The origin of runner’s high

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.

 

 

 

Proline rides again !

Proline is a kinky amino acid.  Kinky in the sense that it is only one of the twenty with a fixed configuration of its alpha carbon because of the ring (which may be why there is more of it in organisms living at high temperature) and kinky in the sense that when present in alpha helices it produces a kink.  The previous post shows how it is used to schlep the body weight’s worth of ATP we make each day out of our mitochondria — https://luysii.wordpress.com/2019/01/30/3939/.

Well here it is in one of the marijuana receptors (CB1).  Binding of delta9 THC in the 7 transmembrane alpha helix bundles of the G Protein Coupled Receptor (GPCR) causes an alteration in the kink allowing transmembrane helix 6 (TM6) to move outward toward the cytoplasm, creating a cavity on the intracellular side, where the G protein trimer can bind.

You can read much more about this in an exquisite paper [ Cell vol. 176 pp. 448 – 458 `19 ] describing the CB1 receptor bound to a synthetic ligand 20 times more potent that delta-9 tetrahydrocannabinol (delta9 THC).  It is a cryoEM study which used 177,000 projections to come up with a 3 Angstrom resolution structure of CB1 bound to MBDB-FUBINACA in complex with its G protein trimer.  They had to use a single chain variable fragment (scFv6) along with a positive allosteric modulator (PAM) called ZCZ-011 to stabilize the complex.

MBDB-FUBINACA is a story in itself.  It is presently the fentanyl of synthetic cannabinoids, which “has been linked to thousands of hospitalizations and numerous fatalities”  [ New England Journal of Medicine vol. 376 pp. 235 – 242 ’17 ].  I’m surprised I’ve never heard of it — have you? But then I’ve been retired from clinical practice for some time. Perhaps the mainstream press, pushing marihuana legalization as it has been, kept it quiet, or more likely there have been no further episodes of mass intoxication from the AMB-FUBINACA (aka the zombie drug) since 2017.

I’ve never knowingly used marihuana.  Frankly it scares me — for why please see — https://luysii.wordpress.com/2014/05/13/why-marihuana-scares-me/.

There are 4 molecular switches buried in GPCRs [ Current Med. Chem. vol. 19 pp. 1090 – 1109 ’12 ]

1. The ionic lock switch between the D/E R Y sequence at the cytoplasmic end of TM3 and E286 at the cytoplasmic end of TM6 (single letter amino acid code used) –http://130.88.97.239/bioactivity/aacodefrm.html

2. TM3 – TM7 lock switch.  In rhodopsin it is between the protonated Schiff base of lysine and a glutamic acid and it broken on light activation,.=

3. Toggle switch linked with the n P x x Y motif in TM7 (x stands for any amino acid) — much more about this later in the post.

4. Transmission switch — produced by agonist binding, the outward movement of TM6 to to ligand binding creating a hole fo the G protein to bind to the receptor on the cytoplasmic side.

So why did I call the Cell paper exquisite?  Because of the molecular detail it provides about just how MDMB FUBINACA activates CB1.  Here’s the structure of AB-FUBINACA — https://en.wikipedia.org/wiki/AB-FUBINACA.   Both look like drugs designed by a committee.  They both have a para-iodophenyl group, an amide, and a fused indole ring with an extra nitrogen (imidazole ring — I never could keep heterocyclic nomenclature straight).    MDMB has a methyl ester (in place of the amide) and a tertiary butyl group (in place of the isoPropyl group).

I don’t have time to look up how Pfizer came up with it.  The FUBINACAs do not resemble delta9 THC at all — https://en.wikipedia.org/wiki/Tetrahydrocannabinol.

The pictures in the paper show how the hydrophobic aromatic side chains of FIVE phenylalanines and 2 tryptophans create a nice oily space for delta9 THC and MBDB-FUBINACA to bind.

F200 (phenylAlanine 200) and W356 are the toggle twin switch which stabilize the inactive conformation of CB1.  The rotation of F200 to interact with the imidazole of FUBINACA, allows W356 to rotate outward, changing the kink produced the the proline #358  in TM6 allowing the helix to straighten and rotate outward toward the cytoplasm, creating a cavity for the G protein to bind to.

Definitely a tour de force for the blind watchman.

