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 inB _max 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 care-fully 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 theB _max of [³H]-IMI binding in platelets of untreated depressed patients when compared with controls. There was no significant modification ofK _d 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.     

High affinity binding of3H-paroxetine and3H-imipramine to rat neuronal membranes

Paroxetine is the most potent and one of the most specific serotonin uptake inhibitors. High-affinity³H-paroxetine and³H-imipramine binding was compared in rat neuronal membranes. TheK _d value for³H-paroxetine binding to neuronal membranes was 0.08 nM, which is exactly the same value as with platelet membranes. TheK _d value for³H-imipramine binding to neuronal membranes was about 4 nM, which is higher than theK _d value for³H-imipramine binding to platelet membranes (0.5 nM). The results indicated that the³H-paroxetine binding site is identical in neuronal membranes and in platelet membranes; this binding site is probably located on the serotonin transport mechanism. In addition, part of the highaffinity³H-imipramine binding to neuronal membranes is probably located on the serotonin transport mechanism, but another part is located elsewhere. Furthermore the polypeptides containing the³H-imipramine binding sites may not be identical in neuronal and platelet membranes.     

3H-Imipramine binding and serotonin uptake in platelets from untreated depressed patients and control volunteers

Human platelets possess specific high-affinity binding sites for ³H-imipramine which have similar characteristics to the sites previously described in human and animal brain. In a group of untreated depressed patients, the B_max of ³H-imipramine binding and the V _max of serotonin uptake in their platelets were found to be significantly lower than in a group of control volunteers. There was no significant difference in the K _d values for ³H-imipramine binding but the K _m values of ³H-serotonin uptake were decreased in the depressed patients. When the measurements of ³H-imipramine binding and ³H-serotonin uptake were compared in the same individual, however, there was no correlation between the individual B _max and V _max values or the K _d and K _m values. These results suggest that although the ³H-imipramine binding site and the mechanism for serotonin uptake are associated, they are not identical.     

Drug inhibition indicates a single-site model of the 5-HT uptake site/antidepressant binding site in rat and human brain

Drug inhibition against [³H]paroxetine binding to rat cortex and human putamen was investigated in saturation experiments. The addition of 5-HT, imipramine, citalopram and clomipramine all produced changes in apparent binding affinity (K_d) without changes in the number of binding sites (B_max). These data suggest that there is no heterogeneity of specific [³H]paroxetine binding, supporting a single site model of the 5-HT uptake site and antidepressant binding site.     

Platelet 5-HT uptake sites in depression: three concurrent measures using [3H] imipramine and [3H] paroxetine

Platelet 5-HT uptake sites were measured in 40 depressed patients and 40 controls using [³H] imipramine binding, defined with desmethylimipramine (DMI) and Na⁺ dependence, and [³H] paroxetine binding. In control subjects the B_max of DMI defined [³H] imipramine binding was significantly higher than both Na⁺ dependent [³H] imipramine (by 30%) and [³H] paroxetine binding (by 22%). The B_max of Na⁺ dependent [³H] imipramine and [³H] paroxetine binding did not differ significantly. The K_d of Na⁺ dependent [³H] imipramine binding was significantly lower than the K_d of DMI defined [³H] imipramine binding. The binding of DMI defined and Na⁺ dependent [³H] imipramine and [³H] paroxetine did not differ significantly between depressed patients and controls in the total group, in those depressed patients who had never taken antidepressants or in those depressed patients who had been recently with-drawn from antidepressants. This study provides no support for the view that the number of platelet 5-HT uptake sites are reduced in depression.     

Characterization of nicotinic acetylcholine receptors on cultured bovine adrenal chromaffin cells using modified l-[3H]nicotine binding assay

