The skull has no spare room
Here is the basic problem with any injury inside the head. The skull is a sealed box. When tissue in most of your body swells, it pushes outward and the skin stretches. When the brain swells, there is nowhere to go.
Pressure rises. Blood struggles to get in. Tissue that survived the first injury can be crushed by the swelling that follows it. That is why, after a brain bleed, swelling is not a side issue. It is one of the main ways damage spreads.
The Hainan paper measured two linked things: how much water built up in the brain, and how leaky the blood-brain barrier became.
What the blood-brain barrier does
Your brain is fussy about what it lets in. The blood vessels that feed it are lined with cells packed tightly together, sealed in a way vessels elsewhere in the body are not. This seal is the blood-brain barrier.
It keeps out most large molecules, many toxins and most of the cells and proteins floating in your blood. That protects neurons, which are delicate and do not regenerate well.
After a bleed, that barrier starts to fail. The seals loosen. Fluid and proteins from the blood seep into brain tissue, and water follows them. A leaky barrier and a swollen brain are two halves of the same problem.
How they measured the leak
The team used a classic test with a blue dye called Evans blue.
Evans blue sticks to albumin, a large protein in the blood. Normally, albumin cannot cross a healthy blood-brain barrier, so neither can the dye. If you find blue dye in brain tissue, the barrier has been breached.
The method went like this:
- At 24 hours after the bleed, dye was injected into a vein in the leg and allowed to circulate for an hour.
- The rat was then flushed through with a salt solution to wash out any dye still inside blood vessels.
- The brain tissue was tested to see how much blue had been left behind, measured with a light-reading instrument.
More blue means a leakier barrier.
What they found
The bleed caused a striking increase in dye leaking into the temporal lobes. Untreated and vehicle-treated rats leaked heavily, with no real difference between the two.
Rats given 5 mg/kg of SS-31 leaked significantly less. In the authors' words, SS-31 treatment markedly reduced the barrier damage caused by the bleed at 24 hours.
Combine that with the brain water result from Part 4, where 5 mg/kg SS-31 gave the biggest drop in swelling, and you get a consistent picture. Less leak, less water, less swelling.
Why a mitochondrial peptide would help a leaky barrier
This is the part I find most interesting, and it is easy to miss.
The barrier is not a wall of plaster. It is made of living cells, the cells lining the blood vessels, and keeping those tight seals in place costs energy. Those cells have mitochondria too.
When oxygen radicals build up after a bleed, they damage the cells that form the barrier as well as the neurons behind it. Seals loosen when the cells holding them are under oxidative attack.
So a peptide that protects mitochondria and limits oxidative damage could, in principle, help the barrier cells hold their seals. The paper does not test this mechanism directly. It measures the outcome, less leak, and connects it to the wider antioxidant effects we will see in Part 7. But the link is plausible, and it is a good example of how one mechanism can show up in several places at once.
Swelling and behaviour line up
There is a quiet strength in this part of the paper. The behavioural scores from Part 4 and the swelling and leak measures here point the same way.
That matters because they are very different kinds of measurement. One is two blinded vets watching a rat eat and walk. The other is weighing dried tissue and reading the colour of a dye. When completely different methods agree, it is less likely that the result comes from a quirk in one of them.
It does not make the result certain. It does make it more believable than any single measure would on its own.
Keeping it in proportion
All of this was measured at one time point, 24 hours. We do not know from this study whether the barrier stayed better protected at three days or a week, or whether the effect faded once the single dose wore off.
And again, the rats were treated 30 minutes after the bleed. Whether the barrier can still be protected when treatment comes much later is a question for future work.
Next, we go down to the level of individual cells, and look at how the team counted neurons that were dying, and which signals inside those cells were switched on.
This series discusses a peer-reviewed laboratory study in rats. The results have not been shown in people. Nothing here is medical advice, and SS-31 is discussed for research purposes only.
