Immunohistochemical localization of DARPP32 in striatal projection neurons and striatal interneurons in pigeons

DARPP32 is a D1-receptor associated signaling protein found in striatal projection neurons in mammals, including both substance P-containing (SP+) neurons and enkephalinergic (ENK+) projection neurons. The present study used immunohistochemical single- and double-labeling to examine the cellular localization of DARPP32 in pigeon striatum. Single-label studies revealed that DARPP32 is present in numerous medium-sized striatal perikarya and DARPP32+ axons and terminals were seen to profusely innervate the two major striatal projection targets, the pallidum and the substantia nigra. The single-labeling studies indicated that about 60% of all striatal perikarya labeled for DARPP32+ in striatum, which exceeds the abundance of either SP+ or ENK+ perikarya. Single-labeling studies also showed that the abundance of DARPP32+ fibers and terminals in pallidum exceeds that of either SP+ or ENK+ fibers and terminals in pallidum. Double-labeling found that 30-50% of striatal SP+ perikarya and 7-24% of ENK+ striatal perikarya labeled for DARPP32 in pigeon, and confirmed that DARPP32 was found in both SP+ and ENK+ fibers and terminals in pallidum. In contrast to its prevalence in striatal projection neurons, DARPP32 was virtually absent from cholinergic and NPY+ striatal interneurons, as also true in mammals. Our data are consistent with the interpretation that many SP+ neurons and many ENK+ neurons in avian striatum possess D1-type dopamine receptors and use a DARPP32 signalling pathway, although this may be more common for SP+ than for ENK+ neurons.