The effects of low doses of morphine on the activity of dopamine-containing cells and on behavior
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Updated: Jul 19, 2026

Assessment of Morphine-induced Hyperalgesia and Analgesic Tolerance in Mice Using Thermal and Mechanical Nociceptive Modalities
Published on: July 29, 2014
Abstract:
Intravenous (IV) administration of 10 ug/kg of morphine sulfate increased the firing rate of Type A dopamine neurons but decreased the extracellular activity of Type B dopamine cells in the substantia nigra pars compacta and ventral tegmental area of anesthetized male rats. The morphine-produced increase in Type A cells was antagonized by 0.10 mg/kg of naloxone, whereas the suppression of B cells was blocked only by pretreatment with 5.0 mg/kg of naloxone. This same low dose of morphine (IV) also produced increased locomotor activity in awake male rats within 10 min of administration.
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