The stepping test and its learning process in different degrees of unilateral striatal lesions by 6-hydroxydopamine in rats.

Four different levels of the nigrostriatal dopamine system lesions were produced by injections of 6-hydroxydopamine at one-, two-, three-, or four-sites in the striatum and drug-induced rotational movement and stepping test were performed to evaluate behavioral impairments in the rat model of Parkinson's disease. A dose-dependent progressive loss of tyrosine hydroxylase-positive cells in the substance nigra pars compacta was observed in rats with striatal lesion from one- to four-sites. Though the differences in the rotational behavior and stepping test between the lesioned and control rats were highly significant, there were no differences in those behaviors among four groups of lesioned rats. During observation of these behavioral tests, the authors found that the times of trials required for acquisition of the stepping test on the first day of training, which reflected learning acuity, increased in a dose-dependent manner in the lesioned rats as compared with the controls. On the contrary, the times of trials on the next day and in the next week, which reflected retention of the acquired memories, were not different among the groups. In conclusion, the rotational movement and stepping test were not sensitive enough to distinguish severity of the striatal lesions, and learning acuity, but not retention of memories, was disturbed by the striatal lesions.