A Novel Subpopulation of Prepositus Hypoglossi Nucleus Neurons Projecting to the Cerebellar Anterior Vermis and Hemisphere in Rats.

The prepositus hypoglossi nucleus (PHN), involved in horizontal gaze control, contributes to this function via cooperation with the vestibulocerebellum (VC). Furthermore, some PHN neurons have been observed to project to cerebellar regions outside the VC. We previously reported a neuronal population in the ventral caudal PHN that projects to lobules III-V of the anterior vermis or to the cerebellar hemispheric crus. Because the properties of these neurons have not been clarified, this study aimed to determine their localization, projections, and electrophysiological and morphological characteristics in male rats. Tracing experiments revealed that these neurons were clustered within the ventral caudal PHN, approximately between the bregma -12.72 and -12.00 mm, and did not project to the uvula/nodulus (UN), which is part of the VC. Whole-cell recordings and morphological experiments revealed that these PHN neurons exhibited high input capacitance, low input resistance, low-frequency firing, prominent voltage sag, and a multipolar shape. These results indicate that a cluster of neurons in the ventral caudal PHN projecting to lobules III-V of the anterior vermis and hemispheric crus share distinct electrophysiological and morphological properties. Furthermore, these PHN neurons are likely to constitute a distinct subpopulation from PHN neurons projecting to the VC in terms of their projection targets. While previous research has focused on PHN projections to the VC and their role in ocular motor control, this study suggests that this subpopulation may be involved in other motor functions, as the cerebellar anterior vermis and hemisphere are known to contribute to broader motor control.