Psych 215Lecture 5a: Serotonin

Lecture 5a: 5-HT Objectives• Explain the derivation of the name “serotonin”• Describe the chemical features of 5-HT that distinguish it from catecholamines• Describe the two general 5-HT systems in the brain in terms of their projections• Distinguish between the projections of the D and M ascending 5-HT systems based on

morphology and sensitivity to toxins• Outline the pathway through which 5-HT is made from tryptophan, indicate which enzyme is the

rate-limiting step in 5-HT synthesis and highlight known regulatory mechanisms• Explain why altering dietary tryptophan levels can affect 5-HT levels• Describe the mechanisms of action of PCPA, fenfluramine, methamphetamine, MDMA (ecstasy),

LSD, reserpine, ondansetron, fluoxetine, paroxetine and sertraline as they relate to 5-HT transmission

• Explain how one could demonstrate that serotonin releasers such as fenfluramine act on non-vesicular stores of 5-HT

• Explain the effects of agonists and antagonists at 5-HT1A and/or 5-HT1B/1D receptors upon extracellular levels of 5-HT

• Describe the intracellular signaling pathway for 5-HT1 receptors and relate this signaling to their role in regulating 5-HT release.

• Describe the intracellular signaling pathway for 5-HT2 receptors and relate this signaling to their hallucinogenic profile.

• Describe some of the oddities observed regarding the regulation of 5-HT2 receptor expression by agonists and antagonists

• Distinguish 5-HT3 receptors from other 5-HT receptors in terms of structure, function, and localization in brain

• Explain the neuronal consequences of the repeated administration of 5-HT releasers and predict which brain regions/behaviors would be most likely affected

Indolamine-Serotonin

Isolated from blood and induced muscle contraction in heart=“serum” + “tone”=serotonin

Isolated from intestinal mucosa and induced muscle contraction=enteramine (secreted by enterochromaffin cells in GI tract)

-90% in GI; 8%-10% in blood platelets & 1-2% in brain (but several 100, 000 5-HT neurons in your brain)

-cannot cross the BBB

catecholamines

5-HT projections in brain

Nine 5-HT containing cell body groups located in the brain stem (B1-9)Two systems: caudal versus rostral systems

Caudal system: B1-4; medulla and central pons; project down spinal cord in several pathways (ventral horn, dorsal horn, preganglionic sympathetic cells rolein sensory, motor and autonomic functioning)Rostral system: B5-9; rostral pons and mesencephalon (dorsal and medial raphe, B6-B8, provide 80% of forebrain 5-HT) innervate cortex, basal ganglia, habenula, thalamus, hypothalamus, limbic system

Synthesis Tryptophan hydroxylase=rate-limiting step-made in cell bodies and transported to terminal for 5-HT synthesis-tryptophan crosses BBB and can be regulated by diet-regulated by PKA, CaMKII-gene regulated by cAMP/CREB although no CRE site-no end-product feedback-PCPA=irreversible antagonist (2 weeks)

AADC=very rapid enzyme (same as for converting DOPA to dopamine; found in terminal)

5-HT metabolism:2 processes:1-deamination & 2-oxidation

1. MAO-A: oxidative deamination of catecholamines-enzyme localized in synapse as well as in terminal; cytosolicMAO responsible for majority of break-down after re-uptake

2. Aldehyde dehydrogenase:5-HIAA which diffuses out of

terminals and into CSF-spinal tap for index of 5-HT release/reuptake

MAO-Ainhibitors

5-HT projections in brain

2 types of 5-HT fibers in the brain:

1-Thin; very numerous; many varicositiesthe “D” system (from dorsal raphe)More vulnerable to toxins (e.g., PCA & MDMA)Make less direct synaptic contact (varicosities) modulatory effects upon frontal cortex, striatal structures

2-Thick, large varicositiesthe “M” system (from medial raphe)Make more conventional synaptic contacts

hippocampus and septum

Terminals & Release:Resirpine: Blocks VMATDepletes 5-HT stores

5-HT releasers: PCA and fenfluramine, MDMA, methamphetamine and at very high doses amphetamine

-non-vesicular mechanism of releasing action because not blocked by reserpine-involves transporter because blocked by re-uptake inhibitors

5-HT toxicity:“D” fibers are particularly sensitive to releaser-type neurotoxins:

Releaser Releaser

5,7-DHT

Terminal degeneration; cell body sparing; get both M and D systems; Regrowth; Super-sensitivity to agonists

MDMA or fenfluramine: Selective for D system; Regrowth?

