Implication Of Serotonin And Hyperalgesia/migraines

DuggaDugga

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Just starting to research this topic, so I figured I'd shared what I've been reading to solicit feedback from you all.

Virtually without question, but with some potential exception to receptor-specificity, serotonin has been implicated in the initiation of hibernation in animals:
Brain serotonin metabolism in hibernation. - PubMed - NCBI
Induction of unseasonable hibernation and involvement of serotonin in entrance into and maintenance of its hibernation of chipmunks T. asiaticus. - PubMed - NCBI
Does serotonin play a role in entrance into hibernation? - PubMed - NCBI
Same with melatonin, which seems to regular glucose homeostasis (insulin resistance), fat accumulation in preparation and endurance of hibernation, along with regulating sensitivity to light:
Plasma melatonin concentrations in hibernating marmots: absence of a plasma melatonin rhythm. - PubMed - NCBI
Melatonin modulates glucose homeostasis during winter dormancy in a vespertilionid bat, Scotophilus heathi. - PubMed - NCBI


This got me to thinking about the role of serotonin in mediating torpor/"hibernation" in animals, along with histamine and melatonin, modified sensitivity to light, sound, pain, and brain inflammation. It seems logical that increased serotonin may play a role in the reclusive, irritability that is experienced by many individuals chronically, especially those suffering from migraines (hyper-sensitivity of 5HT3R?).

From wikipedia (5-HT3 receptor - Wikipedia):
As with other ligand gated ion channels, the 5-HT3 receptor consists of five subunits arranged around a central ion conducting pore, which is permeable to sodium (Na), potassium (K), and calcium (Ca) ions. Binding of the neurotransmitter 5-hydroxytryptamine (serotonin) to the 5-HT3 receptor opens the channel, which, in turn, leads to an excitatory response in neurons. The rapidly activating, desensitizing, inward current is predominantly carried by sodium and potassium ions.[2] 5-HT3 receptors have a negligible permeability to anions.[1] They are most closely related by homology to the nicotinic acetylcholine receptor.

Researchers compared acute and persistent pain responses in wild-type and mutant (lacking the 5-HT3R-A subunit) mice.

(The 5-HT3 Subtype of Serotonin Receptor Contributes to Nociceptive Processing via a Novel Subset of Myelinated and Unmyelinated Nociceptors)

We next examined the animals in a model of persistent pain using the formalin test. In this paradigm, a dilute formalin solution is injected into the plantar surface of the hindpaw, and pain-related behavior (licking) is scored in two phases (for review, see Tjølsen et al., 1992). The first phase (∼0–10 min) is produced by direct activation of nociceptors and therefore provides a measure of acute chemical pain. The second phase results in part from a delayed inflammatory response in the injected paw and thus serves as a model of persistent pain resulting from tissue injury. Consistent with acute pain being intact in null mutant animals, we found that first-phase pain behavior did not differ in wild-type and mutant mice. In contrast, the second phase of pain behavior was significantly reduced in the mutant animals, indicating that 5-HT3Rs are important contributors to the nociceptive circuits that produce persistent pain (Fig. 3 a).

As for the behavioral profile, formalin evoked two phases of dorsal horn neuronal firing as shown previously in rats (Dickenson and Sullivan, 1987). The first phase consisted of an immediate increase in the firing rate that lasted for ∼2 min; there was no difference in the total number of spikes evoked in the first phase (0–10 min) between wild-type (mean ± SEM; 1236 ± 473) and mutant (1770 ± 617) mice (Fig.3 b,c). After a quiescent period, the wild-type mice exhibited a second phase of firing in which there was a marked increase in total spikes (4082 ± 1726), which mirrored the second phase of pain behavior. Consistent with the observed deficit in second phase pain behavior in null mutant mice, the magnitude of neuronal firing during the second phase in the knock-out animals was dramatically reduced (784 ± 410; p = 0.0175).

