Dopaminergic Signalling

Dopamine is made from tyrosine by tyrosine hydroxylase, the rate-limiting step, then decarboxylated; it is cleared from the synapse mainly by the dopamine transporter and degraded by monoamine oxidase and catechol-O-methyltransferase. Its receptors fall into two G protein-coupled families: D1-like couple to Gs and raise cyclic AMP, D2-like couple to Gi and lower it. Anatomy matters as much as pharmacology, because there are four largely independent pathways: nigrostriatal for movement, mesolimbic for reward learning, mesocortical for executive control, and tuberoinfundibular, where hypothalamic dopamine tonically inhibits pituitary prolactin release.

The clinical facts follow the anatomy. Parkinsonian motor signs appear only after a large fraction of nigrostriatal neurons is already lost, so the disease is well advanced at diagnosis. Antipsychotics that block D2 receptors raise prolactin, because they remove the tuberoinfundibular brake, a pathway entirely separate from the one their antipsychotic effect targets. Animal electrophysiology shows phasic dopamine bursts encoding reward prediction error, the mismatch between expected and received outcome, rather than pleasure as such.

A claim about dopamine with no pathway attached is therefore not testable. A compound can change prolactin without touching motivation, or alter reward reactivity with no motor effect, and tonic versus phasic release matters as much as the total amount.

The popular misreadings are large. Dopamine detox protocols treat the transmitter as a depletable reservoir of pleasure, which is not how a recycled synaptic monoamine behaves. In the peptide field, the reduction in intrusive food thoughts on incretin agonists is often attributed to direct dopaminergic action; human imaging shows altered reward reactivity, but the causal mechanism remains unsettled.