Abstract
The effects of cutaneous noxious heat stimuli and intrathecal administration of morphine on the oxidation current of 5-hydroxytryptamine (serotonin: 5-HT) in the nucleus raphe magnus (NRM) were examined in anesthetized rats. An oxidation current of 5-HT was seen at 320–340 mV using differential normal pulse voltammetry with nafion-coated carbon fiber electrodes. The signal was decreased by 28.5 ± 5.7 and by 12.7 ± 4.1% after cutaneous noxious heat stimuli of 52 and 45°C, respectively. These decreases lasted for 5–10 min. Non-noxious stimuli (37°C) did not affect the 5-HT signal. Intrathecal administration of morphine (2.5, 5.0, 10.0 and 15.0 μg) in the absence of cutaneous stimulation did not change the signal significantly. However, low doses of morphine (2.5 or 5.0 μg, i.t.) potentiated the decrease in the 5-HT signal induced by noxious stimuli, and high doses (10.0 or 15.0 μg, i.t.) attenuated it. Both effects of morphine at low and high doses were antagonized by naloxone (0.5 mg/kg, i.p.). These results indicate that the intrathecal administration of morphine affects the cutaneous noxious heat stimulus-induced decrease of serotonin release in the NRM.
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