Raymond Wieboldt

Determination of the Chemical Mechanism of Neurotransmitter Receptor‐mediated Reactions by Rapid Chemical Kinetic Methodsa

How do nerve cells receive information from within and without the organism? How can they store information for short and long periods of time? What role do chemical reactions play in the perception and storage of information? Signal initiation between cells of the nervous system is regulated to a large extent by membranebound proteins, the neurotransmitter receptors. The proteins in the membrane of one cell bind chemical signals, neurotransmitters, released by a neighboring cell. The reactions involve ligand-binding steps and conformational changes of the protein. Upon binding a specific neurotransmitter, the receptors form transiently open transmembrane channels. Receptors for excitatory neurotransmitters allow sodium and potassium ions to pass through the open channels. Receptors for inhibitory neurotransmitters allow the passage of chloride ions. The properties of the receptormediated reactions are fundamental to the ability of a neuron to function. They determine the changes in transmembrane voltage that trigger signal transmission between neurons. Many of the receptor-mediated reactions occur on a sub-millisecond timc scale. Until recently, chemical kinetic techniques suitable for use with membrane-bound receptor proteins that must be studied in intact cells and vesicles in the submillisecond time region were not available. When it was observed that desensitization (the transient and reversible inactivation) of a neurotransmitter receptor can occur in the millisecond time such techniques were developed.” They allow one to determine (1) the ligand-binding properties of the active form both before and after the receptor has dcsensitized,S (2) the rate constants for receptor desensitization,5 and ( 3 ) the rate constants for the formation of the open transmembrane ~hannel.89~ Here we describe three aspects of our work. First, we outline why we use chemical kinetic approaches to study receptor-mediated reactions on the submillisecond time scale. Then we describe a newly developed chemical kinetic mcthod, laser-pulse photolysis of caged neurotransmitters, with a time resolution on