Morphology and Dye-Coupling of Cells in the Pigeon Isthmo-Optic Nucleus

Abstract

Ground-feeding birds such as pigeons possess the most developed isthmo-optic nucleus in all classes of vertebrates. A previous study showed that this centrifugal or retinopetal nucleus modulates visual activity in tectal cells of pigeons; the present study aimed at revealing the morphology and possible dye-coupling of neurons in the isthmo-optic nucleus and in the ectopic cell region by intracellular injections of Lucifer yellow into neurons in slices. One hundred and twelve successfully labeled cells of the isthmo-optic nucleus were classified into bipolar (83%) and multipolar (17%) types, each of which was further divided into two subtypes, B and P and M and N, respectively. Neurons of B- and P-types are similar in that they have apical dendrites and axons usually arising from the opposite pole of piriform perikarya, but they differ in the length (20–120 vs. 10–20 μm) of their dendritic stems; M- and N-types possess polygonal perikarya giving rise to two to five primary dendrites either in the same orientation (M) or in a radiation fashion (N), and their axons originate from perikarya or occasionally from dendritic stems. Twelve single-injections resulted in the labeling of 26 cells, including 11 pairs and 1 quadruple labeling. About half of these are closely apposed ‘twin-cells’. Dye-coupling was found only between neighboring cells in the cell lamina. Thirteen cells in the ECR rostroventral to the ION were labeled and could be grouped into large or L- (46%) and small or S- (54%) types, mainly depending on the dendritic field size and the number of primary dendrites. No dye-coupling was observed between the presumptive ECR cells. The functional role of the ION and the significance of dye-coupling between neurons are discussed.

The centrifugal visual system of a palaeognathous bird, the Chilean Tinamou (Nothoprocta perdicaria)

“…It characteristically consists of small monopolar neurons ∼15 µm in diameter, which in Neognathae with well‐developed ION form a lamina around a central neuropil containing their dendrites (Clarke & Caranzano, ; Cowan, ; Güntürkün, ; Li & Wang, ). The morphologically different ectopic centrifugal neurons, which lie outside the proper ION, are more heterogeneous in size and generally multipolar (Clarke & Cowan, ; Cowan & Clarke, ; Hayes & Webster, ; Li & Wang, ; Médina, Repérant, Miceli, Bertrand, & Bennis, ; O'Leary & Cowan, ; Weidner, Desroches, Repérant, Kirpitchnikova, & Miceli, ; Weidner, Repérant, Desroches, Miceli, & Vesselkin, ; Wolf‐Oberhollenzer, ). The ION presents a high variety of morphologies with different levels of complexity, but in most species (with some noteworthy exceptions such as Procellariiformes and Pelicaniformes) it is at least clearly distinguishable as a nucleus (Gutiérrez‐Ibáñez et al, ).…”

Centrifugal visual system of Crocodylus niloticus: A hodological, histochemical, and immunocytochemical study

“…The majority of the neurons are monopolar with flask‐shaped or pyriform somata, 15–25 μm in diameter, from the apical pole of which two to four primary dendritic trunks emerge to divide into parallel branches directed toward the neuropil. Their axons arise either from the opposite basal pole of the cell body or from one of the apical dendritic trunks (Cowan, 1970; Miceli et al, 1995; Li and Wang, 1999a). The NIO also contains a small population of GABAergic interneurons (Miceli et al, 1995).…”

Dual actions of isthmic input to tectal neurons in a reptile, Gekko gekko

“…These differences may imply that tectoisthmic cells are at least in part not those that receive isthmic afferents (Gruberg et al, 1994), but the observed differences could also be due to selective labeling or small sample. Dye-coupling has been found in various brain regions in different species, including, for example, NI in teleosts (Williams et al, 1983) and in amphibians Wiggers, 1998), the optic tectum in amphibians (Wang & Matsumoto, 1990;Wiggers, 1998), the isthmo-optic nucleus in birds (Li & Wang, 1999), and the striatum, neocortex, and hippocampus in rats (Peinado et al, 1993;Onn & Grace, 1994;Velazquez et al, 1997). Dye-coupling has been suggested to be indicative of electrotonic connections (Peinado et al, 1993) and0or of extensive connections via chemical synapses between cells (Wiggers, 1998).…”