Neurosteroid-induced plasticity of immature synapses via retrograde modulation of presynaptic NMDA receptors

Pregnenolone sulfate enhances spontaneous glutamate release by inducing presynaptic Ca2+-induced Ca2+ release

Pregnenolone sulfate (PS) acts as an excitatory neuromodulator and has a variety of neuropharmacological actions, such as memory enhancement and convulsant effects. In the present study, we investigated the effect of PS on glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs) in acutely isolated dentate gyrus (DG) hilar neurons by use of a conventional whole-cell patch-clamp technique. PS significantly increased sEPSC frequency in a concentration-dependent manner without affecting the current amplitude, suggesting that PS acts presynaptically to increase the probability of spontaneous glutamate release. However, known molecular targets of PS, such as α7 nicotinic ACh, NMDA, σ1 receptors and voltage-dependent Ca(2+) channels, were not responsible for the PS-induced increase in sEPSC frequency. In contrast, the PS-induced increase in sEPSC frequency was completely occluded in a Ca(2+)-free external solution, and was significantly reduced by either the depletion of presynaptic Ca(2+) stores or the blockade of ryanodine receptors, suggesting that PS elicits Ca(2+)-induced Ca(2+) release (CICR) within glutamatergic nerve terminals. In addition, the PS-induced increase in sEPSC frequency was completely occluded by transient receptor potential (TRP) channel blockers. These data suggest that PS increases spontaneous glutamate release onto acutely isolated hilar neurons via presynaptic CICR, which was triggered by the influx of Ca(2+) through presynaptic TRP channels. The PS-induced modulation of excitatory transmission onto hilar neurons could have a broad impact on the excitability of hilar neurons and affect the pathophysiological functions mediated by the hippocampus.

Presynaptic NMDA Receptors and Spike Timing-Dependent Depression at Cortical Synapses

It has recently been discovered that some forms of timing-dependent long-term depression (t-LTD) require presynaptic N-methyl-d-aspartate (NMDA) receptors. In this review, we discuss the evidence for the presence of presynaptic NMDA receptors at cortical synapses and their possible role in the induction of t-LTD. Two basic models emerge for the induction of t-LTD at cortical synapses. In one model, coincident activation of presynaptic NMDA receptors and CB1 receptors mediates t-LTD. In a second model, CB1 receptors are not necessary, and the activation of presynaptic NMDA receptors alone appears to be sufficient for the induction of t-LTD.

Modulatory metaplasticity induced by pregnenolone sulfate in the rat hippocampus: a leftward shift in LTP/LTD-frequency curve

We recently have found that an acute application of the neurosteroid pregnenolone sulfate (PREGS) at 50 muM to rat hippocampal slices induces a long-lasting potentiation (LLP(PREGS)) via a sustained ERK2/CREB activation at perforant-path/granule-cell synapses in the dentate gyrus. This study is a follow up to investigate whether the expression of LLP(PREGS) influences subsequent frequency-dependent synaptic plasticity. Conditioning electric stimuli (CS) at 0.1-200 Hz were given to the perforant-path of rat hippocampal slices expressing LLP(PREGS) to induce long-term potentiation (LTP) and long-term depression (LTD). The largest LTP was induced at about 20 Hz-CS, which is normally a subthreshold frequency, and the largest LTD at 0.5 Hz-CS, resulting in a leftward-shift of the LTP/LTD-frequency curve. Furthermore, the level of LTP at 100 Hz-CS was significantly attenuated to give band-pass filter characteristics of LTP induction with a center frequency of about 20 Hz. The LTP induced by 20 Hz-CS (termed 20 Hz-LTP) was found to be postsynaptic origin and dependent on L-type voltage-gated calcium channel (L-VGCC) but not on N-methyl-D-aspartate receptor (NMDAr). Moreover, the induction of 20 Hz-LTP required a sustained activation of ERK2 that had been triggered by PREGS. In conclusion, the transient elevation of PREGS is suggested to induce a modulatory metaplasticity through a sustained activation of ERK2 in an L-VGCC dependent manner.

Alterations in neonatal neurosteroids affect exploration during adolescence and prepulse inhibition in adulthood

