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Weakly electric fish use of a self-generated electric field to probe their environment, this behaviour is known as electrolocation. This study investigated two aspects of electrolocation in two species of knifefish (Apteronotus leptorhynchus and Eigenmannia virescens). First, we characterized the ability to track moving objects and found that tracking performance did not differ among speeds tested in either species. Second, we characterized a motion-related cue for distance perception, similar to visual parallax, for which rapidly moving objects would be perceived as closer than slowly moving objects. During tracking experiments, the fish remained centered between the moving objects. We hypothesized that the fish use electrosensory parallax to perform this centering behaviour. Thus, we predicted that if one object moved slightly slower than the other, the fish would perceive the slower-moving object as farther away, and would move towards the slower object to remain “centered.” Indeed, our results supported our hypothesis with E. virescens moving towards the slower object to an extent that increased with the relative decrease in speed.

Effective exploration of the environment is a critical aspect of adaptive behaviour, enabling animals to identify food sources, potential mates, refuge locations, and other important resources. The particular strategies used during exploratory behaviours depend on a variety of factors including context, personality traits and natural ecology. Weakly electric fish rely specifically on a short-range electric sense to search and locate objects in their environment in low-light conditions. However, little is known about the exploratory strategies used. We characterized the exploratory movements of two species of weakly electric fish, Apteronotus leptorhynchus and Apteronotus albifrons, in a laboratory setting. Our results suggest that there are behavioural differences between species in their exploratory strategies. Apteronotus albifrons spent more time in the open, travelled at a slower speed when out in the open, and had a higher total feeding time. Interestingly, Apteronotus leptorhynchus had a higher total displacement and preference for wall-following. A subsequent study on the behavioural function of wall-following in the two species suggested that wall-following is used for exploration in weakly electric fish, rather than for protection, and is not an artifact of restricted movement and tank shape.

Weakly electric fish use of a self-generated electric field to probe their environment, this behaviour is known as electrolocation. This study investigated two aspects of electrolocation in two species of knifefish (Apteronotus leptorhynchus and Eigenmannia virescens). First, we characterized the ability to track moving objects and found that tracking performance did not differ among speeds tested in either species. Second, we characterized a motion-related cue for distance perception, similar to visual parallax, for which rapidly moving objects would be perceived as closer than slowly moving objects. During tracking experiments, the fish remained centered between the moving objects. We hypothesized that the fish use electrosensory parallax to perform this centering behaviour. Thus, we predicted that if one object moved slightly slower than the other, the fish would perceive the slower-moving object as farther away, and would move towards the slower object to remain “centered.” Indeed, our results supported our hypothesis with E. virescens moving towards the slower object to an extent that increased with the relative decrease in speed.

Eigenmannia virescens is a gymnotiform weakly electric fish which uses a quasi-sinusoidal electric organ discharge (EOD) to sense their environment. EOD frequency (EODF) is individual-specific. In conspecific interactions, each fish perceives the EODF of the conspecific as a periodic amplitude modulation (AM) of their own discharge. When both fish are stationary, the depth of this AM is constant, but it varies when fish are swimming. We hypothesized that AM variations during swimming act as a noise source that could have no effect on, hinder, or enhance EODF identification. To test this, we quantified the jamming avoidance response (JAR) (a natural behaviour wherein fish are required to accurately determine one another’s EODF) in response to stimuli of varying depths of noise. These experiments demonstrated that swimming noise does not impair the ability of E. virescens to identify conspecific EODF, and actually improves its ability to detect the presence of a neighbouring fish.

