α-Bungarotoxin Binds to Low-Affinity Nicotine Binding Sites in Rat Brain

Reported differences in the pharmacology and distribution of 3H]nicotine and 125I]alpha-bungarotoxin binding sites in mammalian brain suggest that these ligands label separate receptor sites. Affinity purification of an alpha-bungarotoxin binding protein from rat brain failed to copurify the high-affinity nicotine binding site, which remained in the nonbound soluble fraction after the affinity chromatography step. This confirms the independence of these putative receptor sites. Nevertheless, the binding of 125I]alpha-bungarotoxin to P2 membranes was inhibited by (-)-nicotine (Ki = 9 X 10(-6) M), and this sensitivity was preserved after affinity purification. It is proposed that alpha-bungarotoxin binds to a population of low-affinity nicotine binding sites. Comparison of the enantiomers of nicotine in competition studies at both radioligand binding sites revealed an 80-fold preference for the (-) form at the high-affinity 3H]nicotine binding site, whereas the site labelled by 125I]alpha-bungarotoxin displayed little stereoselectivity. In this respect, the brain alpha-bungarotoxin binding site resembles the nicotinic acetylcholine receptor from Torpedo electric organ.