The Unknown Eye - ipRGCs

Here's a question for you: what are the most important cells, nanogram for nanogram, in your body? Here's a hint: until only a few years ago we didn't know they existed, so you unless you read a lot of medical and biological research for fun, you can probably stop guessing. They're called “dots” or ipRGCs, and they are found in your eyes. There are only a scant few thousand of these cells in each eye - which is why they weren't found earlier - but they ultimately control nearly the whole of the human hormone system. And as we've said, that day/night hormone system in turn controls, in one way or another, nearly everything else that goes on inside our bodies: our liver, kidneys, brains, immune systems, and everything else.

We used to think, and teach, that human eyes contained two and only two different kinds of light sensors: rods and cones. The cones give us color vision, and the rods very sensitive night vision in black and white, that is, night vision. That much is still true, of course, but now we know something else. We all have a third kind of light sensor that is at least as important: the dots or ipRGCs, “intrinsically photosensitive retinal ganglion cells”. You probably haven't heard of them yet, since they were only discovered in 2002, four years ago as I write this. These cells tell us whether it's day or night. They are neither rods nor cones, and are present in all sorts of creatures: even in very primitive creatures without eyes. They evolved long before eyes did, and turn out to be fundamental to how bodies work.

Just how new, and neglected, is this discovery? Well, as of today, June 14, 2006 there are only a grand total of 12,700 mentions of them on the web, according to Google, plus less than a thousand more mentions of the abbreviations “ipRGC” and “ipRGCs”. Not much excitement, and yet you can expect this discovery to be absolutely critical in reversing the rise of obesity and chronic disease within a decade or two. Why? Because this new discovery helps tell us why the Photoperiod Effect is occurring, and what we can do about it. Good news for politicians, but bad news for investors in drug companies: our health premiums and government budgets for health services will start to decline, within a decade or two, reversing their spectacular rise for decade after decade. The discovery of ipRGCs will help tip the tide; because this is the last piece of the main puzzle explaining how the human hormone system works. Our increasing scientific knowledge will soon change the superstition that we are the only life on earth not affected by our photoperiod, and give us all much better long-term health as a consequence. Just removing the burden imposed on the health care system by the obesity epidemic and the consequences of that, including vastly inflated rates of diabetes and heart disease, will suffice to reverse all the cost trends in health care in one stroke: releasing society's resources to be spent in far more productive ways. And the evidence, while still scattered, is pointing more and more, to excess light exposure as the single greatest risk factor for obesity.

So what do intrinsically photosensitive retinal ganglion cells do, in our eyes, exactly? These ipRGC “dots”, as I'm going to call them rather informally, aren't sensitive to red light (for example, the last glowing embers of a campfire.) Red light doesn't affect them at all. They are sensitive to high green and extremely sensitive to blue light (and therefore, sensitive to white light which is a mix of light of all colors including blue.) When they sense much green or blue light they use their direct connection to an area within the brain (the SCN inside the hypothalmus, but we'll get to that) to yell “Wake up! It's Day! Send the hormone system into reverse! Get out the aldosterone! Stop making serotonin! Get out the steroids! Crank up your mood and go at it!”. When they cease to detect green or blue light – even fairly small quantities that you might get looking at your clock-radio's LED display – they shout “Night-time! Roll out the night hormones! Start the antioxidants! Stop releasing steroids! Begin the nightly immune cycle! Get out the melatonin! Start making serotonin (etc)!” To oversimplify a bit, this message is coordinated by the SCN, in our brain, and sent on to the pineal gland which begins our nights (if we are in darkness) by starting to release melatonin, “the master hormone”, which controls the levels of so many other hormones. Conversely, as morning light begins (and somewhat before) the daytime steroid (hormone) cycle begins. This is known as the Awakening Cortisol Response: “In healthy adults salivary free cortisol concentrations increase by between 50 and 160% in the first 30 min immediately post-awakening” [15204030]