Binding of [3H]paroxetine to serotonin uptake sites and of [3H]lysergic acid diethylamide to 5-HT2A receptors in platelets from women with premenstrual dysphoric disorder during gonadotropin releasing hormone treatment

Changes in serotonergic parameters have been reported in psychiatric conditions such as depression but also in the premenstrual dysphoric disorder (PMDD). In addition, hormonal effects on serotonergic activity have been established. In the present study, binding of [3H]paroxetine to platelet serotonin uptake sites and binding of [3H]lysergic acid diethylamide ([3H]LSD) to platelet serotonin (5-HT)2A receptors were studied in patients with PMDD treated with a low dose of a gonadotropin releasing hormone (GnRH) agonist (buserelin) or placebo and compared to controls. The PMDD patients were relieved of premenstrual symptoms like depression and irritability during buserelin treatment. The number of [3H]paroxetine binding sites (Bmax) were significantly higher in the follicular phase in untreated PMDD patients compared to controls. When treated with buserelin the difference disappeared. No differences in [3H]LSD binding between the three groups were shown. The present study demonstrated altered platelet [3H]paroxetine binding characteristics in women with PMDD compared to controls. Furthermore, [3H]paroxetine binding was affected by PMDD treatment with a low dose of buserelin. The results are consistent with the hypothesis that changes in serotonergic transmission could be a trait in the premenstrual dysphoric disorder.     

Alterations in platelet serotonin transporter binding in women with postpartum onset major depression

There is considerable debate as to whether postpartum depression (PPD) is biologically distinct from other depressive syndromes. Although abnormalities in serotonergic neural systems have repeatedly been reported in depression, few such studies have been conducted in PPD. In the present study, platelet serotonin transporter (SERT) binding was assessed using [(3)H]paroxetine in 14 depressed pregnant women, 31 normal healthy pregnant women, 39 depressed postpartum women, and 27 normal healthy postpartum women; all of the subjects were drug-free. Significant differences were detected among the 4 groups with respect to the dissociation constant (Kd) of platelet binding sites for [(3)H]paroxetine with the highest Kd values among those with PPD. The density (Bmax) of platelet binding sites for [(3)H]paroxetine did not differ between the study groups. These data suggest that PPD may be associated with unique alterations in serotonergic function that are specific to the puerperium.     

High affinity [3H]imipramine and [3H]paroxetine binding sites in suicide brains

Summary Specific binding of [3H]imipramine and [3H]paroxetine was simultaneously examined in human brains (frontal cortex, temporal cortex, cingulate cortex, hypothalamus, hippocampus and amygdala) from 11 controls and 11 depressed suicide victims. A single saturable high affinity site was obtained for both radioligands. Age was not related to significant changes in [3H]imipramine and [3H]paroxetine binding parameters, which indicates the stability of the brain serotonergic system with increasing age.A major finding of the present study concerns the existence of a significant decrease in the maximum number (Bmax) of [3H]imipramine binding sites in hippocampus from depressed suicides as compared with the control group, without changes in the binding affinity (Kd). In contrast, when [3H]paroxetine was used as radioligand, no changes in either Bmax or Kd were detected in any of the brain regions studied. These findings suggest that [3H]imipramine may be a better marker than [3H]paroxetine when alterations in the presynaptic serotonergic uptake site are to be detected.     

Serotonergic markers in platelets of patients with major depression: upregulation of 5-HT2 receptors.

The uptake of [3H]5-HT and the density (Bmax) as well as affinity (Kd) of 5-HT uptake sites labelled with [3H]paroxetine and of 5-HT2 receptors labelled by [3H]LSD were determined in platelets from 25 medication-free patients with major depression and 20 normal controls. The density (Bmax) of 5-HT2 receptors was found to be significantly increased (by 52%) in platelets from depressed patients, particularly females. No changes were found in the affinity (Kd) of 5-HT2 receptors and in 5-HT uptake or [3H]paroxetine binding parameters. Density of 5-HT2 receptors positively correlated with that of [3H]paroxetine sites in control but not in depressed subjects. No correlation was found between the HAMD scores and Bmax of [3H]LSD binding. The results suggest that upregulation of platelet 5-HT2 receptors is a useful biological marker in major depression, particularly in females.     

