Neurochemical and structural studies on the mechanism of action of hemicholinium-3 in central cholinergic synapses

Quantal analysis of action of hemicholinium-3 studied at a central cholinergic synapse of Aplysia

The effects of hemicholinium-3 (HC-3) on cholinergic transmission were studied on central identified inhibitory (H-type post-synaptic cell, Cl- channels) and on excitatory (D-type post-synaptic cell, cationic channels) synapses of Aplysia californica. In the H-type post-synaptic cell, the amplitude and the decay time of miniature post-synaptic currents (m.p.s.c.s.) were calculated by statistical analysis of long duration induced post-synaptic current (l.d.i.p.s.c.) due to 3 s depolarizations of the presynaptic neurone in the presence of tetrodotoxin. On H-type receptors, with respect to acetylcholine (ACh), HC-3 acted as an agonist and a blocker whereas on D-type receptors, it acted only as a blocker. At low concentration of bath-applied HC-3, in the H-type synapse, the decay time of the evoked inhibitory post-synaptic current (i.p.s.c.) as well as that of the m.p.s.c. was lengthened. These changes were rapidly reversible by wash. The decay time of excitatory post-synaptic current (e.p.s.c.) at the D-type synapse was not affected. On the inhibitory synapse, HC-3 applied in the bath at the concentration of 10(-5) M, reduced considerably the size of the m.p.s.c.s whereas the evoked i.p.s.c.s and the l.d.i.p.s.c.s were only slightly affected pointing to an increase of the quantal content of both responses. After wash, both i.p.s.c.s and l.d.i.p.s.c.s showed a clear facilitation which persisted for several tens of minutes. The presence of presynaptic receptors was considered. Similar facilitation of e.p.s.c.s by HC-3 was observed at the D-type synapse. The comparison of the degree of depression by HC-3 of the m.p.s.c.s and of the responses to ionophoretically applied ACh, indicated that the size of the quantum was not changed. Intracellular injection of HC-3 into the presynaptic neurone of the H-type synapse led to a decrease of transmitter release which affected solely the quantal content of the responses. As the synaptic transmission could not be restored by injection of exogenous ACh into the presynaptic neurone, it was concluded that the depression of transmission was not due to a decrease of ACh synthesis.

Evaluation of 4-methylpiperidine analogs of hemicholinium-3

A series of substituted piperidine analogs of hemicholinium-3 was evaluated for their ability to inhibit neuromuscular transmission, to decrease acetylcholine content of caudate slices, to inhibit choline acetyltransferase activity, and to produce toxicity. Quaternary and tertiary amine derivatives of 4-methyl- and 4-hydroxyl-substituted piperidine analogs containing beta-carbonyl or beta-hydroxyl substitutions in the phenylethyl spacing moiety were tested. 4-Methyl piperidine derivatives maintained potent hemicholinium-3 like activity. Reduction of activity was seen with the 4-hydroxyl piperidine analogs. Compounds with beta-hydroxyl substitution were more potent than those with beta-carbonyl substitution. The tertiary amine, 4-methyl piperidine derivative with a hydroxyl group on the beta-carbon of the ethyl side chain also possessed hemicholinium-3 like activity. However, tertiary amine analogs were substantially less potent than hemicholinium-3 or their quaternary amine analogs.

An in vitro study of hemicholinium-3 on phospholipid metabolism of Krebs II ascites cells

With [Me-14C]choline as marker and after separation of choline and phosphocholine by ion-exchange column chromatography or thin layer chromatography on alumina, it is shown that 40 microM hemicholinium-3 (HC-3) inhibits the cytosolic choline-kinase of rat liver and Krebs cells. This inhibition is competitive (Km different, Vm similar) in the first case and mixed in the second (Km and Vm different). Despite this general inhibition of the phosphocholine formation, the synthesis of phosphatidylcholine (PC) by post-nuclear supernatants of rat liver and Krebs cells is different when tested with HC-3. It is unaffected in rat liver; however, HC-3 induces a PC deficiency in Krebs cells which is time-course dependent between 15 and 120 min and proportional to the drug concentrations in the interval 5-40 microM. Incorporation of AT-[gamma 32P] or [2-14C]ethanolamine into phospholipids shows that the sequential methylation pathway is not detectable in Krebs cells. These results are discussed in relation to those established concerning HC-3 action on phospholipid metabolism in other tissues.

