NEUROANATOMICAL DISTRIBUTION AND VARIATIONS ACROSS THE REPRODUCTIVE-CYCLE OF AROMATASE-ACTIVITY AND AROMATASE-IMMUNOREACTIVE CELLS IN THE PIED FLYCATCHER (FICEDULA-HYPOLEUCA)

The anatomical distribution and seasonal variations in aromatase activ ity and in the number of aromatase-immunoreactive cells were studied i n the brain of free-living male pied flycatchers (Ficedula hypoleuca). A high aromatase activity was detected in the telencephalon and dienc ephalon but low to negligible levels were present in the optic lobes, cerebellum, and brain stem. In the diencephalon, most aromatase-immuno reactive cells were confined to three nuclei implicated in the control of reproductive behaviors: the medial preoptic nucleus, the nucleus o f the stria terminalis, and the ventromedial nucleus of the hypothalam us. In the telencephalon, the immunopositive cells were clustered in t he medial part of the neostriatum and in the hippocampus as previously described in another songbird species, the zebra finch. No immunoreac tive cells could be observed in the song control nuclei. A marked drop in aromatase activity was detected in the anterior and posterior dien cephalon in the early summer when the behavior of the birds had switch ed from defending a territory to helping the female in feeding the nes tlings. This enzymatic change is presumably controlled by the drop in plasma testosterone levels observed at that stage of the reproductive cycle. No change in enzyme activity, however, was seen at that time in other brain areas. The number of aromatase-immunoreactive cells also decreased at that time in the caudal part of the medial preoptic nucle us but not in the ventromedial nucleus of the hypothalamus (an increas e was even observed), suggesting that differential mechanisms control the enzyme concentration and enzyme activity in the hypothalamus. Take n together, these data suggest that changes in diencephalic aromatase activity contribute to the control of seasonal variations in reproduct ive behavior of male pied flycatchers but the role of the telencephali c aromatase in the control of behavior remains unclear at present. (C) Academic Press.