Dextromethorphan attenuated the higher vulnerability to inflammatory thermal hyperalgesia caused by prenatal morphine exposure in rat offspring

Abstract

Background: Co-administration of dextromethorphan (DM) with morphine during pregnancy and throughout lactation has been found to reduce morphine physical dependence and tolerance in rat offspring. No evidence was presented, however, for the effect of DM co-administered with morphine during pregnancy on inflammatory hyperalgesia in morphine-exposed offspring. Therefore, we attempt to investigate the possible effect of prenatal morphine exposure on the vulnerability to hyperalgesia and the possible therapeutic effect of DM in the present study.

Methods: Fifty μl of carrageenan (20 mg/ml) was injected subcutaneously into the plantar surface of the right hind paw in p18 rats to induce hyperalgesia. Mean paw withdrawal latency was measured in the plantar test to index the severity of hyperalgesia. Using Western blotting and RT-PCR, the quantitative analyses of NMDA receptor NR1 and NR2B subunits were performed in spinal cords from different groups of animals.

Results: In the carrageenan-induced hyperalgesia model, rat offspring passively exposed to morphine developed a severe hyperalgesia on postnatal day 18 (p18), which also had a more rapid time course than those in the controls. Co-administration of DM with morphine in the dams prevented this adverse effect of morphine in the offspring rats. Western blot and RT-PCR analysis showed that the levels of protein and mRNA of NMDA receptor NR1 and NR2B subunits were significantly higher in the lumbar spinal cords of rats (p14) exposed to prenatal morphine; the co-administration of DM could reverse the effect of morphine on NR1 and attenuate the effect on NR2B.

Conclusions: Thus, DM may have a great potential in the prevention of higher vulnerability to inflammatory thermal hyperalgesia in the offspring of morphine-addicted mothers.

Figure 1

(A) The paw withdrawal latency was obtained by plantar tests before carrageenan injection in the offspring rats (P18) of different groups. (B) The time-course of the hyperalgesia effect induced by carrageenan injection (50 μl, 20 mg/ml). In (B), the different marks and lines represent the results from different groups (filled circle: Control, open circle: M, open square: M + DM, filled square: DM). Data are presented as mean ± SEM. One-way ANOVA followed by the Newman-Keuls test was used to analyze the data. (A: **p < 0.001 for the morphine group vs the control group; B: **p < 0.001, for the morphine group vs the control group) (Control group: n = 8; M group: n = 8; M + DM group: n = 7; DM group: n = 10).

Figure 2

(A), (C): The blots show the examples of immunobands against NR1 (A) and NR2B (C) and β-actin antibodies on the membrane, which was performed on the membrane protein prepared from the lumbar spinal cords of the offspring (P14) rats in different groups. (B), (D): The quantitative change of NR1 (B) and NR2B (D) protein expression. The values of NR1 and NR2B subunits in the control group were arbitrarily set at 100%. Data are presented as mean ± SEM. One-way ANOVA followed by the Newman-Keuls test was used to analyze the data. (B: *p < 0.05 for the morphine group vs the control group; D: **p < 0.001 for the morphine group vs the control group; *p < 0.05 for the M + DM group vs the control group; ¶p < 0.01 for the M + DM group vs the morphine group, n = 3).

Figure 3

(A), (C): An example of RT-PCR results for the levels of mRNA of NMDA receptor NR1 (A) and NR2B (C) subunits and GAPDH. (B), (D): The quantitative RT-PCR results for the levels of mRNA of NMDA receptor NR1 (B) and NR2B (D) subunit of the offspring rats (P14) in different groups. In (A), each white letter at the bottom of the lane represents the result from certain group (a: Control, b: M, c: M + DM, d: DM). The values of NR1 and NR2B subunits in the control group were arbitrarily set at 100%. Data are presented as mean ± SEM. One-way ANOVA followed by the Newman-Keuls test was used to analyze the data. (B: **p<0.01 for the morphine group vs the control group; D: **p < 0.01, for the morphine group vs the control group, n = 3).