A Lacertilian Dorsal Retinorecipient Thalamus: A Re-Investigation in the Old-World Lizard <i>Podarcis hispanica</i> (Part 1 of 2)

Abstract

The aim of this work is to delineate the retinorecipient cell groups of the dorsal thalamus of lizards and to study some of the differential connections in order to help to understand the evolution of the visual system in tetrapods. Tract-tracing and immunohistochemical techniques were applied to the retinorecipient dorsal thalamus of the lizard Podarcis hispanica. The retina of Podarcis projects to four areas of the dorsal thalamus: nucleus ovalis (Ov), intergeniculate leaflet (IGL), dorsal lateral geniculate nucleus (GLD) and dorsolateral anterior nucleus (DLA). Nucleus ovalis shows a clear cell plate/neuropile organization and projects to the ventral thalamus. Thus, it seems to belong to the ventral rather than to the dorsal thalamus. The IGL contains large cells reactive for GABA and/or NPY immunohistochemistry. It is interconnected with the supra/retrochiasmatic hypothalamus and projects to the opposite thalamus and to the ipsilateral tectum. The caudal DLA, which lacks both GABA- and NPY-like immunoreactive cells is reached by a few thin retinal fibers, although distal dendrites of DLA cells enter the GLD, suggesting an important retinal input. The DLA projects to the medial and dorsal telencephalic cortices. The GLD is the main retinorecipient thalamic structure that projects to the telencephalon. It shows a crude laminar organization in which cell plate neurons project to the ipsilateral pallial thickening, but it does not receive a descending projection from the visual telencephalon and thus differs from the GLD of other amniotic vertebrates. In the context of present knowledge, these results suggest that an IGL homologue is present in all tetrapods studied, whereas Ov seems to be restricted to diapsid vertebrates. Moreover, our data suggest that a unimodal visual projection to the telencephalon (arising from the GLD) first appeared in reptiles by segregation from a limbic (multimodal) thalamo-telencephalic pathway.

Retinal projections in two crocodilian species, Caiman crocodilus and Crocodylus niloticus

“…These have been repeatedly confirmed with numerous experimental techniques Herguéta et al 1995;Reiner et al 1996). The OV, which appears to be a constant primary visual center in all reptilian orders, has been described by a variety of names or even included in the LGC in spite of the fact that it arises from the ventral thalamus (Herguéta et al 1993Reiner et al 1996) and has no relationships with the telencephalon (Kenigfest et al 1995(Kenigfest et al , 1997.…”

“…A retinal projection to the GLM has been described in many lacertilian and chelonian species Repérant et al 1992;Bennis et al 1994;Reiner et al 1996;Kenigfest et al 1997), and in most cases recognized as a medial subunit of the GLD. However, it has recently been shown, in the lizard Podarcis hispanica (Kenigfest et al 1997), that this thalamic retino-recipient structure does not form part of the thalamo-telencephalic visual pathway, since it makes no connections with the telencephalon.…”

“…The absence of retrograde degeneration produced in the diencephalon of the lizard Lacerta vivipara (Powell and Kruger 1960) or the crocodile Alligator mississippiensis after different telencephalic lesions (Kruger and Berkowitz 1960) has led these authors to conclude that the GLD does not project to the telencephalon and is thus part of the ventral thalamus. However, more recent studies, using axonal tracing, have shown in squamates and chelonians that the GLD almost acts as a thalamic relay for visual information destined to the telencephalon by way of the thalamofugal optic tract, and hence that the GLD is a dorsal thalamic structure Kenigfest et al 1995Kenigfest et al , 1997. This relationship remains, however, to be demonstrated in crocodiles.…”

“…Retinal projections to the dorsal thalamus, corresponding to the LGC of crocodilians, have been described in all other reptilian orders Herguéta et al 1995;Reiner et al 1996;Kenigfest et al 1997Kenigfest et al , 1998. However, this constant and important target of retinal fibers shows considerable interspecific cytoarchitectural variations that make detailed comparisons somewhat difficult.…”

The distribution of NPY-like immunoreactivity in the chameleon brain

“…Many immunolabeled cells were observed lateral to the nucleus dorsolateralis, in the nucleus ovalis, the nucleus geniculatus lateralis dorsalis, and in the belt surrounding the nucleus rotundus. The last cell group, which receives a massive retinal input (Bennis et al 1994b), may be compared to the mammalian intergeniculate leaflet (reviewed Belekhova et al 1997;Kenigfest et al 1997). In the rostroventral hypothalamus of the chameleon, the nucleus suprachiasmaticus, which receives a massive retinal innervation (Bennis et al 1994b), is also richly endowed with NPY-immunolabeled fibers that probably arise from the neurons of the perirotundal belt as in the lizard Gallotia (Medina et al 1991).…”