Exogenous opioids influence the microcirculation of injured peripheral nerves

Opioids and opioid receptors orchestrate wound repair

We have previously shown that topical opioids including morphine and its congeners promote healing of full thickness ischemic wounds in rats. We examined the contribution of mu opioid receptor (MOPr)-mediated healing of full thickness ischemic wounds using MOPr and delta or kappa opioid receptor knockout (KO) mice. Wound closure in the early (day 5) as well as later phases was delayed in topical morphine or PBS-treated MOPr-KO mice compared with reciprocal treatments of wounds in wild-type (WT) mice. MOPr expression was significantly upregulated at 30 min in the wound margins and colocalized with wound margins and vasculature in the epidermal and dermal layers of the skin. We next examined whether neuropeptide expression was involved in the mechanism of MOPr-mediated wound closure. Substance P (SP) and calcitonin gene-related peptide immunoreactivity (ir) was significantly increased in the skin of MOPr-KO mice as compared with WT mice. Neuropeptide-ir was increased significantly in PBS-treated wounds of MOPr and WT mice, but morphine treatment reduced neuropeptide immunoreactivity in both as compared with PBS. Wounding of keratinocytes led to the release of opioid peptide beta-endorphin (β-END) in conditioned medium, which stimulated the proliferation of endothelial cells. MOPr-selective (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2, CTOP) and nonselective OPr antagonist naloxone-inhibited endothelial proliferation induced by wounded keratinocyte-conditioned medium. In addition, accelerated wound area closure in vitro by morphine was suppressed by methylnaltrexone, a nonselective OPr antagonist with high affinity for MOPr. Morphine and its congeners stimulated the proliferation of endothelial cells from WT mice but not those from MOPr-KO mice. Furthermore, morphine-induced mitogen-activated protein kinase/extracellular signal-regulated kinase phosphorylation in endothelial cells was significantly decreased in MOPr-KO mice as compared with WT mice. Collectively, these data suggest that MOPr plays a critical role in the proliferation phase with the formation of granulation tissue during wound healing.

Vascular mechanism of axonal degeneration in peripheral nerves in hemiplegic sides after cerebral hemorrhage: An experimental study

Background

Though retrograde neuronal death and vascular insufficiency have been well established in plegics following intracerebral hemorrhage, the effects of plegia on arterial nervorums of peripheral nerves have not been reported. In this study, the histopathological effects of the intracerebral hemorrhage on the dorsal root ganglions and sciatic nerves via affecting the arterial nervorums were investigated.

Methods

This study was conducted on 13 male hybrid rabbits. Three animals were taken as control group and did not undergo surgery. The remaining 10 subjects were anesthetized and were injected with 0.50 ml of autologous blood into their right sensory-motor region. All rabbits were followed-up for two months and then sacrificed. Endothelial cell numbers and volume values were estimated a three dimensionally created standardized arterial nervorums model of lumbar 3. Neuron numbers of dorsal root ganglions, and axon numbers in the lumbar 3 nerve root and volume values of arterial nervorums were examined histopathologically. The results were analyzed by using a Mann-Whitney-U test.

Results

Left hemiplegia developed in 8 animals. On the hemiplegic side, degenerative vascular changes and volume reduction in the arterial nervorums of the sciatic nerves, neuronal injury in the dorsal root ganglions, and axonal injury in the lumbar 3 were detected. Statistical analyses showed a significant correlation between the normal or nonplegic sides and plegic sides in terms of the neurodegeneration in the dorsal root ganglions (p < 0.005), axonal degeneration in the lumbar 3 nerve roots (p < 0.005), endothelial cell degeneration in the arterial nervorums (p < 0.001), and volume reduction in the arterial nervorums (p < 0.001).

Conclusion

Intracerebral hemorrhage resulted in neurodegeneration in the dorsal root ganglion and axonolysis in the sciatic nerves, endothelial injury, and volume reduction of the arterial nervorums in the sciatic nerves. The interruption of the neural network connection in the walls of the arterial nervorums in the sciatic nerves may be responsible for circulation disorders of the arterial nervorums, and arterial nervorums degeneration could result in sciatic nerves injury.

Evidence that nitric oxide- and opioid-containing interneurons innervate vessels in the dorsal horn of the spinal cord of rats

In the dorsal horn of the spinal cord, activation of small fibre nociceptive afferents leads to the release of nitric oxide and enkephalins by interneurons. In this work we encountered unexpected relationships among local spinal cord dorsal horn blood flow, specific forms of afferent input, nitric oxide and intrinsic opioids. Selective rises in rat lumbar dorsal cord blood flow using laser Doppler flowmetry and microelectrode hydrogen clearance polarography were generated by ipsilateral, 'nociceptive' low (3 Hz) frequency stimulation of sciatic afferents. Inhibitors of nitric oxide synthase (NOS) prevented rises in flow during stimulation without influencing baseline flow. Ipsilateral hindpaw intradermal injection of capsaicin, a nociceptive activator, also generated large rises in flow sensitive to NOS inhibition. During NOS blockade or morphine administration there were unexpected acute declines in the dorsal cord blood flow strictly confined to low frequency stimulation epochs. This acute vasoconstrictive effect was prevented by administration of an opioid receptor antagonist. Using immunohistochemistry, terminals apparently innervating dorsal spinal cord blood vessels were labelled with antibodies against neuronal NOS and met-enkephalin. We conclude that local nitric oxide and opioids, probably from interneurons, have competitive actions on dorsal horn microvessels once interneurons are activated during a nociceptive barrage. Collateral innervation of blood vessels may explain this property.