Tinna M. Laughlin

Single intrathecal injections of dynorphin A or des-Tyr-dynorphins produce long-lasting allodynia in rats: blockade by MK-801 but not naloxone.

&NA; 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.

Spinally administered dynorphin A produces long-lasting allodynia : involvement of NMDA but not opioid receptors

Abstract The endogenous opioid peptide dynorphin A has non‐opioid effects that can damage the spinal cord when given in high doses. Dynorphin has been shown to increase the receptive field size of spinal cord neurons and facilitate C‐fiber‐evoked reflexes. Furthermore, endogenous dynorphin levels increase following damage to the spinal cord, injury to peripheral nerves, or inflammation. In this study, sensory processing was characterized following a single, intrathecal injection of dynorphin A (1‐17) in mice. A single intrathecal injection of dynorphin A (1‐17) (3 nmol, i.t.) induced mechanical allodynia (hind paw, von Frey filaments) lasting 70 days, tactile allodynia (paint brush applied to flank) lasting 14 days, and cold allodynia (acetone applied to the dorsal hind paw) lasting 7 days. Similarly, dynorphin A (2‐17) (3 nmol, i.t.), a non‐opioid peptide, induced cold and tactile allodynia analogous to that induced by dynorphin A (1‐17), indicating the importance of non‐opioid receptors. Pretreatment with the NMDA antagonists, MK‐801 and LY235959, but not the opioid antagonist, naloxone, blocked the induction of allodynia. Post‐treatment with MK‐801 only transiently blocked the dynorphin‐induced allodynia, suggesting the NMDA receptors may be involved in the maintenance of allodynia as well as its induction. We have induced a long‐lasting state of allodynia and hyperalgesia by a single intrathecal injection of dynorphin A (1‐17) in mice. The allodynia induced by dynorphin required NMDA receptors rather than opioid receptors. This result is consistent with results in rats and with signs of clinically observed neuropathic pain. This effect of exogenously administered dynorphin raises the possibility that increased levels of endogenous dynorphins associated with spinal cord injuries may participate in the genesis and maintenance of neuropathic pain.

Spinal analgesic actions of the new endogenous opioid peptides endomorphin-1 and -2

TWO highly-selective μ-opioid receptor agonists, endomorphin-1 and -2, were recently purified from bovine brain and are postulated to be endogenous μ-opioid receptor ligands. We sought to determine the effects of these ligands at the spinal level in mice. Endomorphin1 and -2 produced short acting, naloxone-sensitive antinociception in the tail flick test and inhibited the behavior elicited by intrathecally injected substance P. Both endomorphin-1 and -2 were anti-allodynic in the dynorphin-induced allodynia model. Although acute tolerance against both endomorphins developed rapidly, endomorphin-1 required a longer pretreatment time before tolerance was observed. We conclude that the endomorphins are potent spinal antinociceptive and anti-allodynic agents and that they or related compounds may prove therapeutically useful as spinal analgesics.

Cytokine involvement in dynorphin-induced allodynia

Abstract Dynorphin A is an endogenous opioid peptide, which has previously been shown to produce a long‐lasting allodynia and hyperalgesia in mice, behavioral states consistent with signs of clinically observed neuropathic pain. This dynorphin‐induced allodynia was used as a pharmacological, central model of neuropathic pain. In this study, we examined the involvement of the cytokine IL‐1&bgr;, the transcription factor nuclear factor kappa B (NF‐&kgr;B), and de novo protein synthesis in the development of allodynia induced by intrathecal (i.t.) administration of dynorphin in male ICR mice. Pretreatment with the protein synthesis inhibitor cycloheximide (0.3–85 nmol), the NF‐&kgr;B inhibitor pyrrolidinedithiocarbamate (PDTC) (0.001–1000 pmol), the IL‐1 receptor antagonist (IL‐1ra) protein (0.01–100 ng), the caspase‐1 inhibitor (YVAD) (0.1–300 pmol), and the anti‐inflammatory cytokine IL‐10 (0.1–300 ng) all dose‐dependently reduced the induction of dynorphin‐induced allodynia. Finally, IL‐10 administered within the first 24 h after the dynorphin insult prevented the development of chronic allodynia. These results demonstrate that the anti‐inflammatory cytokines IL‐10 and IL‐1ra impede the development of dynorphin‐induced allodynia. These results also suggest that production of new proteins through NF‐&kgr;B activation is required for the induction of allodynia. We speculate that IL‐1ra, IL‐10, PDTC and cycloheximide interfere with the central pro‐inflammatory cascade. Modulation of cytokine activity in the spinal cord may therefore prove to be an effective therapeutic strategy for the treatment of chronic pain.