L06 Serotonin

Serotonin

• Serotonin (or 5-hydroxytryptamine/5-HT) is a monoamine neurotransmitter (along w/DA + NA)

• Like DA: well-known but misunderstood

---

• things that act on monoamine will act on 5HT

• monoamine but NOT catecholamine

• overstates levels of interventions to affect 5-HT

The reality

• 5-HT’s role in affect is complex, not unidirectional

  • Any disruption could be “bad”

• Besides affect, 5-HT mediates other functions

  • social behavior (aggression), arousal, cognition and eating

• Many disorders might involve imbalances in 5-HT

  • Depression, autism spectrum disorder and more

• 5-HT may contribute to depression but is not sole cause

---

• plays a broader role in social behaviour etc., not just mood

• plays a role in aversive processing specifically in text

Serotonergic neurons

• CNS: most are in the raphe nuclei (dorsal, median)

• Most 5-HT is in the gut (~90%) rather than the brain

---

• brain stem packed with clusters of neurons that project thru rest of brain

• drugs can affect serotonergic transmission elsewhere (e.g., gut) since it is also present in the gut

Serotonergic neurons

• Synthesis (from precursor, via enzymes)

• Vesicular storage (VMAT2)

• Metabolism (MAO)

• Transport (SERT)

• Receptors and autoreceptors

Serotonin synthesis

• Precursor is tryptophan

  • Acquired via diet, variation may affect 5HT levels

• Two key enzymes:

  • tryptophan hydroxylase (TPH2 in neurons), rate-limiting

  • aromatic L-amino acid decarboxylase (AADC)

• Higher 5-HT synthesis in males is suspected

---

• tryptophan usually obtained from diet (so differences in diet may lead to differences in 5-HT; but more tryptophan doesn’t exactly lead to more 5-HT)

• >1 synthesis enzyme; but there is a rate-limiting enzyme (TPH2)

Tryptophan intake and diet

• Ratio of amino acids affects tryptophan entry

  • High protein, low carb diets reduce entry + 5-HT (trp depletion diet)

  • Low protein, high carb diets increase entry + 5-HT (trp loading diet)

---

• enters CNS but is competitive with large AAs

• diet w lots of large neutral AA, Trp doesn’t get in, therefore low synthesis

• change diet > AA > Trp entry

Effects of depletion and loading

• Variable effects depending upon subject and protocol

• With Trp depletion, vulnerable individuals might relapse into depression (healthy unaffected)

---

• some capacities improved/impaired w diff levels of TRP/LNAA ratios (away from average)

5-HT metabolism

• Metabolized by monoamine oxidase (MAO), final product is 5- hydroxyindoleacetic acid (5-HIAA)

  • MAO inhibitors are also used to treat depression

  • 5-HIAA used as a surrogate measure of 5-HT

---

• monoamine inhibitors may cause anti-depression effects

• 5-HIAA is a metabolite measured as index of overall serotonergic function (often this case)

Vesicular storage

• 5-HT is transported into vesicles via vesicular monoamine transporter 2 (VMAT2)

• Reserpine (L04 – L06) inhibits VMAT2, preventing 5-HT release*

  • Produces sedation + depression-like behavior

---

• we can affect the ability of NT to move into vesicles

5-HT Transport

• Occurs via the 5-HT transporter (SERT)

• SERT knockout mice have many anatomical, physiological + psychological anomalies

• SERT is the principal target of selective serotonin reuptake inhibitors (SSRIs, such as fluoxetine), the first-line drugs for treating depression

---

• picks up 5-HT from extracellular to intracellular space in cell

• DNM list in image

• many roles of 5-HT peripherally and centrally

• SSRI but transporting (although name implies inhibitor)

Serotonergic receptors

• 14 types have been identified with many subtypes

• All are metabotropic GPCRs except 5-HT3 (ionotropic)

---

• 5-HT1 and 5-HT2 focused on - therapeutic + recreational drugs target these

• most are metabotropic, slow

• GABA = ionotropic

5-HT1A/B Receptors

• 5-HT1A: somatodendritic, inhibit action potentials

  • Role in antidepressant response

  • Delayed desensitization w/SSRIs (how receptor responds to agonist)

• 5-HT1B: presynaptic, NT release

  • Social behavior and other roles

  • KO mouse shows more aggression and impulsivity

---

• usually inhibitor receptors? (only for action potentials and not any other functions)

