synaptic exam 3

ionotropic receptors

ligand gated ion channels, NT (ligand) binds to binding site on receptor and causes a conformational change in receptor structure, opening the channel

metabatropic receptors

cause second messanger mediated effects in cell after NT binding, NT binds, conformational change, signaling cascade in cytoplasm can be activated on their own

GTP- binding Protein coupled receptors

when ligand binds to G-protein coupled receptors, conformational change, activation of heterotrimeric GTP binding protein, intiating signaling cascade.

receptor tyrosine kinase

most common type of growth factor

serine/theronine

specific protein kinase

guanylate cyclase receptor

causes relaxation of smooth muscle

cytoplasmic receptors

located in cytoplasm, binds ligands that diffuse across plasma membrane, after binding ligand-receptor complex changes conformation to be active transcription factor that moves into nucleus and binds to DNA changing expression of genes target gene transcription

speed of receptors

ionotropic: rapid onset of effects

metabotropic: slow onset of effects

termination of receptors

ionotropic: rapid termination of effects

metabatropic: slow termination of effects

ratio of action and response

ionotropic: 1:1 relationship between action and response

metabatropic: >1:1 amplification of response by G-protein and second messengers

effect variation of receptors

ionotropic: limited by type of ion channel that is part of the receptor protein

metabatropic: diverse effects from single NT because of multitude of second messenger-mediated signaling pathways

binding range of receptors

ionotropic: really close

metabatropic: can be farther than ionotropic

site of NT release of receptors

ionotropic: near site of NT release

metabatropic: some distance from site of NT release

dimmer switch

always some baseline level of activity

types of metabatropic receptors

• G-protein coupled

• enzyme linked

• cytoplasmic receptors

enzyme linked

• receptor tyrosine kinases, seronine/theronine kinases, guanylate cyclase receptors

• ligand binding initiates kinase cascade

competitive antagonist

binds to same site as agonist (orthosteric binding) but doesnt activate receptor, reduces/prevents activation of channel by agonist

non-competitive antagonist

binds to receptor at different site from agonist (allosteric binding) but prevents/reduces activation of receptor , NAM

NAM

negative allosteric modulator

agonist

compound that elicits same biological effects as endogenous (naturally occuring) ligand when it binds to receptor

antagonist

compound that reduces or eliminates effect of agonist when bound to receptor

nAChR agonist

• nicotine

• acetylcholine

nAChT antagonist

• tubocuranine

• alphabungarotoxin

reversible antagonist

binds non-covalently to receptor so it can come off receptor when washing off

irreversible antagonist

binds covalently to the receptor, so it cannot come off by competing ligands or by washing off

ionotropic receptors

• selective ligand binding site

• selective pore that can be opened

• EP is determined by electrochemical signal gradient of permeant ions

4 families of ionotropic receptors

1. pentameric (cys-loop) receptor family -> pentamer

2. glutamate receptor family -> tetramer

3. trimeric receptor family -> trimer

4. TRP receptor family -> tetramer

pentamer

• 5 proteins

• nACH receptors

• 5-HT3 receptors

• GABA a receptors

• gylcine receptors

• ZAC

tetramer

• 4 proteins

• NMDA

• AMPA

• kainate receptors

trimer

• 3 proteins

• ATP (P2X) receptors

• ASIC (H+) receptors

tetramer (TRP)

• 4 proteins

• TRP receptors

pentameric receptors structure

extracellular loop with 2 cystein amino acids

• 5 separate subunits

-- 4 membrane spanning segments (M1-M4)

M2 forms pore

nAChR subunits

acetylcholine binds to alpha subunit

• 2 molecules must bind to gate receptor

permeable to sodium, potassium, and calcium

EP= -10 mV

homomeric nAChR

alpha only

heteromeric nAChR

alpha and beta subunits

neuronal AChR

1. different subunit composition

2. varying affinity for acetylcholine and nicotine

3. pass more calcium than muscle AChR

4. presynaptic in CNS

5. susceptible to desensitization

ligand binding

• probabilistic

• binding frequency binds on concentration of ligand

• amount of time ligand spends bound to receptor binding site depends of ligands Kd

dissociation constants Kd

• measure of strength binding affinity

• ratio of unbinding rate over binding rate

• concentration at with 50% of binding sites are occupied

• Kd and affinity are inversely related

-- higher affinity lower Kd

-- lower affinity higher Kd

nAChR in neurons

higher affinity

nAChR in muscle

lower affinity

5 HT3 receptor

non-selective monovalent cation channel

5 subunits (A-E)

