Serotonergic Systems and Receptors Flashcards
Seratonergic neurons project from a number of locations in the brainstem and innervate the entire forebrain. Most interesting are located in the
raphe nuclei (~165,000 seratonergic cells
Caudal raphe nuclei Seratonergic projections to
the cerebellum and descending projections to the spinal cord.
Seratonergic projections to the cerebellum and descending projections to the spinal cord. mediate
sensory, motor, and autonomic functions of 5-HT
Rostral raphe nuclei Ventral projections to
basal ganglia, substantia nigra, VTA, limbic system and cortex.
Rostral raphe nuclei Dorsal projections to
midbrain (tegmentum and tectum).
Rostral raphe nuclei Cerebellar projections to
cerebellar cortex and deep cerebellar nuclei
para-chloroamphetamine, MDMA both have
neurotoxic effects selectively on seratonergic pathways
MDMA is
neurotoxic at high doses or via microinjection
5,7-dihydroxytryptamine (5,7-DHT) is a
BBB impermeable selective toxin that can induce robust serotonergic injury
Seratonergic lesions produce
deficits in food intake, reproductive behaviour, pain sensitivity, anxiety, learning, memory, and motor function
Microelectrodes implanted into the dorsal raphe nuclei in free moving cats
While awake a steady firing pattern is observed Sudden sensory stimulus causes an abrupt cessation of output from the dorsal raphe nuclei
Seratonergic firing patterns in the dorsal raphe suggests
activity during repetitive movement, Proposed to suppress sensory processing and facilitate repetitive tasks
5-HT is colloquially thought to contribute to
happiness, Antidepressants target 5-HT reuptake and improve mood
5-HT binding capacity has been suggested to correlate with tendency towards
spirituality
5-HT1 – GPCR – Gi
(↓ cAMP, GIRK)
5-HT2 – GPCR – Gq/11
(PLC, ↑ Ca2+)
5-HT3
Ligand gated Na+ and K+ channel
5-HT4 – GPCR – Gs
(↑ cAMP)
5-HT5 – GPCR – Gi
(↓ cAMP , GIRK)
5-HT6 – GPCR – Gs
(↑ cAMP)
5-HT7 – GPCR – Gs
(↑ cAMP)
5-HT1A Agonists
Buspirone, Cannabidiol - partial agonist)
Agonists at 5-HT1A cause
hyperphagia
Agonists decrease 5-HT release
Prevents
attenuation of appetite
5-HT1A Concentrated in
hippocampus, amygdala, and septum
Agonists at 5-Inhibitory neurotransmitter
Signals through Giα to inhibit adenylate cyclase
Signals through Gβγ to specialized inhibitory K+-channels (GIRK – G-protein coupled inward rectifying K+-channel)
WAY-100,635
Experimental selective 5HT1A receptor agonist
WAY-100,635 used as
as a PET ligand to identify 5HT1A receptors in the human brain
Spirituality
Proposed 5-HT system was the source of individual variation in spiritual zeal
5-HT1B and 1D
Inhibitory through cAMP and GIRK
1B not expressed in
humans, instead there are two 1D variants (1Dα and 1Dβ)
5-HT1B and 1D are found
on intracranial blood vessels
5-HT1B and 1D Agonists include
anti-migraine medicationq
anti-migraine medication
Sumatripan (Imitrex) agonized 1B and 1D receptors
Migraine pathogenesis
Cortical spreading depression Expanding pulse of activity followed by hypoactivity
migraines often originate
originates in occipital cortex (causing perception of auras)
Migraine pathogenesis - Cortical spreading depression accompanied by
constrictions in blood flow
Reactive vasodilation causes
pain
Reactive vasodilation causes pain
Sensory fibers release vasodilating peptides
Peptides promote a sterile inflammatory response
Inflammation causes sensitization of sensory fibers of the dura mater and meningeal blood vessels which are well innervated by pain fibers
5-HT modulates
neuropeptide release
5HT1D receptors are
inhibitory
Agonists of 5HT1D inhibit release of
vasodilating peptides
