Response properties of electrosensory neurons in the lateral mesencephalic nucleus of the paddlefish

Many fishes and amphibians are able to sense weak electric fields from prey animals or other sources. The response properties of primary afferent fibers innervating the electroreceptors and information processing at the level of the hindbrain is well investigated in a number of taxa. However, there are only a few studies in higher brain areas. We recorded from electrosensory neurons in the lateral mesencephalic nucleus (LMN) and from neurons in the dorsal octavolateral nucleus (DON) of the paddlefish. We stimulated with sine wave stimuli of different amplitudes and frequencies and with moving DC stimuli. During sinusoidal stimulation, DON units increased their firing rate during the negative cycle of the sine wave and decreased their firing rate to the positive cycle. Lateral mesencephalic nucleus units increased their rate for both half cycles of the sine wave. Lateral mesencephalic nucleus units are more sensitive than DON units, especially to small moving dipoles. Dorsal octavolateral nucleus units respond to a moving DC dipole with an increase followed by a decrease in spike rate or vice versa, depending on movement direction and dipole orientation. Lateral mesencephalic nucleus units, in contrast, increased their discharge rate for all stimuli. Any change in discharge rate of DON units is converted in the LMN to a discharge rate increase. Lateral mesencephalic nucleus units therefore appear to code the presence of a stimulus regardless of orientation and motion direction.     

Medullary electrosensory processing in the little skate

1. Previous studies have demonstrated that the resting activity of electrosensory ALLN fibers is modulated by the animal's own respiratory activity and that all fibers innervating a single ampullary cluster are modulated with the same amplitude and phase relationship to ventilation. We demonstrate that ALLN fibers in the skate are modulated in this common-mode manner bilaterally, regardless of receptor group, orientation, or position of the receptor pore on the body surface (Fig. 2). 2. Ascending efferent neurons (AENs), which project to the electrosensory midbrain from the DON, are modulated through a much smaller portion of their dynamic range. AENs give larger responses to an extrinsic local electric field than to the respiratory driving, indicating that a mechanism exists for suppressing ventilatory electrosensory reafference. 3. In paralyzed animals no modulation of resting activity or of responses of extrinsic electric fields could be observed with respect to the animal's respiratory motor commands in the absence of electrosensory reafference. 4. Cells of the dorsal granular ridge (DGR) project to medullary AENs via the DON molecular layer. A majority of proprioceptive DGR neurons are modulated by ventilatory activity, however, in a given fish the modulation is not in the same phase relationship to ventilation among DGR units. 5. The modulation of AENs during respiration was increased following transection of the contralateral ALLN (Fig. 9). Resting activity and responses to excitatory stimuli were inhibited by simultaneous stimulation of the transected contralateral ALLN indicating that a common-mode rejection mechanism is mediated via the commissural interconnections of the DONs.     

Receptive field organization of electrosensory neurons in the paddlefish (Polyodon spathula)

Paddlefish use their electrosense to locate small water fleas (daphnia), their primary prey, in three-dimensional space. High sensitivity and a representation of object location are essential for this task. High sensitivity can be achieved by convergence of information from a large number of receptors and object location is usually represented in the nervous system by topographic maps. However the first electrosensory center in the brain, the dorsal octavolateral nucleus in the hindbrain, is neither topographically organized nor does it show a higher sensitivity than primary afferent fibers. Here, we investigated the response properties of electrosensory neurons in the dorsal octavolateral nucleus (DON), the lateral mesencephalic nucleus (LMN) and the tectum mesencephali (TM). LMN units are characterized by large receptive fields, which suggest a high degree of convergence. TM units have small receptive fields and are topographically arranged, at least in the rostro-caudal axis, the only dimension we could test. Well-defined receptive fields, however, could only be detected in the TM with a moving DC stimulus. The receptive fields of TM units, as determined by slowly scanning the rostrum and head with a 5 Hz stimulus, were very large and frequently two or more receptive fields were present. The receptive fields for LMN units were located in the anterior half of the rostrum whereas TM units had receptive fields predominantly on the head and at the base of the rostrum. A detailed analysis of the prey catching behavior revealed that it consists of two phases that coincide with the location of the receptive fields in LMN and TM, respectively. This suggests that LMN units are responsible for the initial orienting response that occurs when the prey is alongside the anterior first half of the rostrum. TM units, in contrast, had receptive fields at locations where the prey is located when the fish opens its mouth and attempts the final strike.     

