EFFECTS OF TESTOSTERONE AND ITS METABOLITES ON AROMATASE-IMMUNOREACTIVE CELLS IN THE QUAIL BRAIN - RELATIONSHIP WITH THE ACTIVATION OF MALEREPRODUCTIVE-BEHAVIOR

The enzyme aromatase converts testosterone (T) into 17 beta-estradiol and plays a pivotal role in the control of reproduction. In particular , the aromatase activity (AA) located in the preoptic area (POA) of ma le Japanese quail is a limiting step in the activation by T of copulat ory behavior. Aromatase-immunoreactive (ARO-ir) cells of the POA are s pecifically localized within the cytoarchitectonic boundaries of the m edial preoptic nucleus (POM), a sexually dimorphic and steroid-sensiti ve structure that is a necessary and sufficient site of steroid action in the activation of behavior. Stereotaxic implantation of aromatase inhibitors in but not around the POM strongly decreases the behavioral effects of a systemic treatment with T of castrated males. AA is decr eased by castration and increased by aromatizable androgens and by est rogens. These changes have been independently documented at three leve ls of analysis: the enzymatic activity measured by radioenzymatic assa ys in vitro, the enzyme concentration evaluated semi-quantitatively by immunocytochemistry and the concentration of its messenger RNA quanti fied by reverse transcription-polymerase chain reaction (RT-PCR). Thes e studies demonstrate that T acting mostly through its estrogenic meta bolites regulates brain aromatase by acting essentially at the transcr iptional level. Estrogens produced by central aromatization of T there fore have two independent roles: they activate male copulatory behavio r and they regulate the synthesis of aromatase. Double label immunocyt ochemical studies demonstrate that estrogen receptors (ER) are found i n all brain areas containing ARO-ir cells but the extent to which thes e markers are colocalized varies from one brain region to the other. M ore than 70% of ARO-ir cells contain detectable ER in the tuberal hypo thalamus but less than 20% of the cells display this colocalization in the POA. This absence of ER in ARO-ir cells is also observed in the P OA of the rat brain. This suggests that locally formed estrogens canno t control the behavior and the aromatase synthesis in an autocrine fas hion in the cells where they were formed. Multi-neuronal networks need therefore to be considered. The behavioral activation could result fr om the action of estrogens in ER-positive cells located in the vicinit y of the ARO-ir cells where they were produced (paracrine action). Alt ernatively, actions that do not involve the nuclear ER could be import ant. Immunocytochemical studies at the electron microscope level and b iochemical assays of AA in purified synaptosomes indicate the presence of aromatase in presynaptic boutons. Estrogens formed at this level c ould directly affect the pre- and post-synaptic membrane or could dire ctly modulate neurotransmission namely through their metabolization in to catecholestrogens (CE) which are known to be powerful inhibitors of the catechol-O-methyl transferase (COMT). The inhibition of COMT shou ld increase the catecholaminergic transmission. It is significant to n ote, in this respect, that high levels of 2-hydroxylase activity, the enzyme that catalyzes the transformation of estrogens in CE, are found in all brain areas that contain aromatase. On the other hand, the syn thesis of aromatase should also be controlled by estrogens in an indir ect, transynaptic manner very reminiscent of the way in which steroids indirectly control the production of LHRH. Fibers that are immunoreac tive for tyrosine hydroxylase (synthesis of dopamine), dopamine beta-h ydroxylase (synthesis of norepinephrine) or vasotocine have been ident ified in the close vicinity of ARO-ir cells in the POM and retrograde tracing has identified the origin of the dopaminergic and noradrenergi c innervation of these areas. A few preliminary physiological experime nts suggest that these catecholaminergic inputs regulate AA and presum ably synthesis.