ch 6: serotonin

draw the serotonin synthesis pathway

• tryptophan → 5-HTP (rate-limiting step) → 5-HT

• synthesis can be triggered by tryptophan or 5-HT

two forms of tryptophan hydroxylase (TPH)

• TPH2: only in serotonergic neurons

• TPH1: in non-neuronal cells in the gut + pineal gland

neuroendocrine tumors in the gut secrete ____ → elevated blood lvls cause _____ which can be treated w telotristat → a ______

1. 5-HT 

2. carcinoid syndrome

3. nonselective TPH inhibitor → does not cross the BBB → only blocks peripherally expressed TPH1

what effect does a high-protein meal vs low-protein meal have on tryptophan lvls in the blood vs brain (3)

• a high-protein meal will not cause increase in tryptophan or 5-HT in the brain but lvls in blood do increase

• A low-protein, high-carbohydrate meal will increase ratio of tryptophan to competing large neutral amino acids (LNAA) since insulin stimulates uptake of AAs except tryptophan

• therefore: ratio of tryptophan to other AA competitors for transport across the BBB determines whether serotonin synthesis is stimulated or not

2 methods of 5-HT depletion to asses its role in bhvr:

1. para-chlorophenylalanine (PCPA): selectively blocks 5-HT synthesis by irreversibly inhibiting TPH

2. acute tryptophan depletion (ATD): large doses of amino acids minus tryptophan 

   1. impair memory consolidation of verbal info → little-no influence on WM or attention

   2. reduced sleep duration + quality

what are methods of increasing tryptophan and why might this be important? (3)

• tryptophan loading: administration of pure tryptophan 

• supplemented w alpha-lactalbumin or other high-tryptophan proteins

• elevating tryptophan availability has been shown to enhance cognitive functions ie. memory, attention, elevate mood, improve sleep

___% of ingested tryptophan is metabolized to _____ rather than 5-HT which is then further metabolized to the kynirenine pathway → _____ and inflammation can _____. kyurenine metabolites have been implicated in several ___ and ____

1. 95%

2. kynurenine 

3. stress

4. stimulate enzymes involved

5. psychiatric disorders

6. neurodegenerative diseases

5-HT is transported into synaptic vesicles by ______ → ______ (a VMAT blocker) depletes 5-HT which is broken down when _____. _____ directly inhibit 5-HT release + ____ autoreceptors inhibit release by ____________

1. vesicular monoamine transporter 2 (VMAT2)

2. reserpine

3. not protected in vesicles 

4. terminal autoreceptors

5. somato-dendritic

6. slowing the rate of nerve firing

release of 5-HT can be stimulated by (2)

• drugs that are based on the structure of amphetamine

• ie. para-chloroamphetamine, fenfluramine, MDMA (ecstasy)

how does MDMA affect behaviour and what are its drawbacks? (4)

• heightened arousal, euphoria, enhanced perceptual awareness, prosocial effect (an entactogen)

• low + controlled doses: beneficial as an adjunct to psychotherapy in chronic PTSD

• potential toxic effects → serotonin syndrome → can lead to death

  • ↑ MDMA = 5-HT depletion in experimental animals + serotonergic deficits

how is serotonergic transmission terminated and how is it mediated? why is this protein important? (3)

• transmission terminated by reuptake of 5-HT from the extracellular fluid

• process mediated by 5-HT transporter

• SERT is an important target for several antidepressant drugs (SSRIs) → ie. Prozac which inhibit 5-HT reuptake

serotonin is eventually broken down by ____ to form ____

1. MAO-A

2. 5-hydroxyindoleacetic acid (5-HIAA)

serotonin neurons in CNS are found along __1__ loosely associated with the _2_ → the __3__ raphe nucleus + __4__ raphe nucleus give rise to most of the serotonergic fibres in the__5__ → in the cortex, nerve fibers are _6_ at the surface which has a _7_ density of __8__ + __8__

