Pharmacological subtraction of the sensory controls over grasping in rats

Catecholamine-depletion-induced catalepsy isolates and leaves intact an aggregate of allied reflexes (e.g., righting, standing still, bracing, and clinging) which involve all the body and limb segments in defending stable static equilibrium [15]. Because other movement subsystems (locomotion, orienting, scanning, directed use of mouth or forepaws) are depressed, such animals cling in a vertical position for an abnormally long period of time. As a consequence, grasping reflexes may be studied independently of other responses. Haloperidol, a dopamine antagonist, abolishes visually elicited reaching and grasping, but leaves intact tactile and proprioceptive control of grasping. The grasping of haloperidol-treated rats can be further simplified by the pharmacological removal of the remaining sensory controls. The addition of morphine to haloperidol abolishes tactile grasping, while the addition of diazepam to haloperidol abolishes both tactile and proprioceptive (traction-elicited) grasping. Although visual, tactile, and proprioceptive grasping are abolished by haloperidol-plus-diazepam, some vestibular input to clinging remains: such rats, in response to being held vertically upright in the air, flex their digits with sufficient strength to allow them to cling vertically. The strength of forepaw digit flexion is severely diminished by labyrinthectomy, but the digits of the hindpaws appear to be unaffected. This residual non-labyrinthine digit gripping appears to be induced by proprioceptive inputs from the head, neck and torso in response to the vertical body position. Wrapping an elastic bandage snugly around the head and neck of a labyrinthectomized rat given haloperidol-plus-diazepam further diminishes the strength of forepaw digit flexion, and to a lesser degree hindpaw digit flexion. Application of an elastic bandage around the abdomen diminishes the strength of hindpaw digit flexion and to a lesser extent that of the forepaws. The application of both bandages simultaneously completely abolishes the digit flexion of both fore and hindpaws. Therefore, there appears to be a residual digit flexion in the haloperidol-plus-diazepam-treated rat which is controlled by labyrinthine and proprioceptive (head, neck and torso) mechanisms.