Physical dependence on opiate-like peptides

Methionine-enkephalin and β-endorphin, endogenous peptides with activities similar to those of opiates, were infused for 70 hours into the periaqueductal gray-fourth ventricular spaces of the rat brain. When challenged with a naloxone, a specific opiate antagonist, these animals manifested a typical morphine-like withdrawal syndrome. These results show that such peptides can cause physical dependence.