Quiz #1 (1/6-1/8) Flashcards
Stages of synaptic transmission at the nicotinic receptor
- Depolarization of the nerve terminal by an action potential.
- Activation of voltage sensitive Ca2+channels in the terminal causing influx of Ca2+from extracellular medium.
- Fusion of synaptic vesicles with the terminal plasma membrane, causing exocytotic release of ACh.
- Diffusion ofACh across the synaptic cleft and binding of ACh to nicotinic ACh receptors (nAChR) in the muscle postsynaptic membrane. The distance across the synaptic cleft is small.
- Activation of nAChR-dependent ion channels which transiently increase the permeability of the muscle membrane to Na+and K+ions, depolarizing the endplate region. The receptor then becomes desensitized.
- Initiation of a muscle action potential near endplate region, which spreads through the muscle, and eventually leads to muscle contraction if the threshold is reached.
- Termination of synaptic transmission by hydrolysis of ACh to acetate and choline by AChE
Somatic Neurons
Polysynaptic Pathway: Afferent central processes synapse onto interneurons in the ventral horn of the the spinal column.
Monosynaptic pathway: synapse directly on to motoneurons.
Renshaw cells: Inhibitory interneurons which synapse onto and inhibit motor neurons.
Axons from motor neurons synapse onto and activateskeletal muscle.
Choline Uptake and Therapeutic Target
Choline is brought into the nerve terminal through a Na+ dependent transporter.
Hemicholinium-3 (HC-3) inhibits uptake of choline, depleting ACh aat synapses. Not used clinically
ACh synthesis and therapeutic target
Synthesized by the enzyme choline acetyltransferase (CAT) in the cytoplasm: Choline + AcetylCoA –> ACh + CoA.
Inhibitors of CAT include choline mustard aziridinium ion and ethylcholine mustard aziridinium ion
Vesicle Fusion therapeutic target
Fusion is inhibited by Botulinus toxin which acts as a protease to hydrolyze SNAP-25.
Botox is used to treat dystonias (involuntary muscle spasms) including blepharospasm (eyes), wrinkle treatment, hyperhidrosis (under arm sweating), healing of anal fissures, and migraine headaches
NMJ Structures (4)
Muscle membrane: highly folded location of very dense ACh receptors.
Synaptic vesicles: across the post-synaptic muscle membrane where the acetylcholine receptors are concentrated.
Basal lamina: basement membrane structure that surrounds each muscle fiber. When there is muscle damage the basal lamina serves as a scaffold for the regenerating muscle to insure the proper orientation and imparts information concerning the specificity and “memory” of synapse formation.
Acetylcholine esterase: present on the basal lamina. Hydrolyzes ACh, and is very active.
Nicotinic receptors
Nicotinic receptors: ACh activated receptors at the NMJ, ligand-gated, fast response
Desensitization of nAChR
Continuous ACh decreases the response of ACh, this process happens in a matter of seconds and is an intrinsic property of the receptor. Conformational changes of desensitization:
Resting state (R): Channel is closed and ligand is not bound
Activated state (O): Depolarization of the membrane, channel is opened and ligand is bound
Desensitized state (D): Channel is closed but ligand remains bound
De-innervation Supersensitivity of nAChR
presence of receptors in non-synaptic regions of the muscle following muscle degeneration or de-innervation. This skeletal muscle becomes supersensitive to pharmacologic cholinergic agonists. This phenomenon stops after re-innervation
Muscarinic ACh Receptor
G-protein coupled receptor that acts in the ANS
Neuromuscular Blocker Therapeutic Uses
Endotracheal intubation, muscle relaxants during surgery, resetting of fractures, electroshock therapy to prevent physical trauma, intensive care to prevent high airway resistance and to abolish muscle rigidity, and state executions (in a cocktail of drugs)
Competitive Non-Depolarizing Antagonists at nAChR Actions
Block muscle contractions
Competitive Non-Depolarizing Antagonists at nAChR Examples
Natural compounds: d-Tubocurarine (curare)
Synthetic compounds: Gallamine and pancuronium
Competitive Non-Depolarizing Antagonists at nAChR Toxic Effects
action at nicotinic receptor in the autonomic ganglia which can produce autonomic effects including cardiac arrhythmias and decreased blood pressure.
