ALL OF CLINICAL NEUROOOOO

2 classes of synapses

electrical

• passive electrical flow (ion movement) from one neuron to another

• transmission: gap junctions, connexon channels

• quick, bidirectional → better synchrony

chemical

• presynaptic release of chemicals across synaptic cleft, bind to postsynaptic receptors

• transmission: synapses, chemicals

• slower (synaptic delay) → greater complexity

but also see mixed & heterogenous synapses

structure of electrical synapse

connexins form connexon channel (6 connexins/1 hemichannel)

pores are large, can allow metabolites, second messenger, ion movements

where do we find electrical synapses in mammalian brains?

spinal cord, brainstem, hippocampus mossy fibers, thalamic/cerebellar internerouns, glia cross-talk

discovering the chemical synapse

found by Loewi’s frog heart experiment

• stimulate heart, transfer solution form heart to new frog heart → other heart was stimulates

• conclusion: electrical stimulation resulted release of a chemical (acetylcholine) into fluid that affected transmission

synaptic vesicle journey

mobilization: released from reserve pool, trafficking to active zone

docking/priming: SnaRe formation

Ca influx; AP depolarizes nerve terminal, VG Ca channels open

Fusion/exocytosis

Vesicle recycling

neurotransmitters

chemical substance, released @ end of nerve fiber by AP, causes transfer of the impulse to another fiber/structure

synaptic vesicle synthesis

synthesized in golgi appartus & ER in soma, moved down axon to nerve terminal

synaptic vesicle reserve pool

NTs held in SV reserve pools

synapsin proteins bind to & tether SVs to each other

prior to action potential, CaMKII phosphorylates synapsin, releases it from SV → vesicles move to membrane

SV docking/priming

maturation process, allows SV to be released quickly in Ca dependent fashion

requires SNARE complex

SNARE complex

compilation of proteins that allow for vesicular membrane fusion to presynaptic membrane in active zone

• SNARE (NSF attachment protein receptor) binds SNAP (soluble NSF attachment protein)

• T SNARE = presynaptic membrane = syntaxin/SNAP25

• V SNARE = vesicle membrane = synaptobrevin

Ca & vesicular release

AP open VG Ca channels → Ca influx, determines amount of NT release (inhibition of Ca channels = inhibition of NT release)

Ca is NECESSARY AND SUFFICIENT FOR NT RELEASE/EXCITING POSTSYNAPTIC CELL

synaptotagmins = proteins embedded in vesicular membrane, Ca sensors

• integrate into SNARE complex, binds Ca → at high enough concentration, signals for vesicular fusion

• inserts into presynaptic membrane, creates curvature → fusion!

vesicle recycling / HRP movement

extra membrane removed via endocytosis

• proteins bind to vesicle (clathrin forms cage-like structure), form coated pits on vesicles

• coated vesicles are trafficked from membrane via acting, clathrin coat is removed

• vesicle is recycled in endosome,

presynaptic terminal disorders

disruptions in vesicle size, fusion, recycling, SNARE proteins, Ca channels

postsynaptic receptor families: ionotropic

direct

ligand-gated ion channels

transmitter binding/channel function together

fast transmission

postsynaptic receptor families: metabotropic

indirect

GPCRs

ion movement depends on intracellular metabolic steps

• NT binds to receptors, causes G protein dissociation → other proteins activated that open/close channel (G-proteins = transducers)

slow activation, but long-lasting effects

DREADDS = genetically modified GPCRs

how we know NTs contribute to membrane permeability

NMJ: ACh released, stimulates postsynaptic terminals → opens ligand-gated Na channel

more channels = more current flow

reversal potential

membrane potential voltage at which the current flow is reversed

determining inhibitory vs excitatory synapse

what channel NT receptor is bound to, concentration of permeant ions in/outside cell → change in current & generating E/IPSP is determined by Erev & threshold to generate an AP

ways to stop NT actions

diffusion, enzyme degradation, presynaptic reuptake, removal by glial cells

small molecule vs neuropeptide NT

synthesis: cytosol vs rough ER/golgi apparatus of soma

vesicles: small/clear core vs large/dense core

speed: fast vs slow

site of action: close vs distant

duration: short vs long

specificity: yes vs no

removal: reuptake vs enzymatic

acetylcholine: basics

synthesis: acetyl CoA + choline -(choline acetyltransferase)> acetylcholine

degradation in synapse: acetylcholine -(acetylcholinesterase)> acetate + choline

vesicle transporter: VAChT

transporter: ChT

excitatory

sarin gas

organophosphate, inhibits AChE → ACh builds up in synapse, continuously stimulates post-synaptic cell

leads to paralysis, bradycardia, seizures

cholinergic systems

basal forebrain

• attention, learning, memory, motivation, cognition

brainstem

• sleep-wale, sensory processing, attention, motor control

nicotinic ACh receptors

ionotropic

non-selective ligand-gated ion channel → allow Na/K to flow rapidlyy, generatae EPSP

toxins:

