Nematocyst discharge in Hydra vulgaris: Differential responses of desmonemes and stenoteles to mechanical and chemical stimulation

The effects of mechanical and chemical stimulation on desmoneme and stenotele discharge in Hydra vulgaris were quantified, in situ, in isolated tentacles and single tentacles with attached hypostomes. Cnidocils of desmonemes and stenoteles were stimulated 24 h after tentacle ablation. Single impacts with maximum calculated forces of 3.7 × 10− 3 to 3.8 × 10− 2 Newtons were delivered by piezoelectrically-driven glass capillary probes. Video analysis revealed that desmonemes discharged at forces of 3.7 × 10− 3 N; stenoteles required forces of 1.9 × 10− 2 N. Desmonemes not directly stimulated discharged if another desmoneme was adequately stimulated; the effect was carried through to at least two adjacent battery cells. Tentacles responded to desmoneme stimulation, by bending at the stimulation site. These findings imply afferent excitatory pathways between nematocytes onto other nematocytes and myonemes. Locomotory nematocysts (atrichous isorhizas) discharged only in hypostome-attached tentacles contacting a substrate; desmonemes and stenoteles did not discharge during substrate attachment, implying differential neuronal inhibition. At low concentrations, bath-applied mucin, a prey-associated glycosylated protein, lowered desmoneme and stenotele firing thresholds, abolishing the force dose-dependency in stenoteles, and allowing them to discharge at previously below threshold forces. At higher concentrations, mucin inhibited discharge, suggesting an involvement in prey-induced feeding inhibition.