Differential effects of pentobarbital, ethyl alcohol, and chlorpromazine in modifying reticular facilitation of visually evoked responses in the cat

Cortical visually evoked responses (VER) elicited by electrical stimulation of the ipsilateral optic tract were dramatically facilitated by stimulation of the midbrain reticular formation. This facilitation depended on the experimental conditions used, such as the intensity and time course of reticular (RFs) and optic tract stimulation (OTs). Reticular facilitation of the VER was most intense at 8 times the EEG activating threshold with a 50 msec interval between the RFs and OTs. The effects of increasing accumulative doses of pentobarbital, ethyl alcohol, and chlorpromazine given i.v. on reticular facilitation of the VER were observed. In general. these agents did not alter the presynaptic component of the VER except for 32 mg/kg of pentobarbital which increased it. On the other hand, pentobarbital had a marked depressant effect on both the cortical postsynaptic components and relicular influences on them. However. pentobarbital did not depress reticular facilitation of the VER as much as the non-facilitated VER. This data would suggest that pentobarbital has a neocortical depressant effect which is somewhat greater than its effect on the midbrain reticular formation. Ethyl alcohol had a similar cortical depressant effect but produced no significant depression of reticular facilitation of the VER. In fact, RFs restored the VER almost to control. Chlorpromazine (0.5 mg/kg, i.v.) reduced slightly the cortical postsynaptic components of the VER but had no effect on its facilitation by RFs.

These results suggest that reticular facilitation of the VER is more resistant to depression by pentobarbital and ethyl alcohol than the VER alone. The postsynaptic components of the VER are quite sensitive to the effects of these drugs in contrast to its presynaptic component. In marked contrast to the actions of pentobarbital and ethyl alcohol, chlorpromazine showed much less of a postsynaptic neocortical depressant effect even when massive doses (up to 16 mg/kg) were used.