Phaclofen, or phosphonobaclofen, is a selective

antagonist An antagonist is a character in a story who is presented as the chief foe of the protagonist. Etymology The English word antagonist comes from the Greek ἀνταγωνιστής – ''antagonistēs'', "opponent, competitor, villain, enemy, riv ...

for the GABA_B receptor. It was the first selective GABA_B antagonist discovered, but its utility was limited by the fact that it does not cross the blood brain barrier.

References

GABAB receptor antagonists Phosphonic acids Chlorobenzenes {{nervous-system-drug-stub