Summary To characterize the properties of nicotinic acetylcholine receptors (nAChRs) in autonomic ganglia, we examined l-[³H]nicotine binding to membrane fraction prepared from cultured bovine adrenal chromaffin cells, using a modified filtration method. Binding of l-[³H]nicotine to non-treated glass fiber filters interfered with the detection of specific binding to the membrane fraction. Presoaking glass fiber filters in 3% or higher concentrations of polyethyleneimine (PEI) solution (sixty times higher than earlier used concentration) for at least 5 h could reduce the binding of l-[³H]nicotine to the filters to the background level. Specific l-[³H]nicotine binding to the membrane fraction was detected only when the membrane fraction was prepared in Ca²⁺- and Mg²⁺ (EDTA, EGTA and protease inhibitors were added)-free buffer. Specific binding of l-[³H]nicotine was saturable and reversible. Both computer program and Scatchard analysis revealed a single class of high affinity binding sites with an average K_d of 8.9 nM and a B_max of 42.5 fmol/mg protein. The Hill coefficient was 0.98. In inhibition studies, both cholinergic agonists (carbachol and l-nicotine) and ganglionic agonists (lobeline and 1,1-dimethyl-4-phenylpiperazinium iodide) were much effective in inhibiting l-[³H]nicotine binding, whereas both neuromuscular blocking (-bungarotoxin and d-tubocurarine) and ganglionic blocking agents were less effective. These results suggest that high affinity nicotinic binding sites on adrenal chromaffin cells are nAChRs of the ganglion-type, which have properties different from nAChRs on the neuromuscular junction but similar to nAChRs in the brain. Send offprint requests to K. Lee at his present address     

Increased [3H] raclopride binding sites in postmortem brains from schizophrenic violent suicide victims

The specific binding of the D₂-dopamine receptor antagonist radioligand [³H] raclopride was quantitated in the postmortem caudate and frontal cortex from schizophrenic suicide victims and control subjects. In schizophrenic suicides the density of binding sites (B_max) was higher (40%,P<0.05) in the caudate, whereas it did not change in the cortex as compared to those in controls. The apparent dissociation constants (K _d ) were also found increased both in caudate (24%) and cortex (75%) from schizophrenics, but these apparent decreases in receptor affinity did not reach statistical significance. The mean B_max value in drug-free schizophrenic suicides (n=3) did not differ from the B_max value in neuroleptic drug-treated schizophrenics (n=7) but it was found increased in respect to control subjects (n=9). No differences in [³H] raclopride binding were observed between non-schizophrenic suicide victims (n=4) and matched controls (n=4), suggesting that the modifications of D₂-dopamine receptors in schizophrenia are not related to suicide.     

A pharmacological comparison of [3H]-granisetron binding sites in brain and peripheral tissues of the mouse

The affinities of a range of structurally diverse 5-HT₃ receptor agonists and antagonists for [³H]-granisetron binding sites have been measured in membrane homogenates prepared from central and peripheral tissues of the mouse. By comparing the affinities of compounds across these tissues, the question of whether intea-species 5-HT₃ receptor subtypes exist in the mouse has been addressed.In entorhinal cortex and brainstem, [³H]-granisetron bound to a single high affinity saturable binding site (K_d 0.47 ± 0.14 and 0.60 ± 0.05 nM; B _max 20 ± 6 and 7 ± 2 fmol (mg protein)^–1 respectively; mean ±SEM; n = 3). In distal and proximal colon, the specific binding of [³H]-granisetron was best fitted to a 2-site model. K_d values obtained for the high affinity site were similar to those obtained in brain tissue (distal colon: 0.47 ± 0.09 nM, n = 4; proximal colon: 0.39 ± 0.09 nM, n = 4). In salivary gland, 2-sites were evident in 2 out of 4 experiments. The K_d value (calculated from the high affinity site in the 2-site model) was approximately 10-fold less than in brain or colon (3.3 ± 1.1 nM, n = 4). B _max values were 7 ± 2, 4 ± 1 and 71 ± 16 fmol (mg protein)^–1 for distal colon, proximal colon and salivary gland respectively. For all tissues the estimated affinity of the low affinity site was variable, and B _max values could not be reliably calculated.Extensive comparative studies performed with 17 different 5-HT₃ receptor agonists and antagonists in the five tissues did not reveal differences in affinity for any compound between the entorhinal cortex and the brainstem nor between the two regions of the colon. However, MDL72222, R-zacopride, d-tubocurarine, and GR80284 apparently had significantly lower affinity for colon than brain binding sites. Also, MDL72222, 2-methyl-5-HT, GR80284, 1-(m-chlorophenyl)-biguanide, metoclopramide, and granisetron had significantly lower affinity for the salivary gland binding sites than the brain binding sites. In an attempt to replicate these observations, we conducted a second study using the compounds which had shown the largest inter-tissue differences in affinity keeping as many variables as possible constant. Simultaneous comparative assays on entorhinal cortex, colon and salivary gland homogenates taken from the same mice showed that the differences that were apparent in the initial comparative study were not maintained. In conclusion, we can find no clear evidence for the existence of tissue-specific subtypes of the 5-HT₃ high affinity binding site for [³H]-granisetron in the mouse in the tissues tested. However, a low affinity binding site for [³H]-granisetron was detected in peripheral tissues.     

