Biosynthesis of progesterone derived neurosteroids by developing avian CNS: in vitro effects on the GABAA receptor complex

Early embryonic modification of maternal hormones differs systematically among embryos of different laying order: A study in birds

Vertebrate embryos are exposed to maternal hormones that can profoundly affect their later phenotype. Although it is known that the embryo can metabolize these maternal hormones, the metabolic outcomes, their quantitative dynamics and timing are poorly understood. Moreover, it is unknown whether embryos can adjust their metabolic activity to, for example, hormones or other maternal signals. We studied the dynamics of maternal steroids in fertilized and unfertilized rock pigeon eggs during early incubation. Embryos of this species are naturally exposed to different amounts of maternal steroids in the egg according to their laying position, which provides a natural context to study differential embryonic regulation of the maternal signals. We used mass spectrometric analyses to map changes in the androgen and estrogen pathways of conversion. We show that the active hormones are heavily metabolized only in fertilized eggs, with a corresponding increase in supposedly less potent metabolites already within one-fourth of total incubation period. Interestingly, the rate of androgen metabolism was different between embryos in different laying positions. The results also warrant a re-interpretation of the timing of hormone mediated maternal effects and the role of the supposedly biologically inactive metabolites. Furthermore, the results also provide a potential solution as to how the embryo can prevent maternal steroids in the egg from interfering with its sexual differentiation processes as we show that the embryo can metabolize most of the maternal steroids before sexual differentiation starts.

Neurosteroid biosynthesis: enzymatic pathways and neuroendocrine regulation by neurotransmitters and neuropeptides

Neuroactive steroids synthesized in neuronal tissue, referred to as neurosteroids, are implicated in proliferation, differentiation, activity and survival of nerve cells. Neurosteroids are also involved in the control of a number of behavioral, neuroendocrine and metabolic processes such as regulation of food intake, locomotor activity, sexual activity, aggressiveness, anxiety, depression, body temperature and blood pressure. In this article, we summarize the current knowledge regarding the existence, neuroanatomical distribution and biological activity of the enzymes responsible for the biosynthesis of neurosteroids in the brain of vertebrates, and we review the neuronal mechanisms that control the activity of these enzymes. The observation that the activity of key steroidogenic enzymes is finely tuned by various neurotransmitters and neuropeptides strongly suggests that some of the central effects of these neuromodulators may be mediated via the regulation of neurosteroid production.

Birth and hatching: Key events in the onset of awareness in the lamb and chick

The neuroanatomical and neurophysiological development of the embryo and fetus and unique features of the physiological environment of the fetal brain, features which are lost at birth, support recent conclusions that under normal circumstances awareness (or consciousness) is probably not exhibited by the ovine embryo-fetus before birth and that it appears for the first time only after birth. However, there has apparently been no evaluation of whether or not similar mechanisms modulate awareness-related functions in domestic chicks before and after hatching. This comparative review, in seeking to rectify this, arrived at the following conclusions. First, the neural apparatus of both lambs and chicks appears to be too immature to support any states resembling awareness during at least the first half of pregnancy or incubation. Second, electroencephalographic (EEG) activity, which evolves subsequently, shows that states of sleep-like unconsciousness are likely to be continuously present in lambs until after birth, and that such states at least predominate in chicks until after hatching. Third, as in fetal lambs, epochs of so-called 'wakefulness' previously reported in chick embryos do not seem likely to represent short periods of awareness in ovo. Fourth, several neurosuppressive mechanisms, with some unique features, also operate or have the potential to operate in chicks before hatching, but a dearth of published information currently hinders a full comparison with those demonstrated to operate in fetal lambs. Fifth, contradicting the intuitive perception that vocalisation pre-hatching by the chick indicates the presence of awareness, published evidence suggests that vocalisation before and during hatching occurs mostly during EEG states indicating sleep-like unconsciousness. Sixth, as seems to be the case for newborn lambs after birth, it is possible that demonstrable awareness may appear for the first time only after hatching in chicks, presumably through waning neurosuppression and burgeoning neuroactivation, but such awareness seems to take longer to manifest itself. However, additional research in chicks is recommended to further assess this suggestion. Particular attention should be given to the status of vocal interactions between hen and chick which begin several days before hatching, and to the operation of neurosuppressive and neuroactivating mechanisms throughout the last 40% of incubation and during and after hatching.

