The role of acetylcholine in the function of the nucleus raphe magnus and in the interaction of this nucleus with the periaqueductal gray.

The nucleus raphe magnus (NRM) plays an important role in the inhibition of pain. Although this region receives afferents from several areas of the brain, the afferent input from the periaqueductal gray (PAG) has been shown to have significant physiological importance. Together, these two sites constitute the major component of a descending network involved in pain inhibition. In this study the role of acetylcholine (ACh) in the function of the NRM was investigated and the possibility that ACh may be a transmitter between the PAG and the NRM was tested. ACh was applied iontophoretically. Scopolamine and gallamine were used to test the type of cholinergic receptors that are present in the NRM. The results of this study shows the following. (1) The majority of the cells in the NRM are excited by ACh. (2) This response to ACh is partially or totally blocked by scopolamine whereas gallamine does not block the response. (3) There is no correlation between the excitatory response to stimulation of PAG and to ACh. There are cells that respond to PAG stimulation by inhibition but are excited by ACh and there are a few cells that are inhibited by ACh but are excited by PAG stimulation. (4) Scopolamine, at a dose that blocks the ACh response, does not block the response to PAG stimulation. (5) There is no correlation between the response to ACh and the type of projection (direct or indirect) to the spinal cord, as tested by stimulation of the dorsolateral funiculus. From these results it is concluded that ACh is an excitatory transmitter at the NRM region but this transmitter does not mediate the interaction between the PAG and NRM.