Intracellular manganese enhanced MRI signals reflect the frequency of action potentials in Aplysia neurons.

Background: Manganese-enhanced magnetic resonance imaging (MEMRI) is an increasingly popular alternative to standard functional MRI methods in animal studies. The contrast in MEMRI images is based on the accumulation of Mn ions inside neurons, and, since manganese can serve as calcium analogue, this accumulation reflects calcium dynamics providing versatile information about brain neuroarchitecture and functionality. However, despite its use as a functional imaging tool, the exact relationship between the MEMRI signal and neuronal activity remains elusive.

New Method: In order to better understand the mechanisms underlying Mn accumulation resulting in MEMRI signal enhancement we investigated single neuron responses of isolated Aplysia buccal ganglia subjected to chemical (dopamine) or electrical stimulation of an input nerve (oesophageal nerve). The elicited electrical activity that represents a fictive feeding was recorded with electrophysiological methods and the Mn uptake in individual neurons was evaluated with MEMRI at 17.2T.

Results & Comparison With Existing Method(s): We show a positive correlation between bursts of electrical activity and MEMRI signal intensity in identified neurons and demonstrate that the MEMRI signal reflects mainly fast and high membrane depolarization processes such as action potentials, and it is not sensitive to slow and small membrane depolarizations, such as post-synaptic potentials.