Pretectotectal pathway: An ultrastructural quantitative analysis in cats

Abstract

Both the pretectum (PT) and the superior colliculus (SC) play an important role in directing eye movements and in sensorimotor coupling. A reciprocal connection between the PT and the SC has been described, which suggests a strong interplay between these two structures. We injected the cat SC with retrograde tracers and examined the labeled pretectotectal (PTT) cells at the light and electron microscopic level. PTT cells were distributed mostly in the nucleus of the optic tract and 93.1% contained gamma amino butyric acid (GABA). We also observed that PTT cells are located outside of pretectal regions distinguished by dense retinal terminals and clusters of cells that contain calbindin. This suggests that the GABAergic PTT cells are distinct from the GABAergic pretectogeniculate cells that have been previously described as being distributed within these regions. Finally, to determine the synaptic targets of PTT terminals, we injected the PT with anterograde tracers and examined terminals labeled in the SC at the ultrastructural level. The labeled PTT terminals were beaded fibers that were distributed mainly within the stratum griseum superficiale (SGS) of the SC. Using postembedding immunocytochemistry, 94.5% were found to be GABAergic. The PTT terminals were mostly small in size and primarily contacted GABA-negative dendrites (88.1%) and in some cases somata (4.7%). The remainder terminated on GABAergic dendrites (7.2%). Our results suggest that the PTT cells constitute a separate population of GABAergic efferent cells in the PT, which may function to inhibit the activity of non-GABAergic SC efferent cells in the SGS.

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

“…However, the superficial layers of the SC contain a wide variety of cell types, including interneurons and cells that project to the parabigeminal nucleus, contralateral SC, pretectum, ventral and dorsal lateral geniculate nuclei, and lateral posterior nucleus (for reviews, see Huerta and Harting, 1984;May, 2006). SC interneurons, tectopretectal, and tectotectal neurons have been found to be GABAergic (Appell and Behan, 1990;Mize, 1992;Schmidt et al, 2001, Baldauf et al, 2003. Thus, these cell types may be ruled out as the primary targets of area 17 terminals.…”

Modulation of GABAergic inhibition in the rat superior colliculus by a presynaptic group II metabotropic glutamate receptor

“…The three classes of GABA receptor are also relatively abundant and each plays distinct roles in collicular neuronal processing (Binns & Salt, 1997;Schmidt et al 2001). The SC also receives GABAergic input from the substantia nigra, zona incerta, pretectum and the contralateral colliculus (Mize, 1992;Baldauf et al 2003;Born & Schmidt, 2004). The variability of the IPSC paired-pulse behaviour under control conditions (prior to Group II agonist application) is consistent with the notion that there is a substantial variation of inhibitory circuits, but it is nevertheless noteworthy that in all cases IPSCs were affected in the same way by Group II receptor activation.…”

Cortico-fugal output from visual cortex promotes plasticity of innate motor behaviour

“…Instead, we observed labelled axons in the vLGN, superior colliculus and striatum. As the vLGN and superior colliculus project to the NOT-DTN 3,27 (Extended Data Fig. 6a), they could also contribute to the increased drive of the NOT-DTN.…”