Ca(2+)-permeable AMPA receptors are densely expressed in the spinal dorsal horn, but their functional significance in pain processing is not understood. By disrupting the genes encoding GluR-A or GluR-B, we generated mice exhibiting increased or decreased numbers of Ca(2+)-permeable AMPA receptors, respectively. Here, we demonstrate that AMPA receptors are critical determinants of nociceptive plasticity and inflammatory pain. A reduction in the number of Ca(2+)-permeable AMPA receptors and density of AMPA channel currents in spinal neurons of GluR-A-deficient mice is accompanied by a loss of nociceptive plasticity in vitro and a reduction in acute inflammatory hyperalgesia in vivo. In contrast, an increase in spinal Ca(2+)-permeable AMPA receptors in GluR-B-deficient mice facilitated nociceptive plasticity and enhanced long-lasting inflammatory hyperalgesia. Thus, AMPA receptors are not mere determinants of fast synaptic transmission underlying basal pain sensitivity as previously thought, but are critically involved in activity-dependent changes in synaptic processing of nociceptive inputs.

Background Glutamate receptors of the AMPA type (AMPArs) mediate fast excitatory transmission in the dorsal horn and are thought to underlie perception of both acute and chronic pain. They are tetrameric structures made up from 4 subunits (GluR1-4), and subunit composition determines properties of the receptor. Antigen retrieval with pepsin can be used to reveal the receptors with immunocytochemistry, and in this study we have investigated the subunit composition at synapses within laminae I–III of the dorsal horn. In addition, we have compared staining of AMPArs with that for PSD-95, a major constituent of glutamatergic synapses. We also examined tissue from knock-out mice to confirm the validity of the immunostaining. Results As we have shown previously, virtually all AMPAr-immunoreactive puncta were immunostained for GluR2. In laminae I–II, ~65% were GluR1-positive and ~60% were GluR3-positive, while in lamina III the corresponding values were 34% (GluR1) and 80% (GluR3). Puncta stained with antibody against the C-terminus of GluR4 (which only detects the long form of this subunit) made up 23% of the AMPAr-containing puncta in lamina I, ~8% of those in lamina II and 46% of those in lamina III. Some overlap between GluR1 and GluR3 was seen in each region, but in lamina I GluR1 and GluR4 were present in largely non-overlapping populations. The GluR4 puncta often appeared to outline dendrites of individual neurons in the superficial laminae. Virtually all of the AMPAr-positive puncta were immunostained for PSD-95, and 98% of PSD-95 puncta contained AMPAr-immunoreactivity. Staining for GluR1, GluR2 and GluR3 was absent in sections from mice in which these subunits had been knocked out, while the punctate staining for PSD-95 was absent in mice with a mutation that prevents accumulation of PSD-95 at synapses. Conclusion Our results suggest that virtually all glutamatergic synapses in laminae I–III of adult rat spinal cord contain AMPArs. They show that synapses in laminae I–II contain GluR2 together with GluR1 and/or GluR3, while the long form of GluR4 is restricted to specific neuronal populations, which may include some lamina I projection cells. They also provide further evidence that immunostaining for AMPAr subunits following antigen retrieval is a reliable method for detecting these receptors at glutamatergic synapses.

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NO has been identified recently as the prime islet-toxic product of inflammatory macrophages. The adverse effects of IL-1 on isolated islets also have been reported to involve NO. We now show that exposure of an islet cell suspension to the NO donor nitroprusside or to activated macrophages leads to DNA strand breaks. Macrophages did not induce DNA damage in the presence of the NO synthase inhibitor NG-methyl-L-arginine. DNA strand breaks were demonstrated at the level of single cells by a modified nick-translation procedure and confirmed by analysis of DNA fragmentation by gel electrophoresis. DNA strand breaks occurred within 1 h and preceded islet cell lysis. DNA damage could not be prevented by inhibitors of endogenous endonucleases. We conclude that islet cell DNA is an early target of NO action.