The 5-HT2A receptor is mainly expressed in nociceptive sensory neurons in rat lumbar dorsal root ganglia

Abstract

Several lines of evidence indicate that peripheral 5-HT2A receptors are involved in the development of inflammatory and neuropathic pain. However, their localization in sensory cell bodies is not accurately known. We therefore studied 5-HT2A receptor distribution in rat lumbar dorsal root ganglia using immunocytochemistry. Forty percent of L3 lumbar dorsal root ganglion cells were immunoreactive for 5-HT2A receptor. Most were small- to medium-sized cell bodies. Double-labeled experiments revealed that they expressed various chemical phenotypes. The smaller 5-HT2AR cell bodies often bind the isolectin B4 although some 5-HT2AR cell bodies also express substance P (SP). Many 5-HT2A-positive small dorsal root ganglion cells expressed the capsaicin receptor transient receptor potential vanilloid type 1 receptor (TRPV1), confirming their nociceptive nature. In addition, a few large cell bodies were labeled for 5-HT2A, and they also expressed NF200 suggesting that they were at the origin of Aδ or Aβ fibers. A total absence of double labeling with parvalbumin showed that they were not proprioceptors. 5-HT2A immunoreactivity in dorsal root ganglia cells was found in the cytoplasm and along the plasma membrane at the interface between sensory cell and the adjacent satellite cells; this distribution was confirmed under the electron microscope, and suggested a functional role for the 5-HT2A receptor at these sites. We therefore investigated the presence of 5-HT and 5-HIAA in lumbar dorsal root ganglia by high performance liquid chromatography. There were 5.75±0.80 ng 5-HT and 3.19±0.37 ng 5-hydroxyindoleacetic acid (5-HIAA) per mg of protein with a ratio 5-HIAA/5-HT of 0.67±0.10, similar to values typically observed in brain tissues. These findings suggest that 5-HT, via the 5-HT2AR, may be involved in the peripheral control of sensory afferents, mainly unmyelinated nociceptors and to a lesser extent neurons with Aδ or Aβ fibers, and in the control of cellular excitability of some dorsal root cell bodies through a paracrine mechanism of action.