Codeine and 6-acetylcodeine analgesia in mice

Abstract

1. Acetylation of morphine at the 6-position changes its pharmacology. To see if similar changes are seen with codeine, we examined the analgesic actions of codeine and 6-acetylcodeine. 2. Like codeine, 6-acetylcodeine is an effective analgesic systemically, supraspinally and spinally, with a potency approximately a third that of codeine. 3. The sensitivity of 6-acetylcodeine analgesia to the mu-selective antagonists beta-FNA and naloxonazine confirmed its classification as a mu opioid. However, it differed from the other mu analgesics in other paradigms. 4. Antisense mapping revealed the sensitivity of 6-acetylcodeine to probes targeting exons 1 and 2 of the mu opioid receptor gene (Oprm), a profile distinct from either codeine or morphine. Although heroin analgesia also is sensitive to antisense targeting exons 1 and 2, heroin analgesia also is sensitive to the antagonist 3-O-methylnaltrexone, while 6-acetylcodeine analgesia is not. 5. Thus, 6-acetylcodeine is an effective mu opioid analgesic with a distinct pharmacological profile.

Fig. 1.

Timecourse of 6-acetylcodeine analgesia. A group of mice (n=10) received 6-acetylcodeine (20 mg/ kg, s.c.) and analgesia was assessed at the indicated time.

Fig. 2.

Dose–response studies of 6-acetylcodeine and codeine analgesia. Groups of mice (n=20) received the indicated doses of 6-acetylcodeine or codeine (A) systemically, (B) supraspinally or (C) spinally and were assessed in the tailflick assay after 30, 15 or 15 min, respectively. Systemic ED₅₀ values with 95% confidence limits for 6-acetylcodeine and codeine are 12.3 mg/kg (10.1, 15) and 3.91 mg/kg (3.02, 5.07), respectively. The supraspinal ED₅₀ values with 95% confidence limits for 6-acetylcodeine and codeine are 3.53 μg (2.55, 4.90) and 0.86 μg (0.62, 1.19), respectively. Intrathecal ED₅₀ values with 95% confidence limits for 6-acetylcodeine and codeine are 3.45 μg (2.44, 4.90) and 1.06 μg (0.69, 1.62), respectively.

Fig. 3.

Sensitivity of codeine and 6-acetylcodeine to antagonists. Groups of mice (n≥20) received codeine and 6-acetylcodeine and either saline, naltrexone, norBNI, 3-O-methylnaltrexone, β-FNA or naloxonazine. β-FNA and naloxonazine were administered 24 h prior to agonists. The others were given immediately prior to the agonists. Only naltexone, β-FNA and naloxonazine significantly reversed the analgesic actions of the drugs.

Fig. 4.

Codeine analgesia in opioid receptor knockout mice. Groups of mice with the indicated gene disruption were backcrossed to generation F10 into a C57/Bl6 background. A cumulative dose–response curve was carried out with codeine and analyzed quantally. Graded responses analysis give similar results.

Fig. 5.

Antisense mapping 6-acetylcodeine and codeine analgesia. Groups of mice (n≥20) received the indicated antisense (5 μg in 2 μl, i.c.v.) or saline on days 1, 3, 5 and were tested with either codeine (2 μg, i.c.v.) or 6-acetylcodeine (7.5 μg, i.c.v.) supraspinally on day 6 and analgesia was assessed 15 min post-injection. Antisense targeting exon 1 showed a reduction in analgesia for agents, while antisense targeting exon 2 showed a reduction in only for 6-acetylcodeine. Saline and mismatch treated mice showed no reduction in analgesic response.