Lectures 3 & 4 - Neurotransmitter Lifecycle and Neuropharmacology Flashcards
What are agonists?
Molecules that can bind to receptors and activate them = produce the same biological response as the endogenous ligand
What are antagonists? Eg?
Molecules that bind the receptor and are inherently inert except that they may block the functionality of an agonist
Eg: Magnesium at the NMDA receptor
What are the 6 ways in which a drug can act as an agonist in the NT lifecyle?
- Serve as a precursor for an NT
- Stimulate the release of NT vesicles
- Stimulate postsynaptic receptors
- Block autoreceptors to increase the synthesis/release of NTs
- Block NT reuptake
- Inactivate enzyme breaking down NTs
What are the 5 ways in which a drug can act as an antagonist in the NT lifecyle?
- Prevent storage of NTs in vesicles
- Inhibit the release of NT vesicles
- Block postsynaptic receptors
- Inactivate the enzyme responsible for synthesizing the NT
- Stimulate autoreceptors to inhibit the synthesis/release of NTs
What is an example of a drug serving as a precursor for an NT?
Drug given to Parkinson’s disease patients is L-dopa, which serves as a precursor to dopamine
What is an example of a drug stimulating the release of NT vesicles?
The black widow spider venom stimulates the release of ACh
What is an example of 2 drugs stimulating postsynaptic receptors?
Nicotine and muscarine stimulate ACh receptors
What is an example of a drug blocking autoreceptors to increase the synthesis/release of NTs?
Clonidine increases the synthesis/release of norepi
What is an example of a drug blocking NT reuptake?
Cocaine blocking dopamine reuptake by blocking DAT
What is an example of a drug inactivating the enzyme breaking down NTs?
Physostigmine inactivates acetylcholinesterase
What is an example of a drug preventing the storage of NTs in vesicles?
Reserpine inhibits the storage of monoamines in vesicles by blocking VMAT
What is an example of 2 drugs blocking postsynaptic receptors?
Curare and atropine block ACh receptors
What is an example of a drug inactivating the enzyme responsible for synthesizing the NT?
PCPA inactivating the enzyme that synthesizes serotonin
What is an example of a drug stimulating autoreceptors to inhibit the synthesis/release of NTs?
Apomorphine to inhibit the synthesis/release of dopamine
Describe NT secretory vesicles.
Very specialized organelles with lipid bilayers
What is an example of a drug inhibiting the release of NT vesicles? Purpose?
Botulinum toxin (botox) inhibiting ACh release to prevent wrinkling
What are the 2 types of ACh postsynaptic receptors?
- Nicotinic
2. Muscarinic
Why is it very important to respect drug dosages?
Because all drugs are poisonous and can have serious side effects including death
What are NT autoreceptors? Eg?
Receptors that bind the NT in the presynaptic nerve terminal serving as part of a negative feedback loop in signal transduction
eg: Muscarinic ACh receptors in presynaptic terminal
What ion does NT reuptake depend on?
Na+
What do anti-depressants do? What are they called?
Block the reuptake of serotonin by blocking SERT
SSRIs = selective serotonin reuptake inhibitors
What are the 3 criteria for defining a molecule as a neurotransmitter? Example for 1 and 3? Do you need all 3 to be an NT?
- The substance must be produced and stored by the presynaptic neuron (eg: glutamate and glycine are non-essential AAs so are found in all cells, but in some neurons they are NTs)
- The substance must be released in an activity (Ca2+) dependent manner
- Specific receptors for the substance must be present on the postsynaptic cell (eg: application of exogenous NT mimics the postsynaptic effect of presynaptic stimulation)
YES, NEED ALL 3
What is also packaged in vesicles on top of NTs? Eg?
Co-transmitters that can have multiple effects upon release
Eg: ATP
How to classify NTs by size? List them and provide examples.
