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The Journal of Neurophysiology Vol. 87 No. 6 June 2002, pp. 2726-2733
Copyright ©2002 by the American Physiological Society
1Department of Anesthesiology and 2Department of Neuroscience and Anatomy, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033-0850
Chen, Shao-Rui and
Hui-Lin Pan.
Hypersensitivity of Spinothalamic Tract Neurons Associated With
Diabetic Neuropathic Pain in Rats. J. Neurophysiol. 87: 2726-2733, 2002. Diabetic neuropathic pain is
often considered to be caused by peripheral neuropathy. The involvement
of the CNS in this pathological condition has not been well documented.
Development of hypersensitivity of spinal dorsal horn neurons is
involved in neuropathic pain induced by traumatic nerve injury. In the
present study, we determined the functional changes of identified
spinothalamic tract (STT) neurons and their correlation to diabetic
neuropathic pain. Diabetes was induced in rats by intraperitoneal
injection of streptozotocin. Hyperalgesia and allodynia were assessed
by the withdrawal responses to pressure, radiant heat, and von Frey
filaments applied to the hindpaw. Single-unit activity of STT neurons
was recorded from the lumbar spinal cord in anesthetized rats. The
responses of STT neurons to mechanical and thermal stimuli and the
sensitivity to intravenous morphine were determined in diabetic and
normal rats. In 12 diabetic rats, mechanical allodynia and
hyperalgesia, but not thermal hyperalgesia, developed within 2 wk after
streptozotocin injection and lasted for 
7 wk. Compared to the 32 STT
neurons recorded in normal animals, the 37 STT neurons in diabetic rats displayed a higher spontaneous discharge activity and enlarged receptive fields. Also, the STT neurons in diabetic rats exhibited lower thresholds and augmented responses to mechanical stimulation. Intravenous injection of 2.5 mg/kg of morphine suppressed significantly the responses of STT neurons to noxious stimuli in 12 nondiabetic rats.
However, such an inhibitory effect of morphine on the evoked response
of STT neurons was diminished in 14 diabetic animals. This
electrophysiological study provides new information that development of
hypersensitivity of spinal dorsal horn projection neurons may be
closely related to neuropathic pain symptoms caused by diabetes.
Furthermore, the attenuated inhibitory effects of morphine on evoked
responses of STT neurons in diabetes likely accounts for its reduced
analgesic efficacy in this clinical form of neuropathic pain.
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