Packet information encoding in a cerebellum-like circuit.

Packet information encoding of neural signals was proposed for vision about 50 years ago and has recently been revived as a plausible strategy generalizable to natural and artificial sensory systems. It involves discrete image segmentation controlled by feedback and the ability to store and compare packets of information. This article shows that neurons of the cerebellum-like electrosensory lobe (EL) of the electric fish Gymnotus omarorum use spike-count and spike-timing distribution as constitutive variables of packets of information that encode one-by-one the electrosensory images generated by a self-timed series of electric organ discharges (EODs). To evaluate this hypothesis, extracellular unitary activity was recorded from the centro-medial map of the EL. Units recorded in high-decerebrate preparations were classified into six types using hierarchical cluster analysis of post-EOD spiking histograms. Cross-correlation analysis indicated that each EOD strongly influences the unit firing probability within the next inter-EOD interval. Units of the same type were similarly located in the laminar organization of the EL and showed similar stimulus-specific changes in spike count and spike timing after the EOD when a metal object was moved close by, along the fish's body parallel to the skin, or when the longitudinal impedance of a static cylindrical probe placed at the center of the receptive field was incremented in a stepwise manner in repetitive trials. These last experiments showed that spike-counts and the relative entropy, expressing a comparative measure of information before and after the step, were systematically increased with respect to a control in all unit types. The post-EOD spike-timing probability distribution and the relatively independent contribution of spike-timing and number to the content of information in the transmitted packet suggest that these are the constitutive image-encoding variables of the packets. Comparative analysis suggests that packet information transmission is a general principle for processing superposition images in cerebellum-like networks.

Species variation in steroid hormone-related gene expression contributes to species diversity in sexually dimorphic communication in electric fishes.

Sexually dimorphic behaviors are often regulated by gonadal steroid hormones. Species diversity in behavioral sex differences may arise as expression of genes mediating steroid action in brain regions controlling these behaviors evolves. The electric communication signals of apteronotid knifefishes are an excellent model for comparatively studying neuroendocrine regulation of sexually dimorphic behavior. These fish produce and detect weak electric organ discharges (EODs) for electrolocation and communication. EOD frequency (EODf), controlled by the medullary pacemaker nucleus (Pn), is sexually dimorphic and regulated by androgens and estrogens in some species, but is sexually monomorphic and unaffected by hormones in other species. We quantified expression of genes for steroid receptors, metabolizing enzymes, and cofactors in the Pn of two species with sexually dimorphic EODf (Apteronotus albifrons and Apteronotus leptorhynchus) and two species with sexually monomorphic EODf ("Apteronotus" bonapartii and Parapteronotus hasemani). The "A." bonapartii Pn expressed lower levels of androgen receptor (AR) genes than the Pn of species with sexually dimorphic EODf. In contrast, the P. hasemani Pn robustly expressed AR genes, but expressed lower levels of genes for 5α-reductases, which convert androgens to more potent metabolites, and higher levels of genes for 17ß-hydroxysteroid dehydrogenases that oxidize androgens and estrogens to less potent forms. These findings suggest that sexual monomorphism of EODf arose convergently via two different mechanisms. In "A." bonapartii, reduced Pn expression of ARs likely results in insensitivity of EODf to androgens, whereas in P. hasemani, gonadal steroids may be metabolically inactivated in the Pn, reducing their potential to influence EODf.

Electrosensory and metabolic responses of weakly electric fish to changing water conductivity.

Weakly electric gymnotiform fishes use self-generated electric organ discharges (EODs) to navigate and communicate. The electrosensory range for these processes is a function of EOD amplitude, determined by the fish's electric organ (EO) output and the electrical conductivity of the surrounding water. Anthropogenic activity, such as deforestation, dams and industrial/agricultural runoff, are known to increase water conductivity in neotropical habitats, likely reducing the electrosensory range of these fish. We investigated whether fish modulate EO output as means of re-expanding electrosensory range after a rapid increase in water conductivity in the pulse-type Brachyhypopomus gauderio and the wave-type Eigenmannia virescens. Furthermore, because EOD production incurs significant metabolic costs, we assessed whether such compensation is associated with an increase in metabolic rate. Following the conductivity increase, B. gauderio increased EOD amplitude by 20.2±4.3% over 6 days but with no associated increase in metabolic rate, whereas the EOD amplitude of E. virescens remained constant, accompanied by an unexpected decrease in metabolic rate. Our results suggest that B. gauderio uses a compensation mechanism that requires no metabolic investment, such as impedance matching, or a physiological trade-off wherein energy is diverted from other physiological processes to increase EO output. These divergent responses between species could be the result of differences in reproductive life history or evolutionary adaptations to different aquatic habitats. Continued investigation of electrosensory responses to changing water conditions will be essential for understanding the effects of anthropogenic disturbances on gymnotiforms, and potential physiological mechanisms for adapting to a rapidly changing aquatic environment.

