Search

In order to investigate the hearing capacities of adult zebrafish (Danio rerio), wild type zebrafish were conditioned to both sound pressure and particle motion in a respondent conditioning paradigm. Sound fields were generated by five underwater loudspeakers in a cylindrical tank, which allows separate control of sound pressure and particle motion. Sound stimuli were soft-switched sound pulses having a strength (RMS) of 0.4 to 5 Pa sound pressure (112 to 134 dB re 1 μPa), 6.7 × 10−7 to 6.7 × 10−6 m/s particle velocity or a combination thereof. Frequencies used were 800 Hz or 250 Hz. During the test a fish was placed in the acoustic centre of the tank, confined in a soft nylon mesh fitted with two silver chloride recording electrodes to measure both changes in position and the ventilatory response. Each sound presentation was followed by a brief mechanical jerk of the fish cage (unconditioned stimulus). The startling response to the unconditioned stimulus was thus coupled to the sound signal (conditioned stimulus). Responses were scored as changes in fish position and/or ventilatory rhythm, occurring during or after the sound pulses, but before the unconditioned stimulus. In this way, we found that wildtype zebrafish respond to artificial sounds having either predominantly sound pressure or particle motion. Discrimination of directionally different sounds could not be established. The method is suited well to probe other auditory capabilities, and to test zebrafish mutants lacking one or more otoliths.

A brief sketch is given of the changing interpretation of Academic Freedom at the universities in The Netherlands during the last half of the twentieth century, by discussing the discoveries of electroreception in catfish, Ameiurus nebulosus, and dogfish, Scyliorhinus canicula. It is concluded that science is no longer hypothesis-driven, but money-driven. Whereas innovation and discoveries belong typically to the domain of the individual scientist and flourish in a bottom-up organisation, the funding agencies of today advocate a top-down approach to stimulate creativity. Apparently the resultant institutionalization has not the desired result.

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.

Abstract Spontaneous firing of neurons plays an essential part in the detection of sensory stimuli. Spontaneous firing of primary afferents of ampullary electroreceptor organs in the catfish Ameiurus nebulosus (Lesueur, 1819) was studied in relation to the distribution, thresholds, and frequency characteristics of the electroreceptor organs. The spontaneous firing rate was correlated with the place on the skin. The mean inter-spike interval in 55 dorsal and 49 ventral ampullary organs in five specimens was 16.8 ms +/- 0.41 SEM and 20.5 ms +/- 0.48 SEM, corresponding to firing rates of 59.5 and 48.7 s-1 respectively. The concomitant coefficients of variation were 0.33 and 0.29. Approximately half of the dorsal ampullae were innervated by two fibres. The firing rates of each of the two fibres was lower than the firing rate of organs innervated by a single neuron. Responses to stimuli as weak as 10 pA could be recovered from the noisy average firing level provided the number of averaging sweeps was sufficiently large. This was equivalent to a stimulus of 0.025 μV/cm and was lower than the behavioural threshold of 1 μV/cm. The gain of the frequency response was enhanced at the carrier frequency, at twice the carrier frequency, and in the range from 75-90 Hz. The results revealed that the occurrence of spontaneous activity improved the signal to noise ratio of responses to electrical stimuli by reduction of the coefficient of variation, absence of a threshold, and phase locking.

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 (

Abstract Heart rate deceleration (HRD) after exposure to novel stimuli is part of the orienting reflex, and can be used as a tool to investigate the susceptibility of various organisms to sensory stimuli. HRD as response criterion was used in unrestrained catfish, Ameiurus (Ictalurus) nebulosus (Lesueur, 1819) to investigate its susceptibility to electrical stimuli. HRD in catfish occurs after stimulation with light, mechanical stimuli, and electrical stimuli. HRD shows habituation and correlates with stimulus strength. The response to sinusoidal electrical stimuli from 70 to 700 μV/cm p-p was determined in the range from 0.1 to 1000 Hz. Using HRD as response criterion we found that at 85 μV/cm catfish react to stimuli from 0.1 to 3 Hz. In the absence of stimuli, the heart rate develops an ultradian rhythm with periods of 7 to 15 min. About twice a day cardiac arrest of 1 min occurs. During anaesthesia oscillations with a period of about 1 min are recorded. Comparison of this study with others supports the notion that there exist at least two neural channels for processing electrical stimuli. One channel is involved in predation, namely processing the fast potential changes accompanying the passage of a bioelectric dipole; another is involved in processing uniform DC fields used for navigation.

