There is a great moment (for the neuropharmacologist) in “Postcards from the Edge” with Meryl Streep. She’s walking along with the bimbo who she just found out seduced the guy who seduced her, when the bimbo blurts out that she feels great because of her endolphins.
Well exercise may raise endorphins in the blood which many regarded as an explanation of the runner’s high. But almost as soon as the endorphins were discovered, it was found that they don’t get into the brain when injected into the blood. (If you’re wondering how we can know this, it is based on a synthetic endorphin containing a radioactive atom — injecting it into the blood stream shows it doesn’t get into the brain.
This shouldn’t be surprising, the brain is quite selective about what it lets in. Consider the first useful treatment of Parkinson’s disease, L-DOPA (L DihydrOxy PhenylAlanine) which does get into the brain, which then breaks it down to dopamine losing two oxygens in the process, which doesn’t get into the brain (even though dopamine is a smaller and less complicated molecule). Functionally, this is known as the blood brain barrier (BBB).
So maybe exercise raises endorphrins in the brain, but a better explanation for the runner’s high is now at hand [ Nature vol. 612 pp. 633 – 634, 739 – 747 ’22 ]. You won’t believe the answer, which involves the organisms in your gut, but the evidence is quite good, as you are about to read.
First, the composition of the gut microbiome predicts how much mice voluntarily run on exercise wheels or treatmills. Treatment with antibiotics which diminishes the amount of microbiota diminishes exercise endurance. Adding the gut microbiome from high exercise mice to germ free mice (gnotobiotic mice) raises running capacity to that of the donor.
Increased levels of dopamine are considered rewarding or pleasurable. Cocaine prevents it from being taken up after neurons release it, an antidepressant (Monamine Oxidase — MAO) prevents it from being destroyed. etc. etc.
It is known that exercise increases the levels of dopamine in an area of the brain called the striatum. Dopamine gets to the striatum by the axons of neurons in the ventral tegmental area (VTA). Inhibition of neurons in the VTA decreases dopamine in the striatum and decreases the amount of exercise a mouse will do.
What does the gut microbiota have to do with this?
Well, germfree (gnotobiotic) mice didn’t change MAO levels in the striatum on exercise, and the dopamine surge and striatal neural activity were blunted. And germfree mice don’t run as much.
Well, clearly the little bugs down there are producing some sort of signal which IS getting to the brain, not an easy feat getting past the blood brain barrier given the example of L-DOPA above.
We know the bugs produce all sorts of metabolites, the body uses. One example is vitamin K, which is crucial in the biochemical maturation of coagulation factors, deficiencies of which produce hemorrhagic disease of the newborn. This may explain why the ritual circumcision of Jewish males occurs 8 days after birth, after the gut bacteria have had a chance to make it.
The work cited above shows that the bugs produce fatty acid amides (FAAs) which bind to the type I cannabinoid receptor (CB1) which binds marihuana.
Like just about everything else in the body, there are sensory nerves from the gut going to the spinal cord. The FAAs activate some of these nerves by binding to CB1. Giving FAAs to germfree mice increases physical activity.
Gut sensory nerves containing CB1 also have another protein called TRPV1. Stimulating these nerves with a TRPV1 ligand increases physical activity. This is true even in germfree mice.
Well we know marihuana has no trouble getting pCast the BBB, so why couldn’t the FAAs produced by the bugs do the same and increase exercise. Well, it could but it doesn’t. Severing the sensory nerve before it gets to the spinal cord abolishes the effects of the microbiome (which is still there) on exercise.
So, clearly the continuity of the nerve is crucial for the effect of gut bacteria on exercise, as are FAAs and the CB1 receptor found on the nerve.
Well the sensory nerve from the gut gets into the spinal cord, but there is a lot more work to be done, to determine the pathway by which stimulation of the nerve changes MAO levels in the striatum (as the striatum is a long way from the spinal cord). So like all great experiments, it suggests further questions and work required to resolve them.
A beautiful series of experiments. Could brain ‘endolphins’ still play a role in exercise. Sure, but whether they do or not, doesn’t detract from the work here.
One could study the effect of exercise on brain (not blood) endorphins and the effect of cutting the sensory nerve from the gut on their brain levels.
Chemiotics II 
Comments
>Increased levels of dopamine are considered rewarding or pleasurable
Not really. Increased levels of dopamine are considered to increase the incentive salience of any stimuli. They could be forward reward prediction or maybe retrocausual reward prediction, but it’s reward prediction, not reward. It’s wanting not liking.
Liking/reward/pleasure in mammals does not require dopaminergic populations that predict reward to be active or involved. Pleasure at least requires the involvement of the glutamergic neurons in the shell of the nucleus accumbens and their projections to the ventral pallidum glutamergic populations.
The ventral pallidum does have dopaminergic populations too, but they don’t have to be involved for the observable and reportable correlates of pleasure in mammals.
Superkuh: Sorry for the delay in responding. The theory that dopamine release is inherently pleasurable is ancient neuropharmacology involving amphetamine and cocaine like drugs.