Ketamine coadministration attenuates morphine tolerance and leads to increased brain concentrations of both drugs in the rat

Abstract

Background and purpose: The effects of ketamine in attenuating morphine tolerance have been suggested to result from a pharmacodynamic interaction. We studied whether ketamine might increase brain morphine concentrations in acute coadministration, in morphine tolerance and morphine withdrawal.

Experimental approach: Morphine minipumps (6 mg·day(-1) ) induced tolerance during 5 days in Sprague-Dawley rats, after which s.c. ketamine (10 mg·kg(-1) ) was administered. Tail flick, hot plate and rotarod tests were used for behavioural testing. Serum levels and whole tissue brain and liver concentrations of morphine, morphine-3-glucuronide, ketamine and norketamine were measured using HPLC-tandem mass spectrometry.

Key results: In morphine-naïve rats, ketamine caused no antinociception whereas in morphine-tolerant rats there was significant antinociception (57% maximum possible effect in the tail flick test 90 min after administration) lasting up to 150 min. In the brain of morphine-tolerant ketamine-treated rats, the morphine, ketamine and norketamine concentrations were 2.1-, 1.4- and 3.4-fold, respectively, compared with the rats treated with morphine or ketamine only. In the liver of morphine-tolerant ketamine-treated rats, ketamine concentration was sixfold compared with morphine-naïve rats. After a 2 day morphine withdrawal period, smaller but parallel concentration changes were observed. In acute coadministration, ketamine increased the brain morphine concentration by 20%, but no increase in ketamine concentrations or increased antinociception was observed.

Conclusions and implications: The ability of ketamine to induce antinociception in rats made tolerant to morphine may also be due to increased brain concentrations of morphine, ketamine and norketamine. The relevance of these findings needs to be assessed in humans.

Figure 1

The design of the morphine tolerance and withdrawal experiments. In experiment sets 1 (blue) and 2 (red), 48 and 24 Sprague–Dawley rats were used respectively.

Figure 2

Development of antinociceptive tolerance after morphine minipump implantations. On day 0, minipumps delivering morphine 6 mg·day^–1 or vehicle were implanted s.c. under brief isoflurane anaesthesia. The means (±SEM) of the tail flick (A) and hot plate (B) latencies are shown after 1, 3 and 5 days of pump implantation. The mean raw nociceptive latencies are presented to allow assessment of the data quality. No extra morphine doses were given before the behavioural measurements. ^*P < 0.05, ^***P < 0.001; significantly different from the vehicle control group; n = 8 in the vehicle group and n = 16 in the morphine group.

Figure 3

Effects of an acute low dose of ketamine on antinociception in morphine-tolerant rats under chronic morphine treatment. Rats under an ongoing morphine (6 mg·day^–1, Mo) or vehicle (Veh) pump treatment received an acute s.c. dose of ketamine (10 mg·kg^–1, Ket) or vehicle (Veh) on day 5. Antinociception was measured using tail flick (A) and hot plate (B) tests. The mean of the maximum possible effect (MPE%) ±SEM is plotted. In the rotarod test (C), the mean (±SEM) survival time (seconds) is plotted. From separate animals having had the same pretreatments, whole brain, serum and liver samples were collected after 90 min of ketamine administration. The tissue concentrations and serum levels of the experimental drugs and their main metabolites (morphine, MO; M3G; ketamine, KET; norketamine, NORKET) in the tissues were quantified and the means of the groups (±SEM) are presented in graphs D–I. The percentage difference between the treatment groups is shown for each compound. n = 8. ^**P < 0.01, ^***P < 0.001; significantly different from the vehicle control group. #P < 0.05, ##P < 0.01, ###P < 0.001; significantly different from the morphine pump treated group that received acute vehicle. §§P < 0.01, §§§P < 0.001; significantly different from the vehicle-pretreated group that received acute ketamine.

Figure 4

Effects of an acute low dose of ketamine, morphine and their combination on antinociception in morphine-tolerant rats under morphine withdrawal. Rats had morphine pumps that delivered morphine (Mo) or vehicle (Veh) 6 mg·day^–1 for 5 days in total. On the evening of day 5, the morphine pumps were removed. On day 7, rats in morphine withdrawal received s.c. doses of morphine, ketamine or a combination. Control animals received vehicle or ketamine. Morphine was administered 15 min before ketamine. Antinociception was measured using tail flick (A) and hot plate (B) tests. The mean of the maximum possible effect (MPE%) ±SEM is plotted. In the rotarod test (C), the mean (±SEM) survival time (seconds) is plotted. Whole brain, serum and liver samples were collected after 90 min of ketamine administration. The tissue concentrations and serum levels of the experimental drugs and their main metabolites (morphine, MO; M3G; ketamine, KET; norketamine, NORKET) in the tissues were quantified and the means of the groups (±SEM) are presented in graphs D–I. The percentage difference between the treatment groups is shown for each compound. n = 8. *P < 0.05, **P < 0.01, ***P < 0.001; significantly different from the vehicle control group. #P < 0.05, ###P < 0.001; significantly different from the morphine pump treated group that received acute morphine only. §P < 0.05, §§P < 0.01, §§§P < 0.001; significantly different from the vehicle-pretreated group that received acute ketamine.

Figure 5

Effects of an acute low dose of ketamine on antinociception in acute coadministration with morphine. Rats received an acute s.c. dose of ketamine (10 mg·kg^–1, Ket) or vehicle (Veh). Morphine was given 15 min before ketamine. Antinociception was measured using tail flick (A) and hot plate (B) tests. The mean of the maximum possible effect (MPE%) ±SEM is plotted. In the rotarod test (C), the mean (±SEM) survival time (seconds) is plotted. Whole brain, serum and liver samples were collected after 90 min of ketamine administration. The tissue concentrations and serum levels of the experimental drugs and their main metabolites (morphine, MO; M3G; ketamine, KET; norketamine, NORKET) in the tissues were quantified and the means of the groups (±SEM) are presented in graphs D–I. The percentage difference between the treatment groups is shown for each compound. n = 8, except n = 7 in the morphine-treated group. *P < 0.05, **P < 0.01, ***P < 0.001; significantly different from the vehicle control group. #P < 0.05, ##P < 0.01, ###P < 0.001; significantly different from morphine only. §P < 0.05, §§§P < 0.001; significantly different from acute ketamine only.