Vestibulotrigeminal and vestibulospinal projections in rats: retrograde tracing coupled to glutamic acid decarboxylase immunoreactivity.

Immunohistochemical experiments were performed using glutamic acid decarboxylase (GAD) to identify gamma-aminobutyric acid (GABA)ergic neurons in the vestibular nuclei (VN). VN neurons projecting to the sensory trigeminal complex (STC) or to the C1-C2 segments of the spinal cord were identified by injection of wheat germ agglutinin-apo-horseradish peroxidase coupled to colloidal gold (gold-HRP), a retrogradely transported tracer, in these structures. The experiments combining injection of gold-HRP in spinal cord and GAD immunohistochemistry revealed the existence in the medial, inferior and lateral VN of GAD immunoreactive neurons projecting to the spinal C1-C2 level. Experiments combining injection of gold-HRP in the STC and GAD immunohistochemistry demonstrated that, at least, 30-50% of the vestibulo-trigeminal neurons also contained GAD. Injections of two different retrograde tracers (gold-HRP and Biotinylated dextran amine) in the STC and the spinal cord demonstrated that some VN neurons project by axon collaterals to both structures. Because of the GABAergic spinal and STC vestibular projections we assume that these VN neurons with collateral projection are GABAergic. Therefore primary afferents from the face, neck or hindlimb could be modulated by inhibitory influences from GABAergic vestibular neurons.