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      Large Somal Size Is Associated with the Expression of Galanin but Not with Neuronal Birthdate in the Sexually Dimorphic Male Nucleus of Ferret’s Preoptic Area/Anterior Hypothalamus

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          Using Nissl and Golgi stains, a sexually dimorphic male nucleus (MN) comprised of a cluster of large cells with large dendritic arbors has been identified in the dorsal preoptic area/anterior hypothalamus (POA/AH) of male ferrets. The MN-POA/AH is formed only in males by the action of estradiol derived from the neural aromatization of testosterone during the last quarter of a 41-day gestation. The ferret’s dorsal POA/AH is also characterized by a sex difference in the expression of the neuropeptide galanin which first arises in males around embryonic day (e) 34. We asked whether the male-typical phenotype of large somal size is related to birthdate and/or the capacity of dorsal POA/AH neurons to express galanin. In experiment 1 we labeled cohorts of cells born on E20, E24, or E28 by injecting the amniotic sacs of individual fetuses with the thymidine analogue bromodeoxyuridine (BrdU). On postnatal day 20, BrdU-immunoreactive cells were visualized immunohistochemically, counterstained with cresyl violet, and their somal sizes were measured. BrdU-immunoreactive cells were significantly larger in the males’ MN-POA/AH than in a comparable region of females, regardless of when they were born between E20 and E28. In experiment 2 galanin-immunoreactive cells in the dorsal POA/AH of adult ferrets were visualized immunohistochemically, and their somal sizes were measured. Somal areas of galanin-immunoreactive cells were significantly larger in the MN-POA/AH of intact, breeding, or castrated and testosterone-treated males than in the corresponding area of females. Our results suggest that cells in the males’ MN-POA/AH are more likely to be larger than cells in females’ corresponding region, regardless of birthdate. Finally, in adulthood the male-typical phenotype of large Nissl-stained somal areas of MN-POA/AH cells may, in part, reflect their increased galanin expression.

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          Most cited references 8

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          Spatiotemporal patterns of secretomotor neuron generation in the parvicellular neuroendocrine system 1Published on the World Wide Web on 3 June 1997. 1

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            Sexual dimorphism in the preoptic/anterior hypothalamic area of ferrets: effects of adult exposure to sex steroids.

            The organization of neuronal cell bodies in the caudal preoptic area (POA) and rostral anterior hypothalamic area (AH) was studied in Nissl-stained brain sections from adult male and female ferrets. Computer-assisted image-analysis procedures were developed to help estimate the areas of cellular density and the sizes of individual perikarya. At the junction of the POA and AH, a bilateral dorsal-medial group of neurons was apparent only in male ferrets (dorsal nucleus). At the same coronal level, a ventral-medial group of neurons was apparent bilaterally in both males and females (ventral nucleus). The mean somal area of cells in the dorsal nucleus of males was significantly greater than the mean somal area of cells in the corresponding dorsal region of females or in the ventral nucleus of both sexes. The dorsal nucleus was clearly discernible in adult males regardless of their hormonal status, although cells in the dorsal nucleus were larger in intact breeding males or gonadectomized males given testosterone, estradiol or dihydrotestosterone than in gonadectomized males given no gonadal hormones or given progesterone. Neither the grouping of large cells nor the steroid-induced increase in cell size, characteristic of the male dorsal nucleus, was seen in the comparable dorsal region of females. The sex difference in cellular organization observed in the ferret at the junction of the POA and AH is the first difference of this type to be seen in the POA/AH of a non-rodent mammalian species. Its identification will, hopefully, aid in the analysis of the neural mechanisms that control various sex-specific behaviors in this species.
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              Neurogenesis and cell migration into the sexually dimorphic preoptic area/anterior hypothalamus of the fetal ferret.

              A sexually dimorphic male nucleus (MN) of the preoptic area/anterior hypothalamus (POA/AH), comprising large, estradiol-receptor containing neurons, is formed in male ferrets due to the action of estradiol, derived from the neural aromatization of circulating testosterone, during the last quarter of a 41-day gestation. Two experiments were conducted to compare the birthdates and the migration pattern of cells into the sexually dimorphic portion of the dorsomedial POA/AH as well as the nondimorphic ventral nucleus (VN) of the POA/AH of males and females. In experiment 1 the thymidine analog, bromodeoxyuridine (BrdU), was injected into the amniotic sacs of fetuses of different mothers between embryonic (E) days 18 and 30. Kits from all mothers were sacrificed on E38, and brains were processed to localize BrdU immunoreactivity (IR) for determining the birthdates of neurons in the POA/AH. Cells in the MN-POA/AH of males and in a comparable region of females were born between E22 and E28; cells in the nondimorphic VN-POA/AH of both sexes were born between these same ages. These results suggest that cells in the sexually dimorphic as well as the nondimorphic subdivision of the ferret POA/AH are born during the same embryonic period. This is well before the ages (E30-E41) when administering testosterone to females can stimulate, and blocking androgen aromatization in males can inhibit, MN-POA/AH differentiation. In experiment 2 BrdU was injected on E24, and kits from different litters were perfused on E30, E34, or E38. Brains were processed for BrdU-IR as well as glial fibrillary acidic protein (GFAP), which served as a marker for radial glial processes. The orientation of radial glial processes in fetal brains of both sexes suggested that cells migrate into the dorsomedial POA/AH from proliferative zones lining the lateral as well as the third ventricles. Quantitative, computer-assisted image analysis of BrdU-IR in groups of male and female brains supported this hypothesis. There were no significant sex differences in the distribution of BrdU-IR over the three ages studied, suggesting that formation of the MN-POA/AH in males cannot be attributed to an effect of estradiol on the migration of those cells born on E24 into this sexually dimorphic structure. Finally, total BrdU-IR did not change significantly in the POA/AH of male and female kits killed at E30, E34, or E38 while the area of the POA/AH increased more than 2.5-fold over this period, suggesting that few of the POA/AH cells born on E24 die during this period in either sex. In the absence of evidence that formation of the male ferret's MN-POA/AH depends on steroid-induced changes in neurogenesis, cell migration, or death, we suggest that the specification of a particular neuronal phenotype (e.g., large somal size; capacity to produce some undetermined neurotransmitter or neuropeptide) may be responsible.

                Author and article information

                S. Karger AG
                October 1998
                14 October 1998
                : 68
                : 4
                : 235-243
                a Department of Biology, Boston University, Boston, Mass.; b Department of Biomedical Sciences, The Shriver Center, Waltham, Mass., USA
                54371 Neuroendocrinology 1998;68:235–243
                © 1998 S. Karger AG, Basel

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                Page count
                Figures: 5, References: 22, Pages: 9
                Reproductive Neuroendocrinology


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