Pacemaker activity in hydra is modulated by glycine receptor ligands

In the mammalian central nervous system, the neurotransmitter, glycine, acts both on an inhibitory, strychnine-sensitive receptor (GlyR) and an excitatory, strychnine-insensitive site at the NMDA receptor. Here we present electrophysiological evidence that the strychnine-sensitive glycine agonists, glycine and taurine, and the antagonist, strychnine, affect the endodermal rhythmic potential (RP) system and that the ectodermal contraction burst (CB) pacemaker system is modulated by glycine and strychnine in hydra. The RP and CB pacemaker systems are responsible for the respective elongation and contraction of hydra's body column. Activity of the CB system, quantified by the rate of contraction bursts (CBs), the number of pulses per contraction burst (P/CB), and the duration of bursts, was decreased by glycine. Glycine, coadministered with the strychnine-insensitive glycine site blocker, indole-2-carboxylic acid (I2CA), decreased RPs but not CBs or P/CB. The effect was mimicked by taurine. Strychnine increased the duration of RP production, and decreased CB duration. The effect of glycine with I2CA was counteracted by strychnine. The results support the idea that a vertebrate-like GlyR may be involved in modulating activity of the endodermal RP system and suggest that a glycine site on an NMDA receptor may be involved in the CB system.