Cortical pyramidal tract interneurones and their sensitivity to L-glutamic acid.

1. Pyramidal tract interneurones, defined as neurones which are activated synaptically as a result of pyramidal tract stimulation, have been identified in the rat cerebral cortex. The number of evoked spikes depended upon stimulus strength, and stimulation in a specific thalamic nucleus produced a burst of activity lasting for up to 1 sec.2. These cells are readily excited by a brief (50 msec) pulse of glutamate applied by micro-iontophoresis. Other, unidentified cells are not so responsive.3. Synaptically evoked spikes resulting from pyramidal tract stimulation can be blocked by the iontophoretic and I.P. administration of substances shown to antagonize glutamate excitation of cells.4. The results support suggestions that glutamic acid is a neurotransmitter in the cerebral cortex. The evidence presented further indicates that glutamic acid could be the transmitter released by the pyramidal tract.