Functional correlates of exaggerated oscillatory activity in basal ganglia output in hemiparkinsonian rats

Abstract

Exaggerated beta range (13-30Hz) synchronized activity is observed in the basal ganglia of Parkinson's disease (PD) patients during implantation of deep brain stimulation electrodes and is thought to contribute to the motor symptoms of this disorder. To explore the translational potential of similar activity observed in a rat model of PD, local field potentials (LFPs) and spiking activity in basal ganglia output were characterized in rats with unilateral dopamine cell lesion during a range of behaviors. A circular treadmill was used to assess activity during walking; hemiparkinsonian rats could maintain a steady gait when oriented ipsiversive to the lesioned hemisphere, but were less effective at walking when oriented contraversive to lesion. Dramatic increases in substantia nigra pars reticulata (SNpr) LFP oscillatory activity and spike-LFP synchronization were observed within the beta/low gamma range (12-40Hz) in the lesioned hemisphere, relative to the non-lesioned hemisphere, with the dominant frequency of spike-LFP entrainment and LFP power varying with behavioral state. At 3weeks postlesion, the mean dominant entrainment frequency during ipsiversive treadmill walking and grooming was 34Hz. Other behaviors were associated with lower mean entrainment frequencies: 27-28Hz during alert non-walking and REM, 17Hz during rest and 21Hz during urethane anesthesia with sensory stimulation. SNpr spike-LFP entrainment frequency was stable during individual treadmill walking epochs, but increased gradually over weeks postlesion. In contrast, SNpr LFP power in the 25-40Hz range was greatest at the initiation of each walking epoch, and decreased during walking to stabilize by 6min at 49% of initial values. Power was further modulated in conjunction with the 1.5s stepping rhythm. Administration of l-dopa improved contraversive treadmill walking in correlation with a reduction in SNpr 25-40Hz LFP power and spike synchronization in the dopamine cell lesioned hemisphere. These effects were reversed by the serotonergic 1A agonist, 8-OH-DPAT. While the prominent spike-LFP phase locking observed during ongoing motor activity in the hemiparkinsonian rats occurs at frequencies intriguingly higher than in PD patients, the synchronized activity in the SNpr of this animal model has much in common with oscillatory activity recorded from the basal ganglia of the PD patients. Results support the potential of this model for providing insight into relationships between synchronization of basal ganglia output induced by loss of dopamine and motor symptoms in PD.

Keywords: 6-Hydroxydopamine; Basal ganglia; Beta frequency; Dopamine; Gait; Local field potentials; Motor cortex; Oscillations; Parkinson's disease; Serotonin; Substantia nigra pars reticulata.

