Supervised learning algorithm for analysis of communication signals in the weakly electric fish Apteronotus leptorhynchus.

Signal analysis plays a preeminent role in neuroethological research. Traditionally, signal identification has been based on pre-defined signal (sub-)types, thus being subject to the investigator's bias. To address this deficiency, we have developed a supervised learning algorithm for the detection of subtypes of chirps-frequency/amplitude modulations of the electric organ discharge that are generated predominantly during electric interactions of individuals of the weakly electric fish Apteronotus leptorhynchus. This machine learning paradigm can learn, from a 'ground truth' data set, a function that assigns proper outputs (here: time instances of chirps and associated chirp types) to inputs (here: time-series frequency and amplitude data). By employing this artificial intelligence approach, we have validated previous classifications of chirps into different types and shown that further differentiation into subtypes is possible. This demonstration of its superiority compared to traditional methods might serve as proof-of-principle of the suitability of the supervised machine learning paradigm for a broad range of signals to be analyzed in neuroethology.

The effect of urethane and MS-222 anesthesia on the electric organ discharge of the weakly electric fish Apteronotus leptorhynchus.

Urethane and MS-222 are agents widely employed for general anesthesia, yet, besides inducing a state of unconsciousness, little is known about their neurophysiological effects. To investigate these effects, we developed an in vivo assay using the electric organ discharge (EOD) of the weakly electric fish Apteronotus leptorhynchus as a proxy for the neural output of the pacemaker nucleus. The oscillatory neural activity of this brainstem nucleus drives the fish's EOD in a one-to-one fashion. Anesthesia induced by urethane or MS-222 resulted in pronounced decreases of the EOD frequency, which lasted for up to 3 h. In addition, each of the two agents caused a manifold increase in the generation of transient modulations of the EOD known as chirps. The reduction in EOD frequency can be explained by the modulatory effect of urethane on neurotransmission, and by the blocking of voltage-gated sodium channels by MS-222, both within the circuitry controlling the neural oscillations of the pacemaker nucleus. The present study demonstrates a marked effect of urethane and MS-222 on neural activity within the central nervous system and on the associated animal's behavior. This calls for caution when conducting neurophysiological experiments under general anesthesia and interpreting their results.

The effect of eugenol anesthesia on the electric organ discharge of the weakly electric fish Apteronotus leptorhynchus.

Eugenol, the major active ingredient of clove oil, is widely used for anesthesia in fish. Yet virtually nothing is known about its effects on CNS functions, and thus about potential interference with neurophysiological experimentation. To address this issue, we employed a neuro-behavioral assay recently developed for testing of water-soluble anesthetic agents. The unique feature of this in-vivo tool is that it utilizes a readily accessible behavior, the electric organ discharge (EOD), as a proxy of the neural activity generated by a brainstem oscillator, the pacemaker nucleus, in the weakly electric fish Apteronotus leptorhynchus. A deep state of anesthesia, as assessed by the cessation of locomotor activity, was induced within less than 3 min at concentrations of 30-60 µL/L eugenol. This change in locomotor activity was paralleled by a dose-dependent, pronounced decrease in EOD frequency. After removal of the fish from the anesthetic solution, the frequency returned to baseline levels within 30 min. Eugenol also led to a significant increase in the rate of 'chirps,' specific amplitude/frequency modulations of the EOD, during the 30 min after the fish's exposure to the anesthetic. At 60 µL/L, eugenol induced a collapse of the EOD amplitude after about 3.5 min in half of the fish tested. The results of our study indicate strong effects of eugenol on CNS functions. We hypothesize that these effects are mediated by the established pharmacological activity of eugenol to block the generation of action potentials and to reduce the excitability of neurons; as well as to potentiate GABAA-receptor responses.

The Neurosphere Simulator: An educational online tool for modeling neural stem cell behavior and tissue growth.

Until very recently, distance education, including digital science labs, served a rather small portion of postsecondary students in the United States and many other countries. This situation has, however, dramatically changed in 2020 in the wake of the COVID-19 pandemic, which forced colleges to rapidly transit from face-to-face instructions to online classes. Here, we report the development of an interactive simulator that is freely available on the web (http://neurosphere.cos.northeastern.edu/) for teaching lab classes in developmental biology. This simulator is based on cellular automata models of neural-stem-cell-driven tissue growth in the neurosphere assay. By modifying model parameters, users can explore the role in tissue growth of several developmental mechanisms, such as regulation of mitosis or apoptotic cell death by contact inhibition. Besides providing an instantaneous animation of the simulated development of neurospheres, the Neurosphere Simulator tool offers also the possibility to download data for detailed analysis. The simulator function is complemented by a tutorial that introduces students to computational modeling of developmental processes.

Modeling of sustained spontaneous network oscillations of a sexually dimorphic brainstem nucleus: the role of potassium equilibrium potential.

