N. Ya. Lukomskaya

Effects of Ionotropic Glutamate Receptor Channel Blockers on the Development of Pentylenetetrazol Kindling in Mice

Experiments on mice were performed to study the ability of monocationic and dicationic adamantane and phenylcyclohexyl derivatives to prevent the development of kindling induced by i.p. administration of pentylenetetrazol (Corasol, 35 mg/kg). The monocationic phenylcyclohexyl derivative IEM-1921 effectively slowed the development of kindling, this being seen over a wide range of doses (0.0001–0.1 µmol/kg). A monocationic adamantane derivative (memantine), also a selective non-competitive blocker of NMDA receptors, produced a similar effect at doses 100 times higher. The anticonvulsive activity of the dicationic phenylcyclohexyl derivative IEM-1925, which could block both types of glutamate receptors, differed from the activity of the monocationic derivative by having a more complex dose-response relationship. Thus, the development of kindling was suppressed by essentially the same doses as needed for the monocation IEM-1921 (0.001 µmol/kg). However, on reducing the dose by a factor of 10 (0.0001 µmol/kg), IEM-1925 facilitated the development of kindling. This difference in the prophylactic activities of selective NMDA receptor blockers and substances able to block both NMDA and AMPA receptors provides evidence that the mechanism of kindling involves both types of ionotropic glutamate receptor and the effects of compounds depend not only on the ratio of the contributions of these receptors, but also on the kinetic characteristics of the blocking action.

Studies of the Roles of NMDA and AMPA Glutamate Receptors in the Mechanism of Corasole Convulsions in Mice

Experiments on mice were performed to study the ability of monocationic and dicationic adamantane and phenylcyclohexyl derivatives to prevent convulsive syndrome induced by i.p. corasole (pentylenetetrazole; 80 mg/kg). Monocationic phenylcyclohexyl derivatives, which are selective blockers of NMDA glutamate receptor channels, along with memantine and MK-801, effectively prevented the appearance of the clonic and tonic components of convulsions at micromolar concentrations. Their dicationic analogs, which block both NMDA and AMPA receptor channels, were ineffective against clonic convulsions, but prevented corasole-induced tonic convulsions at much lower concentrations. The convulsive action of corasole, whose primary target is weakening of the inhibitory action of GABA, appears to be mediated by glutamatergic synaptic transmission. NMDA receptors have a much greater involvement than AMPA receptors in the genesis of clonic convulsions, while AMPA receptor activation appears to be an important component of tonic convulsions.

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 (IÉM-1754 and IÉM-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 IÉM-1754 and IÉM-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.

Blockade and spontaneous recovery of ganglionic transmission after treatment by irreversible inhibitors of cholinesterase

1. 1. Intra-arterial injection of 0.2 ml of three organophosphorus anticholinesterase drugs, armin (0.05–0.15 μM), Gd-42 (0.01–0.03 μM) and GT-165 (0.05–0.1 μM), induced blockade of transmission through the repetitively stimulated cat cervical sympathetic ganglion followed by spontaneous recovery of transmission. 2. 2. During the blockade of transmission induced by armin (4 × 10−5 M), depolarization of the rabbit isolated sympathetic ganglion was observed, while the subsequent recovery was accompanied by an increase in the ganglionic resting potential. 3. 3. Reactivators of cholinesterase TMB-4 (0.02 μM or 2 × 10−5 M and 2-PAM (0.75 μM) or the cholinolytic agent, hexamethonium (0.1 μM), restored the transmission depressed by inhibitors. 4. 4. It is concluded that the depolarization of neurons by the long-term elevated concentration of acetylcholine is the main factor in blockade, while repolarization (spontaneous or induced by drugs) may restore transmission in spite of lasting inhibition of cholinesterase.

The Ability of New Non-Competitive Glutamate Receptor Blockers to Weaken Motor Disorders in Animals

The ability of mono- and dicationic phenylcyclohexyl derivatives, which are non-competitive glutamate antagonists, to prevent convulsions induced in mice by intragastric NMDA or kainate, to weaken catalepsy induced in rats by haloperidol, and to exert their own influences of movement activity and behavior in animals was studied. The actions of study compounds were compared with those of the known NMDA antagonists memantine and dizocilpine. NMDA-induced convulsions were effectively prevented by both mono- and dications, while only dications were effective against kainate convulsions. Anticataleptic activity was significantly more marked in monocations, which lacked the ability to block non-NMDA receptors. Side effects on motor coordination were less marked with study compounds than with dizocilpine. Thus, the effects of phenylcyclohexyl derivatives in in vivo experimental models correlate with their anti-NMDA and anti-AMPA activity. They can be regarded as potential agents for treating parkinsonism and other motor disorders.

Action of snake venom polypeptides on cholinergic receptors of the isolated rabbit sympathetic ganglion

Neurotoxins ( a -bunga ro tox in , najatoxin, etc.) , i solated f rom the venom of snakes of the cob ra family have the p rope r ty of se lec t ive ly and i r r e v e r s i b i y bIoeking the ef fec ts of acetylehoIine. According to exis t ing data , neurotoxins act on cei ls with n ico t ine sens i t ive ehol inergic r e c e p t o r s [5, 7]. In some c a s e s , however , it has been shown that not all n ico t ine sens i t ive cel ls a re blocked by neurotoxins . F o r instance, in expe r i ments on the do r sa l musc le of the leech, neurotoxins did not a l t e r the effects of n ieo t inomimet ies , although the choi inergic r e c e p t o r s of this musc le a re usually r egarded as typical n ico t ine -sens i t ive s t r u c t u r e s [2, 10]. Nor was sens i t iv i ty to neurotoxins found in expe r imen t s on the hea r t s and musc l e s of mol Iuses and ech inode rms , even though these objec ts have s i m i l a r pha rmaco log ica l c h a r a c t e r i s t i c s [3].

