Physical dependence produced by central morphine infusions: an anatomical mapping study

Morphine sulfate (1.5 nmoles/h) was chronically infused into various brain regions in rats. After 72 h of continuous infusions, an intraperitoneal injection of naloxone hydrochloride (5 mg/kg) was given to determine if physical dependence had developed. Various withdrawal signs were present following the naloxone challenge in rats chronically infused with morphine into the periventricular gray region. These withdrawal signs included escape behavior, wet-dog shakes, and teeth chattering. Several other brain regions (e.g., amygdala, caudate nucleus, lateral hypothalamus, nucleus accumbens, thalamus) were also tested, but morphine infusions into these areas produced only slight physical dependence. The magnitude of physical dependence produced by morphine infusions into the rostral aspect of the periventricular gray was comparable to that seen following repeated systemic morphine injections, while the physical dependence produced by caudal infusions into the vicinity of the locus coeruleus was considerably less. These data confirm the importance of the periventricular gray region in the development of physical dependence on morphine and reveal that a drug action in the more rostral aspect of this brain region produces the strongest signs of physical dependence.