Platelet serotonin functions in untreated major depression

The uptake of [14C]5-HT, [3H]paroxetine and [3H]LSD binding was determined in platelets from 30 untreated patients with major depression and compared with corresponding variables from 30 healthy age-, sex- and season-matched control subjects. The maximum velocity (Vmax) for the 5-HT uptake was significantly decreased in patients (P = 0.014) compared to control subjects. Depressed women had significantly lower Vmax than female control subjects. In men, Vmax did not differ between patients and control subjects. Vmax was significantly lower in male inpatients compared with male outpatients (P = 0.05). The density (Bmax) of 5-HT uptake sites was found to be significantly increased in patients (P < 0.05) compared to control subjects and male patients had significantly higher Bmax than male control subjects, but there was no difference between female control subjects and female patients. No significant difference was found in Bmax of 5-HT2-receptors between patients and control subjects. A positive correlation was found between Bmax of 5-HT2-uptake sites and the degree of anxiety and between Bmax of 5-HT2 receptors and MADRS scores. Bmax of 5-HT2-receptors was positively correlated with the degree of suicidality. The results in the present study indicate that there may be a gender difference in serotonergic dysfunction in depression.     

High affinity [3H]paroxetine binding to serotonin uptake sites in human brain tissue

[3H]Paroxetine binding to human brain tissue was characterized. Competition studies in the putamen and frontal cortex revealed single-site binding models for binding sensitive to 5-hydroxytryptamine (5-HT) (Ki 1-3 microM) and citalopram (Ki 0.6 nM), which displaced the same amount of binding. However, desipramine, norzimeldine and fluoxetine displaced additional binding (10-20%) and these competitors fitted two-site binding models with high affinity components in the nanomolar range and low affinity components in the micromolar range. The high affinity components approximated the 5-HT- and citalopram-sensitive binding fraction. Most of the [3H]paroxetine binding sites were protease-sensitive, but the low-affinity (microM) sites appeared to be protease-resistant. Based on these findings, only the [3H]paroxetine binding representing the fraction sensitive to 30 microM 5-HT (or e.g. 0.3 microM norzimeldine), was regarded as specific binding. This binding fraction was saturable with an apparent binding affinity (Kd) of 0.03-0.05 nM throughout the brain. The highest binding densities were obtained in the hypothalamus and substantia nigra (Bmax 500 fmol/mg protein). The basal ganglia reached intermediate densities (Bmax 200 fmol/mg protein), whereas cortical areas had low Bmax values (less than 100 fmol/mg protein). The lowest B max value was noted in cerebellar cortex (30 fmol/mg protein). The [3H]paroxetine binding was competitively inhibited by low concentrations of 5-HT, imipramine and norzimeldine, suggesting that the substrate recognition site for 5-HT uptake was labeled. Compounds active at dopaminergic, noradrenergic, histaminergic, 5-HT1, 5-HT2 and cholinergic muscarinic sites did not affect the binding at 100 microM concentrations. It is concluded that [3H]paroxetine is a marker for the 5-HT uptake site in the human brain, provided that an adequate pharmacological definition of specific binding is performed.     

