Frequency coding by central olfactory neurons in the sphinx moth Manduca sexta

Sexually receptive female moths and many other insects release chemical attractants (sex pheromones) to lure conspecific mates. Recent evidence indicates, moreover, that the odor plume formed downwind from the female possesses a discontinuous structure that appears to provide the searching male with orientationcues. Using intracellular methods, we find that many central olfactory neurons in male moths (Manduca sexta) can track pulsed pheromonal stimuli precisely. The cells respond to each brief odor pulse with a similarly brief burst of action potentials, and the separation between response bursts is aided by inhibitory synaptic input. Furthermore, these neurons appear to participate in at least two levels of 'feature detection': they respond selectively to pheromonal stimuli, and they follow pulsed stimulation only in a limited range of frequencies Above the frequency limit, the cells respond as if the male is stimulated by a prolonged, uniform concentration of pheromone. The ability of these neurons to encode changes in the temporal characteristics of pheromonal stimuli may provide the male with positional cues to help him locate the pheromone source over long distances.