Allopregnanolone (AlloP) is a neurosteroid that plays an important role during neural development. Alterations of endogenous neonatal allopregnanolone levels alter the localisation and function of GABA neurons in the adult brain and affect behaviour in adulthood. We have carried out research into the effects of an increase (AlloP administration) or a decrease (administration of finasteride, inhibitor of the AlloP synthesis) of neonatal AlloP levels during the fifth to ninth postnatal days in male Wistar rats on the novelty exploration (Boissier test) at adolescent ages (40 and 60 days old), and on the prepulse inhibition achievement in adulthood (85 days). We also investigated the role of a GABA(A) modulator (midazolam, 1, 1.75 or 2.5mg/kg body weight) in the long-lasting behavioural changes in adulthood (85 days). Results indicate that neonatal finasteride decreases both novelty-exploration (head-dipping and locomotion) and anxiety-relevant scores (the distance travelled in and the number of entries into the central zone) at adolescent age, along with a reduction in body weight and general locomotion. Also, neonatal AlloP administration decreases prepulse inhibition in adulthood. Prepulse inhibition disruption was only partially reproduced decreasing the neonatal AlloP levels by means of finasteride administration. Although there was no interaction between neonatal neurosteroid manipulation and adult benzodiazepine treatments, the effects of midazolam were dose-dependent: the lowest dose of midazolam increased whereas the highest disrupted the expected progressive reduction of the startle response (and the consequent improvement of the PPI percentage) after the gradual increase in prepulse intensity. Reduced prepulse inhibition of startle provides evidence of deficient sensorimotor gating in several disorders, including schizophrenia. Alterations of AlloP levels during maturation could partly explain the inter-individual differences shown by adult subjects in response to novelty (exploration) and in the sensorimotor gating and prepulse inhibition. Also, abrupt changes in neonatal levels of AlloP could be related to a susceptibility to neurodevelopmental disorders.

Neuronal MHC class I molecules are involved in excitatory synaptic transmission at the hippocampal mossy fiber synapses of marmoset monkeys

Several recent studies suggested a role for neuronal major histocompatibility complex class I (MHCI) molecules in certain forms of synaptic plasticity in the hippocampus of rodents. Here, we report for the first time on the expression pattern and functional properties of MHCI molecules in the hippocampus of a nonhuman primate, the common marmoset monkey (Callithrix jacchus). We detected a presynaptic, mossy fiber-specific localization of MHCI proteins within the marmoset hippocampus. MHCI molecules were present in the large, VGlut1-positive, mossy fiber terminals, which provide input to CA3 pyramidal neurons. Furthermore, whole-cell recordings of CA3 pyramidal neurons in acute hippocampal slices of the common marmoset demonstrated that application of antibodies which specifically block MHCI proteins caused a significant decrease in the frequency, and a transient increase in the amplitude, of spontaneous excitatory postsynaptic currents (sEPSCs) in CA3 pyramidal neurons. These findings add to previous studies on neuronal MHCI molecules by describing their expression and localization in the primate hippocampus and by implicating them in plasticity-related processes at the mossy fiber-CA3 synapses. In addition, our results suggest significant interspecies differences in the localization of neuronal MHCI molecules in the hippocampus of mice and marmosets, as well as in their potential function in these species.

Ethanol acutely inhibits ionotropic glutamate receptor-mediated responses and long-term potentiation in the developing CA1 hippocampus

BACKGROUND

Developmental ethanol (EtOH) exposure damages the hippocampus, causing long-lasting alterations in learning and memory. Alterations in glutamatergic synaptic transmission and plasticity may play a role in the mechanism of action of EtOH. This signaling is fundamental for synaptogenesis, which occurs during the third trimester of human pregnancy (first 12 days of life in rats).

METHODS

Acute coronal brain slices were prepared from 7- to 9-day-old rats. Extracellular and patch-clamp electrophysiological recording techniques were used to characterize the acute effects of EtOH on alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor (AMPAR)- and N-methyl-D-aspartate receptor (NMDAR)-mediated responses and long-term potentiation (LTP) in the CA1 hippocampal region.

RESULTS

Ethanol (40 and 80 mM) inhibited AMPAR- and NMDAR-mediated field excitatory postsynaptic potentials (fEPSPs). EtOH (80 mM) also reduced AMPAR-mediated fEPSPs in the presence of an inhibitor of Ca2+ permeable AMPARs. The effect of 80 mM EtOH on NMDAR-mediated fEPSPs was significantly greater in the presence of Mg2+. EtOH (80 mM) neither affected the paired-pulse ratio of AMPAR-mediated fEPSPs nor the presynaptic volley. The paired-pulse ratio of AMPAR-mediated excitatory postsynaptic currents was not affected either, and the amplitude of these currents was inhibited to a lesser extent than that of fEPSPs. EtOH (80 mM) inhibited LTP of AMPAR-mediated fEPSPs.

CONCLUSIONS

Acute EtOH exposure during the third-trimester equivalent of human pregnancy inhibits hippocampal glutamatergic transmission and LTP induction, which could alter synapse refinement and ultimately contribute to the pathophysiology of fetal alcohol spectrum disorder.

