Spontaneous locomotion in a cage used for videotaping was assessed over a 30 minute duration recording session prior to MPTP treatment and at 3 months after treatment. Total distance traveled was computed using the Ethovision software program. At baseline, young adult, middle-aged, and aged monkeys were progressively less active (). All monkeys were unilaterally depleted of striatal DA by MPTP treatment to achieve the behavioral endpoint of complete disuse of the contralateral arm and hand. All monkeys held the affected arm in a flexed posture and would not retrieve food with this hand. Overt impairment of the contralateral lower limb was not detected, but not explicitly tested, nor did subjects exhibit spontaneous rotational behavior. Statistical analysis revealed significant differences attributable to age (F(2,5)=6.91, p<0.04), treatment (pre- and post-MPTP, F(1,5)=41.09, p<0.002) and the age × treatment interaction (F(2,5)=9.89, p<0.02). Post-hoc analysis indicated that significant differences were attributable to the large decrease in baseline spontaneous activity in aged monkeys compared to young monkeys (-79%, p=0.005). After MPTP treatment, further reductions in activity were observed for all age groups, but none of these changes reached statistical significance.
Figure 2 Spontaneous locomotor activity progressively decreases with advancing age. Data represents total distance traveled (cm) in a video recording cage over a thirty minute interval prior to MPTP treatment (Pre) and 3 months after induction of unilateral parkinsonism (more ...)
Measurement of DA and the metabolite HVA from both intact and DA-depleted caudate nucleus and putamen revealed aging-related changes. For the intact hemisphere, both caudate and putamen showed statistically significant declines in DA that began in middle-age (caudate -55%, putamen –54%) and were sustained in old age (caudate -43%, putamen –30%) () (Caudate: F(5,42)=41.98, p<0.0001; p<0.0001 for comparison of young intact to middle-aged and aged intact; Putamen: F(5,42)=32.90, p<0.0001; p<0.0001 for comparison of young intact to middle-aged and aged intact). HVA also was significantly reduced from young adult levels in the caudate nucleus and putamen of middle-aged (caudate -46%, putamen –50%) and aged monkeys (caudate –20%, putamen -30%)()(Caudate: F(5,42)=29.83, p<0.0001; p<0.05 for comparison of young intact to middle-aged and aged intact; Putamen: F(5,42)=24.19, p<0.0001; p≤0.0001 for comparison of young intact to middle-aged and aged intact). No differences among age groups were detected for the HVA/DA ratio in either striatal structure.
Figure 3 With advancing age, striatal DA content declines and the capacity to increase DA activity in response to insult is lost in the oldest monkeys. Significant aging-related decreases in DA content of the caudate nucleus and putamen of the intact hemisphere (more ...)
In the DA-depleted striatum, there were the expected dramatic decreases in DA (-92-99%) and HVA (-85-94%) in all age groups as compared to their intact hemisphere ()(Caudate and Putamen DA and HVA p<0.0001 for comparison of lesioned hemisphere to intact hemisphere). No statistically significant differences in the extent of depletion were found between the age groups. This confirms that equivalent damage to the DA system was achieved in animals of all age groups. The most dramatic aging-related change in DA biochemistry was found for the HVA/DA ratio. In both caudate nucleus and putamen, the anticipated significant increase in DA activity in response to MPTP-induced degeneration was present in young and middle-aged monkeys, but completely absent in the oldest age group ()(Caudate: F(5,41)=7.42, p<0.0001; p<0.005 for comparison of intact to lesioned hemispheres, p<0.009 for comparison of middle-age lesion to old lesion, p<0.04 for comparison of young lesion to both middle-age and old lesion; Putamen: F(5,41)=5.85, p<0.0005; p<0.005 for comparison of intact to lesioned hemispheres, p<0.003 for comparison of young and middle-age lesion to old lesion).
