The response of the 5-hydroxyindole oxidation current to noxious stimuli in the spinal cord of anesthetized rats: modification by morphine.
The effects of cutaneous noxious heating and of systemic morphine on serotonergic activity in the spinal cord were examined in anesthetized rats. An oxidation current of 5-hydroxyindole signal was seen at 280-300 mV with differential normal pulse voltammetry. Noxious heat stimuli produced a mean signal increase over control values of 15.5 +/- 3.4% at 52 degrees C, and 7.2 +/- 5.5% at 45 degrees C. These increases lasted for 5-10 min. Non-noxious stimuli (37 degrees C) did not affect the 5-hydroxyindole signal. Morphine (0.5, 2.0 and 5.0 mg/kg, i.p.) in the absence of cutaneous stimulation did not change the signal significantly. Systemic morphine alone did not significantly modify the 5-hydroxytryptamine (5-HT) metabolism, as observed in in vivo voltammetry, in the spinal cord of anesthetized rat. However, a low dose of morphine (0.5 mg/kg, i.p.) attenuated the increase in the signal modified by noxious stimuli, and high doses (2.0 or 5.0 mg/kg, i.p.) enhanced it. Both effects of morphine were antagonized by naloxone (0.5 mg/kg, i.v.). It is likely that morphine with noxious stimuli modify the sensitivity of serotonergic descending inhibitory system. It is concluded that noxious heating of the skin increases the 5-HT metabolism in the spinal cord of anesthetized rats and that systemic administration of morphine modulates this 5-HT metabolism.
Taguchi
K
K
Department of Pharmacology, Showa College of Pharmaceutical Sciences, Tokyo, Japan.
Suzuki
Y
Y
D000704:Analysis of Variance; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004563:Electrochemistry / Q000379:methods; D006358:Hot Temperature; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D010084:Oxidation-Reduction; D010146:Pain / Q000503:physiopathology*; D051381:Rats; D011919:Rats, Inbred Strains; D012701:Serotonin / Q000378:metabolism*; D013116:Spinal Cord / Q000187:drug effects / Q000378:metabolism* / Q000503:physiopathology; D013997:Time Factors
D012701:Serotonin; D009270:Naloxone; D009020:Morphine