Diffusion of nerve growth factor in rat striatum as determined by multiphoton microscopy

Neurotrophins such as nerve growth factor (NGF) may be useful for treating diseases in the central nervous system; our ability to harness the potential therapeutic benefit of NGF is directly related to our understanding of the fate of exogenously supplied factors in brain tissue. We utilized multiphoton microscopy to quantify the dynamic behavior of NGF in coronal, 400-mum thick, fresh rat brain tissue slices. We administered a solution containing bioactive rhodamine nerve growth factor conjugate via pressure injection and monitored the dispersion in the striatal region of the coronal slices. Multiphoton microscopy facilitated repeated imaging deep (similar to200 mum) into tissue slices with minimal photodamage of tissue and photobleaching of label. The pressure injection paradigm approximated diffusion from a point source, and we therefore used the corresponding solution to the diffusion equation to estimate an apparent diffusion coefficient in brain tissue (D-b(34degreesC)) of 2.75 +/- 0.24 X 10(-7) cm(2)/s (average +/- SE). In contrast, we determined a corresponding free diffusion coefficient in buffered solution (D-f(34degreesC)) of 12.6 +/- 0.9 X 10(-7) cm(2)/s using multiphoton fluorescence photobleaching recovery. The tortuosity, defined as the square root of the ratio of D-f to D-b, was 2.14 and moderate in magnitude.