Substance P receptor (SPR)-expressing spinal neurons were ablated with the selective cytotoxin substance P-saporin. Loss of these neurons resulted in a reduction of thermal hyperalgesia and mechanical allodynia associated with persistent neuropathic and inflammatory pain states. This loss appeared to be permanent. Responses to mildly painful stimuli and morphine analgesia were unaffected by this treatment. These results identify a target for treating persistent pain and suggest that the small population of SPR-expressing neurons in the dorsal horn of the spinal cord plays a pivotal role in the generation and maintenance of chronic neuropathic and inflammatory pain.

Neuropathic pain states are accompanied by increased sensitivity to both noxious and non-noxious sensory stimuli, characterized as hyperalgesia and allodynia, respectively. In animal models of neuropathic pain, the presence of hyperalgesia and allodynia are accompanied by neuroplastic changes including increased spinal levels of substance P, cholecystokinin (CCK), and dynorphin. N-Methyl-D-aspartate (NMDA) receptors appear to be involved in maintaining the central sensitivity which contributes to neuropathic pain. In addition to its opioid activities, dynorphin has been suggested to act at the NMDA receptor complex. In an attempt to mimic the increased levels of spinal dynorphin seen in animal models of neuropathic pain, rats received a single intrathecal (i.t.) injection of dynorphin A(1-17), dynorphin A(1-13), dynorphin A(2-17) or dynorphin A(2-13) through indwelling catheters. Tactile allodynia was determined by measuring response threshold to probing with von Frey filaments. Dynorphin A(1-17) administration evoked significant and long-lasting tactile allodynia (i.e. > 60 days). Likewise, the i.t. administration of dynorphin A(1-13) or dynorphin A(2-17) or dynorphin A(2-13) also produced long-lasting tactile allodynia. Intrathecal pretreatment, but not post-treatment, with MK-801 prevented dynorphin A(1-17)-induced development of allodynia; i.t. administration of MK-801 alone had no effect on responses to tactile stimuli. In contrast, i.t. pretreatment with naloxone did not affect the development of tactile allodynia induced by dynorphin A(1-17) or alter sensory threshold when given alone. These results demonstrate that a single dose of dynorphin A, or its des-Tyr fragments, produces long-lasting allodynia which may be irreversible in the rat. Further, this effect appears to be mediated through activation of NMDA, rather than opioid, receptors. While the precise mechanisms underlying the development and maintenance of the allodynia is unclear, it seems possible that dynorphin may produce changes in the spinal cord, which may contribute to the development of signs reminiscent of a "neuropathic' state. Given that levels of dynorphin are elevated following nerve injury, it seems reasonable to speculate that dynorphin may have a pathologically relevant role in neuropathic pain states.

Inflammatory pain involves the sensitization of both primary afferent and spinal cord neurons. To explore the neurochemical changes that contribute to inflammatory pain, we have examined the expression and ligand-induced internalization of the substance P receptor (SPR) in the spinal cord in acute, short-term, and long-term inflammatory pain states. These inflammatory models included unilateral injection of formalin (8-60 min), carrageenan (3 hr), and complete Freund's adjuvant (CFA; 3 d) into the rat hindpaw as well as adjuvant-induced polyarthritis (21 d). In acute inflammatory pain there is ongoing release of substance P (SP) as measured by SPR internalization in lamina I neurons at both 8 and 60 min after formalin injection. Although there is no tonic release of SP in short-term inflammatory pain, at 3 hr after carrageenan injection, SP is released in response to both noxious and non-noxious somatosensory stimulation with SPR internalization being observed in neurons located in both laminae I and III-IV. In long-term inflammatory pain models (CFA and polyarthritis) the same pattern of SP release and SPR activation occurs as is observed in short-term inflammation with the addition that there is a significant upregulation of the SPR in lamina I neurons. These results suggest that SPR internalization might serve as a marker of the contribution of ongoing primary afferent input in acute and persistent pain states. These stereotypical neurochemical changes suggest that there are unique neurochemical signatures for acute, short-term, and long-term inflammatory pain.

