Tag Archives: Substantia nigra

How flat can a 100 amino acid protein be?

Alpha-synuclein is of interest to the neurologist because several mutations cause Parkinson’s disease or Lewy Body dementia.  The protein accumulates in the Lewy Bodies of these diseases.  These are concentric hyaline inclusions over 15 microns in diameter found in pigmented brain stem nuclei (substantia nigra, locus coeruleus).

The protein contains 140 amino acids.  It is ‘natively unfolded’ meaning that it has no ordered secondary structure (alpha helix, beta sheet).  No one is sure what it does.  Mouse knockouts are normal, so the mutations must produce something new.

Alpha-synuclein can form amyloid fibrils, which are basically stacks of pancakes made of flattened segments of proteins one on top of the other.

Would you believe that the 100 amino terminal amino acids of alpha-synuclein can form an absolutely flat structure.  Well it does and there are pictures to prove it in PNAS vol. 117 pp. 20305 – 20315 ’20.  Here’s a link if you or your institution has a subscription — https://www.pnas.org/content/pnas/117/33/20305.full.pdf.

This isn’t the usual alpha-synuclein, as it was chemically synthesized with phosphorylated tyrosine at amino acid #39.  Who would have ever predicted that 100 amino acids could form a structure like this?  I wouldn’t. The structure was determined by cryoEM and all the work was done in China.  Very state of the art world class work.  Bravo.

The four hour cure for depression: what is Ketamine doing?

It is a sad state of affairs when you look forward to writing a post on depression.

https://www.fox32chicago.com/news/79-shot-15-fatally-over-fourth-of-july-weekend-in-chicago

https://www.straitstimes.com/asia/south-asia/indian-grooms-wedding-funeral-leave-more-than-100-infected-with-coronavirus

From Nature 2 July — “G4 a type of swine flu virus from China can proliferate in human airway cells.  34/338 pig farm workers in China have antibodies to it.  In ferrets G4 causes lung inflammation and coughing.”

Well that’s enough reason to flee to the solace of the basic neuroscience of depression.

 

 

The drugs we use for depression aren’t great.  They don’t help at least a third of the patients, and they usually take several weeks to work for endogenous depression.  They seemed to work faster in my MS patients who had a relapse and were quite naturally depressed by an exogenous event completely out of their control.

Enter Ketamine which, when given IV, can transiently lift depression within a few hours.  You can find more details and references in an article in  Neuron ( vol. 101 pp. 774 – 778 ’19)  written by the guys at Yale who did some of the original work. However, here’s the gist of the article.  A single dose of ketamine produced antidepressant effects that began within hours peaked in 24 – 72 hours and dissipated within 2 weeks (if ketamine wasn’t repeated).  This occurred in 50 – 75% of people with treatment resistant depression.  Remarkably one third of treated patients went into remission.

This simply has to be telling us something very important about the neurochemistry of depression.

Naturally there has been a lot of work on the neurochemical changes produced by ketamine, none of which I’ve found convincing ( see https://luysii.wordpress.com/2019/10/27/how-does-ketamine-lift-depression/ ) until the following paper [ Neuron  vol. 106 pp. 715 – 726 ’20 ].

In what follows you have to have some basic knowledge of synaptic structure, but here’s a probably inadequate elevator pitch.  Synapses have two sides, pre- and post-.  On the presynaptic side neurotransmitters are enclosed in synaptic vesicles.  Their contents are released into the synaptic cleft when an action potential arrives from elsewhere in the neuron.  The neurotransmitters flow across the very narrow synapse to bind to receptors on the postsynaptic side, triggering (or not) a response of the postsynaptic neuron.  Presynaptic terminals vary in the number vesicles they contain.

Synapses are able to change their strength (how likely an action potential is to produce a postsynaptic response).  Otherwise our brains wouldn’t be able to change and learn anything.  This is called synaptic plasticity.

One way to change the strength of a synapse is to adjust the number of synaptic vesicles found on the presynaptic side.   Presynaptic neurons form synapses with many different neurons.  The average neuron in the cerebral cortex is post-synaptic to thousands of neurons.

We think that synaptic plasticity involves changes at particular synapses but not at all of them.

Not so with ketamine according to the paper.  It changes the number of presynaptic vesicles at all synapses of a given neuron by the same percentage — this is called synaptic scaling.  Given 3 synapses containing 60  50 and 40 vesicles, upward synaptic scaling by 20% would add 12 vesicles to the first 10 to the second and 8 to the third.   The paper states that this is exactly what ketamine does to neurons using glutamic acid (the major excitatory neurotransmitter found in brain).  Even more interesting, is the fact that lithium which treats mania has the opposite effects decreasing the number of vesicles in each synapse by the same percentage.

