Nucleus raphe magnus modulation of response of rat dorsal horn neurons to unmyelinated fiber inputs: partial involvement of serotonergic pathways

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Abstract
1. In the rat under N2O-halothane anesthesia, stimulation of the nucleus raphe magnus (NRM) with stimulus parameters similar to those used to obtain analgesia in freely moving animals strongly inhibits the responses of dorsal horn convergent neurons due to A-delta- and C-afferents. 2. Responses to noxious radiant heat were also depressed, and pronounced post-effects were frequently observed. 3. Comparison between coupled sites of stimulation in NRM and in adjacent bulbar reticular formation (BRF) on responses to C-fibers revealed the preeminent effects of NRM; these were systematically encountered (93% of neurons), much more pronounced, and of longer duration. 4. The latency of these inhibitory effects (around 20 ms) suggests the participation of myelinated axons in such descending action and, consequently, we question the involvement of unmyelinated serotonergic fibers. 5. However, descending inhibitory influences from NRM on responses to C-fibers are reduced after 5-hydroxytryptamine (5-HT) depletion by p-chlorophenylalinine (pCPA), thus demonstrating the implication of both serotonergic and nonserotonergic pathways. 6. In addition, after pCPA pretreatment, long-lasting and sustained excitatory effects from NRM were observed in 35% of convergent neurons; their possible origin is discussed.