Neuroimaging of the serotonin reuptake site requires high-affinity ligands

Numerous attempts have been made to develop suitable radiolabeled tracers for positron emission tomography or single photon emission computed tomography imaging of the serotonin transporter (SERT), but most often, negative outcomes are reported. The aim of this study is to define characteristics of a good SERT radioligand and to investigate species differences. We examined seven different selective serotonin reuptake inhibitors (SSRIs) and that except for one all have been previously tested as emission tomography ligands. The outcome of the ligands as emission tomography tracers was compared in relation with receptor density (Bmax) and/or ligand affinity (Kd) in rat and monkey cerebrum and cerebellum (reference region) membranes. [3H]-(S)-Citalopram and [3H]-(+)-McN5652 display statistically significantly lower affinity, whereas [3H]paroxetine displays statistically significantly higher affinity for SERT in monkey cortex when compared with the rat cerebrum. The affinity of [3H]MADAM, [123I]ADAM, and [11C]DASB for SERT obtained with rat cerebrum and monkey cortex are similar. In monkey cortex, Kd and Bmax could not be determined with [3H]fluoxetine. Of the seven SSRIs, [3H]-(S)-citalopram, [3H]MADAM, and [11C]DASB displayed significant specific binding to SERT in monkey cerebellum, with Bmax cortex:cerebellum ratios being 17, 3, and 4, respectively. In rat brain tissue the ratios were 12, 6, and 3, respectively. In conclusion, it can be estimated that imaging of the human SERT in a high-density region requires radioligands with Kd values between 0.03 and a maximum of 0.3 nM (at 37 degrees C). The differential specific cerebellar binding raises the question of the suitability of cerebellum as a reference region for nonspecific binding.     

Regional and subcellular localization in human brain of [3H]paroxetine binding, a marker of serotonin uptake sites

The characteristics of the binding of [3H]paroxetine, a selective serotonin (5-HT) uptake blocker, were investigated in human brain. The Kd value was 0.23 +/- 0.07 nM, and the Bmax value was 190 +/- 39 fmol/mg protein in the putamen. The capacity of various antidepressive drugs to inhibit [3H]paroxetine-specific binding in human brain was well correlated with their capacity to inhibit [3H]5-HT uptake in rat brain. The highest concentrations of [3H]paroxetine-specific binding sites were found in the substantia nigra, hypothalamus, and hippocampus. Lower values were obtained in the basal ganglia and the thalamus. The specific binding was very low in cerebral and cerebellar cortices. The regional distribution of [3H]paroxetine binding sites differs from that of [3H]ketanserin binding to S2 serotonin receptors. The subcellular distribution of the [3H]paroxetine-specific binding sites obtained by differential centrifugation revealed a synaptosomal enrichment in the frontal cortex and striatum, whereas an enrichment in the microsomal fraction was found in striatum. The results show that [3H]paroxetine is a ligand of choice to label the 5-HT uptake molecular complex in human brain.     

Binding of [(3)H]lysergic acid diethylamide to serotonin 5-HT(2A) receptors and of [(3)H]paroxetine to serotonin uptake sites in platelets from healthy children, adolescents and adults

Possible age effects on binding of [(3)H]lysergic acid diethylamide ([(3)H]LSD) to serotonin 5-HT(2A) receptors and of [(3)H]paroxetine to serotonin uptake sites were studied in platelets from healthy children (11-12 years of age), adolescents (16-17 years of age) and adults. Significant overall age effects were found both for the number of binding sites (B(max)) for [(3)H]LSD binding (p < 0.001), the affinity constant (K(d)) for [(3)H]LSD binding (p < 0.001), B(max) for [(3)H]paroxetine binding (p < 0.001) and K(d) for [(3)H] paroxetine binding (p = 0.006). In general, there was a decrease in B(max) with increasing age, which predominantly occurred between the ages 11-12 years and 16-17 years for the 5-HT(2A) receptor, and after 16-17 years of age for the serotonin uptake site. These developmental changes might have an impact on the effect of treatment with serotonergic drugs in children and adolescents. When the platelet serotonin variables investigated are employed in studies in children or adolescents, age matching or, alternatively, introduction of age control in the statistical analysis should be performed.     

Platelet [3H]paroxetine and [3H]lysergic acid diethylamide binding in seasonal affective disorder and the effect of bright light therapy.

