Neurosteroid modulation of the GABAA receptor in the developing guinea pig cerebral cortex

Injury of the developing cerebellum: a brief review of the effects of endotoxin and asphyxial challenges in the late gestation sheep fetus

The vulnerability of the fetal and newborn brain to events in utero or at birth that cause damage arising from perturbations of cerebral blood flow and metabolism, such as the accumulation of free radicals and excitatory transmitters to neurotoxic levels, has received considerable attention over the last few decades. Attention has usually been on the damage to cerebral structures, particularly, periventricular white matter. The rapid growth of the cerebellum in the latter half of fetal life in species with long gestations, such as the human and sheep, suggests that this may be a particularly important time for the development of cerebellar structure and function. In this short review, we summarize data from recent studies with fetal sheep showing that the developing cerebellum is particularly sensitive to infectious processes, chronic hypoxia and asphyxia. The data demonstrates that the cerebellum should be further studied in insults of this nature as it responds differently to the remainder of the brain. Damage to this region of the brain has implications not only for the development of motor control and posture, but also for higher cognitive processes and the subsequent development of complex behaviours, such as learning, memory and attention.

Neurosteroids in the fetus and neonate: Potential protective role in compromised pregnancies

Complications during pregnancy and birth asphyxia lead to brain injury, with devastating consequences for the neonate. In this paper we present evidence that the steroid environment during pregnancy and at birth aids in protecting the fetus and neonate from asphyxia-induced injury. Earlier studies show that the placental progesterone production has a role in the synthesis and release of neuroactive steroids or their precursors into the fetal circulation. Placental precursor support leads to remarkably high concentrations of allopregnanolone in the fetal brain and to a dramatic decline with the loss of the placenta at birth. These elevated concentrations influence the distinct behavioral states displayed by the late gestation fetus and exert a suppressive effect that maintains sleep-like behavioral states that are present for much of fetal life. This suppression reduces CNS excitability and suppresses excitotoxicity. With the availability of adequate precursors, mechanisms within the fetal brain ultimately control neurosteroid levels. These mechanisms respond to episodes of acute hypoxia by increasing expression of 5alpha-reductase and P450scc enzymes and allopregnanolone synthesis in the brain. This allopregnanolone response, and potentially that of other neurosteroids including 5alpha-tetrahydrodeoxycorticosterone (TH-DOC), reduces hippocampal cell death following acute asphyxia and suggests that stimulation of neurosteroid production may protect the fetal brain. Importantly, inhibition of neurosteroid synthesis in the fetal brain increases the basal cell death suggesting a role in controlling developmental processes late in gestation. Synthesis of neurosteroid precursors in the fetal adrenal such as deoxycorticosterone (DOC), and their conversion to active neurosteroids in the fetal brain may also have a role in neuroprotection. This suggests that the adrenal glands provide precursor DOC for neurosteroid synthesis after birth and this may lead to a switch from allopregnanolone alone to neuroprotection mediated by allopregnanolone and TH-DOC.

Neurosteroid regulation of central nervous system development

Neurosteroids are a relatively new class of neuroactive compounds brought to prominence in the past 2 decades. Despite knowing of their presence in the nervous system of various species for over 20 years and knowing of their functions as GABA(A) and N-methyl-d-aspartate (NMDA) ligands, new and unexpected functions of these compounds are continuously being identified. Absence or reduced concentrations of neurosteroids during development and in adults may be associated with neurodevelopmental, psychiatric, or behavioral disorders. Treatment with physiologic or pharmacologic concentrations of these compounds may also promote neurogenesis, neuronal survival, myelination, increased memory, and reduced neurotoxicity. This review highlights what is currently known about the neurodevelopmental functions and mechanisms of action of 4 distinct neurosteroids: pregnenolone, progesterone, allopregnanolone, and dehydroepiandrosterone (DHEA).

A 17beta-derivative of allopregnanolone is a neurosteroid antagonist at a cerebellar subpopulation of GABA A receptors with nanomolar affinity

BACKGROUND AND PURPOSE

High-affinity, subtype-selective antagonists of the neurosteroid binding sites of GABA(A) receptors are not available. We have characterized an allopregnanolone derivative as an antagonist of cerebellar GABA(A) receptors with nanomolar affinity.

