Glutamatergic transmission between antagonistic motor neurones in the locust

The pharmacology of the direct central connections between the fast extensor and flexor motor neurones of a locust (Schistocerca gregaria) hind leg was studied. A spike in the fast extensor produces an EPSP in the flexor motor neurones. Glutamate depolarized the flexor motor neurones when injected into the neuropil. Quisqualate, but not by kainate or NMDA, also depolarized the flexor motor neurones. The fast extensor was also depolarized by glutamate, and also by kainate, but not by quisqualate, AMPA or NMDA. The glutamate response in the flexor motor neurones and the EPSP evoked by a spike in FETi both had similar reversal potentials. The FETi-evoked EPSP was blocked by bath application of the glutamate antagonist glutamic acid diethyl ester. The responses of extrasynaptic somata receptors to glutamate were compared to the neuropil responses. Glutamate usually hyperpolarized the somata of FETi and the flexor motor neurones. The response of a flexor motor neurone to glutamate was abolished at potentials less negative than -90 mV. The results provide evidence for glutamate transmission at central synapses in the locust, and show that presumed synaptic receptors in the neuropil differ to the extrasynaptic soma response