NUCLEI WITHIN THE ROSTRAL VENTROMEDIAL MEDULLA MEDIATING MORPHINE ANTINOCICEPTION FROM THE PERIAQUEDUCTAL GRAY

The relative contributions of nuclei within the rostral ventromedial m edulla (RVM) involved in mediating morphine induced antinociception fr om the periaqueductal gray (FAG) were examined. Lidocaine injections ( 4%) at the time of morphine's maximal response were used to provide a localized neural block and were administered in the nucleus raphe magn us/reticularis gigantocellularis pars alpha (RMg/GiA; commonly referre d to as RMg), reticularis gigantocellularis (Gi) and reticularis parag igantocellularis lateralis (LPGi). Microinjection of morphine (6 nmol; 0.5 mu 1) into the FAG of awake rats produced an inhibition of the ta il-flick reflex that was maximal after 30 min. This response was unaff ected by a single medial lidocaine injection (0.5 mu l) into the RMg/G iA or Gi, bilateral injections into the Gi or LPGi or triple injection s that included both the RMg/GiA and LPGi. A partial, non-significant block of morphine's response was observed either by bilateral injectio ns (0.5 mu l) into both the Gi and LPGi (% inhibition = 16.4 +/- 24.8) or by bilateral injections in the LPGi and a single medial injection into the Gi (% inhibition = 41.5 +/- 29.8). However, injection of a gr eater volume of lidocaine (1 mu l) into the RMg/GiA or bilaterally int o the LPGi affected adjacent medial and lateral tissue, and completely inhibited morphine's response. Furthermore, triple injections of lido caine (0.5 mu l) into the Gi or bilateral injections (0.5 mu l) into t he Gi and a single medial injection into the RMg/GiA completely blocke d morphine's antinociceptive response. These results indicate that mor phine antinociception from the FAG is mediated by a large volume of ti ssue in the RVM containing nuclei located both medially and laterally. Additionally, the principal nuclei involved in this response appear t o be the Gi and RMg/GiA.