The influence of tonic serotonergic modulation on the responses of spinal dorsal horn neurons to natural peripheral stimulation was examined in physiologically intact, awake, drug-free cats. Systemically administered methysergide (maximum cumulative dose 2 mg/kg) caused significant changes in responses of some dorsal horn neurons to both mildly noxious and non-noxious stimulation. Individual changes provide evidence, in this model, for tonic 5-HT modulation of many aspects of sensory transmission at the level of the spinal cord. Taken together, the changes demonstrate the significant degree of plasticity that exists for some spinal dorsal horn neurons. It is clear that the plasticity of some spinal dorsal horn neurons allows for a much broader response profile than would be apparent under the restricted circumstances of a normal neurophysiologic study. Removal of tonic inhibition on responses to noxious stimuli may be an aspect of neuronal plasticity that functions to provide an immediate change in the way that the nervous system responds to a noxious stimulus.
∗Correspondence to: J.G. Collins, Ph.D., Department of Anesthesiology, Yale University School of Medicine, 333 Cedar Street, New Haven, CT 06510, U.S.A.
Submitted May 11, 1989; revised August 4, 1989; accepted September 5, 1989.
© Lippincott-Raven Publishers.