Curare can also cause histamine release resulting in bronchospasm,
Cardiovascular collapse: hypertension and increased BP
Respiratory paralysis
Competitive Non-Depolarizing Antagonists at nAChR Overcoming Toxic Effects
Anti-AChE to increase ACh at synaptic cleft
Depolarizing Partial Agonists at nAChR Actions
twitching, and blockade of muscle contraction
Depolarizing Partial Agonists at nAChR Examples
decamethazoinum-C10 and succinylcholine
Depolarizing Partial Agonists at nAChR MOA
bind to the receptor, activate the receptor and antagonize ACh binding. The receptor becomes desensitized due to the prolonged exposure.
Succinylcholine
Depolarizing Partial Agonists at nAChR. short half-life and is hydrolyzed by serum enzyme pseudo- or plasma- choline esterase. mutants for this enzyme don’t degrade succinylcholine rapidly and have prolonged apnea
Depolarizing Partial Agonists at nAChR Treating Toxic effects
Cannot treat with Anti-AChE because this enhances desensitization. Only treatment is artificial respiration
AChE
Located in the basal lamina
Active Site: anionic site interacts with the quaternary ammonium group of ACh and the esteratic site is where ester hydrolysis occurs and contains a His and Ser
Non-covalent and reversible AChE inhibitors
Example is edrophonium which interacts with the anionic site and hydrogen bonds with the histidine at the esteratic site.
Covalent and reversible AChE inhibitors
Examples are the carbamate esters, physostigmine and neostigmine. The covalent complex is slowly reversible (t ½ 30 minutes, effective 3-4 hours)
Covalent and irreversible AChE inhibitors
Examples are the organophosphates, DFP (diisoproplyflurophosphate), parathion and echothiophate. DFP is a nerve gas. Reactivation can occur with 2-PAM (pralidoxime) which forms an oxide-phosphonate. However, if the enzyme is “aged” the intermediate is extremely stable and cannot be reactivated. Aging takes 2-3 days with DFP but only 3 minutes with Soman (nerve gas)
Physiological Effects of Anti-AChE
Act at nAChR at the NMJ, and in sympathetic and parasympathetic ganglia.
At the NMJ this decreases the force of muscle contraction and in high doses causes receptor desensitization.
They also indirectly activate muscarinic receptors in the parasympathetic nervous systems and in sweat gland, increase GI smooth muscle motility and decrease heart rate
Myasthenia Gravis
autoimmune disease in which patients make Abs against their own nAChR in muscle causing neuromuscular weakness and muscular fatigue
Myasthenia Gravis Treatment
AChE inhibitor, neostigmine, which prolongs the time the patient can produce muscular contractions because the drug increases the lifetime and effective concentration of ACh at the neuromuscular junction.
Atropine in Myasthenia Gravis
muscarinic antagonist, to suppress stimulation of ACh in the ANS
Myasthenia Gravis Diagnosis
short-acting AChE inhibitor, edrophonium. Patients with Myasthenia Gravis will show a transient increase in strength lasting several minutes.
Functions of ANS
Regulates the functions of the heart, blood vessels, GI tract, various types of smooth muscle and strives to maintain the internal environment against a constantly changing external environment
ANS vs NMJ
- At the NMJ, the actions of somatic nerves are voluntary while control of the autonomic neurons is generally involuntary
- The target of the somatic neurons, the skeletal muscle is quiescent (not stimulated) in the absence of neuronal stimulation while the target organs of the ANS show tonic activity. E.g. your heart is always beating and you can either slow down the rate or increase the rate through the autonomic nervous system.
- Axons for neurons innervating skeletal muscle are found in the spinal cord. In the autonomic system, sensory information is carried by afferent pathways to the CNS where the controlling and integrating centers for the ANS are located. Efferent nerves of the ANS provide all innervated structures of the body with the exception of the skeletal muscle which is controlled by somatic neurons.
- ANS has presynaptic receptors to regulate the release of neurotransmitters, whereas the NMJ doesn’t
Parasympathetic vs Sympathetic: location
Parasympathetic preganglion neurons originate in the midbrain, medulla and sacral portions of the spinal cord. Ganglia are closer to the target and often only innervate one effector organ.