- Small: AAs, monoamines, and ACh
2. Large: peptides, derived from larger precursor proteins (eg: enkephalin, substance P)
What is the difference between how small and large NTs are synthesized? Which is faster? Why?
- Small: enzymes synthesized in cell body slowly travel down the axon and synthesize NTs from NT precursors that have been transported from the synaptic cleft to the axon terminal. Packaging of NTs in vesicles then happens at the axon terminal. => FASTER because recycling of previously released NTs
- Large: enzymes and NT precursors are synthesized in the cell body are transported in vesicles down microtubule tracks in the axon. Once they get to the axon terminal the enzymes modify the precursors inside the vesicles
What are neuromodulators?
Molecules which may not fit all of the 3 criteria to be NTs, but influence the electrical activity of postsynaptic neurons
What kind of NTs are opioid analgesics?
Peptides
What are 3 endogenous opioids?
- Endorphins
- Enkephalins
- Dynorphins
To what receptors do opioids bind?
Mu-opioid GPCR (MORs)
What are opioids degraded by?
Extracellular peptidases
What are 4 exogenous opioids? What is another name for these?
- Morphine
- Codeine
- Oxycodone
- Heroin
= Analgesics
What NTs are released after exercise?
Opioids
What is a common and dangerous side-effect of exogenous opioids? What does this lead do?
Euphoria leading to abuse leading to the disinhibition of the ventral tegmental area of the brain releasing dopamine into the forebrain
How do opioids control pain?
Opioid releasing neurons inhibit C-fibers (type of nociceptors) which synapse onto neurons ascending to the dorsal horn of the spinal cord, thereby inhibiting our central perception of pain
What are C-fibers?
Fibers within our CNS that represent our ability to detect pain
What are 3 other side-effects of exogenous opiates? Explain each.
- Itching due to a generalized release of histamines by mast cells
- Slowed breathing due to a decreased sensitivity of medullary chemoreceptive neurons to CO2
- Nausea, vomiting, and constipation due to the activation of gut MORs
Where are AA NTs most prevalent?
CNS
What is the precursor to the NT GABA? What is the enzyme?
Glutamic acid decarboxylase (GAD): glutamate => GABA + CO2
What are the 2 AA NTs?
- Glutamate
2. Glycine
Is glutamate excitatory or inhibitory?
Excitatory
How is glutamate removed from the synaptic cleft?
Excitatory AA transporter (EAAT) on:
- Pre/Post-synaptic membranes
- Astrocytes
How is glutamate degraded intracellularly? In what cells?
Glutamine synthase: glutamate + ATP => glutamine + ADP + Pi
In astrocytes and neurons
How is glutamine transported to the presynaptic terminal?
SNAT7
What enzyme synthesizes glutamate?
Glutaminase
What transporter repackages Glu in vesicles in the presynaptic terminal?
Vesicular glutamate transporter (VGLUT)
Describe the speed of the Glu lifecycle.
Really fast
What are the 2 main types of Glu receptors? List the subtypes for each.
- Ionotropic receptors: AMPA, NMDA, kainate
2. Metabotropic GPCRs: mGluR1-8
Describe ionotropic and metabotropic NT receptors. What is the mechanism for each called? Which is faster?
- Ionotropic receptors form an ion channel pore = direct gating (FASTER)
- Metabotropic receptors are indirectly linked with ion channels on the plasma membrane of the cell through signal transduction mechanisms, often G proteins = indirect gating
What does AMPA stand for?
Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor
What do all of the ionotropic glutamate receptors have in common?
Non-selective cation channels
What can block the NMDA receptor?
Mg++
What ions are permeable in the AMPA receptors?
Na+/K+
What ions are permeable in the NMDA receptors?
Na+, K+, and Ca++
What ions are permeable in the Kainate receptors?
Cations
What does NMDA stand for?
N-methyl-D-aspartate
What are the metabotropic glutamate receptors very critical for?
