Dual mechanisms contribute to enhanced voltage dependence of an electric fish potassium channel.

The voltage dependence of different voltage-gated potassium channels, described by the voltage at which half of the channels are open (V1/2), varies over a range of 80 mV and is influenced by factors such as the number of positive gating charges and the identity of the hydrophobic amino acids in the channel's voltage sensor (S4). Here we explore by experimental manipulations and molecular dynamics simulation the contributions of two derived features of an electric fish potassium channel (Kv1.7a) that is among the most voltage-sensitive Shaker family potassium channels known. These are a patch of four contiguous negatively charged glutamates in the S3-S4 extracellular loop and a glutamate in the S3b helix. We find that these negative charges affect V1/2 by separate, complementary mechanisms. In the closed state, the S3-S4 linker negative patch reduces the membrane surface charge biasing the channel to enter the open state while, upon opening, the negative amino acid in the S3b helix faces the second (R2) gating charge of the voltage sensor electrostatically biasing the channel to remain in the open state. This work highlights two evolutionary novelties that illustrate the potential influence of negatively charged amino acids in extracellular loops and adjacent helices to voltage dependence.

Mixed selectivity coding of sensory and motor social signals in the thalamus of a weakly electric fish.

High-level neural activity often exhibits mixed selectivity to multivariate signals. How such representations arise and modulate natural behavior is poorly understood. We addressed this question in weakly electric fish, whose social behavior is relatively low dimensional and can be easily reproduced in the laboratory. We report that the preglomerular complex, a thalamic region exclusively connecting midbrain with pallium, implements a mixed selectivity strategy to encode interactions related to courtship and rivalry. We discuss how this code enables the pallial recurrent networks to control social behavior, including dominance in male-male competition and female mate selection. Notably, response latency analysis and computational modeling suggest that corollary discharge from premotor regions is implicated in flagging outgoing communications and thereby disambiguating self- versus non-self-generated signals. These findings provide new insights into the neural substrates of social behavior, multi-dimensional neural representation, and its role in perception and decision making.

A new fluorescent probe for sensing of biothiols and screening of acetylcholinesterase inhibitors.

A new N2O-type BODIPY probe (LF-Bop) has been proposed for the selective and sensitive detection of biologically relevant small molecular thiols. This detection is based on the Michael addition reaction between the thiol and nitrostyrene groups in the probe, which decreases the quenching effect from the nitro group, thus resulting in the recovery of the deep-red fluorescence from the BODIPY structure. The results show that LF-Bop is able to detect all tested free thiols through a fluorescence turn-on assay. The lowest limit of detection (LOD) for glutathione was found to be down to nanomolar levels (220 nM). Based on this probe, we have developed a new fluorescence assay for the screening of acetylcholinesterase inhibitors. In total, 11 natural and synthetic alkaloids have been evaluated. Both experimental measurements and theoretical molecular docking results reveal that both natural berberine and its synthetic derivative dihydroberberine are potential inhibitors of acetylcholinesterase.

A time-stamp mechanism may provide temporal information necessary for egocentric to allocentric spatial transformations.

Learning the spatial organization of the environment is essential for most animals' survival. This requires the animal to derive allocentric spatial information from egocentric sensory and motor experience. The neural mechanisms underlying this transformation are mostly unknown. We addressed this problem in electric fish, which can precisely navigate in complete darkness and whose brain circuitry is relatively simple. We conducted the first neural recordings in the preglomerular complex, the thalamic region exclusively connecting the optic tectum with the spatial learning circuits in the dorsolateral pallium. While tectal topographic information was mostly eliminated in preglomerular neurons, the time-intervals between object encounters were precisely encoded. We show that this reliable temporal information, combined with a speed signal, can permit accurate estimation of the distance between encounters, a necessary component of path-integration that enables computing allocentric spatial relations. Our results suggest that similar mechanisms are involved in sequential spatial learning in all vertebrates.

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.

Local vasotocin modulation of the pacemaker nucleus resembles distinct electric behaviors in two species of weakly electric fish.