Sympathetic preganglion neurons originate in the intermediate spinal cord. Ganglia are in the spinal cord and often innervate many effector organs
Parasympathetic vs Sympathetic: Targets
Most organs are innervated by sympathetic and parasympathetic branches which often act in an antagonist manner.
Adrenal Medulla
part of the sympathetic system. Contains secretory cells (chromaffin cells) that release catecholamines, e.g. epinephrine and norepinephrine in response to neuronal stimulation
Sympathetic Effects
“fight or flight”
1. Heart: increased heart rate, increased contractile force
- Arterioles (but not skeletal muscle) and veins: vasoconstriction (increased blood pressure)
- Eye: dilation of pupils (mydriasis) and relaxation of ciliary muscle for far vision
- Lungs: dilation of bronchioles
- Liver: increased blood sugar (glycogen breakdown)
- Sweat Glands: secretion (no parasympathetic innervation)
- GI tract: inhibition of peristalsis and secretion
Parasympathetic Effects
“rest and digest”
- Heart: bradycardia and reduces contractile force in the atria
- Eye: miosis of pupils (constriction) and contraction of ciliary muscle for near vision
- Lungs: Bronchial Constriction
- GI tract: stimulation of peristalsis and secretion
5 .Arterioles and veins (not skeletal muscle): only receive sympathetic innervation
- Lachrymal Glands (tear glands): secretion (no sympathetic innervation)
- Salivary glands: secretion (little sympathetic innervation that causes secretion)
Synaptic Transmission through Autonomic Sympathetic Ganglia
- An initial large depolarization, EPSP, due to the activation of the ganglionic nicotinic receptor of the postganglionic neuron by ACh. Responsible for transmission through the ganglion
- A small hyperpolarization, slow IPSP, due to the activity of an interneuron. Serves to modulate synaptic transmission
- A slow or late EPSP due to activation of mAChR on the post-ganglionic neuron
Synaptic Transmission through Parasympathetic Ganglia
Less is known. Studies with amphibian heart suggest the presence of a fast EPSP due to activation of nACh-R and a slow IPSP due to activation of mACh-R.
The main pathway uses ACh activating nicotinic receptors post-synaptically.
Ganglionic Stimulating Drugs
- Nicotinic: Examples
Nicotine, DMPP, and TMA
Ganglionic Stimulating Drugs - Nicotinic: MOA
low concentrations activate nAChR in sympathetic and parasympathetic ganglia and the adrenal medulla. High concentrations desensitize the nicotinic ACh-R in ganglionic synapses and cause blockade
Nicotine concentration effects
Ganglionic stimulating drugs.
Low concentrations: sympathomimetic cardiovascular effects which include increased inotropic and chronotropic effects and parasympathetic GI effects to increase GI motility
High concentrations: convulsions
Ganglionic Stimulating Drugs - Muscarinic
Examples: muscarine, pilocarpine
MOA: stimulation of sympathetic nervous system by late EPSP
Effects: direct sympathetic effects on the cardiovascular system The experimental drug, McN-A-343, acts preferentially to stimulate mACh-R in the sympathetic ganglia and in the adrenal medulla compared to heart or vascular smooth muscle through receptor subclass specificity
Ganglionic Blockers
Examples
Pentolinium, hexamethonium, and triethyl ammonium (TEA)
Ganglionic Blockers: MOA
competitive antagonists that block ganglionic transmission by blocking nAChR without a change in the membrane potential of the ganglionic cells. Inhibit parasympathetic and sympathetic ganglia
Ganglionic Blockers: Effects
depends on the predominant tone
Sympathetic-
Arterioles and veins: cause vasodilation
Sweat glands: anhidrois, decreases sweating
Parasympathetic-
Heart: tachycardia
Eye: mydriasis (pupillary dilation) and cycloplegia (paralysis of the intraocular muscle)
Salivary glands: Xerostomia
GI tract: relaxation of intestinal muscle, constipation
Bladder: difficulty voiding the bladder
Ganglionic Blockers - Therapeutic uses
were used as hypotensive agents due to dilation of arteries and veins
Muscarinic Synapses
- At the NM junction there is a highly specialized structure with a highly differentiated nerve terminal, there are discrete terminals; in the ANS the muscarinic receptors are more spread out to give a uniform response
- At the NM the synapse gap is very small 20 nm to 50 nm. At Muscarinic synapses the distance is generally larger, 20 nm to 2 µm.
