Opiate Analgesia

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Abstract
It is generally accepted that systemically administered opiates produce analgesia through an action in the central nervous system. Indeed, mu, delta, and appa opiate receptors and receptor-selective opioid peptides, although widely distributed in the central nervous system, are concentrated in several sites that have been implicated in the regulation of nociceptive messages. The analgesia produced by the microinjection of opioids at these sites is hypothesized to result from activation of a descending antinociceptive pathway, which originates in the midbrain periaqueductal gray matter and inhibits the transmission of nociceptive messages at the level of the spinal cord.1 Injection of opiates at . . .