The effects on Schild regressions of antagonist removal from the receptor compartment by a saturable process

Drug Design Targeting the Muscarinic Receptors and the Implications in Central Nervous System Disorders

There is substantial evidence that cholinergic system function impairment plays a significant role in many central nervous system (CNS) disorders. During the past three decades, muscarinic receptors (mAChRs) have been implicated in various pathologies and have been prominent targets of drug-design efforts. However, due to the high sequence homology of the orthosteric binding site, many drug candidates resulted in limited clinical success. Although several advances in treating peripheral pathologies have been achieved, targeting CNS pathologies remains challenging for researchers. Nevertheless, significant progress has been made in recent years to develop functionally selective orthosteric and allosteric ligands targeting the mAChRs with limited side effect profiles. This review highlights past efforts and focuses on recent advances in drug design targeting these receptors for Alzheimer’s disease (AD), schizophrenia (SZ), and depression.

Clearance of oxytocin and its potentially enzyme resistant analogues in the OXT-receptor compartment of the potassium depolarized rat myometrium

The time–response behaviour of a group of oxytocin analogues structurally modified on potential sites of oxytocin splitting by tissue inactivation enzymes (“enzyme probes”) was investigated ex vivo on the potassium depolarized rat myometrium (at 30°C) and compared with the data obtained in the in vivo experiments. The modified oil‐immersion method by Kalsner and Nickerson was used to record time profiles after cessation of a steady state myometrium contraction triggered by analogues in a high potassium tissue medium. An exchange of the aqueous medium for mineral oil enables to suppress return diffusion of the peptide and to record its irreversible clearance near the corresponding receptor compartment. Response records were analysed by a nonlinear numeric procedure based on combination of steady state and kinetic terms that allows concomitant estimations of affinities from time–response measurements, in the given case for analogues on depolarized myometrium. Potential inactivation‐sensitive sites in the oxytocin chain are the Ν‐terminal peptide bond Cys¹‐Tyr² (aminopeptidase splitting), the intramolecular disulphide bridge (reduction and formation of the practically inactive linear peptide) and the C‐terminal Leu⁸‐GlyNH₂⁹ or the Pro⁷‐Leu⁸ (postprolin cleaving enzyme) bond, respectively. Clearance rate constants of single peptides in the OXT‐receptor compartment were in an interval of 0.025 to 0.28 min⁻¹. The fragment contribution analysis reveals a significant linear additivity of individual structural changes and thus a predictivity of irreversible inactivation rate in the receptor compartment. The most potent inactivation of oxytocin is associated with aminopeptidase splitting; other enzymes may play some though nondecisive role. Less significant differences within the peptide group were found for rate constants for peptide transport between receptor compartment and its external aqueous medium. Besides rate constants, the evaluation of time–response data yields affinity values of the tested peptides and indicates a 25‐times desensitation of depolarized compared with a native state.

Kinetics of oxytocin and deaminooxytocin displacement from the OXTR-receptor compartment in rat uterus ex vivo

The oil immersion method suggested earlier by Kalsner and Nickerson for analysing actions of sympathomimetic drugs upon smooth muscle tissues was applied to isometric preparations of rat myometrium stimulated by oxytocin and deaminooxytocin. An exchange of the aqueous medium by mineral oil allows monitoring the displacement of the peptides from their receptor compartment in absence of free diffusion transport between tissue and organ medium. Exponential analysis of the data from the uterotonic decay phase allows several inferences to be drawn: 1) Transport rate constants (roughly equal for the two peptides) are higher than rate constants of (irreversible) elimination from the receptor compartment. 2) The response decay rate in the oil immersion phase is proportional solely to the peptide elimination and thus offers estimates of elimination rate constants. 3) Peptide elimination kinetics in the receptor compartment is only insignificantly influenced by the kinetics of ligand-receptor binding. 4) As expected, the elimination rate constant of deaminooxytocin is considerably lower than for oxytocin. The apparent concentration of receptors in the paracellular space of the myometrium ("apparent", since receptor molecules are embedded in the cell membrane and hence not exposed to a diffusive flux), estimated from histometric parameters, appears rather high: 7 and 120 μM for high and low affinity receptors, respectively. Concentration-response curves for rat uterus stimulated by oxytocin or deaminooxytocin indicate that only about 0.25 to 5 per cent of the available receptors are involved in eliciting a maximal uterus contraction. The remnant receptor pool is likely to behave as a receptor reserve ("spare receptors").

