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The analgesic effect of morphine in the tail immersion test was studied in rats three and ten days after intracerebroventricular 5,7-dihydroxytryptamine (5,7-DHT) given to selectively destroy serotonergic neurons. Morphine analgesia was reduced three but not ten days after the neurotoxin. Ten days after 5,7-DHT, the inhibiting effect of metergoline, a serotonin antagonist, on morphine analgesia was still present, suggesting that functional recovery of the serotonergic system may partly explain the different results.
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