INTRODUCTION

Mice have been widely used in biomedical research because of their genetic similarity to humans, availability of genetically altered strains, and disease models [1, 2]. The full extent of neural processing in the olfactory system and visual system of mice has not been fully observed by fMRI (e.g., no reports about neural activations in the piriform cortex of mice). Cerebral blood volume (CBV) fMRI with the intravascular contrast agent of ultrasmall superparamagnetic iron oxide nanoparticles (USPIO) has a higher sensitivity compared to BOLD fMRI. In this study, CBV fMRI was used to study simultaneous odorant and visual stimulation-induced neural activations in the olfactory and visual systems of mice.

METHODS

All animal procedures were approved by our internal IACUC committee. Wild type C57BL/6J mice (n=8) were studied. Anesthetics: isoflurane (0.3%) + dexmedetomidine (0.4mg/kg bolus + 0.8mg/kg/hour infusion) + ketamine (16mg/kg/hour). MRI: 7T Bruker Biospec system. Coils: 2-cm receive-only surface coil + 72-mm transmit volume coil. fMRI data was acquired by single-shot GE EPI; matrix size=64×64; TE=11ms; FOV=2×2cm2. fMRI slice selection: sixteen consecutive coronal slices covering from the rostral edge of olfactory bulb to the caudal edge of visual cortex. Odorant stimulation: isoamyl acetate (2870 ppm). Visual stimulation: 100 Watt light bulb with 1-sec on and 1-sec off. Each fMRI measurement includes 40-sec baseline, 40-sec stimulation, and 80-sec recovery (Fig. 1A). For each mouse, the olfactory fMRI and visual fMRI were interleaved (Fig. 1B), and twenty fMRI measurements were performed during a 1-h experimental session. USPIO (5mg/ml, 20mg/kg, 4ml/kg) was injected through tail vein during the preparation. The EPI images from each mouse were registered to template images. The data from the olfactory fMRI measurements and visual fMRI measurements of all animals were averaged to map the respective fMRI activations (p<0.01, cluster size=4).

RESULTS

Odorant stimulation induced fMRI activations in the olfactory bulb (OB), anterior olfactory nucleus (AON), and piriform cortex (PC) (Fig. 2A). Visual stimulation induced fMRI activations in the visual cortex, lateral geniculate nuclei (LGN). Interestingly, cingulate cortex also shows activations during the visual stimulation (Fig. 2B). The temporal characteristics of the CBV fMRI responses to repeated odorant stimuli were examined. Figs. 3A, 3C, and 3E show the time courses of the fMRI signals for the ten olfactory fMRI measurements in the OB, AON, and PC, respectively. Robust CBV responses were observed for all the ten odor stimulations in all three structures. For each structure, the amplitudes of the CBV responses to the repeated odor stimulations were similar, indicating that the olfactory responses do not exhibit a significant adaptation effect in these structures of mice during the repeated stimulations. The lack of adaptation in the OB observed in mice is similar to the observation in NHPs [3]. Similar to olfactory response, the temporal characteristics of the CBV responses to repeated visual stimuli in the activated visual structures were examined. Figs. 3B, 3D, and 3F show the time courses of the fMRI signals for the ten visual fMRI measurements in the visual cortex, lateral geniculate nucleus, and cingulate cortex, respectively. The visual responses in these structures also do not exhibit a significant adaptation effect in mice. The temporal patterns of the CBV responses during stimulations were examined by averaging the time courses from the respective 10 olfactory or visual fMRI measurements for each mouse. Fig. 5 shows the time courses from each individual animal. For the olfactory responses, the temporal pattern in the OB is similar to the observations in the OB of monkeys [3] and rats [4], and no response attenuation during the continuous 40-sec stimulation. In the AON and PC, two peaks are observed with a prominent initial peak following odor onset and a second peak following odor offset. The first peak indicates fast adaption during odor stimulation, and the second peak is caused by “post-inhibition excitation rebound.” A similar two-peak pattern has been observed in the olfactory response of piriform cortex neurons in rats by electrophysiology recordings [5]. For the visual responses, while the temporal pattern during the stimulation is similar to previous observations in the visual cortex of cats [6], post-stimulus undershoots can be observed in all the three structures.

Conclusion

The novel findings of this study are: 1) the olfactory response in the AON and PC cortex in mice can be robustly observed by fMRI and 2) the visual response is observed in more regions than previous reports in rodents. CBV fMRI with its higher sensitivity should be a valuable technique to investigate the effects of biology interventions on the neural processing in mice.

Acknowledgements

No acknowledgement found.