The possible role of conformational isomerism in the biological actions of acetylcholine

The pharmacology of molluscan neurons

It is commonly accepted that the basic physiological properties of the neurons as well as the nature of transmitter substances have remained relatively unchanged through evolution, while brain size and neuron number have greatly increased. Among invertebrates the molluscs, due to the large size of their neurons and lesser complexity of the neural networks controlling specific behavior, have proved to be especially useful for studying elementary properties of single neurons, network organization as well as various forms of learning and memory. The study of putative neurotransmitters has indicated that molluscs use the same low molecular-weight substances and peptides or their metabolites and cyclic nucleotides as transmitters and second messengers as the other species of various phyla. At the same time the receptors of neurotransmitters were found to have certain characteristic properties in the molluscs. The large molluscan neurons have permitted the isolation of individual identifiable nerve cells, and the subsequent analysis of quantities of the transmitters and their metabolic enzymes. These studies have demonstrated that single neurons frequently can contain more than one putative neurotransmitter. It can be expected that this model will contribute to an understanding of the role of multiple transmitters within a single neuron assuring the plasticity of the nervous system. The cellular mechanisms of plasticity have been demonstrated first in molluscan nervous systems. It was proved in identified Aplysia neurons that the same transmitter (ACh) can be released from an interneuron onto two or more follower neurons and can excite one and inhibit another or evoke a biphasic response on a third type of cell. The biphasic response of the molluscan neurons to neurotransmitters was the first demonstration of the plastic synaptic changes. The discovery of individual neurons with their groups of follower cells acting as chemical units has provided an insight into the organization of various behavioral acts. Study of the gastropod molluscs has also shown that the giant serotonergic cells can act as peripheral modulator neurons, as well as interneurons, and in this way they can affect their target organs at more than one level. The molluscan studies have provided more information on transmitter receptors as it was shown that molluscan neurons have at least six different 5HT receptors, three Ach receptors which can be separated pharmacologically. This type of study has led to the discovery of numerous new antagonists and poisons.(ABSTRACT TRUNCATED AT 400 WORDS)

Acetylcholine-like molecular arrangement in psychomimetic anticholinergic drugs

A study of the relation between the psychotropic activity and the antagonism to acetylcholine observed for some heterocyclic amino esters and compounds of the phencyclidine series suggests some common molecular structural requirements for their properties. Criteria obtained from quantum mechanical calculations of acetylcholine-like molecules indicate that their molecular reactivity with the cholinergic receptor site follows a certain dynamic interaction pattern. This pattern suggests a certain molecular arrangement essential for the interaction, which is based on the electronic properties of the molecules and therefore remains valid for the evaluation of compounds which lack any apparent similarity to acetylcholine. This type of molecular arrangement is shown to be shared by both activators and inhibitors of the acetylcholine receptor discussed here, thus supporting the hypothesis of their binding to a common receptor. The differences in biological activity are attributed to the effect of molecular structural factors which are not commonly included in the molecular arrangement based on the active groups of acetylcholine. The role of such factors is revealed by a study of the observed differences in the cholinergic and psychomimetic activities of related pairs of isomers and enantiomers of the molecules investigated. Structural factors which interfere with the conformational changes occurring in the receptor protein induced by an activator are characterized through differences obtained by the comparative investigation of the activities of the agonist acetate and the antagonist benzilate amino esters of quinuclidine, tropine, and pseudotropine. The same factors are shown in studies of the phencyclidine series to contribute to the antagonism to acetylcholine activity that is closely related to the psychomimetic activity of these drugs in the central nervous system. Similarly, phencyclidine derivatives in which the characteristic acetylcholine-like molecular arrangement is modified by various substitutions are shown to loose both anticholinergic and psychotropic behavior. This close correlation is supported by the identification of molecular regions which will generate the proper molecular arrangement in local anesthetics and morphine, compounds which are known to be involved in cholinergic mechanisms.

Esters of choline and beta-trimethylammoniopropionic acid: geometrical isomerism and anti-acetylcholinesterase activity

The preparation of the choline esters of cis- and trans-4-t-butylcyclohexanecarboxylic acid is reported. The similar inhibitory potency displayed by these isomers towards the acetylcholinesterase catalysed hydrolysis of acetylcholine is explained on the basis of the binding of a thermodynamically unstable conformation of the cis-isomer to the active site. Similar studies employing the β-trimethylammoniopropionate esters of cis- and trans-4-t-butylcyclohexanol suggest that the “reverse esters” do not bind to the active site in an identical manner to the acylcholines.

The possible role of conformational isomerism in the biological actions of acetylcholine: spectral studies

Infrared studies of the NC-H stretching frequencies of selected trimethylammonium salts, including acetylcholine, were made in an attempt to obtain evidence that intramolecular NC-H—O hydrogen bonding is responsible for the existence in solution of a quasi-ring conformation for acetylcholine. Acetylcholine and related compounds were also examined by nuclear magnetic resonance spectroscopy. The carbonyl stretching frequencies of an homologous series of acetoxy alkyl sulphones were measured in dioxan solution to gain information on the effect of electron withdrawing substituents in the absence of intermolecular hydrogen bonding. The observed high ester carbonyl absorption frequency in acetylcholine may be interpreted in terms of the inductive effect from the onium nitrogen atom but the results are not incompatible with the existence of a quasi-ring conformation involving C-H—O hydrogen bonding between one of the N-methyl groups and the acetoxy oxygen atom.