Finally, to identify the contribution of peripheral versus central 5-HT3Rs to sustained formalin-evoked pain behaviors, we used a pharmacological approach to inhibit receptor function in specific sites. We found that peripheral (intraplantar) injection of the 5-HT3R antagonist ondansetron reduced second, but not first, phase pain behavior in wild-type mice (Fig. 4 a). Because 5-HT3Rs are also found on the central (spinal) terminals of primary afferents and on dorsal horn interneurons (Hamon et al., 1989; Kia et al., 1995), we also examined the effect of ondansetron administered directly into the CSF (intrathecally). As observed after peripheral injection, intrathecal ondansetron dose dependently suppressed the second-phase paw-licking behavior in the formalin test (Fig. 4 b) but had no effect on the first (acute pain) phase (data not shown). These pharmacological results indicate that the reduced second-phase formalin behavior in the knock-out mice likely reflects loss of peripheral and central (spinal) 5-HT3R activity (Oyama et al., 1996)

As predicted, despite the differences in pain behavior, the edema produced by intraplantar serotonin did not differ in mutant and wild-type mice (Fig. 5). Paw injections of the 5-HT3R agonists mCPBG (1.0 μg/10 μl) or 2-methylserotonin (10 μg/10 μl) also produced intense paw licking in the wild-type mice. Importantly, however, the 5-HT3R agonists did not evoke significant paw swelling in either wild-type or null mutant mice (Fig. 5). Together, these results indicate that, when serotonin is released in the setting of tissue injury, it contributes to nociceptive processing and edema, but only the former is influenced by activation of the 5-HT3R.

The authors conclude:
Release of serotonin in the setting of injury thus has multiple consequences. Serotonin directly activates nociceptive afferents to increase the barrage of impulses transmitted to the spinal cord, resulting in an increase in behaviors indicative of pain. It also contributes to peripheral neurogenic inflammation, via activation of small-diameter peripheral afferents and release of proinflammatory peptides, such as SP from peripheral terminals. The latter induces extravasation of proteins from postcapillary venules, which in turn contributes to peripheral edema (Lembeck et al., 1982). The contributions of serotonin to nociception–pain and swelling, however, are readily dissociable according to the receptors that are activated. We found that peripheral injection of serotonin produced both pain behavior and swelling of the hindpaw in wild-type mice, but only the former was reduced in the 5-HT3R null mutant mice.

This got me interested in whether any 5-HT3R antagonists has been investigated as treatments for migraines.
I was able to locate some research, but it was limited largely by the toxic effects of the experimental treatments.
https://www.ncbi.nlm.nih.gov/pubmed/2045832
https://www.ncbi.nlm.nih.gov/pubmed/10563226
https://www.ncbi.nlm.nih.gov/pubmed/15515404

Curious what everyone else thinks. Please post any thoughts and studies you have to advance the conversation.
I'm still trying to sort out the differentiation of different serotonin "receptors", what agonizes and antagonizes them respectively, and what we know about the energetic state of the cell potentiates their respective affinities.
 
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DuggaDugga

DuggaDugga

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Serotonergic Neuromodulation of Peripheral Nociceptors

Administering exogenous 5-HT also elicits inflammation and hyperalgesia in humans [1213, 1921] and rats [2225]. Further, injecting 5-HT evokes itching [2627] and may play an additional role in pain perception [28]. Local injection of 5-HT into the rat hindpaw evokes a transient thermal hyperalgesia that peaks between 10–15 minutes and returns to basal responses within 30 minutes [23, 25, 29]. These effects of 5-HT are likely to be physiologically relevant. Thermal injury induces a local release of 5-HT [17] and peripheral inflammation induces a 4-fold increase in 5-HT [1516, 30]. Moreover, intraplantar administration of 5-HT also induces significant edema [25]

Investigating the mechanism, the authors introduce transient receptor potential cation channel subfamily V member 1(TrpV1) , a protein responsible for sensing and regulating temperature. I thought it was quite interesting that the protein responds to oxidized linoleic acid. Something worth considering is the potential role of unsaturated oils in stimulating hyperalgesia.

One mechanism whereby neuromodulators modify nociceptive signals is to alter the threshold for noxious stimuli. TRPV1 channels play a critical role in nociception by transducing thermal and chemical stimuli [3134] and responding to oxidized linoleic acid metabolites [5, 35] and inflammatory mediators [3637]. Capsaicin, the pungent ingredient in chili peppers, is an agonist for TRPV1. This compound is often used in functional studies to identify nociceptors and when applied to the skin, elicits itch, pain, and the sensation of heat. During inflammation, activation of TRPV1 by endogenous ligands leads to thermal hyperalgesia [3637]. Neuromodulators such as 5-HT may regulate the stimulus threshold for sensory neurons by changing the properties of TRPV1 channels [3839].

