Developmental fluoxetine exposure and prenatal stress alter sexual differentiation of the brain and reproductive behavior in male rat offspring.

Depression during pregnancy and postpartum is a significant health problem and affects up to 20% of women. While selective serotonin reuptake inhibitor (SSRI) medications are the drug of choice for treatment of maternal depression, the combined effect of maternal depression and perinatal SSRI exposure on offspring development is poorly investigated. Our aim was to determine the role of exposure to fluoxetine during development on sexual behavior and sexually dimorphic brain structures in male offspring using a rodent model of maternal adversity. Sprague-Dawley rat dams were stressed during gestation and were chronically treated throughout lactation with either fluoxetine or vehicle beginning on postnatal day 1. Four groups of offspring were used: (1) Control+Vehicle, (2) Control+Fluoxetine, (3) Prenatal Stress+Vehicle, and (4) Prenatal Stress+Fluoxetine. We show here that developmental fluoxetine treatment decreases the anogenital distance in juvenile male offspring. In adult male offspring, maternal fluoxetine treatment results in a decrease in the number of intromissions, a longer latency to the first intromission, and a longer latency to the first ejaculation. Furthermore, developmental fluoxetine and/or prenatal stress decrease the area of the sexually dimorphic nucleus of the preoptic area (SDN-POA). Prenatal stress, but not exposure to developmental fluoxetine, decreases the number of tyrosine hydroxylase (TH)-positive cells in anteroventral periventricular nucleus (AVPv) and the volume of the posterior bed nucleus of the stria terminalis (pBST) in male offspring. These results provide important evidence for the long-term impact of maternal adversity and maternal fluoxetine use on the development of primary endocrinology systems in juvenile and adult male offspring.

Congenital hydrocephalus and abnormal subcommissural organ development in Sox3 transgenic mice.

Congenital hydrocephalus (CH) is a life-threatening medical condition in which excessive accumulation of CSF leads to ventricular expansion and increased intracranial pressure. Stenosis (blockage) of the Sylvian aqueduct (Aq; the narrow passageway that connects the third and fourth ventricles) is a common form of CH in humans, although the genetic basis of this condition is unknown. Mouse models of CH indicate that Aq stenosis is associated with abnormal development of the subcommmissural organ (SCO) a small secretory organ located at the dorsal midline of the caudal diencephalon. Glycoproteins secreted by the SCO generate Reissner's fibre (RF), a thread-like structure that descends into the Aq and is thought to maintain its patency. However, despite the importance of SCO function in CSF homeostasis, the genetic program that controls SCO development is poorly understood. Here, we show that the X-linked transcription factor SOX3 is expressed in the murine SCO throughout its development and in the mature organ. Importantly, overexpression of Sox3 in the dorsal diencephalic midline of transgenic mice induces CH via a dose-dependent mechanism. Histological, gene expression and cellular proliferation studies indicate that Sox3 overexpression disrupts the development of the SCO primordium through inhibition of diencephalic roof plate identity without inducing programmed cell death. This study provides further evidence that SCO function is essential for the prevention of hydrocephalus and indicates that overexpression of Sox3 in the dorsal midline alters progenitor cell differentiation in a dose-dependent manner.

Emx and Nfi genes regulate cortical development and axon guidance in the telencephalon.

The Emx and Nuclear Factor One (Nfi) genes encode transcription factors that regulate numerous embryonic developmental processes. The two mammalian Emx genes, Emx1 and Emx2, are expressed in the embryonic cortex and regulate the specification of the cortex into different sensory and motor areas along the rostrocaudal axis. To date, few developmental processes have been attributed specifically to Emx1, with most analyses demonstrating a redundancy of function between Emx1 and Emx2, with Emx2 being most essential for development. Here we provide evidence that Emx1 and Emx2 regulate different developmental processes during corpus callosum formation and review how both genes function in cellular migration and the formation of cortical axon projections. The Nfi gene family is made up of four members, Nfia, Nfib, Nfic and Nfix. Expression analyses show that Nfia, Nfib and Nfix are expressed in the developing telencephalon. They play roles in patterning, glial development, cortical cell migration and axon guidance. We review the role of these genes in cortical cell migration, glial development and the formation of cortical axon projections, and examine the overlapping mutant phenotypes between the Emx and Nfi gene families.

