Rat subthalamic nucleus and zona incerta share extensively overlapped representations of cortical functional territories

Abstract

The subthalamic nucleus (STN) and the zona incerta (ZI) are two major structures of the subthalamus. The STN has strong connections between the basal ganglia and related nuclei. The ZI has strong connections between brainstem reticular nuclei, sensory nuclei, and nonspecific thalamic nuclei. Both the STN and ZI receive heavy projections from a subgroup of layer V neurons in the cerebral cortex. The major goal of this study was to investigate the following two questions about the cortico-subthalamic projections using the lentivirus anterograde tracing method in the rat: 1) whether cortical projections to the STN and ZI have independent functional organizations or a global organization encompassing the entire subthalamus as a whole; and 2) how the cortical functional zones are represented in the subthalamus. This study revealed that the subthalamus receives heavy projections from the motor and sensory cortices, that the cortico-subthalamic projections have a large-scale functional organization that encompasses both the STN and two subdivisions of the ZI, and that the group of cortical axons that originate from a particular area of the cortex sequentially innervate and form separate terminal fields in the STN and ZI. The terminal zones formed by different cortical functional areas have highly overlapped and fuzzy borders, as do the somatotopic representations of the sensorimotor cortex in the subthalamus. The present study suggests that the layer V neurons in the wide areas of the sensorimotor cortex simultaneously control STN and ZI neurons. Together with other known afferent and efferent connections, possible new functionality of the STN and ZI is discussed.

Keywords: AB_10000343; AB_221569; AB_2298772; cerebral cortex; subthalamic nucleus; subthalamus; zona incerta.

Fig. 1

A: A drawing of a rat coronal section shows subdivisions of the diencephalon. B and C: A parvalbumin (PV) immunostained coronal section showing the main structures in the subthalamus including the subthalamic nucleus (STN), dorsal and ventral subdivision of zona incerta (ZId and ZIv), and the Forel field H2 (H2). VM, ventromedial thalamic nucleus; cp, cerebral peduncle; ml, medial lemniscus.

Fig. 2

A: A low magnification photomicrograph of a coronal section of the lateral agranular cortex showing a lentivirus injection site. The section was doubly immunostained for green fluorescent protein (GFP, dark blue) and neuron specific nuclear protein (NeuN, brown). B: A higher magnification photomicrograph of the area marked in A shows that virus transfected cells are pyramidal and that many other cells stained with NeuN were not transfected.

Fig. 3

Photomicrographs of a coronal section doubly immunostained for GFP and NeuN showing GFP-labeled axons and boutons in STN and ZI after injection of lentiviruses in the lateral agranular cortex. Thin GFP-labeled axons traverse the subthalamus from the cerebral peduncle (cp) in a dorsomadial direction and form terminal fields in STN, ZIv, and ZId.

Fig. 4

A-C: Three examples of lentivirus injection in different rostrocaudal levels of the lateral agranular cortex (AGl). A1-C1: For this and some subsequent figures, the sections showing the virus injection sites in the top row were immunostained for GFP and NeuN. A= is the distance from the interaural line. A2-C2: For each case, boutons in five coronal sections spaced 160μm (i.e, ones in every 4th consecutively cut sections) covering the rostrocaudal extent of the subthalamic nucleus (STN) were plotted. Each red dot represents approximately 50 boutons in 40μm sections. AGm, medial agranular cortex; cp, cerebral peduncle; H2, Forel field H2; ml, medial lemniscus; VM, ventromedial thalamic nucleus; ZId and ZIv, dorsal and ventral subdivision of zona incerta.

Fig. 5

A-C: Three examples of lentivirus injection in different rostrocaudal levels of the medial agranular Cx (AGm). D and E; Two examples of lentivirus injection in the anterior cingulate Cx (ACg). PrL, prelimbic Cx. Refer to the Fig. 4 legend for other details.

Fig. 6

Results of lentivirus injections into the prelimbic Cx (PrL) (A), infralimbic and dorsal peduncular cortex Cx (IL+ DP) (B), and into the lateral orbital Cx (LO) (C and D). VO, ventral orbital Cx. Refer to the Fig. 4 legend for other details.

Fig. 7

Results of lentivirus injections into the primary somatosensory Cx (S1) (A and B) and the anterior and posterior parietal Cx (PtA and PtP) (C and D). V2, secondary visual Cx. Refer to the Fig. 4 legend for other details.

Fig. 8

Photomicrographs of a coronal section doubly immunostained for GFP and NeuN showing labeled axons and boutons in the subthalamus after injection of viruses in S1. The low magnification photomicrograph (A) shows axons leaving the cerebral peduncle (cp), traversing through lateral part of ZI, and innervating the ventral posteromedial thalamic nucleus (VPM). Some of the axons in the ZI emitted very thin collaterals innervating to ZI (B). C and D show fine axons with boutons innervating to STN and ZI, respectively.

Fig. 9

Summary diagram shows that the projections from the sensory, motor, and anterior limbic cortices form large, cloud-like projection zones with fuzzy borders and extensive overlaps that encompass entire subthalamus. The diagram also shows other main afferent and efferent projections to the each subdivision of the subthalamus. Cx, cerebral cortex; Th, thalamic nuclei; STN, subthalamic nucleus; ZId and ZIv, dorsal and ventral subdivision of zona incerta.