Behaviour-related effects of physostigmine on the rat visual evoked potential.

The present study compares behaviour-dependent and physostigmine-induced changes of the visual evoked potential (VEP) in unrestrained rats to provide further indications on the role of acetylcholine in the behavioural VEP modulation. On 30 rats the VEPs on the pial surface of the primary visual cortex were investigated during five spontaneous behavioural states. Physostigmine, carbachol and nicotine were intraperitoneally applied on 7 rats. Several VEP parameters were found changing in dependence of the arousal level: the VEP latency, the amplitude of the second negative component (N41), and the voltage of the positive component between 60 and 80 ms after the light flash. Physostigmine caused amplitude alterations like during high arousal states. The VEP latency, however, was altered in an opposite way. Nicotine specifically altered the N41 amplitude whereas carbachol delayed the latency of the VEP. The second negative component (N41) is assumed cholinergically modulated. Moreover, the negative component between 60 and 80 ms latency that emerged in high arousal states may be cholinergically generated and may represent a marker of the activation degree of the cholinergic system.