Changes in limb tone produced by regional injections of opiates into rat brain.

A mechanical apparatus was used to measure the resistance of the hindlegs to flexion in conscious rats. Systemically-administered morphine (7.5-12.5 mg/kg) caused increased muscle tone in the limbs. In rats with intracerebral cannulae, intrastriatal morphine (5-15 micrograms) had no effect on limb tone, whereas in increase in limb tone was induced by morphine (1-30 micrograms) in the globus pallidus. A strong and sustained increase in limb tone developed when morphine (0.2-2.5 micrograms) and the mu opiate receptor agonist DAGO (1.5-2.5 micrograms) were administered into the periaqueductal gray region of the midbrain, whereas the delta opiate receptor agonist DADLE (2.5-8 micrograms) was ineffective. The increased limb tone induced by opiates may be generated by mu-type receptors in at least two areas of brain.