AMPAR-mediated synaptic transmission in the CA1 hippocampal region of neonatal rats: unexpected resistance to repeated ethanol exposure

Alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate glutamatergic receptors (AMPAR) mediate most of the fast excitatory synaptic transmission in mature neurons. In contrast, a number of developing synapses do not express AMPARs; these are gradually acquired in an activity-driven manner during the first week of life in rats, which is equivalent to the third trimester of human pregnancy. Neuronal stimulation has been shown to drive high conductance Ca(2+)-permeable AMPARs into the synapse, strengthening glutamatergic synaptic transmission. Alterations in this process could induce premature stabilization or inappropriate elimination of newly formed synapses and contribute to the hippocampal abnormalities associated with fetal alcohol spectrum disorder. Previous studies from our laboratory performed with hippocampal slices from neonatal rats showed that acute ethanol exposure exerts potent stimulant effects on CA1 and CA3 neuronal networks. However, the impact of these in vitro actions of acute ethanol exposure is unknown. Here, we tested the hypothesis that repeated in vivo exposure to ethanol strengthens AMPAR-mediated neurotransmission in the CA1 region by means of an increase in synaptic expression of Ca(2+)-permeable AMPARs. We exposed rats to ethanol vapor (serum ethanol concentration approximately 40 mM) or air for 4h/day from postnatal day (P) 2-6. In brain slices prepared at P4-6, we found no significant effect of ethanol exposure on input-output curves for AMPAR-mediated field excitatory postsynaptic potentials (fEPSPs), the contribution of Ca(2+)-permeable AMPARs to these fEPSPs, or the acute effect of ethanol on fEPSP amplitude. These results suggest that homeostatic plasticity mechanisms act to maintain glutamatergic synaptic strength and ethanol sensitivity in response to repeated developmental ethanol exposure.

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.

Pregnenolone sulfate potentiates the inwardly rectifying K channel Kir2.3

Background

Neurosteroids have various physiological and neuropsychopharmacological effects. In addition to the genomic effects of steroids, some neurosteroids modulate several neurotransmitter receptors and channels, such as N-methyl-D-aspartate receptors, γ-aminobutyric acid type A (GABA_A) receptors, and σ₁ receptors, and voltage-gated Ca²⁺ and K⁺ channels. However, the molecular mechanisms underlying the various effects of neurosteroids have not yet been sufficiently clarified. In the nervous system, inwardly rectifying K⁺ (Kir) channels also play important roles in the control of resting membrane potential, cellular excitability and K⁺ homeostasis. Among constitutively active Kir2 channels in a major Kir subfamily, Kir2.3 channels are expressed predominantly in the forebrain, a brain area related to cognition, memory, emotion, and neuropsychiatric disorders.

Methodology/Principal Findings

The present study examined the effects of various neurosteroids on Kir2.3 channels using the Xenopus oocyte expression assay. In oocytes injected with Kir2.3 mRNA, only pregnenolone sulfate (PREGS), among nine neurosteroids tested, reversibly potentiated Kir2.3 currents. The potentiation effect was concentration-dependent in the micromolar range, and the current-voltage relationship showed inward rectification. However, the potentiation effect of PREGS was not observed when PREGS was applied intracellularly and was not affected by extracellular pH conditions. Furthermore, although Kir1.1, Kir2.1, Kir2.2, and Kir3 channels were insensitive to PREGS, in oocytes injected with Kir2.1/Kir2.3 or Kir2.2/Kir2.3 mRNA, but not Kir2.1/Kir2.2 mRNA, PREGS potentiated Kir currents. These potentiation properties in the concentration-response relationships were less potent than for Kir2.3 channels, suggesting action of PREGS on Kir2.3-containing Kir2 heteromeric channels.

Conclusions/Significance

The present results suggest that PREGS acts as a positive modulator of Kir2.3 channels. Kir2.3 channel potentiation may provide novel insights into the various effects of PREGS.

Presynaptic NMDA receptors: newly appreciated roles in cortical synaptic function and plasticity

Many aspects of synaptic development, plasticity, and neurotransmission are critically influenced by NMDA-type glutamate receptors (NMDARs). Moreover, dysfunction of NMDARs has been implicated in a broad array of neurological disorders, including schizophrenia, stroke, epilepsy, and neuropathic pain. Classically, NMDARs were thought to be exclusively postsynaptic. However, substantial evidence in the past 10 years demonstrates that NMDARs also exist presynaptically and that presynaptic NMDA receptors (preNMDARs) modulate synapse function and have critical roles in plasticity at many synapses. Here the authors review current knowledge of the role of preNMDARs in synaptic transmission and plasticity, focusing on the neocortex. They discuss the prevalence, function, and development of these receptors, and their potential modification by experience and in brain pathology.

Nanomolar concentrations of pregnenolone sulfate enhance striatal dopamine overflow in vivo

The balance between GABA-mediated inhibitory and glutamate-mediated excitatory synaptic transmission represents a fundamental mechanism for controlling nervous system function, and modulators that can alter this balance may participate in the pathophysiology of neuropsychiatric disorders. Pregnenolone sulfate (PS) is a neuroactive steroid that can modulate the activity of ionotropic glutamate and GABA(A) receptors either positively or negatively, depending upon the particular receptor subtype, and modulates synaptic transmission in a variety of experimental systems. To evaluate the modulatory effect of PS in vivo, we infused PS into rat striatum for 20 min via a microdialysis probe while monitoring local extracellular dopamine (DA) levels. The results demonstrate that PS at low nanomolar concentrations significantly increases extracellular DA levels. The PS-induced increase in extracellular DA is antagonized by the N-methyl-d-aspartate (NMDA) receptor antagonist, d-AP5 [d-(-)-2-amino-5-phosphonopentanoic acid], but not by the sigma receptor antagonist, BD 1063 [1(-)[2-(3,4-dichlorophenyl)-ethyl]-4-methylpiperazine]. The results demonstrate that exogenous PS, at nanomolar concentrations, is able to increase DA overflow in the striatum through an NMDA receptor-mediated pathway.

