Brain inputs to the vestibular nuclei in lampreys.

INTRODUCTION: To avoid image blurring, the vestibulo-ocular (VOR) and the optokinetic (OKR) reflexes stabilize gaze. In all vertebrates, the VOR is mediated via direct projections from the vestibular nuclei to the motor nuclei that control the extraocular muscles. Lampreys show three vestibular nuclei that are well characterized in terms of their projections and sensory inputs, but much less is known about their inputs from other brain regions and the connectivity between them. METHODS: We used tracer injections and extracellular electrophysiological recordings, to analyze the inputs to each vestibular nucleus. RESULTS: The lamprey vestibular nuclei are largely interconnected, while their inputs from other brain regions are scarce. The main rostral areas projecting to the vestibular nuclei are the pretectum and the ventral tier of the thalamus, which send ipsilateral inputs to the three vestibular nuclei. DISCUSSION: These findings show that the interconnectivity of the vestibular nuclei was already stablished at the beginning of vertebrate evolution, and that the pretectal input provides visual information to all the vestibular nuclei, indicating that visuovestibular integration for gaze and body stabilization was already important at the base of vertebrate evolution.