[Structural characteristics of ionotropic glutamate receptors revealed by channel blockade]

The topography of the channel binding site in glutamate receptors (AMPA and NMDA types of rat brain neurons, receptors of molluscan neurons and insect muscle), and in two subtypes of nicotinic cholinoreceptors (in frog muscle and cat sympathetic ganglion), has been investigated by comparison of the blocking effects of mono- and dicationic derivatives of adamantane and phenylcyclohexyl. The channels studied can be divided into two groups. The first one includes AMPA receptor and glutamate receptors of mollusc and insect, and is characterised by the absence of activity of monocationic drugs and the strong dependence of dicationic once on the internitrogen distance in the drug molecule. The second group includes NMDA receptor and both nicotinic cholinoreceptors. Contrary, here the blocking potency of monocations and dications are practically equal irrespective of molecule length. The data obtained suggest that hydrophobic and nucleophilic components of the binding site are located close to each other in the channels of the NMDA receptor type but are separated by approximately 10 A in the AMPA receptor channel.