Octopaminergic Regulation of an Insect Visceral Muscle

Abstract

Octopamine is now well established as a naturally occurring biogenic amine in the nervous system of invertebrates and vertebrates. While its physiological role within the vertebrate nervous system is still only speculative, its importance as a chemical messenger in the invertebrates is well established (see Orchard, 1982; Evans, 1985). However, despite the overwhelming body of knowledge covering its role as a neurotransmitter, neurohormone and neuromodulator, identification of octopamine containing neurons has only been made in a very limited number of cases. In insects, identified octopaminergic neurons have been shown to belong to a group of specialised cells located on the dorsal mid-line of the thoracic and abdominal ganglia (see Evans, 1985). These cells have bifurcating axons which project symmetrically into left and right peripheral nerve roots of the ganglion and consequently have been called DUM neurons. Examples of such neurons include DUMETi, in the metathoracic ganglion of locusts (Evans and O’Shea, 1978), and four DUM cells in the VIIIth abdominal ganglion of the firefly (Christensen et al., 1983). DUMETi modulates neuromuscular transmission and myogenicity of the extensor tibiae muscle in the hind leg, while the Photuris DUM cells regulate larval luminescence. While DUM neurons appear to be distributed extensively throughout the ventral nerve cord little is known of their function except from studies of the identified neurons mentioned above.