EXPERIMENTAL APPROACH

Receptor binding and electrophysiological methods were used for the allosteric modulation of cerebellar GABA(A) receptors by an allopregnanolone derivative, (20R)-17beta-(1-hydroxy-2,3-butadienyl)-5alpha-androstane-3alpha-ol (HBAO). GABA(A) receptors of rat cerebellar membranes were labelled with the chloride channel blocker [(3)H]ethynylbicycloorthobenzoate (EBOB). The ionophore function of GABA(A) receptors was studied by whole-cell patch clamp electrophysiology in cultured rat cerebellar granule and cortical cells.

KEY RESULTS

Partial displacement of cerebellar [(3)H]EBOB binding by nanomolar HBAO was attenuated by 0.1 mM furosemide, an antagonist of alpha(6) and beta(2-3) subunit-containing GABA(A) receptors. Displacement curves of HBAO were reshaped by 30 nM GABA and shifted to the right. However, the micromolar potency of full displacement by allopregnanolone was not affected by 0.1 mM furosemide or 30 nM GABA. The nanomolar, but not the micromolar phase of displacement of [(3)H]EBOB binding by GABA was attenuated by 100 nM HBAO. Submicromolar HBAO did not affect [(3)H]EBOB binding to cortical and hippocampal GABA(A) receptors. HBAO up to 1 microM did not affect chloride currents elicited by 0.3-10 microM GABA, while it abolished potentiation by 1 microM allopregnanolone with nanomolar potency in cerebellar but not in cortical cells. Furosemide attenuated cerebellar inhibition by 100 nM HBAO.

CONCLUSIONS AND IMPLICATIONS

HBAO is a selective antagonist of allopregnanolone, a major endogenous positive modulator via neurosteroid sites of cerebellar (probably alpha(6)beta(2-3)delta) GABA(A) receptors.

Stress in pregnancy activates neurosteroid production in the fetal brain

Neurosteroids such as allopregnanolone are potent agonists at the GABA(A) receptor and suppress the fetal CNS activity. These steroids are synthesized in the fetal brain either from cholesterol or from circulating precursors derived from the placenta. The concentrations of allopregnanolone are remarkably high in the fetal brain and rise further in response to acute hypoxic stress, induced by constriction of the umbilical cord. This response may result from the increased 5alpha-reductase and cytochrome P-450(SCC) expression in the brain. These observations suggest that the rise in neurosteroid concentrations in response to acute hypoxia may represent an endogenous protective mechanism that reduces excitotoxicity following hypoxic stress in the developing brain. In contrast to acute stress, chronic hypoxemia induces neurosteroidogenic enzyme expression without an increase in neurosteroid concentrations and, therefore, may pose a greater risk to the fetus. At birth, the allopregnanolone concentrations in the brain fall markedly, probably due to the loss of placental precursors; however, stressors, including hypoxia and endotoxin-induced inflammation, raise allopregnanolone concentrations in the newborn brain. This may protect the newborn brain from hypoxia-induced damage. However, the rise in allopregnanolone concentrations was also associated with increased sleep. This rise in sedative steroid levels may depress arousal and contribute to the risk of sudden infant death syndrome. Our recent findings indicate that acute hypoxic stress in pregnancy initiates a neurosteroid response that may protect the fetal brain from hypoxia-induced cell death, whereas the decline in allopregnanolone levels after birth may result in greater vulnerability to brain injury in neonates.

Nanomolar allopregnanolone potentiates rat cerebellar GABAA receptors

The ionophore function of gamma-aminobutyric acid A (GABA(A)) receptors was studied by whole-cell patch clamp electrophysiology in primary cultures of rat cerebellar cortex. Chloride currents elicited by 1 microM GABA were potentiated by allopregnanolone with a plateau of high affinity (EC(50) = 14 nM) and a peak of potentiation around 1 microM allopregnanolone. Furosemide (0.1 mM) eliminated the high affinity phase and increased the EC(50) to 685 nM. GABA(A) receptors of rat cerebellar synaptosomal membranes were labelled with [(3)H]ethynylbicycloorthobenzoate (EBOB). Allopregnanolone displaced [(3)H]EBOB binding with IC(50) = 320 nM. The displacing potency of allopregnanolone was strongly enhanced (IC(50) = 39 nM) in the presence of 400 nM GABA and 60 nM SR 95531. Nanomolar potentiation by allopregnanolone can be associated with cerebellar GABA(A) receptors containing alpha(6), beta(2-3) and delta subunits. This might be suitable for physiological modulation of tonic inhibitory neurotransmission via extrasynaptic GABA(A) receptors in cerebellar granule cells by neurosteroids.

