Vmax) was not significantly different in OCD patients and normal volunteers. There were trends for the Slowness Subscale of the Maudsley Obsessional-Compulsive Inventory (MOCI) to be positively correlated with the Km of 5-HT uptake (p = 0.094), whereas the Global Scale, Checking Subscale, and Doubting Conscientiousness Subscale of MOCI were negatively correlated with the Kd of IB (p = 0.066, p = 0.08, and p = 0.062, respectively). The results provide further evidence for the dysfunction of the serotonergic system in OCD.

Non-specific inhibition of imipramine binding argues against an endogenous ligand

We have attempted to characterize the product(s) inhibiting [3H]imipramine binding from rat cortical membranes. Fractions inhibiting [3H]imipramine binding were analysed by gas chromatography-mass spectrometry (GC-MS) and high performance liquid chromatography-mass spectrometry (HPLC-MS). Tissue extracts showed marked inhibitory activity (two different peaks); however, extracts containing no brain tissue (blanks) also exhibited inhibitory activity (one peak after HPLC pre-purification). The chemical composition of the compound that inhibited [3H]imipramine binding was identical in both cases (tissue and blank extracts) and consisted of zinc chelates, probably formed by the use of zinc sulfate during the extraction procedure. The second inhibitory peak in the tissue extracts was 5-hydroxytryptamine (serotonin, 5-HT). These results suggest that the inhibitory activity on [3H]imipramine binding attributed to a low-molecular weight putative endacoid is non-specific and due to ions introduced in the sample during purification.

Alteration of 5-HT uptake by plasma fractions in the premenstrual syndrome

The effects of plasma and an aqueous plasma fraction from patients with premenstrual syndrome (PMS) and control subjects on the uptake of 5-hydroxytryptamine (5-HT) in washed human platelets and rat forebrain synaptosomes were studied. Pre- and postmenstrual samples of unextracted plasma from the control group significantly enhanced platelet uptake of 5-HT. In contrast, an aqueous fraction following extraction of the plasma with organic solvents caused a dose-dependent decrease of 5-HT uptake. Plasma obtained from patients with PMS caused less stimulation of 5-HT uptake compared to plasma from the control group. The aqueous fraction of premenstrual plasma from patients tended to inhibit 5-HT uptake to a greater extent than a similar plasma fraction from controls. The inhibition of 5-HT uptake was associated with an increase in Km. Aqueous plasma fractions from both groups also inhibited 5-HT uptake in brain synaptosomes. However, there were no significant differences between groups. The results of the platelet study suggest that there may be quantitative differences in the plasma concentration of endogenous factors that affect 5-HT uptake between patients with PMS and control subjects and that such differences may explain the previously reported alteration of platelet 5-HT uptake and content associated with PMS symptoms.

Binding of some antidepressants to the 5-hydroxytryptamine transporter in brain and platelets

Antidepressant agents with properties to inhibit 5-hydroxytryptamine (5-HT, serotonin) uptake in brain tissue and platelets bind with high affinities to neuronal and platelet membranes. [3H]Imipramine, [3H]paroxetine and [3H]citalopram label specific binding sites related to the 5-HT transporter. [3H]Paroxetine and [3H]citalopram appear to be better ligands than [3H]imipramine. The former label a homogenous population of binding sites, whereas the displaceable binding of [3H]imipramine is heterogenous. Recent observations in several laboratories, which have taken the heterogeneity of [3H]imipramine binding into account, indicate that the binding of antidepressants to the 5-HT transporter probably occurs to the same site that binds 5-HT for transport and not to a separate site as previously suggested. Additional bonds to subsites in close vicinity to the 5-HT recognition site may contribute to the binding. No convincing evidence has been presented of the existence of an endogenous ligand other than 5-HT itself that binds to the [3H]imipramine binding site. Recent studies also suggest that repeated treatment of rats with antidepressant agents does not produce any alterations of the binding of [3H]imipramine or [3H]paroxetine to membranes of cerebral cortex. It is also doubtful whether the density of the 5-HT uptake site in platelets measured with these ligands is decreased in affective disorders as first reported.

