Synaptic transmission in the ampullary electroreceptor of the transparent catfish, Kryptopterus

1. Ampullary electroreceptors of the transparent catfish Kryptopterus bicirrhus were studied in situ and in an isolated fin preparation. A microelectrode in the ampulla or beneath the receptor epithelium recorded discharges of the afferent fiber. The receptors are sensitive tonic receptors and the tonic discharge was markedly modified by stimuli as low as 0.1 mV applied at the ampullary opening, anodal (outside positive) stimuli being excitatory and cathodal stimuli being inhibitory (Fig. 1). 2. Strong anodal stimuli excited the fiber with a minimum delay of about 1.3 ms (Fig. 2). Further increase in stimulus strength led to a jump in latency to less than 0.3 ms, ascribable to direct stimulation of the afferent fiber. Thus, there is a minimum synaptic delay of about 1 ms. A brief strong excitatory stimulus was followed by an after-discharge that could not be blocked by strong inhibitory stimuli (Fig. 3). These data are difficult to explain by electrical transmission from receptor celt to afferent fiber, but are consistent with chemical transmission. 3. Provided the skin barrier to penetration of solutes was disrupted, high Mg⁺² solutions (15-30 mM) produced a rapid and reversible depression of spontaneous and evoked discharges (Figs. 4-5). High Ca⁺² solutions (15 mM) had little effect. 4. L-glutamate at a concentration of 1 μM strongly excited the afferent fiber. The excitation also occurred when spontaneous and stimulus evoked activity was blocked by high Mg⁺² solution. L-glutamate at 1 mM transiently excited, then blocked the afferent fiber, probably by excessive depolarization of the terminals (Figs. 6, 7). 5. GABA, L-aspartate, D-glutamate (Fig. 8) and cholinergic drugs had little effect. 6. We conclude that transmission from receptor cell to afferent fiber is chemically mediated by a Ca dependent process, and that L-glutamate is a candidate transmitter for this synapse.