Beta frequency synchronization in basal ganglia output during rest and walk in a hemiparkinsonian rat

Abstract

Synchronized oscillatory neuronal activity in the beta frequency range has been observed in the basal ganglia of Parkinson's disease patients and hypothesized to be antikinetic. The unilaterally lesioned rat model of Parkinson's disease allows examination of this hypothesis by direct comparison of beta activity in basal ganglia output in non-lesioned and dopamine cell lesioned hemispheres during motor activity. Bilateral substantia nigra pars reticulata (SNpr) recordings of units and local field potentials (LFP) were obtained with EMG activity from the scapularis muscle in control and unilaterally nigrostriatal lesioned rats trained to walk on a rotary treadmill. After left hemispheric lesion, rats had difficulty walking contraversive on the treadmill but could walk in the ipsiversive direction. During inattentive rest, SNpr LFP power in the 12-25 Hz range (low beta) was significantly greater in the dopamine-depleted hemisphere than in non-lesioned and control hemispheres. During walking, low beta power was reduced in all hemispheres, while 25-40 Hz (high beta) activity was selectively increased in the lesioned hemisphere. High beta power increases were reduced by l-DOPA administration. SNpr spiking was significantly more synchronized with SNpr low beta LFP oscillations during rest and high beta LFP oscillations during walking in the dopamine-depleted hemispheres compared with non-lesioned hemispheres. Data show that dopamine loss is associated with opposing changes in low and high beta range SNpr activity during rest and walk and suggest that increased synchronization of high beta activity in SNpr output from the lesioned hemisphere during walking may contribute to gait impairment in the hemiparkinsonian rat.

Figure 1

Photograph of the circular treadmill. Rats were trained to walk in the treadmill at a rate of 9 rpm in ipsiversive and contraversive directions. During recordings, rats would walk for 5 min in the rotating treadmill and then rest in the stationary treadmill for 10–20 min between walking epochs.

Figure 2

Immunohistochemical results and electrode microwire placement. (A) Coronal section of the substantia nigra pars compacta (SNpc) illustrating immunohistochemistry of tyrosine hydroxylase (TH). Note the lack of TH staining in the SNpc of the lesioned (left) hemisphere compared with the non-lesioned (right) hemisphere. Location of section: bregma = −5.04 mm (Paxinos and Watson, 1986). Mean loss of TH+ stained = 86%, range = 76–97%. (B) Coronal section of the substantia nigra pars reticulata (SNpr) illustrating microwire placement. Sections were stained with cresyl violet and potassium ferricyanide to reveal the iron deposits; recording sites are indicated by arrows. Note the small electrolytic lesion clearly evident in the right section. Location of section: left figure, bregma = −5.02 mm and right figure, bregma = −5.28 mm (Paxinos and Watson, 1986). Scale bars: 1 mm

Figure 3

SNpr spike train and LFP recordings during rest and walk on a circular treadmill from a unilaterally dopamine cell lesioned rat. Representative spike trains (blue) and LFPs (green) were recorded simultaneously from the SNpr of the non-lesioned (A) and lesioned (B) hemispheres during rest (treadmill stationary), preparation to walk (treadmill adjustment) and walking (treadmill on). EMGs (green, middle panels) were also recorded simultaneously from the scapularis muscles; the EMGs contralateral to the SNpr recording site are presented accordingly. EMG activity and video taped behavioral analyses directed the selection of epochs to analyze for rest and walk epochs. Wavelet scalograms (bottom panels) represent the time-frequency plots of spectral power. Spectral power was plotted on a log₁₀ scale with greater power represented by redder colors. SNpr LFP low beta power during rest was greater than low beta power during walking, as illustrated in the scalograms from the non-lesioned and lesioned hemispheres. Notably, SNpr LFP low beta power in the lesioned hemisphere was greater than low beta power in the non-lesioned hemisphere (before the first arrow on the time axis in bottom panel). When the experimenter approached the treadmill, thus alerting the rat, a change from low beta frequency power to high beta frequency (25–40 Hz) power in the lesioned hemisphere (first arrow) was observed. When the treadmill motor was turned on (second arrow) and adjustments to the rat’s posture were made to position him in the correct walking direction, high beta power appeared more prominently in the lesioned hemisphere relative to the non-lesioned hemisphere. With the onset of walking (third arrow), a prominent band of high beta SNpr LFP activity emerged in the lesioned hemisphere.

