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

Effect of aging and sex on the [3H]-paroxetine binding to human platelets

Since there exist conflicting results with regard to the possible effect of aging on platelet [3H]-imipramine binding, taken as a peripheral marker of the serotonin (5-HT) transporter, we reinvestigated this matter by comparing the binding of the more selective ligand [3H]-paroxetine in 20 aged and 23 young subjects. The results showed that neither the maximum binding capacity nor the dissociation constant (Kd) were significantly different in the two groups. When the subjects were compared according to sex, the young females revealed a statistically significant lower Kd than the males, while the contrary was true for the aged females. The Kd was significantly and negatively correlated to age in males. In addition, a significant age x gender interaction was also observed. Therefore, the sex of a subject would seem to provoke significant age-related changes in the Kd of [3H]-paroxetine binding to platelet membranes. This might indicate modifications in the 5-HT transporter that could form the basis of a sex-related difference in vulnerability to disorders, such as depression, where a dysfunction at this level is hypothesized.

Autoradiographic characterization of [3H]imipramine and [3H]citalopram binding in rat and human brain: species differences and relationships to serotonin innervation patterns

The neuroanatomical distribution of binding sites for [3H]imipramine and [3H]citalopram was assessed by in vitro autoradiography in select regions of the rat and human forebrain. To determine involvement of serotonin-containing terminals in the binding of [3H]imipramine and [3H]citalopram, binding of these compounds was measured in rats after destroying serotonin-containing neurons with 5,7-dihydroxytryptamine (5,7-DHT). Treatment with this neurotoxin decreased serotonin content by 90% and reduced [3H]citalopram binding to a similar extent. These results demonstrate that [3H]citalopram binding is a reliable marker for serotonin-containing terminals. Binding of [3H]imipramine was reduced by only 15-35% after 5,7-DHT treatment. These latter results suggest that only a small fraction of [3H]imipramine binding to brain sections is associated with serotonergic terminals under standard conditions used in autoradiographic studies with the ligand. Dose-response effects of fluoxetine and desipramine on displacement of [3H]imipramine binding in forebrain regions indicate that the ligand labels predominantly high capacity, low affinity binding sites. To determine the utility of the rat brain as a model for [3H]imipramine and [3H]citalopram binding in the human brain, binding of the ligands was compared in human and rat hypothalamus, amygdala, and hippocampus. The pharmacological characteristics of [3H]imipramine and [3H]citalopram binding were similar in the rat and human brain. However, substantial species differences were observed in topographic patterns of [3H]imipramine binding within the hippocampus and hypothalamus. The distribution of [3H]citalopram binding sites within the amygdala and hypothalamus were also strikingly different in rats compared to humans. This work provides the first demonstration that marked species differences exist in the topography of serotonergic innervation and in the distribution of [3H]imipramine binding sites within the rat and human brain regions examined.

Peripheral markers of serotonin and dopamine function in obsessive-compulsive disorder

In an attempt to clarify the possible role of a serotonergic and dopaminergic dysfunction in obsessive-compulsive disorder (OCD), we measured platelet 3H-imipramine (3H-IMI) binding, serotonin uptake, and platelet sulfotransferase activity in 17 drug-free OCD patients and an equal number of healthy controls. Serotonin uptake and 3H-IMI binding sites in platelets have been shown to constitute peripheral markers of those present in presynaptic serotonergic neurons. Sulfotransferase, an enzyme involved in the catabolism of phenolic compounds and of cathecholamines such as dopamine, has similar kinetic characteristics in brain and platelets. Our results showed a lower number of 3H-IMI binding sites and a higher level of sulfotransferase activity in OCD patients compared with those in controls. These preliminary results suggest involvement of both the serotonin and dopamine systems in OCD.