A binds serotonin

• found in GI tract and brainstem

GABA receptors

GABA B is metabatropic

GABA A: pentameric with nAChR

• ligand gated chloride channels

• EP= -70mV

• alpha, beta, and gamma subunits

hyperpolarizing inihibition

DF on GABA channels are determined on Ecl because they are chloride channels

DF=VM-Ecl

• chloride current through GABA will always move EP towards calcium

• although it is depolarizing, it is still hyperpolarizing because -70mV is considered hyperpolarizing

shunting inihibition

depolarization is lowered

glycine receptor

inhibitory transmitter in brainstem and SC

permeant to chloride

faster opening and closing than GABA

Zinc activated channel receptors ZAC

can antagonize or modulate other receptors (GABA or glutamate)

• zinc specific responses

-- nonselective monovalent cation channels

-- present in some mammals not others

• can be proton receptors

Glutamate receptors

• most common type of NT

NMDA, AMPA, kainate

• four subunits, 3 transmembrane domains each has large extracellular loop (2 domains that form a clamshell)

domains of glutamate clamshell

1: amino terminal domain: binds antagonists

2: ligand binding domain: binds glutamate and agonists

conductance of glutamate receptors

AMPA and kainate: low

NMDA: high

gating speed of glutamate receptors

AMPA and kainate: fast

NMDA: slow

Ion permeability of glutamate receptors

AMPA and kainate: sodium, potassium and sometimes calcium

NMDA: sodium, potassium and calcium

Mg block of glutamate receptors

AMPA and kainate: no

NMDA: yes

NMDA

• regulator of synaptic plasticity

• two GluN1 and two GluN2 subunits

-- glycine binds to GluN1 (co-agonist)

-- glutamate binds to GluN2 (activator)

• larger percentage of current is calcium than nAChR current

• blocked by Mg at rest

coincidence detector

NMDA because presence of glutamate and depolarization

AMPA

• fast and excitatory

• GluA1-4 subunits

• GluA2 blocks calcium so those without A2 are permeable to calcium

• no magnesium, no glycine needed

kainate

• fast and excitatory

• less common than AMPA and NMDA

• found in hippocampus, cerebellum, amygdala, retina, and SC

• GluK1-5

• a little slower than AMPA, not as slow as NMDA

metabatropic

• link between NT and cellular signaling cascades

• extracellular binding of NT to receptor is first signal

• second signal is inside the cell

-- cAMP

discovery of second messengers Experiment 1

liver homogenate with cytoplasm and cell membranes received epinephrine which increased liver phosphorylase activity

discovery of second messengers Experiment 2

liver homogenate no longer has cell membranes, received epinephrine and no change in liver phorsphorylase activity

discovery of second messengers Experiment 3

product homogenate from experiment 1 (increased LP activity) removed membranes and combined with homogenated from experiment 2 (no change LP activity) without membranes resulting in increased liver phosphorylase activity

discovery of second messengers

external signal (first messenger) --> receptor (transducer/primary effector)--> secondary messenger/secondary effector

metabatropic receptor structure

• lots of G-protein receptors

• lots of ligands (high variation in ligand binding site)

• 7 transmembrane domains

-- 3 extracellular loops
-- 3 intracellular loops

activation of G-protiens

1. ligand binds to receptor

2. G alpha unit on receptor unbinds GDP and binds GTP

3. G alpha unit and Gbeta/gamma unit separate

4. G alpha unit excites adenyl cyclase generating cAMP

5. G beta/gamma unit binds to protein / phosphorylates

deactivation of G-proteins

1. GTPase is activates and unbinds GTP from G alpha

2. GDP binds to G alpha unit

3. Full unit binds to Gbeta/gamma again

4. full unit binds to receptor again

G proteins are diverse

• 16 genes for alpha subunit

• 5 genes for beta subunit

• 14 genes for gamma subunit

antisense oligonucleotide knockdown

determine what proteins are present for given receptor

• prevents translation of mRNA into protein

termination of metabatropic signaling

1. GTPase activity of G alpha subunit

2. interaction with primary effector can speed up GTP hydrolysis

3. regulation of G-protein signaling proteins

how to metabatropic receptors affect neuronal activity

• alter calcium channel function

• alter potassium channel function

• release calcium from intracellular stores

• alter probability of release

• alter post synaptic receptor sensitivity

• protein upregulation

direct pathway

acetylcholine --> mAChR (Gbeta-gamma/ potassium channels)

cAMP pathway

norepinephrine--> beta adrenergic receptor --> Galpha s --> adenyl cyclase --> cAMP --> cAMP-dependent protein kinase A (PKA)

phosphinositol pathway

acetylcholine --> muscarinic AChR --> Galpha q --> PLC --> IP3 (calcium release) / DAG (PKC)

arachidonic acid pathway

histamine --> histamine receptor --> Galpha i/o --> PLA2 --> achidonic acid --> 5-lipoxygenase/ 12-lipoxygenase/ cyclooxygenase

modulating GIRK channels

beta/gamma dimers activate GIRK channels, Galpha dont

transducer binding sites on GIRK

• direct pathway: subcompartment in local cell environments that bind elements together

