RECEPTIVE FIELD EXPANSION AND FUNCTIONAL CHANGES OF NEURONS IN
THE VENTROPOSTEROLATERAL THALAMIC NUCLEUS DURING SPINAL
STRYCHNINE-INDUCED ALLODYNIA IN THE URETHANE-ANESTHETIZED RAT
Stephen E. Sherman, Lei Luo, and Jonathan O. Dostrovsky
Department of Physiology, Medical Sciences Building, University of Toronto,
Toronto Ontario, Canada, M5S 1A8, Astra Pain Control AB, Preclinical R&D, Novum Unit, S-141 57, Huddinge, Sweden
APStracts 4:108N, 1997.
ABSTRACT
Allodynia is an unpleasant sequela of neural injury or neuropathy which is
characterized by the inappropriate perception of light tactile stimuli as
pain. This condition may be modelled experimentally in animals by the
intrathecal (i.t.) administration of strychnine, a glycine receptor
antagonist. Thus, after i.t. strychnine, otherwise innocuous tactile stimuli
evoke behavioural and autonomic responses which are normally elicited only by
noxious stimuli. The current study was undertaken to determine how i.t.
strychnine alters the spinal processing of somatosensory input by examining
the responses of neurons in the ventroposterolateral (VPL) thalamic nucleus.
Extracellular, single-unit recordings were conducted in the lateral thalamus
of 19 urethane-anaesthetized, male, Wistar rats (342 ñ 44 g). Receptive fields
and responses to noxious and innocuous cutaneous stimuli were determined for
19 units (one per animal), prior to and immediately following i.t. strychnine
(40 æg). Eighteen of the animals developed allodynia as evidenced by the
ability of otherwise innocuous brush or air jet stimuli to evoke
cardiovascular and/or motor reflexes. All (3) of the nociceptive specific (NS)
units became responsive to brush stimulation after i.t. strychnine, and one
became sensitive to brushing over an expanded receptive field. Expansion of
the receptive field, as determined by brush stimulation, was also exhibited by
all of the low-threshold mechanoreceptive (LTM) units (14) and wide dynamic
range (WDR) units (2) after i.t. strychnine. The use of air jet stimuli at
fixed cutaneous sites also provided evidence of receptive field expansion,
since significant unit responses to air jet developed at 13 cutaneous sites
(on 7 animals) where an identical stimulus was ineffective in evoking a unit
response prior to i.t. strychnine. However, the magnitude of the unit response to cutaneous air jet stimulation was not changed at sites that had already
been sensitive to this stimulus prior to i.t. strychnine. The onset of
allodynia corresponded with the onset of the altered unit responses (i.e.
lowered threshold/receptive field expansion) for the majority of animals (9),
but the altered unit response either terminated concurrently with symptoms of
allodynia (6) or, more frequently, outlasted the symptoms of allodynia (10) as
the effects of strychnine declined. The present results demonstrate that the
direct, receptor-mediated actions of strychnine on the spinal processing of
sensory information are reflected by changes in the receptive fields and
response properties of nociceptive and non-nociceptive thalamic neurons. These
changes are consistent with the involvement of thalamocortical mechanisms in
the expression of strychnine-induced allodynia and moreover suggest that i.t.
strychnine produces also changes in innocuous tactile sensation.
Received 24 March 1997; accepted in final form 19 June 1997.
APS Manuscript Number J250-7
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 15 July 1997