Dopamine

neurotransmitter requirements

-localization at appropriate sites

-compound is released onto innervated structure following stimulation of nerve

-compound produces response identical to nerve stimulation

-antagonists block responses

dopamine synthesis

1) tyrosine --> DOPA by TH

2) DOPA --> DA by AADC (DDC)

3) DA enters vesicles via VMAT

TH (tyrosine hydroxylase)

enzyme that converts tyrosine to DOPA; rate-limiting enzyme in catecholamine synthesis

AADC (aromatic amino acid decarboxylase)

enzyme that converts DOPA to DA

ways to increase neurotransmission

-increase synthesis

-inhibit reuptake (block transporter)

-reverse transporter direction

-inhibit metabolism (inhibit enzymes)

-agonize postsynaptic receptor

-antagonize presynaptic autoreceptors

ways to decrease neurotransmission

-inhibit synthesis

-deplete neurotransmitter

-antagonize postsynaptic receptor

-agonize presynpatic autoreceptors

steps of monoaminergic neurotransmission

1) synthesis

2) packaging into vesciles

3) release from terminal into synapse

4) bind to receptors

5) reuptake by transporter

6) recycling into vesicles or breakdown by MAO

dopamine metabolism

- DA -> DOPAC by MAO then -> HVA by COMT

or

- DA -> 3-methoxytyramine by COMT then -> HVA by MAO

MAO-A

MAO isoform primarily located in the brain, gut, liver, placenta, and skin that metabolizes DA, NE, and 5-HT

MAO-B

MAO isoform primarily located in the brain, platelets, and lymphocytes that metabolizes DA

nigrostriatal pathway

dopaminergic pathway from the substantia nigra to the striatum and forebrain

mesolimbic pathway

dopaminergic pathway from the VTA to the NAc, amygdala, hippocampus, and olfactory tubercle

mesocortical pathway

dopaminergic pathway from VTA to mPFC, cingulate cortex, and entorhinal cortex

tuberoinfundibular pathway

dopaminergic pathway from the arcuate nucleus to the pituitary (inhibiting prolactin release)

dopamine functions in the brain

voluntary movement, reward, sleep regulation, feeding, mood, attention, cognitive function, impulse control, decision making, olfaction, vision, and hormonal regulation

VMAT (vesicular monoamine transporter)

actively transports monoamines from cytosol into vesicles against concentration gradient using a proton electrochemical gradient generated by vesicular H+-ATPase in membranes of vesicles (antiporter); mutations can cause developmental delay, dystonia, and parkinsonism

VMAT2, VMAT1

_____ is mainly expressed in the CNS, while _____ is expressed peripherally and in neuroendocrine cells

reserpine

competitive nonselective VMAT inhibitor that prevents loading of monoamines into vesicles, resulting in them being metabolized by MAO over time (irreversible depletion)

tetrabenazine (TBZ)

non-competitive VMAT2 inhibitor that modulates DA while maintaining proper exocytotic DA release and signal termination; used to treat Huntington's disease and tardive dyskinesia (both produce hyperactivity/unwanted motor activity)

DAT (dopamine transporter)

symporter that couples transport of DA from synapse with 2 Na+ and 1 Cl- down their concentration gradients into the cell; localized to dendrites, axons, and soma of mesencephalic DA neurons (highest in dorsal striatum and NAc); target of many psychostimulants including cocaine, methamphetamine, amphetamine, methylphenidate, etc.

steps of DA reuptake

-outward-facing DAT binds Na+ and then DA, which changes conformation from outward to inward-facing

-inward-facing DAT releases Na+ and DA into cytoplasm and returns to original conformation

DAT S1 binding site

DAT binding site with a high affinity for substrates, ions, and competitive inhibitors that is flanked by extra- and intra-cellular gates that control solute movement

DAT S2 binding site

DAT binding site believed to allosterically modulate the S1 binding site

DAT regulation

-endocytic trafficking (can be recycled or degraded)