Inhibition of MAO activity, 3H-imipramine binding, 3H-paroxetine binding and 3H-5-HT uptake by human cerebrospinal fluid

Addition of small amount of human cerebrospinal fluid (CSF) inhibited both types of MAO in monkey brain mitochondria. The specific binding of 3H-paroxetine decreased remarkably with increasing CSF volumes, while 3H-imipramine binding was slightly inhibited. Scatchard analysis of 3H-paroxetine binding in the presence and absence of CSF indicated that the inhibitory effect was associated with a decreased Bmax without an appreciable change in Kd. Addition of CSF induced an inhibition of uncompetitive 3H-5-HT uptake to monkey cerebral membranes. These results indicate that the materials in human CSF inhibit 3H-paroxetine binding, and modulate the uptake system for 5-HT.

Diurnal concordance of human platelet serotonin content and plasma alpha-1-acid glycoprotein concentration

The diurnal rhythm of platelet serotonin (5-HT) content and plasma alpha-1-acid glycoprotein (AGP) was investigated in twelve healthy male volunteers aged 19-30 years. Platelet 5-HT content and plasma AGP concentration varied across a twenty-four hour sampling period in a manner consistent with a circadian pattern. Platelet 5-HT levels rose from baseline at 5.00 hr to a peak at 14.00 hr and slowly declined to baseline during the night-time hours. A concordant pattern was observed for plasma AGP with peak concentrations occurring between 8.00 hr and 14.00 hr. These findings support the notion that AGP is a positive endogenous allosteric stimulator of platelet 5-HT transport, since there is a direct relationship between platelet 5-HT levels and plasma AGP concentration. A theoretical model is presented to explain the diurnal effects of AGP on the platelet 5-HT transporter.

Endogenous monoamine oxidase inhibitor-like substances in monkey brain

The extraction and partial purification of endogenous "monoamine oxidase (MAO) inhibitor-like" material from the monkey brain are described. The endogenous material (F-1 and F-2) obtained after Bio-Gel P-2 gel filtration and silica column chromatography inhibited MAO in the monkey brain mitochondria toward 5-hydroxytryptamine (5-HT), beta-phenylethylamine (beta-PEA), tyramine and dopamine as substrates. The inhibitory effects of F-1 and F-2 were non-linear concentration dependent, and F-1 non-competitively inhibited A-form MAO, while F-2 inhibited A-form MAO competitively and inhibited B-form MAO non-competitively. These substances were more potent inhibitors of A-form than of B-form MAO. F-2 was heat stable but liable to the treatment with pepsin and trypsin. F-1 was not inactivated by heat treatment and digestion with pepsin and trypsin. F-1 may be a low molecular weight (less than 1350) compound, including certain monoamines or their metabolites or other unidentified compounds, while F-2 was a low molecular weight (about 2500) peptide.

Alteration of platelet serotonergic mechanisms and monoamine oxidase activity in premenstrual syndrome

Platelet uptake and content of 5-hydroxytryptamine (5-HT), platelet monoamine oxidase (MAO) activity, and plasma free and total tryptophan levels were determined in patients diagnosed with premenstrual syndrome (PMS) and in control subjects. The Vmax of 5-HT uptake and 5-HT content in platelets of PMS patients were significantly decreased during the premenstrual phase (cycle days -9 to -1) compared to control subjects. Platelet MAO activity was significantly lower postmenstrually (cycle days 5-9) in PMS patients compared to the premenstrual phase. There were no differences in plasma free and total tryptophan levels between PMS patients and control subjects during either interval. As platelets are believed to be a peripheral model for central serotonergic neurons, the results suggest that PMS symptomatology may be related to alterations in serotonergic neuronal mechanisms.

