A fully noninvasive and robust experimental protocol for longitudinal fMRI studies in the rat

Abstract

Functional magnetic resonance imaging (fMRI) is a unique tool to study brain activity and plasticity changes. Combination of blood–oxygen level-dependent (BOLD) fMRI and electrical forepaw stimulation has been used as a standard model to study the somatosensory pathway and brain rehabilitation in rats. The majority of fMRI studies have been performed in animals anesthetized with α-chloralose as functional-metabolic coupling is best preserved under this anesthesia. However, α-chloralose is not suitable for survival procedures due to side effects, limiting its use to single time point studies of the same animal. We therefore developed a new, totally noninvasive fMRI protocol, using sedation with the α 2-adrenoreceptor agonist medetomidine in combination with transcutaneous monitoring of blood gases. The continuous subcutaneous administration of medetomidine resulted in stable physiological conditions over a long time and all animals tolerated the repetitive fMRI experiments well. A robust and reproducible, significant BOLD signal increase was observed upon forepaw stimulation in the contralateral primary somatosensory cortex in two consecutive medetomidine sessions in all rats, which was similar to the BOLD signal increase observed in the same animals under α-chloralose during a third independent session. Activation in the secondary somatosensory cortex was observed less frequently under both medetomidine and α-chloralose. No head motion artifacts or nonspecific brain activation was present. Sedation was quickly reversed by the administration of the antagonist atipamezole after the fMRI experiment. These results demonstrate that longitudinal fMRI studies can be performed safely under sedation with medetomidine to study functional recovery processes upon therapeutical treatment.