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Ten microsatellite loci were isolated from a species of the Neotropical electric eel, Eigenmannia, a genus of freshwater fish characterized by small populations and low vagility. Nine microsatellites were polymorphic, the number of alleles ranging from seven to 27, and values of observed heterozygosity ranging from 0.327 to 0.741. These loci were developed for population genetic studies of Eigenmannia sp. 2, however, cross-amplification carried out with other species of this genus as well as other Gymnotiformes genera indicate that these molecular markers are also potentially useful for population-level studies in closely related species.

The diencephalic prepacemaker nucleus (PPn) of gymnotiform electric fish projects to the medullary pacemaker nucleus and modulates its regular firing frequency. Each firing of the pacemaker, in turn, drives an electric organ discharge (EOD). Two types of PPn neurons were retrogradely labeled from the pacemaker with HRP in Eigenmannia and Apteronotus. In both species, smaller ovoidal cells were found in the dorsomedial part of the PPn (PPn-G), and larger multipolar cells were found in the ventrolateral part of the PPn (PPn-C). This morphological distinction between the two subnuclei in the PPn was paralleled by a functional dichotomy. Microiontophoresis of L-glutamate in the PPn-G of both species elicited slow and gradual accelerations of EOD frequency characterized by a time constant on the order of seconds. The elicited frequency modulations were similar to those observed during the jamming avoidance response and during courtship. Glutamate stimulation of the PPn-C, in contrast, produced fast and abrupt frequency modulations characterized by a time constant on the order of milliseconds. These abrupt modulations resembled "chirps" observed during courtship and aggression. Similar behavior was produced by intracellular current injection into a PPn-C neuron of Apteronotus, and intracellular labeling of this neuron with Lucifer Yellow revealed a multipolar PPn-C neuron similar to those retrogradely labeled with HRP.

The dorsal torus semicircularis (torus) of the gymnotiform fish Eigenmannia was examined in Golgi-impregnated material. These results were correlated with those of a previous HRP study which used retrograde labelling techniques to identify the efferent cell types of the torus (Carr et al., '81, J. Comp Neurol. 203:649-670). The torus is a laminated midbrain nucleus of the electrosensory system. It receives somatotopically ordered electrosensory input from the medulla and caudal lobe of the cerebellum, proprioceptive input from the descending nucleus of the trigeminal nerve, and input from the optic tectum. The torus projects to the nucleus praeeminentialis, the optic tectum, nucleus electro-sensorius, parts of the central posterior thalamus, the pretectum, the lateral mesencephalic reticular formation (LMRA), the reticular formation, and the inferior olive. The torus has 12 laminae and 48 cell types by Golgi criteria. There are three major orientations to the dendritic fields of the toral neurons: purely horizontal neurons with dendrites confined to a single lamina, multipolar neurons whose dendrites often do not respect laminar boundaries, and vertical cells with dendrites that travel in the vertical bundles of dendrites and axons which pierce the torus at regular intervals. There are four major groups of vertically oriented neurons. The first has a predominantly horizontal dendritic tree with one or two vertical dendrites which connect the cell to a distant lamina. The second consists of "U"-shaped neurons with a horizontal arbor and two major dendrites which ascend in adjacent vertical bundles. The third group is made up of bilaminar neurons which receive input from two vertically separated dendritic arbors, and the fourth group is purely vertical in orientation. A group of four tegmental cell types in the LMRA also send their dendrites into the efferent tracts of the torus, and into lamina IX. The torus is similar in complexity and number of cell types to the mammalian inferior colliculus. The large number of cell types in these midbrain sensory nuclei, compared to the number of afferent inputs (seven or more for the torus) is notable and may reflect the parcellation of function associated with the parallel processing of these inputs.

Eigenmannia is a weakly electric fish that emits a constant-frequency electric organ discharge (EOD). Probability coder (P unit) and phase coder (T unit) electroreceptive afferents differentially encode changes in EOD amplitude and phase, respectively. physiologically identified T and P units were intracellularly labelled with HRP and their terminals were examined with electron microscopy to determine their postsynaptic targets. This technique reveals that phase and amplitude are relayed to first-order electrosensory neurons by two parallel but not independent pathways. P-type afferents terminate on granular interneurons, basilar pyramidals, and polymorphic cells, electrosensory lateral line lobe targets that monitor amplitude modulations, but P-type afferents do not contact spherical cells. T-type afferents relay phase information to spherical cells and thus form a separate afferent pathway. T unit terminals do not synapse directly on basilar pyramidal cells. Collateral branches from T-type afferents, however, were also found to terminate on granule and polymorphic cells, thereby adding phase information into the amplitude channel. P- and T-type afferents exhibit cellular specificity by forming synaptic junctions with different subsets of post synaptic targets in the deep neuropil. The afferent terminals make either asymmetric chemical or gap junction synapses depending on the identity of the post synaptic target. T units contacting granule cells or polymorphic cells had not been previously described. Two possible roles of adding phase to amplitude information are discussed in terms of electrolocation.

