To understand what might be happening, we have to back up and talk about obsessions and compulsions and where they exist in the brain. For everything we are has a place in the brain (right click in new tab for a song).
Compulsions mean performing unpleasantly repetitive or seemingly unnecessary acts in order to prevent perceived negative consequences ("step on a crack, break your mother's back"). Scratching, hair pulling and twisting do fit under that definition, as people often perform the behavior to stave off anxiety. Impulsivity is the predisposition toward rapid, thoughtless acts without regard for the eventual negative consequences (such as gambling or uncontrolled aggression), and hair-pulling (trichotillomania) and picking may also fall under this category. At first glance, compulsion and impulsivity may seem opposites, but they run in parallel neuronal tracts, and people with symptoms of one will often have symptoms of the other.
The compulsive and impulsive psychiatric diseases are among the most highly inherited diseases in psychiatry. Obsessive compulsive disorder, ADHD, Tourette's, autism, and substance abuse disorders indisputably run in families, suggesting (more even than for other psychiatric disorders) physical brain differences between those susceptible to the disorders and those who are not. Children with autism, for example, are likely to have tics or repetitive physical movements consistent with Tourette's or OCD, or impulsivity consistent with ADHD, or both. Those obsessively driven to drink or gamble are also more likely to be impulsive as well. There are all sorts of subcategories of different kinds of obsessive and impulsive behavior that exist along different neuronal tracts - the complexity, therefore, is immense.
However, one can conceptualize the brain as doing two things for impulsive behavior and compulsions. On one hand, the brain has tracts that promote these behaviors - these tracts generally run from the center, more "primal" parts of the brain up to the outer "civilized" cortex. The cortex tends to work in the opposite direction, inhibiting compulsions and impulsivity. Therefore one can become vulnerable to these behaviors via two means - either by problems that increase the "primal" signals from the center of the brain, or by problems that block the "civilizing" influence of the outer shell of the brain.
In terms of neurotransmitters, serotonin deficiency is thought to be responsible in part for anxiety driven behaviors, obsessions, skin-picking, hair-pulling, etc. Dopamine deficiency in the cortex is responsible for impulsivity (such as in ADHD). However, dopamine excess is thought to be responsible for motor tics, tapping in autism, for example, and other obsessive physical acts.
But let's bring it back to Garner's mice. They are genetically prone to hair-pulling behavior (Jaminet postulates this is due to a predominant Th1 immune/inflammation response to infection), and the hair-pulling worsened with increasing serotonin turnover in the brain, despite the "general rule" that anxiety and picking/hair-pulling behaviors are due to serotonin deficiency.
(In the mood for more opera? If so, how about this classic?)
But not so fast. There may not be as much of a mystery as we like to think. In his paper, Garner brought up the fact that SSRIs (selective serotonin reuptake inhibitors), while used to treat trichotillomania, can also induce skin-picking behaviors in some. His idea (I'm stretching, a bit) by the end of the paper was that this particular strain of mice would be a bad candidate for SSRI treatment. He may be right, but he may be wrong, because increasing serotonin turnover in the brain via diet is not the same as administering an SSRI.
The problem with thinking we know anything about the human brain is that the brain is immensely complex. Remember - a hundred billion neurons with 10,000 interconnections EACH who fire up to 1000 times per second. And when we strictly limit ourselves to serotonin, we are talking 20 different flavors of receptors, each doing subtly different things. Garner's mouse diet increased serotonin turnover overall in the brain, presumably jacking up the general serotonin signal. Well, there are mouse models for everything, and some have shown that increasing the activation of the serotonin 2C receptor (5-HT2C receptor) increases compulsive behavior, just like in Garner's mice.
SSRIs work a little differently than just blanket administration of serotonin in the brain. SSRIs result in the downregulation of the post-synaptic receptors, among them 5-HT2C. Therefore they will (in general) decrease the overall signal away from compulsion, perhaps relieving symptoms. SSRIs will tend to favor serotonin signal to another receptor, the serotonin 2A receptor. Activating this receptor tends to reduce anxiety and compulsions.
Psychiatrists use a whole host of SSRIs (prozac, paxil, zoloft, celexa, lexapro ect. ect. yawn yawn) but only one serotonin receptor activator (buspar, a 5HT2A agonist which may well be one of the most useless medicines we have in my experience). For better or worse, and for all their limitations, SSRIs do a better job of modulating the anxiety/compulsion symptoms than straight-up administration of serotonin (via a tryptophan promoting diet or tryptophan supplements).
Again, the medicines are not my interest in this blog, but our knowledge of how they work, and the animal models, give us clues as to how our brains work, and that I find interesting. Are our brains better off with a diet of whole foods with just enough omega 3s, not too many omega 6s, no weird industrial designer pseudofood or toxic plant proteins? Most likely. And Garner's mice tell us that straying from the design parameters of the diet our brain likes may well cause major problems. And that sugar and tryptophan supplementation may cause major problems, if we share genetic vulnerabilities with his mice.
That is one theory. I also question whether kyurnetic and inflammation in general were escalated with Garner's mice's experimental diet. From my comments on Dr. Jaminet's post:
Just want to add to my speculation about increasing inflammation (which I think is the more likely scenario) – tryptophan is [a] precursor for both serotonin and kyrunetic. The former helps modulate major brain circuitry communication, the latter seems to encourage excitatory communication, and in excess will be neurotoxic. Kyurnetic is elevated in cases of inflammation. SSRIs increase serotonin initially, but then after 2 weeks, the post synaptic receptors are down regulated, so the overall effect is not so much to increase serotonergic transmission, but to make what transmission there is go through more efficiently. SSRIs also seem to favor the metabolic pathway of tryptophan to serotonin rather than kyrurnetic.So, take your pick. But there are many plausible methods by which increasing tryptophan can increase picking/hair-pulling behaviors, and more and more reasons to eat well from the beginning.
It is possible that by massively increasing the tryptophan uptake into the brain in the context of the baseline inflammatory diet and a genetically vulnerable mouse population, there were increases in kyurnetic, leading to neurotoxicity and mouse psychopathology. I’m not a big fan of l tryptophan and 5 htp for that reason – I know you are not a fan due to tryptophan's role in the infectious theory of neurotoxicity. Since this scenario probably occurs in the human brain as well, it makes one wonder about our USDA diets.