Electroreceptor Cells Research Articles

Cellular patterning and cyto-architectural organization of the skin of electric catfish (Malapterurus electricus, Siluriformes) with a particular emphasis on its ampullary electroreceptor

The functional morphology of the skin of Malapteruridae is presumably evolved to cope with a diversified range of ambient physiological, environmental, and behavioral conditions. Herein, we firstly characterized the microstructures and intriguing patterning of the skin of twelve adult electric catfish (Malapterurus electricus, Malapteruridae) using histological, histochemical, immunofluorescent, and ELISA standard methodology. The skin comprises three sequentially–oriented layers: the epidermis, dermis, and hypodermis with a significantly increased thickness of the former. The epidermis contains four types of cells: the surface epithelial cells, mucous cells, granular cells, and club cells. We defined distinctive ampullary electroreceptors in the outer epidermis that possess flask-shaped sensory crypt containing electroreceptor cells together with vertical collagen rods. Dermis and hypodermis are composed of connective tissue; however, the former is much more coarse and dense with comparable reactivity for Masson–Goldner trichrome (MT). Placing our data in the context of the limited body of previous work, we showed subtle changes in the expression of mucin subunits together with cytoskeletal fractions of collagens, myosin, F-actin, keratins, and tubulins. Taken as a whole, our results convincingly showed that the skin of M. electricus shares some structural similarities to other Siluriformes, however, it has some functional modifications that are implicated in protection, defense, and foraging behavior.

Behavioural relevance of AC and DC in prey detection by the brown bullhead, Ameiurus nebulosus

Abstract A large range of aquatic vertebrates employs passive electroreception to detect the weak bioelectric fields that surround their prey. Bioelectric fields are dynamic in strength and frequency composition, but typically consist of a direct current (DC) and an alternating current (AC) component. We examined the biological relevance of these components for prey detection behaviour in the brown bullhead by means of a preference test. We gave each fish the choice between two small dipoles emitting a DC step or AC stimulus of variable strength, respectively. We used AC stimuli that were either representative for ventilatory movements by prey (1 Hz sine wave) or optimal for the ampullary electroreceptor cells (10 Hz sine wave). In an attempt to present a more complex stimulus, we also used slightly modified recordings of bioelectric prey fields, but this yielded no results. Brown bullheads prefer DC stimuli to 10 Hz sine waves if the stimulus intensity of either component is much larger. When the stimulus presentation consists of DC versus 1 Hz, most fish will choose randomly unless the stimulus intensities differ greatly. Then, they favour the component that had a higher amplitude during training. Our results suggest an intrinsic behavioural preference for very low frequency signals (<10 Hz) as well as plasticity in prey detection behaviour.

Open lumina of microampullary electroreceptor organs in the transparent catfish, Kryptopterus bicirrhis (Valenciennes 1840), contain 50 mM potassium: why?

Abstract Administration of the cell impermeant fluorescent K+ and Na+ probes potassium-binding fluorescent indicator (PBFI) and sodium-binding fluorescent indicator (SBFI) to the lumina of ampullary electroreceptor organs in the transparent catfish Kryptopterus bicirrhis (Valenciennes 1840), demonstrated an unexpected high concentration of K+ ions: 50 mM. Since the lumina of the ampullary organs are in open contact with the surrounding water, such a high K+ concentration inside the lumen can be maintained only by heavy metabolic transport. The implications of this finding for stimulus transduction in freshwater ampullary electroreceptor cells are discussed.

Evidence for transmitter operated electrical synapses (TOES) in ampullary electroreceptor organs

Afferent fibres of ampullary electroreceptor organs in electrosensitive fish fire spontaneously, that is, they fire without external stimulus. In the past it has been postulated that the spontaneous activity originates from a sustained level of neurotransmitter release delivered by the electroreceptor cells. The spontaneous activity can be modulated by electrical stimuli. Blocking of the chemical synapse, however, reduces the susceptibility to electrical stimuli to 2% or less, but the spontaneous activity to 60% only. By evaluating existing experimental evidence it is concluded that spontaneous firing of afferents is based on two processes. (1) A membrane bound oscillator, which does not depend on transmitter release, is almost free of frequency fluctuations, and is described by Hodgkin/Huxley-equations (HH-equations). (2) Release of neurotransmitter, which increases the firing level, adds frequency noise, and raises the susceptibility of the afferent to electrical stimuli. There is evidence that neurotransmitter release acts as a gating process, which makes the generator area of the afferents directly accessible to electrical stimuli from the outside. Apparently, the activated synapse behaves as a transmitter operated electrical synapse (TOES).

