chapter 6 opener. synthesis and metabolism of ach synthesis –precursors choline – from dietary...

Chapter 6 Opener

Synthesis and metabolism of ACh

• Synthesis– Precursors

• choline – from dietary fat• acetyl coenzyme A (acetyl CoA) – from fats and sugars

– Enzyme• choline acetyltransferase

• Metabolism– Enzyme

• acetylcholinesterase– Metabolites

• choline• acetic acid

Drugs that affect ACh storage and release

• ACh is stored in vesicles– Put there by vesicular ACh transporter (VAChT)– Vesamicol

• Blocks VACHT– Less ACh in vesicles

– Thus, less available for release

• Suppresses REM sleep– Cholinergic system interacts with Thalamus to influence sleep

» Increased cholinergic activity = awake and REM

» Decreased cholinergic activity = nREM sleep

6.2 A cholinergic synapse

Acetylcholinesterase (AChE)

• One form is in the presynaptic cell to break down excess intracellular ACh

• Another form is present on the postsynaptic membrane.– Breaks down ACh in synapse

• Another form is neuromuscular junction– Muscle cells secrete the enzyme to breakdown the

ACh released by the corresponding neuron• Immediately after ACh causes muscle contraction AChE

removes it.

Choline transporter

• Choline is reuptaken by choline transporter

• Hemicholine-3 (HC-3)– Blocks choline transporter– Reduces ACh production– Impairs attention in rats

Block metabolism

• Drugs that block AChE (reversible effects)– Physostigmine– Increases levels of ACh

• Slurred speech• Hallucinations• convulsions

Neostigmine and pyridostigmine

• Synthetic analogs of physostigmine– Do not cross blood-brain barrier– Also reversible– Used to treat myasthenia gravis

• Autoimmune disorder• Antibodies attach to ACh receptors in muscle• Eventually the receptors are broken down• Lack of receptors leads to insensitivity to ACH

– Severe weakness– Fatigue

– Blocking AChE prolongs ACh activity which stimulates the remaining receptors.

6.4 Myasthenia gravis, an autoimmune disorder

Irreversible AChE inhibitors

• Insecticides– Weak versions

• Nerve Gas– Sarin and Soman

• Strong irreversible AChE inhibitors

– over-activates cholinergic synapses• Sweating• Salivating• Vomiting• Convulsions• Death by asphyxiation due to paralysis of diaphram muscles

Antidote for nerve gas

• Pyridostigmine bromide (PB)– Reversible AChE inhibitor protects against nerve gas– Apparently the reversible inhibition protects from the irreversible

inhibition.– Must be administered ahead of time

• During first gulf war soldiers were instructed to take 3 tablets daily when at risk for nerve gas

• Animal studies had shown low risk for crossing blood brain barrier (BBB).

• Unfortunately it appears that stress can increase the level at which this drug crosses the BBB– Forced swim test in rats.– Gulf war syndrome?

Forced Swim Test

6.5 Stress increases pyridostigmine entry into the brain

Botulinum Toxin

• Blocks release of ACh at neuromuscular junction– Stops the vesicles from cleaving to the presynaptic

membrane– Very potent poison– Muscle weakness– Paralysis– Can lead to death by asphyxiation

• Rarely

• Now drugs have been created to treat muscle spasms– Purified botulinum toxin A (Botox)– Also used as a cosmetic treatment to treat wrinkles

Box 6.1 Botulinum Toxin—Deadly Poison, Therapeutic Remedy, and Cosmetic Aid

Cell bodies of cholinergic neurons in brain

• Interneurons in striatum (caudate and putamen)– Balance between ACh and DA affects

movement• If ACh activity in the striatum outweighs DA activity

that can lead to the symptoms of Parkinson’s disease

– Thus, anticholinergic drugs are sometimes used to treat Parkinson’s in the early stages of the disease.

6.7 Anatomy of cholinergic pathways in the brain

Basal forebrain cholinergic system (BFCS)

• These are cholinergic brain regions that send their axons to the forebrain, particularly frontal cortex regions.– Also hippocampus and Amygdala.

