Transduction and transmission in ampullary electroreceptors of catfish

0300-9629~92$5.00 + 0.00 Q 1992 PergamonPressLtd

Camp. Biochem. Physiot.Vol. 103A, No. 2, pp. 245-252, 1992 Printedin Great Britain

MINI REVIEW TRANSDUCTION AND TRANSMISSION IN AMPULLARY ELECTRORECEPTORS OF CATFISH F.

BRETSCHNEIDERand

R. C.

PETERS

Laboratory of Comparative Physiology, Padualaan 8, NL-3584 CH Utrecht, The Netherlands (Receined 2 March 1992; accepted I April 1992) Abstract-l. 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. Spike generation~r se and the m~uiation of spike frequency by electrical stimuh behave differently with respect to a number of experimental factors. 3. Stimulus current enters presumably through non-voltage-sensitive or non-specific ion channels. 4. Fluctuations of the spike frequency may be used as a measure for proper functioning of this sense organ.

Ampullary electroreceptors constitute one of the two classes of sense organs that respond to a wide variety of electric fields in the habitat of electrosensitive fish. The other class, tuberous receptors, respond to the specific electric signals generated by the so-called electric organs of electric fish. The ampullary electroreceptors are further divided into two morphologically and taxonomically different subtypes. The first type, called Ampulla of Lorenzini (Lorenzini, 1678; see Zakon, 1988), is a large sense organ, with thousands of sensory cells and a canal that may attain a length of 3Ocm. It occurs in most species of elasmobranchs. The second type, called microampulla, has more modest dimensions: about 25 sensory cells in a neuromast, fitted with a short (100 pm at most) canal. They are found ao. in silurid fishes (catfish), lungfishes and sturgeons. This paper deals exclusively with microampullae, the name of which stems from the general shape of the organ in a sagittal view. Herrick (1901) was the first to describe the microscopical anatomy; he named it small pit organ to distinguish this small sensor (whose function was unknown at the time) from the larger, also pit-shaped neuromasts of the mechanosensitive lateral line system. M&linger (1964) described the ~trastructure, and found a number of peculiarities mentioned later on. The main components of the signal route are depicted in Fig. 1: the sensory cells, bearing microvilli at the apical membranes, are exposed to the outer medium; the chemical synapses and nude dendrite branches, innervating each receptor cell at more than one site, merge into one myelinated nerve fibre. If the input and output of an electroreceptor are both considered to be electrical signals, then the complete process shows a substantial voltage gain: some electroreceptors react to external voltages in the order of microvolts (Dijkgraaf and Kalmijn,

1966; Bennett 1978), whereas the magnitude of action potentials is in the well-known order of 100 mV. Despite the fact that the sensory information is in the frequency of the action potentials rather than in their amplitude, the output signal is far more powerful than the stimulus. Thus, the principle of transduction in these receptors is posing an interesting problem, comparable with transduction in systems such as vision and hearing. As to the use of the term “transduction” in electroreceptors, one could argue that there is no transduction at all, since stimulus and receptor potential share the same modality: an electrical potential difference. On the other hand, one of the matters of interest is how the stimulus current, essentially an electric field in the water surrounding the fish, is diverted through the receptor ceils, thus enabling transmission of coded information as a neural signal to the brain. Therefore we consider it is useful to use the term “transduction” to indicate the first step in the sensory chain, the change from stimulus in the environment to the intracellular potential that is usually called receptor potential, despite the fact that in this case no change of modality is involved. In a similar way, the next step, i.e. passing the signal from the receptor cell to the nerve fibre, is called (synaptic) transmission, whereas the conversion of postsynaptic potential to spike frequency is usually called encoding. The corresponding decoding takes place in the central nervous system. THE CURRENT

MODEL

OF TRANSDUCTION

Transduction and coding in the different types of electroreceptor proved to be very diverse; the differences could sometimes, but not always, be correlated with the function of a particular receptor type. For instance, the detection of the short and relatively strong electric organ discharges of an electric fish itself needs other sensor properties than the detection 245

246

F.

