Noncompetitive inhibition of N-methyl-D-aspartate by conantokin-G: evidence for an allosteric interaction at polyamine sites

Conantokins T and G are polypeptide toxins present in snails of the genus Conus. These substances were recently reported to act as N-methyl-D-aspartate (NMDA) antagonists. In the present study, we examined the possible mechanisms producing this antagonism. Conantokin-G inhibited spermine- and spermidine-stimulated [3H]MK-801 binding to extensively washed rat forebrain membranes in a noncompetitive manner with IC50 values of approximately 507 and approximately 946 nM, respectively. In contrast, glutamate-enhanced [3H]MK-801 binding was unaffected by conantokin-G concentrations of less than or equal to 20 microM. At concentrations greater than or equal to 5 microM, conantokin-G effected a modest, noncompetitive inhibition of glycine-stimulated [3H]MK-801 binding and also produced a small enhancement of basal [3H]MK-801 binding. Conantokin-G reduced (IC50 approximately 1.08 microM) the NMDA-stimulated accumulation of cyclic GMP in cerebellar granule cell cultures to basal values, but did not affect kainate-mediated increases in cyclic GMP. These findings indicate that conantokin-G acts as a noncompetitive NMDA antagonist through an allosteric inhibition of polyamine responses. The neurochemical profile of this polypeptide is distinct from previously described noncompetitive NMDA antagonists.

[Septic cavernous sinus thrombosis after nasal surgery]

A case of septic cavernous sinus thrombosis after surgical removal of nasal polyps is presented. To the best of our knowledge, this is the second report of this rare and serious complication of nasal surgery. The clinical features are briefly discussed and the importance of early diagnosis and treatment is emphasized.

Chronic treatment with 1-aminocyclopropanecarboxylic acid desensitizes behavioral responses to compounds acting at the N-methyl-D-aspartate receptor complex

Functional antagonists at the N-methyl-D-aspartate (NMDA) receptor complex produce anti-depressant-like actions in preclinical models. Thus, an injection of a glycine partial agonist (1-aminocyclopropanecarboxylic acid; ACPC), a competitive NMDA antagonist (2-amino-7-phosphonoheptanoic acid; AP-7) or a use-dependent cation channel blocker (MK-801) reduced immobility in the forced swim test (FST) with efficacies comparable to imipramine (Trullas and Skolnick 1990). Seven daily injections of ACPC (200-400 mg/kg) abolished the effects of both this compound (200-1200 mg/kg) and AP-7 (200-300 mg/kg) in the FST. The loss in effectiveness of ACPC required 7 days of treatment to become fully manifest, and was reversed by discontinuing treatment. Other agents active in the FST (e.g. MK-801, imipramine, and nifedipine) were unaffected by this regimen. Moreover, ACPC and AP-7 remained active in the FST following repeated injections of MK-801, AP-7, or imipramine. Chronic treatment with ACPC did not affect its actions in the elevated plus-maze, but significantly attenuated the convulsant and lethal effects of NMDA (125 mg/kg). Tissue levels of ACPC indicate the modified behavioral responses produced by chronic treatment are not attributable to pharmacokinetic factors. These findings suggest repeated administration of ACPC may effect an "uncoupling" of NMDA and glycine receptors, resulting in an apparent desensitization of the behavioral actions of substances acting at these sites.

1-aminocyclopropanecarboxylates exhibit antidepressant and anxiolytic actions in animal models

1-Aminocyclopropanecarboxylic acid (ACPC) is a high affinity ligand at strychnine-insensitive glycine receptors that exhibits partial agonist properties in both biochemical and electrophysiological measures. While ACPC was reported active in animal models commonly used to evaluate potential antidepressants (forced swim) and anxiolytics (plus-maze), the zwitterionic character of this compound could limit both penetration into the central nervous system and oral availability. The present experiments were designed to determine the duration of action of ACPC, its efficacy following oral administration, and to compare these effects with the more lipophilic ACPC methyl ester. Parenterally and orally administered ACPC were equipotent in reducing immobility in the forced swim test, an action manifested for at least 6 h. Both orally and parenterally administered ACPC methyl ester were approximately 3.3-fold more potent than ACPC in the forced swim test. In the elevated plus-maze, both ACPC and ACPC methyl ester were active for 1-2 h after parenteral administration. These findings suggest that 1-aminocyclopropanecarboxylates may constitute a novel class of antidepressant/anxiolytic agents.