Ionic and neuromodulatory regulation of burst discharge controls frequency tuning

Sensory neurons encode natural stimuli by changes in firing rate or by generating specific firing patterns, such as bursts. Many neural computations rely on the fact that neurons can be tuned to specific stimulus frequencies. It is thus important to understand the mechanisms underlying frequency tuning. In the electrosensory system of the weakly electric fish, Apteronotus leptorhynchus, the primary processing of behaviourally relevant sensory signals occurs in pyramidal neurons of the electrosensory lateral line lobe (ELL). These cells encode low frequency prey stimuli with bursts of spikes and high frequency communication signals with single spikes. We describe here how bursting in pyramidal neurons can be regulated by intrinsic conductances in a cell subtype specific fashion across the sensory maps found within the ELL, thereby regulating their frequency tuning. Further, the neuromodulatory regulation of such conductances within individual cells and the consequences to frequency tuning are highlighted. Such alterations in the tuning of the pyramidal neurons may allow weakly electric fish to preferentially select for certain stimuli under various behaviourally relevant circumstances.     

A Biophysical Model of Adaptive Noise Filtering in the Shark Brain

Sharks detect their prey using an extremely sensitive electrosensory system that is capable of distinguishing weak external stimuli from a relatively strong background noise generated by the animal itself. Experiments indicate that part of the shark’s hindbrain, the dorsal octavolateralis nucleus (DON), is responsible for extracting the external stimulus using an adaptive filter mechanism to suppress signals correlated with the shark’s breathing motion. The DON’s principal neuron integrates input from afferents as well as many thousands of parallel fibres transmitting, inter alia, breathing-correlated motor command signals. There are a number of models in the literature, studying how this adaptive filtering mechanisms occurs, but most of them are based on a spike-train model approach. This paper presents a biophysically based computational simulation which demonstrates a mechanism for adaptive noise filtering in the DON. A spatial model of the neuron uses the Hodgkin–Huxley equations to simulate the propagation of action potentials along the dendrites. Synaptic inputs are modelled by applied currents at various positions along the dendrites, whose input conductances are varied according to a simple learning rule. Simulation results show that the model is able to demonstrate adaptive filtering in agreement with previous experimental and modelling studies. Furthermore, the spatial nature of the model does not greatly affect its learning properties, and in its present form is effectively equivalent to an isopotential model which does not incorporate a spatial element.     

Information-processing demands in electrosensory and mechanosensory lateral line systems.

The electrosensory and mechanosensory lateral line systems of fish exhibit many common features in their structural and functional organization, both at the sensory periphery as well as in central processing pathways. These two sensory systems also appear to play similar roles in many behavioral tasks such as prey capture, orientation with respect to external environmental cues, navigation in low-light conditions, and mediation of interactions with nearby animals. In this paper, we briefly review key morphological, physiological, and behavioral aspects of these two closely related sensory systems. We present arguments that the information processing demands associated with spatial processing are likely to be quite similar, due largely to the spatial organization of both systems and the predominantly dipolar nature of many electrosensory and mechanosensory stimulus fields. Demands associated with temporal processing may be quite different, however, due primarily to differences in the physical bases of electrosensory and mechanosensory stimuli (e.g. speed of transmission). With a better sense of the information processing requirements, we turn our attention to an analysis of the functional organization of the associated first-order sensory nuclei in the hindbrain, including the medial octavolateral nucleus (MON), dorsal octavolateral nucleus (DON), and electrosensory lateral line lobe (ELL). One common feature of these systems is a set of neural mechanisms for improving signal-to-noise ratios, including mechanisms for adaptive suppression of reafferent signals. This comparative analysis provides new insights into how the nervous system extracts biologically significant information from dipolar stimulus fields in order to solve a variety of behaviorally relevant problems faced by aquatic animals.     

