Modulation by nicotine on binding of cerebral muscarinic receptors with muscarinic agonist and antagonist
Abstract
AIM: To study the modulatory effects of nicotine on the binding of brain muscarinic receptors.
METHODS: The binding of brain muscarinic receptors with the agonist [3H] oxotremorine-M or the antagonist l-[3H]QNB was determined in the presence/absence of nicotine.
RESULTS: Pre-incubation of the membrane fraction derived from rat cerebral cortex with nicotine 1.0 mumol.L-1 led to a decrease in the dissociation constant (Kd) for [3H] oxotremorine-M binding to muscarinic receptors, while the maximal binding value (Bmax) was unchanged. The Kd value for binding of the muscarinic antagonist l-[3H]QNB was concentration-dependently increased by preincubation with nicotine 0.1 nmol.L-1-10.0 mumol.L-1, with Bmax unchanged. The effect of nicotine on the Kd for l-[3H]QNB binding was prevented by mecamylamine 10 nmol.L-1, but was enhanced by dithiothreitol 10 mumol.L-1, which by itself was also capable of increasing the Kd value.
CONCLUSION: Nicotine increases the affinity of brain muscarinic receptors for muscarinic agonist, but decreases the affinity of brain muscarinic receptors for muscarinic antagonist.
Keywords:
METHODS: The binding of brain muscarinic receptors with the agonist [3H] oxotremorine-M or the antagonist l-[3H]QNB was determined in the presence/absence of nicotine.
RESULTS: Pre-incubation of the membrane fraction derived from rat cerebral cortex with nicotine 1.0 mumol.L-1 led to a decrease in the dissociation constant (Kd) for [3H] oxotremorine-M binding to muscarinic receptors, while the maximal binding value (Bmax) was unchanged. The Kd value for binding of the muscarinic antagonist l-[3H]QNB was concentration-dependently increased by preincubation with nicotine 0.1 nmol.L-1-10.0 mumol.L-1, with Bmax unchanged. The effect of nicotine on the Kd for l-[3H]QNB binding was prevented by mecamylamine 10 nmol.L-1, but was enhanced by dithiothreitol 10 mumol.L-1, which by itself was also capable of increasing the Kd value.
CONCLUSION: Nicotine increases the affinity of brain muscarinic receptors for muscarinic agonist, but decreases the affinity of brain muscarinic receptors for muscarinic antagonist.