High field BOLD response to forepaw stimulation in the mouse.

We have established a robust protocol for longitudinal fMRI in mice at high field MRI using a medetomidine anesthesia. Electrical forepaw stimulation in anesthetized animals is widely used to produce BOLD contrast in the primary somatosensory cortex. To preserve neuronal activity, most fMRI experiments used alpha-chloralose to produce sedation, but severe side effects make this procedure unsuitable for survival experiments. As advantageous alternative, the alpha(2)-adrenergic receptor agonist medetomidine has been applied successfully to permit longitudinal fMRI studies in rats. With the advent of transgenic technology, mouse models have become increasingly attractive raising the demand for implementation of a suitable fMRI protocol for mice. Therefore, we investigated the use of medetomidine for repetitive fMRI experiments in C57BL/6 mice. We evaluated the optimal medetomidine dose for subcutaneous application. Somatosensory evoked potentials (SSEPs) in the contralateral somatosensory cortex were recorded to assess brain activity under medetominidine following forepaw stimulation. Repetitive administration of medetomidine, the requirement for longitudinal brain activation studies, was well tolerated. Using the forepaw stimulation paradigm, we observed BOLD contrast in the contralateral somatosensory cortex in approximately 50% of the performed scans using gradient echo-echo planar imaging (GE-EPI). However, imaging the small mouse brain at high field strength is challenging and we observed strong susceptibility artifacts in GE-EPI images in the cortex. We have developed an agar gel cap for successful compensation of these artifacts as prerequisite for successful mouse fMRI at 11.7T. The established protocol will be suitable for brain activation studies in transgenic animals and for studies of functional deficit and recovery after brain injury in mice.