Ampullary electroreceptors in the sturgeon Scaphirhynchus platorynchus (rafinesque)

1. Ampullary receptors in the skin of the shovelnose sturgeon, Scaphirhynchus platorynchus, were examined morphologically and physiologically. As in the chondrostean paddlefish, Polyodon (Jørgensen etal., 1972), and in the ampullae of Lorenzini of elasmobranchs (Waltman, 1966; Szabo, 1972; Szamier and Bennett, 1980) each receptor cell has a cilium on its apical (lumenal) surface (Figs. 2, 3). Several synaptic projections from the basal part of the receptor cell fit into invaginations of the innervating nerve terminals (Fig. 3). A dense ribbon extends into each projection and the ribbons are covered with vesicles above the projections. Each ampulla is innervated by a single nerve fiber. 2. The primary afferent fibers innervating these receptors are spontaneously active (20-60 impulses/s) and this activity is modulated by electrical stimuli of less than 1 mV applied at the receptor opening. Cathodal stimuli, which make the outside of the skin negative with respect to the inside, accelerate the resting nerve discharge (Fig. 4). As a cathodal stimulus is increased beyond the level at which the maximum nerve discharge is evoked, the nerve response decreases in frequency until it is completely blocked (Figs. 5, 7). Anodal stimuli of increasing strength decelerate and eventually block the resting nerve discharge (Fig. 4). Increasing the strength of an anodal stimulus beyond the level at which the resting discharge is blocked results in a progressive return of nerve impulses (Figs. 5, 7). 3. Application of 10 mM CoCl2 or MgCl2 to the receptor openings usually produces a rapid and reversible block of both the resting and evoked nerve discharge (Fig. 8). Recovery from this suppression is facilitated by application of 10 mM CaCl2. 4. These results are similar to those obtained with elasmobranch ampullae of Lorenzini and suggest that a similar mode of operation is present (Clusin and Bennett, 1979a). We conclude that the ampullary organs of chondrostean fishes are electroreceptors. Their morphological and functional similarities to the ampullary receptors in elasmobranchs suggest that they should be classified as ampullae of Lorenzini.