Differential effects of typical and atypical neuroleptics on alpha-noradrenergic and dopaminergic post synaptic receptors.

Mezilamine (2-methylamino-4-N-methylpiperazino-5-methylthio-6-chloropyrimidine), a new antidopaminergic agent, is more effective, in vitro and in vivo, in competing for the binding of [³H]-haloperidol in rat tuberculum olfactorium than in rat striatum.Such effects, similar to atypical neuroleptics (clozapine, sulpiride), are opposite to classical neuroleptics like chlorpromazine or UK 177 (2-benzilamino-4-N-methylpiperazino-5-methylthio-6-chloropyrimidine). Mezilamine displaces 10 times more [³H]-clonidine binding than [³H]-WB 4101 binding in rat cerebral cortex and such a preference for α-agonist rather than antagonist sites is also opposite to most neuroleptics, including UK 177.A combination of phenoxybenzamine and mezilamine causes catalepsy and produces a preferential acceleration of striatal dopamine turnover whereas mezilamine alone is more effective on the mesolimbic system. Such results suggest that α-agonist activity in combination with antidopaminergic properties may limit the extrapyramidal effects and induce a selective neuroleptic action in the mesolimbic system.     

EFFECT OF THYROID STATUS ON β-ADRENORECEPTORS AND MUSCARINIC RECEPTORS IN THE RAT LUNG

• 1 The effects of thyroid status on the specific binding of the muscarinic ligand (–)-[³H] quinuclidinyl benzilate (QNB) and of the β-adrenoreceptor ligand (–)-[³H] dihydroalprenolol (DHA) in the adult rat lung were investigated.
• 2 The specific binding of (–)-[³H] quinuclidinyl benzilate (QNB) to lung membranes was saturable and the equilibrium dissociation constant (K_D) determined from Scatchard analysis was 54 pM. Kinetic analysis of the binding of [³H] QNB yielded a K_D of 42 pM. [³H] QNB binding was inhibited by muscarinic agonists and antagonists, the order of their potency was l-hyoscyamine>atropine>scopolamine>oxotremorine>carbachol. These data were consistent with [³H] QNB binding to the muscarinic receptor.
• 3 Adult male rats treated for 2 weeks with the antithyroid agent 3-amino-1,2,4-triazole (ATZ) showed a 52% and 80% reduction in the serum concentration of triiodothyronine (T₃) and thyroxine (T₄) respectively. These hypothyroid rats also had a 39% decrease in the concentration of lung β-adrenoreceptors and a 37% decrease in the concentration of lung muscarinic receptors as compared to euthyroid controls. Concurrent treatment of rats with ATZ and T₄ for 2 weeks resulted in a reduction of 15% and 20% in the concentration of lung β-adrenoreceptors and muscarinic receptors respectively. The K_D values for [³H] DHA and [³H] QNB binding did not change with the ATZ or ATZ + T₄ treated groups.
• 4 Administration of T₄ (500 μg/kg/day) to male rats for 12 days did not result in any significant change in the concentration of either β-adrenoreceptors or muscarinic receptors compared to euthyroid controls. No change in the K_K values for [³H] DHA or [³H] QNB binding were detected.
• 5 The results show that hypothyroid rats have a reduced lung concentration of both β-adrenoreceptors and muscarinic receptors whereas in hyperthyroid rats these receptors do not significantly change from euthyroid controls.

