Binding of [3H]Ro 16-6491, a Reversible Inhibitor of Monoamine Oxidase Type B, to Human Brain Mitochondria and Platelet Membranes

The reversible inhibitor of monoamine oxidase type B (MAO-B) [3H]Ro 16-6491 binds specifically and with high affinity to a single population of binding sites in human frontal cortex crude mitochondria and platelet membranes. In both tissues binding equilibrium was reached after 1 h incubation at 20 degrees C. Dissociation of bound radioactivity was relatively fast at 20 degrees C (t1/2 = 90-120 min) whereas at 0 degrees C [3H]Ro 16-6491 showed the characteristics of a slowly dissociating ligand. Inhibitors and substrates of MAO-B inhibited binding of [3H]Ro 16-6491, whereas MAO-A blockers were much less potent. Ro 16-6491 was also a substrate for MAO-B and a stable unidentified intermediate of the oxidation of Ro 16-6491 possessing high affinity for the enzyme may account for the marked MAO-B inhibitory effect of the drug. According to this hypothesis Ro 16-6491 would behave as a mechanism-based reversible inhibitor. In conclusion, [3H]Ro 16-6491 binds selectively to MAO-B and represents an excellent new radioligand probe for studying the regional tissue distribution of this enzyme in normal and pathological conditions.     

Comparison of Tryptic Peptides of Benzodiazepine Binding Proteins Photolabeled with [3H]Flunitrazepam or [3H]Ro 15–4513

When rat brain membranes were incubated with the benzodiazepine agonist [3H]flunitrazepam or the partial inverse benzodiazepine agonist [3H]Ro 15-4513 in the presence of ultraviolet light one protein (P51) was specifically and irreversibly labeled in cerebellum and at least two proteins (P51 and P55) were labeled in hippocampus. After digestion of the membranes with trypsin, protein P51 was degraded into several peptides. When P51 was photolabeled with [3H]Ro 15-4513, four peptides with apparent molecular weights of 39,000, 29,000, 21,000, and 17,000 were observed. When P51 was labeled with [3H]flunitrazepam, only two peptides with apparent molecular weights of 39,000 and 25,000 were obtained. Protein P55 was only partially degraded by trypsin, and whether it was labeled with [3H]flunitrazepam or [3H]Ro 15-4513 it yielded the same two proteolytic peptides with apparent molecular weights of 42,000 and 45,000. These results support the existence of at least two different benzodiazepine receptor subtypes associated with proteins P51 and P55. The different receptors seem to be differentially protected against treatment with trypsin. In addition, these results indicate that in the benzodiazepine receptor subtype associated with P51 benzodiazepine agonists and partial inverse benzodiazepine agonists irreversibly bind to different parts of the molecule.     

[3H]Citalopram Binding to Brain and Platelet Membranes of Human and Rat

Citalopram, a selective serotonin (5-HT) uptake inhibitor with antidepressant properties, was found to bind with high affinity to the 5-HT transporter from human neuronal and platelet membranes. At 20 degrees C, KD was about 1.5 nM in both tissues. [3H]Citalopram bound to rat neuronal membranes with higher affinity than to human neuronal and platelet membranes; at 20 degrees C KD was about 0.7 nM. The Bmax value for the binding of [3H]citalopram to platelet membranes was close to that found using the 5-HT uptake inhibitors [3H]imipramine and [3H]paroxetine, suggesting that all three 5-HT uptake inhibitors bind to the 5-HT transporter. The dissociation rate of [3H]citalopram increased twofold with each 4-5 degree C increase in temperature in both human and rat membranes, although at any given temperature, the dissociation rate was about four times faster in the human neuronal and platelet membranes than in rat neuronal membranes.     

