Cross-Inhibition of NMBR and GRPR Signaling Maintains Normal Histaminergic Itch Transmission

Abstract

We previously showed that gastrin-releasing peptide receptor (GRPR) in the spinal cord is important for mediating nonhistaminergic itch. Neuromedin B receptor (NMBR), the second member of the mammalian bombesin receptor family, is expressed in a largely nonoverlapping pattern with GRPR in the superficial spinal cord, and its role in itch transmission remains unclear. Here, we report that Nmbr knock-out (KO) mice exhibited normal scratching behavior in response to intradermal injection of pruritogens. However, mice lacking both Nmbr and Grpr (DKO mice) showed significant deficits in histaminergic itch. In contrast, the chloroquine (CQ)-evoked scratching behavior of DKO mice is not further reduced compared with Grpr KO mice. These results suggest that NMBR and GRPR could compensate for the loss of each other to maintain normal histamine-evoked itch, whereas GRPR is exclusively required for CQ-evoked scratching behavior. Interestingly, GRPR activity is enhanced in Nmbr KO mice despite the lack of upregulation of Grpr expression; so is NMBR in Grpr KO mice. We found that NMB acts exclusively through NMBR for itch transmission, whereas GRP can signal through both receptors, albeit to NMBR to a much lesser extent. Although NMBR and NMBR ϩ neurons are dispensable for histaminergic itch, GRPR ϩ neurons are likely to act downstream of NMBR ϩ neurons to integrate NMB-NMBR-encoded histaminergic itch information in normal physiological conditions. Together, we define the respective function of NMBR and GRPR in itch transmission, and reveal an unexpected relationship not only between the two receptors but also between the two populations of interneurons in itch signaling.

Facilitation of TRPV4 by TRPV1 is required for itch transmission in some sensory neuron populations

“…Scratching behaviors were performed as previously described (51, 52). All behavior was performed during the light cycle.…”

A central role for spinal dorsal horn neurons that express neurokinin-1 receptors in chronic itch

“…The upper cervical spinal cord was harvested and post-fixed in 4% paraformaldehyde followed by 30% sucrose. Thirty-micrometer sections of the cervical spinal cord were immunostained with either anti-rabbit GRPR antibody (1:500; LS-A831, Medical & Biological Laboratories International, Woburn, MA) [16; 36] or anti-rabbit NK1R antibody (1:500; AB5060, Millpore, Billerica, MA) using Tyramide Signal Amplification kits (Life Technologies). Briefly, they were incubated with 0.2% Triton X-100 for 10 minutes at room temperature, followed by incubation with peroxidase quenching buffer (PBS + 3% H 2 O 2 ) for either 30 minutes for GRPR antibody or 15 minutes for NK1R at room temperature.…”

“…Histaminergic and non-histaminergic itch require TRPV1 and TRPA1, respectively [18; 33]. In the spinal cord, glutamate as well as neuropeptides including substance P, gastrin releasing peptide (GRP), neuromedin B, and natriuretic polypeptide B (Nppb) are involved in the transmission of itch signals [6; 8; 21; 27; 36]. Neurons expressing the substance P neurokinin-1 receptor (NK1R) represent the majority of ascending somatosensory projection neurons from the spinal and medullary dorsal horn, and are implicated in acute itch [14; 29].…”

Descending Control of Itch Transmission by the Serotonergic System via 5-HT1A-Facilitated GRP-GRPR Signaling

“…Gastrin-releasing peptide (GRP), an endogenous neuropeptide for itch, is expressed in a subset of dorsal root ganglion (DRG) and trigeminal ganglion (TG) cells and mediates GRPR activation in the spinal cord (Sun and Chen, 2007; Takanami et al, 2014; Zhao et al, 2013b). While confusion has arisen about GRP expression in DRG neurons, the issue has recently been clarified (Liu et al, 2014; Takanami et al, 2014; Zhao et al, 2013b; Zhao et al, 2014). The GRP-GRPR pathway is primarily engaged in transducing nonhistaminergic acute itch sensation, and may play a relatively minor role in histaminergic itch (Akiyama et al, 2013; Sun et al, 2009; Zhao et al, 2014).…”

“…While confusion has arisen about GRP expression in DRG neurons, the issue has recently been clarified (Liu et al, 2014; Takanami et al, 2014; Zhao et al, 2013b; Zhao et al, 2014). The GRP-GRPR pathway is primarily engaged in transducing nonhistaminergic acute itch sensation, and may play a relatively minor role in histaminergic itch (Akiyama et al, 2013; Sun et al, 2009; Zhao et al, 2014). Enhanced GRP and GRPR expression is positively correlated with the intensity of chronic itch manifested by increased scratching bouts of animals (Nattkemper et al, 2013; Tominaga et al, 2009; Zhao et al, 2013b).…”