Antimyoclonic properties of S2 serotonin receptor antagonists in the rat

The capacity of the putative S2 serotonin receptor antagonists, pirenperone, pipamperone, ketanserin and cinanserin, to block the myoclonic syndrome produced by 30 mg/kg of L-5-hydroxytryptophan (5-HTP) [after lesioning 5-hydroxytryptamine (serotonin, 5-HT)-containing neurons with 5,7-dihydroxytryptamine (DHT)] or 15 mg/kg of fenfluramine (FF) or p-chloroamphetamine (PCA) was tested in adult male Sprague-Dawley rats. S2 antagonists inhibited limb (arrhythmic and asynchronous) and axial (truncal) myoclonus in a dose-dependent manner in the rank order of potency: pirenperone greater than pipamperone greater than ketanserin = cinanserin. Abnormal movements (myoclonus, lateral head weaving) of the myoclonic syndromes were better antagonized than postural abnormalities (hindlimb abduction, hunching of back). Centrally acting drugs, selective for S2 receptors (pirenperone, pipamperone), exhibited greater antimyoclonic properties than the non-selective 5-HT antagonist methysergide, which was as effective as ketanserin and cinanserin. Significant non-specific reduction in myoclonus without the improvement of other behavioral responses followed treatment with sedative/neuroleptic drugs, such as haloperidol (but not the non-neuroleptic dopamine antagonist sulpiride), clonazepam and diazepam. The anticonvulsants valproic acid (100 and 300 mg/kg), adrenocorticotrophic hormone (ACTH; 100 and 300 U/kg), diphenylhydantoin (15 mg/kg), and phenobarbital (20 mg/kg) and drugs which do not act principally at S2 receptors were ineffective in these models. These data support the hypothesis that myoclonus in behavioral models induced by 5-HT is S2 receptor mediated. S2 antagonists could have a role in the treatment of human myoclonus.