Zambotti F, Zonta N, Parenti M, Tommasi R, Vicentini L, Conci F, Mantegazza P
Naunyn Schmiedebergs Arch Pharmacol. 1982 Mar;318(4):368-9. doi: 10.1007/BF00501180.
Microinjections of muscimol, a GABA receptor agonist, into the periaqueductal gray matter (PAG) counter-acted the antinociceptive effect of morphine in rats, as measured by the "tail-flick" method. Muscimol's effect was partially reversed by bicuculline.