VGLUT1 mRNA and protein expression in the visual system of prosimian galagos (Otolemur garnetti)

Abstract

The presynaptic storage and release of glutamate, an excitatory neurotransmitter, is modulated by a family of transport proteins known as vesicular glutamate transporters. Vesicular glutamate transporter 1 (VGLUT1) is widely distributed in the central nervous system of most mammalian and nonmammalian species, and regulates the uptake of glutamate into synaptic vesicles as well as the transport of filled glutamatergic vesicles to the terminal membrane during excitatory transmission. In rodents, VGLUT1 mRNA is primarily found in the neocortex, cerebellum, and hippocampus, and the VGLUT1 transport protein is involved in intercortical and corticothalamic projections that remain distinct from projections involving other VGLUT isoforms. With the exception of a few thalamic sensory nuclei, VGLUT1 mRNA is absent from subcortical areas and does not colocalize with other VGLUT mRNAs. VGLUT1 is similarly restricted to a few thalamic association nuclei and does not colocalize with other VGLUT proteins. However, recent work in primates has shown that VGLUT1 mRNA is also found in several subcortical nuclei as well as cortical areas, and that VGLUT1 may overlap with other VGLUT isoforms in glutamatergic projections. In order to expand current knowledge of VGLUT1 distributions in primates and gain insight on glutamatergic transmission in the visual system of primate species, we examined VGLUT1 mRNA and protein distributions in the lateral geniculate nucleus, pulvinar complex, superior colliculus, V1, V2, and the middle temporal area (MT) of prosimian galagos. We found that, similar to other studies in primates, VGLUT1 mRNA and protein are widely distributed in both subcortical and cortical areas. However, glutamatergic projections involving VGLUT1 are largely limited to intrinsic connections within subcortical and cortical areas, as well as the expected intercortical and corticothalamic projections. Additionally, VGLUT1 expression in galagos allowed us to identify laminar subdivisions of the superior colliculus, V1, V2, and MT.

Figure1

Coronal sections through the lateral geniculate nucleus stained for (A) CO, (B) Nissl, (C and E) VGLUT1 mRNA, and (D and F) VGLUT1 protein. Notes: Scale bar is 500 µm for panels (A–D), 250 µm for panels (E–F). Thalamic midline is to the left. Abbreviations: PE, external parvocellular; KE, external koniocellular; KI, internal koniocellular; PI, internal parvocellular; MI, internal magnocellular; ME, external magnocellular.

Figure 2

Coronal sections through the pulvinar complex stained for (A) Nissl, (B) CO, (C) VGLUT1 mRNA, and (D) VGLUT1 protein. (E–J) High magnification images of subdivisions of the pulvinar complex stained for (E–G) VGLUT1 mRNA and (H–J) VGLUT1 protein. Notes: Scale bar is 500 µm for panels (A–D), 100 µm for panels (E–J). Thalamic midline is to the right. Abbreviations: PI, inferior pulvinar; PL, lateral pulvinar; PM, medial pulvinar.

Figure 3

Coronal sections through the superior colliculus stained for (A) CO, (B) Nissl, (C and E) VGLUT1 protein, and (D and F) VGLUT1 mRNA. The stratum zonale or zonal layer (SZ) is a thin band that runs across the dorsal surface of the SC; immediately below is the stratum griseum superficiale or superficial gray layer (SGS) that is divided into upper and lower sublayers (uSGS and ISGS respectively); the stratum opticum or optic layer (SO) lies below the SGS and separates the superficial layers from the intermediate layers; below the SO lies the stratum griseum intermediale or intermediate gray layer (SGI), which is divided into three sublayers (SGla, SGlb, and SGlc form dorsal to ventral); the stratum album intermediale or intermediate white layer (SAI) lies below the SGI and separates the intermediate layers from the deep layers; below the SAI lies the stratum griseum profundum or deep gray layer (SGP) and lastly; ventral to the SGP lies the stratum album profundum or deep white layer (SAP), which borders the periaqueductal gray. Notes: Scale bar is 0.5 mm for panels (A–D), 100 µm for panels (E–F).

Figure 4

Low magnification images of coronal sections through V1 (area 17) and V2 (area 18) stained for (A) CO, (B) Nissl, (C) VGLUT1 mRNA, and (D) VGLUT1 protein. Arrowheads mark the V1/V2 border. Dorsal surface of cortex is up, ventral surface is down, hemispheric midline is to the left. Note: Scale bar is 2 mm.

Figure 5

High magnification images of the laminar organization of V1 in sections stained for (A) CO, (B) Nissl, (C) VGLUT1 mRNA, and (D) VGLUT1 protein. Laminar divisions for layers I–VI are presented from dorsal to ventral in each image. Note: Scale bar is 250 µm.

Figure 6

High magnification images of the laminar organization of V2 in sections stained for (A) CO, (B) Nissl, (C) VGLUT1 mRNA, and (D) VGLUT1 protein. Laminar divisions for layers I–VI are presented from dorsal to ventral in each image. Note: Scale bar is 250 µm.

Figure 7

Low magnification images of coronal sections through the middle temporal area (MT) stained for (A) CO, (B) Nissl, (C) VGLUT1 mRNA, and (D) VGLUT1 protein. Dorsal is up, hemispheric midline is to the right. Note: Scale bar is 1 mm.

Figure 8

High magnification images of the laminar organization of middle temporal area in sections stained for (A) CO, (B) Nissl, (C) VGLUT1 mRNA, and (D) VGLUT1 protein. Laminar divisions for layers I–VI are presented from dorsal to ventral in each image. Note: Scale bar is 250 µm.

Figure 9

Cortical and subcortical visual connections of the (A) lateral geniculate nucleus, (B) pulvinar complex, and (C) superior colliculus. Shading intensity reflects levels of expression for both VGLUT1 mRNA and protein. Summarized from the literature.,,–,–,– Abbreviations: PI, inferior pulvinar; PL, lateral pulvinar; PM, medial pulvinar; MT, middle temporal area; LGN, lateral geniculate nucleus; lSGS, lower superficial gray layer; uSGS, upper superficial gray layer; C retina, contralateral retina; I retina, ipsilateral retina.

Figure 10

Visual connections of V1, V2, and MT in prosimian galagos. Shading intensity reflects levels of expression for both VGLUT1 mRNA and protein. Brodmann’s divisions listed in gray on the right side of each layer for V1. Summarized from the literature.,–,,,–,,,– Abbreviations: PI, inferior pulvinar; PL, lateral pulvinar; MT, middle temporal area; SGS, superficial gray layer; LGN, lateral geniculate nucleus.