Life sciences
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The effects of delta-receptor antagonists on cocaine- and methamphetamine-induced place preferences were examined in rats. Cocaine- and methamphetamine-induced place preferences were significantly attenuated by naltrindole (NTI: a non-selective delta-opioid receptor antagonist). Furthermore, naltriben (NTB: a selective delta 2-opioid receptor antagonist), but not 7-benzylidenenaltrexone (BNTX: a selective delta 1-opioid receptor antagonist), attenuated the cocaine- and methamphetamine-induced place preferences. These results suggest that delta-opioid receptors, particularly delta 2-opioid receptors, may be involved in the reinforcing effects of cocaine and methamphetamine.
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Loperamide effects on hepatobiliary function, analgesia and gut transit were studied in mice. Varying doses of the antidiarrheal drug, loperamide, were administered to mice by intracerebroventricular, intravenous, subcutaneous and intragastric routes. Gut motility was determined by intestinal transit of India ink, analgesia by warm water tail flick latency, and hepatobiliary function by retention of the anionic dye, sulfobromophthalein in plasma and liver. ⋯ Intragastric loperamide at one fortieth its LD50 caused marked elevation of sulfobromophthalein levels and gut slowing, but no analgesia. Sulfobromophthalein elevation and gut slowing by intragastric loperamide were not affected by spinal cord transection but were reversed by naltrexone, an opiate antagonist. Non-toxic doses of loperamide slow gut transit and modify hepatobiliary function in mice by opiate actions at peripheral sites.
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The alpha 2 agonist clonidine has been shown to be anxiolytic in a number of preclinical anxiety models. Interestingly, intravenous infusion of the alpha 2 antagonists idazoxan at 10 mg/kg and rauwolscine at 2.24 mg/kg significantly disinhibited lick-shock conflict responding in rats similar to the alpha 2 agonist clonidine (0.022 mg/kg) and the benzodiazepine diazepam (0.5 mg/kg). However, the alpha 2 antagonists yohimbine and piperoxan, the alpha 2 agonists medetomidine, guanfacine, and guanabenz, the non-specific alpha antagonist phentolamine, and the alpha 1 antagonist prazosin did not disinhibit conflict responding in the Vogel lick-shock paradigm. ⋯ Idazoxan (10 mg/kg) significantly decreased water consumption by 45%. Therefore, idazoxan increased conflict responding for water reward at a dose (10 mg/kg) which also decreased water consumption in a non-conflict paradigm. These data suggest that agents with selective antagonism at the alpha 2 receptor site may be anxiolytic while agents with less specificity at this site such as yohimbine, piperoxan, and phentolamine are not anxiolytic.
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The purpose of this study was to examine the roles of brain opioid receptors, using the opioid receptor antagonist naloxone, and brain alpha 2 adrenergic and imidazole receptors, using their agonist clonidine, in the hypertension and tachycardia induced by electrical stimulation of the AHA and PHA area. Unanesthetized and unrestrained Wistar rats 300-400 g that had previously had catheters inserted into the lateral cerebral ventricle and femoral artery and electrodes in AHA or PHA areas received intracerebral (ICV) administration of naloxone or clonidine prior to hypothalamic stimulation. AHA and PHA stimulation with current strength from 0.5 to 2.0 mA produced a significant (p < 0.05) and dose dependent increase in BP and heart rate. ⋯ In contrast to AHA stimulation, naloxone did not alter the BP increase produced by PHA stimulation while clonidine prevented the effects of PHA stimulation. Heart rate did not increase with PHA stimulation. These data suggest that (i) the mechanisms involved in the hypertensive response to AHA are different from that of PHA. (ii) the endogenous opioid system is more operative in mediating the BP elevation produced by AHA but not PHA stimulation (iii) activation of the central alpha adrenergic or imidazole receptors can suppress hypertensive response to both AHA and PHA but is more effective for PHA than AHA stimulation.
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Comparative Study
The antinociceptive potencies of oxycodone, noroxycodone and morphine after intracerebroventricular administration to rats.
The antinociceptive activity of noroxycodone (NOR) was determined and its potency compared with that of oxycodone (OXY) and morphine (MOR) after intracerebroventricular (icv) administration to Sprague-Dawley rats. The antinociceptive potencies of OXY and NOR relative to MOR were 0.44 and 0.17, respectively. ⋯ NOR, administered by the icv route, also produced excitatory effects throughout the antinociceptive dose range, the severity of which was reduced but not abolished by prior administration of naloxone (55 nmol icv). As excitatory effects have not been observed in patients receiving OXY, it is unlikely that NOR contributes to the analgesic activity of OXY administered systemically to humans.