Hemicholinium impairs septo-hippocampal facilitatory action

Intraventricular injections of hemicholinium-3 led to a sharp reduction in hippocampal acetylcholine content (by 79% on the average). This was associated with the following changes in population spikes evoked in area CA1 by commissural stimulation: (1) a tendency to progressive increase, over 1-2 hours; (2) in a few cases (3 out of 12), a striking depression of the normal strong facilitation produced by brief tetanic stimulation of the medial septum (typically 10 pulses at 50-100 Hz); (3) a much more consistent tendency towards fading of the septal facilitatory effect during repeated applications of such brief septal tetani (in 10 cases out of 12); as well as, (4) diminished facilitation by sustained, lower frequency septal tetanic stimulation (20-50 Hz). The reduced efficiency of septal action--especially during repetitive stimulation--was not accompanied by a consistent reduction of the facilitation produced by local applications of acetylcholine; it is, therefore, best explained by the diminished availability of acetylcholine, and so provides further evidence that septo-hippocampal facilitation is mediated by a cholinergic mechanism.

The pharmacological actions of some polymethylene-bis-(hydroxyethyl)-dimethyl-ammonium compounds on cholinergic transmission

Some polymethylene-bis-(hydroxyethyl)-dimethyl-ammonium (dicholine) compounds containing from 7 to 10 methylene groups between the quaternary nitrogen atoms, have been studied for activity at various sites of cholinergic transmission. The post-junctional activity of the compounds was investigated on the frog rectus abdominis muscle. Small doses of C8-, C9-, and C10-dicholine potentiated ACh contractions, however larger doses of all the dicholine compounds blocked the nicotinic receptor sites of the frog rectus muscle. The dicholine compounds blocked neuromuscular transmission in the rat phrenic nerve diaphragm. These actions at the rat neuromuscular junction are compared with the purely post-junctional actions on the frog rectus muscle. Although the compounds exert anticholinesterase activity in vitro it is suggested that this effect plays little part in the action of the drugs at the neuromuscular junction. It is concluded that the dicholine compounds have both pre- and post-junctional activity at the neuromuscular junction. The dicholine compounds are acetylated at varying rates by partially purified choline acetyltransferase (ChAc) although all are acetylated less readily than choline. The rate of acetylation of the dicholine compounds by ChAc parallels their activity in blocking neuromuscular transmission and it is suggested that other quaternary ammonium compounds containing hydroxyl or hydroxyethyl groupings may be acetylated by ChAc and this may affect their blocking action at the neuromuscular junction.

Renal tubular excretion of triethylcholine (TEC) in the chicken: enhancement and inhibition of renal excretion of choline and acetylcholine by TEC

1. [3H]-triethylcholine (TEC) was actively transported by the renal tubule of the chicken at a rate 85% that of simultaneously administered p-aminohippuric acid (PAH). 2. TEC was demonstrated to be transported by the organic cation transport system in the kidney through inhibition with quinine and the bio-cation choline. 3. When the infusion of TEC was increased to 2 times 10(-6) mol kg(-1) min(-1) reaching the infused kidney, the transport of [3H]-TEC was inhibited, suggesting that an excretory transport maximum for TEC in the renal tubules had been reached. 4. The excretion of both choline and acetylcholine was enhanced by TEC loads as low as 1 times 10(-18) mol kg(-1) min(-1). Enhancement continued as TEC infusion was increased up to approximately 1 times 10(-7) mol kg(-1) min(-1) at which point this enhancement was converted to inhibition. 5. Possible mechanisms for the biphasic effect of TEC on organic cation transport are discussed.