Reinterpretation of research on the electric sense in aquatic organisms with ampullary organs results in the following conclusions. The detection limit of limnic vertebrates with ampullary organs is 1 microV cm(-1), and of marine fish is 20 nV cm(-1). Angular movements are essential for stimulation of the ampullary system in uniform d.c. fields. Angular movements in the geomagnetic field also generate induction voltages, which exceed the 20 nV cm(-1) limit in marine fish. As a result, marine electrosensitive fish are sensitive to motion in the geomagnetic field, whereas limnic fish are not. Angular swimming movements generate a.c. stimuli, which act like the noise in a stochastic resonance system, and result in a detection threshold in marine organisms as low as 1 nV cm(-1). Fish in the benthic space are exposed to stronger electric stimuli than fish in the pelagic space. Benthic fish scan the orientation plane for the maximum potential difference with their raster of electroreceptor organs, in order to locate bioelectric prey. This behaviour explains why the detection threshold does not depend on fish size. Pelagic marine fish are mainly exposed to electric fields caused by movements in the geomagnetic field. The straight orientation courses found in certain shark species might indicate that the electric sense functions as a simple bisensor system. Symmetrical stimulation of the sensory raster would provide an easy way to keep a straight course with respect to a far-field stimulus. The same neural mechanism would be effective in the location of a bioelectric prey generating a near-field stimulus. The response criteria in conditioning experiments and in experiments with spontaneous reactions are discussed.

The variability of the bioelectric field of the electrosensitive catfish, Ictalurus nebulosus, was investigated by recording the potential variation occurring when the fish passed a stationary electrode, and by recording the field of a stationary fish by a 15-electrode array. A good first order approximation of the recorded field of a 20 cm long fish is a dipole dc source with the source and sink about 7 cm apart, carrying a current of about 1 microA in water with a specific resistivity of 3.3 kohm cm. At 5 cm distance from the dipole axis such a source generates an electric potential swing in the order of 50 microV in free space, head negative, tail positive. Superimposed on the basic component are respiration related fluctuations, and fluctuations related to the activity of the alimentary canal, gills, and skin. Novel stimuli, or stressors like investigators approaching the aquarium, evoke sudden increases in field strength. which last about 15 min. Demineralization of the aquarium water causes changes in field strength and reversal of field polarity. The administration of food causes field variations in the vicinity of the anal opening. The bioelectric field shows diurnal fluctuations of 100 microV. The peak is at about 04:00, the dip at 14:00. The fluctuations of the bioelectric field are sufficiently strong and specilic to serve as electrical stimuli to other electrosensitive catfish. It is suggested that the field changes allow a simple form of electrocommunication. i.e. inform conspecifics about some physiological properties of the field source. The cellular mechanisms underlying the fluctuations of the bioelectric field are homeostatic processes mediated by ion pumps and ion channels.

Saffan®, previously known as CT1341, is an injectable steroid anesthetic for use in cats and monkeys. It is also suited to induce anesthesia in fishes. It has the convenient property to induce anesthesia of the central nervous system, and to leave the sensory system of the integument operational. The active constituents are alphaxalone and alphadolone. A dose of 24 mg/kg i.m. is sufficient for 2 to 3 hours narcosis in fish of several hundreds of grams. For small specimens administration via bath-water in doses of 2 to 4.8 mg/l for induction and half this dose for maintenance are advisable. In catfish, Ictalurus nebulosus and I. melas, of 100 g or more the induction time after i.m. injection with 21 mg/kg is about 20 min and independent of weight or temperature. The recovery time varies with temperature from 213 min at 13°C to 19 min at 19°C. In 1000 g catfish, Clarias gariepinus, the onset of surgical anesthesia occurs after 26 min on average at doses of 24 mg/kg at 24°C. In 5 g catfish, Kryptopterus bicirrhis, 4.8 mg/l bath water at 25°C makes the opercular movements disappear after 5 min. For 15 cm TL Gnathonemus petersii a good induction dose is 2 mg/l bathwater. After 20 minutes at 23°C surgical narcosis is reached.

