Facial injections of pruritogens or algogens elicit distinct behavior responses in rats and excite overlapping populations of primary sensory and trigeminal subnucleus caudalis neurons

J Neurophysiol. 2011 Sep;106(3):1078-88. doi: 10.1152/jn.00302.2011. Epub 2011 Jun 8.

Abstract

In the present study, we investigated whether intradermal cheek injection of pruritogens or algogens differentially elicits hindlimb scratches or forelimb wipes in Sprague-Dawley rats, as recently reported in mice. We also investigated responses of primary sensory trigeminal ganglion (TG) and dorsal root ganglion (DRG) cells, as well as second-order neurons in trigeminal subnucleus caudalis (Vc), to pruritic and algesic stimuli. 5-HT was the most effective chemical to elicit dose-dependent bouts of hindlimb scratches directed to the cheek, with significantly less forelimb wiping, consistent with itch. Chloroquine also elicited significant scratching but not wiping. Allyl isothiocyanate (AITC; mustard oil) elicited dose-dependent wiping with no significant scratching. Capsaicin elicited equivalent numbers of scratch bouts and wipes, suggesting a mixed itch and pain sensation. By calcium imaging, ∼ 6% of cultured TG and DRG cells responded to 5-HT. The majority of 5-HT-sensitive cells also responded to chloroquine, AITC, and/or capsaicin, and one-third responded to histamine. Using a chemical search strategy, we identified single units in Vc that responded to intradermal cheek injection of 5-HT. Most were wide dynamic range (WDR) or nociceptive specific (NS), and a few were mechanically insensitive. The large majority additionally responded to AITC and/or capsaicin and thus were not pruritogen selective. These results suggest that primary and second-order neurons responsive to pruritogens and algogens may utilize a population coding mechanism to distinguish between itch and pain, sensations that are behaviorally manifested by distinct hindlimb scratching and forelimb wiping responses.

Publication types

  • Comparative Study
  • Research Support, N.I.H., Extramural

MeSH terms

  • Animals
  • Behavior, Animal / drug effects
  • Behavior, Animal / physiology*
  • Face / physiology
  • Injections, Intradermal
  • Male
  • Pain / chemically induced
  • Pain / physiopathology*
  • Pruritus / chemically induced
  • Pruritus / physiopathology*
  • Rats
  • Rats, Sprague-Dawley
  • Sensory Receptor Cells / drug effects
  • Sensory Receptor Cells / physiology*
  • Serotonin / administration & dosage
  • Serotonin / toxicity
  • Trigeminal Caudal Nucleus / drug effects
  • Trigeminal Caudal Nucleus / physiology*

Substances

  • Serotonin