Structural characteristics of ionotropic glutamate receptors as identified by channel blockade

Characterization of ionotropic glutamate receptors in insect neuro-muscular junction

Pharmacological properties of ionotropic glutamate receptors from Calliphora vicina larvae neuro-muscular junction (C. vicina iGlurs) were studied by two-electrode voltage-clamp technique. Characteristics of the ion channel pore were analyzed using a 26-member series of channel blockers, which includes mono- and dicationic derivatives of adamantane and phenylcyclohexyl. Structure-activity relationships were found to be markedly similar to the Ca2+-permeable AMPA receptors (AMPAR) but not NMDA receptors (NMDAR) channel subtype seen in vertebrates. Like AMPARs the channels of C. vicina iGlurs are sensitive mainly to dicationic compounds with 6-7 spacers between hydrophobic headgroup and terminal aminogroup. Study of the voltage dependence of block demonstrated that, like AMPARs, the C. vicina iGlur channels, are permeable to organic cations with dimensions exceeding 10 A. Concentration dependence of block suggests the presence of two distinct channel populations with approximately 20-fold different sensitivity to cationic blockers. The recognition domain properties are more complex. Besides glutamate, the channels can be activated by kainate, quisqualate and domoate. Competitive antagonists of AMPAR and NMDAR are virtually inactive against the C. vicina iGlurs as well as allosteric modulators GYKI 52466 and PEPA. Surprisingly, the responses were potentiated 3 times by 100 mkM of cyclothiazide. We conclude that the channel-forming domain of C. vicina iGlurs is AMPAR-like, whereas the recognition domain is specific.