Reorganization of the corticotectal projections introduced by neonatal monocular enucleation in the Monodelphis opossum and the influence of serotoninergic depletion.

The influence of neonatal serotoninergic lesion (performed with s.c. injection of 5,7-dihydroxytryptamine) on the plasticity of the developing corticotectal projection was studied in the gray short-tailed opossum (Monodelphis domestica). As a first step, the placement and density of neurons projecting from the visual cortical areas to the superior colliculus was established in the adult opossum. Injections of retrogradely transported fluorescent dyes into the superior colliculus of intact three-month-old animals labeled neurons of cortical layer V. In this species, there are three visual areas: the striate area and two secondary areas, the laterally placed peristriate area and the medial visual area. The population of the labeled neurons was denser in peristriate and medial visual areas than in the striate area. Secondly, the influence of neonatal monocular enucleation on the extent of this projection was investigated, alone or in combination with a serotoninergic lesion. Injection of dyes into the superior colliculi of three-month-old animals that were unilaterally enucleated on the second postnatal day also labeled neurons of cortical layer V. However, the density of the cortical neurons projecting to the superior colliculus contralateral to the remaining eye was much lower. This reduction was most profound in the striate visual area. No significant modifications of this projection were found on the side ipsilateral to the remaining eye. In another group of opossums, unilateral enucleation on the second postnatal day was combined with serotoninergic lesion. Brains of some of the treated pups were immunostained for serotonin on the fifth postnatal day. At this age, 70-80% of serotoninergic axons in the brain were missing. However, in about three weeks these axons had regrown, and their density in the neocortex was approximately the same as in the control animals. We conclude that severe reduction of the serotoninergic innervation during the early postnatal period did not influence the plastic changes induced in the corticotectal projection by unilateral enucleation.

Localization of the 5-HT1A receptors in the brain of opossum Monodelphis domestica.

This paper describes the distribution of 5-HT1A receptors in the brain of opossum Monodelphis domestica. They were visualized by immunohistological staining with an antibody against the amino acid sequence (170-186) of this receptor that was previously successfully used in the rat and monkey. As in Eutherians, high levels of immunostaining were present in the septum, hippocampus, raphe nuclei and some other brain stem nuclei. Neocortex, several thalamic nuclei and hypothalamus showed moderate density of the labeled structures. Moderate levels of 5-HT1A receptors were also observed in the caudate nucleus and putamen, unlike in the rat, in which labeling in these nuclei was almost absent. Another difference with the rat was observed in the neocortex: in the opossum immunostaining was absent in the layer 4 of many cortical areas. In general, distribution and density of this important receptor in the opossum is very similar to that described in the rat and monkey and therefore it follows a general mammalian pattern.