     

Binding of [3H]quinuclidinyl benzilate to intestinal mucus: An artifact in identification of epithelial cell muscarinic receptors

The widely used muscarinic receptor ligand [³H]quinuclidinyl benzilate ([³H]QNB) was found to bind in a site-specific but artifactual manner to rat intestinal mucus, obscuring specific binding to muscarinic receptors on intestinal epithelial cells. Atropine inhibited [³H]QNB binding to mucus with an apparent IC₅₀ of 2.1 × 10^?7 M, compared to an IC₅₀ of 1.4 × 10^?8 M obtained with a homogenate of intestinal epithelial cells. Unlabeled QNB also inhibited binding of [³H]QNB to mucus but the apparent IC₅₀(4 × 10^?7 M) was about 300-fold greater than the IC₅₀ determined with a control tissue, heart muscle (IC₅₀, 1.2 × 10^?9M). [³H]QNB binding was saturable over the concentration range of 1–7 nM in the heart, with an apparent k_D of 0.76 nM. As expected from the high IC₅₀ for QNB in the mucus binding experiments, binding to mucus was not saturable over the 1–15 nM concentration range. Based on pH profiles and temperature dependency of binding, it seems unlikely that mucin, the primary component of mucus, was responsible for [³H]QNB binding to the mucus. The findings have implications for studies which involve binding of [³H]QNB in particular and other ligands in general to mucus-secreting epithelial tissues.     

Kainic acid lesions of striatum and decortication reduce specific [3H]sulpiride binding in rats,so D-2 receptors exist post-synaptically on corticostriate afferents and striatal neurons

Unilateral kainic acid lesions of rat striatum reduced specific striatal [³H]spiperone and [³H]sulpiride binding sites (Bmax) by 52 and 67% respectively compared with the intact side. The dissociation constant (K_D) for [³H]spiperone binding was unchanged but that for [³H]]sulpiride binding was reduced. Specific striatal [³H]spiperone and [³H]sulpiride binding was reduced by 22 and 37% respectively in unilateral decorticate animals, but there was no change in K_D. Unilateral 6-hydroxydopamine lesions of the medial forebrain bundle caused no change in striatal [³H]spiperone binding sites or K_D value, but produced a 27% increase in [³H]sulpiride binding sites with no change in K_D. These data support the hypothesis of D-2 receptors located on cortico-striate glutamate fibres, but also indicate the presence of both D-1 and D-2 receptors on the cell bodies of striatal neurons.     

BIOCHEMICAL CHARACTERISTICS OF MUSCARINIC CHOLINORECEPTORS IN SWINE TRACHEAL SMOOTH MUSCLE

• 1 The tritiated muscarinic cholinoreceptor antagonist quinuclidinyl benzilate, [³H]QNB, was used to characterize the muscarinic receptors associated with homogenized membrane of the smooth muscle from swine trachea. Based on receptor binding assays, the homogenate had specific, saturable, high-affinity receptors for [³H]QNB.
• 2 Specific binding was time- and temperature-dependent. The association of [³H]QNB with the muscarinic receptor reached equilibrium much sooner at 37°C than 25°C at a [³H]QNB concentration of 180 pM (30 min and 2 h, respectively). Equilibrium at both temperatures was attained within 5 min at a [³H]QNB concentration of 1800 pM. All remaining experiments were performed at 37°C.
• 3 Binding was saturable with respect to [³H]QNB and tissue concentrations. Analysis of binding isotherms yielded an apparent equilibrium dissociation constant (K_D) of 51±20 pM and a maximum receptor density (B_max) of 2.17±0.27 pmole/mg protein. The Hill coefficient for [³H]QNB binding was 1.07±0.16. The association (K₁) and dissociation (K_-1) rate constants were determined to be (5.51±0.16) × 10⁸ M^?1 min^?1 and (1.41±0.18) × 10^?2 min^?1, respectively. K_D calculated from the ratio of K₁ and K_-1 was 26.3±3.8 pM; this value is close to the value of K_D calculated from Scatchard plots of binding isotherms.
• 4 The density of muscarinic receptor binding sites was 10-fold greater in tracheal smooth muscle than in tracheal epithelium (0.20±0.03 pmole/mg protein). There is no difference between weanling and young adult swine in the density of muscarinic receptors in tracheal smooth muscle.
• 5 The nonselective muscarinic antagonists atropine, scopolamine and quinuclidinyl benzilate (QNB) competitively inhibited [³H]QNB binding to the homogenate with Hill coefficients of 0.9-1.0 and inhibition constants (K_i) of nanomolar range.
• 6 Competition with selective muscarinic antagonists pirenzepine and 3-quinuclidinyl xanthene-9-carboxylate (QNX) gave K_i values, 0.26 M and 0.78 nM, respectively, and Hill coefficients of approximately 1. There was a single population of [³H]QNB binding sites of the M₂ subtype for all tested muscarinic antagonists.
• 7 Competition with selective muscarinic agonists pilocarpine and carbachol yielded K_i values of micromolar range, Hill coefficients of less than 1, and revealed the existence of two binding sites (P < 0.01).