BRET~CHNEIDERand

eoidermis

fresh

-axon

water

Fig. 1. Schematic diagram of a catfish ampullary electroreceptor. The outer medium, freshwater, is at the left, the inside of the fish at the right. axon: myelinated nerve fibre; i: inward, hence excitatory current; s.c.: sensory cells; asterisk: supposed site of spike generation. One of the sensory cells is depicted with microvilli, which are present in a low

density on the receptor cells and, still more sparse, on the accessory cells (not depicted). of the weak, low-frequency alternating current stemming from a distant prey animal. The transduction mechanism of ampullary electroreceptors in freshwater fish was formulated by Bennett (1967, 1971). Details may also be found in more recent reviews (Bennett and Clusin, 1979; Obara et al., 1981; Bennett and Obara, 1986; Zakon, 1986, 1988; Bodznick, 1989). The current model may be summarized as follows. The apical membrane is the boundary between the cytoplasm of the receptor cell and fresh water, a medium with a low and unpredictable salt content. It is thought to act as a series resistance that is high enough to segregate the cytoplasm from the freshwater medium, but low enough to allow the largest part of the stimulus voltage to appear at the basal, i.e. innervated membrane. The synapses are of the chemically mediating type. They are thought to form the step that is responsible for most of the gain in the whole system. Therefore, this type of synapse was named “high-gain synapse” (Obara and Higuchi, 1980). A number of other synapses, such as the motor end-plate and the synapses of spinal motoneurons, have a voltage gain below one. A continuous flow of neurotransmitter substance constantly depolarizes the afferent terminals, which in turn causes a relatively high spontaneous activity (spike frequency) in the afferent nerve. External electric fields modulate the transmitter flux, and hence the spike frequency. This model was based mainly on the presence of chemical synapses, which are evident from both morphological and physiological data. Mullinger (1964) described, in Zctalurus, elements typical for chemical synapses, such as clear vesicles, synaptic bodies and membrane specializations. Wachtel and Szamier (1969) found, in Kryptopterus, the same features characteristic for chemically mediating synapses. Long delay times, strongly dependent on temperature, were found by Bennett (1967, 1968) and by Roth (1978). In addition, measurement of the intracellular potential in electroreceptors of freshwater catfish Kryptopterus yielded no regenerative response, i.e. no presynaptic spike or any other potential change that appears to be amplified by the

R. C. PETERS

positive feedback inherent in voltage-sensitive ion channels. Although the functioning of chemical synapses is presumed to need an influx of calcium via voltage-dependent calcium channels, this inherent excitability might be either too small to elicit measurable intracellular responses, or, equivalently, might be shunted heavily by leakage channels. The stimulus in the water appears, probably only slightly attenuated, at the basal, i.e. innervated membrane. Other evidence stems from the postsynaptic potentials, measured in the ampullary electroreceptors of the marine catfish Plotosus (Obara and Bennett, 1972). They seem to have normal amplitudes, i.e. about 10 mV. Thus, by the apparent lack of a “transducer”, the ampullary electroreceptor seems to consist mainly of a parallel array of synapses with a high gain, exposed, via a passive membrane, to the stimulus in the water, and followed by a spike generating and propagating nerve fibre (Bennett and Clusin, 1979). On the basis of recent research, however, a more detailed and subtle formulation is needed. RECENT RESEARCH ON AMPULLARY ELECTRORECEPTORS OF CATFISH

In the last decade, new data on the functioning of the catfish electroreceptors became available, that either detail and/or extend the model or question the simplicity of the proposed mechanism. Some of this work is summarized below. Sensitivity

