Subcellular localization of dopamine receptors

Dopamine receptor labeling in terminals presynaptic to asymmetrical synapses 

A similar picture applies to D2 labeling. Generally, a small number of D2 immunoreactive terminals forming asymmetrical synapses have been observed (Sesack et al., 1994 Hersch et al., 1995 Yung et al., 1995), or none at all (Levey et al., 1993). Unfortunately there has been no quantification of the fraction of such terminals which are labeled. However, as for the D1 receptors, their qualitative descriptions suggest only a small fraction of corticostriatal terminals to be D2 positive. 

Hersch et al. (1995) found that D1 immunoreactive terminals presynaptic to symmetrical synapses were exceedingly rare whereas the D2 immunoreactive terminals were quite frequent. Synapses formed by D2 immunoreactive terminals were not easy to identify due to a lack of pronounced pre or postsynaptic densities, but many D2 positive presynaptic terminals made symmetrical synapses with dendritic shafts and spines. Consistent with this, many presynaptic D2 receptors were also seen in terminals which were not positive for TH, suggesting they may be heteroreceptors (Sesack et al., 1994). This is confirmed by the demonstration of the D2 positive GABA axon terminals presynaptic to symmetrical synapses (Delle Donne et al., 1997). 

Levey et al. (1993) found that axon terminals immunoreactive for D1 and D2 receptor proteins formed symmetrical synapses exclusively, and primarily with unlabeled dendritic shafts. In cultures, D1 and D2 receptors have been colocalized to terminals of intrinsic neurons (Wong et al., 1999). Functional D1 receptors have also been demonstrated on the terminals of striatal cells in the substantia nigra (Fiorillo and Williams, 1998). Collectively, the results for D1 and D2 receptors suggest their presence on the terminals of intrinsic GABA neurons. 

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