Bimodal integration was tested for the catfish Ictalurus nebulosus in a psychophysical experiment. The modalities tested were an electric direct current field and a light spot. In a two-alternative-forced-choice experiment the photopic photoreception threshold was 0.5 lux and the electroreception threshold 5.5 μV/cm. Bimodal thresholds were 0.2 lux and 4.5 μV/cm. For both electroreception and photoreception the difference between unimodal and bimodal threshold data were significant. This indicates that the catfish can integrate information from electroreception and photoreception. Attempts to determine tresholds under scotopic conditions succeeded for photoreception but not for electroreception. The scotopic photoreception threshold was 2.5 x 10-2 lux. Scotopic photoreception thresholds were significantly lower than photopic photoreception thresholds. No scotopic bimodal tests were performed.

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.

An excitatory amino acid, most probably l -glutamate, acts as a neurotransmitter at the receptor cell–afferent fibre synapses in the ampullary electroreceptor organs of the freshwater catfish Ictalurus nebulosus . In the present study, we have used an electrophysiological approach to investigate the presence of N -methyl- d -aspartate receptors at this level. N -Methyl- d -aspartate, dissolved in an Mg 2+ -containing (normal) solution, had no effect on afferent activity, not even at 5 mM. However, addition of 5 mM N -methyl- d -aspartate to an Mg 2+ -free solution evoked an enduring increase in firing rate. The application of N -methyl- d -aspartate combined with electrical sine wave stimulation produced a firing increase in the primary afferents, even in the presence of Mg 2+ (1.5 mM). Glycine (0.01–0.001 mM) significantly potentiated the N -methyl- d -aspartate responses. Addition of antagonists of the actions of N -methyl- d -aspartate, 7-chlorokynurenic acid, dl -2-amino-5-phosphonovaleric acid and ketamine in concentrations of 0.5–2.0 mM led to a decrease in resting and stimulus-evoked activity. 7-Chlorokynurenic acid also blocked the responses to application of N -methyl- d -aspartate. The glycine agonist d -serine (0.01 mM) prevented the 7-chlorokynurenic inhibitory effect. These results suggest the involvement of N -methyl- d -aspartate receptors in mediating the actions of l -glutamate at the afferent synapses of the electroreceptor organs of the catfish.

We report the results of a search for evidence of periodic unstableorbits in the electroreceptors of the catfish. The function of thesereceptor organs is to sense weak external electric fields. Inaddition, they respond to the ambient temperature and to the ioniccomposition of the water. These quantities are encoded by receptorsthat make use of an internal oscillator operating at the level of themembrane potential. If such oscillators have three or more degreesof freedom, and at least one of which also exhibits a nonlinearity,they are potentially capable of chaotic dynamics. By detecting theexistence of stable and unstable periodic orbits, we demonstratebifurcations between noisy stable and chaotic behavior using theambient temperature as a parameter. We suggest that the techniquedeveloped herein be regarded as an additional tool for the analysisof data in sensory biology and thus can be potentially useful instudies of functional responses to external stimuli. We speculatethat the appearance of unstable orbits may be indicative of a stateof heightened sensory awareness by the animal.

Spontaneous afferent activity was recorded from 26 single ampullary electroreceptive organs of freshwater catfish (Ictalurus nebulosus LeS) at various temperatures. Regular grouping of action potentials was apparent in this secondary sensory system at 35 degrees C and occasionally at 30 degrees C. Impulse groups consisted of up to seven impulses. The precise timing of impulse generation and the temporal sequence of impulses indicated that oscillating processes are involved. Expectation density functions were calculated for records of afferent activity obtained at various temperatures below 35 degrees C. In the majority of records the function was periodic. Impulse grouping and expectation density functions became more distinct in units exhibiting extremely high thresholds (i.e. being insensitive) to electrical stimuli. The results suggest that the oscillations originate from the postsynaptic membrane. The temporal pattern of impulse generation within impulse groups of ampullary electroreceptor organs and of specific warm and cold receptors was compared and found to be similar. Application of cadmium and menthol, which both reduce calcium entry, suppressed spontaneous activity in normal and insensitive electroreceptor systems, attenuated the sensitivity of normal receptors and modified the periodic pattern. This indicates that calcium is implicated in sensory transduction and in postsynaptic mechanisms. The data suggest that an oscillating process is one component of signal transmission in ampullary electroreceptor organs of teleost fish.

