Synapses and Neurotransmitters 1 - 3 Flashcards
synapse is
junction between 2 neurons allowing signals to pass evidence for neurons: golgi stain study of reflexes electron microscopy
electrical synapse
formed of gap junction that allows current to pass directly between neurones
directly connects cytoplasms
made of connexin proteins that join to make connexons then gap junctions
how to tell if neurons are connected by gap junctions
small molecules e.g. dyes diffuse from one neuron to other
researchers filled green neuron with red dye and saw another neuron fill with red dye
both hyper and depolarising stimuli passed from one neuron to other - blocked by deleting a connexin gene
electrical synapses are good for …
fast communication, synchronising neurons
first evidence of a chemical synapse
2 isolated frog hearts stimualte vagus nerve heart rate slows remove fluid add to other heart heart rate slows
prototypical chemical synapse
post synapse can be another neuron or non neuronal
e.g.
motor neuron — skeletal muscle
autonomic neuron —- hormonal gland, smooth muscle or heart
steps in chemical synaptic transmission
1 - package neurot in vesicle, put at pre synaptic terminal
2 - action potenital arrives —-> voltage gated Ca channels opens
3 - Ca influx —–> vesicles fuse to membrane, neurot released
4 - neurot diffuses across cleft, activate receptors on post syn membrane —-> further signalling
5 - neurot removed from cleft
synaptic vesicles vs secretory granules
‘clear’, small (40-50nm) —– ‘dense’, large (100nm)
small molecule neurot —– peptide neurot
filled by transporter proteins at pre-syn terminals —– created and filled by ER/ Golgi secretory apparatus
recycled by endocytosis —– ‘one and done’
fusion of vesicles
via SNAREs
Vesicle SNAREs or Target SNAREs
when vesicle is ready to fuse, SNAREs join together to anchor it down
synaptotagmin forms complex with SNAREs, it has Ca binding sites so when Ca enters it binds and changes synaptotagmins shape = makes SNARE ‘zip’ together forcing vesicle to fuse to plasma membrane
SNAREs are targets for toxins e.g. botulinum / tetanus toxin
bidning at post synaptic receptors
ligand gated ion channels (ionotropic) - directly depolarise or hyperpolarise post-syn cell
GPCR (metabotropic) - more complex effects
removal of neurotransmitter
different methods:
1 - diffuse away
2 - actively took up by transporters for recycling (into pre syn or glia)
3 - destroyed in synaptic cleft by enzymes
electrical vs chemical synapses
signals pass in both directions —– one direction
passed directly, cna only be attenuated —– can be radically transformed
fast —– slower
both are ‘plastic’ - can be modified, but chemical more so allowing summing up of inputs by post neuron
most synapses = chemical
neuromuscular junction
fast and reliable transmission
mtotor neuron action potenitals always cause muscle cell action potential
uses ACh
one of largest synapses
presyn - large no of active zones
postsyn - motor end plate, contains juncitonal folds that precisely align with active zones, densely filled with neurot receptors
how did we find that neurots come from vesicles?
stimulate motor nerve and record from muscle
- evoked response were all integer multiple of spontaneous potenitals
- statistical distribution showed neurot come in quantal packets
CNS synapses
axodendritic
axoaxonic
axosomatic
dendrosomatic
dendrodendritic
neurotransmitters should be …
in pre synaptic terminals
released in respnse to stimulation
acting on post syn
experiments for determining neurot molecules -
is it there?
immunohistochemistry e.g. antibody testing
experiments for determining neurot molecules -
does the cell express enzymes to synthesise it or transporter proteins to store it?
immunostaining - in situ hybridisation
experiments for determining neurot molecules -
is it released?
collect fluid around neruons after stimulating (difficult)
experiments for determining neurot molecules -
does it affect post syn cell?
test if molecule mimics effect of stimulating pre syn cell
experiments for determining neurot molecules -
does it block neurotransmitters?
