However, those studies did not determine whether the GRPR neurons are interneurons or projection neurons. The dramatic loss of GRPR neurons in the TR4 cKO mice, in which projection neurons were preserved, suggests instead that the GRPR neuron are, in fact, Galunisertib excitatory interneurons that must trigger itch by engaging

projection neurons. Figure 8 illustrates this circuit, but this schematic also includes a population of GRP-positive interneurons in the superficial dorsal horn, loss of which would also impact pruritogen-induced itch. Although most studies indicate that GRP is expressed by a subset of peptidergic and TRPV1-expressing primary afferents (Akiyama et al., 2013; Sun and Chen, 2007), our in situ analysis (and see also Bröhl et al., 2008; Mishra et al., 2012) suggests that pruritogen-responsive unmyelinated afferents may check details not only engage the GRPR circuits directly, but also indirectly, via GRP-expressing excitatory interneurons. For pain and itch

to be segregated, it follows that the GRPR-positive population of interneurons and a different (pain-provoking) population must engage different populations of projection neurons. In other words, there must be one labeled line for itch and another for pain. However, based on the concurrent reduction of pain and itch after deletion of the relatively large NK1 receptor-expressing subset of projection neurons Carstens et al. (2010) concluded the opposite, namely that there is convergence of pain and itch circuits upon a common output system. Figure 8 illustrates both until scenarios, one circuit in which there are labeled lines for pain and itch and another in which “itch”- and “pain”-selective excitatory interneurons converge onto NK1R-expressing projection neurons. If the latter circuit indeed exists, then the differential perception

of pain and itch must involve distinguishable firing codes generated by spinal cord projection neurons, codes that must be “read” by the brain. The fact that the great majority, if not all superficial dorsal horn neurons, responds to noxious heat and mechanical stimuli, as well as to pruritogens, certainly points to a convergent circuit. Clearly it is critical to determine the extent of convergence of GRPR-positive and negative interneurons upon projection neurons. Figure 8 also highlights the fact that the extent of specificity in the processing of pain and itch messages must consider the contribution of the primary afferent. Consistent with several previous studies (Akiyama et al., 2009; Davidson et al., 2012), we found that all capsaicin and histamine responsive dorsal horn neurons are noxious heat responsive. It follows that they all receive input from TRPV1-expressing primary afferents. However, that conclusion does not eliminate the possibility that there is physiological specificity in the central connections of these TRPV1 afferents.