ABSTRACT
Spikes in the octopaminergic dorsal unpaired median (DUM) neurone which innervates the extensor tibiae muscle of the locust metathoracic leg (DUMETi) produce direct and indirect effects on muscle tension.
Direct effects include a slowing of an intrinsic rhythm of contraction and relaxation, a relaxation of muscle tone and a small hyperpolarization of the muscle membrane potential. The latter two effects are weak and variable. All three effects are mimicked by superfusion of octopamine and are mediated by octopamine receptors on the muscle fibres.
Indirect effects are found when the DUMETi neurone is stimulated at the same time as the motoneurones innervating the extensor muscle. They include (a) potentiation of tension generated in the extensor muscle by spikes in the slow excitatory motoneurone (SETi), (b) reduction in duration of each twitch contraction generated by SETi due to an increase in the rate at which the muscle relaxes, (c) increase in the amplitude of the synaptic potential generated by SETi. These various effects have a time course of several minutes and far outlast the duration of DUMETi stimulation. They can be mimicked by superfusion of octopamine.
The effect of DUMETi on neuromuscular transmission is mediated by receptors with a high affinity for octopamine located both on the muscle and on the terminals of the slow motoneurone. The presence of the presynaptic receptors is revealed by the increase in the frequency of spontaneous miniature end plate potentials recorded in the muscle in the presence of octopamine.
DUMETi is a member of a group of similar aminergic neurones and it is suggested that they may share a role in modulating transmission at peripheral neuromuscular synapses, and possibly central synapses.
Not all of the neurones with spiking cell bodies in the dorsal median group conform to the same pattern as DUMETi (unpublished observations and C. S. Goodman, personal communication). Some are confined to the ganglion and do not have peripheral axons, while others are bilaterally asymmetrical. There is a question, therefore, as to whether the DUM neurones are all really ‘unpaired’ in the adult. The term unpaired is, however, appropriate if one considers the unique origin of the neurones from the single, unpaired median neuroblast (C. S. Goodman, personal communication). At this time, therefore, a change of generic name is not necessary but may become necessary later as more is discovered about individuals of this group of neurones.
