Assessment of behavior during labor in rats and effect of intrathecal morphine.
BACKGROUND
Efficacy of analgesics varies with the type of pain. Little is known in this regard concerning labor pain, given the ethical barriers to study in humans and the lack of surrogate animal models. To address this, the authors classified and quantified spontaneous behaviors during labor and delivery in rats and examined the effects of a known analgesic, intrathecal morphine.


METHODS
Pregnant rats were video recorded for 72 h surrounding the time of anticipated labor and delivery. Specific behaviors were identified and classified into general activities, phasic stretching behaviors, and maternal attention activities. Rats received intrathecal infusion of saline or morphine, 0.035-3.5 microg/h, beginning approximately 1 day before delivery, and effects on behaviors and response to noxious heating of the paw were quantified.


RESULTS
Phasic stretching behaviors occurred with high frequency before delivery of the first pup and were rare after delivery of the last pup. Intrathecal morphine at infusion rates greater than 0.035 microg/h abolished these behaviors without affecting general or maternal behaviors or the timing or duration of labor and delivery. Morphine was also antinociceptive to noxious heat, but only at infusion rates of 1.0 microg/h or higher.


CONCLUSIONS
Phasic stretching behaviors are observed after distension or inflammation of pelvic viscera in rats, and similar behaviors occur during labor and delivery. Selective and dose-related blockade by intrathecal morphine of only these behaviors suggests that they reflect nociception and that this simple monitoring method can be used to study therapies for the pain of labor and delivery.
Tong
Chuanyao
C
Department of Anesthesiology, Wake Forest University School of Medicine, Winston-Salem, North Carolina 27157-1009, USA.
Conklin
Dawn R
DR
Liu
Baogang
B
Ririe
Douglas G
DG
Eisenach
James C
JC
D016362:Analgesia, Obstetrical / Q000379:methods; D000701:Analgesics, Opioid / Q000008:administration & dosage / Q000494:pharmacology*; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D005260:Female; D006614:Hindlimb; D006358:Hot Temperature; D007278:Injections, Spinal; D048949:Labor Pain / Q000188:drug therapy*; D007743:Labor, Obstetric / Q000187:drug effects*; D023421:Models, Animal; D009020:Morphine / Q000008:administration & dosage / Q000494:pharmacology*; D009425:Nesting Behavior / Q000187:drug effects; D010147:Pain Measurement / Q000187:drug effects; D010812:Physical Stimulation / Q000379:methods; D011247:Pregnancy; D051381:Rats; D017207:Rats, Sprague-Dawley; D012965:Sodium Chloride / Q000008:administration & dosage; D013997:Time Factors
D000701:Analgesics, Opioid; D012965:Sodium Chloride; D009020:Morphine