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Genetically encoded calcium indicators (GECIs) such as GCaMP are invaluable tools 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 adeno-associated viral (AAV) transduction procedures, we describe spatially confined aberrant Ca 2+ microwaves slowly travelling through the hippocampus following expression of GCaMP6, GCaMP7, or R-CaMP1.07 driven by the synapsin promoter with AAV-dependent gene transfer in a titre-dependent fashion. Ca 2+ microwaves developed in hippocampal CA1 and CA3, but not dentate gyrus nor neocortex, were typically first observed at 4 wk after viral transduction, and persisted up to at least 8 wk. The phenomenon was robust and observed across laboratories with various experimenters and setups. Our results indicate that aberrant hippocampal Ca 2+ microwaves depend on the promoter and viral titre of the GECI, density of expression, as well as the targeted brain region. We used an alternative viral transduction method of GCaMP which avoids this artefact. The results show that commonly used 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+ microwaves, and we provide a potential solution., Competing Interests: NM, MM, EG, DO, FD, PR, FH, RY, MF, HB, MW, TK No competing interests declared, (© 2024, Masala et al.)

Genetically encoded calcium indicators (GECIs) such as GCaMP are invaluable tools 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 adeno-associated viral (AAV) 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, in a titre-dependent fashion. 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 promoter and viral titre of the GECI, density of expression as well as the targeted brain region. We used an alternative viral transduction method of GCaMP which avoids this artifact. The results show that commonly used 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., Competing Interests: Conflict of Interest None of the authors have a conflict of interest.

The hippocampal dentate gyrus is an important relay conveying sensory information from the entorhinal cortex to the hippocampus proper. During exploration, the dentate gyrus has been proposed to act as a pattern separator. However, the dentate gyrus also shows structured activity during immobility and sleep. The properties of these activity patterns at cellular resolution, and their role in hippocampal-dependent memory processes have remained unclear. Using dual-color in vivo two-photon Ca 2+ imaging, we show that in immobile mice dentate granule cells generate sparse, synchronized activity patterns associated with entorhinal cortex activation. These population events are structured and modified by changes in the environment; and they incorporate place- and speed cells. Importantly, they are more similar than expected by chance to population patterns evoked during self-motion. Using optogenetic inhibition, we show that granule cell activity is not only required during exploration, but also during immobility in order to form dentate gyrus-dependent spatial memories., Competing Interests: MP, NN, AH, TN, NM, FD, OB, JM, LE, KG, HB No competing interests declared, (© 2021, Pofahl et al.)

The pituitary adenylate cyclase-activating polypeptide (PACAP)-27 modulates various biological processes, from the cellular level to function specification. However, the cardiac actions of this neuropeptide are still under intense studies. Using control (+|+) and mice lacking (-|-) either R-type (Ca v 2.3) or T-type (Ca v 3.2) Ca 2+ channels, we investigated the effects of PACAP-27 on cardiac activity of spontaneously beating isolated perfused hearts. Superfusion of PACAP-27 (20nM) caused a significant increase of baseline heart frequency in Ca v 2.3(+|+) (156.9±10.8 to 239.4±23.4 bpm; p<0.01) and Ca v 2.3(-|-) (190.3±26.4 to 270.5±25.8 bpm; p<0.05) hearts. For Ca v 3.2, the heart rate was significantly increased in Ca v 3.2(-|-) (133.1±8.5 bpm to 204.6±27.9 bpm; p<0.05) compared to Ca v 3.2(+|+) hearts (185.7±11.2 bpm to 209.3±22.7 bpm). While the P wave duration and QTc interval were significantly increased in Ca v 2.3(+|+) and Ca v 2.3(-|-) hearts following PACAP-27 superfusion, there was no effect in Ca v 3.2(+|+) and Ca v 3.2(-|-) hearts. The positive chronotropic effects observed in the four study groups, as well as the effect on P wave duration and QTc interval were abolished in the presence of Ni 2+ (50μM) and PACAP-27 (20nM) in hearts from Ca v 2.3(+|+) and Ca v 2.3(-|-) mice. In addition to suppressing PACAP's response, Ni 2+ also induced conduction disturbances in investigated hearts. In conclusion, the most Ni 2+ -sensitive Ca 2+ channels (R- and T-type) may modulate the PACAP signaling cascade during cardiac excitation in isolated mouse hearts, albeit to a lesser extent than other Ni 2+ -sensitive targets., (Copyright © 2016. Published by Elsevier Ltd.)