Abstract
Previously we demonstrated that Mn-enhanced MRI (MEMRI) can be used to detect sound-evoked activity in the mouse auditory midbrain. To further investigate this noninvasive neuroimaging technique, we applied MEMRI to map tonotopic activity in the mouse inferior colliculus (IC). Our previous studies established that MEMRI signal enhancement is optimal at postnatal week 3, when an adult-like tonotopic map is already present. We therefore injected (IP) 3-week old mice with MnCl2 (0.4mmol/kg body weight) and exposed them to either of two different frequency band sound stimuli (High frequency (HF), 20-60kHz; Low frequency (LF), 2-6kHz) for 24-h prior to imaging. For the HF stimulated mouse group, signal enhancement was confined to the caudal-ventral part of central IC. In contrast, the signal enhancement in the LF stimulated mouse IC was found in the dorso-rostral region. To further confirm this result, we performed experiments to determine if the observed frequency-dependent enhancement patterns could be shifted in individual mouse. Mice were first exposed to the HF stimulus for 24-h after MnCl2 injection and imaged to analyze IC activity. These mice were then maintained in a quiet environment for 24-h with no defined sound stimulation, and imaged again to confirm Mn clearance from the ventral IC, after which they were injected again with MnCl2 (0.2mmol/kg), exposed to 24-h of LF stimulation and imaged with MRI. Each individual mouse displayed an appropriate switch in the frequency-specific activation pattern. These results demonstrate that MEMRI has sufficient spatial resolution and sensitivity to detect tonotopic activity pattern in the mouse IC. This approach should be useful in future studies to examine the tonotopic map establishment in mice IC or altered tonotopic representations in genetically-altered mice with defects in IC development.
