An altered neonatal behavioral phenotype in Mecp2 mutant mice.

We examined somatic growth, somatosensory reflexes, and ultrasonic calls from postnatal day 3 to day 18 in Mecp2 mutant mice, a mouse model of Rett syndrome. Both Mecp2 null male and Mecp2 heterozygous female mice exhibited normal somatic growth, but transient delays in the development of some reflexes relative to sex-matched wild-type mice. Both Mecp2 null male and heterozygous female mice exhibited dramatic increases in ultrasonic vocalizations in response to social isolation; these differences were evident as early as postnatal day 5. These results suggest very early abnormalities in sensory reflex development and behavioral responsiveness in the Mecp2 mutants that may provide a target for early therapeutic intervention.