April 2009
Volume 50, Issue 13
Free
ARVO Annual Meeting Abstract  |   April 2009
Comparison of Inhibitory Mechanism for 2-Aminoethyl Methylphosphonate (2-AEMP) and TPMPA at GABAC Receptors
Author Affiliations & Notes
  • A. Xie
    Ophthalmology & Visual Sciences, University of Illinois at Chicago, Chicago, Illinois
  • F. Feng
    Ophthalmology & Visual Sciences, University of Illinois at Chicago, Chicago, Illinois
  • J. Yan
    Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee
  • R. F. Standaert
    Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee
  • D. R. Pepperberg
    Ophthalmology & Visual Sciences, University of Illinois at Chicago, Chicago, Illinois
  • H. Qian
    Ophthalmology & Visual Sciences, University of Illinois at Chicago, Chicago, Illinois
Investigative Ophthalmology & Visual Science April 2009, Vol.50, 1016. doi:
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      A. Xie, F. Feng, J. Yan, R. F. Standaert, D. R. Pepperberg, H. Qian; Comparison of Inhibitory Mechanism for 2-Aminoethyl Methylphosphonate (2-AEMP) and TPMPA at GABAC Receptors. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1016.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Purpose: : We previously identified 2-aminoethyl methylphosphonate (2-AEMP) as a competitive antagonist to the GABAC receptors (refs. 1,2). In the present study we tested the kinetics of inhibitory action of 2-AEMP and compared the results obtained with the action of TPMPA, a commonly used GABAC receptor antagonist.

Methods: : Whole-cell patch-clamp was used to record GABA-induced currents from SHP-5 neuroblastoma cells stably transfected with human GABA ρ1 subunit.

Results: : 2-AEMP competitively inhibited GABA-induced currents elicited from the GABAC-expressing cells. The IC50 was 18 µM, a value similar to that observed for TPMPA (7 µM). Consistent with an action as a competitive antagonist, 2-AEMP had no effect on the deactivation phase of the GABA-elicited response. However, there was a large difference in the kinetics of action of 2-AEMP and TPMPA. The OFF kinetics of the compounds were examined when the receptor was activated by GABA. Both compounds inhibited the GABA-elicited current, and the conclusion of test compound application in the presence of GABA led to a release from inhibition. The time constant of inhibition release was 0.96 s and 7.6 s for 2-AEMP and TPMPA, respectively. Pre-treatment with either 2-AEMP or TPMPA slowed onset of the GABA-elicited response. The relationship between the pre-treatment period and the amplitude of the current at a fixed time in the GABA response was used to determine the ON rate of the antagonist. At concentrations representing the respective IC50 values, 2-AEMP exhibited an ON time constant of 0.066 s, whereas TPMPA's ON time constant was much slower (1.5 s). The two compounds also differed in their ability to reduce the current response elicited by a brief (10 ms) application of high (1 mM) GABA concentration. Both drugs efficiently inhibited the response to prolonged application (16 s) of low (1 µM) GABA concentration. However, with brief application of high GABA concentration, the residual response observed with 2-AEMP signficantly exceeded that observed with TPMPA.

Conclusions: : Although 2-AEMP and TPMPA have comparable affinities for the GABAC receptor, 2-AEMP exhibits faster ON and OFF kinetics than TPMPA. This difference could provide a useful tool for elucidating GABAC receptor function in the retina, e.g., differentiating the activities of synaptic vs. extra-synaptic GABAC receptors. (1) Chowdhury et al. (2007) Bioorg. Med. Chem. Lett. 17:3745-3748. (2) Xie et al., 2008 Soc. Neurosci., abst. 608.10.

Keywords: inhibitory receptors • neurotransmitters/neurotransmitter systems • ion channels 
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