Light-dark differences in [3H]-paroxetine binding to rabbit platelet membranes

Summary [³H]-Paroxetine binding to rabbit blood platelet membranes from samples obtained under light and dark conditions was examined. Animals were kept on a 14 h light (L) — 10 h dark (D) schedule and blood samples were collected at L + 7 and D + 5 h. Significant differences were found for B _max values of [³H]-paroxetine binding, with low B _max values during the light period and high B _max values during the dark period. The K _d values were not significantly different. These results confirm previous observations on light-dark differences of [³H]-imipramine binding in rabbit blood platelets suggesting the existence of a circadian rhythm for the 5-HT transporter complex.Send offprint requests to S. Z. Langer at the above address     

Quantitative [3H] dipyridamole autoradiography: evidence for adenosine transporter heterogeneity in guinea pig brain

Summary [³H] Dipyridamole binding in guinea pig brain slices has been characterized. Binding of [³H] dipyridamole to guinea pig forebrian slices was found to be rapid, reversible and saturable. Saturation experiments revealed a class of high affinity binding sites with a B _max value of 592 ± 118 fmol/mg protein and K _d value of 10.8 nM ± 2.1 nM in the analysed concentration range. In competition experiments, the adenosine transport inhibitors hexobendine and dipyridamole itself were the most potent displacers (inhibition constants of 4.6 nM ± 1 nM and 11.5 nM ± 3 nM) with pseudo-Hill coefficients close to 1. Competition curves with nitrobenzylthioinosine, another adenosine transport inhibitor, however, showed a biphasic profile with a pseudo-Hill coefficient of 0.33 ± 0.04. Just 42% ± 4% of [³H] dipyridamole binding were inhibited by nanomolar concentrations of nitrobenzylthioinosine and only micromolar concentrations displaced the remainder. Subsequent quantitative autoradiography demonstrated regional differences in the inhibition of [³H] dipyridamole binding by submicromolar concentrations of nitrobenzylthioinosine. While in cortical areas of cerebrum and cerebellum 500 nM nitrobenzylthioinosine displaced binding of [³H] dipyridamole to only about one-third of its sites (in the Purkinje cell layer less than 10%), it showed similar potency as dipyridamole in various areas of the brainstem and hypothalamus. This biphasic and regionally heterogenous interaction of nitrobenzylthioinosine with [³H] dipyridamole binding sites in guinea pig brain slices strongly suggests heterogeneity of adenosine transporters.     

Circadian rhythm of the B max of [3H]-imipramine binding in rabbit platelets

Summary [³H]-imipramine binding was measured in rabbit blood platelet membranes on a 24 h cycle. Animals were kept on a 14 h light (L) 10 h dark (D) schedule, and blood samples were collected at L + 2, L + 8, D + 2, D + 8 and L – 2 h on a following cycle. Significant differences were found for B_max values of [³H]-imipramine binding, with highest values during the dark phase and lowest during the light phase. No significant differences were found in K _d values. These results suggest the existence of a circadian rhythm for the B_max of [³H]-imipramine binding in blood platelets. Send offprint requests to S. Z. Langer     

Effects of acute and chronic desipramine treatment on somatostatin receptors in brain

The effects of acute (5 mg/kg, IP twice daily for 2 days) and chronic (5 mg/kg IP twice daily for 21 days) administration of desipramine (DMI) on [¹²⁵I]-Tyr¹¹-somatostatin binding sites in brain were examined. There was no change in [¹²⁵I]Tyr¹¹-somatostatin binding in membranes prepared from the frontal cortex, striatum, and hippocampus of rats acutely or chronically treated with DMI as compared to non treated animals. [¹²⁵I]Tyr¹¹-Somatostatin binding was increased in membranes prepared from the rat nucleus accumbens only after chronic DMI administration. Scatchard analysis of the binding data from the nucleus accumbens showed that [¹²⁵I]Tyr¹¹-somatostatin labels a single population of somatostatin binding sites with an affinity constant, K_d, of 1.8±0.60 nM and a B_max of 330±90 fmol/mg protein. Chronic treatment with DMI increased the B_max (500±140 fmol/mg protein) but had no effect on the K_d. This finding shows a regional effect of DMI on [¹²⁵I]Tyr¹¹-somatostatin binding sites in rat brain and suggests that somatostatin may play a role in the pathophysiology of depression.     