Selective enhancement of fentanyl-induced antinociception by the delta agonist SNC162 but not by ketamine in rhesus monkeys: Further evidence supportive of delta agonists as candidate adjuncts to mu opioid analgesics

Mu-opioid receptor agonists such as fentanyl are effective analgesics, but their clinical use is limited by untoward effects. Adjunct medications may improve the effectiveness and/or safety of opioid analgesics. This study compared interactions between fentanyl and either the noncompetitive N-methyl-D-aspartate (NMDA) glutamate receptor antagonist ketamine or the delta-opioid receptor agonist SNC162 [(+)-4-[(alphaR)-alpha-[(2S,5R)-2,5-dimethyl-4-(2-propenyl)-1-piperazinyl]-(3-phenyl)methyl]-N,N-diethylbenzamide] in two behavioral assays in rhesus monkeys. An assay of thermal nociception evaluated tail-withdrawal latencies from water heated to 50 and 54°C. An assay of schedule-controlled responding evaluated response rates maintained under a fixed-ratio 30 schedule of food presentation. Effects of each drug alone and of three mixtures of ketamine+fentanyl (22:1, 65:1, 195:1 ketamine/fentanyl) or SNC162+fentanyl (59:1, 176:1, 528:1 SNC162/fentanyl) were evaluated in each assay. All drugs and mixtures dose-dependently decreased rates of food-maintained responding, and drug proportions in the mixtures were based on relative potencies in this assay. Ketamine and SNC162 were inactive in the assay of thermal antinociception, but fentanyl and all mixtures produced dose-dependent antinociception. Drug interactions were evaluated using dose-addition and dose-ratio analysis. Dose-addition analysis revealed that interactions for all ketamine/fentanyl mixtures were additive in both assays. SNC162/fentanyl interactions were usually additive, but one mixture (176:1) produced synergistic antinociception at 50°C. Dose-ratio analysis indicated that ketamine failed to improve the relative potency of fentanyl to produce antinociception vs. rate suppression, whereas two SNC162/fentanyl mixtures (59:1 and 176:1) increased the relative potency of fentanyl to produce antinociception. These results suggest that delta agonists may produce more selective enhancement than ketamine of mu agonist-induced antinociception.

The presence of atropinesterase activity in animal plasma

The enzyme atropinesterase (EC 3.1.1.10) causes the rapid hydrolysis of tropane alkaloids such as atropine and scopolamine. This enzyme is known to occur in a certain proportion of rabbits and some plants, although its presence in other animal species remains controversial. The potential presence in some animals but not others of an enzyme which can rapidly hydrolyse compounds such as atropine is a potential unwanted experimental variable in many experiments. Because of the uncertainty surrounding the enzyme and the paucity of data, it was decided to examine whether we could detect and characterise atropinesterase activity in the plasma of dogs, goats, guinea-pigs, humans, pigs, rabbits and rhesus by separating and quantitating the substrate (atropine) and one of the products (tropic acid) by high performance liquid chromatography (HPLC). It was found that plasma from some but not all rabbits possessed a capacity to breakdown large quantities of atropine; an effect that was apparently enantiomer-specific. Plasma from other rabbits, and plasma from all other species investigated, proved capable of hydrolysing atropine at a rate exceeding that of non-specific breakdown. It remains to be determined whether this effect is due to a low expression of atropinesterase or an alternative hydrolysing enzyme.

The design of novel methoctramine-related tetraamines as muscarinic receptor subtype selective antagonists

Several novel methoctramine-related tetraamines were designed, and their biological profiles at muscarinic receptor subtypes were assessed by functional experiments in isolated guinea pig and rat atria (M2) and smooth muscle (ileum and trachea, M3) and by binding assays in rat cortex (M1), heart (M2), and submaxillary gland (M3) homogenates and NG 108-15 cells (M4). Tripitramine, a nonsymmetrical tetraamine, resulted in the most potent and the most selective muscarinic M2 receptor antagonist of the series (pA2 = 9.14-9.85; pKi = 9.54). Spirotramine (FC 15-94), a symmetrical tetraamine, was able to differentiate between muscarinic M1 receptors (pKi = 7.88) and the other subtypes (M2, pKi = 6.20; M3, pKi = 5.81; M4, pKi = 6.27). Thus, tripitramine and spirotramine could be valuable tools for the pharmacological classification and characterization of muscarinic receptor subtypes.