Serotonin may act as a neuromodulator of nociception by altering TRPV1 during inflammation. For example, in sensory neurons 5-HT increases excitability to thermal stimuli and enhances capsaicin- and heat-evoked currents [4041]. Heat and capsaicin alike stimulate TRPV1. Depleting 5-HT attenuates visceral pain and reduces TRPV1 activation [42]. Repeated application of capsaicin desensitizes TRPV1 and reduces afferent transmission of painful stimuli [43]. Inflammatory mediators may act by altering sensitization and desensitization of TRPV1 channels. For example, 5-HT stimulates nociceptors for a greater duration than inflammation alone [4445] and this may be mediated by alterations in TRPV1. However, detailed functional studies such as electrophysiology or calcium imaging are required to determine whether 5-HT alters TRPV1 desensitization.

Co-expression of multiple subtypes of 5-HT receptors with TRPV1 is critical for understanding how 5-HT modulates nociceptor activity. Several cell signaling pathways engaged by 5-HT are known to alter TRPV1 activity, possibly by altering phosphorylation of the channel. For example, 5-HT2 receptors are excitatory G protein-coupled receptors that increase PLC/PKC signaling via Gαq coupling. Activating this pathway can sensitize TRPV1. 5-HT3 receptors are ligand-gated cation channels (ionotropic receptors) that directly excite nociceptors [5253]. In cultured sensory neurons, 5-HT2A and 5-HT3 receptors antagonists attenuate calcium signaling and reduce the ability of 5-HT to enhance capsaicin-evoked CGRP release [46]

The authors point to receptors type 2 and 3 as being of primarily responsible for edema and pain.
Local injection of the 5-HT2A receptor antagonist ketanserin attenuates 5-HT-evoked edema [25, 29], while granisetron, a 5-HT3 receptor antagonist, has no effect on edema [25]. These results indicate that distinct 5-HT receptors control different mechanisms of pain and inflammation. This conclusion is supported by reports of 5-HT2A receptor expression in blood vessels [61] and 5-HT3 receptor expression on nociceptors [50]. Collectively, the studies indicate that the complexity of the peripheral 5-HT system in different pain states may be due, in part, to activation of a broad range of 5-HT receptor subtypes that are co-localized with TRPV1 on nociceptor sensory neurons.

Based on the current literature, it can be concluded that peripherally acting 5-HT enhances nociceptor activity as observed by (a) increased calcium influx in nociceptive sensory neurons, (b) increased proinflammatory peptide release from sensory neurons, and (c) enhanced behavioral hyperalgesia. There are a number of possible mechanisms through which 5-HT evokes pain, including recent evidence of a modulatory role of 5-HT on the nociceptors that express TRPV1 (Figure 2). While the specific mechanisms by which 5-HT and other neuromodulators affect nociception remain to be elucidated, it is important to continue to investigate these pathways to aid in the development of new and better therapeutics for pain conditions.

@Travis -- I know you've spent a good amount of time researching specific gene expression. What's your opinion on the various serotonin receptors?
 

Travis

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What's your opinion on the various serotonin receptors?
I had decided not to say anything about serotonin receptors before reading about G protein. This sits at the interface between microtubules and the cell membrane, and so it shouldn't be too surprising that histamine and niacin both release prostaglandins through G protein receptors.

I think all of these small planar molecules like serotonin, melatonin, dopamine, and epinephrine must act on the microtubules more-or-less directly. Microtubules are found inside of nerves running parallel the their length, surrounded by myelin composed mostly of pregnenolone and progesterone. I think it would be fair to say that microtubules basically are the nervous system, and also that the cell's microtubule network (cytoskeleton) is essentially the cell's miniature nervous system.

"Considerable evidence suggests that there is an interaction between G proteins and the cytoskeleton." ―Mark M. Rasenick

Micrtotubules have been shown histologically to run through mitrocondria. I think all theories of consciousness, and how serotonin works, should be based on microtubules.

When I had read previously about serotonin I didn't think too deeply about how it actually works, physically. I now think that understanding G protein will help to understand serotonin in general—and also histamine . . . and perhaps even the catecholamines.
 
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DuggaDugga

DuggaDugga

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I had decided not to say anything about serotonin receptors before reading about G protein. This sits at the interface between microtubules and the cell membrane, and so it shouldn't be too surprising that histamine and niacin both release prostaglandins through G protein receptors.