Active sensing: Eavesdropping on your neighbor to locate prey.

When animals using active sensing, e.g., sonar or an electric organ discharge, cooperate while foraging, the emitted sound or electric field is available to neighboring conspecifics. Experimental and modelling studies have shown that an electric fish can use the discharge of neighbors to extend their own electrosensory prey detection range.

Differential expression of steroid-related genes across electrosensory brain regions in two sexually dimorphic species of electric knifefish.

The production of communication signals can be modulated by hormones acting on the brain regions that regulate these signals. However, less is known about how signal perception is regulated by hormones. The electrocommunication signals of weakly electric fishes are sexually dimorphic, sensitive to hormones, and vary across species. The neural circuits that regulate the production and perception of these signals are also well-characterized, and electric fishes are thus an excellent model to examine the neuroendocrine regulation of sensorimotor mechanisms of communication. We investigated (1) whether steroid-related genes are expressed in sensory brain regions that process communication signals; and (2) whether this expression differs across sexes and species that have different patterns of sexual dimorphism in their signals. Apteronotus leptorhynchus and Apteronotus albifrons produce continuous electric organ discharges (EODs) that are used for communication. Two brain regions, the electrosensory lateral line lobe (ELL) and the dorsal torus semicircularis (TSd), process inputs from electroreceptors to allow fish to detect and discriminate electrocommunication signals. We used qPCR to quantify the expression of genes for two androgen receptors (ar1, ar2), two estrogen receptors (esr1, esr2b), and aromatase (cyp19a1b). Four out of five steroid-related genes were expressed in both sensory brain regions, and their expression often varied between sexes and species. These results suggest that expression of steroid-related genes in the brain may differentially influence how EOD signals are encoded across species and sexes, and that gonadal steroids may coordinately regulate central circuits that control both the production and perception of EODs.

Collective sensing in electric fish.

A number of organisms, including dolphins, bats and electric fish, possess sophisticated active sensory systems that use self-generated signals (for example, acoustic or electrical emissions) to probe the environment1,2. Studies of active sensing in social groups have typically focused on strategies for minimizing interference from conspecific emissions2-4. However, it is well known from engineering that multiple spatially distributed emitters and receivers can greatly enhance environmental sensing (for example, multistatic radar and sonar)5-8. Here we provide evidence from modelling, neural recordings and behavioural experiments that the African weakly electric fish Gnathonemus petersii utilizes the electrical pulses of conspecifics to extend its electrolocation range, discriminate objects and increase information transmission. These results provide evidence for a new, collective mode of active sensing in which individual perception is enhanced by the energy emissions of nearby group members.

Ecologically mediated differences in electric organ discharge drive evolution in a sodium channel gene in South American electric fishes.

Active electroreception-the ability to detect objects and communicate with conspecifics via the detection and generation of electric organ discharges (EODs)-has evolved convergently in several fish lineages. South American electric fishes (Gymnotiformes) are a highly species-rich group, possibly in part due to evolution of an electric organ (EO) that can produce diverse EODs. Neofunctionalization of a voltage-gated sodium channel gene accompanied the evolution of electrogenic tissue from muscle and resulted in a novel gene (scn4aa) uniquely expressed in the EO. Here, we investigate the link between variation in scn4aa and differences in EOD waveform. We combine gymnotiform scn4aa sequences encoding the C-terminus of the Nav1.4a protein, with biogeographic data and EOD recordings to test whether physiological transitions among EOD types accompany differential selection pressures on scn4aa. We found positive selection on scn4aa coincided with shifts in EOD types. Species that evolved in the absence of predators, which likely selected for reduced EOD complexity, exhibited increased scn4aa evolutionary rates. We model mutations in the protein that may underlie changes in protein function and discuss our findings in the context of gymnotiform signalling ecology. Together, this work sheds light on the selective forces underpinning major evolutionary transitions in electric signal production.

