Microinjections of tubocurarine, leptazol, strychnine and picrotoxin into the cerebral cortex of anaesthetized cats.

1. In cats anaesthetized with intravenous chloralose, microinjections of tubocurarine, leptazol, strychnine or picrotoxin, in a volume of 1 mul, were made into the grey matter of the cerebral cortex and the electrical activity was recorded from the site of injection with the microinjection cannula which, insulated except at its tip, served as recording electrode.2. Routinely the injections were made into the gyrus splenialis or into the underlying gyrus cinguli close to the mid-line, because the injections would then most likely be in grey and not in white matter. Injected in this way all four drugs set up foci of excitation which gave rise to synchronous firing of a large number of neurones with the result that high voltage negative spikes were recorded from the microinjection cannula.3. On injection into the gyrus splenialis the threshold dose was about 0.04 mug for picrotoxin, about 0.2 mug for tubocurarine, about 5 mug for strychnine and 25 to 50 mug for leptazol. Following the injection of larger doses the spike discharge continued for a few hours after picrotoxin and tubocurarine, for over an hour after strychnine, but for a few minutes only after leptazol. On injection into the gyrus cinguli the threshold doses were slightly greater and with larger doses the spikes occurred at greater frequency but were of lower voltage than in the gyrus splenialis.4. With large doses of picrotoxin injected into the gyrus splenialis the spikes developed an after-positivity and an after-discharge which sometimes passed into a short period of fast activity.5. The foci of excitation set up by the drugs were restricted to the site of injection because on raising or lowering the microinjection cannula the spikes recorded from it quickly decreased in voltage and then disappeared. When the injections were made close to a sulcus and the microinjection cannula, on being lowered, traversed the sulcus, the spikes changed their polarity.6. The spike discharge appears to be a consistent response to the injections of the drugs into grey matter of any part of the cerebral cortex since it was also obtained on their injection into the pyriform cortex, amygdala and area retrolimbica anterior, but not on their injection into white matter or caudate nucleus, thalamus or hypothalamus.