Comparison of responses of primate spinothalamic tract neurons to pruritic and algogenic stimuli.
We investigated the role of mechanosensitive spinothalamic tract (STT) neurons in mediating 1) the itch evoked by intradermal injection of histamine, 2) the enhanced sense of itch evoked by innocuous stroking (alloknesis), and 3) the enhanced pain evoked by punctate stimulation (hyperalgesia) of the skin surrounding the injection site. Responses to intradermal injections of histamine and capsaicin were compared in STT neurons recorded in either the superficial or the deep dorsal horn of the anesthetized monkey. Each neuron was identified by antidromic activation from the ventral posterior lateral nucleus of thalamus and classified by its initial responses to mechanical stimuli as wide dynamic range (WDR) or high-threshold (HT). Approximately half of the WDRs and one of the HTs responded weakly to histamine, some with a duration > 5 min, the maximal time allotted. WDRs but not HTs exhibited a significant increase in response to punctate stimulation after histamine consistent with their possible role in mediating histamine-induced hyperalgesia. Neither type of neuron exhibited significant changes in response to stroking, consistent with their unlikely role in mediating alloknesis. Furthermore, nearly all STT neurons exhibited vigorous and persistent responses to capsaicin, after which they became sensitized to stroking and to punctate stimulation. We conclude that the STT neurons in our sample are more likely to contribute to pain, allodynia, and hyperalgesia than to itch and alloknesis.
Simone
Donald A
DA
Department of Oral Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA. simon003@umn.edu
Zhang
Xijing
X
Li
Jun
J
Zhang
Jun-Ming
JM
Honda
Christopher N
CN
LaMotte
Robert H
RH
Giesler
Glenn J
GJ
D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000704:Analysis of Variance; D000818:Animals; D002211:Capsaicin; D004594:Electrophysiology; D005071:Evoked Potentials; D006632:Histamine; D006358:Hot Temperature; D006930:Hyperalgesia / Q000139:chemically induced / Q000209:etiology / Q000503:physiopathology*; D007271:Injections, Intradermal / Q000379:methods; D007796:Laminectomy / Q000379:methods; D008252:Macaca fascicularis; D010812:Physical Stimulation / Q000379:methods; D020671:Posterior Horn Cells / Q000187:drug effects / Q000502:physiology*; D011537:Pruritus / Q000139:chemically induced / Q000503:physiopathology*; D011930:Reaction Time; D012867:Skin / Q000187:drug effects / Q000294:innervation; D013133:Spinothalamic Tracts / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D013268:Stimulation, Chemical; D013788:Thalamus / Q000033:anatomy & histology / Q000502:physiology; D013997:Time Factors
D006632:Histamine; D002211:Capsaicin