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Losses of cone opsin genes are noted in animals that are nocturnal or rely on senses other than vision. We investigated the cone opsin repertoire of night-active South American weakly electric fish. We obtained opsin gene sequences from genomic DNA of 3 gymnotiforms (Eigenmannia virescens, Sternopygus macrurus, Apteronotus albifrons) and the assembled genome of the electric eel (Electrophorus electricus). We identified genes for long-wavelength-sensitive (LWS) and medium-wavelength-sensitive cone opsins (RH2) and rod opsins (RH1). Neither of the 2 short-wavelength-sensitive cone opsin genes were found and are presumed lost. The fact that Electrophorus has a complete repertoire of extraretinal opsin genes and conservation of synteny with the zebrafish (Danio rerio) for genes flanking the 2 short-wavelength-sensitive opsin genes supports the supposition of gene loss. With microspectrophotometry and electroretinograms we observed absorption spectra consistent with RH1 and LWS but not RH2 opsins in the retinal photoreceptors of E. virescens. This profile of opsin genes and their retinal ex-pression is identical to the gymnotiform's sister group, the catfish, which are also nocturnally active and bear ampullary electroreceptors, suggesting that this pattern likely occurred in the common ancestor of gymnotiforms and catfish. Finally, we noted an unusual N-terminal motif lacking a conserved glycosylation consensus site in the RH2 opsin of gymnotiforms, a catfish and a characin (Astyanax mexicanus). Mutations at this site influence rhodopsin trafficking in mammalian photoreceptors and cause retinitis pigmentosa. We speculate that this unusual N terminus may be related to the absence of the RH2 opsin in the cones of gymnotiforms and catfish. [ABSTRACT FROM AUTHOR]

When startled by sound, goldfish make large turns away from a rostral stimulus and small responses away from caudal stimuli, suggesting that rostral startling stimuli recruit larger pools of reticulospinal neurons in the Brainstem Escape Network (BEN) than do caudal stimuli. Consistent with this idea, the zebrafish Mauthner (M-) cell fires when the fish is startled by tail-directed stimuli, but the M-cell homologues (MiD2cm and MiD3cm) are also recruited when the fish is startled by displacing the head. Because vision is known to modulate M-cell activity, a nonstartling, modulatory sensory ‘signal’ conveyed to the reticular formation may be stronger if the visual sensory image is from a rostral vs. caudal spatial location and could account for a differential neuron pool recruitment and response magnitude. In this study, electrophysiological recordings from cichlid Mauthner neurons showed that visual stimulation of the caudal retina (by a rostral cue) generates a depolarization that is about 1.5 times the amplitude of that generated by stimulation of the rostral retina (by a caudal cue). In behavioral testing, where fish were stimulated visually for 30 ms and then startled by sound, fish startled in the presence of a rostral visual stimulus performed larger amplitude and faster turns than when startled in the presence of a caudal visual stimulus. Thus, M-cell potentials might reflect the strength of visual input to the BEN in general. For a particular visual spatial location, the relative strength of descending visual input appears to contribute to a recruitment of a reticulospinal neuron population that generates a turn magnitude appropriate to the visual cue, and suggests that a retinotopic representation is preserved in the BEN. Copyright © 2006 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

The ability to discriminate the sign of the difference in frequency (DF) between two wavelike signals is integral to the jamming avoidance response (JAR) of weakly electric gymnotiform fish such as Eigenmannia. ‘Whole-cell’ intracellular recordings from neurons in the nucleus electrosensorius (nE) of Sternopygus, a gymnotiform that lacks the JAR, revealed that this nucleus receives information from both the ampullary and the tuberous electrosensory systems. Most tuberous units responded to DF stimuli, and many of these cells were DF sign-sensitive, i.e., they responded differently for one sign of DF. Although the distribution of ampullary units was somewhat restricted, sign-sensitive units were found in all areas of the nE in Sternopygus. Whole-cell recordings made in the nE of Eigenmannia revealed that, as in Sternopygus, sign-sensitive cells were not restricted to areas associated with control of the JAR. The diverse neurophysiology and connectivity of the nucleus electrosensorius suggests that, besides its role in the JAR of some members of the order, this nucleus likely serves as an interface between sensory input and neural circuits controlling other behaviors and endocrinological states in other gymnotiforms as well. The discovery of sign sensitivity in the nE of Sternopygus indicates that this property is not uniquely associated with the presence of a JAR; rather, the ability to discriminate the sign of DF may be relevant to many other behavioral contexts in gymnotiforms. Existing evidence indicates that the JAR evolved more than once in this group; the presence of sign sensitivity in ancestral gymnotiforms may have made this parallelism more likely. Copyright © 2004 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

No abstract available Copyright © 2012 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]