Dopamine neurotransmission

Question 1

Dopamine

Answer 1

Tyrosine hydroxylase (TH) converts tyrosine to L-DOPA using tetrahydrobiopterin (BH4) as a cofactor. This is rate-limiting.
Aromatic L-amino acid decarboxylase (AADC) then makes dopamine.
Catecholamines can inhibit TH in the cytosol. Presynaptic auto-receptors also cause feedback inhibition.

Question 2

Transporters

Answer 2

DAT (SLC6A3) - 12 TM protein involved in DA reuptake using the ion gradient created by Na/K ATPase.
Binding site for the inhibitors cocaine, amphetamine and methylphenidate, which increase synaptic DA.

DA is taken up into vesicles by vesicular monoamine transporter protein VMAT-2, which extrudes H+ while taking up DA, using the hydrogen concentration gradient to accumulate dopamine.
Reserpine blocks VMAT-2 and depletes DA in vesicles, resulting in a Parkinson’s phenotype.

Question 3

6-hydroxydopamine (6-OHDA)

Answer 3

Neurotoxin that can be taken up by DAT and target dopaminergic neurons.
Auto-oxidation of this compound produces hydrogen peroxide, which causes oxidative stress. Inhibits the mitochondrial respiratory chain, leading to apoptosis.
6-OHDA can be injected into the striatum to produce a model of PD.

Question 4

Metabolism

Answer 4

Dopamine metabolism involves monoamine oxidase (MAO-B) on the inner mitochondrial membrane and catechol-o-methyltransferase (COMT), found in the cytosol. These can act in either order.
If MAO acts first, the intermediate DOPAC is formed. If COMT acts first, 3-MT is formed.
Homovanillic acid (HVA) formed at the end.

Question 5

Dopamine conversion

Answer 5

In noradrenergic neurons, dopamine β-hydroxylase, converts dopamine to noradrenaline.
In adrenergic neurons, PNMT converts NA to adrenaline.

Question 6

Receptors

Answer 6

D1-like receptors (D1, D5) are Gs coupled and increase intracellular cAMP. They also cause PKC activation which results in IP3 synthesis and calcium mobilisation. The cell depolarises and neurotransmitter release is initiated.
These receptors have a large C-terminus

D2-like receptors (D2, D3, D4) are Gi coupled and decrease cAMP and so decreased activation of voltage-gated Ca2+ channels as K+ currents increase. The cell is hyperpolarised.
These receptors have a large intracellular loop-3.

Question 7

Dopamine pathways

Answer 7

Only 1% of neurons in the brain are dopaminergic.
Exposing brain tissue to formaldehyde vapour converts dopamine to a fluorescent isoquinone that can be seen as yellow or green under the microscope. Tyrosine hydroxylase immunohistochemistry produces even better visualisation of dopamine pathways.
Dopamine cell bodies cluster in three specific regions.
The nigrostriatal pathway originates in the substantia nigra, which innervates the basal ganglia to control voluntary movement. Degeneration of these neurones in Parkinson’s disease leads to motor impairment.
The mesolimbic pathway initiates in the ventral tegmental area (VTA) of the midbrain and innervates the nucleus accumbens, which is part of the limbic system controlling emotion, pleasure, reward and goal-directed behaviour. Over-activity contributes to psychosis, delusions and hallucinations, which make up the positive symptoms of schizophrenia.
The mesocortical pathway originates in the VTA, and innervates areas of the frontal cortex, affecting emotion and motivation. Under-activity of this pathway may contribute to the negative symptoms of schizophrenia, such as social withdrawal. Cognitive side-effects may also be mediated by this pathway.
The tuberoinfundibular pathway originates in the arcuate nucleus of the hypothalamus and projects to the median eminence in the anterior pituitary gland. DA release inhibits prolactin secretion. Prolactin is involved in lactation and maternal behaviour, so it has neuroendocrine function.