The binding of acidic amino acids to snail, Helix aspersa, periesophagic ring membranes reveals a single high-affinity glutamate/kainate site

Characterization of a domoic acid binding site from Pacific razor clam

The Pacific razor clam, Siliqua patula, is known to retain domoic acid, a water-soluble glutamate receptor agonist produced by diatoms of the genus Pseudo-nitzschia. The mechanism by which razor clams tolerate high levels of the toxin, domoic acid, in their tissues while still retaining normal nerve function is unknown. In our study, a domoic acid binding site was solubilized from razor clam siphon using a combination of Triton X-100 and digitonin. In a Scatchard analysis using [3H]kainic acid, the partially-purified membrane showed two distinct receptor sites, a high affinity, low capacity site with a KD (mean +/- S.E.) of 28 +/- 9.4 nM and a maximal binding capacity of 12 +/- 3.8 pmol/mg protein and a low affinity, high capacity site with a mM affinity for radiolabeled kainic acid, the latter site which was lost upon solubilization. Competition experiments showed that the rank order potency for competitive ligands in displacing [3H]kainate binding from the membrane-bound receptors was quisqualate > ibotenate > iodowillardiine = AMPA = fluorowillardiine > domoate > kainate > L-glutamate. At high micromolar concentrations, NBQX, NMDA and ATPA showed little or no ability to displace [3H]kainate. In contrast, Scatchard analysis using [3H]glutamate showed linearity, indicating the presence of a single binding site with a KD and Bmax of 500 +/- 50 nM and 14 +/- 0.8 pmol/mg protein, respectively. These results suggest that razor clam siphon contains both a high and low affinity receptor site for kainic acid and may contain more than one subtype of glutamate receptor, thereby allowing the clam to function normally in a marine environment that often contains high concentrations of domoic acid.

NMDA glutamate receptor antagonists selectively affect the synaptic mechanisms of nociceptive sensitization in snails

Defensive behavior command neurons LP11 and RP11 were studied in semi-intact snail preparations to investigate the effects of N-methyl-D-aspartate (NMDA) receptor antagonists on the mechanisms of nociceptive sensitization. Application of sensitizing stimuli to the heads of control snails led to membrane depolarization and increased excitability, and also depressed the responses of neurons to tactile and chemical sensory stimuli in the short-term stage and facilitated responses in the long-term stage of sensitization. Development of sensitization in conditions of exposure to the NMDA receptor antagonists AP5 or MK-801 produced changes in the membrane potential and membrane excitability of command neurons similar to those seen in neurons of control sensitized snails. In addition, changes in the responses of command neurons to tactile stimulation of the head and foot and chemical stimulation of the foot in these conditions were also similar to those seen in neurons of control animals. Acquisition of sensitization during administration of NMDA receptor antagonists led to pronounced depression of responses to chemical test sensory stimulation of the snails' heads in both the short-term and long-term stages of sensitization. Thus. in sensitized snails. NMDA glutamate receptor antagonists selectively acted on the mechanisms of induction of plasticity the synaptic "inputs" of command neurons mediating excitation from chemical sensory stimuli from the animal's head.

Identification and characterization of an N-methyl-D-aspartate-specific L-[3H]glutamate recognition site in synaptic plasma membranes

Conditions have been developed for an L-[3H]glutamate binding assay in which 85-95% of the specific binding is to a site that corresponds to the N-methyl-D-aspartate subclass of acidic amino acid receptors. Incubation of synaptic plasma membranes with L-[3H]glutamate in 50 mM Tris/acetate, pH 7.4, for 2-20 min at 2 degrees C results in binding with pharmacological characteristics of the electrophysiologically defined N-methyl-D-aspartate receptor. The fraction of glutamate binding to this subclass of receptors, relative to the total, decreases with both increased time and temperature. This binding is reversible, is concentrated in the synaptic plasma membrane fraction, has a pH optimum of 7.0-7.4, and is linear with respect to tissue protein concentration. The binding is unaffected by 1 mM concentrations of the anions sulfate, chloride, bromide, thiocyanate, phosphate, acetate, nitrate, or carbonate and the monovalent cations potassium or ammonium. However sodium and the divalent cations copper, cobalt, zinc, cadmium, and manganese decrease binding to this N-methyl-D-aspartate site.