and spontaneous

activity

Although sensitivity and spontaneous activity are functionally coupled to convey external electrical events to the central nervous system, they seem to originate independently, since there is no significant correlation found between the spontaneous frequency (F) of a microampulla and the modulation of the spike frequency caused by the adequate stimulus, called “electrosensitivity” (S) for short (Bretschneider et al., 1980). In addition, firing frequency and modulation react differently to a number of natural or experimental factors. F, for instance, reacts differently to changes in temperature than S. Peters and Bretschneider (1980) found a smooth decrease in spontaneous activity upon cooling, whereas the sensitivity remained constant over almost the entire temperature range. SchHfer et al. (1990) found the frequency characteristics to be strongly temperature-dependent. A strong outward current (a few PA/cm*) abolishes S while leaving F approximately normal (Peters et al., 1975; Bretschneider et al., 1979, 1980), whereas denervation first affects F whilst leaving S normal (Peters et al., 1988; Teunis et al., 1989). This is the main reason for us to express the electrosensitivity S as spike frequency modulation per stimulus strength (dimension Hz per A per cm*, or Hz. cm2/nA) instead of the familiar relative measure, which is dimensionless, and expressed as a percentage. Although used frequently in relation with frequency modulation (FM), the relative modulation depth would be useless here, since it would go to infinity at vanishing “carrier” frequency (spontaneous activity), and thus be uninformative, whereas the absolute modulation has a finite value.

241

Electroreceptors of catfish The maximum sensitivity to electric current that can be attained by ampullary electroreceptors (Kalmijn et al. 1976; Peters and van Wijland, 1974) is the value cited most in conjunction with functional or evolutionary aspects, since this defines the limits to the type of electric phenomena and/or the distance across which electroreception may play a part. In addition, however, the range of sensitivities of individual receptors may be equally interesting. Bretschneider et al. (1980) measured spontaneous activity and electrosensitivity of a number of receptor organs by in-uivo recording from single units of Ictalurus. The sensitivity S is scattered over more than two orders of magnitude: from about 0.01 Hz. cm*/nA up to 3 Hz. cm’/nA. At first sight, this might look as a so-called range fractionation, improving the range of stimulus amplitudes that the fish can cope with, and that might compensate for the lack of efferent control. For other receptor systems, such as the lateral line, inhibition via efferent synapse back to the receptor cells is proposed as a method to control the sensitivity, and hence to extend the range of mechanical stimulus amplitudes, although there might be other, hidden functions (Giirner, 1967; Roberts and Meredith, 1989). In case of the microampullae the occurrence of organs with a sensitivity lower than maximum is less obvious, since the strengths of naturally occurring electric fields are very low, often scarcely above threshold (compare the above-mentioned range of receptor sensitivities with the strength of natural electric phenomena found by Peters and Bretschneider, 1972). Even if we include the d.c. electric field usually associated with the intestinal tract and other epithelia of the fish itself, and the modulation thereof called “breathing potential”, the highest sensitivity found would be appropriate in virtually all stimulus situations, i.e. most receptor organs would operate within the approximately linear range of the operating characteristic. Excluding the detection of strong currents emanating from rusty bicycles and other waste products of modern society, units with a lower than maximum sensitivity seem useless. In addition to scattering between units, the electrosensitivity of each receptor might fluctuate in time, so that the statistical distribution of sensitivities is merely a distribution of snapshots of a varying quantity. Teunis et al. (1989) found approximately synchronous fluctuations of the sensitivity of a control group of electroreceptors used in denervation experiments. This might be caused by changes in the metabolic homeostasis of the fish, e.g. the vasodilatory state of the skin. Moreover, the spontaneous activity of different organs in one fish proved to fluctuate synchronously too.

was found to be intermediate between the 6 db per octave roll-off of the familiar first-order filter and no filtering at all. This has been described as a half-order curve or, more general, as a curve of fractional order (Thorson and Biederman-Thorson, 1974). The frequency characteristics of a fractional-order high-pass filter of order 0.5 is depicted in Fig. 2, in comparison with the well-known first-order high-pass (RC) filter. In our opinion, this type of description deserves more attention in biology: fractional-order characteristics may arise by either a serial chain of closely coupled processes or a parallel convergent arrangement, both of which are likely to occur in a number of biological systems. In the case of the electroreceptor, the source of the fractional-order characteristic still has to be found. Transduction

