Mice heterozygous for the Rosa26-lsl-GCaMP3 conditional allele have a loxP-flanked STOP cassette prevents transcription of the downstream GCaMP3 fusion gene. The donating investigator reports that the presence of the neomycin resistance cassette did not have any adverse effects on the expression of GCaMP3 or the health and survival of these mice. Because the CAG promoter driven reporter construct was targeted for insertion into the Gt(ROSA)26Sor locus, GCaMP3 expression is determined by which tissue(s) express Cre recombinase. When bred to mice that express Cre recombinase, the resulting offspring will have the STOP cassette deleted in the cre-expressing tissues; resulting in expression of the fluorescent calcium indicator protein, GCaMP3. The donating investigator reports that mice do not express GCaMP3 prior to introduction of Cre recombinase. Following exposure to Cre recombinase, GCaMP3 expression (EGFP fluorescence) is detected in the cre-expressing tissues. Following calcium binding (such as neuronal activation) bright EGFP fluorescence is observed.

GCaMP3 is a fluorescent calcium indicator. The GCaMP3 fusion gene contains the chicken smooth muscle M13 fragment of myosin light chain kinase (MYLK), a circularly permutated EGFP sequence (with several amino acid substitutions designed to increase dynamic range and baseline fluorescence), and a rat calmodulin (Calm) sequence modified to increase the fluorescence change for small calcium transients. In the absence of calcium binding, dim EGFP fluorescence is expected as there is a pore from the outside of its barrel into the chromophore. Upon calcium binding, this pore becomes occupied and fluorescence is increased.

When bred to STOCK Tg(Slc1a3-cre/ERT)1Nat/J mice (Stock No. 012586), expressing tamoxifen-inducible Cre Recombinase in astrocytes of the brain, resulting mice express GCaMP3 in 35% of cortical astrocytes, and 100% of Gergmann glia cells following tamoxifen administration. In vivo two-photon imaging was used to define the activity patterns of cortical and cerebellar astrocytes during increased Ca²⁺ release during locomotion.