9 - drug action in the CNS Flashcards
what is the input region of a neuron, containing lots of receptors?
dendrites
how are neurotransmitters stored?
in synaptic vesicles
what are the 2 methods of inactivation/reuptake of neurotransmitters?
- transporters in presynaptic terminal
- enzymes break down neurotransmitter
describe the steps in the release of a neurotransmitter, its action and recycling
- when an AP arrives at the presynaptic terminal it causes the opening of voltage gated Ca++ channels and Ca++ enters the cell
- Ca++ causes the vesicles to bind to the presynaptic membrane
- neurotransmitter is released into the synapse by exocytosis
- the neurotransmitter binds to receptors on the postsynaptic membrane. the receptor determines if an excitatory or inhibitory signal is passed on
- the neurotransmitter is then broken down or directly taken back up into the presynaptic terminal by transporters where it is recycled
what are the precursors of acetylcholine (ACh)?
choline + acetyl coenzyme A
describe the synthesis of ACh
choline + Acetyl coenzyme A —> coenzyme A + acetylcholine (ACh)
what enzyme is used in the synthesis of ACh?
choline acetyltransferase (ChAT)
what does ChAT do?
transfers acetate ion from acetyl-CoA to choline
what is special about ChAT?
- specific to cholinergic neurons
- present in neuronal terminal in excess (ie. enzyme is not saturated)
ie. with enough precursor you can always make more ACh
how is ACh stored?
in synaptic vesicles in the axonal terminal
how is ACh released?
- released into the synaptic cleft upon the arrival of an AP and influx of Ca++
( synaptic vesicles bind with the presynaptic membrane and push the transmitter out into the cleft )
what does ACh bind to?
postsynaptic receptors on postsynaptic neuron or muscle — muscarinic (M1-M5) and nicotinic
produces either an inhibitory or excitatory signal
what enzyme is responsible for the breakdown of ACh and what is it broken down into?
Acetylcholinesterase (AChE)
breaks down ACh into choline + acetate molecule
once ACh has been broken down, what is taken back up into the presynaptic terminal?
choline
what is responsible for the reuptake of choline?
choline transporter
choline is recycled and made into ACh — stored in vesicles
the basal forebrain contains what 2 groups of cholinergic neurons?
- medial septal group
- nucleus basalis group
the cholinergic pontomesencephalon neurons project onto what?
hindbrain, thalamus, hypothalamus and basal forebrain
what is a neuromuscular junction (NMJ)?
a chemical synapse between a motor neurone and skeletal muscle fibre
communication at a NMJ is carried out by what?
ACh
what initiates the release of ACh at a NMJ?
the arrival of an AO propagating along the axon of the motor neuron
______ of the nerve endings leads to opening of presynaptic voltage-gated ___ channels and transmitter release ___-dependent vesicle _____
- depolarisation
- Ca++
- Ca++- dependent
- exocytosis
how does ACh binding at a receptor in a NMJ lead to depolarisation?
postsynaptic ligand-gated ion channels (“nicotinic” ACh receptors) open (conformational change) and let Na+ ions into the muscle cell, thus causing depolarisation
what happens after depolarisation in a muscle cell due to ACh?
AP is generated on the membrane of the skeletal muscle cell (due to depolarisation from Na+ entry) — this allows Ca++ entry — muscle contraction
____ entry into the muscle cell leads to muscle contraction
Ca++
action of ACh is terminated by what?
AChE = acetylcholine esterase
name a disorder of the NMJ
myasthenia gravis
describe myasthenia gravis
- an autoimmune condition that affects the nerves and muscles
- the immune system produces antibodies that block or damage muscle ACh receptors — prevents the muscles contracting (nowhere for ACh to act)
- this prevents messages being passed from the nerve endings to the muscles — results in the muscles not contracting (tightening) and becoming weak
what are commonly affected in myasthenia gravis
the eye and facial muscles and those that control swallowing
what can be prescribed for myasthenia gravis? how do they work? when do they work best?
medications such as Pyridostigmine — inhibitor of AChE — prevent the breakdown of ACh — increase synaptic availability of ACh
these medicines tend to work best in cases of mild myasthenia gravis. they can improve muscle contractions and strength in the affected muscles
in schizophrenia where do each of the following symptoms come from in the brain?