The dorsodorsal division (DD) of the teleost telencephalon has been implicated in memory processes similar to those associated with the mammalian hippocampus. The network connectivity and neural activity underlying this involvement have remained unclear. This thesis attempts to elucidate both. Attempts have been made to record the neural activity of DD neurons, but none have succeeded in correlating the recorded firing with any meaningful stimuli. In this thesis, I present single-unit electrophysiological recordings of DD neurons that reveal persistent activity in the form of up-states which are evoked by two modalities of naturalistic sensory stimuli – visual and electrosensory. The anatomy of DD was a little better understood than the neural activity. Recent anatomical work has shown that DD is strongly inter-connected with the cortical-like dorsolateral division (DL) of the pallium, re-inforcing its similarity to mammalian hippocampal structures. This same work has also revealed much of DD’s extrinsic connectivity. It was not, however, of a resolution fine enough to disambiguate the connections of the various DD subregions, nor to clarify the existence and structure of intrinsic DD connectivity. In this thesis, I further elucidate the connectivity of DD, by isolating its subregions. This was done by means of very small and precise neurotracer injections. These injections revealed strong recurrent connectivity within individual DD subregions, multiple pathways between DD and DL, and striking similarities between the connectivities of DL and DD and those of the mammalian dentate gyrus and CA3 respectively. From the results of these investigations I propose a model of homology between the teleost DD-DL loop and the putative pattern separation and completion networks contained in the mammalian cortico-hippocampal circuitry, as well as a role for the observed persistent activity in DD within this model. I further propose the dorsal teleost telencephalon as an excellent model system for the further study of the network mechanisms of pattern separation and completion.

The weakly electric fish Sternopygus macrurus emits an electric organ discharge (EOD) composed of a series of pulses. The EOD pulse is mainly shaped by sodium currents. There are two sodium channel α subunits orthologs of the mammalian Nav1.4 expressed in the EO of Sternopygus. Previous studies identified a novel splice variant of the Nav1.4b (Nav1.4bL), in which an extra 51-amino acid occurs in the N terminal end. Nav1.4bL currents inactivate and recover from inactivation significantly faster than Nav1.4bS. The voltage-dependence of steady-state inactivation of smNav1.4bL shifts to hyperpolarized potential. Structural analysis predicts an α-helix in the middle of the extended N terminus. Removal of a proline right after the α-helix significantly slows down current decay but has no effect on channel recovery from inactivation, suggesting inactivation and recovery have independent mechanism. Mutagenesis analysis of the extended N terminus showed that the short helical region, especially the positive charges in the helix, is an important determinant for channel voltage-dependence of steady-state inactivation. However, other residues outside the helical region are required for regulation of fast inactivation and recovery form inactivation. Functional and structural analysis provides evidence for the importance of the C terminus in fish Nav1.4b channel properties. Chimera in which the C terminus of smNav1.4bS was substituted by the human Nav1.4 C terminus, shows an 11 mV positive shift in voltage-dependence of activation and a -16 mV negative shift in inactivation. Deletion of the distal half of smNav1.4bS negatively shifted voltage-dependence of inactivation and significantly accelerated channel recovery from inactivation. In the deletion mutant, the regulation by the N segment is missing. Substitution of the C terminus mutant retains wild type channel inactivation and recovery properties and can be regulated by N segment again. My study provides evidence that the extended N terminus of smNav1.4bL binds the distal part of C terminal tail to modulate channel inactivation properties. This is the first time to show the distal C terminus is involved in channel recovery from inactivation. Studies in the fish sodium channel properties provide useful information to understand function and structure of voltage-gated sodium channels. / text