• block = a-bungarotoxin, a-conotoxin

• stimulate = arecoline, nicotine

composed of 5 subunits

• 2a, 1B, 1d, 1y/e (peripherral)

• 3a, 2B (neuronal)

  • ACh binding site on ALPHA SUBUNIT (both must be occupied!!!!!)

muscarinic ACh receptors (mAChR)

metabotropic

opens variety channels to exert excitatory vs inhibitory effects (hippocampus = +, striatum = -)

antagonists

• atropine, scopolamine, ipratropium

7 transmembrane domains

• 5 types: 1/3/5 = stimulatiory (Gq), 2/4 = inhibitory (Gi)

  • 1 binding site

Glutamate

vital for normal brain function, can’t cross BB so synthesized from precursors

synthesis: glutamine -(glutaminase)> glutamate

vesicle loader: VGLUT

transporters: EAAT (GLAST, GLT1) & SAT2

glutamine-glutamate cycle

• maintain Glu supply

• remove Glu from synapse

ionotropic Glu receptors

nonselective ligand-gated cation channels → produce EPSPs

3 types:

• AMPA (fast)

• NMDA (slow)

• Kainate (middle)

  • presynaptic = feedback mechanism

  • postynaptic = EPSPs

AMPA-Rs

largest EPSCs of Glu receptors, primary mediator of excitatory signaling in CNS

tetramers → GluA104, each has ligand binding & transmembrane domain

ligand binds → LBD shuts, pulls on gate helices in TMD → pore opens

NMDA-Rs

EPSCs longer acting than AMPA, allows Na/K/Ca conductance

voltage activated (opens only during AP)

• depolarization-dependent removal of Mg2+ from channel pore

• co-agonist = glycine needed to activate receptor

tetramers (GluN1-3)

• GluN2 (2 subunits) bind glu, GluN1/3 (2 subunits) bind glycine

• Glu/glycine bind → conformational change → pore opens

LTP: AMPA/NMDA-R working together

AMPA-R activation depolarizes membrane, removes Mg2+ block from NMDA-R

Ca enters cell, acts as secondary messenger, recruits more AMPA-R to surface

glutamate excitotoxicity

pathological process, nerve cells are damaged/killed b/c of excessive stimulation of Glu receptors _> high levels of Ca & Na

metabotropic GluR

slower PSC than ionotropic, can inhibit or excite

3 classes:

• mGluR1/5: stimulatiory (Gq), LTP

• mGluR2.3: inhibitory (Gi), autoreceptors & astrocytes, LTD

• mGlu4/6-8: inhibitory (Gi)

dimers

• 2 identical subunits → venus flytrap domain connected ia linker domain to transmembrane domaine

• Glu binds → flytrap closes, transmembrane domain twists, channel opens

GABA

inhibitory NT

breakdown requires mitochondrial enzymes GABA transaminase, succinic semidaldehyde DH → succinate → tricarboxylic acid

GABAa receptors

ionotropic

anion channel (mostly Cl), fast IPSP

pentamers (2a, 2b, 1y)

drugs that act on GABAa-R

benzodiazapines (valium, librium): increase GABA

barbituates (phenobarbital, pentobarbital) increase GABA

ketamine

ethanol

ADDICTIVTEEETIEITNEITNEN

GABAb receptors

metabotropic

inhibitory (K+ activation, block Ca channels), slow IPSP

heterodimers (B1/B2 subunits) → GABA binds to B1 domain

use venus fly trap domain like mGluR

Glycine

inhibitory

ligand-gated Cl channels

• similar to GABAa-R

• strychnine blocks pore, co-agonist for NMDA-R

50% inhibitor spinal cord synapses

pentamers (4a, 1B)

biogenic amines

modulate neuronal function in CNS/PNS

slow, diffuse action away from synaptic cleft

5 transmitters:

