Acetylcholine. Kevin K. Caldwell, Ph.D.

Size: px

Start display at page:

Download "Acetylcholine. Kevin K. Caldwell, Ph.D."

Annabel Poole
5 years ago
Views:

1 Acetylcholine Kevin K. Caldwell, Ph.D.

2 Classification of Neurotransmitters Peptides Adenosine ATP

3 Criteria for defining a neurotransmitter Purves, D. et al. Neuroscience, 4th edition, 2007

4 Synaptic vesicle release and recycling ATP ATP

5 ACETYLCHOLINE Biochemistry Anatomy Physiology Pharmacology Pathology

6 Acetyl-CoA + Choline Acetylcholine Synthesis Acetylcholine = CH 3 -C=O-O-CH 2 -CH 2 -N(CH 3 ) 3 choline acetyltransferase ChAT + Acetylcholine (ACh) Mitochondria Diet Re-uptake PC PC PLD PA + Choline

7 Acetylcholine Degradation: acetytlcholiesterase CH 3 -C=O-O-CH 2 -CH 2 -N(CH 3 ) 3 + AChE-OH HO-CH 2 -CH 2 -N(CH 3 ) 3 + AChE-O-C=O CH 3 H 2 O Acetylated enzyme intermediary Nerve gases, pesticides, and therapeutic drugs AChE-OH + HO-C=O CH 3 H2O AChE turnover number: 25,000 molecules/second

There are two types of cholinesterases Acetylcholinesterase ( (AChE), also known as RBC cholinesterase, erythrocyte cholinesterase, or (most formally) acetylcholinesterase.

In the mammalian brain the majority of AChE occurs as a tetrameric, G4 form with much smaller amounts of a monomeric G1 (4S) form Pseudocholinesterase ( (BChE or BuChE), also known as plasma

8 There are two types of cholinesterases Acetylcholinesterase ( (AChE), also known as RBC cholinesterase, erythrocyte cholinesterase, or (most formally) acetylcholinesterase. It is found primarily in the blood and neural synapses. Acetylcholinesterase exists in multiple molecular forms. In the mammalian brain the majority of AChE occurs as a tetrameric, G4 form with much smaller amounts of a monomeric G1 (4S) form Pseudocholinesterase ( (BChE or BuChE), also known as plasma cholinesterase, butyrylcholinesterase, or (most formally) acylcholine acylhydrolase, found primarily in the liver AChE butyrylcholinesterase Butyrylcholinesterases are important for the degradation of succinylcholine, a short-acting depolarizing neuromuscular blocking agent used as a muscle relaxant.

septal nuclei, striatum and in the dorsal tegmental area (DTA) Acetylcholine is also present in the autonomic

9 Localization of Acetylcholine in the CNS, PNS, and ANS Acetylcholine is the neurotransmitter in the neuromuscular junction Central cholinergic neurons are localized in the basal forebrain cholinergic complex, septal nuclei, striatum and in the dorsal tegmental area (DTA) Acetylcholine is also present in the autonomic ganglia: pre-ganglionic neurons in sympathetic ganglia and pre- and post- ganglionic neurons in the parasympathetic system

10 Physiology of Cholinergic Transmission

11 Physiology of Cholinergic Transmission

12 Acetylcholine receptors (AChRs) 2 types: Nicotinic (nachr): 1. Agonist: nicotine 2. Antagonist: d-tubocurarine Muscarinic (machr): 1. Agonist: muscarine, pilocarpine 2. Antagonist: atropine

13 Nicotinic Receptors nicotinic receptors are classified as: muscle neuronal ligand-gated ion channels: permeable to Na +, K + and sometimes Ca 2+ ions. pentameric structures that are homomeric or heteromeric combinations of seventeen (α1-α10, β1-β4, γ, δ,ε) similar, but genetically distinct, subunits. α4 and β2 are the most abundant nachr subunits in the brain nachrs containing α4β2 subunits are the most abundant high affinity nicotinebinding nachrs in the brain and mediate the rewarding and reinforcing effects of nicotine neuronal nachrs are localized both presynaptically (where they regulate the release of several neurotransmitters) and postsynaptically (where they reside extrasynaptically and influence cell excitability) The α-subunit constitutes the principal component of the binding site, whereas the adjacent subunit provides the complementary site.

14 Structural features of the nachr (Figure 13-9 from Brady et al. 8th ed.)

15 Cholinergic Receptors

16 Figure from Brady textbook

17 Nicotinic (ligand-gated channel) Ach- -ACh In the neuromuscular junction, the embryonic form of nicotinic receptors is composed of α1, β1, δ, and γ subunits in a 2:1:1:1 ratio, or the adult form composed of α1, β1, δ, and ε subunits in a 2:1:1:1

18 Toxins affecting Nicotinic receptors

19 2. Diseases Affecting Transmission at the NMJ A. Autoimmune Disorders Myasthenia gravis (MG) (Ag: nachr)

20 Muscarinic (G-protein coupled)

21 GPCRs GPCRs all posses a common structure: 1. an extracellular amino terminus 2. seven hydrophobic transmembrane (TM) segments 3. an intracellular carboxyl terminus. The GPCRs can be divided into five families based on sequence similarity within the 7 TM segments: 1. the rhodopsin family (~80-90% of all GPCRs, inlcuding the machrs) 2. the adhesion family 3. the frizzled/taste family 4. the glutamate family 5. the secretin family

22 machrs in brain M1 is the predominant form of the machr in the CNS; it is found postsynaptically (principally on dendrites) in the cerebral cortex, hippocampal formation, striatum and thalamus. M2 receptors are enriched in the brainstem and thalamus, but are also found in the cerebral cortex, hippocampal formation and striatum. Many M2 are presynaptic- on cholinergic terminals- serve as autoreceptors. M3 and M5 are expressed at much lower levels than M1 and M2. M4 are widely disributed throughout the brain; highest expression in the striatum, where they regulate DA release.

