Dopaminergic brainstem disconnection is common to pharmacological and pathological consciousness perturbation Lennart R. B. Spindlera,b,1, Andrea I. Luppia,b, Ram M. Adapaa, Michael M. Craiga,b, Peter Coppolaa,b, Alexander R. D. Peattiea,b, Anne E. Manktelowa, Paola Finoiaa,c, Barbara J. Sahakiand,e, Guy B. Williamsb,f, Judith Allansonb,g, John D. Pickardb,c,f, David K. Menona,f, and Emmanuel A. Stamatakisa,b,1 aUniversity Division of Anaesthesia, Addenbrooke’s Hospital, University of Cambridge, Cambridge CB2 0SP, United Kingdom; bDepartment of Clinical Neurosciences, Addenbrooke’s Hospital, University of Cambridge, Cambridge CB2 0SP, United Kingdom; cDivision of Neurosurgery, School of Clinical Medicine, Addenbrooke’s Hospital, University of Cambridge, Cambridge CB2 0SP, United Kingdom; dDepartment of Psychiatry, School of Clinical Medicine, University of Cambridge, Cambridge CB2 0SZ, United Kingdom; eBehavioural and Clinical Neuroscience Institute, University of Cambridge, Cambridge CB2 3EB, United Kingdom; fWolfson Brain Imaging Centre, University of Cambridge, Cambridge CB2 0QQ, United Kingdom; and gDepartment of Neurosciences, Addenbrooke’s Hospital, Cambridge University Hospitals National Health Service Foundation, Cambridge CB2 0SP, United Kingdom Edited by Emery N. Brown, Massachusetts General Hospital, Boston, MA, and approved May 21, 2021 (received for review December 21, 2020) Clinical research into consciousness has long focused on cortical of anesthesia (5, 6). In both pharmacologically and pathologi- macroscopic networks and their disruption in pathological or cally perturbed consciousness, key DMN nodes in the posterior pharmacological consciousness perturbation. Despite demonstrat- cingulate cortex (PCC) and precuneus (7, 8) show disrupted ing diagnostic utility in disorders of consciousness (DoC) and brain-wide functional connectivity associated with loss of con- monitoring anesthetic depth, these cortico-centric approaches sciousness (9). These macroscopic phenomena have demon- have been unable to characterize which neurochemical systems strated some diagnostic and prognostic value for DoC patients may underpin consciousness alterations. Instead, preclinical exper- and form empirical bases for contemporary theories of con- iments have long implicated the dopaminergic ventral tegmental sciousness (4, 10–13). However, these cortico-centric perspec- area (VTA) in the brainstem. Despite dopaminergic agonist efficacy tives have remained inherently unable to address whether in DoC patients equally pointing to dopamine, the VTA has not specific neurochemical systems may mediate perturbed con- NEUROSCIENCE been studied in human perturbed consciousness. To bridge this sciousness and the associated macroscopic network changes. The translational gap between preclinical subcortical and clinical delineation of these neurochemical drivers of consciousness is cortico-centric perspectives, we assessed functional connectivity key to the identification of amenable therapeutic targets in DoC changes of a histologically characterized VTA using functional and for the understanding of anesthetic mechanisms—and as MRI recordings of pharmacologically (propofol sedation) and path- ologically perturbed consciousness (DoC patients). Both cohorts demonstrated VTA disconnection from the precuneus and poste- Significance rior cingulate (PCu/PCC), a main default mode network node widely implicated in consciousness. Strikingly, the stronger VTA– Understanding the neural bases of consciousness is of basic PCu/PCC connectivity was, the more the PCu/PCC functional con- scientific and clinical importance. Human neuroimaging has nectome resembled its awake configuration, suggesting a possible established that a network of interconnected brain regions neuromodulatory relationship. VTA-PCu/PCC connectivity in- known as the default mode network disintegrates in anes- creased toward healthy control levels only in DoC patients who thesia and after brain damage that causes disorders of con- behaviorally improved at follow-up assessment. To test whether sciousness. However, the neurochemical underpinnings of this VTA–PCu/PCC connectivity can be affected by a dopaminergic ag- network change remain largely unknown. Motivated by pre- onist, we demonstrated in a separate set of traumatic brain injury clinical animal work and clinical observations, we found that patients without DoC that methylphenidate significantly increased across pharmacological (sedation) and pathological (disorders this connectivity. Together, our results characterize an in vivo do- of consciousness) consciousness perturbation, the dopaminer- paminergic connectivity deficit common to reversible and chronic gic source nucleus, the ventral tegmental area, disconnects