The differential contribution of spinopetal projections to increases in vocalization and motor reflex thresholds generated by the microinjection of morphine into the periaqueductal gray.

The capacity of morphine microinjected into the ventrolateral periaqueductal gray (vPAG) to elevate the thresholds of spinal motor reflexes (SMRs), vocalizations during shock (VDSs) and vocalization afterdischarges (VADs) was challenged by the intrathecal administration of receptor antagonists to serotonin (methysergide), norepinephrine (phentolamine) and mu-opiates (naloxone). Methysergide and phentolamine were equipotent in reversing increases in SMR thresholds. The efficacy of these antagonists to reduce increases in VDS and VAD thresholds was dependent on the dose of morphine administered into the vPAG. These results indicate that the dose of morphine administered into the vPAG determines the contribution of spinopetal projections in inhibiting dorsal horn neurons involved in reflex generation versus the rostral transmission of pain information. A hypothesis is offered regarding the mechanisms by which vPAG administered morphine suppresses nociceptive transmission through different levels of the neuraxis.