Gene and Allele-Specific Expression Underlying the Electric Signal Divergence in African Weakly Electric Fish.

In the African weakly electric fish genus Campylomormyrus, electric organ discharge signals are strikingly different in shape and duration among closely related species, contribute to prezygotic isolation, and may have triggered an adaptive radiation. We performed mRNA sequencing on electric organs and skeletal muscles (from which the electric organs derive) from 3 species with short (0.4 ms), medium (5 ms), and long (40 ms) electric organ discharges and 2 different cross-species hybrids. We identified 1,444 upregulated genes in electric organ shared by all 5 species/hybrid cohorts, rendering them candidate genes for electric organ-specific properties in Campylomormyrus. We further identified several candidate genes, including KCNJ2 and KLF5, and their upregulation may contribute to increased electric organ discharge duration. Hybrids between a short (Campylomormyrus compressirostris) and a long (Campylomormyrus rhynchophorus) discharging species exhibit electric organ discharges of intermediate duration and showed imbalanced expression of KCNJ2 alleles, pointing toward a cis-regulatory difference at this locus, relative to electric organ discharge duration. KLF5 is a transcription factor potentially balancing potassium channel gene expression, a crucial process for the formation of an electric organ discharge. Unraveling the genetic basis of the species-specific modulation of the electric organ discharge in Campylomormyrus is crucial for understanding the adaptive radiation of this emerging model taxon of ecological (perhaps even sympatric) speciation.

Understanding daily rhythms in weakly electric fish: the role of melatonin on the electric behavior of Brachyhypopomus gauderio.

Living organisms display molecular, physiological and behavioral rhythms synchronized with natural environmental cycles. Understanding the interaction between environment, physiology and behavior requires taking into account the complexity of natural habitats and the diversity of behavioral and physiological adaptations. Brachyhypopomus gauderio is characterized by the emission of electric organ discharges (EOD), with a very stable rate modulated by social and environmental cues. The nocturnal arousal in B. gauderio coincides with a melatonin-dependent EOD rate increase. Here, we first show a daily cycle in both the EOD basal rate (EOD-BR) and EOD-BR variability of B. gauderio in nature. We approached the understanding of the role of melatonin in this natural behavior through both behavioral pharmacology and in vitro assays. We report, for the first time in gymnotiformes, a direct effect of melatonin on the pacemaker nucleus (PN) in in vitro preparation. Melatonin treatment lowered EOD-BR in freely moving fish and PN basal rate, while increasing the variability of both. These results show that melatonin plays a key role in modulating the electric behavior of B. gauderio through its effect on rate and variability, both of which must be under a tight temporal regulation to prepare the animal for the challenging nocturnal environment.

Diet in phenotypically divergent sympatric species of African weakly electric fish (genus: Campylomormyrus)-A hybrid capture/HTS metabarcoding approach.

Ecological speciation within the mormyrid genus Campylomormyrus resulted in sympatric species exhibiting divergence in their feeding apparatus and electric organ discharge (EOD). This study documents the overall diet of the genus Campylomormyrus and examines the hypothesis that the Campylomormyrus radiation is caused by adaptation to different food sources. We performed diet assessment of five sympatric Campylomormyrus species (C. alces, C. compressirostris, C. curvirostris, C. tshokwe, C. numenius) and their sister taxon Gnathonemus petersii with markedly different snout morphologies and EODs using hybrid capture/HTS DNA metabarcoding of their stomach contents. Our approach allowed for high taxonomic resolution of prey items, including benthic invertebrates, allochthonous invertebrates and vegetation. Comparisons of the diet compositions using quantitative measures and diet overlap indices revealed that all species are able to exploit multiple food niches in their habitats, that is fauna at the bottom, the water surface and the water column. A major part of the diet is larvae of aquatic insects, such as dipterans, coleopterans and trichopterans, known to occur in holes and interstitial spaces of the substrate. The results indicate that different snout morphologies and the associated divergence in the EOD could translate into different prey spectra. This suggests that the diversification in EOD and/or morphology of the feeding apparatus could be under functional adaptation.

Electric organ discharge from electric eel facilitates DNA transformation into teleost larvae in laboratory conditions.