Simultaneous measurements of calcium mobilization and afferent nerve activity in electroreceptor organs of anesthetized Kryptopterus bicirrhis

The transduction pathway of ampullary electroreceptor organs involves ionic currents. It has been shown that calcium, as well as sodium and potassium play important parts in this process. In this study we examine the stimulus-evoked changes in the Fura-2 ratio in electroreceptor cells. Furthermore, we recorded stimulus-evoked Fura-2 ratio changes while Na + and K + channels were blocked by amiloride and TEA. Simultaneously, extracellular recordings of the afferent spike activity were made. The results show the presence of stimulus evoked fluctuations in the Fura-2 ratio. These fluctuations can be abolished by the application of Cd 2+, TEA, and amiloride. The stimulus-evoked activity of the afferent nerve was decreased due to application of these drugs. We conclude that the transduction current is carried by Na +, K +, and probably Ca 2+. This fits the existing model on transduction in electroreceptors.

Waveform tuning of electroreceptor cells in the weakly electric fish, Gnathonemus petersii

Electroreceptive afferents from A- and B-electroreceptor cells of mormyromasts and Knollenorgans were tested for their sensitivity to different stimulus waveforms in the weakly electric fish Gnathonemus petersii. Both A- and B-mormyromast cells had their lowest sensitivity to a waveform similar to the self-generated electric organ discharge (EOD) (around 0° phase-shift). Highest sensitivities, i.e. lowest response thresholds, in both A- and B-cells were measured at phase shifts of +135°. Thus, both cell types were inversely waveform tuned. The sensitivity of B-cells increased sharply with increasing waveform distortions. Their tuning curves had a sharp minimum of sensitivity at +7° phase shift. A-cells had a much broader waveform tuning with a plateau level of low sensitivity from +24° to −15°. Across a 360° cycle of phase-shifts, the range of thresholds was 16 dB for individual B-cells and 4.5 dB for individual A-cells. Knollenorgan afferents were tuned to 0° phase-shifted EODs and had a dynamic range of 12 dB. Lowest sensitivities were measured at a phase shift of +165°. Experiments with computer-generated stimuli revealed that the strong sensitivity of mormyromast B-cells of EOD waveform distortions cannot be attributed to any of the seven waveform parameters tested. In addition, EOD stimuli must have the correct duration for B-cells to respond to waveform distortions. Thus, waveform tuning appears to be based on the specific combination of several waveform parameters that occur only with natural EODs.

Converging electroreceptor cells improve sensitivity and tuning

We studied the effect of convergent clustering of ampullary electroreceptor organs on stimulus transduction and transmission in the catfish Icalurus melas by electrophysiologically recording primary afferent activity of single ampullae (singlets) and pairs (doublets) innervated by the same afferent. Doublets were twice as sensitive as singlets, and showed sharper tuning around the best frequency. The slope of the phase curve in the doublets was slightly steeper than in the singlets. The spontaneous activity and scatter in interspike interval were not correlated with clustering. The implications of these findings for signal averaging in sensory neurons and their relevance for behaviour are discussed.

Caffeine reduces the efficacy of electroreceptor cell synapses: an electrophysiological single-unit in vivo study

Ampullary electroreceptor organs of catfish, Ictalurus melas, were exposed apically to caffeine solutions at concentrations of 0, 5, 7.5, 10, and 15 mM. Recording sinusoidally-modulated activity of single-unit afferents reveals a dose-dependent decrease in mean afferent activity and sensitivity. A rebound effect of average activity occurs after caffeine is washed out. After 25 min exposure to 15 mM caffeine the peak of the gain curve shifts from 8 Hz to 4 Hz. The corresponding phase characteristic shows an increased phase lag with a maximum shift of 35 degrees at 20 Hz. The latency between stimulus and response increases from 12 to 19 ms; the recovery time after onset of the pulse decreases with 60 ms. The most probable explanation for the recorded effects is that caffeine reduces the availability of intracellular Ca 2+ by blocking of the inositol triphosphate receptors in the endoplasmic reticulum. This in turn would affect many intracellular properties and processes. The unavailability of Ca 2+ could reduce the synaptic efficacy and increase latency by suppressing fusion of synaptic vesicles with the presynaptic membrane and by depressing vesicle transport. The change in frequency response corresponds in part to reduction of the apical membrane surface area of the receptor cells, and in part to the increased latency. Accumulation of glutamate-containing vesicles could account for the higher mean activity and modulation amplitude in the lower frequency range after caffeine is washed out. Caffeine might act postsynaptically by inducing hyperpolarization of the terminals of the primary afferents.

