Ontogeny of bidirectional connections between the medial nucleus of the amygdala and the principal bed nucleus of the stria terminalis in the rat

Nuclei in the amygdala and bed nuclei of the stria terminalis (BST) form functionally organized units that are linked by topographically organized connections. The posterodorsal part of the medial nucleus of the amygdala (MEApd) and the principal nucleus of the BST (BSTpr) share strong birectional connections that project primarily through the stria terminalis. The presence of structural and neurochemical sexual dimorphisms in both the MEApd and BSTpr suggests that connections between the nuclei may develop during the postnatal critical period for sexual differentiation. In this study, 1,1′dioctadecyl‐3,3,3′‐tetramethylindocarbocyanine perchlorate (DiI) axonal labeling was used to define the ontogeny of this bidirectional pathway. Placement of DiI crystals into the MEApd of rats perfused on embryonic day (E) 20 resulted in DiI‐labeled fibers with axonal morphology in the BSTpr, but similar labeling was not evident in the MEApd until after birth. However, as early as E14, tracer implants into the caudal MEA (the presumptive MEApd) labeled elongated cellular processes in the region of the stria terminalis that extended into the presumptive BSTpr. Based on the correspondence of these DiI‐labeled processes with immunostaining for vimentin, these cellular processes are probably derived from glial cells. Implants of DiI into the posterior BST also labeled cellular processes that extended through the medial part of the stria terminalis, but they remained confined to the molecular layer of the MEApd from E14 through P1. Labeled axons derived from the BSTpr were not observed in the MEApd until P5, demonstrating that the bidirectional connections that exist between the MEApd and BSTpr in mature rats do not develop simultaneously. The density of connections between the BSTpr and MEApd increased during the postnatal period and was similar to that of adults by P15. These findings suggest that projections from the MEApd through the stria terminalis to the BSTpr may be specified initially by a glial substrate and that return projections to the amygdala from the BSTpr develop secondary to its innervation by the MEApd. J. Comp. Neurol. 489:42–58, 2005. © 2005 Wiley‐Liss, Inc.