Synthesis and biological evaluation of a series of novel N- or O-fluoroalkyl derivatives of tropane: potential positron emission tomography (PET) imaging agents for the dopamine transporter

A series of novel fluoroalkyl-containing tropane derivatives was synthesized, and their binding affinities for the dopamine transporter (DAT), serotonin transporter (SERT), and norepinephrine transporter (NET) were determined via competitive binding assays. Among these derivatives, the fluoropropyl ester of beta-CIT (19), the fluoroethyl ester of beta-CIT (20), the N-fluoropropyl derivative of beta-CBT (12), and the fluoropropyl ester of beta-CMT (18) displayed higher affinity and greater selectivity for the DAT versus SERT and NET than FP-CIT, which indicates that they are attractive candidates for the development of (18)F-labeled PET imaging agents for the DAT.

Fatty acid derivatives of clozapine: prolonged antidopaminergic activity of docosahexaenoylclozapine in the rat

Stable amides of clozapine derived from fatty acids prominent in cerebral tissue might enhance the central activity of clozapine and reduce its exposure to peripheral tissues. Such derivatives might enhance the safety of this unique drug, which is the only agent with securely established superior antipsychotic effectiveness, but with a risk of potentially lethal systemic toxicity. Amide derivatives of clozapine were prepared from structurally varied fatty acid chlorides and evaluated for ability to inhibit behavioral arousal in rat induced by dopamine agonist apomorphine and to induce catalepsy. Their duration-of-action and potency were compared to free clozapine, and concentrations of clozapine were assayed in brain and blood. Selected agents were also evaluated for affinity at dopamine receptors and other potential drug-target sites. Clozapine-N-amides of linoleic, myristic, oleic, and palmitic acids had moderate initial central depressant activity but by 6 h, failed to inhibit arousal induced by apomorphine. However, the docosahexaenoic acid (DHA) derivative was orally bioavailable, 10-times more potent (ED(50) 5.0 micromol/kg) than clozapine itself, and very long-acting (>/= 24 h) against apomorphine, and did not induce catalepsy. DHA itself was inactive behaviorally. Clozapine showed expected dopamine receptor affinities, but DHA-clozapine was inactive at these and other potential target sites. After systemic administration of DHA-clozapine, serum levels of free clozapine were very low, and brain concentrations somewhat lower than after administering clozapine. DHA-clozapine is a long-acting central depressant with powerful and prolonged antidopaminergic activity after oral administration or injection without inducing catalepsy, and it markedly reduced peripheral exposure to free clozapine. It lacked the receptor-affinities shown by clozapine, suggesting that DHA-clozapine may be a precursor of free, pharmacologically active clozapine. Such agents may represent potential antipsychotic drugs with improved central/peripheral distribution, and possibly enhanced safety.

Alkylation of rat dopamine transporters and blockade of dopamine uptake by EEDQ

Effects of the alkylating agent EEDQ (N-ethoxycarbonyl-2-ethoxy-1, 2-dihydroquinoline) on levels of dopamine transporter (DA(T)) and function were examined in caudate-putamen (CPu) tissue from rat brain. EEDQ produced profound, dose-dependent decreases in DA(T) binding in homogenates (IC(50)=78 microM) and frozen sections (IC(75)=200 microM) that were not reversed by washing. EEDQ also blocked uptake of [(3)H]DA in CPu synaptosomes (IC(50)=17 microM). However, single (10 mg/kg) or repeated administration of EEDQ in vivo (15 mg/kg/day x 3) did not alter DA(T) levels or DA uptake in CPu. Pretreatment of rats with alpha-methyl-p-tyrosine and reserpine to deplete endogenous dopamine also failed to lower DA(T) levels in CPu after injections of EEDQ. EEDQ is an effective alkylating agent for DA(T) in vitro, but not to evaluate metabolic turnover or function of DA(T) in vivo. The results encourage development of selective and in vivo-active DA(T)-alkylating agents.

Synthesis and monoamine transporter affinity of 2beta-carbomethoxy-3beta-(2''-, 3''- or 4''-substituted) biphenyltropanes

A series of 11 novel 3beta-substituted biphenyltropanes was synthesized and evaluated by selective radioligand binding assays for affinity to monoamine transporters. Both 5-HTT potency and selectivity for 5-HTT over DAT was greatest with electron withdrawing group at the 3''-position.

Cyclopentadienyltricarbonylrheniumbenzazepines: synthesis and binding affinity

Analogues of the benzazepine dopamine D1 receptor antagonist SCH-23390 incorporating the cyclo-pentadienyltricarbonyl-rhenium (CPTR) moiety were synthesized and evaluated pharmacologically. The CPTR derivatives retained affinity (0.3-2.9 nM) and D1 selectivity of the parent compound, supporting their use as neuropharmacological surrogates for 99mTc-labeled SPECT radiopharmaceuticals.

[3H]beta-CIT: a radioligand for dopamine transporters in rat brain tissue

[3H]2-beta-carbomethoxy-3-beta-[4'-iodophenyl]tropane (beta-CIT) was prepared and evaluated. With rat forebrain tissue, [3H]beta-CIT showed high affinity for dopamine transporters (DAT), with selectivity for DAT over norepinephrine transporters, but not serotonin transporters, as well as DAT-stereoselectivity with beta-CIT, amphetamine and methylphenidate. Affinity and selectivity for 53 compounds assayed with [3H]beta-CIT and standard DAT radioligand [3H]GBR-12935 were highly correlated (r0.95). [3H]beta-CIT is proposed as a useful, high-affinity DAT radioprobe.

Effects of alkylating agents on dopamine D(3) receptors in rat brain: selective protection by dopamine

Dopamine D(3) receptors are structurally highly homologous to other D(2)-like dopamine receptors, but differ from them pharmacologically. D(3) receptors are notably resistant to alkylation by 1-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), which readily alkylates D(2) receptors. We compared EEDQ with N-(p-isothiocyanatophenethyl)spiperone (NIPS), a selective D(2)-like receptor alkylating agent, for effects on D(3) and D(2) receptors in rat brain using autoradiographic analysis. Neither agent occluded D(3) receptors in vivo at doses that produced substantial blockade of D(2) receptors, even after catecholamine-depleting pretreatments. In vitro, however, D(3) receptors were readily alkylated by both NIPS (IC(50)=40 nM) and EEDQ (IC(50)=12 microM). These effects on D(3) sites were blocked by nM concentrations of dopamine, whereas microM concentrations were required to protect D(2) receptors from the alkylating agents. The findings are consistent with the view that alkylation of D(3) receptors in vivo is prevented by its high affinity for even minor concentrations of endogenous dopamine.

Selective alkylatation of dopamine D2 and D4 receptors in rat brain by N-(p-isothiocyanatophenethyl)spiperone

Effects of the D2-like receptor alkylating agent NIPS (N-[p-isothiocyanatophenethyl]spiperone) on dopamine receptors in rat brain were characterized by radioreceptor assays and quantitative autoradiography. NIPS alkylated D2 and D4 receptors concentration-dependently in brain sections and transfected cells. NIPS also alkylated both receptors dose-dependently in vivo, with no effect on dopamine D1-like or serotonin 5-HT2 receptors at a dose that occluded 75% of D2 and D4 receptors. Pretreatment with D2-like receptor selective antagonist haloperidol completely blocked the effects of NIPS. The findings demonstrate that NIPS selectively alkylates D2 and D4 receptors, indicating its potential utility for studies of these receptors.

