LEADING ROLE OF THALAMIC OVER CORTICAL-NEURONS DURING POSTINHIBITORY REBOUND EXCITATION

The postinhibitory rebound excitation is an intrinsic property of thal amic and cortical neurons that is implicated in a variety of normal an d abnormal operations of neuronal networks, such as slow or fast brain rhythms during different states of vigilance as well as seizures. We used dual simultaneous intracellular recordings of thalamocortical neu rons from the ventrolateral nucleus and neurons from the motor cortex, together with thalamic and cortical field potentials, to investigate the temporal relations between thalamic and cortical events during the rebound excitation that follows prolonged periods of stimulus-induced inhibition. Invariably, the rebound spike-bursts in thalamocortical c ells occurred before the rebound depolarization in cortical neurons an d preceded the peak of the depth-negative, rebound field potential in cortical areas. Also, the inhibitory-rebound sequences were more prono unced and prolonged in cortical neurons when elicited by thalamic stim uli, compared with cortical stimuli. The role of thalamocortical loops in the rebound excitation of cortical neurons was shown further by th e absence of rebound activity in isolated cortical slabs. However, whe reas thalamocortical neurons remained hyperpolarized after rebound exc itation, because of the prolonged spike-bursts in inhibitory thalamic reticular neurons, the rebound depolarization in cortical neurons was prolonged, suggesting the role of intracortical excitatory circuits in this sustained activity. The role of intrathalamic events in triggeri ng rebound cortical activity should be taken into consideration when a nalyzing information processes at the cortical level; at each step, co rticothalamic volleys can set into action thalamic inhibitory neurons, leading to rebound spike-bursts that are transferred back to the cort ex, thus modifying cortical activities.