The effect of hyperosmotic treatment on the functioning of ampullary electroreceptor organs.

The condition of the microvilli of the apical membrane in ampullary electroreceptor organs of the freshwater catfish, Ictalurus nebulosus, was manipulated by administration of hyperosmotic solutions (500 mM) in order to investigate to what extent the apical surface area determines the sensitivity and the shape of the frequency curves. Electrophysiological recordings demonstrated that hyperosmotic sucrose and glucose solutions reduce the sensitivity by 80%, and alter the shape of the frequency curves. The synaptic delay and the average spontaneous activity remained almost unchanged. Calculations with an equivalent electric circuit of an electroreceptor cell indicate that a reduction in apical surface area can account for these effects. Apparently, the sensitivity and the frequency curves in ampullary organs in freshwater catfish depend on the apical surface area. A hyperosmotic urea solution proved to be less suitable to investigate the effect of manipulations of the apical surface area. The almost total loss in sensitivity caused by administration of urea is caused by both shunting of the stimulus by leaky tight junctions and a reduced synaptic efficacy. The repetitive activity observed after exposure to glucose or sucrose must be attributed to malfunctioning of postsynaptic parts of the receptor organ.

Vincristine disturbs spontaneous firing of the afferent nerve fibre in ampullary electroreceptor organs

Ampullary electroreceptor organs of the catfish were apically exposed to 0.3 mM vincristine in order to investigate the part played by the microtubular system in stimulus transduction. The main effects were repetitive firing of the afferent fibre, a reduction of the mean spontaneous activity and a reduction of the spike amplitude two to four days after exposure to vincristine. The mean sensitivity was less susceptible to vincristine than the spontaneous activity. Since the shape of the frequency curves remained unchanged and similar effects as described above were also observed after denervation, we conclude that vincristine most likely does not affect electroreceptor cell functioning, but causes degeneration of the afferent fibre.

Microvilli in electroreceptor organs in Ictalurus nebulosus play a part in signal filtering

Ampullary electroreceptor organs of catfish show a band-pass-filter characteristic on sinusoidal electric stimulation. The structures and processes which are responsible for the frequency characteristics are not fully understood. To investigate the role of the apical membrane and its microvilli in signal filtering, the ampullary organs were apically exposed to the actin filament disrupting agent cytochalasin B. Electrophysiological data showed that cytochalasin B treatment reduced the absolute sensitivity to about 20% over the whole frequency range. The decrease in sensitivity at 20 Hz, however, was less than at other frequencies. The phase lags at 14 and 20 Hz became less negative, indicating a relatively better transduction at high frequencies. Calculations with an electric equivalent circuit of an electroreceptor cell indicated that a reduction in apical surface area in combination with a reduction of the number or the conductivity of apical ion channels can explain such effects. We conclude that, although only the basal membrane is thought to be involved in stimulus transduction, the apical membrane contributes considerably to the frequency characteristics of ampullary electroreceptor organs.

Effects of denervation upon receptor cell survival and basal cell proliferation in tuberous electroreceptor organs of a weakly electric fish

Weakly electric fish generate electric fields for the purposes of electrolocation and communication. These fields are detected by specialized receptor organs: the tuberous organs. In the present study we investigated the effects of denervation upon receptor cell survival and progenitor (basal) cell proliferation rate. The left, infraorbital, anterior lateral line nerve of brown ghosts (Apteronotus leptorhynchus) was sectioned, and the proximal stump was dipped in ricin to prevent regrowth. In groups of four, the animals were given two daily injections of the cell proliferation marker bromodeoxyuridine (BrdU) for 2 days at 1, 2, 3, or 4 weeks following denervation. At the completion of the BrdU injection schedule, a piece of cheek skin, rostroventral to the eye, was removed from the left (denervated) and the right (intact) sides and processed for light microscopy or immunocytochemistry. Our results show: (1) there is progressive receptor cell death and tuberous organ degeneration following denervation; (2) basal cell proliferation increases steadily with time after denervation and tuberous organ degeneration; and (3) despite denervation, some proliferating basal cells differentiate into receptor cells, but these new receptor cells eventually die. These results suggest that innervation is essential for tuberous electroreceptor cell survival and that the rate at which basal cells proliferate is regulated by receptor cell health, locally released factors, or both.

