Sniffing out insect control

How do insects smell? Badly, according to a study, if they lack a certain kind of protein critical to their unique ability to detect and interpret pheromones--the insect equivalent of "smelling." Researchers at the University of Texas Southwestern Medical Center, Dallas, have discovered how an olfactory binding protein links incoming pheromone signals and specific nerve cells in an insect's brain, which in turn translates those signals. Pheromones are chemical signals given off by animals that, when detected by others of the same species, mediate a variety of behaviors, such as feeding, mating, and colonizing.

Scientists have discovered that fruit flies lacking a certain protein are unable to process pheromone signals properly into the appropriate behavior. Research by Dean Smith, associate professor of pharmacology, is the first to link pheromone-induced behavior directly with the activity of olfactory binding proteins, or OBPs, and may aid in the development of new insect control methods.

The nerve cells, or neurons, in insects responsible for picking up on pheromone signals have been studied for decades, as have pheromones themselves. However, the biochemical mechanism by which pheromones and other odorants selectively activate those sensory neurons is poorly understood. "We've known about OBPs for 20 years, but, until now, their function and significance was unclear," indicates Smith, who works in the Center for Basic Neuroscience. Olfactory binding proteins are produced by non-neural cells and are secreted into the fluid bathing the dendrites, or nerve endings, of olfactory neurons.

Smith's research group found that an OBP in fruit flies called LUSH is required for olfactory neurons to smell the pheromone 11-cis vaccenyl acetate, or VA. Mutant flies lacking the gene that codes for the LUSH protein are unable to detect the VA pheromone and do not display the behavior associated with that pheromone, which normally signals the flies to aggregate in groups.

When the VA pheromone contacts a tiny hair on a fly's antenna, it binds with the LUSH protein. Once bound, the LUSH protein changes shape so it can fit into a receptor on the surface of a specific olfactory neuron inside the hair, which sends the appropriate behavior signal to the bug. "Without LUSH as a bridge, this pheromone can't get its signal to the neuron and the fly doesn't behave normally," Smith notes.

"This reduction in spontaneous activity was a surprising finding. Our results indicate that LUSH, and not the pheromone, is what directly activates the chemosensory neurons. It is likely that OBPs in other insects also work this way, although the pheromones are different in different species. We think that OBPs might be new targets for insect control and repellents."

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