Week 11 - Monoamine Neurotransmission

Question 1

serotonin (5-HT)

Answer 1

The amino acid tryptophan is actively
transported by carrier protein across the BBB, and enzymatically converted to serotonin (5- HT) in serotonergic neurons.

Question 2

dopamine and noradrenaline

Answer 2

Tyrosine is the precursor

Tyrosine -> L-DOPA -> Dopamine -> noradrenaline -> adrenaline

*Dopaminergic neurons lack DbH, so
cannot make NA

Question 3

Common Features of Monoamine Synapses

Answer 3

Presynaptic neuron
- VMAT-2 sequesters neurotransmitters into synaptic vesicles
- presynaptic receptors (activated)
- Autoreceptors located presynaptically;

Postsynaptic neuron
- postsynaptic receptors (activated)

• Use of presynaptic reuptake transporters (DAT, NET, and SERT) which share some homology (main targets for pharmacological agents);
• Areas of specificity include the location of synapses and expressed receptors.

Question 4

Metabolism of serotonin (5-HT)

Answer 4

Enzymatic breakdown of 5-HT is principally done by monoamine oxidase (MAO) followed by
conversion to 5- hydroxyindole acetic acid (5-HIAA), which is actively transported out of the CNS for ultimate excretion in the urine.

Question 5

Catecholamine synthesis and metabolism

Answer 5

Tyrosine - (Tyrosine hyroxylase) -> L-DOPA - (DOPA decarboxylase) -> Dopamine - (Dopamine hydroxylase) ->Noradrenaline - (phenylethanolamine M-methyltransferase (PNMT)) -> Adrenaline

Dopamine - (MAO-A) -> DOPAC - (COMT) -> Homovanillic acid

Noradrenaline - (MAO-B + COMT) -> nor-metanephrine - Vanyllilmandelic acid (VMA)

Notice that COMT and MAO enzymes are involved in metabolism of these two neurotransmitters (DA and NA).
Metabolites, including HVA and VMA, can be measured in the urine as indications of systemic monoamine
concentrations. These enzymes can be effective targets for affecting CNS concentrations in addition to interfering with the reuptake transporters.

Question 6

Noradrenergic pathways in the CNS

Answer 6

1. Cell bodies in pons and medulla, and the discrete medial forebrain nerve bundle (MFB, which may release NA diffusely);
2. Locus coeruleus (LC, pons) is involved in descending pain control (spinal cord). LC is quiet during sleep, but activity increases prior to waking up – thought to be involved with arousal.
3. Very small population of neurons has the enzyme for conversion of NA to
   adrenaline.

Question 7

Noradrenergic receptors at play

Answer 7

Virtually all forms for the adrenergic
receptors are present in the CNS, except for b3, and all are GPCR coupled.

Alpha1 receptor roles in CNS are poorly understood and are on neurons and glial cells. They may have involvement in motor control, fear and cognition. Alpha2 receptors are linked more to blood pressure control, sedation and analgesia.

Beta1 receptors are in the higher brain (cortex, striatum and hippocampus) while Beta2 receptors are largely found in the cerebellum.

Question 8

Serotonin (5-HT) in the CNS

Answer 8

5-HT is placed into vesicles by VMAT2 and is cleared by SERT in the presynaptic membrane.

Note autoreceptors present as well as various GPCR receptor types, plus ionotropic receptors.

Question 9

5-HT Autoreceptors

Answer 9

Different types of 5-HT autoreceptors in different sites:
Somatodendritic 5-HT1A autoreceptors decrease neuronal firing when activated by 5-HT released from self and other neurons.

In humans, 5-HT1D and in rodents, 5-HT1B, are similarly located presynaptically, where they can modulate release of 5-HT.

Question 10

Signal transduction of 5-HT receptor subtypes

Answer 10

5-HT receptors are generally classified into 7 families (5-HT1 to 5-HT7 ) and at least 14 subtypes according to their signal cascades.
5-HT receptors, except for 5-HT3 receptors, are G protein-coupled receptors with a 7 TMS structure, mediating intracellular signal transduction via coupling to individual G proteins
(e.g., Gi/o , Gs and Gq ), while 5-HT3 receptors form a Na+ and Ca2+ pore (ionotropic).

