Imaging Calcium in Hippocampal Presynaptic Terminals With a Ratiometric Calcium Sensor in a Novel Transgenic Mouse.

Area of Science:

• Neuroscience
• Molecular Biology
• Genetics

Background:

• Genetically encoded calcium indicators (GECIs) are crucial for measuring neuronal activity.
• Low expression levels in transgenic mice often limit GECI sensitivity.
• Targeting GECIs to presynaptic terminals can improve sensitivity and specificity.

Purpose of the Study:

• To develop a transgenic mouse line with enhanced GECI expression and presynaptic targeting.
• To characterize the sensitivity and dynamic range of the new calcium sensor (SyGCaMP2-mCherry) in hippocampal neurons.
• To investigate the role of presynaptic calcium in synaptic transmission and plasticity using the new mouse line.

Main Methods:

• Development of the SyG37 transgenic mouse line expressing SyGCaMP2-mCherry.
• In vivo electrophysiological recordings and multiphoton microscopy in hippocampal CA1 and CA3 regions.
• Pharmacological manipulation (DNQX) to differentiate synaptic response origins.
• Induction of long-term potentiation (LTP) to study synaptic plasticity.

Main Results:

• SyGCaMP2-mCherry showed linear responses up to 10 stimuli at 100 Hz and detected single-stimulus responses.
• Single bouton responses were measurable, with ensemble amplitude correlating with stimulus number and bouton count.
• Calcium clearance was faster in CA3 boutons than CA1 boutons.
• AMPA receptor-mediated transmission contributed 14% to CA1 responses.
• LTP induction increased the proportion of postsynaptic connections to 76%.

Conclusions:

• The SyG37 mouse line enables stable optical detection of synaptic activation and connectivity at the single bouton level.
• This model is valuable for characterizing presynaptic calcium's contribution to synaptic transmission and plasticity.
• The findings advance our understanding of neuronal circuit function and plasticity mechanisms.