The anticonvulsant actions of sigma receptor ligands in the Mg2+-free model of epileptiform activity in rat hippocampal slices.

1. The anticonvulsant potency of a series of structurally-dissimilar compounds which possess nanomolar affinities for high-affinity sigma binding sites was examined in the Mg2+-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. 2. Applied at sigma site-selective (i.e. nanomolar) concentrations, dextromethorphan, ditolylguanidine, caramiphen and opipramol failed to modify Mg2+-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 (IC50 values in microM): 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 Mg2+-free medium. 3. Co-application of ropizine (10 microM), an allosteric modulator of dextromethorphan binding to high-affinity sigma receptors, failed to endow dextromethorphan 10 nM with anticonvulsant properties and did not modify the anticonvulsant potency of 10 microM dextromethorphan. 4. The effects of dextrorphan (10 microM), ifenprodil (20 microM), loperamide (50 microM) and caramiphen (100 microM) were examined in the presence of external Mg2+ 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. 5. Because anticonvulsant activity was observed only at micromolar concentrations of the sigma ligands tested, the results indicate that their anticonvulsant actions should not be ascribed to their occupancy, observed at nanomolar concentrations, of high-affinity sigma binding sites. Rather, anticonvulsant activity more likely reflects functional NMDA receptor antagonism and/or blockade of high voltage-activated Ca2+ 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.