The neural bases of social behavior diversity in vertebrates have evolved in close association with hypothalamic neuropeptides. In particular, arginine-vasotocin (AVT) is a key integrator underlying differences in behavior across vertebrate taxa. Behavioral displays in weakly electric fish are channeled through specific patterns in their electric organ discharges (EODs), whose rate is ultimately controlled by a medullary pacemaker nucleus (PN). We first explored interspecific differences in the role of AVT as modulator of electric behavior in terms of EOD rate between the solitary Gymnotus omarorum and the gregarious Brachyhypopomus gauderio. In both species, AVT IP injection (10µg/gbw) caused a progressive increase of EOD rate of about 30%, which was persistent in B. gauderio, and attenuated after 30min in G. omarorum. Secondly, we demonstrated by in vitro electrophysiological experiments that these behavioral differences can be accounted by dissimilar effects of AVT upon the PN in itself. AVT administration (1µM) to the perfusion bath of brainstem slices containing the PN produced a small and transient increase of PN activity rate in G. omarorum vs the larger and persistent increase previously reported in B. gauderio. We also identified AVT neurons, for the first time in electric fish, using immunohistochemistry techniques and confirmed the presence of hindbrain AVT projections close to the PN that might constitute the anatomical substrate for AVT influences on PN activity. Taken together, our data reinforce the view of the PN as an extremely plastic medullary central pattern generator that not only responds to higher influences to adapt its function to diverse contexts, but also is able to intrinsically shape its response to neuropeptide actions, thus adding a hindbrain target level to the complexity of the global integration of central neuromodulation of electric behavior.

Preparing for the unpredictable: adaptive feedback enhances the response to unexpected communication signals.

To interact with the environment efficiently, the nervous system must generate expectations about redundant sensory signals and detect unexpected ones. Neural circuits can, for example, compare a prediction of the sensory signal that was generated by the nervous system with the incoming sensory input, to generate a response selective to novel stimuli. In the first-order electrosensory neurons of a gymnotiform electric fish, a negative image of low-frequency redundant communication signals is subtracted from the neural response via feedback, allowing unpredictable signals to be extracted. Here we show that the cancelling feedback not only suppresses the predictable signal but also actively enhances the response to the unpredictable communication signal. A transient mismatch between the predictive feedback and incoming sensory input causes both to be positive: the soma is suddenly depolarized by the unpredictable input, whereas the neuron's apical dendrites remain depolarized by the lagging cancelling feedback. The apical dendrites allow the backpropagation of somatic spikes. We show that backpropagation is enhanced when the dendrites are depolarized, causing the unpredictable excitatory input to evoke spike bursts. As a consequence, the feedback driven by a predictable low-frequency signal not only suppresses the response to a redundant stimulus but also induces a bursting response triggered by unpredictable communication signals.

The role of The Gentleman's Magazine in the dissemination of knowledge about electric fish in the eighteenth century.

Although torpedoes and Malopterurus, a Nile catfish, had been described and even used medically in antiquity, their discharges were poorly understood before the second half of the eighteenth century. It was then that their actions, along with those of certain South American "eels," became firmly associated with electricity. The realization that an animal could produce electricity marked a turning point in the history of neurophysiology, which had long described nerve actions with recourse to animal spirits. By examining The Gentleman's Magazine during the period when electric fish were becoming electrical, one can begin to appreciate how new discoveries about these unusual creatures captured the imagination of scientists and were filtered down to the literate public.

Fin-fishes in Yorùbá natural healing practices from southwest Nigeria.

Natural healing treatment recipes from southwest Nigeria were examined for fin-fishes of ethnomedical importance. Fifteen families of 19 genera were identified with members found either in marine, fresh or brackish waters. Two thousand two hundred and eighty-five recipes were examined, 9.11% contained fin-fish or its derivative as component. Fertility/conception-related medical conditions have highest proportion of recipes (26.26%) with fish/derivatives as inputs. One hundred and forty-one recipes were examined for fertility-related conditions, the highest for any medical condition during this study. Clarias sp. (Clariidae) (catfish) recorded the highest rate of use (78.94%) followed by Malapterurus electricus (Malapteruridae) (electric catfish) with 5.26%. Fin-fishes/derivatives were observed to have both physical and spiritual activities in treatment preparations. Most of the fish species were in need of conservation interventions.

