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Glutamatergic Transmission in Hydra: NMDA/D-Serine Affects the Electrical Activity of the Body and Tentacles of Hydra vulgaris (Cnidaria, Hydrozoa)

Department of Biological Sciences, University of Rhode Island, Kingston, Rhode Island 02881

^* To whom correspondence should be addressed. E-mail: kass.simon{at}uri.edu

Previous electrophysiological studies on the early-evolved metazoan Hydra vulgaris provided evidence that glutamate, acting through -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate receptors, affects hydra's pacemaker systems; immunocytochemical studies showed that N-methyl-D-aspartate (NMDA) receptors were present in hydra tentacles; behavioral studies demonstrated that NMDA/D-serine affected mouth opening induced by reduced glutathione, and with AMPA/kainate, discharge of nematocysts. In this study, extracellular recordings were made from the tentacle and peduncle of hydra during bath application of NMDA and D-serine (both at 1 x 10^–5 mol l^–1 to 1 x 10^–9 mol l^–1) in the presence of 1 x 10^–7 mol l^–1 AMPA or kainate. NMDA/D-serine produced a significant increase in tentacle activity, increasing the rate of tentacle pacemaker pulses (TPs) at 1 x 10^–7 mol l^–1, and small, behaviorally uncorrelated tentacle pulses (SUTPs) at 1 x 10^–5 mol l^–1. The NMDA antagonist, D-2-amino-5-phosphonopentanoic acid (D-AP5), counteracted the effects. NMDA/D-serine (1 x 10^–7 mol l^–1) also caused a potentially significant (trend) decrease in the rate of small, behaviorally uncorrelated electrical body pulses (SUBPs) and rhythmic potentials (RPs). The effect was counteracted by D-AP5. The ectodermal contraction burst (CB) pacemaker system was unaffected by NMDA/D-serine. Our results indicate that glutamate, acting on NMDA/AMPA-kainate receptors, may cause opposing effects on the coordinating systems of tentacle and body—exciting the tentacle effectors and potentially causing an inhibition in the body column.

Abbreviations: AMPA, -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid • bT, blocking period • BVC, bicarbonate versene culture solution • CB, contraction burst • Con A, concanavalin A • D-AP5, D-2-amino-5-phosphonopentanoic acid • FANOVA, Friedman two-way analysis of variance • GABA, -amino-butyric acid • iGluR, ionotropic glutamate receptor • LTD, long-term depression • LTP, long-term potentiation • m. ± s.d., mean ± standard deviation • m. ± s.e., mean ± standard error • m.d. ± i.q.r, median ± inter-quartile range • mGluR, metabotropic receptor • NMDA, N-methyl-D-aspartate • NMDAR, NMDA receptor • P/CB, average number of pulses per contraction burst • PLP, pre-locomotor pulse • pT, pre-treatment period • RP, rhythmic potential • SUBP, small, uncorrelated body pulse • SUTP, small, uncorrelated tentacle pulse • T1, treatment period 1 • T2, treatment period 2 • TP, tentacle pulse