Metanicotine: A nicotinic agonist with central nervous system selectivity?in vitro and in vivo characterization

Abstract

A growing body of evidence suggests that disruption of nicotinic cholinergic systems may be an important factor in the etiology of a number of different diseases, ranging from neurodegenerative diseases, such as Alzheimer's and Parkinson's, to ulcerative colitis. The mechanistic basis for such diverse nicotinic effects is likely to lie in the ever growing number of potential receptor subtypes. Therefore, the development of receptor subtype-selective probes is essential to understand the emerging complexity of nicotinic cholinergic systems and the mechanisms underlying diseases that may involve these systems. Toward this end, we have evaluated the nicotinic agonist metanicotine, (E)-N-methyl-4-(3-pyridinyl)-3-butene-1-amine, using the following in vitro and in vivo methods: 1) receptor binding and up-regulation, 2) neurotransmitter release and ion flux in synaptosomes/cells, 3) in vivo microdialysis in rats, 4) reversal of scopolamine-induced amnesia in a step-through passive-avoidance paradigm, 5) water maze performance in mice, 6) radial-arm maze performance in brain-lesioned rats, 7) changes in heart rate and blood pressure, and 8) physiological depression of body temperature, locomotor activity, acoustic startle, and respiration rate. Our in vitro results indicate that metanicotine binds with high affinity to the major receptor subtype in brain (α4β2), evokes dopamine release from striatal synaptosomes and Rb+ efflux from thalamic synaptosomes, but does not activate ganglionic, muscle, or other peripheral type nicotinic receptors. These results suggest that metanicotine is selective for α4-containing central nervous system (CNS) nicotinic receptors and has reduced selectivity for peripheral nervous system (PNS) receptor subtypes. These conclusions are further supported by in vivo studies with metanicotine showing enhanced cognitive effects and significantly lower peripheral effects. Our in vivo results indicate that metanicotine increases the release of acetylcholine, norepinephrine, dopamine, and serotonin in cortex and is equal to or better than nicotine on measures of cognitive enhancement. By comparison, metanicotine is significantly less potent than nicotine in increasing heart rate and blood pressure and in causing physiological depression. These results are consistent with in vitro data indicating metanicotine's CNS receptor selectivity, and they suggest that this ligand may be a suitable tool for probing the relationships that underlie the complex central and peripheral pharmacology of nicotinic cholinergic systems. Furthermore, metanicotine may be a good lead candidate for developing nicotinic agonists as CNS therapeutics with reduced peripheral side effects. Drug Dev. Res. 38:169–176 © 1996 Wiley-Liss, Inc.