591 © 2011 Universities Federation for Animal Welfare The Old School, Brewhouse Hill, Wheathampstead, Hertfordshire AL4 8AN, UK Animal Welfare 2011, 20: 591-596 ISSN 0962-7286 Voluntary ingestion of buprenorphine in mice KR Jacobsen* † , O Kalliokoski † , J Hau † and KSP Abelson †‡ † Department of Experimental Medicine, University of Copenhagen, Blegdamsvej 3B, 2200 Copenhagen N, Denmark ‡ Department of Neuroscience, Division of Comparative Medicine, Uppsala University, Uppsala, Sweden * Contact for correspondence and requests for reprints: kiroja@sund.ku.dk Abstract Buprenorphine is a widely used analgesic for laboratory rodents. Administration of the drug in a desirable food item for voluntary ingestion is an attractive way to administer the drug non-invasively. However, it is vital that the animals ingest the buprenorphine- food-item mix as desired. The present study investigated how readily female and male mice (Mus musculus) of two different strains consumed buprenorphine mixed in a commercially available nut paste (Nutella®), and whether variation between genders and strains would affect the subsequent serum concentrations of buprenorphine. Buprenorphine at different concentrations mixed in Nutella® was given to male and female C57BL/6 and BALB/c mice in a complete cross-over study. Pure Nutella® or buprenorphine (1.0–3.0 mg kg –1 bodyweight [bw]) mixed in 10 g kg –1 bw Nutella® were given to the mice at 1500h. The mice were video recorded until the next morning, when blood was collected by submandibular venipuncture. The concentration of buprenorphine in the Nutella® mix did not affect the duration of ingestion in any of the groups. However, female mice consumed the Nutella® significantly faster than males. Repeated exposure significantly reduced the start time of voluntary ingestion, but not the duration of eating the mixture. These differences did not however affect the serum concentration of buprenorphine measured 17 h post administration. Keywords: analgesia, animal welfare, buprenorphine, mice, refinement, voluntary ingestion Introduction Pain in animals subjected to invasive procedures may be considered a ‘contingent inhumanity’ which is almost always detrimental to the object of the experiment (Russell & Burch 1959). Peri-operative treatment with an appropriate analgesic is thus an important refinement of invasive procedures. Buprenorphine is a highly potent opioid. It acts as a partial agonist on the μ receptor subtype and is widely used as an analgesic in laboratory rodents subjected to mild or moderate invasive surgical procedures (Flecknell 2001). The recommended route of subcutaneous injection requires dosing every 8–12 h (Roughan & Flecknell 2002; Hedenqvist & Hellebrekers 2003; Flecknell 2009) which may stress the animals and result in fluctuating serum concentrations if not injected at correct intervals. In general, small animals subjected to injections with needles, often display symptoms of distress, and alternative non- invasive routes of administrations should be welcomed (Russell & Burch 1959). Furthermore, the duration of subcutaneously administered buprenorphine varies widely in different publications, and according to Gades et al (2000) the analgesic effect of buprenorphine only lasts 3–5 h in mice (Mus musculus) measured by tail-flick and hot-plate tests. In contrast, oral dosing of buprenorphine has been shown to result in a high and constant concentra- tion in the circulation of mice (Kalliokoski et al 2011). However, oral dosing by gavage requires restraint of the animals. The potential stress of this procedure can be elim- inated by allowing the animal to voluntarily consume the drug, a method which has gained some acceptance as an analgesic regimen in rats (Rattus norvegicus) (Liles et al 1998; Flecknell et al 1999; Goldkuhl et al 2010). Voluntary ingestion of buprenorphine in rats has, however, had varying degrees of success. Doses 100× higher than those recommended for subcutaneous injection seem necessary to induce serum concentrations of buprenorphine providing effective analgesia in analgesiometric tests (Thompson et al 2004). Furthermore, some studies, using fruit-flavoured gel as the food item, demonstrated that oral administration of buprenorphine in concentrations inducing appreciable analgesia resulted in unpalatable mixtures not voluntarily consumed by the rats (Martin et al 2001; Thompson et al 2006). Despite variation in pain sensitivity according to the stage of the oestrous cycle, Thompson et al (2006) concluded that only the recommended dose of 0.05 mg kg –1 bw buprenorphine given by subcutaneous injection is successful in increasing the latency time measured by the hot-water tail-flick test. In contrast, Goldkuhl and colleagues (2008, 2010) found that oral doses of 0.4 mg kg –1 bw dissolved in 2 g kg –1 bw Nutella® reduced Universities Federation for Animal Welfare Science in the Service of Animal Welfare