Altered Recruitment of Motor Cortex Neuronal Activity During the Grasping Phase of Skilled Reaching in a Chronic Rat Model of Unilateral Parkinsonism

Abstract

Parkinson's disease causes prominent difficulties in the generation and execution of voluntary limb movements, including regulation of distal muscles and coordination of proximal and distal movement components to achieve accurate grasping. Difficulties with manual dexterity have a major impact on activities of daily living. We used extracellular single neuron recordings to investigate the neural underpinnings of parkinsonian movement deficits in the motor cortex of chronic unilateral 6-hydroxydopamine lesion male rats performing a skilled reach-to-grasp task the. Both normal movements and parkinsonian deficits in this task have striking homology to human performance. In lesioned animals there were several differences in the activity of cortical neurons during reaches by the affected limb compared with control rats. These included an increase in proportions of neurons showing rate decreases, along with increased amplitude of their average rate-decrease response at specific times during the reach, suggesting a shift in the balance of net excitation and inhibition of cortical neurons; a significant increase in the duration of rate-increase responses, which could result from reduced coupling of cortical activity to specific movement components; and changes in the timing and incidence of neurons with pure rate-increase or biphasic responses, particularly at the end of reach when grasping would normally be occurring. The changes in cortical activity may account for the deficits that occur in skilled distal motor control following dopamine depletion, and highlight the need for treatment strategies targeted toward modulating cortical mechanisms for fine distal motor control in patients.SIGNIFICANCE STATEMENT We show for the first time in a chronic lesion rat model of Parkinson's disease movement deficits that there are specific changes in motor cortex neuron activity associated with the grasping phase of a skilled motor task. Such changes provide a possible mechanism underpinning the problems with manual dexterity seen in Parkinson's patients and highlight the need for treatment strategies targeted toward distal motor control.

Keywords: 6-OHDA lesion; Parkinson's; extracellular recording; grasping; single neuron; skilled reaching.

Copyright © 2019 the authors.

Figures

Figure 1.

Histological and behavioral effects of the unilateral 6-OHDA lesion. A, Schematic top view of a rat showing naming conventions. Example is for a rat in which the left paw was dominant as assessed before sham or lesion surgery. Intracerebral injections of vehicle or 6-OHDA, and recording electrode placement, were contralateral to the presurgery dominant paw, which in the remainder of the paper is referred to as the “contralateral” paw. B, Example images from coronal sections of ventral midbrain including the substantia nigra and ventral tegmental area show immunohistochemical staining for TH (green). Scale bar 0.5 mm. C, Proportion of reaches performed postsurgery with the contralateral paw in control and lesioned animals. ***p < 0.001, Mann–Whitney U test. D, Mean duration of the terminal part of the reach, executed by the contralateral (Contra; solid line) and ipsilateral (Ipsi; dashed line) paws from control and lesioned rats. ***p < 0.001 for group × paw interaction (X), two-way ANOVA, ## p < 0.01 Holm–Sidak post hoc tests. E, Same data as D, split according to most (solid line) and least used (dashed line) paw. C, Contralateral paw; I, ipsilateral paw. ***p < 0.001 for main effects of group and usage, two-way ANOVA. F, Frequency distributions (peak normalized) of reaching durations (5 ms bin) for least used (top) and most used paws (bottom) in control (purple) and lesioned (green) rats. Solid lines, contralateral paw (contra); dashed lines, ipsilateral paw (ipsi). S, Skewness; K, kurtosis.

Figure 2.

Characteristics of neuronal recordings. A, Lines on atlas sections at indicated anteroposterior positions relative to bregma show positions of recording tracks in control (purple) and 6-OHDA lesioned rats (green). Section at +2.04 mm is merged with a neutral-red stained coronal section from the equivalent histological section for the track which penetrated deeper into cortex. M1, M2: subregions of motor cortex as defined in atlas of Paxinos and Watson (2007). B, Representative spike train activity and average action potential waveforms of a broad (top) and narrow spike neuron (bottom). C, Points in scatter plot show the relationship between mean firing rate, action potential (AP) width and AP amplitude for each neuron in control rats. Narrow and broad waveforms are indicated by open and filled symbols, respectively. D, As for C, for neurons from lesioned rats. E, Scatter plots show recording depth of each neuron against AP width (top), AP amplitude (middle), and mean firing rate (bottom). Purple triangles and green circles indicate neurons from control and lesioned animals, respectively. Dashed horizontal lines indicate the approximate depth of borders between cortical layers 2/3, 5, and 6.

