Postnatal development of the vasopressinergic system in golden hamsters

Adult golden hamsters, as compared to rats, lack several parvicellular vasopressinergic cell groups. We looked at the development of the vasopressinergic system in hamsters to draw comparisons with maturing rats. Arginine-vasopressin-immunoreactive (AVP-ir) neurons, their fibers and associated AVP binding sites were observed at several intervals after birth. Different rates of maturation were observed between different populations of vasopressinergic neurons. Within the suprachiasmatic nucleus (SCN), small AVP-ir neurons, their fibers and related binding sites maturated gradually during the first month after birth. In comparison, large AVP-ir neurons were apparent in newborn animals. Similarly, AVP-ir fibers and AVP binding sites were also present in the brain of newborns within areas not related to small vasopressinergic neurons from the SCN, such as the central amygdala (CeA) or the cerebral cortex. During the following weeks, a heterogenous pattern of development was observed within such areas. As the neurosecretory vasopressinergic system appeared to develop gradually, projections to the brain and their associated binding sites developed rapidly during the first week of life. Transient patterns of maturation were observed within certain sites. Indeed, some of the labelling observed in newborns regressed later. As similar reports were made in rats, our observations draw analogies between the vasopressinergic systems of these two species, beside their apparent dissimilarities in adult animals. Furthermore, our data also reinforce the concept that large vasopressinergic neurons do not constitute a homogenous population.