Why marihuana scares me

There’s an editorial in the current Science concerning how very little we know about the effects of marihuana on the developing adolescent brain [ Science vol. 344 p. 557 ’14 ]. We know all sorts of wonderful neuropharmacology and neurophysiology about delta-9 tetrahydrocannabinol (d9-THC) — http://en.wikipedia.org/wiki/Tetrahydrocannabinol The point of the authors (the current head of the Amnerican Psychiatric Association, and the first director of the National (US) Institute of Drug Abuse), is that there are no significant studies of what happens to adolescent humans (as opposed to rodents) taking the stuff.

Marihuana would the first mind-alteraing substance NOT to have serious side effects in a subpopulation of people using the drug — or just about any drug in medical use for that matter.

Any organic chemist looking at the structure of d9-THC (see the link) has to be impressed with what a lipid it is — 21 carbons, only 1 hydroxyl group, and an ether moiety. Everything else is hydrogen. Like most neuroactive drugs produced by plants, it is quite potent. A joint has only 9 milliGrams, and smoking undoubtedly destroys some of it. Consider alcohol, another lipid soluble drug. A 12 ounce beer with 3.2% alcohol content has 12 * 28.3 *.032 10.8 grams of alcohol — molecular mass 62 grams — so the dose is 11/62 moles. To get drunk you need more than one beer. Compare that to a dose of .009/300 moles of d9-THC.

As we’ve found out — d9-THC is so potent because it binds to receptors for it. Unlike ethanol which can be a product of intermediary metabolism, there aren’t enzymes specifically devoted to breaking down d9-THC. In contrast, fatty acid amide hydrolase (FAAH) is devoted to breaking down anandamide, one of the endogenous compounds d9-THC is mimicking.

What really concerns me about this class of drugs, is how long they must hang around. Teaching neuropharmacology in the 70s and 80s was great fun. Every year a new receptor for neurotransmitters seemed to be found. In some cases mind benders bound to them (e.g. LSD and a serotonin receptor). In other cases the endogenous transmitters being mimicked by a plant substance were found (the endogenous opiates and their receptors). Years passed, but the receptor for d9-thc wasn’t found. The reason it wasn’t is exactly why I’m scared of the drug.

How were the various receptors for mind benders found? You throw a radioactively labelled drug (say morphine) at a brain homogenate, and purify what it is binding to. That’s how the opiate receptors etc. etc. were found. Why did it take so long to find the cannabinoid receptors? Because they bind strongly to all the fats in the brain being so incredibly lipid soluble. So the vast majority of stuff bound wasn’t protein at all, but fat. The brain has the highest percentage of fat of any organ in the body — 60%, unless you considered dispersed fatty tissue an organ (which it actually is from an endocrine point of view).

This has to mean that the stuff hangs around for a long time, without any specific enzymes to clear it.

It’s obvious to all that cognitive capacity changes from childhood to adult life. All sorts of studies with large numbers of people have done serial MRIs children and adolescents as the develop and age. Here are a few references to get you started [ Neuron vol. 72 pp. 873 – 884, 11, Proc. Natl. Acad. Sci. vol. 107 pp. 16988 – 16993 ’10, vol. 111 pp. 6774 -= 6779 ’14 ]. If you don’t know the answer, think about the change thickness of the cerebral cortex from age 9 to 20. Surprisingly, it get thinner, not thicker. The effect happens later in the association areas thought to be important in higher cognitive function, than the primary motor or sensory areas. Paradoxical isn’t it? Based on animal work this is thought to be due pruning of synapses.

So throw a long-lasting retrograde neurotransmitter mimic like d9-THC at the dynamically changing adolescent brain and hope for the best. That’s what the cited editorialists are concerned about. We simply don’t know and we should.

Having been in Cambridge when Leary was just getting started in the early 60’s, I must say that the idea of tune in turn on and drop out never appealed to me. Most of the heavy marihuana users I’ve known (and treated for other things) were happy, but rather vague and frankly rather dull.

Unfortunately as a neurologist, I had to evaluate physician colleagues who got in trouble with drugs (mostly with alcohol). One very intelligent polydrug user MD, put it to me this way — “The problem is that you like reality, and I don’t”.