Effect of desipramine treatment on 3H-imipramine binding in the blood platelets of depressed patients

Platelet imipramine binding (IB) was studied in depressed patients before and after treatment with desipramine for 17-28 days. Administration of desipramine was associated with a significant increase in Bmax. There was a trend for an increase in Kd, but it did not reach statistical significance. The net result of the changes in Bmax and Kd was an increase in IB. There were significant correlations between the change in depression ratings and pretreatment Kd, as well as the change in Kd during treatment. These results suggest that decreased IB is not a trait-dependent marker, but a state-dependent marker for depression.

Studies on the serotonin transporter in platelets

[3H]-Imipramine and [3H]-paroxetine label with high affinity a recognition site which is associated with the serotonergic transporter in blood platelets. The pharmacological profile of [3H]-imipramine and [3H]-paroxetine binding is highly correlated with the potency of drugs to inhibit the uptake of serotonin. Dissociation kinetic experiments suggest that the substrate recognition site for serotonin may be different from the modulatory site which is labeled with [3H]-imipramine or [3H]-paroxetine. The existence of an endocoid acting on the imipramine receptor to modulate the serotonin transporter has been proposed by several laboratories. In clinical studies most laboratories have reported a decrease in Bmax of [3H]-imipramine binding in platelets from depressed untreated patients when compared with matched healthy volunteers. The Bmax of [3H]-imipramine binding in platelets appears to be a state-dependent biological marker in depression.

Seasonal variation of imipramine binding in the blood platelets of normal controls and depressed patients

Imipramine binding (IB) was studied in the blood platelets from normal controls and depressed patients over a 4-year period (1981-1984) to determine if seasonal variation was present in Bmax or KD. Bimonthly variation in the Bmax of IB was found in normal controls studied longitudinally. No such variation was found when individual values from normal controls were examined on a monthly or seasonal basis. Bmax in depressed patients showed a significant seasonal, but not monthly, variation. KD of IB varied in normal controls using monthly or seasonal data, but not in the probably more reliable bimonthly data. These results suggest that IB studies comparing groups of subjects should match groups for season of the year or, for greater accuracy, month of the year.

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 antidepressant effect of beta-adrenoreceptor subsensitivity: a brief review and clinical implications

This paper reviews briefly the very common finding in chronic antidepressant use, of subsensitivity of the beta-adrenoreceptor-linked cyclic AMP system. This subsensitivity is observed with a number of different antidepressant treatments, including pharmacological, electrical and sleep deprivation. The subsensitivity requires intact noradrenergic and serotonergic systems, functionally linking the two neurotransmitters most often implicated in depression. Thyroid hormones and estrogens also cause subsensitivity, while the opposite effect is seen with Reserpine and Propranolol. A modified conditioning/sensitization model is proposed, implicating psychosocial stressors with a biological inability to down-regulate beta-adrenoreceptors.

Upregulation of imipramine binding and serotonin uptake by estradiol in female rat brain

This report describes the effect of chronic estradiol treatment on the serotonin transporter in the female rat brain. Both [3H]imipramine binding and [3H]serotonin uptake increased by 20-30% in the frontal cortex and hypothalamus of ovariectomized rats after 12 days of 17 beta-estradiol treatment. No differences were observed in the binding and uptake parameters as a function of the rats' estrous cycle or in untreated ovariectomized rats, as compared to controls. Estradiol in vitro, inhibits [3H]imipramine binding as well as serotonin uptake in rat brain and human platelets. Like serotonin, estradiol decreases the dissociation rate in vitro of [3H]imipramine from its binding site in a dose-dependent manner.