The detection threshold of the electrosensory system in catfish, Ictalurus nebulosus, was assessed psychophysically with the 'staircase method' in a two-alternative forced-choice experiment over a period of 6 months. Detection thresholds were assessed at stimulus frequencies of 0.03, 0.1, 1, 3, 10, 30, and 100 Hz at temperatures of 5, 10, 15, 20, and 25 °C. The water conductivity was kept between 0.23 and 0.27 mS/cm. The behavioural response curve has a band pass filter shape with maximum transfer at frequencies in the range from 1 to 3 Hz. The lowest value of the threshold was 0.8 μV/cm at 1 Hz, 25 °C, and 0.26 mS/cm. At lower frequencies, e.g. 0.03 Hz, the mean threshold ranged from 20 to 100 μV/cm; at 100 Hz the mean threshold ranged from 500 to 1000 μV/cm. The overall performance of the fish improved during the course of the experiment as was demonstrated by the decrease of the mean threshold at 1 Hz from 35 to 10 μV/cm. Threshold-temperature curves were incongruous with electro-physiologically made sensitivity recordings of electroreceptor organ primary afferents. At frequencies above 3 Hz temperature effects were most noticeable, mainly as an increased sensitivity. Temperature effects at frequencies less than 3 Hz were negligible. The results serve as reference values for future studies on water pollution where the electric sense serves as a biomonitor system. We conclude that such studies should be conducted at stimulus frequencies between 0.1 and 3 Hz.

1. Subcutaneous injection of vincristine (more than 3 μg) in the catfish, Ictalurus nebulosus , produces a black spot on the skin with a grey surround. Doses between 1 and 3 μg give a less pronounced discolouration. 2. Two to three days after injection of 5 μg vincristine, repetitive activity is detected in primary afferents of unstimulated electroreceptor organs close to the site of injection. 3. Vincristine increases the phase-lag of the modulation of afferent activity in electroreceptor organs during electrical stimulation without a clear effect on the sensitivity of catfish electroreceptor organs. 4. The amplitude of the action potentials of the primary afferents begins to decrease after 2 days; after 3 days they gradually disappear in the background noise. 5. Application of Org 2766, a potentially neurotrophic compound, at 2 days, but not 1 day, before vincristine application prevents vincristine effects on the phase shift. 6. Preliminary electron-microscopical studies of the synapse shows a severe depletion of glycogen granules in the afferent nerve fibre after vincristine application, 7. It is concluded that electroreceptor organs can be used to study neuropathies caused by vincristine, and that Org 2766 may be useful for preventive treatment of such neuropathies.

1. 1. Transduction and transmission in catfish ampullary electroreceptors is mediated by sensory cells bearing microvilli, chemically mediating synapses, nerve terminals and one axon. Although some aspects still remain to be clarified, a number of properties have been found. 2. 2. Spike generation per seand the modulation of spike frequency by electrical stimuli behave differently with respect to a number of experimental factors. 3. 3. Stimulus current enters presumably through non-voltage-sensitive or non-specific ion channels. 4. 4. Fluctuations of the spike frequency may be used as a measure for proper functioning of this sense organ.

1. Ampullary electroreceptors of the freshwater catfish, Ictalurus nebulosus, were examined for the effects of bath-applied l -glutamate ( l -GLU) and its agonists quisqualate (Q), kainate (KA) and n -methyl- d -aspartate (NMDA). Resting discharge rate and stimulus-evoked activity in single afferent fibers were recorded in an in vitro preparation. 2. l -GLU (0.1–1 mM), Q (0.8–40 μM) and KA (10–250 μM) strongly increased both resting activity and stimulus-evoked activity in the afferent fibres. NMDA had no effect, even at a concentration of 5.0 mM. 3. The potencies of l -GLU and its agonists, arbitrary defined as the concentrations which gave 50% of maximal frequency increase, were of the order of 7 μM (Q), 25 μM (KA) and 0.6 mM ( l -GLU). 4. The excitatory effects of l -GLU persisted in receptors suppressed by high Mg2+, indicating that l -GLU was acting at the postsynaptic site. 5. The data presented are consistent with our current concept that the action of l -GLU in Ictalurus electroreceptors is mediated via Q/AMPA- and KA-types, but not the NMDA-type, of membrane receptors.