Although the processes used for temporarily storing and manipulating neural information have been extensively studied at the synaptic level far less attention has been given to the underlying cellular and molecular mechanisms that contribute to change in the intrinsic excitability of neurons. More importantly, how do these mechanisms of plasticity integrate with ongoing mechanisms of regulation of neural intrinsic excitability and, in turn, homeostasis of entire neural circuits? In this dissertation I describe the underlying mechanisms that contribute to persistent neural activity and, more globally, sensorimotor adaptation using weakly electric fish as my model system. Weakly electric fish have evolved a behavior adaptation known as the jamming avoidance response (JAR), and it is this adaptation that allows the organism to elevate its own electrical discharge in response to intraspecific interactions and subsequent distortions of the animal’s electric field. The elevation operates over a wide range and in vivo can last tens of hours upon cessation of a jamming stimulus. I demonstrate that the underlying mechanisms of the adaptation are mediated by calcium-dependent signaling in the pacemaker nucleus and that calcium-mediated phosphorylation plays an important role in the regulation of the long-term frequency elevation (LTFE). I demonstrate using an in vitro brain slice preparation from the weakly electric fish, Apteronotus leptorhynchus that the engram of memory formation depends on the cooperativity of calcium-dependent protein kinases and protein phosphatases. In addition, I show that the memory formation (in the form of LTFE) does not depend on the continued flux of calcium, but rather the phosphorylation events downstream of NMDA receptor activation. Moreover, I describe the differences in the expression of protein phosphatases and protein kinases as they relate to species-specific differences in sensorimotor adaptation. It is important to note that this is the first time that the cooperativity between different isoforms of protein kinase C (PKC) have been shown to play a role in graded long-term change in neuronal activity and, in turn, providing the neural basis of species-specific behavior. The neural adaptation of the electromotor system in weakly electric fish provides an excellent model system to study the underlying cellular and molecular events of vertebrate memory formation. / text