apply drugs, delete genes encoding enzyme/transporters/receptors
types of neurotransmitters
amino acids
amines
peptides
neurons usually release only one kind of neurot , but can release more
often peptide releasing neurons also release small mol transmitter = co transmitter
amino acid and amine neurot
small molecules
stored in vesicles
bind to ligand gated ion channels or GPCR
peptide neurot
large molecules
stored in secretory granules
only bind to GPCRs
types of neurot receptors
lignag gated ion channel - ionotropic
directly depolarise/hyperpolarise post syn cell
GPCR - metabotropic
multiple possible 2nd messengers - allow amplification
convergence and divergence
allow flexibility
each transmitter can activate multiple receptors = divergence
each receptor can have different downstream effectors
different transmitters/receptors can activate same downstream effectors = convergence
glutamate
most common excitatory transmitter in CNS
amino acid
found in all neurons
3 ionotropic glutamate receptor subtypes
selective agonists
action is terminated by selective uptake into presyn terminal and glia
3 subtypes of glutamate receptors
AMPA
NMDA
kainate
AMPA receptors
mediate fast exitatory transmission
Ca cant pass through
channel opens and lets Na/K in
results in excitatory post syn potential - EPSP
at rest cell is mostly permeable to K so cells mem pot lies quite close to Nernst pot for K, when channel opens it allows permeablility to both so mem pot becomes more positive
NMDA receptors
often co exist with AMPA receptors
voltage dependant Mg block
only open when neuron is already depolarised
let Ca in —> downstream signalling
function as coincidence detectors:
at rest Mg is attracted by -ve voltage in cell which blocks opening of pore, if cell was previously depolarised Mg wont block so glutamate binds and it opens which lets Na, K and Ca in
glutamate also activates metabotropic glutamate receptors
mGluRs - allows glutamate to sometimes by inhibitory
ionotropic vs metabotropic glutamate receptors
- receptor
- example
- mechanism
- speed
4 subunits forming gated ion channel —– GPCR
AMPA, NMDA —– mGluR1/2
open ion channel —– activiate G proteins, trigger cascade
fast (msec) —– slow (sec/min)
GABA ( gamma amino butyric acid)
not an amino acid used to syntheisise proteins
made from glutamate by glutamic acid decarboxylase
action is terminated by selective uptake in presyn terminal or glia
normally an inhibitory neurot , most common one in CNS
produces IPSPs via GABA-gated chloride channels (GABAa receptors), if membrane potenital is above Nernst pot for Cl-
too much inhibition = coma/loss of conciousness (silencing too many neurons)
too little = seizures (over excited neurons)
modulation of GABAa receptors
other chemicals can bind to this receptor and modulate response to GABA binding
have no effect without GABA binding (allosteric binding)
ethanol
benzodiazepines
neurosteroids
barbiturates
GABA also acts via metabotropic GABAb receptors
GPCRs
act in diverse ways in different cells but may:
open K channels, close Ca channels, trigger other 2nd messengers like cAMP
often presynaptic or autoinhibitory - feedback loop
glycine
inhibits neurons via glycine gated chloride channels - opens channel Cl enters cekk which suppresses post syn from firing
or
binds to NMDA glutamate receptors, sometimes binding of glycine and glutamate needed for receptor to open
dendritic integration
multiple EPSPs in dendrites, current travels passively
spike initiation zone at cell body makes action potential and travels along axon
spatial arrangement
an inhibitory signal can block propagation of an EPSP towards soma
GABAa receptor dont always produce IPSP e.g if Vm is near Cl nernst potential
in this case they act by shunting inhibition
opening Cl conductance decreases membrane resistance - current leaks out of membrane
if arranged differently inhibitory signal would have no effect on action potential
presynaptic inhibition
GABA blocks output of another neurons presynaptically
GABA causes inactivation of Ca channels so less Ca enters and less neurots are released which causes a reduced effect on post syn membrane
acetylcholine
made by ChAT - from acetyl CoA and choline = good marker for cholinergic neurons
ACh broken down in cleft by acetylcholinesterase into acetic acid and choline
choline is re taken up by choline transporter to make more ACh
acts on nicotinic and muscarininc receptors
ACh and nicotinic receptors
ACh gated Na/Ca channel, found at NMJ in CNS
ACg and muscarininc receptors
5 types if GPCRs, found in CNS and ANS - vagustoff from experiment was ACh
how do different molecules affect ACh?