Transient receptor potential M3 channels are ionotropic steroid receptors in pancreatic beta cells

Transient receptor potential (TRP) cation channels are renowned for their ability to sense diverse chemical stimuli. Still, for many members of this large and heterogeneous protein family it is unclear how their activity is regulated and whether they are influenced by endogenous substances. On the other hand, steroidal compounds are increasingly recognized to have rapid effects on membrane surface receptors that often have not been identified at the molecular level. We show here that TRPM3, a divalent-permeable cation channel, is rapidly and reversibly activated by extracellular pregnenolone sulphate, a neuroactive steroid. We show that pregnenolone sulphate activates endogenous TRPM3 channels in insulin-producing beta cells. Application of pregnenolone sulphate led to a rapid calcium influx and enhanced insulin secretion from pancreatic islets. Our results establish that TRPM3 is an essential component of an ionotropic steroid receptor enabling unanticipated crosstalk between steroidal and insulin-signalling endocrine systems.

The role of NMDA receptor subtypes in short-term plasticity in the rat entorhinal cortex

We have previously shown that spontaneous release of glutamate in the entorhinal cortex (EC) is tonically facilitated via activation of presynaptic NMDA receptors (NMDAr) containing the NR2B subunit. Here we show that the same receptors mediate short-term plasticity manifested by frequency-dependent facilitation of evoked glutamate release at these synapses. Whole-cell patch-clamp recordings were made from layer V pyramidal neurones in rat EC slices. Evoked excitatory postsynaptic currents showed strong facilitation at relatively low frequencies (3 Hz) of activation. Facilitation was abolished by an NR2B-selective blocker (Ro 25-6981), but unaffected by NR2A-selective antagonists (Zn(2+), NVP-AAM077). In contrast, postsynaptic NMDAr-mediated responses could be reduced by subunit-selective concentrations of all three antagonists. The data suggest that NMDAr involved in presynaptic plasticity in layer V are exclusively NR1/NR2B diheteromers, whilst postsynaptically they are probably a mixture of NR1/NR2A, NR1/NR2B diheteromers and NR1/NR2A/NR2B triheteromeric receptors.

Presynaptic and postsynaptic NMDA receptors mediate distinct effects of brain-derived neurotrophic factor on synaptic transmission

In addition to its effects on neuronal survival and differentiation, brain-derived neurotrophic factor (BDNF) plays an important role in modulating synaptic transmission and plasticity in many brain areas, most notably the neocortex and hippocampus. These effects may underlie a role for BDNF in learning and memory as well as developmental plasticity. Consistent with localization of the tropomyosin-related kinase B receptor to both sides of the synapse, BDNF appears to have pre- and postsynaptic effects, but the underlying cellular mechanisms are unclear and it is not known whether pre- and postsynaptic modulations by BDNF occur simultaneously. To address these issues, we recorded dual-component (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA] and N-methyl-D-aspartate [NMDA]) miniature excitatory postsynaptic currents (mEPSCs) from cortical and hippocampal pyramidal neurons and dentate gyrus granule cells from acute brain slices. BDNF had no effect on the fast component of mEPSC decay or on the peak amplitude, suggesting that BDNF did not modulate postsynaptic AMPA receptors, although BDNF rapidly modulated NMDA receptors, as seen by an enhancement of the slow component of mEPSC decay that was prevented by blocking postsynaptic NMDA receptors. At the same time, BDNF acted presynaptically to enhance mEPSC frequency. Surprisingly, the effect on frequency was also NMDA receptor dependent, but required activation of presynaptic, not postsynaptic, NMDA receptors. BDNF also enhanced action potential-dependent glutamate release via presynaptic NMDA receptors, an effect that was unmasked when voltage-gated calcium channels were partially inhibited. Our results indicate that BDNF acutely modulates presynaptic release and postsynaptic responsiveness through simultaneous effects on pre- and postsynaptic NMDA receptors.

