A few weeks ago I had the opportunity of observing something really special, somehting I had never seen, something that left me with an open mouth, something that made nine hours of experimental work worth it, even if we couldn’t actually finish, even if it meant we lost our “patient”. The other day I saw how a brain dies. From the inside. And you could as well.
This video is part of one of my daily experiments. A chunk of mouse visual cortex where the bright circles are the labeled cell bodies of neurons filled with a fluorescent calcium indicator (OGB1) that signals changes in intracelular calcium concentration and works as a proxy for neuronal activity, as I already explained in a previous post.
Why is calcium such a good indicator of neuronal activity? It is because transmission of excitatory signals mediated by the neurotransmitter glutamate involve 2 kinds of specific receptors AMPA-R y NMDA-R, the former being permeable to calcium ions. Under normal conditions calcium is a very common second messenger not only in the brain but in many other cell types and calcium waves have been described for astrocytes (1) and during brain development (2). What happens when there is over-excitation of the system is that this calcium channels remain open and there’s a big surge of calcium going in the cell, producing toxic species (ROS) and collapsing the cell’s mitochondria. In short: neurotoxicity and in turn, cell death.
Now, how did this happen during my experiment? My hypothesis is that since it was a very long one and I had the animal under anesthesia for over 7-8 hours, maybe the degree of depression of inhibitory circuits, mainly due to anesthesia (a cocktail of opiates, analgesics and sedatives) which affects mainly GABA (gammaaminobutiric acid) receptors,those typical of inhibitory neurons, was SO high it allowed for the over-excitation and subsequent calcium wave that can be observed around the middle of the video and taking place around the animal’s death. Of course, this is only my educated guess…
BTW I guees you wonder about the left edge of the image where a miniature reflection can be easily appreciated. I will explain just like my good colleague Ron Jortner (Dr.) did: our microscope sees only in one direction, just like we only read in one direction, at least consciously. Because, in fact our eyes, just like our microscope, read each line twice. Only one of them they do much faster: that’s why the reflection is smaller and in case of our eyes, even unconscious. I could have edited out that part but I liked the explanation and also learning about that detail I haven’t been aware of before: that we always read twice.
I really hope you have found this calcium wave as fascinating as I and some of my colleagues did as not to feel the freakiest of the bunch 😉
*Addendum: @Raven_neo has kindly linked me to the work of a dutch group that after decapitation of rats found a similar wave of cell activity in EEG.