Projections from the vestibular nuclei to the hypothalamic paraventricular nucleus: morphological evidence for the existence of a vestibular stress pathway in the rat brain.

Although it has been reported by several laboratories that vestibular stress activates the hypothalamo-pituitary-adrenocortical axis (HPA), the existence of neuronal connections between vestibular and hypothalamic paraventricular neurons has not yet been demonstrated. By the use of a virus-based retrograde trans-synaptic tracing technique in the rat, here we demonstrate vestibular projections to the paraventricular nucleus (PVN). Pseudorabies virus (Bartha strain, type BDR62) was injected into the PVN, and the progression of the infection along synaptically connected neurons was followed in the pons and the medulla, 3 and 4 days post-inoculation. Virus-infected neurons were revealed mainly in the medial vestibular nucleus. Labeled cells were scattered in the spinal, and very rarely in the superior nuclei, but none of them in the lateral vestibular nucleus. Injections of cholera toxin B subunit, a monosynaptic retrograde tracer into the PVN failed to label any cells in the vestibular nuclei. These results provide anatomical evidence for the existence of a vestibulo-paraventricular polysynaptic pathway and support the view that the HPA axis is modulated by vestibular stress.