In Vivo Trans-Synaptic Tract Tracing From the Murine Striatum and Amygdala Utilizing Manganese Enhanced MRI (MEMRI)

Abstract

Small focal injections of manganese ion (Mn^(2+)) deep within the mouse central nervous system combined with in vivo high-resolution MRI delineate neuronal tracts originating from the site of injection. Previous work has shown that Mn^(2+) can be taken up through voltage-gated Ca^(2+) channels, transported along axons, and across synapses. Moreover, Mn^(2+) is a paramagnetic MRI contrast agent, causing positive contrast enhancement in tissues where it has accumulated. These combined properties allow for its use as an effective MRI detectable neuronal tract tracer. Injections of low concentrations of MnCl_2 into either the striatum or amygdala produced significant contrast enhancement along the known neuronal circuitry. The observed enhancement pattern is different at each injection site and enhancement of the homotopic areas was observed in both cases. Ten days postinjection, the Mn^(2+) had washed out, as evidenced by the absence of positive contrast enhancement within the brain. This methodology allows imaging of neuronal tracts long after the injection of the ion because Mn^(2+) concentrates in active neurons and resides for extended periods of time. With appropriate controls, differentiation of subsets of neuronal pathways associated with behavioral and pharmacological paradigms should be feasible.