A chimera based on the chicken alpha 4 nicotinic acetylcholine receptor (nAChR) subunit containing an insert from loop B to the N-terminus of the Drosophila melanogaster D alpha 2 (=SAD) subunit was constructed and co-expressed with the chicken beta 2 nAChR subunit in Xenopus laevis oocytes. The actions of the neonicotinoid insecticide imidacloprid were examined. Replacement of the region loop B to the N-terminus of the alpha 4 subunit by the corTesponding region of the D alpha 2 subunit had little effect on the concentration-response curve for imidacloprid. However, replacement of Glu219 by proline in the YXCC motif in loop C of the chimeric alpha 4 subunit resulted in a marked displacement to the left of the concentration-response curve for imidacloprid not seen when an equivalent mutation was made in the alpha 4 beta 2 nAChR. The results suggest that the region loop B to the N-terminus in the D alpha 2 subunit contributes to the high imidacloprid sensitivity of the hybrid D alpha 2 beta 2 nAChR. (c) 2005 Elsevier Ireland Ltd. All rights reserved.