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Nerve ligation injury in rats results in reduced nociceptive and non-nociceptive thresholds, similar to some aspects of clinical conditions of neuropathic pain. Since underlying mechanisms of hyperalgesia and allodynia may differ, the present study investigated the pharmacology of morphine and MK-801 in rats subjected to a tight ligation of the L5 and L6 nerve roots or to a sham-operation procedure. Response to acute nociception was measured by (a) withdrawal of a hindpaw from a radiant heat source, (b) withdrawal of the tail from a radiant heat source or (c) the latency to a rapid flick of the tail following immersion in water at different noxious temperatures. Mechanical thresholds were determined by measuring response threshold to probing the hindpaw with von Frey filaments. Nerve ligation produced a significant, stable and long-lasting decrease in threshold to mechanical stimulation (i.e., tactile allodynia) when compared to sham-operated controls. Standardization of the diameter of the filaments (to that of the largest filament) did not alter the response threshold in nerve-injured animals. Nerve ligation produced decreased response latency of the ipsilateral paw (i.e., hyperalgesia) when compared to that of sham-operated rats. Tail-flick latencies to thermal stimuli induced by water at constant temperatures (48 degrees, 52 degrees or 55 degrees C) or by radiant heat were not significantly different between nerve-injured and sham-operated groups. At doses which were not behaviorally toxic, MK-801 had no effect on tactile allodynia. At these doses, MK-801 blocked decreased paw withdrawal latency to radiant heat in nerve-injured rats, but did not significantly elevate the response threshold of sham-operated rats. Systemic (i.p.) or intracerebroventricular (i.c.v.) doses of morphine previously shown to be antiallodynic in nerve-ligated rats did not affect the response to probing with von Frey filaments in sham-operated controls. Intrathecal (i.t.) morphine did not change paw withdrawal thresholds elicited by von Frey filaments of either nerve-ligated rats (as previously reported) or of sham-operated rats at doses maximally effective against thermal stimuli applied to the tail or foot. Spinal morphine produced dose-dependent antinociception in both nerve-injured and sham-operated groups in the foot-flick test but was less potent in the nerve-injured group. Presuppression of hyperalgesia of the foot with i.t. MK-801 in nerve-injured animals did not alter the potency of i.t. morphine. I.t. morphine was also active in the tail-flick tests with decreased potency in nerve-injured animals and, at some stimulus intensities, with a decreased efficacy as well. These data emphasize the distinction between the inactivity of morphine to suppress mechanical withdrawal thresholds (as elicited by von Frey filaments) and the activity of this compound to block the response to an acute thermal nociceptive stimulus in sham-operated or nerve-injured rats. It appears that nerve ligation injury produces a thermal allodynia/hyperalges...

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Neuropathic pains arising from peripheral nerve injury can result in increased sensitivity to both noxious and non-noxious stimuli and are accompanied by a number of neuroplastic alterations at the level of the spinal cord including upregulation of neurotransmitters including dynorphin, cholecystokinin and neuropeptide Y. Additionally, such pain states appear to be associated with activation of excitatory amino acid receptors including the N-methyl-D-aspartate (NMDA) receptor. Neuropathic pains have often been classified as 'opioid resistant' in both clinical and laboratory settings. As it is known that dynorphin produces 'non-opioid' effects through interaction with NMDA receptors and this peptide is upregulated after peripheral nerve injury, the present studies were undertaken to determine the possible importance of this substance in the neuropathic state. Nerve injury was produced in rats by tight ligation of the L5 and L6 spinal roots of the sciatic nerve. Catheters were inserted for the intrathecal (i.t.) delivery of drug to the lumbar spinal cord. Tactile allodynia was determined by measuring responses to probing the plantar surface of the affected limb with von Frey filaments, and acute nociception was determined in the 55 degrees C hot-water tail-flick test in nerve-ligated and sham-operated subjects. Intrathecal administration of MK-801 or antisera to dynorphin A (1-13) did not alter the tactile allodynia associated with nerve-ligation injury or the baseline tail-flick latency in either sham-operated or nerve-injured animals. As previously reported, i.t. morphine did not alter tactile allodynia and showed reduced potency and efficacy to block the tail-flick reflex in nerve-injured animals. Co-administration, however, of i.t. morphine with MK-801, or i.t. antisera to dynorphin A (1-13) given prior to morphine elicited both a full antiallodynic response and a complete block of the tail-flick reflex in nerve-injured animals. These results suggest that tonic activation of NMDA receptors, following peripheral nerve injury, is involved with the attenuation of the effectiveness of spinal morphine in a model of neuropathic pain. Additionally, this tonic NMDA activity may be mediated, in part, by increased levels of endogenous dynorphin associated with peripheral nerve injury.