The torus semicircularis of Gymnotiform fish is an enlarged laminated midbrain structure which receives lemniscal input from electrosensory, mechanoreceptive lateral line, and auditory systems. The electrosensory input in confined to the dorsal torus, while the auditory and mechanoreceptive systems project to the ventral torus. Anterograde and retrograde techniques were used were used to determine the connections of the dorsal torus in Apteronotus and Eigenmannia. The dorsal torus can be divided into nine major laminae, each of which has distinct afferent and efferent connections. The dorsal torus receives five afferent inputs: (1) A contralateral topographic input from the posterior lateral line lobe (PLLL) projects to laminae III, V, VI, VII, VIIIB, and VIIID. (2) Eurydendroid cells of the caudal lobe of the cerebellum project contralaterally to lamina VIIIB. (3) A portion of the descending nucleus of V projects to laminae VIIIA, VIIIC, and IX. (4) Lamina I is a cap of fine myelinated fibers which may originate in the torus longitudinalis. They project to laminae II and III. (5) The ipsilateral optic tectum projects to the dorsal torus. The dorsal torus projects to six major targets: (1) Laminae VII, VIII, and IX project bilaterally to a lateral region of the diencephalon above n. preglomerulosus, herein named n. electrosensorius. An area below the dorsal thalamus receives a smaller ipsilateral projection. (2) Laminae II, V, VIvn, VII, VIII, and IX project topographically to the deeper laminae of the ipsilateral optic tectum. This projection is in spatial register with the visual map in the superficial layers of the tectum. (3) Lamina VIIID projects ipsilaterally to the lateral reticular formation. (4) All laminae other than I, VI, and VIIIB project topographically to ahe ipsilateral n. praeeminentialis, which provides a powerful descending projection to the PLLL. (5) Lamina IX projects to a dorsal pretectal area. (6) The ipsilateral inferior olive receives a projection from the dorsal torus.

Age-related changes in ampullary and tuberous receptor organ morphology were studied in six species of gymnotiform weakly electric fish. Cheek skin was silver-stained, whole-mounted, and viewed under Nomarski differential interference contrast optics. The ampullary receptor units of all species show an increasing number of receptor organs per afferent fiber with fish size, presumably the result of addition of newly formed receptor organs. Ampullary units composing over a dozen organs were observed in large specimens of a few species. Receptor cells were also added in the tuberous receptor system of all species, but in different ways. As previously reported for Sternopygus, small specimens of Eigenmannia had only a single tuberous receptor organ per afferent. Fish of increasing size retained a population of afferents that innervated only a single receptor organ and, in addition, had a population of afferents that innervated a cluster of receptor organs. The mean number of receptor organs per cluster increased in fish of increasing size. In addition, the mean number of sensory receptor cells per organ increased. New organs presumably derive from older ones, which divide under the stimulus of continued addition of new receptor cells. Apteronotus, Adontosternarchus, and Hypopomus all added more receptor cells to their tuberous organs. In these species, every afferent innervated only a single tuberous organ and there was no indication of division of receptor organs. Gymnorhamphichthys and Gymnotus were intermediate in that they added new receptor cells to each receptor organ, and, in larger fish, these were segregated into discrete patches within a single receptor organ. It is likely that the addition of new receptor cells aids in increasing sensitivity of both ampullary and tuberous receptors as fish grow.

Ampullary organs of the transparent catfish, Kryptopterus bicirrhus, are present in large numbers on the head and in a regular pattern of lines on the body and fins. The organs lie in the epidermis, and have a pore that opens to the surface. Flattened cells form a roof and walls. On the floor of the organ there are a “sensory hillock,” composed of spherical receptor cells and columnar supporting cells, and a “secretory hillock” composed of columnar secretory cells. The receptor cells are nonciliated and have only afferent innervation. The organ cavity is filled with jelly. The organs are compared with ampullary organs of the weakly electric fish Eigenmannia, ampullae of Lorenzini of Raja, and small pit organs of Amiurus. Structural characteristics of the ampullary organs of Kryptopterus make them especially suitable for electrophysiological studies.

Ampullary receptor organs of the South American weakly electric gymnotid fish Eigenmannia virescens consist of a pore at the surface of the skin, a canal through the epidermis, and the expanded basal end of the canal in the corium. The cavity of the organ contains a jelly that is filled with fine fibers. The canal wall consists of three to six layers of flattened cells that appear to be derived from the adjacent skin. Along the lumen of the organ the cells are joined by tight junctions. Usually there are four spherical receptor cells in the base of the organ. They are innervated by single neural terminals. These organs are compared to tuberous receptor organs found in the same species, and the functional significance of the fine structure observed in these cells is discussed.