The response of individual nucleus raphe magnus neurons to microinjections of met-enkephalin at midbrain and at bulbar loci: evidence for midbrain-bulbar convergence on individual raphe neurons.

Local microinjection of met-enkephalin into both the nucleus reticularis paragigantocellularis (Pgc) and into the midbrain periaqueductal gray (PAG) altered the spontaneous discharge of many nociresponsive units in the nucleus raphe magnus (NRM) of the urethane-anesthetized rat. The activity of the majority of units (11 of 14) was suppressed following microinjections into both the PAG and Pgc. Two units showed an increase in spontaneous activity after PAG microinjections and a decrease after Pgc microinjections. One cell showed a decrease in activity after microinjection into Pgc and was unaffected by the PAG microinjection. These findings demonstrate that many nociresponsive units in NRM are affected by microinjections of met-enkephalin into both Pgc and the PAG, indicating a convergent influence of these two regions on single NRM neurons.