Platelet 5-HT uptake sites,labelled with [3H] paroxetine,in controls and depressed patients before and after treatment with fluoxetine or lofepramine

Platelet [³H] paroxetine binding was measured in 73 depressed patients and in 64 healthy volunteers. No differences were found in B_max or K_d either overall, or when the 61 depressed subjects who had never received psychotropic drugs were analysed separately. Within the depressed group, no differences in B_max or K_d were found between subgroups divided on the basis of endogenicity, suicidal thoughts or severity of depression. None of the subgroups differed significantly from controls. Forty of the depressed subjects were retested after 6 weeks' treatment with fluoxetine (n=22) or lofepramine (n=18). Treatment was not associated with any change in B_max but a similar and significant increase in K_d was noted following treatment with either antidepressant. Neither pre- nor post-treatment platelet binding parameters appeared to relate to clinical response to treatment.     

Binding of Pinoline on the 5-Hydroxytryptamine Transporter: Competitive Interaction with [3H] Citalopram

Abstract: Pinoline (6-methoxy-1,2,3,4-tetrahydro-β-carboline) is a naturally occurring compound in the mammalian body which inhibits 5-hydroxytryptamine (5-HT) uptake and exerts antidepressant-like behavioural effects in rats. The present study investigates the effects of pinoline on [³H]citalopram binding to the 5-HT transporter on rat brain. Our experiments revealed that pinoline inhibits [³H]citalopram binding with IC₅₀ 1255±167 nM and K_i 572±76 nM; Hill coefficient for inhibition was close to 1. In saturation experiments, pinoline co-incubated with [³H]citalopram, increased dose-dependently the K_d value but had no effect on the B_max value of [³H]citalopram binding. Micromolar concentrations of pinoline did not have influence on the dissociation rate of specifically bound [³H]citalopram. Binding parameters of [³H]citalopram did not differ significantly in cerebral cortex and hippocampus of rats treated for 10 days with pinoline or vehicle. These results indicate that pinoline did not have any modulative influence on the activity of 5-HT transporter and it interacts competitively with citalopram on the substrate recognition site of the 5-HT transporter.     

Evidence for high-affinity binding sites for the adenosine A2A receptor agonist [3H] CGS 21680 in the rat hippocampus and cerebral cortex that are different from striatal A2A receptors