I found this rather depressing when I first read it, as I realized that there was no chemical process intrinsic to a neuron which could possibly work quickly enough to change all the synapses at once.  To do this you need a drug which goes everywhere at once.

But you don’t. There are certain brain nuclei which send their processes everywhere in the brain.  Not only that but their processes contain varicosities which release their neurotransmitter even where there is no post-synaptic apparatus.  One such nucleus (the pars compacta of the substantia nigra) has extensively ramified processes so much so that “Individual neurons of the pars compact are calculated to give rise to 4.5 meters of axons once all the branches are summed”  — [ Neuron vol. 96 p. 651 ’17 ].  So when that single neuron fires, dopamine is likely to bathe every neuron in the brain.  We think that something similar occurs in the locus coeruleus of the lower brain which has only 15,000 neurons and releases norepinephrine, and also in the raphe nuclei of the brainstem which release serotonin.

It should be less than a surprise that drugs which alter neurotransmission by these neurotransmitters are used to treat various psychiatric diseases.  Some drugs of abuse alter them as well (Cocaine and speed release norepinephrine, LSD binds one of the serotonin receptors etc, etc.)

The substantia nigra contains only 450,000 neurons at birth, so you don’t need a big nucleus to affect our 80 billion neuron brains.

So the question before the house, is have we missed other nuclei in the brain which control volume neurotransmission by glutamic acid?   If they exist, could their malfunction be a cause of mania and/or depression?  There is plenty of room for 10,000 to 100,000 neurons to hide in an 80 billion neuron brain.

Time to think outside the box people. Here is an example:  Since ketamine blocks activation of one receptor for glutamic acid, could there be a system using volume neurotransmission which releases a receptor inhibitor?

Addendum 7 July — I sent a copy of the post to the authors and received this back from one of them. “Thank you very much for your kind words and interest in our work. Your explanation is quite accurate (my only suggestion would be to replace “vesicles” with “receptors”, as the changes we propose are postsynaptic). Reading your blog reassures us that our review article accomplished its main goal of reaching beyond the immediate neuroscience community to a wider audience like yourself.”

 

Why don’t serotonin neurons die like dopamine neurons do in Parkinson’s disease

Say what ?  “This proportion will likely be higher in rat dopaminergic neurons, which have even larger axonal arbors with ~500,000 presynapses, or in human serotonergic neurons, which are estimated to extend axons for 350 meters” – from [ Science vol. 366 3aaw9997 p. 4 ’19 ]

I thought I was reasonably well informed but I found these numbers astounding, so I looked up the papers.  Here is how such statement can be made with chapter and verse.

“The validity of the single-cell axon length measurements for dopaminergic and cholinergic neurons can be independently checked with calculations based on the total volume of the target territory, the density of the particular type of axon (axon length per volume of target territory), and the number of neuronal cell bodies giving rise to that type of axonThese population analyses are made possible by the availability of antibodies that localize to different types of axons: anti-ChAT for cholinergic axons (also visualized with acetylcholine esterase histochemistry), anti-tyrosine hydroxylase for striatal dopaminergic axons, and anti-serotonin for serotonergic axons.

The human data for axon density and neuron counts have been published for forebrain cholinergic neurons and for serotonergic neurons projecting from the dorsal raphe nucleus to the cortex, and cortical volume estimates for humans are available from MRI analyses; forebrain cholinergic neuron data is also available for chimpanzees. These calculations lead to axon length estimates of 107 m and 31 m, respectively, for human and chimpanzee forebrain cholinergic neurons, and an axon length estimate of 170–348 meters for human serotonergic neurons.”

H. Wu, J. Williams, J. Nathans, Complete morphologies of basal forebrain cholinergic neurons in the mouse. eLife 3, e02444 (2014). doi: 10.7554/eLife.02444; pmid: 24894464

How in the world can these neurons survive as long as they do?

Not all of them do–  At birth there are 450,000 neurons in the substantia nigra (one side or both sides?), declining to 275 by age 60.  Patients with Parkinsonism all had cell counts below 140,000 [  Ann. Neurol. vol. 24 pp. 574 – 576 ’88 ]. Catecholamines such as dopamine and norepinephrine are easily oxidized to quinones, and this may be the ‘black stuff’ in the substantia nigra (which is latin for black stuff).

