The Mechanism Of Pain Modulation By Estrogen In Basal Pain Threshold And Discogenic Back Pain

BACKGROUNDAs one of the widespread sex hormones in the body,the estrogen's regulation of pain has been affirmed by various studies.Life experience suggests that the female may be more sensitive to pain than the male and the incidence of pain-related diseases is higher.This naturally makes us think that estrogen is a cause of allergic factors in pain,however it cannot explain the clinical phenomenon that female patients with IBS feel the severest pain during menstrual while the estrogen is at the lowest level.It is clearly that the pain modulation of estrogen also depends on the types of pain,estrogen's concentration or stability.Besides,for another common pain-related disease-the lumbar disc pain,the incidence of postmenopausal women is also significantly higher than men.Many clinical doctors and researchers used to think that the disc herniation is the main cause of lumbar disc pain,while in the last decades many studies have confirmed that the intervertebral disc degeneration(IDD)is the pathological basis of the following disc herniation and lumbar spondylolisthesis.Due to the extensive biological function of estrogen,it is likely that the estrogen is strongly related to the IDD.With the social development and progress of medical care,people live much longer than before and the aging trend of population has become increasingly prominent.How to improve the quality of postmenopausal life of women is a vary noteworthy issue.We have been interested in the relative deficiency of estrogen after menopause and how the estrogen works in pain modulation and IDD.In this present study,by giving the ovariectomized mice estrogen replacement therapy,we observe the changes of basal pain threshold and explore the possible mechanism of lumbar disc pain induced by IDD in order to prevent or delay the occurrence of lumbar disc pain.And we hope this study will provide an inspiring revelation for the clinical treatments of postmenopausal women with various types of painrelated diseases.METHODAll the studies used 12-week-old female C57 mice.We measured the basal pain threshold and conducted the following morphological and molecularbiological experiments by giving the ovariectomized(OVX)mice estrogen replacement therapy(ERT).All the mice were divided into five groups randomly: the normal group(group N),the sham operation group(group S),the OVX group(group O),the OVX+ERT group(group E)and the OVX+placebo group(group P).First,we observed the effect of receiving exogenous estrogen on the basal pain thresholds of OVX mice to explore the possible mechanism of estrogen in pain modulation.The main experimental method was to give the OVX mice ERT at the first week after operation.And we measured the basal pain thresholds including the mechanical hyperalgesia paw withdraw threshold(PWT)and the thermal hyperalgesia paw withdraw latency(PWL)at 1 day before and 3 days,1 week,2 weeks,3 weeks,4 weeks and 5 weeks after OVX.Second,we observed the estrogen's effect in the process of intervertebral disc degeneration(IDD)and the relationship of estrogen and p38 MAPK pathway to explore the possible mechanism of IDD.The main experimental method: levels of inflammatory cytokines in serum were measured at 3 days and 5 weeks postoperatively;5 weeks after operation,we made the HE stained paraffin slices of the spinal columns to observe the IDD and the lumbar intumescentia of spinal cord was taken to analyze the expression of p38 by Western blot.RESULTThe estrogen's effects on basal pain thresholds: PWT and PWL of OVX mice which did not receive ERT(group O and group P)continued to decrease after the operation.The PWT and PWL were different from those of group N and group S statistically(P<0.05),and the PWL's difference was more significant(P<0.01).One week after OVX,the PWT and PWL of group E which received ERT have been maintained or slightly improved but were still lower than group N and group S(P<0.01).About 3 weeks after OVX,the ERT has an obvious effect and group E was different from group O and group P significantly(P<0.01).Paraffin slices of HE staining showed that the morphological performances of the IVD in group N and group S were nearly normal at lower magnifications(100�)and there were no obvious signs of degeneration.The AF structure was intact and regular,collagen fibers were arranged tightly,and there were clear boundaries between the NP and the AF.While in the OVX groups without ERT(group O and group P),the NP and the AF were much looser,and the AF showed some atrophies and fractures.After receiving ERT,the IDD degree of group E was lighter than the other two OVX groups.While at higher magnifications(400�),we saw the collagen arranged parallel to each other and the fiber thickness was uniform in normal NP.Some AF may be fractured but were basically symmetry and regular.The NP of OVX groups were loose and messy;the collagen fiber arrangement in degenerative NP was uneven and disorganized.There was also some necrosis liquefaction and absorption which looked like some honeycomb-like or vesiclelike cavities.The degenerated CEP showed fibrosis,ossification and rupture and part of the CEC nuclei dissolved,accompanied by vascular invasion where appeared some red blood cells.The CEC in group N and group S were regularly arranged in the CEP while the density of CEC was decreased in the OVX groups with cells in a "squeezed-like" shape and an uneven distribution.The CEC were still more than those in group O and group P and partly were still distributed regularly which meant the degree of CEP degeneration was less than that of the other two OVX groups.ELISA of three inflammatory cytokines had similar results.The inflammatory cytokines' concentrations of OVX mice(group O,group E and group P)were much higher than both group N and group S(P < 0.01).After receiving ERT,group E decreased significantly compared with group O and group P(P<0.01)but still higher than group N and group S.The western blot results at 5 weeks after OVX showed that although the content of p38 in group S was higher than that in group N,there was no significant difference(P>0.05).After ERT,the p38 in group E was less than group O and group P but was only significantly different from group O(P<0.05).Meanwhile,the p38 in group E was still more than group N or group S,and was significantly more than group N(P<0.05).CONCLUSIONWe can draw the following conclusions based on the experimental results: 1.The estrogen has a definite analgesic effect on the modulation of basal pain thresholds in OVX mice.2.The estrogen may have an anti-inflammatory effect in OVX mice through p38 MAPK pathway which may delay the process of IDD in OVX mice.3.Receiving ERT is beneficial for postmenopausal women to prevent various types of pain-related diseases.4.How to modulate the p38 MAPK pathway more precisely so as to reduce the incidence of discogenic back pain induced by IDD,we still have much work to do.