Glutamate, acetylcholine, and ?-aminobutyric acid as transmitters in the pyloric system of the stomatogastric ganglion of a stomatopod, Squilla oratoria

Abstract

The neurotransmitters mediating the synaptic interactions in the pyloric system of the stomatogastric ganglion of a stomatopod, Squilla oratoria, were examined. Putative transmitters were applied iontophoretically to the pyloric cells. Glutamate and GABA produced inhibitory responses in all motoneurons but acetylcholine did not. These inhibitory responses were due to increases in conductance to either K+ or Cl− or both, and blocked by picrotoxin. The inhibitory postsynaptic potentials evoked by the constrictor and dilator neurons were different in their time courses, reversal potentials, ion selectivities, and picrotoxin sensitivities. Glutamate is a transmitter candidate for inhibitory synapses made among the pyloric cells as well as for their neuromuscular junctions. In some cells, glutamate and acetylcholine evoked excitatory responses which were blocked by joro spider toxin and by tubocurare, respectively. They mediated the extrinsic inputs to modulate the pyloric rhythm. The transmitter, glutamate, is conserved in the ganglion neurons between stomatopods and decapods during evolution. Use of two transmitters, glutamate and acetylcholine, may have evolved in decapods, while the ionic mechanism is preserved in both orders. The neuromodulators, acetylcholine and γ-aminobutyric acid, are conserved between both orders. Glutamate may be used as the neuromodulator in stomatopods.