Involvement of ionotropic glutamate receptors in the appearance of arecoline tremor in mice.

Administration of the muscarinic cholinoreceptor agonist arecoline (6 mg/kg, s.c.) to mice induced long-lasting tremor. The ability of non-competitive antagonists of ionotropic glutamate receptors to suppress the onset of tremor was studied. These antagonists, i.e., adamantane and phenylcyclohexyl derivatives, selectively blocked NMDA-type receptor channels (monocations) or both NMDA-and AMPA-type channels (dications). Both types of blocker weakened arecoline tremor, though the dose-response relationships were different for mono-and dications. The effects of dications appeared only at low blocker doses (0.0001–0.01 µmol/kg) but gradually disappeared on dose elevation. These data lead to the conclusion that the mechanism of pathogenesis of arecoline tremor predominantly involves NMDA-type receptors. Moderate blockade of AMPA-type receptors could potentiate the preventive effect of mixed-action antagonists (anti-NMDA + anti-AMPA), though predominance of blocking action against AMPA-type receptors prevented this effect.

Mechanism of the ganglion-blocking action of bis-ammonium compounds

The action of polymethylene bis-ammonium compounds on acetylcholine-activated channels was investigated on voltage-clamped neurons of the isolated rabbit superior cervical ganglion. The kinetics of complex formation by the compound and the open channel was determined from shortening of decay of the fast excitatory postsynaptic current, the rate of which corresponds to the rate of channel closure, whereas the kinetics of dissociation of this complex was determined by recovery of the second response to application of two pulses of acetylcholine. With membrane hyperpolarization complex formation was found to be accelerated, and its dissociation retarded. The potential-dependence of interaction between compound and channel increases with lengthening of the polymethylene chain. Variation of one of the two cationic groups in a compound with a tetramethylene chain does not affect potential-dependence. In a series of bis-ammonium compounds, ganglion-blocking activity, determined on the cat superior cervical ganglionin situ was found to correlate with the velocity constant of binding with the open channel. It is concluded that the ganglion-blocking action of bis-ammonium compounds is determined by their channel-blocking activity.

Mechanisms of the blockade of glutamate channel receptors: Significance for structural and physiological investigations

The mechanism of the blocking action of phenylcyclohexyl derivative IEM-1925 on ionotropic NMDA and AMPA glutamate receptors was studied. Experiments on isolated rat brain neurons (hippocampal pyramidal cells and striatal cholinergic interneurons) were performed using local voltage clamping in the “whole cell” configuration. In equilibrium conditions at a membrane potential of −80 mV, there was no selectivity in the action of IEM-1925 on the open channels of either type of glutamate receptor. However, data were obtained showing significant differences in the mechanisms of the blocking actions. Although IEM-1925 was unable to penetrate into closed channels of either receptor type, molecules were able to leave closed AMPA receptor channels but not closed NMDA receptor channels. In hyperpolarization, the departure of the blocker from open NMDA receptor channels was slowed, while departure from open and closed AMPA receptor channels was accelerated. The blocker thus appeared able to penetrate AMPA receptor channels to enter cells, the gating mechanism of these channels being located above the blocker binding site. The actions of IEM-1925 on NMDA and AMPA receptors were compared with its ability to suppress tremor in mice induced with s.c. doses of arecoline. The results indicated that both types of receptors have a role in producing tremor. The differences in the mechanisms of action on AMPA and NMDA receptors may explain the ambiguous nature of the effects of the glutamate channel blocker in experimental therapy.

Bioelectrical Activity in the Sleep–Waking Cycle in Rats after Pilocarpine-Induced Status Epilepticus

Chronic recording of electrograms of the somatosensory, auditory, and visual cortex, the hippocampus, and the caudate nucleus in Krushinskii–Molodkina and Wistar rats was used to study bioelectrical activity in the sleep–waking cycle during the month following pilocarpine-induced status epilepticus. The processes of normalization of behavior and the organization of sleep and waking were completed in animals over the three days following status epilepticus. From day 2 to day 4 after administration of pilocarpine, electrograms of the study structures in all rats at all stages of the sleep–waking cycle showed epileptiform activity in the form of sharp waves, multiple peaks, and peak-wave discharges. Interhemisphere asymmetry in the manifestations of paroxysmal activity was seen in some cases. Spontaneous epileptiform activity on the EEG, in the form of single or grouped peak-wave discharges, was recorded in rats only on the background of waking and slow-wave sleep. The simultaneous presence of different types of epileptiform activity on EEG patterns and the duration of paroxysms on the EEG during the post-status period indicated that exposure to pilocarpine can induce the formation of multiple foci of epileptogenesis in the rat brain, with production of stable paroxysmal syndrome, which has clear similarity with the clinical picture of nonconvulsive status epilepticus in humans.