Synaptic plasticity, learning and memory, and information storage
What 4 things can synaptic plasticity refer to?
- Synapse formation
- Synapse refinement
- Activity dependent plasticity
- Synaptic competition = selection of particular spines to activate
Describe the mechanism of activity dependent plasticity? What does it arise from? What is this important for?
The more you activate a postsynaptic membrane, the more you will have a flux of AMPA/NMDA receptors that are contributing to the size and shape of dendritic spines and upon binding of glutamate within specific patterns (slow/fast activation) we can change the synaptic gain of the synapse. These will also stay longer.
Arises from the use of cognitive functions and personal experience so it is the biological basis for learning and the formation of new memories
Is GABA excitatory or inhibitory?
Inhibitory
Describe the synthesis of GABA.
GAD: glutamate => GABA
By what is GABA packaged into vesicles in the presynaptic terminal?
Vesicular GABA transporter (VGAT)
By what is GABA removed from the synaptic cleft?
Presynaptic/astrocyte GABA transporter (GAT)
What are the 2 types of GABA receptors? List them.
- Ionotropic: GABAa, GABAc
2. GPCRs: GABAb
What are GABA a and c receptors permeable to? Direction of flux?
Cl-
Inward
Where are GABAc receptors found?
Retina
Which GABA receptors are more prevalent: a or c?
a
Where are postsynaptic densities (PSDs) found? What are they made of?
Glutamatergic synapses
Electron dense regions filled with glutamate receptors and other proteins
What is CaMKII? Where is it found? Role?
Enzyme that can detect Ca++ flow at a synapse found in PSDs
Contributes to LTP by phosphorylating substrates
How have researchers tried to understand how info is stored in our CNS?
Used an animal model of an acute brain slice preparation (hippocampus) and record the long-term potentiation of excitatory postsynaptic potential (EPSP) with extracellular field electrodes to axon stimulation at variable frequencies => the EPSP is amplified after the tetanus for long periods of time
What is a tetanus?
High frequency stimulation
Describe the long-term potentiation induction with glutamate.
Glutamate binds to its receptors at resting potential: only the AMPA receptor is activated because NMDA is blocked by Mg++ and once depolarization of postsynaptic membrane is reached, the NMDA receptors are also activated so Ca++ influx starts
Which receptor is called the coincidence detector?
NMDA receptor
What 2 things are required for plasticity/LTP?
- Depolarization of postsynaptic membrane = glutamate
2. Ca++ entry = depolarization
What is the opposite of LTP? What does it do?
Long Term Depression (LTD)
Depresses EPSP for long periods of time
When does LTD occur?
Sleep
LTP stimulation frequency?
Brief, high frequency
LTD stimulation frequency?
Low-frequency for 10-15 min
Are NMDARs activated in both LTP and LTD?
YUP
LTP Ca++ rise?
Large
LTD Ca++ rise?
Small
LTP: AMPARs inserted or removed?
Inserted
LTD: AMPARs inserted or removed?
Removed
What 2 drugs blocks NMDARs?
- Ketamine
2. PCP
What enzymes are involved in LTP?
Kinases
What enzymes are involved in LTD?
Phosphatases
What are GABAb receptors a drug target for?
Autism
Where are GABAb receptors located?
Both the pre and post synaptic cells
How does ambient GABA affect the presynaptic terminal? What is this called?
It binds to GABAb receptors (GABA autoreceptors) and beta and gamma subunits of the receptor inhibit further GABA release by inhibiting VG CA++ channels
What are the 3 subunits of GPCRs? What do we call them?
Heterotrimer: alpha, beta, and gamma subunits
Describe GABA binding to GABAb receptors on postsynaptic cell.
It binds to GABAb receptors and beta and gamma subunits of the receptor activate G-protein inwardly rectifying K+ channels (GIRK)
What are the 2 ways in which GABA hyperpolarizes the post synaptic cell?
