dopamine Flashcards
catecholamines
epinepherine, norepinepherine, dopamine
epinepherine operates in the
PNS
Catecholamine
neurotransmitters have common structure (with individual variations)
dopamine synthesis
L-tyrosine – Tyrosine hydroxylase -> L-DOPA – DOPA decarboxylase -> dopamine
when AMPT attaches to TH what does it do
inhibits the synthesis of dopamine, norepinepherine, epineperine
VMAT
is the transporter that loads dopamine into synaptic vesicles
Reserpine
Reserpine inhibits VMAT and depletes DA and NE as cytosolic catecholamines are rapidly degraded
Reserpine treatment causes
sedation in animals and induces depression in humans
intracellular pathway of dopamine
dopamine —MAO–> DOPAC — COMPT –> HVA
Extracellular pathway of dopamine
dopamine —COMPT–> 3MT — MAO –> HVA
Cocaine and amphetamine affect
DAT functions
Cocaine & amphetamines inhibits
DAT preventing dopamine reuptake
preventing dopamine uptake causes what ?
Increases dopamine in the synapse
Prolongs dopamine signalling
Hyperactivity of dopaminergic circuits
Presynaptic cell rich in
anabolic enzymes (TH, DOPA decarboxylase)
VMAT expressed on vesicles for
loading dopamine
Dopamine receptors in
postsynaptic membrane
Autoreceptors in
presynaptic membrane for feedback inhibition
Dopamine transporter (DAT) responsible for
reuptake
D1
family [D1, D5] – G-protein coupled receptors signalling through Gsα to ↑cAMP (Excitatory)
D2
family [D2, D3, D4] – G-protein coupled receptors signalling through Giα to ↓ cAMP (Inhibitory)
Unlike classical synapses, dopamine can often synapse onto the
neck of dendritic spines
synapsing onto the neck of dendritic spines
allows dopamine to modulate the activity of the synapse
Dopamine can gate the signals at dendritic spines –
– increasing or decreasing signal transmission
Dopamine accounts for 90% of
catecholamine neurotransmission in the CNS
Nigrostriatal system
projects from substantia nigra and ventral tegmental area to striatum (caudate and putamen)
Tuberoinfundibular system
projects from the hypothalamus to the medial eminence to stimulate the pituitary
prolactin secretion
Mesolimbic/mesocortical system
Projects from the ventral tegmental area to the limbic system, nucleus accumbens, mesial frontal, anterior cingulate, and entorhinal cortex
Dopaminergic lesions Bilateral nigrostriatal lesion
sensory neglect, motivational deficits, motor impairment.
Unilateral lesion of nigrostriatal pathway results in
postural asymmetry and turning
6-hydroxydopamine (6-OHDA) is a
selective neurotoxin.
Nigrostriatal system
Projects to the striatum
Involved in motor control
D1 and D2 family receptors
Degradation in Parkinson’s leads to motor symptoms
treatment of parkinsons disease includes
includes L-DOPA, precursor to dopamine
MPTP
) is a neurotoxin that degrades dopaminergic neurons in the Substantia nigra and produces Parkinson’s symptoms and is resistant to L-dopa treatment
Degeneration of dopaminergic neurons in the nigrostriatal system
central to the pathophysiology of Parkinson’s disease
target receptors in parkinsons
Targets enriched with D1 and D2 receptors in the basal ganglia
DAT knockout causes
hyperactivity - Decreased re-uptake prolongs DA signalling at the synapse
Cocaine (inhibiting DAT activity) has comparable effects on
locomotion to DAT knockout.
D1 receptor knockout removes
cocaine’s hyperlocomotion
Mesolimbic dopaminergic pathways Targets enriched in
D1, D2 family receptors
Mesolimbic dopaminergic pathways Limbic connections are proposed to
mediate memory, learning, and affect
Mesolimbic dopaminergic pathways The nucleus accumbens is proposed to act to modify
salience of information flow, implicated in motivation & addictions (motivational salience), and psychosis (sensory salience)
Schizophrenia and Psychotic disorders positive symptoms
Delusions
Hallucinations
Disorganized speech
Grossly disorganized or catatonic motor behaviour
Schizophrenia and Psychotic disorders negative symptoms
Avolition
Social deficits
Flattened affect
Cognitive deficits
Schizophrenia and Psychotic disorders Psychosis proposed to result from
altered dopaminergic signalling
Hyperactivity in mesolimbic system leads to
positive symptoms
Nucleus accumbens in SCZ Mesolimbic dopamine is proposed to mediate
salience
Motivational salience
addictions
Sensory salience
– sensory gating
Nucleus accumbens in SCZ Excess dopamine activity leads the patient to
perceive voices, sounds, and imagery as inappropriately salient
Nucleus accumbens in SCZ - False significance assigned to
internal and external stimuli are interpreted as delusions and hallucinations
Typical antipsychotics inhibit
D1 and D2 family dopamine receptors
Chlorpromazine
(first discovered neuroleptic)
Haloperidol
(still widely used front-line antipsychotic)
Antipsychotic efficacy is correlated with
D2 binding potential
Stimulants (esp. amphetamine) can induce
psychosis at sufficient dose
Extrapyramidal side effects (nigrostriatal):
akinesia, akathesia, acute dystonic reaction, Pseudoparkinsonism,
Akinesia
inability to initiate movement
Akathisia
inability to remain motionless
Acute dystonic reaction
– sustained muscle contraction, twisting and repetitive movements
Pseudoparkinsonism
– fixed (non-progressive) Parkinsonism without degeneration of dopaminergic neurons
Extrapyramidal side effects Tuberofundibular:
Hyperprolactinaemia
Hyperprolactinaemia
can result from antipsychotic treatment
Amenorrhea (♀), infertility(♂/♀), sexual dysfunction (♂/♀), hypogonadism (♂), spontaneous lactation (♂/♀)
Dopamine levels in post-mortem SCZ brains are
are elevated in the striatum
SCZ - PET and SPECT imaging of dopamine receptors show
show increased basal levels of dopamine
Basal dopamine levels are predictive of
responsiveness to antipsychotic therapy
typical antipsychotics targeting dopamine only address
positive symptoms
in SCZ what is the dopamine activity in the cortex?
hypoactivity of dopamine seen in cortex
Gaba neuron is
inhibitory – take it away and we lose control
Addictive behaviour is linked to
impulsive traits in humans
Impulse control
is a manifestation of inhibitory control (component of executive function)
Impulse control Involves structures such as
anterior cingulate, dorsolateral prefrontal cortex, lateral orbital prefrontal cortex, and motor/premotor cortex
Inhibitory control can be considered a
gating event
Impulsive rats show increased premature responses and have
increased self-administration of cocaine than low impulsive rats (○)
PET imaging of a dopamine receptor D2/3 antagonist showed high
impulsive rats have reduced binding potential in the ventral striatum.
Reduced D2/3 binding potential correlates with high
impulsivity and addictive behaviour (cocaine self-administration).
SCZ patients have very high comorbidity with
addictions
Cannabis or amphetamine intoxication is a frequent precipitant of the first
episode of psychosis
Early substance use / abuse (teenage use) correlates with
SCZ onset and severity but is neither necessary nor sufficient