Supraoptic oxytocin and vasopressin neurones show differential sensitivity to the neurosteroid pregnenolone sulphate

Prolactin induces a hyperpolarising current in rat paraventricular oxytocinergic neurones

Prolactin and oxytocin are important reproductive hormones implicated in several common adaptive functions during pregnancy, pseudopregnancy and lactation. Recently, extracellular recordings of supraoptic neurones have shown that prolactin may modulate the electrical activity of oxytocinergic neurones. However, no study has been conducted aiming to establish whether prolactin directly influences this activity in oxytocinergic paraventricular neurones. In the present study, we addressed this question by studying the effects of prolactin on the electrical activity and voltage-current relationship of identified paraventricular neurones in rat brain slices. Whole-cell recordings were obtained and neurones were classified on the basis of their morphological and electrophysiological fingerprint (i.e. magnocellular or parvicellular) and neuropeptide phenotype (i.e. oxytocinergic or non-oxytocinergic). We report that prolactin elicited a hyperpolarising current in 37% of the neurones in this nucleus, of which the majority (67%) were identified as putative magnocellular oxytocin neurones and the reminder (33%) were regarded as oxytocin-negative, parvicellular neuroendocrine neurones. Our results suggest that, in addition to the well-established negative feedback loop between prolactin-secreting lactotrophs and dopaminergic neurones in the arcuate nucleus, an inhibitory feedback loop also exists between lactotrophs and oxytocinergic paraventricular neurones.

Pregnenolone sulfate enhances long-term potentiation in CA1 in rat hippocampus slices through the modulation of N-methyl-D-aspartate receptors

Among the different steroids found in the brain, pregnenolone sulfate (3beta-hydroxy-5-pregnen-20-one-3-sulfate; PREGS) is known to enhance hippocampal-associated memory. The present study employs rat hippocampal slices to investigate the ability of PREGS to modulate long-term potentiation (LTP), a phenomenon considered as a model of synaptic plasticity related to memory processes. LTP (3 x 100 Hz/1 sec within 2 min), implicated essentially glutamatergic transmission, for which the different synaptic events could be pharmacologically dissociated. We show that PREGS enhances LTP in CA1 pyramidal neurons at nanomolar concentrations and exhibits a bell-shaped concentration-response curve. The maximal effect of PREGS on both induction and maintenance phases of LTP is observed at 300 nM and requires 10 min of superfusion. Although PREGS does not change the N-methyl-D-aspartate (NMDA) component of the field potentials (fEPSPs) isolated in the presence of 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in Mg2+-free artificial cerebrospinal fluid, PREGS does enhance the response induced by NMDA application (50 microM, 20 sec). PREGS does not modify the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) component of the fEPSPs isolated in the presence of 100 microM DL-2-amino-7-phosphopentanoic acid (DL-AP5) or its potentiation induced by a single tetanic stimulation and the response induced by AMPA application (10 microM, 10 sec). Furthermore, PREGS does not affect the recurrent inhibition of the fEPSPs mediated by gamma-aminobutyric acid type A (GABA(A)) receptor. In conclusion, this study shows the ability of PREGS to enhance LTP in CA1 by accentuating the activity of NMDA receptors. This modulation of LTP might mediate the steroid-induced enhancement of memory.

Excitatory effects of oxytocin and cholecystokinin on oxytocin neurones: differences between virgin, pregnant and lactating rats

Extracellular recordings were made from supraoptic oxytocin (OT) neurones in rat brain slices to investigate the effect of reproductive state on their excitation by OT and cholecystokinin (CCK). Stable background activity was induced by local glutamate application, and OT neurones were identified by their continuous firing and opioid inhibition. The excitatory effect of OT (indicated by percentage neurones excited, peak increment in firing, and response magnitude) was maximal in lactating rats, intermediate in virgin, and lowest in pregnant animals. Excitation by CCK was high in both lactating and virgin rats but, as with OT tests, was minimal in the pregnant group. These adaptations in the responses to OT and CCK may help to ensure that the firing of OT neurones is appropriately regulated to meet changing demands for OT release.

Neuroactive steroid mechanisms and GABA type A receptor subunit assembly in hypothalamic and extrahypothalamic regions

Gonadal- and neuronal-derived steroids are capable of altering brain functions through two basic mechanisms: slow (genomic) and rapid (novel nongenomic membrane) types of activities. The genomic activities that are circumscribed to the numerous neuronal and glial expressed receptor actions involve transcriptional processes regulated largely by classical steroids. On the other hand, rapid nongenomic activities are linked to the stereoselective interactions of potent neuroactive steroids. It appears that both of these steroid mechanisms can be successfully evoked at the ligand-gated heteroligomeric GABA type A receptor. However, the precise structural prerequisites and type of molecular steroid interactions implicated in this neuronal target have not been fully investigated. This article reviews the most common subunits (alpha, beta, and gamma) of the native GABA type A receptor involved in signaling pathways of slow and rapid steroidal mechanisms. Different beta-containing compositions (alpha1beta1-3gamma2) are necessary for the slow type of mechanism, whereas different alpha-containing constructs (alpha2-6beta 1/2 gamma2/2L) are linked to the rapid type. Because of the major role played by neuroactive steroids in GABA-dependent neuroendocrine and sociosexual events, distinction of the specific subunit combination is essential not only for elucidating neuronal communicative expressions during such events but also for elucidating their potential neuroprotective role in neurodegenerative disorders.