- Binds GABAa receptors to let Cl- in
2. Binds GABAb receptors to let K+ out
What is spillover GABA? How would it affect a glutamate neuron? What is this called?
When there are many APs flowing down a GABAnergic neuron, a lot of GABA will be released and some of it, spillover GABA, can affect a glutamate neuron by binding GABAb heteroreceptors on the presynaptic terminal to inhibit the Ca++ channels to decrease the release of glutamate
Where are NT receptors located on a neuron?
Dendritic spines and shafts
What do sedatives do generally?
They decrease arousal
What are the 3 kinds of sedatives?
- Barbiturates
- Benzodiazepine
- Ethanol
What are 2 barbituate drugs? What receptor do they target?
- Sleeping pills
- Anti-epileptics
GABAa receptor agonists
What is the mechanism of action of benzodiazepine and barbituate?
Allosteric modulators and potentiators of the GABAa receptor to alter the effects of GABA binding = increase the flux of Cl- through the ion channel
What are the 4 mechanisms of action of ethanol?
- NMDA receptor allosteric inhibitor
- GABAa receptor agonist
- 5-HT3 receptor allosteric activator
- Blocks VG-Ca++ channels
Why is ethanol dangerous in terms of overdoses and withdrawal?
Because of its widespread effects
Can ethanol cross the BBB?
Yes
Why does alcohol cause memory loss (aka blackouts)?
Because its an NMDA receptors antagonist (allosteric inhibition) and will inhibit the formation of synaptic plasticity
Which act faster the AA NTs or the monoamines?
AA
What are the 5 monoamine NTs?
- Dopamine
- Norepi
- Epi
- Serotonin
- Histamine
What is the shorthand for serotonin?
5-HT
Is serotonin a catecholamine?
NOPE
What is another name for epinephrine?
Adrenaline
What are the 3 catecholamines? Why are they called like this?
- DA
- NE
- EPI
Because they contain a catechol group: aromatic ring with 2 OH groups next to one another and 1C away from ring attachment
Describe the structure of dopamine.
Catechol-CH2-CH2-NH2
Describe the structure of norepi.
Catechol-CH-(OH)-CH2-NH2
Describe the structure of epi.
Catechol-CH-(OH)-CH2-NH-CH3
What are the 2 main types of neurons in our CNS?
GABAnergic and glutamatergic neurons
What is the role of monoamine NTs in the CNS?
Diffuse modulatory role
What is the role of monoamine NTs in the PNS?
NTs of the sympathetic NS
Describe the synthesis of catecholamines (4 steps). Which is the rate limiting step?
- Tyrosine hydroxylase: tyrosine = L-dopa (OH added on aromatic ring) - RATE LIMITING
- Dopa decarboxylase: L-dopa = dopamine (remove COOH)
- Dopamine beta-hydroxylase: dopamine = NE (OH added on C1 of side chain)
- Phentolamine N-methyltransferase(PNMT): NE = EPI (CH3 added on N)
What does dopa stand for?
Dihydroxyphenylalanine
What is dopamine critical for? 3 things
- Reward
- Evaluating the saliency of our day to day lives
- Learning mechanisms
Where are dopamine neurons found?
- Ventral tegmental area
- Substantia negra
- Other structures deep within our midbrain
What NT is addiction to alcohol and opiates associated with? Describe it.
Dopamine system: aberrant learning of substance abuse that you know is bad for you
What is the other name of serotonin?
5-hydroxytryptamine
Describe the synthesis of serotonin (2 steps)
- Tryptophan hydroxylase: trypotophan = 5-hydroxytryptophan (OH added on aromatic ring)
- 5-HTP decarboxylase: 5-hydroxytryptophan = 5-hydroxytryptamine (COOH removed)
How are all monoamine NTs transported into vesicles?
Vesicular monoamine transporter (VMAT)
Describe the dopamine CNS system.
