Serotonin depletion prevents electrocortical synchronization following acute midbrain deactivation

Bilateral midbrain cooling (5-15 degrees C) was performed in cats pretreated with p-chlorophenylalanine (PCPA). This was done in order to examine the possible role of 5-hydroxytryptamine (5-HT) systems in the process of electrocortical synchronization which normally accompanies midbrain reticular deactivation. Prior to treatment with PCPA, midbrain cooling always produced tonic electrocortical synchronization and behavioral unresponsiveness. Cooling following PCPA treatment on the other hand produced behavioral unresponsiveness which was not accompanied by tonic electrocortical synchronization; in that case electrocortical desynchronization persisted in spite of midbrain reticular deactivation. These results lead to the hypothesis that two distinct, tonically active, desynchronizing systems exist. During the acute blockade of the classical midbrain reticular activating system it is proposed that 5-HT systems normally operate to inhibit or otherwise prevent the expression of the second brain stem electrocortical desynchronizing mechanism. It is proposed, therefore, that 5-HT acts indirectly to produce electrocortical synchronization. In the absence of 5-HT, midbrain deactivation is not sufficient to guarantee synchronization is assured only when both desynchronizing systems are inactivated as would be the case in the acute cerveau isolé.