Astrocyte-Neuron Lactate Shuttle and Hemodynamic Brain Scans
Hemodynamic brain scans such as Positron Emission Tomography (PET) and functional Magnetic Resonance Imaging (fMRI) have been of immense utility in the neuroscience fields in illuminating the components and modulation of brain function. However, these hemodynamic brain scans have afforded utility without brain scientists truly understanding the mechanism by which they produce signals. Previously, it had been accepted without verification that the images produced by such techniques were direct results of neuronal activity. However, new techniques in studying brain function have revealed that when astrocyte activity is inhibited, hemodependent brain scans do not function (Halber 2008). Additionally, other studies have demonstrated that astrocytes display finely tuned responses to stimuli (Halber 2008) which are amenable to communication with each other in networks via calcium-wave propagation. In light of these new findings in regards to astrocyte function and communication, their influence on hemodynamic brain scans would offer much lucidity to our current understanding of neuropathology and neuromodulation.
General Findings of Astrocytic Influence on Brain Scans
Both PET and fMRI scans are hemodependent brain scans. However, the mechanism by which each works do differ from each other by which fundamental principle of blood flow they measure. In fMRI, a BOLD-signal (blood-oxygen-level-dependent) measurement is utilized which is then translated into working images. PET scans require a radioactive labeled tracer which accumulates in areas of activation. These active areas are then expressed in imaging techniques that interpret the level of activation. However, neurons are in direct contact with neither the neurovasculature nor the microcirculation. Because of this information, coupled with the knowledge that astrocytic endfeet are in direct contact with neurovasculature, their interpolation between neurovasculature and neurons leads them to be suspect in potentially influencing the cerebral microcirculation. It has been proposed that, with these credentials, astrocytes might indirectly support, modify, and in fact directly reflect the brain activity that is being measured in hemodynamic imaging (Schummers 2008). Consequently, several studies have purported similar findings that astrocytes regulate blood flow, suggesting that these functional cells are the key determinants of the signals detected by fMRI (Mo 2008). In much accord with this regulatory function, neurons and astrocytes, by interacting to utilize glucose and burn oxygen, therefore both influence PET imaging technique (Segelken 2004).
Metabolic Functions in Relation to Neuroimaging
Much accreditation to the germinal efforts of investigation of the astrocyte-neuron lactate shuttle has been conducted by James Schummers and colleagues in their investigation of the visual tuning and mapping of astrocytes in...