Effect of selected excitant and depressant agents on the cortical response to midline thalamic stimulation in the rabbit

Abstract

The effects of depressant and excitant agents given intravenously were tested upon the pre-central agranular cortical response to repetitive midline thalamic stimulation in 78 adult rabbits. After preparation under ether, the animals were locally anesthetized and transferred to flaxedil and artificial respiration for the trials. Non-polarizable electrodes and DC recording were used, and the following observations made: 1. 1. The cortical response consists of four components. Two belong to the primary (negative) potential of the recruiting series and are decremental and incremental waves. Following the primary spike is an after-positivity which is trailed by a negative deflection of 250–400 msec duration, the slow negativity. 2. 2. Depressants (thiopental, methohexital, paraldehyde and chloral hydrate) behaved similarly in significantly reducing both slow negativity and incrementing wave at a lower concentration than that required to affect the decrementing wave. For methohexital and paraldehyde, after-positivity is significantly reduced as well. The effect of nitrogen inhalation on the recruiting response is presented and discussed with respect to mode of action depressants. 3. 3. Excitants used fall into two groups. One (including metrazol, picrotoxin and benadryl) increases slow negativity while diminishing the amplitude of the primary potential of the recruiting series. The others (procaine and caffeine) increase the primary potential while abolishing slow negativity. These results suggest an intimate relation between primary potential and slow negativity such as a common origin in the cortical substrate which produces the potentials. 4. 4. Differences between the effects of these agents on the direct cortical and recruiting responses are discussed and are thought to arise from differences in the route by which the responses are evoked.