Background: Electric eels (Electrophorus sp.) are known for their ability to produce electric organ discharge (EOD) reaching voltages of up to 860 V. Given that gene transfer via intense electrical pulses is a well-established technique in genetic engineering, we hypothesized that electric eels could potentially function as a gene transfer mechanism in their aquatic environment. Methods: To investigate this hypothesis, we immersed zebrafish larvae in water containing DNA encoding the green fluorescent protein (GFP) and exposed them to electric eel's EOD. Results and Discussion: Some embryos exhibited a mosaic expression of green fluorescence, in contrast to the control group without electrical stimulation, which showed little distinct fluorescence. This suggests that electric eel EOD has the potential to function as an electroporator for the transfer of DNA into eukaryotic cells. While electric eel EOD is primarily associated with behaviors related to sensing, predation, and defense, it may incidentally serve as a possible mechanism for gene transfer in natural environment. This investigation represents the initial exploration of the uncharted impact of electric eel EOD, but it does not directly establish its significance within the natural environment. Further research is required to understand the ecological implications of this phenomenon.

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.

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.

Receptive field sizes and neuronal encoding bandwidth are constrained by axonal conduction delays.

Studies on population coding implicitly assume that spikes from the presynaptic cells arrive simultaneously at the integrating neuron. In natural neuronal populations, this is usually not the case-neuronal signaling takes time and populations cover a certain space. The spread of spike arrival times depends on population size, cell density and axonal conduction velocity. Here we analyze the consequences of population size and axonal conduction delays on the stimulus encoding performance in the electrosensory system of the electric fish Apteronotus leptorhynchus. We experimentally locate p-type electroreceptor afferents along the rostro-caudal body axis and relate locations to neurophysiological response properties. In an information-theoretical approach we analyze the coding performance in homogeneous and heterogeneous populations. As expected, the amount of information increases with population size and, on average, heterogeneous populations encode better than the average same-size homogeneous population, if conduction delays are compensated for. The spread of neuronal conduction delays within a receptive field strongly degrades encoding of high-frequency stimulus components. Receptive field sizes typically found in the electrosensory lateral line lobe of A. leptorhynchus appear to be a good compromise between the spread of conduction delays and encoding performance. The limitations imposed by finite axonal conduction velocity are relevant for any converging network as is shown by model populations of LIF neurons. The bandwidth of natural stimuli and the maximum meaningful population sizes are constrained by conduction delays and may thus impact the optimal design of nervous systems.

Hormonal coordination of motor output and internal prediction of sensory consequences in an electric fish.

Steroid hormones remodel neural networks to induce seasonal or developmental changes in behavior. Hormonal changes in behavior likely require coordinated changes in sensorimotor integration. Here, we investigate hormonal effects on a predictive motor signal, termed corollary discharge, that modulates sensory processing in weakly electric mormyrid fish. In the electrosensory pathway mediating communication behavior, inhibition activated by a corollary discharge blocks sensory responses to self-generated electric pulses, allowing the downstream circuit to selectively analyze communication signals from nearby fish. These pulses are elongated by increasing testosterone levels in males during the breeding season. We induced electric-pulse elongation using testosterone treatment and found that the timing of electroreceptor responses to self-generated pulses was delayed as electric-pulse duration increased. Simultaneous recordings from an electrosensory nucleus and electromotor neurons revealed that the timing of corollary discharge inhibition was delayed and elongated by testosterone. Furthermore, this shift in the timing of corollary discharge inhibition was precisely matched to the shift in timing of receptor responses to self-generated pulses. We then asked whether the shift in inhibition timing was caused by direct action of testosterone on the corollary discharge circuit or by plasticity acting on the circuit in response to altered sensory feedback. We surgically silenced the electric organ of fish and found similar hormonal modulation of corollary discharge timing between intact and silent fish, suggesting that sensory feedback was not required for this shift. Our findings demonstrate that testosterone directly regulates motor output and internal prediction of the resulting sensory consequences in a coordinated manner.

The sensory effects of light on the electric organ discharge rate of Gymnotus omarorum.

