An immunocytochemical analysis of the lateral geniculate complex in the pigeon (Columba livia)

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Abstract
The lateral geniculate complex (GL) of pigeons was investigated with respect to its immunohistochemical characteristics, retinal afferents, and the putative transmitters/modulators of its neurons. The distributions of serotonin‐, choline acetyltransferase‐, glutamic acid decarboxylase‐, tyrosine hydroxylase‐, neuropeptide Y‐ (NPY), substance P‐ (SP), neurotensin‐ (NT), cholecystokinin‐ (CCK), and leucine‐enkephalin‐ (L‐ENK) like immunoreactive perikarya and fibers were mapped. Retinal projections were studied following injections of Rhodamine‐B‐isothiocyanate into the vitreous. Transmitter‐specific projections onto the visual Wulst and the optic tectum were studied by simultaneous double‐labelling of retrograde tracer molecules and immunocytochemical labelling. The GL can be divided into three major subdivisions, the n. geniculatus lateralis, pars dorsalis (GLd; previously designated as the n. opticus principalis thalami, OPT), the n. marginalis tractus optici (nMOT), and the n. geniculatus lateralis, pars ventralis (GLv). All three subdivisions are retinorecipient. The GLd can be further subdivided into at least five components differing in their immunohistochemical characteristics: n. lateralis anterior (LA); n. dorsolateralis anterior thalami, pars lateralis (DLL), n. dorsolateralis anterior thalami, pars magnocellularis (DLAmc); n. lateralis dorsalis nuclei optici principalis thalami (LdOPT); and n. suprarotundus (SpRt). The LdOPT consists of an area of dense CCK‐like and NT‐like terminals of probable retinal origin. Three subnuclei (DLL, DLAmc, SpRt) were shown to project to the visual Wulst. Cholinergic and cholecystokinergic relay neurons participated in this projection. The nMOT occupies a position between the GLd and GLv and encircles the rostral pole of n. rotundus and the LA. It is characterized mainly by medium sized NPY‐like perikarya which were shown to project onto the ipsilateral optic tectum. Bands of NPY‐like fibers in the tectal layers 2, 4, and 7 could at least in part be due to this projection of the nMOT. Most of the antisera used revealed transmitter/modulator‐specific fiber systems in the GLv which often showed a layer‐specific distribution. Perikaryal labelling was only obtained with glutamic acid decarboxylase. On the basis of its chemoarchitectonics, topography, and connectional pattern, the GLd complex of pigeons is most directly equivalent to the mammalian GLd. However, although the different subdivisions of the avian GLd may represent functionally different channels within the thalamofugal pathway similar to the lamina‐specific differentiation within the mammalian geniculostriate projection, direct comparison of subnuclei of birds and mammals is not justified at this time. The nMOT appears similar to the intergeniculate leaflet (IGL) and the avian GLv clearly corresponds in many features to the mammalian GLv.