• catecholamines (DA/NE/EPI)

• histamine

• 5HT

catecholamines

tyrosine -(tyrosine hydroxylase)> DOPA -(DOPA decarboxylase)> DA -(DA-B hydroxylase)> NE -(PANMT)> EPI

TH = rate limiting, co-substrate O2, co-factor BH4

Dopamine

4 pathways:

• mesocortical

• mesolimbic

• nigrastriatal

• tuberinfundibular

vesicular packaging: VMAT

reuptake: DAT (Na dependent)

catabolism: MAO, COMT

receptors: metabotropic, 5 domains, Gi or Gs

Norepinephrine

vesicular packaging: VMAT

reuptake: NET (Na dependent), also uptake DA

catabolism: MAO, COMT

receptors: metabotropic, 2 types

• a or B

  • a1 = slow depolarization, K inhibition

  • a2 = slow hyperpolarization, K activation

  • B1/2 = inhibitory

Epinephrine

low brain levels, in medulla/lateral tegmental that project to hypothalamus

vesicular packaging: VMAT

uptake: NET

catabolism: MAO, COMT

receptors: same as NE

histamine

synthesized in hypothalamus

general roles: arousal, attention → inflammation in allergic reactions

vesicular packaging: VMAT

uptake: plasma membrane MAT

catbolism: methyltransferase, MAO

receptors: metabotropic, H1-4

• H1 antagonists = motion sickness, allergies, H2 antagonists = GI disorders

Serotonin

synthesis: tryptophan -(tryptophan hydroxylase)> 5-hydroxytryptophan -(AADC)> 5-HT

vesicular packaging: VMAT

uptake: SERT

catabolism; MAO

receptors: metabotropic, except for 5HT-3 which is ligand-gated ion channel

ATP as a NT

purine

co-released with conventional NTs

can change neuron electrical properties → excitatory in spinal cord motor, dorsal horn, sensory ganglia, hippocampus

catabolism: enzymatic reaction to adenosine

receptors:

• ionotropic: P2X-R (trimeric, non-selective cation channel)

• metabotropic: P1-R, P2Y-R → xanthines/theophylline

neuropeptides

peptides that can act as hormones AND NTs

synthesis: rER propeptide → golgi apparatus peptides

catabolism: peptidases

receptors: metabotropic

5 categories: brain-gut, opioid, pituitary, hypothalamic release hormones, all others

endocannabinoids

unconventional NT b/c retrograde signaling

• produced in post-synapse in Ca dependent manner, diffuse across membrane

catabolism: FAAH

receptors: CB1 (Gi, THC bind site, inhibit presynaptic GABA), CB2 (Gi, peripheral)

nitric oxide

unconventional NT b/c gas

byproduct of NOS conversion of arginine into citrulline → Ca dependent

diffuse thru membrane & extracellular space → short-lived

acts directly on intracellular targets (second messenger)

3 components of a chemical synapse

molecular signal, receptor molecule, effector molecule

3 classes of chemical signaling molecules

cell-impermeant (most NTs), cell-permeant, cell-associated

4 classes of signal transduction

ligand-gated ion channels: nACh, AMPA/KA, NMDA, GABAa

GPCRs: mACh, DA, NE, 5HT, GABAb, peptides

enzyme-linked: NGF/BDNF receptors (Trk) (protein kinases)

Intracellular: steroid hormone (gene transcription)

how do the 4 classes alter gene transcription

ionotropic: Ca enters cell, acts as 2nd messenger

metabotropic: couple to 2nd messenger systems

enzyme-linked: tyrosine kinases

intracellular: bind to/edit mRNA in nucleus

kinase

enzyme that adds phosphate groups to other molecules → ATP = donor

phosphatase

enzyme that removes phosphate groups → water is recipient

guanine nucleotide binding proteins 2 classes

heterotrimeric

• 3 subunits: A/B/y

• a binds GDP, BY joins inactive

• signal binds, GDP→GTP

• a dissociates from STP, all active

• B/y dissociate, proteins act on downstream

  • inactivate by GTP -(GAP)>GDP

monomeric

• single subunit

• bound to GDP = ianctive

• GDP -(GEE)> GTP once signal binds to receptor, facilitated by adaptor proteins

Gs proteins

effector: adenylyl cyclase

2nd messenger: cAMP

late effector: PKA

Gq proteins

effector: PLC

2nd messenger: DAG, IP3

late effector: PKC, Ca

Gi proteins

effector: adenylyl cyclase

2nd messenger: cAMP

late effector: PKA

BUT INSTEAD OF ACTIVATING, IT INHIBITS EFFECTOR

ampligication

individual signaling molecules can generate a larger number of products thru several enzymatic reactions

main advantage: control over cellular behavior across longer time course

PLC

effector for Gq proteins

results in activation

mechanism:

