Using Manganese Enhanced MRI to Reveal Active Olfactory Circuitry in Response to Odorant Stimuli

Introduction Olfactory stimuli activate different combinations of olfactory sensory neurons (OSNs) in the olfactory epithelium. Since OSNs expressing the same odorant receptor (OR) project axons to the main olfactory bulb (MOB) and innervate the same set of glomeruli in a stereotypic fashion, the activation of OSNs elicited by odorants is transformed to a spatial pattern in the MOB, i.e., an odor map, in the glomerular layer (GL) [1]. Current methods (i.e., 2-deoxyglucose, optical imaging, or BOLD fMRI [2,3]) for mapping odorant activation in GL are limited by invasive procedures, field-of-view, or spatial resolution. Manganese enhanced MRI (MEMRI) is a new method for mapping neuronal function [4]. Manganese ion (Mn2+) can enter neurons through voltage-gated Ca-channel and be transported to projecting neurons. While many techniques that have been used to map the bulb struggle to resolve individual glomeruli, it may be possible to achieve glomerular level resolution with MEMRI. Previously, it has been shown that Mn2+ delivered to the nose of mice with an odor led to specific enhancement in the MOB due to accumulation in odor activated OSNs and then tracking of the Mn2+ to the MOB [5]. Neurons activated by an odorant take up Mn2+ at a faster rate than other neurons, therefore the fastest enhancement in the MOB represents the input activity into the glomerular layer. Since the tracing of Mn2+ is relatively slow the enhancement can be detected using high-resolution, dynamic MEMRI. Here we demonstrate a much simpler protocol than previously used and develop a dynamic imaging procedure that enables odor maps to be made from the glomerular layer of the bulb. Furthermore, it is demonstrated that individual glomeruli can be detected by comparison of MEMRI activation to anatomical images of the specific glomeruli expressing green fluorescent protein (GFP).