Abstract
2-Aminoethyl methylphosphonate (2-AEMP), an analog of γ-aminobutyric acid (GABA), was recently found to exhibit antagonist activity at ρ1 GABAC receptors. The present study was undertaken to elucidate the mechanism of 2-AEMP’s action and to test the activities of 2-AEMP analogs. Whole-cell patch-clamp techniques were used to record membrane currents in neuroblastoma cells stably transfected with human ρ1 GABAC receptors. The action of 2-AEMP was compared with that of 1,2,5,6-tetrahydropyridin-4-yl methylphosphinic acid (TPMPA), a commonly used GABAC antagonist. With 10 µM GABA, 2-AEMP’s IC50 (18 μM) differed by less than 2.5-fold from that of (TPMPA) (7 µM), and results obtained were consistent with a primarily competitive mode of inhibition by 2-AEMP. Terminating the presentation of 2-AEMP or TPMPA in the presence of GABA produced a release from inhibition. However, the rate of inhibition release upon the termination of 2-AEMP considerably exceeded that determined with termination of TPMPA. Moreover, when presented at near their respective IC50 concentrations, the pre-incubation period associated with 2-AEMP’s onset of inhibition was much shorter than that for TPMPA. Analogs of 2-AEMP possessing a benzyl or n-butyl rather than a methyl substituent at the phosphorus atom, as well as analogs bearing a C-methyl substituent on the aminoethyl side chain, exhibited reduced potency relative to 2-AEMP. Of these analogs, only (R)-2-aminopropyl methylphosphonate significantly diminished the response to 10 µM GABA. Structure–activity relationships are discussed in the context of molecular modeling of ligand binding to the GABA binding site of the ρ1 GABAC receptor.