Control of Cnida Discharge: I. Evidence for Two Classes of Chemoreceptor

Appropriate chemical stimulation of cnidocytes along with mechanical stimulation is required to trigger discharge of cnidae. It has been generally assumed that such chemosensitization is mediated via specific chemoreceptors. Such chemoreceptors and their complementary ligands have never been identified. We now identify two groups of naturally occurring substances that chemosensitize cnida discharge in the feeding tentades of the sea anemone, Aiptasia pallida. In addition, using a novel technique to quantify cnida discharge we demonstrate that these chemosensitizers act through at least two distinct classes of receptors. One class is broadly specific toward a variety of amino and imino acids and histamine (K0.5 = 11-30 nM), but is competitively inhibited by antihistamines (Ki = 0.1-7.4 µM). A second class is specific for N-acetylated sugars (K0.5 = 0.1-1.5 µM), but not affected by antihistamines. Presumably, these chemoreceptors detect specific substances from potential prey. Thus, cnidocytes are sensitized to discharge their cnidae in response to mechanical stimuli originating from the prey.