In Experiment 1, neonates were injected daily from PND1-7 with 10 mg/kg of melanotan-II (MTII) and either tested for juvenile play behavior (Sal, n = 11f, 16m; MTII, n = 11f, 15m) or adult partner preference (Sal, n = 15f, 10m; MTII, n = 18f, 12m). In Experiment 2, PND6-7 neonates were sacrificed 1-hr after an acute injection of 10mg/kg MTII or saline, or handling only (H, handled). Either immunohistochemistry (IHC) for hypothalamic neuropeptide activation (n = 6H, 4 sal, 4MTII) was performed or plasma corticosterone was assayed (n = 8H, 8sal, 9MTII). In experiment 3, neonates were injected daily with a melanocortin-4 agonist (MC4R-A, PF-446687, Pfizer) and tested for partner preference in adulthood (Vehicle, n = 9f, 9m; 0.1 mg/kg, n = 10f, 15m; 1 mg/kg, n = 9f, 15m). Animal numbers are outlined in tables on the right. F, female; M, male. PPT, Partner preference test.