Effects of trunk-to-head rotation on the labyrinthine responses of rat reticular neurons

Vestibulospinal reflexes elicited by head displacement become appropriate for body stabilization owing to the integration of neck input by the cerebellar anterior vermis. Due to this integration, the preferred direction of spinal motoneurons' responses to animal tilt rotates by the same angle and by the same direction as the head over the body, which makes it dependent on the direction of body displacement rather than on head displacement. It is known that the cerebellar control of spinal motoneurons involves the reticular formation. Since the preferred directions of corticocerebellar units' responses to animal tilt are tuned by neck rotation, as occuring in spinal motoneurons, we investigated whether a similar tuning can be observed also in the intermediate station of reticular formation. In anaesthetized rats, the activity of neurons in the medullary reticular formation was recorded during wobble of the whole animal at 0.156 Hz, a stimulus that tilted the animal's head by a constant amplitude (5°), in a direction rotating clockwise or counter clockwise over the horizontal plane. The response gain and the direction of tilt eliciting the maximal activity were evaluated with the head and body axes aligned and during a maintained body-to-head displacement of 5-20° over the horizontal plane, in either direction. We found that the neck displacement modified the response gain and/or the average activity of most of the responsive neurons. Rotation of the response direction was observed only in a minor percentage of the recorded neurons. The modifications of reticular neurons' responses were different from those observed in the P-cells of the cerebellar anterior vermis, which rarely showed gain and activity changes and often exhibited a rotation of their response directions. In conclusion, reticular neurons take part in the neck tuning of vestibulospinal reflexes by transforming a head-driven sensory input into a body-centred postural response. The present findings prompt re-evaluation of the role played by the reticular neurons and the cerebellum in vestibulospinal reflexes.