N-acetylneuraminic acid (NANA) stimulates in situ cyclic AMP production intentacles of sea anemone (Aiptasia pallida): Possible role in chemosensitization of nematocyst discharge

Cnidocytes, the stinging cells of cnidarians, optimally discharge nematocys ts in response to combined physical contact and stimulation of specific che moreceptors, In the tentacles of certain sea anemones, the primary chemorec eptors bind N-acetylated sugars, such as N-acetylneuraminic acid (NANA), Se nsitization with NANA predisposes contact-sensitive mechanoreceptors (CSMs) to trigger discharge in response to physical contact, In the ectoderm of s ea anemone tentacles, cnidocyte/supporting cell complexes (CSCCs) control a nd trigger nematocyst discharge, Previous findings have implicated cyclic A MP (cAMP) as a second messenger in NANA-sensitized nematocyst discharge. Ho wever, no reports have directly demonstrated that the cAMP content of tenta cles changes in response to NANA stimulation. We now show that NANA elevate s in situ cAMP levels in a dose-dependent manner in the ectoderm of tentacl es from the sea anemone Aiptasia pallida, However, the endoderm of tentacle s shows no detectable cAMP response to NANA, The effect of NANA on the cAMP content of the ectoderm is biphasic, Micromolar NANA increases the in situ cAMP level, with a maximal response occurring at 1.8x10(-5)mol l(-1) NANA, At higher NANA concentrations, the cAMP content decreases to that of contr ols. Because the cAMP dose/response curve to NANA coincides precisely with the dose/response curves of NANA-sensitized nematocyst discharge and nemato cyst-mediated adhesive force, a second-messenger role for cAMP in NANA-sens itized nematocyst discharge is strongly suggested, The addition of isobutyl -l-methylxanthine (IBMX) to the medium with sea anemones increases tissue c AMP levels both in the absence and in the presence of NANA, However, anesth etizing anemones in sea water containing high levels of Mg2+ blocks the NAN A-stirnulated cAMP response of the ectoderm. In addition, our results sugge st that NANA-stimulated cAMP may activate endogenous cAMP-dependent protein kinase (PKA) in broken cell preparations of tentacles, Thus, NANA-stimulat ed cAMP may function as a second messenger in the NANA chemosensory signali ng pathway controlling nematocyst discharge.