3H-paroxetine binding in brains of alcoholics

High affinity 3H-paroxetine binding was studied in human frontal cortex and hippocampus obtained from normal controls and alcoholics. On the basis of Scatchard analyses, a significant decrease in the maximal number of binding sites (Bmax) was found in the hippocampus of alcoholics (n = 8) as compared with that of controls (n = 10) (mean +/- SD = 63 +/- 35 vs. 114 +/- 70 fmoles/mg protein). There was no significant difference in the dissociation constants (Kd) between the two groups. The presumed effect of chronic alcohol abuse on 3H-paroxetine binding may be region-specific since no significant difference in either Bmax or Kd for 3H-paroxetine binding was found in the frontal cortex between normal controls and alcoholics. No significant correlation of 3H-paroxetine binding with age or postmortem interval was observed. The decrease in 3H-paroxetine binding in the hippocampus of alcoholics is probably indicative of reduced density of serotonergic nerve terminals either as a preexisting condition or as a result of neuronal damage caused by ethanol or the sequelae of alcoholism, such as nutritional deficiencies.

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

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

Fetal ethanol exposure diminishes hippocampal beta-adrenergic receptor density while sparing muscarinic receptors during development

Because of ostensible effects of fetal exposure to ethanol on cardiac and memory functions, beta-adrenergic and muscarinic receptor binding were surveyed in hippocampus and heart in 8- and 17-day-old rat pups. Pregnant, multiparous rats were intubated with either 6 g/kg ethanol or isocaloric dextrose twice daily from gestational days 10-16. At birth, offspring were fostered to untreated mothers. Pups exposed to ethanol had diminished birth weights, although there was no difference in the amount of weight gain by ethanol and control dams during gestation, nor in litter size. Ethanol pups remained smaller than control pups, but this difference was significant only until 8 days of age. At 17 days of age, ethanol pups had fewer hippocampal beta-adrenergic receptors than age-matched controls; muscarinic receptors and CA1 cell densities were not disparate. Parallel studies suggested that approximately 50% of the hippocampal beta-adrenergic receptors in 8-day-olds were of the beta 1 and beta 2 subtypes, while by 17 days of age approximately 70% of the receptors were beta 1. There was an ontogenetic increment in both beta-adrenergic and muscarinic binding from 8 to 17 days of age in hippocampus. No differences between age or drug groups were found in the binding measures in heart tissue. The present findings indicate that fetal ethanol treatment affects developmental measures and beta-adrenergic receptors in the hippocampus in a quasi-selective manner, but not hippocampal CA1-cell density.

Methodological issues in the preparation and assay of platelet 3H-imipramine binding

Several methodological factors in the preparation of platelets and the determination of platelet 3H-imipramine (3H-IMI) binding were examined. The ionic composition of the assay significantly affected platelet 3H-IMI binding. Approximately 25% of the specific binding of 3H-IMI to intact platelet preparations was retained in the absence of sodium and chloride ions. The addition of sodium ions enhanced the specific binding of 3H-IMI, but the addition of chloride in the presence of sodium had a more pronounced effect, enhancing binding approximately five-fold over that observed with the addition of sodium. Sodium was the only cation tested that enhanced binding. Only halides enhanced binding in the presence of sodium with the following order of potency: Cl- greater than Br- greater than I- = F-. Ions increased the density of binding sites (Bmax) and did not affect the affinity of the binding sites for 3H-IMI. In the presence of sodium and chloride, the use of serotonin (5HT) to define nonspecific binding in saturation experiments resulted in lower binding densities (Bmax) than when desipramine was used to define nonspecific binding. The component of binding that was insensitive to 5HT was roughly equal to the Bmax of 3H-IMI binding obtained in the absence of sodium and chloride using desipramine to define nonspecific binding. Overall, these data suggest that not all 3H-IMI binding that is displaced by desipramine is related to serotonergic mechanisms, and suggest that 5HT is a better choice than desipramine for the determination of the nonspecific binding of 3H-IMI. In addition, the binding of 3H-IMI to different platelet preparations was compared. The binding of 3H-IMI to intact platelets was less than that obtained using lysed platelet membranes when data were expressed per mg protein. The Coomassie Blue dye-binding method to determine platelet protein resulted in greater Bmax values than were obtained with the Folin phenol reagent method. The method of platelet preparation that is commonly used to prepare platelets for 3H-IMI binding resulted in similar binding values when compared to a method that prepares the entire platelet population. The results suggest that some, but not all, variations in laboratory methods used to prepare platelets and assay for platelet 3H-IMI binding may affect clinical studies examining this measure.

