Apelin-13 enhances contextual fear extinction in rats

Neuroprotective Roles of Apelin-13 in Neurological Diseases

Apelin is a natural ligand for the G protein-coupled receptor APJ, and the apelin/APJ system is widely distributed in vivo. Among the apelin family, apelin-13 is the major apelin isoform in the central nervous system and cardiovascular system, and is involved in the regulation of various physiopathological mechanisms such as apoptosis, neuroinflammation, angiogenesis, and oxidative stress. Apelin is currently being extensively studied in the nervous system, and apelin-13 has been shown to be associated with the onset and progression of a variety of neurological disorders, including stroke, neurodegenerative diseases, epilepsy, spinal cord injury (SCI), and psychiatric diseases. This study summarizes the pathophysiological roles of apelin-13 in the development and progression of neurological related diseases.

Apelin-13 reduces lipopolysaccharide-induced neuroinflammation and cognitive impairment via promoting glucocorticoid receptor expression and nuclear translocation (Manuscript-revision)

Neuroinflammation is usually associated with cognitive decline, which is involved in neurodegenerative diseases. Apelin, a neuropeptide, exerts various biological roles in central nervous system. Recent evidence showed that apelin-13, an active form of apelin, suppresses neuroinflammation and improves cognitive decline in diverse pathological processes. However, the underlying mechanism of apelin-13 in neuroinflammation remains largely unknown. The present study aimed to determine underlying mechanism of apelin-13 on neuroinflammation-related cognitive decline. The lipopolysaccharide (LPS) intracerebroventricular (i.c.v.) to is used to establish a rat model of neuroinflammation-related cognitive decline. The results showed that apelin-13 inhibits LPS-induced neuroinflammation and improves cognitive impairment. Apelin-13 upregulates the GR level and nuclear translocation in hippocampus of rats. Moreover, glucocorticoid receptor inhibitor RU486 prevents apelin-13-mediated neuroprotective actions on cognitive function. Taken together, apelin-13 could exert a protective effect in neuroinflammation-mediated cognitive impairment via the activation of GR expression and nuclear translocation.

Intravenous Administration of Pyroglutamyl Apelin-13 Alleviates Murine Inflammatory Pain via the Kappa Opioid Receptor

Apelin is an endogenous neuropeptide, which has wide distribution in central nervous system and peripheral tissues. Pyroglutamyl apelin-13 [(pyr)apelin-13] is the major apelin isoform in human plasma. However, the role of peripheral (pyr)apelin-13 in pain regulation is unknown. The aim of this study was to investigate the effect of the peripheral injection of (pyr)apelin-13 on inflammatory pain using the formalin test as well as to evaluate the mechanistic basis for the effect. Results showed intravenous (i.v.) injection of (pyr)apelin-13 (10, 20 mg/kg) to significantly decrease licking/biting time during the second phase of the mouse formalin test. In contrast, i.v. injection of apelin-13 had no influence on such effect. Intramuscular injection of (pyr)apelin-13 reduced licking/biting time during the second phase only at a dose of 20 mg/kg. The antinociception of i.v. (pyr)apelin-13 was antagonized by the apelin receptor (APJ, angiotensin II receptor-like 1) antagonist, apelin-13(F13A). (pyr)apelin-13 (i.v. 20 mg/kg) markedly upregulated Aplnr and Adcy2 gene expression in the prefrontal cortex, whereas Fos gene expression was downregulated. The antinociception of i.v. (pyr)apelin-13 was blocked by the opioid receptor antagonist naloxone and the specific kappa opioid receptor (KOR) antagonist nor-binaltorphimine (nor-BNI). (pyr)Apelin-13 upregulated the dynorphin and KOR gene expression and protein levels in the mouse prefrontal cortex, not in striatum. (pyr)Apelin-13 did not influence the motor behavior. Our results demonstrate that i.v. injection of (pyr)apelin-13 induces antinociception via the KOR in the inflammatory pain mouse model.