INTRODUCTION

The striatum receives excitatory inputs from cortex and thalamus, serves as the major hub of the basal ganglia system, and is important in cognition, motivation, reward, sensorimotor function and many neuropsychiatric disorders. A recent PubMed search identified ~12,000 studies examining striatal function using fMRI. Data from our lab and others indicates that applying traditional neurovascular coupling to striatum may be erroneous,1,9 and we have discounted explanations of the striatum response by major theories of negative fMRI signal such as deactivation,1,2 vascular steal,1,2,3,4 or excessive oxygen consumption.3,5 This prompts the question: What if interpretations of striatal BOLD have been wrong, or even diametrically opposed to underlying neuronal activity? To further illuminate the mechanisms of fMRI signal generation in the striatum, we performed optogenetic fMRI of 7 distinct circuits related to the striatum, recorded electrophysiological signals, measured dopamine release and manipulated neurotransmission using concurrent intracranial pharmacology and fMRI, performed multispectral fiberphotometry to assess striatal neurovascular coupling, and finally, conducted human translational studies.

METHODS

Our investigations into fMRI signal origins in the striatum utilized established procedures and neuroscience tools including: optogenetic-fMRI,6-9 electrophysiology,10 electrical deep-brain stimulation (DBS),6,7 and fast-scan cyclic voltammetry (FSCV)11,12 in wildtype male Sprague-Dawley or TH-Cre Long-Evans rats, and dual-spectral fiber-photometry for GCaMP and CBV measurement in awake A2A-Cre C57B/6 mice.13,14 Further, we developed an intracranial pharmacological-fMRI system for 0.5uL/10min microinfusions of drug via plastic cannula into striatum using a MR-compatible setup. We performed a human fMRI paradigm with transcutaneous electrical nerve stimulation (TENS)15,16 and transcranial magnetic stimulation (TMS).17 Rat fMRI studies were performed on a Bruker 9.4-Tesla/30-cm scanner. Rats were maintained on light sedation.6,19,20 Feraheme (30mg/kg i.v.) was used for CBV-fMRI, acquired by a gradient echo-EPI sequence. fMRI data was preprocessed and analyzed using established pipelines.6-8,21,22

RESULTS and DISCUSSION:

Our optogenetic-fMRI studies (Figure 1) revealed several circuits capable of driving robust negative CBV changes in the striatum, including: stimulation of direct excitatory input to striatum via the parafascicular thalamus (Pf) and primary motor cortex (M1),23 indirect excitatory input from the anterior insula,24 direct activation of striatal medium spiny neurons (MSNs), antidromic activation of D1-receptor-expressing-MSNs (D1MSNs) via terminals in substantia nigra pars reticulata (SNr),25 and activation of external globus pallidus (GPe) principle neurons. Only stimulation of DAergic neurons in the ventral tegmental area(VTA)/substantia nigra pars compacta in TH-Cre rats evoked a positive CBV response. We used acute electrophysiology to record neuronal activity during a subset of optogenetic stimulation procedures (Figure 2). Increased local field potential (LFP) power in both striatum and respective stimulation sites was observed during stimulation of all targets (M1, striatum, SNr and GPe), and all areas except striatum during GPe stimulation displayed predominantly excitatory single-unit activity. These findings exclude the existence of conventional neurovascular coupling in the striatum. To probe the involvement of DA and other vasoactive neurotransmitters in striatal neurovascular coupling, we utilized intracranial pharmacological-fMRI and FSCV (Figure 3). Negative striatal CBV-fMRI responses were created with antidromic stimulation of D1MSNs via the SNr to bypass confounding light/heating artifacts in fMRI.26,27 Compared to vehicle, only oxotremorine-M, an M4 muscarinic ACh receptor agonist,28,29 attenuated the negative response. ACh infusions had no effect, suggesting that MSN activity is responsible. Furthermore, antagonists of the vasoconstrictors somatostatin or neuropeptide-Y, released by persistent, low-threshold spiking (LTS) interneurons,30-33 had no effect. D1R+D2R antagonist cocktail also had no effect. FSCV revealed that optogenetic antidromic SNr-D1MSN stimulation and M1 DBS evoke negative tissue oxygen without DA concentration changes, while VTA DBS evokes positive oxygen and positive DA. Thus, negative striatal fMRI signals are likely downstream of MSN activity, but not cholinergic or LTS interneurons or DA, and positive signals are downstream of DA. We used multispectral fiber-photometry to measure striatal CBV and D2MSN activity simultaneously in awake A2A-Cre mice (Figure 4). Spontaneous locomotor activityevoked simultaneous increases in D2MSN activity with local CBV decreases, indicating that negative striatal CBV can be downstream of D1MSN and D2MSN activity. Finally, we found a bilateral negative striatal fMRI response to bilateral median nerve TENS, and an ipsilateral negative response in striatum to TMS at the left posterior middle frontal gyrus in humans (Figure 5). This indicates that atypical striatal neurovascular coupling is conserved across conditions and species, and should translate to humans.

CONCLUSION

Our results suggest revising interpretation of positive fMRI responses in the striatum from neuronal activation to DA release, and negative fMRI responses in the striatum from neuronal inhibition to activation. These findings should apply to other brain areas with atypical coupling, and more broadly to neuroimaging techniques using vascular responses as a surrogate marker of neuronal activity.

Acknowledgements

We thank members of the Shih lab for valuable discussions concerning the studies described in this abstract. Our team is supported by NIMH R01MH111429, R41MH113252, R21 MH106939, NINDS R01NS091236, NIAAA U01AA020023, R01AA025582, NICHD U54HD079124, NINDS T32NS007431, NINDS F31NS087909, NIDA F31041104, NHLBI T32HL069768 American Heart Association 15SDG23260025, and Brain & Behavior Research Foundation.

References

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