Figure 3.

Effect of 6-OHDA lesion on neuronal activity measures averaged over 30 s of recording during task and no-task epochs. A, Mean firing rate in task (filled circle and solid line) and no-task epochs (open triangle and dashed line), in control (purple) and lesioned rats (green). X* indicates p < 0.05, for interaction (ANOVA); #p < 0.05 for Holm–Sidak's post hoc contrast for rest vs reaching in lesioned rats. B, Interspike interval coefficient of variation (ISI CV). C, Percentage of neurons classifed as bursting.*p < 0.01, main effect of epoch (ANOVA). D, Mean number of bursts per second in neurons defined as being bursty. *p < 0.05, main effect of epoch (mixed-effects model). E, Mean number of spikes per burst in bursting cells. F, Mean percentage spikes in bursts in bursting cells.

Figure 4.

Characteristics of reach-related activity. A, Example of individual motor cortex neuron that showed an increase in firing rate during reaches by the contralateral paw, recorded in a control rat. PETH are aligned (time 0) to end of the extension phase of the reach and onset of food grasping. Bin width 20 ms, smoothed with three-bin rolling average. Dotted horizontal lines show the ± 2 SD thresholds for detection of a modulation. B, Example of a rate-decrease (lesioned rat). C, Example of a biphasic modulation (control rat). D, Heat plot raster lines show z-score-normalized firing rate (smoothed with 3-bin rolling average) for all neurons in control animals with a significant firing rate increase during reaching. Neurons with a first increase as part of a biphasic response are included. Data are ordered by onset time. Open rectangles mark period of first significant rate increase for each neuron, determined using original unsmoothed PETHs. Dotted ellipses demarcate qualitative groupings of responses with similar temporal characteristics. E, Heat plot raster for neurons in control animals with a firing rate decrease. Dotted ellipses demarcate two groups of neurons with particularly strong responses and similar temporal characteristics. F, Colored traces show control animal population average PETH, for neurons with either only a rate-increase (+, red), only a decrease (−, blue), or a biphasic response (±, orange). To emphasize timing relationships traces have been scaled to the same maximal positive or negative amplitude. Colored vertical panels show approximate times of movement phases, and black horizontal lines show timing of activation in selected muscles, previously described in video and EMG recording studies using the same task. Figure is adapted from Hyland and Jordan (1997).

Figure 5.

Comparison of proportions of motor cortex neurons responding during reaches by the contralateral paw in control and lesioned animals. A, Overall proportions. Rate increase (+), red; decrease (−), blue; no change (0), white. Data for control animals calculated from PETH constructed only from the first 58 trials. *p < 0.05, χ2 test. B, Proportions responding as a function of baseline firing rate in control (left) and lesioned animals (right). Numbers above columns show numbers of neurons in each bin.

Figure 6.

Effect of lesion on reach-related neural activity timing and amplitude. A, Heat plots showing z-score normalized PETH from neurons in control rats, using only first 58 trials. Top, Rate-increase cells. Bottom panel, Rate-decrease cells. Other conventions are as for Figure 4, D and E. The dotted ovals span the same time ranges as in Figure 4. B, Heat plots for neurons in lesioned animals. Dashed line box highlights a group of rate-increase neurons with peak activity as part of a biphasic responses, not seen in controls. C, Traces show population average z-scored firing rate ± SEM (solid and dotted lines, respectively), for neurons classified as having pure rate increases in control (purple) and lesioned (green) animals. Data were smoothed with a three-bin rolling average. Vertical dashed lines demarcate subperiods 1–6 used for statistical comparisons. Inset graph shows data after excluding one outlier neuron with a very large amplitude response from the lesioned group. D, As for C, for neurons with pure rate-decreases. *p < 0.05, Mann–Whitney U test comparing average area under the curve in each subperiod. E, As for D, for neurons with biphasic responses. Only one neuron in control animals had a significant biphasic response component in PETH with reduced trial number, so no statistical contrasts were possible.