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

Nicotine's modulation of hippocampal noradrenergic neurotransmission 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 [3H]nore-pinephrine release from synaptosomes. The results indicate that at least two populations of nAChRs contribute to this release: a novel alpha6(alpha4)beta2beta3beta4 subtype and an alpha6(alpha4)beta2beta3 subtype. These are distinct from subtypes that modulate synaptosomal norepinephrine release in the rat hippocampus in which an alpha6/beta2 and/or alpha6/beta4 ligand binding interface is not present. Whereas alpha-conotoxin MII fully inhibits nicotine-evoked [3H]norepinephrine release in mouse, it is ineffective in blocking [3H]norepinephrine release in rat. Block of [3H]norepinephrine release by alpha-conotoxin BuIA, a toxin that kinetically distinguishes between beta2- and beta4-containing nAChRs, was partially reversible in mouse but irreversible in rat. This indicates that in contrast to rat, mouse nAChRs are made of both beta4 and non-beta4-containing populations. Results from beta2 and beta4 null mutant mice confirmed this conclusion, indicating the presence of the beta2 subunit in all nAChRs and the presence of the beta4 subunit in a subpopulation of nAChRs. In addition, both alpha4 and beta3 subunits are essential for the formation of functional nAChRs on mouse noradrenergic terminals. Cytisine, a ligand formerly believed to be beta4-selective, was a highly effective agonist for alpha6beta2-containing nAChRs. The sum of these results suggests a possible novel nAChR subtype that modulates nor-adrenergic neurotransmission within the mouse hippocampus.