Medullary electrosensory processing in the little skate

1. Ampullary electroreceptors in elasmobranchs are innervated by fibers of the ALLN, which projects to the dorsal octavolateralis nucleus (DON). The purpose of this study is to examine the response characteristics of ALLN fibers and DON neurons to weak D.C. and sinusoidal electric field stimuli presented as local dipole fields. 2. ALLN fibers respond to presentation of D.C. fields with a phasic burst, followed by a more slowly adapting period of firing. Ascending efferent neurons (AENs) in the DON respond to stimuli with a similar initial burst, which adapts more quickly. 3. Type 1, 2, and 3 neurons are possible local interneurons or commissural DON neurons. Type 1 neurons demonstrate response properties similar to those of AENs. Type 2 cells demonstrated slowly adapting responses to excitatory stimuli, the duration of the response increased with the amplitude of the stimulus. Type 3 neurons demonstrated an increased rate of firing, but the response lacked any specific temporal characteristics. 4. ALLN fibers typically have receptive fields consisting of a single ampulla. The receptive field sizes of DON neurons exhibited varying degrees of convergence for different cell types. 5. Responses of ALLN fibers and DON neurons to weak sinusoidal stimuli demonstrated very similar frequency response characteristics for all cell types. The peak sensitivity of electrosensory neurons was between 5-10 Hz.     

Vision and electroreception: Integration of sensory information in the optic tectum of the weakly electric fishApteronotus albifrons

Summary The responses of single neurons to visual and electrosensory stimulation were studied in the optic tectum of the weakly electric fishApteronotus albifrons. Most of the cells recorded in the region of the tectum studied, the anterior medial quadrant, were poorly responsive or completely insensitive to flashes of light or to bursts of AC electrical stimuli applied to the entire fish. However, these cells gave vigorous responses to moving visual or electrosensory stimuli. Most cells showed differences in their response contingent upon the direction of the stimulus movement and most received input from both the visual and electrosensory systems. Electrosensory responses to moving stimuli were depressed by jamming stimuli, 4 Hz amplitude modulation of the animal's electric organ discharge, presented simultaneously with the moving stimulus. However, the jamming signal presented alone typically evoked no response. Moving visual stimuli, presented simultaneously with the electrosensory, were usually able to restore the magnitude of a response toward its value in the unjammed situation. For most of the cells studied the receptive fields for vision and electroreception were in register. In some cases the visual and electrosensory components could be separated by presenting the two types of stimuli separately, or by presenting both simultaneously but with some amount of spatial separation, which causes the two to be misaligned relative to the fish. In other cases the individual responses could not be separated by spatial manipulations of the two stimuli and in these cases differences in the alignment of the two types of stimuli could cause changes in the intensity of the cells' responses.Abbreviations AM amplitude modulation - EOD electric organ discharge - PLLL posterior lateral line lobe     

Electric organ corollary discharge pathways in mormyrid fish

Corollary discharge signals associated with the motor command that elicits the electric organ discharge are prominent in the electrosensory lobe of mormyrid fish (Gnathonemus petersii). Central pathways and structures that convey these signals from the motor command nucleus to the electrosensory lobe are known anatomically, but these structures and their contributions to the various corollary discharge phenomena have not been examined physiologically. This study examines one such structure, the mesencephalic command associated nucleus (MCA).Recordings from MCA cells show a highly stereotyped two spike response. The first spike of the response has a latency of about 2.5 ms following the initiation of the electric organ discharge (EOD) motor command which is about 5.5 ms before the occurrence of the EOD.Results from stimulation and lesion experiments indicate that MCA is responsible for: 1) the gate-like corollary discharge-driven inhibition of the knollenorgan pathway; 2) the gate-like corollary discharge-driven excitation of granule cells in the mormyromast regions of the electrosensory lobe; and 3) various excitatory effects on other cells in the mormyromast regions.Some corollary discharge phenomena are still present after MCA lesions, including the earliest corollary discharge effects and the plasticity that follows pairing with electrosensory stimuli. These phenomena must be mediated by structures other than MCA.Abbreviations BCA bulbar command associated nucleus - C EOD motor command - C3 central cerebellar lobule 3 - COM EOD motor command nucleus - DLZ dorsolateral zone of ELL cortex - EGa eminentia granularis anterior - EGp eminentia granularis posterior - ELa nucleus exterolateralis anterior - ELL electrosensory lobe - ELLml molecular layer of ELL cortex - EOD electric organ discharge - gang ganglion layer - gran granule layer - jlem juxtalemniscal region - JLl lateral juxtalobar nucleus - JLm medial juxtalobar nucleus - lat nucleus lateralis - ll lateral lemniscus - MCA mesencephalic command associated nucleus - mol molecular layer - MOml molecular layer of the medial octavolateral nucleus - MRN medullary relay nucleus - MZ medial zone of ELL cortex - nALL anterior lateral line nerve - NELL nucleus of the electrosensory lobe - nX cranial nerve X (vagus) - OT optic tectum - PCA paratrigeminal command associated nucleus - pee praeeminentialis electrosensory tract - plex plexiform layer - prae nucleus praeeminentialis - sublem sublemniscal nucleus - TEL telencephalon - VLZ ventrolateral zone of ELL cortex - vped valvular peduncle     

