PPA Module 2 (Lectures 2.6-2.9)

Catecholamine Synthesis

NE Synthesis

Oxidized from dopamine by dopamine β-hydroxylase in cytosol

o Reaction rate limited only by supply of dopamine, coming from a multi-step process

Movement of NE across membranes requires ___________________

Active Cotransport

NE is stored in synaptic vesicles by

-vesicular monoamine transporter (VMAT1 or 2)

− Ion co-transport used, exchanging H+ pumped into vesicle by vesicular-ATPase for NE

Reuptake of NE

-Reuptake from the synaptic cleft by the norepinephrine transporter (NET) terminates signaling

~90% recovery. Ion co-transport required, coupling NE entry with influx of 1 Na + + 1 Cl

Key neurotransmitter of the sympathetic nervous system

Epinephrine

EPI synthesis

-Methylation from NE by phenylethanolamine N-methyltransferase (PNMT)

-PNMT found mostly in endocrine cells. Stress indirectly increases EPI synthesis

-PNMT reaction rate limited only by supply of norepinephrine

EPI transport

Transport: EPI storage and reuptake is performed by VMAT and NET as described with NE

NE and EPI are neurotransmitters for _______receptors

Adrenergic (all metabotropic 7-TM GPCR-class receptors)

Alpha 1 Receptors

-apha1A, alpha1B, alpha1D

-Gq

-Adrenergic receptors

-Excitatory

Alpha 2 Receptors

-alpha2A,alpha2B,alpha2C

-Gi

-inhibitory

-dec cAMP

-Adrenergic receptors

Beta Receptors

-Beta1,Beta2,Beta3

-Excitatory

-Gs

-inc cAMP

-Adrenergic receptors

Alpha 1 Drugs

-Phenylephrine:alpha 1 agonist

-Prazosin: alpha 1 blocker

Alpha 2 Drugs

-Clonidine: alpha 2 agonist

-Yohimbine: alpha 2 antagonist

Beta Drugs

-Isoproternol: beta agonist

-Propranolol: Beta blocker, Beta antagonist

Uptake Inhibitors

block amine transporters to ↑NT, ADHD medication Atomoxetine

False Transmitters

packaged like and replace NT, but weak (or no) receptor affinity (phenylethylamine)

Degredation of NE and EPI

NE, EPI metabolized by monoamine oxidase (MAO) and catechol-O-methyl transferase (COMT)

EPI in CNS located in

medullary reticular formation

NE location in CNS

containing neurons concentrated in two pons-medulla tracts:

1)Locus Coeruleus:

-Regulate blood pressure via α2A centers in brain stem, ↑ baroreceptor reflex

-Project widely in brain, with wide ranging effects!

2) Lateral Tegmentum: Regulate nociceptive input: (analgesia): descending path activates SC interneurons to inhibit pain input

NE action on arousal

NE from the reticular activating system (RAS) activates α2A in the prefrontal cortex

Activation increases attention and focus and enhances sensory processing, increases cognition

NE action on memory

Enhances formation and retrieval of both long-term and working memory

NE action on cognitive performance

Enhances sensory processing, and brain's response time to input

NE action on reward

Effects mixed. Increases substance reward, but α1 activation decreases appetite.

NE action on mood

α2 agonists (e.g., clonidine) reduce anxiety, panic and PTSD. α2 antagonists may

be antidepressant (ex: yohimbine). MAO inhibitors ↑NE+DA: these are established antidepressants

Tyramine

Another tyrosine metabolite, also found in

fermented foods: beer, cheese, sausage, red wine

-Acts as false transmitter (weak "NT")

-MAO inhibitors counter indicated: MAO-I ↑ effect

Phenylephrine

α1 agonist. Decongestant.

Prazosin

α1 blocker; acts as inverse agonist. Treatment for high blood pressure; maybe alcoholism?

