Evidence that insect dorsal unpaired medican (DUM) neurons are octopaminergic

The dorsal unpaired median neuron innervating the extensor tibiae (DUMETi) of locusts and grasshoppers, as well as some other DUM neurons, cause long-term inhibition of the slow intrinsic rhythm of contraction of the muscle by releasing a neurohumoral agent. Large, (600 - 1900 A), dense-core vesicles are present in the nerve terminals. The inhibitory action is mimicked by infusioninto the leg of a minute (0.01 ml.) drop of locust saline containing a very low concentration (10(-6)M) of dopamine or noradrenaline, or an extraordinarily low concentration of octopamine (2.5 x 10(-9) M). The amplitude and frequency of the contractions slowly return to normal. Neither the cell body nor the neuron fluoresces with the Falck-Hillarp treatment, even after a period of ligature has caused an accumulation of vesicles. Octopamine does not fluoresce with this treatment. Accordingly, it is suggested that this neuron, and the other efferent DUM neurons, are octopaminergic.?23Author