• diff receptors related to diff social behaviours

5-HT2A Receptors

• Predominantly excitatory and targeted by many drugs

  • 5-HT2B ~ involved with more peripheral effects

  • 5-HT2C ~ psychological functions and drug responses

---

• clozapine, risperidone (5-HT2C)

5-HT in behavior

• Anxiety and depression (aversive processing)

• Behavioral inhibition (impulsivity)

• Social behavior (aggression)

• Arousal and consciousness (psychedelics)

• Hunger/eating (not covered)

• Memory/cognition (not covered)

• Pain (not covered)

---

Anxiety

• Feeling of worry, nervousness or unease

• Adaptive to an extent, but problems arise when dysregulated

• Role of 5-HT is complex

w

5-HT signaling + anxiety

• Duration matters (i.e. how long)

• Increases sometimes associated w/increased anxiety

  • During stress, acute elevations in 5HT2

  • SSRIs might increase anxiety (jitteriness syndrome); inhibits 5-HT transport → anxiety

  • SERT KO mice have increased anxiety

• Selective activation of raphe nuclei of special interest

• image: Acute but not long-term activation increases anxiety

---

• selective activation of dorsal raphe neurons → brief activation vs chronic (chronic was not associated w anxiety)

Depression

• Loss of interest or pleasure even when performing activities normally enjoyed (e.g. Anhedonia)

• Decreased energy (e.g. Fatigue)

• Feelings of guilt or low self-worth

• Disturbed sleep, appetite and activity

• Inability to concentrate

• Thoughts of suicide

Causes of depression

• Like most psychological disorders, multiple environmental and genetic factors contribute to risk

• Our job is to discuss the neurochemical ones (as we are in PSY396)

• Several major theories in this area:

  • Monoamine hypothesis

  • Neuroendocrine hypothesis

  • Inflammation hypothesis

The monoamine hypothesis

• ↓ Monoamine NTs (noradrenaline + serotonin) → ↓ Mood (depression)

• This is where the much-maligned ideas of the “happy transmitter” and “chemical imbalance” originated.

---

• monoamines includes serotonin and noradrenaline

Supporting evidence

1. Reserpine depletes 5-HT and other monoamines, produces depression-like behavior

2. Low levels of 5-HT (i.e. in blood), 5-HT metabolites (e.g. 5-HIAA) and other monoamine markers in depression

3. Tryptophan depletion can trigger an episode of depression in vulnerable individuals

4. Most drugs that treat depression target the monoamine system (inhibiting transport/metabolism)

---

• making inferences about periphery → caveat regarding 5-HT metabolites

• found drugs that work → came up with theory from them

Antidepressant drugs

• Surprising effect: some re-uptake of 5-HT by DAT, co-release of 5-HT with DA (not routine)

---

History of antidepressant drugs

• MAO inhibitors were first widely-used class (starting late 50s)

• Tricyclic antidepressants shortly afterward

  • Transporters and receptors

• SSRIs followed (popularity rose over the 90s)

• Numerous atypical drugs since:

  • Selective NAT + 5HTT inhibitors (SNRIs), DAT + NAT inhibitors (Bupropion)

---

• more recently, more mixed ones + those targeting noradrenaline

Antidepressant controversy

• Debate about over-prescription (for off-label use and mild depression) and discontinuation effects (~15-60% estimated; severe ~ 3% of cases)

Serotonin syndrome

• Psychological, autonomic and neurophysiological changes (tremor + seizures)

• Linked to antidepressant medications (5HT1A and 5HT2B action)

• Uncommon without polypharmacy (i.e. many drugs at once)

---

• affects mood, autonomic = organs, tremor (motor)

Problems (evidence against)

• Drugs not always effective (~30% may not respond)

• Benefits small over placebo

• Though drugs increase serotonin levels immediately, mood is not improved for weeks

• Drugs may increase anxiety (jitteriness syndrome)

• Drugs don’t reverse depression directly but instead might reduce symptoms through other mechanisms (e.g. emotional blunting)

---

• depression sometimes arises from diff mechanisms, where drugs may not be effective (that inhibits serotonin reuptake)

• secondary long-term effect of drugs

• drugs may help manage depression by indirectly affecting other mechanisms, doesn’t have to directly affect serotonin

The debate rages on

Even if serotonin imbalances do not cause depression, altering serotonin levels may still help treat it. Why?

