Pruriception and neuronal coding in nociceptor subtypes in human and nonhuman primates

Area of Science:

• Neuroscience
• Primate sensory physiology
• Molecular biology

Background:

• Intradermal administration of beta-alanine (ALA) and bovine adrenal medulla peptide 8-22 (BAM8-22) induces itch in humans.
• The specific human dorsal root ganglion (DRG) neurons expressing receptors for these pruritogens (MRGPRD and MRGPRX1) and the cutaneous afferents activated remain unidentified.
• Understanding these pathways is crucial for deciphering itch mechanisms.

Purpose of the Study:

• To identify the human DRG neurons expressing MRGPRD and MRGPRX1.
• To determine which primate cutaneous afferents are activated by ALA and BAM8-22.
• To investigate the combined effects of different pruritogens on sensory perception.

Main Methods:

• In situ hybridization was used to examine the expression of MRGPRD and MRGPRX1 in human DRG neurons.
• Electrophysiological recordings were performed in nonhuman primates (Macaca nemestrina) to assess neuronal activation.
• Human volunteers received combined administration of ALA, BAM8-22, and histamine to evaluate sensory responses.

Main Results:

• MRGPRD and MRGPRX1 were found to be co-expressed in a subpopulation of TRPV1+ human DRG neurons.
• Subtypes of polymodal C-fiber nociceptors in nonhuman primates were preferentially activated by ALA and BAM8-22, showing significant overlap.
• Combined administration of different pruritogens in humans resulted in itch and nociceptive sensations comparable to single pruritogen application.

Conclusions:

• The study identifies specific DRG neurons and primate afferents involved in ALA- and BAM8-22-induced itch.
• Primate pruriceptive processing exhibits differences compared to rodents.
• The findings do not support the spatial contrast theory for the coding of itch and pain.