Decreased [ 3H]-naloxone binding and elevated dynorphin-A( 1–8) content in Zucker rat brain

We have previously reported that female obese Zucker rats are hypersensitive to painful stimuli and are resistant to the analgesic effects of morphine. In continuation we hypothesized that these phenomena are possibly the result of diminished population of opioid receptors, or an overabundance of dynorphin interfering with morphine analgesia. We now report that female obese Zucker rats have decreased concentrations of mu opioid receptors in whole brain and elevated levels of Dynorphin A( 1–8) (DYN) in a brain area known to be associated with responses to nociceptive stimuli.