Sensory processing in the fast electrosensory pathway of pulse gymnotids studied at multiple integrative levels

Living life with an electric touch

The electric organ discharges (EODs) produced by weakly electric fish have long been a source of scientific intrigue and inspiration. The study of these species has contributed to our understanding of the organization of fixed action patterns, as well as enriching general imaging theory by unveiling the dual impact of an agent's actions on the environment and its own sensory system during the imaging process. This Centenary Review firstly compares how weakly electric fish generate species- and sex-specific stereotyped electric fields by considering: (1) peripheral mechanisms, including the geometry, channel repertoire and innervation of the electrogenic units; (2) the organization of the electric organs (EOs); and (3) neural coordination mechanisms. Secondly, the Review discusses the threefold function of the fish-centered electric fields: (1) to generate electric signals that encode the material, geometry and distance of nearby objects, serving as a short-range sensory modality or 'electric touch'; (2) to mark emitter identity and location; and (3) to convey social messages encoded in stereotypical modulations of the electric field that might be considered as species-specific communication symbols. Finally, this Review considers a range of potential research directions that are likely to be productive in the future.

Time-Comparator Neural Circuits of Gymnotiform Electric Fishes

Weakly electric fish possess electrosensory neural systems that are dedicated to detect microsecond time differences between sensory signals. Many features of this timing system, such as electroreceptor encoding, time-locked responses, and time-comparator neural circuit, are shared by closely related as well as distantly related electric fishes. The appearance and location of the time-comparator neural structures, however, are different among species. The timing systems of different electric fish species are compared.

Electroreception, electrogenesis and electric signal evolution

Electroreception, the capacity to detect external underwater electric fields with specialised receptors, is a phylogenetically widespread sensory modality in fishes and amphibians. In passive electroreception, a capacity possessed by c. 16% of fish species, an animal uses low-frequency-tuned ampullary electroreceptors to detect microvolt-range bioelectric fields from prey, without the need to generate its own electric field. In active electroreception (electrolocation), which occurs only in the teleost lineages Mormyroidea and Gymnotiformes, an animal senses its surroundings by generating a weak (< 1 V) electric-organ discharge (EOD) and detecting distortions in the EOD-associated field using high-frequency-tuned tuberous electroreceptors. Tuberous electroreceptors also detect the EODs of neighbouring fishes, facilitating electrocommunication. Several other groups of elasmobranchs and teleosts generate weak (< 10 V) or strong (> 50 V) EODs that facilitate communication or predation, but not electrolocation. Approximately 1.5% of fish species possess electric organs. This review has two aims. First, to synthesise our knowledge of the functional biology and phylogenetic distribution of electroreception and electrogenesis in fishes, with a focus on freshwater taxa and with emphasis on the proximate (morphological, physiological and genetic) bases of EOD and electroreceptor diversity. Second, to describe the diversity, biogeography, ecology and electric signal diversity of the mormyroids and gymnotiforms and to explore the ultimate (evolutionary) bases of signal and receptor diversity in their convergent electrogenic-electrosensory systems. Four sets of potential drivers or moderators of signal diversity are discussed. First, selective forces of an abiotic (environmental) nature for optimal electrolocation and communication performance of the EOD. Second, selective forces of a biotic nature targeting the communication function of the EOD, including sexual selection, reproductive interference from syntopic heterospecifics and selection from eavesdropping predators. Third, non-adaptive drift and, finally, phylogenetic inertia, which may arise from stabilising selection for optimal signal-receptor matching.

Postnatal brain development of the pulse type, weakly electric gymnotid fish Gymnotus omarorum

