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1) We determined the frequency response of the electroreceptive microampullae in the African mudfish, Clarias lazera (C. and V.) at temperatures of 10, 15, 20, 25 and 30°C. Maximum modulation of the spontaneous activity was found at stimulus frequencies of 10 to 30 Hz. 2) The sensitivity expressed as the change in spike rate caused by the gross current through one microampulla was found to be about 0.4 Hz/pA at 25°C for stimuli of 10 Hz. This corresponds to a sensitivity of 0.7 Hz/nA/cm2 at ‘macrostimulation’ via a horseshoe-shaped electrode (Fig. 1.). Fig. 1. Diagram of the experimental setup. To stabilize the stimulus current through the recording electrode we used an a.c. voltage source and a series resistance of 1 GΩ. The other current source consisted of an a.c. voltage source and an op-amp voltage-to-current converter. We used the compensation signal to suppress the stimulus artefact. Figure options Download full-size image Download as PowerPoint slide 3) The difference between the temperature dependence of the spontaneous activity and the modulation of the spontaneous activity suggests two different underlying biochemical mechanisms. We suggest that receptor cells detect stimuli by measuring the amount of metabolical energy that is needed to compensate electrochemical disturbances—which is what stimuli are—in order to maintain intracellular homoiostasis. 4) The presence of microampullae was verified histologically. 1% Neutral red in tapwater proved to be a helpful stain in tracing microampullae. Surprisingly cupulae of free neuromasts were coloured brown by neutral red; this is ascribed to the precipitation of neutral red by the intracupular potassium ions. We also found neutral red to precipitate in microampullae.

Denervation-induced changes in the synaptic efficacy of single electroreceptors in catfish (Ictalurus nebulosus, Teleostei) were studied in vivo under alfadolone anaesthesia. At 16 degrees C the following effects were found 48 h post-operatively: (1) the average amplitude of the extra-dermally recorded spikes decreased from 100 to less than 20 microV; (2) the average resting discharge decreased from 40 to less than 25 spikes/s; (3) neither the sensitivity nor the frequency characteristic changed. The results indicate that the resting discharge and the modulation mechanism of sensory synapses are controlled by different biochemical mechanisms. The resting discharge seems to be related to the trophic function of the afferent nerve and to its generator region, whereas the modulation mechanism is apparently associated with the receptor cell.