Binding of [3H]Ro 5–4864 and [3H]PK 11195 to Cerebral Cortex and Peripheral Tissues of Various Species: Species Differences and Heterogeneity in Peripheral Benzodiazepine Binding Sites

The binding of [3H]PK 11195 and [3H]Ro 5-4864 to membrane preparations from cerebral cortex and peripheral tissues of various species was studied. [3H]PK 11195 (0.05-10 nM) bound with high affinity to rat and calf cerebral cortical and kidney membranes. [3H]Ro 5-4864 (0.05-30 nM) also successfully labeled rat cerebral cortical and kidney membranes, but in calf cerebral cortical and kidney membranes, its binding capacity was only 3 and 4%, respectively, of that of [3H]PK 11195. Displacement studies showed that unlabeled Ro 5-4864, diazepam, and flunitrazepam were much more potent in displacing [3H]PK 11195 from rat cerebral cortex and kidney membranes than from calf tissues. The potency of unlabeled Ro 5-4864 in displacing [3H]PK 11195 from the cerebral cortex of various other species was also tested, and the rank order of potency was rat = guinea pig greater than cat = dog greater than rabbit greater than calf. Analysis of these displacement curves revealed that Ro 5-4864 bound to two populations of binding sites from rat and calf kidney and from rat, guinea pig, rabbit, and calf cerebral cortex but to a single population of binding sites from cat and dog cerebral cortex. Using [3H]PK 11195 as a ligand, the rank order of binding capacity in cerebral cortex of various species was cat greater than calf greater than guinea pig greater than rabbit greater than dog greater than rat, whereas when [3H]Ro 5-4864 was used, the rank order of binding capacity was cat greater than guinea pig greater than rat greater than rabbit greater than calf greater than dog.     

[3H]Neurotensin(8–13) Binds in Human Brain to the Same Sites as Does [3H]Neurotensin but with Higher Affinity

The binding of [3H]neurotensin(8-13) to membranes from human frontal cortex at 0 degree C was time dependent, specific, saturable, and reversible. Saturation isotherms provided an equilibrium dissociation constant (KD) of 0.52 nM, and the maximal number of binding sites (Bmax) was 3.5 pmol/g original wet weight of tissue. Scatchard analysis yielded a straight line, and the Hill coefficient was equal to 1, a result indicating that [3H]neurotensin(8-13) bound to single, noncoopertive sites. The KD values of several analogs of neurotensin determined in competition with [3H]neurotensin(8-13) were similar to those previously determined in competition with [3H]neurotensin. The regional distribution of binding sites for [3H]neurotensin(8-13) was also similar to that for [3H]neurotensin. These results suggest that [3H]neurotensin(8-13) binds to the same sites as [3H]neurotensin and that [3H]neurotensin(8-13) has a higher affinity than [3H]neurotensin for these sites in human brain.     

k-Binding and Degradation of [3H]Dynorphin A (1–8) and [3H]Dynorphin A (1–9) in Suspensions of Guinea Pig Brain Membranes

Following incubation of [3H]dynorphin A (1-8) and [3H]dynorphin A (1-9) with suspensions of guinea pig brain membranes, analysis of the supernatants by HPLC has shown that both peptides are degraded at 25 degrees C and at 0 degrees C. Bestatin and captopril reduce degradation at 0 degrees C but for a similar degree of protection at 25 degrees C arginine-containing dipeptides are also required. The effects of these peptidase inhibitors on the degradation profiles indicate that [3H]dynorphin A (1-8) has three main sites of cleavage: the Tyr1-Gly2, Arg6-Arg7, and Leu5-Arg6 bonds. With [3H]dynorphin A (1-9) as substrate the Arg7-Ile8 and Ile8-Arg9 bonds are also liable to cleavage. In binding assays, in contrast to the effects of peptidase inhibitors on the degradation of unbound [3H]dynorphin A (1-8) and [3H]dynorphin A (1-9), bestatin and captopril have little effect on the binding characteristics of the tritiated dynorphin A fragments at the kappa-site at 0 degrees C. However, at 25 degrees C binding is low in the absence of peptidase inhibitors. When binding at mu- and delta-sites is prevented, the maximal binding capacities of [3H]dynorphin A (1-8), [3H]dynorphin A (1-9), and [3H](-)-bremazocine at the kappa-site are similar; [3H]dynorphin A (1-9) has 5-10 times higher affinity for the kappa-site than [3H]dynorphin A (1-8). Comparison of the effects of peptidase inhibitors on unbound dynorphin A fragments with their effects in binding assays suggests that the bound peptides are protected from the action of peptidases.     