Teleosts are a numerous and diverse group of fish showing great variation in body shape, ecological niches and behaviors, and a correspondent diversity in brain morphology, usually associated with their functional specialization. Weakly electric fish are a paradigmatic example of functional specialization, as these teleosts use self-generated electric fields to sense the nearby environment and communicate with conspecifics, enabling fish to better exploit particular ecological niches. We analyzed the development of the brain of the pulse type gymnotid Gymnotus omarorum, focusing on the brain regions involved directly or indirectly in electrosensory information processing. A morphometric analysis has been made of the whole brain and of brain regions of interest, based on volumetric data obtained from 3-D reconstructions to study the growth of the whole brain and the relative growth of brain regions, from late larvae to adulthood. In the smallest studied larvae some components of the electrosensory pathway appeared to be already organized and functional, as evidenced by tract-tracing and in vivo field potential recordings of electrosensory-evoked activity. From late larval to adult stages, rombencephalic brain regions (cerebellum and electrosensory lateral line lobe) showed a positive allometric growth, mesencephalic brain regions showed a negative allometric growth, and the telencephalon showed an isometric growth. In a first step towards elucidating the role of cell proliferation in the relative growth of the analyzed brain regions, we also studied the spatial distribution of proliferation zones by means of pulse type BrdU labeling revealed by immunohistochemistry. The brain of G. omarorum late larvae showed a widespread distribution of proliferating zones, most of which were located at the ventricular-cisternal lining. Interestingly, we also found extra ventricular-cisternal proliferation zones at in the rombencephalic cerebellum and electrosensory lateral line lobe. We discuss the role of extraventricular-cisternal proliferation in the relative growth of the latter brain regions.

Time disparity sensitive behavior and its neural substrates of a pulse-type gymnotiform electric fish, Brachyhypopomus gauderio

Roles of the time coding electrosensory system in the novelty responses of a pulse-type gymnotiform electric fish, Brachyhypopomus, were examined behaviorally, physiologically, and anatomically. Brachyhypopomus responded with the novelty responses to small changes (100 μs) in time difference between electrosensory stimulus pulses applied to different parts of the body, as long as these pulses were given within a time period of ~500 μs. Physiological recording revealed neurons in the hindbrain and midbrain that fire action potentials time-locked to stimulus pulses with short latency (500-900 μs). These time-locked neurons, along with other types of neurons, were labeled with intracellular and extracellular marker injection techniques. Light and electron microscopy of the labeled materials revealed neural connectivity within the time coding system. Two types of time-locked neurons, the pear-shaped cells and the large cells converge onto the small cells in a hypertrophied structure, the mesencephalic magnocellular nucleus. The small cells receive a calyx synapse from a large cell at their somata and an input from a pear-shaped cell at the tip of their dendrites via synaptic islands. The small cells project to the torus semicircularis. We hypothesized that the time-locked neural signals conveyed by the pear-shaped cells and the large cells are decoded by the small cells for detection of time shifts occurring across body areas.

Perceptions of Latin American scientists about science and post-graduate education: Introduction to the 5th issue of CBP-Latin America

Although science and engineering (S&E) publications and doctoral degree awards in Latin America had experienced an impressive growth in the past decades, a qualitative evaluation of this increased output must be performed. Previous studies have indicated that growth in visibility of Latin American science - determined by ratio of citations per paper - has not kept pace with the increase in number of publications. In the present editorial, we analyzed - by means of a 12-item questionnaire - the individual perceptions of forty senior researchers involved in CBP-Latin America (29 Brazilians and 11 non-Brazilians) plus a special group composed by six extraordinary Latin American scientists (the "masters"). The questionnaire - using 6-point Likert-like scale for quantification of perception - focused on issues surrounding doctoral educational system as well as the governmental educational policies and publication pressure from funding agencies. In general, the most striking result was the perception (by 82% of respondents) of lack of job opportunities for people holding a PhD diploma in the field of comparative biochemistry and physiology. Other major trends include (i) lack of satisfaction with governmental policies for science and post-graduate education due to policies promoting mass production for papers and PhD diplomas (65-77% of respondents felt that way) (ii) that current PhD students are doing an adequate job, but have not improved in quality as compared to those from 10 years ago (the same was observed for PhD thesis in terms of present versus past), and (iii) that research infrastructure and the curricula of post-graduate courses do not constitute a problem, but (iv) recent-PhDs are not as fit as they should be in paper-writing skills, especially as perceived by Brazilian respondents. The general perceptions were very similar among Brazilians, non-Brazilians and "masters". The use of a larger study-population, with scientists of more diverse fields is the next logical step to best evaluate the level of satisfaction about science and post-graduate policies in the continent. Finally, this fifth and last special issue of CBP-Latin America celebrates the contribution of 20 new manuscripts, which adds up to 118 published studies highlighting the depth, breadth and enthusiasm of Latin American comparative biochemistry and physiology - enjoy.