Synthesis and photochemical properties of a kainate precursor and activation of kainate and AMPA receptor channels on a microsecond time scale.

Kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolylpropionate (AMPA) receptors are transmembrane proteins that can form ion channels upon binding a specific ligand. The receptors are located at major excitatory synapses in the mammalian central nervous system. Kainate and AMPA receptors participate in many physiological activities of the brain, including learning and memory, and are involved in many neurological disorders. Elucidation of the mechanisms of receptor transmembrane channel formation, inhibition, and regulation is important in understanding fundamental central nervous system function and in designing potential therapeutic agents. Kainate can activate both kainate and AMPA receptors, leading to channel opening in the microsecond to millisecond time region. A newly developed laser pulse photolysis technique, with a microsecond time resolution, has been successfully used to study the chemical reactions of receptor proteins in the microsecond to millisecond time region. To apply the technique to kainate and AMPA receptors, a photolabile kainate precursor in which a caging group, the alpha-carboxy-2-nitrobenzyl group, is attached to the gamma-carboxyl group of kainic acid has been synthesized. The photolytic release of free kainate from the caged kainate on the microsecond time scale, initiated by a pulse of laser light at 308 nm, was measured. The quantum yield is 0.34 at pH 6.8 and room temperature. The half-life of the major component (approximately 86%) of the photolytic reaction is 45 microseconds, while that of the minor component (approximately 14%) is 0.7 ms. The effects of the caged kainate on kainate and AMPA receptors endogenously expressed in rat hippocampal neurons were also evaluated. Caged kainate (750 microM) did not activate the receptor channels, nor did it potentiate or inhibit the kainate response. Photolysis of the caged kainate by a pulse of 333-nm laser light resulted in a rapid rise (with a t1/2 of 0.4 ms) in the whole-cell current due to the opening of kainate-activated receptor channels. The results presented demonstrate that this kainate precursor is suitable for rapid chemical kinetic investigations of the kainate and AMPA receptors in the microsecond to millisecond time region.