Contributions of signaling by dopamine neurons in dorsal striatum to cognitive behaviors corresponding to those observed in Parkinson's disease

Abstract

Although the cardinal features of Parkinson's disease (PD) are motor symptoms, PD also causes cognitive deficits including cognitive flexibility and working memory, which are strongly associated with prefrontal cortex (PFC) functions. Yet, early stage PD is not characterized by pathology in the PFC but by a loss of dopaminergic (DA) projections from the substantia nigra to the dorsal striatum. Moreover, the degree to which PD symptoms can be ascribed to the loss of DA alone or to the loss of DA neurons is unknown. We addressed these issues by comparing mouse models of either chronic DA depletion or loss of DA projections to the dorsal striatum. We achieved equal levels of striatal DA reduction in both models which ranged from mild (~25%) to moderate (~60%). Both models displayed DA concentration-dependent reductions of motor function as well as mild deficits of cognitive flexibility and working memory. Interestingly, whereas both motor function and cognitive flexibility were more severely impaired after mild ablation of DA neurons as compared to mild loss of DA alone, both models had equal deficits after moderate loss of DA. Our results confirm contributions of nigro-striatal dopamine signaling to cognitive behaviors that are affected in early stage PD. Furthermore, our findings suggest that the phenotype after ablation of DA neurons accrues from factors beyond the mere loss of DA.

Keywords: 6-Hydroxydopamine; Dopamine-deficient mice; Executive function; Neurodegeneration; Tyrosine hydroxylase; Viral inactivation.

Fig. 1

Expression of TH and DAT in the midbrain. Expression of TH and DAT was visualized in coronal midbrain sections of sham control, Th-inactivated and DA neuron ablated mice by immunostaining for TH (red) and DAT (green). A–C: TH expression pattern in sections through the SNpc and VTA of sham control (A), Th-inactivated (B) and DA neuron-ablated mice (C). Expression of TH is similarly reduced in the SNpc of Th-inactivated and DA neuron-ablated mice. D–F: DAT expression pattern in sections through SNpc and VTA of sham control (D), Th-inactivated (E) and DA neuron-ablated mice (F). Expression of DAT is not altered in Th-inactivated mice and clearly reduced in the SNpc of DA neuron-ablated mice.

Fig. 2

Tissue content of catecholamines in the striatum. A–D: Shows data for sham control (N = 29), and Th-inactivated (N = 35) and DA neuron-ablated mice (N = 13) with mild depletion of DA (> 70 % of sham level) in the dorsal striatum. A: DA content in the dorsal and ventral striatum. B: DOPAC levels in the dorsal striatum. C: Ratio of DOPAC/DA in the dorsal striatum. D: Norepinephrine levels in the dorsal striatum. E–H: Shows data for sham control (N = 29), and Th-inactivated (N = 21) and DA neuron-ablated mice (N = 17) with moderate depletion of DA (< 70 % of sham level) in the dorsal striatum. E: DA content in the dorsal and ventral striatum. F: DOPAC levels in the dorsal striatum. G: Ratio of DOPAC/DA in the dorsal striatum. H: Norepinephrine levels in the dorsal striatum. Significant effects are marked with an asterisk (★ ★p < 0.01, ★ p < 0.05) or with a diamond (p < 0.05, comparison between Th-inactivated and DA-neuron-ablated mice). All data are shown as means ± SEM.

Fig. 3

Motor functions. A–C: Show data for sham control and mice with mild depletion of DA in the dorsal striatum for Th-inactivated and DA neuron-ablated mice. (A) Latency to fall in the hanging-wire/four-limb hang test by sham control (N = 33), Th-inactivated (N = 32) and DA neuron ablated mice (N = 10). (B) Latency to fall in an accelerated rotarod test over a 12-trial testing period by sham control (N = 33), Th-inactivated (N = 25) and DA neuron-ablated mice (N = 13). (C) Latency to remove a circular adhesive placed on the animal’s forehead by sham control (N = 40), Th-inactivated (N = 9) and DA neuron-ablated mice (N = 10). D–F: Show data for mice with moderate depletion of DA in the dorsal striatum. (D) Latency to fall in the hanging-wire/four-limb hang test by sham control (N = 33), Th-inactivated (N = 18) and DA neuron-ablated mice (N = 15). (E) Latency to fall in an accelerated rotarod test over a 12-trial testing period by sham control (N = 33), Th-inactivated (N = 21) and DA neuron-ablated mice (N = 15). (F) Latency to remove a circular adhesive placed on the animal’s forehead by sham control (N = 40), Th-inactivated (N = 13) and DA neuron-ablated mice (N = 15). Significant effects are marked with an asterisk (★ ★ p < 0.01, ★ p < 0.05). All data are shown as means ± SEM.

