Nitric oxide regulates swimming in the jellyfish Aglantha digitale

The cnidarian nervous system is considered by many to represent neuronal organization in its earliest and simplest form. Here we demonstrate, for the first time in the Cnidaria, the neuronal localization of nitric oxide synthase (NOS) in the hydromedusa Aglantha digitale (Trachylina). Expression of specific, fixative-resistant NADPH-diaphorase (NADPH-d) activity, characteristic of NOS, was observed in neurites running in the outer nerve ring at the base of the animal and in putative sensory cells in the ectoderm covering its tentacles. At both sites, diphenyleneiodonium (10^-4 M) abolished staining. Capillary electrophoresis confirmed that the NO breakdown products NO₂^- and NO₃^- were present at high levels in the tentacles, but were not detectable in NADPH-d–negative areas. The NADPH-d–reactive neurons in the tentacles send processes to regions adjacent to the inner nerve ring where swimming pacemaker cells are located. Free-moving animals and semi-intact preparations were used to test whether NO is involved in regulating the swimming program. NO (30–50 nM) and its precursor L-arginine (1 mM) stimulated swimming, and the effect was mimicked by 8-Br-cGMP (50–100 μM). The NO scavenger PTIO (10–100 μM) and a competitive inhibitor of NOS, L-nitroarginine methyl ester (L-NAME, 200 μM), significantly decreased the swimming frequency in free-moving animals, while its less-active stereoisomer D-nitroarginine methyl ester (D-NAME, 200 μM) had no such effect. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one (ODQ, 5–20 μM), a selective inhibitor of soluble guanylyl cyclase, suppressed spontaneous swimming and prevented NO-induced activation of the swimming program. We suggest that an NO/cGMP signaling pathway modulates the rhythmic swimming associated with feeding in Aglantha, possibly by means of putative nitrergic sensory neurons in its tentacles.