Receptors Flashcards
What is a receptor?
It is a specific protein that is the site for binding of a signaling molecule.
- Can also be: Enzymes, Na+, K+ ATPase pump, nucleic acids
- Two types
- Transmembrane
- Intracellular (some nucleic)
Ligand
Is a compound that is specific fo each receptor
- Activates the receptor causing biological response
- Endogenous substances
Agonist
A compound that binds to a receptor causing activation and EXPECTED effect
- Effect can be natural or synthetic
- Two main types:
- Full
- Partial
Partial Agonist
A drug that binds to a receptor (usually at agonist site) that activates the receptor, but not as much as a full agonist.
- When a partial agonist is administered with a full agonist, drug effect of full agonist is decreased
- Partial agonist can have antagonistic activity: agonist- antagonist
- Occupy the same number of receptors as full agonist, but effect is decreased
Inverse Agonist
A compound that binds to the receptor creating OPPOSITE of expected effect
- Binds to same sites as agonist (competing with it)
- “Turns off” activity of the receptor
*It is NOT an ANTAGONIST
Antagonist
A compound that when bound to a receptor causes INACTIVATION, preventing expected (normal) response.
- Usually reversible
- Two Types:
- Competitive
- Noncompetitive
- When ligand is bound
- Interact with receptor but DO NOT CHANGE receptor
- Have affinity but NO EFFICACY
- Block the action of other drugs
- Effect observed ONLY IN THE PRESENCE OF AGONIST
Competitive Antagonist
COMPETES with AGONIST for receptor binding sites
- Concentration and receptor affinity affect response
- Can reverse the effects of ANtagonist with large dose of AGONIST
Noncompetitive Antagonist
Binds to receptor at DISCRETE SITES [allosteric] (different from Agonist) and alters maximal response
- Partial Response: Decreases maximum efficacy of drug
Receptor States
Multiple receptors in the cell and they change in number
- Not all receptors are active or inactive
- Agonist–Fully Active
- Partial Agonist–Some Active
-
Antagonist–Changes balance so receptor is inactive
- “active” but blocked
-
Inverse Agonist–Favors inverse of full agonist
- acts as antagonist
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Agonists
vs
Antagonists
AGONIST:
Enzyme activation/Inhibition
Ion channel modulation
DNA transcription
ANTAGONIST:
Effect of Agonist blocked
Non-Polar Molecule
A molecule that has a neutral charge; no net positive or negative charge
Polar Molecule
- Do not have a net charge, but certain regions have partial negative and positive charge
- Soluable in H2O–water is polar
Occurs in the absence of a ligand. Cell will increase the number of receptors in an attempt to sense missing ligand
Up-Regulation
Occurs with increase stimulation by a ligand. Cell will reduce the number of receptors specific to that ligand.
Down-Regulation
Name the four types of cell receptors.
- Ion channel linked
- G-protein coupled
- Intrinsic enzyme linked
- Intracellular
- Respond to fast neurotransmitters such as Ach.
- Nicotinic and glutamate receptors (Excitatory)
- GABAa and glycine receptors (Inhibitory)
Ligand-gated Ion Channel
- Present in the membranes of excitable nerves, cardiac, and skeletal muscle cells
- Open/Close in response to voltage changes in cell
- Named for ion permeability
- Play an important role in muscle contraction/propagation of action potentials
- Some excitatory, some inhibitory
- Local anesthetics bind to the intracellular domain of this receptor
Voltage-gated ion Channel
- Cellular response illicited with these receptors through protein that stimulates an increase or decrease in protein synthesis. (Ex: Catecholamines)
- Most numerous receptor type
- Contains three subunits that influence enzyme activity and ion channels
G-Protein Coupled Receptors
- Subunit that stimulates, increases adenylcyclase, makes cAMP from ATP.
- cAMP is important 2nd messenger
Gs
Protein Subunit
- Subunit that inhibits Adenylcyclase
G<span>i</span>
Protein Subunit
- Activates phospholipase C, increasing DAG and IP3, which increases Ca2+ from cells
Gq
Protein Subunit
Prolonged exposure of a receptor to an agonist causing subsequent dosing with agonist to produce reduced maximal effects.
–Barash Chpt 11, pg 254
Desensitization
Prolonged exposure of receptors to antagonist.
Can occur with long term use of beta-blockers and abrupt discontinuation.
