Octopaminergic Modulation of Flight Muscle in the Locust

The modulatory actions of octopamine on neurally induced twitch tension in the dorsal longitudinal flight muscles of the locust are described. Octopamine increases the amplitude of twitch tension, the rate of twitch contraction and the rate of twitch relaxation in this fast twitch muscle. The specificity of the receptors mediating these octopamine responses is also described. Evidence is presented to suggest that the dorsal unpaired median neurone to the locust dorsal longitudinal flight muscles (DUMDL) is an octopaminergic neurone whose activation mimics the application of exogenous octopamine to the muscle. The effects of both DUMDL and octopamine on the dorsal longitudinal muscle depend upon the frequency of stimulation of motor neurone input to the muscle. The results are discussed in terms of the behavioural significance of the release of octopamine during the first few minutes of locust flight. It is suggested that such a release is likely to be an important modulatory factor influencing the kinetics of contraction of the dorsal longitudinal muscles, resulting in an increase in the force generated by each muscle contraction together with an energy-saving adaptation due to a reduced overlap in the duration of twitches in antagonistic muscles.