central nervous system L14-16 Flashcards
Parkinsonism
drug/stroke/infection induced symptoms
motor symptoms of parkinsons
resting tremor
muscle rigidity
suppression of voluntary movements
post-mortem neuropathology of Parkinson’s
loss of DA cell bodies in substantia nigra
(PET images)
pathology of Parkinson’s disease
degeneration of DAergic neurones of nigrostriatal tract
therefore loss of DA transmission in striatum
parkinsonism pathology
any condition in which DA transmission is lost in striatum via D2
pharmacological blockade/ brain lesion
4 dopamine pathways in brain
nigrostriatal
mesocortical
mesolimbic
tuberoinfundibular
nigrostriatal
substantia nigra to striatum
mesolimbic
ventral tegmental area to frontal cortex
tuberoinfundibular
arcuate nucleus to pituitary gland
pharmacotherapeutic aim of Parkinson’s
increase DA transmission in striatum
dopamine synthesis
tyrosine > L-DOPA > dopamine
via tyrosine hydrolase and then DOPA decarboxylase
dopamine storage
vesicles
VMAT transports DA to vesicles
dopamine metabolism
monoamine oxidase (MAO) will metabolise DA outside the vesicles
DA metabolism process
DA > DOPAL > DOPAC > HVA (Homovanillic acid)
monoamine oxidase isoforms
MAOa and MAOb
different gene products
both on X chromosome
MAOb mainly metabolises DA
DA release
depolarization of terminal causing exocytosis
v-gated Ca2+ channels allow Ca2+ into terminal
vesicles fuse w membrane
DA receptors
G-protein coupled
D1-like > stimulate adenylate cyclase/ excitatory
D2-like> stimulate inhibit adenylate cyclase/ open K+channel
DA reuptake
via transporter/ DAT
L-DOPA
levodopa
precursor bypassing rate-limiting enzyme
^DA synthesis
DA polarity
can’t cross membranes
sympathomimetic effects
metabolised in gut by MAO
MAO inhibitor
selegiline
MAOb inhibitor
blocks intraneuronal metabolism of DA
^DA content of vesicles
less intraneuronal breakdown
D2 Receptor agonists
bromocriptine
or apomorphine/ lisuride
(inhibit striatal output neurones directly)
Parkinson’s therapeutics
L-DOPA w carbidopa
given in combo, effects wear off as neurons degenerate
:( psychosis/ cognitive dysfunction/ addiction
latroigenic/ drug-induced parkinsonism
antipsychotic drugs block D2 receptors in all 4 pathways
depression symptoms
poor concentration
altered appetite
anhedonia
despair
depression pathology
unknown
pharma suggests dysfunction of 5-HT/NA systems (noradrenaline/ 5-hydroxytoyptamine)
locus coerulus
NAergic cell bodies projecting to hypothalamus and midbrain/ hippocampus and cortex
dorsal and median raphe nuclei
contain seretonergic pathways
NA production
tyrosine> (tyrosine hydroxylase)> L-DOPA> (DOPA decarboxylase)> dopamine> (dopamine beta-hydroxylase)> noradrenaline
5-HT production
tryptophan> (tryptophan hydroxylase)> 5-HTP> (5-HTP decarboxylase)> 5-HT
NA/ 5-HT storage
in vesicles protecting from MAO
intraneural metabolism by MAO
NA/5-HT metabolism
MAO
present in DA/NA/5-HT
transmitter breakdown in cytosol
MAOa and MAOb metabolize both NA/5-HT
NA metabolism process
NA>(MAO/aldehyde dehydrogenase)> DOPEG>(catechol-o-methyl transferase)> MHPG
5-HT metabolism process
5-HT>(MAO)>aldehyde>(aldehyde dehydrogenase)> 5-HIAA
NA/5-HT release and receptors
depolarization causes exocytotic release
NA receptors
G-protein coupled receptors
alpha/ beta
a1> stimulate P1 cycle
a2> inhibit adenylate cyclase/ open K+ channels/ hyperpolarize/ ^cAMP
B1,2,3> stimulate adenylate cyclase
5-HT receptors
G-protein coupled
5-HT(a-f) > inhibit adenylate cyclase
5-HT(1a)> open K+ channel
5-HT2(a-c)> stimulate P1 cycle
ligand gated ion channel
5-HT3
autoreceptors
NA and 5-HT release
inhibit transmitter release
NA>a2
5-HT>5-HT1b
5-HT/ NA termination
high affin reuptake transporter removes from synaptic cleft
*monoamine reuptake transporters > distinct transporters proteins/ preferentially take up NA/5-HT
monoamine theory of depression
relative decrease in NA/5-HT neurotransmission underlies depression symptoms
present antidepressant targets
^NA/5-HT
MAO inhibitors
reuptake inhibitors
MAOIs
inhibit MAO and intracellular 5-HT/NA metabolism
^ vesicular 5-HT/NA content/ release
MAOI effects
stimulant ^DA transmission
^sympathomimetic amine actions (“cheese reaction”)
interact w reuptake inhibitors
TCA
tricyclic antidepressants
H1/ACh/a1 antagonism > postural hypotension/ dry mouth/ sedation
e.g. imipramine/ amitryptiline
SSRI’s
selective serotonin reuptake inhibitors
inhibit 5-HT reuptake selectively
e.g. prozac, sertraline/zoloft, paroxetine
antidepressants delayed therapeutic actions
autoreceptor desensitization
synaptic remodelling
reward pathway
DA neurones w cell bodies in ventral tegmental area projects to n.accumbens> activation releases DA> acts on inhibitory D2 receptors, inhibitting n.accumbens
amphetamine mechanism
- enter nerve terminal via PM transporter
- vMAT2 transport
- neurotransmitter release into cytosol
- membrane transporter reverses neurotrans release in cleft
- neurotrans reuptake blocked
cocaine mechanism
- binds to DAT PM transporter
- DA reuptake blocked
- ^DA duration in cleft
opioid mechanism
via G-protein coupled receptors (mu/ delta/ k)
mu (gut/ spinal chord/ brain stem) > motility/analgesia/resp depression)
n.accumbens mediates euphoria
depends on route of admin/ relative affinity for receptors
heroine
more lipophilic than morphine/ converted to morphine/ act on mu receptors
heroine mechanism
- activate mu receptors
- DA disinhibited
- mu activated/ n.accumbens inhibitted
ketamine
glutamate neurotransmitter acting on CNS neurones
mediated by excitatory ligand-gated ion channel receptors
blocks NDMA receptors
CNS glutamatergic neurones
long projection neurones/ principal cells connecting cortex and subcortical structures
glutamate NMDA receptor
allosteric change opens Na+ channel tetrameric
GluN1/GluN2/GluN3 dimers
nicotine
activates DA neurones directly/ indirectly w glu inputs
agonist at nicotinic ACh receptors
nicotinic receptors
pentamer
ligand-gated ion channel
Na+/K+ permeable when open
cannabinoid receptors
THC interaction w G-protein coupled receptors > CB1/CB2
CB1 (expressed in CNS)
CB2 (expressed in immune tissues)
retrograde signalling
- released neurotrans activates receptor on postsyn neurone
- cannabinoid released/ CB1 activated on presyn neurone
- neurotrans from presyn neurone decreases
THC
mimics endocannabinoids and decreases presyn neurotrans release
decreases neurotransmission
GABA
neurotransmitter for widespread neurones
GABAa/GABAb receptor mediation
benzodiazepines receptor complex
allosteric change opens Cl- channel
BDZs enhance GABAa