1. midline of the brainstem (medulla, pons, midbrain)

2. raphe nuclei

3. dorsal raphe nucleus (DRN)

4. median raphe nucleus (MRN)

5. forebrain

6. dense 

7. ↑

8. dendrites + synaptic connections 

how does DRN serotonin neuron firing relate to sleep–wake state, and what do different patterns do? (3)

• Natural states: awake = tonic/regular, SWS = reduced/irregular, REM = near-silent

• cats: when awake = tonic firing (regular rate) → irregular w slow-wave sleep → stop firing during REM sleep

• DRN/MRN KO mice: slow tonic 5-HT drive → promotes NREM sleep; burst/phasic drive → promotes wakefulness
  🧠 Takeaway: DRN 5-HT controls arousal; rate + pattern determine sleep vs wake.

most 5-HT receptors are ___, except ___ which is _____.

• metabotropic

• 5-HT3

• excitatory ionotropic receptor

compare 5-HT_1A to 5-HT_2A receptor subtypes (5)

• 5-HT_1A: in hippocampus, septal area, parts of amygdala, dorsal raphe nucleus

  • reduce cAMP synthesis by inhibiting adenylyl cyclase 

  • ↑sed opening of K+ channels + membrane hyperpolarization = ↓sed firing of either post-synaptic cell or the serotonergic cell itself

• 5-HT_2A: in cerebral cortex: striatum, nucleus accumbens

  • activate the PIP2 messenger system = ↑sed Ca2+ lvls + activates PKC 

what is the hypothesized connection btw serotonin + hallucinogenic drugs? how can this be useful when treating disease like schizophrenia? (3)

• DOI: widely used 5-HT2A agonist 

• effects of LSD believed to stem from its ability to stimulate 5-HT2A receptors

• blockade of 5-HT2A receptors + D2 antagonism leads to symptom improvement in patients with schizophrenia while minimizing the side effects associated w previous drugs

what receptor subtype is targeted by triptan drugs + what effect do they have? (2)

• 5-HT_1B/1D: causes constriction of the blood vessels w/in the brain → providing relief from migraine symptoms

  • + receptor agonist blocking of pain signals transmitted by the trigeminal nerve

how are 5-HT₃ receptors related to chemotherapy-induced nausea + how can vomiting/nausea be counteracted (3)

• 5-HT₃ receptors present on the peripheral terminals of the vagus nerve → induces activation of vomiting centre in the brainstem

• 5-HT₃ antagonists can counteract the nausea/vomiting

  • ie. ondansetron (Zofran), granisetron (Kytril), palonosetron (Aloxi)

methodological approaches in human clinical research to assess serotonergic function (3)

• comparing lvls of 5-HIAA in CSF or 5-HT + 5-HIAA in postmortem brain regions w bhvrl traits or neuropsychiatric disorders

• assess responses to SSRIs or receptor agonists + antagonists

• identify associations btw psychiatric disorders + polymorphisms in the genes for SERT or serotonergic receptors

pharmacological tools for probing the the serotonergic system specifically 5,7-DHT (5)

• serotonergic neurotoxins → para-choloamphetamine, MDMA, 5,7-dihydroxytryptamine (5,7-DHT) → doesn’t readily cross BBB so has to be given directly into the brain

  • lesioning results in ∆s in hunger/eating bhvr, anxiety, pain sensitivity, learning/memory 

  • causes massive damage to serotonergic axons + nerve terminals in the forebrain but in the raphe nuclei are usually spared

• drug challenges w selective receptors agonists + antagonists

what do TPH2 knockout mice reveal about serotonin function (3)

• TPH-2 KO mice have complete loss of 5-HT in the brain → preservation of lvls in bloodstream 

  • helps regulate the development of its own fibre system 

  • ↓ 5-HT = large ↑ in aggressive bhvr 

how can loss of 5-HT in the brain be restored? temporarily (3) vs fully rerstored (3)