Dopamine neurons in the substantia negra and ventral tegmental area send within ascending dopaminergic fibers into areas of the brain that are responsible for detection of reward: basal ganglia, frontal lobe, striatum
What do all monoamine NTs stimulate?
Arousal
What area of the brain is degenerative in Parkinson’s disease?
Substantia negra
What is serotonin important for?
Mood
Describe the serotonin CNS system.
Rafi nuclei secrete serotonin and there is a very diffuse innervation across the CNS: cerebellum, hypo, temporal lobe, thalamus, neocortex, basal ganglia
Describe the norepi CNS system.
Locus coeruleus spontaneously secretes norepi to cerebellum, thalamus, neocortex, hypo, temporal lobe
Describe the epi CNS system.
Medullary neurons secrete epi into lower brain structures: medulla, hypo, cerebellum, corpus callosum, thalamus, hypo, pons, cerebral cortex
Rank the prevalence of nerves of the monoamines.
- Serotonin
- Dopamine
- Norepi
- Epi
How are the actions of monoamine NT stopped? 2 ways
- Removal by specific transporters (reuptake): DAT, NET (NE and EPI), and SERT
- Degraded in presynaptic terminal by monoamine oxidase or catecholamine-O-methyltransferase (COMT)
What are 2 examples of SSRIs?
- Paxil
2. Flouxetine
How do tricyclic antidepressants work?
Block the reuptake of ALL monoamine NTs
What is the biological issue of people who are chronically depressed?
They do not have enough monoamines in their CNS synapses in particular areas of the brain
What are the 3 classes of antidepressants? Which is the most prevalently used and efficacious?
- Tricyclic
- SSRIs***
- MAOIs
How do MAOI antidepressants work?
They inhibit monoamine oxidase to inhibit the degradation of monoamines at the synapse
What enzyme synthesizes ACh? Describe the reaction.
Choline acetyltransferase (ChAT): choline + acetyl CoA = ACh
How is ACh transported into vesicles?
Vesicular transporter AChT
How is ACh broken down?
Acetylcholinesterase (AChE) in the basal lamina of the postsynaptic cell
What are the 2 postsynaptic ACh receptors? Describe each
- Nicotinic = ionotropic non-selective cation channel
2. Muscarinic = GPCR activating K+ channels
How is choline transported into the presynaptic cleft for ACh synthesis?
Na+/Choline symporter
What is special about acetylcholinesterase?
One of the fastest enzymes known
What kind of drugs are AChE inhibitors? 3 kinds
- Organophosphate sarin nerve gas and pesticides
- Myasthenia gravis to promote contraction at the NMJ
- Dementia in Alzheimer’s and Parkinson’s patients (small pop of patients)
How can organophosphate pesticides be absorbed?
ALL routes: inhalation, ingestion, and dermal absorption
Describe sarin nerve gas.
Oderless and colorless
What are the initial symptoms of sarin nerve gas absorption? What do these mimic?
Mimic parasympathetic activation:
- Pupillary constriction
- Glandular hypersecretion
- Cognitive and mood effects
- Increased salivation and drooling
- Bladder and bowel emptying
What is the ultimate symptom of sarin nerve gas?
Death due to suffocation from lung muscle paralysis because the muscle will be depolarized for an extended period of time which will cause constriction of the lung muscles
What is the role of ACh in the PNS?
NT of the parasympathetic NS
What are 2 sarin nerve gas antidotes? Explain how each works.
- Atropine: muscarinic receptor antagonist (called duodote)
2. Pralidoxime: regenerates AChE (only if given within 5 hrs because can prevent covalent bonding of gas with AChE)
Describe the disease myasthenia gravis. Symptoms?
Autoimmune disorder caused by antibodies (IgG) binding and blocking nicotinic receptors at the NMJ
Impaired facial expressions and speech and swallowing
What is another name for myasthenia gravis?
Grave muscle weakness
How are AChE inhibitors adminstered to MG patients?
