The Effect of Spinal Analgesia on Visceral Nociceptive Neurons in Caudal Medulla of the Rat

• Published in: Anesthesia and analgesia Vol. 89; no. 3; p. 721
• Main Authors: Ness, T J, Piper, J G, Follett, K A
• Format: Journal Article
• Language: English
• Published: Hagerstown, MD International Anesthesia Research Society 01.09.1999; Lippincott
• Subjects: Analgesia; Analgesics; Analgesics, Opioid - administration & dosage; Analgesics, Opioid - pharmacology; Anesthetics, Local - administration & dosage; Anesthetics, Local - pharmacology; Animals; Biological and medical sciences; Drug Tolerance; Electric Stimulation; Evoked Potentials; Injections, Intravenous; Injections, Spinal; Lidocaine - administration & dosage; Lidocaine - pharmacology; Male; Medical sciences; Medulla Oblongata - cytology; Medulla Oblongata - drug effects; Microelectrodes; Morphine - administration & dosage; Morphine - pharmacology; Neurons - drug effects; Neurons - physiology; Neuropharmacology; Nociceptors - drug effects; Pharmacology. Drug treatments; Rats; Rats, Sprague-Dawley; Nociception; Animal model; Spinal cord; Intravenous administration; Rat; Rodentia; Central nervous system; Electrophysiology; Nervous system; Morphine; Opiates; Narcotic analgesic; Intrathecal administration; Neurophysiology; Local anesthetic; Analgesia; Vertebrata; Mammalia; Regional anesthesia; Pain; Treatment; Animal; Organic amide; Lidocaine
• DOI: 10.1097/00000539-199909000-00036
• ISSN: 0003-2999

A population of neurons resident in the caudal ventrolateral medulla are excited by noxious cutaneous and visceral stimuli from large portions of the body. These neurons act as monitors of ascending nociceptive information, and we hypothesized that they would be inhibited by spinally administered analgesics in a clinically relevant fashion. Rats were anesthetized with oxygen/halothane. The caudal medulla was surgically exposed, and a catheter placed into the intrathecal space overlying the lower thoracic spinal cord via the surgical site. Single medullary neurons were characterized for responses 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 used as a pretreatment. Colorectal distension-evoked responses of medullary nociceptive neurons were inhibited in a dose-dependent, naloxone-reversible fashion by intrathecal and IV morphine (50% effective dose values3.5 and 440 μg/kg, respectively). Intrathecal lidocaine abolished responses to colorectal distension and produced a spinal level at doses producing minimal effects when administered systemically. Prior treatment with an infusion of morphine produced tolerance to the effects of subsequent intrathecal morphine administration. These findings support the use of this preparation as a neurophysiologic model of spinal analgesia. IMPLICATIONSNeurons in the brainstem, isolated electrophysiologically, were used as whole body monitors of pain-related activity in the rat. As a neurophysiologic model of nociception, this preparation may prove useful for the study of regionally administered analgesics and local anesthetics.