OCTOPAMINERGIC INNERVATION AND MODULATION OF A LOCUST FLIGHT STEERINGMUSCLE

We demonstrate that the meso- and metathoracic pleuroaxillary flight s teering muscle (M85 mesothorax, M114 metathorax) of the migratory locu st are each innervated by a single dorsal unpaired median neurone (DUM 3,4,5a), The soma of this neurone can be localized by retrograde stain ing of the motor nerve with Neurobiotin, but not with cobalt salts, Th e 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 branche s, Other muscle targets include the second tergal remoter coxa (M120) and the posterior rotator coxae (M122, M123, M124), but not the first tergal remoter coxa (M119) and subalar (M129) flight muscles. Octopami ne-like immunoreactive varicosities occur on the pleuroaxillary muscle s, Stimulation of DUM3,4,5a and octopamine (10(-6) mol l(-1)) superfus ion increased the amplitude and the relaxation velocity of neurally ev oked twitch contractions of this muscle, Octopamine also significantly reduced the tonic tension that this muscle develops when stimulated a t flight frequency (20 Hz), while increasing the amplitude of each pha sic twitch, A catch-like tension is also reduced in the presence of oc topamine. Simulations of the motor pattern experienced by the pleuroax illary muscles during roll manoeuvres suggest that transient changes i n tension underlying corrective steering could be doubled in the prese nce of octopamine.