Pregnenolone and dehydroepiandrosterone administration in neonatal rats alters the immunoreactivity of hippocampal synapsin I, neuropeptide Y and glial fibrillary acidic protein at post-puberty

It is well documented that neurosteroids administered during the neonatal period influence the development of several brain systems. In our previous study, pregnenolone administered to rats during the neonatal period altered adenosinergic and dopaminergic functions in the striatum and cerebral cortex. The present study examined the effects of the treatment with pregnenolone and dehydroepiandrosterone (DHEA) from the postnatal day (P) 3-P7 on synapsin I (a marker for presynaptic terminals) and glial fibrillary acidic protein (GFAP: a marker for astroglia) levels in the hippocampus of Sprague-Dawley rats at 3 and 7 weeks of age. In addition, neuropeptide Y and dynorphin A immunoreactivity was measured. The administration of pregnenolone and DHEA to neonatal rats significantly altered the expression of synapsin I in the dentate gyrus and CA3 region at post-puberty but not at pre-puberty. A significantly greater expression of GFAP-immunoreactive astrocytes or processes was demonstrated in the pregnenolone- and DHEA-treated groups at both pre-puberty and post-puberty. A significant increase in the number and size of GFAP-immunoreactive astrocytes and in the extension of arborization was seen in the overall hippocampus. The number of neuropeptide Y-positive cells in the hilus region was also significantly increased in the neurosteroid-treated group at post-puberty. No differences were detected in dynorphin A immunoreactivity among the experimental groups. These results of this study suggest that pregnenolone and DHEA play an important role in the development of hippocampus.

Increased neurosteroid sensitivity of hippocampal GABAA receptors during postnatal development

To investigate developmental changes in neurosteroid modulation of GABA(A) receptors, whole-cell currents were elicited by applying GABA with allopregnanolone or pregnenolone sulfate (PS) to dentate granule cells (DGCs), acutely isolated from 7-14-day-old and adult rats. GABA evoked larger currents from dentate granule cells acutely isolated from adult rats (adult DGCs) than from neonatal DGCs, due to increased efficacy (1662+/-267 pA in adult DGCs versus 1094+/-198 pA in neonatal DGCs, P=0.004), and current density (0.072+/-0.01 pA/microm(2) in neonatal rat DGCs to 0.178+/-0.02 pA/microm(2) in adult DGCs), but unchanged potency (EC(50) was 18.5+/-2 microm in adult DGCs, and 26.6+/-7.9 microm in neonatal DGCs, P=0.21). Allopregnanolone sensitivity of GABA(A) receptor currents increased during development due to an increased potency (21.1+/-4.7 nM in adult DGCs versus 94.6+/-9 nM in neonatal DGCs, P=0.0002). The potency and efficacy of PS inhibition of GABA(A) receptor currents were remained unchanged during development (13+/-6 microm and 13.2+/-5.9 microm, P=0.71 and 85.5%+/-3.5% and 83.6%+/-0.8%, P=0.29, respectively). To investigate possible mechanism of developmental changes in GABA(A) receptor properties, in situ hybridization for alpha1, alpha4 and gamma2 subunit mRNAs was performed in dentate gyrus of the two age groups. Qualitatively, alpha1 subunit mRNA was expressed at low levels in neonatal rats while it was well expressed in adult rats. The alpha4 and gamma2 subunits were well expressed in the dentate gyrus of adult and neonatal rats. Immunohistochemical staining for alpha1 subunit in hippocampal slices from neonatal and adult rats was examined under confocal laser scanning microscope. This demonstrated that cell bodies and dendrites of granule cells are moderately positive for the alpha1 staining in adult rats but weakly so in neonatal rats. Higher-magnification images demonstrate large number of clusters of alpha1-subunit in the cell bodies of dentate granule cells of adult rat but rare clusters in granule cells of neonatal rats. Maturation of GABA(A) receptors in DGCs is characterized by increased number of GABA(A) receptors that are more sensitive to endogenous neurosteroid allopregnanolone, which might be related to increased expression of alpha1 subunit.

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.