<p class="first" id="d4962878e137">As altricial infants gradually transition to adults,
their proximate environment changes.
In three short weeks, pups transition from a small world with the caregiver and siblings
to a complex milieu rich in dangers as their environment expands. Such contrasting
environments require different learning abilities and lead to distinct responses throughout
development. Here, we will review some of the learned fear conditioned responses to
threats in rats during their ontogeny, including behavioral and physiological measures
that permit the assessment of learning and its supporting neurobiology from infancy
through adulthood. In adulthood, odor–shock conditioning produces robust fear learning
to the odor that depends upon the amygdala and related circuitry. Paradoxically, this
conditioning in young pups fails to support fear learning and supports approach learning
to the odor previously paired with shock. This approach learning is mediated by the
infant attachment network that does not include the amygdala. During the age range
when pups transition from the infant to the adult circuit (10–15 d old), pups have
access to both networks: odor–shock conditioning in maternal presence uses the attachment
circuit but the adult amygdala-dependent circuit when alone. However, throughout development
(as young as 5 d old) the attachment associated learning can be overridden and amygdala-dependent
fear learning supported, if the mother expresses fear in the presence of the pup.
This social modulation of the fear permits the expression of defense reactions in
life threatening situations informed by the caregiver but prevents the learning of
the caregiver itself as a threat.