Antibodies specifically produced against pain transduction proteins can be used to visualize nociceptors and to examine the expression patterns of proteins involved in the generation of the pain signal.

The sensory endings of nociceptors are thin fibers with diameters near 1 µm. The image below shows such a fiber in the skin, growing along the interface between epidermis (the outer skin) and dermis (the true skin). The epidermis is composed of several layers of cells (here the cell nuclei are stained blue) and topped by a layer of dead cells which forms the body surface. The epidermis, which is composed of various cell types and connective tissue, is the black area below the red pain fiber. The pain fiber was stained with an antibody against TRPV1, a heat-activated ion channel that mediates the perception of noxious heat. The antibody was visualized by a second antibody which carried a red fluorescent label.

Immunohistochemistry can be used to identify distinct populations of pain cells within dorsal root ganglia. The fluorescence image below shows cells expressing the heat-gated ion channel TRPV1, visualized using TRPV1-specific antibodies (red). The blue DAPI stain of cell nuclei in this 10 µm-thick section of a DRG results mostly from non-neuronal satellite cells which form a protective cell layer around each neuron. The unstained neurons themselves leave elliptic, black patches on the image – some are marked by white circles. TRPV1-positive cells are small-to-medium diameter neurons. The green stain results from the binding of the lectin IB4 – labeled with a green fluorophore – to a subpopulation of DRG neurons. Neurons expressing both TRPV1 and IB4 show yellow fluorescence.

Pain cells form synaptic connection in the dorsal horn of the spinal cord. The dorsal horn receives sensory input from somatosensory neurons, whereas the ventral horn is the origin of the motoroutput that controls muscle movements. To understand the neural circuitry in the spinal cord one first has to find out in which layer of the dorsal horn each subpopulation of pain cells forms their synapses. The image on the right shows that most TRPV1-expressing pain cells terminate in the most superficial layer of the dorsal horn (layer 1), while IB4-expressing pain cells form their synapses within layer 2, the substantia gelatinosa. In the outer part of layer 2, yellow immunofluorescence indicates that both antigens are localized in the same synapses.