Effects of peripheral inflammatory hyperalgesia on expression of GABA and its receptors in the rat spinal dorsal horn

Objective To study the changes in the expression of GABA and its receptors in the dorsal horn of rat spinal cord induced by hind paw formalin injection. Methods Twelve healthy male SD rats weighing 300-325 g were randomly divided into two groups: (A) control group (n=6) and (B) peripheral inflammatory hyperalgesia group in which 5% formalin 50μl was injected subcutaneously into the plantar region of right hind paw to produce persistent pain(n=6). The animals were killed 24h after formalin injection. L_(4-6) segment of the spinal cord was removed. The number of GABA immuno-reactive cells was examined by immuno-cytochemistry technique and the exprpession of GABA_(Aβ3) and GABA_(B1) receptor mRNA in the spinal dorsal horn were determined by in situ hybridization. Results In control group(A), GABA immuno-reactive cells and GABA_(Aβ3) and GABA_(B1) receptor mRNA were observed throughout the lumbar spinal dorsal horn. The density of GABA immuno-reactive cells and GABA_(B1) receptor mRNA was highest in the superficial laminae Ⅰ-Ⅲ; while the GABA_(Aβ3) receptor mRNA was evenly distributed in the spinal dorsal horn. In group B, 24h after formalin injection there was a significant increase in the GABA immuno-reactive cells in the ipsilateral spinal dorsal horn. The expression of GABA_(Aβ3) and GABA_(B1) receptor mRNA were significantly increased in bilateral dorsal horn compared to group A. The three parameters were positively correlated and their locations in the spinal dorsal horn were not significantly different from those in group A. Conchusion GABA and its receptors may play an important role in nociceptive modulation.