Intrinsic oscillators in the central nervous system play a preeminent role in the neural control of rhythmic behaviors, yet little is known about how the ionic milieu regulates their output patterns. A powerful system to address this question is the pacemaker nucleus of the weakly electric fish Apteronotus leptorhynchus. A neural network comprised of an average of 87 pacemaker cells and 20 relay cells produces tonic oscillations, with higher frequencies in males compared to females. Previous empirical studies have suggested that this sexual dimorphism develops and is maintained through modulation of buffering of extracellular K+ by a massive meshwork of astrocytes enveloping the pacemaker and relay cells. Here, we constructed a model of this neural network that can generate sustained spontaneous oscillations. Sensitivity analysis revealed the potassium equilibrium potential, EK (as a proxy of extracellular K+ concentration), and corresponding somatic channel conductances as critical determinants of oscillation frequency and amplitude. In models of both the pacemaker nucleus network and isolated pacemaker and relay cells, the frequency increased almost linearly with EK, whereas the amplitude decreased nonlinearly with increasing EK. Our simulations predict that this frequency increase is largely caused by a shift in the minimum K+ conductance over one oscillation period. This minimum is close to zero at more negative EK, converging to the corresponding maximum at less negative EK. This brings the resting membrane potential closer to the threshold potential at which voltage-gated Na+ channels become active, increasing the excitability, and thus the frequency, of pacemaker and relay cells.

Cellular automata modeling suggests symmetric stem-cell division, cell death, and cell drift as key mechanisms driving adult spinal cord growth in teleost fish.

Adult neurogenesis - the generation of neurons during adulthood - is intensively studied, yet little is known about its consequences at the tissue level. In the teleost fish Apteronotus leptorhynchus, morphometric analysis has revealed that the total number of cells in the spinal cord increases continuously throughout adulthood, driven by the activity of neurogenic stem/progenitor cells in both the ependymal layer at the central canal and in the radially located parenchyma. This net increase in cell numbers demonstrates cellular addition, as opposed to cellular turnover which appears to be the common outcome of adult neurogenesis in mammals. Grounded on a comprehensive set of quantitative data generated through high-resolution mapping of stem cells and their progeny, we constructed a cellular automata model of the stem-cell-driven growth of the spinal cord. Simulations based on this model suggest that three cellular mechanisms play a critical role for promoting sustained tissue growth and acquisition of correct form of the spinal cord, including the development of the ependymal layer and the parenchyma: the number of symmetric stem-cell divisions versus asymmetric divisions; the probability of the progeny of progenitor cells to undergo cell death; and the radial drifting of cells.

Glia-mediated modulation of extracellular potassium concentration determines the sexually dimorphic output frequency of a model brainstem oscillator.

Sexual dimorphism in behavior is widespread among animals, but the cellular mechanisms underlying neural control of this phenomenon are largely unknown. One behavior that has provided some important clues about how such sex differences might develop is the electric organ discharge of Apteronotus leptorhynchus. In this weakly electric fish, the mean discharge frequencies of males and females are 880 Hz and 740 Hz, respectively, with little overlap of the two frequency bands. The discharges are controlled, in a one-to-one fashion, by the neural oscillations of the pacemaker nucleus in the medulla oblongata. Experimental evidence has shown that the astrocytic syncytium associated with the neural network that generates these oscillations is significantly larger, and stronger coupled via gap junctions, in females than in males. In the present study, modeling of this network was performed to test the hypotheses that the sex-dependent differences in the structure and properties of the astrocytic syncytium mediate better buffering of extracellular potassium in females than in males, which in turn causes, via a lowering of the potassium equilibrium potential, a decrease in the oscillation frequency. Simulations of the neural activity of the pacemaker nucleus and its individual components demonstrated that under both spontaneous and induced conditions the oscillation frequency and the potassium equilibrium potential are strongly positively correlated. These simulations predict that sufficient separation of the electric organ discharge frequencies for establishment of the sexual dimorphism can be achieved by rather minor alterations in the concentration of the extracellular potassium concentration in the pacemaker nucleus.

Cellular Automata Modeling of Stem-Cell-Driven Development of Tissue in the Nervous System.

Mathematical and computational modeling enables biologists to integrate data from observations and experiments into a theoretical framework. In this review, we describe how developmental processes associated with stem-cell-driven growth of tissue in both the embryonic and adult nervous system can be modeled using cellular automata (CA). A cellular automaton is defined by its discrete nature in time, space, and state. The discrete space is represented by a uniform grid or lattice containing agents that interact with other agents within their local neighborhood. This possibility of local interactions of agents makes the cellular automata approach particularly well suited for studying through modeling how complex patterns at the tissue level emerge from fundamental developmental processes (such as proliferation, migration, differentiation, and death) at the single-cell level. As part of this review, we provide a primer for how to define biologically inspired rules governing these processes so that they can be implemented into a CA model. We then demonstrate the power of the CA approach by presenting simulations (in the form of figures and movies) based on building models of three developmental systems: the formation of the enteric nervous system through invasion by neural crest cells; the growth of normal and tumorous neurospheres induced by proliferation of adult neural stem/progenitor cells; and the neural fate specification through lateral inhibition of embryonic stem cells in the neurogenic region of Drosophila.