Light-activated compounds as probes for nicotinic acetylcholine receptors

Nanoengineering Ion Channels for Optical Control

“…Using a tethered ligand compound of similar length, but bearing an azobenzene in the tether (QBr; FIGURE 1B), allowed the agonist to bind the receptor and activate it in the trans configuration, whereas in the cis configuration the tether was too short for the agonist to bind (5). Photoisomerization and subsequent induction of nAChR currents could be achieved reversibly with millisecond light pulses (12,28,30), but it was found that QBr acts as a partial and a quaternary ammonium blocking group at the other end (MAQ) (FIGURE 1D) was found to conjugate E422C and block the channel in the trans state, and blocking was relieved in the cis state under UV illumination (4). In this modified K + channel, termed SPARK, the extended configuration of azobenzene allows the ligand to reach the binding site, whereas the short form does not (FIGURE 1C), as was the case in nAChR, described above.…”

“…Pulses of light lasting only a millisecond, and having intensities that are typically used in confocal microscopy (i.e., which are well tolerated by cells and live animals), are long enough to open and close the channels and trigger significant changes in membrane potential, and thus to elicit action potentials (43). The photoisomerization rate of azobenzene is faster still, in the picosecond range (41), with a quantum yield of 0.2-0.4 (8), which has allowed kinetic studies of gating mechanisms (28,30). In the dark, cistrans relaxations depend on substitutions in and around the azobenzene, and rates have been reported between milliseconds to days for spontaneous relaxation into the more stable trans state.…”