The role(s) of astrocytes and astrocyte activity in neurometabolism, neurovascular coupling, and the production of functional neuroimaging signals

Data acquired with functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) are often interpreted in terms of the underlying neuronal activity, despite mounting evidence that these signals do not always correlate with electrophysiological recordings. Therefore, considering the increasing popularity of functional neuroimaging, it is clear that a more comprehensive theory is needed to reconcile these apparent disparities and more accurately explain the mechanisms through which various PET and fMRI signals arise. In the present article, we have turned our attention to astrocytes, which vastly outnumber neurons and are known to serve a number of functions throughout the central nervous system (CNS). For example, astrocytes are known to be critically involved in neurotransmitter uptake and recycling, and empirical data suggests that brain activation increases both oxidative and glycolytic astrocyte metabolism. Furthermore, a number of recent studies imply that astrocytes are likely to play a key role in regulating cerebral blood delivery. Therefore, we propose that, by mediating neurometabolic and neurovascular processes throughout the CNS, astrocytes could provide a common physiological basis for fMRI and PET signals. Such a theory has significant implications for the interpretation of functional neuroimaging signals, because astrocytic changes reflect subthreshold neuronal activity, simultaneous excitatory/inhibitory synaptic inputs, and other transient metabolic demands that may not elicit electrophysiological changes. It also suggests that fMRI and PET signals may have inherently less sensitivity to decreases in synaptic input (i.e. 'negative activity') and/or inhibitory (GABAergic) neurotransmission.