Specificity of stimulus-evoked fMRI responses in the mouse: The influence of systemic physiological changes associated with innocuous stimulation under four different anesthetics

Functional magnetic resonance (fMRI) in mice has become an attractive tool for mechanistic studies, for characterizing models of human disease, and for evaluation of novel therapies. Yet, controlling the physiological state of mice is challenging, but nevertheless important as changes in cardiovascular parameters might affect the hemodynamic readout which constitutes the basics of the fMRI signal. In contrast to rats, fMRI studies in mice report less robust brain activation of rather widespread character to innocuous sensory stimulation. Anesthesia is known to influence the characteristics of the fMRI signal. To evaluate modulatory effects imposed by the anesthesia on stimulus-evoked fMRI responses, we compared blood oxygenation level dependent (BOLD) and cerebral blood volume (CBV) signal changes to electrical hindpaw stimulation using the four commonly used anesthetics isoflurane, medetomidine, propofol and urethane. fMRI measurements were complemented by assessing systemic physiological parameters throughout the experiment. Unilateral stimulation of the hindpaw elicited widespread fMRI responses in the mouse brain displaying a bilateral pattern irrespective of the anesthetic used. Analysis of magnitude and temporal profile of BOLD and CBV signals indicated anesthesia-specific modulation of cerebral hemodynamic responses and differences observed for the four anesthetics could be largely explained by their known effects on animal physiology. Strikingly, independent of the anesthetic used our results reveal that fMRI responses are influenced by stimulus-induced cardiovascular changes, which indicate an arousal response, even to innocuous stimulation. This may mask specific fMRI signal associated to the stimulus. Hence, studying the processing of peripheral input in mice using fMRI techniques constitutes a major challenge and adapted paradigms and/or alternative fMRI readouts should also be considered when studying sensory processing in mice.