Hello Steve, it has been a while since I have worked in this area.. But this review paper (Neural Plasticity

Volume 2013 (2013), Article ID 654257, 19 pages

http://dx.doi.org/10.1155/2013/654257

Review Article

Ionotropic Glutamate Receptors and Voltage-Gated Ca2+ Channels in Long-Term Potentiation of Spinal Dorsal Horn Synapses and Pain Hypersensitivity

Dong-ho Youn,Gábor Gerber, and William A. Sather)

indicates that when C-fibers terminate in Lamina I or II they either use glutamate or substance P.  For glutamate, the post synaptic receptors are ionotropic glutamate receptors.

So if you know which system you want to look at (glutamate or substance P), you could colocalize your protein of interest with an antibody for a specific receptor class on the interneurons. As below, ionotropic glutamate receptors have multiple subunits. So, you can make your staining a little more specific if you stain for a subunit of the receptor subtype that you are sure is there.

There are many types of GluRs. Further, they underwent a terminology change after their discovery. So the earlier papers use older terminology. For this area, you just need to consider AMPA receptors (GluA1-GluA4).

There is a good review paper on glutamate receptors that lays out their structure and terminology, just look at the AMPA receptor sections: Traynalis et al.,  Pharmacol Rev. 2010 Sep; 62(3): 405–496.

doi:  10.1124/pr.109.002451

PMCID: PMC2964903

Glutamate Receptor Ion Channels: Structure, Regulation, and Function

They say: "The glutamate receptors assemble as tetrameric complexes of subunits (Laube et al., 1998; Mano and Teichberg, 1998; Rosenmund et al., 1998; Greene, 2001; Matsuda et al., 2005; Nakagawa et al., 2005; Sobolevsky et al., 2009), and functional receptors are formed exclusively by assembly of subunits within the same functional receptor class (Partin et al., 1993; Kuusinen et al., 1999; Leuschner and Hoch, 1999; Ayalon and Stern-Bach, 2001; Ayalon et al., 2005). Glutamate receptors are grouped into four distinct classes based on pharmacology and structural homology, including the AMPA receptors (GluA1–GluA4), the kainate receptors (GluK1–GluK5), the NMDA receptors (GluN1, GluN2A–GluN2D, GluN3A, and GluN3B), and the δ receptors (GluD1 and GluD2). The AMPA receptor subunits GluA1 to GluA4 can form both homo- and heteromers. "

In terms of the distribution of the receptors on the Rexed Lamina I or II interneurons, I am sure there are  newer papers than the ones below (ones that use the newer terminology for the glutamate receptors) that can tell you what specific type of glutamate receptor is found on the interneurons. Then you can use an antibody for a specific subunit in that receptor on those cells.(the receptor itself is a tetramer, and you can stain for just one part of it)

R. C. Spike, D. J. Maxwell, and A. J. Todd, “GluR1 and GluR2/3 subunits of the AMPA-type glutamate receptor are associated with particular types of neurone in laminae I–III of the spinal dorsal horn of the rat,” European Journal of Neuroscience, vol. 10, no. 1, pp. 324–333, 1998. View at Publisher · View at Google Scholar · View at Scopus

H. S. Engelman, T. B. Allen, and A. B. MacDermott, “The distribution of neurons expressing calcium-permeable AMPA receptors in the superficial laminae of the spinal cord dorsal horn,” Journal of Neuroscience, vol. 19, no. 6, pp. 2081–2089, 1999.

I hope this helps. feel free to call me if you have questions. 703-371-2733