Fig. 4

Visuospatial function and spatial reference memory. Data were obtained using the Morris water maze procedure with a 4-day training procedure and 4 trials per day. A–E: Show data for sham control (N = 34), and Th-inactivated (N = 22) and DA neuron-ablated mice (N = 8) with mild depletion of DA in the dorsal striatum. (A) Latency to climb onto the hidden platform during training. (B) Swim speed in the maze during training. (C) Path length traveled during training sessions. (D) Time spent searching in the quadrants of the Morris water maze after 4 days of training. (F) Average proximity to the exact platform position after 4 days of training. F–J: Show data for sham control (N = 34), and Th-inactivated (N = 15) and DA neuron ablated mice (N = 15) with moderate depletion of DA in the dorsal striatum. (F) Latency to climb onto the hidden platform during training. (G) Swim speed in the maze during training. (H) Path length traveled during training sessions. (I) Time spent searching in the quadrants of the Morris water maze after 4 days of training. (J) Average proximity to the exact platform position after 4 days of training. Significant effects are marked with an asterisk (★ ★ p < 0.01, ★ p < 0.05). All data are shown as means ± SEM.

Fig. 5

Spatial working memory. Working memory was recorded using a modified Morris water maze procedure. Animals received 4 blocks of a 4-day training procedure with 2 trials per day and the position of the platform was changed every day. The time between trial pairs was constant within each block and varied between blocks (60 s, 300 s, 900 s and back to 60 s). Working memory was calculated for each block as average difference between escape latencies on the trial pairs (Trial 1 – Trial2). A positive Δ-score that was significantly higher than zero was interpreted as intact working memory. A: Δ-scores by sham control (N = 29), and Th-inactivated (N = 8) and DA neuron-ablated mice (N = 12) with mild depletion of DA in the dorsal striatum. B: Δ-scores by sham control (N = 29), and Th-inactivated (N = 10) and DA neuron-ablated mice (N = 15) with moderate depletion of DA (< 70 % of sham level) in the dorsal striatum. Significant effects are marked with an asterisk (★ p < 0.05). All data are shown as means ± SEM.

Fig. 6

Cognitive flexibility and cue-dependent learning. Animals were first trained to acquire a turn-based water-escape strategy and then had to learn a new cue-based water-escape strategy. A-B: Show cognitive flexibility and cue-dependent learning data for sham control (N = 18), and Th-inactivated (N = 15) and DA neuron-ablated mice (N = 12) with mild depletion of DA in the dorsal striatum. Percentage of correct trials during 3-day training of turn-based water escape (A) and percentage of correct trials during 5-days of training a cue-based water-escape condition by animals that previously learned turn-based water-escape (B). C: Percentage of correct trials during 3-day training of cue-based water-escape by animals that were not previously trained to learn turn-based water-escape escape (sham N = 14, mild DA depletion: Th-inactivated N = 5 and DA neuron-ablated N = 5). D–E: Shows data for sham control (N = 18), and Th-inactivated (N = 10) and DA neuron-ablated mice (N = 15) with moderate depletion of DA in the dorsal striatum. Percentage of correct trials during 3-day training of turn-based water escape (D) and percentage of correct trials during 5-days of training a cue-based water-escape condition by animals that previously learned turn-based water-escape (E). F: Percentage of correct trials during 3-day training of cue-based, water-escape by animals that were not previously trained to learn turn-based water-escape (sham N = 14, moderate DA depletion: Th-inactivated N = 5 and DA neuron-ablated N = 6). All data are shown as means ± SEM.

Fig. 7

Correlation analysis of striatal DA and cognitive behaviors. Correlation coefficients (Spearman’s r) were calculated for levels of striatal DA and performance on the following cognitive tests. A: Spatial reference memory (measures of quadrant preference and average proximity to exact platform position). B: Spatial working memory (Δ-scores for all ITI conditions: 60-s, 300-s, 900-s and Re-60-s). C: Cognitive flexibility (percentage of correct trials on 5 days following rule shift). Significant correlations are marked with an asterisk (★ ★ p < 0.01, ★ p < 0.05).