consciousness perturbation. This noninvasive assessment of the from the main nodes of the default mode network. As the dopaminergic system bridges preclinical and clinical work, associ- severity of this dopaminergic disconnection was associated ating dopaminergic VTA function with macroscopic network alter- with default mode network disintegration, we propose that ations, thereby elucidating a critical aspect of brainstem–cortical dopaminergic modulation may be a central mechanism for interplay for consciousness. consciousness maintenance. brainstem | consciousness | disorders of consciousness | dopamine | Author contributions: L.R.B.S., D.K.M., and E.A.S. designed research; L.R.B.S., R.M.A., A.E.M., P.F., B.J.S., G.B.W., J.A., J.D.P., D.K.M., and E.A.S. performed research; L.R.B.S., neurotransmitter A.I.L., M.M.C., P.C., A.R.D.P., and E.A.S. analyzed data; L.R.B.S. and E.A.S. wrote the paper; and L.R.B.S., A.I.L., R.M.A., M.M.C., P.C., A.R.D.P., A.E.M., P.F., B.J.S., G.B.W., J.A., J.D.P., elineating the neural underpinnings of consciousness is of D.K.M., and E.A.S contributed to the revision. Dbasic scientific and clinical importance, both to understand The authors declare no competing interest. its reversible suppression during sedation/anesthesia (1) and to This article is a PNAS Direct Submission. allow informed treatment choices for patients with chronic dis- This open access article is distributed under Creative Commons Attribution License 4.0 (CC BY). orders of consciousness (DoC) (2). Striking parallels between See online for related content such as Commentaries. these perturbed states of consciousness have been characterized 1To whom correspondence may be addressed. Email: [email protected] or eas46@cam. using human neuroimaging, which converge in particular on the ac.uk. default mode network (DMN). This prominent large-scale net- This article contains supporting information online at https://www.pnas.org/lookup/suppl/ work progressively loses functional connectivity integrity with doi:10.1073/pnas.2026289118/-/DCSupplemental. increasing severity in DoC (3, 4) and also with increasing depth Published July 23, 2021. PNAS 2021 Vol. 118 No. 30 e2026289118 https://doi.org/10.1073/pnas.2026289118 | 1of11 Downloaded by guest on September 29, 2021 such for the formulation of an integrated clinical account of substantia nigra) can even reverse the sedative effects of pro- consciousness. pofol (19)—whereas lesions to the VTA lengthen recovery times To this end, preclinical work has focused in on the brainstem following propofol anesthesia in rodents (20). More broadly, neurotransmitter nuclei based on findings across a wide range of methylphenidate-induced reversal of propofol anesthesia in rats experimental animal work utilizing anesthetic drugs and lesion is also suggested to act primarily via a dopamine-dependent approaches. The transmitter systems studied in this context mechanism (21, 22). These preclinical findings implicate do- range from glutamate and glycine, over orexin and histamine, to paminergic signaling from the VTA as a tonic regulator of acetylcholine and the biogenic amines (14). Among them, in wakefulness. particular the dopaminergic system, has emerged as a candidate While equivalent experiments cannot be carried out in humans neurochemical driver of consciousness due to its consistent im- due to their invasiveness, the relevance of dopamine has equally plication in preclinical animal studies and clinical DoC contexts. been revealed in clinical settings through reports of beneficial Preclinically, Palmiter and colleagues demonstrated that effects of various dopaminergic agonists in DoC patients. These dopamine-deficient mice were behaviorally unconscious but that drugs include levodopa (23), bromocriptine (24), methylpheni- retroviral restoration of dopaminergic signaling reversed asso- date (25), and amantadine (26, 27), with further promising re- ciated behavioral deficits (15). The source of the relevant do- sults from past and ongoing small-scale trials of apomorphine paminergic signaling was subsequently identified to be the (28). The corresponding and long-standing idea of dopami- ventral tegmental area (VTA), the main dopaminergic brainstem nergic dysfunction in DoC (29) is corroborated by a recent nucleus. Both optogenetic (16) and pharmacological VTA acti- 11C-raclopride positron emission tomography (PET) study which vation (17) acutely promote wakefulness in rodents. Critically, indicated impaired presynaptic dopamine release in minimally electrical and optogenetic (18) stimulation of the VTA (but not conscious state (MCS) patients (30). However, despite this Fig. 1. VTA disconnects stepwise and reversibly from PCu/PCC in propofol sedation. (A) In awake participants, the VTA ROI showed