• a subunit activates PLC, PLC hydrolyzes PIP2 into DAG + IP3

• IP3 acts on ER receptors, releases CA

• DAG interacts w/ Ca to activate PKC

Ca as a second messanger

most common

sources: ion channels, intracellular organelles

bind to things like calmodulin & synaptotagmin

removal: pumped out of cell/into intracellular storage, binds to buffer proteins

cAMP

effector for Gs/Gi proteins

produced cAMP & cGMP → cAMP binds to PKA, cGMP binds to PKG → both bind to/open ligand gated ion channels

PKA

Gs/Gi pathways, primary effector of cAMP

2 reg/2 catalytic subunits

catalytic domain has specific amino acid sequence → targets specific proteins

• kinase anchoring proteins (AKAPs) can localize PKA to specific locations

PKC

Gq-protein, primary effector of DAG/Ca

Ca makes it move to plasma membrane, DAG bind regulatory domains → reg domains separate, diffuses thru cell & phosphorylates targets

CaMKII

Ca/calmodulin dependent

most abundant component of post synaptic density

12 subunits, each has catalytic/regulatory domain

• low Ca, domains binds together

• high Ca disinhibits catalytic domain

undergoes autophosphorylation

protein tyrosine kinases two classes

receptor tyrosine

• enzyme-linked, transmembrane, extracellular binds ligan & intracellular = catalytic

non-RTK

• cytoplasmic/membrane associated

• indirectly activated by extracellular signals

MAPK

activated by other kinases, part of cascade:

• extracellular growth factor → RTK → monomeric G proteins

also activated by heat shock/stress

activation: phosphorylation of active loop, conformation change activates catalytic domain

common phosphatases

PP1: activated by PKA, dephos AMPA/NMDA-R, K/Ca channels

PP2A: constitutively active

PP2B (calcineurin): acutely controlled by intracellular Ca2+, AMPA phosphorylation in LTD

2nd-messenger mediated regulation of gene expression

phosphorylation state of creb can increase/decrease transcription

• creb = ubiquitous transcription factor, binds to CRE on DNA, creb-sensitive genes = cfos, BDNF, TH, neuropeptides, involved in LTP

nuclear (intracellular) receptors

receptors for membrane permeable ligands

different reg mechs

• glucocorticoid: bind to cytoplasmic receptor, cause unfolding & movement into nucleus, binds to DNA & activated RNA complex

• thyroid hormone: receptor binds to DNA & suppresses transcription, TH binds causing a conformation change, promoter sequence is open

c-fos

immediate early gene, transcription factor

active for stimulus-induced tasks (30-60 min), activated second-order (delay response) genes

regulatory regions: cytokines/ciliary neurotopic factors, growth factors, CREB

NGF/TrkA signal transduction pathway

enzyme-linked receptor

NGF = neurotrophic growth factor, need for differentiation/survival/synaptic connectivity

NGF binds TrkA → neurite outgrowth (ras/PLC), survival (PI3)

LTD in cerebellar purkinje cells

presynaptic parallel fibers realease Glu into synapse → activate AMPA-R & mGluR (Gq)

if climbing fibers activate at same time…

• stronger EPSP/Ca influx

• increased sensitivity of IP3-R increases Ca near parallel fiber synapse

• PKC activates → AMPA-R is phosphorylated, leads to internalization of receptor

net results: less AMPA-R near parallel fiber synapse → less Glu signaling → LTD at synapse

phosphorylation of tyrosine hydroxylase

phosphorylating TH causes conformational change that increase catalytic domain activity → increased catecholamine production

