The role of alpha 2-adrenoceptors of the medullary lateral reticular nucleus in spinal antinociception in rats.

We attempted to find out the role of alpha 2-adrenoceptors of the medullary lateral reticular nucleus (LRN) in antinociception in rats. Spinal antinociception was evaluated using the tail-flick test, and supraspinal antinociception using the hotplate test. Antinociceptive effects were determined following local electric stimulation of the LRN, and following microinjections of medetomidine (an alpha 2-adrenoceptor agonist; 1-10 micrograms), atipamezole (an alpha 2-adrenoceptor antagonist; 20 micrograms) or lidocaine (4%) into the LRN. The experiments were performed using intact and spinalized Hannover-Wistar rats with a unilateral chronic guide cannula. Electric stimulation of the LRN as well as of the periaqueductal gray produced a significant spinal antinociceptive effect in intact rats. Medetomidine (1-10 micrograms), when microinjected into the LRN, produced no significant antinociceptive effect in the tail-flick test in intact rats. However, following spinalization, medetomidine in the LRN (10 micrograms) produced a significant atipamezole-reversible antinociceptive effect in the tail-flick test. In the hot-plate test, medetomidine (10 micrograms) in the LRN produced a significant atipamezole-reversible increase of the paw-lick latency in intact rats. Microinjection of atipamezole (20 micrograms) or lidocaine alone into the LRN produced no significant effects in the tail-flick test. The results are in line with the previous evidence indicating that the LRN and the adjacent ventrolateral medulla is involved in descending inhibition of spinal nocifensive responses. However, alpha 2-adrenoceptors in the LRN do not mediate spinal antinociception but, on the contrary, their activation counteracts antinociception at the spinal cord level.(ABSTRACT TRUNCATED AT 250 WORDS)