     

Irreversible blockade of striatal dopamine receptors in vivo by a derivative of α-flupenthixol

Flupenthixyl chloride (FPT-Cl), a derivative of the dopamine (DA) receptor antagonist and neuroleptic α-flupenthixol possessing a cloroethyl side chain, has been prepared and evaluated for use in vivo in affinity labeling of DA receptors. Binding of [³H]spiperone to rat striatal DA receptors was markedly altered up to 12 h after intraventricular injection of FPT-Cl as compared with controls, while Scatchard plots of [³H]spiperone binding obtained on rat striatal homogenates 24 and 48 h after injection of FPT-Cl had values of B_max significantly lower than in controls. The results suggest that the administration of FPT-Cl leads to irreversible and possibly covalent blockade of a portion of the spiperone binding sites in rat striatum. A second population of receptors appears to be blocked reversibly and presumably noncovalently by FPT-Cl, and these spiperone binding sites are partially reactivated in vivo after 48 h.     

Selective blockade of dopamine D-1 receptors by SCH 23390 discloses striatal dopamine D-2 receptors mediating the inhibition of adenylate cyclase in rats

l-Glutamate but not methyl-D-aspartate (NMDA) or quisqualate (Quis) (10^?6 M) in vitro with or without preincubation increased significantly the K_D value of the [³H]N-propylnorapomorphine ([³H]NPA) binding sites by 21 and 36% respectively in striatal membranes of rat without influencing the striatal [³H]spiperone binding sites. The number of striatal [³H]NPA binding sites was not changed by l-glutamate (10^?6 and 10^?5 M) in vitro. There may thus exist interactions between striatal glutamate receptors — not related to excitatory amino-acid receptors of the NMDA or the QUIS type - and high affinity striatal DA receptors.     

Properties of the muscarinic cholinergic receptors in rat atrium

Summary 1. Properties of the muscarinic cholinergic receptor sites in the rat atrial homogenate and microsomal fraction were studied by the use of tritium labelled 3-quinuclidinyl benzilate ([³H]-QNB), a potent muscarinic antagonist. 2. The specific [³H]-QNB binding to the receptor sites displayed saturability, stereospecificity as well as reversibility. 3. The competition studies showed that muscarinic antagonists were more potent than muscarinic agonists. 4. Certain neuromuscular blocking agents, antipsychotics, antiarrhythmics and antihistamines also were capable of interacting with the [³H]-QNB binding sites. However, - and -adrenergic agents, calcium antagonist (D-600) and ionophore (A-23187) failed to show any effect. 5. Analyses by the double reciprocal plot, Hill plot and Scatchard plot of the dependence of the specific [³H]-QNB binding on the concentration of QNB suggested that binding was occurring to a single population of receptor sites in the atrial homogenate or microsomal fraction. Further, there was no evidence of any detectable site to site interactions (positive or negative cooperative type). From the Scatchard plot, the equilibrium dissociation constant (K_D) of 1.1 nM was calculated and the Hill coefficient was close to 1.0 6. Interaction of the muscarinic antagonists with the [³H]-QNB sites showed the Hill coefficients close to 1.0 whereas for the agonists, the coefficients were much less than 1.0 indicating that agonist-receptor site interactions have some different characteristics from those following antagonist-receptor site interaction. 7. The rate and the maximal level of QNB binding to the receptor sites was markedly influenced by the temperature; various cations, on the other hand, displayed no effect either on the association or dissociation of QNB binding. The specific QNB binding exhibited a broad pH optimum from pH 6.0–8.5. 8. Treatment of the membrane fraction (or homogenate) with either phospholipase A or C and with p-chloromercuribenzoate caused significant inhibition of [³H]-QNB binding. 9. The QNB binding site was resistant to tryptic digestion. Even when about 40% of the membrane proteins were removed by the tryptic proteolysis, the [³H]-QNB binding ability of the membrane remained unaffected; in fact, the removal of tryptic proteolytic products by centrifugation markedly increased the specific QNB binding to the membrane.     

