Hydrozoan nematocytes send and receive synaptic signals induced by mechano-chemical stimuli

Nematocytes, the stinging cells of Hydrozoa, can be considered as prototypic mechanosensory hair cells bearing a concentric hair bundle, the cnidocil apparatus. These cells produce typical mechanoreceptor potentials in response to deflection of their cnidocil. Here we show that mechanosensory signals are relayed to neighbouring nematocytes via chemical neurotransmission and that nematocytes receive synaptic input from surrounding nematocytes, hair cells and probably from epithelial cells. Intracellular voltage recordings from stenotele nematocytes of capitate hydroid polyps showed two distinct types of responses when other nematocytes within the same tentacle were mechanically stimulated: (i) graded depolarizations of variable duration ('L-potentials'), and (ii) uniform impulse-like, often repetitive depolarizations ('T-potentials') that occurred in correlation with contractions of epitheliomuscular cells. Voltage clamp experiments showed that despite the stereotyped time course of T-potentials, their generation did not involve electrically excitable conductances. Instead, time course, post-stimulus delay, susceptibility to blockers of neurotransmission and gap junctions, and induction by electrical stimulation of other nematocytes indicate that L- and T-potentials are postsynaptic, most likely glutamatergic potentials. Both result from different presynaptic pathways: L-potentials are induced monosynaptically by presynaptic receptor potentials, T-potentials are most likely triggered by presynaptic action potentials propagating through the ectodermal epithelium via gap junctions. Moreover, contact-chemosensory (phospholipid) stimulation of the presynaptic nematocyte is a positive modulator of the nematocyte's afferent synaptic efficacy and of cnidocyst discharge, both triggered by mechanoreceptor potentials. The results reveal that hydrozoan nematocytes act as bimodal sensory cells, signalling coincident chemical and mechanical stimuli indicative of prey, and receive signals from other nematocytes and sensory cells.