• phosphorylated by PKA, CamK11, MAPK, and PKC

synaptic plasticity

experience/activity dependent changes in synaptic tranmission

2 types

• short term: lasts for a few minutes

  • 4 types: facilitation, augmentation, potentiation, depression

• long term: 30 mins to years

  • 2 types: LTP vs LTD

  • involves regulation of gene expression

hebbian theory of plasticity

cells that fire together wire together

facilitation

rapid increase in synaptic strength due to 2 APs occurring in close succession → time dependent

due to prolonged presynaptic Ca level increase → more vesicular release

synaptotagmin7 = trigger for enhanced vesicular release

augmentation

enhances Ca dependent fusion of vesicles to presynaptic membrane

lasts a few seconds

occurs by Ca enhancing action of presynaptic SNARE reg-protein munc13 → SUPER PRIMING = more vesicular release

potentiation

post-tectonic potentiation

enhances Ca dependent fusion of vesicles to presynaptic membrane

occurs of 10secs - 1 min, can outlast the stimulus

Ca activation of kinases

depression

progressive depletion of vesicular reserve pool

increase activity = increase vesicles release = increase depletion of vesicles available = increase depression

• as activity decreases, vesicle numbers recover & see less depression

vesicle depletion hypothesis

until the release pool is replenished by the reserve pool, there will be a decrease in synaptic strenth → less synapsin = more depression

interaction between short-term plasticity mechanisms: NMJ

repeated stimulus = increased calcium

• facilitation occurs, then augmentation

vesicles depleted → depression occurs

vesicle pool is replenished, another stimulus causes potentiation b/c lingering Ca in terminal

habituation

process by which an organism becomes less responsive to a repeated stimulus

sensitization

generalization of an aversive response elicited by a noxious sitmulus to other non-noxious stimuli

mechanism of habituation

Glu transmission between sensory & motor neuron decreases due to synaptic depression

mechanism of sensitization

sensory neurons synapse onto presynaptic mechanosensory terminals, release 5Ht to stimulate NT release

synaptic mechanism of short-term sensitization

modulatory interneuron releases 5HT onto sensory

Gs proteins activate (increase adenylyl cyclase/cAMP)

cAMP activated PKA

K channels are phosphorylated (decrease chance of opening during AP → prolongs presynaptic portion of AP)

more Ca channels open

more NT release

synaptic mechanisms of LTP require changes in gene expression (in aplysia)

repeated tail shocks: PKA activated CREB

CREB activates enzyme ubiquitin hydrolase, degrades PKA reg subunits → persistent PKA activation

CREB activated T.F. C/EBP → promotes addition of more synaptic terminals

LTP

activity dependent strengthening of synaptic transmisison

LTD

activity-dependent weakening of synaptic transmission

synaptic plasticity in mammals: hippocampus

high frequency stimulation of hippocampus increases synaptic transmission

properties of LTP

specificity (restricted to activated synapse), associativity (links 2 or more independent processes), coincidence detection (presense of 2+ simultaneous signals) → NMDA-R = coincidence detectors

requirements for induction of LTP

postsynaptic depolarization

• HFS or associative induction of LFS

• AMPA/KA-R mediated depolarization

NMDA-R activation

• Mg block released when synapses are active & Glu is released

Ca influx

• restricted to dendrites of individual spines

*induction = NMDA, expression = AMPA

expression of LTP (1-2 hr)

AMPA.KA-R activation causes postsynaptic depolarization, Mg block removed from NMDA-R

Glu binds NMDA-R, Ca enters cell

AMPA-R recruitment from recycling endosome to membrane mediated by synaptotagmins

Ca binds calmodulin, CaMKII/PKC activation

• facilitate AMPA delivery to extrasynaptic areas

increased sensitivity to Glu

LTP late phase

relies of PKA, activation of TF like CREB → probably involves creating new dendritic spines

silent synapses

observation: no postsynaptic EPSPs when Glu was stimulated, but robust potential when membrane was depolarized

synapses only contain NMDA-R

prevalent in development & SUD

LTD process

solution to problem of continuous strengthening

result of LFS

decreased EPSPs for several hours, synapse specific

expression of LTD

small & slow rise in Ca

activation of phosphatases

• PPI, PP2B (Ca dependent)

removal of AMPA-R in clathrin-dependent endocytosis manner to endosome

late phase requires protein synthesis

LTD in cerebellar purkinkje cells vs hippocampus

Purkinje involves kinases, no Ca thru NMDA-R

similarities: AMPA-R internalization, CREB activity in late phase

sensation

conversion of sensory info into a neural signal