The median raphe region (MRR) is thought to be serotonergic and plays an important
role in the regulation of many cognitive functions. In the hippocampus (HIPP), the
MRR exerts a fast excitatory control, partially through glutamatergic transmission,
on a subpopulation of GABAergic interneurons that are key regulators of local network
activity. However, not all receptors of this connection in the HIPP and in synapses
established by MRR in other brain areas are known. Using combined anterograde tracing
and immunogold methods, we show that the GluN2A subunit of the NMDA receptor is present
in the synapses established by MRR not only in the HIPP, but also in the medial septum
(MS) and in the medial prefrontal cortex (mPFC) of the mouse. We estimated similar
amounts of NMDA receptors in these synapses established by the MRR and in local adjacent
excitatory synapses. Using retrograde tracing and confocal laser scanning microscopy,
we found that the majority of the projecting cells of the mouse MRR contain the vesicular
glutamate transporter type 3 (vGluT3). Furthermore, using double retrograde tracing,
we found that single cells of the MRR can innervate the HIPP and mPFC or the MS and
mPFC simultaneously, and these double-projecting cells are also predominantly vGluT3-positive.
Our results indicate that the majority of the output of the MRR is glutamatergic and
acts through NMDA receptor-containing synapses. This suggests that key forebrain areas
receive precisely targeted excitatory input from the MRR, which is able to synchronously
modify activity in those regions via individual MRR cells with dual projections.
