Thomas P. Jacobs

Dynorphin-related peptides cause motor dysfunction in the rat through a non-opiate action

1 We compared effects on motor function of four peptides belonging to the dynorphin family ‐dynorphin‐(1–17) (DYN‐(1–17)), dynorphin‐(1–13) (DYN‐(1–13)), dynorphin‐(1–8) (DYN‐(1–8)) and α‐neo‐endorphin (αNE). 2 After intrathecal administration, each of these peptides produced dose‐related, flaccid, hindlimb paralysis, with the order of potency being DYN‐(1–17) > DYN‐(1–13) > αNE ≏DYN‐(1–8). 3 This motor dysfunction was not reversed or blocked by the opiate receptor antagonist naloxone and was not produced by a variety of other κ‐selective agonists. 4 However, paralysis was produced by des‐Tyr‐dynorphin (DYN‐(2–17)), which does not act at the opioid receptor. 5 Taken together, the present studies show that dynorphin‐related peptides, uniquely amongst opioids, produce motor dysfunction, an action which does not appear to be mediated by opioid receptors.

Dynorphin induces partially reversible paraplegia in the rat

Endogenous opioids have previously been implicated in the pathophysiology of spinal cord injury, but the specific opioid(s) involved has not been determined. Here we report that dynorphin administered intrathecally causes dose-related, partially reversible, hindlimb paralysis in the unanesthetized rat. Other opioids, including mu-selective and delta-selective synthetic enkephalins or beta-endorphin, failed to induce motor dysfunction. These findings suggest that dynorphin or dynorphin-related peptides may play a role in spinal cord injury.

Deteriorating stroke model: Histopathology, edema, and eicosanoid changes following spinal cord ischemia in rabbits

Secondary motor dysfunction is often observed following ischemic episodes in the central nervous system. To study potential mechanisms of postischemic motor deterioration, we developed a rabbit spinal cord ischemia model that has characteristics similar to the clinical condition termed deteriorating stroke. In this model, 70% of the rabbits regained substantial motor function by 4 hours after complete hindlimb paralysis during lumbar spinal cord ischemia; however, over the next 20 hours motor function steadily declined to the point where only 30% of the rabbits had minimal hopping function. The role of eicosanoids in spinal cord ischemia was studied by radioimmunoassay of several prostaglandins (6-keto-PGF1 alpha, PGE2, and TxB2) in the spinal cord. After 5 minutes of reperfusion, TxB2 levels were markedly elevated (p less than 0.05) while 6-keto-PGF1 alpha levels did not change. The TxB2:6-keto-PGF1 alpha ratio was also significantly increased. After 30 minutes of reperfusion, PGE2 levels were also elevated (p less than 0.05). Tissue edema measured by microgravimetry was also increased after 30 minutes of reperfusion in both gray and white matter. By 4 hours of reperfusion, rabbits regained near-normal hindlimb motor function while PGE2, 6-keto-PGF1 alpha, TxB2, and tissue water content were back to normal. However, by 18 hours of reperfusion, when hindlimb function was deteriorating, TxB2 levels were elevated again, and edema in gray and white matter was increased as was the number of necrotic neurons observed by light microscopy. These results suggest that the secondary deterioration of motor neurologic function was due to the excess formation of TxA2 primarily in the late reperfusion phase. However, further studies are necessary to elucidate the relation of TxA2 with ischemic neural injury.

Increased dynorphin immunoreactivity in spinal cord after traumatic injury.

Opiate antagonists, at high doses, have been shown to improve physiological variables and outcome after experimental spinal injury. Dynorphin appears to be unique amongst opioids in producing hindlimb paralysis after intrathecal injection. Taken together, these findings suggest a possible pathophysiological role for endogenous opioids, particularly dynorphin, in spinal injury. In the present studies we examined the relationship between changes in dynorphin immunoreactivity (Dyn-ir) in rat spinal cord after traumatic injury and the subsequent motor dysfunction. Trauma was associated with significantly increased Dyn-ir at the injury site, but not distant from the lesion. Dyn-ir was found elevated as early as 2 h and as late as 2 weeks after trauma, and was significantly correlated with the degree of injury. These data are consistent with the hypothesis that dynorphin systems may be involved in the secondary injury that follows spinal trauma.

