Low doses of alpha 2-adrenoceptor antagonists augment spinal morphine analgesia and inhibit development of acute and chronic tolerance

Abstract

Background and purpose: Ultra-low doses of opioid receptor antagonists augment spinal morphine antinociception and block the induction of tolerance. Considering the evidence demonstrating functional and physical interactions between the opioid and alpha(2)-adrenoceptors, this study investigated whether ultra-low doses of alpha(2)-adrenoceptor antagonists also influence spinal morphine analgesia and tolerance.

Experimental approach: Effects of low doses of the competitive alpha(2)-adrenoceptor antagonists-atipamezole (0.08, 0.8 ng), yohimbine (0.02, 2 ng), mirtazapine (0.02 ng) and idazoxan (0.08 ng) were investigated on intrathecal morphine analgesia, as well as acute and chronic morphine antinociceptive tolerance using the rat tail flick and paw pressure tests.

Key results: At doses markedly lower than those producing alpha(2)-adrenoceptor blockade, atipamezole, yohimbine, mirtazapine and idazoxan, prolonged the antinociceptive effects of morphine. When co-administered with repeated acute spinal injections of morphine, all four agents blocked the induction of acute tolerance. Co-injection of atipamezole with morphine for 5 days inhibited the development of tolerance in a chronic treatment paradigm. Spinal administration of atipamezole also reversed established antinociceptive tolerance to morphine as indicated by the restoration of morphine antinociceptive potency. The effects of atipamezole on spinal morphine tolerance were not influenced by treatment with 6-hydroxydopamine.

Conclusions and implications: Low doses of competitive alpha(2)-adrenoceptor antagonists can augment acute morphine analgesia and block or reverse tolerance to spinal administration of morphine. These actions are interpreted in terms of their interaction with an opioid-alpha(2)-adrenoceptor complex, whose activity may have a function in the genesis of analgesic tolerance.

Figure 1

Effects of intrathecal administration of α₂-adrenoceptor antagonists, atipamezole (5, 10 μg) yohimbine (30 μg), mirtazapine (2 μg) and idazoxan (10 μg) on the acute antinociceptive action of a maximal dose of the α₂-adrenoceptor agonist clonidine (13 μg) in the tail flick test. The antagonists were administered with clonidine as a single injection. Nociceptive testing was performed every 10 min after the injection for the first hour and every 30 min for the following 2-h test period. The data are expressed as mean±s.e.mean for four to six animals. Significant difference from the action of clonidine alone at the corresponding time point: ^*P<0.05, ^**P<0.01, ^***P<0.001.

Figure 2

Time course of the effects of ultra-low doses of intrathecal (a) atipamezole (0.08, 0.8 ng), (b) yohimbine (0.2 ng), mirtazapine (0.02 ng) and idazoxan (0.08 ng) on the acute antinociceptive action of a maximal dose of morphine (Mor, 15 μg) in the tail flick test. Each agent was administered with morphine as a single injection. Nociceptive testing was every 10 min post-injected for the first 60 min and every 30 min for the following 2 h period. The data are expressed as mean±s.e.mean for four to seven animals. Significant difference is presented as difference from the action of morphine at the corresponding time point. ^*P<0.05, ^**P<0.01, ^***P<0.001.

Figure 3

Time course of the effects of ultra-low doses of intrathecal (a) atipamezole (0.08, 0.8 ng), (b) yohimbine (0.2 ng), mirtazapine (0.02 ng) and idazoxan (0.08 ng) on the acute antinociceptive action of a maximal dose of morphine (Mor, 15 μg) in the paw pressure test. Each agent was administered with morphine as a single injection. Nociceptive testing was every 10 min post-injected for the first 60 min and every 30 min for the following 2 h period. The data are expressed as mean±s.e.mean for four to seven animals. Significant difference is presented as difference from the action of morphine at the corresponding time point. ^*P<0.05, ^***P<0.001.