BACKGROUND: Seasonal affective disorder (SAD) has been regarded as a melatonin disorder, but the pathophysiological mechanisms of SAD are to a large extent unclarified. Serotonergic mechanisms have also been studied, but they have shown inconsistent results. METHODS: We have compared [3H]paroxetine and [3H]lysergic acid diethylamide (LSD) binding in platelets from 23 SAD patients and 23 controls. Then SAD patients had 4 weeks of light therapy. On the last treatment day new blood samples were drawn. Symptoms before and after light treatment were measured by SIGH-SAD. RESULTS: Bmax for paroxetine binding before light treatment was higher in SAD patients compared to controls and also higher in responders than in nonresponders. Bmax decreased significantly during light treatment. We also found a negative correlation between the two Bmax values before but not after light treatment. There was a negative correlation between Bmax for paroxetine binding before treatment and clinical status after treatment. Patients with reduced Bmax for LSD binding after treatment had a better clinical treatment response. CONCLUSIONS: The present study indicates that serotonin receptor parameters might be suitable in the prediction of clinical response to light treatment.     

Platelet serotonin uptake and paroxetine binding among allelic genotypes of the serotonin transporter in alcoholics

Expression rates of long (L) and short (S) alleles of the serotonin (5-HT) transporter (5-HTT) gene have been shown to differ under various circumstances. We compared 5-HTT uptake (function) level and paroxetine binding (density) in platelets of alcoholics as indices of 5-HTT expression rate among LL, LS, and SS genotypes. Concentration curves of [3H]5-HT and [3H]paroxetine were used to quantify the equilibrium constant (Km) and maximum 5-HT uptake rate (Vmax) for 5-HTT uptake into intact platelets and the dissociation constant (Kd) and maximum specific binding density (Bmax) for paroxetine binding to platelet membranes, respectively. Genotypes were determined using electrophoresis with fluorescent markers. Vmax for 5-HTT uptake did not correlate with Bmax for paroxetine binding (r=-0.095, P=0.415). Means of Vmax and Bmax did not differ in a statistically significant manner among LL, LS, and SS genotypes in these alcoholic subjects. However, Vmax for LL and SS appeared to have a bimodal distribution, so the percentage of subjects with Vmax <200 fmol/min-10(7) platelets was statistically significantly higher in LL than in SS (51.5% vs. 22.7%, respectively), with an odds ratio of 3.6 (P<0.05). The percentage of Vmax <200 fmol/min-10(7) platelets for LS was 39.3% (not significant vs. LL or SS). Previous studies of healthy human controls have shown that 5-HTT density in raphe nuclei and 5-HTT uptake in platelets are higher in the LL genotype than in S carriers. Our findings in currently drinking alcoholics support the hypothesis that those with the LL genotype of the 5'-HTTLPR region of the 5-HTT gene have reduced 5-HTT function.     

Effect of increasing estradiol levels on platelet peripheral benzodiazepine binding sites in women undergoing human menopausal gonadotropin treatment

The effect of increasing serum estradiol levels on platelet [3H]PK 11195 binding was investigated to determine the influence of gonadal steroids on platelet peripheral benzodiazepine binding sites. Serum estradiol and platelet [3H]PK 11195 binding were determined in 10 anovulatory, but otherwise healthy women before and after treatment with human menopausal gonadotropin. Despite a 30-fold increase in posttreatment estradiol, the kinetic parameters of platelet [3H]PK 11195 binding (Kd, Bmax) remained essentially unchanged. These data indicate that peripheral benzodiazepine binding sites on platelets are not influenced by elevated concentrations of estradiol. Thus, it is suggested that variations in circulating levels of this female gonadal steroid are likely not responsible for the modifications in platelet peripheral benzodiazepine binding sites seen in association with various disease states.     