Auto-inhibition: Gi-coupled 5-HT receptors

-cell body level: 5-HT1A-terminal level: 5-HT1B/1D

Re-uptake:

Non-selective re-uptake inhibitors-bind DAT, NET and SERT

SSRI

Paroxetine=Paxil, Aropax, Oxetine, Aroxat, Cebrilin, Deroxat, Motivan, Paroxetina, Optipar, Paroxat, Pondera, Seroxat, Posivil, Pexot, Paraxyle, Plasare, Paradise CR

Sertraline=Zoloft, Sertralin, Lustral, Apo-Sertral, Asentra, Gladem, Serlift, Stimuloton, Xydep, Serlain, Concorz

Fluoxetine=Prozac

5-HT receptor subtypes:

5-HT1 receptors:

Anti-depressant Rx’s can reduce 5-HT1A and 5-HT1D/1B autoreceptor function

5-HT2 receptors:Antagonists Agonists

Agonists very rapid down-regulation; chronic state of super-sensitivity???Antagonists down-regulation of receptor function???inverse agonists?

5-HT3 receptors:

Antagonists

SSRIMDMA

methamphetaminefenfluramine

MDMAmethamphetamine

fenfluramine

reserpine

PCPA

MAO-A I’s*/*

*/*@ somatodendritic5-HT1A receptors

*/**/*LSD/anti-psychotics

ondansetron

Lecture 5b: Acetylcholine

Lecture 5b: ACh Objectives• Describe how Acetylcholine (ACh) is synthesized. • Describe the sources for the Acetyl CoA and Choline used for ACh

synthesis and discuss what limits ACh synthesis in brain.• Explain the role of acetylcholinesterase (AChE) in terminating ACh activity,

describe the factors regulating it and describe the neurochemical and behavioral effects of inhibition

• Compare and contrast muscarinic and nicotinic receptors in terms of their effectors systems and distribution and provide examples of agonists and antagonists of each

• Explain the structure of the nicotinic receptor and how it can give rise to at least 9 different subtypes.

• Explain and illustrate how drugs may interfere with ACh activity by effecting its synthesis, release, degradation or receptor activity.

• Explain why atropine can ameliorate the effects of AChE inhibitors.

Lecture 5c: Histamine Objectives• Describe how histamine can be synthesized • Compare and contrast the 3 histamine receptors and explain (1) why H1

antagonists make you drowsy and (2) why H2 compounds cannot be used currently to treat neuropsychiatric disorders

Acetylcholine (ACh)

The first neurotransmitter to be discovered.Otto Loewi, 1921

– All muscular movement is accomplished by the release of acetylcholine.

– Appears to be involved in regulating REM sleep, perceptual learning, and memory.

ACh Synthesis• ACh is synthesized in the cytoplasm from

Acetyl CoA and choline in a reaction catalyzed by the enzyme cholineacetyltransferase (ChAT).

Sources of Acetyl CoAand choline

Choline• Obtained from foods such as egg yolks, kidney, liver, seeds, and

various vegetables. It is also naturally produced by the liver.

• Free Cholie and lipid-bound choline circulating in blood plasma readily

crosses the BBB.

• Cholinergic nerve terminals uptake choline by a either a high affinity or

a low affinity choline transport system.

• The high affinity system is sodium-dependent and carrier mediated and

is unique to cholinergic cells. It is inhibited when the cell is

depolarized.

• Low affinity choline transport operate by passive diffusion; provides

many non-neuronal cells with the choline they require to carry out

processes such as the synthesis of various choline-containing

phospholipids

ACh synthesis

• Factors regulating ACh synthesis:

• negative feedback: ACh binds to ChAT and inhibits its catalytic ability.

• Law of mass action: ACh concentration is proportional to the availability of

Acetyl CoA, choline and choline acetylcholietransferase.

• The most important limiting factor is the rate of choline high affinity uptake.

• Drugs inhibiting Ach synthesis

• Juglone: ChAT inhibitor

• Triethylcholine (TEC): Inhibits high affinity choline uptake

• The latter is more effective, reflecting the major function of choline uptake in

synthesis

Termination of ACh activity

-Upon release, acetylcholine is hydrolyzed into choline and acetate by acetylcholinesterase (AChE), and, to a much lesser extent, by other nonspecific cholinesterases.

• AChE has an anionic subsite which attracts the positive charge of ACh and an estertic site which possess the serine residue that breaks the ester bond.

• Extremely efficient enzyme, hydrolyzes 5000 molecules of ACh per molecule of enzyme per one second.

