Octopaminergic Innervation and Modulation of a Locust Flight Steering Muscle

We demonstrate that the meso- and metathoracic pleuroaxillary flight steering muscle (M85 mesothorax, M114 metathorax) of the migratory locust are each innervated by a single dorsal unpaired median neurone (DUM3,4,5a). The soma of this neurone can be localized by retrograde staining of the motor nerve with Neurobiotin, but not with cobalt salts. The primary neurite projects in the superficial DUM cell tract, and the axons run in nerve roots 3, 4 and 5 and in all their secondary branches. Other muscle targets include the second tergal remotor coxa (M120) and the posterior rotator coxae (M122, M123, M124), but not the first tergal remotor coxa (M119) and subalar (M129) flight muscles. Octopamine-like immunoreactive varicosities occur on the pleuroaxillary muscles. Stimulation of DUM3,4,5a and octopamine (10(-6) mol l-1) superfusion increased the amplitude and the relaxation velocity of neurally evoked twitch contractions of this muscle. Octopamine also significantly reduced the tonic tension that this muscle develops when stimulated at flight frequency (20 Hz), while increasing the amplitude of each phasic twitch. A catch-like tension is also reduced in the presence of octopamine. Simulations of the motor pattern experienced by the pleuroaxillary muscles during roll manoeuvres suggest that transient changes in tension underlying corrective steering could be doubled in the presence of octopamine.