Question 11

Serotonergic pathways in the CNS

Answer 11

The neurons using 5-HT and their CNS projections resemble the pattern of noradrenergic neurons.

Cell bodies are grouped in the pons and upper medulla (raphe nuclei), with projections to various regions.

Question 12

5-HT activity or loss is correlated with:

Answer 12

• Arousal and mood:
  LC release of NA is linked to attention. These neurons on silent during sleep, with increased activity occurring with arousal. Depletion of 5-HT by inhibiting tryptophan hydroxylase activity produces insomnia. Treatment with tryptophan accelerates sleep onset and increases sleep duration.
• Aggression:
  low CSF [5-HIAA] is correlated with violent impulsive behaviour. Knockout mice without 5-HT1B exhibit extreme agro behaviour.
• Appetite:
  inhibitors of SERT can be used as appetite suppressant to treat obesity.
• Migraine:
  5-HT agonists are used for acute therapy

Question 13

Serotonin/noradrenaline and
the treatment of depression

Answer 13

Major Depressive Disorder is linked to an imbalance or reduced levels of activity of serotonin and possibly noradrenaline.

Antidepressant medication is geared to increasing synaptic concentrations of these two monoamines by either antagonising monoamine uptake transporters or by antagonizing the metabolism of serotonin and noradrenaline.

Question 14

Pharmacological use of 5-HT infrastructure

Answer 14

• buspirone , 5-HT 1A receptor agonist used to treat anxiety;
• ‘triptans’ (e.g. sumatriptan), 5-HT1D agonists used to treat migraine;
• 5-HT 2 antagonists (e.g. pizotifen) used for migraine prophylaxis;
• selective serotonin uptake inhibitors (e.g. fluoxetine) used to treat depression;
• ondansetron, a 5-HT 3 antagonist, used to treat chemotherapy-induced emesis;
• MDMA (ecstasy), a substrate for the 5-HT transporter. It then displaces 5-HT from nerve terminals onto 5-HT receptors to produce its mood-altering effects.

Question 15

Dopaminergic (DA) pathways in the brain

Answer 15

DA pathways in the brain include the mesocortical pathway from DA neurons in ventral tegmental area (VTA) to cortex, the mesolimbic pathway from VTA to nucleus accumbens, the nigrostriatal pathway from substantia nigra to the striatum, and the tuberoinfundibular pathway
from hypothalamic nuclei (arcuate nucleus and periventricular nucleus) to the pituitary.

Question 16

Dopamine synthesis

Answer 16

diagram on my ipad
- L-Tyrosine
- L-DOPA
- dopamine

Question 17

Dopamine metabolism

Answer 17

diagram on ipad
- COMT +MAO
- aldehyde dehydrogenase
- COMT
and stuff

Question 18

Dopamine reuptake transporters
(DATs)

Answer 18

• This is an energy-dependent process.
• Na+/K+-ATPase creates a concentration gradient of ions across the pre-synaptic membrane that drives the opening of the transporter and co-transport of Na+ and Cl- ions and dopamine from the synaptic cleft.
• K+ ions binding to the transporter enable it to return to the outward position.
• The dopamine transporter (DAT) is then available to bind another dopamine molecule for re-uptake

Question 19

D1 and D2 receptor structures

Answer 19

D2-like receptors have a shorter (1/7)
COOH-terminal tail and a bigger 3rd
intracellular loop (I3).
Amino acid residues involved in
dopamine binding are indicated in
transmembrane domains.
Differences between loops and carboxy termini indicate differing second messenger mechanisms of
action.

Question 20

Signal transduction of dopamine receptors

Answer 20

D1 and D5 receptors link through Gs to stimulate adenylyl cyclase and activate protein kinase A (PKA). PKA mediates many of the effects of D1 and D5 receptors by phosphorylating a wide array of proteins, including voltage-activated Na, K and Ca channels, as well as ionotropic glutamate and GABA receptors. D2 , D3 , and D4 receptors link through Gi /Go and activate K channels as well as inhibiting Ca channels and adenylyl cyclase, and can also affect other cellular second messenger cascades. Dimers too!

Question 21

Location of dopamine receptors in synapses

Answer 21

• D1 receptors are almost exclusively
  postsynaptic.
• D2 and D3 receptors can be both
  pre- and post-synaptic.

