Fine structure of the dorsal latreral geniculate nucleus of the turtle, Emys orbicularis: A Golgi, combined hrp tracing and GABA immunocytochemical study

Abstract

The afferent and efferent cortical projections of the dorsal lateral geniculate nucleus (GLD) of adult specimens of the turtle Emys orbicularis were investigated after intraocular or intracortical injections of horseradish peroxidase (HRP), and the distribution of gamma aminobutyric acid (GABA) immunoreactivity in the nucleus was carried out by immunocytochemical techniques, both techniques being combined with light and electron microscopy. In addition, some specimens were prepared for double-labeling of HRP and GABA immunoreactivity, and additional samples impregnated by a rapid Golgi technique. On purely morphological grounds, four types of neurons can be distinguished by light microscopy: two types of large cells in the cell plate which project to the cortex, and two types of smaller cells in the neuropil and optic tract which do not. The small cells are consistently GABA-immunoreactive, while the former are, with extremely rare exceptions, immunonegative for GABA. The supposition that the small neurons of the neuropil are interneurons is supported by electron microscopic observations; these strongly GABA-immunoreactive cells have large plicated nuclei surrounded by a thin layer of cytoplasm poorly endowed with organelles. The dendrites of these cells may contain pleomorphic synaptic vesicles (DCSVs) and appear to be presynaptic to other dendritic profiles. These DCSVs are occasionally contacted by GABA-immunoreactive axon terminals, and more frequently by retinal terminals consistently immunonegative for GABA. The latter, frequently organized in glomeruli, also make synaptic contacts with immunonegative dendrites arising from corticopetal neurons of the cell plate. Two major categories of GABA-immunoreactive axon terminals can be distinguished, and we are led to the conclusion that one of these represents an intrinsic GABAergic innervation of the GLD, while the second is tentatively interpreted as an extrinsic source of GABA to the nucleus, possibly from ventral thalamic structures. The fine structure of the dorsal lateral geniculate nucleus of Emys orbicularis thus shows many similarities with that of mammals.

Pretectal connections in turtles with special reference to the visual thalamic centers: A hodological and ?-aminobutyric acid-immunohistochemical study

“…Retrogradely labeled neurons were observed in the SGC of the TO, as in Group 1 specimens, and also in the stratum griseum et fibrosum superficiale (SGFS). These results are consistent with existing findings in turtles (Hall and Ebner, 1970b;Ulinski, 1983Ulinski, , 1990Belekhova et al, 1993;Kenigfest et al, 1995).…”

“…Similar to other reptiles (Bennis et al, 1991;Rio et al, 1992;Pritz and Stritzel, 1994b;Pritz, 1995), the GLd of the turtle contains both intrinsic GABAergic neurons (Belekhova et al, 1991(Belekhova et al, , 1993Rio et al, 1992;Kenigfest et al, 1995) and fibers of extrageniculate origin. Our data indicate that at least some of the latter arise from neurons in the Lm and possibly in the Gt and Ptv (Belekhova et al, 1999;present results).…”

“…Thus, there are some grounds for suggesting a common function of the pretectum in all amniotes (Fite, 1985;McKenna and Wallman, 1985), although the problem of the homologies of the pretectal nuclei remains unsolved (Northcutt, 1984;Reiner, 1994;Reiner et al, 1996). We previously described (Belekhova et al, 1991(Belekhova et al, , 1993Kenigfest et al, 1995) GABA immunoreactivity in the GLd and Rot of turtles. In the GLd, some of this immunoreactivity is due to the presence of intrinsic GABAergic interneurons that make symmetrical axodendritic and dendrodendritic synaptic contacts with their targets (Kenigfest et al, 1995).…”

“…We previously described (Belekhova et al, 1991(Belekhova et al, , 1993Kenigfest et al, 1995) GABA immunoreactivity in the GLd and Rot of turtles. In the GLd, some of this immunoreactivity is due to the presence of intrinsic GABAergic interneurons that make symmetrical axodendritic and dendrodendritic synaptic contacts with their targets (Kenigfest et al, 1995). In this latter study, we also described asymmetrical axodendritic and axosomatic GABApositive synapses, suggesting that, as in mammals (Cucchiaro et al, 1991(Cucchiaro et al, , 1993Wahle et al, 1994;Reimann and Schmidt, 1996), these have an extrageniculate origin, possibly in the GABA-immunopositive neurons of the pretectal nuclei and reticular thalamic nucleus.…”

Fine structure of the visual dorsolateral anterior thalamic nucleus of the pigeon (Columba livia): A hodological and GABA‐immunocytochemical study

“…It is important to point out here that these different afferent supplies to the avian DLA, with the exception of the accessory optic system, have also been described in the GLd of reptiles (cf. Kenigfest et al, 1995Kenigfest et al, , 1998Kenigfest et al, , 2000Kenigfest et al, , 2004Kenigfest et al, , 2005Kenigfest et al, , 2006Belekhova et al, 2003) and mammals (cf. Niimi et al, 1970Niimi et al, , 1971Pasik et al, 1988Pasik et al, , 1990Raczkowski and Fitzpatrick, 1989;Papadopoulos et al, 1990;Harting et al, 1991a,b;Cucchiaro et al, 1991aCucchiaro et al, ,b, 1993Diamond et al, 1991;van der Want et al, 1992;Beaulieu and Cynader, 1992;Robson, 1993;Wilson, 1993;Feig and Harting, 1994;Montero, 1994;Vidnyanszky and Há mori, 1994;Feig et al, 1998;Wang et al, 2001Wang et al, , 2002Uhlrich et al, 2003).…”