Preliminary investigation of the thermal antinociceptive effects of codeine in cats

The aim of this study was to evaluate the potential thermal antinociceptive effects of oral administration of a single dose of codeine in cats compared with positive (buprenorphine) and negative (saline 0.9%) controls.

Six adult healthy cats weighing 5.14 ± 0.6 kg were used. Skin temperature and thermal thresholds (TTs) were evaluated using a wireless device (Topcat Metrology) at baseline, 0.5, 1, 3, 6 and 10 h after treatment. In period 1, TTs were evaluated after subcutaneous administration of saline 0.9%. In period 2, cats were administered either oral codeine (10 mg total, 2.0 ± 0.2 mg/kg) or buccal buprenorphine (0.04 mg/kg) in a cross-over, blinded study design. Half of the volume of buprenorphine was administered into each cheek pouch. Δ TT (difference between TTs after and before treatment) was used for data comparison. Mean ± SD data were analyzed using one-way ANOVA followed by Dunnett's or Tukey's test when appropriate (P <0.05).

Adverse effects did not occur in any group. Skin temperature was not different between groups nor over time. Temporal changes in TTs were not observed after saline or codeine. Buprenorphine increased Δ TT at 3 h (2.7 ± 3.3°C) when compared with baseline or saline (P <0.05). For buprenorphine, TTs were not >47.6°C at any time point in four cats. The mean highest temperature recorded in the two other cats in that group was 54.5 and 52.8°C at 3 h.

At the dose administered, codeine did not produce thermal antinociception. Mild increases in TT after buccal buprenorphine might be related to the first-pass effect after drug swallowing, drug spillage during administration and/or individual variability. These factors should be taken in to consideration when administering buprenorphine by this route in the clinical setting.