Characterization of the interaction of zamifenacin at muscarinic receptors in vitro

The interaction of zamifenacin ((3R)-(+)-diphenylmethoxy-1-(3,4)-methylenedioxyphenethyl)pi peridine) at muscarinic receptor subtypes was studied using radioligand binding and functional techniques, in vitro. In radioligand binding studies, zamifenacin acted as a competitive antagonist, with the following pKi values; rat cerebral cortex (M1) 7.90 +/- 0.08, myocardium (M2) 7.93 +/- 0.13, submaxillary gland (M3) 8.52 +/- 0.04 and rabbit lung (M4) 7.78 +/- 0.04. In functional studies zamifenacin acted as a surmountable antagonist, exhibiting the following apparent affinity values; canine saphenous vein (putative M1) 7.93 +/- 0.09, guinea-pig left atria (M2) 6.60 +/- 0.04, guinea-pig ileum (M3) 9.31 +/- 0.06, guinea-pig oesophageal muscularis mucosae (M3) 8.84 +/- 0.04, guinea-pig trachea (M3) 8.16 +/- 0.04, and guinea-pig urinary bladder (M3) 7.57 +/- 0.15. Therefore, zamifenacin is selective for muscarinic M3 receptors in guinea-pig ileum, oesophageal muscularis mucosae, trachea and bladder over muscarinic M2 receptors in atria. The degree of muscarinic M3/M2 receptor selectivity depends upon the muscarinic M3 receptor preparation studied.

In vitro characterization of tripitramine, a polymethylene tetraamine displaying high selectivity and affinity for muscarinic M2 receptors

1. The antimuscarinic effects of tripitramine were investigated in vitro in isolated driven left (force) and spontaneously beating right (force and rate) atria as well as in the ileum of guinea-pig and rat and in the trachea and lung strip of guinea-pig and compared with the effects of methoctramine. 2. Tripitramine was a potent competitive antagonist of muscarinic M2 receptors in right and left atria. The pA2 values ranged from 9.14 to 9.85. However, in the guinea-pig and rat left atria but not in guinea-pig right atria, tripitramine at lower concentrations (3-10 nM) produced a less than proportional displacement to the right of agonist-induced responses owing to the presence of a possible saturable removal process. 3. Tripitramine was about three orders of magnitude less potent in ileal and tracheal than in atrial preparations (pA2 values ranging from 6.34 to 6.81) which makes it more potent and more selective than methoctramine. 4. Another intriguing finding was the observation that the pA2 value of 7.91 observed for tripitramine in guinea-pig lung does not correlate with that found at both muscarinic M2 and M3 receptor subtypes, which clearly indicates that the contraction of guinea-pig lung strip is not mediated by these muscarinic receptor subtypes. 5. A combination of tripitramine with atropine resulted in addition of the dose-ratios for left atria as required for two antagonists interacting competitively with the same receptor site, whereas the same combination gave a supra-additive antagonism on guinea-pig ileum which suggests that tripitramine interacts with a second interdependent site. 6. Tripitramine was more specific than methoctramine since, in addition to muscarinic receptors, it inhibited only frog rectus abdominis muscular (pIC50 value of 6.14) and rat duodenum neuronal (pIC50 value of 4.87) nicotinic receptors among receptor systems investigated, namely alpha 1-, alpha 2-, and beta 1-adrenoceptors, H1- and H2-histamine receptors, and muscular and neuronal nicotinic receptors.

Pharmacological analysis of agonist-antagonist interactions at acetylcholine muscarinic receptors in a new urinary bladder assay

1. Agonist-antagonist interactions at acetylcholine (ACh) muscarinic receptors have been analysed by use of an improved urinary bladder assay, isolated and intact, from the mouse. With 5-methylfurmethide as agonist, validated cumulative concentration-effect curves were obtained in less than 7 min by re-dosing before the response plateaux began to fade. 2. The pKB value estimated for pirenzepine was 6.76. The pKB values estimated for atropine and N-methylatropine from data obtained at concentrations which produced dose-ratios greater than 20 and 60 were 8.90 and 9.58, respectively. 3. The deviation from simple competitive behaviour at low dose-ratios with atropine and N-methylatropine was consistent with the operation of saturable antagonist removal processes. The deviation observed with atropine was corrected by pre-incubation with methylbutyrate, an alternative substrate for 'atropine esterase'. 4. The simple competitive behaviour of N-methylatropine was restored following pre-incubation with the neuronal choline uptake blocker hemicholinium-3 (HC-3). However, the pKB estimated for N-methylatropine under these conditions was low. This latter result could be accounted for by the observed behaviour of HC-3 as a competitive antagonist of ACh muscarinic receptors (pKB = 4.01). 5. We conclude that the modified mouse urinary bladder assay is suitable for the quantitative analysis of muscarinic receptor interactions. In addition, we postulate the existence of a previously undescribed uptake mechanism for quaternary muscarinic receptor antagonists.