Electroreception is a well-established sensory faculty in aquatic vertebrates. The general physiology of the receptor organs is comprehensively documented. The transduction mechanism of the receptor cells and the synaptic transmission mechanism are less well understood. Research has been hampered by the inaccessibility of the synaptic site. This paper describes how to prepare an in vitro preparation of ampullary electroreceptor organs which allows exposure of both the mucosal and the serosal sides of the receptor cells to superfusion of test solutions. The preparation is quite robust and has been shown to function reliably for more than 8 h. Furthermore, the use of in vitro electroreceptors organs as a model for pharmacological studies is evaluated.

The process of regeneration of skin patch denervated empullary electroreceptor organs of the African catfish Clarias gariepinus has been investigated at an ambient temperature of 28°C with both electrophysiological and histological methods. At day 1 after denervation none of the receptor organs on the skin patch showed afferent activity. At this stage none of the ampullary organs previously recorded showed a normal appearance. Degenerative changes consisted of a decreased number of receptor cells and an often invisible lumen. At day 7 regeneration seems to start with a high density of primordial ampullary organs, more than a seven-fold increase compared to controls. In these units, the level of spontaneous activity is very low: compared to controls, more than a two-fold increase in mean interspike interval. At this stage, the sensitivity to electrical stimuli is already at the level of untreated control organs. At day 15 there is a lower, i.e. approximately normal, density of ampullary organs with a normal morphology. In these units both spontaneous firing and sensitivity returned to normal. It can be concluded that the functional dichotomy between spontaneous firing and sensitivity that was found in degenerating ampullary electroreceptor organs is also found during the process of their regeneration, although the underlying cellular changes may be totally different. The speed of recovery suggests that only regeneration of the distal part of the sectioned nerve fibers takes place.

The sequence of interspike intervals of the spontaneous activity in denervated electroreceptor organs of the catfish is analysed with several statistical techniques: visual inspection of dot displays, interval histograms, serial correlograms, tests for stationarity and trends, and tests for renewal properties, based on the spectrum of intervals. The interspike interval train of primary afferents can usually be treated as a renewal process. Following denervation, a number of non-renewal properties emerge. The interval histogram of the renewal spike trains can be fitted well with a gamma probability density function; non-renewal cases need a more complex approach. We propose that the stochastic fluctuations in interval duration arise from randomly occurring quantal depolarizations (Stein's model of stochastic neuronal excitation). Two important properties with regard to synaptic transduction are: afferent firing results from capture of a limited number of depolarizing quanta; and the quantal input rate and the threshold for firing appear to be correlated.

The interspike interval histogram of spontaneously firing electroreceptor organs of freshwater catfish (Ictalurus nebulosus LeS.) fits well with the probability density function of a gamma distribution. The shape parameter r of this probability density function can be used as a measure for the firing threshold of the spike generator, and the scale parameter lambda provides a measure for the input rate of synaptic quanta. Here, we have studied the physiological meaning of the two parameters of the fitted gamma probability density function by manipulating the mean firing rate. This was done in two ways: "frequency clamp" stimulation and a change in temperature. Frequency clamp stimulation, compensating for adaptation of the response, keeps the afferent firing rate at a fixed value for a limited period. We show that within such a period not only the mean spike rate remains constant, but these spike trains can be regarded as renewal series: the spike generator is functionally uncoupled from the frequency-dependent parts of the transduction path. Concerning the gamma parameters, two types of responses are found: at strong excitation lambda increases, r does not change; with small stimuli r is inversely proportional to the stimulus strength, lambda remains unchanged. This is difficult to explain when stimulus transduction acts via the synaptic input rate only. Stimulation seems to influence the firing threshold of the spike generator directly. At strong inhibition a regular firing pattern suddenly takes over; the major part of the variability suddenly disappears from the spike train. A change in temperature causes a shift in lambda (Q10 approximately 2.3), but not in r. This points to a temperature effect on the input rate only.

1. 1. Exposure of catfish, Ictalurus nebulosus, to sublethal concentrations of cadmium deteriorates electro-orientation performance. 2. 2. Cadmium, at a concentration of 40 μg l , doubles the behavioural threshold for electric stimuli within 48 hr of exposure; both prolonged exposure and higher concentrations result in higher thresholds. The effect is reversible. 3. 3. Electro-orientation performance can be used to monitor the quality of surface water.

