Serotonin modifies the sensitivity of the male silkmoth to pheromone.

Serotonin is known to modulate the response of neuronal populations in the primary olfactory center of the moth olfactory system, the antennal lobe. Here, we analyzed the effects of serotonin on the behavior related to the restricted pheromone olfactory pathway of the male silkmoth, Bombyx mori. In order to understand the effects of serotonin at the behavioral level, we applied serotonin (10(-5) mol l(-1), 10(-4) mol l(-1) and 10(-3) mol l(-1)) to the brain and found that 10(-4) mol l(-1) serotonin increases the sensitivity to female pheromone whereas 10(-3) mol l(-1) serotonin had the opposite effect. Levels of serotonin in the brain were determined using HPLC with electrochemical detection. Inhibitory effects were observed after applying the serotonin antagonists mianserin (10(-4) mol l(-1)) and ketanserin (10(-3) mol l(-1)). Additionally, we quantified the circadian variation of serotonin in the brain using HPLC with electrochemical detection. Further, this variation correlated well with a circadian variation of the male sensitivity to pheromone. These results show that the serotonin-related enhancement of neuronal responses at the antennal lobe level is expressed at the behavioral level as a modulation of pheromone sensitivity and that the circadian variation of serotonin levels in the brain correlates with changes in the moth's pheromone sensitivity.