Effect of atropine and carbachol on vestibular compensation in squirrel monkeys

The central cholinergic system's involvement in vestibular compensation has been clearly demonstrated in the infra-human primate model. In the squirrel monkey, long after unilateral labyrinthectomy, atropine injection produced significant reduction of slow-phase eye velocity of the spontaneous nystagmus. The atropine effect was also depicted through the dynamic vestibular inputs, reducing the maximum slow-phase eye velocity of damped pendular rotation nystagmus. Contrary results (increased slow-phase eye velocities) were found after carbachol injection. After atropine injection, the locomotor deviation count increased significantly, more to the nonlesion side. Carbachol injection engendered the opposite result. After bilateral labyrinthectomy, no significant effect was found from drug injection. Thus, our findings suggest that atropine's major site of action after unilateral labyrinthectomy is the vestibular nuclei on the intact side.