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Distribution of gastrin‐releasing peptide in the rat trigeminal and spinal somatosensory systems

Gastrin‐releasing peptide (GRP) has recently been identified as an itch‐specific neuropeptide in the spinal sensory system in mice, but there are no reports of the expression and distribution of GRP in the trigeminal sensory system in mammals. We characterized and compared GRP‐immunoreactive (ir) neurons in the trigeminal ganglion (TG) with those in the rat spinal dorsal root ganglion (DRG). GRP immunoreactivity was expressed in 12% of TG and 6% of DRG neurons and was restricted to the small‐ and medium‐sized type cells. In both the TG and DRG, many GRP‐ir neurons also expressed substance P and calcitonin gene‐related peptide, but not isolectin B4. The different proportions of GRP and transient receptor potential vanilloid 1 double‐positive neurons in the TG and DRG imply that itch sensations via the TG and DRG pathways are transmitted through distinct mechanisms. The distribution of the axon terminals of GRP‐ir primary afferents and their synaptic connectivity with the rat trigeminal sensory nuclei and spinal dorsal horn were investigated by using light and electron microscopic histochemistry. Although GRP‐ir fibers were rarely observed in the trigeminal sensory nucleus principalis, oralis, and interpolaris, they were predominant in the superficial layers of the trigeminal sensory nucleus caudalis (Vc), similar to the spinal dorsal horn. Ultrastructural analysis revealed that GRP‐ir terminals contained clear microvesicles and large dense‐cored vesicles, and formed asymmetric synaptic contacts with a few dendrites in the Vc and spinal dorsal horn. These results suggest that GRP‐dependent orofacial and spinal pruriceptive inputs are processed mainly in the superficial laminae of the Vc and spinal dorsal horn. J. Comp. Neurol. 522:1858–1873, 2014. © 2013 Wiley Periodicals, Inc.