Down-regulation of 3H-imipramine binding sites in rat cerebral cortex after prenatal exposure to antidepressants

Several antidepressant drugs were given to pregnant rats in the last 15 days of gestation and 3H-imipramine binding (3H-IMI) was subsequently measured in the cerebral cortex of the offspring. The selective serotonin (5-HT) uptake blockers chlorimipramine and fluoxetine as well as the selective monoamine oxidase (MAO) inhibitors clorgyline and deprenyl induced, after prenatal exposure, a down-regulation of 3H-IMI binding sites at postnatal day 25. The density of these binding sites was still reduced at postnatal day 90 in rats exposed in utero to the MAO inhibitors. The antidepressants desipramine and nomifensine were ineffective in this respect. After chronic treatment of adult animals, only chlorimipramine was able to down-regulate the 3H-IMI binding sites. Consequently, prenatal exposure of rats to different antidepressant drugs affecting predominantly the 5-HT systems induces more marked and long-lasting effects on cortical 3H-IMI binding sites. The results suggest that the developing brain is more susceptible to the actions of antidepressants.

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.

Differences between sodium-dependent and desipramine-defined [3H]imipramine binding in intact human platelets

Measurements of sodium-dependent [3H]imipramine binding to intact human platelets from 20 human volunteers were made and compared to desipramine-defined binding, a method commonly employed in population studies of platelet [3H]imipramine sites. The density (Bmax) of sodium-dependent [3H]imipramine sides in platelets was significantly lower (449 +/- 36 sites/platelet) and the affinity (Kd) significantly higher (1.15 +/- 0.12 nM) than those obtained when excess desipramine was used to define specific binding (Bmax 654 +/- 33 sites/platelet, p less than 0.001; Kd 1.52 +/- 0.11 nM, p less than 0.001). There was no significant correlation between the density (Bmax) of sodium-dependent and desipramine-defined binding in individual subjects, suggesting that a different proportion of sites are labeled under the two assay conditions. No age-dependent variation was found in either Kd or Bmax values of sodium-dependent or desipramine-defined [3H]imipramine binding. The results suggest determination of sodium-dependent [3H]imipramine binding to intact platelets may be a useful measure for the estimation of [3H]imipramine recognition sites relevant to the serotonin uptake in studies of patients with affective disorders.

Platelet tritiated imipramine binding and MAO activity in Alzheimer's disease patients with agitation and delusions

Decreased platelet 3H-imipramine binding density and decreased monoamine oxidase (MAO) activity have been considered as biological characteristics of several neuropsychiatric disorders, and may be related to central serotonin defects. Since serotonin system defects occur in Alzheimer's disease (AD), and decreased brain 3H-imipramine binding density, and increased brain and platelet MAO activity are reported also, we studied platelet 3H-imipramine binding density (Bmax) and platelet MAO activity in AD outpatients without antecedent psychiatric disorder. AD subjects with significant symptomatic behavioral disorder, predominantly agitation and delusions, and AD subjects without symptomatic behaviors were compared with controls. Age, sex, mini-mental state examination score, and illness duration did not distinguish the two AD groups. The agitated/delusional group showed significantly lower Bmax values than uncomplicated AD subjects or controls. MAO activity was significantly increased among female AD subjects without symptomatic behaviors compared to those who were agitated or to controls. These results indicate that 3H-imipramine binding and MAO activity may distinguish AD subjects with agitation or delusions from those without symptomatic behaviors, and suggest the existence of a biologically based Alzheimer's behavioral subtype.

