Electron microscopic analysis of D1 and D2 dopamine receptor proteins in the dorsal striatum and their synaptic relationships with motor corticostriatal afferents

Abstract

The precise localization of Dl and D2 dopamine receptors within striatal neurons and circuits is crucial information for further understanding dopamine pharmacology. We have used subtype specific polyclonal and monoclonal antibodies against Dl and D2 dopamine receptors to determine their cellular and subcellular distributions, their colocalization, and their differential connectivity with motor cortical afferents labeled either by lesion-induced degeneration or by anterograde transport of biotinylated dextrans. Dl and D2 are primarily expressed in medium-sized neurons and spiny dendrites. Axon terminals containing Dl were rare whereas DS-immunoreactive axon terminals forming symmetrical synapses with dendrites and spines were common. In 2 p.m sections, Dl was localized to 53% of neurons, and D2 to 48% of neurons, while mixing Dl and D2 antibodies labeled 78%. By electron microscopy, Dl was localized to 43% of dendrites and 38% of spines while D2 was localized to 38% of dendrites and 48% of spines. Combining Dl and D2 antibodies resulted in the labeling of 88.5% of dendrites and 92.6% of spines. Using different chromogens for Dl and D2, colocalization was not observed. lpsilateral motor corticostriatal afferents were primarily axospinous and significantly more synapsed with Dl than DS-positive spines (65% vs 47%). Contralateral motor corticostriatal afferents were frequently axodendritic and no difference in their frequency of synapses with Dl and D2 dendrites and spines was observed. These findings demonstrate differential patterns of expression of Dl and D2 receptorsin striatal neurons and axon terminals and their differential involvement in motor corticostriatal circuits.[

Dopaminergic signaling in dendritic spines

“…Postsynaptically, D2 receptors are concentrated in shafts and spines of both cortical pyramidal neurons and striatopallidal MSNs. While displaying similar postsynaptic localization profiles, D1 and D2 receptors appear to be distributed in distinct populations of spines, at least in the striatum [12]. This distribution may reflect the segregation of D1 and D2 into the direct and the indirect striatal pathways [38], respectively, rather than a differential transport of these receptors to specific spines within the same neurons.…”

“…However, a unique, disproportionately large population of DA receptors is localized in dendritic spines [11][12][13][14], where most of the excitatory glutamatergic synapses are formed. Moreover, a subset of DA receptor-containing spines is innervated simultaneously by both glutamatergic terminals and dopaminergic terminals [15][16][17].…”

“…The expression and localization of D2-class receptors have also been investigated at the cellular and subcellular levels [11][12][13][35][36][37][39][40]. D2 receptors, the predominant subtype of this class, are localized in both pre-and postsynaptic structures.…”

“…the striatonigral neurons that are associated with the direct pathway of the basal ganglion circuitry [38]. These receptors are localized primarily in the spine heads, shafts, and less commonly soma of MSNs [11][12][13][14]. In the cortex and hippocampus, D1 receptors are present in a subpopulation of interneurons [33], but are primarily expressed in pyramidal neurons, with a predominant subcellular localization in the spines of apical dendrites [14].…”

“…D5 receptors, while co-expressed with D1 receptors in cortical pyramidal neurons, are predominately localized in shafts [14], where inhibitory gamma-aminobutyric acid (GABA)-containing neurons form major postsynaptic contacts [34]. The segregation of D1 and D5 at the subcellular level suggests that these closely related receptors may mediate distinct excitatory and inhibitory functions, respectively.The expression and localization of D2-class receptors have also been investigated at the cellular and subcellular levels [11][12][13][35][36][37][39][40]. D2 receptors, the predominant subtype of this class, are localized in both pre-and postsynaptic structures.…”