The binding of the adenosine A_2A receptor selective agonist 2-[4-(2-p-carboxyethyl) phenylamino]-5-N-ethylcarboxamidoadenosine (CGS 21680) to the rat hippocampal and cerebral cortical membranes was studied and compared with that to striatal membranes. [³H] CGS 21680, in the concentration range tested (0.2–200 nM), bound to a single site with a K _d of 58 nM and a B _max of 353 fmol/mg protein in the hippocampus, and with a K _d of 58 nM and a B _max of 264 fmol/mg protein in the cortex; in the striatum, the single high-affinity [³H] CGS 21680 binding site had a K _d of 17 nM and a B _max of 419 fmol/mg protein. Both guanylylimidodiphosphate (100 M) and Na⁺ (100 mM) reduced the affinity of [³H] CGS 21680 binding in the striatum by half and virtually abolished [³H] CGS 21680 binding in the hippocampus and cortex. The displacement curves of [³H] CGS 21680 binding with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), N ⁶-cyclohexyladenosine (CHA), 5-N-ethyl-carboxamidoadenosine (NECA) and 2-chloroadenosine (CADO) were biphasic in the hippocampus and cortex as well as in the striatum. The predominant [³H]CGS 21680 binding site in the striatum (80%) had a pharmacological profile compatible with A_2A receptors and was also present in the hippocampus and cortex, representing 10–25% of [³H]CGS 21680 binding. The predominant [³H]CGS 21680 binding site in the hippocampus and cortex had a pharmacological profile distinct from A_2A receptors: the relative potency order of adenosine antagonists DPCPX, 1,3-dipropyl8-{4-[(2-aminoethyl)amino]carbonylmethyloxyphenyl} xanthine (XAC), 8-(3-chlorostyryl) caffeine (CSC), and (E)-1,3-dipropyl-8-(3,4-dimethoxystyryl)-methylxanthine (KF 17,837) as displacers of [³H] CGS 21680 (5 nM) binding in the hippocampus and cerebral cortex was DPCPX > XAC CSC KF 17,837, and the relative potency order of adenosine agonists CHA, NECA, CADO, 2-[(2-aminoethylamino)carbonylethylphenylethylamino]-5-N-ethylcar-boxamidoadenosine (APEC), and 2-phenylaminoadenosine (CV 1808) was CHA NECA CADO > APEC CV1808 > CGS 21680. In the presence of DPCPX (20 nM), [³H] CGS 21680 (0.2-200 nM) bound to a site (A_2A-like) with a K _d of 20 nM and a B _max of 56 fmol/mg protein in the hippocampus and with a K _d of 22 nM and a B _max of 63 fmol/mg protein in the cortex. In the presence of CSC (200 nM), [³H]CGS 21680 (0.2–200 nM) bound to a second high-affinity site with a K _d of 97 nM and a B _max of 255 fmol/mg protein in the hippocampus and with a K _d of 112 nM and a B _max of 221 fmol/mg protein in the cortex. Two pharmacologically distinct [³H]CGS 21680 binding sites were found in synaptosomal membranes of the hippocampus and cortex and in the striatum, one corresponding to A_2A receptors and the other to the second high-affinity [³H]CGS 21680 binding site. In contrast, the pharmacology of [³H]CHA binding was similar in synaptosomal membranes of the three brain areas. The present results establish the existence of at least two high-affinity [³H]CGS 21680 binding sites in the CNS and demonstrate that the [³H]CGS 21680 binding site predominant in the hippocampus and cerebral cortex has different binding characteristics from the classic A_2A adenosine receptor, which predominates in the striatum.     

High-affinity binding of [3H]doxepin to histamine H1-receptors in rat brain: Possible identification of a subclass of histamine H1-receptors

The binding of the radioactively labeled tricyclic antidepressant, [³H]doxepin, to rat brain tissie was examined. Scatchard plots of specific [³H]doxepin binding indicated the presence of two distinct binding sites. The equilibrium dissociation constant (K_D) of the high-affinity site was 0.020 nM with a maximal binding capacity (B_max) of 13.7 fmol/mg protein. The corresponding values for the low-affinity site were 3.6 nM and 740 fmol/mg protein, respectively. The high-affinity site was sensitive to competition by pharmacologically relevant concentrations of histamine H₁ antagonists such as pyrilamine (K_D = 1.0 nM), diphenhydramine (K_D = 20 nM), d-chlorpheniramine (K_D = 1.7 nM), and 1-chlorpheniramine (K_D = 97 nM). The B_max for [³H]doxepin binding in the high-affinity H₁-receptor, however, was approximately 10% of the B_max obtained using [³H]pyrilamine to label the H₁-receptor. Various tricyclic antidepressants were very potent inhibitors at the high-affinity [³H]doxepin site. Their potencies, however, did not correlated with their potencies previously reported for the H₁-receptor. The regional distribution of [³H]doxepin high-affinity sites correlated with the known distribution of H₁-receptors in the rat brain. These results suggest that [³H]doxepin is binding to a subclass of histamine H₁-receptors.     