Hypersensitization
- Associated with sypathetic response of the autonomic nervous system (ANS)
- Located in CNS and peripheral tissues
- Characterized by neurotransmitter response
- Epi, Norepi, Dopamine
- Largely excitatory response; Fight or Flight
Adrenergic Receptor
Name the three major subtypes of Adrenergic Receptors.
- Alpha
- Beta (ß)
- Dopaminergic (D or DA)
- Receptors located in vascular smooth mm, GU smooth mm, the liver, and CNS post-synaptically (excitatory)
- Activation causes contsriction of smooth mm (vasoconstriction) and positive inotropism in the myocardium
- GPCR
- Gq subunit - activates phospolipase C, increasing DAG and IP3
Alpha-1
Adrenergic Receptors
Alpha-1 Adrenergic
Agonists
- Phenylepherine, Norepinepherine, Epinepherine, Dopamine
- Causes smooth mm contraction, vasoconstriction, antidiuresis; increases IP3 and DAG
Alpha-1 Adrenergic ANtagonists
- Phenoxybenzamine, phentolamine (Regitine-used as a “reversal” for Epi, NE infiltration in PIV), Labetalol, Doxazosin (Cardura), Prazosin (Minipress)
- Vasodilation, relaxation of smooth mm
- Receptors located in pancreatic islet (ß) cells, platelets, nerve terminals, CNS, and vascular smooth mm
- Presynaptic activation decreases cAMP which inhibits NE release, resulting in decreased SVR, decreased CO, decreased inotropism, and decreased HR
- Postsynaptic activation causes vasoconstriction and platelet aggregation
- GPCR
- Gi subunit - Inhibits adenylate cyclase, Ca+ and K<span>+</span> ion channels.
Alpha-2
Adrenergic Receptors
Alpha-2 Adrenergic
Agonists
- Clonidine, NorEpi, Epi, Phenylepherine
-
Presynaptically: Clonidine; dexmedetomidine (Precedex)
- Reduced Peripheral Vascular Resistance (PVR)
- Inhibit release of NorEpi
- CNS depression, Sedation
-
Postsynaptically: NorEpi, Phenylepherine, Epi
- Vasoconstriction
- Inhibition of insulin release, decreased GI motility, inhibition of ADH, platelet aggregation
Alpha-2 Adrenergic
Antagonists
- Rarely used medically
- Similar to Alpha-1 Antagonists-inhibit release of NE
- Receptors located in the heart (SA node, myocardium, ventricular conduction system) and renal juxtaglomerular cells
- Activation increases cAMP
- Increased HR, positive inotropic and chronotropic effects, renal renin release, relaxed coronaries
- Postsynaptic, sensitive to NE and Epi equally
- GPCR
- Gs subunit - stimulates adenylate cyclase and Ca+ ion channels
Beta (ß)-1
Adrenergic Receptors
Beta (ß)-1 Adrenergic Agonists
- Isoproterenol, Epi, NorEpi, Dopamine, Dobutamine
- Increased HR, Increase contractility
Beta (ß)-1 Adrenergic Antagonists
- Commonly called “Beta-Blockers”
- Sudden cessation can elicit rebound tachycardia and myocardial ischemia r/t upregluation of B-1 receptors
- Metoprolol, Propanolol, Esmolol
- Postsynaptic: Vasodilation, decreased HR, decrease inotropy
- Receptor located:
- presynaptically in the myocardium and SA node
- postsynaptically in vascular, bronchial, GI, and GU smooth mm
- Activation increases cAMP:
- Sensitive to NE presynaptically:
- constriction, accelerated NE release
-
Epi postsynaptically:
- Positive inotropism, chronotriopism, dialation of smooth mm
- Sensitive to NE presynaptically:
- Gs subunit - Stimulates adenylate cyclase and Ca+ ion channels
Beta (ß)-2 Adrenergic Receptor
Beta (ß)-2 Adrenergic Agonists
- Commonly used for bronchodilatory effects
- Isoproterenol (used for bronchospasm during anesthesia), Albuterol (SABA), Epi, NE, Dopamine
-
Presynaptic: Increased HR, accelerates NE release
- Postsynaptic: Vasodilation, bronchodilation, Gi and GU relaxation, uterine relaxation, insulin secretion, amylase secretion
-
Presynaptic: Increased HR, accelerates NE release
Beta (ß)-2 Adrenergic Antagonists
- ß2 receptors are typically antagonized with nonselective ß1 Blockers
- Butoxamine (selective–used primarly in experimentation to identify ß2 receptors), Propanolol, Alprenolol (antihypertensive/angina med; older med), Esmolol, Nadolol, Timolol, Labetolol
- small degree of peripheral vascular bed vasoconstriction occurs with selective antagonism
- Receptors located within the CNS, vascular smooth mm, kidneys, and postganglionic sympathetic nerves
- Has 5 subtypes, two of which are predominant
- GPCR
Dopaminergic Receptors
- Receptors located in vascular smooth mm, renal and mesentery BV, rental tubules, juxtaglomerular cells, and sypathetic ganglia
- Similar in structure to subtype 5, but located in different areas.
