Changes in the Arginine-Vasopressin Immunoreactive Systems in Male Mice Lacking a Functional Aromatase Gene L. Plumari,* C. Viglietti-Panzica,* F. Allieri,* S. Honda,† N. Harada,† P. Absil,‡ J. Balthazart‡ and G. C. Panzica* *Laboratory of Neuroendocrinology, Rita Levi Montalcini Centre for Brain Repair, Department of Anatomy, Pharmacology and Forensic Medicine, University of Torino, Torino, Italy. †Molecular Genetics, Fujita Health University, Toyoake, Japan. ‡Centre for Cellular and Molecular Neurobiology, University of Lie `ge, Lie `ge, Belgium. Key words: testosterone, oestradiol, bed nucleus of the stria terminalis, lateral septum, paraventricular nucleus. Abstract In male rodents, the arginine-vasopressin-immunoreactive (AVP-ir) neurones of the bed nucleus of the stria terminalis (BNST) and medial amygdala are controlled by plasma testosterone levels (decreased after castration and restored by exogenous testosterone). AVP transcription in these nuclei is increased in adulthood by a synergistic action of the androgenic and oestrogenic metabolites of testosterone and, accordingly, androgen and oestrogen receptors are present in both BNST and medial amygdala. We used knockout mice lacking a functional aromatase enzyme (ArKO) to investigate the effects of a chronic depletion of oestrogens on the sexually dimorphic AVP system. Wild-type (WT) and ArKO male mice were perfused 48 h after an i.c.v. colchicine injection and brain sections were then processed for AVP immunocytochemistry. A prominent decrease (but not a complete suppression) of AVP-ir structures was observed in the BNST and medial amygdala of ArKO mice by comparison with the WT. Similarly, AVP-ir fibres were reduced in the lateral septum of ArKO mice and but not in the medial preoptic area, a region where the AVP system is not sexually dimorphic in rats. No change was detected in the supraoptic and suprachiasmatic nuclei. However, a decrease in AVP-ir cell numbers was however, detected in one subregion of the paraventricular nucleus. These data support the hypothesis that the steroid-sensitive sexually dimorphic AVP system of the mouse forebrain is mainly under the control of aromatized metabolites of testosterone. In rodents, parts of the arginine-vasopressin (AVP) neuronal system are sexually dimorphic and sensitive to sex steroids (1). For example, the bed nucleus of the stria terminalis (BNST) and the medial amygdala (2) contain more AVP- immunoreactive (AVP-ir) cells in males than in females and projections from these nuclei to the lateral septum are denser in males than in females (3, 4). The AVP cells and fibres located in these brain regions are involved in the control of a variety of social behaviours in mammals (5, 6). In both male and female adult rat, AVP expression in the BNST and medial amygdala is enhanced by testosterone: gonadectomy inhibits mRNA expression and AVP immuno- reactivity in both nuclei and they are restored by treatment with exogenous testosterone (7). The contribution of testo- sterone metabolites to the regulation of the AVP system has also been investigated in the adult rat. Oestradiol (E 2 ), when given in association with dihydrotestosterone (DHT), strongly enhances the AVP mRNA expression in the medial amygdala and BNST, whereas E 2 alone is less effective and DHT has no effect (8, 9). These studies, as well as studies performed on other classes of vertebrates (10, 11), suggest that the brain aromatization of testosterone is a key process controlling the action of this steroid on the limbic vasopressin/vasotocin systems. This hypothesis is also supported by the presence of oestrogen receptors of the alpha and beta subtype in these brain regions and, specifically, in AVP neurones (12–14). The sexual dimorphism of the AVP system is organized perinatally by testosterone secreted by the male gonads (15). It has been assumed that the aromatization of testosterone into oestrogen also plays a key role in these organizational effects, but this notion had not been experimentally tested until recently (16). The aim of the present study was to test the Correspondence to: Professor GianCarlo Panzica, Laboratory of Neuroendocrinology, Department Anatomy, Pharmacology and Forensic Medicine, c.so M. D’Azeglio 52, I-10126 Torino, Italy (e-mail: giancarlo.panzica@unito.it). Journal of Neuroendocrinology, 2002, Vol. 14, 971–978 # 2002 Blackwell Science Ltd