AROMATASE IN AXONAL PROCESSES OF EARLY POSTNATAL HYPOTHALAMIC AND LIMBIC AREAS INCLUDING THE CINGULATE CORTEX

It has been shown that sexual dimorphic morphology of certain hypothal amic and limbic areas underlie gender-specific sexual behavior and neu roendocrine mechanisms. The key role played by locally formed estrogen in these developmental events has been revealed during a critical per inatal period. In this study, we aimed to document the presence of est rogen-synthetase (aromatase)-immunoreactive elements in the involved l imbic system and hypothalamus of the developing rat brain. On postnata l day 5, animals of both sexes were perfusion-fixed, and sections from the forebrain and hypothalamus were immunolabelled for aromatase usin g an antiserum that was generated against a 20 amino acid sequence of placental aromatase. Aromatase-immunoreactivity was present in neurona l perikarya and axonal processes in the following limbic structures: t he central and medial nuclei of the amygdala, stria terminalis, bed nu cleus of the stria terminalis (BNST), lateral septum, medial septum, d iagonal band of Broca, lateral habenula and all areas of the limbic (c ingulate) cortex. In the hypothalamus, the most robust labelling was o bserved in the medial preoptic area, periventricular regions, ventrome dial and arcuate nuclei. The most striking feature of the immunostaini ng with this antiserum was its intracellular distribution. In contrast to the heavy perikaryal labelling that can be observed with most of t he currently available aromatase antisera, in the present experiments, immunoperoxidase was predominantly localized to axons and axon termin als. All the regions with fiber staining corresponded to the projectio n fields of neuron populations that have previously been found to expr ess perikaryal aromatase. Our results confirm the presence of aromatas e-immunoreactivity in developing Limbic and hypothalamic areas. The ma ssive expression of aromatase in axonal processes raises the possibili ty that estrogen formed locally by aromatase may not only regulate the growth, pathfinding and target recognition of its host neuronal proce sses, but may also exert paracrine actions on structures in close prox imity, including the target cells. (C) 1997 Elsevier Science Ltd.