ADAPTIVE CHANGES IN VESTIBULAR SYSTEM

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Flatfish constitute a natural paradigm for investigating adaptive changes in the central nervous system, because their vestibular and oculomotor coordinate systems undergo a 90 degrees relative displacement during metamorphosis. This displacement requires the introduction of a new conversion factor for the transmission of signals coded in vestibular coordinates into motor commands coded in eye muscle coordinates. Our previous studies revealed a dramatic rewiring at the level of secondary vestibular neurons connected to the horizontal canal system, exhibiting an extensive termination pattern in vertical oculomotor neuron pools via connections which are absent in all other vertebrates studied so far. Experiments are planned to give a full description of the pre- and postmetamorphic vestibulo-ocular reflex (VOR) system in the flatfish. These include, first, extracellular and intercellular HRP applications at the level of primary and secondary vestibular afferents of both labyrinths including a quantitative analysis of the peripheral nerves supplying vestibular end-organs in order to demonstrate symmetry of the labyrinthine connections as well as double intracellular recordings to ascertain the excitatory or inhibitory nature of the identified secondary vestibular neurons within the push-pull system of the VOR system. Secondly, extracellular recordings during optokinetic stimulation will provide information about the organization of the visual space of these animals. Thirdly, experiments in the alert behaving flatfish will reveal the structure-function relationship of the morphologically described connections to the actual eye movements of the fish. Fourth, experiments in premetamorphic and metamorphosing animals and in fish which are locked in certain stages of the metamorphosis will reveal the nature of the occurrance of these newly formed connections, whether they originate from newly born neurons or from synaptic connections which have been there previously. These experiments involve the creation of hyperdimensional larvae by environmental and pharmacological manipulations, as well as the usage of autoradiographic techniques. The proposed experiments would not only be a comprehensive way to describe the VOR system in the adult flatfish, but also give information about the mechanisms involved in an adpating brain, especially in the context of adaptive plasticity during experimental perturbation.