Photoaffinity Labeling of Benzodiazepine Receptor Proteins with the Partial Inverse Agonist [3H]Ro 15–4513: A Biochemical and Autoradiographic Study

Photolabeling of the benzodiazepine receptor, which to date has been done with benzodiazepine agonists such as flunitrazepam, can also be achieved with Ro 15-4513, a partial inverse agonist of the benzodiazepine receptor. [3H]Ro 15-4513 specifically and irreversibly labeled a protein with an apparent molecular weight of 51,000 (P51) in cerebellum and at least two proteins with apparent molecular weights of 51,000 (P51) and 55,000 (P55) in hippocampus. Photolabeling was inhibited by 10 microM diazepam but not by 10 microM Ro 5-4864. The BZ1 receptor-selective ligands CL 218872 and beta-carboline-3-carboxylate ethyl ester preferentially inhibited irreversible binding of [3H]Ro 15-4513 to protein P51. Not only these biochemical results but also the distribution and density of [3H]Ro 15-4513 binding sites in rat brain sections were similar to the findings with [3H]flunitrazepam. Thus, the binding sites for agonists and inverse agonists appear to be located on the same proteins. In contrast, whereas [3H]flunitrazepam is known to label only 25% of the benzodiazepine binding sites in brain membranes, all binding sites are photolabeled by [3H]Ro 15-4513. Thus, all benzodiazepine receptor sites are associated with photolabeled proteins with apparent molecular weights of 51,000 and/or 55,000. In cerebellum, an additional protein (MW 57,000) unrelated to the benzodiazepine receptor was labeled by [3H]Ro 15-4513 but not by [3H]flunitrazepam. In brain sections, this component contributed to higher labeling by [3H]Ro 15-4513 in the granular than the molecular layer.     

Opposite Age-Dependent Changes of α2A-Adrenoceptors and Nonadrenoceptor [3H]Idazoxan Binding Sites (I2-Imidazoline Sites) in the Human Brain: Strong Correlation of I2 with Monoamine Oxidase-B Sites

Abstract: In the postmortem human brain (27 specimens of frontal cortex, Brodmann area 9), the specific binding of the antagonists [³H]RX 821002 (2-methoxyidazoxan) to α_2A-adrenoceptors and that of [³H]idazoxan to l₂-imidazo-line sites (a nonadrenoceptor mitochondrial site) were determined in parallel to study the effect of aging (range, 4–89 years) on both brain proteins. The density of α_2A-adrenoceptors and age were negatively correlated (r=-0.71; p < 0.001). In contrast, the density of l₂-imidazo-line sites was positively correlated with aging (r= 0.59; p < 0.005). The ratio of receptor densities (α_2A/l₂) also showed a marked negative correlation with age (r=-0.76; p < 0.001). In an age-selected group (range, 10–89 years), the density of monoamine oxidase (MAO)-B sites labeled by [³H]Ro 19–6327 (lazabemide) also showed a positive correlation with age (r= 0.80; p < 0.005). In these subjects, the density of l₂-imidazoline sites correlated well with the density of MAO-B sites (r= 0.70; p < 0.005). The ratio of the density of these sites (MAO-B/l₂) did not correlate with the age of the subject at death (r=-0.15). In the human frontal cortex, idazoxan displayed very low affinity (K_i= 89 μM) against the binding of [³H]Ro 19–6327 to MAO-B, which discounted a direct interaction of [³H]idazoxan with the active center of the enzyme and indicated that the l₂-imidazoline site cannot be identified with MAO-B. However, l₂-imidazoline sites and MAO-B show a clear coexpression not only in the human frontal cortex during the process of aging, but also in various brain regions of the human and rat brains. It is suggested that the l₂-imidazoline site has a specific location on glial (astrocyte) cells.     

Comparison of the In Vitro Receptor Binding Properties of N-[3H]Methylspiperone and [3H]Raclopride to Rat and Human Brain Membranes