Multiple high affinity binding sites for 5-hydroxytryptamine: a new class of sites distinct from 5-HT1 and S2

Two different classes of binding sites probably related to serotonergic receptors have already been reported: 5-HT1 binding sites recognize [3H]5-hydroxytryptamine with a high affinity (Kd = 3 nM) and S2 binding sites recognize [3H]spiroperidol and [3H]ketanserine. An additional population of sites has been observed in crude membrane preparations or fractions enriched with synaptosomal membranes obtained from rat brain cortex. This population was observed as a single class of sites in a synaptosomal fraction (L fraction--according to Laduron (1977)). It corresponded to a dissociation constant Kd = 13-15 nM, and Bmax = 0.80 +/- 0.15 pmol/mg protein. Displacement experiments showed that it recognized preferentially the 5-HT structure (bufotenin, 5-MeO-tryptamine). Tryptamine was a weak displacer and 5,7-dihydroxytryptamine totally inefficient. Neither 8-OH-DPAT, nor quipazine had any effect. Methiothepin, cinanserin and cyproheptadine displaced 5-HT from these sites whereas ergot derivatives did not. Contrary to 5-HT1 binding, this recently observed binding was not altered by GTP; alpha-MSH reduced the corresponding Bmax whereas Leu-enkephalin did not. The degenerative lesion of the serotonergic fibers led to a slight increase in the Bmax of the binding without altering the Kd which means that corresponding sites are not located on serotonergic fibers and might be postsynaptically located.     

Changes in the serotonin transporter in the hippocampus of subjects with schizophrenia identified using [3H]paroxetine

Summary [³H]paroxetine binding to membrane from hippocampus, obtained at autopsy, from 24 schizophrenic and 24 non-schizophrenic subjects has been measured. The affinity of [³H]paroxetine binding to hippocampal membrane was decreased in subjects with schizophrenia (Kd=0.50 ± 0.04 vs. 0.24 ± 0.02nM; mean ± S.E.M. p < 0.001) but was not different in schizophrenic subjects who had or had not committed suicide (Kd=0.50 ± 0.07 vs. 0.50 ± 0.04nM). The density of [³H]paroxetine binding sites did not differ between the schizophrenic and non-schizophrenic subjects. For the schizophrenic subjects, there was no relationship between ante-mortem neuroleptic drug treatment and [³H]paroxetine binding to the hippocampal membrane. Finally, this study has shown that neuroleptic drug treatment of rats does not alter [³H]paroxetine binding to the hippocampal membranes. Thus, it would seem that the changes in the affinity of [³H]paroxetine binding to the hippocampus of schizophrenic subjects are not likely to be due to neuroleptic drug treatment but may be involved in the pathology of the illness.     

A PET study of 5-HT1A receptors at different phases of the menstrual cycle in women with premenstrual dysphoria

The cause of premenstrual dysphoric disorder (PMDD) is largely unknown. It has been hypothesized that normal ovarian function triggers PMDD-related biochemical events within the brain and that serotonin plays an important role. In the present study, positron emission tomography (PET) and [carbonyl-¹¹C]WAY-100635 were used to examine serotonin 5-HT_1A receptors in a control group of women and in a group of women with PMDD. Two PET examinations were performed in each subject, one before (follicular phase) and one after ovulation (luteal phase). Each subject's menstrual cycle was confirmed by ultrasonography of the ovaries as well as with hormone levels in blood and urine. The 5-HT_1A binding potential was measured in six regions of interest and calculated according to the simplified reference tissue model. In the raphe nuclei, the 5-HT_1A binding potential changed from the follicular to the luteal phase of the menstrual cycle in asymptomatic controls. In women with PMDD, the observed change between phases was significantly smaller. The results are in concordance with previously reported challenge studies of 5-HT_1A receptor-mediated effects indicating different serotonergic responses between women with PMDD and controls. The study principally provides new support, in vivo, for a serotonergic dysregulation in women with PMDD.     

Pulsatile LH secretion in women with premenstrual syndrome (PMS): evidence for normal neuroregulation of the menstrual cycle.

