Central sites of naloxone-precipitated shaking in the anesthetized, morphine-dependent rat

Naloxone hydrochloride, an opiate antagonist, administered via the intracranial or parenteral route precipitates shaking behavior in the morphine-dependent rat. We made localized bilateral injections of naloxone HCl, 1.5 mug/rat, into 60 subcortical sites of the pentobarbital-anesthetized, morphine-dependent rat and found that two circumscribed areas of the brain, the medial hypothalamus and the periaqueductal-4th ventricular spaces, were selectively sensitive to naloxone-precipitated shaking. In the nondependent rat, morphine injections into the anterior diencephalon inhibited the shaking response to ice water; injections of morphine into the medial diencephalon were less effective. However, naloxone antagonized the morphine-inhibited shaking more effectively when injections of naloxone were made in the medial diencephalon than when injections were made in the anterior diencephalon. These results suggest that the reciprocal relationship of morphine and morphine-naloxone effects on shaking behavior may be regulated by topographically different structures in the diencephalon.