Two populations of neurokinin 1 receptor-expressing projection neurons in lamina I of the rat spinal cord that differ in AMPA receptor subunit composition and density of excitatory synaptic input

Neuroscience and Molecular Pharmacology, Faculty of Biomedical and Life Sciences, West Medical Building, University Avenue, University of Glasgow, Glasgow G12 8QQ, UK.
Neuroscience (Impact Factor: 3.36). 03/2010; 167(4):1192-204. DOI: 10.1016/j.neuroscience.2010.03.028
Source: PubMed


Lamina I of the spinal cord contains many projection neurons that express the neurokinin 1 receptor (NK1r). It has been reported that these cells can undergo long-term potentiation (LTP), which may result from insertion of AMPA-type glutamate receptors (AMPArs) containing GluA1 or GluA4 subunits. We therefore investigated synaptic AMPAr expression on these cells with immunocytochemistry following antigen-retrieval. We also examined their density of glutamatergic input (by analysing AMPAr synaptic puncta and contacts from glutamatergic boutons), and phosphorylation of extracellular signal-regulated kinases (pERKs) following noxious stimulation. Our results indicate that there are two populations of NK1r-expressing projection neurons: large GluA4(+)/GluA1(-) cells with a high density of glutamatergic input and small GluA1(+)/GluA4(-) cells with a much lower input density. Results from pERK experiments suggested that the two groups may not differ in the types of noxious stimulus that activate them. Glutamatergic synapses on distal dendrites of the large cells were significantly longer than those on proximal dendrites, which presumably compensates for the greater attenuation of distally-generated excitatory postsynaptic currents (EPSCs). Both types of cell received contacts from peptidergic primary afferents, however, on the large cells these appeared to constitute over half of the glutamatergic synapses, and were often associated with elongated AMPAr puncta. This suggests that these afferents, which probably contain substance P, provide a powerful, secure synaptic input to large NK1r-expressing projection neurons. These results demonstrate the importance of GluA4-containing AMPArs in nociceptive transmission and raise the possibility that different forms of LTP in lamina I projection neurons may be related to differential expression of GluA1/GluA4.

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    • "Among these subunits, GluR1 and GluR2 were found to be highly expressed in the superficial DH (laminae I–II), where primary afferents carrying nociceptive inputs make synapses to the spinal second-order nociceptive neurons. GluR3 and GluR4 are weakly expressed in the DH (Furuyama et al., 1993), although GluR4 was recently detected in large neurokinin-1 receptor-expressing projection neurons in lamina I (Polgar et al., 2010). Thus, GluR1 and GluR2 represent the two most abundant subunits forming the AMPAR channel in the DH. "
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    • "Additional primary antibodies directed against different epitopes were not available to serve as controls for the ion channel immunohistochemistry assessments. However, in particular for the GluR1 antibody, staining has been shown to be absent in the relevant knockout mice when processed using antigen retrieval (Polgar et al., 2010). Control sections stained only with secondary antibodies were included in all experiments and showed no or only minimal fluorescence, indicating low autofluorescence of ON (data not shown; Fritschy, 2008). "
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    • "Although Nichols et al. demonstrated measurable loss of lamina III neurons at 30 days after SP-Sap and throughout the dorsal horn at 100 and 200 days post infusion of the toxin [22]; Wiley et al., using the SSP-Sap at the same dose and survival time (seven days) that we employed, demonstrated a selective loss of lamina I neurons with no neuronal decrease in laminae III-VI or × [7]. An exciting recent paper from Todd's group describes two populations of NK1 receptor bearing projection neurons, larger neurons with GluA4 AMPA receptor subunits and medium sized neurons that were positive for GluA1, rather than GluA4 [23]. They also identified a third population of smaller neurons, having lower NK1 receptor densities, that were not projection cells [21]. "
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