Structural and functional connections between the median and the ventrolateral preoptic nucleus.

The median preoptic nucleus (MnPO) and the ventrolateral preoptic nucleus (VLPO) are two brain structures that contain neurons essential for promoting non-rapid eye movement (NREM) sleep. However, their connections are still largely unknown. Here, we describe for the first time a slice preparation with an oblique coronal slicing angle at 70° from the horizontal in which their connectivity is preserved. Using the in vivo iDISCO method following viral infection of the MnPO or ex vivo biocytin crystal deposition in the MnPO of mouse brain slices, we revealed a strong axonal pathway from the MnPO to the VLPO. Then, to further explore the functionality of these projections, acute 70° slices were placed on multielectrode arrays (MEAs) and electrical stimulations were performed near the MnPO. Recordings of the signals propagation throughout the slices revealed a preferential pathway from the MnPO to the VLPO. Finally, we performed an input-output curve of field responses evoked by stimulation of the MnPO and recorded in the VLPO. We found that field responses were inhibited by GABAA receptor antagonist, suggesting that afferent inputs from the MnPO activate VLPO neuronal networks by disinhibition.