C.: Characterization of nicotine binding in mouse brain and comparison with the binding of alpha-bungarotoxin and quinuclidinyl

The binding of [3Hjnicotine to mouse brain has been measured and subsequently compared with the binding of [‘251]a-bungarotoxin (a-BTX) and L-[3H]quinucidinyl benzilate (QNB). The binding of nicotine was saturable, reversible, and stereospecific. The average lID and Bmax were 59 flM and 88 fmoles/mg of protein, respectively. Although the rates of association and dissociation of nicotine were temperature-dependent, the incubation temperature had no effect on either KD or Bmax. When measured at 20#{176}or 37#{176}, nicotine appeared to bind to a single class of binding sites, but a second, very low-affinity, binding site was observed at 4#{176}.Nicotine binding was unaffected by the addition of NaCl, KC1, CaCl2, or MgSO4 to the incubation medium. Nicotinic cholinergic agonists were potent inhibitors of nicotine binding; however, nicotinic antagonists were poor inhibitors. The regional distribution of binding was not uniform: midbrain and striatum contained the highest number of receptors, whereas cerebellum had the fewest. Differences in site densities, regional distribution, inhibitor potencies, and thermal denaturation indicated that nicotine binding was not the same as either QNB or a-BTX binding, and therefore that receptors for nicotine may represent a unique population of cholinergic receptors