The response properties of ampullary electroreceptors have been studied in the catfish Ictalurus nebulosus at skin temperatures between 5 and 35 degrees C. A unimodal relationship between spontaneous activity and temperature was obtained. Mean (+/- SEM) peak discharge rate was 57.3 +/- 1.8 impulses s-1 at 25 degrees C; the receptors were active at 5 degrees C (15.0 impulses s-1) and at 35 degrees C (31.5 impulses s-1). There were no dynamic responses to temperature changes in either the warming or cooling direction. The shape of the frequency characteristic depended on temperature: the peak of the gain curve shifted to low frequencies at low temperatures. There was a concomitant change of the phase characteristic: the intersection at zero degree phase angle shifted to higher frequencies with an increase of temperature, thus increasing the lead at lower frequencies and decreasing the lag at higher frequencies. Latency after combined excitatory and inhibitory impulse stimulation was temperature dependent, ranging from 16.4 ms (5 degrees C) to 5.6 ms (35 degrees C). Application of the specific calcium channel blocker menthol (0.2 mM) suppressed spontaneous activity, the effect becoming more prominent at higher temperatures. Sensitivity to sinusoidal electrical stimulation was also impaired, but to a lesser degree and mainly at lower temperatures. We conclude that the filter properties of the receptor organ can be modelled by a band-pass filter in series with a latency, both of which are temperature dependent. These filter properties might be partially based on the activation kinetics of the transduction channels.

1. 1. The results of previous denervation experiments indicate the important role of synaptic transmission in the transduction process in ampullary electroreceptor organs of the catfish. 2. 2. One of the classical ways to characterize a transduction system is by means of its pulse response. We have used an input pulse with properties that minimize the influence of differences in sensitivity between different organs. 3. 3. The response to this pulse is characterized with time to onset, or latency, duration of the response, and size of the response, or the number of action potentials elicited per input pulse: (a) Pulse latency changes with degeneration from the control value of 10msec, up to approx. 15msec at 36 hr after denervation; (b) the duration and the size of the response are not correlated with time after denervation; (c) the position of the peak of the response is shifted with the same amount as the latency of the onset. 4. 4. This leads us to the conclusion that in a degenerating electroreceptor organ the pulse response is only shifted to a larger latency, but is unaffected in every other aspect. 5. 5. The development of a threshold in the sensitivity of the transmitter releasing mechanism to the intracellular free calcium concentration seems to provide the best explanation for the present data.

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).

The functioning of electroreceptor organs of Ictalurus sp. was investigated by inhibiting synaptic transmission by the administration of tetanus toxin in vitro. A piece of Ictalurus skin of about 20 mm diameter was mounted in an Ussing-type chamber. After establishing the normal functioning of the organ, tetanus toxin (TeTx) was applied basolaterally for 150 min in 66.7 pM and 400 pM concentrations, while the single unit nerve activity was recorded extracellularly. Spontaneous spike activity and the sensitivity of the electroreceptor organs were measured. The results show that TeTx reduces sensitivity to less then 20% of its original value, whereas the spontaneous activity is unaffected by the treatment. This indicates that the afferent nerve is capable of generating impulses independent of receptor cell neurotransmitter release. In the discussion we suggest two alternative mechanisms for the emergence of the spontaneous spike activity.

The ototoxic side-effects of the anti-cancer drug cisplatin ( cis -diaminedichloroplatinum) have been widely investigated. However, the exact site of action remains unclear. In this study, the electroreceptor organ of the freshwater catfish Ictalurus melas is used as a model for examining the acute effects of cisplatin. The sensory cells in the electroreceptor organ are homologous to the inner hair cells in the cochlea of mammals. The effects of cisplatin administration can be investigated by in vivo recording of the spike trains from the electroreceptor organ primary afferents. Exposure of electroreceptor organs to 330 μM cisplatin for 1 h causes the spontaneous activity to drop, the overall sensitivity to diminish and the shape of the frequency characteristics to change. These effects persist in the week after administration. Control levels have returned at day 22. These results demonstrate an acute and, with considerable hysteresis, reversible cisplatin effect on the electroreceptor organs, which is to a large extent consistent with the cisplatin-induced effects in isolated hair cells in mammals. The time-course of the effect supports the hypotheses that ion channels are blocked immediately by cisplatin administration, and that cisplatin metabolites disturb enzymatic cellular processes.