Hypothalamic imprinting by gonadal steroid hormones.

The results of more than four decades of research on different mammalian species have established that the brain, like the rest of the reproductive system, is esentially basically female. For the male to develop structural and functional characteristics typical of his species, his brain must be exposed to testicular hormones during a critical period, or critical periods, of development. As mammals, human beings are most likely subject to this process of the hormone-dependent sexual differentiation of the brain, but proving it will be difficult. Common sense ethics preclude experimental procedures such as castration of neonatal infants or exposing the female fetus to testosterone perinatally. Thus, scientists are restricted to the retrospective study of "Experiments of Nature." The results of such studies support to a degree a meaningful role of hormones in the development of the human brain. The concept of the sexual differentiation of brain structure and function has a potentially profound influence on clinical decisions with respect to sex assignment and clinical management of infants with ambiguous or poorly developed external genitalia. Because of the importance of a baby's sex in our culture, parents of such infants must be given consideration, but so should the infant whose hormonal environment prenatally may well have produced permanent changes in the structure and functional potential of his/her brain.

The murine Otp homeobox gene plays an essential role in the specification of neuronal cell lineages in the developing hypothalamus.

Hypothalamic nuclei, including the anterior periventricular (aPV), paraventricular (PVN), and supraoptic (SON) nuclei strongly express the homeobox gene Orthopedia (Otp) during embryogenesis. Targeted inactivation of Otp in the mouse results in the loss of these nuclei in the homozygous null neonates. The Otp null hypothalamus fails to secrete neuropeptides somatostatin, arginine vasopressin, oxytocin, corticotropin-releasing hormone, and thyrotropin-releasing hormone in an appropriate spatial and temporal fashion, and leads to the death of Otp null pups shortly after birth. Failure to produce these neuropeptide hormones is evident prior to E15.5, indicating a failure in terminal differentiation of the aPV/PVN/SON neurons. Absence of elevated apoptotic activity, but reduced cell proliferation together with the ectopic activation of Six3 expression in the presumptive PVN, indicates a critical role for Otp in terminal differentiation and maturation of these neuroendocrine cell lineages. Otp employs distinct regulatory mechanisms to modulate the expression of specific molecular markers in the developing hypothalamus. At early embryonic stages, expression of Sim2 is immediately downregulated as a result of the absence of Otp, indicating a potential role for Otp as an upstream regulator of Sim2. In contrast, the regulation of Brn4 which is also expressed in the SON and PVN is independent of Otp function. Hence no strong evidence links Otp and Brn4 in the same regulatory pathway. The involvement of Otp and Sim1 in specifying specific hypothalamic neurosecretory cell lineages is shown to operate via distinct signaling pathways that partially overlap with Brn2.

ARNT2 acts as the dimerization partner of SIM1 for the development of the hypothalamus.

One major function of the hypothalamus is to maintain homeostasis by modulating the secretion of pituitary hormones. The paraventricular (PVN) and supraoptic (SON) nuclei are major integration centers for the output of the hypothalamus to the pituitary. The bHLH-PAS transcription factor SIM1 is crucial for the development of several neuroendocrine lineages within the PVN and SON. bHLH-PAS proteins require heterodimerization for their function. ARNT, ARNT2, and BMAL1 are the three known general heterodimerization partners for bHLH-PAS proteins. Here, we provide evidence that Sim1 and Arnt2 form dimers in vitro, that they are co-expressed in the PVN and SON, and that their loss of function affects the development of the same sets of neuroendocrine cell types within the PVN and SON. Together, these results implicate ARNT2 as the in vivo dimerization partner of SIM1 in controlling the development of these neuroendocrine lineages.