Physiological roles for the neurosteroid allopregnanolone in the modulation of brain function during pregnancy and parturition

Allopregnanolone is a well-established allosteric modulator of the GABAA receptor but its physiological roles within the nervous system remain unclear. Derived principally from circulating progesterone, allopregnanolone achieves its highest concentrations within the nervous system during late pregnancy and recent studies have now begun to elucidate its roles at this time in the rat. At the molecular level it is clear that the regulation of GABAA receptor subunit gene expression by progesterone and its derivatives occurs in a subunit- and a neuron-specific manner and that both progesterone and allopregnanolone are involved. At the cellular level, the increasing concentrations of allopregnanolone with advancing pregnancy can be shown to have important physiological actions in repressing the electrical activity of specific neuronal phenotypes such as the magnocellular oxytocin neurons. The marked fall in progesterone and allopregnanolone concentrations prior to parturition equally appears to have a substantial impact upon GABAA receptor signaling in the hippocampus, frontal cortex and oxytocin neurons. Together, studies at a basic level suggest that the rise and fall in allopregnanolone concentrations during pregnancy are likely to exert a powerful regulatory influence upon neurotransmission in a variety of brain networks. The temporal correlation between these events and the observed cognitive, psychiatric and physiological changes associated with pregnancy and the peri-partum period in humans is striking and warrants close attention.

Brain preparations for maternity--adaptive changes in behavioral and neuroendocrine systems during pregnancy and lactation. An overview

Pregnancy, parturition and lactation comprise a continuum of adaptive changes necessary for the development and maintenance of the offspring. The endocrine changes that are driven by the conceptus and are essential for the maintenance of pregnancy and are involved in the preparations for motherhood are outlined. These changes include large increases in the secretion of sex steroid hormones, and the secretion of peptide hormones that are unique to pregnancy. The ability of these pregnancy hormones to alter several aspects of brain function in pregnancy is considered, and the adaptive importance of some of these changes is discussed, for example in metabolic and body fluid adjustments, and the induction of maternal behavior. The importance of sex steroids in determining the timing of the various adaptive changes in preparing for parturition and maternal behavior is emphasized, and the concept that the actions of prolactin and oxytocin, quintessential mammalian motherhood neuropeptides, can serve to coordinate a spectrum of adaptive changes is discussed. The part played by oxytocin neurons and their regulatory mechanisms is reviewed to illustrate how neural systems involved in maternity are prepared in pregnancy via changes in phenotype, synaptic organization and in the relative importance of their different inputs, to function optimally when needed. For oxytocin neurons secreting from the posterior pituitary, important in parturition and essential in lactation, these changes include mechanisms to restrain their premature activation, and adaptations to support synchronized burst firing for pulsatile oxytocin secretion in response to stimulation via afferents from the birth canal, olfactory system or suckled nipples. Within the brain, expression of oxytocin receptors permits centrally released oxytocin to facilitate the expression of maternal behavior. Changes in other neuroendocrine systems are similarly extensive, leading to lactation, suppression of ovulation, reduced stress responses and increased appetite; these changes in lactation are driven by the suckling stimulus. The possible link between these adaptations and changes in cognition and mood in pregnancy and post partum are considered, as well as the dysfunctions that lead to common problems of depression and puerperal psychoses.

Neurosteroid modulation of GABA IPSCs is phosphorylation dependent

The neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one (allopregnanolone) facilitates GABA(A) receptor-mediated ionic currents via allosteric modulation of the GABA(A) receptor. Accordingly, allopregnanolone caused an increase in the slow decay time constant of spontaneous GABA-mediated IPSCs in magnocellular neurons recorded in hypothalamic slices. The allopregnanolone effect on IPSCs was inhibited by a G-protein antagonist as well as by blocking protein kinase C and, to a lesser extent, cAMP-dependent protein kinase activities. G-protein and protein kinase C activation in the absence of the neurosteroid had no effect on spontaneous IPSCs but enhanced the effect of subsequent allopregnanolone application. These findings together suggest that the neurosteroid modulation of GABA-mediated IPSCs requires G-protein and protein kinase activation, although not via a separate G-protein-coupled steroid receptor.