Modulation of tension production by octopamine in the metathoracic dorsal longitudinal muscle of the cricket Teleogryllus Oceanicus

Application of octopamine to the metathoracic dorsal longitudinal muscle (DLM) of the cricket Teleogryllus oceanicus produced dose-dependent increases of twitch amplitude, contraction rate and relaxation rate. The threshold for octopamine effects was between 10−8 and 10−7mol l−1, while maximal effects were seen at approximately 10−5 mol l−1. 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 octopamine2A or octopamine2B receptors in this preparation. Octopamine had both presynaptic and postsynaptic effects, since it increased both miniature end-plate potential (mEPP) frequency and muscle relaxation rate. Ata calcium concentration of 11 mmol l−1, octopamine did not affect muscle membrane potential, input resistance or EJP amplitude, but the EJP duration at half amplitude (T1/2) was slightly increased. In low-calcium saline (1.8 mmol l−1), octopamine did not affect membrane potential or T1/2, but EJP amplitude was increased. Stimulation of the octopaminergic dorsal unpaired median neuron (DUMDL), which innervates the metathoracic DLM, increased twitch amplitude in about 25% of the preparations. Failure in the other preparations was apparently due to spike conduction failure within the metathoracic ganglion. These results show that octopamine can be an important modulator of metathoracic DLM tension production.