Electrophysiological and spectral properties of second-order retinal neurons in the eel

Analysis of visual discrimination in Japanese eel (Anguilla japonica)

Japanese eels were trained to discriminate between a checkerboard panel and a plain gray panel in a circular pool with three pipes. One of the pipes was open, whereas the others were closed. The correct choice of discriminative stimulus was reinforced by entering the pipe. When the panels were displayed vertically (on the wall), the eels successfully learned discrimination, but they were unable to acquire the task when the panels were presented horizontally (on the floor). Enucleation of the retina impaired discrimination, whereas ablation of the olfactory plates did not. In the second experiment, the eels underwent three tests after discriminative training with vertical stimuli displayed. When plain black or white panels were presented instead of a checkerboard panel, the eels could not discriminate. Thus, the discriminative stimulus must have both black and white components. The eels exhibited a generalization gradient along the fines of the checkerboard. Finally, the pallium was damaged by coagulation, and the eels did not maintain the discrimination after the lesions. The behavioral deficits were classified into successful relearning and no relearning. Damage to the dorso-lateral (DL) or dorso-central (DC) pallium was associated with severe impairment (no relearning), although it was not possible to isolate the particular brain area or combination of brain areas which was required. The DL damage probably causes memory deficits, but the deficits caused by the DC damage might be motor or motivational deficits.

Impairments in spatial learning by telencephalic lesions in Japanese eels (Anguilla japonica)

This study aimed to use Japanese eels (Anguilla japonica) as subjects to examine the effects of telencephalic lesions on spatial learning. Ten Japanese eels were trained on a Morris-type spatial learning task. Four pipes were placed in a pool; however, the eels could hide in only one of these pipes. The learning task ensured that the eels learned about the position of the open pipe. Subsequently, their telencephalons were damaged. The lesioned eels could not maintain their learning and demonstrated deficits in re-learning as some of them were unable to relearn the task. An analysis of the lesion sizes revealed that while damage to the dorsolateral pallium correlates with maintenance of learning, damage to the dorsomedial pallium correlates with re-learning.

Spatial Learning in Japanese Eels Using Extra- and Intra-Maze Cues

Japanese eels (Anguilla japonica) were trained on a spatial-learning paradigm in a pool placed in an experimental room where several extra-maze cues were present. Four tubes were placed in the pool, of which one was open and could be entered by the eels. The open tube was placed at a fixed position in the pool and contained a triangular block that served as an intra-maze cue. The eels learned to identify the open tube, and their performance was maintained when the pool was rotated. However, they were unable to maintain their performance in a dark room, which suggests that spatial learning is based on visual cues. To determine the influence of the extra- and intra-maze cues, the tube with the triangle was moved to a new position and another open tube was kept in its place. The eels chose either the tube at the original position or the tube with the triangle at its new position, suggesting that spatial discrimination may be based on either extra- or intra-maze cues. We thus conclude that the eels employed an adjunctive strategy of multiple cues. In the next experiment, the eels were trained to visually discriminate the position of the stimulus (triangle), which changed in every trial. After the training, the eels were submitted to a test in which, in addition to the triangular pattern, a rectangular pattern was introduced. The eels discriminated between the tubes with the triangular and rectangular patterns, suggesting that they had the ability to discriminate visual patterns.

Spatial learning in Japanese eels (Anguilla japonica)

Japanese eels (Anguilla japonica) were trained on a Morris-type spatial learning task. There were four tubes in a pool, but the eels could hide in only one of these. The eels learned the position of the open tube, and maintained their performance when the pool was rotated to remove possible intra-maze cues. The eels could not maintain their performance in a dark room, suggesting that spatial learning involved extra-maze visual cues. When the position of the open tube was randomly changed every day, the performance of the eels in finding the open tube did not improve.