Time coding in the midbrain of mormyrid electric fish. I. Physiology and anatomy of cells in the nucleus exterolateralis pars anterior

In mormyrid electric fish, species-specific electric organ discharge waveforms are thought to be analyzed by the Knollenorgan electroreceptor subsystem. The midbrain anterior and posterior exterolateral nuclei (ELa and ELp) are thought to be the sites of this analysis. This paper is an electrophysiological study of the properties of the neurons in ELa. We recorded intracellularly from three classes of cells within ELa: the afferent axons from the nucleus of the electrosensory lateral line lobe (NELL), the large interstitial cells of ELa and an unidentified cell type. The large cells and the NELL axons were identified by intracellular injection of biocytin and are physiologically similar. Cells in ELa responded to square pulse stimuli with one or more time-locked action potentials with 2.8–3.0 ms latency. Both large cells and NELL axons arborized extensively in ELa and contacted numerous small cells. Based on the pattern of arborizations, we constructed a counter- current flow model of temporal coding by the small cells of ELa. We postulate that individual small cells are not selectively tuned for specific stimulus durations, but rather, the firing patterns of groups of small cells must be analyzed by neurons further up in the sensory hierarchy to determine the stimulus duration. Accepted: 25 June 1997     

Orientation in the dark: brain circuits involved in the perception of electric signals in mormyrid electric fish.

Weakly electric fish produce electric signals with a specialised organ in their tail. In addition, they are electrosensitive and can perceive their self-generated signals (for electrolocation) and electric signals of other electric fishes (for electrocommunication). Mormyrids possess three types of peripheral electroreceptor organs, one used for electrocommunication and two types involved in electolocation. They are innervated by afferent fibres, which project to different zones in the electrosensory lateral line lobe (ELL) in the medulla. Brain circuits for electrolocation and electrocommunication are separated almost throughout the whole brain. Electrolocation pathways run from the ELL-cortex to the torus semicircularis of the midbrain and then via the valvula cerebelli towards the telencephalon. Pathways involved in electrocommunication run from the nucleus of the ELL to another part of the torus and from there through the isthmic granule nucleus to the valvula. In addition, a pathway via the preglomerular complex to the telencephalon might exist. In both the electrolocation and the electrocommunication circuits, prominent recurrent pathways are present.     

Response properties of diencephalic neurons to visual, acoustic and hydrodynamic stimulation in the goldfish, Carassius auratus

We studied the responses to sensory stimulation of three diencephalic areas, the central posterior nucleus of the dorsal thalamus, the anterior tuberal nucleus of the hypothalamus, and the preglomerular complex. Units sensitive to acoustic (500 Hz tone burst), hydrodynamic (25 Hz dipole stimulus) and visual (640 nm light flash) stimuli were found in both the central posterior and anterior tuberal nucleus. In contrast, unit responses or large robust evoked potentials confined to the preglomerular complex were not found. In the central posterior nucleus, most units were unimodal. Many units responded exclusively to visual stimulation and exhibited a variety of temporal response patterns to light stimuli. In the anterior tuberal nucleus of the hypothalamus, most units responded to more than one modality and showed a stronger response decrement to stimulus repetitions than units in the central posterior nucleus. Our data suggest that units in the central posterior nucleus are primarily involved in the unimodal processing of sensory information whereas units in the anterior tuberal nucleus of the hypothalamus may be involved in multisensory integration.     

