Effect of BM-5, a presynaptic antagonist-postsynaptic agonist, on cortical acetylcholine release

Adenosine selectively depresses muscarinic compared with non-muscarinic receptor mediated depolarisation of the rat superior cervical ganglion

1. A grease gap d.c. recording technique was used to measure electrophysiological responses of the isolated rat superior cervical ganglion. 2. Adenosine at 100 microM depressed depolarisations to the muscarinic agonists carbachol, muscarine and methylfurmethide. In contrast adenosine (100 microM) did not alter depolarisations to 1,1-dimethyl-4-phenylpiperazinium, 2-methyl-5-hydroxytryptamine and potassium and enhanced depolarisations to 5-hydroxytryptamine and gamma-aminobutyric acid. 3. Adenosine-induced depressions of the depolarisations to carbachol, muscarine, and methylfurmethide tended to be increased in the presence of 0.3 microM methoctramine (a muscarinic receptor antagonist with slight selectivity for M2 receptors). The increase was statistically significant (P < 0.01) for carbachol. 4. Medium containing 0.1 mM Ca2+ and 0.3 microM pirenzepine augmented the hyperpolarising phase of the response to carbachol. Adenosine (10-300 microM) hyperpolarised ganglia and did not significantly alter the hyperpolarisation to 0.3 or 1 microM carbachol but selectively reduced the depolarisation response to 3 microM carbachol. 5. Adenosine-induced hyperpolarisations (100 microM) were enhanced when applied during depolarisations to muscarinic agonists (muscarine, pilocarpine, N-methyl-N-(l-methyl-4-pyrrolidine-2-butynyl)acetamide (BM-5)), and other M-current inhibitors, barium and eledoisin-related-peptide. Adenosine induced hyperpolarisations were not affected by D-Ala6-luteinizing-hormone-releasing-hormone or uridine 5'-triphosphate which produced small depolarisations. 6. It is concluded that adenosine acts selectively in opposing mechanisms of depolarisation of the rat SCG that are due to the action of muscarinic agonists (acting via M1-receptors) and by other M-current inhibitors.

Modification of the duration of action and pharmacodynamics of arecoline by tetraisopropylpyrophosphoramide

Tetraisopropylpyrophosphoramide (ISO-OMPA) pretreatment in the mouse was examined for its ability to prolong the time course of arecoline (ARE) concentration using brain ARE concentration as an index. Brain ARE levels were also correlated with acetylcholine (ACh) and choline concentrations. Pretreatment with 40 mg kg-1 ISO-OMPA increased ARE brain concentrations from 3 nmoles g-1 to 76 nmoles g-1 15 min after i.p. administration of 25 mg kg-1 ARE. ARE (25 mg kg-1) alone significantly increased brain ACh and choline levels. In time course studies, administration of ARE (15 mg kg-1) produced a peak brain level of 7.9 nmoles g-1 at 3 min. ISO-OMPA pretreatment increased the peak level of ARE to 46.5 nmoles g-1 and the peak time to 7-15 min. The time of maximum brain elevation of ACh and choline produced by ARE alone lagged slightly behind the peak level time observed for ARE. ARE alone produced a maximal increase in ACh and choline levels to 34.1 and 57.1 nmoles g-1, respectively. In the presence of ISO-OMPA, ARE further increased ACh and choline levels to 48.2 and 103 nmoles g-1, respectively. After i.p. administration the highest concentration of ARE was found in the cortex, followed by the subcortex, and cerebellum. A significant elevation of ACh was observed in the cortex.

Stereoselective binding and activity of oxotremorine analogs at muscarinic receptors in rat brain

The activities of the enantiomers of BM-5 were examined to measure muscarinic cholinergic selectivity in the central nervous system. Autoradiographic studies assessed the ability of each enantiomer to inhibit the binding of [3H]-(R)-quinuclidinyl benzilate ([3H]-(R)-QNB) to muscarinic receptors in the rat brain. (+)-(R)-BM-5 inhibited [3H]-(R)-QNB binding to rat brain sections at concentrations below 1.0 microM, while 100-fold higher concentrations of (-)-(S)-BM-5 were required for comparable levels of inhibition. Analysis of the autoradiograms indicated that both stereoisomers had a similar distribution of high affinity binding sites. Each enantiomer displayed higher affinity for muscarinic receptors in the superior colliculi and lower affinity for receptors in the cerebral cortex and hippocampus. (+)-(R)-BM-5 and oxotremorine inhibited adenylyl cyclase activity in the cerebral cortex with efficacies comparable to that for acetylcholine. (+)-(R)-BM-5 was 26-fold more potent than (-)-(S)-BM-5 in inhibiting adenylyl cyclase. Oxotremorine-M and carbamylcholine stimulated phosphoinositide turnover in the cerebral cortex. Oxotremorine had lower activity and (+)-(R)-BM-5 was essentially inactive at comparable concentrations. The difference in activity of the two enantiomers indicates a remarkable stereochemical selectivity for muscarinic receptors. The stereoselectivity index is comparable for both the autoradiographic assays (48) and measures of adenylyl cyclase activity (26) in the cerebral cortex.

An improved synthesis of the enantiomers of BM-5 and their effects on the central in vivo release of acetylcholine

Racemic N-methyl-N-(1-methyl-4-pyrrolidino-2-butynyl)acetamide (BM-5), a putative postsynaptic agonist and presynaptic antagonist at muscarinic receptors, was resolved into the enantiomers by a new method suitable for large scale preparation. The method involves a chemoselective N-debenzylation as the key step. The enantiomers of BM-5 were obtained after six separate steps in 25% overall yield. The ability of the enantiomers to release acetylcholine was evaluated in vivo by use of brain dialysis. (R)-BM-5 was the more potent enantiomer in this assay.