Retinofugal Projections Into Visual Brain Structures in the Bat Artibeus planirostris: A CTb Study

Abstract

A well-developed visual system can provide significant sensory information to guide motor behavior, especially in fruit-eating bats, which usually use echolocation to navigate at high speed through cluttered environments during foraging. Relatively few studies have been performed to elucidate the organization of the visual system in bats. The present work provides an extensive morphological description of the retinal projections in the subcortical visual nuclei in the flat-faced fruit-eating bat (Artibeus planirostris) using anterograde transport of the eye-injected cholera toxin B subunit (CTb), followed by morphometrical and stereological analyses. Regarding the cytoarchitecture, the dorsal lateral geniculate nucleus (dLGN) was homogeneous, with no evident lamination. However, the retinal projection contained two layers that had significantly different marking intensities and a massive contralateral input. The superior colliculus (SC) was identified as a laminar structure composed of seven layers, and the retinal input was only observed on the contralateral side, targeting two most superficial layers. The medial pretectal nucleus (MPT), olivary pretectal nucleus (OPT), anterior pretectal nucleus (APT), posterior pretectal nucleus (PPT) and nucleus of the optic tract (NOT) were comprised the pretectal nuclear complex (PNT). Only the APT lacked a retinal input, which was predominantly contralateral in all other nuclei. Our results showed the morphometrical and stereological features of a bat species for the first time.

Keywords: chiropteran; cholera toxin subunit b; lateral geniculate nucleus; phyllostomidae; pretectal region; retinal projections; superior colliculus; visual system.

Figure 1

Artibeus planirostris (Photo/Image courtesy of Frederico Horie Silva).

Figure 2

Photomicrographs of the brain sections of flat-faced fruit-eating bat showing the dLGN in bright field stained by Nissl technique at rostral, middle and caudal levels (right) and drawings (left; A–C), respectively. The boxed area in (B) are shown in high magnification in (D), illustrating the detailed morphology of the cells in the dLGN. Black arrows indicating rounded shape neurons in the dLGN. Scale bar 100 μm (A–C) and 10 μm (D). Abbreviations: see list.

Figure 3

Photomicrographs of the brain sections of flat-faced fruit-eating bat showing the PNT in bright field stained by Nissl technique at rostral, middle and caudal levels (right) and drawings (left; A–C), respectively. Scale bar 100 μm (A–C). Abbreviations: see list.

Figure 4

Photomicrographs of the brain sections of flat-faced fruit-eating bat showing the PNT in bright field stained by Nissl technique at middle and caudal levels (A,B). The boxed area in (A,B) are shown in high magnification in (C–F), illustrating the detailed morphology of the cells in the MPT, OPT, NOT and PPT respectively. Black arrows indicating elliptical shaped neurons and black arrow heads indicating round shaped neurons. Scale bar 100 μm (A,B) and 10 μm (C–F). Abbreviations: see list.

Figure 5

Photomicrographs of the brain sections of flat-faced fruit-eating bat showing the SC in bright field stained by Nissl technique at middle level (right) and drawing (left; A) The boxed areas in (B) are shown in high magnification in (C,D), illustrating the detailed morphology of the cells in the SC. Black arrows indicating elliptical shaped neurons in the ZS and black arrow heads indicating round shaped neurons in the SGS. Scale bar 100 μm (A,B) and 10 μm (C,D). Abbreviations: see list.

Figure 6

Photomicrographs of the dLGN coronal sections of flat-faced fruit-eating bat at rostral (A), middle (B) and caudal (C) levels, illustrating the distribution pattern of retinal projections in the ipsi and contralateral sides. The boxed areas in (D) are shown in high magnification in (E,F) respectively, illustrating the detailed morphology of the retinal axons in the superficial (E), and deep layers in the contralateral side of the dLGN. White arrows indicating R1and R2-like terminals in the superficial layer (E), and black arrows indicating simple endings (F). Scale bar 100 μm (A–D) and 10 μm (E,F). Abbreviations: see list.

Figure 7

Relative optical density (ROD) values in the dLGN layers of flat-faced fruit-eating bat (n = 5). To compare across layers, the ROD in each layer was schematically displayed as levels of gray in the drawings (A) through rostrocaudal length. The bars represent the means (± standard error) of the mean ROD values of individual animals by layer (B). The dashed and solid lines represent the overall mean of ROD for the deep and superficial layers, respectively. General Linear Mixed Model (GLMM) revealed ROD significant difference between the layers analyzed (F_(1,109) = 60.6, p < 0.001). Different letters represent significant individual differences (Bonferroni post hoc test, p < 0.05). Scale bar 500 μm in (A). Abbreviations: see list.

Figure 8

Photomicrographs of the PNT coronal sections of flat-faced fruit-eating bat at middle (A) and caudal (B) levels, illustrating the distribution pattern of retinal projections. The boxed areas in (A,B) are shown in high magnification in (C–E), respectively. The boxed area in (A) is shown in high magnification in (C), illustrating the detailed morphology of the retinal axons in the NOT. The boxed areas in (B) are shown in high magnification in (D,E), respectively, illustrating the detailed morphology of retinal axons in the PPT (D,E). Black arrows indicating R2-like terminals; Black arrow heads indicating R1 like-terminals; White arrows indicating simple ending-like terminals; and White arrow heads indicating string-like terminals. Scale bar 100 μm (A,B) and 10 μm (C–E). Abbreviations: see list.

Figure 9

ROD values in the PNT of flat-faced fruit-eating bat (n = 5). To compare among nuclei, the ROD in each nucleus was schematically displayed as levels of gray in the drawings (A) through rostrocaudal length. The bars represent the means (± standard error) of the mean ROD values of individual animals by nuclei (B). GLMM did not reveal ROD significant difference among nuclei analyzed (F_(3,78) = 1.61, p = 0.193). Different letters represent significant individual differences (Bonferroni post hoc test, p < 0.05). Scale bar 500 μm in (A). Abbreviations: see list.

Figure 10

Photomicrographs of the SC coronal sections of flat-faced fruit-eating bat at rostral (A), middle (B) and caudal (C) levels, illustrating the distribution pattern of retinal projections in the ipsi and contralateral sides. The boxed areas in (D) are shown in high magnification in E (SGS) and F (ZS), respectively. White arrows indicate R2 and R1-like terminals; and Black arrows indicating string-like terminals. Scale bar 100 μm (A–D) and 10 μm (E,F). Abbreviations: see list.

Figure 11

ROD values in the SC of flat-faced fruit-eating bat (n = 5). To compare across layers, the ROD in each layer was schematically displayed as levels of gray in the drawings (A) through rostrocaudal length. The bars represent the means (± standard error) of the mean ROD values of individual animals by layer (B). The dashed and solid lines represent the overall mean of ROD for the SGS and ZS layers, respectively. GLMM revealed ROD significant difference between the layers analyzed (F_(1,117) = 117.4, p < 0.001). Different letters represent significant individual differences (Bonferroni post hoc test, p < 0.05). Scale bar 500 μm in (A). Abbreviations: see list.