Electrosensitive spatial vectors in elasmobranch fishes: implications for source localization

The electrosense of sharks and rays is used to detect weak dipole-like bioelectric fields of prey, mates and predators, and several models propose a use for the detection of streaming ocean currents and swimming-induced fields for geomagnetic orientation. We assessed pore distributions, canal vectors, complementarity and possible evolutionary divergent functions for ampullary clusters in two sharks, the scalloped hammerhead (Sphyrna lewini) and the sandbar shark (Carcharhinus plumbeus), and the brown stingray (Dasyatis lata). Canal projections were determined from measured coordinates of each electrosensory pore and corresponding ampulla relative to the body axis. These species share three ampullary groups: the buccal (BUC), mandibular (MAN) and superficial ophthalmic (SO), which is subdivided into anterior (SOa) and posterior (SOp) in sharks. The stingray also has a hyoid (HYO) cluster. The SOp in both sharks contains the longest (most sensitive) canals with main projections in the posterior-lateral quadrants of the horizontal plane. In contrast, stingray SO canals are few and short with the posterior-lateral projections subsumed by the HYO. There was strong projection coincidence by BUC and SOp canals in the posterior lateral quadrant of the hammerhead shark, and laterally among the stingray BUC and HYO. The shark SOa and stingray SO and BUC contain short canals located anterior to the mouth for detection of prey at close distance. The MAN canals of all species project in anterior or posterior directions behind the mouth and likely coordinate prey capture. Vertical elevation was greatest in the BUC of the sandbar shark, restricted by the hammerhead cephalofoil and extremely limited in the dorsoventrally flattened stingray. These results are consistent with the functional subunit hypothesis that predicts specialized ampullary functions for processing of weak dipole and geomagnetic induced fields, and provides an anatomical basis for future experiments on central processing of different forms of relevant electric stimuli.     

The Spatial Structure of Stimuli Shapes the Timescale of Correlations in Population Spiking Activity

Throughout the central nervous system, the timescale over which pairs of neural spike trains are correlated is shaped by stimulus structure and behavioral context. Such shaping is thought to underlie important changes in the neural code, but the neural circuitry responsible is largely unknown. In this study, we investigate a stimulus-induced shaping of pairwise spike train correlations in the electrosensory system of weakly electric fish. Simultaneous single unit recordings of principal electrosensory cells show that an increase in the spatial extent of stimuli increases correlations at short () timescales while simultaneously reducing correlations at long () timescales. A spiking network model of the first two stages of electrosensory processing replicates this correlation shaping, under the assumptions that spatially broad stimuli both saturate feedforward afferent input and recruit an open-loop inhibitory feedback pathway. Our model predictions are experimentally verified using both the natural heterogeneity of the electrosensory system and pharmacological blockade of descending feedback projections. For weak stimuli, linear response analysis of the spiking network shows that the reduction of long timescale correlation for spatially broad stimuli is similar to correlation cancellation mechanisms previously suggested to be operative in mammalian cortex. The mechanism for correlation shaping supports population-level filtering of irrelevant distractor stimuli, thereby enhancing the population response to relevant prey and conspecific communication inputs.     

Distribution of Kv1-like potassium channels in the electromotor and electrosensory systems of the weakly electric fish Apteronotus leptorhynchus

The electromotor and electrosensory systems of the weakly electric fish Apteronotus leptorhynchus are model systems for studying mechanisms of high-frequency motor pattern generation and sensory processing. Voltage-dependent ionic currents, including low-threshold potassium currents, influence excitability of neurons in these circuits and thereby regulate motor output and sensory filtering. Although Kv1-like potassium channels are likely to carry low-threshold potassium currents in electromotor and electrosensory neurons, the distribution of Kv1 alpha subunits in A. leptorhynchus is unknown. In this study, we used immunohistochemistry with six different antibodies raised against specific mammalian Kv1 alpha subunits (Kv1.1-Kv1.6) to characterize the distribution of Kv1-like channels in electromotor and electrosensory structures. Each Kv1 antibody labeled a distinct subset of neurons, fibers, and/or dendrites in electromotor and electrosensory nuclei. Kv1-like immunoreactivity in the electrosensory lateral line lobe (ELL) and pacemaker nucleus are particularly relevant in light of previous studies suggesting that potassium currents carried by Kv1 channels regulate neuronal excitability in these regions. Immunoreactivity of pyramidal cells in the ELL with several Kv1 antibodies is consistent with Kv1 channels carrying low-threshold outward currents that regulate spike waveform in these cells (Fernandez et al., J Neurosci 2005;25:363-371). Similarly, Kv1-like immunoreactivity in the pacemaker nucleus is consistent with a role of Kv1 channels in spontaneous high-frequency firing in pacemaker neurons. Robust Kv1-like immunoreactivity in several other structures, including the dorsal torus semicircularis, tuberous electroreceptors, and the electric organ, indicates that Kv1 channels are broadly expressed and are likely to contribute significantly to generating the electric organ discharge and processing electrosensory inputs.     

