Morphological and electrophysiological consequences of unilateral pre- versus postganglionic vestibular lesions in the frog.

The combined removal of the labyrinthine sense organs and of the ganglion of Scarpa on one side (postganglionic section) resulted in a degeneration of afferent fibres in the eighth nerve of the frog (Rana temporaria) within 2-4 days. If the eighth nerve was sectioned more peripherally (preganglionic section) and its distal part was removed together with the labyrinthine organs degeneration of afferent fibres was absent or restricted to very few fibres. Electrical stimulation of vestibular afferents in vitro evoked monosynaptic field potentials in the ipsilateral and via commissural fibres di- and polysynaptic field potentials in the contralateral vestibular nuclei. Afferent-evoked field potentials recorded on the intact side of chronic frogs (> or = 60 days) with a pre- or postganglionic lesion and afferent-evoked field potentials recorded on the operated side of chronic frogs with a preganglionic lesion had amplitudes that were very similar to those recorded in control frogs. Commissurally evoked field potentials recorded on the operated side of chronic frogs with pre- or postganglionic lesions were significantly increased (by about 90%) with respect to control amplitudes. In both groups the time-course of this increase was very similar, started between 15 and 30 days and saturated for survival periods longer than 60 days. Unilateral inactivation of vestibular afferents, but not degeneration, is the likely common denominator of the central process leading to the reported neural changes. A reactive supersensitivity of central vestibular neurons on the operated side for glutamate as a possible mechanism is unlikely, since converging afferent and commissural inputs are both glutamatergic and only one of them, the commissural input, was potentiated. Comparison of the time-courses of neural changes in the vestibular nuclei and postural recovery in the same individuals excludes a causal relation between both phenomena.