A role for dopamine-mediated learning in the pathophysiology and treatment of Parkinson's disease

Abstract

Dopamine contributes to corticostriatal plasticity and motor learning. Dopamine denervation profoundly alters motor performance, as in Parkinson's disease (PD); however, the extent to which these symptoms reflect impaired motor learning is unknown. Here, we demonstrate a D2 receptor blockade-induced aberrant learning that impedes future motor performance when dopamine signaling is restored, an effect diminished by coadministration of adenosine antagonists during blockade. We hypothesize that an inappropriate corticostriatal potentiation in striatopallidal cells of the indirect pathway underlies aberrant learning. We demonstrate synaptic potentiation in striatopallidal neurons induced by D2 blockade and diminished by application of an adenosine antagonist, consistent with behavioral observations. A neurocomputational model of the basal ganglia recapitulates the behavioral pattern and further links aberrant learning to plasticity in the indirect pathway. Thus, D2-mediated aberrant learning may contribute to motor deficits in PD, suggesting new avenues for the development of therapeutics.

Figure 1. Dissociating learning and performance effects of dopamine receptor blockade on acquisition and maintenance of a motor skill

To dissociate learning and performance effects we used a multi-phase rotarod design in which initial learning under either a dopamine-normal or dopamine-impaired condition is paired with a subsequent testing phase in the opposite condition. A two and three phase design (shown above graphs) was used to assess the effects of dopamine receptor blockade on initial acquisition (two-phase design) or continued performance of an established skill (three-phase design), respectively. Each daily session consisted of 5 trials. In all figures, session means averaged across the 5 trials are reported. Graphs show latency to fall in wild-type C57BL/6 mice administered either a cocktail of dopamine antagonists (0.16 mg/kg eticlopride + 0.1 mg/kg SCH23390, filled gray triangles) or saline (filled red circles) during either (A) initial acquisition of rotarod performance or (B) after performance is established through 12 days prior training. A control group was administered the antagonist cocktail and returned to their homecage without rotarod training and then subsequently tested without drug (A, open gray squares). Each point represents the mean of 5 trials during daily sessions. Between each treatment phase, there was a 72 hour break without training. (C) distance traveled in the open field (OF) for three 45 min sessions on 3 consecutive days for a group that previously received cocktail administration and rotarod training (open triangles), a naive group (red circles) and a group administered 0.16 mg/kg eticlopride 30 min prior to testing. (D) Average rearing/vertical time across all three OF sessions. (F) Average percent of time mice (same groups as D/E) remained in the forward two-thirds of track on treadmill during two 20s test trials at 15 and 20 cm/s. A, N= 7 (homecage controls, open square, N=5); B, N=6; C/D, N=7; E, N=8. Error bars, s.e.m.

Figure 2. Effect of D1 or D2 antagonism on acquisition and subsequent drug-free recovery

Latency to fall in wild-type C57BL/6 mice administered an antagonist of either (A) D1 (SCH23390) or (B) D2 (Eticlopride) during 5 days of initial acquisition of rotarod performance and subsequent non-drug performance. Red traces indicate saline controls (shown in both plots). Bar graphs show the average of last three days of drug training (black bars) followed by the first 5 days of drug-free recovery averaged across all doses for (C) D1 and (D) D2 blockade. Each point represents the average of 5 trials during daily sessions. A 72 hour break occurred between treatment and non-treatment phases. N= 4/dose, ***, p < .001. Error bars, s.e.m.

Figure 3. Effect of D1 or D2 antagonism on established performance

Latency to fall in wild-type C57BL/6 mice administered an antagonist of either (A) D1 (SCH23390) or (B) D2 (Eticlopride) following 5 days of initial training under non-drug conditions and subsequent recovery. Each point represents the average of 5 trials during daily sessions. (C/D) Average performance across sessions in each phase plotted by dose for (C) D1 and (D) D2 blockade. Bar graph insets show first three recovery days averaged across all doses of SCH23390 and for 0.16 and 0.64 mg/kg eticlopride. (E) Comparison on D1 and D2 blockade (SCH23390, 0.1 mg/kg and eticlorpide, 0.16 mg/kg, respecitvely) after 12 days of initial training. (F) Effect of initial training length on subsequent D2 blockade (eticlopride, 0.16 mg/kg) and recovery showing 12 (same data as E, 6 and 3 days of training). Throughout, red traces indicate saline controls. A 72 hour break occurred between treatment and drug-free recovery phases. N= 4/dose for A-D and N=6 for E-F, statistics reported in text. Error bars, s.e.m.

