[Effect of melatonin on the content of β-endorphin in the hypothalamic arcuate nucleus and periaqueductal grey of midbrain in morphine withdrawal mice].

The present study was undertaken to investigate the effect of melatonin on the content of β-endorphin (β-EP) in the hypothalamic arcuate nucleus (Arc) and periaqueductal grey (PAG) of midbrain in morphine withdrawal mice. Male Kunming mice were injected subcutaneously (s.c.) with an increasing dose of morphine continuously for 8 d to establish morphine dependence model. Withdrawal response was induced by naloxone (3 mg/kg body weight, s.c.). The potency of withdrawal response was evaluated according to the jumping times and body weight loss. Ninty minutes prior to the precipitation of naloxone, 80 mg/kg body weight of melatonin (MEL) was injected intraperitoneally (i.p.) to observe its antagonistic effect on the withdrawal response in morphine-dependent mice. After behavioral observation, radioimmunoassay was used to determine the content of β-EP in the PAG of midbrain, and immunohistochemical assay was used to observe the intensity of β-EP-like immunoreactivity in the Arc in mice. It was shown that MEL inhibited the naloxone-precipitated withdrawal responses in mice significantly (P<0.05). In the meantime, MEL increased the content of β-EP in the PAG of midbrain significantly (P<0.05) and attenuated the intensity of β-EP-like immunoreactivity in the Arc in mice (P<0.05). The results suggest that MEL increases the content of β-EP in the PAG of midbrain, decrease the content of β-EP in the Arc in morphine withdrawal mice.