Forskolin modulates acetylcholine receptor gating by interacting with the small extracellular loop between the M2 and M3 transmembrane domains.

1. Forskolin acts as an allosteric modulator of muscle-type nicotinic acetylcholine receptors. Receptors from mouse muscle and Torpedo electroplax demonstrate differential sensitivity to inhibition by forskolin. Previous work from this laboratory suggested that the gamma subunit is responsible for this differential sensitivity. 2. We have used a series of mouse/Torpedo species-chimeric gamma subunits to further define the site of forskolin interaction with the gamma subunit. Analysis of the patterns of forskolin inhibition of receptors containing mouse/Torpedo chimeric gamma subunits along with the mouse alpha, beta, and delta subunits suggests that forskolin interacts with the small extracellular domain that links the M2 and M3 transmembrane domains (the M2-M3 linker). 3. We suggest that the M2-M3 linker domain plays an important role in the transduction of ligand binding to the conformational changes that result in channel opening.