Laser-Pulse Technique Could Aid Drug Design

The same technique, called laser-pulse photolysis, already has identified a cocaine-like analog compound to block the effects of cocaine poisoning on the nervous system, Hess said at the annual meeting of the American Association for the Advancement of Science (AAAS), held last week in Denver.

"Mechanism-based drug design can proceed more rationally, knowing the exact roles and timing of all the chemical players at the junctions between neurons and muscle cells or between neurons and other neurons," Hess said. The recent discovery of cocaine analogs, one application of the laser-pulse photolysis technique, will be reported in the scientific literature, Hess said. "These compounds are not ready for clinical use, but they do provide a lead for pharmaceutical development."

Laser-pulse photolysis allows neuroscientists to look at entire ensembles of molecules--not just single channels or single molecules -- during split-second chemical reactions that relay electrical signals through the nervous system, Hess said. "These reactions can be over and done in 0.3 millisecond. To observe them in detail, we need to equilibrate the receptor with the neurotransmitter in much shorter time frames," he said. One way to beat the clock and test the effects of a potential therapeutic agent is to present the compound as a "caged neurotransmitter," a specially constructed molecule that has no effect on receptor proteins so it can be mixed with a cell without triggering reactions.

Once the caged neurotransmitter is in place, according to Hess, a single pulse of laser light can cleave the protective cage within microseconds, allowing the neurotransmitter to bind to receptors. Then, as the freshly exposed neurotransmitter opens transmembrane channels through which electrical currents flow, investigators can watch the millisecond shifts -- between open and closed states of channels -- and determine whether a drug, such as a cocaine analog, is having the desired effect.

With GEFS epilepsy, a genetic mutation is believed to be responsible for a single, inappropriate amino acid in the so-called GABA receptor in brain cells. (The disorder is called febrile because, out of the approximately 5 percent of young children who experience seizures during a high fever, a small proportion with a genetic predisposition later develop epilepsy.) Using laser-pulse photolysis, Hess and his students discovered the reason for the receptor malfunction: a shift in the equilibrium from the open-channel form toward the closed-channel form. They also tested several compounds with high molecular weights that can shift the channel-opening equilibrium -- an encouraging indication that small-molecule drugs can be found to overcome mutations in the GABA receptor and halt the raging "electrical storms" that characterize epilepsy.

Also valuable would be a treatment to short-circuit the electrochemical effects of cocaine, Hess said. "There are more than 5 million cocaine users in the United States alone, at an estimated cost to society of $37 billion annually," he said, citing a 1999 report by the US Office of National Drug Control Policy. "During the past two decades, many attempts have been made to find compounds that prevent cocaine's inhibition of proteins that are essential in brain function, but compounds that alleviate cocaine inhibition have not been identified -- until now."

The expectation is, Hess says, "that the effects of disease-causing mutations of receptors that control signal transmission between nerve cells will be better understood -- and potentially useful treatments will be identified and tested -- using this new technique."

Studies leading to the development of laser-pulse photolysis and tests of its applications were supported, in part, by grants from the National Institutes of Health.

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