Complementary δ2-protocadherin expression delineates parallel basal ganglia circuits in primates

The basal ganglia (BG) form anatomically and functionally segregated yet integrative parallel circuits, but the molecular mechanisms specifying them remain unclear. We immunohistochemically mapped the expression of three δ2-protocadherin (δ2-PCDH) cell adhesion molecules-PCDH10, PCDH17, and PCDH19-in the BG of macaques. Within the striatum, each PCDH exhibited regional gradients of expression along the rostro-caudal and ventromedial-dorsolateral axes. The three PCDHs showed complementary distributions that continuously delineated molecular boundaries corresponding to functional subdivisions in a graded fashion. Such complementary distributions were also observed in the BG output nuclei. Given that neurons expressing the same δ2-PCDH in distinct BG structures preferentially connect with each other, the three δ2-PCDH expression patterns could define functional territories within parallel BG circuits. Together, the complementary expression of PCDH10, PCDH17, and PCDH19 broadly aligns with the distinct BG circuits, respectively, suggesting molecular codes underlying the segregated yet integrative parallel organization of the primate BG.