Seasonal changes in neurosteroid concentrations in the amphibian brain and environmental factors regulating their changes

Up to now the regulatory mechanisms, which govern the concentrations of neurosteroids in the brain, are unclear. Seasonal breeders may serve as excellent models to understand physiological changes in neurosteroid levels and their regulatory mechanisms. The present study first investigated immunohistochemically the localization of cytochrome p450 side-chain cleavage enzyme (p450scc) and 3beta-hydroxysteroid dehydrogenase/delta(5)-delta(4)-isomerase (3betaHSD) in the brain of the newt Cynops pyrrhogaster, a seasonally breeding amphibian. Both p450scc- and 3betaHSD-like immunoreactive cells were restricted to the preoptic area. Seasonal changes in neurosteroid concentrations were then examined using adult males. Pregnenolone concentrations in the brain showed marked changes during annual breeding cycle and a maximal level in August, independent of the plasma steroid levels which were all low throughout the year. Progesterone concentrations in the brain, which were lower than pregnenolone levels, also showed peaks in April and August. In contrast, the pregnenolone sulfate level was low and its change was less pronounced. To clarify environmental factors that induce seasonal changes in neurosteroid levels, adult males were further subjected to different photoperiods and ambient temperatures for 5 weeks. Both pregnenolone and progesterone concentrations in the brain were significantly higher in the long day (LD) group than in the short day (SD) group, whereas no significant effects of different ambient temperatures on neurosteroid levels were detected. These results suggest that the newt brain possesses steroidogenic enzymes p450scc and 3betaHSD and exhibits seasonal changes in the pregnenolone and progesterone concentrations. Photoperiod may be a more important environmental factor than temperature for the regulation of neurosteroid levels in the brain.

In ovo chronic neurosteroid treatment affects the function and allosteric interactions of GABAA receptor modulatory sites

We investigated the effects of in ovo chronic administration of the endogenous neurosteroid epipregnanolone (5beta-pregnan-3beta-ol-20-one) on the GABA(A) receptor complex present in chick optic lobe synaptic membranes. Chronic epipregnanolone treatment failed to exert any effect on the chick optic lobe total protein content and wet weight at the different doses tested. [3H]Flunitrazepam control binding remained unaltered after neurosteroid exposure, however, the positive allosteric modulation of this ligand by 4 microM allopregnanolone was reduced in a dose-dependent manner by neurosteroid treatment. Embryo exposure to 30 microM epipregnanolone decreased allopregnanolone EC(50) and E(max) values. Analyses of saturation binding isotherms disclosed that such administration had no effect on K(d) and B(max) values for [3H]flunitrazepam and [3H]GABA binding. [3H]GABA binding modulation disclosed an increase in allopregnanolone EC(50) value with a decrease in its E(max) value. Diazepam EC(50) and E(max) values were enhanced, while low affinity sodium pentobarbital EC(50) value was reduced by epipregnanolone treatment. The investigation of the GABA(A) receptor function revealed that administration of this neurosteroid reduces the efficacy of GABA to induce 36Cl(-) influx into microsacs prepared from chick optic lobe. These results indicate that endogenous neurosteroid epipregnanolone chronically administered in ovo produces homologous uncoupling between steroid modulatory sites, and those corresponding to benzodiazepine and GABA receptors. Thus epipregnanolone is able to induce heterologous changes in the allosteric linkage between benzodiazepine and barbiturate modulatory sites, and the GABA receptor site. Taken jointly with results on epipregnanolone enhancing effects on [3H]flunitrazepam and [3H]GABA binding, in the context of its endogenous synthesis, our present findings support this neurosteroid as the endogenous modulator of GABA(A) receptor sites and function during chick optic lobe development.

Novel brain function: biosynthesis and actions of neurosteroids in neurons

Peripheral steroid hormones act on brain tissues through intracellular receptor-mediated mechanisms to regulate several important brain neuronal functions. Therefore, the brain is considered to be a target site of steroid hormones. However, it is now established that the brain itself also synthesizes steroids de novo from cholesterol. The pioneering discovery of Baulieu and his colleagues, using mammals, and our studies with non-mammals have opened the door of a new research field. Such steroids synthesized in the brain are called neurosteroids. Because certain structures in vertebrate brains have the capacity to produce neurosteroids, identification of neurosteroidogenic cells in the brain is essential to understand the physiological role of neurosteroids in brain functions. Glial cells are generally accepted to be the major site for neurosteroid formation, but the concept of neurosteroidogenesis in brain neurons has up to now been uncertain. We recently demonstrated neuronal neurosteroidogenesis in the brain and indicated that the Purkinje cell, a typical cerebellar neuron, actively synthesizes several neurosteroids de novo from cholesterol in both mammals and non-mammals. Pregnenolone sulfate, one of neurosteroids synthesized in the Purkinje neuron, may contribute to some important events in the cerebellum by modulating neurotransmission. Progesterone, produced as a neurosteroid in this neuron only during neonatal life, may be involved in the promotion of neuronal and glial growth and neuronal synaptic contact in the cerebellum. More recently, biosynthesis and actions of neurosteroids in pyramidal neurons of the hippocampus were also demonstrated. These serve an excellent model for the study of physiological roles of neurosteroids in the brain, because both cerebellar Purkinje neurons and hippocampal neurons play an important role in memory and learning. This paper summarizes the advances made in our understanding of neurosteroids, produced in neurons, and their actions.