Apically administered cytochalasin B and D decreases sensitivity of electroreceptor organs in the North-American catfish, Ictalurus nebulosus

The receptor cells of the ampullary electroreceptor organs of Ictalurus nebulosus bear microvilli on the apical membrane. Whereas microvilli in mechanoreceptive hair cells and in chemoreceptor cells have a transduction function, the function of these membrane specializations in electroreceptor cells is not fully understood. To study the role of the microvilli of the electroreceptor cells, the ampullary electroreceptor organs were apically exposed to the microfilament-disrupting agents cytochalasin B and D. Electrophysiological measurements showed that cytochalasin caused a high decrease in sensitivity and a slight decrease in spontaneous activity. Exposure to cytochalasin B resulted in a striking disorganization of the microvilli on the apical membrane of the electroreceptor cells. The most plausible explanation for the results is that treatment with cytochalasin mainly affects the actin filaments of the microvilli causing an increase of the resistance of the apical membrane. A high apical resistance results in a decrease of the voltage over the basal membrane, which in turn reduces the sensitivity. The conclusion is that intact apical microvilli are necessary for proper functioning of ampullary electroreceptor organs. Alterations in microvillar properties, like surface area and ion channel conductancy might play a considerable role in the regulation of the sensitivity.

Electrical Properties of Electroreceptor Cells Isolated from Skate Ampulla of Lorenzini.

Electrical Properties of Electroreceptor Cells Isolated from Skate Ampulla of Lorenzini.

The Sensing of Electrical Capacitances by Weakly Electric Mormyrid Fish: Effects of Water Conductivity

ABSTRACT Weakly electric fish can perceive electric properties of objects by monitoring the responses of their epidermal electroreceptors (mormyromasts) to their own electric organ discharges (EOD), a process known as active electrolocation. Mormyrid fish can distinguish capacitative from resistive properties of objects. It is mainly animate objects that possess capacitative properties. Water conductivity is a critical environmental factor that varies widely from season to season and has strong effects on the emitted EOD. The two goals of this study were: (1) to investigate the ability of Gnathonemus petersii to detect the properties of capacitative objects in waters of different ion content and (2) to test a recently formulated hypothesis which states that the detection of the features of a capacitative object depends on a comparison of the inputs from the two types of mormyromast primary afferents. Individuals of G. petersii were tested in a conditioned electrolocation procedure. With increasing water conductivities from 50 to 1100 μS cm−1, EOD amplitude decreased and the detection threshold for small capacitances increased. At 50 μS cm−1, the smallest detectable capacitative value was below 0.5nF; this increased to about 20nF at 800 μS cm−1. When conductivity approached about 1000 μS cm−1, fish were no longer able to electrolocate, probably because of the reduction in EOD amplitude at high conductivities. The fish’s ability to discriminate a capacitative object unequivocally from every resistive object was also tested at different conductivities. Below about 800 μS cm−1, all fish could do so. Above that conductivity, however, fish could no longer discriminate between capacitative and resistive objects of similar impedance, although they could still discriminate between objects of different impedances. The two types of receptor afferents (from the ‘A’ and ‘B’ cells) of mormyromast electroreceptor organs have different thresholds, with the B afferents being more sensitive. I suggest that only the B receptor cells remain active at about 800 μS cm-1, when the EOD amplitude is much reduced. With input from B afferents only, an unambiguous capacitance detection was no longer possible. This supports the hypothesis that capacitance detection is achieved by comparing inputs of A and B electroreceptor cells.

Neuropharmacological analysis of synaptic transmission in the Lorenzinian ampulla of the skate Raja clavata

Dissected ampullae of Lorenzini of the skate (Raja clavata) were studied with the aim of determining the synaptic transmitter between electroreceptor cell and afferent fibre. Resting activity and stimulus-evoked activity in response to electrical pulses were recorded in single afferent units at constant perfusion with normal and test solutions containing different putative neurotransmitters. Presynaptic transmitter release was blocked by Mg2+ (up to 50 mM) to investigate the effects of the test substances upon the postsynaptic membrane. L-Glutamate (L-GLU) and L-aspartate (L-ASP), both at concentrations between 10(-7) and 10(-3) M, enlarged strongly resting and stimulus-evoked discharge frequency in the afferent fibre. If transmission was blocked by high Mg2+, resting discharge frequency could be restored by L-GLU or L-ASP. The glutamate agonists quisqualate (10(-8)-10(-5) M) and N-methyl-D-aspartate (10(-5)-10(-3) M) enlarged spontaneous activity in the afferent fiber. The same was found for kainic acid (10(-9)-10(-5) M). Taurine at concentrations between (10(-5) and 10(-3) M caused a concentration-dependent decrease in afferent activity. The same was found for gamma-aminobutyric acid (GABA; 10(-5)-10(-4) M), and for the catecholamines adrenaline and noradrenaline, both in concentrations between 10(-5) and 10(-3) M. Serotonine (10(-5)-10(-3) M) and dopamine (10(-5)-10(-3) M) had no effect on resting or evoked activity in the Lorenzinian ampulla afferents.(ABSTRACT TRUNCATED AT 250 WORDS)