Neuropharmacological assessment of potential dopamine D4 receptor-selective radioligands

Radiolabeled dopamine D4 receptor-selective agents ([3H]1-benzyl-4-[ N-(3-isopropoxy-2-pyridinyl)-N-methyl]-aminopiperidine maleate; [3 H]PNU-101958. and [125I]1-[4-iodobenzyl]-4-[ N-(3-isopropoxy-2-pyridinyl)-N-methyl]-aminopiperidine; [125I]RBI-257) were prepared and characterized. With D4.2- and D2L receptor-transfected cell membranes, [3H]PNU-101958 showed high dopamine D4 receptor affinity and selectivity, and potent inhibition by dopamine D4 receptor-selective compounds. However, its binding with rat brain homogenates showed little regional selectivity, and pharmacology inconsistent with selective dopamine D4 receptor labeling. Autoradiography indicated partial displacement of [3H]PNU-101958 by unlabeled dopamine D4 receptor ligands without regional selectivity, and lack of selective labeling with [125I]RBI-257. The results encourage further efforts to develop better dopamine D4 receptor-selective radioligands.

Local injection of alkylating and nonalkylating dopamine receptor antagonists into rat basal forebrain: autoradiographic assessment of D2-like and D3 sites

N-chloroethyl derivatives of 7-hydroxy-1,2,3,4-tetrahydronaphthalene (7-OH-DPAT), 1-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), or fluphenazine were microinjected into rat nucleus accumbens (Acc), and receptor binding quantified autoradiographically after 24 h. EEDQ reduced [3H]nemonapride (D2-like receptors) binding in Acc (by 84%) and islands of Calleja (IC; 44%), without affecting [3H](+)-7-OH-DPAT (D3); N-chloroethyl-7-OH-DPATs blocked both radioligands in Acc and IC (30%-70%); fluphenazine had no effect.

Regional distribution of dopamine D4 receptors in rat forebrain

Binding of the D2-like (D2/D3/D4) radioligand [3H]nemonapride under selective conditions (with 300 nM S[-]-raclopride and other masking agents to occlude D2/D3 receptors and non-specific binding sites) revealed a subset of raclopride-insensitive binding sites considered D4-like receptors. These sites were stereoselective to R(-)-N-n-propylnorapomorphine (NPA) over its S(+)-NPA in a similar fashion to cloned D4 receptors expressed in cell lines. In addition, the highly D4-selective agent L-745,870 displaced 74-83% of these sites in rat brain regions, suggesting that most were D4 receptors. These apparent D4 receptors represented a relatively high proportion of D2-like receptors in hippocampus, dorsolateral frontal, medial prefrontal and entorhinal cortex, but fewer in caudate-putamen and nucleus accumbens.

Long-term effects of S(+)N-n-propylnorapomorphine compared with typical and atypical antipsychotics: differential increases of cerebrocortical D2-like and striatolimbic D4-like dopamine receptors

Changes in D2-like dopamine (DA) receptor binding in rat brain regions were compared by quantitative in vitro receptor autoradiography after 21-d treatment with a typical (fluphenazine), atypical (clozapine), or candidate atypical antipsychotic (S[+]-N-n-propylnorapomorphine, [+]-NPA). Fluphenazine treatment significantly increased binding of the D2,3,4 radioligands [3H]nemonapride and [3H]spiperone in caudate-putamen (CPu: 22%, 32%), nucleus accumbens (ACC: 67%, 52%), olfactory tubercle (OT: 53%, 43%), and medial prefrontal cerebral cortex (MPC: 46%, 47%) but not dorsolateral frontal cortex (DFC). D2-like binding in MPC was also increased by (+)-NPA (49%, 39%) and clozapine (60%, 40%), but not in DFC, CPu, ACC, or OT. Binding of D2,3-selective [3H]raclopride increased less after fluphenazine in ACC (27%) and CPu (16%) than with the nonselective radioligands, and not after clozapine or (+)-NPA. D3-selective binding of [3H]R (+)-7-OH-DPAT was not changed with any treatment or region including islands of Calleja. Binding of [3H]nemonapride or [3H]spiperone under D4-selective conditions (with 300 nM S[-]-raclopride and other masking agents, at sites occluded by D4 ligand L-745,870), was increased by fluphenazine, (+)-NPA, clozapine in ACC (120%, 76%, 70%, respectively), and CPu (54%, 37%, 35%), but not in OT, DFC or MPC. These results support the hypothesis that cerebrocortical D2-like and striatolimbic D4-like receptors contribute to antipsychotic actions of both typical and atypical drugs and encourage further consideration of S(+)aporphines as potential atypical antipsychotics.

RBI-257: a highly potent dopamine D4 receptor-selective ligand

RBI-257 (1-[4-iodobenzyl]-4-[N-(3-isopropoxy-2-pyridinyl)-N-methyl]-aminopiperid ine), the p-iodobenzyl analog of U-101,958 (1-benzyl-4-[N-(3-isopropoxy-2-pyridinyl)-N-methyl]-aminopiperidine) had a lower dissociation constant (Ki = 0.3 vs. 2.7 nM) and higher selectivity than U-101,958 at dopamine D4 receptors, over dopamine D2 and D3 receptors in transfected cell membranes and D2-like sites in rat forebrain. Dopamine D4 receptor affinity of iodo-isomers of RBI-257 ranked: para > meta > ortho. RBI-257 had much lower affinity at D1 and D5 dopamine receptors in transfected cells, as well as dopamine D1-like receptors, alpha1, alpha2 or beta(1,2) adrenoceptors, sigma(1,2) receptors and 5-HT1A or 5-HT2A receptors, and transporters for dopamine, norepinephrine or serotonin in rat forebrain tissue. RBI-257 may be a useful probe or radioligand for brain dopamine D4 receptors.

Role of dopamine in behavioral effects of serotonin microinjected into rat striatum

Bilateral local microinfusion of serotonin (5-hydroxytryptamine; 5-HT) into the ventrolateral striatum (VLS) of the rat forebrain induces quantifiable stereotyped orofacial behaviors. The role of presynaptic dopamine (DA) and structural requirements of indoles for expression of this behavioral effect and for inhibition of neuronal transport of [3H]DA were examined. Bilateral local injection of 6-OHDA (8 micrograms/side) into VLS depleted DA and markedly diminished the behavioral effects of 5-HT. Intracerebral pretreatment with the potent DA transport inhibitors GBR-12909 (6 micrograms/side) or nomifensine (4 micrograms/side) also markedly decreased behavioral responses to 5-HT. A series of indoles and tyramine were examined for ability to induce stereotypy following infusion into the VLS. Of compounds tested, only p-tyramine, 5-HT, tryptamine and L-5-hydroxytryptophan (5-HTP) elicited strong orofacial behaviors; indoles lacking a free amino group or containing other substituents were virtually inactive in vivo, and the effect of 5-HTP was prevented by systemic pretreatment with the decarboxylase inhibitor NSD-1015, indicating its required conversion to 5-HT. Uptake of [3H]DA (0.1 microM) into rat striatal synaptosomes was inhibited in a concentration-dependent manner in the following apparent rank-order: p-tyramine, N-methyl-5-HT, tryptamine, 5-HT, N-methyltryptamine (IC50 = 44-718 nM), other indoles (IC50 = 10-100 microM). These results support the conclusion that oral stereotypy induced by microinjection of 5-HT or other aromatic amines into rat VLS is mediated by local release of endogenous DA. These results extend previous findings indicating that this effect of 5-HT was not blocked by 5-HT receptor antagonists, and suggest mediation by a neuronal transport process involved in the uptake or storage of DA.

