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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.

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.

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.

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.

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.