THE ACTION OF GABA-B ANTAGONISTS IN THE TRIGEMINAL NUCLEUS OF THE RAT

The iontophoretic administration of the GABA(B) antagonists (P-(3-aminopropyl)-P-di-ethocymethylphosphinic acid (CGP 35348) and 2-hydroxy-saclofen blocked the action of iontophoretic-ally applied L-baclofen on neurons in the trigeminal nucleus of rats, anesthetized with halothane. The substance CGP 35348 appeared to be more potent than 2-hydroxy-saclofen. The iontophoretic administration of GABA resembled L-baclofen in depressing excitatory transmission and facilitating segmental inhibition in the trigeminal nucleus. The depression of excitatory transmission was also blocked by CGP 35348 and the facilitation of segmental inhibition produced by GABA was partially blocked. These observations indicate that CGP 35348 is not only a baclofen antagonist but actually a GABA(B) receptor antagonist and the baclofen was acting at GABA(B) receptors in the trigeminal nucleus. The portion of the effect of GABA, not blocked by CGP 35348, was probably mediated by GABA(A) receptors, since it was previously found that segmental inhibition in the trigeminal nucleus could be modulated by GABA(A) agonists and antagonists as well.