To elucidate the role of the apical membrane, the influence of the ion composition of the water has been investigated (Roth, 1971; Bauswein, 1977). Although calcium seems to play a part, most effects of altered ion composition can be both mimicked and counteracted by appropriate electrical offset currents (Zhadan and Zhadan, 1975; Peters et al., 1975) indicating that the main effect of altering the medium might be an electrical offset. The magnitude of this offset may amount to about 5 PA in the water, or some l&20 mV across the skin. It is still not known if an inward (exciting) stimulus is carried through the apical membrane in the form of an inward calcium current, an inward current carried by an outflux of anions or, alternatively, consists of a slight decrease of a steady outward potassium or sodium (leakage) current. However, the ampullary electroreceptor of freshwater fish has a remarkable power to adapt to different media. Changes in ion composition that are likely to occur in nature produce electrical offsets of the above-mentioned magnitude, i.e. more than a thousand times the detection threshold. Although sudden changes in ion contents cause an inhibition of the electroreceptors that may last for up to 30 min (Peters et al., 1975, 1989), the sense organs do not seem to be demanding as to the steadystate composition: catfish kept in St Petersburg (Leningrad; ca 0.4mM Ca2+; Zhadan and Zhadan, 1975) Utrecht (ca 1 mM Ca2+; Peters et al., 1975)

Frequency characteristics

Put in classical terms, ampullary electroreceptors of catfish proved to be very “tonic”. Although the spike frequency shows, in response to an electrical step stimulus, substantial adaptation in a few seconds (Roth, 1971) the frequency characteristics of the response extend to very low frequencies (Peters and Buwalda, 1972; Bretschneider et al., 1985). The slope of the high-pass side (i.e. the low frequency roll-off)

-

- first order

-

half order

Fig. 2. Frequency characteristics of first-order and halforder filters compared.

248

F. BRETSCHNEIDER and R. C. PETERS

and Munich (ca 2 mM Ca*+; Roth, 1971) seem to perform equally well. To probe the properties of the apical membrane further, Peters er al. (1989) exposed a part of the skin of catfish, containing several small pits, to blockers of calcium channels via a soft rubber ring, gently pressed onto the dorsal skin of the head region of intact, live fish. The response to blockers as diverse as verapamil (blocks voltage-sensitive calcium channels), tetra-ethyl ammonium (TEA; blocks voltagesensitive potassium channels) and tetrodotoxin (TT’X; blocks voltage-sensitive sodium channels) suggests that these “classical” ion channels are not involved in conduction of the stimulus current. The channels involved might be non-voltage sensitive and/or non-specific cation channels instead. Cadmium, also a potent calcium channel blocker, does have a marked effect on electroreceptor sensitivity. This ion, however, proved to permeate and accumulate in the receptor cells (Zwart er al., 1989). Additional information on the properties of the apical membrane of the receptor cell follows from measurement of the stimulus current that really traverses the receptor, in other words, the “input impedance” of the small pit. The degree to which the current in the water acts as a stimulus depends on the ratio of the resistances of the water and the receptor circuit. If the voltage across the skin is known, the current through one electroreceptor can be calculated from the input resistance of that sense organ. The stimulus strength is reported by most authors as current density in the water (Peters and Bretschneider, 1972; Kalmijn, 1974; Finger, 1986). In approximately homogeneous fields, generated by a pair of large electrodes in a tank with a uniform cylindrical or rectangular cross section, this is indeed the most reliable measure, since it depends only on the total current and the cross-sectional area of the tank. If the conductivity is known, the voltage gradient can be determined by Ohm’s law. The approximate voltage across the skin may then be determined by the size of the fish, since the core of a freshwater fish, through the relatively low specific resistance of the body fluid, assumes an average potential (Kalmijn, 1974; Peters et al., 1974). For the same reason, the potential difference across the active part of an electroreceptor is virtually the same as the skin potential. In one study of the brown bullhead, Ictalurus nebulosus, the potential difference across the skin was measured, by a subcutaneous cannula, synchronous with stimulus current and receptor response (Bretschneider et al., 1979). This showed that, in a fish of this size, a current density of 2 x lo-’ A/cm* causes a transcutaneous voltage of 2 mV. Extrapolation to the behaviourally determined threshold value of 5 x lo-” A/cm2 yields a threshold voltage of 5pV across the skin. In an attempt to estimate the “true” current sensitivity of the catfish electroreceptor, Peters and Mast (1983) compared stimulation via a macroscopic silver electrode with local stimulation of a single receptor organ via the (glass capillary) recording electrode. Because neighbouring receptors responded almost as strongly as the one that was stimulated, the authors concluded that the largest part of the stimulus current

