Short-term high-resolution imaging of developing hippocampal neurons in culture.

Dissociated cell cultures of the rodent hippocampus have become a standard model for studying many facets of neural development. The cultures are quite homogeneous and it is relatively easy to express green fluorescent protein (GFP)-tagged proteins by transfection. Because the cultures are essentially two dimensional, there is no need to acquire images at multiple focal planes. For capturing rapid subcellular events at high resolution, as described here, one must maximize weak signals and reduce background fluorescence. Thus, these methods differ in several respects from those used for time-lapse imaging. Lipofectamine-mediated transfection yields a higher level of expression than does transfection with a nucleofection device. Images are usually collected with a spinning-disk confocal microscope, which improves the signal-to-noise ratio. In addition, we use an imaging medium designed to minimize background fluorescence rather than to enhance long-term cell survival. It is also important to select cultures at an appropriate stage of development. In our hands, lipofectamine-based transfection works best on cells between 3 and 10 d after plating. GFP-based fluorescence can be observed as early as 4 h after adding the DNA/lipid complexes to the cells, but expression usually increases over the next ∼12 h and remains steady for days. The ratio of DNA to lipid is critical; to lower expression levels of the tagged construct, we use a combination of expression vector and empty plasmid, keeping the DNA amount constant. An example is included to illustrate the imaging of the microtubule-based vesicular transport of membrane proteins.