In brain gray matter, neurovascular coupling (NVC) maintains brain metabolism homeostasis by modulating blood flow according to neuronal activity. In white matter, the energy cost of information transmission along myelinated axons is reduced and the need for NVC is unknown. Here, we used two-photon imaging through chronically-implanted GRIN lenses in mice and high-field BOLD fMRI (17.2 T) in rats to investigate NVC along the entire length of the optic nerve, a unique model of a myelinated axonal tract. We found that flickering light and drifting grating stimulations increased blood flow in the retina, the unmyelinated optic nerve head, and at the level of the nerve synaptic terminals. However, it did not affect blood flow and oxygenation in the myelinated part of the optic nerve, i.e., the intracranial optic nerve and the optic tract. We conclude that during natural visual stimulation, action potential propagation in activated myelinated axons does not trigger NVC. Neurovascular coupling is essential for gray matter function, yet its role in white matter is unclear. Here, the authors demonstrate that visual stimulation does not elicit neurovascular coupling in the myelinated portion of the rodent optic nerve.
Suarez-Baquero et al. (Sat,) studied this question.