Decreased platelet 3H-imipramine binding in primary major depression compared with depression secondary to medical illness in elderly outpatients

Platelet 3H-imipramine binding and monoamine oxidase (MAO) activity were investigated in elderly outpatients with primary major depression, and in a group with depression secondary to medical illness (organic mood disorder, depressed by DSM-III-R criteria) in a multidisciplinary geriatric clinic. The density of the binding of 3H-imipramine (Bmax) was decreased significantly in subjects with major depression compared to subjects with secondary depression, and to controls. There was no difference in Bmax values between subjects with secondary depression and controls. MAO activity was increased in the group with secondary depression, but not in the group with primary major depression. These results provide preliminary evidence for the relative specificity of platelet 3H-imipramine binding as a marker for primary major depressive disorder compared to secondary depression in medically ill elderly people, supports the concept of biological heterogeneity in secondary depression, and extends the findings of decreased Bmax values in two previous studies in non-medically ill depressed elderly patients.

Quantitative autoradiography of serotonin uptake sites in rat brain using [3H]cyanoimipramine

The binding of [3H]cyanoimipramine to serotonin uptake sites in rat brain slices was studied using quantitative autoradiography. Binding was of high affinity and was to a single class of binding site. This is in contrast to results previously obtained by others with [3H]imipramine where two binding sites were observed. The sites labeled by [3H]cyanoimipramine had properties consistent with this ligand labeling serotonin uptake sites, as: (1) binding is displaced by drugs which are potent inhibitors of serotonin uptake but not by drugs which are weak inhibitors of uptake; (2) binding is dependent on the presence of sodium ions as is the uptake of serotonin; (3) binding is almost completely eliminated in the brains of rats lesioned by the serotonin neurotoxin 5,7-dihydroxytryptamine; (4) the distribution of binding sites throughout the rat brain is highly correlated with that found previously for [3H]indalpine, a potent serotonin uptake inhibitor, and for [3H]imipramine. The properties of binding of [3H]cyanoimipramine make it an ideal ligand for the quantitative autoradiography of serotonin uptake sites.

Inhibition of sodium-dependent [3H]imipramine binding in rat brain by serotonin and serotonin uptake inhibitors

The sodium-dependent component of [3H]imipramine binding in rat cerebral cortex was completely displaced by serotonin and non-tricyclic (fluoxetine) as well as tricyclic (clomipramine) serotonin uptake inhibitors. The competition curves were monophasic yielding pseudo-Hill coefficient close to unity and IC50 values of 89, 10 and 6 nM, respectively for serotonin, fluoxetine and clomipramine. The Ki of serotonin for the sodium-dependent [3H]imipramine binding was within the range of its Km for the uptake process. These results suggest that sodium-dependent [3H]imipramine sites relevant for serotonin uptake may be the same as the substrate recognition sites on the serotonin transporter.

Movement initiation characteristics in young adult rats in relation to the high- and low-affinity agonist states of the striatal D2 dopamine receptor

Changes in the speed of movement initiation as a function of age, brain damage, or rat strain are associated with altered characteristics of nigrostriatal dopamine (DA) neurons and of striatal D2 DA receptors. In the present study we investigated the relationship between movement initiation (response parameters: percent of successful responses and response latency) and the agonist binding states of the D2 DA receptor in corpus striatum in 3-month-old Sprague-Dawley rats (n = 51). In contrast to the typical experimental procedure, the variances of the behavioral and receptor binding data were intentionally made as small as possible to provide the most stringent test of putative relationships among variables. Rats were trained to release a lever as rapidly as possible in response to a light/buzzer (CS) combination in order to avoid a mild footshock (UCS). Percent avoidance scores, latencies of the fastest successful trials (successful latencies) and mean latencies for all responses (mean latencies) were collected for 1000-, 500-, 300- and 200-ms CS-UCS intervals. Twenty-four hours following the last behavioral test, animals were euthanized for measurements of the high- and low-affinity binding of DA to D2 receptors in corpus striatum. The standard errors of the mean for both behavioral and receptor binding parameters were, generally, less than 10%. The tightness of the receptor binding data appeared to be related to a lack of biological variance in the animals rather than to an artifact associated with the behavioral testing procedure, since a parallel experiment indicated that different numbers of behavioral shaping sessions had no effects on striatal D2 binding characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)

Aminergic systems in Alzheimer's disease and Parkinson's disease

Biochemical markers for serotoninergic and catecholaminergic neurons in frontal and temporal poles were examined post mortem in brains of patients with Alzheimer's disease, Parkinson's disease, and the two combined. Binding of [3H] citalopram to serotoninergic uptake sites and levels of serotonin were decreased by 40 to 50% in brains of persons in each disease category. In contrast, significant reductions of catecholaminergic markers were not detected. In all three disease groups, the choline acetyltransferase activity was reduced by 50 to 60%. Binding sites for adenosine (A1), muscarinic cholinergic, phencyclidine, beta-adrenergic, and calcium antagonist receptors were unchanged. We conclude that substantial damage to serotoninergic neurons occurs in persons with Parkinson's and Alzheimer's diseases.