- Activation increases cAMP:
-
Postsynaptic:
- Vasodilation, diuresis, renin release, sodium excretion, minor inhibition of sypathetic ganglial nerves
-
Postsynaptic:
- GPCR
- Gs subunit - stimulates adenylate cyclase and Ca+ ion channels
Dopamine (DA,D)1
Receptor
DA1
Agonists
- Fenoldopam, Dopamine, Epi
- Vasodilation, diuresis, nausea, vomiting, dizziness
DA1
Antagonist
- Haloperidol, Droperidol, Phenothiazines (Thorazine), Metoclopramide (Reglan)
- Relief of n/v (Reglan, Haloperidol, Droperidol), increased gastric motility (Reglan), mesenteric smooth mm constriction,
- Receptor located in presynaptic postganglionic sympathetic nerves and postsynaptic renal and mesenteric vascular smooth mm.
- Similar in structure to subtypes 3 and 4
- Activation decreases cAMP
- Presynaptic: inhibition of NE, secondary vasodilation
- Postsynaptic: vasoconstriction, inhibition of aldosterone relase from adrenal cortex
- GPCR
- Gi subunit - inhibits adenylate cyclase, Ca+ and K+ ion channels
Dopamine (DA,D)2
Receptor
DA2
Agonists
- Dopamine, Bromocriptine
-
Presynaptic:
- Inhibition of NE release, secondary vasodilation
-
Postsynaptic:
- Vasoconstriction of renal and mesenteric vasculature
DA2
Antagonists
- Domperidone
- GI prokinesis, prolactin release, antiemetic effects
- Associated with the parasympathetic region of the CNS, Rest and Digest
- Has excitatory and inbitory,
- Two main types:
- Nicotinic
- Muscarinic
Cholinergic Receptors
- Ligand-gated ion channel receptors
- Two subtypes
- m - found in postsynaptic skeletal neuromuscular junction
- n - found in the autonomic ganglia (postganglionic; ANS) and adrenal medulla
- Activated by Acetycholine, opening Na+ and K+ depolarizing ion channel
- action mediated by ion
Nicotinic Cholinergic Receptor
Nicotinic
Agonists
- Acetycholine, Nicotine
- Nn and Nm
-
Succinylcholine
- Nm : depolarizing NMB, opens ligand ion channel, depolarizing and inhibiting neurotranmission
Nicotinic
Antagonists
-
Nn
- Dextromethorphan
- Trimethaphan - inhibits sympathetic and parasympathetic autonomic activity, used for BP control in aortic dissection
- vasodilation
-
Nm
- Vecuronium - competitive antagonist, blocks Ach at the NM junction; muscle relaxation, paralysis.
