ABSTRACT
Purpose To observe the changes of dynorphin-like immunoreactivities of neurons in some rat brain nuclei that are related to analgesia following exogenous administration of melatonin. Methods The experimental rats were divided into two groups, injected intraperitoneally with melatonin 110 mg/kg and with vehicle, respectively. One hour after the injection, the rat brain was processed for coronal sections. The sections were stained with immunohistochemical ABC technique. The integral optical density (IOD) of the stained section was measured by the computer-assisted image processing technique. Results Dynorphin-like immunoreactivities in the supraoptic nucleus and nucleus raphe dorsalis showed obvious reduction following the single injection of melatonin.IOD values in the above nuclei were decreased significantly (P<0.01) with the melatonin treatment. In the paraventricular nucleus of hypothalamus, periaqueductal gray and nucleus raphe magnus, there was no difference (P>0.05) about the IOD values between melatonin-treated group and vehicle-treated group. Conclusions Melatonin may result in the decrease of dynorphin content in the supraoptic nucleus and nucleus raphe dorsalis.