Melatonin attenuates the intensity of beta-endorphin immunoreactivity in the arcuate nucleus of rat hypothalamus.

The present study was undertaken to observe the effect of exogenously administered melatonin on the intensity of beta-endorphin (beta-Ep) immunoreactivity of the neuron in the arcuate nucleus of rat hypothalamus with an aim to explore the possible mechanisms of the analgesic effect of melatonin. The experimental rats were divided into two groups, one injected intraperitoneally with melatonin (110 mg/kg) and the other with only a vehicle. One hour after injection, the brain was processed for coronal sections, which were stained with immunohistochemical ABC technique. The integral optical density (IOD) and mean optical density (OD) of the stained sections were measured with a computer-assisted image-processing and analytical system. beta-Ep immunoreactivity was much decreased in the sections treated with melatonin and the IOD and OD were also decreased significantly (P<0.01; P<0.05). The above results suggest that melatonin may result in a decrease of beta-Ep content in the arcuate nucleus, as a result of increased beta-Ep release induced by administration of melatonin. It is likely that the analgesic effect of melatonin may be in part mediated by the release of beta-endorphin from the arcuate nucleus.