I-125-alpha-conotoxin MII identifies a novel nicotinic acetylcholine receptor population in mouse brain

alpha-Conotoxin MII (CtxMII), a peptide toxin from the venom of the predato ry cone snail Conus magus, displays an unusual nicotinic pharmacology. Spec ific binding of a radioiodinated derivative (I-125-alpha-CtxMII) was identi fied in brain region homogenates and tissue sections. Quantitative autoradi ography indicated that I-125-alpha-CtxMII binding sites have an unique phar macological profile and distribution in mouse brain, being largely confined to the superficial layers of the superior colliculus, nigrostriatal pathwa y, optic tract, olivary pretectal, and mediolateral and dorsolateral genicu late nuclei. Expression of alpha-CtxMII binding sites in the nigrostriatal pathway, combined with evidence for alpha-CtxMII-sensitivity of nicotine-in duced [H-3]dopamine release in rodent striatal preparations indicates that I-125-alpha-CtxMII binding nicotinic acetylcholine receptors are likely to be physiologically important. Unlabeled alpha-CtxMII potently (K-i < 3 nM) competed for a subset of [H-3]epibatidine binding sites in mouse brain homo genates, but weakly (IC50 > 10 mu M) interacted with I-125-alpha-bungarotox in and (-)-[H-3]nicotine binding sites, confirming this compound's novel ni cotinic pharmacology. Quantitative autoradiography revealed that alpha-CtxM II binds with high affinity at a subset of [H-3]epibatidine binding sites w ith relatively low cytisine affinity ("cytisine-resistant" sites), resolvin g [H-3]epibatidine binding into three different populations, each probably corresponding to a receptor subtype. The majority population seems to corre spond to that which binds nicotine and cytisine with high affinity ("cytisi ne-sensitive" sites). Comparison of the cytisine-resistant population's dis tribution with that of alpha 3 subunit mRNA expression suggests that the fr actions both more and less sensitive to alpha-CtxMII probably contain the a lpha 3 subunit, perhaps in combination with different beta subunits.