Neurosteroids: biosynthesis and function of these novel neuromodulators

Localization of p450scc and 5alpha-reductase type-2 in the cerebellum of fetal and newborn sheep

Prenatally, neuroactive steroids that modulate GABAergic activity may be synthesized de novo within the fetal brain. We have examined changes in immunoreactivity staining for the steroidogenic enzymes cholesterol P450 side-chain cleavage (P450scc), and 5alpha-reductase type-2 in the cerebellum of late gestation (130-145 days gestation) fetal sheep and newborn lambs (1-4 weeks of age). Both enzymes were predominantly localized in the Purkinje cell body and dendrites of the fetal and newborn cerebellum, with weaker immunoreactivity in a few cells of the inner granular layer. P450scc immunoreactivity was present in Purkinje neurons expressing either of the neuronal microtubule associated proteins MAP1b/5 or MAP2a/b, but was absent from GFAP and HNK-1 positive cells. Soma of Purkinje neurons were also immunopositive for 5alpha-reductase type-2 in the fetuses, but expression decreased to just detectable levels in the 1-2 and 2-4 week old lambs. Both MAP1b/5- and MAP2a/b-positive Purkinje neurons showed 5alpha-reductase type-2 expression in the fetus, whereas the residual 5alpha-reductase staining in the newborn lamb was present only in MAP2a/b-positive Purkinje neurons. Allopregnanolone in the cerebellum decreased from 21.8+/-1.9 ng/g wet weight in fetuses at 140-145 days gestation to 6.7+/-0.5 ng/g in 2-4 week old lambs (P<0. 05). Thus, synthesis of neuroactive steroids from cholesterol is possible in cerebellar neurons in late gestation and persists into neonatal life, 5alpha-reductase type-2 expression is greater in the fetus compared to the neonate, and allopregnanolone concentrations in the cerebellum decrease significantly after birth.

Modulatory actions of steroid hormones and neuropeptides on electrical activity in brain

Electrophysiological studies over the past decades have shown that many compounds in addition to 'classical' neurotransmitters affect electrical activity in the brain. These compounds include neuropeptides synthesized in brain as well as compounds which are released from peripheral sources and subsequently enter the brain compartment, such as corticosteroid hormones from the adrenal gland. In the present review, this principle is illustrated by describing the effects of two substances, i.e. vasopressin and corticosterone. Neuropeptides and corticosteroid hormones add at least two essential aspects to information processing in the brain. First, they both act conditional, i.e. they modulate the actions of 'classical' neurotransmitters, rather than changing basal neuronal activity by themselves. Second, the time-frame in which modulation of electrical properties takes place differs from that generally seen with 'classical' neurotransmitters. Neuropeptides modulate electrical activity over a period of minutes, while effects of corticosteroid hormones usually become apparent after at least an hour but then last for hours. In this way, neuropeptides and steroid hormones expand the repertoire of responses through which the brain reacts to environmental challenges.

RSH/Smith-Lemli-Opitz syndrome: a multiple congenital anomaly/mental retardation syndrome due to an inborn error of cholesterol biosynthesis

The RSH/Smith-Lemli-Opitz syndrome (RSH/SLOS) is an autosomal recessive multiple congenital anomaly/mental retardation syndrome caused by an inborn error of cholesterol biosynthesis. The RSH/SLOS phenotypic spectrum is broad; however, typical features include microcephaly, ptosis, a small upturned nose, micrognathia, postaxial polydactaly, second and third toe syndactaly, genital anomalies, growth failure, and mental retardation. RSH/SLOS is due to a deficiency of the 3beta-hydroxysterol Delta(7)-reductase, which catalyzes the reduction of 7-dehydrocholesterol (7-DHC) to cholesterol. This inborn error of cholesterol biosynthesis results in elevated serum and tissue 7-DHC levels. The 3beta-hydroxysterol Delta(7)-reductase gene (DHCR7) maps to chromosome 11q12-13, and to date 66 different mutations of this gene have been identified in RSH/SLOS patients. Identification of the biochemical basis of RSH/SLOS has led to development of therapeutic regimens based on dietary cholesterol supplementation and has increased our understanding of the role cholesterol plays during embryonic development.