Serotonin immunocytochemistry of physiologically characterized raphe magnus neurons.

To determine if PAG stimulation activates serotonin containing neurons in RM, intracellular techniques were used to identify raphe magnus (RM) cells that are excited by antinociceptive periaqueductal gray (PAG) stimulation in the cat. RM neurons that received a monosynaptic EPSP from PAG shock were intracellularly labeled with ethidium bromide. Intracellularly stained RM cells were large, medium and small sized neurons. To determine if these cells contain serotonin, sections containing intracellularly labeled RM neurons were processed for serotonin immunocytochemistry. None of the RM cells (N = 32) which were intracellularly stained with ethidium bromide contained immunoreactive serotonin. These results are evidence that non-serotonergic RM cells are activated by antinociceptive PAG stimulation; these cells may be important in the relay of PAG induced antinociception.