Leptin regulates olfactory-mediated behavior in ob/ob mice

We have investigated olfactory-mediated pre-ingestive behavior in leptin (ob/ob) and leptin receptor (db/db) mutant mice compared to age- and gender-matched wild-type (wt) mice. Olfactory-mediated behavior was tested using a buried food paradigm 5 times/day at 2-h intervals for 6 days. Mean food-finding times of ob/ob and db/db mice were approximately 10 times shorter than those of wt mice. To test the effect of leptin replacement in ob/ob mice, leptin (1 or 5 μg/g body weight in sterile saline) or carrier was injected i.p. once daily prior to testing. Mean food finding times in ob/ob mice injected with carrier or with 1 μg/g leptin were similar and were 2–3 times faster than in wt mice. Mean food finding times in ob/ob mice injected with 5 μg/g leptin tripled compared to carrier-injected ob/ob mice and were of the same order of magnitude as those of wt mice, suggesting functional leptin replacement. A 3-factor repeated measures ANOVA demonstrated significant differences between the 6 cohorts (P = 0.0001), food finding times (P ≤ 0.0001), and cohort by day interaction (P ≤ 0.0001). Post hoc tests suggested that the ob/ob + 5 μg/g leptin cohort performed more like the wt cohort in the food-finding test than like the ob/ob or ob/ob + carrier cohort. Potential local sites of leptin production and action were identified with immunohistochemistry and in situ hybridization in epithelial and gland cells of the olfactory and nasal mucosae. Our results strongly suggest that leptin acting through leptin receptors modulates olfactory-mediated pre-ingestive behavior.