Sterochemical preferences for curarimimetic neuromuscular junction blockade III: enantiomeric bisquaternary amines related to benzoquinonium as probes

Macromolecular sites for specific neurotoxins and drugs on chemosensitive synapses and electrical excitation in biological membranes

The present review deals with the molecular mechanisms and elementary phenomena underlying the activation of the voltage- and chemo-sensitive membrane macromolecules: sodium- and potassium-ion channels and nicotinic ACh receptors and their associated ion channel. To achieve an understanding of their various kinetics and conformational states, a number of novel alkaloids, BTX, HTXs, gephyrotoxins, and certain psychotomimetic drugs such as phencyclidine, and many other pharmacologically active agents have been used. Biochemical assays and various electrophysiological techniques have been used in a number of biological preparations--e.g., Torpedo membranes, brain synaptosomes, amphibian and mammalian neuromuscular preparations--to describe the action of such agents. The availability of BTX and scorpion toxins together with aconitine and veratridine as activators and TTX and STX as antagonists of the voltage-sensitive sodium channels, made possible the identification and the physiological and pharmacological characterization of these channels. These studies provided the basis for understanding the mechanisms underlying electrical excitability and culminated, more recently, in the purification and reconstitution of sodium channels from rat brain and in the successful cloning of these channels with the elucidation of their primary structure. We now know that the sodium channel has a molecular mass of 316,000 daltons, consists of five subunits, and has multiple sites for various ligands. In contrast to sodium channels, various classes of potassium channels (inward and outward rectifier potassium channels and Ca(2+)-activated potassium channels) have been described. Unlike the sodium channels, there are no known specific activators for potassium channels. However, a number of potassium channel blockers such as 4-aminopyridine, HTX, histamine, and norepinephrine have been identified which complement the varying types of potassium channels in different neurons. One class of potassium channel blockers with profound medical and social implications comprises PCP and its analogues. The blockade of the potassium-induced 86Rb+ efflux from brain cells, the resulting prolongation of muscle and nerve action potentials, and the increase in transmitter release observed with PCP and some analogues are all highly suggestive of a role for the potassium channel in the behavioral effects of these drugs and its potential involvement in schizophrenia. A number of toxic principles of both plant and animal origin played a significant role in the development of our knowledge about the nAChR.(ABSTRACT TRUNCATED AT 400 WORDS)

The ionic channel of the nicotinic acetylcholine receptor is unable to differentiate between the optical antipodes of perhydrohistrionicotoxin

The enantiomers of perhydrohistrionicotoxin were studied in their effects on endplate currents recorded at the junctional region of sartorius muscles of Rana pipiens. The two optical antipodes progressively decreased the peak amplitude of the endplate currents and were indistinguishable from each other at all times. The enantiomers shortened equally the time constants for endplate current decay, but did not alter their voltage sensitivities. Although perhydrohistrionicotoxin contains 4 chiral centers, complete steric inversion does not alter its effects on the acetylcholine receptor-ion channel complex. By contrast the recognition site of the AcChR is extremely sensitive to any change in the chirality of agonists.

Stereochemical preferences for curarimimetic neuromuscular junction blockade IV: Monoquaternary ammonium probes possessing carbon and nitrogen asymmetry

Two enantiomeric pairs of neuromuscular junction blocking agents were prepared in which an asymmetric carbon atom is adjacent to an asymmetric quaternized nitrogen moiety. The blocking agents were obtained from the enantiomers of laudanosine by stereoselective quaternization with benzyl and ethyl iodides. Curarimimetic potencies were measured with an in vivo cat hypoglossal nerve-tongue muscle preparation. The studies suggest that the asymmetry present in these structures does not lead to significant differences in blocking potency between enantiomers.

Neuromuscular junction blocking agents: an initial screening bioassay

A simple, rapid and sensitive in vivo bioassay for the initial screening of neuromuscular junction (NMJ) blocking agents has been accomplished. The i.v. retro-orbital plexus (IVROP) mode of injection was utilized, for the first time, in conjunction with the mouse inclined screen bioassay.

Hofmann elimination with diazomethane on curare bases and selected quaternary tetrahydroisoquinoline alkaloids

Use of a large excess of alkali-free diazomethane resulted in a Hofmann elimination with selected curare bases and some other quaternary tetrahydroisoquinoline alkaloids. (+)-Tubocurarine chloride provided a monostilbene methine, O,O-dimethyltubocurinemethine, and a monostilbene--monostyrene compound, O,O-dimethyltubocurinedimethine. The major elimination products of (+)-isotubocurarine chloride and (+)-carnegine methiodide were monostyrene methines, O,O-dimethyltubocurineisomethine and carneginemethine, respectively. Treatment of (+)-laudanosine methiodide with potassium hydroxide, under the conditions of Hofmann degradation, or alkali-free diazomethane solution provided the same stilbene compound, laudanosinemethine. The structures of the elimination compounds were further confirmed by catalytic reduction and quaternization with methyl iodide.