Animals
Experiment 1 used 72 C57BL/6J male mice (Charles River, UK) weighing a mean 24.9 g (SEM ±0.1) at the start of the experiment. Experiment 2 used 33 male mice bred at the University of Sussex (18 WTs; 15 KOs) weighing a mean 25.9 g (SEM ±0.4) at the start of the experiment. One week prior to food deprivation, animals were single housed in a controlled environment held at 21±2°C and 50±15% relative humidity with a 12∶12 h light-dark period (lights on at 07∶00 h). One week before commencing behavioural training, animals were food deprived to 85–90% of their ad libitum weight. During this week, animals were handled daily for 5–10 min after which 3–4 sucrose pellets were placed in each home-cage to reduce neophobia. On the last day of the week, animals in Experiment 1 received a sham saline injection (4 ml/kg) for habituation to the injection procedure. Animals were fed 2.5–3.0 g daily of standard laboratory chow (Special Diet Service Ltd, Witham, UK) 1 h after completion of behavioural training and testing. The experiments were licensed under the UK Animals (Scientific Procedures) Act 1986 (Project Licence 70/6654) following approval by the University of Sussex, Local Ethical Review Committee.