Octopamine modulates the activity of motoneurons related to calling behavior in the gypsy moth Lymantria dispar.

A morphofunctional investigation of the different neuronal subpopulations projecting through each of the nerves IV-VI emerging bilaterally from the terminal abdominal ganglion (TAG) was correlated with the octopaminergic activity in the ganglion that controls the ovipositor movements associated with calling behavior in the female gypsy moth Lymantria dispar. Tetramethylrodamine-dextran backfills from nerve stumps resulted in a relatively low number of TAG projections, ranging from 12 to 13 for nerve pair IV, 12 to 14 for nerve pair V, and 8 to 9 for nerve pair VI. Furthermore, as assessed by electrophysiological recordings, a number of fibers within each of these nerves displays spontaneous tonic activity, also when the ganglion is fully disconnected from the ventral nerve cord (VNC). Octopamine (OA) applications to the TAG strongly enhanced the activity of these nerves, either by increasing the firing rate of a number of spontaneously firing units or by recruiting new ones. This octopaminergic activity affected calling behavior, and specifically the muscle activity leading to cycling extensions of the intersegmental membrane (IM) between segments VIII and IX (ovipositor). Our results indicate that in the female gypsy moth the octopaminergic neural activity of the TAG is coupled with extensions and retractions of IM for the purpose of releasing pheromone, where motor units innervated by nerve pair IV appear antagonistic with respect to those innervated by nerve pair V.