Median Raphe Nonserotonergic Neurons Modulate Hippocampal Theta Oscillations

Hippocampal theta oscillations (HTOs) during rapid eye movement (REM) sleep play an important role in mnemonic processes by coordinating hippocampal and cortical activities. However, it is not fully understood how HTOs are modulated by subcortical regions, including the median raphe nucleus (MnR). The MnR is thought to suppress HTO through its serotonergic outputs. Here, our study on male mice revealed a more complex framework indicating roles of nonserotonergic MnR outputs in regulating HTO. We found that nonselective optogenetic activation of MnR neurons at theta frequency increased HTO amplitude. Granger causality analysis indicated that MnR theta oscillations during REM sleep influence HTO. By using three transgenic mouse lines, we found that MnR serotonergic neurons exhibited little or no theta-correlated activity during HTO. Instead, most MnR GABAergic neurons and Vglut3 neurons respectively increased and decreased activities during HTO and exhibited hippocampal theta phase-locked activities. Although MnR GABAergic neurons do not directly project to the hippocampus, they could modulate HTO through local Vglut3 and serotonergic neurons as we found that MnR GABAergic neurons monosynaptically targeted Vglut3 and serotonergic neurons. Additionally, pontine wave recorded from the MnR during REM sleep accompanied nonserotonergic activity increase and HTO acceleration. These results suggest that MnR nonserotonergic neurons modulate hippocampal theta activity during REM sleep, which regulates memory processes.SIGNIFICANCE STATEMENT The MnR is the major source of serotonergic inputs to multiple brain regions including the hippocampus and medial septal area. It has long been thought that those serotonergic outputs suppress HTOs. However, our results revealed that MnR serotoninergic neurons displayed little firing changes during HTO. Instead, MnR Vglut3 neurons were largely silent during HTO associated with REM sleep. Additionally, many MnR GABAergic neurons fired rhythmically phase-locked to HTO. These results indicate an important role of MnR nonserotonergic neurons in modulating HTO.