Olfactory classical conditioning in newborn mice

Determining the behavioural phenotype of genetically altered mice is a valuable approach for elucidating the function of genes and their role in cognitive disorders. Methods for phenotyping newborn mice are scarce and generally confined to sensorimotor reflexes. Here, we describe a simple method for assessing associative abilities in newborn mice. We used a two-odour-choice classical conditioning paradigm in mice from the day of birth (post-natal age 0, P0) to P6. Acquisition required 20 trials: 10 trials during which the pups were placed over the conditioned stimulus (CS+) odour (lemon or peppermint) for 30 s and simultaneously stroked gently with a paintbrush and 10 trials during which the pups were placed over the other odour (CS−) for 30 s, without stroking. Then, the pups were subjected to five odour-preference trials to test for conditioning. This sequence of five trials was repeated after 5 and 24 h to assess retention of the conditioned odour preference. During the immediate post-acquisition sequence, the pups spent significantly more time over the CS+ than over the CS− (p < 0.0001). No extinction of the conditioned preference was observed during this test. No preference was observed after 5 or 24 h, indicating that the conditioned response was promptly lost. Conditioning was effective as soon as P0-P1. Thus, conditioning may emerge in newborn mice sooner than previously reported. This paradigm is well suited to phenotyping of large samples of genetically altered mice and may shed light on the role for genes in paediatric cognitive impairments.