Synthesis and binding properties of the fluorinated substituted benzamide [3H]NCQ 115, a new selective dopamine D2 receptor ligand.

[3H]NCQ 115 [R)-5-bromo-2,3-dimethoxy-N-[1-([2,5-3H]-4- fluorobenzyl)-2-pyrrolidinyl)methyl)benzamide) was prepared by acylation of (R)-(2-aminomethyl)-1- ([2,5-3H]-4-fluorobenzyl)pyrrolidine, which was obtained in a stereo-conservative synthesis from (R)-prolinamide. Purification by reversed phase high performance liquid chromatography (HPLC) gave [3H]NCQ 115 with a radiochemical purity of greater than 99% and a specific activity of 0.97 GBq/mumol (36 Ci/mmol). Saturation analyses, association and dissociation kinetics as well as binding competition with several compounds of various classes were performed with [3H]NCQ 115 in rat striatal homogenates. Saturation analyses in vitro showed that [3H]NCQ 115 bound to a single binding site with a Kd = 214 pM and Bmax = 35.4 fmol/mg. The binding of [3H]NCQ 115 was dependent upon sodium ions, since the number of binding sites was altered when sodium ions were excluded from the incubation medium. NCQ 115 inhibited the binding of [3H]raclopride to dopamine D2 receptors with high affinity (Ki = 147 pM), having much lower affinity for other receptors. The affinity of this substituted 1-benzyl-2-pyrrolidinylmethyl benzamide was confined to the (R)-enantiomer, which contrasts with that of the corresponding N-ethyl derivatives such as FLB 457, raclopride, eticlopride, sulpiride and NCQ 298, where the pharmacological activity is found in the (S)-enantiomer. It can be concluded that [3H]NCQ 115 binds to dopamine D2 receptors in the rat striatum with high affinity and high selectivity. [3H]NCQ 115 can also be used for in vivo binding studies of the brain. [18F]NCQ 115 may be a suitable ligand for positron emission tomography (PET) studies of the human brain in vivo.

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.

Serotonin receptors in the brain following total hepatectomy in rats treated with branched-chain amino acids.

Serotonin concentrations and receptor binding characteristics were investigated in rats subjected to total hepatectomy, portacaval shunt (PCS) or sham-operation. The animals were infused for 5 hr with a 10% glucose solution or the same solution enriched with 0.24 M branched-chain amino acids (BCAA). Hepatectomized animals were in grade-two coma at the end of the experiment independent of infusion. Indoleamines in mesencephalon-pons and diencephalon were analyzed by high-pressure liquid chromatography with electrochemical detection. Serotonin receptors (5-HT1 and 5-HT2) were investigated in the cortex and hippocampus by radioligand binding studies using 3H-serotonin for analysis of 5-HT1-receptors and 3H-ketanserin for analysis of 5-HT2-receptors. Concentrations of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were significantly increased after hepatectomy compared with controls. Treatment with BCAA significantly decreased 5-HT and 5-HIAA levels in hepatectomized animals. The affinity and the number of binding sites for the 5-HT1 and 5-HT2 receptors were found to be similar in all groups. The present study indicates that PCS for 1 week and the anhepatic state for 5 hr in rats do not influence brain serotonin receptors in contrast to previous studies in pigs with liver ischemia or rabbits with galactosamine-induced liver damage. In addition, infusion of BCAA for 5 hr did not alter the binding characteristics.

Effects of temperature on the in vitro binding of 3H-raclopride to rat striatal dopamine-D2 receptors.

The influence of temperature on the in vitro binding of 3H-raclopride to rat striatal dopamine-D2 receptors was investigated. The KD-values obtained in Scatchard plots were approximately 1.6 nM at temperatures between 15 degrees and 30 degrees. At 37 degrees the KD-value was found to be 3.0 nM, indicating a lower affinity without affecting the number of the receptors (Bmax). The rate of association of 3H-raclopride to the receptors was decreased with decreasing temperatures, and at 6 degrees more than 150 min. incubation was needed to reach the steady state level. From the association constants, the activation energy of the binding reaction was calculated to be 80 kJ/mol. The driving forces of the binding reaction was suggested to be a change in entropy at temperatures up to 30 degrees but a change in enthalpy at 37 degrees.

