Population characteristics of preproenkephalin mRNA-containing neurons in the globus pallidus of the rat.

Anatomical, neurochemical and electrophysiological evidence indicates the presence of multiple neuronal subpopulations within the rodent globus pallidus (GP). One subpopulation that has not been well characterized is GP neurons that express preproenkephalin mRNA (PPE+ cells). The present study seeks to further characterize GP subpopulations by determining whether the PPE+ GP neurons express parvalbumin immunoreactivity (PV-IR) and where their axons project by retrogradely labeling pallidal neurons with the tracer FluoroGold (FG). Using combined PV immunocytochemistry (ICC) and PPE mRNA in situ hybridization, we observed that PV-IR and PPE mRNA identify predominantly separate pallidal cell populations. Combined FG ICC and PPE mRNA in situ hybridization also revealed that this neuropeptide mRNA is more often found in FG-labeled pallidostriatal than pallidosubthalamic neurons. Our data support a growing body of evidence that suggests the GP is more heterogeneous than accounted for by current functional models of the basal ganglia.