Antagonism of maximal metrazol seizures in rats and its relevance to an experimental classification of antiepileptic drugs.

The antagonism of various components of maximal Metrazol-seizures (MMS) (i.e. tonic hindpaw extension, tonic backward extension of the forepaws, generalized clonic seizures and tremors), ataxia and loss of righting reflex-activity have been studied in a standardized procedure comparing 41 antiepileptics and related compounds. Appropriate analyses (Cluster Analysis and Principal Component Analysis) resulted in the distinction of seven clusters which could be considered along two continua. A first continuum is characterized by a progressive strengthening of loss of righting reflex-inducing properties, a decreasing dissociation of ataxia and loss of righting reflex and the disappearance of a selective anticonvulsant effect. The second continuum is characterized by an increasing relative potency of ataxia-inducing properties, an increasing dissociation of ataxia and loss of righting reflex and a decreasing antagonism of tremors. Three main types of anticonvulsants could be defined: drugs with a complete anti-MMS effect antagonizing both clonic and tonic seizures; drugs selectively abolishing tonic seizures, i.e., tonic extension of hind- and forepaws; and drugs exclusively blocking tonic hind-paw extension. The neurological and clinical significance of these different types of anticonvulsant activity has been discussed. Finally, the described modification of the maximal Metrazol-seizures test is proposed for routine screening purposes.