• All of these areas of the brain are involved in learning and memory

• Atropine and Scopolamine (anticholinergic drugs; muscarinic receptors) – Interfere with learning and memory in many types of

learning tasks

Lesions of BFCS disrupt cognitive functioning

• 192 IgG-saporin– 192 IgG is an antibody that binds specifically

to basal forebrain cholinergic neurons.– Saporin is a neurotoxin

• When injected into the ventricular system the BFCS neurons take in this substance.

• Those neurons are selectively destroyed• Affects learning and memory

– i.e., Berger-Sweeney et al. (1994)

4.20 The Morris water maze

6.8 Cholinergic lesions

• Note in the previous study that ventricular exposure was more disruptive– This exposure would affect the entire BFCS,

rather than just one part (as in injection only in the nucleus basalis).

Alzheimer’s disease

• It has been proposed that a portion of the cognitive decline that is seen in aging may be due to dysfunction of the BFCS

• Perhaps Alzheimer’s disease as well.– There is severe damage to the BFCS in Alzheimer’s disease.

• As well as other cortical regions and the hippocampus

• Early medications for Alzheimer’s disease were Cholinergic agonists– Tacrine (Cognex)– Donepezil (Aricept)– Rivastigmine (Exelon)

• These are all AChE inhibitors– Only somewhat effective

Acetylcholine receptor subtypes

• Nicotinic receptors– Respond to nicotine (ACh agonist)

• Concentrated at neuromuscular junction• Also in the sympathetic and parasympathetic nervous

system and parts of the brain

• Ionotropic receptors– ACh binding opens Na+ and Ca++ channels– Depolarizing effects– Thus, excitatory

• Muscles = contract• Neuron = increase in firing rate

Nicotinic receptors

• They have two binding sites for ACh– Both must be activated for the channel to

open.

• The affinity of nicotinic receptors in the brain and autonomic nervous system are greater than the affinity of nicotinic receptors on muscle cells.– Smokers

• Enjoy a smoke without having muscle spasms.

6.9 Structure of the nicotinic ACh receptor

Curare

• D-tubocurarine = active ingredient• Nicotinic receptor antagonist

– High affinity for receptors in muscles

• Used by south American Indians in poison darts.– Paralyzes animal and causes death because

of respiration failure

• Good for horror stories and movies – Person would be paralyzed, but still aware.

• Muscarinic receptors– Respond to muscarine (derived from a

particular mushroom; Amanita muscaria).• Metabotropic• 5 subtypes (m1-m5)

– Different subtypes activate different second messengers

– Often cause K+ channels to open

• Widespread throughout the brain.

M5 muscarinic receptor and opiate reward

• M5 may be related to the rewarding properties of morphine– Mutant mice that lack M5 receptors show

lowered place conditioning effects to morphine

• Could lead to pharmacological treatment of opiate conditioning

4.22 Place-conditioning apparatus

6.11 Genetic deletion of the M5 muscarinic receptor reduces the rewarding effects of morphine

Many drugs used to treat psychological disorders produce muscarinic side effects

• Muscarinic receptors common in autonomic nervous system– Particularly parasympathetic– Agonists = parasympathomimetic – mimic parasympathetic

action• Slows heart rate• Controls secretory responses

– Salivation– Sweating– Tearing

– Antagonists = parasympatholytic – prevent parasympathetic action

• Muscarinic blockade– Lack of salivation

» Tooth decay

• Many drugs used to treat psychological disorders produce muscarinic side effects– Pharmacologists are working to make drugs that are

less likely to activate muscarinic receptors• Atropine is a muscarinic antagonist commonly

used to dilate pupils – as we discussed last time.– Causes blurred vision

• Derived from Atropa Belladona – deadly nightshade– Very poisonous plant– Women placed juice of the berries in their eyes

• Cosmetic – dilated pupils

6.12 The deadly nightshade (Atropa belladonna)

Serotonin

• Serotonin = 5-hydroxytryptamine (5-HT)

• Synthesis– Trytophan (precursor)

• Converted by tryptophan hydroxylase

– To 5-hydroxytryptophan (5-HTP)• Converted by aromatic amino acid decarboxylase

(AADC)

– To 5-hydroxytryptamine (5-HT)

6.13 Synthesis of serotonin

• The first step is the rate limiting step.– Takes the longest

• Like we talked about with dopamine

– Thus, tryptophan hydroxylase is the rate limiting enzyme

• Only serotonergic neurons contain this enzyme.• Thus, labeling for tryptophan hydroxylase is one way to

identify serotonergic neurons

• Also notice that the enzyme in the second step is the same for catecholamines and indolamines– Aromatic amino acid decarboxylase (AADC)

Turkey Dinner Effect?

