Parafascicular thalamic nucleus activity in a rat model of Parkinson's disease

Abstract

Parkinson's disease is associated with increased oscillatory firing patterns in basal ganglia output, which are thought to disrupt thalamocortical activity. However, it is unclear how specific thalamic nuclei are affected by these changes in basal ganglia activity. The thalamic parafascicular nucleus (PFN) receives input from basal ganglia output nuclei and directly projects to the subthalamic nucleus (STN), striatum and cortex; thus basal ganglia-mediated changes on PFN activity may further impact basal ganglia and cortical functions. To investigate the impact of increased oscillatory activity in basal ganglia output on PFN activity after dopamine cell lesion, PFN single-unit and local field potential activities were recorded in neurologically intact (control) rats and in both non-lesioned and dopamine lesioned hemispheres of unilateral 6-hydroxydopamine lesioned rats anesthetized with urethane. Firing rates were unchanged 1-2 weeks after lesion; however, significantly fewer spontaneously active PFN neurons were evident. Firing pattern assessments after lesion showed that a larger proportion of PFN spike trains had 0.3-2.5 Hz oscillatory activity and significantly fewer spike trains exhibited low threshold calcium spike (LTS) bursts. In paired recordings, more PFN-STN spike oscillations were significantly correlated, but as these oscillations were in-phase, results are inconsistent with feedforward control of PFN activity by inhibitory oscillatory basal ganglia output. Furthermore, the decreased incidence of LTS bursts is incompatible with inhibitory basal ganglia output inducing rebound bursting in PFN after dopamine lesion. Together, results show that robust oscillatory activity observed in basal ganglia output nuclei after dopamine cell lesion does not directly drive changes in PFN oscillatory activity.

Figure 1

Schematic diagram of PFN and STN recording sites. A. Left panel shows recording sites (n=102) in the control hemisphere from brains reconstructed in the coronal plane on standard atlas sections (Paxinos and Watson, 1997). Right panel shows recording sites (n=116 and 32, respectively) of PFN neurons in dopamine lesioned (left) and non-lesioned (right) hemispheres. The stained tissue shows deposit of Pontamine blue dye (arrows) in both hemispheres and their locations in the schematic diagram are indicated by X. B. STN recording sites in control (left panel) and lesioned (right panel) hemispheres. Abbreviations: CM central median thalamic nucleus, fr fasciculus retroflexus, ic internal capsule, LH lateral hypothalamic area, ml medial lemniscus, PFN parafascicular nucleus, Po posterior thalamic nucleus group, STN subthalamic nucleus, ZID zona incerta dorsal, ZIV zona incerta ventral.

Figure 2

Characteristics of PFN spike trains and LFPs. A. Simultaneously recorded PFN spike trains and LFPs from control (top) rats, and non-lesioned (middle) and dopamine lesioned (bottom) hemispheres of unilateral 6-OHDA lesioned rats. Firing rates of these 3 neurons were 2.3 Hz, 2.3 Hz and 1.5 Hz, respectively. The dots and asterisks above spikes in spike trains indicate LTS bursts (all doublets, see methods). LTS bursts with the asterisk above are shown in greater detail to the right of each spike train. B. Bar graphs of mean firing rate (top) and the number of spontaneously active neurons per recording track (bottom) in dopamine lesioned (black bars), non-lesioned (NL, grey bars) and control (white bars) hemispheres. Dopamine lesion did not affect firing rate. Significantly fewer spontaneously active neurons per track occurred in the lesioned hemisphere compared to the control. The non-lesioned hemisphere did not differ significantly from control or lesioned hemispheres. C. Bar graphs of incidence of bursty firing pattern (with burstiness index ≥0.5, left) and ISI CV (middle) showed no significant differences between control, non-lesioned and dopamine lesioned hemispheres. Right bar graph shows significantly more PFN spike trains exhibited significant 0.3–2.5 Hz oscillatory activity in the dopamine lesioned than control hemisphere. The non-lesioned hemisphere did not differ significantly from control or lesioned hemispheres. D. Spectral analyses of PFN spike train autocorrelograms from spike trains illustrated in A above show significant periodic oscillatory activity with main peaks of 0.7–0.8 Hz from recordings in control (left), non-lesioned (middle) and lesioned hemispheres (right). Dashed horizontal lines in the spectra indicate the p=0.05 level of significance. E. Bar graph shows total power in LFPs in the 0.3–2.5 Hz range did not differ significantly between control, non-lesioned and lesioned hemispheres. *Significant difference of p<0.05 compared to the control hemisphere.

