Activation of Serotonergic Neurons in the Raphe Magnus Is Not Necessary for Morphine Analgesia

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Abstract
A wealth of pharmacological and behavioral data suggests that spinally projecting serotonergic cells mediate opioid analgesia. A population of medullary neurons, located within raphe magnus (RM) and the neighboring reticular nuclei, contains serotonin and is the source of serotonin in the spinal dorsal horn. To test whether serotonergic neurons mediate opioid analgesia, morphine was administered during recordings from medullary cells that were physiologically characterized as serotonergic (5HTp) by their slow and steady discharge pattern in the lightly anesthetized rat. Selected 5HTpcells (n= 14) were intracellularly labeled, and all contained serotonin immunoreactivity. The discharge of most 5HTpcells was not affected by an analgesic dose of systemic morphine. In a minority of cases, 5HTpcells either increased or decreased their discharge after morphine administration. However, morphine altered the discharge of some 5HTpcells in the absence of producing analgesia and conversely did not alter the discharge of most 5HTpcells in cases in which analgesia occurred. RM cells with irregular discharge patterns and excitatory or inhibitory responses to noxious tail heat were classified asonandoffcells, respectively. Allonandoffcells that were intracellularly labeled (n= 9) lacked serotonin immunoreactivity. Alloncells were inhibited, and mostoffcells were excited by systemic morphine. Because 5HTpcells do not consistently change their discharge during morphine analgesia, they are unlikely to mediate the analgesic effects of morphine. Instead, nonserotonergic cells are likely to mediate morphine analgesia in the anesthetized rat. In light of the sensitivity of morphine analgesia to manipulations of serotonin, serotonin release, although neither necessary nor sufficient for opioid analgesia, is proposed to facilitate the analgesic effects of nonserotonergic RM terminals in the spinal cord.

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