Effect of the anticonvulsant U-54494A on cortical neuron excitability: comparison to the kappa agonist U-50488H.

U-54494A, a 1,2-diamine anticonvulsant, and U-50488H, a structurally related agonist for opiate kappa receptors, were tested for effects on spontaneous and glutamate-evoked firing rates in cerebral cortex of urethane-anesthetized male Sprague-Dawley rats. Iontophoretic application of 1,2-diamines, glutamate diethyl ether (GDEE), or procaine depressed spontaneous and amino acid-induced firing of cortical neurones. With continued ejection of 1,2-diamines or procaine, firing was silenced completely, but GDEE could maintain a partial suppression. A rapid rebound of excitation followed cessation of procaine ejections, but not of other agents. Procaine, but not U-54494A, blocked axonal conduction of rabbit sciatic nerve. Intravenous U-54494A and U-50488H significantly depressed spontaneous firing rates of cortical neurones, but only the U-50488H effects were antagonized by naloxone. It is concluded that U-54494A inhibits neuronal excitability by a mechanism independent of the analgesic kappa receptor. Biochemical and physiological studies have demonstrated that U-54494A and the kappa opioid agonist U-50488H (a structurally related diamine) (1) have anticonvulsant activity (2, 3). U-54494A lacks kappa analgesic and sedative properties, and it has been suggested that the mechanism of action of this compound may be mediated by a subtype of kappa opioid receptor (3). The effects of kappa analgesics on neuronal firing in nociceptive pathways have been described (4, 5). However, no previous electrophysiological studies on U-54494A have been done. Since U-54494A antagonizes amino acid-induced seizures (3), the interactions of this compound with glutamate are of interest. In the present study, the antagonist efficacies of U-54494A and U-50488H for inhibiting spontaneous and 1-glutamate stimulated neurons of the rat prefrontal cerebral cortex were assessed after i.v. and microiontophoretic administration of the compounds. Effects observed with these routes of administration allow the observation of neuronal changes occurring immediately after administration and take advantage of the high temporal resolution provided by the electrophysiological recording techniques of single cells. A preliminary account of portions of this work have been previously disclosed (6).