• serotonin is contributing factor, but is one of many

• and depression is complex

Long-term SSRIs + Autoreceptors

• With chronic use, 5-HT autoreceptors downregulated

• Downregulation increases 5-HT release

• Effect is accompanied by desensitization of other receptors (particularly 5-HT1A)

---

• downregulation of presynaptic receptor many inhibit NT release

Long-term MAOIs have effects too!

• Loss of post-synaptic receptors for monoamines (L05)

---

• less receptors, receptor desensitization, or change in NT release

Long-term effects of 5-HT drugs (2): neurogenesis

Postnatal NG + depression

• In most of the mammalian brain, new neurons are not generated after birth

• The hippocampus might be an exception and show postnatal neurogenesis

• New hippocampal cells could affect mood, learning + more

NG + depression

• Antidepressants increase NG, at least in animals

• If NG is blocked, antidepressants are less effective

• Time course of antidepressants corresponds to time course of maturation of newborn neurons (~4 weeks)

• Most major work is done in animals (generalize with care)

---

• more excitable, plastic → more easily activated → long-term potentiation

• note animal models → NG procedures invasive in humans

• add more notes

Levels of analysis for depression

Which level do we choose?

---

Beyond the brain

• SSRIs might induce personality changes

  • Neuroticism decreased, extraversion increased

• SSRIs might induce “emotional blunting” (valence of positive + negative states reduced)

  • Might be therapeutic (if negative states are a problem) but comes with side effects (positive states are less fun)

  • In reinforcement learning tasks, people taking antidepressants may respond less to negative outcomes and not modify their behavior

---

• antidepressants may not be treating, but may be tuning system down for lots of emotions (may be useful for very aversive states, but note positive states?)

Other disorders treated by SSRIs

• Generalized anxiety disorder

• Social anxiety disorder

• Panic disorder

• Bipolar depression*

• Premenstrual dysphoric disorder

• Post-traumatic stress disorder

• Bulimia nervosa

• Obsessive-compulsive disorder

---

• antidepressants broadly used for other conditions, not just depression

On a related note…

• As SSRIs complex effects, there is interest in alternative drugs

• In humans, the selective serotonin releasing agent (SSRA, see L03) fenfluramine was associated with:

  • Reduced sensitivity for outcomes in aversive contexts (reinforcement learning task > similar to SSRIs)

  • Enhanced inhibition, bias towards impulse control (no-go task)

  • Improved verbal memory

---

Opponency theory: 5HT vs. DA?

• both forms of transmission actually strong in any given state

• both in play, both interacting

Aggression

• Behavior intended to cause harm/pain (whether psychological or physical)

• Affective defense aggression (reactive)

  • response to a perceived threat, emotional, SNS activation

• Predatory attack aggression (instrumental)

  • planned, goal-directed, lacks emotion or SNS activation

---

• distinct mechanisms for reactive + instrumental

Aggression, Impulsivity and 5HT

• Disrupting 5-HT signaling affects aggression and impulsivity

• TPH2-KO, MAO-KO, 5HTR1b-KO mice are more aggressive

  • For TPH2-KO: supplementation to increase 5-HT levels reduces aggression

• SERT-KO and 5HTR1a-KO mice are less aggressive

---

• KO = synthesis problem (without gene, can’t make it)

• WT = wild type

• latency to attack low = attack sooner + more attacks

• data suggests deletion of TPH2 limits serotonin and increases aggression

• supplement removed to allow for serotonin?

• predicting that high serotonin levels = high aggression difficult to do

  • diff levels → diff amounts of aggression

Generalization to humans?

• Rare gene variants linked w/aggression in humans

  • MAOA1 (case of James Fallon), 5HT2B2

• Low 5-HIAA levels associated with anti-social behavior

• Interaction of 5-HT system with home environment

  • Relevant to blunting of 5-HT system with stress in animals?