N-substituted analogs of 2 beta-carbomethoxy-3 beta- (4'-iodophenyl)tropane (beta-CIT) with selective affinity to dopamine or serotonin transporters in rat forebrain

This report concerns the synthesis and chemical characterization of novel series of N-substituted 2 beta-carbomethoxy-3 beta-(4'-iodophenyl)tropane (beta-CIT, 2) analogs and their neuropharmacological evaluation for affinity at dopamine (DAT), serotonin (5-HTT), and norepinephrine membrane transporters in rat brain tissue. N-Substituted analogs of beta-CIT with a 2 beta-carbomethoxy ester moiety showed lower DAT affinity than beta-CIT for the DAT, and some were more selective for the 5-HTT over the DAT. 2 beta-Carbomethoxy(iodophenyl)nortropane analogs of beta-CIT with the N-substituents difluoroethyl, mesoxypropyl, iodopropyl, and methylpropionyl all yielded > 10-fold lower DAT affinity than beta-CIT itself, whereas the N-(fluoropropyl)-2 beta- isopropyl ester analog (1) of beta-CIT exceeded beta-CIT (2, an N-methyl-2 beta-carbomethoxy ester) in DAT affinity. Several N-haloalkyl-substituted beta-CIT analogs yielded high 5-HTT affinity (Ki < 0.6 nM), ranking: N-fluoropropyl (5) > N-chloropropyl (4) > or = N-bromopropyl (3) > beta-CIT (2) > N-3'-phtalimidopropyl (11), with particularly high (ca. 30-fold) 5-HTT-over-DAT selectivity found in the N-fluoropropyl (5) and N-fluoroethyl (6) compounds, compared to only 3.o-fold 5-HTT selectivity in beta-CIT itself. Highly 5-HTT selective agents such as 5 and 6 may be useful as brain-imaging ligands for serotonin neurons or as mood-elevating drugs, while the high affinity and selectivity for the DA transporter found in N-(fluoropropyl)-2 beta-(carboxyisopropyl)-3 beta-(4'-iodophenyl)-nortropane (1) and N-(fluoropropyl)-2 beta-carboxymethoxy-3 bet-(4'-iodophenyl)nortropane (FP-beta-CIT, 5) support their use as improved markers for DA neurons.

Distribution of fluphenazine and its metabolites in brain regions and other tissues of the rat

Rats were given 5, 10, or 20 mg/kg oral doses of fluphenazine (FLU) dihydrochloride daily for 15 days. FLU and its sulfoxide (FL-SO), 7-hydroxy (7-OH-FLU) and N4'-oxide (FLU-NO) metabolites were assayed in plasma, liver, kidney, fat, whole brain, and brain regions by specific and sensitive radioimmunoassays (RIA). All metabolites were detected in tissues at higher levels than in plasma, and the levels increased with dose. FLU was 10- to 27-fold higher in brain regions than in plasma. Brain vs plasma levels of FLU correlated more closely than levels of its metabolites. Liver contained the highest levels of all analytes at all doses. FLU-SO was the major metabolite in brain regions (24% to 96% of FLU) and accumulated in fat 43 to 75 times more than FLU. Levels of 7-OH-FLU and FLU-NO were very low in brain (1% to 20% of FLU). FLU-SO and FLU-NO had only 1% to 3% the affinity for D1 and D2 receptors, but 7-OH-FLU had 20% the D2 and 5% the D1 affinity of FLU. The low affinity for dopamine receptors and low brain-levels of metabolites of FLU indicate that they are not likely to contribute importantly to pharmacologic responses of FLU. Also, the estimated relative "activity factor" for these compounds in the brain indicated that the contribution to neuropharmacologic activity by metabolites is less than 1% of FLU. Consequently, clinical monitoring of plasma FLU alone may be sufficient.

1-Phenyl-3-amino-1,2,3,4-tetrahydronaphthalenes and related derivatives as ligands for the neuromodulatory sigma 3 receptor: further structure-activity relationships

A series of 1-phenyl-3-amino-1,2,3,4-tetrahydronaphthalenes (1-phenyl-3-aminotetralins, PATs) previously was found to stimulate tyrosine hydroxylase activity and dopamine synthesis in rat brain through interaction with a novel sigma 3 receptor. Specifically, the trans-1R,3S-(-) isomer of H2-PAT showed highest affinity for sigma 3 receptors and also produced maximal stimulation of tyrosine hydroxylase activity and dopamine synthesis, as compared to the trans-1S,3R-(+) isomer. Affinity for sigma 3 receptors and functional potency at stimulating dopamine synthesis were attenuated either by altering the position or dimethyl substitution pattern of the amino group or by hydroxylating the tetralin aromatic ring. A preliminary binding model can accommodate many PAT analogs and several non-PATs with a wide range of affinities for the sigma 3 receptor. Here, we report the synthesis and evaluation of additional analogs in order to expand previous structure-activity relationship studies. Further molecular modifications include synthesis of 1-phenyl-1-methyl-3-amino, 1-phenyl-2-amino, 1-phenyl-3-(trimethylammoniumyl), and 1-phenyl-3-(phenylalkyl) analogs, as well as ring-expanded tetrahydrobenzocycloheptenes. In general, the above modifications decreased sigma 3 receptor affinity and, in some cases, caused a reversal of the sigma 3 binding selectivity of trans- versus cis-PATs found previously. Most analogs were selective for sigma 3 receptors and showed little or no affinity for either sigma 1/sigma 2 or dopamine D1, D2, and D3 receptors. N-Phenylalkyl substituents, such as N-phenylethyl, however, endowed the 1-phenyl-3-aminotetralins with enhanced sigma 1/sigma 2 and dopamine receptor affinity while decreasing sigma 3 affinity, thus abolishing sigma 3 selectivity.