Dopamine and serotonin systems in human and rodent brain: effects of age and neurodegenerative disease

The nonpathological age-related changes in the dopamine- and serotonin-containing neurotransmitter systems in human and rodent brain are reviewed. The dopamine system exhibits age-related declines both presynaptically and postsynaptically. Presynaptically, both the levels of dopamine and the number of midbrain dopamine-containing neurons decline by up to 50% at advanced ages in the absence of neurological disease. Postsynaptically, the density of D-2 dopamine receptors decreases by 40%, while D-1 dopamine receptors either increase (man) or remain stable (rodents). Additional reductions of dopamine levels and D-2 receptors have been reported in Alzheimer's disease (AD), but these changes are relatively small, and not consistently observed. The levels of serotonin appear stable during normal aging, and presynaptic markers such as (3H)imipramine binding may actually increase. In human brain, the two major classes of serotonin receptor (S-1 and S-2) decrease by 30 to 50% over the lifespan. In AD, both presynaptic and postsynaptic markers of the serotonin system are reduced, including a loss of the serotonin-containing raphe neurons. The additional loss of serotonin receptors in AD approaches 80% when compared with young normals. A hypothesis is presented to explain the typically young age at onset of schizophrenia (usually before 30 years of age) and the older age at onset of parkinsonism (rarely before 50 years of age) within the context of normal age-related declines in the dopamine system occurring in the absence of neurological disorders. The possibility that chronic cocaine abuse might accelerate the development of parkinsonism is discussed.

Pitfalls in demonstrating an endogenous ligand of imipramine recognition sites

The recognition sites for the 5-hydroxytryptamine (5-HT) uptake inhibitors imipramine and paroxetine may represent receptors for a presently unknown endogenous ligand, whose function would be to modulate 5-HT uptake. Attempts to isolate such a factor from rat brain tissue are described, following a published procedure. It is shown that chromatographic fractions found to inhibit the binding of [3H]imipramine and [3H]paroxetine to rat brain membranes consisted of material essentially unretained by the reverse-phase HPLC column, and they were of high osmolarity. Thus, the activity was probably unspecific in nature, and the presence in rat brain of the factor has not been unequivocally demonstrated.

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 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.

Association of [3H]-imipramine and [3H]-paroxetine binding with the 5HT transporter in brain and platelets: relevance to studies in depression

[3H]-Imipramine and [3H]-paroxetine label with high affinity a site which is associated with the serotonergic transporter in brain and platelets. The pharmacological profile of inhibition by drugs of [3H]-imipramine and [3H]-paroxetine binding is highly correlated with the potency of the drugs to inhibit the uptake of 5HT. Denervation of serotonergic neurons by electrolytic lesions or with 5,7-dihydroxytryptamine produces marked decreases in the density of [3H]-imipramine as well as [3H]-paroxetine binding. Dissociation kinetic experiments support the view that the substrate recognition site for 5HT is different from the modulatory site which is labelled by [3H]-imipramine or [3H]-paroxetine. The existence of an endogenous ligand acting on the [3H]-imipramine recognition site to modulate the 5HT transporter was proposed by several laboratories. [3H]-Imipramine binding in platelets appears to be a biological marker in depression. Studies carried out in several laboratories report a significant decrease in the Bmax of platelet [3H]-imipramine binding without changes in Kd, when severely depressed untreated patients are compared with healthy volunteers matched for age and sex. The Bmax of platelet [3H]-imipramine binding appears to be a state-dependent biological marker in depression. It is tempting to speculate that the endocoid of the [3H]-imipramine recognition site may play a role in the pathogenesis of depression.

Reduced 3H-imipramine binding but unaltered 3H-serotonin uptake in platelets of adolescent enuretics

High affinity 3H-imipramine binding and 3H-serotonin uptake to platelets was evaluated in nine untreated adolescent enuretics (ages 13-18) and nine age- and sex-matched controls. A significant decrease in the maximal binding of 3H-imipramine (Bmax) was observed in the enuretics as compared to the controls. No alteration in the affinity of 3H-imipramine to its binding sites (Kd) or in serotonin uptake kinetic parameters (Vmax, Km) was detected. The lack of correlation between Bmax and Vmax values might indicate that the binding sites for imipramine and the sites for serotonin uptake are not identical.

