Pheromone-triggered flip-flopping interneurons in the ventral nerve cord of the silkworm moth,Bombyx mori

Abstract

1. A group of extracellularly recorded descending interneurons in the ventral nerve cord of the male silkworm mothBombyx mori share a common flip-flopping input. In response to repeated stimuli these flip-flopping interneurons switch back and forth between long lasting high and low firing rates (Figs. 1, 2). 2. Changes in the level of the female pheromone bombykol in an airstream directed at the antennae were the most effective stimuli in eliciting flip-flopping of the interneurons. Changes in light intensity, up or down, were the only other effective stimuli (Fig. 2, Table 1). 3. The low or high firing frequency states following a single stimulus presentation lasted as long as 4 min, although with some decrement (Fig. 3a). 4. The high and low states corresponded to two different positions of the antennae. Transitions between states were mirrored by changes in antennal position. Immobilizing the antennae did not eliminate flip-flopping (Fig. 5). 5. In intact animals, the two antennal positions corresponded to left and right turning movements of the legs and to left and right ‘ruddering’ of the abdomen (Fig. 6). 6. When bombykol-laden air puffs were presented to a single antenna, the neurons sometimes showed flip-flopping responses, but often remained in a ‘favored’ state between stimuli (Fig. 7). 7. When different concentrations of bombykol were presented to the two antennae, the interneurons assumed a high or low firing rate, consistent with a turn toward the antenna receiving the higher concentration. Different concentrations of bombykal to the two antennae (with equal, constant bombykol levels to the two antennae) elicited the opposite effect, i.e., firing rates consistent with turning away from the higher bombykal concentration (Fig. 8). 8. Most interneurons responded in graded fashion to other stimuli besides those which elicited flip-flopping (Fig. 9). Six response types were discriminated, based upon exclusive combinations of sensory inputs and upon the direction of the correlation between flip-flopping states and animal turning (Table 2). 9. All of the results support the hypothesis that these interneurons carry left and right turning instructions for pheromone-directed turns in the upwind approach of maleBombyx to the female.