The influence of guanyl-5′-yl imidodiphosphate and sodium chloride on the binding of the muscarinic agonist, [3H] cis methyldioxolane

[³H]Quinuclydinyl benzylate([³H]QNB) binding was carried out on crude synaptosomal membranes isolated from cat cerebral cortex. The specific binding showed a single type of site with K_D 0.25 nM, Hill number 0.89 and B_max 114 pmol/g protein. The local anesthetics procaine, tetracaine, and the adrenergic antagonists phentolamine and propranolol, in concentrations between 1 nM and 500 μM, inhibited [³H]QNB binding with K_i varying between 9 μM for procaine and 80 μM for propranolol. The Hill coefficients obtained from logit/log plots suggested that there was no cooperativity between the binding sites for local anesthetics. At various concentrations the inhibition by procaine and propranolol may appear as competitive or non-competitive. The Hill numbers obtained from the saturation curves suggest that there was no cooperativity between anesthetics and [³H]QNB binding sites. Neither 1 mM Ca²⁺ nor Mg²⁺ affected [³H]QNB binding or the action of the drugs. The effect of local anesthetics and adrenergic antagonists was reversible and these drugs did not protect the muscarinic receptor from the deleterious effect of Triton X-100 as was the case with muscarinic agents. Our findings suggest that local anesthetics inhibit [³H]QNB binding to the muscarinic receptor by acting at some accessory site but not on the true receptor site. The possible mechanism of action of local anesthetics on synaptic transmission is discussed.     

Binding studies with [3H]cis-methyldioxolane in different tissues

Summary Special conditions - tricine buffer containing Ca²⁺ and Mg²⁺, 22°C (TCM) — allow to label a much higher proportion of muscarinic receptors by [³H]cis-methyldioxolane (CD) than hitherto described (Vickroy et al. 1984 a). Taking the maximum number of binding sites, B _max, of [³H]QNB as 100%, B _max of [³H]CD amounts to 83% in the rat heart instead of the reported 17%, 33% in the cerebral cortex instead of 6%, 20% in hippocampus and 55% in pons/medulla. In the salivary glands specific binding was negligible. The affinities of a number of muscarinic agonists and antagonists to [³H]CD and [³H]QNB binding sites in different tissues of the rat are compared. Apparent affinities of agonists are much higher in the [³H]CD system, affinities of antagonists are slightly higher in the [³H]QNB system. In both assay systems receptors of heart and pons/ medulla membranes seem to have similar drug specificity. They differ somewhat from those in the cortex. Receptors in the salivary glands, however, seem to be completely different from those in the other three tissues. In the heart [³H]CD binding can be abolished almost completely by GppNHp. In the cortex about half of the [³H]CD binding is susceptible to GppNHp. The reduction of binding in the cortex is due to a change in B _max and not in the dissociation constant K _D. Competition of unlabelled pirenzepine with [³H]CD: In heart and pons/medulla only low affinity sites for pirenzepine (M₂-receptors) are labelled by [³H]CD. In regions rich in M₁ receptors like hippocampus (80% M₁ receptors) or cortex (65–70% M₁ receptors) the proportion of M₁ receptors labelled by [³H]CD is smaller than expected considering the concentration of M₁ receptors present in these tissues. Thus [³H]CD, under the conditions described in this paper, seems to label preferentially but not exclusively M₂ receptors in their agonist high affinity form. Send offprint requests to A. Closse at the above address     