• Human effects are in general less clear/compelling

---

• rare for genes to be connected with something

• all we say is that certain variations are associated with aggression

5-HT + the ultimatum game

A. Dictator extends offers, player accepts or refuses

• perceived fairness a factor

B. After many offers, the player might elect to “punish” the dictator

---

• common in finance + economic behaviours

• spend own money to hurt dictator (after history w dictator)

5-HT + other social behaviors

• Tryptophan depletion is associated with increased rejection of unfair offers and punishment

• Conversely, drugs increasing 5-HT levels (e.g. SSRIs) may increase acceptance of unfair offers

• Results are complex, subject to interpretation

• Genes in the 5-HT + DA system weakly related to performance

---

• 1st isn’t a problem, extreme forms are harmful

5-HT + Disordered Social Behavior – Focus on Autism- Symptoms

• Poor social interaction

  • Fails to respond to name, poor eye contact, resists cuddling, prefers playing/being alone, aggression in certain cases

  • May not recognize/respond to social cues

• Repetitive behaviors/Difficulty switching behaviors

  • Arranging objects, making sounds, hand flapping, head rolling and body rocking

  • Special interests

• Slow language development

  • Starts later than age of 2 (may remain non-verbal), repetition of words/phrases (echolalia), abnormal tone/rhythm

---

Studying ASD today

ASD ~ Neurodevelopmental

• Characterized by abnormalities in brain growth and/or development

• Deficits in emotion, learning, self-control + memory

• Class also includes schizophrenia (SZ), attention deficit hyperactivity disorder (ADHD), learning disabilities and more

• Alterations in brain structure may be associated with imbalances in serotonin at key periods

---

• what defines the neurodevelopmental category

• not acquired

5-HT + Autism

• Associated with hyperserotonemia (high 5-HT peripherally, ~25% of male cases)

• Imbalances early in life may affect neurodevelopment and subsequent adult behavior

• 2nd image:

• SERT KO mice show autistic-like behavior; tryptophan depletion normalizes behavior; several other transgenic studies are similar

• Some have proposed treating autism w/5-HT drugs

---

• very high serotonin levels in periphery in some cases

• levels of 5-HT during development affects wiring in early stage + effects are long-lasting

• low/high levels of 5-HT associated w diff wiring + conditions

• KO/depletion has opposite effects on mice

5-HT Selective drugs

Many relevant drugs beyond SSRIs

---

• skipped

Beyond SSRIs…

• Atypical antipsychotics for schizophrenia (clozapine, risperidone) act on 5-HT2 receptors (5HT2A ~ therapeutic effects, 5HT2C ~ side effects)

• Psychedelics are thought to involve 5HT2 receptors

  • Lysergic acid dimethyl amide (LSD), psilocybin, ayahuasca, mescaline are good examples; ketamine is not (mechanism not 5HT)

• 5-HT transmission is also central to the effects of 3,4- methylenedioxymethamphetamine (MDMA; ecstasy)

---

• psychedelics have similar structure to 5-HT

• ketamine not a lot of action on 5-HT

• may ask for MDMA acronym but DNM long name

Psychedelics

Psychedelics + 5-HT

• LSD is structurally similar to 5-HT

• In humans, the perceived intensity of the psychedelic experience correlates with neural 5-HT2R binding (e.g. in the cingulate cortex)

• In humans and animals, behaviors associated with psychedelics are blocked by 5-HT2R antagonists

  • In animals, the head twitch and discriminative states (L03) to psychedelics are both diminished by 5-HT2 blockers

---

• binds receptor more, and receptors not available, effects are greater

Psychedelic use

• Interest in treating mental health problems

  • Alternative treatment, early trials suggest it is not superior to other drugs

  • Could be combined with other methods

  • *note: not a lot of research surrounding this as treatment

• Interest in microdosing for perceived benefit

• Research is tightly regulated due to legal status; funding hard to get

• Placebo-controlled trials are complicated, placebo effects likely

  • Placebo responses in trials when confederates are present

  • Microdosing studies also show placebo effects

• Perceived benefits may not reflect real drug effects

  • Recent studies have suggested complex effects on creativity (acute impairment, long-term enhancement)

---

• breaking blind…

• always know if you’re in treatment group for drugs → effects are strong

• dose too low → can’t effect things of interest

Doing Better Psychedelic Research

• Inference issues

• Conflict of interest

• Safety

• Lack of control

• Sample size

• Selection bias

• Study duration

• Breaking blind

• Placebo effects

• Mechanisms

---

• drug studies must be carefully observed + safe

MDMA + 5-HT

• Potently increases 5-HT release (also affects catecholamines)

• Associated with euphoria, warmth, energy and empathy

• High doses associated w/lasting damage to the 5-HT system in animals, cognitive deficits and other problems

• Therapeutic roles being explored

---

• MDMA/ecstasy in medical context → explored effects in various conditions