block release
block aceytlcholinesterase
activate ACh receptors
block ACh receptors
blocking release of ACh
botulinum toxin - by destroying SNAREs to stop fusion of vesicles therefore diapraghm doesnt work = die
black widow venom - causes large Ca influx = release ACh but stops further release
blocking ACh enzyme
nerve gas - ACh isnt broken down so activation of muscles is messed up
organophosphate pesticides - ACh stays in cleft too long
alzheimers treatment - cholinergic neurons die, so stop exsisting ACh from being taken up may help
activating ACh receptors
nicotine and muscarine
neonicotinoid pesticides
blocking ACh receptors
nicotinic - curare, alpha-bungarotoxin (snake venom)
muscarin - atropine
monoamines
synthesised from amino acids
catecholamines: dopamine, norepinephrine, epinephrine
seratonin: made from tryptophan
storage and removal of monoamines
packed into vesicles by vesicular monoamine transporters
removed from cleft by reuptake transporters - specific for each monoamine
destroyed by monoamine oxidase (MAO) and catechol-o-methyltranferase (COMT) on post syn cell - only for catechols not serotonin
monoamine receptors
mostly GPCR
different receptors activate different G protein effectors
neurotransmitter and its receptor
dopamine —– D1 like receptors: D1, D5 and D2 like receptors: D2,3,4
nor/epinpehrine —– adrenergic: alpha and beta types
seratonin —– 7 receptors - 6 GPCRs and 1 ligand gated Na/K channel
dopamine in motor control
dopaminergic neurons are in the nucleus substantia nigra and project to striatum
nigrostriatal pathway facilitates initiation of voluntary movement
these neurons die in parkinsons disease
treating parkinsons
by increasing dopamine
tyrosine –(tyrosine hydorxylase - rate limiting step)– L dopa —- dopamine
so can give L dopa to patients as it crosses blood brain barrier
MOA-B inhibitors
MOA-B is an enzyme that destroys dopamine
so can enhance remaining dopamines actions
dopamine in reward
these neurons live in ventral tegmental area and project into cortex and limbic system
mesolimbic pathway mediates reward/motivation
noradrenergic neurons
regulate arousal
small number in locus coeruleus innervate whole brain
sleep/wake, attention, arousal, mood
serotonergic neurons
regulate sleep/wake, mood
live in Raphe nuclei, project all over brain
drugs affecting monoamines
cocaine, amphetamines - block reuptake of dopamine (increase of dopamine in cleft = rewarding) and norepinephrine
antipsychotics - block dopamine receptors (possible side effect: Parkinson-like effects)
antidepressants:
tricyclics - block reuptake of NE, seratonin
SSRIs - selective, only block seratonin receptors
MOA-A inhibitors - stops seratonin being destroyed
other important neurotransmitters
opioid peptides
ATP
endocannabinoids
nitric oxide
opioid peptides
bind to opioid receptors - GPCRS
regulate pain perception, emotion etc
regulate coughing, GI tract
opioid receptors = target of morphine and heroin
ATP
often a co transmitter
P2X2 - ATP gated ion channels
P2Y2 - GPCRs
endocannabinoids
lipid soluble, not in vesicles
Ca triggers synthesis, not vesicle fusion
retrograde signalling ( postsyn—>pressyn)
bind to GPCRs - target of THC
nitric oxide
gas - membrane permeable
acts on soluble guanylate cyclase, not membrane receptor
