Gabrα6 knockdown increases nociception in non-inflamed and inflamed temporomandibular joint

OBJECTIVES: Women report greater temporomandibular joint (TMJ) pain when the concentration of estrogen is diminishing. Like in humans, the nociceptive response is lowest in rats at proestrus, when estrogen concentrations are highest. Previous studies showed expression of the GABAA receptor subunit α6 (Gabrα6) was significantly upregulated in the trigeminal ganglion (TG) when compared to rats given high (i.e., proestrus) versus low (i.e. diestrus) concentrations of estrogen. Activation of the GABA receptors appears to inhibit activity of nociceptive neurons. We hypothesize that cyclical plasma levels of 17 β-estradiol in female rats modulate, in part, TMJ nociception through changes in GABAA receptor expression in the trigeminal ganglia. METHODS: To initially address this hypothesis the TG of male rats was infused with siRNA complexed to polyethylenimine. The siRNA had homology to the Gabrα6 gene or the siRNA had homology to no gene (i.e. control siRNA). Forty-eight hours later these two groups were subdivided further; the rats were given bilateral TMJ injections of 15 μg complete Freund's adjuvant (CFA) or saline within the superior joint space of TMJ. To determine how changes in gene expression effect the nociceptive response in a male rat with a non-inflamed TMJ (arthralgia) or with a inflamed TMJ (arthritic) we measured the activity of small and medium to large neurons or satellite glial cells in the TG. For this experiment TG tissue was double stained with phospho-ERK antibodies and antibodies for neuronal or glial markers. RESULTS: In rats with and without TMJ inflammation knock-down of Gabrα6 increased phospho-ERK levels in the TG and increased the electrical activity of second order neurons. CONCLUSION: These findings suggest that Gabrα6 can modulate TMJ nociception.  Future studies will block Gabrα6 expression in cycling female rats, NIDCR grants DE016059-01 (LLB) and DE15372 (PRK).