Frequency tuning in the electroreceptive periphery

Our studies are concerned with the frequency tuning that is provided by the electrical resonance of tuberous electroreceptors. Frequency selectivity had previously been measured in the electroreceptor's afferent fibers, and resonant conductances in the electroreceptor cell membrane had been implicated in producing the selectivity. With transdermal application of sinusoidal current, we measured the frequency dependence of the impedance of small areas of the electroreceptor/skin structure of the weakly electric fish Sternopygus and Eigenmannia, and used our data to make a quantitative linear model of the structure. The qualitative form of the model was proposed by Bennett (1). The quantitative model allows us to estimate the frequency selectivity of the voltage across the innervated membrane of the electroreceptor cells. The frequency selectivity of electroreceptor cell voltage derived from our data are as sharp as the neural selectivity at frequencies close to the most sensitive frequency. Many of our measurements supported the linear system model. However, spontaneous electroreceptor voltage oscillations were detected in some of our specimens, suggesting that the electroreceptors can operate in a regime of active nonlinearity. A simple explanation for the observed oscillations is that they arise when damping in the electroreceptor cell's resonant membrane is negative for a limited span of membrane voltage surrounding the resting voltage. The response of oscillating units to sinusoidal current was compatible with this explanation. We report experimental observations bearing on the consequences of active nonlinearity for the frequency tuning of a resonant system.

Transduction at electroreceptor cells manipulated by exposure of apical membranes to ionic channel blockers

1. Stimulus transduction in catfish electroreceptors was investigated electrophysiologically during local exposure to low-Ca 2+ water and the ionic channel blockers verapamil, bepridil, Cd 2+ TTX and TEA. 2. Current through receptor cells is not carried by Ca-ions passing through voltage sensitive Ca-channels in the apical membranes. Ca-channel Mockers act on compartments inside the receptor cells. 3. Resting discharge rate and modulation of resting discharge can be manipulated independently. This is inconsistent with the concept that both parameters are controlled by neurotransmitter release only; more receptor cell functions must be involved.

Effects of some agonists of excitatory amino acids on synaptic transmission in the skate electroreceptors (ampullae of Lorenzini)

The effects on synaptic transmission of blockers of amino acids at postsynaptic receptors were investigated by means of external perfusion of the basal membrane of electroreceptor cells in ampullae of Lorenzini and by recording of spike activity from individual nerve fibers in the skateRajaclavata: glutamic acid diethyl ester (DEE-GLU), glutamic acid dimethyl ester (DME-GLU), D-α-aminoadipic acid (DAA), kynurenic acid (KYN), and cis-2,3-piperidine dicarboxylic acid (PDA). The most effective blocker was found to be DEE-GLU, producing reversible blockade of postsynaptic amino acid response. It was found that DAA did not change synaptic transmission in the electroreceptors, while PDA largely affected response produced by applying L-aspartic acid (L-ASP); KYN affected response produced by L-glutamic acid (L-GLU). It is deduced that L-GLU and L-ASP are the most effective agonists at the afferent synapse of ampullae of Lorenzini and that their excitatory effect is produced by activating quisqualate receptors.

Effects of glutamic acid diethylester on synaptic transmission in the skate ampullae of Lorenzini

The effects on synaptic transmission of glutamic acid diethylester (GDEE), a glutamate receptor blocker, were investigated by recording spike activity from single nerve fibers in the electroreceptor cells of the skate (Raja clavata) ampullae of Lorenzini. It was found that adding GDEE to the bathing medium led to a concentration-dependent reduction in or complete blockade of background and evoked receptor activity; 10−6 M GDEE was the minimum effective concentration. It was also shown that GDEE reversibly blocked postsynaptic response produced by excitatory amino acids: L-glutamate (L-GLU) and L-asparate (L-ASP). Findings suggest the involvement of L-GLU or a related substance in synaptic transmission in the ampullae of Lorenzini.