摘要：

Reported differences in the pharmacology and distribution of [3H]nicotine and [125I]α‐bungarotoxin binding sites in mammalian brain suggest that these ligands label separate receptor sites. Affinity purification of an α‐bungar‐otoxin binding protein from rat brain failed to copurify the high‐affinity nicotine binding site, which remained in the nonbound soluble fraction after the affinity chromatogra‐phy step. This confirms the independence of these putative receptor sites. Nevertheless, the binding of [125I]α‐bungaro‐toxin to P2membranes was inhibited by (‐)‐nicotine (Ai= 9 × 10‐6M), and this sensitivity was preserved after affinity purification. It is proposed that α‐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]α‐bungarotoxin displayed little stereoselectivity. In this respect, the brain α‐bungarotoxin binding site resembles the nicotinic acetyl‐

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