[D-Ala2, D-Leu5] Enkephalin Inhibits TLR4/NF-κB Signaling Pathway and Protects Rat Brains against Focal Ischemia-Reperfusion Injury

Abstract

Background. Cerebral ischemia-reperfusion (I/R) injury is the main cause of acute brain injury, which is a life-threatening disease due to the lack of effective treatments. [D-Ala2, D-Leu5] enkephalin (DADLE) is a synthetic delta-opioid receptor agonist that is reported to confer neuroprotective effect; however, the underlying mechanism is still being explored. The purpose of the present study is to determine whether DADLE administrated intracerebroventricularly could attenuate the cerebral I/R injury, to determine if this is through inhibiting the toll-like receptor 4 (TLR4)/nuclear factor kappa B (NF-κB) signaling pathway and therefore inhibiting neuroinflammation in an ischemic stroke model. Methods. Rats were subjected to 120 minutes of ischemia by transient middle cerebral artery occlusion (MCAO). At 45 minutes after ischemia, DADLE or control vehicle (artificial cerebrospinal fluid, ACSF) was given to the rats intracerebroventricularly. Neurological deficit, cerebral infarct volume, and histopathological changes were assessed at 24 hours after reperfusion. Brain inflammation was assessed by measuring tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) in the ischemic penumbra by ELISA. The expression of TLR4 was determined by immunohistochemistry staining and western blotting. The expression of NF-κB was investigated by western blotting. Results. Compared with the vehicle-treatment (ACSF), DADEL improved neurological deficit ( 9.6 ± 2.1 versus 13.8 ± 1.9 ), reduced cerebral infarct volume ( 18.74 ± 3.30 % versus 10.57 ± 2.50 % ), and increased the number of normal neurons ( 29.72 ± 8.53 % versus 51.37 ± 9.18 % ) after cerebral I/R injury in rats (all P < 0.05 ). Expressions of inflammatory molecules including TNF-α and IL-6 were highly expressed in the vehicle-treated rats, whereas treatment with DADLE downregulated these expressions ( P < 0.05 ). Additionally, cerebral I/R injury significantly increased the TLR4 and NF-κB expression in vehicle-control group, which was markedly inhibited by DADLE ( P < 0.05 ). Conclusions. DADLE, administrated intracerebroventricularly at 45 minutes after cerebral ischemia, significantly ameliorated I/R-induced brain damage in rats. This kind of neuroprotective effect appears to be related to the downregulation of TLR4-mediated inflammatory responses.

δ-opioid Receptor, Microglia and Neuroinflammation

“…An early study demonstrated that activating DOR using DOR agonist SNC-121 attenuated TNF-α induced matrix metalloproteinase-2 (MMP-2) secretion in the glia cells, and the effects of DOR were realized by negatively regulating the p38 MAPK and NF-κB signaling [ 125 ]. More recently, a group of researchers established a MCAO rat model, in which cerebral ischemia-reperfusion injury increased TLR4 and NF-κB expression in the rat’s brain, which was remarkably inhibited by DOR-specific agonist DADLE [ 126 ]. NF-κB was regarded as one of the crucial signaling targets in DOR-mediated long-term neuroprotection.…”

Loss of lncRNA UCA1 ameliorates the injury managed by cerebral ischemia-reperfusion by sponging miR-18a-5p

“…The model of transient focal MCAO rat was established in this experiment, which was based on a previous study [7]. 10% chloral hydrate 300 mg/kg was injected intraperitoneally for anaesthesia, and no response of the rat tail was taken as an indication of successful anaesthesia.…”

Promising Application of D-Amino Acids toward Clinical Therapy

“…At present, D-peptide is frequently used, and it is recognized as a potent and promising drug. Enkephalin (D-Ala2, D-Leu5) is effective to attenuate cerebral I/R injury [154]. D-Trp-6-LH-RH is used for the treatment of advanced prostatic cancer/advanced ovarian cancer, etc.…”