23 machr antagonists in the treatment of Parkinson s Disease PD is characterized by reduced number of dopaminergic neurons in the substantia nigra. This leads to an imbalance in striatal dopaminergic and choliergic NT, which is required for coordinated locomotor control. The cholinergic dominance is treated using an machr antagonist, such as bentropine or trihexyphenidyl.

24 TABLE EFFECTS OF ACETYLCHOLINE (ACh) STIMULATION ON PERIPHERAL TISSUES Mediated primarily by Muscarinic receptors Tissue Vasculature (endothelial cells) Eye iris (pupillae sphincter muscle) Ciliary muscle Salivary glands and lacrimal glands Bronchi Heart Gastrointestinal tract Urinary bladder Sweat glands Reproductive tract, male Uterus Effects of ACh Release of endothelium-derived relaxing factor (nitric oxide) and vasodilation Contraction and miosis Contraction and accommodation of lens to near vision Secretion thin and watery Constriction, increased secretions Bradycardia, decreased conduction (atrioventricular block at high doses), small negative inotropic action Increased tone, increased gastrointestinal secretions, relaxation at sphincters Contraction of detrusor muscle, relaxation of the sphincter Diaphoresis (excessive sweating) Erection Variable, dependent on hormone influence

Nerve gases Structure of VX VX gas is one of the most dangerous chemicals created. VX gas was developed in England in 1952. The "V" of VX signifies it long persistence.

In the liquid form of VX, it is absorbed through the eyes or the skin of the victim. It takes an hour or two to take effect and its effects result in death.

25 Nerve gases Structure of VX VX gas is one of the most dangerous chemicals created. VX gas was developed in England in The "V" of VX signifies it long persistence. So it is more dangerous and toxic than its cousins of the "G" variety like GA (Tabun) and GB (Sarin), which dissipate quickly and have only short-term effects. In the liquid form of VX, it is absorbed through the eyes or the skin of the victim. It takes an hour or two to take effect and its effects result in death. The gaseous form is more deadly than the liquid form and acts almost immediately on the victim. Since it does no have an odor, victims do not know they are exposed until it is usually too late. The LD50 can be as little as 10mg for humans 12 people died and more than 5,0o0 subway riders were injured in the sarin gas attack in Tokyo, March 20, 1995.

26 Clinical Signs of a Cholinergic Crisis resulting from nerve gas exposure Parasympathetic NS (Muscarinic AChR) Bronchospasm Nausea, vomiting, diarrhea Miosis, blurring of vision Hypersecretions of nasopharynx, trachea, conjunctivae and GI tract Bradycardia NMJ (Nicotinic) Fasciculations, Flaccid paralysis (NMJ)

Treatment of Mild to Moderate Exposure: Atropine Mild People exposed to a low or moderate dose of VX by inhalation, ingestion (swallowing), or skin absorption may experience some or all of the

27 Treatment of Mild to Moderate Exposure: Atropine Mild People exposed to a low or moderate dose of VX by inhalation, ingestion (swallowing), or skin absorption may experience some or all of the following symptoms within seconds to hours of exposure: Runny nose Watery eyes Small, pinpoint pupils Eye pain Blurred vision Drooling and excessive sweating Cough Chest tightness Rapid breathing Diarrhea Increased urination Confusion Drowsiness Weakness Headache Nausea, vomiting, and/or abdominal pain Slow or fast heart rate Abnormally low or high blood pressure Moderate Respiratory distress, wheezing Muscle fasciculations Urinary incontinence Fecal Incontinence A reversible AChE inhibitor Pyridostigme was also given as protective a agent during First Gulf war

28 Treatment of Severe Exposure to Nerve gases Exposure to a large dose of VX by any route may result in these additional health effects: Loss of consciousness Convulsions Paralysis Respiratory failure possibly leading to death Treatment 6 mg of atropine by autoinjector 2 ampules of 600 mg of 2-PAM Cl * by autoinjector Diazepam 10 mg IM *2-Pyridine Aldoxime Methylchloride (Pralidoxime) Oximes remove agent from enzyme unless aging has occurred Aging: agent-enzyme complex changes T ½ Aging times: Soman 2 minutes, Sarin 3-4 hours, others longer

29 Therapeutic agents affecting ACh function Pyridostigmine AChE inhibitors (reversible) Myasthenia gravis (Pyridostigmine) Alzheimer s Disease (Donezepil and Galantamine) Galantamine

Cholinergic receptors( cholinoceptors ) are two families muscarinic and nicotinic depending on their affinities to cholinomimetic agents(agents that

Cholinergic receptors( cholinoceptors ) are two families muscarinic and nicotinic depending on their affinities to cholinomimetic agents(agents that Cholinergic drugs Cholinergic drugs Are drugs act on receptors that are activated by acetylcholine(ach) which is the neurotransmitter of the parasympathetic nervous system. ACH is synthesized in the cholinergic