Conformational analysis, pharmacophore identification, and comparative molecular field analysis of ligands for the neuromodulatory sigma 3 receptor

Molecular modeling studies were carried out on a series of 1-phenyl-3-amino-1,2,3,4-tetrahydronaphthalenes (phenylaminotetralins, PATs), several PAT structural analogs, and various non-PAT ligands that demonstrate a range of affinities for a novel sigma 3 receptor linked to stimulation of tyrosine hydroxylase and dopamine synthesis in rodent brain. In an effort to develop a ligand-binding model for the sigma 3 receptor, a pharmacophore mapping program (DISCO) was used to identify structural features that are common to ligands that exhibit moderate to high binding affinity for sigma 3 sites. DISCO then was utilized to propose a common pharmacophoric region that included one low-energy conformation of each compound in the training set. The resulting alignment was utilized in a comparative molecular field analysis (CoMFA) study in an attempt to correlate the steric and electrostatic fields of the molecules with the respective binding affinities at the sigma 3 receptor. A suitably predictive model was obtained from the CoMFA analysis which will be employed in the development of additional PAT analogs that could potentially display high affinity and selectivity for the sigma 3 receptor. The excluded volumes which resulted from comparing molecular volumes of active and inactive compounds were visualized to examine the limits of steric tolerance imposed by the sigma 3 receptor.

S-(+)-aporphines are not selective for human D3 dopamine receptors

1. Our aim was to test the hypothesis that selectivity for D3 dopamine (DA) receptors may contribute to limbic anti-DA selectivity of S-(+)-aporphine DA partial agonists. 2. Affinity was tested with 3H-emonapride, using human D3 receptors in mouse fibroblasts and D2 receptors in rat striatal tissue. 3. D3 receptors showed a picomolar affinity for 3H-emonapride, Na+ dependence, and reversible saturability, as well as stereoselectivity. Confirmatory or novel D3/D2 pharmacologic selectivity was found with several benzamides, thioxanthenes, buspirone, GBR-12909, and DA agonists including hydroxyaminotetralins [ADTN, (+)-7-OH-DPAT, (-)-PPHT and its fluorescein derivative], (-)-N-propylnorapomorphine, (-)-3-PPP, (-)-quinpirole, and SDZ-205-502, but neither aminoergoline nor (+)-aporphine partial agonists. 4. The results extend pharmacologic characterization of D3-transfected cell membranes but fail to account for the high limbic anti-DA selectivity of S-(+)-aporphines.

Receptor affinities of aporphine enantiomers in rat brain tissue

R(-) and S(+) enantiomers of apomorphine, N-n-propylnorapomorphine and 11-hydroxy-N-n-propylnorapomorphine were screened for affinity at over 40 representative sites in rat brain tissue that included amine, purine, amino acid and peptide receptors, transporters, ion channels, and effector components; only dopamine receptors and alpha-adrenoceptors showed appreciable affinity that was quantified further. The aporphines showed R(-) > S(+) isomeric selectivity as well as D2 > D1 selectivity at dopamine receptors. While R(-) isomers preferred alpha 2-adrenoceptors, S(+)-aporphines were alpha 1-selective, with similar affinity at alpha 1-adrenoceptors and dopamine D2 receptors. Interactions of S(+)-aporphines at alpha 1-adrenoceptors as well as dopamine D2 receptors may contribute to their unusual behavioral properties.

New sigma-like receptor recognized by novel phenylaminotetralins: ligand binding and functional studies

Several novel phenylaminotetralins (PATs) cause functional changes in brain that are associated with binding to saturable, high affinity sites that are not identical to any known central nervous system receptor. These PATs were tested for their ability to cause receptor-mediated functional effects on tyrosine hydroxylase activity in corpus striatum from rat and guinea pig brain. (+/-)-(trans)-1-Phenyl-3-dimethylamino-6-chloro-7-hydroxy-1,2,3,4- tetrahydronaphthalene (CI, OH-PAT) increased tyrosine hydroxylase activity (by approximately 30-40%) at 0.1 microM. Higher concentrations inhibited enzyme activity by indirect mechanisms that may include displacement of intraneuronal dopamine. The 6,7-unsubstituted congener (+/-)-(trans)-1-phenyl-3-dimethylamino-1,2,3,4- tetrahydronaphthalene stimulated tyrosine hydroxylase by as much as 50-60% over basal activity, without displacement of dopamine. Similarly to certain (+)-benzomorphan sigma receptor ligands, the effects of both PATs to activate tyrosine hydroxylase were blocked competitively by the putative sigma antagonist BMY-14802. Radiolabeled [3H]CI,OH-PAT bound saturably and with high affinity to guinea pig brain membranes (Kd = 31 pm, Bmax = 6.5 fmol/mg of protein). The pharmacological profile of these binding sites was inconsistent with those of known sigma 1, sigma 2, dopaminergic, serotonergic, adrenergic, opioid, N-methyl-D-aspartate, or several other characterized central nervous system recognition sites. Together, these data suggest that these PATs may be agonists at a novel sigma-like site that has neuromodulatory activity that results in increases of brain catecholamine synthesis via activation of tyrosine hydroxylase.

Synthesis and pharmacological evaluation of 1-phenyl-3-amino-1,2,3,4-tetrahydronaphthalenes as ligands for a novel receptor with sigma-like neuromodulatory activity

Certain novel 1-phenyl-3-amino-1,2,3,4-tetrahydronaphthalenes (1-phenyl-3-aminotetralins, PATs) produced stimulation (ca. 30% above basal levels) of tyrosine hydroxylase (TH) activity at 0.1 microM concentrations in rodent brain tissue. This effect on TH was blocked by the putative sigma-receptor antagonist BMY-14802, suggesting involvement of a novel neuromodulatory sigma-like receptor. Within the new phenylaminotetralin series, a correlation was found between the ability to stimulate TH and the potency to compete for binding sites labeled by (+/-)-[3H]1-phenyl-3-(N,N-dimethylamino)-6-chloro-7-hydroxy-1,2,3,4- tetrahydronaphthalene ([3H](+/-)-4). trans-Catechol analogs had low affinity for [3H]4 sites, and although they inhibited TH activity, this effect was not blocked by known sigma or dopamine antagonists. Analogs with dihydroxy substituents (catechols), as well as nitrogen substituents larger than methyl, had little affinity for [3H]4 binding sites and did not significantly affect TH activity. The pharmacology of the [3H]4 binding site is unique from that of any known sigma or dopamine receptor, thus the effects appear to be mediated by a previously uncharacterized binding site/receptor. The site has stereoselectivity for the (1R,3S)-(-)-isomer of 1-phenyl-3-(N,N-dimethylamino)-1,2,3,4-tetrahydronaphthalene; this isomer is also more active at stimulating TH. Thus, certain 1-phenyl-3-amino-1,2,3,4-tetrahydronaphthalenes appear to be selective probes of a novel receptor type that mediates sigma-like neuromodulatory activity and may have pharmacotherapeutic utility in conditions in which modulation of dopamine function is important.

Isomeric selectivity at dopamine D3 receptors

Racemic 7-hydroxy-N,N-dipropylaminotetralin (7-OH-DPAT) shows greater affinity for limbic-selective dopamine D3 receptors than for more ubiquitous dopamine D2 receptors. R(+)-7-OH-DPAT was prepared and evaluated in radioreceptor assays using membranes of fibroblasts expressing the human dopamine D3 receptor as well as rat striatal membranes containing dopamine D2 receptors. This enantiomer had 2-fold greater D3 affinity than the racemate and similarly greater D3 vs. D2 selectivity (64-fold). The results may facilitate development of D3 selective agents and evaluation of functions of these receptors.

