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

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Abstract
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 μg) had no effect on limb tone, whereas an increase in limb tone was induced by morphine (1–30 μg) in the globus pallidus. A strong and sustained increase in limb tone developed when morphine (0.2–2.5 μg) and the mu opiate receptor agonist DAGO (1.5–2.5 μg) were administered into the periaqueductal gray region of the midbrain, whereas the delta opiate receptor agonist DADLE (2.5–8 μg) was ineffective. The increased limb tone induced by opiates may be generated by mu-type receptors in at least two areas of brain.