Pregnenolone sulfate induces NMDA receptor dependent release of dopamine from synaptic terminals in the striatum

Neuromodulators that alter the balance between lower-frequency glutamate-mediated excitatory and higher-frequency GABA-mediated inhibitory synaptic transmission are likely to participate in core mechanisms for CNS function and may contribute to the pathophysiology of neurological disorders such as schizophrenia and Alzheimer's disease. Pregnenolone sulfate (PS) modulates both ionotropic glutamate and GABA(A) receptor mediated synaptic transmission. The enzymes necessary for PS synthesis and degradation are found in brain tissue of several species including human and rat, and up to 5 nM PS has been detected in extracts of postmortem human brain. Here, we ask whether PS could modulate transmitter release from nerve terminals located in the striatum. Superfusion of a preparation of striatal nerve terminals comprised of mixed synaptosomes and synaptoneurosomes with brief-duration (2 min) pulses of 25 nM PS demonstrates that PS increases the release of newly accumulated [3H]dopamine ([3H]DA), but not [14C]glutamate or [3H]GABA, whereas pregnenolone is without effect. PS does not affect dopamine transporter (DAT) mediated uptake of [3H]DA, demonstrating that it specifically affects the transmitter release mechanism. The PS-induced [3H]DA release occurs via an NMDA receptor (NMDAR) dependent mechanism as it is blocked by D-2-amino-5-phosphonovaleric acid. PS modulates DA release with very high potency, significantly increasing [3H]DA release at PS concentrations as low as 25 pM. This first report of a selective direct enhancement of synaptosomal dopamine release by PS at picomolar concentrations via an NMDAR dependent mechanism raises the possibility that dopaminergic axon terminals may be a site of action for this neurosteroid.

Presynaptic glutamate receptors: physiological functions and mechanisms of action

Glutamate acts on postsynaptic glutamate receptors to mediate excitatory communication between neurons. The discovery that additional presynaptic glutamate receptors can modulate neurotransmitter release has added complexity to the way we view glutamatergic synaptic transmission. Here we review evidence of a physiological role for presynaptic glutamate receptors in neurotransmitter release. We compare the physiological roles of ionotropic and metabotropic glutamate receptors in short- and long-term regulation of synaptic transmission. Furthermore, we discuss the physiological conditions that are necessary for their activation, the source of the glutamate that activates them, their mechanisms of action and their involvement in higher brain function.

Neonatal finasteride induces anxiogenic-like profile and deteriorates passive avoidance in adulthood after intrahippocampal neurosteroid administration

Recent findings indicate that neurosteroids could act as important keys during the brain development. Fluctuations in neonatal allopregnanolone (AlloP) could result in altered pharmacological properties of the GABA(A) receptor system in adulthood. Recent studies demonstrated that neurosteroids play a critical role in regulating normal neurodevelopment in the hippocampus. The aim of the present work is to screen whether developmentally altered neurosteroid levels influence the behavioral response to adult intrahippocampal administration of AlloP, a GABA(A) positive modulating neurosteroid, and pregnenolone sulfate (PregS), a GABA(A) negative modulator in rats. For this purpose, pups received AlloP (10 mg/kg, s.c.), a 5alpha-reductase inhibitor (finasteride, 50 mg/kg, s.c.) or vehicle from the fifth to the ninth postnatal day. At maturity (i.e. 90 days old) a bilateral cannula was implanted into the hippocampus. After recovery from surgery, animals received an administration of AlloP (0.2 microg/0.5 microl), PregS (5 ng/0.5 microl) or vehicle in each hippocampus 5 min before they were tested in the elevated plus maze (EPM) and immediately after the passive avoidance training session, and retention was tested 24 h later. Results indicated that neonatal finasteride treatment deteriorated passive avoidance retention and elicited an anxiogenic-like effect in the EPM test in adulthood, as seen by the reduction of open arm entries and in the time spent in the open arms. Intrahippocampal PregS administration also disrupted passive avoidance, possibly related to its anxiogenic profile. Fluctuations in neonatal AlloP affect the aversive learning and the anxiety-related behavior in adulthood, and this effect could be in part mediated by alterations of the mature functions of the hippocampus, possibly via the GABA(A) receptor. These data point to the role of GABAergic neurosteroids in critical periods of vulnerability that influence normal development of GABAergic pathways in the CNS.

Regional differences in the decay kinetics of GABA(A) receptor-mediated miniature IPSCs in the dorsal horn of the rat spinal cord are determined by mitochondrial transport of cholesterol

We examined the possibility of a differential spatial control in the endogenous production of 3alpha5alpha-reduced steroids and its consequences on GABA(A) receptor-mediated miniature IPSCs (mIPSCs) in laminas II and III-IV of the rat spinal cord dorsal horn (DH). Early in postnatal development [younger than postnatal day 8 (P8)], mIPSCs displayed slow decay kinetics in laminas II and III-IV resulting from a continuous local production of 3alpha5alpha-reduced steroids. This was mediated by the tonic activity of the translocator protein of 18 kDa (TSPO), which controls neurosteroid synthesis by regulating the transport of cholesterol across the mitochondrial membrane system. TSPO activity disappeared in laminas III-IV after P8 and was functionally downregulated in lamina II after P15, resulting in a marked reduction of mIPSC duration in these laminas. TSPO-mediated synthesis of 3alpha5alpha-reduced steroids was spatially restricted, because, at P9-P15, when their production was maximal in lamina II, no sign of spillover to laminas III-IV was apparent. Interestingly, after P8, the enzymes necessary for the synthesis of 3alpha5alpha-reduced steroids remained functional in laminas III-IV and could produce such steroids from various precursors or after a single subcutaneous injection of progesterone. Moreover, induction of an acute peripheral inflammation by intraplantar injection of carrageenan, restored a maximal TSPO-mediated neurosteroidogenesis in laminas III-IV. Our results indicate that the decay kinetics of GABA(A) receptor-mediated mIPSCs in the DH of the spinal cord are primarily controlled by 3alpha5alpha-reduced steroids, which can be produced from circulating steroid precursors and/or in a spatially restricted manner by the modulation of the activity of TSPO.

