The Effect Of Estrogen On The MRNA Expression Of Estrogen Receptor And Myosin Heavy Chain In Rat Masseter Muscle

Temporomandibular disorder (TMD), characterised by pain and recurrence, is one of the most commonly seen diseases in dentistry clinic. The prevalence of TMD, mostly identified as functional disturbances in the masticatory system, has been well documented to be higher in young females than in males. Estrogen, a steroid compound with multifunctions, is one of the most important hormones in women. The effects of estrogen on the composition and metabolism of myofiber in limb skeletal muscles have been investigated extensively in recent years. However, the effect of estrogen on masticatory muscles has been rarely studied. This study is therefore undertaken to examine this aspect and to provide new data regarding the possible role of estrogen in the development of TMD.Sixty twelve-week old female Sprague-Dawley rats were randomly divided into 3 groups: (1) control (Sham surgery), (2) ovariectomy without estrogen replacement (Ovx) treatment, (3) ovariectomy with estradiol valerate replacement (Ovx/EV) treatment. Half of the animals were sacrificed at 4 and 8 weeks, respectively, after surgery and the masseter muscles were removed for analysis. Real time PCR quantification with SYBR Green I was employed to study the mRNA expression of estrogen receptor alpha (ERα) and beta (ERβ), myosin heavy chain (MHC) I,IIa,IIx and IIb .The results showed:1. Expression of ERαand ERβwas detected in normal rat masseter muscle with ERαmainly in myonuclei while ERβin both myonuclei and endothelial cell nuclei of blood capillaries. ERαmRNA expression was stronger than ERβmRNA.2. Strong expression of MHC- IIb mRNA in masseter of rat was found.3. Ovx/EV is a good animal model for studying the effect of estrogen on rat masticatory function.4. At week 4, the expression of ERαmRNA was significantly higher in the Ovx group than in the Ovx/EV group.5. The expression of ERβmRNA in the Ovx group at week 4 was significantly higher than at week 8. The expression of ERβmRNA in the Ovx group at week 4 was also significantly higher than the sham and Ovx/EV groups at both week 4 and week 8 time points.6. MHC-I mRNA expression did not differ among groups.7. MHC-IIa mRNA expression was higher in the Ovx group at week 8 than its expression in the Sham, Ovx, Ovx/EV groups at week 4 and the sham group at week 8.8. MHC-IIx mRNA expression was higher in the Ovx group at week 8 than its expression in the Sham, Ovx, Ovx/EV groups at week 4 and the sham group at week 8.9. MHC-IIb mRNA expression was not significantly different among groups.The results suggest that the rat masseter muscle is one of the targets of estrogen.In response to estrogen level decrease, the expression of ERαand ERβmRNA tend to be upregulated initially and then be restoreded, while the expression of MHC- IIa and IIx mRNA tend to be upregulated constantly. It is concluded that estrogen does have effect on the masticatory function of female rats.