Corticostriatal dysfunction underlies diminished striatal ascorbate release in the R6/2 mouse model of Huntington's disease

Abstract

A behavior-related deficit in the release of ascorbate (AA), an antioxidant vitamin, occurs in the striatum of R6/2 mice expressing the human mutation for Huntington's disease (HD), a dominantly inherited condition characterized by striatal dysfunction. To determine the role of corticostriatal fibers in AA release, we combined slow-scan voltammetry with electrical stimulation of cortical afferents to measure evoked fluctuations in extracellular AA in wild-type (WT) and R6/2 striatum. Although cortical stimulation evoked a rapid increase in AA release in both groups, the R6/2 response had a significantly shorter duration and smaller magnitude than WT. To determine if corticostriatal dysfunction also underlies the behavior-related AA deficit in R6/2s, we measured striatal AA release in separate groups of mice treated with d-amphetamine (5 mg/kg), a psychomotor stimulant known to release AA from corticostriatal terminals independently of dopamine. Relative to WT, both AA release and behavioral activation were diminished in R6/2 mice. Collectively, our results show that the corticostriatal pathway is directly involved in AA release and that this system is dysfunctional in HD. Moreover, because AA release requires glutamate uptake, a failure of striatal AA release in HD is consistent with an overactive glutamate system and diminished glutamate transport, both of which are thought to be central to HD pathogenesis.

Fig. 1

Locations of electrode placement. (A) Diagram of coronal sections showing locations of voltammetry electrode placement in striatum (0.5 mm anterior, 2 mm lateral to bregma and 3.2 to 3.6 mm ventral from skull surface; light gray) and stim electrode in cortex (M1; 1.0 mm anterior, 1.5 mm lateral from bregma, 0.5 mm ventral from skull surface; dark gray). (B) Photomicrograph of coronal section from lesioned R6/2 mouse shows voltammetry electrode tract and proper placement in striatum.

Fig. 2

Striatal ascorbate (AA) release in response to cortical stimulation (stim). (A) Sample voltammograms obtained from a WT mouse and a R6/2 mouse. Baseline AA peak (solid line) represents a single scan taken immediately before stim. The post stim peak (dashed line) represents the first single scan taken 10 s after cortical stim of 10 s duration. The AA oxidation peak occurred between -100 and -200 mV versus reference. Note that the AA peak increases after cortical stim in the WT mouse but does not in the R6/2 mouse. (B) Mean percent change in striatal AA release across time. Baseline (b) represents the last baseline scan before stim. Dashed lines denote baseline on y-axis and stim on x-axis. Single scans at each time point were compared to baseline. (** p < 0.001, * p < 0.01) n = 23 sessions in 5 WT mice and 25 sessions in 5 R6/2 mice.

Fig. 3

Striatal ascorbate (AA) release in behaving mice. (A) Overall mean percent change in vehicle (VEH)-treated animals. R6/2 mice exhibited a diminished release of AA compared to wild-type (WT) mice (* p < 0.05 compared to R6/2 mice; n = 13 sessions in 11 WT mice and 15 sessions in 11 R6/2 mice). (B) Overall mean percent change in striatal AA release in amphetamine (AMPH)-treated animals. WT mice showed an increase in striatal AA release in response to AMPH that was not observed in R6/2 mice (* p < 0.05 compared to R6/2 mice; n = 14 WT mice and 12 R6/2 mice). (C) Mean percent change in striatal AA over time from baseline taken 5 min before injection of AMPH. WT mice increased AA release in response to AMPH, while R6/2 mice did not. (* p < 0.05 compared to R6/2 mice; n = 14 WT mice and 12 R6/2 mice).

Fig. 4

Effects of amphetamine (AMPH) on behavior of WT and R6/2 mice. Behavior was coded for a 3-min period during the pre-amphetamine baseline and at 45 min after injection around the time of peak motor activity. (A) Percent of time spent in locomotor activity. AMPH significantly increased locomotor activity in WT but not R6/2 mice (**p < 0.01, *p < 0.05). (B) Percent of time spent scratching. AMPH significantly increased time spent scratching in WT but not R6/2 mice (***p < 0.001, **p < 0.01). (C) Instances of scratching. AMPH significantly increased instances of scratching in WT but not R6/2 mice (X² = 123.33, p < 0.001). Please note that instances of specific behaviors occurred minimally in R6/2 mice. Thus X² analyses were used and standard error is not applicable for this and the following figures. (D) Number of rears. The number of rears significantly increased in response to AMPH in WT but not R6/2 mice (X² = 82.78, p < 0.001). (E) Number of circles. AMPH increased the number of circles in WT mice (X² = 103.22, p < 0.001). No circling behavior was observed in R6/2 mice. (F) Instances of head bobbing. Instances of head bobbing significantly increased in response to AMPH in WT but not R6/2 mice (X² = 58.00, p < 0.001). n = 17 sessions in 15 WT mice treated with vehicle (VEH), 15 sessions in 15 WT mice treated with AMPH, 14 sessions in 13 R6/2 mice treated with VEH, 14 sessions in 14 R6/2 mice treated with AMPH.