Chemical Diversity and spatial distribution of neurons in the entopeduncular nucleus of the mouse

Yuta Miyamoto and Takaichi Fukuda
Dept Anat and Neurobiol, Grad Sch Med Sci, Kumamoto Univ


The entopeduncular nucleus (EPN) in the rodent brain corresponds to the internal segment of the globus pallidus in primates. In spite of its importance in the circuitry of the basal ganglia, the cellular composition of EPN has been only partially understood and its internal 3D structure remains to be clarified. Previous studies have divided EPN roughly into the rostral and caudal halves, with the former containing somatostatin (SOM)-immunoreactive neurons whereas the latter dominated by parvalbumin (PV)-immunoreactive neurons, but the validity of this simple rostrocaudal segmentation needs to be tested. In this study the cytoarchitecture of EPN of the mouse was analyzed by multi-labeled immunohistochemistry and stereology-based quantification. Immunostaining of substance P (SP), which is contained in axon terminals of striatal direct pathway neurons, determined the rostrocaudal extent of EPN, ranging from 980 to 1780μm caudal to the bregma. PV neurons were distributed in the caudal two-thirds of this range. PV negative neurons were abundant in the rostral half but were also located caudally, where they surrounded the PV neuron-rich core. This core-surround organization closely matched the dual SP immunoreactivity that was more intense in the surround. The total numbers of PV positive and negative neurons in EPN were 537±116 and 1152±220, respectively. Disector analysis showed the percentages of SOM(+)/PV(-), SOM(-)/PV(+), and SOM(-)/PV(-)-neurons to be 45.7±1.5%, 28.6±4.4%, and 25.7±3.8%, respectively. Part of the last population was thought to be presumptive excitatory neurons based on the observations that 13.0% of EPN neurons did not show immunoreactivity for glutamic acid decarboxylase while ChAT-immunoreactive neurons accounted for only 3.6% of EPN neurons. These observations combined with the 3D reconstruction from the serial sections yielded the conclusion that the mouse EPN takes the form of the two-layered, eccentric ellipsoids consisting of PV-rich/SP-poor core and PV-poor/SP-rich surround, with the center of the core positioned caudally at 1600μm from the bregma.