Response of acetylcholine receptors to rapid photochemically produced increases in agonist concentration

SYNAPTIC transmission to muscle fibres and electroplaques takes place on a millisecond timescale. To study the molecular events leading to activation of the cholinergic receptor channel, we have developed a method to produce similarly rapid, spatially uniform steps of agonist concentration near intact postsynaptic membranes. Electrophorus electroplaques in a voltage-clamp apparatus were exposed to a solution containing a photochromic compound. Initially the compound was in a predominantly cis form, which had little effect on the membrane. During a brief light flash, some of the compound was isomerised to the trans isomer, which is a cholinergic agonist. As a result the membrane conductance increased along a timecourse which reflects the rate processes underlying activation of the cholinergic receptor channel