-PKC decreases DAT expression

-post-translational modifications

-D2R activation increases DAT function and expression

effects of DAT-targeting drugs

sympathetic stimulation, arousal, insomnia, agitation, euphoria, increased attention, appetite suppression, psychosis

examples of DAT-targeting drugs

cocaine, amphetamine, methamphetamine, methylphenidate, and cathinones

cocaine

monoamine transporter (including DAT) inhibitor that stabilizes the transporters in the outward-facing direction, resulting in blocking of reuptake of monoamines which leads to increased levels in the synapse or junction which allows for continued postsynaptic signaling

amphetamine

competitive DAT and VMAT substrate that enters neurons through DAT and enters VMAT to displace DA from vesicles, resulting in reversal of DAT and DA efflux into synapse

also acts as MAO and COMT inhibitors, decreasing DA metabolism

methamphetamine

psychostimulant similar to amphetamine but more lipophilic; low doses can improve mood and coggnitive function, but sustained and excessive dosing can produce neurotoxicity

DA and DAT in striatum

methamphetamine causes reduced _____ that can recover with prolonged abstinence

cathinone

an amphetamine-like stimulant derived from the khat plant

cathinone derivatives

bupropion, methcathinone, and mephedrone

methcathinone

cathinone derivative that is a DAT and NET blocker

mephedrone

cathinone derivative that is a non-selective transporter substrate/releaser

evidence of DAT's role in rewarding/locomotor-stimulating effect of psychostimulants

-drugs that bind only at DAT have similar reinforcing properties to those that bind DAT/NET/SERT

-DAT binding in NAc correlates with reported "high" of stimulants in humans

-DAT KO animals are unresponsive to locomotor-stimulating effects of stimulants

MPTP (MPP+) and 6-OHDA

DAT substrates that act as neurotoxins by inhibiting mitochondrial respiratory chain; can be used in animal models of Parkinson's disease

DAT knockout phenotype

delayed extracellular DA clearance resulting in elevated extracellular DA in the striatum and downregulation of pre- and post-synaptic DA receptors

causes hyperactivity, impaired motor function, mild cognitive deficits, and increased motivation for reward

DAT and SERT

double KO mice that have ___ and ___ knocked out have no conditioned place preference to cocaine, suggesting indirect modulatory action of SERT on DA neurons

caudate and NAc, frontal cortex

DA uptake in ___ and ___ depends mostly on DAT, whereas DA uptake in the ___ depends mostly on NET

D1-like receptors

D1 and D5; coupled to Gs (or Golf in striatum)

D2-like receptors

D2, D3, and D4; Gi/o-coupled

lower DA concentrations

D2 autoreceptors are generally activated by _____ than postsynaptic D2 receptors

DA receptor KO phenotype

impaired motor activity, reward, and learning; schizophrenia-related model

D1 receptor

Gs-coupled DA receptor that is the most highly expressed DA receptor in the CNS, especially in neocortical, mesolimbic, and nigrostriatal regions

D2 receptor

Gi-coupled DA receptor mostly expressed in the dorsal striatum, NAc, substantia nigra, VTA, and mesolimbic regions

somatodendritic D2 autoreceptors

presynaptic DA receptors located on soma or dendrites of VTA and substantia nigra neurons that activate GIRK K+ channels to cause hyperpolarization/inhibition

axonal D2 autoreceptors

presynaptic DA receptors located on nerve terminals, inhibiting vesicular DA release, increasing DAT expression and activity, and inhibiting TH via PKA to decrease DA synthesis

D3 receptor

Gi-coupled DA receptor that is primarily expressed in limbic regions, especially the NAc; plays a role in reward, emotion, movement, and cognition

D4 receptor

Gi-coupled DA receptor that is mainly expressed in the frontal cortex and limbic system; plays a role in impulse control and is associated with ADHD and SUDs

