Binding parameters for 125I-epibatidine and [125I]A-85380 at striatal high- and low-affinity α -CtxMII-sensitive, and α -CtxMII-resistant sites
Binding of 125I-epibatidine and [125I]A-85380 to striatal membranes was performed as described in text. Three populations of nicotinic binding sites were identified with each radioligand. These sites had similar affinities for the radioligands but could be distinguished by their distinctly different affinities for α -CtxMII. Each value represents the mean ± S.E.M. of four animals. The Ki values for α -CtxMII inhibition of [125I]epibatidine binding for the high- and low-affinity sites were significantly different from one another (two-way ANOVA, p < 0.001), with similar results for [125I]A-85380 binding. No significant differences were detected when the binding densities of each of the sites were compared between the two radioligands (one-way ANOVA, p > 0.05). As well, the α -CtxMII Ki values obtained at the high-affinity α -CtxMII–sensitive sites were statistically indistinguishable when determined using either of the radioligands; similar results were obtained for the low-affinity sites.
| Ligand | Parameter | α-Ctx-MII—Sensitive Sites | α-Ctx-MII—Resistant Site | |
|---|---|---|---|---|
| High Affinity | Low Affinity | |||
| 125I-Epibatidine | Bmax (fmol/mg of protein) | 14.0 ± 1.9 | 14.6 ± 2.2 | 48.9 ± 6.2 |
| n h | 1.41 ± 0.25 | 1.07 ± 0.07 | 0.95 ± 0.06 | |
| Kd (pM) | 21.7 ± 5.5 | 57.5 ± 26.7 | 88.8 ± 29.6 | |
| Ki for α-CtxMII (nM) | 0.15 ± 0.04 | 149 ± 85 | ||
| [125I]A-85380 | Bmax (fmol/mg of protein) | 16.1 ± 4.8 | 10.6 ± 2.0 | 39.6 ± 4.6 |
| n h | 1.03 ± 0.07 | 1.10 ± 0.16 | 1.33 ± 0.32 | |
| Kd (pM) | 59.5 ± 28.8 | 23.7 ± 5.0 | 56.1 ± 29.8 | |
| Ki for α-CtxMII (nM) | 0.11 ± 0.06 | 98.3 ± 66.1 | ||