Functional circuit mapping of striatal output nuclei using simultaneous deep brain stimulation and fMRI

Abstract

The substantia nigra pars reticulata (SNr) and external globus pallidus (GPe) constitute the two major output targets of the rodent striatum. Both the SNr and GPe converge upon thalamic relay nuclei (directly or indirectly, respectively), and are traditionally modeled as functionally antagonistic relay inputs. However, recent anatomical and functional studies have identified unanticipated circuit connectivity in both the SNr and GPe, demonstrating their potential as far more than relay nuclei. In the present study, we employed simultaneous deep brain stimulation and functional magnetic resonance imaging (DBS-fMRI) with cerebral blood volume (CBV) measurements to functionally and unbiasedly map the circuit- and network level connectivity of the SNr and GPe. Sprague-Dawley rats were implanted with a custom-made MR-compatible stimulating electrode in the right SNr (n=6) or GPe (n=7). SNr- and GPe-DBS, conducted across a wide range of stimulation frequencies, revealed a number of surprising evoked responses, including unexpected CBV decreases within the striatum during DBS at either target, as well as GPe-DBS-evoked positive modulation of frontal cortex. Functional connectivity MRI revealed global modulation of neural networks during DBS at either target, sensitive to stimulation frequency and readily reversed following cessation of stimulation. This work thus contributes to a growing literature demonstrating extensive and unanticipated functional connectivity among basal ganglia nuclei.

Keywords: Deep brain stimulation; External globus pallidus; Rat; Striatum; Substantia nigra pars reticulata; fMRI.

Figure 1

(A) Schematic of the experimental setup, including custom surface coil and microwire DBS electrode. (B–C) Electrode tip placements within the SNr (B) and GPe (C) of all experimental subjects. Tip placements were estimated using T₂-weighted anatomical scans, which we deemed satisfactory given the relatively large size (including anteroposterior distance) of our targets, as well as the minimal electrode artifact.

Figure 2

Functional activation maps of CBV modulation by SNr- and GPe-DBS. Two DBS stimulation frequencies are shown for each target: SNr- DBS at 40 or 130 Hz (A and B, respectively), GPe-DBS at 40 or 130 Hz (C and D, respectively). Notable observations include CBV decreases in the striatum at both targets, as well as large, ipsilateral frontal cortical modulation by GPe-DBS. At both targets, stimulation responses were largely ipsilateral and stronger at 130 Hz compared to 40 Hz. 12 slices were acquired in each scan, with numbers below slices denoting relative distance from bregma (in mm). Color bar denotes t score values obtained by GLM analyses, with a significance threshold of p < 0.05. Functional activation maps for all additional tested frequencies are located in Supplemental Figures S4–5.

Figure 3

SNr-DBS evoked CBV changes at select, anatomically-defined regions of interest. CBV traces (10–400 Hz; yellow bar denotes stimulation epoch; note different Y axis scales across ROIs) are accompanied by bar graphs displaying percent changes in CBV amplitude changes during the stimulation period (mean ± SEM CBV values for the DBS stimulation period). * denotes significant differences in CBV amplitude from 10 Hz (p < 0.05). Insert depicts representative slice example for each pre-defined ROI (note that most ROIs encompassed multiple slices). Unless otherwise denoted, all ROIs are ipsilateral to the DBS hemisphere. (A) External globus pallidus (B) Substantia nigra (C) Ipsilateral dorsolateral striatum (D) Contralateral dorsolateral striatum (E) Posterior hypothalamus (F) Somatosensory cortex. Additional ROIs (total = 19) are located in Supplemental Figure S6.

Figure 4

GPe-DBS evoked CBV changes at select, anatomically-defined regions of interest. CBV traces (10–400 Hz; yellow bar denotes stimulation epoch; note different Y axis scales across ROIs) are accompanied by bar graphs displaying percent changes in CBV amplitude changes during the stimulation period (mean ± SEM CBV values for the DBS stimulation period). * denotes significant differences in CBV amplitude from 10 Hz (p < 0.05). Insert depicts representative slice example for each pre-defined ROI (note that most ROIs encompassed multiple slices). Unless otherwise stated, all ROIs are ipsilateral to the DBS hemisphere. (A) External globus pallidus (B) Substantia nigra (C) Ipsilateral dorsolateral striatum (D) Contralateral dorsolateral striatum (E) Prelimbic cortex (F) Somatosensory cortex. Note that the displayed ROI’s for Panel E differ between Figures 3 and 4. Additional ROIs (total = 19; including Posterior hypothalamus) are located in Supplemental Figure S7.

Figure 5

fcMRI Modulation via DBS of the SNr and GPe. (A) Mean correlation matrices (SNr n = 6, GPe n = 7) for each stimulus condition (Rest₁, Stim 40 Hz, Rest₂, Stim 130 Hz, Rest₃) using 45 region-of-interests (ROIs: 1–45 Ipsilateral, 46–90 Contralateral, see Figure Key). (B) Post-hoc comparison of significantly modulated connections. Significance of individual connections (see Supporting Material: Supplemental Tables 1–2) was determined using repeated measures analysis of variance across animals (rANOVA, p ≤ 0.01 uncorrected, Δ_{Z-correlation} ≥ 0.10). Connections were grouped according to modulation direction (Enhanced: increased correlation; Suppressed: increased anti-correlation) and then two-sample t-tests (see Supporting Material: Table S3) were used to statistically compare stimulus conditions (Rest, 40 Hz, and 130 Hz, see Figure Key). Data plotted as mean ± standard error of the mean (SEM). * denotes pair-wise significant (p < 0.001) differences. Abbreviations: PLC: Prelimbic Cortex; ILC: Infralimbic Cortex; OFC: Orbitofrontal Cortex; CC: Cingulate Cortex; Insula: Insular Cortex; NAc: Nucleus Accumbens; AS; Anterior Striatum; vPAll: Ventral Pallidum; Sept: Septum; lHyp: Lateral Hypothalamus; Amyg: Amygdala; BNST: Bed Nucleus of the Stria Terminalis; MDT: Mediodorsal Thalamus; vHipp: Ventral Hippocampus; VTA: Ventral Tegmental Area; AC: Auditory Cortex; AOB: Accessory Olfactory Bulb; DLS: Dorsolateral Striatum; DMS: Dorsomedial Striatum; ENT: Entorhinal Cortex; GPe: External Globus Pallidus; Motor: Motor Cortex (Primary and Secondary); OT: Olfactory Tubercle; PAG: Periaqueductal Grey; PPTg: Pedunculopontine Tegmental Nucleus; PC: Parietal Cortex; Piriform: Piriform Cortex; pHyp: Posterior Hypothalamus; pThal: Posterior Thalamus; S2: Secondary Somatosensory Cortex; SN: Substantia Nigra; Somato: Primary Somatosensory Cortex; STN: Subthalamic Nucleus; TeA: Temporal Association Cortex; VL: Ventrolateral Thalamus; VPL: Ventral Posterolateral Thalamus; Visual: Visual Cortex (Primary and Secondary); ZI: Zona Incerta; dHipp: Dorsal Hippocampus; dRaphe: Dorsal Raphe Nucleus; lHab: Lateral Habenula; mPOA: Medial Preoptic Area; SC: Superior Colliculus; vHyp: Ventral Hypothalamus

Figure 6

Robust Functional Connectivity Modulation. Enhanced connections demonstrating more robust modulation (ΔZ-Corr > 0.20) overlaid on volume rendering of the rat brain (also see Tables S1–2). Green – SNr-DBS, Orange – GPe-DBS.