Studying the function of long-range projecting VIP-expressing neurons in the facial motor nucleus

The vestibular system is critical for maintaining balance and postural equilibrium in most vertebrates, and the medial vestibular nucleus (MVN) is a key component in the integration of vestibular sensory inputs. However, it is not fully clear how MVN and its related brain regions contribute to vestibular equilibrium. In this study, we identified a population of vasoactive intestinal peptide (VIP)-expressing neurons in the facial motor nucleus (FMN) of the brainstem that provides long-range inputs to MVN. While the lower motor neurons in FMN mediate specific orofacial and vibrissal muscle activities, the role of FMN-VIP neurons is unknown. To study the functional role of these neurons, we utilised cell-type specific ablation and viral tracing techniques to dissect how FMN-VIP neurons contribute to the vestibular activity. Our results indicate that the majority of FMN-VIP neurons are GABAergic and ablating these neurons through taCasp3-TEVp expression altered the animal’s postural maintenance and balance performance in the horizontal beam task (HBT). Interestingly, animals with MVN neuronal ablation displayed the similar vestibular phenotype. Furthermore, using trans-synaptic Rabies tracing, we observed extensive monosynaptic inputs from the reticular formation and the vestibular nuclei to FMN-VIP neurons, which further demonstrate that FMN-VIP neurons are involved in the regulation of balance and postural equilibrium by partaking in the vestibular circuitry. Altogether, our ongoing study reveals a previously unknown functional connection between FMN and MVN, the two phylogenetically old and independent structures in the brain.