Absolutely nothing...? Not quite, but it may be good for a lot less than anyone thought. At least according to a recent paper in PLoS One describing what happens to mice given genetic knockout which left them almost completely unable to produce the neurotransmitter serotonin (5HT).
The mice lacked either one, or both, of two genes called TPH1 and TPH2, which code for two related enzymes called tryptophan hydroxylase-1 and tryptophan hydroxylase-2. These are necessary for the production of serotonin from the amino acid tryptophan (which you get from eating turkey... and also most other foods). No tryptophan hydroxylase, no serotonin.
Tryptophan hydroxylase-1 is mostly responsible for making serotonin outside the brain, while tryptophan hydroxylase-2 predominates in neurones. So the mice lacking both enzymes ("double knockouts") should have had no serotonin at all, anywhere. In fact, chemical analysis revealed a small amount present in the brains, but it was >99% less than normal, and even this may have been some kind of contaminant rather than serotonin:
Reduction of 5-HT in TPH2KO mice ranged from 67.5% (cerebellum) to 96.9% (striatum), while 5-HT reduction in DKO mice ["double knockouts" who lacked both TPH1 and TPH2] ranged from 94.4% (cerebellum) to 99.2% (cortex). 5-HT levels were lower in DKO mice than in TPH2KO mice in all brain regions examined. The percentage of 5-HIAA reduction paralleled changes in 5-HT. No generalized changes were noted in other neurotransmitter levels.
So, what happened to these serotonin-less animals? The big story is - remarkably little. They were alive, for one thing. They weren't writhing in pain thinking "Every moment I live is agony!" like that mutant on The Simpsons. The double knockout mice were slightly smaller and leaner than usual (less body fat), but only by a few % points. Otherwise, they were normal on almost every measure. This is very surprising, given that serotonin is one of the oldest neurotransmitters in evolutionary terms. Even insects use serotonin as a transmitter. Even some single-celled organisms have serotonin. There are at least 14 different types of serotonin receptor in the mouse body (same for humans). What are they all doing? Nothing especially important, clearly.
The results dramatically indicate that 5-HT is not essential for overall development and that its role in behavior is modulatory rather than essential. Initial phenotypic analysis of these mutants revealed no differences in a range of measures of physical health including assays for cardiac, immune system, endocrine, and ophthalmic function (unpublished observations).
However, that's not the end of the story. The mice were also tested in a battery of standard behavioural tests used to measure anxiety levels and such like; these are commonly used to measure the effects of antidepressants and other such drugs in rodents. Given that antidepressants such as Prozac are supposed to work by increasing serotonin levels in the brain, you'd expect that mice with no serotonin would be "depressed".
The TPH1 knockout animals showed no differences at all - no surprise since, as you'll recall, they only lacked serotonin outside the brain e.g. in the intestines, where it seems to play a role in digestion - although presumably not a vital one. So, no surprise there. The TPH2 knockouts, and the TPH1/TPH2 double knockouts were remarkably normal too, showing no differences on most of the behavioural tests
For the TPH2KO and DKO, there were no differences between the KO or DKO and WT littermate control mice in motor coordination, acoustic startle response and sensorimotor gating, tonic inflammatory pain sensitivity, and learning and memory as assessed in inverted screen, pre-pulse inhibition, formalin paw, and trace fear conditioning assays, respectively
But they did show differences in the marble burying test, the forced swim test, and the tail suspension test. The double-knockouts generally showed the most profound effects. But here's the twist - far from being "depressed", the knockout mice were less "depressed" on the forced swim test (i.e. the genetic knockout had the same effect to that seen with antidepressants.) That is, they showed more struggling and less immobility. This is the exact opposite of what you might have expected.
On the other hand, the knockouts showed increased immobility on the tail suspension test, which is generally taken to be a depressive behaviour, and they buried more marbles in the marble burying test, which is opposite to the effects of Prozac. It's not clear what if anything burying more marbles means; some have suggested that the frantically burying mice are showing OCD-like symptoms. Hmm.
So, what these results show is that a) mice can live almost normal lives without serotonin, or at best with trace amounts, and b) the main effects of having no serotonin are upon "depression-like" behaviours, but whether the knockouts are more or less depressed is unclear (the authors push the idea that they're more depressed, but really it's impossible to say.) Still, this is a bit more evidence that the serotonin hypothesis of depression isn't quite dead.
To my mind, though, the most interesting result by far is that serotonin is so dispensible. Mice can live essentially normal lives without it, which is not true for most other neurotransmitters. Bear in mind, though, that just because serotonin is not necessary for normal functioning doesn't mean that if you do have serotonin, it isn't doing anything. It might be that in the knockout mice, other systems had taken over the roles normally played by serotonin.
Finally, this study was run by Lexicon Pharmaceuticals, who use genetic knockout technology to discover new drugs. They end by saying...
Our results strongly support targeting the 5-HT system to treat affective disorders and the use of knockout mice as a tool to tease apart mechanisms involved in the etiology of these disorders.
Katerina V. Savelieva, Shulei Zhao, Vladimir M. Pogorelov, Indrani Rajan, Qi Yang, Emily Cullinan, Thomas H. Lanthorn (2008). Genetic Disruption of Both Tryptophan Hydroxylase Genes Dramatically Reduces Serotonin and Affects Behavior in Models Sensitive to Antidepressants PLoS ONE, 3 (10) DOI: 10.1371/journal.pone.0003301