The effect of spinal analgesia on visceral nociceptive neurons in caudal medulla of the rat

A population of neurons resident in the caudal ventrolateral medulla are ex cited by noxious cutaneous and visceral stimuli from large portions of the body. These neurons act as monitors of ascending nociceptive information, a nd we hypothesized that they would be inhibited by spinally administered an algesics in a clinically relevant fashion. Rats were anesthetized with oxyg en/ halothane. The caudal medulla was surgically exposed, and a catheter pl aced into the intrathecal space overlying the lower thoracic spinal cord vi a the surgical site. Single medullary neurons were characterized for respon ses to cutaneous and visceral (colorectal distension) stimuli. The effects of IV and intrathecally administered morphine and lidocaine were determined . The intrathecal infusion of morphine for 6 days before testing was also u sed as a pretreatment. Colorectal distension-evoked responses of medullary nociceptive neurons were inhibited in a dose-dependent, naloxone-reversible fashion by intrathecal and TV morphine (50% effective dose values: 3.5 and 440 mu g/kg, respectively). Intrathecal lidocaine abolished responses to c olorectal distension and produced a spinal level at doses producing minimal effects when administered systemically. Prior treatment with an infusion o f morphine produced tolerance to the effects of subsequent intrathecal morp hine administration. These findings support the use of this preparation as a neurophysiologic model of spinal analgesia. Implications: Neurons in the brainstem, isolated electrophysiologically, were used as whole body monitor s of pain-related activity in the rat. As a neurophysiologic model of nocic eption, this preparation may prove useful for the study of regionally admin istered analgesics and local anesthetics.