Further indications of a chemical synapse in the electroreceptors of the catfish

Summary

• 1. The ampullary electroreceptors in the skin of catfish consist of 10 to 20 secondary sensory cells which are in contact with the fresh water around the fish at their apical membrane, and which have synaptic contacts with an afferent nerve fiber at their basal membrane. The afferent nerve fiber generates 50 to 70 impulses per second spontaneously in “fresh water” conditions, i.e. with 2 to 10 mM CaCl₂ solutions over the skin of the fish. If the “fresh water” medium is made hypertonic by adding 500 mM glycerine or sucrose per liter, the spontaneous activity of the afferent fiber increases to 120 imp/s within 150 s (Fig. 1).

• 2. The afferent fiber responds to the onset of a rectangular anodal current pulse traversing the receptor with an impulse frequency increase. With high stimulus intensities and at high temperatures (24 °C) the response begins not less than 5 ms after the stimulus onset; with low intensities and at low temperatures (7 °C) this latency is prolonged to 60 ms (Fig. 2).

• 3. With a series of repetitive rectangular stimuli, the afferent fiber responses decrease within seconds (Fig. 3A_1,2, C_1,2). At the end of an (impulse-frequency-increasing) anodal series, spontaneous activity is reduced (Fig. 3B_1,2), whereas after the (impulse-suppressing) cathodal series, impulse activity is increased (Fig. 3D_1,2).

• 4. These findings support the hypothesis that the synapses between the sensory cells and the afferent nerve fiber are chemical and that the transmitter is excitatory.