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Abstract
Neuromodulators are generally understood to act through expansive projections that broadcast signals relatively indiscriminately, in stark contrast to the precise, synapse-bound organization of fast neurotransmitters like glutamate and GABA. This dichotomy has left the local architecture of neuromodulatory release and receptor engagement largely obscure. Here, using dopamine as a model system, we reveal that neuromodulatory signaling can be guided by precise molecular specificity. We show that dopamine axons exhibit a tropism for neurons expressing D1 or D2 dopamine receptors. Single-bouton-resolved optical imaging of release shows that dopamine is not broadcast stochastically but is highly localized to varicosities that form direct, synapse-like appositions with receptor-expressing somata and dendrites. To map endogenous receptor organization relative to release sites, we generated ALFA-tagged D1 and D2 receptor knock-in mice (ALFADoR mice). In cultures and intact tissue, D1 and D2 receptors formed discrete puncta rather than exhibiting diffuse membrane distributions. Across striatum, amygdala, and prefrontal cortex, receptor puncta were preferentially positioned near dopamine varicosities, and apposed receptor puncta were larger than nonapposed puncta. These findings reveal a synapse-like architecture for dopamine signaling in which release sites and receptor clusters are spatially coupled to support a precise and efficient neuromodulation.