A bioeletrogenese tem atraído a atenção da ciência desde a antiguidade. Capazes de produzir campos elétricos e também de sentir estes campos, os peixes elétricos pulsadores de campo fraco são um modelo de estudo praticamente com características únicas em neuroetologia, ja que permitem ao experimentador medir de maneira não invasiva os sinais espaco-temporais envolvidos em pelo menos duas capacidades complexas do sistema nervoso do animal: a eletrolocalizacao (em que e produzida uma imagem elétrica das proximidades) e a eletrocomunicacao (em que os padrões de pulsos são usados para identificar conspecificos, seu sexo, tamanho, resolver disputas de território, etc). Entretanto, como os pulsos geralmente são idênticos em indivíduos de uma mesma espécie e a amplitude do sinal medido depende da distancia dos animais aos eletrodos usados, experimentos com animais livres para se movimentar são muito difíceis de realizar, mais ainda experimentos com mais de um animal interagindo. Por isso, na maioria dos trabalhos encontrados na literatura o comportamento elétrico dos animais e registrado durante curtos intervalos de tempo em que seus movimentos eram bastante restritos ou limitados a agua bem rasa. Além disso, os estímulos eram geralmente compostos por pulsos quadrados ou períodos senoidais apresentados a intervalos regulares. Os protocolos experimentais usados eram sempre unidirecionais, ou seja, não dependiam nem se adaptavam a atividade dos peixes. Para lidar com estas limitações, que acreditamos tornarem o comportamento dos animais muito diferente do que ocorre na natureza, desenvolvemos aparatos experimentais para registrar e estudar o comportamento elétrico e motor de peixes elétricos pulsadores nadando livremente por longos períodos de tempo e que podem ser facilmente adaptados para o estudo de diversas espécies. Utilizamos técnicas de interação em tempo real entre computadores e sistema nervoso vivo, adaptado de protocolos do tipo dynamic clamp, para produzir estímulos elétricos realistas e também estímulos luminosos. Mostramos protocolos de estimulação clássicos unidirecionais bem como bidirecionais, dependentes da atividade dos animais. Aplicamos técnicas de analise de dados baseadas na teoria da informação que permitiram associar a entropia da serie de pulsos do órgão elétrico a movimentação do animal. Aplicamos estes aparatos e técnicas para estudar peixes elétricos de campo fraco de espécies que pertencem a ordens diferentes e, portanto, são o resultado de historias evolutivas distintas: o peixe sul americano G. carapo, da ordem dos Gymnotiformes e o peixe africano G. petersii, da ordem dos Mormyriformes. Obtivemos evidencias de comunicação dos animais e estudamos quais os padrões mais prováveis de disparo em diferentes condições. Uma das espécies apresentou um longo transiente quando exposta a um novo ambiente, evidenciando que as técnicas tradicionais de restringir periodicamente o movimento do peixe não são adequadas para o estudo do comportamento desta espécie. Nossos resultados apresentaram varias evidencias de que os animais são capazes de distinguir estímulos realistas (gravados de conspecificos), de estímulos aleatórios com propriedades estatísticas semelhantes e que há 2 valores de echo response validando a necessidade dos métodos de estimulo desenvolvidos. Também pudemos mostrar que protocolos de estimulação em tempo real bidirecionais, são mais efetivos em interagir com o código do peixe quando comparados com os protocolos unidirecionais tradicionais e que os animais são capazes de aprender a controlar seu comportamento motor e também sua frequência de disparo para evitar estímulos indesejados. / Bioloelectrogenesis is known since ancient times. Weakly electric fish are a wonderful model in Neuroethology because they produce and sense eletric fields. These unique features allow non invasive experiments to access complex spatio-temporal signals involved in 2 tasks called electrocommunication and electrolocation. Electrolocation is the ability to see the surrounding areas /objects by analyzing changes in the fish\'s own electric field and electrocommunication is the ability to identify conspecifics, fight for dominance etc. In this last task fish have their electric field distorted by conspecifics\' eletric organ discharges. Usually, within species, pulse-type weakly electric fish discharge pulses with similar waveform and the amplitude of the pulse depends on the distance to the recording electrodes being very difficult to measure the discharges in freely swimming animals, specially when 2 or more animals are interacting. For these reasons, most studies found in the literature are done with restrained animals or in shallow tanks. The most commom stimuli used are square/sine waves or very short pre-recorded discharges in classic protocol where the stimuli do not depend on the fish\'s activity. To overcome these issues trying to perform more naturalistic experiments, we developed experimental setups to record the electric and motor behavior in freely pulse-type electric fish for long periods. Our setups have also the advantage of being easy to adapt making possible to study several species. We performed real time experiments with realistic electric and light stimuli using dynamic clamp techniques adapted to Neuroethology. We show both classic unidirectional protocols as well as bidirectional closed loop interaction, taking into account the fish\'s dynamic activity. Analyzes based on Information Theory revealed that the entropy of the electric organ discharges are correlated to the their movement. We performed experiments using the setups and techniques mentioned before in 2 species that have evolved independently: G. Carapo (Gymnotidae) from South America and G. petersii (Mormyridae) from Africa. We show evidence of real communication and we study the inter pulse discharge probability in different behavioral circumstances. One specie showed a long transient behavior when introduced in new environment, hence, the traditional experiments with restrained animals might not be suitable to study natural behavior. Our results show several evidences that the fish can distinguish between realist stimuli from conspecifics and random ones, that there are 2 values of echo response instead of 1, demonstrating the importance of our new setup and protocols. We could also show that closed loop protocols were more effective to stimulate and interact with the fish\'s activity and that the animals are able to control their motor and electric behavior avoiding possibly harmful stimulation.

Behavioural syndromes, defined as suites of correlated behaviours across different contexts, are used to characterize individual variability in behaviours. Males of the weakly electric fish species, Apteronotus leptorhynchus, produce electro-communication signals called chirps. Chirps are thought to be involved in agonistic signalling, as their relative incidence increases during agonistic conspecific interactions. However, high levels of individual variability in aggression obscure the role of chirps in mediating aggression. Here, I tested the presence of an aggression-boldness behavioural syndrome, and then considered the implications such a syndrome would have on chirping behaviours. Behavioural tests in anti-predation, object novelty, feeding, conspecific intrusion and novel environment exploration contexts revealed a syndrome involving only object novelty and feeding. We found no correlation between chirping behaviour and the assessed behaviours. Our results demonstrate that chirps represent a more complex communication system than previously suggested.