Agonists of 5HT1D inhibit release of vasodilating peptides
Promotes vasoconstriction rather than vasodilation
Agonists of 5HT1D inhibit release of vasodilating peptides
Promotes vasoconstriction rather than vasodilation Leads to
decreased excitation of trigeminal nerve and decreased effect on nausea centres
5-HT2 receptor family
Gqα signal to PLC resulting in increased Ca2+ and PKC activation
5-HT2 receptor family Mostly function as
postsynaptic receptors
5-HT2 receptor family High densities in
nucleus accumbens, striatum, cortex (esp. frontal)
5HT2A Experimental agonists DOI
DOI administration to rodents leads to a characteristic head twitch (brief and periodic)
Antagonists of 5HT2A
ketanserin, clozapine (Cozaril) and risperadone (Risperdal)
ketanserin
antihypertensive and useful as a PET radioligand for 5HT2A receptors
Atypical antipsychotics
clozapine (Cozaril) and risperadone (Risperdal)
Psychedelics affecting 5HT2 receptors
LSD, mescaline, psilocybin are all agonists at 5HT2 receptors (particularly 5HT2A)
5HT2A agonists are considered
potent hallucinogens
5-HT2B and 2C receptors affect appetite
fenfluramine (norfenfluramine) is an agonist for 5-HT2B and 5HT2C (and 2A at sufficient dose)
Lorcaserin (Belviq)
is a specific agonist for 5-HT2C and anorectic drug in limited use to treat obesity
5-HT2C is also expressed in the
choroid plexus and regulates CSF composition and volume in response to 5-HT in the CSF
5-HT2C in the amygdala is
anxiogenic and proposed to be the cause of acute side effects of SSRIs
5-HT3 receptors
only 5-HT ligand-gated ion channel
5HT3 receptors are expressed in the
chemoreceptor trigger zone and in the gut
Agonists of 5-HT3 receptors are
vomit inducing
Agonists of 5-HT3
2-methyl-5-HT
Antagonists of 5-HT3 receptors are
anti-emetic
Ondasetron (Zofran)
is a prescription anti-emetic used to treat chemotherapy-related nausea
Subdivisions of the DRN have been implicated in
anxiety, panic, and mood disorders
anxiety in the Dorsal Raphe nucleus
DRN, dorsal part (DRD)
DRN, caudal part (DRC)
Panic in the Dorsal Raphe nucleus
DRN, ventrolateral (DRVL)
ventrolateral periaqueductal gray (VLPAG)
Serotonergic neurons in the DRN are active in response to
anxiogenic stimuli
Serotonergic neurons in the DRN are active in response to anxiogenic stimuli Anxiogenic agents
(caffeine) increase activity
Social defeat paradigms increase
DRN activity
DRD activated in
fear-potentiated startle
DRC activated by
unpredictable noise stress
Antidepressants targeting serotonergic neurotransmission are effective
anxiolytics
Antidepressants targeting serotonergic neurotransmission are effective anxiolytics
MAOI, TCA antidepressants are highly effective but with severe side effects
Antidepressants targeting serotonergic neurotransmission are effective anxiolytics ones with fewer side effects
SSRI and SNRI are effective and well tolerated
5-HT depletion (via tryptophan depletion)
increases depressive symptoms
5-HT1A receptors function primarily as
autoreceptors
Autoreceptors
drive negative feedback to inhibit 5HT release
5-HT1A agonists are effective
anxiolytics
5-HT1A agonists are effective anxiolytics
Cannabidiol (5-HT1A partial agonist)
Buspirone (5-HT1A partial agonist)
Polymorphisms of the 5-HT1A receptor gene
are implicated in risk of depression and anxiety
Predicts poor response to SSRI therapy
In the presence of SSRI antidepressants 5-HT is
broadly increased
In the basolateral amygdala
5-HT activates 5-HT2C receptors
5-HT2C activation is
anxiogenic
5-HT2C receptors are
downregulated slowly with SSRI use
In the basolateral amygdala 5-HT activates 5-HT2C receptors Proposed to play a role in the
early anxiety and depression in initial SSRI use