- aggressive
- affective
- positive
- negative
- cognitive
- aggressive = orbitofrotnal cortex + amygdala
- affective = ventromedial prefrontal cortex
- positive = striatum
- negative = mesocortical/prefrontal cortex, nucleus accumbens reward circuits
- cognitive = dorsolateral prefrontal cortex
antipsychotics working in the dorsal striatum can cause what SE?
extrapydramidal symptoms — parkinsonism
what does D2 normally do in the tuberoinfundibular pathway?
inhibit prolactin release
seizure control in management of epilepsy is achieved by what?
getting a balance between factors that influence excitatory postsynaptic potential (EPSP) and those that influence inhibitory postsynaptic potential (IPSP)
too much excitation = SEIZURES
what excitatory and inhibitory factors need to be in balance in treating epilepsy?
excitatory:
- EPSPs
- Na+ influx
- Ca++ currents
- paroxysmal depolarisation
inhibitory:
- IPSPs
- K+ efflux
- Cl- influx
- pumps
- low pH
what 4 types of drugs are currently available anticonvulsants in epilepsy treatment?
- drugs that inhibit Na+ channels
- drugs that inhibit calcium channels
- drugs that enhance GABA-mediated inhibition
- drugs that inhibit glutamate receptors
explain drugs that inhibit Na+ channels in epilepsy
- prevent the return of Na+ channels to the active state by stabilising the in the inactive state
(normally Na+ depolarises the cell causing AP generation and thus excitation — this is prevented)
explain drugs that inhibit Ca++ channels in epilepsy
- calcium channels are important in regulating neurotransmitter release
- inhibiting these results in reduced current though the T-Type calcium channels
- stops neurotransmitter release
what are calcium channel inhibitors particularly useful in controlling?
absence seizures
what are high voltage activated channels involved in?
neurotransmitter release
explain drug that enhance GABA-mediated inhibiton in epilepsy
- GABA receptor agonists — activate GABAR
- GABA reuptake inhibitors — increase synaptic GABA
- GABA transaminase inhibitors — reduce breakdown of GABA
what mediates the synthesis of GABA?
glutamic acid decarboxylase (GAD)
how is GABA stored?
packed into presynaptic vesicles by a transporter (VGAT)
GABA is released into the synaptic cleft in response to what?
an AP and the presynaptic elevation of intracellular Ca++
how is GABA released into the synaptic cleft?
by fusion of GABA-containing vesicles with the presynaptic membrane
neurons and glia take up GABA via specific GABA transporters (GATs) — what 4 have been identified?
GAT-1, GAT-2, GAT-3 and GAT-4 — each have a characteristic distribution in the CNS
how is GABA broken down within cells?
metabolised by the widely distributed mitochondrial enzyme GABA-transaminase (GABA-T)
what are the ionotropic glutamate receptors?
AMPA, Kainate & N -methyl-D-aspartate (NMDA)
AMPA + Kainate vs NMDA
- AMPA + Kainate — these sites open a channel throguh the receptor, allowing Na+ and small amounts of calcium to enter. fast acting — important in FAST Glu transmission
- NMDA — this site opens a channel that allows large amounts of calcium to enter along with the sodium ions. open later than AMDA
what is the NMDA channel blocked by in the result state?
magnesium
what site facilities the opening of the NMDA receptor channel?
glycine site
the metatropic Glu site is regulated by complex reactions and its response is mediated by ______?
second messengers
explain drugs that inhibit glutamate receptors in epilepsy
inhibit the action of glutamate = less excitation
where do CNS stimulants (drugs of abuse) have their main effect?
mesolimbic pathway:
ventral tegmental area (VTA : A10) in midbrain to limbic regions associated with reward, motivation, affect and memory
—> reward pathway
what does cocaine do?
inhibits DA transporter — inhibits reuptake — increases synaptic DA — intensifies and prolongs the stimulation of postsynaptic neurons in the brains pleasure circuits, causing a cocaine “high”
how does methamphetamine work?
increases release and inhibits transporter of DA
how does nicotine work?
activates presynaptic receptors — increases DA release in reward pathway
what are the current treatments for Alzheimer’s disease?
cholinesterase inhibitor — prevents ACh breakdown — increase ACh in synapse = good for cognition
they restore symptoms but don’t prevent degeneration of ACh neurons (cause)
what are some novel approaches to treating Alzheimer’s?
- secretase modulators — decrease AB42 production
- anti-aggregants — prevent AB aggregation
- immunotherapies — clear AB deposition
prevent amyloid buildup and neuroinflammation
what is AB in alzheimer’s?
AB peptide is precursor to amyloid plaques seen in AD
— AB deposition and neurodegeneration
