Responses of Rat Spinal Dorsal Horn Neurons to Intracutaneous
Microinjection of Histamine, Capsaicin and Other Irritants.
E. Carstens.
Section of Neurobiology, Physiology and Behavior, University of California,
Davis, Davis, CA 95616, Tel: 916-752-7805, Fax: 916-752-5582, e-mail:
eecarstens@ucdavis.edu.
APStracts 4:0050N, 1997.
ABSTRACT
To investigate the spinal processing of cutaneous pruritic and algesic
stimuli, single-unit recordings were made from wide dynamic range-type lumbar
spinal dorsal horn neurons in pentobarbital-anesthetized rats. Neuronal
responses to mechanical and noxious thermal stimuli were recorded, as well as
to microinjection (1 æl) of histamine (0.01-10% = 9x10-1 - 9x10-4 M),
capsaicin (0.1% = 3.3x10-3 M) or other algesic chemicals into skin within the
receptive field via intracutaneously placed needles. Most (84%) of the 89
neurons responded to intracutaneous (i.c.) microinjection of histamine with a
brief phasic discharge followed by an after-discharge of variable (seconds to
minutes) duration. Ten min following i.c. microinjection of histamine (but not
following NaCl), there was a significant increase in the mean area of the low-
threshold (but not high-threshold) portion of unit mechanical receptive
fields. However, responses to graded pressure stimuli were not significantly
affected following histamine. Responses did not exhibit significant
tachyphylaxis when histamine microinjections were repeated at 5 or 10 min
intervals. Unit responses significantly increased in a dose-related manner to
microinjection of histamine at concentrations ranging across 4 orders of
magnitude. Within 30 sec after i.c. microinjection of the H1 antagonist,
cetirizine, unit responses to i.c. histamine delivered at the same skin site
were significantly attenuated. Unit responses to histamine, as well as to
noxious thermal stimulation, were significantly reduced following systemic
administration of morphine (3.5 mg/kg i.p.) in a naloxone-reversible manner.
Application of a mechanical rub, scratch or a noxious heat stimulus during the
unit's ongoing response to i.c. histamine produced a brief and marked
excitation, often followed by a period of reduced ongoing discharge. Unit
responses to histamine were markedly suppressed by electrical stimulation in
the midbrain periaqueductal gray (PAG). Most (79%) histamine-responsive units
tested also responded to i.c. microinjection of capsaicin. Following the
initial microinjection of capsaicin, subsequent responses to histamine and
capsaicin microinjections were significantly reduced. Units also responded to
i.c. ethanol (capsaicin vehicle) in a dose-related manner, and showed
tachyphylaxis to repeated i.c. ethanol at 80% but not at 8%. The mean response
to 80% ethanol was significantly smaller than to 0.1% capsaicin. All units
tested also responded to topical application of mustard oil (50%) and i.c.
serotonin (30 æg). The results are discussed in terms of theories that attempt
to reconcile psychophysical and clinical observations of pain and itch
sensation.
Received 11 March 1996; accepted in final form 22 January 1997.
APS Manuscript Number J192-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 19 February 1997