Hippocampal norepinephrine (NE) is secreted by neurons projecting from the locus coeruleus (LC) to the hippocampus; LC nicotinic receptors (NAchRs) are involved in the effects of systemic nicotine on this pathway. To clarify the NAchR subtypes, NAchR antagonists, termed alpha-conotoxins, were microinjected into the LC before nicotine; MII and AuIB were used to assess the potential involvement of alpha3beta2 and alpha3beta4 subunit-containing NAchRs, respectively. Nicotine dose-dependently stimulated hippocampal NE release (P < 0.01). MII (>0.25 pmol) reduced the NE response to nicotine (67% decrease; P < 0.05), as did AuIB (44% reduction by 25 pmol; P < 0.05). Administered together, however, MII and AuIB were no more effective than MII. Thus, MII and AuIB are capable of interacting with NAchR subtypes other than those previously defined as alpha3beta2 and alpha3beta4, respectively. NAchRs containing both beta2 and beta4-subunits may be involved.