Behavioral asymmetries and recovery in rats with different degrees of unilateral striatal dopamine depletion

Abstract

A detailed behavioral analysis during the first postoperative week was performed in rats which had sustained various degrees of unilateral neostriatal dopamine (DA) lesions by administration of the neurotoxin 6-hydroxydopamine into the substantia nigra. These animals were assigned to different groups according to their residual DA levels in the damaged neostriatum (as percentage of the intact side). On the first day after toxin injection into the substantia nigra, turning asymmetries (tight turns) toward the side of the lesion were observed in animals with a mean residual DA level of 32% or less. Out of these, the strongest asymmetries were observed in animals with a mean residual DA of 3%. After one week, the asymmetry in tight turns had totally recovered except in those groups with mean residual DA levels of 17% or less. Partial recovery was found in animals with mean residual DA of 9 and 17%, whereas no indication for recovery was found in animals with the most severe lesions (mean residual DA 3%). Measurement of thigmotactic scanning also revealed an asymmetry for the side of the lesion on the first post-operative day. This asymmetry was observed over a wider range of DA lesion than that observed in turning, namely up to a mean residual DA level of 78%. Furthermore, recovery to symmetry was observed in all lesion-groups except in those with more severe lesions (mean residual DA 17% or less). In contrast to turning, the strongest asymmetries were not displayed by the animals with the most severe lesions. Furthermore, locomotor activity was affected by the lesion, since on the first postoperative day locomotion was reduced in animals with mean residual DA of 39% or less. On day 7, this lesion-dependent deficit had recovered to control levels. Finally, the analysis of net turns allowed the prediction of lesion size in animals with residual DA levels of less than 15%. These results are discussed with respect to mechanisms of recovery, the role of lesion size, and the value of different behavioral measures to predict the degree of DAergic lesion.