Robert C. Peters

Influence of Direct Current Stimulation On the Ion-Induced Sensitivity Changes of the Electroreceptors (Small Pit Organs) of the Brown Bullhead, Ictalurus Nebulosus LeS

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 electroreceptors 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.

Resting Discharge and Sensitivity of Ampullary Electroreceptors in Clarias gariepinus Related to Convergence Ratio during Ontogeny

The ampullary electroreceptor organs of the teleost fish Clarias gariepinus converge to single afferent neurons. During the first 4 months of ontogeny the convergence ratio (CR) increases from 1 to 3. Extracellular single-unit recordings show that a 3-fold increase in CR results on average in a 3.6-fold increase in sensitivity measured at stimulus frequencies of 2, 8 and 20 Hz, and in a small but significant 1.1-fold increase in resting discharge. Adaptation of the primary afferents could account for the differences between changes in sensitivity and resting discharge.

Saffan®: A review and some examples of its use in fishes (Pisces: Teleostei)

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.

The bioelectric field of the catfish Ictalurus nebulosus

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.

The Octavo-Lateralis Percepton in Lower Aquatic Vertebrates: Clustered Versus Dispersed Sensor Configurations

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.

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

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.

Ultradian Rhythms in Fishes

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.

Spontaneous firing in primary afferent neurons of ampullary electroreceptor organs as attribute of bandwidth, threshold, and topology

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.

Evoked bradycardia as response criterion for the detection of electrical stimuli in catfish

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.

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

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.