D1- and D2-like dopamine receptors are co-localized on the presynaptic varicosities of striatal and nucleus accumbens neurons in vitro

The neuromodulatory actions of dopamine in the striatum and nucleus accumbens are likely to depend on the distribution of dopamine receptors on individual postsynaptic cells. To address this, we have visualized D1- and D2-like receptors on living medium-spiny GABAergic neurons in cultures from the striatum and nucleus accumbens using receptor antagonist fluoroprobes. We labeled D1-like receptors with rhodamine-SCH23390, D2-like receptors with rhodamine-N-(p-aminophenethyl)spiperone and synaptic sites with K+-stimulated uptake of the activity-dependent endocytic tracer FM-143. The fluoroprobes were applied in sequence to assess co-localization. We found that D1- or D2-like receptors were present on about two-thirds of the cells, and co-localized on 22+/-3% (mean +/- S.E.M.) of striatal and 38+/-6% of nucleus accumbens cells. On either D1 or D2 labeled cells, postsynaptic labeling continuously outlined the cell body membrane and extended to proximal dendrites, but not axons. About two-thirds of synaptic varicosities showed D1 or D2 labeling. D1- and D2-like receptors were co-localized on 21+/-4% of striatal and 27+/-3% of nucleus accumbens varicosities. Presynaptic labeling was typically more intense than postsynaptic labeling. The distribution of presynaptic dopamine receptors contrasted with that of postsynaptic GABA(A) receptors, which were clustered in longer patches on neighboring postsynaptic membranes. The extensive presence of D1- and D2-like receptors on presynaptic varicosities of medium-spiny neurons suggests that the receptors are likely to play an important and interacting role in the presynaptic modulation of inhibitory synaptic transmission in the striatum and nucleus accumbens. The significant overlap in labeling suggests that D1-D2 interactions, which occur at the level of individual postsynaptic cells, the circuit level and the systems level, may also be mediated at the presynaptic level. Finally, the ability to visualize dopamine, as well as GABA(A), receptors on the individual synapses of living neurons now makes possible physiological studies of individual mesolimbic system synapses with known receptor expression.