Deficits in gamma frequency brain oscillations (30-100+Hz) are observed in Alzheimer’s Disease (AD) and are associated with AD cognitive deficits. Restoration of these oscillations by non-invasive brain stimulation at 40Hz entrains neurons and improves cognitive performance, with modulation of microglial phenotype in the hippocampus of AD mouse models. However, it is unclear whether the biological mechanism of this 40Hz stimulation is a direct effect on microglia, or an indirect effect via neurons or other brain cells.
To investigate this, microglia and microglial-neuronal co-cultures were stimulated in vitro with 40Hz for 1-4 hours followed by treatment with varied inflammatory stimuli for 4-24 hours. Production of the cytokines IL-6 and TNF, phagocytosis, and gene expression were assessed for modulation of microglial phenotype. APPNL-G-F mice were also stimulated with 40Hz, via TI hippocampal stimulation, for 1 hour per day for 8 days. Microglia and Aβ numbers, area and co-localisation were assessed for microglial modulation.
Direct 40Hz stimulation of microglia in vitro did not modulate microglial phenotype. However, 40Hz stimulation of microglial-neuronal co-cultures reduced microglial production of IL-6 relative to sham stimulated co-cultures. Moreover, this reduction in microglial IL-6 production was observed following treatment of microglia with conditioned medium from 40Hz stimulated neurons and was not observed using 1 or 8Hz control frequencies. Size filtration of conditioned medium prior to transfer to microglia suggested this effect was mediated by a small molecule (<10kDa) released by neurons following 40Hz stimulation. In vivo, following 40Hz TI hippocampal stimulation of APPNL-G-F mice, no modulation of microglial phenotype was observed. This data provided evidence that microglial phenotype is not directly modulated by 40Hz stimulation, but rather is modulated indirectly via neuronal communication. Further work is needed to address whether this in vitro effect on cytokine production occurs in vivo.