D5 receptor

Gs-coupled DA receptor mainly expressed in the hippocampus; plays a role in cognitive function

medium spiny neurons (MSNs)

neurons containing many dendrites that are located in ventral striatum and release GABA when depolarized; most either express D1 or D2 receptors

deltaFosB

gene marker expressed primarily in D1 MSNs in response to drugs of abuse

D1 MSNs

MSNs that stimulate movement and promote reward via the direct pathway

direct pathway

pathway stimulated by GABAergic D1 MSNs within the striatum, resulting in inhibition of the GABAergic substantia nigra, disinhibiting the substantia nigra's inhibition of the thalamus, thalamus sends glutamate to frontal cortex to increase motor activity

indirect pathway

pathway in which GABAergic D2 MSNs within the striatum inhibit the GABAergic globus pallidus, causing disinhibition of the glutamatergic subthalamic nucleus, resulting in activation of the GABAergic substantia nigra, which inhibits the thalamus from sending glutamate to frontal cortex (decreases motor activity)

Parkinson's disease

neurodegenerative disease caused by substantial loss of nigrostriatal DA neurons and presence of Lewy bodies (intracellular aggregates), resulting in bradykinesia, muscular rigidity, resting tremor, and impairment of postural balance

direct pathway (Parkinson's)

loss of dopaminergic input to D1 MSNs in the striatum decreases _____ stimulation resulting in increased substantia nigra inhibition of thalamic excitation of cortex, producing decreased motor control

indirect pathway (Parkinson's)

loss of dopaminergic input to D2 MSNs in the striatum decreases _____ inhibition resulting in increased substantia nigra inhibition of thalamus

Parkinson's treatment strategies

1) increase L-DOPA levels to increase dopamine synthesis and restore dopaminergic tone in striatum

2) inhibit L-DOPA metabolism and peripherally inhibit synthesis of DA from DOPA so to enhance uptake across BBB

carbidopa

peripheral AADC inhibitor used to treat Parkinson's that peripherally reduces conversion of DOPA to DA allowing for more DOPA to cross BBB and also reduces peripheral side effects associated with increased peripheral DA

tolcapone

COMT inhibitor used to treat Parkinson's that reduces L-DOPA metabolism to increase the amount that can enter the brain

selegiline

irreversible MAO-B inhibitor used to treat Parkinson's that decreases dopamine metabolism

nonselective DA receptor agonists

drugs that nonselectively activate DA receptors that can be used to treat Parkinson's, RLS, and hyperprolactinemia but can cause side effects such as hallucinations, confusion, and nausea; ex: bromocriptine

ropinirole and pramipexole

D2/D3 receptor agonists used with L-DOPA in treating Parkinson's disease (especially later stages) or alone in treating restless leg syndrome

rotigotine

D1/D2 agonist used to treat Parkinson's disease

schizophrenia

psychiatric disorder characterized by positive symptoms (hallucinations, delusions, disorganized speech or behavior) believed to be caused by mesolimbic DA hyperactivity, negative symptoms (anhedonia, lack of motivation, lack of speech, apathy) believed to be caused by mesocortical DA hypoactivity, and cognitive deficits

original dopamine hypothesis

hypothesis that schizophrenia results from hyperactive DA transmission (so DA antagonists should treat it, but they only treat the positive symptoms)

D2 antagonists

First-generation antipsychotics that target the positive symptoms of schizophrenia (mesolimbic pathway hyperactivity), but can cause extrapyramidal side effects and anhedonia or tardive dyskinesia with chronic use; ex: haloperidol and chlorpromazine

supersensitivity

chronic D2 antagonist administration results in D2 _____ and tardive dyskinesia

tardive dyskinesia (TD)

stereotyped, repetitive, involuntary movements of the face, extremities, or trunk that results from nigrostriatal D2 supersensitivity and upregulation; can be treated with VMAT2 inhibitors

revised dopamine hypothesis

hypothesis that the positive symptoms of schizophrenia arise from hyperactive DA in the mesolimbic pathway and the negative symptoms of schizophrenia arise from hypoactive DA in the mesocortical pathway

schizophrenia treatment strategy

decrease DA activity in mesolimbic pathway and increase DA activity in cortical areas