Prolonged D2 antidopaminergic activity of alkylating and nonalkylating derivatives of spiperone in rat brain

Alkyl and arylalkyl derivatives of the dopamine (DA) D2 antagonist spiperone were prepared and characterized chemically and pharmacologically. They included the N-methyl, N-phenethyl (NPS), and N-p-aminophenethyl (NAPS) derivatives, as well as the alkylating isothiocyanato (NIPS), bromacetamido, and ethylfumaramido p-substituted N-phenethylspiperones. These compounds showed high lipophilicity (log P up to 6.0 with NIPS), as well as very high in vitro D2 affinity (Ki = 35-280 pM) and D2 versus D1 selectivity (540-9000-fold) in radioreceptor assays with corpus striatum of rat brain. Of the alkylating series, NIPS showed the highest D2 affinity (57 pM) and D2 versus D1 selectivity (2040-fold) and so was selected for further evaluation. NPS, NAPS, and NIPS showed little or no affinity for 34 non-DA binding sites defined by radioligand assays for monoamine, amino acid, and peptide neurotransmitters, ion channels, peptide growth factors, and transmission mediators but did show low alpha 2 and moderate alpha 1 and 5-hydroxytryptamine (5-HT2) affinity with rat forebrain tissue in vitro; NIPS showed a marked gain in D2 versus 5-HT2 selectivity, compared with spiperone (1520- versus 26-fold). Systemic injections of NIPS induced marked decreases in rat striatal D2 binding sites 24 hr later, with little effect on D1, 5-HT2, or alpha 1 sites; NIPS and NAPS lowered apparent Bmax values at D2 receptors with little change in ligand affinity, ex vivo as well as in vitro. NPS, NAPS, and NIPS all induced dose-dependent lowering of D2 binding ex vivo (ID50 = 1-9 mumol/kg, intraperitoneally) and blocked the behavioral effects of the DA agonist apomorphine (0.9 mumol/kg) potently (ID50 = 0.3-0.5 mumol/kg) at 24 hr. Recovery from these anti-DA actions required about 1 week after equimolar (15 mumol/kg) and similarly effective doses of NPS and NAPS, as well as NIPS. Thus, highly selective and avidly bound lipophilic D2 affinity ligands with similarly avid in vitro and prolonged in vivo anti-DA activities can be derived from N-phenethylspiperones with or without an alkylating moiety present. Such affinity ligands may represent useful additions to previously used, generally less selective, D2 affinity ligands.

Interactions of fluoxetine with metabolism of dopamine and serotonin in rat brain regions

Given evidence of inhibitory effects of serotonin on dopaminergic neurotransmission, a series of experiments sought neurochemical evidence of interactions between the selective serotonin transport inhibitor fluoxetine and the metabolism of dopamine (DA) or serotonin (5-HT) in regions of rat brain that might account for extrapyramidal side-effects associated with clinical use of fluoxetine. There were significant inhibitory effects of acute or repeated fluoxetine treatment on the turnover of 5-HT (accumulation of 5-hydroxytryptophan, or ratio of [5-hydroxyindoleacetic acid]/[5-HT]) in striatum, nucleus accumbens and frontal cerebral cortex, but only minor effects on metabolism of DA (accumulation of dihydroxyphenylalanine, or [homovanillic acid]/[DA] ratio), even at high doses or with repeated treatment, and no significant inhibition of the DA metabolism-increasing actions of haloperidol.

Rate of recovery of D1 and D2 dopaminergic receptors in young vs. adult rat striatal tissue following alkylation with ethoxycarbonyl-ethoxy-dihydroquinoline (EEDQ)

The rate of recovery of D1 and D2 receptor binding sites in rat striatal tissue labeled with [3H]SCH-23390 or [3H]YM-09151-2 was followed daily after irreversible blockade of these receptors with the alkylating agent ethoxycarbonyl-ethoxy-dihydroquinoline (EEDQ). These rates were significantly higher in young post-weaning rats than in young adults (14% vs. 7% per day). The findings suggest that rates of synthesis of new D1 and D2 receptor proteins may be increased during a phase of neurodevelopment in the first postnatal month when the accumulation of both receptor types, especially of D1 receptors, is about maximal.

Stereoisomeric probes for the D1 dopamine receptor: synthesis and characterization of R-(+) and S-(-) enantiomers of 3-allyl-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine and its 6-bromo analogue

Substituted 1-phenyl-3-benzazepines (e.g., SKF 38393 and fenoldopam) exhibit stereoselectivity in moderately high-affinity binding to and partial agonist activation of D1 dopamine receptors. The 3-allyl (APB) and the 3-allyl-6-chloro (6-Cl-APB) analogues of SKF 38393 are reported to have higher affinity and selectivity for the D1 DA receptor and higher in vivo central neuropharmacologic activity than SKF 38393. We recently reported the corresponding 3-allyl-6-bromo analogue (6-Br-APB) also to be a high-affinity D1 agonist. We now describe the synthesis and characterization of the R-(+) and S-(-) enantiomers of both APB and 6-Br-APB and their comparison with corresponding enantiomers of SKF 38393 with respect to D1 receptor binding affinity and D1 and D2 selectivity. The R-(+) enantiomers of both novel substituted 1-phenyl-3-benzazepines bound to the D1 receptor sites in rat forebrain tissue with much higher affinity and selectivity than their S-(-) antipodes. R-(+)-3-Allyl-6-bromo-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3- benzazepine [(R)-(+)-6-Br-APB, 18] exhibits the highest affinity of the reported 1-phenyl-3-benzazepine D1 agonists.

R and S enantiomers of 11-hydroxy- and 10,11-dihydroxy-N-allylnoraporphine: synthesis and affinity for dopamine receptors in rat brain tissue

The R-(-)- and S-(+)-enantiomers of 11-hydroxy-N-allyl (4), and 10,11-dihydroxy-N-allyl (3) congeners of 11-hydroxy-N-n-propylnorapomorphine (11-OH-NPa, 2) or N-n-propylnorapomorphine (NPA, 1) were synthesized. Binding affinity of these compounds at dopamine (DA) receptor sites was evaluated with a membrane preparation of corpus striatum from rat brain. The R/S enantiomeric receptor affinity ratio was enhanced by allylic substitution of 3 and 4 and their R isomers had high DA receptor affinity similar to that of the N-n-propyl congeners. These N-allylnoraporphines are proposed as useful precursors to the preparation of their tritiated N-n-propyl enantiomers.

Lack of increase in dopamine transporter binding or function in rat brain tissue after treatment with blockers of neuronal uptake of dopamine

Rats were pretreated daily for 10 days with a dopamine (DA) uptake blocker ([+]amphetamine, benztropine, cocaine, GBR-12909, mazindol, or nomifensine) or control vehicle and, after 1-4 days of no treatment, striatal tissue was fractionated to provide synaptosomes and membranes for assays of transport of 3H-DA or binding of 3H-GBR-12935. There were no significant increases of apparent maxima for uptake (Vmax) or binding (Bmax) or consistent changes in ligand affinity. Pharmacologic characterization of 3H-GBR-12935 binding extended the impression that this ligand has high affinity and selectivity for many agents which block neuronal uptake of DA uptake and much less for those which interact with DA receptors or other amine transporters. The results suggest that dopamine transporters are not regulated in the same way as receptors, nor influenced similarly toward upregulation and supersensitization by repeated treatment with antagonists.

