Björn Ringdahl

Pharmacological properties of oxotremorine and its analogs

The mechanism of action of oxotremorine is reviewed. A number of analogs of oxotremorine, including agonists, partial agonists and antagonists, are described. Many of these are potent and highly specific in their central actions. The possible basis for this specificity is outlined.

STEREOCHEMICAL REQUIREMENTS FOR CENTRAL AND PERIPHERAL MUSCARINIC AND ANTIMUSCARINIC ACTIVITY OF SOME ACETYLENIC COMPOUNDS RELATED TO OXOTREMORINE

1 The enantiomers of some analogues of the central muscarinic agent, oxotremorine, were prepared and investigated for tremorogenic and tremorolytic activity in intact mice and for muscarinic and antimuscarinic activity on the isolated ileum of the guinea‐pig. 2 The R‐isomers were more potent than the S‐isomers both in vivo and in vitro regardless of whether the compounds are agonists, partial agonists or competitive antagonists. 3 It is suggested that in the oxotremorine series, agonists and antagonists interact with a common receptor site, in contrast to classical muscarinic antagonists which are believed to bind also to accessory receptor areas, located close to the agonist binding site.

Dissociation constants and relative efficacies of acetylcholine, (+)- and (-)-methacholine at muscarinic receptors in the guinea-pig ileum

1 Dissociation constants (KA) and relative efficacies of acetylcholine, (+)‐methacholine and (‐)‐methacholine at muscarinic receptors in the guinea‐pig isolated ileum were determined in the absence and presence of the cholinesterase inhibitor diisopropylfluorophosphate. The method used involved analysis of dose‐response data before and after fractional inactivation of receptors with propylbenzilylcholine mustard. 2 The KA values, estimated after cholinesterase inhibition, of acetylcholine, (+)‐ and (‐)‐methacholine were 1.7, 2.0 and 620 μM, respectively. 3 The large (730 fold) difference in spasmogenic activity between (+)‐ and (‐)‐methacholine is due primarily to a difference in affinity for ileal muscarinic receptors although differences in efficacy (2 to 4 fold) also contribute. 4 It is suggested that the methyl group at the chiral centre of (+)‐methacholine has no apparent effect on the binding to muscarinic receptors, whereas the corresponding methyl group of (‐)‐methacholine interferes with binding, presumably by stabilizing a conformation of the drug which does not fit the receptor very well.

Muscarinic receptor occupation and receptor activation in the guinea-pig ileum by some acetamides related to oxotremorine

1 The dissociation constants (KD values) and relative efficacies of seven acetamide analogues of oxotremorine, including two enantiomeric pairs, at muscarinic receptors in the guinea‐pig isolated ileum were determined. The method used involved analysis of dose‐response data before and after fractional inactivation of receptors with propylbenzilylcholine mustard. 2 All of the compounds studied had lower affinities than oxotremorine, but some had substantially higher relative efficacies. 3 Replacement of the pyrrolidine ring of N‐methyl‐N‐(4‐pyrrolidino‐2‐butynyl)acetamide(I), the parent compound in the series, by a dimethylamino or a trimethylammonium group decreased the affinity 32 and 4.5 fold, respectively, whereas the relative efficacy increased 5.7–8.3 times. 4 There was no correlation between relative efficacies and affinities of the compounds. The structural requirements for high affinity and high efficacy appeared to be quite different.

Muscarinic activity of some secondary and tertiary amines and quaternary ammonium salts structurally related to oxotremorine

A number of secondary and tertiary amines as well as quaternary ammonium compounds, obtained by structural modification of the amino group of oxotremorine (1) and its acetamide analogue (14), were investigated for muscarinic and antimuscarinic activity in vivo and in vitro. For the quaternary ammonium analogues, decrease of in vitro muscarinic potency is well correlated with increase of the size of the quaternary ammonium group, as estimated from increments in apparent molal volumes. A similar decrease of muscarinic potency with increasing substitution at the nitrogen atom is generally observed for the secondary and tertiary amines. For the latter the reduction in muscarinic activity appears to be due to loss of efficacy, since the higher homologues are partial agonists or antagonists. There is a highly significant correlation between muscarinic activity in vitro and central tremorogenic activity of the tertiary amines.

