Statistical mapping of functional olfactory connections of the rat brain in vivo

The olfactory pathway is a unique route into the brain. To better characterize this system in vivo, rat olfactory functional connections were mapped using magnetic resonance (MR) imaging and manganese ion (Mn2+) as a transport-mediated tracer combined with newly developed statistical brain image analysis. Six rats underwent imaging on a 1.5-T MR scanner at pre-administration, and 6, 12, 24, 36, 48, and 72 h and 5.5, 7.5, 10.5, and 13.5 days post-administration of manganese chloride (MnCl2) into the right nasal cavity. Images were coregistered, pixel-intensity normalized, and stereotactically transformed to the Paxinos and Watson rat brain atlas, then averaged across subjects using automated image analysis software (NEUROSTAT). Images at each time point were compared to pre-administration using a one-sample t statistic on a pixel-by-pixel basis in 3-D and converted to Z statistic maps. Statistical mapping and group averaging improved signal to noise ratios and signal detection sensitivity. Significant transport of Mn2+ was observed in olfactory structures ipsilateral to site of Mn2+ administration including the bulb, lateral olfactory tract (lo) by 12 h and in the tubercle, piriform cortex, ventral pallidum, amygdala, and in smaller structures such as the anterior commissure after 24 h post-administration. MR imaging with group-wise statistical analysis clearly demonstrated bilateral transsynaptic Mn2+ transport to secondary and tertiary neurons of the olfactory system. The method permits in vivo investigations of functional neuronal connections within the brain.