Characterization of nicotinic acetylcholine receptors that modulate nicotine-evoked [3H]norepinephrine release from mouse hippocampal synaptosomes

Nicotine's modulation of hippocampal noradrenergic neuro-transmission may contribute to its mnemonic properties, but the nicotinic acetylcholine receptor ( nAChR) subtypes that modulate terminal release of norepinephrine are unknown. In the present study, we used a number of subtype-selective alpha-conotoxins in combination with nicotinic receptor subunit-deficient mice to characterize nAChRs that modulate [H-3] norepinephrine release from synaptosomes. The results indicate that at least two populations of nAChRs contribute to this release: a novel alpha 6(alpha 4)beta 2 beta 3 beta 4 subtype and an alpha 6(alpha 4)beta 2 beta 3 subtype. These are distinct from subtypes that modulate synaptosomal norepinephrine release in the rat hippocampus in which an alpha 6/beta 2 and/or alpha 6/beta 4 ligand binding interface is not present. Whereas alpha-conotoxin MII fully inhibits nicotine-evoked [ 3H] norepinephrine release in mouse, it is ineffective in blocking [ H-3] norepinephrine release in rat. Block of [ 3H] norepinephrine release by alpha-conotoxin BulA, a toxin that kinetically distinguishes between beta 2- and beta 4-containing nAChRs, was partially reversible in mouse but irreversible in rat. This indicates that in contrast to rat, mouse nAChRs are made of both beta 4 and non-beta 4-containing populations. Results from beta 2 and beta 4 null mutant mice confirmed this conclusion, indicating the presence of the beta 2 subunit in all nAChRs and the presence of the beta 4 subunit in a subpopulation of nAChRs. In addition, both beta 4 and beta 3 subunits are essential for the formation of functional nAChRs on mouse noradrenergic terminals. Cytisine, a ligand formerly believed to be beta 4-selective, was a highly effective agonist for alpha 6 beta 2-containing nAChRs. The sum of these results suggests a possible novel nAChR subtype that modulates noradrenergic neurotransmission within the mouse hippocampus.