The horizontal optokinetic reflex of the opossum (Didelphis marsupialis aurita): physiological and anatomical studies in normal and early monoenucleated specimens.

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.

Electrocorticographic and behavioral features of turtle parenteral penicillin epilepsy.

The epileptogenic action of parenteral penicillin was studied in turtles with electrodes chronically implanted in both cerebral hemispheres. Penicillin was administered intraperitoneally at doses of 200,000 to 1,600,000 IU/kg. Doses higher than 600,000 IU/kg caused the appearance on the electrocorticogram of bilateral sharp waves or biphasic spikes, sometimes more pronounced in one hemisphere. These events were often accompanied by clonic activity of the neck muscles and mouth movements in an epileptic automatism. Intravenous penicillin (200,000 to 1,000,000 IU/kg) also evoked changes of the basic pattern of the electrocorticogram. Doses up to 350,000 IU/kg induced bilateral sharp waves and/or spikes sometimes accompanied by the seizures described in item 2. Doses above 400,000 IU/kg produced bilateral synchronous spike or polyspike discharges with a clonic-tonic seizure pattern. The same dose of penicillin induced more marked changes in the electrocorticogram when injected intravenously than intraperitoneally. Since larger doses of parenteral penicillin were required to evoke epileptic activity in turtles than in cats, the present results are consistent with the concept that the brain of phylogenetically lower animals is less susceptible to epileptogenic agents than the brain of higher animals.

Horizontal optokinetic reflex in the opossum Didelphis marsupialis aurita.

Electro-oculographic recordings were performed in 10 opossums. The optokinetic reflex was elicited by projecting a random dot stimulus on a cylindrical screen moving horizontally from left to right or right to left at various constant speeds. Binocular stimulation yielded the same response as the temporal to nasal monocular condition. The nasal to temporal monocular response was always less than that to the opposite direction: 50% at 3 degrees/s and 15% at 18 degrees/s. These results are discussed in a comparative context.

The nucleus of the optic tract (NOT) and the dorsal terminal nucleus (DTN) of opossums (Didelphis marsupialis aurita).

Wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was injected unilaterally into the pretectocollicular region of opossums (Didelphis marsupialis aurita), primarily to investigate the existence of a commissural subcortical pathway but also to reveal afferents and efferents of the nucleus of the optic tract (NOT) and dorsal terminal nucleus (DTN) in this species. Labelled cells and terminals were observed in the contralateral NOT-DTN. Furthermore, HRP was injected bilaterally in the region of the inferior olive (IO) to verify if the distribution of labelled cells in the NOT-DTN overlapped the region of commissural labelled cells. The two subpopulations of retrogradely labelled cells coincided, being distributed within the retinal terminal field attributed to the NOT-DTN, as revealed by contralateral eye injections of HRP. The commissural cells were located slightly more ventral than the olivary cells in the optic tract. The pretectocollicular WGA-HRP injections also labelled cells and terminals bilaterally in the lateral terminal nucleus (LTN), interstitial nucleus of the superior fasciculus, posterior fibers (INSFp), ventral lateral geniculate nucleus (vLGN), and superior colliculus (SC) and ipsilaterally in the medial terminal nucleus (MTN). In addition, further caudally, labelled cells and terminals were observed bilaterally in the nuclei prepositus hypoglossi (PH) and in the medial (MVN) and lateral (LVN) vestibular nuclei. Labelled terminals were found in the ipsilateral nucleus reticularis tegmenti pontis (NRTP) and in the IO with ipsilateral predominance. This study allowed an anatomical delimitation of the NOT-DTN in this opossum species, as defined by the olivary and commissural subpopulations, as well as a hodological evaluation of this region. The existence of some common anatomical aspects with other mammalian species is discussed.

Horizontal optokinetic reflex in the opussum Didelphis marsupialis aurita

Electro-oculographic recordings were performed in 10 opossums. The optokinetic reflex was elicited by projecting a random dot stimulus on a cylindrical screen moving horizontally from left to right or right to left at various constant speeds. Binocular stimulation yelded the same response as the temporal to nasal monocular condition. The nasal to temporal monocular response was always less than to the opposite direction: 50% at 3 degrees/s and 15% at 18 degress/s. These results are discussed in a comparative context