Clonidine

α2 agonist. Treatment for high blood pressure, ADHD, anxiety, PTSD; modest analgesic

Yohimbine

α2 antagonist (and others). Possible antidepressant; studied for erectile dysfunction.

Isoproterenol

nonselective β agonist. Treatment for bradycardia; historically used to treat asthma

Propanolol

"beta blocker"; nonselective β antagonist. Treatment for high blood pressure; migraine, anxiety.

Atomoxetine

NET inhibitor, treatment for ADHD.

The only small molecule neurotransmitter not an amino acid or synthesized from one.

Acetylcholine

The major NT at neuromuscular junctions and ANS, acts as CNS NT and neuromodulator

Acteylcholine

Cholinergic agonists

Nicotine

Muscarine

Cholinergic agonist toxic stimulant from Solanaceae

Nicotine

Cholinergic Agonist A convulsant from A. muscaria:

Muscarine

Synthesis of ACh

-acetylation of choline at the -OH catalyzed by choline acetyltransferase (ChAT)

-Consumes Acetyl coenzyme A, a key molecule in fatty acid metabolism + citric acid cycle

-While the only biosynthetic step, the limit on rate is availability of choline

-Choline is obtained primarily from diet. Used in cell membrane synthesis, but N+ charge prevents passage.

-Choline taken up into cell by Na + cotransport via choline transporter (CHT1)

ACh Storage

-Presynaptic ACh stored in vesicles by vesicular acetylcholine transporter(VAChT),

-using ion co-transport to exchange pumped H+ for acetylcholine

ACh Transport

Once released into the synaptic cleft, charged acetylcholine cannot re-enter the

cell, and is NOT transported.

ACh signaling termination

Acetylcholine signaling is terminated by metabolism in the synaptic cleft

Metabolism of ACh

Acetylcholinesterase converts ACh into choline and acetate

-Acetylcholinesterase (AChE) is abundant in synaptic cleft and works extremely fast (< 1msec)

-A number of neurotoxins (Sarin, VX) and pesticides inhibit AChE, causing paralysis + heart failure

Nerve Agents (normal function)

1)nerve endings release ACh

2)when ACh touches receptor on relaxed muscle tissue it contracts

2)AChE sticks to ACh

4)AChE disables ACh causing muscle to relax

Never Agents

1)nerve agents disrupt balance of ACh system

2)ACh released as normal

3)nerve agents stick to and disable AChE

4)result is ACh build up muscles go into overdrive

Which Cholingergic Receptor:

-ion channel

-excitatory

-↑Na+ influx,↑K+ efflux; some ↑Ca2+ influx

-Depolarizes very fast (msec)

-Nicotine an agonist

-Composed of 5 subunits forming pentameric channel w/pore

-ACh-gated

-17 subunits known to exist: 10α, 4β, γ, δ, ε

-Brain: ratio typically 2 α + 3 β

-Composition dictates drug affinity, Ca 2+ permeability, kinetics

-Non-selective cation channel, bu

Nicotinic acetylcholine receptors: (nAChR)

M1 mAChRs

M 1: Excitatory: Gq-linked: ↑PLC, ↑IP3, DAG

M2 mAChRs

M 2: Inhibitory: Gi-linked: ↓AC, ↓cAMP

M3 mAChRs

M 3: Excitatory: Gq-linked: ↑PLC, ↑IP3, DAG

M4 mAChRs

M 4: Inhibitory: Gi-linked: ↓AC, ↓cAMP

M5 mACHRs

M 5: Excitatory: Gq-linked: ↑PLC, ↑IP3, DAG

Scopolamine

Non-selective mAChR antagonist

Atropine

Non-selective mAChR antagonist

Curare

non-selective nAChR antagonist

Dihydro-β-erythrodine

(α4) 2(β4) 3 antagonist

α-bungarotoxin

Antagonist at (α7) 5 nAChR

Type (α7) 5 (nAChR)