Synapse-specific expression of functional presynaptic NMDA receptors in rat somatosensory cortex

Presynaptic NMDA receptors (NMDARs) modulate release and plasticity at many glutamatergic synapses, but the specificity of their expression across synapse classes has not been examined. We found that non-postsynaptic, likely presynaptic NR2B-containing NMDARs enhanced AMPA receptor-mediated synaptic transmission at layer 4 (L4) to L2/3 (L4-L2/3) synapses in juvenile rat barrel cortex. This modulation was apparent at room temperature when presynaptic NMDARs were activated by elevation of extracellular glutamate or application of exogenous NMDAR agonists. At near physiological temperatures, modulation of transmission by presynaptic NMDARs occurred naturally, without the need for external activation. Blockade of presynaptic NMDARs depressed unitary and extracellularly evoked EPSCs at L4-L2/3 synapses, accompanied by increases in paired-pulse ratio and coefficient of variation, indicative of a decrease in presynaptic release probability. NMDAR agonists increased the frequency of miniature EPSCs in L2/3 neurons, without altering their amplitude or kinetics. Focal application of NMDAR antagonist revealed that the NMDARs that modulate L4-L2/3 transmission are located in L2/3, not L4, consistent with localization on terminals or axons of L4-L2/3 synapses, rather than on the somatodendritic compartment of presynaptic L4 neurons. In contrast, presynaptic NMDARs did not modulate L4-L4 synapses, which originate from the same presynaptic neurons as L4-L2/3 synapses, or cross-columnar L2/3-L2/3 horizontal projections, which synapse onto the same postsynaptic target neurons. Thus, presynaptic NMDARs selectively modulate L4-L2/3 synapses, relative to other synapses made by the same neurons. Existence of these receptors may support specialized processing or plasticity by L4-L2/3 synapses.

Developmental switch in the contribution of presynaptic and postsynaptic NMDA receptors to long-term depression

NMDA receptor (NMDAR) activation is required for many forms of learning and memory as well as sensory system receptive field plasticity, yet the relative contribution of presynaptic and postsynaptic NMDARs over cortical development remains unknown. Here we demonstrate a rapid developmental loss of functional presynaptic NMDARs in the neocortex. Presynaptic NMDARs enhance neurotransmitter release at synapses onto visual cortex pyramidal cells in young mice [before postnatal day 20 (P20)], but they have no apparent effect after the onset of the critical period for receptive field plasticity (>P23). Immunoelectron microscopy revealed that the loss of presynaptic NMDAR function is likely attributable in part to a 50% reduction in the prevalence of presynaptic NMDARs. Coincident with the observed loss of presynaptic NMDAR function, there is an abrupt change in the mechanisms of timing-dependent long-term depression (tLTD). Induction of tLTD before the onset of the critical period requires activation of presynaptic but not postsynaptic NMDARs, whereas the induction of tLTD in older mice requires activation of postsynaptic NMDARs. By demonstrating that both presynaptic and postsynaptic NMDARs contribute to the induction of synaptic plasticity and that their relative roles shift over development, our findings define a novel, and perhaps general, property of synaptic plasticity in emerging cortical circuits.

