Intrathecal morphine reduces the visceromotor response to acute uterine cervical distension in an estrogen-independent manner.
BACKGROUND
Acute uterine cervical distension (UCD) forms the basis for obstetric and some gynecologic pain. Systemic morphine inhibits the visceromotor response to UCD in rats by an action in the central nervous system, but the effect of morphine is blocked by exposure to estrogen. The purpose of the present study was to determine whether this estrogen blockade of the action of morphine reflects a spinal mechanism.


METHODS
Virgin Sprague-Dawley rats received estrogen or placebo treatment for 1 week after ovariectomy. Rats were then anesthetized, and the electromyographic response in the rectus abdominis muscle to UCD was recorded in the absence and presence of cumulative dosing with intrathecal morphine.


RESULTS
Estrogen treatment did not alter the stimulus- response relationship between UCD and reflex muscle contraction. Intrathecal morphine reduced the visceromotor reflex response to UCD in a dose-dependent manner that was unaffected by estrogen treatment.


CONCLUSIONS
These data suggest that intrathecal morphine is effective in reducing the visceromotor response to UCD and that the reduction in efficacy of systemic morphine in this model is unlikely to reflect a reduction of the efficacy of morphine at the spinal level. These data agree with clinical studies that indicate that systemic morphine, in doses that reduce acute postoperative pain, have minimal to no effect in women in labor, yet intrathecal injection of opioids provides rapid, complete analgesia.
Shin
Sang-Wook
SW
Department of Anesthesiology and Center for the Study of Pharmacological Plasticity in the Presence of Pain, Wake Forest University School of Medicine, Winston-Salem, North Carolina 27157-1009, USA.
Eisenach
James C
JC
D000701:Analgesics, Opioid / Q000008:administration & dosage / Q000627:therapeutic use*; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D001835:Body Weight / Q000187:drug effects; D002584:Cervix Uteri / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D004576:Electromyography; D004958:Estradiol / Q000494:pharmacology; D004967:Estrogens / Q000502:physiology*; D005260:Female; D007278:Injections, Spinal; D009020:Morphine / Q000008:administration & dosage / Q000627:therapeutic use*; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000503:physiopathology*; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D010052:Ovariectomy; D010146:Pain / Q000188:drug therapy* / Q000209:etiology / Q000503:physiopathology*; D010812:Physical Stimulation; D051381:Rats; D017207:Rats, Sprague-Dawley; D014599:Uterus / Q000502:physiology*
D000701:Analgesics, Opioid; D004967:Estrogens; D009292:Narcotic Antagonists; D009270:Naloxone; D004958:Estradiol; D009020:Morphine