Aberrant hippocampal Ca(2+) micro-waves following synapsin-dependent adenoviral expression of Ca(2+) indicators

Genetically encoded calcium indicators (GECIs) such as GCaMP are an invaluable tool in neuroscience to monitor neuronal activity using optical imaging. The viral transduction of GECIs is commonly used to target expression to specific brain regions, can be conveniently used with any mouse strain of interest without the need for prior crossing with a GECI mouse line and avoids potential hazards due to the chronic expression of GECIs during development. A key requirement for monitoring neuronal activity with an indicator is that the indicator itself minimally affects activity. Here, using common adenoviral transduction procedures, we describe spatially confined aberrant Ca(2+) micro-waves slowly travelling through the hippocampus following expression of GCaMP6, GCaMP7 or R-CaMP1.07 driven by the synapsin promoter with AAV-dependent gene transfer. Ca(2+) micro-waves developed in hippocampal CA1 and CA3, but not dentate gyrus (DG) nor neocortex, were typically first observed at 4 weeks after viral transduction, and persisted up to at least 8 weeks. The phenomenon was robust, observed across laboratories with various experimenters and setups. Our results indicate that aberrant hippocampal Ca(2+) micro-waves depend on the viral titre of the GECI under the synapsin promoter. Further, neuronal subtype, and density of GECI expression may play a role. We used an alternative viral transduction method of GCaMP which avoids this artifact. The results show that commonly used syn-Ca(2+)-indicator AAV transduction procedures can produce artefactual Ca(2+) responses. Our aim is to raise awareness in the field of these artefactual transduction-induced Ca(2+) micro-waves and we provide a potential solution for this problem.