Nociceptive stimulation induces glutamate receptor down-regulation in the trigeminal nucleus

The dorsal horn of the subnucleus caudalis of the spinal trigeminal nucleus is a relay of ore-facial pain transmission; increase in subnucleus caudali s neuronal activity in response to tissue injury affects the level of chemi cal mediators participating in nociceptive processing. We investigated, by means of immunocytochemistry, the expression of N-methyl-D-aspartate and al pha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) glutamate recepto r subunits in this nucleus in a model of inflammation Rats injected with fo rmalin in the whisker pad were compared with saline-injected control rats. One and two days after formalin injection, the immunostaining of cell bodie s and neuropil of the AMPA receptor subunits GluR1 and GluR2/3 was markedly decreased in the ipsilateral superficial laminae of the subnucleus caudali s compared to the contralateral side. Side differences were not evident in the saline-treated animals. The down-regulation of AMPA GluR1 and GluR2/3 w as no longer detectable in the subnucleus caudalis three days after formali n injection. No side difference was detected in the N-methyl-D-aspartate re ceptor subunit NR2A/B immunoreactivity of the subnucleus caudalis at any li me-point in the formalin-injected animals. The modulation of AMPA receptor may be related to the decrease of hyperalgesia evident Ih after formalin in jection, in spite of the increasing perioral inflammation evident later on and characteristic of the formalin model. The present findings point out a selective down-regulation of AMPA receptor subunits in the transduction of trigeminal pain. These data also support t he involvement of glutamate receptor subunits in the processing of trigemin al inflammation induced by noxious chemical stimulation (C) 1999 IBRO. Publ ished by Elsevier Science Ltd.