Responses of cells in the mormyrid electrosensory lobe to EODs with distorted waveforms: implications for capacitance detection

Mormyrid fish (Gnathonemus petersii) can discriminate between ohmic and capacitive electrical objects during active electrolocation. The neural basis of this ability was investigated by recording cells in the dorsolateraland medial zones of the electrosensory lobe. Natural electric organ discharges (EODs) distorted by capacitive objects and EODs with computer generated phase shifts were used as stimuli.Cells in the dorsolateral zone were very sensitive to phase shifted EODs with constant amplitude spectra. Phase shifts as small as 1 were effective. Cells in this zone also responded more to EODs with capacitance induced distortions than to non-distorted EODs. These effects were very similar to the effects on B-type primary afferents from mormyromast electroreceptors which project to this zone.Cells in the medial zone were not sensitive to phase shifted EODs. Capacitance induced waveform distortions were effective, but the effect of such distortions was opposite to the effect on dorsolateral zone cells. These effects were very similar to the effects on A-type primary afferents from mormyromast electroreceptors which project to this zone.The results show that peripheral information about capacitive objects is preserved in the electrosensory lobe, but do not indicate any further processing of capacitive information in the lobe.     

Ultrastructure and cytochemistry of salivary glands of the predator Podisus nigrispinus (Hemiptera: Pentatomidae)

Podisus nigrispinus Dallas (Hemiptera: Pentatomidae) is a zoophytophagous insect with a potential for use as a biological control agent in agriculture because nymphs and adults actively prey on various insects by inserting mouthparts and regurgitating the contents of the salivary glands inside the prey, causing rapid paralysis and death. However, the substances found in saliva of P. nigrispinus that causes the death of the prey are unknown. As a first step to identify the component of the saliva of P. nigrispinus, this study evaluated the ultrastructure and cytochemistry of the salivary glands of P. nigrispinus. The salivary system of P. nigrispinus has a pair of principal salivary glands, which are bilobed with a short anterior lobe and a long posterior lobe, and a pair of tubular accessory glands. The principal gland epithelium is composed of a single layer of cells enclosing a large lumen. Epithelial cells of the principal salivary gland vary from cubic to columnar shape, with one or two spherical and well-developed nuclei. Cells of the anterior lobe of the principal salivary gland have an apical surface with narrow, short, and irregular plasma membrane foldings; apical and perinuclear cytoplasm rich in rough endoplasmic reticulum; and mitochondria with tubular cristae. The basal portion of the secretory cells has mitochondria associated with many basal plasma membrane infoldings that are short but form large extracellular canals. Secretory granules with electron-dense core and electron-transparent peripheral are dispersed throughout the cytoplasm. Cells of the posterior lobe of the principal salivary gland are similar to those of the anterior lobe, except for the presence of mitochondria with transverse cristae. The accessory salivary gland cells are columnar with apical microvilli, have well-developed nucleus and cytoplasm rich in rough endoplasmic reticulum, and have secretory granules. Cytochemical tests showed positive reactions for carbohydrate, protein, and acid phosphatase in different regions of the glandular system. The principal salivary glands of P. nigrispinus do not have muscle cells attached to its wall, suggesting that saliva-releasing mechanism may occurs with the participation of some thorax muscles. The cytochemical and ultrastructural features suggest that the principal and accessory salivary glands play a role in protein synthesis of the saliva.     

Neuroethology and life history adaptations of the elasmobranch electric sense.

