J. Steroid Biochem. Molec. Biol. Vol. 44, No. 4-6, pp. 521-540, 1993 0960-0760/93$6.00 + 0.00 Printed in Great Britain. All rights reserved Copyright© 1993PergamonPress Ltd BRAIN AROMATASE AND THE CONTROL OF MALE SEXUAL BEHAVIOR J. BALTHAZART* and A. FOIDART Laboratory of General and Comparative Biochemistry, University of Li6ge (Bat. LI), 17 place Delcour, 4020 Li6ge, Belgium Summary--The activational effects of testosterone (T) on male copulatory behavior are mediated by its aromatization into estradiol. In quail, we have shown by stereotaxic implantation of steroids and metabolism inhibitors and by electrolytic lesions that the action of T and its aromatization take place in the sexually dimorphic medial preoptic nucleus (POM). The distribution and regulation of brain aromatase was studied in this species by product-formation assays measuring aromatase activity (AA) in microdissected brain regions and by immunocytochemistry (ICC). Aromatase-immunoreactive (ARO-ir) neurons were found in four brain regions: the POM, the septal region, the bed nucleus of the stria terminals (BNST) and the tuberal hypothalamus. ARO-ir cells actually outline the POM boundaries. ARO-ir material is found not only in the perikarya of neurons but also in the full extension of their cellular processes including the axons and the presynaptic boutons. This is confirmed at the light level by the demonstration of immunoreactive fibers and punctate structures in brain regions that are sometimes fairly distant from the closest ARO-ir cells. A lot of ARO-ir cells in the POM and BNST do not contain immunoreactive estrogen receptors (ER-ir) as demonstrated by double label ICC. These morphological data suggest an unorthodox role for the enzyme or the locally formed estrogens. In parallel with copulatory behavior, the preoptic AA decreases after castration and is restored by T to levels seen in sexually mature males. This probably reflects a change in enzyme concentration rather than a modulation of the activity in a constant number of molecules since the maximum enzymatic velocity (Vmax) only is affected while the affinity (Kin) remains unchanged. In addition, T increases the number of ARO-ir neurons in POM and other brain areas suggesting that the concentration of the antigen is actually increased. This probably involves the direct activation of aromatase transcription as demonstrated by RT-PCR studies showing that aromatase mRNA is increased following T treatment of castrates. These activating effects of T seem to result from a synergistic action of androgenic and estrogenic metabolites of the steroid. The anatomical substrate for these regulations remains unclear at present especially in POM where ARO-ir cells do not in general contain ER-ir while androgen receptors appear to be rare based on both [3H] dihydrotestosterone autoradiography and ICC. Transynaptic mechanisms of control may be considered. A modulation of brain aromatase by catecholamines is also suggested by a few pharmacological studies. This notion is further supported by anatomical data demonstrating dense projections of dopamine fl-hydroxylase and tyrosine hydroxylase-immunoreactive fibers around ARO-ir cells. INTRODUCTION Aromatase also called estrogen synthetase, an enzyme of the cytochrome P450 family (P- 450 .... ; EC 1.14.14.1), irreversibly transforms testosterone (T) into estradiol-17fl (E2) or an- drostenedione (A4) into estrone (El). This en- zyme was originally found in the ovary and placenta but in the early seventies, Naftolin and Proceedings of the Third International Aromatase Confer- ence. Basic and Clinical Aspects of Arornatase, Bologna, Italy, 14 17 June 1992. *To whom correspondence should be addressed coworkers [1, 2] were able to identify a low aromatase activity (AA) in the brain of several mammalian species. This enzymatic activity has since been identified in the brain of many species belonging to all vertebrate classes from jawed fishes to mammals. The presence of this enzyme in the brain of cyclosomes is questionable and it has not been detected in invertebrates, such as the lobster and the octopus [3-5]. Brain AA is usually low in higher vertebrates (orders of magnitude below activities normally observed in peripheral structures) but the en- zyme has a critical importance in the control of reproductive processes. There is a wealth of 521