Projections from the supragenual nucleus to the lateral mammillary and dorsal tegmental nuclei.

An organism's sense of direction depends on vestibular input to thalamic and forebrain structures. The supragenual nucleus (SGN) sits at a prime location in the network to convey vestibular signals forward to update the representation of head direction (HD) with ongoing head movement. The SGN receives anatomical projections from the medial vestibular nuclei and nucleus prepositus hypoglossi and prior reports have suggested that the SGN sends projections to both the dorsal tegmental nucleus (DTN) and lateral mammillary nucleus (LMN), two midbrain nuclei crucial in generating a representation of current HD. It is unknown, however, whether distinct or overlapping populations in SGN project to these structures, which has implications for how the SGN plays a role in generating and updating the HD signal. We performed a dual-color retrograde tracer study to determine whether the DTN and LMN projections from SGN arise from the same or different populations of cells. We report that the SGN→DTN projection is markedly stronger than that to LMN, filling most cells within the SGN, while SGN cells projecting to LMN tended to be smaller and sparser. We also found a small population of SGN cells projecting to both DTN and LMN. Further, our results indicate the presence of a large population of neurons in SGN that project only to the contralateral DTN and that these cells have little overlap with the ipsilateral projection to LMN. These results have implications for how the HD signal is generated within the DTN-LMN network.