Pregnenolone sulfate in the brain: a controversial neurosteroid

Pregnenolone sulfate (PREGS) has been shown, either at high nanomolar or at micromolar concentrations, to increase neuronal activity by inhibiting GABAergic and by stimulating glutamatergic neurotransmission. PREGS is also a potent modulator of sigma type 1 (sigma1) receptors. It has been proposed that these actions of PREGS underlie its neuropharmacological effects, and in particular its influence on memory processes. On the other hand, the PREGS-mediated increase in neuronal excitability may become dangerous under particular conditions, for example in the case of excitotoxic stress or convulsions. However, the physiopathological significance of these observations has recently been put into question by the failure to detect significant levels of PREGS within the brain and plasma of rats and mice, either by direct analytical methods based on liquid chromatography/mass spectrometry (LC/MS) or enzyme linked immunosorbent assay (ELISA) with specific antibodies against PREGS, or by indirect gas chromatography/mass spectrometry (GC/MS) analysis with improved sample workup. These recent results have not come to the attention of a large number of neurobiologists interested in steroid sulfates. However, although available direct analytical methods have failed to detect levels of PREGS above 0.1-0.3 ng/g in brain tissue, it may be premature to completely exclude the local formation of biologically active PREGS within specific and limited compartments of the nervous system. In contrast to the situation in rodents, significant levels of sulfated 3beta-hydroxysteroids have been measured in human plasma and brain. Previous indirect measures of steroid sulfates by radioimmunoassays (RIA) or GC/MS had detected elevated levels of PREGS in rodent brain. The discrepancies between the results of different assay procedures have revealed the danger of indirect analysis of steroid sulfates. Indeed, PREGS must be solvolyzed/hydrolyzed prior to RIA or GC/MS analysis, and it is the released, unconjugated PREG which is then quantified. Extreme caution needs to be exercised during the preparation of samples for RIA or GC/MS analysis, because the fraction presumed to contain only steroid sulfates can be contaminated by nonpolar components from which PREG is generated by the solvolysis/hydrolysis/derivatization reactions.

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).

Endogenous alkaline transients boost postsynaptic NMDA receptor responses in hippocampal CA1 pyramidal neurons

In hippocampus, activation of the Schaffer collaterals generates an extracellular alkaline transient both in vitro and in vivo. This pH change may provide relief of the H+ block of NMDA receptors (NMDARs) and thereby increase excitability. To test this hypothesis, we augmented extracellular buffering in mouse hippocampal slices by adding 2 microM bovine type II carbonic anhydrase to the superfusate. With addition of enzyme, the alkaline transient elicited by a 10 pulse, 100 Hz stimulus train was reduced by 33%. At a holding potential (V(H)) of -30 mV, the enzyme decreased the half-time of decay and charge transfer of EPSCs by 32 and 39%, respectively, but had no effect at a V(H) of -80 mV. In current clamp, a 10 pulse, 100 Hz stimulus train gave rise to an NMDAR-dependent afterdepolarization (ADP). Exogenous enzyme curtailed the ADP half-width and voltage integral by 20 and 25%, respectively. Similar reduction of the ADP was noted with a brief 12 Hz stimulus train. The effect persisted in the presence of GABAergic antagonists or the L-type Ca2+ channel blocker methoxyverapamil hydrochloride but was absent in the presence of the carbonic anhydrase inhibitor benzolamide or when the exogenous enzyme was heat inactivated. The effects of the enzyme in voltage and current clamp were noted in 0 Mg2+ media but were abolished when (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine maleate was included in the patch pipette. These results provide strong evidence that endogenous alkaline transients are sufficiently large in the vicinity of the synapse to augment NMDAR responses.

PREGS induces LTP in the hippocampal dentate gyrus of adult rats via the tyrosine phosphorylation of NR2B coupled to ERK/CREB [corrected] signaling

An acute application of neurosteroid pregnenolone sulfate (PREGS) to hippocampal slices from adult rats induced a long-lasting potentiation (LLP PREGS) at the perforant path-granule cell synapse. As a partial mechanism of the LLP PREGS, we previously revealed that PREGS transiently increases the probability of presynaptic glutamate release via a sensitization of alpha7-nicotinic acetylcholine receptor (alpha7nAChR). We herein demonstrate that the LLP PREGS could be separated into two independent processes: the above-mentioned early presynaptic-origin short-term potentiation (STP PREGS) and a delayed postsynaptic N-methyl-d-aspartate receptor (NMDAr)-dependent long-term potentiation termed LTP(PREGS). This study focused on the analysis of the signaling mechanism underlying the LTP PREGS. PREGS increased the tyrosine phosphorylation of NR2B, a subunit of NMDAr, and the NMDAr-mediated Ca2+ influx in the granule cells. The enhanced Ca2+ influx was largely attenuated by the NR2B subunit inhibitor ifenprodil and the Src kinase family inhibitor PP2. PREGS also triggered a persistent phosphorylation of extracellular signal-regulated kinase 2 (ERK2) followed by an ERK-dependent phosphorylation of cAMP-response element-binding protein (CREB), which was crucial for the LTP PREGS induction and was sensitive to ifenprodil. These results suggest that PREGS induces an acute increase in the NR2B tyrosine phosphorylation which enhances the Ca2+ influx through NMDAr, followed by an activation of the ERK/CREB signaling cascade that leads to LTP PREGS.