Structural requirements for affinity and efficacy of N-(4-amino-2-butynyl) succinimides at muscarinic receptors in the guinea-pig ileum and urinary bladder

The muscarinic activities on the isolated guinea-pig ileum and urinary bladder of some N-(4-amino-2-butynyl)succinimides, modified only in the amino group, were resolved into receptor affinity and efficacy components. The structural requirements for high affinity and high efficacy were quite different. Cyclic tertiary amino moieties generally favoured high affinity, while small acyclic amino and ammonium groups favoured high efficacy. On the ileum, dissociation constants and relative efficacies of the succinimides were highly correlated (r = 0.94-0.97) with those of the identically modified N-(4-amino-2-butynyl)-2-pyrrolidones. This observation suggests that N-(4-amino-2-butynyl)succinimides and 2-pyrrolidones bind to and activate muscarinic receptors in a similar fashion. In spite of their agonist properties on the ileum, the succinimides studied were agonists, partial agonists or competitive antagonists on the urinary bladder. However, dissociation constants and relative efficacies of the compounds showed good agreement in the two tissues. It therefore appears that muscarinic receptors in the guinea-pig ileum and urinary bladder are pharmacologically similar. The large differences observed in agonist potency and relative maximal responses between the two tissues were explained by a greater receptor reserve for muscarinic agonists in the ileum than in the bladder.

A comparison of the stimulant activities of oxotremorine analogues on the frog rectus abdominis and the guinea pig ileum

A series of tertiary amines structurally related to the potent muscarinic agent oxotremorine showed weak stimulant activity on the frog rectus abdominis preparation. The corresponding quaternary ammonium salts were more effective stimulants, the most potent being half as active as carbachol. The effects appeared to be due to direct stimulation of post-junctional nicotinic receptors. The relationship between the structures of these compounds and their ability to stimulate nicotinic receptors was compared with the structure-activity relationships in more conventional nicotinic stimulants. Finally, a comparison was made between the observed nicotinic potencies and previously published muscarinic potencies in the isolated guinea pig ileum. The tertiary amines were in general highly selective for muscarinic as opposed to nicotinic receptors, whereas the quaternary derivatives were only slightly more selective for muscarinic receptors than carbachol.

Muscarinic activity in the isolated guinea pig ileum of some carboxamides related to oxotremorine

A series of structural analogues of the potent oxotremorine-like agent N-(4-pyrrolidino-2-butynyl)-N-methyl-acetamide (1) was investigated for muscarinic activity in the isolated guinea pig ileum. Substitution of larger alkyl groups for the acetyl methyl group of 1 results in an attenuation of muscarinic potency. The observation that the agonist N-(4-dimethylamino-2-butynyl)-N-methylpropionamide (6) has a dissociation constant (KA = 5.1 X 10(-5) M), estimated after elimination of spare receptors with dibenamine, similar to that of the antagonist N-(4-dimethylamino-2-butynyl)-N-methyl-2,2-dimethylpropionamide (11) suggests that the decrease in muscarinic agonist activity with increasing substitution is due mainly to a loss of efficacy. The N-methyl group of 1 is essential for muscarinic activity since its replacement by a hydrogen atom or an ethyl group yields antagonists.

Antimuscarinic potency and functional selectivity of oxotremorine analogs at muscarinic receptor subtypes in the rat

The potency of six antimuscarinic oxotremorine analogs at sympathetic ganglionic M1 and brainstem M2 muscarinic receptors was compared in the rat. Inhibition of the pressor effects of McNA343 or physostigmine was used as functional indicators of M1 and M2 blockade, respectively. 50% inhibitory doses (ID50) were calculated against both cholinomimetics. Of the analogs, PCA-10 was the most potent, with ID50 values of 0.16 and 0.11 mumol/kg versus McNA343 and physostigmine, respectively. A correlation analysis indicated that these analogs did not functionally discriminate between responses mediated by neuronal M1 or M2 muscarinic receptors in vivo. In contrast, these analogs antagonized M1 ganglionic responses at doses which produced negligible antagonism at cardiac and vascular M2 receptors.