Protons: Linoleic acid in the hypothalamus

Hi all, just getting my head above water now that we have two or three locums at work to cover some of the (rather difficult) gaps in the rota!



Before we look at the fat mouse study which wins the prize for most miserly hoarding of data, I just wanted to put up a brief post, based on that paper, about breaking your hypothalamus with a high fat diet. Just to re emphasis: This is NOT what happens to a human after 7 days on a high fat diet.

Remember Schwartz's rats? Put them on a high fat diet and this happens to food intake:



Note the very sudden and dramatic spike in the intake of food, shown by the red line which I've added to emphasise the abrupt change from baseline chow consumption. We can ignore the red oval for this post. What happens in the VMH neurons of these rats?

This is what happens:



The dark brown staining cells on the right are dying, the rats have been eating "cookie dough", which they "can't get enough of", for seven days. The nice healthy cells in the left hand photomicrograph are from rats on crapinabag. The basic idea appears to be that feeding rats a bit of fat and sugar makes them eat so much, starting in just one day, that by seven days their VMH is killed by over indulgence. You eat too much, you kill your brain. Simple. This is, of course, absolute bollocks.

At the risk of repetition, we can produce exactly the same lesions in the VMH with MSG or gold thioglucose (or an ice pick if you must be crude and don't want nice pictures). This injury results in fat gain which must be compensated for by overeating. Rats will gain weight more slowly if they are on low fat diets than on high fat diets because of the effects of increased de novo lipogenesis which I've discussed in previous posts.

Want pretty pictures from GTG injured rats? Here's some random immuno from a random paper, there's a lot of it around, only black and white though:



Gold thioglucose on the right, arrow marks the injury area. And I just noticed the same pics, also in black and white, from fat injured rats from elsewhere in the Schwartz paper (mirror imaged compared to the GTG pics, random choice of which side of brain got sectioned!) after just a week on their high fat diet:



So we can produce the pretty black stains of dying cells with gold thioglucose (or MSG if we looked at neonatal immuno) but this injury preceeds the loss of calories in to adipocytes and subsequent "hyperphagia". THE INJURY COMES FIRST.

Let's really look at the bizarre idea that non-forced "overeating" causes subsequent damages your VMH. This is how it works for over eating by a gold thioglucose injected rat, no yummie high fat diet needed: It simply decides to over eat crapinabag because this has suddenly and randomly become delicious and so it becomes obese. We all know overeating CAUSES the VMH injury in fat fed rodents. So how do GTG injured rats get the injury first and over eat secondarily? Gold thioglucose obese rodents might SEEM to have a chemical lesion causing obesity but clearly they get fat first, travel back in time (squeezing in to a time machine as obese chrononaughts) and retrospectively force the researchers to give them the injection of GTG to obtain the lesion in their VMH which they are going to produce in the future by eating too much crapinabag. Got that? You've all watched Back to the Future. I watched parts I and II but never managed part III. It's simple time travel. Ditto MSG and ice-pick (ouch!) obese rodents. Self inflicted injuries using time travel.

Or we could abandon such stupidity and say that high fat diets injure the VMH first and this injury increases fat storage by decreasing sympathetic tone to adipocytes, as it does.

And I suspect it's superoxide, generated by a high F:N ratio (classically derived from palmitic acid at an F:N ratio of 0.47) in POMC neurons, which probably does the damage. You all know POMC neurons, the ones in the VMH with both gluokinase to sense (via metablism) glucose and CD36 to monitor FFA status (via metabolism again). No lactate for the energy status sensing neurons of the VMH...

So the question is, as always, what happens to the VMH of a C57BL/6 mouse (bred to get fat on a high fat diet) when put on a high fat diet which does NOT generate superoxide in POMC neurons? You can do this.

No one has done the necessary immuno staining under these conditions to get the pretty pictures of dying (or non dying) cells, as far as I know. But it's easy to look at the weight gains, which are a reasonable surrogate for POMC injury. Schwartz again using rats:



Not the most lucid graph, but it gives the basic idea. The control weight gains on the left are comparable to the weight gains shown for day 14.

Now, here is what happens if you take a C57BL/6 mouse and put it on to 35% of calories from fat if you keep the F:N ratio of that fat well below 0.47, using omega 6 PUFA with an F:N ratio of 0.42, as the primary source of fat:



Ignore the top two lines (for now) and look at the weight gain of the mice in the bottom two lines. One group weaned on to crapinabag, the other weaned on to 35% of calories from fat, but a fat with a low F:N ratio. There is zero, zilch, nil difference in weight gain over three weeks. There is no excess weight because there is no VMH injury. No one generates significant superoxide from a low F:N ratio fat like linoleic acid. That appears to include the POMC neurons of C57BL/6 mice.

C57BL/6 mice (and Long Evans rats) are specifically bred to get fat on palmitic acid (sometimes plus fructose) based diets. They fail to deal with the absolutely normal levels of superoxide produced in POMC neurons in the VMH which are crucial to energy status sensing. They do not have the luxury of developing insulin resistance as their job is to monitor both glucose and fatty acid levels. They are not allowed to run on lactate with an F:N ratio of 0.2 the way much of the brain does. They take whatever plasma gives them and do their best to cope with it. Or, in the case of rodents bred to become fat on high fat diets, not cope with it.

Before we go looking at the linoleic acid paper a bit more carefully I think it's worth trying to look at energy sensing rather more peripherally than the POMC neurons of the VMH. Then we can come back to the fat mice and try to think about what's going on using the meagre data available. Because it's quite interesting.

Peter