Impairments and compensation in mouth and limb use in free feeding after unilateral dopamine depletions in a rat analog of human Parkinson''s disease

The Montoya Staircase Test has been designed as a simple objective way of measuring changes in skilled movements following motor system damage. In the test, rats reach from a central platform for food pellets located on adjacent staircases and the measure of success is the number of food pellets obtained. As there has been no detailed behavioral analysis of how animals reach in this task, the present study evaluates reaching in the test by combining end point measures (success) with movement analysis based on video recordings. It is found that control rats locate food using olfaction and then reach using an identifiable sequence of movements, including (1) aiming the limb, (2) opening the digits in preparation for grasping, (3) grasping, and (4) supinating the paw, during limb withdrawal, to place food in the mouth. The nonreaching limbs adjust posture during the reaching sequence. Rats with unilateral DA-depletions show (1) severe impairments in success when using their contralateral-to-lesion limb (bad limb), (2) moderate impairments in using their ipsilateral-to-lesion limb (good limb), (3) abnormal reaching movements and posture, and (4) a variety of compensatory movements so as to enhance success. When success produced by compensatory adjustment is subtracted from total success, the DA-depleted rats show no recovery. The results confirm that the Staircase Test is a sensitive measure for motor system damage and demonstrate that when movement analysis is combined with end point measures, the test can dissociate impairment, recovery, and compensation.