The brain is a complex organ composed of numerous populations of cells controlling neuronal activity that are tightly coupled with the cerebrovascular networks. Since more than a century, it is known that the tight coupling between neuronal activity and regional cerebral blood flow (CBF) is essential to the normal brain function. The increase in neuronal activity is associated with an increase of local perfusion, known as functional hyperemia. Despite its physiopathological importance, the cellular and molecular mechanisms of neurovascular coupling remain poorly understood. Nevertheless, it is currently accepted that neuronal activation triggers an increase of CBF that is controlled by the neurogliovascular unit composed of terminals of neurons, astrocytes, glial cells, pericytes, blood vessel muscles and more. How activation of the brain triggers functional hyperemia is an important question to better understand normal brain physiology and whose answer could shed light on several pathologies such as stroke, migraine, Alzheimer’s disease, psychological disorders and others pathologies in which neurovascular coupling is known to be impaired.
Neuronal networks in the brain include glutamatergic principal neurons and GABAergic interneurons. The latter may be a minority cell type, but they are vital for normal brain function because they regulate the activity of principal neurons. In recent years, single-cell reverse transcriptase multiplex PCR (scRT-mPCR) after patch clamp has been used to classify cortical interneurons according to their electrophysiological, morphological and molecular characteristics. By using unsupervised clustering expression profiling of canonical interneuronal markers, four classes of GABAergic interneurons were defined: vasoactive intestinal peptide (VIP), neurogliaform (NG), somatostatin (Sst) and fast-spiking parvalbumin (FS-PV). The reliable identification of molecular markers expressed by specific interneuron subpopulations is of paramount importance because they can be employed in the functional analysis of cortical circuitry such as the one controlling cerebral perfusion.
We recently demonstrated that ChR2 based photo-stimulation of fast spiking neurons expressing PV (FS-PV) gives rise to an effective contraction of penetrating arterioles in acute brain slice by using patch-clamp electrophysiology and IR video-microscopy. PV interneurons are the main GABAergic population in the cortex that represents around 40% of the total cortical interneurons in rodents. Parvalbumin is expressed predominantly by chandelier and basket cells implicated in two cortical functions: 1) the control and shaping of the excitatory response, and 2) the initiation of critical periods for plasticity. Nevertheless, PV interneurons were never described to mediate vascular responses, as they do not contain any known vasoactive peptides although a subpopulation expresses nitric oxide synthase. It is unlikely that PV interneurons regulate directly cerebral blood volume but they could act as a relay to inhibit local or distal interneurons involved in a vasodilation tone. These findings bring new insight to the complex mechanisms of the neurovascular coupling but require further investigation to answer the role of PV neuron straightforwardly combining in vivo optogenetic modulation of specific cell types and precise brain imaging in alive rodents. To fulfill this objective, we are developing a new brain imaging technology called Functional UltraSound (fUS) imaging.
fUS imaging technology relies on a new sequence for power Doppler imaging that is sensitive enough to detect blood flow in very small vessels without the need for contrast agents. The common functional brain imaging modalities, including functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), have excellent depth penetration but do not provide good spatial and temporal resolution, which are critical when imaging transient events that are present in most of the pathologies. Conversely, basic ultrasound imaging achieves good spatiotemporal resolution in depth, but suffer of a lack of sensitivity limiting its use to the imaging of major vessels. To overcome this limitation, we have developed a novel modality called functional ultrasound (fUS) for imaging whole-brain microvasculature dynamics at high spatiotemporal resolution (10µm, 10ms). The benefits of functional ultrasound imaging include excellent safety record (ultrasounds are routinely used in hospitals), portability and affordable price.