I think all of these small planar molecules like serotonin, melatonin, dopamine, and epinephrine must act on the microtubules more-or-less directly. Microtubules are found inside of nerves running parallel the their length, surrounded by myelin composed mostly of pregnenolone and progesterone. I think it would be fair to say that microtubules basically are the nervous system, and also that the cell's microtubule network (cytoskeleton) is essentially the cell's miniature nervous system.

"Considerable evidence suggests that there is an interaction between G proteins and the cytoskeleton." ―Mark M. Rasenick

Micrtotubules have been shown histologically to run through mitrocondria. I think all theories of consciousness, and how serotonin works, should be based on microtubules.

When I had read previously about serotonin I didn't think too deeply about how it actually works, physically. I now think that understanding G protein will help to understand serotonin in general—and also histamine . . . and perhaps even the catecholamines.
Thanks for that, Travis. I'm going read those papers and respond once I've digested them. Coincidentally, I took a break from reading on aldosterone to read your comment and, when I went back to the paper on aldosterone, mention of G protein was in the very next paragraph. A lot of dots are about to get connected for me I think!
 

Travis

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Thanks for that, Travis. I'm going read those papers and respond once I've digested them. Coincidentally, I took a break from reading on aldosterone to read your comment and, when I went back to the paper on aldosterone, mention of G protein was in the very next paragraph. A lot of dots are about to get connected for me I think!
I just made a quick post of my initial findings on the G protein receptors—with nice images.
 

Sucrates

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I've spent a lot of time reading about serotonin and migraine. Yet to come up with a strong argument for nature the relationship. I suspect there are multiple different etiologies. It seems that at least vasodilation from serotonin is almost a constant, though not always in the same cerebral vessels.

I'd recommend looking at substance P, bradykinin, prostaglandins (PGE2 in particular) and aspirin. This book is useful, turns up at a reasonable price every now and then.

https://www.amazon.in/5-Hydroxytryptamine-Mechanisms-Headaches-Frontiers-Headache/dp/0881679275

There are some interesting Japanese remedies I suspect might give some leads, on my "to do list".

Choto-san or chitosan is one. I think there are some animal studies combined with sumatriptan. It's used to treat obesity, kidney failure and crohns too...
 

Travis

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Sucrates brought-up a good point with prostaglanin E₂. The formation of prostaglandin E₂ is a slower process, and could represent the second phase of pain alluded to in the above-quoted experiment:

"Consistent with acute pain being intact in null mutant animals, we found that first-phase pain behavior did not differ in wild-type and mutant mice. In contrast, the second phase of pain behavior was significantly reduced in the mutant animals, indicating that 5-HT₃Rs are important contributors to the nociceptive circuits that produce persistent pain."
Many cytokines are known to activated phospholipase A₂, which then releases arachidonic acid from the cell membrane. This would then go through cyclooxygenase to become prostaglandin H₂, and then further acted-upon to produce derivative prostaglandins. These prostaglandins could then activate certain pain receptors.

"Consistent with the observed deficit in second phase pain behavior in null mutant mice, the magnitude of neuronal firing during the second phase in the knock-out animals was dramatically reduced."
I'm not sure the time interval of this process, but I think it could represent the "second phase" of the pain response is these 5-HT₃R knockout mice. Prostaglanin E₂ seems highly-involved in the pain response: Just take a look at the following titles:
  • Cornelsen, Mary. "Repeated injection of prostaglandin E₂ in rat paws induces chronic swelling and a marked decrease in pain threshold." Prostaglandins (1973)
  • Ferreira, & Moncada. "Prostaglandins and the mechanism of analgesia produced by aspirin‐like drugs." British journal of pharmacology (1997)
And some more elaborate experiments below, with full-text links:
I just got done reading a bit about prostaglandins. What I'd like to know is what determines the prostaglandin E₂/D₂ ratio? You'd think it's probably just the amount and types of enzymes available, but what controls that?

I get the feeling from reading prostaglandin studies that prostaglandin E₂ spikes after acute inflammation, followed by more prostaglandin D₂ during the recovery phase. Perhaps it's prostaglandin E₂ itself which upregulates prostaglandin D₂ synthase through one of the PPAR receptors? Not sure.
 

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