leaked out of the lumen, which means that an electroreceptor must have a very high input impedance. The corrected sensitivity values are in the order of magnitude of 30 HZ/PA. Even without the correction factor the sensitivity amounted to 0.1 f 0.2 Hz/pA. The input resistance (actually the input impedance at a frequency low enough to exclude capacitive current) was estimated by Bretschneider et al. (1991) by inserting a glass pipette into the porus of a Kryptopterus electroreceptor. The impedance proved to be very high; at least about 40 MQ per ampulla. This permits us to calculate the (maximum) amount of electric current necessary for transduction, At the above-mentioned voltage threshold, this is as little as 0.13 pA. It must be kept in mind, however, that this is a comparison of the statistically determined detection limit of whole animals with the current into one of the many hundreds or even thousands of electroreceptors. Thus, an electroreceptor may not be said to “respond” to less than 1 pA, even if the spike train of most receptors will contain a minute signal component. Transmission

The chemical nature of the synapses between receptor cells and nerve fibre was already suggested strongly by electron microscopical work (Mullinger, 1964; Wachtel and Szamier, 1969). It was confirmed physiologically by the two-pulse method (Teeter and Bennett, 1981) and by the very long latency at low temperatures (Roth, 1978; Schafer et al., 1990). In addition, the frequency characteristics, more specifically a comparison of the amplitude curve with the phase curve, indicate a delay of about 10msec at room temperature (Bretschneider ef al., 1985) whereas the optimum in the amplitude curve shifts to lower frequencies at lower temperatures (Schafer et al., 1990). The nature of the transmitter was investigated by applying different candidate transmitter substances to receptors in isolated fins of Kryptopterus (Teeter and Bennett, 1981). They found the best responses with glutamate, a substance that has been proposed earlier as a transmitter in lateral&type sense organs. Synaptic noise

Apart from spontaneous activity and responses to stimuli, the spike train of an electroreceptor shows fluctuations, or noise, that may originate partly or wholly in the receptor organ. Usually, to assess the response of the organ to a man-made stimulus, the magnitude of such fluctuations is reduced by the experimenter through an averaging process. In addition, however, analysis of the noise itself may contain cues about the functioning of the sense organ. Analysis of the spike train from an electroreceptor may be performed with two different goals in mind: analysis of the spike generation process per se and/or analysis of the processes that precede the generation of spikes. In the latter case, one tries, as it were, to probe the “analog” processes in the receptor cells and in the synaptic terminals by “looking through” the spike generator. Obviously, to describe the functioning of the entire receptor organ one must unify the results, but it seems useful to segregate the black box