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

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

Low-affinity binding of [3H]imipramine to primary astrocyte cultures

High-affinity uptake of serotonin (5-HT) by primary cultures of rat cortical astrocytes has been shown recently to be potently inhibited by tricyclic antidepressants in a manner similar to that described for brain synaptosomes [Katz and Kimelberg, J. Neurosci. 5, 1901 (1985)]. Since the high-affinity binding of [3H]imipramine (IMI) to brain membranes has been well correlated with the inhibition of synaptosomal 5-HT uptake, the binding of [3H]IMI to these astrocyte cultures was examined. No evidence for the existence of a high-affinity binding site was detected in either intact astrocytes or membranes prepared from astrocyte cultures. However, a very dense population of low-affinity binding sites was observed using both methods. This site was similar in affinity (0.606 microM for membranes and 0.959 microM for intact cells) to a low-affinity site observed with rat brain membranes (1.79 microM) but was present at a much greater density in astrocytes (1610 pmoles/mg protein for membranes and 672 for intact cells versus 53 pmoles/mg protein in brain), and may have prevented detection of the high-affinity site. Low-affinity binding to astrocytes was sodium independent, as was low-affinity binding to brain membranes. There was a poor correlation between the inhibitory potencies of the drugs tested against imipramine binding and 5-HT uptake. The binding of 15 nM [3H]IMI was nearly equipotently inhibited by all of the antidepressants tested with IC50 values ranging from 0.56 to 2.6 microM. Other receptor ligands such as 5-HT, chlorpheniramine, quipazine, atropine and benztropine were relatively weak inhibitors of [3H]IMI binding, whereas chlorpromazine was more potent than the tricycle antidepressants.

Platelet [3H]imipramine binding in generalized anxiety disorder, panic disorder, and agoraphobia with panic attacks

The density of platelet [3H]imipramine binding sites is reported to be decreased in unipolar depression and, hence, is a putative biological marker. There is considerable evidence for a phenomenological and biological relationship of panic disorder with affective disorder. We studied platelet [3H]imipramine binding site density in unmedicated subjects with generalized anxiety disorder (GAD; n = 55), panic disorder (PD) with and without agoraphobia (n = 52), and normal controls (n = 26) in order to determine whether or not patients with panic disorder differed from controls in this biological assay. We found no differences in binding site density (Bmax) or affinity (Kd) among the PD, PD with agoraphobia, GAD, and control groups. Nor did we find a relationship between Bmax or Kd and the severity of depressive symptoms or the presence of a family history of affective disorder. In view of two conflicting prior studies, the use of [3H]imipramine binding in panic disorder remains problematic.

3H-imipramine binding in depressed elderly: relationship to family history and clinical response

Platelet 3H-imipramine binding (Bmax) was determined in 34 elderly (mean age 64.8) unipolar depressed outpatients who were being treated with either nortriptyline or interpersonal psychotherapy for 10 to 16 weeks, and in nondepressed elderly controls. Bmax values were decreased in the depressed group. In addition, Bmax values were depressed further in subjects with a history of depression in first degree relatives. Good clinical response with either nortriptyline or psychotherapy was associated with lower Bmax compared to those subjects who had a poorer response to treatment. Treatment nonresponders and those with a negative family history of depression had Bmax values that were somewhat decreased but not significantly different from controls. This study extends to the elderly the potential applicability of platelet 3H-imipramine binding as a marker of depressive illness, and proposes a predictor for treatment response in elderly unipolar depressed patients.

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.