Figure 4

SNpr LFP power in the beta frequency range recorded in the non-lesioned and lesioned hemispheres of the unilateral dopamine cell lesioned rat 7 days following 6-OHDA infusion and from neurologically intact (control) rats. Power spectra (A, B) are representative examples from SNpr LFP activity in the non-lesioned (A) and lesioned (B) hemispheres of an individual rat over the frequency range of 11–50 Hz. SNpr LFP activity was greater in the low beta frequency range (12–25 Hz) in both non-lesioned and lesioned hemispheres at rest (solid line) than during walking (dotted line). During walking, a prominent peak of high beta activity emerged in the lesioned hemisphere (B); this was not observed in the non-lesioned (A) or control (data not shown) hemispheres. Total power in the frequency ranges of low beta (12–25 Hz) and high beta (25–40 Hz) are presented (C, D) as the mean ± SEM for LFP recordings in the SNpr of 8 neurologically intact (control) hemispheres and 9–10 non-lesioned and lesioned hemispheres during rest and walk epochs. In the low beta frequency range (C), SNpr LFP power was significantly greater in the lesioned hemispheres compared with SNpr LFP power in the non-lesioned or control hemispheres when the animals were at rest. SNpr LFP power was significantly reduced when the animals were walking compared with epochs of rest in all 3 hemispheric states. There were no significant differences between the SNpr LFP power in the 3 hemispheres during walking. In the high beta frequency range (D), there was no significant differences in SNpr LFP power during rest among the 3 hemispheres. However, during walking high beta SNpr LFP power was significantly increased in the lesioned hemispheres. * p<0.05 significantly different than rest within hemispheric condition + p<0.05 significantly different than control and non-lesioned hemispheres for rest in low beta frequency and walk in high beta frequency

Figure 5

LFP power in the high beta frequency range (25–40 Hz) recorded with electrode arrays placed bilaterally in the SNpr of the lesioned and non-lesioned hemispheres of unilateral dopamine cell lesioned rats. To confirm the results obtained with recordings with electrode bundles and observe the effects of dopamine replacement therapy, experiments were conducted with this electrode configuration in rats 21 days post lesion (n=6 rats). During walk epochs, SNpr LFP power in the high beta range was significantly increased in the lesioned hemispheres, but not in the non-lesioned hemispheres (A). L-DOPA administration (4–5 mg/kg plus 15 mg/kg benserazide, i.p., a dose that improves gait but does not induce rotation) significantly reversed the elevated SNpr LFP power. No significant differences were observed between rest and walk with and without L-DOPA in the non-lesioned hemispheres. Wavelet scalograms (B) illustrate the effect of dopamine replacement with L-DOPA on SNpr LFP high beta frequency activity in the lesioned hemisphere of an individual rat. Before L-DOPA administration, notable LFP power in the 25–40 Hz range is evident during ipsiversive walk epochs (B, top). Following L-DOPA administration (~ 30 min post i.p. injection), LFP power in significantly reduced during ipsiversive walk epochs (B, bottom). Wavelet scalograms represent the time-frequency plots of spectral power with spectral power plotted on a log₁₀ scale with greater power represented by redder colors. * p<0.05 significantly different than walk in non-lesioned hemispheres + p<0.05 significantly different than rest, rest plus L-DOPA, and walk plus L-DOPA in lesioned hemispheres

Figure 6

Phase locking in the SNpr of lesioned and non-lesioned hemispheres of unilateral dopamine cell lesioned rats. Representative examples of individual rats are shown in A, B, D and E. Group results are shown in C and F. (A) SNpr spike trains recorded in the lesioned hemispheres showed increased phase locking of spikes when paired with SNpr LFP in the 12–25 Hz low beta range during rest (solid line) compared with scrambled spikes (dashed line). No phase locking of SNpr spikes with SNpr LFP was observed during walking in the 12–25 Hz range (B). (C) Normalized SNpr spike-triggered SNpr LFP waveform averages (STWA) were obtained by dividing unscrambled STWA peak to trough amplitude values by scrambled STWA amplitude values. Dashed line indicates a ratio = 1, where STWA amplitudes from scrambled and unscrambled are equal. In the low beta frequency range normalized STWAs from the lesioned hemispheres were significantly greater than STWAs from non-lesioned hemispheres during rest. In contrast, during walking, there was no significant difference between the STWAs from lesioned and non-lesioned hemispheres. (D) In the 25–40 Hz high beta range, SNpr spike trains recorded in the lesioned hemispheres demonstrated no significant phase locking of spikes with SNpr LFPs during rest. (E) However, during walking, a marked increase in phase locking compared with scrambled spikes paired with LFPs was demonstrated. (F) In the high beta frequency range normalized STWAs from the lesioned hemispheres were significantly greater than STWAs from the non-lesioned hemispheres during walking. In contrast, during rest, there was no significant difference between the STWAs from lesioned and non-lesioned hemispheres. Data in bar graphs represent 2–3 spike trains per hemisphere from 5 rats. * p<0.05 significantly different than non-lesioned + p<0.05 significantly different than walk for low beta frequencies and rest in high beta frequencies in lesioned hemispheres