The premenstrual syndrome (PMS) has been proposed to result from excessive exposure to and/or withdrawal of brain opioid activity during the luteal phase. Because hypothalamic opioids are believed to modulate GnRH secretion, in part under the influence of ovarian steroids, we performed longitudinal studies of gonadotropin and ovarian steroid secretion across ovulatory, symptomatic cycles of 17 PMS patients and 8 normal volunteers. Pulsatile LH secretion was measured every 10 min for 8 hr at times when central opioid activity was expected to be low (early follicular phase), high (mid-luteal phase; ML), and declining (late luteal phase). In both subject groups, a cycle-phase effect was observed for LH pulse frequency (p = < 0.001) and amplitude (p = 0.002), and for the transverse mean concentrations of LH (p = 0.05), FSH (p < = 0.001), estradiol (E2) (p = < 0.001) and progesterone (P) (p = < 0.001). ML P secretion in PMS patients was pulsatile, and mean concentrations (over 30-60 min) were similar to those of normal controls. The changes in pulsatile LH secretion across the cycle were not different in the PMS patients compared to the normal women, though mean FSH in the ML phase was higher in the PMS group (p = < 0.05). The similar changes in luteal LH pulse frequency fail to provide evidence that GnRH secretion is impaired, thus challenging the view that the neuroregulation of the menstrual cycle in women with PMS is markedly altered.     

Objective sleep interruption and reproductive hormone dynamics in the menstrual cycle

ObjectivesWomen report greater sleep disturbance during the premenstrual phase of the menstrual cycle and during menses. However, the putative hormonal basis of perceived menstrual cycle-related sleep disturbance has not been investigated directly. We examined associations of objective measures of sleep fragmentation with reproductive hormone levels in healthy, premenopausal women.MethodsTwenty-seven women with monthly menses had hormone levels measured at two time points during a single menstrual cycle: the follicular phase and the peri-ovulatory to mid-luteal phase. A single night of home polysomnography (PSG) was recorded on the day of the peri-ovulatory/mid-luteal-phase blood draw. Serum progesterone, estradiol, and estrone levels concurrent with PSG and rate of change in progesterone (PROGslope) from the follicular blood draw to PSG were correlated with log-transformed wake after sleep onset (lnWASO%) and number of wakes/hour of sleep (lnWake-Index) using linear regression.ResultsSleep was more fragmented in association with a steeper PROGslope (lnWASO% p = 0.016; lnWake-Index p = 0.08) and higher concurrent estrone level (lnWASO% p = 0.03; lnWake-Index p = 0.01), but the effect of estrone on WASO was lost after accounting for PROGslope. WASO% and Wake-Index were not associated with concomitant progesterone or estradiol levels.ConclusionsA steeper rate of rise in progesterone levels from the follicular phase through the mid-luteal phase was associated with significantly greater WASO, establishing a link between reproductive hormone dynamics and sleep fragmentation in the luteal phase of the menstrual cycle.     

Ontogenesis of [3H]serotonin binding sites in the hypothalamus of the female rat: relation to serotonin-induced LH release in moxestrol-pretreated rats

The release of luteinizing hormone (LH) evoked by exogenous serotonin (5 mg/kg, i.p.) was measured at different developmental ages in the female rat. Since LH response to serotonin is modulated by estrogen, moxestrol, a synthetic compound which does not bind to alpha-fetoprotein, was administered 48 h before serotonin or saline injections. Serotonin was ineffective in releasing LH in 5-day females pretreated with moxestrol; the response increased abruptly at 12 and 18 days and from then onwards decreased gradually. This is in clear contrast to the development of the serotonin-induced prolactin release, which is absent in the first postnatal week, becomes evident on day 12 and increases gradually as the rat approaches puberty. In a second group of experiments, the ontogenesis of hypothalamic-preoptic suprachiasmatic [3H]serotonin binding was studied in 5-, 16-, 27- and 37-day-old female rats. A specific serotonergic binding site could be quantified at 5 days (Bmax 4.31 : 1.89 fmol/micrograms DNA; Kd: 2.74: 1.17 nM). A gradual increase of both Bmax and Kd was observed as the animal matured: the number of binding sites almost doubled from day 16 to day 27, and increased even further by 37 days (Bmax 24.21 : 5.16 fmol/micrograms DNA). These data indicate: a specific [3H]serotonin binding site in hypothalamic-preoptic suprachiasmatic area is present in the first postnatal week of the female rat; Bmax and Kd values increase from 5 to 37 days of age.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sequential serotonin and noradrenalin associated processes involved in postpartum blues

