Effects of TLR4 and other immune system molecules on sensations

In general pleasant drugs can cause withdrawal which is similar to being sick like headache, pain sensitivity, overactive sweating/shivering, runny nose, possible nausea and unhappy sleepy feeling.
After randomly finding TLR4 (toll like receptor 4) in Wikipedia it suddenly started to make sense to me why almost all drugs i ever used caused withdrawals with such sickly symptoms.

In summary sedatives can activate TLR4 which in turn activates many proteins involved in stimulating cells including blocking protein that breaks up adrenaline causing added activity. Also it increases TNF levels which could increase amount of AMPA glutamate receptors on surface of neurons by ~40% in 2 hours (another 22 hours could add extra 10%) and contribute to serious over stimulation of sensory neurons familiar to those who have got withdrawals from opiates, sleeping pills or ethanol where noises, light, smells, touch and pain can easily feel overwhelming. Tiredness probably comes from activating NF-kB which starts producing inflammation related proteins including protein COMT, which breaks up adrenaline and dopamine causing typical tiredness and lack of motivation common to diseases and withdrawals. Many withdrawal symptoms from opiates and ethanol could be blocked by blocking TLR4 activity by for example opiate antagonist naloxone. Glutamate receptors mediate signals between sensory and motor neurons. Even if i take 0,5 mg of alprazolam i almost always wake up in cold sweats getting easily overwhelmed by all types of sensory stimuli. Any sedative i've used including GHB, opiates, alprazolam, pregabalin and cannabinoids can have aftereffects where skin is either sweaty or shivering and overall feeling is very tense like all sensory and motor neurons were overactive.For example with GHB i woke up in 4 hours also in cold sweats and oversensitive skin that felt too uncomfortable from any pump.
TNF activity can also increase synthesis of NO by stimulating iNOS which dilates blood vessels and cause redness in inflamed region while in brain this dilation tends to cause headaches (especially if touching skull) which are common to almost any withdrawal including withdrawal from caffeine, other stimulants and sedatives.

TNF (tumor necrosis factor) can help immune system remove tumor and infections getting released by active TLR4 but chronic inflammation even by TNF could promote cancer formation as active NF-kB receptor can block cell suicide while making cells grow and divide faster. IL-1 (interleukin 1) gets released in similar situations as TNF and IL-1 also adds pain sensitivity.

As sidenote above image shows LPS which is common component of bacterial cells and it is common stimulator for immune system and TLR4 protein. TLR4 is also activated by saturated fatty acids and in image above saturated fatty acid groups are in black which seems like sign of receptor being too unselective to differentiate between LPS and saturated fatty acid.In one study i found most common saturated fatty acid palmitic acid could increase TNF levels by ~20 times in concentration of ~0,1 grams per liter which is around 8 grams for someone weighing ~80 kg.

TNF and other inflammatory proteins stimulate dorsal horn region of spine which gets signals about pain, touch and temperature.
 
Morphine can activate TLR4 by making glial cells release immune system activating substances. Usually opiates reduce pain by activating cells in PAG located in midbrain which then inhibits activity in dorsal horn of spine (input types shown above). Opioids can also increase release of TNF that stimulates dorsal horn so sensitivity to pain and temperatures increase but at least in early phases of opiate use TNF has less effect than opiates on spine. Withdrawal symptoms start usually 24 hours after last dose and mostly pass in 7-10 days. Morphine also binds with TLR4 on surface of glial cells which causes release of pro-inflammatory cytokines like TNF. Morphine withdrawal releases TNF inside PAG and injecting TNF into PAG causes withdrawal symptoms. TNF stimulates glutamate receptors and sodium channels.   

Illustration about pathway from TLR4 to NF-kB (NF-kappaB).

COMT (catechol-O-methyltransferase) is protein that breaks up adrenaline, noradrenaline and dopamine. Lowered COMT activity increases pain perception and muscle pain. TNF can lower COMT production in at least glial cells. NF-kB increases production of COMT, iNOS and COX-2.

Palmitic acid which may be most common saturated fatty acid in plants and animals caused release of TNF if TLR4 receptors were working but unsaturated omega-3 fatty acid DHA could reduce release of TNF. TNF levels could increase by over 20 times if there was 0,1 grams of palmitic acid per liter. 0,03grams of DHA could block that release of TNF by palmitic acid. TLR4 inhibitor could avoid release of TNF. Mutations in TLR4 could prevent diabetes caused by saturated fatty acids but not by unsaturated fatty acids.  

GPR120 is unsaturated fatty acid receptor that is expressed in hypothalamus. Activated GPR120 blocks activity of TLR4, iNOS and reduces release of TNF.

TNFR1 (TNF receptor 1) deletion reduces amount of AMPA (but not NMDA) glutamate receptors in cortex while TNFR2 didn't seem to have that effect. TNFR1 and TNFR2 both bind with TNF but while TNFR1 starts apoptosis TNFR2 protects cells from excitotoxicity. While TNFR1 can promote production of AMPA it also inhibits production of GABA A receptors on cell surface.

Results are illustrated above. In A part TNFR1 is colored green while TNFR2 is red. B part shows if and how much did TNF add AMPA receptors. C part shows almost no effect of TNF on number of glutamate receptors on cells without TNFR1 but in normal WT cells TNF increased AMPA density on neurons by ~40%. TNF seems to regulate transport and clustering of AMPA glutamate receptors on cells but not much inside cell as in D part if proteins were separated in gel then TNF didn't seem to cause much difference in amount of glutamate receptors throughout cells. 