- Rocuronium, Cisatracurium (Nimbex)
- G-protein coupled receptor
- Has 5 subtypes
-
1,3,5 - Gq subunit: activates Phospholipase C, increasing DAG and IP3
- 1 - present in CNS, autonomic ganglia, glands (salivary, gastric), enteric GI nerves
- 3 - present in CNS, smooth mm, and glands
- 5 - low levels in CNS, associated with Dopamine neurons
-
2, 4 - Gi subunit: inhibits adenylate cyclase
- 2 - present in CNS, heart, smooth mm, and autonomic nerve terminals
- 4 - present in CNS, forebrain
-
1,3,5 - Gq subunit: activates Phospholipase C, increasing DAG and IP3
Muscarinic Cholinergic Receptors
Muscarinic
Agonists
- Acetycholine
- Muscarine
- Toxic compound in mushrooms
- Increased salivation, gastric secretions (M1), decreased HR and atrial contractility (M2), smooth mm contraction, emesis (M3)
Muscarinic
Antagonists
- Scopolamine
- antiemetic properties, reduces secretions
- Atropine
- broad antagonist
- reduces secretions, increases HR and atrial contractility
- Two main types of these receptors
- A type - found primarily in the CNS
- B type - found mainly in peripheral tissue
-
Postsynaptic inhibitory receptor
- A type - CNS depression
- B type - skeletal muscle relaxation
- GPCR
GABA receptors
GABAA
Agonists
- Act on GABAA receptor through potentiation, direct gating and inhibition
- Benzodiazepines
- Propofol
- Etomidate
- volitile anesthetics
- anesthetic steroids
GABAA
Antagonists
- Flumazenil (Romazicon) - competative antagonist
GABAB
Agonists
- Baclofen
- antispasmotic
- peripheral smooth mm relaxant
GABAB
Antagonists
- Saclofen
- competative antagonist
- used in research
- a member of the Serotonin receptor family
- ligand gated ion channel receptor
- targeted for the management of drug induced n/v
- antiemetics (Zofran) antagonize
- can potentiate a midgraine; migraine meds are serotonin receptor agonists
- agonized response n/v
- antiemetics (Zofran) antagonize
5-HT3 Serotonin Receptor
- Receptors located on nerve endings of sensory neurons in spinal cord and in the brain on descending neurons
- Three types
- Mu
- Kappa
- Delta
- When activated, decending neurons release NE and 5HT, the release of Substance P and glutamate is reduced creating less neurotranmission to brain = brain interprets less pain
- GPCR
- Gi subunit - inhibition of adenylate cyclase, reduction of cAMP
Opioid Receptors
- Has different subtypes
- subtype 2 associated with physical dependence
- Responsible for analgesia in spinal and supraspinal areas
- specifically substantia gelatinosa in posterior horn of spinal cord
- brain areas responsible for pain interpretation
- Most lipophilic receptor
- Activation elicits:
- Analgesia, euphoria, miosis, bradycardia, hypothermia, urinary retention, depression of ventilation, constipation
Mu
Opioid Receptor
(MOR)
Mu
Agonists
- Endorphins, Morphine, Synthetic opioids
- Analgesic effect, euphoria, miosis, bradycardia, hypothermia, urinary retention, decreased GI motility (Mu2), respiratory depression (Mu2), physical dependence (Mu2)
- Responsible for analgesia spinal and supraspinal
- dorsal horn of spinal cord
- same areas as Mu
- Activation results in inhibition of neurotransmitter release via N-type Ca+ ion channels.
- Analgesia, sedation, dysphoria, miosis, diuresis, low abuse potential
- Opioid agonist-antagonists act principally on this receptor
- Extensive research conducted on antagonists to combat addiction
Kappa
Opioid Receptor
(KOR)
Kappa
Agonists
- Dynorphins
- endogenous opioid peptide
- Analgesic effect spinal, supraspinal, dysphoria, sedation, pupillary constriction, diuresis, low abuse potential
- Responsible for supraspinal and spinal analgesia
- No current drugs on the market targeting receptor
- Respond to endogenous ligands: enkephalins
- May modulate activity of Mu receptors
- Activation causes modulation of hormone and neurotransmitter release:
- Analgesia, respiratory depression, physical dependence, mild constipation, urinary retention
Delta
Opioid Receptor
(DOR)
Delta
Agonists
- Enkephalins
- No drugs on market
- Spinal and supraspinal analgesia, moderation of Mu receptors, respiratory depression, mild constipation, urinary retention, physical dependence
Opioid Receptor
(MOR, KOR, DOR)
Antagonists
- Naloxone (Narcan)
- Competative antagonist
- increases sympathetic nervous system activity
- tachypnea, tachycardia, hypertension, nausea, vomiting, sudden perception of pain
- Naltrexone
- similar to Naloxone, but has sustained antagonism up to 24h
- reduces euphoric effect
- highly effective orally
- Nalmefene
- pure opioid antagonist
- analogue of naltrexone
- same potency as Naloxone
- Methylnaltrexone
- active at peripheral opioid receptors
- ionized, does not cross blood-brain barrier