Increases in vocalization and motor reflex thresholds are influenced by the site of morphine microinjection: comparisons following administration into the periaqueductal gray, ventral medulla, and spinal subarachnoid space.

The relative influence of morphine microinjected into the periaqueductal gray, ventral medulla (nucleus raphé magnus or nucleus reticularis gigantocellularis), or spinal subarachnoid space on the thresholds of responses organized at spinal (spinal motor reflexes, SMRs), medullary (vocalizations elicited during shock, VDSs), and rhinencephalic-diencephalic (vocalization after discharges, VADs) levels of the neuraxis was assessed. Dose-dependent increases in response thresholds differed with the site of morphine injection. These results indicate that the mu-opiate-receptor-linked systems in the mesencephalon, medulla, and spinal cord exert differential antinociceptive effects on pain behaviors organized at different levels of the neuraxis. A hypothesis is offered regarding the mechanisms through which morphine inhibits nociceptive transmission through various levels of the CNS. VADs are promoted as a model system for analyzing the affective-motivational dimension of the pain experience.