Analgesia following microinjection of phosphodiesterase inhibitors at brainstem sites

Microinjection of RO 20-1724 (0.60 micrograms) at sites in the caudal reticular formation of the rat brainstem resulted in analgesia as measured in the tail flick, paw pinch and hot plate tests. Microinjection of theophylline (0.45 micrograms) at sites in the caudal reticular formation also produced analgesia in the tail flick and hot plate tests, but not in the paw pinch test. Since these agents inhibit phosphodiesterase and elevate cyclic nucleotides in rat brain homogenates, these data suggest that cyclic nucleotides have a functional role in brainstem structures involved in modulation of nociceptive threshold.