[
Cold Spring Harb Symp Quant Biol,
1996]
An animal generates appropriate behavioral responses to a stimulus based on the intrinsic qualities of the stimulus, the context in which it appears, and the animal's previous experience. Olfactory stimuli elicit responses that are often reproducible across different individuals in a species: Many animals display characteristic behaviors in response to the smell of members of their species, food sources, or dangerous conditions. Other sensory modalities can also evoke innate responses, but in the olfactory system, odorants that elicit distinct behaviors are differentiated at the first step of sensory detection by the olfactory receptor neurons. Therefore, the neuronal circuitry responsible for different behaviors can be traced starting from the initial sensory events. Our studies have focused on olfaction and chemosensation in the nematode Caenorhabditis elegans. Chemosensation is the richest mechanism C. elegans has for interacting with its environment, both in the number of different stimuli that are recognized and in the variety of different responses that can be elicited. Virtually all behaviors in C. elegans are modulated by chemical cues. Individual molecules can be attractants or repellents, or they can regulate egg-laying, feeding, or movement. In addition, pheromone cues control development of the animal and mating behavior between males and hermaphrodites. How are these chemical cues recognized and discriminated from one another? The answer to this question lies partly in the biochemical mechanisms that recognize individual odorants, and partly in the neuronal circuitry that drives particular