-- identifying binding sites on ion channels for activated G protein binding

• beta/gamma has 2 binding sites from in vitro binding and displacement with peptides

• Galpha can bind but only to speed rate of GTP hydrolysis

• whole heterotrimer can bind to help colocalize channel near receptor

regulation of G protein coupled receptors

• receptors become desensitized when exposed to ligand for long period

-- can uncouple

-- can remove receptor from membrane

-- phosphorylation can change the G protein that couples and change signaling pathways activated

G protein coupled receptor signaling specificity

1. multiple types of G protein subunits

2. receptor can couple to different signaling pathways

3. signaling pathways can be spatially restricted

basic job of Nervous system

• collect information

• assess

• generate adaptive responses

short term synaptic plasticity: paired pulse facilitation

• weak synapse

• 1st AP: low vesicle depletion

• 2nd AP: high vesicle depletion --> a lot of calcium released

tetanic potentiation

• large stimulus train results in high NT release

tetanic stimulus

train of APs in short succession

RNS

• surface electrodes apply tetanic stimulus to peripheral nerve

• record from connected muscle

-- magnitude of CAMP (compound muscle AP) is proportional to number of muscle fibers activated by stimulus

• in neuromuscular disorders, tetanus can cause different responses

-- LEMS: CMAP is small, and builds throughout tetanus

CNS synapses are weak

• can undergo facilitation easily

• allows neurons to use firing rate without sacrificing strength of communication

-- residual calcium helps release probability even when vesicles are depleting

--- balances strength of post synaptic response during burst firing

• weak synapses are high pass filter

• short term depression is low pass filter

• short term plasticity is important for working memory

high pass filter

AP stimulation is only effective at high frequencies

low pass filter

AP stimulation is only effective at low frequencies

calcium buffering mechanisms

• buffers saturation

• channel modulation

• synaptotagmin 7

buffer saturation

the short-term increase in neurotransmitter release when two action potentials arrive at a synapse in rapid succession

channel modulation

the influx of calcium during a first action potential directly alters or "facilitates" the property of the presynaptic calcium channels themselves, making them open more readily or pass a larger calcium current during a closely following second action potentia

synaptotagmin 7

acts as the high-affinity, slow calcium (Ca²⁺) sensor that mediates paired-pulse facilitation (PPF) and short-term synaptic enhancement at central nervous system synapses

autocrine

• neuron signaling itself

• inhibitory

paracrine

neurons releasing NT into general space instead of specific synapse

endocrine

chemical communication by blood

metabatropic receptors can cause changes in membrane potential

1. acetylcholine released from superior ganglion in chain acts on B cell within ganglion

2. acts on extrasynaptic muscarinic receptors and nAChRs

3. LHRH diffsues from distance and acts on LHRH receptor

causes slow EPSP

habituation

reduction in responsiveness to environmental stimulus after repeated/prolonged exposure

short-term habituation

• increased intracellular calcium during low frequency stimulation activating small GTP binding proteins decreasing amount of vesicle release

• doesn't occur during high frequency stimulation because high intracellular calcium activates PKC which inhibits small GTP binding proteins

m= npq

m = EPSP size

n= changes - small GTP binding proteins shut down release sites

p= no change

q= no change

sensitization

repeated exposure to stimuli can lead to increase in responsiveness

mechanism of sensitization

1. serotonin is released from interneuron activating cAMP pathway

2. PKA increases vesicle release by blocking potassium channels and phosphorylating proteins that increase probability of release

H.M

• lost anterograde declarative memory (recall of facts)

• non declarative memory was still intact (performance)

• lost sense of time

learning to memory pathway through hippocampus

neurons in entorhinal cortex synapse with neurons in dentate gyrus --> send axons along mossy fiber pathway to synapse with pyramidal cells in CA3 region of hippocampus --> send axons along Schaffer collateral pathway to synapse of pyramidal neurons in CA1 region of hippocampus

homosynaptic long term potentiation (LTP)

long term increase in synaptic strength

• perforant pathway from entorhinal cortex --> dentate gyrus neuron --> CA3 pyramidal neuron --> CA1 pyramidal neuron (trisynaptic loop)