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Neuropeptide Y signaling plays an important role in inhibiting chronic pain in the spinal cord of mice. However, little is known about the respective roles of two major neuropeptide Y receptors, Y1R and Y2R, in evoked and spontaneous pain behavior under normal physiological condition. Using intrathecal administration approach, we found that pharmacological inhibition of Y2R, unexpectedly, gave rise to spontaneous pain behavior. In addition, Y2R antagonism also resulted in longlasting mechanical but not thermal hypersensitivity. By contrast, neither overt spontaneous pain behavior nor mechanical and thermal hypersensitivity were detected after pharmacological inhibition of Y1R. Remarkably, the activation of Y1R produced powerful analgesic effect: blocking both evoked and spontaneous pain behavior resulted from Y2R antagonism. These findings highlight the pivotal role of endogenous Y2R in gating mechanical and spontaneous pain transmission. Importantly, our results suggest that Y1R could be a therapeutic target that may be exploited for alleviating spontaneous pain without affecting acute pain transmission. [ABSTRACT FROM AUTHOR]

There are substantial disagreements about the expression of gastrin-releasing peptide (GRP) in sensory neurons and whether GRP antibody cross-reacts with substance P (SP). These concerns necessitate a critical revaluation of GRP expression using additional approaches. Here, we show that a widely used GRP antibody specifically recognizes GRP but not SP. In the spinal cord of mice lacking SP (Tac1 KO), the expression of not only GRP but also other peptides, notably neuropeptide Y (NPY), is significantly diminished. We detected Grp mRNA in dorsal root ganglias using reverse transcription polymerase chain reaction, in situ hybridization and RNA-seq. We demonstrated that Grp mRNA and protein are upregulated in dorsal root ganglias, but not in the spinal cord, of mice with chronic itch. Few GRPþ immunostaining signals were detected in spinal sections following dorsal rhizotomy and GRP+ cell bodies were not detected in dissociated dorsal horn neurons. Ultrastructural analysis further shows that substantially more GRPergic fibers form synaptic contacts with gastrin releasing peptide receptor-positive (GRPR+) neurons than SPergic fibers. Our comprehensive study demonstrates that a majority of GRPergic fibers are of primary afferent origin. A number of factors such as low copy number of Grp transcripts, small percentage of cells expressing Grp, and the use of an eGFP GENSAT transgenic as a surrogate for GRP protein have contributed to the controversy. Optimization of experimental procedures facilitates the specific detection of GRP expression in dorsal root ganglia neurons. [ABSTRACT FROM AUTHOR]

Background A recent study by Mishra and Hoon identified B-type natriuretic peptide (BNP) as an important peptide for itch transmission and proposed that BNP activates spinal natriuretic peptide receptor-A (NPRA) expressing neurons, which release gastrin releasing peptide (GRP) to activate GRP receptor (GRPR) expressing neurons to relay itch information from the periphery to the brain (Science 340:968-971, 2013). A central premise for the validity of this novel pathway is the absence of GRP in the dorsal root ganglion (DRG) neurons. To this end, they showed that Grp mRNA in the DRG neurons is either absent or barely detectable and claimed that BNP but not GRP is a major neurotransmitter for itch in pruriceptors. They showed that NPRA immunostaining is perfectly co-expressed with Grp-eGFP in the spinal cord, and a few acute pain behaviors in Nppb-/- mice were tested. They claimed that BNP is an itch-selective peptide that acts as the first station of a dedicated neuronal pathway comprising a GRP-GRPR cascade for itch. However, our studies, along with the others, do not support their claims. Findings We were unable to reproduce the immunostaining of BNP and NPRA as shown by Mishra and Hoon. By contrast, we were able to detect Grp mRNA in the DRGs using in situ hybridization and real time RT-PCR. We show that the expression pattern of Grp mRNA is comparable to that of GRP protein in DRGs. Pharmacological and genetic blockade of GRPGRPR signaling does not significantly affect intrathecal. BNP-induced scratching behavior. We show that BNP inhibits inflammatory pain and morphine analgesia. Conclusions Accumulating evidence demonstrates that GRP is a key neurotransmitter in pruriceptors for mediating histamine-independent itch. BNP-NPRA signaling has dual roles in itch and pain and does not function upstream of the GRP-GRPR dedicated neuronal pathway. The site of BNP action in itch and pain and its relationship with GRP remain to be clarified (Science 340:968-971, 2013). [ABSTRACT FROM AUTHOR]