Friday, May 18, 2007

High speeding mapping of neural circuits with optical techniques.

Before I was seduced by studying how the brain works, I used to be a membrane biophysics, cellular, molecular biologist, and occasionally I come across a bit of work that is so neat and powerful that I want to mention it.

Wang et al. engineer the genetic delivery into neurons of a light sensitive rhodopsin membrane channel protein (ChR2), from an algae. Illumination of ChR2-positive neurons in cortical slices produces rapid photocurrents that can elicit action potentials. The timing, number, and spatial location of these action potentials can be controlled precisely by light, allowing functional mapping of cortical circuits. Here is their abstract:

To permit rapid optical control of brain activity, we have engineered multiple lines of transgenic mice that express the light-activated cation channel Channelrhodopsin-2 (ChR2) in subsets of neurons. Illumination of ChR2-positive neurons in brain slices produced photocurrents that generated action potentials within milliseconds and with precisely timed latencies. The number of light-evoked action potentials could be controlled by varying either the amplitude or duration of illumination. Furthermore, the frequency of light-evoked action potentials could be precisely controlled up to 30 Hz. Photostimulation also could evoke synaptic transmission between neurons, and, by scanning with a small laser light spot, we were able to map the spatial distribution of synaptic circuits connecting neurons within living cerebral cortex. We conclude that ChR2 is a genetically based photostimulation technology that permits analysis of neural circuits with high spatial and temporal resolution in transgenic mammals.
Fluorescence image of dye-filled layer VI pyramidal neuron; circles indicate locations where light-evoked synaptic responses were evoked.

7 comments:

Murky Thoughts said...

It's cool, but if it's not in Nature or Science, it's got to have some serious limitation(s). I imagine it's still going to be easier to stimulate or drive in vivo cells with microlectrodes, and I don't see how this makes it any easier to map and record where the activity goes, which seems like the harder of the two problems, and something we've seen a few schemes for already in the way of voltage-dependent fluorescent indicators.

Deric Bownds said...

I have to totally disagree with you on "it has to be in Nature or Science". While it is likely to have received appropriate scrutiny, there are a number of journals out there of high quality that have equal or higher standards.

Also, the whole point of the optical technique is that it is potentially vastly easier and more powerful than electrical recordings. An array of optical sensors could report of networks of activity that you could never get at with multiple microelectrode penetrations.

Murky Thoughts said...

I agree and tried to say that recording optically has obvious advantages over electrode recording, but the new paper you cite seems to be about stimulating optically, not recording optically. Or are they using ChR2 itself as a reporter of the voltage change? I hadn't thought of the "cis" to "trans" switch changing the spectral absorption profile of the retinal. But it would be unusually convenient if you could probe that absorption and get a measurable signal over the light noise of background brain tissue I think. If the cis to trans switch is coupled to opening channels, I doubt the photons that you open the channels with have enough energy left afterwards to fluoresce back at you at a wavelength good for measuring. I assumed the authors in this paper had loaded the cells with FURA or some other voltage-sensative dye for the recording aspect of the experiment.

Murky Thoughts said...

Actually, "dye filling" is mentioned in the photo caption.

Deric Bownds said...

I'm sorry, I wasn't paying enough attention before I fired off my brief response. What you would like to have (and they haven't yet done) is something like the ChR2 that will set off a nerve signal when illuminated at its absorption maximum, and ANOTHER voltage sensitive fluorescent membrane dye whose fluorescence changes can be subsequently monitored during irradiating at the fluorescence excitation wavelength...

Murky Thoughts said...

Yes. But also I was saying which one I'd rather have and why--to support my off-the-cuff opinion that what this paper newly demonstrates is merely "cool" or nifty, rather than a seed of change in the way neurophysiologists go about their business.

Murky Thoughts said...

e.g. This report illustrates the recording side of the coin.

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