Effects of morphine on the electrochemical signal modified by noxious stimuli in the nucleus raphe magnus of anesthetized rats.

The effects of noxious stimuli and morphine on the serotonergic system in the nucleus raphe magnus were examined by in vivo voltammetry studies, using anesthetized rats. The normal electrochemical signal of 280-300 mV was essentially due to the presence of 5-hydroxyindoles in the nucleus raphe magnus. Heating or pinching produced mean decreases of 21.9 +/- 5.2% and 18.0 +/- 6.1% of control, respectively in the 5-hydroxyindole signal. Non-noxious (brushing or warm water) stimulation did not affect the 5-hydroxyindole signal. A small dose of morphine (0.5 mg/kg, i.p.) enhanced the inhibition of the signal by noxious stimuli but large doses (2.0 or 5.0 mg/kg, i.p.) resulted in lesser reductions of the signal. The value of the 5-hydroxyindole signal was unaffected by morphine alone (0.5, 2.0 and 5.0 mg/kg). Effects of both small- and large-doses of morphine were antagonized by naloxone (1 mg/kg, i.v.). Allopurinol (20 mg/kg, i.p.), a xanthine oxidase inhibitor, decreased the steady signal (40.7 +/- 16.2%). After pretreatment with allopurinol, noxious stimuli-induced decreases, both with and without administration of morphine, were similar to those in nontreated rats. In brief, noxious stimulation was found to decrease 5-hydroxyindole signal in the nucleus raphe magnus; morphine enhanced or attenuated this decrease in the anesthesized rat.