The Horizontal Optokinetic Reflex of the opossum (Didelphis marsupialis aurita): Physiological and Anatomical Studies in Normal and Early Monoenucleated Specimens

Abstract

In the opossum the symmetrical binocular horizontal optokinetic nystagmus gives way to an asymmetrical monocular reflex: the nasotemporal (NT) stimulation yielding lower gain than the temporonasal (TN). In adults, monocularly enucleated at postnatal days 21-25 (pnd21-25), the gain of NT responses is markedly increased, approaching that of TN. Severe cell loss was detected in the nucleus of the optic tract (NOT) on the deafferented side in early monoenucleated specimens. In normal animals retinal afferents to the NOT are all crossed, while in animals enucleated at pnd21-25 sparse uncrossed retinal elements were observed. Although this abnormal projection might influence the increased NT response in this subgroup, it is argued that the increased symmetry in monoenucleated opossums may be the result of changes mediated by the commissural connection between both NOTs.

Cytochrome oxidase and NADPH‐diaphorase on the afferent relay branch of the optokinetic reflex in the opossum

“…In this and previous studies in the opossum Nasi et al, 1997), it was shown that large injections centered in the IO yielded labeled cells in the NOT-DTN, the APN, and the PPN. Also in the same species, IO projecting cells were shown to be more abundant in the anterior half of the NOT-DTN.…”

“…Moreover, by the use of CTB intraocular injections, sparse ipsilateral terminal fields that went undetected before in Didelphis aurita (Lent et al, 1976;Nasi et al, 1997;Vargas et al, 1996), although described for Didelphis virginiana and Marmosa mitis (Royce et al, 1976), could be identified scattered throughout the NOT. Moreover, by the use of CTB intraocular injections, sparse ipsilateral terminal fields that went undetected before in Didelphis aurita (Lent et al, 1976;Nasi et al, 1997;Vargas et al, 1996), although described for Didelphis virginiana and Marmosa mitis (Royce et al, 1976), could be identified scattered throughout the NOT.…”

“…A detailed description of this procedure was given elsewhere (Nasi et al, 1997;Vargas et al, 1996). A detailed description of this procedure was given elsewhere (Nasi et al, 1997;Vargas et al, 1996).…”

Cytochrome oxidase and NADPH-diaphorase on the afferent relay branch of the optokinetic reflex in the opossum

“…One animal was used to compare the distribution of CO-labeled cells in the NOT-DTN with that of cells that project to the IO. A detailed description of this procedure was given elsewhere (Nasi et al, 1997;Vargas et al, 1996). Briefly, the animal was kept anesthetized with sodium pentobarbital (30 mg/kg per hour, Knoll) injected intraperitoneally after an initial dose twice as large.…”

Tooled for the Task: Vision in the Opossum

“…Researchers have also shown that the opossum, like other mammals, has a well-developed system to stabilize the image in the retina (Nasi et al 1997). Our studies have focused on the nucleus of the optic tract, or NOT (Volchan et al 1989, Pereira et al 1994, 1998, 2001, which detects the horizontal wavering of the retinal image and generates, through several steps downstream, a compensatory eye movement called the horizontal optokinetic reflex.…”