Abstract

α-Conotoxin MII (CtxMII), a peptide toxin from the venom of the predatory cone snail Conus magus, displays an unusual nicotinic pharmacology. Specific binding of a radioiodinated derivative (¹²⁵I-α-CtxMII) was identified in brain region homogenates and tissue sections. Quantitative autoradiography indicated that ¹²⁵I-α-CtxMII binding sites have an unique pharmacological profile and distribution in mouse brain, being largely confined to the superficial layers of the superior colliculus, nigrostriatal pathway, optic tract, olivary pretectal, and mediolateral and dorsolateral geniculate nuclei. Expression of α-CtxMII binding sites in the nigrostriatal pathway, combined with evidence for α-CtxMII-sensitivity of nicotine-induced [³H]dopamine release in rodent striatal preparations indicates that ¹²⁵I-α-CtxMII binding nicotinic acetylcholine receptors are likely to be physiologically important. Unlabeled α-CtxMII potently (K_i < 3 nM) competed for a subset of [³H]epibatidine binding sites in mouse brain homogenates, but weakly (IC₅₀ > 10 μM) interacted with ¹²⁵I-α-bungarotoxin and (−)-[³H]nicotine binding sites, confirming this compound's novel nicotinic pharmacology. Quantitative autoradiography revealed that α-CtxMII binds with high affinity at a subset of [³H]epibatidine binding sites with relatively low cytisine affinity (“cytisine-resistant” sites), resolving [³H]epibatidine binding into three different populations, each probably corresponding to a receptor subtype. The majority population seems to correspond to that which binds nicotine and cytisine with high affinity (“cytisine-sensitive” sites). Comparison of the cytisine-resistant population's distribution with that of α3 subunit mRNA expression suggests that the fractions both more and less sensitive to α-CtxMII probably contain the α3 subunit, perhaps in combination with different β subunits.