Comparisons between the in vitro binding of two substituted benzamides and two butyrophenones to dopamine-D2 receptors in the rat striatum.

The in vitro dopamine-D2 receptor binding profiles of two substituted benzamides (3H-sulpride and 3H-raclopride) and two butyrophenones (3H-spiperone and 3H-domperidone) were compared. 3H-Raclopride, 3H-domperidone and 3H-sulpride labelled approximately the same number of binding sites in the rat striatum, while 3H-spiperone labelled a higher number of binding sites. This latter finding was suggested to be due to the labelling of 5-HT2 receptors in addition to the dopamine-D2 binding sites since the labelling of these 5-HT2 receptors with 3H-spiperone could be avoided by the addition of the 5-HT2 receptor antagonist ketanserin. The inhibition constants (Ki) of the unlabelled compounds were similar to the Kd values obtained in the saturation analyses with 3H-raclopride, 3H-domperidone and 3H-sulpiride but were significantly higher when 3H-spiperone was used as radioligand. Addition of ketanserin to 3H-spiperone made the Ki values more similar to those obtained with the other radioligands. The implications of these findings in testing antidopaminergic activity is discussed.

The effects of manipulation of presynaptic 5-HT nerve terminals on postsynaptic 5-HT1 and 5-HT2 binding sites of the rat brain.

The effects of long-term treatment of rats with alaproclate and amiflamine on the number and kinetics of 5-HT1 and 5-HT2 binding sites were investigated using in vitro receptor binding techniques. Some other studies have reported down-regulatory effects of alaproclate and amiflamine on 5-HT2 binding sites in certain regions of the rat forebrain, but no such effects could be detected in the present study. Induction of a high-affinity binding site for 3H-5-HT after long-term antidepressant treatment, as has been reported elsewhere, was not obtained in the present study. The results are compared to the effects obtained by treatment of rats with para-chloroamphetamine (PCA), which depletes the presynaptic neurons of monoamines. These different types of treatment do not cause any change in the binding properties of the specific 5-HT binding sites. It is thus concluded that such manipulations of the presynaptic 5-HT neurons do not affect the postsynaptic 5-HT1 and 5-HT2 binding sites.

Acute effects of atypical antidepressants on various receptors in the rat brain.

Using in vitro receptor binding techniques, the effects of a number of different antidepressants on rat cerebral cortex receptors were investigated. In contrast to the tricyclic antidepressants, which potently inhibit several postsynaptic receptors, many atypical antidepressants have no or very little affinity for these receptors. Thus bupropion, amineptine, citalopram, fluoxetine, fluvoxamine and viloxazine are devoid of any activity, while amoxapine, doxepin and trazodone exert effects on a number of receptors. The implications of these receptor blocking effects are discussed.

Effects of fibrinogen fragment and proteolytic enzyme on the immune system in mice.

The immunomodulating ability of fibrinogen fragment (Hol-DSK) and proteolytic enzyme with fibrinolytic activity from Aspergillus oryzae was analysed in CBA/Ca, C57Bl, C3H/HeJ and NMRI mice. Suppressive effects on the blastogenic response to mitogens were noted. No consistent suppressive effect, but sometimes an enhancing one, was recorded on the antibody responses to protein antigens. Administration of Hol-DSK or proteolytic enzyme shortly after the immunization resulted in a slight inhibition of the development of delayed hypersensitivity to picryl chloride in C57Bl but not in CBA mice. When similar administration was done simultaneously with administration of the challenging antigen of picryl chloride or sheep red blood cells in immunized mice, significant impairment of the delayed-type hypersensitivity reaction was noted. The modulating effects by Hol-DSK did not alter the in vitro bacteriocidal effects of peritoneal exudate cells from mice on Escherichia coli.