An immunohistochemical study on the distribution of vasotocin neurons in the brain of two weakly electric fish, Gymnotus omarorum and Brachyhypopomus gauderio.

Hypothalamic nonapeptides (arginin vasotocin-vasopressin, oxytocin-isotocin) are known to modulate social behaviors across vertebrates. The neuroanatomical conservation of nonapeptide systems enables the use of novel vertebrate model species to identify general strategies of their functional mechanisms. We present a detailed immunohistochemical description of vasotocin (AVT) cell populations and their projections in two species of weakly electric fish with different social structure, Gymnotus omarorum and Brachyhypopomus gauderio. Strong behavioral, pharmacological, and electrophysiological evidence support that AVT modulation of electric behavior differs between the gregarious B. gauderio and the solitary G. omarorum. This functional diversity does not necessarily depend on anatomical differences of AVT neurons. To test this, we focus on interspecific comparisons of the AVT system in basal non-breeding males along the brain. G. omarorum and B. gauderio showed similar AVT somata sizes and comparable distributions of AVT somata and fibers. Interestingly, AVT fibers project to areas related to the control of social behavior and electromotor displays in both species. We found that no gross anatomical differences in the organization of the AVT system account for functional differences between species, which rather shall depend on the pattern of activation of neurons embedded in the same basic anatomical organization of the AVT system.

Sulfate transporters involved in sulfate secretion in the kidney are localized in the renal proximal tubule II of the elephant fish (Callorhinchus milii).

Most vertebrates, including cartilaginous fishes, maintain their plasma SO4 (2-) concentration ([SO4 (2-)]) within a narrow range of 0.2-1 mM. As seawater has a [SO4 (2-)] about 40 times higher than that of the plasma, SO4 (2-) excretion is the major role of kidneys in marine teleost fishes. It has been suggested that cartilaginous fishes also excrete excess SO4 (2-) via the kidney. However, little is known about the underlying mechanisms for SO4 (2-) transport in cartilaginous fish, largely due to the extraordinarily elaborate four-loop configuration of the nephron, which consists of at least 10 morphologically distinguishable segments. In the present study, we determined cDNA sequences from the kidney of holocephalan elephant fish (Callorhinchus milii) that encoded solute carrier family 26 member 1 (Slc26a1) and member 6 (Slc26a6), which are SO4 (2-) transporters that are expressed in mammalian and teleost kidneys. Elephant fish Slc26a1 (cmSlc26a1) and cmSlc26a6 mRNAs were coexpressed in the proximal II (PII) segment of the nephron, which comprises the second loop in the sinus zone. Functional analyses using Xenopus oocytes and the results of immunohistochemistry revealed that cmSlc26a1 is a basolaterally located electroneutral SO4 (2-) transporter, while cmSlc26a6 is an apically located, electrogenic Cl(-)/SO4 (2-) exchanger. In addition, we found that both cmSlc26a1 and cmSlc26a6 were abundantly expressed in the kidney of embryos; SO4 (2-) was concentrated in a bladder-like structure of elephant fish embryos. Our results demonstrated that the PII segment of the nephron contributes to the secretion of excess SO4 (2-) by the kidney of elephant fish. Possible mechanisms for SO4 (2-) secretion in the PII segment are discussed.

The paraventricular organ of mormyrid fish: uptake or release of intraventricular biogenic amines?

The paraventricular organ of Gnathonemus petersii was investigated with light and electronmicroscopical techniques. It contains high concentrations of dopamine, noradrenaline and serotonin, but the synthesizing enzymes are not or hardly present. Consequently, the cerebrospinal fluid-contacting neurons might pick up their biogenic amines from the ventricular fluid. Dense subependymal axonal plexuses in the everted telencephalon probably release these substances into the ventricle. However, electronmicroscopical observations suggest release rather than uptake by the paraventricular organ. The possible significance of intraventricular release, transport and uptake of biogenic amines is discussed.

The distribution of excitatory amino acid binding sites in the brain of an electric fish, Apteronotus leptorhynchus.

The distribution of three types of exitatory amino acid receptors was examined in the brain of a high frequency weakly electric fish, Apteronotus leptorhynchus, by localizing the binding sites of ligands selective for mammalian kainic acid (KA), quisqualate (AMPA) and N-methyl-D-aspartate (NMDA) receptors. All three binding sites were densest within the forebrain and in certain hypothalamic nuclei (nucleus tuberis anterior, inferior lobe). The core of the dorsal forebrain (dorsal centralis) had a very high density of NMDA binding sites and only moderate levels of AMPA and KA binding sites, while this was reversed for the dorsolateral forebrain. The AMPA and NMDA binding sites were found throughout the brain while KA binding sites were relatively restricted and were absent from most of the brainstem. The cerebellar molecular layer contained a very high density of KA and AMPA binding sites but almost no NMDA binding sites; the granular layer had a low density of AMPA and NMDA binding sites but was lacking in KA binding sites. All three types of binding sites were found within the electromotor system (nucleus electrosensorius and prepacemaker nucleus) at sites where the iontophoresis of glutamate causes species-specific behaviours. KA binding sites were found at only two sites along the electrosensory afferent pathways: (1) in the molecular layer of the electrosensory lateral line lobe, associated with a feedback pathway emanating from granule cells of the overlying cerebellum, and (2) in the lateral nucleus praeminentialis dorsalis, associated with a descending pathway emanating from the torus semicircularis. NMDA and AMPA binding sites are found throughout the electrosensory pathways. Within the electrosensory lateral line lobe the NMDA binding sites were predominantly associated with the feedback pathways terminating in its molecular layer and not with the deep neuropil layer containing primary electroreceptor afferents.