Topographic mapping of the cortical EEG power in the unrestrained rat: peripheral effects of neuroactive drugs.

Different behavioural states of the unrestrained rat are characterized by certain patterns of prominent slow waves in the neocortical EEG. The topographic distribution and the origin of the slow waves within the rat's cortex have been investigated on 15 Long-Evans rats. Fourteen epidural electrodes were arranged on the bulbus olfactorius, on the frontal areas Fr2 and Fr1, on the visual areas 18 and 17, and on the cerebellum of the right hemisphere. Five spontaneous behavioural states were investigated. The second space derivative of the EEG spectral power distribution was calculated. Two regions on the rat's brain surface were found as the probable origin of the slow waves: the frontal areas and area 18. Area 17 does not express prominent EEG frequencies that contribute to the cortical EEG in any of the investigated behavioural states. On the other hand, the short and middle latent components of the visually evoked potential were found to have originated only in the area 17. It is possible that the occipital areas of the rat's neocortex do not prominently contribute to the spectral power of the neocortical EEG. The EEG of the area 18 likely reflects hippocampal activity. Four main slow frequencies were expressed in the investigated cortical areas: two delta and two theta frequencies. Two cortical inputs of slow sleep waves were found: area 18 and the frontal cortex. High-voltage spindles were generated in area Fr2. Furthermore, various cholinergic drugs and adrenaline were intraperitoneally applied. The influence of these neuroactive drugs on the frontal and occipital EEG of relaxed waking immobility was compared with the spontaneous EEG changes during the different behavioural states. Besides the specific substance effects there are unspecific EEG modulations resulting from a change of the general behavioural state. The relative independence of the three behavioural markers respiration rate, EEG power, and general behaviour indicated by the drug effects are discussed.