Serotonin

Question 1

Where are serotonergic neurons found?

Answer 1

Almost all serotonergic neurons in the CNS are found along the midline of the brainstem, associated with the raphe nuclei.
The dorsal and median raphe nuclei give rise to most of the serotonergic fibers in the forebrain.
5-HTcontaining cell groups are designated with a “B.”
5-HT enervation of the forebrain is greater than that of dopamine, comparable to norepinephrine
5-HT-producing cells also exist in the gut (part of the digestive process)

Question 2

What is the chemical/molecular type of serotonin?

Answer 2

5-HT is a monoamine (like DA and NE), but its molecular type is an indolamine

Question 3

How is serotonin synthesized?

Answer 3

Synthesized from the amino acid tryptophan.

1. Tryptophan hydroxylase converts tryptophan to 5-hydroxyl-tryptophan (5-HTP) rate-limiting step in process.
2. Aromatic amino acid decarboxylase(AADC-aka Dopamine Decarboxylase) converts 5-HTP to 5-HT (serotonin).

Question 4

What is one way that we can target/localize serotonergic neurons in the brain?

Answer 4

Tryptophan hydroxylase is found only in serotonergic neurons and thus make good target for markers

Question 5

What is Para-chlorophenylalanine(PCPA)?

Answer 5

irreversibly inhibits tryptophan hydroxylase to block 5-HT synthesis by forming a covalent bond with the protein
Effects can last a number of days because the brain has to make new proteins again

Question 6

What is one way that we can stimulate serotonin synthesis?

Answer 6

5-HT synthesis can be stimulated by administration of large doses of precursor (tryptophan or 5-HTP).
Tryptophan competes with other amino acids for transport across bloodbrain barrier.
Ratio of tryptophan to other amino acids determines stimulation of 5-HT synthesis.

Question 7

What is one way that we can temporarily reduce serotonin synthesis in humans? What symptoms result?

Answer 7

Temporary reductions in 5-HT levels can be achieved in humans by giving an amino acid “cocktail” (minus tryptophan)
Large neutral amino acids inhibit entry of the remaining tryptophan into the brain.
The cocktail also stimulates protein synthesis in liver which further reduces level of plasma tryptophan.
Reducing 5-HT in this manner can cause:
REappearance of symptoms in previously-depressed patients (but not healthy ones)
behavioral changes (e.g.; increased impulsivity) in healthy subjects (Inability to inhibit inappropriate actions)

Question 8

How is serotonin transported into vesicles? What are the implications of this in terms of drugs? (Think back to lectures on catecholamines)

Answer 8

5-HT is transported into vesicles by VMAT2 (like catecholamines).
 Reserpine sensitive (If we give a drug like Reserpine that blocks this particular vesicular transporter, you get a reduction in brain serotonin levels )

Question 9

How is serotonin release modulated?

Answer 9

5-HT autoreceptors on presynaptic terminals inhibit release.
Somatodendritic Autoreceptors (on raphe neural cell bodies) slows firing rates of 5-HT neurons

Question 10

How is serotonin removed from the synapse?

Answer 10

After release, 5-HT is removed from synapse via reuptake by the 5-HT transporter (aka SERT)
Breakdown of 5-HT is catalyzed by MAO to yield 5-hydroxyindoleacetic acid (5-HIAA) (sensitive to MAO inhibitors).

Question 11

How can we measure the activity of 5-HT neurons?

Answer 11

Levels of 5-HIAA in the brain or CSF is used as a measure of the activity of 5-HT neurons.

Question 12

Describe two drugs that enhance serotonin release via amphetamine-like mechanisms (And what do we mean by “amphetamine-like” mechanisms?)

Answer 12

Firstly, what do we mean by amphetamine-like mechanisms?
Amphetamines target transporters and not only block them, but also reverse them (in this case, the drugs take serotonin molecules that are inside the terminal and shoot them out into the synapse)

Fenfluramine: was prescribed for appetite suppression in obese patients.
Side effect: cardiac problems
Some died (dozens, maybe a hundred of millions who took the drug)

3,4-methylenedioxymethamphetamine (MDMA, aka- Ecstasy or Molly): recreational drug
Also acts on DAT and NET, but has higher affinity for SERT than d- or methamphetamine
Often get different reports about the effects of the drugs because street drugs don’t tend to be pure
Intensifies sensory perceptions - some people find this quite enjoyable
Some milder ones work on impulse dependent increases in transmitter release

Question 13

SERT blockers

Answer 13

Many contemporary antidepressants act as selective serotonin reuptake inhibitors (SSRIs), such as fluoxetine(Prozac).
Results in an increase in serotonin levels (acute)
Different acute and chronic effects

Question 14

How many serotonin receptor subtypes are there? Are they ionotropic or metabotropic?

Answer 14

There are at least [14] 5-HT receptors: nearly all are metabotropic.

