The Journal of pharmacology and experimental therapeutics
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J. Pharmacol. Exp. Ther. · Oct 1989
Pharmacological characterization of alpha adrenoceptors involved in the antinociceptive and cardiovascular effects of intrathecally administered clonidine.
The effects on nociception, blood pressure and heart rate of clonidine administered intrathecally to the lumbar level were determined in conscious rats and in rats anesthetized lightly with pentobarbital. In anesthetized rats, intrathecal (i.t.) clonidine (3.2-32.0 micrograms) inhibited the nociceptive tail-flick reflex and had biphasic effects on blood pressure; lesser doses (1.0-10.0 micrograms) produced depressor effects, whereas a greater dose (32.0 micrograms) produced a marked pressor response. Clonidine also produced biphasic effects on blood pressure in conscious rats, with the dose-response function shifted upward and to the left of that observed in anesthetized rats. ⋯ After i.t. injection of 32.0 micrograms of [3H]clonidine, peak levels of radioactivity in the blood were observed at 2 min and corresponded to a blood concentration of 38.8 ng/ml. Injection of an i.v. bolus dose (2.5 micrograms/kg) sufficient to produce these blood levels resulted in a transient pressor response. These results suggest that after i.t. administration of greater doses of clonidine, sufficient amounts of the drug are rapidly redistributed systemically to produce pressor effects by stimulation of vascular alpha adrenoceptors.
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J. Pharmacol. Exp. Ther. · Oct 1989
Kappa-opioid receptor-mediated antinociception in the rat. II. Supraspinal in addition to spinal sites of action.
This study examines whether there is a supraspinal, in addition to spinal, component to the antinociceptive actions against heat and pressure stimuli of kappa-opioid receptor agonists (U-69,593, U50,488H, bremazocine and tifluadom) as compared to mu-opioid receptor agonists (Tyr-D-Ala-Gly-NMe-Gly-ol, fentanyl and morphine) in the rat. The antinociception induced by kappa- and mu-opioids (applied s.c.) was unaffected by systemic quaternary naltrexone (50 mg/kg) revealing that it is mediated in the central nervous system. All kappa- and mu-opioids produced dose-dependent antinociception upon intrathecal application, in each case reversible by naloxone (5 mg/kg s.c.). ⋯ Naltrexone was 10-fold more potent in blocking morphine as compared to U50,488H whereas nor-binaltorphimine, a preferential kappa-antagonist, was 6-fold more potent against U50,488H than morphine. Indeed, whereas a dose of 0.2 mg/kg of naltrexone reversed mu-agonist actions, this dose was inactive against all kappa-agonists: the actions of these could be antagonized only by 2.0 mg/kg. These data indicate that in addition to kappa-receptors in the spinal cord, kappa-receptors in the brain can mediate antinociception against noxious heat and pressure.