The external globus pallidus (GPe) has been historically described as a simple relay nucleus through the basal ganglia, receiving striatal input and projecting output through the subthalamic nucleus¹. However, recent anatomical studies have demonstrated newfound complexity in GPe connectivity, highlighting its potential role as far more than a relay nucleus^2-3. In sum, such studies are providing a paradigm shift in our evaluation of the basal ganglia’s circuit architecture, necessitating closer evaluation of the functional connectivity patterns among its constituent nuclei. Complementary to anatomical studies, circuit stimulation approaches provide an ideal means for mapping the dynamic output patterns of target nuclei, including the directionality of downstream modulation (excitation or inhibition), and alterations in healthy vs. disease states. When combined with unbiased fMRI readouts, brain stimulation tools can be employed to map the global functional connectomes of brain nuclei with a high degree of circuit recruitment specificity. Here, we first used electrical stimulation of the GPe (at a range of stimulation frequencies) combined with evoked and functional connectivity (fc)-fMRI to map the functional connectome of the GPe in healthy rodents. Next, we employed optogenetic tools to more selectively stimulate GPe neurons, mapping the resultant downstream circuit modulation with evoked-fMRI. Finally, we compared the optogenetically-evoked modulation patterns in healthy rodents to those obtained by optogenetically stimulating the GPe in parkinsonian rats. Our results, detailed below, uncover a bewildering complexity in the GPe’s functional connectome, sensitive to stimulation type and frequency, as well as healthy vs. diseased state.Wildtype Sprague-Dawley rats were either implanted with a tungsten microwire electrode unilaterally targeting the GPe for electrical stimulation studies (n=7), or prepared for optogenetic-fMRI studies (n=12) via microinjections with an adeno-associated virus (AAV) carrying the gene encoding channelrhodopsin-2 (ChR2), fused to an enhanced yellow fluorescent protein (EYFP) or only EYFP (synapsin promoter). A subset of animals also received ipsilateral microinjections of 6-hydroxydopamine targeting the medial forebrain bundle, the gold-standard approach for inducing experimental parkinsonism in rodents^3-4. Electrical GPe stimulation was conducted using a current amplitude of 300uA, 500us pulse duration. Optogenetic GPe stimulation was conducted under the following parameters: 10 ms pulse width, 40Hz, 473-nm wavelength, 16-20mW light pulses. Each rat was endotracheally intubated and ventilated with 0.5% isoflurane and medical air. Dexmedetomidine (0.1 mg/ml) and pancuronium bromide (1.0 mg/ml) were infused intravenously for duration of scan. For CBV-weighted MRI, a tail-vein catheter was used to deliver monocrystalline iron oxide contrast agent at a dose of 30 mg Fe/kg. Single shot, single sampled GE-EPI sequences (BW= 300 kHz, TR= 1000 ms, TE= 8.107 ms, 80x80 matrix, FOV= 2.56 x 2.56 cm2, slice thickness= 1 mm) were acquired using a Bruker 9.4T MR scanner and home-made surface coil. Automatic co-registration using SPM codes were applied to realign time-series data within subjects and then again across subjects. Data were analyzed using our established pipelines detailed elsewhere^5-7.The experimental setups for electrical and optogenetic stimulation of the GPe are shown in Fig 1a and 2a, respectively. Electrical stimulation of the GPe resulted in widespread modulation of evoked- and fcMRI signals (Fig 1b-d). Notable findings from our evoked-fMRI dataset include robust CBV increases of the ipsilateral prefrontal cortex, as well as bidirectional, stimulation frequency-dependent modulation of striatal CBV signals (negative at 10Hz, positive at 70Hz and above). fcMRI provided evidence of robust, frequency-dependent global network modulation by electrical GPe stimulation, with more robust network changes observed with 130Hz compared to 40Hz stimulation. Compared to electrical stimulation, optogenetic stimulation at 40Hz resulted in a more spatially restricted pattern of modulation, with prominent CBV increases locally within the GPe, and CBV decreases in the neighboring dorsal striatum (Fig 2b-c). Surprisingly, in Parkinsonian rats, optogenetically-evoked striatal vasoconstriction downstream was substantially reduced. Moreover, a unique prefrontal negative CBV signal emerged during GPe stimulation in these subjects.This study provides an in-depth characterization of the functional connectivity of the GPe, on circuit and network-levels, as well as in healthy and parkinsonian rodents. Our data clearly do not conform to the traditional model of the GPe as a straightforward relay nuclei; modulation of several noncanonical downstream circuits was observed, including prefrontal cortex and striatum. Our optogenetic experiments of GPe connectivity in the healthy and parkinsonian states was prompted by a number of studies demonstrating basal ganglia rewiring in the parkinsonian state^3-4. Our finding of reduced striatal vasoconstriction during GPe stimulation in parkinsonian animals is strongly suggestive of pallidostriatal circuit rewiring in the dopamine-depleted state. Future studies will address the electrophyiological correlates of this disease-altered striatal fMRI signal.