R(-)2-fluoro-N-n-propylnorapomorphine: a very potent and D2-selective dopamine agonist

A series of 2-substituted N-n-propylnorapomorphine (NPA) derivatives were synthesized and compared with other DA agonists for affinity to D1 and D2 dopamine (DA) receptors in rat brain corpus striatum tissue. The 2-substituents tested reduced D1 affinity similarly, but enhanced D2 affinity in the rank order: F greater than OH greater than Br greater than OCH3 greater than H greater than or equal to NH2. The extraordinarily high D2 affinity (Ki = 12 pM) and D2 vs. D1 selectivity (57,500) of 2-F-NPA far-exceeded that of all other DA agonists tested, and it was about 10-times more potent than NPA in vivo.

Effects of isomers of hydroxyaporphines on dopamine metabolism in rat brain regions

The effects of isomers of di- and monohydroxyaporphines on cerebral dopamine (DA) metabolism were evaluated in representative extrapyramidal (corpus striatum) and limbic (nucleus accumbens septi) tissues of rat brain by three methods: (1) changes in the ratio of homovanillic acid (HVA) to DA, (2) accumulation of L-dihydroxyphenylalanine (DOPA) after inhibiting its decarboxylation to DA under "open-loop" conditions, as well as (3) after gamma-butyrolactone (GBL) pretreatment to provide selective effects at presynaptic DA autoreceptors. The DA-agonist R(-) isomers of the aporphines apomorphine (APO), N-n-propylnorapomorphine (NPA), and 11-hydroxy-N-n-propylnoraporphine (11-OH-NPa) showed consistent dose-dependent inhibition of DA synthesis in both brain regions with all models; the neuroleptic haloperidol had the opposite effect in the first two models only, as expected. The S(+) isomers of NPA and 11-OH-NPa have shown behavioral evidence of antidopaminergic activity, especially in the limbic system. Unlike the neuroleptic, S(+)NPA did not show DA-synthesis enhancing actions in accumbens or striatal tissue but, instead, inhibited DA synthesis like its R(-) antipode in all three test paradigms. S(+)11-OH-NPa given alone produced minor changes in the HVA/DA ratio and did not antagonize R(-)11-OH-NPa, weakly increased accumulation of DOPA in the second model, and had no effect in the third--all without regional selectivity. In the test of autoreceptor functioning, the dihydroxyaporphine S(+)NPA, but not S(+)11-OH-NPa, inhibited DA synthesis and this effect, in turn, was largely reversed by haloperidol, as were the inhibitory effects of the three R(-)aporphines tested. In this model, however, neither S(+)NPA nor S(+)11-OH-NPa antagonized the DA-synthesis inhibiting effect of R(-)APO as haloperidol did. Overall, these results are consistent with evidence that R(-)NPA and 11-OH-NPa have high affinity at D-2 receptor sites in rat brain and show behavioral effects of typical DA agonists. The non-stereoselective inhibitory effects of NPA on DA synthesis may reflect its activity as a weak DA agonist with very low intrinsic activity, but may also include a direct "catechol-effect" on tyrosine hydroxylase. In contrast, R(-)11-OH-NPa appears to be a stereoselective D-2 agonist, active at autoreceptors as well as postsynaptic receptors, that lacks the nonstereospecific effects on DA metabolism of its catechol-aporphine congener. It may be a useful probe for the further characterization of dopamine receptors and autoreceptors.

Presynaptic inhibition of dopamine synthesis in rat striatal tissue by enantiomeric mono- and dihydroxyaporphines

Presynaptic autoreceptor-mediated modulation of dopamine (DA) synthesis was evaluated as the inhibition of tyrosine hydroxylase activity by enantiomeric mono- and dihydroxyaporphines with minced striatal tissue from rat brain. The isomers of N-n-propylnorapomorphine (NPA) both inhibited tyrosine hydroxylase activity [IC50 = 0.3 and 1.0 microM for (R)-(-)- and (S)-(+)-NPA, respectively; R/S potency = 3.6]. Their effects were fully blocked by the nonselective DA receptor antagonist fluphenazine, as well as by the D2-selective antagonist spiperone, but not by the D1 antagonist SCH-23390. These results suggest a D2-type autoreceptor-mediated inhibition of DA synthesis, with limited enantiomeric selectivity of this catechol-aporphine. The corresponding monohydroxy analogs, (R)-(-)- and (S)-(+)-11-hydroxy-N-n-propylnoraporphine (11-OH-NPa) were about 100 times less potent (IC50 = 42 and 87 microM, respectively) than the NPA isomers in fully inhibiting the enzyme activity in normal tissue but, after depletion of endogenous DA by acute in vivo pretreatment with reserpine (which did not alter the number of D1 or D2 specific binding sites), (R)-(-)-11-OH-NPa was a highly potent but partial agonist (IC25 = 7 nM). Fluphenazine and spiperone fully antagonized the inhibition of tyrosine hydroxylase by (R)-(-)-11-OH-NPa in reserpinized tissue, but SCH-23390 was ineffective. Actions mediated by endogenous DA probably contribute to the effect of high concentrations of (R)-(-)-11-Oh-NPa to evoke a full inhibition of DA synthesis, but its high potency partial agonist effects appear to be mediated by D2-autoreceptors. (S)-(+)-11-OH-NPa was a very weak partial agonist in reserpinized tissue, with an IC25 = 30 microM (essentially the same as normal tissue); thus, (R)-(-)-11-OH-NPa was greater than 4,000 times more potent than its S-(+)-enantiomer in the absence of endogenous DA. These results demonstrate that NPA, which contains a catechol moiety, acts as a full agonist to inhibit striatal DA synthesis via a presynaptic autoreceptor of the D2 type, with only slight stereoselectivity, and that its monohydroxy analog is a very potent but partial D2 autoreceptor agonist, with very high stereoselectivity.

Synthesis and dopamine receptor affinities of enantiomers of 2-substituted apomorphines and their N-n-propyl analogues

Syntheses of (R)-(-)-2-methoxyapomorphine (R-8), its antipode S-8, and its (R)-(-)-N-n-propyl R-9 derivatives are described. The dopaminergic receptor affinities of these compounds and their 2-unsubstituted counterparts (R)-(-)-apomorphine (R(-)-APO, R-1), (S)-(+)-apomorphine (S(+)-APO, S-1), and (R)-(-)-N-n-propylnorapomorphine (R(-)-NPA, R-2), as well as those of (R)-(-)-2-chloroapomorphine (R(-)-2-Cl-APO, R-6), (R)-(-)-2-bromoapomorphine (R(-)-2-Br-APO, R-6), were determined with tissue membrane preparations of corpus striatum from rat brain. Contribution of both an N-n-propyl and a 2-hydroxy in (R)-(-)-2-hydroxy-N-n-propylnorapomorphine (R(-)-2-OH-NPA, R-7) or a methoxy group in (R)-(-)-2-methoxy-N-n-propylnorapomorphine (R(-)-2-OCH3-NPA, R-9) produced the highest D2 affinity (0.053 and 0.17 nM) and D2 over D1 selectivity (17,300 and 10,500 times) of the compounds evaluated. The structure-affinity relationships of these 2-substituted aporphines suggest that secondary binding sites of D2 receptors interact with 2-substituents on the A ring of aporphines through H-bonding.

