Virus vectors are promising tools for delivering genes to the nervous system and are used for introducing genes which, when expressed, allow for the manipulation and monitoring of neurons. Selecting the best viral vector requires consideration of a number of factors like the duration over which the expression will be needed and the level of toxicity that can be tolerated. One commonly used vector is the adeno-associated virus (AAV) because of its relatively high efficacy of expression and low toxicity, making it excellent for introducing genetically encoded calcium (Ca2+) indicators for imaging neural activity.
Virus injections demand efficiency in stereotaxic surgical procedures where precise delivery of of an appropriate volume of the vector and location for bregma coordinates in a particular brain region of interest are critical to successful expression in the desired cell types. Some of these steps can take months before one can start Ca2+ imaging experiments. Neuroscientists can now rely on Ready-to-Image GCaMP-encoding viruses offered by Inscopix that are concentration optimized and functionally validated in various brain regions in mice. Ready-to-Image viruses will facilitate measurement of neuronal ensemble Ca2+ dynamics with high-quality, reproducible results and shorten the time to publication.
Our Inscopix scientists recommend keeping the following points in consideration when optimizing for stable Ca2+ indicator expression in a particular brain region of interest. If the concentration is too low, chances are the indicator will be underexpressed and if the concentration is too high, it could lead to cytotoxicity.
- Histology: The accuracy of virus injections must be verified by proper histological methods (see image below). This is a crucial step and will help determine if proper targeting was achieved.
- Time Course: The time course of expression for the duration of experimental imaging should be considered, so that experiments are performed when the expression of the transgene is at optimum levels.
- Optimum Volume: The optimum volume of the vector needed to infuse the brain region of interest should be determined empirically. This depends on the virus titer, the desired spread of the virus, and the targeted brain region.
Histology image of GCaMP6 (green) expressing cortical neurons in mouse; DAPI stained nuclei in blue in infragranular layers in somatosensory (S1) cortex
By combining precise gene delivery approaches with our Ready-to-image virus and our miniature microscope platforms nVista or nVoke, neuroscientists can more quickly and easily assess large-scale neural dynamics in brain circuits during behavior.