Effects of octopamine, dopamine, and serotonin on production of flight motor output by thoracic ganglia of Manduca sexta

Abstract

Effects of biogenic amines on a centrally generated motor pattern in Manduca sexta were examined by pressure injecting nanomole to micromole amounts of octopamine, dopamine or serotonin into thoracic ganglia. Motor output was recorded extracellularly from a pair of antagonistic flight muscles and their motor neurons. The monoamines were found to alter production of a motor pattern that produces rhythmic wing flapping (10 Hz) and exhibits phase relationships similar to those in the flight pattern of intact moths. In mesothoracic ganglia with sensory nerves intact, octopamine (4 × 10⁻⁹ mol) injected into lateral regions evoked regular firing of a single motor neuron, whereas a higher dose (4 × 10⁻⁸ mol) often elicited the flight motor pattern. In the absence of sensory input, these doses of octopamine had little effect. Low doses (10⁻¹⁰ mol) greatly enhanced motor responses to electrical stimulation of a wing sensory nerve. Dopamine (2 × 10⁻¹⁰ mol) injected into the medial region of the mesothoracic ganglion elicited the flight motor pattern in the presence or absence of sensory input. Rhythmic output induced by dopamine (5 × 10⁻¹⁰ mol) was suppressed by injecting serotonin (5 × 10⁻¹⁰ mol) into the same region. These findings demonstrate that dopamine, octopamine, and serotonin have different effects on motor output in Manduca and suggest that these amines are involved in initiating, maintaining and terminating flight behavior, respectively. Octopamine may elicit flight production by enhancing the efficacy of sensory transmission thereby increasing excitability or arousal. Dopamine may act on interneurons involved in generating the flight motor pattern.