///2010 Abstract Details
2010 Abstract Details2018-05-01T17:52:49+00:00

Delivery, but not pregnancy, protects against acute and chronic hypersensitivity in rats

Abstract Number: 61
Abstract Type: Original Research

James C. Eisenach M.D.1 ; Baogang Liu M.D., Ph.D.2

Introduction

Vaginal or cesarean delivery acutely injures visceral and somatic tissue, and such injury can cause chronic pain. Chronic pain after childbirth is remarkably rare compared to surgery, suggesting a protective mechanism. We used two methods of acute and chronic sensitization in rats to determined whether pregnancy or delivery itself protects against hypersensitivity, a common facet of chronic pain.

Methods

Following ACUC approval, female rats were anesthetized and the L5 and L6 spinal nerves were ligated (SNL), a nerve injury model of chronic neuropathic pain. In other rats, capsaicin, 10 mcg, was injected into the plantar surface of the hind paw. Withdrawal threshold to tactile stimulation with von Frey filaments was determined. Groups were compared by a two-way repeated measures ANOVA with P < 0.05 considered significant.

Results

SNL produced a significant hypersensitivity in nonpregnant rats, reducing withdrawal threshold from 15 1.1 g before to 2.6 0.3 g 1 week after surgery (n=17). Similar baseline (16 1.2 g) and 1 week-post SNL (3.2 0.4 g) were obtained when SNL surgery was performed in mid-pregnancy (n=20). In contrast, the effect of SNL was markedly lessened if surgery was performed within 24 after delivery (baseline 21 2.1 g, one week later 13 2.2 g; n=7). One month after surgery, withdrawal threshold was significantly greater if SNL was performed shortly after delivery (16 2.5 g) than if performed in mid-pregnancy (2.2 0.4 g) or in nonpregnant animals (2.8 0.3 g).

Prior to capsaicin injection there was no difference in withdrawal threshold between pregnant rats and those 2 days after vaginal delivery. In contrast, withdrawal threshold was reduced more in nonpregnant rats (to 4.4 0.4 g) than in postpartum rats (to 20 2.4 g) 30 min after intradermal capsaicin injection (n=5-6).

Discussion

The SNL model, which produces hypersensitivity to tactile stimuli and presumed chronic neuropathic pain, is widely used to study mechanisms and treatments for this condition. These results suggest that the early postpartum period, but not pregnancy itself, protects against surgery injury induced neuropathic pain. Intradermal capsaicin produces acute and transient allodynia and hyperalgesia to mechanical stimuli in humans, and is used to model acute hypersensitivity that may reflect chronic hypersensitvity in those with neuropathic pain. This acute hypersensitivity is also reduced in early postpartum rats, consistent with a dampening in mechanisms leading to central sensitization. These results support clinical observations and suggest that a process surrounding childbirth protects against generation of chronic pain from tissue injury.

Supported in part by NIH grant GM48085

SOAP 2010