Levodopa affects spike and local field synchronisation in the pedunculopontine nucleus of a rat model of Parkinson's disease

Abstract

The pedunculopontine nucleus (PPN) undergoes significant anatomic and electrophysiological alterations in Parkinson's disease (PD), severely impacting locomotion. However, the effect of 6-hydroxydopamine (6-OHDA) lesion and levodopa (L-DOPA) therapy on the relationships between spike activities and local field potential (LFP) within the PPN is not well-understood. Synchronisation between the spike activity of individual neurones and LFP of neuronal ensembles is a crucial problem in the pathogenesis of PD. In this study, LFP signals and spikes in the PPN of rats in control, lesioned, and L-DOPA groups were recorded synchronously with a multi-unit electrical signal acquisition system and analysed for their coherence value, spike-field coherence, and phase-lock relationship. The spike-LFP relationship in the PPN was markedly increased in specific frequency bands because of the 6-OHDA lesion but differed depending on the animal locomotion state and neuronal type. L-DOPA had a limited therapeutic effect on the 6-OHDA-induced increase in the coherence value. Our study demonstrates that the PPN spike-LFP relationship is involved in the pathogenesis of PD and is critical for the effects of L-DOPA, providing a basis for the clinical treatment of refractory PD symptoms.

Keywords: Parkinson’s disease; coherence; local field potential; pedunculopontine nucleus; spike.

Figure 1

Representation of the experimental timeline. MFB: medial forebrain bundle; APO: apomorphine; 6-OHDA: 6-hydroxydopamine.

Figure 2

(A–D) Exemplar images of cresyl violet-stained coronal sections of the substantia nigra pars compacta of animals with 6-hydroxydopamine (6-OHDA) lesions. (A) control hemisphere, 100×; (B) lesioned hemisphere, 100×; (C) control hemisphere, 400×; (D) lesioned hemisphere, 400×. (E, F) fluorescence immunohistochemical staining of dopaminergic neurones for tyrosine hydroxylase in the substantia nigra pars compacta. (E) Control hemisphere, 200×; (F) lesioned hemisphere, 200×. (G) Schematic representation of electrode positioning in rats of different groups. (H) Image of a coronal section of PPN stained with cresyl violet. The location of the electrode tip is indicated by a red arrowhead. (I) Schematic representation of the electrode path (red line) into the pedunculopontine nucleus (PPN). (J) Walking time along a control ladder in lesioned and levodopa (L-DOPA) rats. (K) Latency in the rotarod test. (L) distance in the rotarod test. (M) Number of apomorphine (APO)-induced rotations after 30 min in lesioned and L-DOPA rats. * p < 0.05, *** p < 0.001, **** p < 0.0001 compared to the control group; # p < 0.05 compared to the lesioned group.

Figure 3

Neuronal classification results in the pedunculopontine nucleus (PPN) (red: Type A; blue: Type B). (A) Averaged waveforms of each neuronal type. (B) Representative signals of each neuronal type. (C) Principal component analysis (PCA) results in 3D view. (D) Spike duration (trough-to-peak) and firing rate of 36 Type A and 21 Type B neurones. (E) Representative inter-spike interval histogram of a Type A neurone. (F) Representative inter-spike interval histogram of a Type B neurone.

Figure 4

Measurement of the coherence value between spikes and local field potential (LFP) in the pedunculopontine nucleus (PPN). (A) Frequency-coherence value plots in resting Type A neurones. (B) Frequency-coherence value plots in resting Type B neurones. (C) Statistical results of coherence value in resting Type A neurones. (D) Statistical results of coherence value in resting Type B neurones. (E) Frequency-coherence value plots of Type A neurones in locomotion state. (F) Frequency-coherence value plots of Type B neurones in locomotion state. (G) Statistical results of coherence value obtained in Type A neurones in locomotion state. (H) Statistical results of coherence value obtained in Type B neurones in locomotion state. The frequency bands with the most noticeable changes are highlighted via blue boxes. *p < 0.05, **p < 0.01 in comparison to the control group.

Figure 5

Spike-field coherence (SFC) in the pedunculopontine nucleus (PPN) and local field potential (LFP). (A) Frequency-SFC plots in resting Type A neurones. (B) Frequency-SFC plots in resting Type B neurones. (C) Statistical results of SFC obtained with resting Type A neurones. (D) Statistical results of SFC obtained with resting Type B neurones. (E) Frequency-SFC plots in Type A neurones of rats in locomotion state. (F) Frequency-SFC plots in Type B neurones of rats in locomotion state. (G) Statistical results of SFC obtained with Type A neurones of rats in locomotion state. (H) Statistical results of SFC obtained with Type B neurones of rats in locomotion state. The frequency bands with significant changes are highlighted via blue boxes. ***p < 0.001, *p < 0.05, relative to the control group; ###p < 0.001, ##p < 0.01, #p < 0.05, relative to the lesioned group.

Figure 6

Phase-lock between spikes and local field potential (LFP) in the pedunculopontine nucleus (PPN). (A) Statistical results of phase-lock obtained with resting Type A neurones. (B) Statistical results of phase-lock obtained with resting Type B neurones. (C) Rose histograms representing the distribution of spike phase angles with oscillation in noticeably altered bands. The circles around the histograms display the phase orientation for each discharge, and the red lines arising from the centre indicate the length and vector of the mean phase angles. (D) Statistical results of phase-lock obtained with Type A neurones of rats in locomotion state. (E) Statistical results of phase-lock obtained with Type B neurones of rats in locomotion state. **p < 0.01 relative to control rats; #p < 0.05 relative to lesioned rats.

Figure 7

Local field potential (LFP) filtering process is illustrated as a representative 10-s signal in resting state with synchronously recorded spike raster and an LFP spectrogram.