LOCOMOTION OF AGED RATS - RELATIONSHIP TO NEUROCHEMICAL BUT NOT MORPHOLOGICAL-CHANGES IN NIGROSTRIATAL DOPAMINERGIC-NEURONS

Spontaneous locomotion and motor coordination was evaluated in young ( 5-6 month old) and aged (24-25 month old) rats. Animals were tested fo r spontaneous locomotor activity in Digiscan Animal Activity Monitors during the nocturnal cycle. Aged animals exhibited a significant hypoa ctivity compared to their young counterparts. Evaluation of the time c ourse of activity revealed that the young animals had a cyclical patte rn of activity during the 12-hour testing period with clear peaks at 2 -4 hours after the initiation of testing and at 8- to 10-hour interval s thereafter. In contrast, the aged animals exhibited a blunted initia l activity peak. During the remainder of the test period the aged anim als activity was stable with no further peaks in activity. Compared to the young animals the aged animals also (a) remained suspended from a horizontal wire for less time, (b) were unable to descend a wooden po le covered with wire mesh in a coordinated manner, (c) fell more rapid ly from a rotating rod and (d) were unable to maintain their balance o n a series of wooden beams with either a square or rounded top of vary ing widths. Histological analysis demonstrated that there was no reduc tion in the number, area, or length of tyrosine hydroxylase-immunoreac tive neurons within the A8, A9, or A10 region of the aged animals. Neu rochemical analysis revealed that while DA and HVA levels were not dec reased in the aged rats, DOPAC levels, as well as the ratios of DA/DOP AC and DA/HVA, were decreased. These results indicate that neurochemic al but not morphological changes within the nigrostriatal dopaminergic system underlie the deficits in motor behavior observed in aged rats.