OBJECTIVE: We investigated whether postpartum blues was related to changes in parameters of noradrenergic and serotonergic functioning. METHODS: From 26 healthy pregnant women blood was collected at the end of pregnancy and 5 days and 6 weeks postpartum. Serotonergic parameters were: platelet serotonin content; paroxetine binding to platelet membranes as an index of serotonin transporter activity; the serotonin precursor tryptophan in proportion to the large neutral amino acids, as an estimate of its cerebral influx. Noradrenergic indices were the noradrenaline precursor tyrosine and its metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG). The Kennerly and Gath blues questionnaire was applied at day five postpartum. RESULTS: The incidence of postpartum blues was 30%. The tryptophan ratio and serotonin content of platelets were decreased (p<0.01) at day five postpartum in all women. Bmax paroxetine at day five was correlated with blues score (beta=0.460; p=0.031). MHPG levels at 6 weeks were increased in women with blues (p<0.001). In a regression model MHPG at 6 weeks was related to blues score (beta=0.477; p=0.002) and MHPG at day five (beta=0.550; p=0.001), explaining >50% of the variation (R2=0.588; p<0.001). CONCLUSIONS: A decreased serotonergic activity was found at the fifth day postpartum in all subjects. Increased SERT activity, reflected by higher paroxetine binding to platelets might be involved in the onset of blues. The elevated MHPG levels in women with blues are compatible with a higher stress sensitivity, or a decreased stress coping in those and is suggested to be involved with the onset of depression.     

Paroxetine as an in vivo indicator of 3,4-methylenedioxymethamphetamine neurotoxicity: a presynaptic serotonergic positron emission tomography ligand?

The present study sought to determine whether [3H]paroxetine, a potent and selective inhibitor of serotonin uptake in vitro, could be used to label the serotonin transporter in the rat brain in vivo such that it might be employed to develop a presynaptic serotonergic positron emission tomography ligand. Tritium labeled paroxetine was administered intravenously to rats by means of tail vein injection. Four hours later, specific [3H]paroxetine binding was determined by subtracting non-specific binding in the cerebellum from total binding in other brain regions of interest. The distribution of specific [3H]paroxetine binding paralleled the distribution of serotonin uptake sites in all brain regions examined. Pretreatment with serotonin re-uptake inhibitors (citalopram or sertraline) reduced in vivo specific [3H]paroxetine binding by as much as 99%. Specific in vivo [3H]paroxetine binding was further characterized through the use of 5,7-dihydroxytryptamine (5,7-DHT), a known serotonergic neurotoxin. 5,7-DHT (200 micrograms, i.c.v.) caused a marked reduction in specific [3H]paroxetine binding, and induced a prolonged depletion of regional brain serotonin. In a final study, the feasibility of using in vivo [3H]paroxetine binding as an indicator of serotonergic damage induced by another neurotoxin (3,4-methylenedioxymethamphetamine, MDMA) was tested. MDMA-treated rats showed a profound reduction in in vivo [3H]paroxetine binding, along with a lasting depletion of regional brain serotonin. These results demonstrate that [3H]paroxetine can be used to label serotonin uptake sites in the rat brain in vivo, and that the damage induced by serotonergic neurotoxins can be detected using in vivo [3H]paroxetine binding as an indicator. Paroxetine (or one of its derivatives) therefore holds promise as a PET ligand for studying serotonergic neurons in the living human brain in health as well as after neurotoxic injury.