Method: Intact females stratified into proestrus (PF) (high sex hormonal level) and diestrus (DF) (low sex hormonal level) phases of estrous cycle; gonadectomized females (OVX); gonadectomized with replacement of estrogen (OVX+E) or progesterone (OVX+P); intact males; gonadectomized males (ORX) and gonadectomized males with replacement of testosterone (ORX+T) were anaesthetized and equi-nociceptive concentrations of formalin plus the KOR agonist U50,488 (3ug/TMJ) were injected into the TMJ region. After 45 minutes of intra-TMJ injection, animals were terminally anesthetized and a trigeminal ganglion was removed. G-protein kinase 3 (GRK3) and KOR mRNA expressions were evaluated by Western Blot and RT-PCR analyses, respectively.
Result: The expression of GRK3 was significantly lower in diestrus females, OVX females and ORX males than proestrus females and males (p<0.05, ANOVA, Tukey test). Replacement with testosterone in males and estrogen or progesterone in females significantly increased the GRK3 expression. The KOR mRNA analyses demonstrate no statistical difference among groups.
Conclusion: Opioid receptors are members of the family of G-protein-coupled receptors that are desensitized following phosphorylation by GRK, especially GRK3. Following that, the results demonstrate that gonodal hormones increase the GRK3 expression in the trigeminal ganglia which may stimulate the internalization of the KOR expressed in the membrane of the nociceptive primary afferent fibers of TMJ resulting in KOR desensitization. These findings explain sexual dimorphism on KOR-mediated antinociception into TMJ.