Neural control mechanisms of the pheromone-triggered programmed behavior in male silkmoths revealed by double-labeling of descending interneurons and a motor neuron.

Male silkmoths, Bombyx mori, exhibit a characteristic zigzagging behavior consisting of straight-line walking, zigzagging turns, and looping. The timing for shifting the turning direction is synchronized to the sideways head movements controlled by neck motor neurons (NMNs) including a cervical ventral NMN (cv1-NMN). It has been suggested that this programmed behavior is instructed by two types of activity patterns descending from the brain and the thoracic ganglion: one is a phasic excitation and the other is a state-dependent activity similar to the flipflop in electric memory circuits. These activities are shown by certain descending interneurons contained in two subsets of DNs, Group-I and -II DNs. However, it is not yet well understood which DNs are directly related to instructing this behavior. In order to understand neural control mechanisms of this programmed behavior, we investigated the morphological relationship between these DNs and the cv1-NMN, which is an index of this programmed behavior. We applied a double-labeling technique combining backfilling of the cv1-NMN and intracellular staining of single DNs. 3D confocal images revealed overlapping regions between the Group-I, -II DNs and the cv1-NMN. Group-IIA and -IID, which showed typical flipflop activities, Group-IIC DNs, which showed phasic excitation, and Group-IB DNs, which showed long-lasting inhibition had many overlapping regions on the cv1-NMNs. Our results indicate that the programmed behavior is instructed by these types of DNs.