Electroreceptors of eat&h into the above-mentioned subsystems wherever possible. It is equally obvious that answering different questions demands different techniques of analysis. The relation of spike frequency to stimulus current (Z/O curve for short) in ampullary electroreceptors of freshwater fish is approximately linear (Gymnotur: Bennett, 1971; Kryptopterus: Bennett, 1971; Zctalurus: Bretschneider et al., 1980); not logarithmic (Bretschneider et al., 1979; Finger, 1986). More precisely, the Z/O curve over the entire stimulus amplitude domain is sigmoidal, but has a virtually linear part in the vicinity of zero stimulus, i.e. at the weak stimuli found in nature. Therefore, measures of spike frequency such as instantaneous frequency or average spike density reflect the “analog” processes that precede spike generation. The spike generation process is, as it were, transparent. Spike density is easily quantified by the well-known post-stimulus time histogram. Because electroreceptors are usually stimulated with continuous waveforms rather than short pulses, it is more correct to call it “post-trigger time histogram” (PTTH). The trigger may be a zero-crossing or any other fixed point on the stimulus waveform. Depicted in this way, the response to a sinusoidal stimulus with moderate intensity is again sinusoidal, which makes the receptor suited to be subjected to a number of systems-analytical methods. This is why the modulation amplitude of the spike frequency divided by the stimulus amplitude may be called “the” sensitivity, S of the receptor organ. Unfortunately, the influences of the mentioned sub-processes cannot always be segregated. Fluctuations of the spike frequency might be caused in part by presynaptic and transsynaptic processes, in part by spike generation processes. Normal functioning electroreceptors show relatively large fluctuations in the spike frequency. Under certain experimental conditions, the scatter is greatly reduced. This was found in media lacking calcium (Roth 1971; Bretschneider et al., 1980; Peters et al., in press) and during strong, outward d.c. stimulation (Peters et al., 1975; Bretschneider et al., 1980). We tentatively explained this by assuming that either of the above-mentioned interventions suppressed synaptic transmission, so that the small scatter that remains reflects random processes in the spike generator region. If this is true, by far the largest part of the spike frequency fluctuations arise in synaptic processes, the assumed quanta1 release of chemical transmitter being the obvious candidate. In addition, some experimental factors appear to increase, rather than to decrease the irregularity of the spike train. This was found for example at extreme temperatures (Schafer et al., 1990), and after denervation (Peters et al., 1988; Teunis et al., 1989). As a first approximation, spike train statistics may be described from the series of spike interval times by the familiar estimators first moment (mean) and second central moment. The latter is known better as standard deviation, but it must be kept in mind that the distribution of spike interval times is often skewed, so that tests based on the normal distribution may not be applied. Deviations from normality may then be assessed by the third central moment (skewness) and the fourth (kurtosis), and are suited to describe and

249

compare spike interval series collected under different circumstances. The analysis gains in power, however, if one tries to fit, wherever possible, a distribution based on a mathematical or statistical model of the processes that may underly the generation of the spike series. Stein (1965, 1967) described a model of spike generation by assuming a randomly varying release of synaptic transmitter substance with a mean rate of rl quanta per second, in combination with a spike threshold at the depolarization where r (r 2 1) synaptic quanta are necessary to elicit a spike. The size of the quanta is supposed to be constant, and each spike is assumed to reset the system. Under these circumstances, the interspike interval series follows a gamma distribution, which has the two mentioned quantities as parameters: the scale parameter 1 (mean quantum rate) and the shape parameter r (number of quanta per spike). This distribution covers interval distributions between two extremes: if each quantum elicits a spike (r = l), spike generation turns into a Poisson process, the interval distribution of which is exponential; if a very large number of quanta is required (r -Bco), the intervals follow a normal distribution. At intermediate values of r (about S-lo), the interval histogram assumes the slightly skewed form that is found most often (see Fig. 3). Teunis et al. (1991a,b) applied the Stein model to spike generation in the catfish electroreceptor, because it is likely that (i) the postsynaptic potential is incremented by synaptic quanta, and (ii) a spike results from the release of one or more quanta in a certain time interval. In a number of situations, interspike interval data of spontaneous activity can be fitted reasonably well to a gamma distribution. Severe deviations, such as a very broad range, double peaks etc., invariably signify malfunction of the receptor, such as degeneration (Teunis et al., 1991) or a very high temperature (Schafer et al., 1991). The skew usually found in the interval histogram (low values of r) implies that a small number of synaptic vesicles, perhaps 10-l 5, is sufficient to elicit a postsynaptic spike. This implies that the spike generator is a sensitive step in the transduction process, thus supplementing the synaptic gain to explain the total sensitivity of the electroreceptor. In addition, analysis of the shape of the interval distribution may be used to assess the “health” of an ampullary electroreceptor. The function of postsynaptic convergence

In elcctrorcceptor organs, a certain number of receptor cells, and thus postsynaptic terminals,

Fig. 3. Shape of gamma distributions with four different values of the shape parameter Y. t, interspike interval time; f. statistical frequency.

250

F.