Orally
Describe how the NMJ is affected in MG patients. 5 steps
- Antibodies bind to nicotinic receptors and cross-link
- Signals are sent to speed up the decomposition of the receptors by phagocytosis
- Lysosomes help with the decomposition
- This results in the smoothing and simplifying of the post-synaptic terminal
- Chances of ACh binding to one of the nicotinic receptors decreases
Describe the ACh CNS system.
Pontomesencephalotegmental complex secretes ACh to medial septel nuclei, basal nucleus of Meynert, neocortex and thalamus
What is the main effect of ACh in the CNS?
Arousal
Which part of the brain are affected in dementia of Alzheimer’s and Parkinson’s disease patients
Neocortex
What types of NTs are co-transmitters?
AAs and monoamines
Do all vesicles in one axon terminal contain the same NT(s)?
NOPE
Can peptide NTs be co-released? With what NTs?
Yes with small NTs but not packaged in same NT vesicles
How many different NTs can be packaged in the same vesicle?
2 or more
Which requires more AP firing/more [Ca++]: release of vesicles with small NTs or vesicles with peptides?
Vesicles with peptides
What do stimulants do generally?
Promote arousal
What are the 3 types of stimulants?
- Amphetamines and cocaine
- Caffeine
- Nicotine
How do amphetamines and cocaine work in the CNS?
DAT and NET antagonists
How does adderall work?
DAT antagonist
How does caffeine work in the CNS?
Adenosine receptor antagonist
What is the role of adenosine in the CNS?
Ubiquitous neuromodulator
How does nicotine work in the CNS?
Nicotinic receptor agonist
Are the nicotinic receptors at the NMJ affected by normal doses of nicotine?
NOPE
What are the receptors to weed? What part of the brain are they found in? How do they worK?
CB1 = cannabinoid 1 receptor = presynaptic GPCR that blocks Ca++ channels and therefore NT release
Cortex, hypo, brain stem, hippocampus, cerebellum, amygdala
What are the highest expressed GPCRs in our brains?
CB1 receptors
Why does weed cause munchies?
Because the hypo has CB1 receptors
How does weed affect the brain stem? 7 ways.
- Nausea relief
- Rapid heart rate
- Reduced BP
- Drowsiness
- Pain reduction
- Reduced spasticity (muscle contraction)
- Reduced tremor
How does weed affect the hippocampus?
Impaired memory
How does weed affect the cerebellum? 2
Reduced spasticity and impaired coordination
How does weed affect the amygdala? 3
Reduced anxiety and blocking of traumatic memories, reduced hostility
What is the active ingredient of weed?
Tetrahydrocannabidol (THC)
What does THC mimic?
The effects of endogenous cannabinoids: 2-AG
What is the endogenous cannabinoid?
2-arachidonoylglycerol (2-AG)
Describe the signaling mechanism of 2-AG.
Retrograde signaling: postsynaptic release of 2-AG due to influx of Ca++ acts at presynaptic CB1 receptor
Is 2-AG lipid or water soluble? What problem does this cause?
Lipid, so we do not know how it crosses the synapse
What kind of NT is CRH?
Peptide NT
Are all NTs agonists?
YUP
What is synaptic plasticity?
The ability of chemical synapses to change their strength
What are the 3 types of GABAb receptors? Where is each located? How does each work?
- On presynaptic neuron: GABAb autoreceptor inhibiting calcium VG channels
- On postsynaptic neuron: GABAb receptor activates GIRK channels
- On neighbor glutamate neuron: GABAb heteroreceptor inhibiting calcium VG channels
What other NT (other than opioids) can inhibit C-fibers?
GABA
What is a mu-receptor antagonist? What can it block?
Naloxone
Blocks placebo effect on pain reduction
What are Schaffer collaterals?
Axon collaterals given off by pyramidal neurons in the hippocampus and are an integral part of memory formation