Muscarinic receptors control frequency tuning through the downregulation of an A-type potassium current.

The functional role of cholinergic input in the modulation of sensory responses was studied using a combination of in vivo and in vitro electrophysiology supplemented by mathematical modeling. The electrosensory system of weakly electric fish recognizes different environmental stimuli by their unique alteration of a self-generated electric field. Variations in the patterns of stimuli are primarily distinguished based on their frequency. Pyramidal neurons in the electrosensory lateral line lobe (ELL) are often tuned to respond to specific input frequencies. Alterations in the tuning of the pyramidal neurons may allow weakly electric fish to preferentially select for certain stimuli. Here we show that muscarinic receptor activation in vivo enhances the excitability, burst firing, and subsequently the response of pyramidal cells to naturalistic sensory input. Through a combination of in vitro electrophysiology and mathematical modeling, we reveal that this enhanced excitability and bursting likely results from the down-regulation of an A-type potassium current. Further, we provide an explanation of the mechanism by which these currents can mediate frequency tuning.

[Central modulation of a sensory system by a motor command. One intention with two results]. / Modulación central de un sistema sensorial por un comando motor. Una intención con dos resultados.

INTRODUCTION AND DEVELOPMENT: Neuronal mechanisms that underlie diverse sensory motor integration processes (SMI) are essential for the motor control and determine the general organization of the nervous system. Spinal cord, sensory relay nucleus of brainstem and thalamus as well as higher motor control structures are some of the levels, of increasing complexity, at which several processes of SMI occurs during the execution of a motor act. The mechanisms that underlie SMI strategies operating at higher hierarchical levels of motor control are poorly understood. Escape response in teleosts fish is an advantageous experimental model for the analysis of the neural basis of behavior and of the mechanisms and functional consequences of diverse strategies of ISM. We describe several levels of ISM that operate in the neural system that organize this response in most teleosts and we deal with a detailed description of a novel strategy that occurs in Gymnotus carapo, a South American weakly electric fish. In this species, the activation of the Mauthner cell, a command neuron for the initial phase of escape, produces a powerful modulation of the sensory system responsible for active electrorreception, its main sensory modality. CONCLUSION: The neural basis of behavior, even those relatively simple, exhibit several strategies of complex SMI that determine its performance and whose cellular mechanisms begin to be unraveled.

Re-evaluation of the afferent connections of the pituitary in the weakly electric fish Apteronotus leptorhynchus: an in vitro tract-tracing study.

The pituitary plays a key role in the interaction between the brain and the endocrine system. We re-examined the afferent connections of the pituitary in the weakly electric fish Apteronotus leptorhynchus using the in vitro application of dextran-tetramethylrhodamine to the pituitary. The resultant retrograde labeling was analyzed. Application of the tracer to the rostral part, but not the caudal part, of the pituitary labels hypothalamic cells in the anterior division of the periventricular nucleus, the suprachiasmatic nucleus, and the nucleus tuberis lateralis pars anterior. Application of the tracer to either the rostral or caudal parts of the pituitary labels hypothalamic cells in the posterior division of the periventricular nucleus (RPPp), the nucleus hypothalamus caudalis (Hc), the nucleus hypothalamus anterioris, the ventral hypothalamic nucleus, and the central nucleus of the inferior lobe. Furthermore, cells in the rostral two-thirds of the brainstem reticular formation (RF) project to the entire rostrocaudal extent of the pituitary. The largest projections to the pituitary are from Hc, PPp, and RF. Of the cells in Hc that project to the pituitary, almost all (96%) are small and the remainder are medium-sized. Of the cells in PPp that project to the pituitary, about half are small or medium-sized (44% and 56%, respectively). In Hc and PPp, about one-third to one-half of the cells that project to the pituitary are markedly elongated. The cells in RF that project to the pituitary are small (4%), medium-sized (89%), or large (7%) and about four-fifths of these cells are markedly elongated. With regard to the RF projections, the pituitary may receive copies of motor instructions and sensory information supplied by collaterals of the descending and ascending projection systems of RF cells. Thus, the ongoing motor activity of the animal and the ensuing sensory feedback from this activity could directly influence the pituitary.

