Expression and distribution of GABAergic system in rat knee joint synovial membrane

The GABAergic system, found in the adult mammalian brain and composed of gamma-aminobutyric acid (GABA), GABA synthesizing enzyme glutamate decarboxylase (GAD) and GABA receptors, is also located in many peripheral nonneuronal tissues. Studies suggest that synovial membranes possess GABA, and that GABA participates in the control of the inflammatory response in rheumatoid arthritis (RA). However, no studies on the GABAergic system in synovial membranes have been done so far. Therefore, expression and distribution of the GABAergic system in the synovial membrane of the normal rat knee joint were investigated by reverse transcription-polymerase chain reaction (RT-PCR) analyses and immunohistochemistry. Results of RT-PCR analysis showed that mRNA encoding the GAD65 and GABAB receptor subunits necessary for the assembly of functional receptors, R1 and R2, are expressed in the synovial membrane. GAD and GABAB receptor subunits were localized in macrophage-like A cells of the synovial membrane. Macrophage-like A cells of the synovial membrane have a GABA production system and GABAB receptors, and GABA seems to play functional roles in the synovial membrane.