The aim of the present investigation was to study and compare the in vitro binding properties of the two radioligands N-[3H]methylspiperone ([3H]NMSP) and [3H]raclopride. These compounds, labeled with 11C, have been extensively used in positron emission tomography studies on central dopamine D2 receptors in schizophrenic patients, although with diverging results. One study (using [11C]NMSP) showed an increased dopamine receptor density in drug-naive schizophrenic patients, whereas in another study (using [11C]raclopride) the density in schizophrenic patients was no different from that in healthy controls. In the present study, using in vitro binding techniques, the density of the binding sites was found to be similar irrespective of which of the two radioligands was used (20 fmol/mg wet weight in rat striatum and 10 fmol/mg in human putamen; the 5-hydroxytryptamine 2 receptors were blocked with 40 nM ketanserin). [3H]NMSP had a 10-fold higher affinity (KD, 0.3 nM in rat striatum and 0.2 nM in human putamen) than [3H]raclopride (KD, 2.1 nM in rat striatum and 3.9 nM in human putamen), which was consistent with the longer dissociation half-life of [3H]NMSP compared with [3H]raclopride (14.8 and 1.19 min, respectively). There was an approximate overall similarity between the inhibition constants for five dopamine antagonists, chlorpromazine, haloperidol, raclopride, remoxipride, and NMSP, when using either radioligand. The Ki values were, however, two- to four-fold higher when using [3H]NMSP as the radioligand, irrespective of inhibiting compound, except for chlorpromazine (and haloperidol in human putamen). NMSP was found to inhibit the binding of [3H]raclopride competitively, whereas raclopride inhibited the binding of [3H]NMSP both competitively and noncompetitively. This difference suggests that part of the binding site is exclusively used by NMSP and can only be allosterically interfered with by raclopride. It is proposed that [3H]NMSP binds to an additional set of accessory binding sites, presumably located more distantly from the agonist binding active site than the sites to which [3H]raclopride binds.     

Labelling of 5-Hydroxytryptamine3 Receptors with a Novel 5-HT3 Receptor Ligand, [3H]RS-42358–197

Abstract: RS-42358–197{(S)-N-(1-azabicyclo[2.2.2]oct-3-yl)-2,4,5,6-tetrahydro-1H-benzo[de]isoquinolin-1-one hydrochloride} displaced the prototypic 5-hydroxytryptamine₃ (5-HT₃) receptor ligand [³H]quipazine in rat cerebral cortical membranes with an affinity (pK_i) of 9.8 ± 0.1, while having weak affinity (pK_i < 6.0) in 23 other receptor binding assays. [³H]RS-42358–197 was then utilized to label 5-HT₃ receptors in a variety of tissues. [³H]RS-42358–197 labelled high-affinity and saturable binding sites in membranes from rat cortex, NG108–15 cells, and rabbit ileal myenteric plexus with affinities (K_D) of 0.12 ± 0.01, 0.20 ± 0.01, and 0.10 ± 0.01 nM and densities (B_max) of 16.0 ± 2.0, 660 ± 74, and 88 ± 12 fmol/mg of protein, respectively. The density of sites labelled in each of these tissues with [³H]RS-42358–197 was similar to that labelled with [³H]GR 65630, but was significantly less than that found with [³H]-quipazine. The binding of [³H]RS-42358–197 had a pharmacological profile similar to that of [³H]quipazine, as indicated by the rank order of displacement potencies: RS-42358–197 > (S)-zacopride > tropisetron > (R)-zacopride > ondansetron > MDL72222 > 5-HT. However, differences in 5-HT₃ receptors of different tissues and species were detected on the basis of statistically significant differences in the affinities of phenylbiguanide, and 1-(m-chlorophenyl)biguanide when displacing [³H]RS-42358-197 binding. [³H]RS-42358–197 also labelled a population (B_max= 91 ± 17 fmol/mg of protein) of binding sites in guinea pig myenteric plexus membranes, with lower affinity (K_D= 1.6 ± 0.3 nM) than those in the other preparations. Moreover, the rank order of displacement potencies of 15 5-HT₃ receptor ligands in guinea pig ileum was found not to be identical to that in other tissues. Binding studies carried out with [³H]RS-42358–197 have detected differences in 5-HT₃ receptor binding sites in tissues of different species and further underscore the unique nature of the guinea pig 5-HT₃ receptor.     