Passes Na +, K+ and Ca2+

Mediates pro-cognitive effect

Type (α4) 2(β4) 3 (nAChR)

Passes Na + and K+ (not Ca2+ )

Mediates rewarding effects of nicotine

hort interneurons in caudate/putamen

basal ganglia

o Interneurons in striatum use Ach; M4+M5 AChR

o Regulate motor output: respond to environment

with established motor responses.

o Inhibited by DA neurons of substantia nigra

o Parkinson's Disease: Loss of DA inhibition

causes excess ACh, producing resting tremors

ACh-containing neurons form two major pathways in brain:

1)Pontomesencephalotegmental complex:

2)Basal forebrain:

Which ACh containing neural pathway:

-Cholinergic cell bodies in brain stem

pedunculopontine nucleus + laterodorsal

tegmental nucleus;

-Activity mediated by M1AChR.

-Increases autonomic outflow; increases secretions

-Regulates pain: M2, M4AChR activity ↓ pain through

nociceptor inhibition. AChE inhibitors ↑ACh, ↑ analgesia

Pontomesencephalotegmental complex:

Which ACh containing neural pathway:

-cholinergic cell bodies originate

from basal optic nucleus of Meynert and septal

nucleus.

-Major brain acetylcholine system;

regulates/modulates most system

-Neuromodulation: AChR autoreceptors decrease NT release

-(α4) 2(β4) 3 nAChR on A10-DA path:

mediate nicotine, drug reward

Basal Forebrain

ACh action: Cognition

Activity of M1, M2 receptors in cerebral cortex and hippocampus increase cognitive

performance and improve memory. Basal forebrain output correlated with improved learning and memory

- (α7) 5 nAChR activity associated with improved cognitive performance.

- Scopolamine: antagonizes mAChR in brain, produces delirium and amnesia.

- Alzheimer's disease: Loss of cholinergic neurons a hallmark of AD. Damage and dysfunction of

cholinergic system in brain correlates with the progression of memory deficits.

ACh action: attention

Increase acetylcholine in cortex improves response time and processing of sensory stimuli

ACh action: arousal

Cholinergic systems regulate waking/sleep; ACh promotes REM sleep.

ACh may enhance sensory perception to facilitate waking. Effect may be neuromodulatory in basal forebrain

Nicotine

Agonist for all nicotinic-family acetylcholine receptors. Stimulant, addictive, pro-cognitive?

Scopolamine

Non-selective mAChR antagonist. WHO med. Used to treat motion sickness, nausea, vomiting

Muscarine

Agonist for all muscarinic-family acetylcholine receptors.

Organophosphates/Sarin

AChE inhibitors. Pesticides; deadly poisons.

Atropine

Non-selective mAChR antagonist. WHO med. ↑HR; counteracts AChE inhibitors. Hallucinogen.

Curare

non-selective nAChR antagonist. Weakens skeletal muscles, paralyzing diaphragm; asphyxiation

A major mediator of anaphylaxis and inflammation; mediates gastric acid secretion. (Autocoid)

Histamine

Histamine Synthesis

histamine is formed by decarboxylation of the essential amino acid histidine,

catalyzed by the enzyme L-histidine decarboxylase

A major mediator of anaphylaxis and inflammation; mediates gastric acid secretion also acts as a neurotransmitter in the CNS.

Histamine

Once formed, histamine is either _____ or ______

rapidly stored or rapidly inactivated

Histamine is rich at sites of potential injury, bound in granules of ______ and ____

mast cells and basophils

________ cells of stomach: release histamine to ↑ stomach acid from parietal cells

Enterochromaffin-like Cells

Where is histamine found in the brain

tuberomammillary nucleus of hypothalamus

Histamine is stored in synaptic vesicles by:

Vesicular Monoamine Transporter (VMAT)

What are the 2 major pathways of Histamine metabolism?