Here are the numbers for serotonin neurons in the few brain nuclei (dorsal raphe nucleus) in which they are found.  Less than dopamine.  A mere 165,000 +/- 34,000 — https://www.ncbi.nlm.nih.gov › pubmed

So being too small to be seen with a total axon length of a football field, they appear to last as long as we do.  Have we missed a neurological disease due to loss of serotonin neurons?

Why should the axons of dopamine, serotonin and norepinephrine neurons be so long and branch so widely?  Because they release their transmitters diffusely in the brain, and diffusion is too slow, so the axonal apparatus must get it there and release it locally into the brain’s extracellular space, no postsynaptic specializations are present in volume neurotransmission — that’s the point.  This is one of the reasons that a wiring diagram of the brain isn’t enough — https://luysii.wordpress.com/2011/04/10/would-a-wiring-diagram-of-the-brain-help-you-understand-it/.

Just think of that dopamine neuron with 500,000 presynapses.  Synthesis and release must be general, as the neuron couldn’t possibly address an individual synapse.

The more we know the more remarkable the brain becomes.

 

Just when you thought you understood neurotransmission

Back in the day, the discovery of neurotransmission allowed us to think we understood how the brain worked. I remember explaining to medical students in the early 70s, that the one way flow of information from the presynaptic neuron to the post-synaptic one was just like the flow of current in a vacuum tube — yes a vacuum tube, assuming anyone reading knows what one is. Later I changed this to transistor when integrated circuits became available.

Also the Dale hypothesis as it was taught to me, was that a given neuron released the same neurotransmitter at all its endings. As it was taught back in the 60s this meant that just one transmitter was released by a given neuron.

Retrograde transmission was just a glimmer in the mind’s eye back then. We now know that the post-synaptic neuron releases compounds which affect the presynaptic neuron, the supposed controller of the postsynaptic neuron. Among them are carbon monoxide, and the endocannabinoids (e. g. what marihuana is trying to mimic).

In addition there are neurotransmitter receptors on the presynaptic neuron, which respond to what it and other neurons are releasing to control its activity. These are outside the synapse itself. These events occur more slowly than the millisecond responses in the synapse to the main excitatory neurotransmitter of the brain (glutamic acid) and the main inhibitory neurotransmitter (gamma amino butyric acid — aka GABA). Receptors on the presynaptic neuron for the transmitter it’s releasing are called autoreceptors, but the presynaptic terminal also contains receptors for other neurotransmitters.

Well at least, neurotransmitters aren’t released by the presynaptic neuron without an action potential which depolarizes the presynaptic terminal, or so we thought until [ Neuron vol. 82 pp. 63 – 70 ’14 ]. The report involves a structure near and dear to the neurologist the striatum (caudate and putamen — which is striated because the myelinated axons of the internal capsule go through its anterior end giving it a striated appearance).

It is the death of the dopamine containing neurons in the substantial nigra which cause Parkinsonism. They project some of their axons to the striatum. The striatum gets input elsewhere (from the cortex using glutamic acid) and from neurons intrinsic to itself (some of which use acetyl choline as their neurotransmitter — these are called cholinergic interneurons).

The paper makes the claim that the dopamine neurons projecting to the striatum also contain the inhibitory neurotransmitter GABA.

The paper also says that the cholinergic interneurons cause release of GABA by the dopamine neurons — they bind to a type of acetyl choline receptor called nicotinic (similar but not identical to the nicotinic receptors which allow our muscles to contract) in the presynaptic terminals of the dopamine neurons of the substantial nigra residing in the striatum. Isn’t medicine and neuroanatomy a festival of terms? It’s why you need a good memory to survive medical school.

These used optogenetics (something I don’t have time to explain — but see http://en.wikipedia.org/wiki/Optogenetics ) to selectively stimulate the 1 – 2% of striatal neurons which use acetyl choline as a neurotransmitter. What they found was that only GABA (and not dopamine) was released by the dopamine neurons in response to stimulating this small subset of neurons. Even more amazing, the GABA release occurred without an action potential depolarizing the presynaptic terminal.

This literally stands everything I thought I knew about neurotransmission on its ear. How widespread this phenomenon actually is, isn’t known at this point. Clearly, the work needs to be replicated — extreme claims require extreme evidence.

Unfortunately I’ve never provided much background on neurotransmission for the hapless chemists and medicinal chemists reading this (if there are any), but medicinal chemists must at least have a smattering of knowledge about this, since neurotransmission is involved in how large classes of CNS active drugs work — antidepressants, antipsychotics, anticonvulsants, migraine therapy. There is some background on this here — https://luysii.wordpress.com/2010/08/29/some-basic-pharmacology-for-the-college-student/