Light microscopy for TH immunoreactive (THir) neurons of the substantia nigra pars compacta (SNC) in the intact hemisphere viewed at low magnification suggested the presence of a dramatic aging-related decline in neuron number and dendritic neuropil (). However, when SNC THir neuron numbers were quantified using unbiased stereology, no differences in neuron counts were found across age groups (). Marked decreases in cell numbers accompanied MPTP treatment ( & ) and these were not different with increasing chronological age () (F(5,16)=40.86, p<0.0001; p<0.0001 for comparison of intact to lesioned hemisphere for all ages).
Figure 4 Low magnification views of TH immunoreactivity in SNC of the intact hemisphere suggest a decrease in cell number and neuropil density accompany advancing age (calibration bar=600μm). However, quantitation of THir cell numbers with stereology indicates (more ...)
Figure 6 Counts of THir neurons using stereology reveal no aging-related change in the intact hemisphere of monkeys. The expected significant loss of THir neurons accompanied exposure to MPTP. The magnitude of this loss was not different among age groups (* p<0.0001 (more ...)
Figure 5 With advancing age, the number of THir SNC neurons that stain lightly and exhibit lower cross-sectional area increase in the intact hemisphere. While THir intensity and soma area are preserved through middle-age, old subjects exhibited increased variability (more ...)
Despite the absence of overt cell loss in SNC with advancing age, analysis of THir neuron staining intensity and soma area revealed some aging-related changes ( & ). These changes accounted for the dramatic difference in appearance of SNC in the intact hemisphere at low magnification. For this analysis, cells were separately sampled in the main cytoarchitectural subdivisions of SNC: the dorsal tier, ventral tier, and pars lateralis (). For dorsal tier and ventral tier neurons, an aging-related decrease in THir intensity was detected in the intact hemisphere of the oldest monkeys as compared to young adult monkeys (dorsal tier: F(5,16)=4.26, p<0.02; ventral tier: F(5,16)=3.84, p<0.02; p<0.02 for comparison of young to old). Neurons in pars lateralis exhibited no statistically significant decline in THir intensity (p=0.053), but showed significant declines in soma area in both middle-aged and aged monkeys (F(5,16)=7.40, p<0.001; p<0.03 for comparison of young to middle-age and old). In general, both THir intensity and soma area decreased in surviving neurons following MPTP treatment, but only some of these changes achieved statistical significance. In pars lateralis, MPTP treatment produced significant declines in soma area in young adult and aged monkeys, with this decrease exaggerated in the oldest group (F(5,16)=7.40, p<0.001; p<0.03 for comparison of intact to MPTP-treated, p<0.04 for comparison of young MPTP treated to old MPTP-treated). No significant changes in THir intensity accompanied the declines in soma area in pars lateralis. In addition, aged monkeys exhibited a significant decrease in THir soma area in dorsal tier neurons after MPTP treatment (F(5,16)=3.43, p<0.03; p<0.01 for comparison of aged intact to aged MPTP-treated).
Figure 7 Aging and exposure to MPTP produce specific reductions in soma area and TH staining intensity in the three major subdivisions of SNC. For dorsal tier and ventral tier neurons of the intact hemisphere, there was an aging-related reduction in intensity (more ...)
Analysis of THir fiber staining in striatum also revealed aging-related changes. With advancing age the intensity of fiber staining decreased in both caudate nucleus and putamen, with these declines being more pronounced and expressed earlier in the putamen ()(F(5,20)=6.37, p<0.002; Caudate: p<0.01 for comparison of young to old; Putamen: p<0.04 for comparison of young to middle-aged and old). THir fibers did not measure above levels of background in central caudate and putamen in the MPTP-treated hemisphere.
Figure 8 The intensity of THir fiber staining decreased in caudate nucleus and putamen with advancing age in the intact hemisphere. Decreased fiber staining was more pronounced and expressed earlier in the putamen (+ Caudate: p<0.01 for comparison of young (more ...)