Gymnotiformes are nocturnal fishes inhabiting the root mats of floating plants. They use their electric organ discharge (EOD) to explore the environment and to communicate. Here, we show and describe tonic and phasic sensory-electromotor responses to light distinct from indirect effects depending on the light-induced endogenous circadian rhythm. In the dark, principally during the night, inter-EOD interval histograms are bimodal: the main peak corresponds to the basal rate and a secondary peak corresponds to high-frequency bouts. Light causes a twofold tonic but opposing effect on the EOD histogram: (i) decreasing the main mode and (ii) blocking the high-frequency bouts and consequently increasing the main peak at the expense of removal of the secondary one. Additionally, light evokes phasic responses whose amplitude increases with intensity but whose slow time course and poor adaptation differentiate from the so-called novelty responses evoked by abrupt changes in sensory stimuli of other modalities. We confirmed that Gymnotus omarorum tends to escape from light, suggesting that these phasic responses are probably part of a global 'light-avoidance response'. We interpret the data within an ecological context. Fish rest under the shade of aquatic plants during the day and light spots due to the sun's relative movement alert the fish to hide in shady zones to avoid macroptic predators and facilitate tracking the movement of floating plant islands by wind and/or water currents.

An internal model for canceling self-generated sensory input in freely behaving electric fish.

Internal models that predict the sensory consequences of motor actions are vital for sensory, motor, and cognitive functions. However, the relationship between motor action and sensory input is complex, often varying from one moment to another depending on the state of the animal and the environment. The neural mechanisms for generating predictions under such challenging, real-world conditions remain largely unknown. Using novel methods for underwater neural recording, a quantitative analysis of unconstrained behavior, and computational modeling, we provide evidence for an unexpectedly sophisticated internal model at the first stage of active electrosensory processing in mormyrid fish. Closed-loop manipulations reveal that electrosensory lobe neurons are capable of simultaneously learning and storing multiple predictions of the sensory consequences of motor commands specific to different sensory states. These results provide mechanistic insights into how internal motor signals and information about the sensory environment are combined within a cerebellum-like circuitry to predict the sensory consequences of natural behavior.

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.

A new genome assembly of an African weakly electric fish (Campylomormyrus compressirostris, Mormyridae) indicates rapid gene family evolution in Osteoglossomorpha.

BACKGROUND: Teleost fishes comprise more than half of the vertebrate species. Within teleosts, most phylogenies consider the split between Osteoglossomorpha and Euteleosteomorpha/Otomorpha as basal, preceded only by the derivation of the most primitive group of teleosts, the Elopomorpha. While Osteoglossomorpha are generally species poor, the taxon contains the African weakly electric fish (Mormyroidei), which have radiated into numerous species. Within the mormyrids, the genus Campylomormyrus is mostly endemic to the Congo Basin. Campylomormyrus serves as a model to understand mechanisms of adaptive radiation and ecological speciation, especially with regard to its highly diverse species-specific electric organ discharges (EOD). Currently, there are few well-annotated genomes available for electric fish in general and mormyrids in particular. Our study aims at producing a high-quality genome assembly and to use this to examine genome evolution in relation to other teleosts. This will facilitate further understanding of the evolution of the osteoglossomorpha fish in general and of electric fish in particular. RESULTS: A high-quality weakly electric fish (C. compressirostris) genome was produced from a single individual with a genome size of 862 Mb, consisting of 1,497 contigs with an N50 of 1,399 kb and a GC-content of 43.69%. Gene predictions identified 34,492 protein-coding genes, which is a higher number than in the two other available Osteoglossomorpha genomes of Paramormyrops kingsleyae and Scleropages formosus. A Computational Analysis of gene Family Evolution (CAFE5) comparing 33 teleost fish genomes suggests an overall faster gene family turnover rate in Osteoglossomorpha than in Otomorpha and Euteleosteomorpha. Moreover, the ratios of expanded/contracted gene family numbers in Osteoglossomorpha are significantly higher than in the other two taxa, except for species that had undergone an additional genome duplication (Cyprinus carpio and Oncorhynchus mykiss). As potassium channel proteins are hypothesized to play a key role in EOD diversity among species, we put a special focus on them, and manually curated 16 Kv1 genes. We identified a tandem duplication in the KCNA7a gene in the genome of C. compressirostris. CONCLUSIONS: We present the fourth genome of an electric fish and the third well-annotated genome for Osteoglossomorpha, enabling us to compare gene family evolution among major teleost lineages. Osteoglossomorpha appear to exhibit rapid gene family evolution, with more gene family expansions than contractions. The curated Kv1 gene family showed seven gene clusters, which is more than in other analyzed fish genomes outside Osteoglossomorpha. The KCNA7a, encoding for a potassium channel central for EOD production and modulation, is tandemly duplicated which may related to the diverse EOD observed among Campylomormyrus species.

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.