Effects of brucine, a plant alkaloid, on M1 muscarinic receptors and α1-adrenoceptors in the rabbit vas deferens preparation

The plant alkaloid brucine is an analogue of strychnine and is known to be an allosteric modulator at cloned M-1 muscarinic receptors. The functional effects of brucine were examined on the M-1 muscarinic receptors in the rabbit isolated vas deferens preparation. Brucine (10-100 muM) enhanced the effects of the muscarinic agonist McN-A-343 at presynaptic M-1 muscarinic receptors in the rabbit isolated vas deferens preparation, but only when brucine was added prior to McN-A-343. This effect is indicative of a positive allosteric action. It was poorly reversed on washing. Brucine did not affect the responses to the mamba venom muscarinic toxins MT2 and MT4, which are also allosteric activators in this preparation. Brucine (10-100 muM) caused a significant decrease in the twitch response to electrical stimulation in the rabbit vas deferens preparation, which was not antagonised by 100 nM pirenzepine (an M-1 muscarinic antagonist). Brucine and MT4, but not MT2, caused significant decreases (p < 0.05) in the responses to noradrenaline in the rabbit vas deferens preparation. Responses to ATP and KCl were not affected. In radioligand binding assays, brucine displaced the <alpha>(1)-adrenoceptor ligand prazosin from its specific binding sites in membranes made from rat cerebral cortex and rat vas deferens. The apparent K-i values were 150 and 3.4 muM in the cortical and vas deferens membranes, respectively. The positive allosterism found with brucine at cloned M-1 receptors seems to be mirrored at native M-1 receptors. However, the unexpected blocking effects at alpha (1)-adrenoceptors indicates that more selective ligands than brucine are required as starting points for the design of specific enhancers of the activity of M-1 receptors with therapeutic potential. (C) 2000 Elsevier Science Ltd. All rights reserved.