Axon terminals immunolabeled for dopamine or tyrosine hydroxylase synapse on GABA-immunoreactive dendrites in rat and monkey cortex.

Dopamine afferents to the cortex regulate the excitability of pyramidal neurons via a direct synaptic input. However, it has not been established whether dopamine also modulates pyramidal cell activity indirectly through synapses on gamma-aminobutyric acid (GABA) interneurons, and whether such inputs differ across cortical regions and species. We sought to address these issues by an immunocytochemical electron microscopic approach that combined peroxidase staining for dopamine or tyrosine hydroxylase (TH) with a pre-embedding gold-silver marker for GABA. In the deep layers of the rat prefrontal cortex and in the superficial layers of the monkey prefrontal and primary motor cortices, terminal varicosities immunoreactive for dopamine or TH formed primarily thin, symmetric synapses on distal dendrites. Both GABA-immunoreactive dendrites as well as unlabeled spines and dendrites were contacted by dopamine- or TH-immunoreactive terminals. Synaptic specializations were detected at some, but not all of these contacts. The relative frequency of these appositional and synaptic contacts did not appear to differ between the rat and monkey prefrontal cortex, or between the monkey prefrontal and motor cortices. Across regions and species, labeled and unlabeled targets of dopamine- or TH-positive terminals received additional synaptic input from unlabeled, and occasionally GABA-immunoreactive terminals. Close appositions between dopamine- or TH-immunoreactive and GABA-positive terminals were observed only rarely. These findings indicate that dopamine afferents provide direct synaptic inputs to GABA local circuit neurons in a consistent fashion across cortical regions and species. Thus, dopamine's cellular actions involve direct as well as modulatory effects on both GABA interneurons and pyramidal projection neurons.