Comparison of the anticonvulsive activities of organic mono- and dications with their abilities to inhibit NMDA and AMPA glutamate receptors.

The abilities of mono- and dicationic adamantane and phenylcyclohexyl derivatives to (a) block open NMDA and AMPA glutamate receptors in isolated rat brain neurons and (b) prevent convulsions induced in mice by intraventricular NMDA or kainate were studied. Monocations inhibited NMDA receptors in vitro and produced corresponding protection against NMDA-induced convulsions in vivo, but lacked the ability to block AMPA receptors or prevent kainate-induced convulsions. Dications (IEM-1754 and IEM-1925), which inhibited both NMDA and AMPA receptors, were highly effective at protecting against kainate convulsions and were more effective than the corresponding monocations in preventing NMDA convulsions. The origin of convulsions induced by NMDA appears to be based on a component mediated by activation of AMPA receptors. The anticonvulsive activity of IEM-1754 and IEM-1925 were comparable with those of the known NMDA receptor blockers memantine and MK-801. This was combined with an almost complete absence of the side effects characteristic of memantine and MK-801. The complete correspondence between the in vitro data and in vivo results seen with some of the study compounds is evidently associated with their pharmacokinetic properties.

Studies of the structure of glutamate receptor ion channels and the mechanisms of their blockade by organic cations.

The structural determinants for blockade of the AMPA and NMDA subtypes of glutamate receptors were studied by analysis of structural-functional relationships in a series of mono- and dicationic compounds. The results showed that the hydrophobic and nucleophilic components of the blocker binding sites are located close to each other in the channel of the NMDA receptor, while they are spatially distant in the channel of the AMPA receptor. Molecular mechanical methods were used to construct models of these channels satisfying these topographic criteria and providing adequate descriptions of the binding of the channel blockers. According to the models, binding of blockers to the NMDA channel occurs in the selective filter of the channel (the N/Q/R site). The nucleophilic region of the AMPA channel is formed by the oxygen atoms of glycine residues in position +2 relative to the selective filter. Identification of the major relationships between the molecular structure of the ion channels of these glutamate receptor subtypes and their blockade by organic cations allows the further synthesis of AMPA and NMDA channel blockers with specified levels of activity and selectivity to be directed.

[Comparison of the anticonvulsant activity of organic mono- and di-cations and their potential to inhibit NMDA and AMPA glutamate receptors]. / Sopostavlenie protivosudorozhnoi aktivnosti organicheskikh mono- i dikationov s ikh sposobnost'iu ingibirovat' NMDA i AMPA glutamatnye retseptory.

Effects of mono- and dicationic derivatives of adamantine and phenylcyclohexyl were studied on: (i) open channels of NMDA and AMPA glutamate receptors in the experiments on the isolated rat brain neurones, and (ii) convulsions induced by intraventricular injections of NMDA or kainate in mice. Monocations inhibited the NMDA receptors in vitro and prevented convulsions induced by NMDA in vivo, but failed to affect both the AMPA receptors and kainite-induced convulsions. Dications (IEM-1754 and IEM-1925) revealed both anti-NMDA and anti-AMPA potency in vitro, were highly effective against kainite-induced convulsions and excelled monocations in preventing the NMDA-induced ones. Evidently some steps connected with the AMPA receptor activity are involved in the genesis of the NMDA-induced convulsions. Anticonvulsant potency of IEM-1754 and IEM-1925 is comparable with those of known NMDA receptor inhibitors: memantine and MK-801. The IEM-1754 and IEM-1925 show no side effects. An incomplete correspondence between the activity in vitro and in vivo found studying some derivatives, may be due to peculiarities of their pharmacokinetics.

Structural characteristics of ionotropic glutamate receptors as identified by channel blockade.

The channels of four types of ionotropic glutamate receptor (NMDA receptors and Ca-permeable AMPA receptors of rat brain neurons, and cation-selective receptors from mollusk neurons and insect postsynaptic muscle membranes) and two subtypes of nicotinic cholinoreceptor (from frog neuromuscular junctions and cat sympathetic ganglia) were studied. The structural characteristics of channels determining their susceptibility to blockade by organic mono- and dications were identified. These studies used homologous series of adamantane and phenylcyclohexyl derivatives. These experiments showed that the receptors studied here could be divided into two groups. The first group included the AMPA receptor and the mollusk and insect receptors. These were characterized by the lack of effect on the part of monocations and a strong relationship between the activity of dications and the distance between nitrogen atoms. The second group included the NMDA receptor and both subtypes of the nicotinic cholinoreceptor (muscular and neuronal). Here, conversely, the activity of monocations and dications, regardless of their lengths, were essentially identical. A model for the binding sites of blockers in channels is proposed, which takes these observations into account.

[Structural characteristics of ionotropic glutamate receptors revealed by channel blockade]. / Strukturnye osobennosti ionotropnykh glutamatnykh retseptorov, vyiavliaemye blokadoi kanalov.

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.