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 Ca2+ by blocking of the inositol triphosphate receptors in the endoplasmic reticulum. This in turn would affect many intracellular properties and processes. The unavailability of Ca2+ 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 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.

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.

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.

Single afferent unit recording in microampullae of the catfish revealed that bath-applied AMPA increases both resting discharge frequency and electrically evoked responses. The potency of AMPA is of the order of 10 microM. DNQX strongly inhibits the excitatory effects of AMPA. The results suggest the presence of AMPA receptors at the synaptic membrane of ampullary electroreceptor organs in the catfish.

Ultradian rhythms are periodic phenomena with periods shorter than 24 hours. At the cellular level they are related to biochemical and neurogenic oscillators . At supracellular levels of organization they appear as rhythmic changes of respiration, heart beat, and locomotor activity. Ultradian rhythms occur both in early ontogenesis and in adult life. Stimulus deprivation, i.e. absence of modulation of rhythmic activity by nonrhythmic ecofactors, seems to be a condition for the occurrence of ultradian rhythms. During ontogeny stimulus ‘deprivation’ would occur because the complete set of sensory-motor connections has not yet been formed. In the adult organism stimulus deprivation would occur in stimulus poor or stereotypical environments such as laboratory observation rooms and cages. Ultradian rhythms in fishes are related to the rhythmic movements of caged zoo animals and autistic human beings.

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.

This paper discusses the biological relevance of the topology of octavo-lateralis sense organs in lower aquatic vertebrates. The sense organs of the octavo-lateralis system are arranged either as a dense cluster or as a dispersed set. The dense cluster is situated deep in the head, near the hind brain, and is mainly involved in the detection of uniform stimulus fields. It collects information about distant stimulus sources. The dispersed set is spread over the integument and meets the demands for the detection and interpretation of non-uniform stimulus fields, usually emitted by nearby stimulus sources. It is argued that both the stimulation of the semicircular canals by rotation and the stimulation of the dispersed ampullary electroreceptors in uniform stimulus fields present paradoxes that nevertheless fit this general view.

Electrical responses were obtained from the olfactory epithelium (electro-olfactogram, EOG) and from the olfactory tract of female African catfish. The EOG response of females to holding water of males depends on the presence and the size of the male's seminal vesicle. Male catfish thus appeared to release odorants originating from the seminal vesicle. The fluid from this exocrine gland is a highly stimulatory odorant because it contains steroid glucuronides. Higher levels of steroid glucuronides in the seminal vesicle fluid of feral spawning males makes the fluid more potent as an olfactory stimulus, whereas removal of the glucuronides diminished the response. Ovulation did not affect the sensitivity to these complex mixtures of glucuronides in the seminal vesicle fluid. The olfactory epithelium of females also responded to solutions of individual synthetic steroid glucuronides. Of the glucuronides tested, 5β-pregnane-3α,17α-diol-20-one-3α-glucuronide was the most potent odorant. It had a detection threshold of approximately 10 −11 M . Thresholds for the other glucuronides ranged between 10 −9 and 10 −6 M . The present data demonstrate the importance of seminal vesicle steroid glucuronides as olfactory cues.

1) The a.c. sensitivity of the electroreceptors of the brown bullhead, Ictalurus nebulosus LeS., could be controlled by applying direct current stimuli with current densities of 2 × 10 -7 to 2 × 10 -5 A/cm 2 . 2) These effects bear a tonic character and may last up to 15 minutes. 3) Increases or decreases in the electroreceptor's sensitivity caused by alterations in the ionic contents of the animals environment, could also be compensated by applying direct current stimuli. 4) The implantation of electrodes in intact animals was strongly improved by the use of cocoa butter for electrical insulation and mechanical fixation. 5) Implications of the voltage controlled sensitivity of the electroreceptors are discussed.