1)Histamine-N-methyltransferase, then monoamine oxidase (primary method, terminates histamine signaling in synaptic cleft)

2)Diamine Oxidase, then phosphoribosyl transferase (less common)

Histamine is a NT for which receptors?

4 Histamine Receptors (H1-H4)

H1 Receptor: Location, Agonists, Antagonists, G-Protein, excitatory or inhibitory?

Location: smooth muscle, tuberomammillary nucleus, PNS: sensory neurons

Agonists: 2-CH3-Histamine

Antagonists:Cetirizine

G-Protein: Gq

Excitatory

H2 Receptor: Location, Agonists, Antagonists, G-Protein, excitatory or inhibitory?

Location: PNS:smooth muscle, parietal cells (stomach)

Agonists: Amthamine

Antagonists: Ramitidine, Famotidine

G-Protein: Gs

Excitatory

H3 Receptor: Location, Agonists, Antagonists, G-Protein, excitatory or inhibitory?

Location: presynaptic CNS, cortex and subcortex

Agonists: (R)-alpha-CH3-histamine

Antagonists: Pitolisant

G-Protein: Gi/Go

Inhibitory

H4 Receptor: Location, Agonists, Antagonists, G-Protein, excitatory or inhibitory?

Location: Basophil, Thymus

Agonists: 4-CH3-histamine

Antagonists: Thioperamide

G-Protein: Gi/Go

Inhibitory

Actions of Histamine cells in periphery

-inflammation response

-Sexual function and libido

-Promotes gastric acid secretion

Histamine action in inflammation response

Mast cell + basophil granules release histamine ↑swelling, vasodilation, otherautacoids. Causes hypersensitivity to pain. ↑bronchoconstriction; mediates allergy responses.

Histamine and H4 receptor agonists strong promotors of pruritus(itching sensation) and burning feeling

Histamine action in sexual function and libido

H2R antagonists produce erectiledysfunction; direct histamine injection reverses

Histamine action on gastric acid secretion

Enterochromaffin-like (ECL) cellsin the stomach release histamine, stimulating H2 receptors onparietal cells, causing release of stomach acid from those cells.

Histamine release dec/inc with pH

decreases

H2R antagonists block → inc/dec acid

decrease

Where in the CNS do H4 receptors cause pruritis?

This effect is mediated by peripheral H4receptors.

Which Histamine receptors are found in the CNS?

Neurons containing H1 and H3 receptors identified in CNS

In the Histamine CNS Tuberomammillary pathway, which histamine receptors are involved?

H1 receptors + histamine as NT

Origin/Projections of the Histamine CNS Tuberomammillary pathway

Originates in tuberomammillary nuclei of hypothalamus, projects to dorsal raphe, locuscoeruleus, cortex, amygdala, nucleus accumbens, pituitary, hypothalamus

How do Projections of the Histamine CNS Tuberomammillary pathway to basal forebrain and pons affect the release of ACh

they increase ACh release

Actions of the Histamine CNS Tuberomammillary pathway

-Increased cortical activity

-Regulates noiciception

-Regulate neuroendocrine, body temperature and cardiovascular activity.

-H1 agonists inhibit appetite

-Neuromodulation

How does the Histamine CNS Tuberomammillary pathway act on cortical activity?

Increase cortical activity: promotes arousal, wakefulness

H1 agonists in cortex improve cognition; antihistamines ↑ sedation

How does the Histamine CNS Tuberomammillary pathway act on noiciception?

Regulates nociception: Activates inhibitory GABA system, thereby suppressing descendingpain control pathway in dorsal raphe and LC

Low concentrations of H1 receptors in PNS sensory nerve endings: promote hypersensitivity to pain

How does the Histamine CNS Tuberomammillary pathway act on neuromodulation?

Presynaptic H3 receptors in brain act as autoreceptors: reducing their own release. Theyalso decrease release of acetylcholine, NE, DA and serotonin to modulate nociception + signal satiety.