Question 15

How many main families of serotonin receptors are there?

Answer 15

There are 7 main families (5-HT 1 through 7)

Question 16

Why is it so difficult to study the different serotonin receptor subtypes and understand each of their functions?

Answer 16

Many 5-HT receptor drugs are relatively non-selective
Our pharmacological tools are not advanced enough - while they can target certain 5-HT receptors, they are not particularly selective and often end up hitting 2, 3, 4 different receptors
Because of the serotonin receptors’ complexity and our lack of selective drugs for many of these receptors, our understanding of the functions of some of them is limited (sometimes two receptors in the same family may have opposite effects on behaviour)
Thus, it’s hard for us to study the different subtypes and understand each of their functions

Question 17

How many subtypes does the 5-HT1 family have?

Answer 17

5-HT1 family has at least 5 subtypes

5-HT1C was changed to a 5-HT2 subtype because it was more similar to 5-HT2 than 5-HT1

Question 18

How many subtypes does that 5-HT2 family have?

Answer 18

5-HT2 family has at least 3 subtypes

Question 19

Where are 5-HT1A receptors concentrated?

Answer 19

Postsynaptically in forebrain regions (hippocampus, septum, amygdala) and-
Somato-dendritic autoreceptors on cell bodies in raphe

Question 20

What is the mechanism of action for 5-HT1A receptors?

Answer 20

Some are postsynaptic, but some are autoreceptors. The autoreceptors inhibit cAMP and/or increase opening of K+ channels which leads to hyperpolarization

Question 21

What is 8-OHDPAT?

Answer 21

5-HT1A receptor full agonist (activates these receptors about as well as serotonin)
Systemic administration stimulates postsynaptic receptors but also reduce 5-HT release

Question 22

What is Buspirone?

Answer 22

5-HT1A receptor partial agonist aka it has affinity for a receptor and stick to it, and it has some efficacy- it was activate the receptor - but it doesn’t activate that receptor as well as the endogenous ligand (serotonin)
Systemic administration stimulates postsynaptic receptors but also reduce 5-HT release

Question 23

What is WAY-100635?

Answer 23

5-HT1A antagonist (increases serotonin levels in the brain because blocking autoreceptor)
Also blocks the effects of postsynaptic 1A receptor stimulation though

Question 24

Describe the 5-HT1B and 5-HT1D receptors.

Answer 24

5-HT1B-and D receptors-serve as presynaptic autoreceptors on 5-HT terminals(but also reside postsynaptically in striatum/nucleus accumbens)

Question 25

Where are 5-HT2A receptors distributed?

Answer 25

Distributed widely in cortex and in striatum/nucleus accumbens - primarily post-synaptic

Question 26

What is the mechanism of action for 5-HT2A receptors?

Answer 26

Activate phosphoinositide 2nd-messenger system.

Increases Ca2+ levels in postsynaptic cells and activates protein kinase C (like α1- adrenergic receptors)

Question 27

What is DOI (1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane?

Answer 27

5-HT2A agonist
DOI and related drugs are hallucinogenic in humans.
Hallucinogenic effects of lysergic acid diethylamide (LSD) are believed to stem from its ability to stimulate 5-HT2A receptors.

Question 28

What is LSD?

Answer 28

5-HT2A agonist
Hallucinogenic effects of lysergic acid diethylamide (LSD) are believed to stem from its ability to stimulate 5-HT2A receptors.

Question 29

What is ketanserin?

Answer 29

5-HT2A antagonist

Question 30

What is special about 5-HT2C?

Answer 30

Some receptors in the same subtype have opposite effects, some have complementary effects
5-HT2C sometimes are localized in the same region as the 2A receptor, but has an opposite effect

Question 31

What are the behavioural functions of serotonin?

Answer 31

Serotonin is one of the evolutionarily oldest neurotransmitter
“Implicated in virtually everything but responsible for nothing”
Getting rid of serotonin is not at catastrophic as getting rid of other NTs like dopamine/glutamate/GABA
A general theme is 5-HT serves to suppress certain patterns of behavior

Some functions 5-HT has been implicated in include
 Sensory filtering
 Temperature regulation
 Sleep/wake cycles
 Pain modulation
 Learning and memory
 Cognitive flexibility

Important ones we went over in class include:

1. Hunger and satiety
2. Anxiety
3. Aggression
4. Impulsivity

Question 32

How is serotonin related to hunger and satiety?

Answer 32

SSRIs or other 5-HT releasers, 5-HT1B or 5-HT2C receptor agonists reduce food intake hypophagia) by:

1. Reducing amount of food consumed per meal (but not # of meals)
2. Shift food preference away from fatty foods

5-HT acts as short-term satiety signals associated with meal consumption. It disinhibits neurons in the paraventricular hypothalamus, which promotes satiety. (When the paraventricular nucleus of the hypothalamus becomes active, it reduces appetite. When you are feeding, other mechanisms are inhibiting the paraventricular nucleus to reduce the suppression of appetite (thus increasing appetite). Serotonin disinhibits the paraventricular nucleus, causing a rebound increase in excitation which reduces appetite.)