The partial substitution of N for P in GaP has a considerable effect on the luminescence quantum efficiency of GaP epitactic layers. The incorporation of N in GaP liquid phase epitactic layers using NH3 vapour doping has been studied at different H2 partial pressures in argon gas. It is shown that the incorporation of N in different ambients is conveniently described by just one parameter. The reaction of NH3 with Ga is found to be an equilibrium process. The N concentration in the layer is directly proportional to the GaN concentration in the melt. From the correlation of the dissociation of NH3 with the formation of GaN it is concluded that the NH3 pressure, as measured in the exit gas, is not representative of the NH3 pressure at the melt. The dissociation of NH3 is found to be strongly retarded by H2 owing to adsorption of H2 on the surface of reactor materials which catalyse the dissociation.

1. 1. The effects of denervation on extracellular primary afferent single unit activity in electroreceptor organs or catfish, Ictalurus nebulosus, were studied under alphaxalone anaesthesia. 2. 2. Compared to nerve sectioning, skin patch excision is a useful method for denervation; effects are found on the same time scale as in nerve-sectioned organs. 3. 3. Resting discharge decreases from 6 hr after denervation until, at 30 hr, no activity can be recorded. Sensitivity remains unaffected by denervation. 4. 4. In the absence of resting discharge, stimulation of electroreceptors can still evoke responses. 5. 5. These effects are consistent with morphological evidence, demonstrating that the first changes are found in the unmyelinated primary afferent endings, whereas the receptor cells and the myelinated parts of the primary afferents remain unaffected.

By using very low excitation densities in low temperature luminescence studies on GaP very narrow donor-acceptor pair bands were obtained. This has resulted in clear phonon replication patterns, including new replications, of the no-phonon pair bands in ‘green pair’ luminescence involving donors at P and Ga-sites. In the ‘red-orange’ luminescence involving the deep Sip-acceptor structure is observed, which is identified with the no-phonon pair band and its phonon replicas.

Decays of the donor-acceptor pair luminescence have been measured as a function of the pair distance R at liquid He temperatures using a pulsed laser beam excitation and applying gated integration techniques to display the decays. The decay time constants were derived from the exponential decays below R ≈ 20 A (on the resolved pair lines) and from the major components of the non-exponential decays at larger R. The decay time depends exponentially on R, the dependence increasing in the “green” pair sequence (S-C), (S-Zn) and (S-Cd) with the characteristic lengths 9.1 A, 7.7 A and 7.0 A. For the “red-orange” pairs (S-Si) no such length could be derived owning to the strong phonon replication. The radiative transition probabilities extrapolated to zero R are ≈ 5 × 105 sec-1 for the “green” pairs and ≈ 105 sec-1 (rough estimate) for the “red-orange” pairs. The zero-phonon transition probabilities were found to be ≈ 4 × 105 sec-1 and ≈ 3 × 103 sec-1 respectively. Possible causes of decay distortion above R ≈ 20 A are discussed as well as thermal effects.

Low temperature near band-edge absorption and luminescence spectra of Si-doped n-type GaP are reported. Electrical and optical evidence is presented that these spectra are due to the creation and decay of excitons bound to neutral Si donors on Ga sites. Several zero-phonon transitions, each with strong replications by momentum conserving phonons, were observed in both absorption and luminescence. From these measurements it is concluded that the ground state of the SiGa donor is split into two sublevels 0.6 meV apart. Crystals containing a sufficiently low concentration of S to be suitable for a reliable analysis of temperature-dependent Hall-effect measurements were selected by combining 300°K Hall-effect data with low temperature S-exciton absorption data. The electrical measurements support the identification of Si as the main shallow donor. Moreover, it is concluded from these measurements that the degeneracy of the ground state of a Ga-site donor is 3 times that of a P-site donor, in agreement with theoretical expectations.

Low temperature absorption and luminescence spectra due to excitons bound to neutral acceptors C, Zn and Cd in GaP are reported in this paper. Most measurements were performed on GaP crystals grown epitaxially on GaP substrates by the H 2 /HCl or the H 2 /H 2 O method. Several zero-phonon (ZP) transitions, each with strong replicas of momentum conserving (MC) phonons were observed in both absorption and luminescence. For the lowest energy transition of excitons bound to neutral acceptors—and also donors—the exciton binding energy and the strenght of the ZP transitions, relative to their phonon replicas, are discussed with respect to the ionization energy of the centre involved. The intrinsic radiative lifetime calculated from absorption is significantly larger than the experimentally found upper limit, showing that a non-radiative parallel process must exist, as is the case for excitons bound to neutral donors. The spectra are also discussed in relation to the methods of crystal growth.