Comparison of [3H]Choline and d-[3H]Aspartate Efflux from Guinea Pig and Human Neocortex

The outflow of [3H]choline ([3H]Ch) evoked by electrical field stimulation and the efflux of D-[3H]Asp induced by 35 mM KCl and 1-10 microM ouabain were studied in human and guinea pig cortical slices, kept under identical experimental conditions. [3H]Ch outflow was significantly lower whereas D-[3H]Asp efflux was significantly higher in humans than in guinea pigs. This suggests a different proportion of the two neuronal systems in these two species. Blockade of muscarinic autoreceptors with atropine increased, whereas stimulation of alpha 2 receptors with norepinephrine (NE) reduced, the evoked [3H]Ch outflow to the same extent in human and guinea pig cortical slices. Conversely, NE did not affect ouabain-induced D-[3H]Asp efflux, suggesting that an alpha 2-mediated control is not operative in the glutamatergic cortical structures. Desmethylimipramine, 2-5 microM, was able to increase [3H]Ch outflow through atropine-like mechanisms only in the human. This drug at 20-50 microM inhibited [3H]Ch and D-[3H]Asp efflux in both species, through mechanisms unrelated to its monoamine reuptake blocking properties. Thus, similarities and differences can be detected between humans and guinea pigs with regard to (a) the relative potency of the cholinergic and acidic amino acidergic signals and (b) the modulation of neurotransmitter outflow by drugs acting on auto- and the heteroreceptors.     

Kinetics of [3H]Ro 15–1788 Binding to Membrane-Bound Rat Brain Benzodiazepine Receptors

Abstract: Ro 15–1788 is thought to interact specifically with the benzodiazepine receptor population in the mammalian CNS as an antagonist. We have compared the kinetics of interaction of this ligand with the benzodiazepine agonist flunitrazepam in the rat cerebellum. The association of [³H]Ro 15–1788 with the benzodiazepine receptor in this brain region is monoexponential, and the dissociation, initiated either by dilution or by displacement with a number of different ligands, also obeys simple monoexponential kinetics. There is no evidence of cooperative interactions with this ligand, and its dissociation is unaffected by the presence of 100 μ M γ-aminobutyric acid and/or 150 m M sodium chloride. In contrast, the dissociation of the agonist [³H]flunitrazepam is biphasic, and the possible interpretation of this data in terms of agonist-induced conformational change is discussed. Evidence is also presented that the mechanism of interaction of Ro 15–1788 with the benzodiazepine receptor population in hippocampal membranes is distinct from that found in the cerebellum.     

[3H]Dopamine Labeling of D3 Dopaminergic Sites in Human, Rat, and Calf Brain

Abstract: The binding of [³H]dopamine to brain regions of calf, rat, and human was investigated. The calf caudate contained the highest density of [³H]dopamine binding sites, with a B_max value of 185 fmol/mg protein, whereas rat and human striatum contained one-third this number of sites. The K_D values for [³H]dopamine in all tissues were 2–3 nM. Dopaminergic catecholamines (dopamine, apomorphine, 6,7-dihydroxy-2-aminotetralin, and N-propylnorapomorphine) inhibited the binding of [³H]dopamine in all three species, at low concentrations, with IC₅₀ values of 1.5 to 6 nM. Neuroleptics, in contrast, inhibited the binding at high concentrations (with IC₅₀ values of 200 to 40,000 nM). The [³H]dopamine binding sites were saturable, heat-labile, and detectable only in dopamine-rich brain regions; these sites differed from D₂ dopamine sites (labeled by [³H]butyrophenone neuroleptics), and from D_l dopamine sites (labeled by [³H]thioxanthene neuroleptics) associated with the dopamine-stimulated adenylate cyclase. We have, therefore, called these high-affinity [³H]dopamine binding sites D₃ sites. [³H]Apomorphine and [³H]ADTN also appeared to label D₃ sites. These ligands however, were less selective than [³H]dopamine, and labeled sites other than D₃ as well. Assay conditions were important in determining the parameters of [³H]dopamine binding. The optimum conditions for selective labeling of the D₃ dopaminergic sites, using [³H]dopamine, required the presence of EDTA and ascorbate.     