Neuropeptides in spinal cord injury: Comparative experimental models

The possible role of endogenous opioids in the pathophysiology of spinal cord injury was evaluated utilizing a variety of experimental models and species. In the cat, we have shown that beta-endorphin-like immunoreactivity was increased in plasma following traumatic spinal injury; such injury was associated with a decrease in spinal cord blood flow (SCBF) which was reversed by the opiate receptor antagonist naloxone. Naloxone treatment also significantly improved functional neurological recovery after severe injury. Thyrotropin-releasing hormone (TRH), possibly through its anti-endorphin actions, was even more effective than naloxone in improving functional recovery in the cat. In a rat model, utilizing a similar trauma method, TRH proved superior to naloxone in improving SCBF after injury. In addition, naloxone at high doses attenuated the hindlimb paralysis produced by temporary aortic occlusion in the rabbit. The high doses of naloxone required to improve neurological function after spinal injury suggest that naloxones actions, if opiate receptor mediated, may be mediated by non-mu receptors. Dynorphin, an endogenous opioid with a high affinity for the kappa receptor, produced hindlimb paralysis following intrathecal administration in rats. Taken together, these findings suggest that endogenous opioids, possibly acting at kappa receptors in the spinal cord, may serve as pathophysiological factors in spinal cord injury.

Effects of traumatic injury on dynorphin immunoreactivity in spinal cord

Traumatic spinal cord injury in rats resulted in a significant eleation of dynorphin A immunoreactivity in spinal cord tissue at the level of, and below, the site of injury. [Leu5]enkephalin levels in the same tissue samples were not significantly altered following severe injury. Dynorphin A immunoreactivity was found in the fraction relatively enriched in synaptosomes after subcellular fractionation of spinal cord tissue. The dynorphin A content of this fraction was not significantly changed following injury, suggesting that dynorphin containing nerve terminals and axons are not severely damaged as a result of the injury.

Changes in substance P and somatostatin in the spinal cord after traumatic spinal injury in the rat

Immunoreactive substance P (SPI) and somatostatin (SOMI) are found in spinal cord but their physiological roles remain speculative. Several classes of neuropeptides, including endogenous opioids and thyrotropin-releasing hormone (TRH), have been implicated in the pathogenesis or recovery from spinal cord injury. In the present studies, changes in SPI and SOMI were examined in the spinal cord after traumatic injury in the rat. Both peptides showed time-dependent, localized decreases at the injury site, which were statistically related to the degree of post-traumatic neurological dysfunction. Such changes differ from those of a number of other peptides after spinal injury and suggest that substance P and/or somatostatin may play a role in the secondary pathophysiological responses which follow trauma to the spinal cord.

Comparison of naloxone and a δ-selective antagonist in experimental spinal stroke

A highly predictive spinal stroke model in the unanesthetized rabbit was utilized to compare the effects of naloxone and the delta-selective opiate antagonist M154,129 on neurological recovery following ischemic injury to the central nervous system. Naloxone treatment protected against both moderate (20 min aortic occlusion) and severe (25 min aortic occlusion) degrees of ischemic spinal injury, whereas treatment with M154,129 failed to improve recovery in either model. These findings confirm that naloxone therapy can alter the pathophysiological sequelae caused by a critical reduction in blood flow to the central nervous system and suggests that its beneficial effects do not relate to actions at the delta-opiate receptor.

Motor dysfunction after spinal cord injury is mediated by opiate receptors

Naloxone, an opiate receptor antagonist, improves neurological outcome following traumatic cervical spinal cord injury in the cat and ischemic lumbar spinal cord injury in the rabbit. However, the doses of naloxone required have been quite high (greater than 1 mg/kg), suggesting that its beneficial actions are either not opiate receptor mediated or mediated by non-mu opiate receptors (which are less naloxone-sensitive). The kappa receptor appears to be the predominant opiate receptor in the spinal cord in a variety of species. For these reasons we evaluated the effects of a somewhat kappa selective opiate receptor antagonist WIN44,441-3 [WIN(-)] on neurological recovery following traumatic spinal injury in the cat and ischemic spinal injury in the rabbit. Animals treated with this more selective antagonist showed improved motor recovery as compared with animals treated with either physiological saline or with the dextroisomer of the WIN compound [WIN44,441-2; WIN(+)]. These findings support the hypothesis that motor dysfunction after spinal cord injury is in part mediated by opiate receptors and indicate that kappa selective opiate receptor antagonists may have particular therapeutic utility in spinal cord injury.

Dopamine partially mediates the cardiovascular effects of naloxone after spinal injury

Following spinal injury, the opiate antagonist naloxone selectively elevates plasma dopamine levels, with the dopamine changes significantly correlated with improved cardiovascular function. Moreover, the cardiovascular effects of naloxone are significantly attenuated by pretreatment with the dopamine antagonist domperidone. From these data, it is concluded that the cardiovascular effects of naloxone after spinal injury are in part dopamine mediated.