• Although many people believe that increasing tryptophan in the diet can increase serotonin levels, it is more complicated than that.– Turkey dinner effect – not true

• The ratio of tryptophan to other large amino acids determines the rate that tryptophan enters the brain– Competition for crossing the blood brain barrier.

• Consequently it is high carbohydrate and low protein diets that increases brain tryptophan levels– Insulin release is stimulated by high carb diet.– Insulin causes most amino acids to be removed from the blood

stream.• But not tryptophan

– Less competition for tryptophan, means that more crosses the BBB

6.14 Tryptophan entry into the brain and 5-HT synthesis

• Tryptophan depletion can cause the return of depressive symptoms in patients that have recovered from depression.– Indication of serotonin’s role in mood regulation.

• Amino acid cocktails – Without tryptophan

• Competition for crossing the BBB depletes brain tryptophan levels

– With tryptophan• Brain levels remain normal

6.15 Rapid tryptophan depletion leads to symptom relapse in recovered depressed patients

Storage and Autoreceptors

• VMAT2 controls storage in vesicles– Just like for the catecholamines– Thus, the drug reserpine disrupts storage of

serotonin in the same way it disrupts storage of catecholamines

• Remember the knocked out rabbits

• 5-HT1a, 5-HT1b or 5-HT1d receptor are the serotonin autoreceptors– Agonists of these receptors decrease serotonin

release

Some drugs can cause release independent of action potentials

• Amphetamine like drugs– Para-choloroamphetamine

• Drug for experimentation

– Fenfluramine• One time a prescribed appetite depressant

– 3,4-methylenedioxymethamphetamine (MDMA) – Ecstasy

Serotonin and eating behavior = Fen-Phen

• Fen – Fenfluramine– Causes release of serotonin

• Phen – phentermine– Thought to increase catecholamine activity

• The drug combination was an effective weight loss drug.

• Unfortunately causes heart problems

Box 6.3 Fen–Phen and the Fight against Fat

5-HT transporter

• Reuptake is performed by– 5-HT transporter

• Cocaine – not selective• Fluoxetine – Prozac – selective = SSRI

6.16 Features of a serotonergic neuron

Serotonergic system

• The Swedish system– 5-HT represented by the letter B

• Raphe nuclei – medulla, pons, midbrain– B7 – Dorsal Raphe– B8 – Median Raphe

• Give rise to most of the serotonergic fibers in the forebrain

• Lesioning the raphe nuclei disrupts things like food intake, reproductive behavior, pain sensitivity, anxiety, and learning and memory

6.17 Anatomy of the serotonergic system

Receptors

• 15 different receptors so far– 5-HT1 family

• 5-HT1a……5-HT1f

– 5-HT2 family• 5-HT2a, 5-HT2b, and 5-HT2c

– 5-HT3

– 5-HT4

– 5-HT5a and 5-HT5b

– 5-HT6

– 5-HT7

5-HT1a and 5-HT2a – best known

• 5-HT1a– Found in hippocampus, septum, amygdala,

and dorsal raphe– Also serve as autoreceptors

• Reduce synthesis of cAMP

• Increase opening of K+ channels

6.19 5-HT1A and 5-HT2A receptors operate through different signaling mechanisms (Part 1)

5-HT2a

• 5-HT2a– Numerous in cerebral cortex, striatum, and nucleus

accumbens– Activates protein Kinase C (second messenger)– Increases Ca++ levels in the cell (also can serve as a

second messenger system)

• Agonists are hallucinogenic– LSD thought to produce hallucinations via this

receptor

• Antagonists used as a treatment for schizophrenia

6.19 5-HT1A and 5-HT2A receptors operate through different signaling mechanisms (Part 2)