D2 receptors are the predominant
“autoreceptor”, and activation of presynaptic D2 receptors leads to
decrease in dopamine release, whereas activation of postsynaptic D2 receptors leads to stimulation.

Question 22

What are these main dopaminergic tracts responsible for?

Answer 22

• Nigrostriatal: controls motor function and movement (PD).
• Mesolimbic: involved in pleasurable
  sensations, the powerful euphoria of
  drugs of abuse, as well as delusions and hallucinations of psychosis.
• Mesocortical: has a role in mediating cognitive symptoms and affective aspects of schizophrenia.
• Tuberoinfundibular/tuberohypophyseal: inhibits prolactin secretion via
  dopamine release. Many antipsychotics, which block D2 receptors, increase prolactin secretion, leading to breast development and lactation, even in males.

Question 23

Dopamine’s role in nausea and vomiting (emesis)

Answer 23

• Vomiting is a reflex that is integrated by a complex loose neuronal network (vomiting centre (VC)) in the medulla.
• The chemoreceptor trigger zone (CTZ) has connections with the VC, and can detect toxins in blood outside the BBB.
• Emesis is a common adverse effects of dopamine agonists, such as bromocriptine or levodopa.
• Receptors involved are thought to be D2 type, located in the CTZ.
• Dopamine D2 type receptor antagonists thus have antiemetic activity.

Question 24

Disorders associated with
dopaminergic tracts and function

Answer 24

Parkinson’s Disease: loss of dopaminergic neurons in substantia nigra (nigrostriatal tract), leading to rigidity, bradykinesia and tremors;

Substance abuse: may be linked to release of dopamine caused by drugs such as nicotine, cocaine, cannabis, heroin, morphine, and many others (next lectures);

Schizophrenia: increase and decrease of dopamine in various areas of the brain, primarily in mesolimbic and mesocortical pathways;

Depression: decrease in dopamine in several brain regions;

ADHD: decrease in dopamine signalling in anterior frontal cortex, a
region associated with attention and concentration.

Question 25

Case study: cocaine, amphetamine, and monoamines

Answer 25

Amphetamine use results in euphoria, insomnia, psychomotor stimulation, anxiety, loss of appetite, decreased fatigue, increased concentration, respiratory stimulation.
It also produces sympathomimetic effects: mydriasis, tachycardia, and hypertension.
The euphoria is largely due to DA effects, whereas the jittery and anxious feelings are largely due to the NA effects.
Both compounds are used medically for ADHD, narcolepsy, and obesity.
Crystal meth (ice) is preferred by abusers as it has fewer NA effects.

Question 26

At the cellular level, cocaine works as
a local anaesthetic

Answer 26

there you go

Question 27

cocaine taken systemically

Answer 27

If taken systemically, cocaine crosses the BBB and blocks reuptake transporters

Increased NA levels result in vasoconstriction (necrosis of septum)
and heart arrhythmias, strokes, seizures, headaches and possible death.
Increased DA levels can result in dependence.

Question 28

Amphetamine

Answer 28

is close in structure to dopamine, and is compatible with DAT, competing with DA for reuptake. Once in the cell, Amph has great affinity for the VMAT and interferes with the loading of synaptic vesicles. Vesicles are thus depleted and cytoplasmic DA concentration increases. Increased [DA] can lead to DAT reversing direction as well as non-vesicular release of DA, further increasing extracellular DA concentrations.

Question 29

Methamphetamine

Answer 29

• Was developed early in 20th century from amphetamine and was used initially in nasal decongestants and bronchial inhalers.
• It causes increased activity and talkativeness, decreased appetite, and euphoric sense of well-being.
• Methamphetamine differs from amphetamine in that, at comparable doses, much greater concentrations cross the BBB, making it a more powerful stimulant, and it also has a much longer effect.

Question 30

Methamphetamine vs cocaine

Answer 30

table in ipad

Question 31

Short and long-term effects of
methamphetamine “ice”

Answer 31

short-term:
- increased attention and decreased fatigue
- decreased appetite
- euphoria and rush
- increased respiration
- increased activity and awakeness

long-term:
- addiction
- psychosis including paranoia, hallucinations, repetitive motor activity
- changes in brain structure and function
- deficits in thinking and motor skills
- memory loss
- weight loss

Dopamine is the key as this is involved in motivation, pleasure and motor function.