Modulación central de un sistema sensorial por un comando motor. Una intención con dos resultados / Central modulation of a sensory system by a motor command. One intention with two results

Introducción y desarrollo. Los mecanismos neuronales que subyacen a los procesos de integración sensoriomotora (ISM) son esenciales para el control motor y pautan la organización general del sistema nervioso. La médula espinal, los núcleos de relevo sensorial en el tronco encefálico y tálamo, así como los centros superiores de control motor, son algunos niveles, de complejidad creciente, en los que se verifican procesos de ISM durante la ejecución de un acto motor. Los mecanismos que subyacen a las estrategias más complejas de ISM, en general, se conocen poco. La respuesta de escape en peces teleósteos constituye un modelo experimental de elección para el análisis de las bases neuronales de la conducta, y particularmente de los mecanismos y consecuencias funcionales de diversas estrategias de ISM. Se describen los niveles de ISM que operan en este sistema en la mayoría de los teleósteos y se refiere en detalle una estrategia novedosa que ocurre en Gymnotus carapo, pez eléctrico sudamericano de descarga débil. En esta especie, la activación de la célula de Mauthner, responsable de la fase inicial del comportamiento de escape, produce una poderosa modulación del sistema sensorial que organiza la electrorrecepción activa, su principal modalidad sensorial. Conclusión. Las claves de organización del sistema nervioso para la elaboración de comportamientos efectores, aún aquellos relativamente sencillos, incluyen estrategias complejas de ISM que determinan de su desempeño y cuyos mecanismos comienzan a develarse (AU)

Introduction and development. Neuronal mechanisms that underlie diverse sensory-motor integration processes (SMI) are essential for the motor control and determine the general organization of the nervous system. Spinal cord, sensory relay nucleus of brainstem and thalamus as well as higher motor control structures are some of the levels, of increasing complexity, at which several processes of SMI occurs during the execution of a motor act. The mechanisms that underlie SMI strategies operating at higher hierarchical levels of motor control are poorly understood. Escape response in teleosts fish is an advantageous experimental model for the analysis of the neural basis of behavior and of the mechanisms and functional consequences of diverse strategies of ISM. We describe several levels of ISM that operate in the neural system that organize this response in most teleosts and we deal with a detailed description of a novel strategy that occurs in Gymnotus carapo, a South American weakly electric fish. In this species, the activation of the Mauthner cell, a command neuron for the initial phase of escape, produces a powerful modulation of the sensory system responsible for active electrorreception, its main sensory modality. Conclusion. The neural basis of behavior, even those relatively simple, exhibit several strategies of complex SMI that determine its performance and whose cellular mechanisms begin to be unraveled (AU)

The otic gasbladder as an ancillary auditory structure in a mormyrid fish.

Mormyrid fishes use acoustic signals for long-distance communication and a weakly electric field for short-distance interaction. Mormyrids are unique in having an otic gasbladder attached directly to the saccule on each side of the inner ear. Karl von Frisch (1938) hypothesized that the tightly coupled otic gasbladder might aid mormyrid hearing. Using the mormyrid fish (Brienomyrus brachyistius), this study manipulated gas in the otic gasbladder to test this hypothesis and histological sections were made to examine the anatomical relationship between the gasbladder and inner ear. The hearing sensitivity curves (audiograms) were obtained with the auditory brainstem response protocol. Audiograms were obtained from normal fish and from fish in which gas was withdrawn from either one or two otic gasbladders. Removal of gas from one otic gasbladder did not result in a significant change in either hearing ability or acoustically evoked brainwaves as compared to the control fish. Bilateral deflation of the otic gasbladders led to significant threshold changes. Histological sections revealed a particularly close coupling between the otic gasbladder and the saccule chamber. These results support von Frisch's hypothesis that the otic gasbladders of mormyrids assist in underwater sound detection.

Transformations of an auditory temporal code in the medulla of a sound-producing fish.

