Premium
The anticonvulsant actions of σ receptor ligands in the Mg 2+ ‐free model of epileptiform activity in rat hippocampal slices
Author(s) -
Thurgur Claire,
Church John
Publication year - 1998
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1038/sj.bjp.0701902
Subject(s) - anticonvulsant , hippocampal formation , neuroscience , chemistry , pharmacology , receptor , biology , biochemistry , epilepsy
The anticonvulsant potency of a series of structurally‐dissimilar compounds which possess nanomolar affinities for high‐affinity σ binding sites was examined in the Mg 2+ ‐free model of epileptiform activity in rat hippocampal slices. Extracellular field potential recordings in the CA1 region were employed to examine the effects of test compounds on spontaneous epileptiform activity and multiple population spikes evoked by stimulation of the Schaffer collateral‐commissural pathway. Applied at σ site‐selective (i.e. nanomolar) concentrations, dextromethorphan, ditolylguanidine, caramiphen and opipramol failed to modify Mg 2+ ‐free epileptiform activity; neither pro‐ nor anticonvulsant effects were observed. However, applied at micromolar concentrations, these and additional test compounds reversibly inhibited orthodromically‐evoked epileptiform field potentials with a rank order potency (IC 50 values in μ M ): dextrorphan (1.5)>ifenprodil (6.3)>dextromethorphan (10)>ditolylguanidine (15)>loperamide (28)>carbetapentane (38)>caramiphen (46)>opipramol (52). Micromolar concentrations of the same compounds also inhibited spontaneous epileptiform bursts recorded during perfusion with Mg 2+ ‐free medium. Co‐application of ropizine (10 μ M ), an allosteric modulator of dextromethorphan binding to high‐affinity σ receptors, failed to endow dextromethorphan 10 n M with anticonvulsant properties and did not modify the anticonvulsant potency of 10 μ M dextromethorphan. The effects of dextrorphan (10 μ M ), ifenprodil (20 μ M ), loperamide (50 μ M ) and caramiphen (100 μ M ) were examined in the presence of external Mg 2+ on field potential input/output (I/O) relationships and paired‐pulse facilitation (PPF) of field excitatory postsynaptic potentials. Only caramiphen elicited effects on these parameters, affecting synaptic transmission at the point of synaptic transfer and depressing PPF ratios to below baseline values. The effects of caramiphen on I/O relationships mimicked those of the established anticonvulsant adenosine; in contrast, adenosine evoked an increase in PPF ratios. Because anticonvulsant activity was observed only at micromolar concentrations of the σ ligands tested, the results indicate that their anticonvulsant actions should not be ascribed to their occupancy, observed at nanomolar concentrations, of high‐affinity σ binding sites. Rather, anticonvulsant activity more likely reflects functional NMDA receptor antagonism and/or blockade of high voltage‐activated Ca 2+ channels, effects which are associated with micromolar concentrations of the test compounds. Modulation of GABAergic inhibitory mechanisms may also contribute to the anticonvulsant properties of caramiphen.