It has been suggested that gamma band oscillations in cortical local field potentials (LFPs) may be related to the information processing which supports higher brain functions including memory, emotion, cognition, and consciousness. Recent evidence suggests that the generation mechanism of gamma rhythms involves inhibitory interneuronal activities mediated by gamma-aminobutyric acid type A receptor.
Another receptor that may be related to these higher brain functions is N-methyl-D-aspartate receptor (NMDAR). Interestingly, NMDAR may also be related to gamma oscillations. For example, NMDAR blockers can increase gamma activities in cats and rodents. However, it has remained unclear whether the same effects can be seen in the primate. Because NMDAR blockers can cause dissociative anesthesia, hallucination, dreaming, and antidepressive effects in the human, NMDAR blocker-induced changes of neural oscillations might be a clue to study the mechanisms of higher brain functions.
In this study, we examined the hypothesis that NMDAR is involved in the regulation of gamma oscillations in the primate brain. Cortical LFPs were recorded using electrodes chronically implanted in the neocortex and hippocampus in two Japanese monkeys. We administered either ketamine or MK-801 as NMDAR blockers, and analyzed the changes of oscillatory activities in LFPs.
We found that NMDAR blockers caused a significant increase in the amplitude of gamma oscillations both in the neocortex and hippocampus in a dose dependent manner. Gamma oscillations induced by NMDAR blockers showed a burst-suppression pattern. The timing of burst and suppression was almost synchronized between the cortical and hippocampal leads.
The results suggest that NMDAR exerts an inhibitory influence on gamma oscillations both in the neocortex and hippocampus in the primate, and that there are some central systems to switch the burst and suppression across the broad regions. Elucidation of these mechanisms might clarify the physiology of higher brain functions and the pathophysiology of mental disorders such as schizophrenia, depression, and memory disorder.