Noradrenergic modulation of neuronal responses to glutamate in the vestibular complex

Increases in firing rate induced in secondary vestibular neurons by microiontophoretic application of glutamate were studied during long-lasting applications of noradrenaline (NA) and/or its antagonists and agonists. Sixty-nine percent of the tested neurons, scattered through all nuclei of the vestibular complex, modified their responsiveness to glutamate in the presence of NA. The effects were depressive in a majority (40%) and enhancing in a minority (29%) of cases. NA application depressed responses to glutamate more often than it increased them in lateral, medial and superior vestibular nuclei, while the reverse was true for the spinal nucleus. The mean intensities of NA-evoked effects were comparable in the various nuclei. The enhancing effects of NA were antagonized by application of the alpha2 receptor antagonist yohimbine, and their depressive effects were enhanced by the beta receptor antagonist timolol. It is concluded that NA exerts a control on the processing of vestibular information and that this modulation is exerted by at least two mechanisms involving alpha2 and beta noradrenergic receptors.