Reduced [H]quinuclidinyl benzilate binding to muscarinic receptors in disulfoton-tolerant mice

The organophosphate, acetylcholinesterase inhibitor, disulfoton, O,O-diethyl S-[2-(ethylthio)ethyl]phosphorodithioate, was given daily for 2 weeks to male mice at two different dosages. Clinical signs of poisoning disappeared in 5 days after the beginning of the treatment, i.e., the animals developed apparent tolerance to disulfoton toxicity. Tolerant mice were less sensitive to a lethal dose of carbachol and exhibited a decrease of [³H]quinuclidinyl benzilate ([³H]QNB) binding in forebrain, hindbrain, and ileum. Scatchard analysis of saturation experiments revealed a decrease in the density of receptors (B_max) in the disulfoton-treated mice, as compared with controls. No significant changes in affinity were found, except in the ileum. A time-course study showed a good parallelism between the decrease of [³H]QNB binding and the development of tolerance. Twenty-one days after the end of the disulfoton treatment AChE activity was still inhibited, but [³H]QNB binding had returned to normal levels. The recovery of [³H]QNB binding appears to be faster in ileum than in forebrain and hindbrain. These findings indicate that the development of tolerance to chronic organophosphate treatment is, at least partially, due to a reduction in the number of cholinergic receptors.     

Interactions of D600 (methoxyverapamil) and local anesthetics with rat brain α-adrenergic and muscarinic receptors

D600 (methoxyverapamil) was found to inhibit the specific binding assayed in rat brain homogenates of the antagonist agents [³H]WB 4101 and [³H]QNB to the α-adrenergic and muscarinic receptors respectively. The IC₅₀ concentrations of D600 in standard binding experiments were 1.7 × 10_?6 M and 1.4 × 10 _?5M, with calculated k₁ values of 0.98 × 10_?6 M and 8.83 × 10_?6M.Scatchard analyses showed these inhibitions to be competitive. Lidocaine and tetracaine also inhibited radioligand binding to these receptors, with K₁ values of 5.25 × 10^?4M and 4.85 × 10^?5 M for the α-receptor and 8.2 × 10^t-5 M and 6.94 × 10^?6 M for the muscarinic receptor; these inhibitions also appeared to be competitive. Increasing the Ca²⁺ concentration in the assays to 10 mM did not influence the effects of D600 or the anesthetics. Analyses of inhibitions of muscarinic receptor binding produced by D600 and lidocaine over a range of pH indicated that the inhibitory species of D600 is the uncharged form, whereas the charged form of lidocaine is inhibitory. Interactions of D600 and lidocaine with the agonist site on the muscarinic receptor were studied by measuring the effects of these agents on the displacement of [³H]QNB by the muscarinic agonist carbachol. Comparison of these results with a theoretical model indicates that carbachol, [³H]QNB, and D600 or lidocaine competitively displace one another at the same agonist site. The binding of labeled naloxone to the opiate receptor was also inhibited by D600, the IC₅₀g being 4 × 10^t?6 M. These inhibitory effects of D600 and the local anesthetics on different receptors suggest that these agents may act by a common mechanism, namely by perturbing membrane structures. These results suggest caution in interpreting experiments in which D600 and verapamil are used analytically as Ca antagonists to assess the involvement of Ca in a biological system.     

Persistent increase in striatal dopamine stimulated adenylate cyclase activity persists for more than 6 months but disappears after 1 year following withdrawal from 18 months cis-flupenthixol intake

Administration of cis-flupenthixol to rats for 18 months enhanced apomorphine-induced stereotyped behaviour, increased the number of specific [³H]spiperone binding sites in striatum and potentiated striatal dopamine stimulated cyclic AMP formation, but did not alter specific [³H]piflutixol binding. Following withdrawal of cis-flupenthixol intake, apomorphine-induced stereotypy returned to control values after 1 month and B_max for [³H]spiperone binding returned to normal after 3 months. In contrast, the increased dopamine stimulated adenylate cyclase activity remained elevated 6 months after drug removal, but was normal 1 year after drug withdrawal.     

