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 pyramida l neurons via a direct synaptic input. However, it has not been establ ished whether dopamine also modulates pyramidal cell activity indirect ly through synapses on gamma-aminobutyric acid (GABA) interneurons, an d whether such inputs differ across cortical regions and species. We s ought to address these issues by an immunocytochemical electron micros copic approach that combined peroxidase staining for dopamine or tyros ine hydroxylase (TH) with a pre-embedding gold-silver marker for GABA. In the deep layers of the rat prefrontal cortex and in the superficia l 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 dend rites as well as unlabeled spines and dendrites were contacted by dopa mine- or TH-immunoreactive terminals. Synaptic specializations were de tected at some, but not all of these contacts. The relative frequency of these appositional and synaptic contacts did not appear to differ b etween the rat and monkey prefrontal cortex, or between the monkey pre frontal and motor cortices. Across regions and species, labeled and un labeled targets of dopamine- or TH-positive terminals received additio nal synaptic input from unlabeled, and occasionally GABA-immunoreactiv e 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 GAB A local circuit neurons in a consistent fashion across cortical region s and species. Thus, dopamine's cellular actions involve direct as wel l as modulatory effects on both GABA interneurons and pyramidal projec tion neurons. (C) 1995 Wiley-Liss, Inc.