Nicotinic acetylcholine receptor chimeras of rat α7 and Drosophila SAD reveal species-specific agonist binding regions
✍ Scribed by van den Beukel, Ingeborg; Klaassen, Remco; Smit, Guus B; van Kleef, Regina G D M; Oortgiesen, Marga
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 88 KB
- Volume
- 55
- Category
- Article
- ISSN
- 1526-498X
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✦ Synopsis
a-BgTx, EPI or IMI. In contrast the dissociation rate was markedly increased when initiated by MLA compared to a-BgTx, EPI or IMI (Fig 1).
Displacement of [ 125 I]a-BgTx, [ 3 H]EPI and [ 3 H]IMI by MLA was with high potency with an IC 50 value close to the K d value of each of these labelled ligands. Displacement of [ 3 H]MLA by a-BgTx, EPI and IMI resulted in low potency, but was characterised by shallow displacement curves.
DISCUSSION AND CONCLUSIONS
Saturation studies in M persicae membranes demonstrate that [ 125 I]a-BgTx, [ 3 H]EPI, and [ 3 H]IMI all label two binding components of differing af®nities in a ratio of 1:3. In each case the sum of the high and low af®nity B max values approximated the B max value of the single site labelled by [ 3 H]MLA. [ 3 H]MLA is a novel radioligand which is highly selective in vertebrate nAChRs, 3 but which appears to be unable to distinguish between a heterogeneous population of nicotinic ligand binding sites in insects.
The increase in dissociation rate of [ 3 H]a-BgTx using MLA instead of either a-BgTx, EPI or IMI is consistent with an allosteric interaction by MLA at a binding site other than that of the high af®nity [ 3 H]a-BgTx binding site.
Displacement studies have shown that MLA is a potent ligand at sites labelled with high af®nity by [ 125 I]a-BgTx, [ 3 H]EPI, and [ 3 H]IMI. The shallow displacement curves obtained when [ 3 H]MLA is displaced by a-BgTx, EPI, and IMI (data not shown) are likely to be due to displacement of [ 3 H]MLA from at least two distinct binding sites, for which the displacing ligands have different af®nities, but which are indistinguishable by [ 3 H]MLA.
Interestingly, the high af®nity binding of [ 3 H]IMI in hemipteran membranes compared to that in nonhemipteran insects may help explain why IMI is particularly useful for the control of sucking pests. REFERENCES 1 Elbert A, Becker B, Hartwig J and Erdelen C, Imidacloprid ± a new systemic insecticide. P¯anzenschutz-