Neuronal activity classified by receptive field, adequate stimulus, and raphe-spinal (R-S) projection in 48 lightly anaesthetized rats was recorded from raphe magnus (RM) area. Effects of nanoinjections of Met-enkephalin in periaqueductal gray (PAG; 2 or 10 micrograms), nucleus reticularis paragigantocellularis (PGC; 1 or 5 micrograms), and both sites (conjointly) were studied. Most of the 78 neurons studied responded to noxious stimulation, some with increased responses ('on-cells') and some with decreased responses ('off-cells').
Major findings: (1) Met-enkephalin in PAG and PGC depressed the response of on-cells and enhanced the response of off-cells; (2) for a given RM or R-S cell, PAG and PGC effects were typically identical; and (3) conjoint injections tended to produce greater effects than single effects, whether enhancing or depressing.