Intraventricular administration of MSH induces hyperalgesia in rats

In a completely crossed, double blind designed study, six rats received intraventricular injections of 0.1, 1.0 and 10 micrograms of alpha-MSH and a placebo. The rats were tested for response to painful thermal stimuli with the tail-flick test. All of the doses of alpha-MSH produced hyperalgesia during the first 20 min of testing. Only the 1.0 microgram dose of alpha-MSH produced hyperalgesia throughout the 80 min course of the experiment. This study, coupled with previous reports that MSH/ACTH fragments may attenuate morphine-induced analgesia, suggest that MSH can have opposite actions from those of the endorphins. It is possible that alpha-MSH and related peptides may be endogenous anti-opiates.