Abstract
Activity-dependent changes in the input-output (I-O) relationship of a neural circuit are central in the learning and memory function of the brain. To understand circuit-wide adjustments, optical imaging techniques to probe the membrane potential at every component of neurons, such as dendrites, axons and somas, in the circuit are essential. We have been developing fast voltage-sensitive dye (VSD) imaging methods for quantitative measurements, especially for single-photon wide-field optical imaging. The long-term continuous measurements needed to evaluate circuit-wide modifications require stable and quantitative long-term recordings. Here, we show that VSD imaging (VSDI) can be used to record changes in circuit activity in association with theta-burst stimulation (TBS)-induced long-term potentiation (LTP) of synaptic strength in the CA1 area. Our optics, together with the fast imaging system, enabled us to measure neuronal signals from the entire CA1 area at a maximum frame speed of 0.1 ms/frame every 60 s for over 12 h. We also introduced a method to evaluate circuit activity changes by mapping the variation in recordings from the CA1 area to coordinates defined by the morphology of CA1 pyramidal cells. The results clearly showed two types of spatial heterogeneity in LTP induction. The first heterogeneity is that LTP increased with distance from the stimulation site. The second heterogeneity is that LTP is higher in the stratum pyramidale (SP)-oriens region than in the stratum radiatum (SR). We also showed that the pattern of the heterogeneity changed according to the induction protocol, such as induction by TBS or high-frequency stimulation (HFS). We further demonstrated that part of the heterogeneity depends on the I-O response of the circuit elements. The results show the usefulness of VSDI in probing the function of hippocampal circuits.
Highlights
Brain function depends on neural circuit activity
We have described and demonstrated an optical recording method with voltage-sensitive dye (VSD) that allows us to record long-term modifications of neural circuits in slice preparations
We measured changes accompanied by long-term potentiation (LTP) induction in the CA1 area of mouse hippocampal slices to demonstrate the usefulness of the method
Summary
Brain function depends on neural circuit activity. Understanding the circuit behavior of in vitro specimens is essential to address the physiology and pathology of the normal and diseased brain. Conventional electrophysiological methods, such as field potential recordings, sharp electrode intracellular recordings and patch clamping, are useful for determining the activity of the cells in VSD Imaging of LTP in CA1 the circuit and the synaptic connections between elements. One must evaluate the activity across the broad span of the circuit to understand neural circuit mechanisms of brain malfunction (Uhlhaas and Singer, 2012; Anticevic and Murray, 2017)
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