Lack of seasonal variation in platelet [3H]imipramine binding in humans

The Bmax of [3H]imipramine (IMI) binding has been reported to be reduced in platelets of depressed untreated patients as compared with normal controls. However, it has also been suggested that this difference could be related to the failure to take into account seasonal variations in the binding parameters for [3H]IMI recognition sites in platelets. For this reason, [3H]IMI binding was studied throughout 1 year in platelet membranes from 11 control volunteers, with blood samples collected once a month. The Bmax and Kd values of [3H]IMI binding showed no significant variation throughout the 12-month period of the study. These results indicate that in the control population, the platelet [3H]IMI binding parameters remain stable, and that the decrease in Bmax observed in depressed untreated patients reflects a genuine difference, which may be considered to be a biological marker in depression.

Subdivision of mouse brain [3H]imipramine binding based on ion dependence and serotonin sensitivity

The specific binding of [3H]imipramine to mouse brain membranes in an assay containing 120 mM NaCl and 5 mM KCl was similar in regional distribution and pharmacological specificity to that reported previously in rat and human brain. However, the absence of ions decreased the density of the specific binding of [3H]imipramine and did not affect the equilibrium dissociation constant. Sodium was the only cation, and halides were the only anions tested that enhanced the specific binding of [3H]imipramine. Chloride did not increase the density of binding in the absence of sodium. The ion-sensitive binding of [3H]imipramine was regionally dependent and was highly correlated with the uptake of 5-hydroxytryptamine (5-HT, serotonin) into synaptosomes from brain regions. 5-HT did not inhibit the binding of [3H]imipramine in the absence of ions. Antidepressants inhibited binding in the absence and presence of ions, but in the presence of ions inhibition curves were shifted to the left and the apparent complexity of inhibition was increased. Quantitative analysis of the inhibition of [3H]imipramine binding by antidepressants conducted in the presence of ions was consistent with two binding sites. Lesion of the serotonergic input to the cerebral cortex by 5,7-dihydroxytryptamine suggested that both the 5-HT-sensitive and ion-sensitive binding of [3H]imipramine were associated with serotonergic nerve terminals. [3H]Imipramine binding displaced by desipramine, but insensitive to 5-HT and ions, was not affected by the lesion. Thus, the binding of [3H]imipramine that is displaced by desipramine, the most common assay for [3H]imipramine binding, includes a component that is not associated with brain serotonergic nerve terminals and 5-HT uptake, and, in addition, a separable component that is highly correlated with serotonergic function. These data have important implications for studies of serotonergic neurons and for the interpretation of imipramine binding data.

Effect of adrenalectomy and corticosterone on [3H]imipramine binding in rat blood platelets and brain

The effect of adrenalectomy and administration of glucocorticoids on [3H]imipramine binding (IB) of rat blood platelets and brain was investigated. Adrenalectomy significantly increased both Kd and Bmax of IB in the blood platelets but not the brain of male Sprague-Dawley rats. Administration of corticosterone acetate, 1 mg/kg i.p. for 7 days, decreased both Kd and Bmax in the blood platelets of sham-operated rats, but only Bmax in adrenalectomized rats. Corticosterone administration also decreased Bmax in frontal cortex and hypothalamus of sham and adrenalectomized rats but had no effect on Kd. These results suggest that glucocorticoids may modulate imipramine binding.

[3H]imipramine displacement and 5HT uptake inhibition by tryptoline derivatives: in rat brain 5-methoxytryptoline is not the autacoid for [3H]imipramine recognition sites

A putative endacoid capable of displacing [3H]imipramine from its high affinity binding site and of inhibiting [3H]serotonin (5HT) uptake has been partially purified from rat brain tissue. It appears to be unevenly distributed in various rat brain structures following a pattern that only partially matches the extent of the serotonergic innervation in the rat brain structures investigated. The highest amounts have been recovered in striatum followed by hippocampus, cerebral cortex, brain stem and less in diencephalon, cerebellum, hypothalamus, olfactory bulb. Virtually no inhibitory activity on [3H]imipramine binding or on [3H]5HT uptake in addition to 5HT has been found in rat pineal extracts. Its absence in the pineal and various chemicophysical properties discussed in this report suggest that the rat brain endacoid for the imipramine binding site is not 5-methoxytryptoline, a compound previous proposed as the candidate for the role of endogenous ligand of [3H]imipramine recognition site. Moreover, the study of a series of tryptoline derivatives indirectly supports these conclusions.

