Effects of photoperiod on the circadian timing of pheromone response in male Trichoplusia ni: relationship to the modulatory action of octopamine

The pheromone-mediated upwind flight of male cabbage looper moths was measured throughout the scotophase in a series of photoperiods, to determine the phase-relationship between the time of peak response to pheromone during the dark and the photoperiod cues. Male upwind flight and source contact was measured to a low dose of pheromone, one that would best reveal the presence of a temporal peak in male sensitivity. Results failed to support the idea that there is a narrow 'window' of maximal sensitivity during the dark that is phase-regulated by either the lights off, or lights on signal. Rather, there was a clear relationship between the lights off signal and the time (2-3h) to reach maximal response levels, regardless of the duration of the dark. Once the point of maximal response was reached, response levels displayed relatively constant levels until the very end of the dark period. Experiments involving shifts in the time of lights off or lights on also supported the idea that the lights off cue is critical for expression and timing of the response rhythm. Other experiments confirmed that a circadian oscillator is involved, evidenced by the rhythmic expression of male behavior mirroring the duration of expected dark periods in two photoperiods when males were tested in constant dark conditions. Also, constant dark and light conditions significantly affected the pattern of change in levels of octopamine observed in the brain in a light:dark regime. Of particular interest, the pattern of decrease in octopamine levels correlated well with the pattern of response to sex pheromone over the dark, supporting an earlier hypothesis that octopamine modulates neural pathways involved in perception of the odor signal, and that this action is triggered by the lights off cue. The results support the conclusion that the lights off photoperiod cue is a critical time in which a number of physiological process are initiated that will affect male response thresholds for a number of behaviors during the dark, including our previously demonstrated modulatory action of octopamine on the sensitivity of male moths to sex pheromone.