TNF can cause apoptosis but melatonin may reduce it. Melatonin also inhibits calmodulin proteins which are involved in memory formation. Melatonin reduced amount of apoptotic proteins caused by hydrogen peroxide, TNF and glutamate toxicity to control levels.

Saturated fatty acid could increase of production of COX-2 with involvement from TLR4. Ibuprofen and aspirin both reduce inflammation by inhibiting COX proteins.

Endocannabinoid 2-AG could inhibit COX-2 when PPAR and retinoic X receptor join, attach to DNA and start regulating gene expression (like other nuclear receptors that need activation to reach DNA). LPS could increase amount of COX-2 and NF-kB by over 6 times but 2-AG could lower those to levels seen without LPS. 

TNF released for example by infections or radiation sickness binds with ryanodine calcium channels of vagal nerve and resulting vagal nerve activity may cause symptoms of malaise like nausea and reduced gastric motility as vagal nerve inhibits activity of intestines and other internal organs. Delayed gastric emptying tends to be perceived as nausea. CB1 receptor agonists may reduce these visceral symptoms of sickness by downregulating ryanodine receptors with protein kinase A (PKA). TNF itself is produced almost throughout body including fat cells, heart cells and connective tissue.

Comparison of vagal nerve responses (measured with fluorescent light) to electric stimulation. TNF almost doubled it. WIN2 is CB1 agonists but WIN3 is its inactive analogue. H89 is PKA antagonist.

Overview of involved proteins.VGCC-voltage gated calcium channel. TNF causes LTP in spine and at least brainstem by activating ryanodine receptors that stimulate release of neurotransmitters.

IL-1 can counteract analgesia from morphine while IL-1 antagonists can increase analgesia from morphine.  IL-1 antagonists can also restore analgesia during morphine withdrawals and mice with deletion of IL-1 receptor seem to have longer lasting analgesia.  

Ethanol activates TLR4 causing activation of Myd88 attached to TLR4 which in turn activates NF-kB receptor. Disrupting GABA A receptor reduces expression of TLR4 receptor and loss of either receptor reduces binge drinking in rats. Deleting TLR4 could reduce anxiety in rats addicted to ethanol. Also blocking TLR4 activity by for example naloxone reduced sedation and motor impairment from drinking ethanol.

While morphine reduces pain after spinal injury it also seems to increase region of damage after spinal injury by about 20-25%. Opiates increase release of pro-inflammatory cytokines which in turn block activity from opiates. Increased damage may partially come from overactivity of glutamate receptors as opiates reduce amount of glutamate re-uptake proteins on glial cells so stimulating glutamate may stay around cells for longer. NMDA glutamate receptor is usual sensory neurotransmitter and NMDA receptors are somewhat blocked by magnesium ions. Opiates activate protein kinase C, which removes this magnesium "plug" therefore making NMDA receptors easier to activate and possibly lead to excessive pain sensitivity. 

Several tricyclic substances including tricyclic antidepressant amitriptyline bind to TLR4 and inhibit activity of TLR4 being able to amplify analgesic effects of morphine.  Tricyclic antidepressants are sometimes also used for pain reduction

NF-kB seems involved in formation of synapses (which may connect immune system with memory formation). Deleting this gene reduces synapse density. NF-kB activity is increased by stimulating neurotransmitters when connections grow and form. Estrogen and GABA A antagonists can also stimulate NF-kB. 

NF-kappaB stimulates cell division and suppresses apoptosis. It is produced more in cancer cells. Pro-inflammatory cytokines like TNF and IL-1 tend to stimulate cancer especially  with chronic inflammation. Nicotine and other carcinogenic substances in tobacco stimulate NF-kB. NF-kB stimulates blood vessel formation by causing cells to release appropriate substances including TNF. COX-1 and COX-2 inhibitor aspirin seems to reduce NF-kB activity. Calcium channel blocker nifedipine has similar effect. Inhibiting NF-kB may help against tumors but it may also cause too weak immune system. Without NF-kB TNF could work more as an apoptosis stimulator than as tumor promoter. Authors tried to see if any NF-kB blocker could help against cancer but didn't find any good example.

Sleepiness

Several components of immune system cause sleepiness or sleep.

Many pro-inflammatory molecules like IL-1 and TNF (maybe also growth hormone releasing hormone) cause sleepiness while many anti-inflammatory substances reduce sleepiness. Sleep deprivation tends to increase release of pro-inflammatory substances. NF-kappaB is present in all cells with nuclei and if activated it reaches nucleus and produces sleepiness inducing pro-inflammatory substances like IL-1 and TNF. NF-kappaB show diurnal rhythm in cortex and sleep deprivation activates it’s production. TNF can activate iNOS which produces NO that could dilate blood vessels. Sleep is reduced if iNOS is inhibited. Levels of iNOS vary through 24 hours and it activates during sleepy period. TNF can also increase COX-2 activity and sleep is reduced if COX-1 and COX-2 get inhibited by acetaminophen. At least influenza virus can increase IL-1 and TNF production for days even if body temperature falls. Vagal nerve seems to promote sleep after detecting cytokines. TNF seems to cause sleeping locally. For example TNF added to one hemisphere causes more NREM sleep like activity in that hemisphere and if injected in cortical columns then this column starts to show sleep like activity.