THE EFFECTS OF PHYSOSTIGMINE ON THE RESPONSE CHARACTERISTICS OF THE CAT VISUAL-EVOKED POTENTIAL

Steady-state pattern visual evoked potentials were recorded from the s urface of the cat primary visual cortex before and after the intraveno us administration of physostigmine, an agent that blocks the enzyme re sponsible for the breakdown of synaptically released acetylcholine. Un der pentobarbital anesthesia, physostigmine increased the amplitude an d changed the phase of the second response harmonic of the visual evok ed potential, whereas the amplitude and phase of the fourth harmonic w ere not affected. These effects persisted for 15 to 45 minutes and wer e blocked by prior treatment with scopolamine or atropine. In addition , scopolamine or atropine administered 5 to 10 minutes after physostig mine returned the visual evoked potential to the baseline state. In co mparison, when nitrous oxide was used, physostigmine caused a marked r eduction in visual evoked potential amplitude, an effect that was reve rsed by subsequent atropine. These results indicate that the cholinerg ic system influences the visual evoked potential via a muscarinic path way and that this influence is strongly affected by the anesthetic reg imen used.