FAST OSCILLATIONS (20-40 HZ) IN THALAMOCORTICAL SYSTEMS AND THEIR POTENTIATION BY MESOPONTINE CHOLINERGIC NUCLEI IN THE CAT

Previous investigations in various motor and sensory cortical areas have shown that fast oscillations (20-80 Hz) of focal electroencephalogram and multiunit activities occur spontaneously during increased alertness or are dependent upon optimal sensory stimuli. We now report the presence of 20- to 40-Hz rhythmic activities in intracellularly recorded thalamocortical cells of the cat. In some neurons, subthreshold oscillations were triggered by depolarizing pulses and eventually gave rise to action potentials. In other neurons, the oscillations consisted of fast prepotentials, occasionally generating full spikes that arose from the resting or even from hyperpolarized membrane potential levels, and leading to trains of spikes at more depolarized levels. The rhythmic nature of these fast prepotentials was confirmed by means of an autocorrelation study, which demonstrated clear peaks at 25-ms intervals (40 Hz). In view of the recent evidence that mesopontine cholinergic nuclei trigger and maintain activation processes in thalamocortical systems, we tested the possibility that stimulation of these brainstem nuclei potentiates the 40-Hz waves on the background of the cortical electroencephalogram. This was indeed the case. The potentiation outlasted the stimulation by 10-20 s. The brainstem-induced facilitation of cortical 40-Hz oscillations was blocked by scopolamine, a muscarinic antagonist. That this facilitation was transmitted by brainstem-thalamic cholinergic projections was confirmed by persistence of the phenomenon after large excitotoxic lesions of the nucleus basalis of Meynert.