VESTIBULAR COMPENSATION IN LAMPREYS - ROLE OF VISION AT DIFFERENT STAGES OF RECOVERY OF EQUILIBRIUM CONTROL

The main motor disorder evoked by unilateral labyrinthectomy (UL) in t he lamprey (Lampetra fluviatilis) is a complete loss of equilibrium an d rolling (rotation about the longitudinal axis) during swimming. A pr evious study has shown that the recovery of equilibrium control in the lamprey takes, on average, 33 days. However, lampreys were able to ma intain equilibrium if UL was combined with removal of the ipsilateral eye ('surgical compensation' of the vestibular deficit). It was sugges ted that tonic excitatory inflow, rather than specific information abo ut the orientation of the animal in space delivered by the remaining e ye, is important for the recovery of equilibrium control. In the prese nt study, a number of experiments were designed to test this hypothesi s. It was found that illumination of the eye contralateral to the UL o r continuous electrical stimulation (10 Hz) of the corresponding optic nerve resulted in immediate restoration of equilibrium control. The s ame result was obtained when the vestibular nerve on the UL side was s timulated. Thus, the roll control system in the lamprey, driven by onl y one labyrinth, is able to maintain equilibrium provided that the lac k of tonic inflow from the missing labyrinth is compensated for by ton ic vestibular or visual input. The present study has also shown that t he importance of visual input for maintaining equilibrium after UL dec reases with time. In animals that achieved a high degree of compensati on, removal of the eyes on day 23 after UL evoked decompensation, wher eas removal on day 70 did not. A reduction of the significance of visu al input was also observed in surgically compensated UL lampreys. In t hese animals, removal of the remaining eye on days 1-3 after the first surgery resulted in a complete loss of equilibrium, removal on day 7 resulted in a partial loss, whereas removal on days 48-55 did not affe ct the postural stability. Three lines of evidence suggest that asymme trical visual input evokes plastic changes in the roll control system. (i) In one group of animals, initially one eye was removed, and then 50 days later the labyrinth ipsilateral to the missing eye and remaini ng eye were removed. These animals exhibited a mild impairment of equi librium control, in contrast to the animals in which both surgeries we re performed simultaneously. (ii) In another group of animals, initial ly one eye was removed, and then 50 days later the remaining eye and b oth labyrinths were removed. These animals exhibited rolling towards t he eye that remained intact for longer. (iii) A short-term electrical stimulation (5-10 min daily for 3 days) of the optic nerve (contralate ral to UL) in blinded animals considerably improved the equilibrium co ntrol compared with that of non-stimulated animals; the improvement wa s observed for 60 days after stimulation.