BRETSCHNEIDER and

converges onto one afferent nerve fibre. In Kryptopterus, about 25 receptor cells converge onto a single afferent (Bretschneider et al., 1991). In this species, each individual organ appears to have its own afferent fibre. In other fish species, however, a second stage of convergence exists: a number of neighbouring electroreceptors, each with one afferent, appears to converge in turn onto the same fibre. If the noise of the spike frequency arises from the quanta1 nature of synaptic transmission, one would expect a higher convergence ratio (i.e. more receptor cells or receptor organs onto one afferent fibre) to cause a reduction of the noisiness. This follows from the fact that addition of organs to one spike generator results in summation of the postsynaptic potentials. More precisely, one might expect averaging rather than summation, since not only the amount of excitatory postsynaptic current, but also the area of passive, leaking membrane surface increases with increasing convergence ratio. Therefore, one expects single electroreceptors to have a spontaneous activity that fluctuates more strongly than the more convergent receptor arrays. Peters and Mast (1983) recorded, in the African catfish Ciarias, from clusters consisting of two to six ampullae, and found a slight increase in the sensitivity with the number of contributing receptors. They argued that the spike initiating zone might grow slightly less than proportional to the number of ampullae converging to it. On the other hand, Peters and van Ieperen (1989) reported, from juveniles of the same species, a much higher gain in sensitivity associated with the convergence ratio. They did not find a concomitant increase in the frequency of the spontaneous activity. Both results are hard to explain. Teunis et al. (1990) found, with adult Clurius closely matched in age and size, no significant effects of convergence, neither in sensitivity nor in spontaneous activity or the fluctuations thereof. They argued that, if neither the sensitivity nor the noise are improved, the only functional value of increased convergence will be the improved robustness against small skin trauma. Sanchez (1988) and Sanchez and Zakon (1987) found, in the tuberous receptors of the weakly electric fish Sternopygus, a substantial increase in sensitivity with convergence. They devised a model for the relation of sensitivity to convergence ratio under the assumption that all branches from all organs, and from all cells within an organ, conduct spikes, and that all spikes are conducted to the final, single afferent. Although they do not take into account any possible retrograde “resetting” of electrically excitable branches by the fastest “pacemaker”, such as described by Pabst (1977) for the lateral line of Xenopus, the increase in sensitivity predicted by the model matches the values they found. Altogether this means that the effect of increasing convergence ratio depends on whether the contributing branches conduct electrotonic signals or spikes. In our opinion, the most plausible situation is that the short, very thin branches within a receptor organ conduct the excitation level by means of an analog signal, i.e. electrotonic currents, whereas the longer distances between neighbouring pit organs are bridged by the conduction of spikes.

R. C. PEERS PROSPECT

Despite the new findings and the relative simplicity of the catfish electroreceptor a number of problems concerning the transduction are still enigmatic, such as the role of the apical membrane. What ion species carry the stimulus current, through what types of ion channel? What is the function of specializations of the apical membrane like microvilli (most catfish species) and cilia (Xenomystis). Is the influence of the ion composition of the water purely an electrical offset, or has calcium a “chemical” influence on the apical membrane? Why is the input impedance so high? What type of amino acid receptor sites mediate the postsynaptic depolarization? Does the noise that causes the large fluctuations in spike frequency indeed stem from the synaptic quanta? Are synaptic quanta constant or dependent on the activity level? How are the contributions of the different receptor cells averaged? Is there one or more than one locus of spike generation? What is the relevance of convergence of more than one receptor onto one afferent fibre? What structures are responsible for the peculiar bandpass frequency characteristics? To answer these questions will need a multifarious approach; physiological and pharmacological methods to pinpoint the involved ion channels, receptor molecules etc., pure morphological as well as histochemical methods to elucidate the role of different cell parts and organelles and, last but not least, bchavioural research to link the recorded single-unit response to biologically significant perception and behaviour. Acknowledgement-We

acknowledge critical comment on the manuscript by Prof. Dr W. A. van de Grind and by Dr P. S. Heijmen, and the draftsmanship of the Design and Image Processing department of the biology faculty.

REFERENCES Bauswein E. (1977) Effect of calcium on the differentiating operation of the ampullary electroreceptor in Ictalurus nebulosus. J. camp. Physiol. 121, 381-394.

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