Dadle

Amyloid β1–42 peptide were purchased from Sigma-Aldrich (Cat: SCP0038, respectively, Bay St. Louis, MS, United States). UFP-512, a highly selective DOR agonist, was synthesized by our research partner (Aguila et al., 2007 (link)). Naltrindole hydrochloride, a DOR antagonist; DADLE, a DOR unspecific agonist; and DAMGO, a MOR agonist were all purchased from Tocris Bioscience (Cat: 0740, 3790, 1171, Bristol, United Kingdom). Cell Counting Kit-8 was from Beyotime, Co. (Cat: C0039, Shanghai, China). BACE1 Rabbit polyclonal antibody were purchased from Proteintech (Cat:12807-1-AP, Rosemont, IL, United States). Anti-β-actin antibody and APP rabbit antibody were purchased from Cell Signaling Technology (Cat: 4272, 76600S, Danvers, CO, United States). BACE1 Activity Kit were purchased from BioVision (Cat: K388-100, Milpitas, CA, United States). Human/Rat β Amyloid(42) ELISA Kit were obtained from Wako (Cat: 292-64501, Osaka, Japan).

After 5 days the animals were individually placed into PhenoMaster cages in the same environmental conditions as the standard home cages. Animals were recorded every 40 min for 2 h for motor activity by the number of squares crossed, the amount of food eaten and water drunk, as well as oxygen consumption and carbon dioxide production. Thirty minutes before the start of experiments, test solutions were injected directly into the stomach with a special metallic probe in a volume of 0.1 ml per 100 g of body weight. A peptide agonist of mu opioid receptors DAMGO [(D-Ala², N-MePhe⁴, Gly-ol)-enkephalin] and a peptide agonist of delta opioid receptors DADLE [(D-Ala², D-Leu⁵)-enkephalin] were used. Peptides rapidly degrade in the gastrointestinal tract and, when introduced directly into the stomach, have no systemic or central effect. The first (control) group of animals (n = 10) was administered vehicle alone (water); the second group (n = 10) was administered 200 mg/kg DAMGO (Tocris Bioscience, Bristol, UK); and the third group (n = 10) was administered 200 mg/kg DADLE (Tocris Bioscience). The dose of the administered substances was selected on the basis of our previous experiments (Trigub et al., 2014 (link); Bogdanova et al., 2015 (link)).