Arousal of cerebral cortex electroencephalogram consequent to high-frequency stimulation of ventral medullary reticular formation

We have theorized that large neurons in the ventral and medial reticular formation of the medulla are critical for both autonomic and cortical arousal. To test this theory, we anesthetized rats with urethane, lowered concentric bipolar stimulating electrodes into the medullary reticular formation, and implanted electroencephalogram (EEG) and ECG recording electrodes. We stimulated in the medulla with pulse frequencies ranging from 50 to 300 Hz while recording cortical EEG and ECG. These female rats were ovariectomized, and one subgroup was administered estradiol. Electrical stimulation at either 200 or 300 Hz among the large medullary reticular neurons in nucleus paragigantocellularis (PGi) caused a significant reduction in the portion of the EEG power spectrum represented by delta-waves (0.1-4 Hz) and theta-waves (4.1-8 Hz). Correspondingly, there were increases in gamma-wave power (22-50 Hz), especially when using 300 Hz. Stimulation at <= 100 Hz produced fewer changes in cortical EEG. The major features of these results were not significantly different according to estradiol-treatment. In contrast, estradiol administration significantly lowered ECG. Electrical stimulation in PGi increased ECG in vehicle control animals, but decreased it in estradiol treated animals. Regarding the EEG, we conclude that, even in anesthetized animals, stimulation of PGi reticular neurons can increase cortical arousal and that high frequencies of stimulation amplify this effect.