BEIJING, April 26 (Xinhuanet) -- Changes in a small section of the midbrain involved in the reward system reveal a single dose of morphine was found to lower the inhibitions of rats, even after the drug had left their systems.

    U.S. researchers said on Wednesday the finding may help scientists better understand addiction in humans.

    Morphine blocked the brain's ability to strengthen connections, or synapses, in rats that ratchet down reward or pleasure, researchers from Brown University reported in the journal Nature.

    "What we have found is that the inhibitory synapses can no longer be strengthened 24 hours after treatment with morphine, which suggests that a natural brake has been removed," said Julie Kauer, a professor of molecular pharmacology, physiology and biotechnology at Brown.

    "This happens 24 hours after the animal had one dose of morphine. There is no morphine left in the brain. It shows that it is a persistent effect of the drug," she said in a telephone interview.

    Kauer said the finding adds to growing evidence suggesting a link between learning and addiction and may help in the development of drugs to treat addiction.

    "Strengthening synapses, we think, is the beginning of the formation of memory," she said.

    By shutting off the natural ability to strengthen connections that inhibit pleasure, the brain may be learning to crave drugs, she said.

    Kauer said the brain has two kinds of neurons -- those that excite the nerve connections and those that inhibit or depress them.

    "If inhibition is reduced, you get runaway excitability," she said.

    (Agencies)