Modulation using racemic and levo-rotary baclofen of the trigeminal reflex in man

Baclofen (Lioresal), a muscle relaxant, exerts a specific action on the trigeminal system by depressing excitatory synaptic transmission in the spinal trigeminal nucleus. To evaluate the effects of racemic and L-baclofen on the human trigeminal reflexes, the area of the blink reflex was measured in seven normal subjects, before and after i.v. administration of racemic baclofen (25 mg.) and oral administration of L-baclofen (15 mg.). The blink reflex is a trigeminal facial reflex consisting of two components (R1 and R2): R1 has a shorter latency and is mediated by an oligosynaptic pontine circuit; R2 has a longer latency and is believed to be relayed via a polysynaptic circuit through the lateral bulbar reticular system. Whereas the R1 response was scarcely affected by administration of racemic baclofen, it was significantly reduced by L-baclofen (P less than 0.01). R2 was depressed by both drugs (P less than 0.01). These results indicate that both racemic and L-baclofen inhibit trigeminal transmission in man, probably because they interfere with excitatory transmission through the interneurons of the lateral reticular formation. In addition, since L-baclofen reduced both R1 and R2 this form of the drug presumably has a more powerful effect than its racemic counterpart, on the few interneurons of the short latency component.