Two novel types of L-glutamate receptors with affinities for NMDA and L-cysteine in the olfactory organ of the Caribbean spiny lobster Panulirus argus

Abstract

A subset of olfactory receptor neurons of the Caribbean spiny lobster Panulirus argus possesses receptors for L-glutamate that can mediate both excitatory and inhibitory responses (P.C. Daniel, M.F. Burgess, C.D. Derby, Responses of olfactory receptor neurons in the spiny lobster to binary mixtures are predictable using a non-competitive model that incorporates excitatory and inhibitory transduction pathways, J. Comp. Physiol. A 178 (1992) 523-536). In this study, we have used biochemical and electrophysiological techniques to understand the role of these receptors in olfactory transduction, and to compare these olfactory glutamate receptors with peripheral and central L-glutamate receptors in other animals. Using a radioligand-binding assay with a membrane-rich preparation from the dendrites of olfactory receptor neurons, we have identified two types of binding sites for L-glutamate. Both sites showed rapid, reversible, and saturable association with radiolabeled L-glutamate, and their Kd values (1 nM and 3 microM) are effective in physiological studies of glutamate-sensitive olfactory neurons, suggesting these binding sites are receptors involved in olfactory transduction. Both sites were completely inhibited by high concentrations of NMDA and L-cysteine, and only partially inhibited by other L-glutamate analogs and odorants. Electrophysiological recordings from L-glutamate-best olfactory receptor neurons showed that NMDA and L-cysteine are both partial agonists and antagonists of glutamate receptors. Together, these results suggest the olfactory L-glutamate receptors of spiny lobsters are novel types of L-glutamate receptors that are functionally important in mediating olfactory responses.