Figure 1

SNpr LFP power, spike-LFP synchronization and dominant frequency in the dopamine cell-lesioned hemisphere, as compared to the non-lesioned hemisphere, during different behaviors and under urethane anesthesia. A: Wavelet-based scalograms of LFP spectral power in the SNpr from dopamine cell-lesioned and non-lesioned hemispheres (left and middle panels) show periods of inattentive rest, alert, ipsiversive treadmill walking, grooming, rapid eye movement (REM) sleep and during urethane anesthesia with sensory stimulation (urethane+SS). Spectral power is shown on a logarithmic scale with greater power being represented by red colors. With the exception of the REM sleep and urethane epochs, examples are from the same rat on day 21 post-lesion. Episodes of REM sleep were recorded infrequently, and an example of one is shown here. Corresponding FFT-based LFP power spectra on the right represent the same epochs as shown in the scalograms. Note the marked increases in high beta/low gamma power in the lesioned hemisphere relative to the non-lesioned hemisphere during alert, treadmill walking and grooming, with increases in a lower beta range under urethane+SS. B: Bar graphs show mean ratios of SNpr LFP power in the dopamine cell-lesioned hemisphere relative to SNpr LFP power in the corresponding intact hemisphere for the ranges of 12–25 Hz (blue), 25–40 Hz (red) and 45–60 Hz (yellow) during inattentive rest (inatt rest), alert non-walking (alert), ipsiversive walking (walk), and grooming (groom) states (n=9), and under urethane+SS (ureth) (n=8). Data were averaged per rat. Dashed line represents a ratio of 1, indicating identical power in lesioned and non-lesioned hemispheres. In the 12–25 Hz range: * p<0.02 compared with alert, walk and grooming. In the 25–40 Hz range: * p<0.003 compared with rest and urethane. # p<0.03 compared with rest, alert and urethane+SS (1-way RM ANOVA. C: Spike-LFP synchronization in the 12–45 Hz range in the dopamine cell-lesioned hemisphere (red) and non-lesioned hemisphere (blue) over a range of behavioral states as reflected by the ratio of STWA-based peak-to-trough amplitudes of unshuffled vs. shuffled multiunit spike trains (4 spike trains over two epochs per behavior per rat, n=9 rats). Dashed line represents a ratio of 1, indicating that STWA peak-to-trough amplitudes from shuffled and unshuffled spike trains are equal. Note large differences in mean STWA peak-to-trough amplitude ratios in the dopamine lesioned hemisphere relative to the non-lesioned hemisphere showing greater phase-locking of spikes with LFP in the dopamine-depleted hemisphere (2-way RM ANOVA, p<0.001). See STWA waveform example for lesioned hemisphere in Fig 7. A D: Bar graph shows mean STWA-based dominant multiunit entrainment frequencies in SNpr LFP activity for spike trains that were significantly phase-locked to LFP (see Methods) in the low beta-low gamma range (12–45 Hz) in the dopamine cell-lesioned hemisphere during the same behavioral states as represented in A. * p<0.05 compared to rest, alert and urethane+SS,; # p<0.05 compared to rest and urethane+SS (1-way RM ANOVA). n= 31,30, 29, 28, and 29 for rest, alert, walk groom and urethane, respectively.

Figure 2

The distribution of dominant frequencies in SNpr LFP activity in the 12–45 Hz range in the dopamine cell-lesioned hemisphere for epochs of alert rest, treadmill walking and under urethane anesthesia with sensory stimuli (ureth+SS). Data for alert and treadmill walking were from days 18–23 and from urethane+SS days 40–50. Dominant frequencies were obtained from SNpr multiunit spike trains significantly phase-locked to LFPs from 4 electrodes over four 30 s representative epochs as described in methods. Data were averaged per rat. Urethane: grey bars, n=8; walking: black bars, n=9; alert: white bars, n=9.

Figure 3

Time-dependent changes in the 25–40 Hz range SNpr LFP power and dominant frequency in the dopamine cell-lesioned hemisphere relative to the non-lesioned hemisphere during ipsiversive treadmill walking. A: Wavelet-based scalogram shows a typical example of SNpr LFP power in the dopamine cell-lesioned hemisphere during the first 5 min of a walking epoch on day 43 post-lesion. Rectified EMG (green upper traces) recorded from the deltoid shoulder muscle shows consistent muscle activity during the walking epoch. Note that the high beta/low gamma range band shows reduction in power over time during the walking period. B: FFT-based SNpr LFP power in the 25–40 Hz range for consecutive 60s segments over an extended period of the same walking epoch as shown in A in the dopamine cell lesioned (red) and non-lesioned hemisphere (blue). Note a reduction in total power in the lesioned hemisphere over the first 4 min of walking. C: Bars show the 25–40 Hz LFP total power range for 1^st and 6^th min of walking for dopamine cell-lesioned vs non-lesioned hemispheres (n=9) at days 18–23 post lesion. * p< 0.05 compared to the non-lesioned hemisphere and 6^th min within the lesioned hemisphere (2-way RM ANOVA). D: SNpr high beta/low gamma range (25–40 Hz) power in the lesioned hemisphere for 5 consecutive 2.5 min walking epochs (averaged over 30 s segments) over a range of treadmill speeds with 1 min rest epochs in between. Power was consistently reduced after the onset of walking, as in A and B. No change in power was observed in the non-lesioned hemisphere. A typical example is shown (N=6 rats). E: STWA-based entrainment frequency in SNpr LFP power spectra for consecutive 60 s segments of the walking epoch shown in A. Unlike power (in B), peak frequency in SNpr LFP spectral power remains stable over time of treadmill walking in the lesioned hemisphere. F, G: Line plots in F (gray lines) show the total LFP power in the 25–40 Hz range for individual rats from post-lesion day 7 up to 50 days. Mean power (black line) remains stable over days (n=9). In contrast, as shown in G, the dominant frequency during walking is significantly increased over the same time period between days 7 (31.0±0.9 Hz), 21 (34.0±0.3 Hz) and 40 post-lesion (36.7±0.4 Hz) (1-way RM ANOVA, * p<0.001, n=9).

