in rats , microinjection of the opioid agonist morphine ( 15-45 nmol/0.5 microliter ) and the excitatory amino acid monosodium glutamate ( 30-60 nmol/0.5 microliter ) into identical brainstem sites within the mesencephalic periaqueductal gray matter ( pag ; 23 sites ) and the rostral ventromedial medulla ( rvm ; 22 sites ) produced an increase of tail flick and hot plate response latencies .
C0006121
brainstem
C0025462
mesencephalic