Anticonvulsant and related effects of U-54494A in various seizure tests

The anticonvulsant activity of (+-)-cis-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benz am ide monohydrochloride (U-54494A), a benzamide derivative chemically related to kappa opioid receptor agonists, was investigated in three selected seizure models of experimental epilepsy. In the maximal electroshock seizure test in mice, U-54494A (ED50 28 mg/kg i.p.) was effective, with a potency somewhat less than phenobarbital. In combination with clinically used antiepileptics, especially phenobarbital and carbamazepine, the anticonvulsant activity of the latter was significantly increased. More detailed studies with phenobarbital showed additive anticonvulsant effects. The anticonvulsant activity of U-54494A was partially antagonized by naloxone. On the other hand, this compound did not elevate the pentylenetetrazol seizure threshold (at high doses a tendency of proconvulsant action was seen). Furthermore, in unrestrained rats with chronically implanted electrodes, U-54494A (> or = 10 mg/kg) significantly reduced the duration of electrically evoked hippocampal afterdischarges. However, the focal stimulation threshold was not markedly increased. With respect to the possible mode of action, whole-cell voltage-clamp experiments on cultured neonatal rat cardiomyocytes showed that U-54494A depressed the fast sodium inward current in a concentration- and frequency-dependent manner. In summary, our results agree with earlier reports that demonstrated marked anticonvulsant effects of U-54494A in grand mal-analogous seizure tests. Moreover, in combination with some standard antiepileptics, additive effects can be found. It is suggested that, in addition to kappa opioid and excitatory amino acid receptor related effects, modulations of Na+ membrane currents may contribute to the mechanisms of action.