Projections to the anterodorsal thalamus and lateral mammillary nuclei arise from different cell populations within the postsubiculum: implications for the control of head direction cells

Abstract

The neural representation of directional heading is encoded by a population of cells located in a circuit that includes the postsubiculum (PoS), anterodorsal thalamus (ADN), and lateral mammillary nuclei (LMN). Throughout this circuit, many cells rely on both movement- and landmark-related information to discharge as a function of the animal's directional heading. The PoS projects to both the ADN and LMN, and these connections may convey critical spatial information about landmarks, because lesions of the PoS disrupt landmark control in head direction (HD) cells and hippocampal place cells [Goodridge and Taube (1997) J Neurosci 17:9315-9330; Calton et al. (2003) J Neurosci 23:9719-9731]. The PoS → ADN projection originates in the deep layers of PoS, but no studies have determined whether the PoS → LMN projection originates from the same cells that project to ADN. To address this issue, two distinct cholera toxin-subunit B (CTB) fluorophore conjugates (Alexa Fluor 488 and Alexa Fluor 594) were injected into the LMN and ADN of the same rats, and PoS sections were examined for cell bodies containing either or both CTB conjugates. Results indicated that the PoS → LMN projection originates exclusively from a thin layer of cells located superficial to the layer(s) of PoS → ADN projection cells, with no overlap. To verify the laminar distribution and morphological characteristics of PoS → LMN and PoS → ADN cells, biotinylated dextran amine was injected into LMN or ADN of different rats, and tissue sections were counterstained with thionin. Results indicated that the PoS → LMN projection arises from large pyramidal cells in layer IV, whereas the PoS → ADN projection arises from a heterogeneous cell population in layers V/VI. This study provides the first evidence that the PoS → ADN and PoS → LMN projections arise from distinct, nonoverlapping cell layers in PoS. Functionally, the PoS may provide landmark information to HD cells in LMN.

Figure 1

Schematic diagram and coronal section stained with thionin, illustrating the position of PoS relative to surrounding brain areas. Lines parallel to the pial surface and Roman numerals indicate PoS lamina. dhc, dorsal hippocampal commissure; fp, forceps posterior of corpus callosum; PoS, postsubiculum; S, subiculum; RSPa, retrosplenial granular a cortex. Scale bar, 400 μm. Rectangle in schematic diagram indicates approximate location of photographed tissue.

Figure 2

Retrograde labeling in PoS following tracer injection into LMN and ADN. A-D, Case #70. A, Cholera toxin-Alex Fluor 488 (CTB-488) injection sites in LMN. The left LMN is partially filled with CTB-488, whereas the right LMN is completely filled with CTB-488. Dashed lines indicate LMN, corresponding to LMN (shaded areas in rectangle) in diagram above. Scale bar, 500 μm; V3, third ventricle; V3m, third ventricle, mammillary recess. B, CTB-594 injection sites in the ADN. CTB-594 is present in the left ADN and surrounding areas including the anteroventral (AVN) and laterodorsal thalamic (LDN) nuclei. CTB-594 is mostly limited to the ADN in the right hemisphere, with little diffusion into surrounding areas. Dashed lines indicate ADN and surrounding nuclei, corresponding to the rectangle in the diagram above. Scale bar, 500 μm; sm, stria medullaris. C-D, Somas of layer IV and V-VI neurons in the left PoS show labeling following CTB-594 injection (red) into the left anterior thalamic nuclei. Somas of layer IV neurons show light labeling from CTB-488 injection (green) into a portion of the left LMN. In the right PoS, retrograde labeling is primarily limited to layer V/VI following CTB-594 injection that did not appear to diffuse outside of ADN. Note that there is little CTB-594 retrograde labeling in layer IV, presumably because there was an absence of CTB-594 injected into the ipsilateral AVN. Somas and proximal dendrites of layer IV neurons showed dense labeling following CTB-488 injection that completely filled the right LMN. Scale bars, 200 μm. E-G, Case #29. E-F, Cholera toxin was injected into a portion of the right LMN and ADN, respectively. Scale bars, 500 μm. G, The ADN injection resulted in light labeling of the deepest layers of the right PoS (red). The LMN injection resulted in moderate labeling of deep pyramidal cell layer (green). Scale bar, 200 μm. Note that for injections into ADN, some CTB-594 diffused into the third ventricle during injection, resulting in fluorescence along the ventricular wall of the fimbria (panel B), as well as of the PoS (panels C, D, G).

Figure 3

Pyramidal cells in PoS labeled by BDA injection into LMN; case #54. A, For two exemplary neurons (denoted by white arrows), large apical dendrites could be followed toward the superficial layers. For the uppermost cell, note the long basal dendrites confined to the deep layer(s) of postsubiculum and axon extending from the lower cell toward the deep white matter. Also, note the morphological homogeneity of labeled cells and their position, which shows some overlap with lamina dissecans (layer IIIa). Scale bar, 150 μm. Right, Schematic reconstruction of layer IV pyramidal cells depicted in A. Apical dendrites of both cells could be followed into layer II. The long basal dendrite that is contained within layer IV originates from the upper cell and could be followed to the lateral boundary of PoS/subiculum. Roman numerals indicate PoS lamina. B, High magnification illustrating the somas of the cells depicted in A. Scale bar, 25 μm. C, BDA injection site in ipsilateral LMN. Solid line indicates boundaries of LMN; dashed line indicates extent of BDA diffusion into adjacent structures. Dark vertical band above LMN indicates track of the injection cannula. Scale bar, 200 μm.

Figure 4

Pyramidal cell in PoS labeled by BDA injection into LMN; case #55. A-C, Adjacent brain sections containing portions of a heavily labeled pyramidal cell. Scale bar, 75 μm. Right, Schematic reconstruction of the labeled cell. Apical dendrite could be followed into layer I, and basal dendrites into layers V/VI. Roman numerals indicate PoS lamina. IIIa represents lamina dissecans.

Figure 5

Multiform cells in PoS layers V/VI labeled by BDA injection into ADN; case #56. A, Most neurons labeled by BDA injection into ADN lacked long apical dendrites and were not characterized by pyramidal shaped soma. Black lines indicate borders of PoS lamina. Scale bar, 100 μm. B, High magnification image of labeled cells from region outlined by rectangle in panel A. Scale bar, 50 μm. C, BDA injection was most concentrated in the ipsilateral ADN (outlined by solid black line), with some diffusion into adjacent nuclei including the anteroventral and anteromedial thalamic nuclei (outlined by dashed black line). Region of BDA diffusion is outlined by dashed black line. Scale bar, 100 μm.

Figure 6

PoS projections to LMN and ADN originate in distinct cortical layers. Neurons terminating in the ADN are located in PoS layers V/VI, and include heterogeneous cell types. In contrast, PoS neurons that terminate in LMN consist of large pyramidal cells that lie in PoS layer IV. These layer IV pyramidal cells give rise to apical dendrites that are in a position to receive visual cortical inputs, which terminate in layers I and III.