Region Specific Effects of Aging and the Nurr1-Null Heterozygous Genotype on Dopamine Neurotransmission

Abstract

The transcription factor Nurr1 is essential for dopamine neuron differentiation and is important in maintaining dopamine synthesis and neurotransmission in the adult. Reduced Nurr1 function, due to the Nurr1-null heterozygous genotype (+/-), impacts dopamine neuron function in a region specific manner resulting in a decrease in dopamine synthesis in the dorsal and ventral striatum and a decrease in tissue dopamine levels in the ventral striatum. Additionally, maintenance of tissue dopamine levels in the dorsal striatum and survival of nigrostriatal dopamine neurons with aging (>15 months) or after various toxicant treatments are impaired. To further investigate the effects of aging and the Nurr1-null heterozygous genotype, we measured regional tissue dopamine levels, dopamine neuron numbers, body weight, open field activity and rota-rod performance in young (3-5 months) and aged (15-17 months) wild-type +/+ and +/- mice. Behavioral tests revealed no significant differences in rota-rod performance or basal open field activity as a result of aging or genotype. The +/- mice did show a significant increase in open field activity after 3 min of restraint stress. No differences in tissue dopamine levels were found in the dorsal striatum. However, there were significant reductions in tissue dopamine levels in the ventral striatum, which was separated into the nucleus accumbens core and shell, in the aged +/- mice. These data indicate that the mesoaccumbens system is more susceptible to the combination of aging and the +/- genotype than the nigrostriatal system. Additionally, the effects of aging and the +/- genotype may be dependent on genetic background or housing conditions. As Nurr1 mutations have been implicated in a number of diseases associated with dopamine neurotransmission, further data is needed to understand why and how Nurr1 can have differential functions across different dopamine neuron populations in aging.

Keywords: Dopamine; NR4A2; Parkinson’s disease.

Figure 1

Approximations of the regions and size of tissue dissected in thick frozen section and used for dopamine neurochemistry: Sections are approximately 1.70 mm (A) and 1.00 mm (B) rostral to Bregma. Sections were modified from Paxinos and Franklin.

Figure 2

Behavior analysis due to aging: Body weight (A), rota-rod performance (B) and total distance traveled in an open field activity field under basal conditions (C) and after 3 min of restraint stress (D) were measured in young and aged wild-type (+/+) and Nurr1-null heterozygous (+/−) mice. No genotype differences in body weight were found, although aging significantly increased with body weight. Rota-rod performance was also significantly impaired with aging. The aged +/+ mice showed a trend toward impaired rota-rod performance, but was not significantly different compared to the aged +/− mice (C). No significant differences were found in basal open field activity, however, +/− mice were significantly more active after 3 min of restraint stress. Bars represent mean ± S.E.M. Brackets represent significant difference between treatments based on ANOVA with Fisher’s PLSD post-hoc comparison with p<0.05.

Figure 3

Tissue dopamine levels: Dopamine levels were measured in tissue punches from the dorsal striatum (A), nucleus accumbens shell (B), and n ucleus accumbens core (C) across young and aged wildtype (+/+) and Nurr1-null heterozygous (+/−) mice. There was a significant reduction in dopamine in the nucleus accumbens shell and core in the aged +/− mice. Bar graphs represent mean ± S.E.M. Brackets represent significant difference between treatments based on ANOVA with Fisher’s PLSD post-hoc comparison with p<0.05.

Figure 4

Tyrosine hydroxylase immunohistochemistry: TH immunohistochemistry was examined in the ventral midbrain of young and aged wild-type (+/+) and Nurr1-null heterozyous (+/−) mice. No noticeable differences in distribution of tyrosine hydroxylase immunoreactive neurons were observed across these groups.