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  • Response characteristics of pruriceptive and nociceptive trigeminoparabrachial tract neurons in the rat.

Response characteristics of pruriceptive and nociceptive trigeminoparabrachial tract neurons in the rat.

Journal of neurophysiology (2014-10-10)
Nico A Jansen, Glenn J Giesler
ABSTRACT

We tested the possibility that the trigeminoparabrachial tract (VcPbT), a projection thought to be importantly involved in nociception, might also contribute to sensation of itch. In anesthetized rats, 47 antidromically identified VcPbT neurons with receptive fields involving the cheek were characterized for their responses to graded mechanical and thermal stimuli and intradermal injections of pruritogens (serotonin, chloroquine, and β-alanine), partial pruritogens (histamine and capsaicin), and an algogen (mustard oil). All pruriceptive VcPbT neurons were responsive to mechanical stimuli, and more than half were additionally responsive to thermal stimuli. The majority of VcPbT neurons were activated by injections of serotonin, histamine, capsaicin, and/or mustard oil. A subset of neurons were inhibited by injection of chloroquine. The large majority of VcPbT neurons projected to the ipsilateral and/or contralateral external lateral parabrachial and Kölliker-Fuse nuclei, as evidenced by antidromic mapping techniques. Analyses of mean responses and spike-timing dynamics of VcPbT neurons suggested clear differences in firing rates between responses to noxious and pruritic stimuli. Comparisons between the present data and those previously obtained from trigeminothalamic tract (VcTT) neurons demonstrated several differences in responses to some pruritogens. For example, responses of VcPbT neurons to injection of serotonin often endured for nearly an hour and showed a delayed peak in discharge rate. In contrast, responses of VcTT neurons endured for roughly 20 min and no delayed peak of firing was noted. Thus the longer duration responses to 5-HT and the delay in peak firing of VcPbT neurons better matched behavioral responses to stimulation in awake rats than did those of VcTT neurons. The results indicate that VcPbT neurons may have important roles in the signaling of itch as well as pain.

MATERIALS
Product Number
Brand
Product Description

Supelco
Capsaicin, analytical standard
USP
Capsaicin, United States Pharmacopeia (USP) Reference Standard
Sigma-Aldrich
Capsaicin, natural
Sigma-Aldrich
Capsaicin, from Capsicum sp., ≥50% (HPLC)
Capsaicin, European Pharmacopoeia (EP) Reference Standard
Sigma-Aldrich
Chloroquine diphosphate salt, powder or crystals, 98.5-101.0% (EP)
Sigma-Aldrich
Capsaicin, ≥95%, from Capsicum sp.
Supelco
Capsaicin, Pharmaceutical Secondary Standard; Certified Reference Material
Supelco
Urethane, analytical standard
Sigma-Aldrich
Histamine dihydrochloride, ≥99.0% (AT)
Sigma-Aldrich
Urethane, ≥99.0% (GC)
Sigma-Aldrich
Serotonin hydrochloride, powder
Sigma-Aldrich
Serotonin creatinine sulfate monohydrate, powder
Sigma-Aldrich
Histamine dihydrochloride, ≥99% (TLC), powder
Supelco
Chloroquine phosphate, Pharmaceutical Secondary Standard; Certified Reference Material
Sigma-Aldrich
Urethane, ≥99%
Histamine dihydrochloride, European Pharmacopoeia (EP) Reference Standard