Comparison of the ultrastructure of cortical and retinal terminals in the rat dorsal lateral geniculate and lateral posterior nuclei

Abstract

We compared the ultrastructure and synaptic targets of terminals of cortical or retinal origin in the rat dorsal lateral geniculate nucleus (LGN) and lateral posterior nucleus (LPN). Following injections of biotinylated dextran amine (BDA) into cortical area 17, two types of corticothalamic terminals were labeled by anterograde transport. Type I terminals, found throughout the LGN and LPN, were small, drumstick-shaped terminals that extended from thin axons. At the ultrastructural level in both the LGN and LPN, labeled type I corticothalamic terminals were observed to be small profiles that contained densely packed round vesicles (RS profiles) and contacted small-caliber dendrites. In tissue stained for gamma amino butyric acid (GABA) using postembedding immunocytochemical techniques, most dendrites postsynaptic to type I corticothalamic terminals did not contain GABA (97%). Type II corticothalamic terminals, found only in the LPN, were large terminals that sometimes formed clusters. At the ultrastructural level, type II terminals were large profiles that contained round vesicles (RL profiles) and contacted large-caliber dendrites, most of which did not contain GABA (98%). Retinogeniculate terminals, identified by their distinctive pale mitochondria, were similar to type II corticothalamic terminals except that 26% of their postsynaptic targets were vesicle-containing profiles that contained GABA (F2 profiles). Our results suggest that type I corticothalamic terminals are very similar across nuclei but that the postsynaptic targets of RL profiles vary. Comparison of the responses to retinal inputs in the LGN and to layer V cortical inputs in the LPN may provide a unique opportunity to determine the function of interneurons in the modulation of retinal signals and, in addition, may provide insight into the signals relayed by cortical layer V.

Comparison of the ultrastructure of cortical and retinal terminals in the rat superior colliculus

“…1C and D). This morphology was noticeably distinct from that of corticothalamic terminals that we analyzed in tissue from the same animals (Li et al, 2003).…”

“…All procedures conformed to National Institutes of Health guidelines for the care and use of laboratory animals and were approved by the University of Louisville Animal Care and Use Committee. Thalamic tissue from these rats was used for a previously published study (Li et al, 2003).…”

“…To avoid sampling the same terminals in multiple sections, only every 10th section was collected on Formvar-coated nickel slot grids. Every third section in this series was stained for the presence of GABA using previously reported postembedding immunocytochemical techniques (Patel and Bickford, 1997;Li et al, 2003). We used a rabbit polyclonal antibody against GABA (Sigma, St. Louis, MO) at a dilution of 1:500 -1:2,000 and a goatantirabbit IgG conjugated to 15 nm gold particles (Amersham, Arlington Heights, IL) to reveal the distribution of GABA.…”

“…The cortical injections used for this study were described and illustrated in our previous study of corticothalamic projections (Li et al, 2003). The injections covered all layers of area 17 and in some cases also included small portions of area 18b.…”

“…We previously examined the ultrastructure of type I and type II terminals in the rat LPN (Li et al, 2003), and tissue from the same experimental animals was used for the present study. We found that type I corticothalamic terminals in the dLGN and LPN are small profiles (dLGN, 0.28 Ϯ 0.13 m 2 ; LPN, 0.34 Ϯ 0.11 m 2 ) and type II corticothalamic terminals in the LPN are much larger (2.27 Ϯ 1.27 m 2 ).…”