European journal of pharmacology
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Adenosine and ATP exert multiple influences on pain transmission at peripheral and spinal sites. At peripheral nerve terminals in rodents, adenosine A1 receptor activation produces antinociception by decreasing, while adenosine A1 receptor activation produces pronociceptive or pain enhancing properties by increasing, cyclic AMP levels in the sensory nerve terminal. Adenosine A3 receptor activation produces pain behaviours due to the release of histamine and 5-hydroxytryptamine from mast cells and subsequent actions on the sensory nerve terminal. ⋯ Endogenous adenosine systems contribute to antinociceptive properties of caffeine, opioids, noradrenaline, 5-hydroxytryptamine, tricyclic antidepressants and transcutaneous electrical nerve stimulation. Purinergic systems exhibit a significant potential for development as therapeutic agents. An understanding of the contribution of adenosine to pain processing is important for understanding how caffeine produces adjuvant analgesic properties in some situations, but might interfere with the optimal benefit to be derived from others.
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Midazolam increased the shorter-response rate and decreased the reinforcement rate of a contingency-controlled timing behavior--a differential-reinforcement-of-low-rate 45-s schedule. The responding rate changes observed were immediately interpretable as functions of midazolam concentration during a 3-h session--a period for investigating the onset, peak, and disappearance of midazolam effect--in rats. ⋯ The stimulation-sedation model suggested that midazolam possesses both stimulatory and sedative effects in a continuous but sequential fashion, and hypothesizes the coexistence of stimulation and sedation components for midazolam; this model may help delineate possible mechanisms for rebound side effects and of tolerance in humans. The reinforcement rate was, then, an index for evaluating the deficit in timing performance.
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ABT-594 [5-((2R)-azetidinylmethoxy)-2-chloropyridine], a novel neuronal nicotinic acetylcholine receptor agonist, produced significant antinociceptive effects in mice against both acute noxious thermal stimulation--the hot-plate and cold-plate tests--and persistent visceral irritation--the abdominal constriction (writhing) assay (maximally-effective dose in each test 0.62 micromol/kg, i.p.). This effect was not stereoselective since the S-enantiomer, A-98593 [5-((2S)-azetidinylmethoxy)-2-chloropyridine], produced similar antinociceptive effects in this dose range. The effect in the hot-plate test peaked at 30 min after i.p. administration and was still present 60 min, but not 120 min, after injection. ⋯ Preliminary safety testing revealed an ED50 for overt seizure production of 1.9 micromol/kg, i.p. and an LD50 of 19.1 micromol/kg i.p. in mice, values 10 and 100 times the minimum effective antinociceptive dose of the compound. ABT-594 increased the duration of ethanol-induced hypnotic effects, tended to increase pentobarbital-induced hypnotic effects (P = 0.0502), and had no effect on pentobarbital-induced lethality. These data indicate that ABT-594 is a centrally acting neuronal nicotinic acetylcholine receptor agonist with potent antinociceptive and anxiolytic-like effects in mice.