Methods for recording and imaging spreading depolarizations and seizure-like events in mouse hippocampal slices

Spreading depolarization (SD) is a sudden and synchronized depolarization of principal cells followed by depression of activity, which slowly propagates across brain regions like cortex or hippocampus. SD is considered to be mechanistically relevant to migraine, epilepsy, and traumatic brain injury. Interestingly, research into SD typically uses SD triggered immediately after a focal stimulus. Here we optimize an in vitro experimental model allowing us to record SD without focal stimulation. This method uses electrophysiological recordings and intrinsic optical imaging in slices. The method is also relatively easy and inexpensive. Acute hippocampal slices from mice or rats were prepared and used for extracellular and whole-cell recordings. Recordings were made in a submerged-style chamber with flow of artificial cerebrospinal fluid (aCSF) above and below the slices. Flow was fast (> 5ml/min), and temperature was 32°C. As soon as slices were placed in the chamber, aCSF containing 0 mM Mg2+ and 5 mM K+ (0 Mg2+/5 K+ aCSF) was used. Two major types of activity were observed: SD and seizure-like events (SLEs). Both occurred after many minutes of recording. Although both mouse and rat slices showed SLEs, only mouse slices developed SD and did so in the first hour of 0 Mg2+/5 K+ aCSF exposure. Intrinsic optical imaging showed that most SDs initiated in CA3 and could propagate into CA1 and dentate gyrus. In dentate gyrus, SD propagated in two separate waves: (1) into the hilus and (2) into granule cell and molecular layers simultaneously. This in vitro model can be used to better understand the mechanisms and relationship between SD and SLEs. It could also be useful in preclinical drug screening. ### Competing Interest Statement The authors have declared no competing interest.