Abstract
Purpose:
- To evaluate a novel theoretical model for in vivo axonal Mn^2+^ transport with MRI data from the rat optic nerve (ON); and 2) to compare predictions from the new model with previously reported experimental data.
Materials and Methods:
Time‐resolved in vivo T~1~‐weighted magnetic resonance imaging (MRI) of adult female Sprague–Dawley rat (n = 9) ON was obtained at different timepoints after intravitreal MnCl~2~ injection. A concentration‐dependent and a rate‐dependent function for the Mn^2+^ retinal ganglion cell (RGC) axon entrance was convolved with three different transport functions and each model system was optimized to fit the ON data.
Results:
The rate‐limited input function gave a better fit to the data than the concentration‐limited input. Simulations showed that the rate‐limited input leads to a semilogarithmic relationship between injected dose and Mn^2+^ concentration in the ON, which is in agreement with previously reported in vivo experiments. A random walk transport model and an anterograde predominant slow model gave a similar fit to the data, both better than an anterograde predominant fast model.
Conclusion:
The results indicate that Mn^2+^ input into RGC axons is limited by a maximum entrance rate into the axons. Also, a wide range of apparent Mn^2+^ transport rates seems to be involved, different from synaptic vesicle transport rates, meaning that manganese does not depict synaptic vesicle transport rates directly. J. Magn. Reson. Imaging 2010;32:551–560. © 2010 Wiley‐Liss, Inc.