Pharmacology (Melega) Flashcards
Steps in neurochemical transmission
Synthesis
Storage
Release (Ca2+ triggers exocytosis)
Receptor interaction
Reuptake (into nerve terminal) or Inactivation (by metabolism)
NE synthesis/storage
Tyr gets into cytosol without regulation –> TH turning Tyr into L-DOPA is rate-limiting –> L-DOPA turned into DA by AAAD rapidly in cytosol –> DA into vesicles by vesicular monoamine transporter-2 (VMAT) –> DA turned into NE by dopamine beta hydroxylase (DBH) in vesicles and stored there
Autoreceptors
Receptor on presynaptic membrane that binds NE after it’s been released into synaptic cleft
Ex: alpha2 autoreceptor on presyn membrane responding to NE
Provides feedback–inhibits NE release
Heteroreceptors
Receptor on presynaptic membrane that responds to input from another neuron (and different NT, ie Ach)
Ex: muscarinic receptor on adrenergic nerve terminal
Inhibitory–reduces NE release
Norepinephrine transporter (NET)
Reuptake of NE, located on presynaptic membrane
Called Uptake 1
Catechol-O-methyltransferase (COMT)
Metabolizes NE –> normetanephrine
Metabolizes Epi –> metanephrine
(Metabolites have lower affinity for binding receptors)
Monoamine oxidase (MAO)
MAO-A and MAO-B
Metabolizes NE by oxidizing it to aldehyde
(Then aldehyde further acted on by aldehyde reductase or aldehyde dehydrogenase)
MHPG and VMA
Terminal metabolites of NE metabolism
MHPG and sometimes VMA used as index of NE turnover when measured in CSF
Can be produced when MAO acts then COMT, or vice versa!
Which receptors does NE bind?
Alpha1
Alpha2
Beta1
Which receptors does Epi bind?
Alpha1
Alpha2
Beta1
Beta2
Which receptors does isoproterenol bind?
Beta1
Beta2
Adrenal medulla
Located in central part of adrenal glands
Site of synthesis and storage of catecholamines
Responds to impulses from preganglionic sympathetic fibers that release Ach and bind nicotinic receptors
Secretes mostly 80% epi and 20% NE directly into circulation via chromaffin cells
Synthesis of epinephrine
DA taken up into vesicles, converted to NE by DBH –> NE transported out of vesicles into cytosol –> NE converted to EPI by PNMT –> EPI transported back into vesicles
Alpha1 adrenergic receptor signal transduction pathway
EPI/NE binds alpha1 receptor –> G-coupled protein activates PLC –> PLC creates DAG and IP3 –> IP3 binds to IP3-receptor gated Ca2+ channel to let Ca2+ into cytoplasm from SR –> Ca2+ binds calmodulin and activates MLCK –> MLCK activates myosin to bind actin and contract
Beta1 adrenergic receptor signal transduction pathway
EPI/NE binds beta1 receptor –> G-coupled protein activates adenylyl cyclase –> increased cAMP –> activation of PKA –> phosphorylation of L-type Ca2+ channels –> muscle CONTRACTION –> heart has increased contractility
Beta2 adrenergic receptor signal transduction pathway
EPI binds beta2 receptor –> G-coupled protein activates adenylyl cyclase –> increased cAMP –> activation of PKA –> phosphorylation of MLCK –> muscle RELAXATION
Note: same pathway for beta1 and beta2 but opposing effects because of LOCALIZED action
EPI effects at low and high concentrations
Low [EPI]: stimulates beta2 > alpha1; vasodilation
High [EPI]: stimulates alpha1> beta2; vasoconstriction
Note: more alpha1 receptors overall, but have lower affinity for EPI. So when enough EPI to bind to alpha1, they bind to lots of alpha1’s and this effect overrides the few beta2 receptors that are occupied by EPI
At physiological concentrations, what do NE and EPI do?
NE = vasoconstriction (via alpha1)
EPI = vasodilation (via beta2)
Both increase HR, contractility (beta1 (and beta2 for EPI))
Direct mechanism of action
Drug binds adrenergic receptor
Indirect mechanism of action
Causes response by provoking release of NE from presynaptic terminal, or by interfering with NE reuptake
Do not have direct actions on postsynaptic receptor
Mixed mechanism of action
Combination of direct and indirect mechanisms
Reuptake inhibitor
Type of Indirectly Acting Sympathomimetic
Drug binds reversibly to uptake transporter (ex: NET), blocking access for NT to be re-uptaken back into presynaptic terminal
Get increase in extracellular NT
Ex: Cocaine
Other ex: methylphenidate (Ritalin for ADHD increase NE, DA), tricyclic antidepressants (increase NE, serotonin), SSRIs (increase serotonin)
Neurotransmitter-releasing
Type of Indirectly Acting Sympathomimetic
Drug is taken up by presynaptic nerve terminals (through reuptake channel like NET) and enters vesicles, displacing NT from the vesicles, so NT gets into cytosol and is then pushed out of presyn membrane through channels (non-exocytosis exit)
Get increase in extracellular NT
Ex: Methamphetamine, Tyramine
Sympatholytic/Adrenolytic
Block NE effects
Direct Receptor Blocking Agents: Drug can be direct (competitive or irreversible) antagonist
Adrenergic Neuronal Blocking Agents: Drug can bind to vesicles and not allow DA in (so can’t be converted/synthesized to NE), or can not allow reuptake of NE into vesicle (so can’t be stored in vesicles and released into synaptic cleft), then DA and NE metabolized in cytosol by MAO
Pheochromocytoma
Rare catecholamine-secreting tumor derived from chromaffin cells of adrenal medulla that produces high NE and EPI, resulting in severe increase in BP
Can use Phentolamine or Phenoxybenzamine (although that not used much anymore because it’s irreversible) to treat hypertension caused by this