The neuropeptides vasopressin (AVP) and oxytocin (OT) have been
implicated in the regulation of numerous social behaviors in adult and juvenile
animals. AVP and OT signaling predominantly occur within a circuit of
interconnected brain regions known collectively as the “social behavior
neural network” (SBNN). Importantly, AVP and OT signaling within the
SBNN has been shown to differentially regulate diverse social behaviors,
depending on the age and/or sex of the animal. We hypothesized that variation in
the display of these behaviors is due in part to age and sex differences in AVP
and OT synthesis within the SBNN. However, a thorough characterization of AVP
and OT-immunoreactive (ir) fibers and cell bodies across age and sex within the
SBNN has been lacking in rats. We therefore quantified AVP- and OT-ir fibers and
cell bodies in 22 subregions of the fore-brain SBNN in juvenile and adult, male
and female rats. We found numerous age (16 subregions) and sex (10 subregions)
differences in AVP-ir fiber fractional areas, and AVP-ir cell body numbers,
which were mainly observed in the medial amygdala/bed nucleus of the stria
terminalis to lateral septum circuit. In contrast to AVP, we observed no age or
sex differences in OT-ir fiber fractional areas or cell bodies in any of the 22
subregions of the forebrain SBNN. Thus, unlike the static pattern observed for
OT, AVP innervation of the forebrain SBNN appears to undergo developmental
changes, and is highly sexually dimorphic, which likely has significant
functional consequences for the regulation of social behavior.