3H]Quinuclidinyl benzilate binding to muscarinic receptors and [3H]WB-4101 binding to alpha-adrenergic receptors in rabbit iris. Comparison of results in slices and microsomal fractions

The binding characteristics of [³H]-(l)-quinuclidinyl benzilate (QNB) and [³H]WB-4101 to microsomal fractions and slices from rabbit iris muscle were compared. [³H] QNB binding to both microsomal fractions and muscle slices was of high affinity and low capacity and was displaced by muscarinic ligands. The equilibrium dissociation constants (K_D) for [³H]QNB binding to microsomes and slices were 0.069 nM and 1.97nM, respectively. This shift to a higher value for the K_p of the microsomal fraction compared with that of the slices was also observed lor the association rate constants (K_I) and inhibition constants (K_J), but not for the dissociation rate constants (K_?1). Kinetic studies on the binding characteristics of [³H]WB-4101 revealed high affinity sites with K_D values of 2.33 and 10.19 nM for microsomal fractions and slices, respectively. The findings of comparable binding patterns for [³H]QNB and [³H]WB-4101 binding to microsomal fractions and intact muscle slices argue against the possibility of alterations in receptor properties following tissue disruption. It is proposed that the differences in receptor-mediated biochemical responses that are seen between intact tissue and cell-free homogenates, such as the ‘phosphoinositide effect’, are more likely to be due to alterations in receptor function, e.g. changes in ionic permeabilities, rather than to actual changes in receptor properties.     

Interaction of antihistaminics with muscarinic receptor (III)

The muscarinic antagonist 1-[benzilic 4,4′-³H]quinuclidinyl benzilate ([³H] QNB) bound to a single class of muscarinic receptors with high affinity in rabbit ileal membranes. The K _D and B _max values for [³H]QNB calculated from analysis of saturation isotherms were 52.5 pM and 154 fmol/mg, respectively. Chlorpheniramine (CHP), histamine H₁ blocker, increased K _D value for [³H]QNB without affecting the binding site concentrations and Hill coefficient. The K _i value of CHP for inhibition of [³H]QNB binding in ileal membranes was 1.44μM and the pseudo-Hill coefficient for CHP was close to unit. In the functional assay carbachol, muscarinic agonist, increased the contractile force of ileum with ED₅₀ value of 0.11μM. CHP caused the rightward shift of the dose-response curve to carbachol. The pA₂ value of CHP determined from Schild analysis of carbachol-induced contraction was 5.77 and the slope was unity indicating competitive antagonism with carbachol. The dissociation constant (K _i ) of CHP obtained in competitive experiments with [³H]QNB was similar to the K _A value (1.69μM) of CHP as inhibitor of carbachol-induced contraction in rabbit ileum. This result suggests that the binding of H₁ blocker, CHP, vs [³H]QNB to muscarinic receptors in ileal membranes represents an interaction with a receptor of physiological relevance.     

Decrease in muscarinic cholinergic response of the rat heart following treatment with 6-hydroxydopa

Pretreatment with 6-hydroxydopa (6-OHDOPA) at birth produced a decrease in the number of [³H]quinuclidinyl benzilate ([³H]QNB) binding sites in adult rat heart homogenates. The treatment caused hyposensitivity to acetylcholine (ACh) but did not alter the maximal negative inotropic action of ACh in isolated atria of the rat. These results suggest that 6-OHDOPA affects the negative inotropic response to ACh by modifying the receptor number or through an effect on a step between receptor activation and biological response.     

