BBB, Gut + Depression

Where is ~90-95% of the body's serotonin made, and does it cross the blood-brain barrier?

Gut enterochromaffin cells, and no, it can't cross. It signals locally to vagal nerve endings.
Mnemonic: it sends a message, not the molecule.

What are the three main SCFAs made when bacteria ferment fibre?

Butyrate, acetate, propionate. Butyrate fuels colonocytes, is anti-inflammatory and strengthens the gut barrier.
Mnemonic: BAP. Fibre in, BAP out.

How do bacteria influence gut serotonin?

They stimulate enterochromaffin cells to make more serotonin, which activates vagal afferents that carry the signal to the brain.
Mnemonic: bug pokes cell, cell pokes vagus, vagus tells brain.

What does inflammation do to tryptophan, and which enzyme drives it?

IDO pushes tryptophan down the kynurenine pathway and away from serotonin. Interferon-gamma is a key trigger.
Mnemonic: inflame, kynurenine. Calm, serotonin.

Which kynurenine metabolite is neurotoxic and which is neuroprotective?

Quinolinic acid is neurotoxic (an NMDA agonist). Kynurenic acid is neuroprotective (it blocks NMDA).
Mnemonic: Quinolinic = quarrel, Kynurenic = kind.

What's the leaky gut and LPS link to depression?

A permeable gut lets bacterial LPS (from Gram-negative bacteria) into the blood. It activates TLR4 on immune cells and raises cytokines (IL-6, TNF-alpha), which can drive neuroinflammation.
Mnemonic: LPS leaks, cytokines shriek.

What do germ-free mice and microbiota transplants suggest?

Germ-free mice have exaggerated stress responses, and transplanting microbiota from depressed patients can induce depressive-like behaviour in rodents. This is animal evidence only.
Mnemonic: no germs, big stress. Sad germs, sad mice.

What are psychobiotics, and how strong is the human evidence?

Probiotics (e.g. Lactobacillus, Bifidobacterium) aimed at mood. Human evidence is modest and inconsistent so far.
Mnemonic: promising mechanisms, modest results.