5-HT1A receptor agonists stimulate somatodendritic autoreceptors, reducing 5-HT release, leading to hyperphagia. (increase in eating)

Question 33

How is serotonin related to anxiety?

Answer 33

Multiple 5-HT receptors regulate anxiety in opposing manners

1. 5-HT1A agonists (which reduce 5-HT release via autoreceptor activation) reduce anxiety: Drugs that target this receptor (eg: buspirone, more details located in another flashcard) are used to treat anxiety disorders in humans
2. 5-HT1A KO mice show increased anxiety (knockout): These effects seem to be instigated during early postnatal development.
3. 5-HT2A/2C agonists increase anxiety-like behavior: 5-HT 2A or 2C KO mice show reduced anxiety

Question 34

How is anxiety measured in serotonin experiments?

Answer 34

Assay -> Elevated Plus (or zero) maze: has enclosed and open narrow walkways (high, open places are scary to rodents). The animal wants to explore new place (start in safe, nice, enclosed wall) but the narrow walkway is scary and causes anxiety. So there are competing wants - wants to explore, but it’s scary.
Amount of time spent in open vs enclosed areas is used as an index of anxiety (less open time = more anxiety)

Question 35

What is aggression?

Answer 35

behaviors meant to cause physical/psychological harm to a victim.

Question 36

What are some types of aggression?

Answer 36

Predatory aggression (complex form of feeding behaviour)
Maternal aggression (parent will defend their offspring)
Defensive aggression (aggressive to defend yourself against attack)
Social aggression (unprovoked attack - the type of aggression that is more relevant to humans in terms of expressing some sort of dominance)

Question 37

How is serotonin related to aggression?

Answer 37

Low 5-HT activity associated with increased aggressive behavior (can depend on genetic background, drug treatment regimen used, type of aggression, etc.)
Increased aggression may be related to 5-HT regulation of impulsivity (rather than increasing aggression directly)

Question 38

Do 5-HT1A/1B agonists increase or reduce aggression in mice? What is the mechanism of action? How do we know?

Answer 38

5-HT1A/1B agonists reduce aggression in mice
Because giving drugs that stimulate 5-HT receptors reduces behaviour, we think that higher 5-HT = less aggression and less 5-HT = more aggression
However, it’s not that simple because of the autoreceptors - how do we know if these results are due to postsynaptic receptors (eg. increasing activity) or autoreceptors (reducing 5-HT release)?

Based on follow-up experiments, the effects seem to be due to stimulating postsynaptic receptors rather than autoreceptors.

1. They have done follow-up studies where you infuse the drugs directly into terminal regions where they won’t necessarily affect the 1A autoreceptor
2. They have done genetic manipulations where they’ve altered how many 5-HT1A receptors an animal expresses. In this particular study they overexpressed 5-HT1A receptors but only in the raphe serotonin neurons. They placed an animal in a cage for a while and let it establish its home. Then they placed an intruder into it and looked at aggressive tendencies. Animals that have way more 5-HT1A autoreceptors on serotonin neurons are showing a lot more aggressive tendencies - so if you increase 1A activation of serotonin cells that lower serotonin activity throughout the rest of the brain, they’re more aggressive. On the other hand, if you give a drug that stimulates these receptors postsynaptically in the targets of the forebrain, there’s less aggression. Thus, 1A and 1B seem to work postsynaptically to attenuate aggressive tendencies.

Question 39

Explain experiments of serotonin and aggressive behaviour in humans

Answer 39

Acute tryptophan depletion in healthy humans can increase aggressive behavior in laboratory tests

1. Subject plays game for points against a fictive (computer) opponent that can deduct points from the subject
2. You can choose to get your points back, defend yourself from losing points, or attack the opponent back
3. Lower 5-HT levels associated with greater number of “attacks” by subject against opponent.

Question 40

What is impulsivity?

Answer 40

Acting without adequate thought; predisposition toward rapid, unplanned reactions without regard to their negative consequences. A number of psychiatric disorders are associated with increases in impulsivity

Question 41

How is impulsivity measured in mice?

Answer 41

Assay for impulsive action: rat performs an attention task (a light flashes briefly in a certain location, rat must nose-poke it quickly to get reward).
During the inter-trial interval, rat must NOT nosepoke (aka: make a premature response) or it gets a “time out” penalty

Question 42

How is serotonin linked to impulsivity?

Answer 42

5-HT is particular important for mediating ‘waiting’, may act as a brake to control impulsive responses
5-HT selective lesions (with 5,7-DHT)impair “waiting” by increasing premature responses
5HT 2C antagonists also impair impulsivity (same as destroying all serotonin in the brain)
5HT 2C agonists reduce impulsive action