Behavioral stimulation without alteration of β and 5-HT receptors and adenylate cyclase activity in rat brain after chronic sertraline administration

Effects of chronic treatment with selective 5-HT reuptake inhibitors (SSRIs) on the monoaminergic functions have not been much investigated in compared with tricyclic antidepressants. Therefore, we compared the effects of 3-week treatment with sertraline, a potent SSRI, to those of imipramine (10 mg/kg, IP, twice a day), on monoamine receptors and adenylate cyclase (AC) activity in rat brain. Two-week treatment with both sertraline and imipramine reduced immobility in the water wheel test to the comparable extent. Sertraline treatment did not affect K_d and B_max of [³H]CGP12177 and [³H]ketanserin bindings or cAMP accumulation by norepinephrine, isoproterenol, 5’-guanylylimidodiphosphate [Gpp(NH)p] and forskolin in the cortical membrane compared with vehicle-treated rats. On the other hand, imipramine treatment decreased B_max of both bindings and norepinephrine- or isoproterenol-stimulated cAMP accumulation. Treatment with either antidepressant induced no apparent changes in [³H]8-OH-DPAT [2-(N, N-dipropylamino)-8-hydroxy-1,2,3,4-tetrahydronaphthalene] binding in the hippocampal membrane. These results suggested that chronic treatment of sertraline induced little effect on monoamine receptors and AC activity in the brain and that the alteration of these functions may not be primarily involved in antidepressive effects of antidepressants, at least of SSRIs. Received: 19 April 1996 /Final version: 29 October 1996     

Inhibitory effects of clozapine and other antipsychotic drugs on noradrenaline transporter in cultured bovine adrenal medullary cells

The effects of clozapine and other antipsychotic drugs on noradrenaline (NA) transport were examined in cultured bovine adrenal medullary cells and in transfected Xenopus laevis oocytes expressing the bovine NA transporter. Incubation of adrenal medullary cells with clozapine (30–1000 ng/ml) inhibited desipramine (DMI)-sensitive uptake of [³H]NA in a concentration-dependent manner (IC₅₀=110 ng/ml or 336 nM). Other antipsychotic drugs such as haloperidol, chlorpromazine, and risperidone also decreased [³H]NA uptake (IC₅₀= 144, 220, and 210 ng/ml or 383, 690, and 512 nM, respectively). Eadie-Hofstee analysis showed that clozapine reduced V_max of uptake of [³H]NA and increased K_m. Furthermore, clozapine inhibited specific binding of [³H]DMI to plasma membranes isolated from bovine adrenal medulla (IC₅₀=48 ng/ml or 146 nM). Scatchard plot analysis of [³H]DMI binding revealed that clozapine decreased both B_max and K_d. Other antipsychotic drugs, including haloperidol, chlorpromazine, and risperidone, also reduced [³H]DMI binding to the membranes. In transfected Xenopus oocytes expressing the bovine NA transporter, clozapine inhibited [³H]NA uptake in a concentration-dependent manner similar to that observed in adrenal medullary cells. These results suggest that clozapine and haloperidol directly inhibit transport of NA by acting on the site of an NA transporter that influences both substrate transport and binding of tricyclic antidepressants. Received: 13 April 1999 / Final version: 2 November 1999     

Platelet [3H]-paroxetine binding in obsessive-compulsive disorder

Platelet [³H]-paroxetine binding was analysed in 26 patients (13F, 13M) with obsessive compulsive disorder and 26 normal controls (13F, 13M). For patients with obsessive compulsive disorder, B_max was 2127 ± 480 fmol/mg protein (mean±SD) compared to control Bmax values of 1926 ± 696 fmol/mg protein. The mean K_d value for the patients was 0.075 ± 0.025 nM and for the controls was 0.076 ± 0.032 nM. Analysis of covariance indicated a significant effect of sex on both K_d and B_max but no effect of diagnosis (obsessive compulsive disorder versus normal controls) or season of sampling. The data provide no evidence for an abnormality of the platelet uptake mechanism as assessed by the measurement of [₃H]-paroxetine binding to the platelet transporter in obsessive compulsive disorder.     

Effect of synthetic purines and purine nucleosides on [3H]diazepam binding in brain

We have compared fifteen synthetic purines and purine nucleosides on their ability to displace [³H]diazepam binding to rat brain membranes. Among these analogs, 6-methylthioguanine was found to be most potent, inhibiting competitively the specific binding of [³H]diazepam with a K_i value of 16 μM. At a concentration of 50 μM, 6-methyl-thioguanine increased the apparent K_D of specific diazepam binding from 4.3 nM to 13.3 nM without affecting the B_max, nor had it any effect on the non-specific binding. Binding with membrane preparations from developing rat brain was slightly less sensitive to 6-methylthioguanine inhibition than that with membranes prepared from mature brain.