Neuroactive steroids and the constraint of memory

Different normo- and pathophysiological conditions are associated with large variations in plasma and brain concentrations of neuroactive steroids. In an attempt to specify the possible role of these steroids in memory processes, we examined the ability of pregnanolone, a positive modulator of the gamma-aminobutyric acid type A (GABAA) receptor complex, to sustain state dependence in rats. Animals treated with either saline or different doses of pregnanolone were trained to complete a fixed ratio 10 (FR10) schedule of lever presses for milk reward within 120 s, and were tested for the retention of this response 48 h later while treated with the same or a different treatment. The data indicate that saline-to-drug as well as drug-to-saline state changes produced robust failures to recall the response. Furthermore, animals trained with pregnanolone showed transfer of the response when tested with the benzodiazepine chlordiazepoxide and vice versa. The partial benzodiazepine inverse agonist N-methyl-beta-carboline-3-carboxamide (FG-7142) antagonized the states produced by both pregnanolone and chlordiazepoxide. State changes constitute a mechanism of action that may operate endogenously; the release of neuroactive steroids in response to various physiological conditions may act to contain but also to constrain memories associated with these events, rendering these memories inaccessible on other occasions. The apparent memory impairment that can so be produced may render the effects of past experience available in a manner that is appropriately selective.

Neurosteroid biosynthesis in vertebrate brains

In mammals, neurosteroids are now known to be synthesized de novo in the brain as well as other areas of the nervous system through mechanisms at least partly independent of the peripheral steroidogenic glands. However, limited information is available on neurosteroids in non-mammalian vertebrates. We therefore have attempted to demonstrate neurosteroid biosynthesis in the brain of birds and amphibians. These vertebrate brains possessed the steroidogenic enzymes, cytochrome P450 side-chain cleavage enzyme (P450scc) and 3beta-hydroxysteroid dehydrogenase/delta5-delta4-isomerase (3beta-HSD), and produced pregnenolone, pregnenolone sulfate ester and progesterone from cholesterol. Significant seasonal changes in neurosteroids in the brain were observed in seasonally breeding vertebrates. In addition, we attempted to identify the cell type involved in neurosteroidogenesis in mammalian and non-mammalian vertebrates in order to understand the physiological role of neurosteroids. Glial cells are generally accepted to be the primary site for neurosteroid formation, but the concept of neurosteroidogenesis in brain neurons has up to now been uncertain. We recently demonstrated neuronal neurosteroidogenesis in the brain and indicated that the Purkinje cell, a typical cerebellar neuron, actively synthesizes several neurosteroids de novo from cholesterol in both mammals and non-mammals. This paper summarizes the advances made in our understanding of neurosteroid biosynthesis, including neuronal neurosteroidogenesis, in a variety of vertebrate types.

GABA-stimulated chloride uptake during avian CNS development: modulation by neurosteroids

In the present report we studied the GABA-stimulated 36Cl- uptake during chick optic lobe development in order to establish the ontogenetic profile of the functional GABAA receptor complex. A concentration-dependent stimulation of 36Cl- influx by GABA was demonstrated, starting at developmental stages as early as 10 days of incubation. The maximal GABA-induced 36Cl- uptake changed significantly during ontogeny with highest values near hatching. However, GABA potency to stimulate ion influx remained unchanged. We also examined the effect of two neurosteroids, allopregnanolone and epipregnanolone, on GABA-stimulated 36Cl- influx at three developmental stages (embryonic day 14, post-hatching day 1 and adult stage). Both steroids enhanced ion uptake in a concentration-dependent manner, exerting greater stimulatory effects at early developmental stages. Allopregnanolone displayed EC50 values lower than epipregnanolone at all three time points and was also more potent at post-hatching stages. Analysis of the GABA concentration-effect curve disclosed that both steroid decreased EC50 values for GABA stimulation while Emax levels were unaffected. In conclusion, our results showed an early appearance of the GABA-associated chloride channel together with the ability of neurosteroids to modulate GABA-gating of such channel.