[3H]Aniracetam Binds to Specific Recognition Sites in Brain Membranes

Abstract: [³H]Aniracetam bound to specific and saturable recognition sites in membranes prepared from discrete regions of rat brain. In crude membrane preparation from rat cerebral cortex, specific binding was Na⁺ independent, was still largely detectable at low temperature (4°C), and underwent rapid dissociation. Scatchard analysis of [³H]aniracetam binding revealed a single population of sites with an apparent K_D value of ～70 nM and a maximal density of 3.5 pmol/mg of protein. Specifically bound [³H]aniracetam was not displaced by various metabolites of aniracetam, nor by other pyrrolidinone-containing nootropic drugs such as piracetam or oxiracetam. Subcellular distribution studies showed that a high percentage of specific [³H]aniracetam binding was present in purified synaptosomes or mitochondria, whereas specific binding was low in the myelin fraction. The possibility that at least some [³H]aniracetam binding sites are associated with glutamate receptors is supported by the evidence that specific binding was abolished when membranes were preincubated at 37°C under fast shaking (a procedure that substantially reduced the amount of glutamate trapped in the membranes) and could be restored after addition of either glutamate or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) but not kainate. The action of AMPA was antagonized by DNQX, which also reduced specific [³H]aniracetam binding in unwashed membranes. High levels of [³H]aniracetam binding were detected in hippocampal, cortical, or cerebellar membranes, which contain a high density of excitatory amino acid receptors. Although synaptosomal aniracetam binding sites may well be associated with AMPA-sensitive glutamate receptors, specifically bound [³H]aniracetam could not be displaced by cyclothiazide or GYKI 52466, which act as a positive and negative modulator of AMPA receptors, respectively.     

Binding of [3H] γ-Aminobutyric Acid and [3H]Muscimol in Purified Rat Brain Synaptic Plasma Membranes and the Effects of Bicuculline

Abstract: The binding of [³H] γ-aminobutyric acid ([³H]GABA) and [³H]muscimol has been studied in purified synaptic plasma membrane (SPM) preparations from rat brain. Scatchard analysis of specific binding (defined as that displaced by 100 μMγ-aminobutyrate) indicated that the binding of both radiolabelled ligands was best described by a two component Langmuir adsorption isotherm. The apparent K_D and B_max values for [³H]GABA at 4°C were K_D1, 20 nM; K_D2,165 nM; B_max1, 0.48 pmol;B_max2, 6.0 pmol. mg^?1; for [³H]muscimol at 4°C they were: K_D1, 1.75 nM; K_D2, 17.5 nM; B_maxl, 0.84 pmol. mg^?1; B_max2, 4.8 pmol.mg^?1; and for [³H]muscimol at 37°C they were: K_D1, 7.0 nM; K_m, 60 nM; B_max], 0.5 pmol-mg^?1; B_max2, 7.2 pmol-mg¹. Under the experimental conditions used, the similar B_milx values for [³H]GABA and [³H]muscimol binding to the SPM preparations suggests that the high- and low-affinity components for the two radiolabeled ligands are identical. The effects of the GAB A antagonist bicuculline on the binding of [³H]muscimol at 4^CC and 37°C were studied. At 4°C, antagonism of muscimol binding appeared to be competitive at the high-affinity site but noncompetitive at the low-affinity site. At 37°C, antagonism was again competitive at the high-affinity site but was of a mixed competitive/noncompetitive nature at the low-affinity site. Assuming that binding to the high-affinity site is associated with the pharmacological actions of bicuculline, the apparent K_D values obtained suggest a pA₂ value of 5.3 against [³H]muscimol at 4°C and 37°C. This figure is in good agreement with several estimates of the potency of bicuculline based on pharmacological measurements. Results from displacement studies using [³H]GABA and [³H]muscimol suggest that [³H]GABA might be a more satisfactory ligand than [³H]muscimol in GABA radioreceptor assays.     

Formation of [3H]Inositol Metabolites in Rat Hippocampal Formation Slices Prelabelled with [3H]Inositol and Stimulated with Carbachol