Figure 4

Modulation of 25–40 Hz LFP power and EMG amplitude during treadmill walking. A: Wavelet-based scalogram shows a time-frequency representation of LFP power in the SNpr over a ten second epoch of treadmill walking. B: Line graph depicts amplitude of the rectified 25–40 Hz LFP from the same epoch as in A and C. C: Line graph shows EMG amplitude from the scapular EMG electrode contralateral to the lesioned hemisphere during the same epoch as in A and B. The treadmill speed during this epoch allowed the rat to walk consistently, stepping with the forelimb approximately once every 1.5 s. D: EMG-LFP cross-correlation (see Methods) shows notable movement-related modulation of the amplitude of 25–40 Hz SNpr LFP oscillatory activity in the dopamine-deprived hemisphere during walking. Data are presented as mean correlation coefficient averaged in 5 rats ±SEM (pink). Dotted lines represent ±3 SD of the mean values between −2.0 to −1.5 and 1.5 to 2.0 s, indicating significant modulation of LFP beta range oscillations in dopamine-depleted hemisphere by the stepping rhythm during walking.

Figure 5

The effect of treatment with L-dopa on 25–40 SNpr LFP power and gait during walking in the circular treadmill. Cartoons above the plots show the relationship between the direction of walking and the orientation of the affected paws (red) in the circular treadmill with respect to the dopamine cell-lesioned hemisphere (red dot). A, B, E, F: Line graphs show step counts for impaired (red, contralateral to lesion) and non-impaired (blue, ipsilateral to lesion) hind paws during contraversive (A, B) and ipsiversive (E, F) walking over a range of treadmill speeds. During contraversive walking (A, impaired paws on the inside of the track) the rat shows unbalanced stepping and progress in walking is disrupted. When the rat is walking ipsiversive to the lesion (E, impaired paws on the outside of the track) the rat is able to walk at a steady pace. L-dopa administration (5 mg/kg) improves stepping during contraversive walking (B), and gait becomes similar to baseline ipsiversive walking (E). C, G: Time-dependent reduction in 25–40 Hz range LFP power and associated improvement in treadmill walking (5 and 9 RPM) following L-dopa administration (n=4). Step ratios (number of steps by the inner limb vs. outer limb, black line) and power ratios (drug vs baseline beta/low gamma range LFP power, gray lines) are plotted vs. time after L-dopa treatment for contraversive (C) and ipsiversive (G) walking. After treatment with L-dopa, stepping ratios approach 1 as the rat walks more steadily in the contraversive direction (C) in conjunction with a reduction in beta power. L-dopa treatment does not affect step count ratios in G as the rat is able to walk effectively in the ipsiversive direction in the absence of treatment. D, H: Scatter plots depict percent change in the 25–40 Hz range LFP power vs ratio-based motor scores during walking in the circular treadmill following treatment with L-dopa, as shown in C and G. Improvements in stepping (ratios approaching 1) correlate with reduction in SNpr LFP power in the lesioned hemisphere during contraversive walking following L-dopa treatment (D; Pearson coefficient, r=0.781, p<0.001). There is no correlation between reduction in SNpr LFP power following L-dopa treatment and step ratios during ipsiversive walking (H; Pearson coefficient r=0.075, p>0.05).

