Chemoreception in Hydra vulgaris (attenuata): initial characterization of two distinct binding sites for l-glutamic acid

Abstract

To elucidate the relationship between l-glutamic acid and the putative chemoreceptor for glutathione, binding of l-[ 3H]glutamate to a crude membrane fraction from Hydra vulgaris (attenuata) has been characterized. The binding of l-[ 3H]glutamate was rapid, reversible and saturable. A Scatchard analysis of the specific binding revealed values of 10 μM for the dissociation constant ( K d) and 170 pmol/mg for the maximal capacity of binding sites ( B max). A maximum of 65% of the specific l-[ 3H]glutamate binding was inhibited by the chemostimulatory peptide, glutathione. This glutathione-sensitive glutamate binding presumably represents the association of glutamate with a putative chemoreceptor which modulates feeding behavior in hydra. The remaining 35% of the specific l-[ 3H]glutamate binding may be due to a second class of glutamate binding sites which is insensitive to glutathione. The identification of glutathione-insensitive glutamate binding is the first indication of a putative glutamate receptor, which may mediate an action independent of the glutathione-induced feeding response. The glutathione-insensitive and glutathione-sensitive sites must have similar affinities for glutamate since these sites were indistinguishable by Scatchard analysis. A preliminary characterization of the glutathione-insensitive site, performed in the presence of saturating levels of glutathione, revealed inhibition of glutathione-insensitive glutamate binding by kainate and quisqualate, but not by N-methyl- d-aspartate. A comparison of glutamate analogue inhibition of total specific l-[ 3H]glutamate binding with analogue inhibition of glutathione-insensitive l-[ 3H]glutamate binding suggests that kainate and α-aminoadipate may be selective ligands for the glutathione-insensitive and glutathione-sensitive glutamate binding sites, respectively.