High affinity [3H]imipramine binding and serotonin uptake to platelets of adolescent females suffering from anorexia nervosa

High affinity [3H]imipramine binding and [3H]serotonin uptake to platelets were investigated in 17 anorexic females aged 15-18 years as compared to 15 healthy females of similar ages. A significant decrease in the density of [3H]imipramine binding sites was observed in anorexics as compared to controls (368 +/- 40 vs 517 +/- 38 fmoles/mg protein, p less than 0.01). No alteration in Kd values or in the kinetic parameters of serotonin uptake (Vmax, Km) were noted. The fact that the decrease in imipramine binding is not accompanied by a parallel reduction in serotonin uptake might indicate that anorexia nervosa is not ultimately related to major depression and that the imipramine binding site is not identical to the serotonin uptake site.

Reduced Bmax of [3H]-imipramine binding to platelets of depressed patients free of previous medication with 5HT uptake inhibitors

The high-affinity binding sites for [3H]-imipramine (IMI) present in human platelets are associated with the neuronal uptake system for 5HT. It was recently demonstrated that previous antidepressant therapy with drugs which inhibit 5HT uptake could down-regulate [3H]-IMI binding and that this effect could persist up to 1 month after the end of treatment. We therefore re-examined the reported differences in Bmax of [3H]-IMI binding in platelets between control and depressed untreated patients, to evaluate the residual influence of previous antidepressant medication. The saturation characteristics of [3H]-IMI binding were compared in platelets from 17 depressed patients carefully selected according to previous antidepressant therapy and washout period, who were closely matched, for age and sex, with a group of control healthy volunteers. The results reveal a significant decrease by 47% in the Bmax of [3H]-IMI binding in platelets of untreated depressed patients when compared with controls. There was no significant modification of Kd values for platelet [3H]-IMI binding between the depressed and the control groups. Our results support the view that platelet [3H]-IMI binding is a useful tool as a biological marker in depression.

Chronic antidepressant treatment and mouse brain 3H-imipramine binding

Chronic pretreatment of mice with the monoamine oxidase type B inhibitor (-)deprenyl resulted in an increase in the density of cerebral cortical 3H-imipramine binding sites and a decrease in the density of cerebral cortical beta-adrenergic receptors. In contrast, pretreatment of mice with the tricyclic antidepressants imipramine and desipramine did not alter the density of cerebral cortical 3H-imipramine binding sites. Imipramine and desipramine treatment decreased the density of beta-adrenergic receptors. Haloperidol pretreatment resulted in an increase in the density of striatal D-2 dopamine receptors, but did not alter the density of cerebral cortical 3H-imipramine binding sites or beta-adrenergic receptors. These data suggest that brain 3H-imipramine binding sites can be regulated by pharmacological pretreatment, but that this regulation may not occur for all antidepressants.

Platelet 3H-imipramine binding distinguishes depression from Alzheimer dementia

Platelet 3H-imipramine binding and serotonin uptake were studied simultaneously in normal subjects and in depressed, parkinsonian and Alzheimer's disease patients to investigate the usefulness of these variables in the diagnosis of depression in the elderly. Whereas Vmax of platelet serotonin uptake was significantly reduced in all patient groups compared to age matched normal subjects, the density of 3H-imipramine binding was reduced in depressed patients only. The lower Bmax values in depressed patients was independent of patient age. These data suggest that platelet 3H-imipramine binding may be a useful laboratory index which discriminates depression from dementia in the elderly.