Regulation of parkinsonian motor behaviours by optogenetic control of basal ganglia circuitry

Abstract

Neural circuits of the basal ganglia are critical for motor planning and action selection. Two parallel basal ganglia pathways have been described, and have been proposed to exert opposing influences on motor function. According to this classical model, activation of the 'direct' pathway facilitates movement and activation of the 'indirect' pathway inhibits movement. However, more recent anatomical and functional evidence has called into question the validity of this hypothesis. Because this model has never been empirically tested, the specific function of these circuits in behaving animals remains unknown. Here we report direct activation of basal ganglia circuitry in vivo, using optogenetic control of direct- and indirect-pathway medium spiny projection neurons (MSNs), achieved through Cre-dependent viral expression of channelrhodopsin-2 in the striatum of bacterial artificial chromosome transgenic mice expressing Cre recombinase under control of regulatory elements for the dopamine D1 or D2 receptor. Bilateral excitation of indirect-pathway MSNs elicited a parkinsonian state, distinguished by increased freezing, bradykinesia and decreased locomotor initiations. In contrast, activation of direct-pathway MSNs reduced freezing and increased locomotion. In a mouse model of Parkinson's disease, direct-pathway activation completely rescued deficits in freezing, bradykinesia and locomotor initiation. Taken together, our findings establish a critical role for basal ganglia circuitry in the bidirectional regulation of motor behaviour and indicate that modulation of direct-pathway circuitry may represent an effective therapeutic strategy for ameliorating parkinsonian motor deficits.

Figure 1. Selective viral-mediated ChR2 expression in striatal direct- or indirect-pathway MSNs

(a) Schematic of the double-floxed Cre-dependent AAV vector expressing ChR2-YFP under control of the EF1α promoter. ITR, inverted terminal repeat; WPRE, woodchuck hepatitis virus post-transcriptional regulatory element. (b) Sagittal mouse brain schematic. Ctx, cortex; Str, striatum; GP, globus pallidus; SNr, substantia nigra pars reticulata; Th, thalamus; Hipp, hippocampus. Box indicates region shown in panels c and d. (c) Sagittal section showing striatal direct-pathway MSNs expressing ChR2-YFP following injection of Cre-dependent AAV into D1-Cre BAC transgenic mice. Direct-pathway MSN axons target the SNr. (d) Expression of ChR2-YFP in striatal indirect-pathway MSNs of D2-Cre BAC transgenic mice. Indirect-pathway MSN axons target the GP. Scale bars in c and d are 1 mm. (e-f) Examples of ChR2-YFP-expressing neurons that do not co-express interneuronal markers PV or ChAT. Scale bars in e-f are 15 μm. (g) Percent of ChAT, PV, or NPY neurons that co-express ChR2-YFP. Error bars are SEM.

Figure 2. ChR2-mediated excitation of direct- and indirect-pathway MSNs in vivo drives activity in basal ganglia circuitry

(a) Whole-cell current-clamp recordings from ChR2-YFP+ neurons in vitro demonstrate normal current-firing relationships consistent with D1-MSNs (red traces) or D2-MSNs (green traces) (D1-Control, n=10; D1-ChR2, n=3; D2-Control, n=7; D2-ChR2, n=3). (b) Firing rate plotted as a function of injected current in D1-MSNs or D2-MSNs expressing either GFP or ChR2-YFP. (c) ChR2-mediated photocurrents (top) and spiking (bottom) in D1 (left) and D2 (right) MSNs. In this and subsequent panels, blue bars indicate illumination time. (d) Summary of ChR2-mediated photocurrents (left) and spiking (right) for D1-ChR2 (n=5) and D2-ChR2 (n=4) cells. (e) Schematic of in vivo optical stimulation and recording in the striatum (Str). Cortex (Ctx), thalamus (Th), substantia nigra pars reticulata (SNr). (f) An example MSN recorded from the striatum of an anesthetized D1-ChR2 mouse that displayed increased firing in response to illumination. Insets in f-g and j-k show spike waveform with illumination (blue) or without illumination (grey). Scale bar applies to insets in f-g and j-k. (g) An example of a light-sensitive MSN from a D2-ChR2 mouse. (h) Normalized change in MSN firing rates in response to striatal illumination in D1-ChR2 (n=16) or D2-ChR2 (n=10) mice. (i) Schematic of in vivo optical stimulation in striatum and recording in SNr. (j) An example of a SNr neuron recorded from a D1-ChR2 mouse that was inhibited by direct pathway activation. (k) An example of a SNr neuron recorded from a D2-ChR2 mouse that was excited by indirect pathway activation. (l) Normalized change in SNr firing rate in response to activation of the direct (D1, n=8) or indirect (D2, n=4) pathways. Error bars are SEM.

Figure 3. In vivo activation of direct or indirect pathways reveals pathway-specific regulation of motor function

(a) Coronal schematic of cannula placement and bilateral fiber optic stimulation. (b) Example of altered motor activity during bilateral striatal illumination in D1-ChR2 (left) or D2-ChR2 (right) mice. Lines represent the mouse's path; dots represent the mouse's location every 300 ms. Grey path represents 20 s of activity prior to illumination; colored paths are 20 s during subsequent illumination. (c) Motor activity before (pre), during (laser), and after (post) bilateral striatal illumination in D1-ChR2 (left, red) or D2-ChR2 (right, green) mice. Effect of illumination on (d) the velocity of fine movements, (e) initiation of ambulatory bouts, (f) ambulation bout duration, (g) ambulation velocity, (h) frequency of freezing, and (i) duration of freezing bouts, in D1-ChR2 (red bars, n=9) and D2-ChR2 (green bars, n=8) mice. (j) No change in gait in response to illumination in D1-ChR2 (red bars, n=4) or D2-ChR2 (green bars, n=5) mice. Error bars are SEM.

Figure 4. Direct pathway activation rescues motor deficits in the 6-OHDA model of Parkinson's disease

(a) Visualization of striatal dopaminergic afferents by tyrosine hydroxylase (TH) staining in coronal slices. Scale bar is 1 mm. (b) Loss of dopaminergic innervation in dorsomedial striatum 1 week after 6-OHDA injection. Arrow marks the injection site. (c) ChR2-YFP expression in dorsomedial striatum of 6-OHDA-lesioned mice. (d) Merged image shows overlap of ChR2 expression with the 6-OHDA lesion. (e) Motor behavior before (left, black bars) and after 6-OHDA lesion (right, red bars) in D1-ChR2 mice (n=10). In 6-OHDA lesioned mice, behavior is shown before (pre), during (laser), and after (post) activation of the direct pathway. Effect of 6-OHDA lesion and direct pathway rescue on (f) fine movement velocity, (g) initiation of ambulatory bouts, (h) ambulation bout duration, (i) ambulation velocity, (j) frequency of freezing, and (k) duration of freezing bouts. (l) No change in gait was observed after 6-OHDA lesion or direct pathway activation. Error bars are SEM.