A collaboration between German, Swiss, and Japanese scientists has developed a method of using genetically encoded voltage-gated calcium channels to visualize live activity of neurons within the brains of mice. The scientists accomplished this by introducing a virus carrying a gene that coded for a fluorescent protein, which in turn was activated and visible during electrical brain activity.
From a Max Planck Society press release:
Neurons communicate with one another via so-called action potentials. During an action potential, voltage-gated calcium channels are opened resulting in rapid calcium ion influx. Because of this tight coupling, fluorescent calcium indicator proteins can visualize action potentials. These proteins have two fluorescent subunits, one of which radiates yellow light and the other blue. When the proteins bind calcium, the proportion of yellow to blue light changes. Colour variation from blue light towards yellow thus reports different calcium levels – which is why the protein has been dubbed a "cameleon".
With the cameleon protein YC3.60, a fairly new variant, the scientists succeeded in recording the reaction of nerve cells to sensory stimuli in the intact brain of mice: every time the whiskers were deflected by a puff of air, there was a change of colour in the cameleon proteins in the nerve cells of the sensory areas of the cortex. It could therefore be deduced that the affected cells had reacted to the stimulus with action potentials.
Cameleon proteins could therefore revolutionize the study of electrical activity in the brain. To date, the only way scientists could do this is by inserting electrodes into the nerve tissue or the cells. This electrode technique is blind to cell identity and it damages the tissue. By contrast, the cameleon protein’s colour changes can be observed in a much less invasive procedure using glass fibres as light conductors or with the help of modern fluorescence microscopes – known as two-photon laser-scanning microscopes. Moreover, cameleon proteins can be formed by cells themselves provided a corresponding section of DNA has been inserted into the genome in advance. In the experiments conducted by the scientists, viruses served as the vehicle for smuggling the genetic information for the cameleon proteins into the nerve cells.
In two earlier studies, an international team of scientists headed by Mazahir Hasan were the first to demonstrate that similar genetic probes can successfully detect natural sensation (such as smell and touch) in the mammalian brain in the form of unique activity patterns (Hasan et al., 2004) and, more importantly, with single-cell, single-action-potential resolution (Wallace et al., 2008). In the current study, they have reached yet another major milestone as they demonstrate that the cameleon YC3.60 can be used to record activity from a large number of nerve cells during behaviour in freely moving mice. Additionally, it is well suited for recording activity from the same nerve cells in the same animals over a long time period and should help scientists to understand how network activity patterns form to code for different experiences and animal behaviour.
Press release: Blinking neurons give thoughts away…
Abstract in Frontiers in Neural Circuits: Optical recording of neuronal activity with a genetically-encoded calcium indicator in anesthetized and freely moving mice