Analysis of Cooperativity of Drug-Receptor Interaction by Quantitative Iontophoresis at Frog Motor End Plates

Drug concentration at receptor sites of the post-synaptic membrane of an end plate can be accurately programmed by bath application of the drug. With bath application, however, several experimental difficulties in most cases prevent the evaluation of important data of drug-receptor interaction. (1) The receptors desensitize, and the onset and mechanism of desensitization is not well understood. (2) The micro-pipettes involved in clamping circuits can deliver only several hundred nA of current; this is not sufficient for clamping the end plate when high concentrations of strong agonists are applied. Thus the saturating conductance cannot be determined. (3) The space clamp may not be sufficient to control the complete end plate area. Although focal iontophoretic application of the drug minimizes these difficulties, it also raises new ones: (a) the time course and amplitude of drug concentration in the synaptic cleft is generally not known; (b) a knowledge of the geometry of...