• BUT expresses “excess substrate inhibition”-inhibited by high concentration of ACh

Irreversible acetylcholinesteraseinhibitors =“nerve gas”

Treatment•PAM: (2 pyridine aldoxime methiodide): displaces the inhibitor from the active site. However ,doesn’t cross the BBB. •Atropine: doesn’t effect the inhibitor at all, but balances the effect of AChE by blocking the muscarinic ACh receptors. Currently the most commonly used.

•Headache•Convulsions•Coma•Respiratory arrest •Confusion•Slurred speech•Depression•Respiratory d

•Reduced Vision•Small pupil size•Drooling•Sweating •Diarrhea•Nausea•Abdominal pain•Vomiting

•Twitching•Weakness•Paralysis•Respiratory failure

Central NervousSystem Effects

Autonomic NervousSystem Effects

Neuromuscular Effects

organophosphorous compounds –which form highly stable phosphorlyated complexes with AChE that persist for hours or more

Other pharmacological goodies• Botulinum toxin A:

prevents ACh

release paralysis

• α-latrotoxin (black widow

spider venom): promotes

massive vesicular ACh

release

• Hemicholinium and

vesamicol: prevent

choline reuptake lowers

ACh synthesis and stores

Cholinergic pathway: CNSAcetylcholine is widely distributed in the peripheral and central nervous

system. In the CNS, cholinergic neurons can be divided to several groups. :

Main groups of ACh groups

and pathways in the CNS

1. Interneurons within striatal

complex; regulated by

glutamate and dopamine

2. Basal forebrain (including

nucleus basalis, medial

septal nucleus, substantia

inominata); source of

projection neurons to

limbic system and cortex

Cholinergic pathway: PNSMotor systemsCholinergic neurons are found in the spinal ventral horn and

in cranial nerves 3-7 and 12. Thus, ACh is secreted in all neuromuscular junctions and stimulates muscle contraction.

Autonomic nervous systemThe principle transmitter by the parasympathetic and

sympathetic pregangalionic fibers.

Released by most of the parasympathetic post ganglionicinnervating glands and smooth muscles which will be excited or inhibited depending on the type of receptor they present.

Cholinergic receptorsCholinergic receptors can be divided into 2

major categories. Muscarinic receptorsoriginally were distinguished from nicotinic receptors by the selectivity of the agonists muscarine and nicotine respectively.

Basic differences between muscarinic and nicotinic receptors

All striated musclesPostganalionic

parasympathetic and sympathetic neurons

CNS

Parasympatheticallyinnervated cardiac and smooth muscles (heart, iris, stomach, bronchitis bladder etc).

Salivary, tear and sweat glands

CNS

distribution

TubocurarineAtropine, ScopolamineTypical antagonists

ExcitatoryMixedExcitatory/inhibitory

9 subtypes M1, M3, M5 =Gq coupledM2, M4=Gi coupled

Receptor subtype

IonotropicMetabotropicReceptor type

Fast (10<ms)Slow (100-250 ms)Response time

NicotineMuscarineAgonist

Nicotinic receptors antagonists: Curare

Curare was used as an arrow poison by indigenous forest peoples of South America to paralyze their prey.

The main active ingredient was tubocurarine-antagonists at the nicotinic receptor.

-alpha forms ACh and nicotinebinding site-cation channel-rapidly desensitizes upon agonist stimulation

Nicotinic ACh Receptor

EpibatidineABT-418

EpibatidineEpibatidineAgonists

α-BungarotoxinMecamylamine

Dihydro-β-erythroidine

Mecamylamine

Hexamethoniumα-BungarotoxinAntagonists

α7,α8,α9α3,α4,β2,β4α3,α5,α7,β2,β4α1,β1,δ,γ(ε)Subunits

CNSCNSAutonomic ganglion

Skeletal muscleReceptor

Nicotinic Acetylcholine Receptors

Muscarinic receptors

PLCβAdenylylcyclaseinhibition

PLCβAdenylylcyclaseinhibition

PLCβIntracellular response

Gαq/11Gi/oGαq/11Gi/oGαq/11G protein

CDD-0097Xanomelin

Selective Agonist

pF-HHSiDAF-DX 116ePirenzepineSelective Antagonists

Substantianigra

NeostriatumExocrine glands, GI tract

HeartCortex, hippocampus

Distribution

M5M4M3M2M1

Muscarinic Acetylcholine Receptors

Nerve gasPAM

vesamicol

NicotineCurare

Mecamylamine

MuscarineAtropine

Scopolamine

botulinum-Aα-latrotoxin

H1 antagonists: “anti-histamines” drowsiness; H2 antagnoists=anti-nausea; H3 antagonists increase wakefulness

AADC