Figure 4. Effect of adenosine antagonists on impairment and recovery from dopamine antagonist cocktail administered during initial acquisition

(A) Latency to fall across consecutive days/sessions for mice co-administered SCH 58261 during cocktail training (red trace, cocktail only; gray darkens with increasing SCH58261) during the initial drug treatment phase (TX) and the drug-free recovery phase (NO TX), with a 72 hour break between phases. (B) Summary of dose-dependent effects of SCH58261 co-administered during initial acquisition on subsequent drug-free recovery. Mean latency to fall averaged across sessions during the drug-free recovery phase plotted by SCH58261 dose (red bar, cocktail only). (C) Mean latency to fall averaged across all drug-free recovery sessions showing dose response for MSX-3 and theophylline co-administered with dopamine antagonists cocktail during initial acquisition (red bar, mice administered cocktail only; darker gray shades represent increasing doses. MSX-3, 1, 2.5, 5 mg/kg; SCH58261, 0.6, 1.2, 2.5, 5 10 mg/kg; theophylline, 10, 30, 60 mg/kg). (D) Summary of the average latency to fall averaged across doses and sessions during initial acquisition (black bars, dopamine antagonists cocktail + adenosine antagonist) and the subsequent drug-free recovery (gray bars) for mice co-administered either MSX-3, SCH59261 or theophylline with a cocktail of SCH23390 (0.1 mg/kg) and eticlopride (0.16 mg/kg) during initial acquisition. Latency to fall in wild-type C57BL/6 mice administered the adenosine antagonist theophylline at the specified dose either (E) during initial acquisition under a cocktail of dopamine antagonists or (F) during the recovery phase subsequent to initial training under cocktail. Red traces represent control mice receiving no theophylline. Each point represents the average of 5 trials during daily sessions with a 72 hour break between phases. (G/H) Mean performance across sessions during the recovery phase plotted by theophylline dose for mice administered theophylline during either (G) initial training under cocktail or (H) during the recovery phase. N= 4/dose, statistics reported in text. *** p < .001. Error bars, s.e.m.

Figure 5. Effect of adenosine antagonist theophylline on eticlopride induced aberrant learning and recovery of an established skill

(A) Latency to fall in wild-type C57BL/6 mice co-administered the A2A antagonist theophylline at the specified dose together with the D2 antagonist eticlopride (0.16 mg/kg) after initial drug free training to asymptotic performance (ie., established skill). (B) Latency to fall in mice administered either theophylline at 80 mg/kg or saline for 5 days initial acquisition. (C) Mean latency to fall across sessions during the different phases of the experiment. N= 4/dose, statistics reported in text. Error bars, s.e.m.

Figure 6. D2 blockade potentiates excitatory inputs to striatopallidal MSNs, which is reduced by theophylline

A. D2-GFP medium spiny neurons in the dorsolateral striatum were held in voltage clamp (Vm=−70mV; Rec = Recording electrode). Stimulating electrodes (Stim) were placed near the corpus callosum, which allowed stimulation of corticostriatal evoked excitatory synaptic currents (EPSCs) at 30s intervals. B. After baseline EPSC amplitude was established, bath application of the D2 receptor antagonist sulpiride potentiated evoked EPSC’s (20 μM; filled symbols; N=4). Co-application of the adenosine antagonist theophylline (1 μM) significantly attenuated the effects of sulpiride on EPSC amplitude (open symbols; N=6). C. Example traces from each recording condition. D. In a separate set of experiments, bath application of sulpiride still potentiated evoked EPSC’s in the absence of any stimulation (solid symbols, N=5). Similar effects were observed with administration of 2 μM sulpiride (open symbols, N=3). *p<0.05. Error bars, s.e.m.

Figure 7. Model performance under conditions recapitulating mouse experiments

(A) shows percentage of correct responses by the model intact (open red symbols) and with dopamine blockade (filled gray triangles) and subsequent recovery with dopamine activity restored (open gray triangles). Each point represents the average of four trials. (B) shows the effect of blocking either the D1/GO layer (filled triangles) or the D2/NOGO layer (filled squares) subsequent to initial intact learning and subsequent recovery when dopamine function is restored (open symbols). (C) the model was trained under dopamine blockade with a reduced learning rate in the D2/NOGO layer (learning set to ½, light gray, set to 0, dark gray) to simulate A2A antagonism either during initial acquisition under dopamine blockade (triangles) or during dopamine restored recovery (squares). (D) Initial model learning and performance under normal dopamine function with (gray symbols) and without (red symbols) D2/NOGO learning rate reduced to 0 to simulate A2A antagonism. (E) Bar graph showing average performance across epochs during the recovery phase grouped by time (during acquisition or recovery) and degree of reduction in D2/NOGO learning rate (ie., ‘theophylline’). (F) Schematic of basal ganglia neurocomputational model. The basal ganglia model includes layers incorporating the direct (GO) and indirect (NOGO) pathways from cortex (2 input layers) through the striatum (GO and NOGO units), to the globus pallidus externa (GPe), the substantia nigra reticulata/globus pallidus interna (GPi), the thalamus, the premotor cortex (PMC) to the output, or motor cortex. SNc dopamine neurons project to both the GO and NOGO layers of the striatum simulating projections to D1 and D2 MSNs (GO/NOGO, respectively, see Supplemental Materials for detailed description). Fast-spiking inhibitory interneurons (not shown) regulate activity in both striatal populations via feed-forward inhibition. At each trial, the network is presented input from each of two input layers (raised cylinders represent example unit activity). Premotor cortical (PMC) units representing the four candidate responses then become noisily activated. Under baseline conditions, the thalamus is inhibited, but a response is selected once a thalamic unit becomes disinhibited and amplifies activity in the corresponding motor units. The role of the BG is to modulate activity in the thalamus according to whether the responses are adaptive in the current sensory state. Each phase of the experiment (ie., analogous to with or without administered drugs, as in the mouse studies) consisted of 20 epochs, equivalent to sessions. Each epoch contained four trials. Model performance is reported as percentage of correct responses. Each data point is the average performance of 20 models initialized with different random weights (ie., N = 20).