Figure 4

Effects of intrathecal (a) atipamezole (0.8 ng), (b) yohimbine (0.02 ng), mirtazapine (0.02 ng) and idazoxan (0.08 ng) on the induction of acute tolerance to morphine (Mor) in the paw pressure test. Tolerance was induced by intrathecal injection of morphine (15 μg) at 0, 90 and 180 min. Morphine was administered with atipamezole, yohimbine, mirtazapine and idazoxan as a singe injection. Nociceptive testing was performed every 30 min post-injection. The data are presented as mean±s.e.mean; n=five to six animals per treatment group. Significant difference from the action of morphine ^**P< 0.01, ^***P< 0.001.

Figure 5

Effects of atipamezole on the induction of tolerance to chronic antinociceptive effects of morphine (Mor) in the tail flick test. (a) Time course of the action of chronic intrathecal morphine (5 μg) delivered in combination with saline or atipamezole (0.08, 0.8 ng) once daily for 5 days. Nociceptive testing was performed 60 min after each injection. The data are expressed as mean±s.e.mean for four to seven animals. ^*P<0.05, ^**P<0.01, ^***P<0.001. (b) Time course of the effect of morphine–saline and morphine–atipamezole combination on day 5 in a subgroup of animals that had received chronic treatment with these drug treatments (panel a). Nociceptive testing was performed every 30 min after the drug injection. The data are expressed as mean±s.e.mean for four to five animals. Significant difference from morphine at the corresponding time point ^***P<0.001.

Figure 6

Effects of atipamezole on the induction of tolerance to chronic antinociceptive effects of morphine (Mor) in the paw pressure test. (a) Time course of the action of chronic intrathecal morphine (15 μg) delivered in combination with saline or atipamezole (0.08, 0.8 ng) once daily for 5 days. Nociceptive testing was performed 60 min after each injection. The data are expressed as mean±s.e.mean for four to seven animals. ^***P<0.001. (b) Time course of the effect of morphine–saline and morphine–atipamezole combination on day 5 in a subgroup of animals that had received chronic treatment with these drug treatments (panel a). Nociceptive testing was performed every 30 min after the drug injection. The data are expressed as mean±s.e.mean for four to five animals. Significant difference from morphine at the corresponding time point ^***P<0.001. Significant difference from saline at the corresponding time point. ^#P<0.05, ^##P<0.01.

Figure 7

Effect of low-dose atipamezole (0.8 ng) on the antinociceptive effect and the morphine (Mor) potency in animals tolerant to the opioid agonist in the tail flick (a) and paw pressure test (b). Time course for the effects of atipamezole on the morphine-induced antinociception in the opioid-tolerant animals. Tolerance was induced by a single daily injection of morphine (15 μg) from day 1 to 5. On day 6, morphine was administered alone or in combination with atipamezole as a single injection until day 10. Nociceptive testing was performed after 60 min after each injection. The data are expressed as mean±s.e.mean of six to seven animals. Significant difference from the morphine alone (10 days). ^*P<0.05, ^**P<0.01, ^***P<0.001.

Figure 8

Anti-nociceptive effect of intrathecal L-noradrenaline (10 μg) in animals pretreated with intrathecal 6-hydroxydopamine (6-OHDA). Animals in the 6-OHDA group received a single injection of the neurotoxin (20 μg) 7 days before assessment of the effects of L-noradrenaline. Control animals received an injection of the vehicle. Anti-nociceptive testing was performed every 10 min for 60 min and every 30 min for 120 min after the drug injection. Significant difference from corresponding response in the vehicle-treated group. ^*P<0.05, ^**P<0.01, ^***P<0.001.

Figure 9

Influence of intrathecal 6-hydroxydopamine (6-OHDA) pretreatment on the induction of acute spinal morphine (Mor) tolerance and the effect of low-dose atipamezole on this response in (a) the tail flick test and (b) the paw pressure test. Morphine (15 μg) or saline was administered at 0, 90 and 180 min to groups of animals pretreated with a single injection of 6-OHDA (20 μg) or the vehicle 14 days before testing. Atipamezole (0.08 ng) was administered with morphine to 6-OHDA pretreated animals as a single injection. Nociceptive testing was performed every 30 min post-injection. The data are presented as mean±s.e.mean; n=six to seven animals per treatment group. Significant difference from the action of morphine alone at the corresponding time point in the 6-OHDA-pretreated animals ^*P<0.05, ^***P<0.001.