The electric sense of elasmobranch fishes (sharks and rays) is an important sensory modality known to mediate the detection of bioelectric stimuli. Although the best known function for the use of the elasmobranch electric sense is prey detection, relatively few studies have investigated other possible biological functions. Here, we review recent studies that demonstrate the elasmobranch electrosensory system functions in a wide number of behavioral contexts including social, reproductive and anti-predator behaviors. Recent work on non-electrogenic stingrays demonstrates that the electric sense is used during reproduction and courtship for conspecific detection and localization. Electrogenic skates may use their electrosensory encoding capabilities and electric organ discharges for communication during social and reproductive interactions. The electric sense may also be used to detect and avoid predators during early life history stages in many elasmobranch species. Embryonic clearnose skates demonstrate a ventilatory freeze response when a weak low-frequency electric field is imposed upon the egg capsule. Peak frequency sensitivity of the peripheral electrosensory system in embryonic skates matches the low frequencies of phasic electric stimuli produced by natural fish egg-predators. Neurophysiology experiments reveal that electrosensory tuning changes across the life history of a species and also seasonally due to steroid hormone changes during the reproductive season. We argue that the ontogenetic and seasonal variation in electrosensory tuning represent an adaptive electrosensory plasticity that may be common to many elasmobranchs to enhance an individual's fitness throughout its life history.     

The coding of signals in the electric communication of the gymnotiform fish Eigenmannia: From electroreceptors to neurons in the torus semicircularis of the midbrain

Summary In the context of aggression and courtship, Eigenmannia repeatedly interrupts its electric organ discharges (EODs) These interruptions (Fig. 1) contain low-frequency components as well as high-frequency transients and, therefore, stimulate ampullary and tuberous electroreceptors, respectively (Figs. 2, 3). Information provided by these two classes of receptors is relayed along separate pathways, via the electrosensory lateral line lobe (ELL) of the hindbrain, to the dorsal torus semicircularis (TSd) of the midbrain. Some neurons of the torus receive inputs from both types of receptors (Figs. 14, 15), and some respond predominantly to EOD interruptions while being rather insensitive to other forms of signal modulations (Figs. 12, 13). This high selectivity appears to result from convergence and gating of inputs from individually less selective neurons.Abbreviations CP central posterior thalamic nucleus - Df frequency difference between neighbor's EOD and fish's own - DPn dorsal posterior nucleus (thalamus) - EOD electric organ discharge - ELL electrosensory lateral line lobe - JAR jamming avoidance response - LMR lateral mesencephalic reticular formation - nE nucleus electrosensorius - nEb nucleus electrosensorius, beat-related area - nE nucleus electrosensorius, area causing rise of EOD frequency - nE nucleus electrosensorius, area causing fall of EOD frequency - nEar nucleus electrosensorius-acusticolateralis area - NPd nucleus praeeminentialis, pars dorsalis - PPn prepacemaker nucleus - PT pretectal nucleus - SE nucleus subelectrosensorius - TeO optic tectum - TSd dorsal (electrosensory) torus semicircularis - TSv ventral (mechano-sensory and auditory) torus semicircularis     

A role for GABAergic inhibition in electrosensory processing and common mode rejection in the dorsal nucleus of the little skate, Raja erinacea

The electrosensory primary afferents in elasmobranchs are responsive to electric potentials created by the animal's own ventilation, while the second-order neurons (AENs) which receive this afferent input in the medulla suppress responses to ventilatory potentials but retain their extreme sensitivity to electric signals in the environment. Ventilatory potentials are common mode signals in elasmobranchs and a common mode rejection mechanism is one way the AENs suppress ventilatory noise. By pressure injecting the GABA-A receptor antagonist SR95531 while extracellularly recording from AENs, we tested the hypothesis that the subtractive circuitry that selectively reduces common mode signals in AENs utilizes GABA, and that a GAB-Aergic component of the dorsal nucleus commissural pathway mediates crossed inhibition of AENs. Local application of SR95531 increased the spontaneous activity and the responsiveness of AENs to electrosensory stimuli. AEN responses to a common mode stimulus were selectively increased compared to responses to a localized stimulus due to SR95531 application. Contralateral inhibition of AENs was blocked by SR95531, indicating that GABAergic commissural cells may inhibit AENs when the contralateral side of the body is stimulated, as with common mode stimulation. We conclude that GABAergic inhibition contributes significantly to the shaping of AEN responses including common mode rejection.Abbreviations AENs ascending efferent neurons - GABA gamma-aminobutyric acid