Toxicity of 2,4,6-TrinitrotoIuene (TNT) in Hispid Cotton Rats (Sigmodon hispidus): Hematological, Biochemical, and Pathological Effects

The contamination of soil and water with munitions chemicals and their degradation products has been reported at certain munitions production waste disposal sites and at certain Army installations. The effects of 2,4,6-trinitrotoluene (TNT) on wild cotton rats ( Sigmodon hispidus) were evaluated to identify target organ toxicity that could be used to develop biomarkers for exposure assessment for ecological and health risks. The oral LD50 values for TNT in corn oil were 607 and 767 mg/kg body weights for male and female cotton rats respectively. Hematological, pathological, and biochemical effects of TNT were determined after daily oral gavage of TNT in corn oil at doses of 0, 75.9, 151.8, and 303.5 (males) or 0, 96, 192, and 384 mg/kg (females) for 7 days. Cotton rats treated with TNT showed an increase in spleen weights in males (303.5 mg/kg) and in females (192 and 384 mg/kg). Cotton rats of both sexes treated for 7 days with TNT had marked hemolytic anemia with reduced erythrocytes, hemoglobin, and hematocrit in high-dose groups; methemoglobin levels were elevated significantly in males at mid and high dose. Hepatic drug metabolizing enzyme analysis revealed that microsomal O-dealkylation of methoxy, ethoxy, and pentoxy resorufin were elevated in male (high dose) cotton rats. The activity of hepatic glutathione S-transferases (GST) was significantly elevated in male (mid and high dose) and female (all doses) cotton rats exposed to TNT. Histopathological analysis of spleen revealed mild to marked splenic congestion with mild extramedullary hematopoiesis, hemosiderosis, and lymphoid hyperplasia in male and female cotton rats treated with TNT (all doses). Liver weights were increased in males (mid and high dose) and in females (high-dose group). In the high-dose groups, histological changes in liver (mild to moderate hepatocellular hypertrophy, increased hemosiderin pigment in Kupffer cells) in both sexes, and in testis (premature exfoliation of spermatozoa from dilated seminiferous tubules) were observed (mid and high dose). These results suggest that hepatic GST and hemolytic anemia may be biomarkers in cotton rats of terrestrial contamination with TNT or other nitroaromatic explosive compounds.

Eel visual pigments revisited: the fate of retinal cones during metamorphosis

During their complex life history, anguilliform eels go through a major metamorphosis when developing from a fresh water yellow eel into a deep-sea silver eel. In addition to major changes in body morphology, the visual system also adapts from a fresh water teleost duplex retina with rods and cones, to a specialized deep-sea retina containing only rods. The history of the rods is well documented with an initial switch from a porphyropsin to a rhodopsin (P523(2) to P501(1)) and then a total change in gene expression with the down regulation of a "freshwater" opsin and its concomitant replacement by the expression of a typical "deep-sea" opsin (P501(1) to P482(1)). Yellow eels possess only two spectral classes of single cones, one sensitive in the green presumably expressing an RH2 opsin gene and the second sensitive in the blue expressing an SWS2 opsin gene. In immature glass eels, entering into rivers from the sea, the cones contain mixtures of rhodopsins and porphyropsins, whereas the fully freshwater yellow eels have cone pigments that are almost pure porphyropsins with peak sensitivities at about 540-545 nm and 435-440 nm, respectively. However, during the early stages of metamorphosis, the pigments switch to rhodopsins with the maximum sensitivity of the "green"-sensitive cone shifting to about 525 nm, somewhat paralleling, but preceding the change in rods. During metamorphosis, the cones are almost completely lost.

Photopic vision in eels: evidences of color discrimination

Several classes of second-order retinal neurons have been studied electrophysiologically in European eel (Anguilla anguilla) from two different localities, Lake Seliger in Russia and the coastal waters of the Adriatic Sea in Montenegro. The majority of L-horizontal cells (68 explored) had both rod and cone inputs, an uncommon phenomenon among teleosts. Pronounced color-opponent properties, often taken as pointing to the capacity of color vision, were identified in one amacrine cell, apparently of the "blue/yellow" (or "blue/green") type. Microspectrophotometric measurements revealed two different spectral classes of cones with absorption maxima at about 525 and 434 nm. The existence of green-sensitive and blue-sensitive cone units was thus revealed by both electrophysiological and microspectrophotometric techniques.

Radoslav K. Andjus (1926-2003): a brief summary of his life and work

Radoslav K. Andjus was a professor of physiology and biophysics at the University of Belgrade, Serbia, from 1953 to 1992. He was an elected member of the Serbian Academy of Sciences and Arts, the International Academy of Astronautics, and the Montenegrin Academy of Sciences and Arts. He published over 190 papers in domestic and international journals and three textbooks. The main field of his research was thermophysiology. He studied hypothermia, suspended animation and resuscitation, hibernation and biological rhythms, temperature adaptation and acclimation, and cryoprotection. Professor Andjus also contributed significantly to the fields of brain metabolism, endocrinology, electroretinography, as well as biophysical modeling and theoretical biology.