Synthesis and structural requirements of N-substituted norapomorphines for affinity and activity at dopamine D-1, D-2, and agonist receptor sites in rat brain

A series of N-substituted analogues of (R)-(-)-norapomorphine were synthesized to study the optimal structural requirements of the N-alkyl side chain to interact with D-1 and D-2 dopaminergic receptors as well as dopamine (DA) agonist binding sites. Evaluations included testing the affinity of these compounds for DA receptor sites in rat striatal tissue and assessing stereotypy as a behavioral index of dopaminergic activity. The electronic, steric, and lipophilic properties of the N-alkyl side chain were found to be related to affinity, D-2 selectivity, and dopaminergic activity. All 11 compounds evaluated had relatively low affinity at D-1 sites. Optimum D-2 and agonist-site affinity as well as agonist activity were exhibited by N-cyclopropylmethyl (7) greater than or equal to N-allyl (8) greater than or equal to N-propyl (4) or N-ethyl (3) substituted compounds. Branching of the N-alkyl side chain as in N-isopropyl (5) and N-isobutyl (6) markedly reduced the D-2 affinity and activity, presumably due to steric effects. The N-trifluoroethyl (10) and N-pentafluoropropyl (11) derivatives had low affinity for all their dopamine receptor sites and no agonistic activity; evidently, the highly electronegative F atoms decrease basicity of the N atom and therefore decrease the ability of the N atom to be cationic at physiological pH, a proposed requirement for high-affinity binding to DA receptors.

Bromocriptine antagonizes behavioral effects of cocaine in the rat

Rats given cocaine or bromocriptine under conditions of low basal arousal showed dose-dependent increases of locomotor activity for less than or equal to 1 hours, followed by depression of activity that diminished gradually over the next 2 hours. Arousal was related biphasically to dose (maximum at ca. 5 mg/kg) but depression increased monophasically with the dose of either agent. Both behavioral arousal and depression induced by cocaine were antagonized by bromocriptine, even at doses lacking behavioral effects alone (ID50 = 1.0 and 0.36 mg/kg [1.3 and 0.5 mumol/kg], respectively). Bromocriptine blocked depression of locomotion even when given after the initial stimulation by cocaine. Bromocriptine induced very weak stereotypy, and neither increased nor blocked stereotypy induced by cocaine or apomorphine. Cocaine, at maximally effective doses, did not deplete catecholamines or serotonin in brain regions at times of maximum behavioral arousal or depression, nor did bromocriptine after metabolic turnover of dopamine. Bromocriptine antagonized arousal induced by direct injection of dopamine into the nucleus accumbens. The ability of bromocriptine to block both the behavioral arousal and depression induced by cocaine may reflect activity of bromocriptine as a mixed agonist-antagonist with limited intrinsic agonistic activity at central dopaminergic D2 receptors, perhaps with particular reference to limbic mechanisms.

Pharmacology of binding of 3H-SCH-23390 to D-1 dopaminergic receptor sites in rat striatal tissue

3H-SCH-23390, a selective antagonist of D-1 dopamine (DA) receptors, was used in a radioreceptor assay with rat brain striatal tissue, optimized biochemically, and extensively characterized pharmacologically with striatal membranes. Nonspecific binding, defined with excess cis(Z)-flupenthixol (300 nM), averaged 20-25% of total counts bound. Specific binding was linearly dependent on the amount of original striatal tissue (0-4 mg) or protein (0-250 micrograms), temperature dependent, saturable and reversible, and appeared to involve a single site at ligand concentrations limited to less than 10 nM. Binding in rat brain regions ranked as: striatum greater than accumbens greater than prefrontal cortex greater than posterior cerebral cortex greater than cerebellum. Association was virtually complete within 30 min at 30 degrees, and the rate of dissociation at 30 degrees was 0.0377 min-1 (half-time = 18.4 min). Affinity (Ka or Kd) determined from association and dissociation rate constants and from concentration isotherms averaged 0.349 and 0.340 nM respectively. Including Na+ at 150 mM increased apparent maximum specific binding (Bmax) by 19%, with a 29% increase in affinity; other monovalent cations alone had small effects on specific binding; Ca2+ and Mg2+ reduced binding by 42%. Agents (N = 85) were tested for potency (Ki or IC50) in competition with the ligand (at 0.30 nM). Those known to have selective effects at D-1 receptors, generally, were most potent and stereoselective. Na+ (150 mM) had little effect on the affinity of cis-thioxanthenes but decreased that of most other agents tested with high D-1 affinity. For antipsychotic agents, the correlation of typical clinical daily doses versus Ki at D-1 sites (r = 0.06) was much lower than at D-2 sites (r = 0.94). (-)Thioridazine was discovered to be D-1 selective, whereas the (+) enantiomer was selective for D-2 sites labeled with 3H-spiperone. Relatively sedating antidepressants had greater D-1 affinity than their less-sedating, secondary amine congeners.

Determination of N-n-propylnorapomorphine in serum and brain tissue by gas chromatography-negative ion chemical ionization mass spectrometry

A method for the determination of the neuroactive compound N-n-propylnorapomorphine (NPA) in biological tissues is described. Isolation of NPA from serum or brain tissue was achieved via liquid-liquid extraction from phosphate-buffered tissue extract (0.25 M, pH 7.2) into ethyl acetate. The NPA, along with a [2H7]NPA analogue serving as internal standard, was converted to the corresponding bis(trifluoroacetyl) ester by treatment with excess trifluoroacetic anhydride at 75 degrees C. The electrophoric derivatives were analyzed by fused-silica capillary gas chromatography-mass spectrometry in the negative ion chemical ionization mode. Selected ion monitoring of the [M-CF3CO]- ions of derivatized NPA (m/z 390) and internal standard [2H7]NPA (m/z 397) permitted the quantitation of NPA in serum and brain samples obtained from rats treated with either free NPA or the prodrug methylenedioxy-NPA (MDO-NPA). Calibration was conducted down to a practical limit of assay sensitivity, at 0.50 ng NPA per ml of serum and 0.50 ng NPA per g of brain. The relative standard deviation for replicate serum samples spiked at 20 ng/ml was 4.2% (n = 5) and for brain samples at 10 ng/g, it was 3.6%. This method revealed differences in the free NPA brain/serum ratios in rats treated separately with the stereoisomers R-(-)-MDO-NPA and S-(+)-MDO-NPA.