The fish auditory system provides important insights into the evolution and mechanisms of vertebrate hearing. Fish have relatively simple auditory systems, without a cochlea for mechanical frequency analysis. However, as in all vertebrates, the primary auditory afferents of fish represent sounds as stimulus-entrained spike trains. Thus, fish provide important models for studying how temporal spiking patterns are used in higher level neural computations. In this paper we demonstrate that one of the fundamental transformations of information in the auditory system of a sound-producing fish, Pollimyrus, takes place in the auditory medulla. We discovered a class of neurons in which evoked spiking patterns were relatively independent of the stimulus fine structure and appeared to reflect intrinsic properties of the neurons. These neurons generated sustained responses but were poorly phase-locked to tones compared with the primary afferents. The interval histograms showed that spike timing was regular. However, in contrast to primary afferents, the mode of the interspike interval distribution was independent of the period of tonal stimuli. The tuning of the neurons was broad, with best sensitivity in the same spectral region where these animals concentrate energy in their communication sounds. The physiology of these neurons was similar to that of the chopper neurons known in the auditory brainstem of mammals. Our findings suggest that this medullary transformation, from phase-locked afferent input to chopper-like physiology, is basic to vertebrate auditory processing, even within lineages that have not evolved a cochlea.

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.

Functional neuroanatomy of auditory pathways in the sound-producing fish Pollimyrus.

We have described the acoustic pathway from the ear to the diencephalon in a sound-producing fish (Pollimyrus) based on simultaneous neurophysiological recordings from single neurons and injections of biotin pathway tracers at the recording sites. Fundamental transformations of auditory information from highly phase-locked and entrained responses in primary eighth nerve afferents and first-order medullary neurons to more weakly phase-locked responses in the auditory midbrain were revealed by physiological recordings. Anatomical pathway tracing uncovered a bilateral array of both first- and second-order medullary nuclei and a perilemniscal nucleus. Interconnections within the medullary auditory areas were extensive. Medullary nuclei projected to the auditory midbrain by means of the lateral lemniscus. Midbrain auditory areas projected to both ipsilateral and contralateral optic tecta and to an array of three nuclei in the auditory thalamus. The significance of these findings to the elucidation of mechanisms for the analysis of communication sounds and spatial hearing in this vertebrate animal is discussed.

Connections of the basal forebrain of the weakly electric fish, Eigenmannia virescens.

The organization of the ventral nucleus of the ventral telencephalon (Vv) was examined in the weakly electric fish, Eigenmannia virescens. This nucleus, which is considered the teleost homologue to the basal forebrain nuclei of other vertebrates, was subdivided into dorsal and ventral subdivisions, based upon cytoarchitectonic, immunohistochemical, and connectional criteria. Afferent projections were observed from the medial olfactory bulb as well as the terminal nerve ganglion. Telencephalic afferents to the Vv were very restricted, consisting of the supracommissural and the dorsal intermediate nuclei of the ventral telencephalon, the nucleus taenia, and the medial region of the posterior nucleus of the dorsal telencephalon. However, the major afferents to the Vv were diencephalic, particularly those originating from the rostral preoptic area and other hypothalamic nuclei. Additional afferents included the posterior tubercular nucleus, the locus coeruleus, the medial perilemniscal nucleus, and the periventricular nucleus of the posterior tuberculum. Relatively weak projections were observed from the ventral thalamus and the dorsal posterior thalamic nucleus. As described previously, the diencephalic complex of the central posterior thalamic nucleus/prepacemaker nucleus (CP/PPn), which also has cells that innervate the pacemaker circuitry controlling the production of an electric organ discharge, projects to the Vv. Terminal fields of the Vv were observed to be coextensive with afferent cell groups in the preoptic area, lateral and caudal hypothalamic nuclei, and thalamus. An additional efferent target of the Vv was the pretectal nucleus electrosensorius. That many cell groups that are connected with the Vv are also connected with the CP/PPn, particularly the preoptic and hypothalamic nuclei, suggests that the electrocommunicatory system is intimately linked with basal forebrain limbic pathways.