Mechanoreception and synaptic transmission of hydrozoan nematocytes

Mechanoelectric transduction and its ultrastuctural basis were studied in the cnidocil apparatus of stenotele nematocytes of marine and freshwater Hydrozoa (Capitata and Hydra) as a paradigm for invertebrate hair cells with concentric hair bundles. The nematocytes respond to selective deflection of their cnidocil with phasic-tonic receptor currents and potentials, similar to vertebrate hair cells but without directional dependence of sensitivity. Ultrastructural studies and the use of monoclonal antibodies allowed correlating the mechanoelectric transduction with structural components of the hair bundle. Two other types of depolarising current and voltage changes in nematocytes are postsynaptic, as concluded from their ionic and pharmacological characteristics. One of these types is induced by mechanical stimulation of distant nematocytes and sensory hair cells. It is graded in amplitude and duration, but different from the presynaptic receptor potential. Adequate chemical stimulation of the stenoteles strongly increases the probability of discharge of their cnidocyst, if the chemical stimulus precedes the mechanical one. Simultaneously, the probability of synaptic signalling induced by mechanical stimulation is increased, reaching nearly 100%. The chemoreception of the phospholipids used could be localized in the shaft of the cnidocil, because of the water-insolubility of the stimulant. This chemical stimulation itself does not cause a receptor potential; its action is classified as a modulatory process. Electron microscopy of serial sections of the tentacular spheres of Coryne revealed synapses that are efferent to nematocytes and hair cells besides neurite-neurite synapses, each containing 3–10 clear and/or dense-core vesicles of 70–150 nm diameter. The only candidates to explain the graded afferent signal transmission of nematocytes and hair cells are regularly occurring cell contacts associated with 1(–4) clear vesicles of 160–1100 nm diameter. Transient fusion and partial depletion of stationary vesicles are discussed as mechanisms to reconcile functional and structural data of many cnidarian synapses.