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Abstract The basic helix-loop-helix (bHLH) transcription factor Scl displays tissue-restricted expression and is critical for the establishment of the haematopoietic system; loss of Scl results in embryonic death due to absolute anaemia. Scl is also expressed in neurons of the mouse diencephalon, mesencephalon and metencephalon; however, its requirement in those sites remains to be determined. Here we report conditional deletion of Scl in neuronal precursor cells using the Cre/LoxP system. Neuronal-Scl deleted mice died prematurely, were growth retarded and exhibited an altered motor phenotype characterized by hyperactivity and circling. Moreover, ablation of Scl in the nervous system affected brain morphology with abnormal neuronal development in brain regions known to express Scl under normal circumstances; there was an almost complete absence of Scl-null neurons in the hindbrain and partial loss of Scl-null neurons in the thalamus and midbrain from early neurogenesis onwards. Our results demonstrate a crucial role for Scl in the development of Scl-expressing neurons, including gamma-aminobutyric acid (GABA)ergic interneurons. Our study represents one of the first demonstrations of functional overlap of a single bHLH protein that regulates neural and haematopoietic cell development. This finding underlines Scl's critical function in cell fate determination of mesodermal as well as neuroectodermal tissues.

The basic helix-loop-helix (bHLH) transcription factor stem cell leukaemia (SCL) is a 'master regulator' of haematopoiesis, where SCL is pivotal in cell fate determination and differentiation. SCL has also been detected in CNS, where other members of the bHLH-family have been shown to be indispensable for neuronal development; however, no detailed expression pattern of SCL has so far been described. We have generated a map of SCL expression in the embryonic and adult mouse brain based on histochemical analysis of LacZ reporter gene expression in sequential sections of brain tissue derived from SCL-LacZ knockin mice. The expression of LacZ was confirmed to reflect SCL expression by in situ hybridisation. LacZ expression was found in a range of different diencephalic, mesencephalic and metencephalic brain nuclei in adult CNS. Co-localisation of LacZ with the neuronal marker NeuN indicated expression in post-mitotic neurons in adulthood. LacZ expression by neurons was confirmed in tissue culture analysis. The nature of the pretectal, midbrain and hindbrain regions expressing LacZ suggest that SCL in adult CNS is potentially involved in processing of visual, auditory and pain related information. During embryogenesis, LacZ expression was similarly confined to thalamus, midbrain and hindbrain. LacZ staining was also evident in parts of the intermediate and marginal zone of the aqueduct and ventricular zone of the fourth ventricle at E12.5 and E14. These cells may represent progenitor stages of differentiating neural cells. Given the presence of SCL in both the developing brain and in post-mitotic neurons, it seems likely that the function of SCL in neuronal differentiation may differ from its function in maintaining the differentiated state of the mature neuron.

1. In the brain, glucocorticoids bind to both the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). These receptors show clearly distinct developmental patterns in the infant rat. 2. Low levels of GRs are present around the time of birth throughout the brain. Concentrations rise slowly, and do not achieve adult levels until the third week of life, approximately. GR affinity for corticosterone is higher perinatally than at later ages. Receptor microdistribution changes dramatically during ontogeny. In particular, certain regions, such as the suprachiasmatic nucleus of the hypothalamus, express high levels of receptor only during the first week of life. GRs may show impaired capacity to undergo transformation and/or nuclear translocation during the second postnatal week. Environmental manipulations during early ontogeny (e.g., early handling) may have permanent effects on GR capacity. 3. MRs are present at very low concentrations in the first days of life. Binding capacity rises rapidly thereafter and resembles that found in the adult by the end of 1 week. Neither binding affinity in vitro nor overall distribution changes with age. As in the adult, low doses of corticosterone, in vivo, bind mainly to the MRs. Levels of corticosterone are low and relatively unperturbable in the intact infant rat. It is likely, therefore, that most of the physiological actions of this hormone during this period are mediated by the MR.

The present study examined the stress responsiveness of the hypothalamic-pituitary-adrenal axis in relation to the properties of corticosteroid receptors in the brain and pituitary in old (30 months) and young (3 months) male Brown Norway rats. The data demonstrate that circulating ACTH rather than the corticosteroid plasma level was elevated under basal conditions and following stress. Furthermore, a reduction of mineralocorticoid receptor (MR) number in the hippocampus and of glucocorticoid receptor (GR) number in the hypothalamus and the pituitary correspond to increased neuroendocrine responsiveness and negative feedback following stress. The changes in receptor binding do not parallel the changes in the amount of MR and GR mRNA measured with in situ hybridization. This suggests that the processing rather than the receptor gene expression is affected in senescence.

The distribution of vasotocin (VT)-immunoreactive neuronal perikarya and fibers in the canary (Serinus canaria) was investigated with immunohistological techniques. The results suggest that most VT-stained cell bodies are located in three diencephalic regions. First, a large number of densely packed neurons are found in the paraventricular nucleus (PVN) and the anterior preoptic nucleus. Neurons here vary widely in size and shape. Small-size rounded neurons and large-size multipolar neurons appear to concentrate in separate subdivisions. Second, a series of loosely organized cell groups of medium- to large-size cells occurs in the lateral parts of the hypothalamus. These aggregates of neurons apparently correspond to subdivisions of the supraoptic nucleus (SON). Third, diffusely distributed, lightly stained cells are found dorsal to the paraventricular nucleus in the dorsal diencephalon. A number of cells of this group seem to be located in the basal septal area and bed nucleus of the stria terminalis. Immunoreactive fibers and varicosities concentrate in brain regions that are associated with neuroendocrine, autonomic, and limbic functions. Axons from the PVN and SON form compact bundles of the hypothalamohypophysial tract in the lateral hypothalamus and then funnel into the internal zone of the medium eminence (ME). Furthermore, a heavy innervation seems to be present in the palisadal, external zone of the ME. A substantial number of fibers appear to leave the PVN toward extrahypothalamic areas. Most extrahypothalamic VT fibers innervate telencephalic and brainstem regions that are thought to be involved in mediation of limbic and autonomic functions. These areas include the lateral and medial septum, the lateral habenula, the substantia grisea centralis, the area ventralis (Tsai), the locus coeruleus, raphe nuclei, the nucleus tractus solitarii, and lateral medulla. In addition, fibers with immunoreactivity for VT innervate structures such as the optic tectum and the nucleus ovoidalis that have been implicated in sensory processing of visual and auditory information. Finally, VT fibers and varicosities occur in centers including the nucleus robustus archistriatalis and nucleus intercollicularis that have been implicated in vocal control.

In vitro cytosol binding, receptor autoradiography with radiolabelled corticosteroid analogs, and immunocytochemistry with monoclonal antibodies have revealed the presence of two receptor systems for corticosteroids in rat and hamster brains. The type I receptor is found mainly in the hippocampal region, and in the hamster it binds cortisol (F) and corticosterone (B) with similar affinity while in the rat (a species which unlike the hamster secretes solely B) the type I receptor shows high affinity to B and not to F. The type II receptor is more widely distributed in the brain and it binds to F (hamster) or B (rat) with affinity 4-6-fold lower than to the type I. in vivo, the hamster type I and II retain F much more than B while those in the rat show the opposite. In conclusion, the present study clearly indicates species-specificity in type I and type II receptor systems in these animals. Furthermore, the type I receptor displays in vivo stringent preference for retention of the animal's predominantly circulating corticosteroid (F in hamster, in B in rat).