Opossum Didelphis Aurita Research Articles

Cortical and subcortical influences on the nucleus of the optic tract of the opossum

In the present work we propose a new phylogenetic hypothesis for the role played by cortical and subcortical afferents to the nucleus of the optical tract, the main visual relay station of the horizontal optokinetic reflex in mammals. The hypothesis is supported by anatomical and physiological data obtained in the South American opossum ( Didelphis aurita) using the following experimental approaches: (i) single-unit recordings in the nucleus of the optic tract and simultaneous electrical stimulation of the contralateral nucleus of the optic tract; (ii) single-unit recordings in the nucleus of the optic tract and simultaneous electrical stimulation of the ipsilateral striate cortex; (iii) injection of cholera toxin subunit B into the striate cortex and subsequent immunohistochemical reaction to reveal the presence of the marker in the thalamus and mesencephalon; and (iv) single-unit recordings in the nucleus of the optic tract both before and after ablation of the ipsilateral visual cortex. The main results are: (i) there is a strong inhibitory reciprocal effect upon the nucleus of the optic tract following stimulation of its contralateral counterpart; (ii) electrophysiological and anatomical data imply that the visual cortex does not project directly to the nucleus of the optic tract. Rather, cortical terminals seem to target the nearby anterior and posterior pretectal nuclei and orthodromic latencies in the nucleus of the optic tract following stimulation of the visual cortex were twice as large as in the superior colicullus; and (iii) ablation of the entire visual cortex did not have any effect upon binocularity of cells in the nucleus of the optic tract. These results strengthen the model proposed here for the role of the interactions between the nuclei of the optic tract under optokinetic stimulation. The hypothesis in the present work is that the cortical influences upon the nucleus of the optical tract, in addition to the subcortical ones, appeared only recently in phylogenesis. In more primitive mammals, such as the opossum, subcortical interactions are thought to play a relatively important role. With the emergence of retinal specializations, such as the fovea, one might suppose that there followed the appearance of new ocular movements, such as the smooth pursuit and certain types of saccades, that came to join the pre-existent optokinetic reflex.

Ontogeny of cholinergic amacrine cells in the opossum (Didelphis aurita) retina

Immunocytochemistry for choline acetyltransferase (ChAT), the synthesizing enzyme for acetylcholine, was used to determine the onset and to follow the maturation of the cholinergic cells in the retina of a marsupial, the South American opossum (Didelphis aurita). ChAT-immunoreactivity was first detected in amacrine cells in the ganglion cell layer by postnatal day 15 (P15) and in the inner nuclear layer by P35. Much later, at P50 a second sub-population of ChAT-immunoreactive cell bodies was evident in the inner nuclear layer. Processes from ChAT-immunoreactive amacrine cells were detected in the two bands of the inner plexiform layer before synaptogenesis. In the adult retina, these two bands correspond to sublamina 2 and 4 of the inner plexiform layer. In flat whole-mounted preparations, cholinergic cell density was 263±13 cells/mm2 in the ganglion cell layer and it was estimated a total of 24,000 cholinergic neurons. ChAT-immunoreactive somata showed a random pattern of distribution.

Fruit consumption, distance of seed dispersal and germination of Solanaceous plants ingested by Common Opossum (Didelphis aurita) in southern Brazil

The role of the Common Opossum, Didelphis aurita, as an effective seed disperser for Solanaceous plants was studied in a forest fragment of southern Brazil for two years. Opossums were captured once per week to collect feces and to find seeds of these plants. Seeds were then tested for germination rates. The trapping locations allowed estimations of minimum dispersal distances to source plants for the species of seeds found in their feces. Opossums feces contained mainly seeds of six species of Solanaceous plants. The average number of seeds in scats varied among species and ranged from 9 to 33. These Solanaceous fruits were consumed opportunistically by opossums, without seed destruction. The average minimum distances of seed dispersal varied among species and ranged from 40 m to 82 m There was a trend for seeds from plants distributed in gaps and at low population densities to be transported at longer distances by opossums. Opossums, therefore, meet some require¬ ments of effective seed dispersers, at least for Solanaceous plants.

Potential dispersal of Cecropia hololeuca by the Common Opossum (Didelphis aurita) in Atlantic forest, south-eastern Brazil

Several authors have recognized the role of vertebrates as seed disperser of Cecropia. Germination rates of Cecropia hololeuca from scats of Didelphis aurita and from trees were tested in different treatments (light and dark). In addition, seeds from trees were separated in three groups for germination tests : fresh-fruit, 15 days after collection, 30 days after collection. Seed germination under light were more successful than in dark for all groups (fresh-fruit, 15 days after, 30 days after, and from scats). Germination rate of seeds from scats were closer to germination rates of the 30 days after collection group than other groups. These results insinuate that D. aurita feeds on fruit of C. hololeuca on the ground. The possibility that D. aurita disperses seeds to areas beyond parent trees, plus the similar germination rates of seeds from scats with seeds from fruiting trees imply that D. aurita is a potential disperser of C. hololeuca.

Novo transmissor silvestre do "Trypanosoma (Schizotrypanum) cruzi" (Chagas, 1909)

In nests of opossum (Didelphis aurita), localized in palm-trees of the species Attalea indaya Dr., the authors found a new tritatoma, the description of which is being made by Dr. H. LENT. They verified that this triatoma had been naturally infected by Trypanosoma (Schizotrhypanum) cruzy. Two guinea-pigs were subsequently infected by peritoneal inoculation of excrements of this new triatoma. The xenodiagnosis of these guinea-pigs, made with normal nymphas of. T. megistus and T. infestans resulted positive after 25 days. Evidence was obtained of being the opossum (Didelphis) one of the sources infection of the new vector, because several specimens of them were found infected, and also a specimen of D. aurita, which contained trypanosomes with the morphology of T. (S.) cruzy in the peripheral blood.