• temporarily restored w 5-HTP → bc newly synthesized NT is gradually metabolized by MAO-A + cannot be replenished in mutant mice 

  • HT gets produced where its not supposed to 

  • works bc 5-HTP is the product of the rxn catalyzed by TPH → bypasses the missing step in the biosynthetic pathway 

• fully restored by 5-HTP plus carbidopa → an AADC inhiitor that doesn’t cross the BBB

  • reverses ↑ lvls of aggressive bhvr in TPH2 KO males

  • blocks conversion of 5-HTP → 5-HT in peripheral + allows for ↑ 5-HTP availability to the brain + prevents adverse consequences of elevated peripheral 5-HT lvls

bhvrs of TPH2-KO mice + physiological deficits (4)

• more impulsive + compulsive

• less anxious → have poor social communication + social bhvrs including maternal deficit

• poor thermoregulation

• abnormal respiration including apnea

5-HT role in regulation of anxiety 

• SSRIs are used to treat anxiety disorders → target 5-HT_1A receptors

Where are 5-HT₁A receptors and how do they affect anxiety? (3)

• Autoreceptors (raphe soma/dendrites): ↓ firing → ↓ global 5-HT

• Postsynaptic (hippocampus/mPFC/amygdala): activation reduces anxiety
  🧠 Takeaway: Autoreceptor = brake; postsynaptic = anxiolysis.

What do optogenetic studies show about 5-HT terminals in anxiety tests (EPM)? (3)

• Excite 5-HT terminals → ↑ open-arm time (anxiolytic)

• Inhibit terminals → ↓ open-arm time (anxiogenic)
  🧠 Takeaway: Local 5-HT release in limbic targets calms anxiety circuits

How does serotonin relate to aggression, and what circuit is involved? (3)

• Low central 5-HT → ↑ reactive/impulsive aggression

• Circuit: amygdala → BNST → hypothalamus → PAG (+ cortex/hipp/striatum) with raphe 5-HT inputs
  🧠 Takeaway: 5-HT acts as a brake on the threat/reactivity network.

What’s the “5-HT deficiency” idea and clinical link? (3)

• Many studies: low 5-HT/5-HIAA ↔ hyperaggressiveness/irritability

• SSRIs (e.g., sertraline) can reduce anger/irritability in several disorders
  🧠 Takeaway: Trend holds, but context/receptor subtype matters.

How does 5-HT suppress appetite via the arcuate nucleus (ARC)? (3)

• 5-HT₂C on POMC → ↑ α-MSH → MC4R (PVN) → ↓ feeding

• 5-HT₁B on NPY/AgRP → inhibits orexigenic neurons → ↓ feeding
  🧠 Takeaway: 5-HT boosts POMC and brakes NPY/AgRP → MC4R-mediated satiety.

Put the appetite circuit together (flow) (3)

• Raphe 5-HT → ARC (POMC ↑ via 5-HT₂C, NPY/AgRP ↓ via 5-HT₁B) → PVN MC4R → autonomic/endocrine outputs → reduced intake

• Drug note: Lorcaserin = 5-HT₂C agonist (mechanistic example)
  🧠 Takeaway: Raphe → ARC → PVN (MC4R) is the serotonergic satiety axis.

most of the body’s 5-HT is located in the _1_→ the _2_ NS + synthesized by _3_ cells using TPH1. 5-HT release is stimulated by __4__ which promotes secretion of _5_ +  ↑ __6__

1. gut

2. enteric

3. enterochromaffin 

4. food in the gut

5. metabolic hormones 

6. peristaltic activity

5-HT and IBS + potential treatment (3)

• ↑ 5-HT = enhanced gut motility → IBS diarrhea 

• ↓ 5-HT = constipation IBS

• 5-HT₃ antagonist alosetron treating IBS but has potentially dangerous side effects