Summary

1. 1. Application of octopamine to the metathoracic dorsal longitudinal muscle (DLM) of the cricket Teleogryllus oceanicus produced dose-dependent increases twitch amplitude, contraction rate and relaxation rate. The threshold for octopamine effects was between 10⁻⁸ and 10⁻⁷moll⁻¹, while maximal effects were seen at approximately 10⁻⁵moll⁻¹

2. 2. The octopamine receptors were classified as octopamine₂ receptors on the basis of the differential responsiveness of the muscle to the octopamine agonists naphazoline, tolazoline, clonidine and the octopamine antagonists metoclopramide and chlorpromazine. It was not possible to distinguish between octopamine_2A or octopamine_2B receptors in this preparation.

3. 3. Octopamine had both presynaptic and postsynaptic effects, since it increased both miniature end-plate potential (mEPP) frequency and muscle relaxation rate

4. 4. At a calcium concentration of 11 mmoll⁻¹, octopamine did not affect muscle membrane potential, input resistance or EJP amplitude, but the EJP duration at half amplitude (T_½) was slightly increased. In low-calcium saline (1.8 mmol⁻¹), octopamine did not affect membrane potential or T_½, but EJP amplitude was increased

5. 5. Stimulation of the octopaminergic dorsal unpaired median neuron (DUMDL), which innervates the metathoracic DLM, increased twitch amplitude about 25% of the preparations. Failure in the other preparations wasapparently due to spike conduction failure within the metathoracic ganglion.6. These results show that octopamine can be an important modulator of metathoracic DLM tension production