Rat hippocampal formation slices were prelabelled with [3H]inositol and stimulated with carbachol for times between 7 s and 3 min. The [3H]inositol metabolites in an acid extract of the slices were resolved with anion-exchange HPLC. Carbachol dramatically increased the concentration of [3H]inositol monophosphate, [3H]inositol bisphosphate (two isomers), [3H]inositol 1,3,4-trisphosphate, [3H]inositol 1,4,5-trisphosphate, and [3H]inositol 1,3,4,5-tetrakisphosphate. The levels of [3H]inositol 1,4,5-trisphosphate rose most rapidly; they were maximally elevated after only 7 s and declined toward control levels in 1 min followed by a more sustained elevation in levels for up to 3 min. When [3H]inositol 1,4,5-trisphosphate was incubated with hippocampal formation homogenates in an ATP-containing buffer it was very rapidly metabolised. After 5 min [3H]inositol 1,4-bisphosphate, [3H]inositol 1,3,4-trisphosphate, and [3H]inositol 1,3,4,5-tetrakisphosphate could be detected in the homogenates. Under similar experimental conditions [3H]inositol 1,3,4,5-tetrakisphosphate is metabolised to [3H]inositol 1,3,4-trisphosphate and an inositol bisphosphate isomer that is not [3H]inositol 1,4-bisphosphate. We conclude that like other tissues the primary event in the hippocampus following carbachol stimulation is the activation of phosphatidylinositol 4,5-bisphosphate selective phospholipase C.     

[3H]WIN 35,065–2: A Ligand for Cocaine Receptors in Striatum

[3H]WIN 35,065-2 binding to striatal membranes was characterized, primarily by centrifugation assay. Like [3H]cocaine, [3H]WIN 35,065-2 binds to both high- and low-affinity sites. [3H]WIN 35,065-2, however, exhibits consistently higher affinities than [3H]cocaine. Saturation experiments indicate a low-affinity binding site with an apparent KD of approximately 160 nM and a Bmax of 135 fmol/mg of tissue. A high-affinity site has also been identified with an apparent KD of 5.6 nM and a Bmax of 5.2 fmol/mg of tissue. The specific-to-nonspecific binding ratios with [3H]WIN 35,065-2 were higher than with [3H]cocaine in both centrifugation and filtration assays. Pharmacological characterization suggests that [3H]WIN 35,065-2 binds to the dopamine transporter. Mazindol, GBR 12909, nomifensine, and (-)-cocaine are potent inhibitors of [3H]WIN 35,065-2 binding. In contrast, the norepinephrine transporter ligand desipramine is a weak inhibitor, and the serotonin transporter ligand citalopram does not inhibit binding. The effect of sodium on binding was examined under conditions in which (a) the low-affinity site was primarily (87%) occupied and (b) approximately 50% of both sites were occupied. The results indicate that both sites are sodium dependent. Injection of 6-hydroxydopamine into the striatum results in a significant loss of both high- and low-affinity sites, a finding suggesting that both sites are on dopaminergic nerve terminals. Taken together, these data are consistent with the presence of multiple cocaine binding sites associated with the dopamine transporter.     

Purification of Neuropathy Target Esterase from Avian Brain After Prelabelling with [3H]Diisopropyl Phosphorofluoridate

Neuropathy target esterase from hen brains was radiolabelled at the active site with [3H]diisopropyl phosphorofluoridate. The labelled protein was purified by differential centrifugation and Nonidet P40 solubilization, detergent phase partitioning, anion exchange, and preparative sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). The volatilizable counts assay and analytical SDS-PAGE were used to monitor the protein. The 150-kDa subunit polypeptide appears as a single band on analytical SDS-PAGE.     

[3H]Tryptamine Binding Sites Are Not Identical to Monoamine Oxidase in Rat Brain

Competition binding studies, subcellular distribution, and in vitro autoradiography were employed to compare the binding in rat brain of [3H]tryptamine with two radioligands for monoamine oxidase (MAO), [3H]pargyline, and [3H]1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine ([3H]MPTP). The MAO inhibitors pargyline, clorgyline, and deprenyl all yielded biphasic competition curves versus [3H]tryptamine. At low concentrations, these drugs stimulated binding by protecting the radioligand from MAO oxidation; at considerably higher concentrations, they inhibited binding by direct competition at the [3H]tryptamine binding site. In subcellular distribution studies, [3H]tryptamine was localized preferentially to the synaptosomal fraction, whereas [3H]pargyline showed greater binding to the mitochondrial fraction. Equilibrium binding studies revealed that the potencies of a series of seven compounds at inhibiting [3H]tryptamine binding were completely different from their potencies at inhibiting [3H]MPTP binding. Finally, the autoradiographic distribution of [3H]tryptamine binding in rat brain was different from that of [3H]MPTP and [3H]pargyline. We conclude that the [3H]tryptamine binding site in rat brain is not equivalent to MAO.