On the action of intravenous picrotoxin in the sensorimotor cortex of the cat.

The action of topical convulsants, such as strychnine, is not exerted equally on all sets of neurons in the sensorimotor cortex of cats. Small-field neurons near the surface are influenced whereas deeper, wide-field neurons are affected only indirectly. The agent, picrotoxin, was used to test whether or not the deeper neurons are spared the effects of topical convulsants simply because of their location. Picrotoxin, injected intravenously, produced enhancement of the corticofugal reflex discharge evoked by stimulation of any of the four paws and by auditory stimulation. The surface-recorded primary evoked response was also enhanced but only slightly, far less than with topical application. The evoked response in the medial lemniscus was not altered by picrotoxin, but that in the thalamic radiations was enhanced. The drug diazepam, injected intravenously, diminished the enhancements of both the corticofugal reflex discharge and the evoked potential, in some cases reducing the responses to amplitudes smaller than control values. Single-cell recordings in other laboratories have shown that intravenous picrotoxin converts small-field cells to wide-field cells, and that diazepam reverses this effect. The increase in the surface-recorded primary response and the size of the increase in the corticofugal reflex suggest that there is also a direct effect on wide-field neurons or on the pathway leading to them.