Figure 6

Effect of a therapeutic dose of L-dopa (5 mg/kg) in rats with unilateral dopamine cell lesions on 25–40 Hz SNpr LFP power during ipsiversive walking and in anesthetized rats, reversal by the 5HT1A agonist 8-OH-DPAT, and step ratio during ipsiversive and contraversive walking. A–B: Representative wavelet-based scalograms with corresponding FFT-based power spectra below, showing LFP power during 4 series of consecutive recordings. A) Scalograms show LFP power during baseline ipsiversive treadmill walking, 20 min after L-dopa (5mg/kg), 5 min after subsequent administration of 8-OH-DPAT (0.2 mg/kg, 40 min after L-dopa) and 3–5 min following injection of the 5HT1A antagonist WAY-100635 (0.3 mg/kg, 20 min after 8-OH-DPAT). B) The same drug treatment protocol as in A administered to a rat anesthetized with urethane (1 g/kg). Recordings were performed during periods of mild sensory stimulation as described in methods (n=5). C: Bar graphs represent mean ratios of 25–40 Hz SNpr LFP power in dopamine cell-lesioned hemisphere relative to non-lesioned hemisphere before and after treatment with drugs as in A during ipsiversive walking (left graph, n=7) and ratios of 15–30 Hz LFP power as in B in rats under urethane+SS (right graph, n=5). * p<0.001 compared to baseline and 8-OH-DPAT in awake rats (1-way RM ANOVA) and p<0.04 compared to baseline and 8-OH-DPAT in anesthetized rats (1-way RM ANOVA). Dotted line, representing a ratio of 1, indicates equal power in lesioned and non-lesioned hemispheres. D: Bar graphs represent step count ratios for the affected vs. unaffected hind limbs during ipsiversive (blue) and contraversive (red) walking at 9 RPM in the same rats before 6-OHDA lesion (intact) and on 21–23 days post-lesion, in control conditions and after L-dopa treatment. In left panel, rats were sequentially treated with L-dopa (5 mg/kg) and the D2 antagonist eticlopride (0.2 mg/kg, 40–60 min after L-dopa, N=5), and in right panel, rats were subsequently treated with 8-OH-DPAT followed by WAY-100635 (n=7)1, as shown in A. Note that increases in step ratios during contraversive walking after L-dopa treatment are reversed by both 8-OH-DPAT and eticlopride treatments. * p<0.001 compared to ipsiversive walking during the same conditions/drug treatment (2-way RM ANOVA). Impairment in gait during contraversive walking is associated with synchronization of LFP activity in the 25–40 Hz frequency range in the dopamine cell-lesioned hemisphere similar to that seen when rats are walking in ipsilateral direction.

Figure 7

The effect of L-dopa, 8-OH-DPAT and WAY-100635 treatments on phase-locking of SNpr spiking to 25–40 Hz LFP activity in dopamine cell-lesioned and non-lesioned hemispheres. A: Representative examples of spike-triggered LFP waveform averages (STWA) for a typical SNpr spike train in the lesioned hemisphere paired with simultaneously recorded SNpr LFP, band-pass filtered at 25–40 Hz. During treadmill walking, the STWA of the SNpr cell shows strong phase locking at ~90°, toward the trough of the SNpr LFP. L-dopa administration dramatically reduced phase-locking of the SNpr spikes, subsequent treatment with 8-OH-DPAT restored the spike-LFP synchronization and finally, treatment with WAY-100635 again reduced synchronization. Black line: unshuffled spike train; gray line: shuffled spike train. B: Bar graphs show the ratio of STWA-based peak-to-trough amplitudes of unshuffled vs. shuffled spike trains (see Methods) reflecting SNpr Spike-LFP synchronization in the dopamine cell lesioned hemisphere (black bars) and non-lesioned hemisphere (grey bars) before and after subsequent treatment with L-dopa, 8-OH-DPAT and WAY-100635. Dashed line represents a ratio of 1, indicating that STWA peak-to-trough amplitudes from shuffled and unshuffled spike trains are equal. Note the reduction in spike-LFP synchronization after L-dopa treatment, recovery following the 8-OH-DPAT and subsequent reduction by WAY-100635. * differences in mean STWA peak-to-trough amplitude ratios in the dopamine lesioned hemisphere (31 cells, 7 rats) relative to the non-lesioned hemisphere (25 cells, 7 rats) showing greater phase-locking of spikes with LFP in the dopamine-depleted hemisphere in baseline and after treatment with 8-OH-DPAT (2-way RM ANOVA, p<0.001). C: Bar graphs show the proportion of spike trains with spike timing significantly correlated to locally recorded LFPs in the 25–40 Hz range during walking (see Methods) before and after subsequent treatment with L-dopa, 8-OH-DPAT and WAY-100635. *p<0.001 compared to percent correlated spike train in the non-lesioned hemisphere in matching conditions (Chi-squared test).