Endogenous opioids upregulate brain-derived neurotrophic factor mRNA through delta- and micro-opioid receptors independent of antidepressant-like effects

Abstract

Systemic administration of delta-opioid receptor (DOR) agonists decreases immobility in the forced swim test (FST) and increases brain-derived neurotrophic factor (BDNF) mRNA expression in rats, indicating that DOR agonists may have antidepressant-like effects. The aim of this study was to investigate the effects of central administration of endogenous opioid peptides on behavior in the FST and on brain BDNF mRNA expression in rats. Effects of endogenous opioids were compared with those produced by intracerebroventricular administration of a selective non-peptidic DOR agonist (+)BW373U86. Antidepressant-like effects were measured by decreased immobility in the FST. BDNF mRNA expression was determined by in situ hybridization. Centrally administered (+)BW373U86 decreased immobility and increased BDNF mRNA expression in the frontal cortex through a DOR-mediated mechanism, because these effects were blocked by the DOR antagonist naltrindole, but not by the micro-opioid receptor (MOR) antagonist naltrexone (NTX) or the kappa-opioid receptor antagonist nor-binaltorphimine. Of all the endogenous opioids tested, only leu- and met-enkephalin produced behavioral effects like those of (+)BW373U86 in the FST. Unlike (+)BW373U86, the enkephalins upregulated BDNF mRNA expression in the hippocampus through DOR- and MOR-mediated mechanisms. beta-Endorphin, endomorphin-1 and endomorphin-2 significantly increased BDNF mRNA levels in the frontal cortex, hippocampus and amygdala without reducing immobility; and most of these effects were reversed by NTX. This study is the first to provide evidence that endogenous opioids can upregulate BDNF mRNA expression through the DOR and MOR, and that leu- and met-enkephalin have similar pharmacological profiles to synthetic DOR agonists in producing antidepressant-like effects.

Fig. 1

Effects of i.c.v. administration of (+)BW373U86 (nmol) on behavior in the FST in rats. Each value represents the mean ± SEM (n = 6). (+)BW373U86 was administered 30 min before the FST. *P < 0.05; **P < 0.01 compared with vehicle.

Fig. 2

Effects of i.c.v. administration of endogenous opioid peptides (nmol) on behavior in the FST in rats. Each value represents the mean ± SEM (n = 6). Opioid peptide was administered 30 min before the FST. *P < 0.05; **P < 0.01 compared with vehicle.

Fig. 3

Effects of opioid receptor antagonists on the behavioral activity of i.c.v. administration of vehicle (sterile water, 10 μL), (+)BW373U86 (100 nmol), β-endorphin (3 nmol), leu-enkephalin (100 nmol) and met-enkephalin (100 nmol) in the FST in rats (n = 6). Sterile water (Veh 1 mL/kg, s.c.), naltrindole (NTI 1 mg/kg, s.c.) or naltrexone (NTX 0.1 mg/kg, s.c.) was administered 15 min before agonist injection. Nor-binaltorphimine (Nor-BNI 10 mg/kg, s.c.) was administrated 24 h before agonist injection. *P < 0.05; **P < 0.01 compared with the vehicle-pretreated group.

Fig. 4

Representative autoradiograms of BDNF mRNA expression in the rat frontal cortex (a and b). BDNF mRNA expression was determined 3 h after i.c.v. administration of (+)BW373U86. Radiolabeled BDNF cRNA probes were used for in situ hybridization. (A) Section of frontal cortex taken from a vehicle-treated animal. (B) Section of frontal cortex taken from an animal treated with (+)BW373U86 (100 nmol, i.c.v.). Quantification of BDNF mRNA signals in different brain regions including frontal cortex (F.Ctx), CA1, CA3 and dentate gyrus (DG) of hippocampus, and amygdala (Amyg) are represented as mean percent of vehicle ± SEM (n = 4–6). **P < 0.01 compared with vehicle.

Fig. 5

Representative autoradiograms of BDNF mRNA expression in the rat hippocampus (a–c). BDNF mRNA expression was determined 3 h after i.c.v. administration of enkephalins. (a) Section of hippocampus taken from a vehicle-treated animal. (b) Hippocampus of an animal treated with leu-enkephalin (100 nmol, i.c.v.). (c) Hippocampus section taken from an animal treated with met-enkephalin (100 nmol, i.c.v.). Quantification of BDNF mRNA signals in brain regions are represented as mean percent of vehicle ± SEM (n = 4–6). *P < 0.05; **P < 0.01 compared with vehicle.

Fig. 6

Effects of opioid receptor antagonists on i.c.v. administered (+)BW373U86 (100 nmol), leu-enkephalin (100 nmol) and met-enkephalin (100 nmol) induced increases in BDNF mRNA expression in rats. Each value represents the mean percent of (veh + veh) ± SEM (n = 4–6). Levels of BDNF mRNA expression were determined 3 h after agonist administration. Subcutaneous administration of sterile water (Veh, 1 mL/kg), naltrindole (NTI, 1 mg/kg) or naltrexone (NTX, 0.1 mg/kg) was conducted 15 min before agonist administration. *P < 0.05 and **P < 0.01 compared with the veh + veh group; #P < 0.05 and ##P < 0.01 compared with the veh + agonist group.

Fig. 7

Representative autoradiograms of BDNF mRNA expression in the rat brain regions (a–h). Levels of BDNF mRNA were determined 3 h after agonist injection. (a and b) Sections taken from a vehicle-treated animal. (c and d) Sections taken from an animal treated with β-endorphin (3 nmol, i.c.v.). (e and f) Sections taken from an animal treated with endomorphin-1 (90 nmol, i.c.v.). (g and h) Sections taken from an animal treated with endomorphin-2 (90 nmol, i.c.v.). Quantification of BDNF mRNA signals in brain regions are represented as mean percent of vehicle ± SEM (n = 4–6). *P < 0.05; **P < 0.01 compared with vehicle.

Fig. 8

Effects of opioid receptor antagonists on i.c.v. administered β-endorphin (3 nmol), endomorphin-1 (EM-1, 90 nmol) and endomorphin-2 (EM-2, 90 nmol) induced increases in BDNF mRNA expression in rats. Each value represents the mean percent of (veh + veh) ± SEM (n = 4–6). Levels of BDNF mRNA expression were determined 3 h after agonist administration. Subcutaneous administration of sterile water (Veh, 1 mL/kg), naltrindole (NTI, 1 mg/kg) or naltrexone (NTX, 0.1 mg/kg) was conducted 15 min before agonist administration. *P < 0.05 and **P < 0.01 compared with the veh + veh group; #P < 0.05 and ##P < 0.01 compared with the veh + agonist group.