- The response at the NM junction is very fast… less than a 100 µsec. At muscarinic synapses the response is slow, slower than 100 msec.
- Muscarinic receptors are coupled to GPCR whereas at the NMJ the receptors are ligand-gated ion channels
Muscarinic Agonist classes
choline esters, cholinomimetic alkaloids
Muscarinic Agonist general effect
mimic parasympathetic nerve stimulation
Muscarinic Agonist, Choline Esters - Examples
acetylcholine, carbachol, methacholine and bethanecol
Muscarinic Agonist, Choline Esters - MOA
act as agonists at mAChR
Muscarinic Agonist, Choline Esters - Effects
vasodilation of vascular beds, decrease in blood pressure (NO dependent), bradycardia, bronchial constriction, pupil constriction
Muscarinic Agonist, Choline Esters - Therapeutic Uses
- Methacholine was used in the past to control, tachycardia but not now because of side effects.
- Bethanechol is used as a GI tract stimulant to relieve a variety of conditions and to relieve urinary tract retention when there is no physical obstruction.
- ACh is used to produce brief periods of miosis during extraction of cataracts.
- Methacholine, bethanecol, and carbachol are used in treatment of glaucoma to cause miosis which enhances drainage of aqueous humor.
- Methacholine is also used for diagnosis of belladonna (a muscarinic antagonist) poisoning, and for diagnosis of bronchial hyperactivity
Muscarinic Agonists, Cholinomimetic Alkaloids: Examples, effects, uses
Examples: muscarine, arecoline, pilocarpine and the synthetics oxotremorine and aceclidine
Effects: Can cross BBB and can have effects in the CNS
Therapeutic uses: pilocarpine is used for the treatment of glaucoma
Mushrooms: Amanita class mushrooms contain muscarine. Ingestion produces a number of symptoms due to over stimulation of autonomic organs, salivation, diarrhea, bradycardia and CNS effects. Atropine is used as an antidote for muscarine poisoning
Muscarinic Antagonists
Belladona Alkaloids and quaternary derivatives
Muscarinic Antagonists: Examples
Atropine, scopolamine, homotropine. Belladonna poisoning (night shade) is a mixture of atropine and scopolamine
Muscarinic Antagonists: MOA
competitive reversible inhibition of mAChR to block parasympathetic stimulation
Muscarinic Antagonists: Effects
mydriasis (pupil dilation), tachycardia (clinical dose atropine), cycloplegia, inhibit GI secretions
Muscarinic Antagonists: quaternary derivatives
Do not cross the BBB and therefore have no CNS side effects
Substantial nicotinic blocking activity and can have side effects due to ganglionic and neuromuscular blockade
Muscarinic Antagonists: Therapeutic Uses
- Preanesthetic: atropine and scopolamine reduce salivation and bronchial secretion as well as dilating bronchial passages
- Ophthalmology: mydriasis and cycloplegia properties. Drugs differ in duration of action and recovery time is significant (days)
- Peptic Ulcer: decreases motility and gastric secretion. Also inhibit intestinal tone or motility
- Bradycardia: Atropine. bradycardia caused by excessive stimulation of vagal tone following MI
- Cold and Hay Fever: Belladonna alkaloids decrease secretions
- Asthma: ipratropium, quaternary derivative, produces bronchodilation but does not inhibit bronchial secretions
- Motion sickness
M1 receptor subtype
main mACh-R in sympathetic ganglia and it is preferentially activated by McN-A-343.
M2 receptor subtype
main subtype in the heart and is coupled to inhibition of cAMP levels
M3 receptor subtype
main subtype in smooth muscle
M4 and M5 receptor subtype
are in the brain
Muscarinic subtype MOA’s
M1, M3, M5 cause Ca2+ mobilization and PKC activation
M2 and M4 cause inhibition of AC through Gi
AChE in ANS: examples, therapeutic uses
Terminates ACh action to produce cholinometric effects at both nicotinic and muscarinic synapses
AChE Examples: Physostigmine, neostigmine, echothiphate and organophosphates
Therapeutic Uses: glaucoma to cause miosis and increase drainage, atony of the urinary bladder by increasing GI and urinary tract motility (neostigmine) and Atropine intoxication (physostigmine).