Atypical Schild plots with histamine H1 receptor agonists and antagonists in the rabbit aorta

Competitive antagonists of histamine H1 receptor were investigated for their effects on histamine-induced responses in the rabbit aorta. Antazoline-induced antagonism gave linear Schild plots with slope equal 1, while the other antagonists [+)-brompheniramine, (+/-)-chlorpheniramine, diphenhydramine and mepyramine) produced 'atypical' plots with slopes generally less than unity in the thoracic aorta. Schild plots obtained with these antagonists were evaluated using a two independent component model. The high affinity parameters thus estimated were compatible with those that have been reported for these antagonists. No such heterogeneity was observed in the abdominal aorta when diphenhydramine was investigated with different H1 agonists. The results suggest the presence of at least two components in H1-mediated responses in the thoracic aorta; these components are equally antagonized by antazoline, but differentially antagonized by the other antagonists used.

Agonist and antagonist characterization of a putative adrenoceptor with distinct pharmacological properties from the alpha- and beta-subtypes

1. Experiments were done to characterize a putative adrenoceptor which functions to inhibit longitudinal muscle tension development in the guinea-pig ileum. Several phenylethylamine based agonists were investigated: BRL 37344, (-)-isoprenaline, (+)-isoprenaline, noradrenaline, adrenaline, and fenoterol. Propranolol and nadolol were tested as antagonists. Agonist-induced inhibition of the contractile response to histamine was measured under equilibrium conditions with alpha-adrenoceptors and muscarinic cholinoceptors inhibited. 2. Inhibitory responses were obtained to (-)-isoprenaline and BRL 37344 that were resistant to beta-adrenoceptor blockage with propranolol (5 microM) and nadolol (10 microM). These resistant responses were antagonized by much higher concentrations of nadolol (30 to 1000 microM) yielding apparent pA2 values for nadolol of 4.31 with (-)-isoprenaline as the agonist, and 4.68 with BRL 37344 as the agonist. Similar apparent pA2 values for nadolol at the putative adrenoceptor were obtained with noradrenaline (4.79), adrenaline (4.68), and fenoterol (4.38). 3. The order and relative potency of agonists at the putative adrenoceptor was: BRL 37344 (20) greater than (-)-isoprenaline (8) greater than noradrenaline (1) greater than adrenaline (0.5) greater than fenoterol (0.35) greater than (+)-isoprenaline (0.27). 4. The resistance to blockade by propranolol (5 microM), the low affinity of nadolol, and the order and relative potency of agonists, suggest the presence of an adrenoceptor with distinct pharmacological characteristics from currently defined alpha- and beta-adrenoceptors.

pA2 values for antagonists of platelet activating factor on aggregation of rabbit platelets

1. The relative potencies, and equilibrium dissociation constants, for nine antagonists of platelet activating factor (Paf) have been determined on rabbit platelets (in diluted platelet-rich plasma (PRP)) in experiments in which the aggregatory response to Paf was measured. 2. Log concentration-response (% maximum) curves to Paf were obtained in the absence (controls) and presence of different concentrations of each Paf antagonist drug. The antagonists shifted the Paf curves to a higher concentration range and the slopes of the Schild plots, constructed from these data, suggested that the drugs were competitive antagonists of Paf. The slopes of the Schild plots for CV-3988 and SRI 63-119 were greater than 1. 3. The pA2 values (pKB values in parentheses) were: WEB 2086 7.31 (7.63); SRI 63-119 6.95; L-652,731 6.71 (6.73); BN 52021 6.38 (6.47); SRI 63-072 6.36 (6.43); CV-3988 5.87; 48740 RP 4.97 (5.07); ketotifen 4.94 (4.95); thiazinamium 4.73 (4.76). 4. This study provides, for the first time, some functional response data for Paf antagonists (pKB values) which are in an appropriate form for use in classifying putative Paf receptors. The study also provides the comparative potencies of these Paf antagonists in inhibiting Paf-induced platelet aggregation. WEB 2086 was the most potent of the drugs examined.