Figure 3

Correlated activity between PFN and STN. A. Representative examples of simultaneously recorded spike trains and LFPs from PFN and STN show coincident oscillations in spiking activity in the control and dopamine lesioned hemisphere. The dots above spikes in spike trains indicate LTS bursts. B. STN-triggered PFN cross-correlograms from representative spike trains from control and dopamine lesioned hemispheres shown in A above. Cross-correlogram in the control hemisphere (left) shows that neuronal spiking was significantly correlated between PFN and STN. The peak in PFN spiking occurred 30 ms prior to STN spikes. For the lesioned hemisphere (right), the peak in PFN spiking occurred 10 ms after STN spikes. C. Population data of paired PFN-STN recordings. Bar graph (left) shows that significantly (p=0.001) more PFN-STN spike train pairs had significantly correlated activity in the lesioned hemisphere (n=29) than in the control hemisphere (n=28). Right bar graph shows that there was a trend for PFN neurons to fire before STN neurons in the control hemisphere, whereas in the lesioned hemisphere PFN neurons tended to fire after STN neurons. *Significant difference of p<0.05 compared to the control hemisphere.

Figure 4

Characteristics and timing of PFN spikes in LTS bursts. A. Bar graph (left) shows the proportion of ISIs that met the criteria indicative of LTS bursts (see Methods). Few of all ISIs in control (n=78,089), non-lesioned (n=25,988) and lesioned (n=109,118) hemispheres were incorporated in LTS bursts. B. The left bar graph shows the proportion of spike trains that had no LTS bursts (black bars), contained only doublets (light grey bars) or contained a combination of 2–5 spikes in LTS bursts (mixed LTS bursts, dark grey bars). The dopamine lesioned hemisphere had significantly fewer spike trains with at least one LTS burst than control or non-lesioned hemispheres. Dopamine cell lesion did not reduce the proportion of spike trains that contained only doublets or mixed types of LTS bursts. Right bar graph shows that dopamine cell lesion did not affect the mean number of LTS bursts in spike trains that contained only doublets or mixed types of LTS bursts. C. Representative spike trains and simultaneously recorded LFPs from control (top) and dopamine lesioned (bottom) hemispheres to illustrate the timing of spikes in LTS bursts and single spikes i.e. spikes not incorporated in LTS bursts. Spike trains from control and lesioned hemispheres contained 258 and 198 LTS bursts and had firing rates of 4.1 Hz and 3.5 Hz, respectively. These spike trains both exhibit LTS bursts containing 2 and 3 spikes. Black dots and asterisks above spikes in spike trains indicate LTS bursts containing triplets and grey dots indicate LTS bursts containing doublets. LTS bursts with the asterisk above are shown in greater detail to the right of each spike train. Note that spikes from LTS bursts are between the peak and trough in LFP activity, whereas single spikes are near the trough of LFP activity. D. Phase of PFN spikes in LTS bursts (black wedges) and PFN single spikes (grey wedges) referenced to PFN LFP. Only spike trains containing at least 50 LTS bursts were included in this analysis. Data were combined for spike trains from control (n=31) and dopamine lesioned (n=30) hemispheres. STWAs show that PFN spikes in LTS bursts and single spikes are significantly clustered (p<0.05) with respect to PFN LFPs. The mean phase of spikes in LTS bursts (124 ± 56°) was significantly lower than the phase of single spikes (152 ± 28°). *Significant difference of p<0.05.

Figure 5

Properties of interspike intervals (ISIs) in LTS bursts. A. Line graphs of mean ISIs in LTS bursts in control (left), non-lesioned (middle) and lesioned (right) hemispheres. The numbers of LTS bursts containing doublets, triplets, quadruplets and quintuplets are shown within each graph. The dashed line for quintuplets in control rats highlights that there were only 4 LTS bursts in this category. B–D. Effects of the position of the ISI in LTS bursts (B), number of ISIs in LTS bursts (C) and lesion (D) on mean ISI. A 3 factor ANOVA revealed that all single factors (lesion, number of ISIs and position of ISIs) were statistically significant (p=0.001). B. Post-hoc analysis revealed that ISI durations differed significantly (p<0.05) based on the position of ISIs in the LTS bursts (values were combined across the other factors (lesion and number of ISIs)). Within LTS bursts, first ISIs (n=10616) were significantly shorter than the second ISIs (n=2597), which were significantly shorter than third ISIs (n=504), which were significantly shorter than fourth ISIs (n=84). C. Post-hoc analysis showed that ISIs (combined across position of ISI and lesion factors) were significantly (p<0.05) longer for doublets (n=8019) than triplets (n=4186) or quadruplets (n=1260), but there were no significant differences between triplets, quadruplets and quintuplets (n=336). D. Post-hoc analysis revealed that ISIs were significantly shorter (p<0.05) in the control hemisphere (n=6357) than non-lesioned (n=2049) or lesioned (n=5396) hemispheres (data combined across the other 2 factors). Significant interactions between factors were; ISIs were significantly shorter in quintuplets at each ordinal position than doublets, triplets and quadruplets (p=0.001), and ISIs were significantly larger for quintuplets in the control hemisphere compared to non-lesioned and lesioned hemispheres (p=0.001). There were no significant interactions between all 3 factors on ISIs in LTS bursts. *Significant difference of p<0.05.