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Glutamate receptors are involved in spinal nociceptive transmission and the development of persistent inflammatory hyperalgesia. It is unclear, however, whether there are changes in glutamate receptor gene expression associated with tissue injury. In the present study, we used reverse transcription-polymerase chain reaction (RT-PCR) to examine the modulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor gene expression in the rat spinal cord by inflammation. Inflammation was introduced into the hindpaw by intraplantar injection of 0.2 ml of complete Freund's adjuvant (CFA). At 2 h-14 d after inflammation, total RNAs from L4,5 spinal cord were used for RT-PCR with primers targeted at eight flip-flop splice variants of the AMPA receptor subunits. It was found that the GluR1-flop mRNA was up-regulated at 2 h-5 h (P<0.05), down-regulated at 3 d (P=0.05), and returned to control levels at 7 d following inflammation. The GluR2-flip and GluR3-flop mRNAs were up-regulated at 5 h-1 d (P<0.05) and returned to control levels at 3 d after inflammation. The GluR1-flip mRNA was not detected in the samples and the mRNAs for other splice variants did not exhibit significant changes. Immunocytochemical analysis of GluR1 and GluR2 subunits indicate that the protein translation products of these subunits were also increased in the spinal cord. These results demonstrate an increased expression of AMPA receptor subunits that correlates with the acute phase of CFA-induced inflammation and hyperalgesia. Selective changes in the expression of the flip-flop splice variants of the AMPA receptor suggest a reorganization of the composition of the AMPA receptor complex and its involvement in the development of inflammatory hyperalgesia.

Preprodynorphin (PPD) and preproenkephalin (PPE) gene expression in a rat model of orofacial inflammation were examined in order to further characterize the neurochemical mechanisms underlying orofacial inflammation and hyperalgesia. Deep and cutaneous orofacial inflammation was produced by a unilateral injection of complete Freund's adjuvant (CFA) into the rat temporomandibular joint (TMJ) or perioral skin (PO), respectively. RNA blot analysis of the tissues including the spinal trigeminal complex revealed that the PPD mRNA level ipsilateral to TMJ inflammation was increased by 56.5+/-14.7% (n=4) when compared to the Naive group, and was significantly greater than the contralateral PPD mRNA level (p<0.05). The distribution of neurons that exhibited PPD mRNA after inflammation was localized by in situ hybridization (naive approximately 0). In TMJ-inflamed rats (n=6) PPD mRNA-positive neurons were found ipsilaterally in the medial portion of laminae I-II of the upper cervical dorsal horn (4.5+/-0.3), the dorsal portion of the subnucleus caudalis and caudal subnucleus interpolaris (5.2+/-0.3), and the paratrigeminal nucleus (6.4+/-1.2). A very localized induction of PPD mRNA was also identified in a group of neurons in the intermediate portion of the subnucleus caudalis (2.4+/-0.4) in PO-inflamed rats (n=6). The distribution of these PPD mRNA-positive neurons was somatotopically relevant to the site of injury. There were no significant changes in PPE mRNA expression in both TMJ- and PO-inflamed rats. These results indicate that TMJ inflammation resulted in a more intense and widespread increase in PPD mRNA expression when compared to PO inflammation. These changes may contribute to persistent central hyperexcitability and pain associated with temporomandibular disorders., (Copyright 1999 Elsevier Science B.V.)