Synthesis and dopamine agonist and antagonist effects of (R)-(-)- and (S)-(+)-11-hydroxy-N-n-propylnoraporphine

The R-(-) and S-(+) enantiomers of 11-hydroxy-N-n-propylnoraporphine, (R)-3 and (S)-3, were synthesized in six steps from 1-(3-methoxy-2-nitrobenzyl)isoquinoline. Neuropharmacological evaluation of the R and S isomers (by affinity to dopamine receptor sites in rat brain tissues, induction of stereotyped behavior, and interaction with motor arousal induced by (R)-apomorphine in the rat) indicated that, similar to the 10,11-dihydroxy congener 2, both enantiomers can bind to dopamine receptors but that only (R)-3 activates them, whereas (S)-3 shows activity as a dopaminergic antagonist.

Effects of cations on high-affinity binding of 3H-ADTN to dopaminergic sites in calf caudate tissue

Effects of cations on binding of 0.1-10 nM 3H-ADTN to calf caudate membranes included decreased apparent Bmax by [Na+] greater than or equal to 100 mM, little effect on Kd or on affinity of other dopamine (DA) agonists (DA and apomorphine), decreased slopes of inhibition curves produced by agonists, but increased affinity of the antagonists (+)butaclamol; in contrast, low (10-20 mM) [Na+] did not decrease Bmax, increased ligand and agonist affinity and specific binding, and gave steep monophasic inhibition curves for DA agonists. K+, Li+, and Rb+ had little effect at a wide range of concentrations. Mg++ and Ca++, in physiologic concentrations, moderately increased binding of 3H-ADTN, as did microM Mn++ or Co++; the latter ions inhibited binding at greater than or equal to 10 mM, as did Cu++ (IC50 = 10 microM). The results extend impressions that physiologic [Na+] favors binding of DA antagonists and diminishes binding of agonists, but optimal agonist binding occurred at low [Na+] (10-20 mM), while divalent cations had complex actions.

S(+)methylenedioxy-N-n-propylnoraporphine: an orally active inhibitor of dopamine selective for rat limbic system

The 10-11-methylenedioxy (MDO) derivative of S(+)N-n-propylnorapomorphine (NPA) was prepared and tested as a possible active prodrug to S(+)NPA, which we have recently found to exert in vivo activity suggestive of selective antagonism of dopamine receptors in the limbic forebrain but not the extrapyramidal basal ganglia. Like S(+)NPA, S(+)MDO-NPA inhibited the behavioral arousal induced by dopamine injected into nucleus accumbens of the rat, but not the head-turning response to dopamine injected into the corpus striatum. However, only MDO-NPA was orally active and it was somewhat longer-acting than NPA. The activity of S(+)MDO-NPA was prevented by pretreatment with the oxidase inhibitor SKF-525A. These properties are analogous to those of R(-)MDO-NPA, which we had previously reported as an orally active prodrug of the dopamine agonist R(-)NPA. Thus the methylenedioxy derivatives of the two entantiomers of NPA have properties desirable in a potentially clinically useful dopamine agonist and limbic dopamine antagonist, respectively.

Synthetic analgesics and other phenylpiperidines: effects on uptake and storage of dopamine and other monoamines mouse forebrain tissue

The neurotoxin N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) can induce degeneration of dopamine (DA) and other central monoamine neurons, leading to Parkinson's disease-like effects in man, monkey, and mouse. MPTP and other substituted phenylpiperidines related to synthetic analgesics including alphaprodine and meperidine were evaluated for potency vs. uptake of 0.1 microM tritiated DA, norepinephrine (NE), or serotonin (5HT) in synaptosomal preparations of mouse striatum or cerebral cortex. The most potent inhibitor of the uptake of 3H-DA was N-methyl-4-phenylpyridinium ion (MPP+; IC50 = 1 microM, Ki = 0.4 microM), a metabolite of MPTP; its effect was competitive and reversible. Other analogs of MPTP: the N-ethylindole AHR-1709, N,N-dimethyl-MPTP, and N-methyl-4-phenylpiperidine were all more potent than MPTP against 3H-DA uptake. N-dealkylation and N-propyl substitution, as well as pyridine ring substitution, decreased affinity for DA uptake while 3',4'-dihydroxyphenyl substitution increased potency and selectivity for catecholamine uptake, and quarternarization of the pyridine ring also increased potency against DA uptake. Active compounds showed higher potency against the uptake of NE than of DA. MPP+ was also more potent than MPTP in releasing endogenous DA from striatal synaptosomes (EC50 = 3 vs. 30 microM), but did not release the cytoplasmic markers tyrosine hydroxylase and lactate dehydrogenase (LDH). In contrast to MPTP, synthetic phenylpiperidine analgesics, their potential metabolites and the experimental neuroleptic agent AHR-1709 all failed to deplete striatal DA in vivo, even if active in vitro against DA uptake.

Effects of isomers of apomorphines on dopamine receptors in striatal and limbic tissue of rat brain

The optical isomers of apomorphine (APO) and N-propylnorapomorphine (NPA) were interacted with three biochemical indices of dopamine (DA) receptors in extrapyramidal and limbic preparations of rat brain tissue. There were consistent isomeric preferences for the R(-) configuration of both DA analogs in stimulating adenylate cyclase (D-1 sites) and in competing for high affinity binding of 3H-spiroperidol (D-2 sites) and of 3H-ADTN (DA agonist binding sites) in striatal tissue, with lesser isomeric differences in the limbic tissue. The S(+) apomorphines did not inhibit stimulation of adenylate cyclase by DA. The tendency for greater activity or higher apparent affinity of R(-) apomorphines in striatum may reflect the evidently greater abundance of receptor sites in that region. There were only small regional differences in interactions of the apomorphine isomers with all three receptor sites, except for a strong preference of (-)NPA for striatal D-2 sites. These results do not parallel our recent observations indicating potent and selective antidopaminergic actions of S(+) apomorphines in the rat limbic system. They suggest caution in assuming close parallels between current biochemical and functional, especially behavioral, methods of evaluating dopamine receptors of mammalian brain.

Prolonged antidopaminergic actions of single doses of butyrophenones in the rat

Rats were treated once with doses of haloperidol or of droperidol below and above the acute ID50 vs the dopamine agonist apomorphine; they were later challenged with an acute dose of apomorphine (0.3 mg/kg, SC) and rated for stereotyped behavioral responses. The two neuroleptics were similar in acute anti-apomorphine potency (ID50 = 0.12 and 0.18 mg/kg for haloperidol and droperidol, respectively). The antidopaminergic effects of droperidol persisted for nearly 1 week and those of haloperidol lasted for 20-40 days, depending on the dose given. The computed half-time of disappearance of their antidopaminergic effects was 7.6 +/- 1.0 days and 0.59 +/- 0.17 days for haloperidol and droperidol, respectively, following a dose of 0.3 mg/kg, and these indices of duration of action did not vary significantly at doses between 0.1 and 1.0 mg/kg. Haloperidol reduced the acute entry of 3H-apomorphine into brain by 21.5% 1 week later. Treatment with apomorphine alone just prior to haloperidol (both at 0.3 mg/kg) prevented the prolonged antidopaminergic effects of the neuroleptic evaluated 1 week later. These results indicate that some neuroleptics may have very prolonged activity or retention in tissue at sites of action, even after moderate, single doses. Caution is recommended in the interpretation of studies which assume "neuroleptic-free" conditions of subjects previously exposed to a neuroleptic agent.