Phencyclidine binding sites in the nucleus accumbens and phencyclidine-induced hyperactivity are decreased following lesions of the mesolimbic dopamine system

[3H]Phencyclidine [( 3H]PCP) binding to rat nucleus accumbens, hippocampal and striatal membranes, and PCP-induced locomotor hyperactivity were assessed following selective lesions of the mesolimbic dopaminergic system. 6-Hydroxydopamine (6-OHDA) injections into the A10 region of the ventral tegmental area or into the accumbens itself resulted in a blockade of PCP's stimulatory effects and a highly significant reduction in the number of [3H]PCP binding sites and dopamine content of the nucleus accumbens. However, destruction of the dopaminergic mesolimbic fibers did not significantly alter hippocampal or striatal [3H]PCP binding. The data suggest that PCP elicits its locomotor stimulating effects via an interaction with PCP binding sites located mostly on mesolimbic dopaminergic terminals within the nucleus accumbens.     

Effect of acute and chronic morphine administration on brain cholinergic muscarinic receptors

The binding of [3H]quinuclidinyl benzilate (QNB) to various brain regions was determined in rats after acute or chronic treatment with morphine. Morphine and naloxone, in vitro, inhibited the binding of [3H]QNB to striatal membranes only at high concentrations. Thirty minutes after a single injection, morphine (5 or 40 mg/kg s.c.) did not alter the Bmax or Kd values for [3H]QNB binding to striatal receptors. The binding of [3H]QNB to membranes of different brain regions was not changed in morphine tolerant-dependent rats or rats undergoing abrupt or naloxone precipitated withdrawal. The results suggest that central cholinergic muscarinic receptors are unaffected by acute or chronic treatment with morphine, or during abstinence.     

Effect of 3-PPP,a putative dopamine autoreceptor agonist,on [3H]ligand binding to dopamine receptors of calf and rat striatum

3-(3-Hydroxyphenyl)-N-n-propyliperidine (3-PPP) is most effective in inhibiting [³H]apomorphine binding in rat striatal membranes, with Ki values of 63 nM. 3-PPP was six to 27 times less effective when it competed with the binding of [³H]dopamine or [³H]spiperone in calf and rat striatal membranes. At concentrations up to 10 μM, 3-PPP failed to substitute for dopamine in the activation of adenylate cyclase in rat striatal membranes. 3-PPP at 4.8-5 μM caused 50% inhibition of catecholamine uptake in synaptosomes of corpus striatum and hypothalamus, therefore appearing to be a relatively weak uptake inhibitor. The higher affinity of 3-PPP for [³H]apomorphine binding sites is consistent with its binding to a subset of dopamine receptors which are characterized by a high affinity for both the agonist and antagonist of dopamine.     

Stereoselective actions of substituted benzamide drugs on cerebral dopamine mechanisms

Apomorphine-induced locomotor activity in reserpine-pretreated mice was antagonized by pretreatment with (-)-sulpiride and (-)-sultopride. The (+)-enantiomers were inactive. Apomorphine- and amphetamine-induced stereotyped behaviour in rats were antagonized by (-)-sultopride but not by the (+)-enantiomer. Neither enantiomer of sulpiride prevented the onset of the stereotyped response. Both (-)-sulpiride and (-)-sultopride induced increases in striatal and mesolimbic HVA and DOPAC concentrations; (+)-sultopride elevated striatal and mesolimbic DOPAC concentrations but not HVA, while (+)-sulpiride had no effect on HVA or DOPAC in either area. Dopamine concentrations were reduced by the enantiomers of sultopride but not by sulpiride. Low concentrations (10^?9 ?10?6⁶ M) of the (-)-enantiomers of both drugs displaced [³ H]spiperone from its specific binding site in rat striatal preparations, but the (+)-enantiomers were 40 and 100 times less active. However, neither enantiomer of either drug anatagonized the dopamine-induced stimulation of adenylate cyclase in rat striatal preparations. The data suggest that the central pharmacological activity of sulpiride and sultopride resides in the (-)-enantiomers and that this activity occurs at cerebral dopamine receptors not dependent on adenylate cyclase for functional activity.