Modulation of glutamatergic transmission by sulfated steroids: role in fetal alcohol spectrum disorder

It is well established that sulfated steroids regulate synaptic transmission by altering the function of postsynaptic neurotransmitter receptors. In recent years, evidence from several laboratories indicates that these agents also regulate glutamatergic synaptic transmission at the presynaptic level in an age-dependent manner. In developing neurons, pregnenolone sulfate (PREGS) increases the probability of glutamate release, as evidenced by an increase in the frequency of AMPA receptor-mediated miniature excitatory postsynaptic currents and a decrease in paired-pulse facilitation. In hippocampal slices from postnatal day 3-5 rats, this effect is mediated by an increase in Ca(2+) levels in the axonal terminal that depends on presynaptic NMDA receptors. This is followed by delayed potentiation of postsynaptic AMPA receptor currents. Importantly, depolarization of postsynaptic neurons, inhibition of hydroxysteroid sulfatase activity and acute exposure to ethanol mimics the effect of exogenous PREGS application. This developmental form of synaptic plasticity cannot be observed in slices from rats older than postnatal day 6, when presynaptic NMDA receptors are no longer expressed in CA1 hippocampal region. Both in the CA1 hippocampal region and the dentate gyrus of more mature rats, PREGS, dehydroepiandrosterone sulfate and hydroxysteroid sulfatase inhibitors increase paired-pulse facilitation, without affecting basal glutamate release probability. This effect depends on activation of sigma(1)-like receptors and G(i/o) and involves a target in the release machinery that is downstream of residual Ca(2+). These presynaptic actions of sulfated steroids could play important roles in physiological processes ranging from synapse maturation to learning and memory, as well as pathophysiological conditions such as fetal alcohol spectrum disorder.

Neurosteroid pregnenolone sulfate enhances glutamatergic synaptic transmission by facilitating presynaptic calcium currents at the calyx of Held of immature rats

Pregnenolone sulfate (PREGS) is an endogenous neurosteroid widely released from neurons in the brain, and is thought to play a memory-enhancing role. At excitatory synapses PREGS facilitates transmitter release, but the underlying mechanism is not known. We addressed this issue at the calyx of Held in rat brainstem slices, where direct whole-cell recordings from giant nerve terminals are feasible. PREGS potentiated nerve-evoked excitatory postsynaptic currents (EPSCs) without affecting the amplitude of miniature EPSCs, suggesting that its site of action is presynaptic. In whole-cell recordings from calyceal nerve terminals, PREGS facilitated Ca2+ currents, by accelerating their activation kinetics and shifting the half-activation voltage toward negative potentials. PREGS had no effect on presynaptic K+ currents, resting conductance or action potential waveforms. In simultaneous pre- and postsynaptic recordings, PREGS did not change the relationship between presynaptic Ca2+ influx and EPSCs, suggesting that exocytotic machinery downstream of Ca2+ influx is not involved in its effect. PREGS facilitated Ba2+ currents recorded from nerve terminals and also from HEK 293 cells expressed with recombinant N- or P/Q-type Ca2+ channels, suggesting that PREGS-induced facilitation of voltage-gated Ca2+ channels (VGCCs) is neither Ca2+ dependent nor VGCC-type specific. The PREGS-induced VGCC facilitation was blocked by the PREGS scavenger (2-hydroxypropyl)-beta-cyclodextrin applied from outside, but not from inside, of nerve terminals. We conclude that PREGS facilitates VGCCs in presynaptic terminals by acting from outside, thereby enhancing transmitter release. We propose that PREGS may directly modulate VGCCs acting on their extracellular domain.

Sulfated steroids as endogenous neuromodulators

Central nervous system function is critically dependent upon an exquisitely tuned balance between excitatory synaptic transmission, mediated primarily by glutamate, and inhibitory synaptic transmission, mediated primarily by GABA. Modulation of either excitation or inhibition would be expected to result in altered functionality of finely tuned synaptic pathways and global neural systems, leading to altered nervous system function. Administration of positive or negative modulators of ligand-gated ion channels has been used extensively and successfully in CNS therapeutics, particularly for the induction of sedation and treatment of anxiety, seizures, insomnia, and pain. Excessive activation of excitatory glutamate receptors, such as in cerebral ischemia, can result in neuronal damage via excitotoxic mechanisms. The discovery that neuroactive steroids exert rapid, direct effects upon the function of both excitatory and inhibitory neurotransmitter receptors has raised the possibility that endogenous neurosteroids may play a regulatory role in synaptic transmission by modulating the balance between excitatory and inhibitory neurotransmission. The sites to which neuroactive steroids bind may also serve as targets for the discovery of therapeutic neuromodulators.

Neurosteroid-induced enhancement of short-term facilitation involves a component downstream from presynaptic calcium in hippocampal slices

We used Magnesium Green AM to measure Ca(2+) transients in Schaffer collateral presynaptic terminals simultaneously with postsynaptic field potentials (fEPSPs) to investigate the mechanism of neurosteroid enhancement of short-term synaptic facilitation. Measurement of [Ca(2+)](i), isolated to presynaptic events, using the fluorescence ratio (DeltaF/F(0)) demonstrated that at a constant stimulus intensity there was no change in the excitability of presynaptic fibres between paired stimuli or between ACSF and 1 mum pregnenolone sulphate (PREGS). Paired-pulse facilitation (PPF) was correlated with residual Ca(2+) ([Ca(2+)](res)), and there was an additional increase in the integralDeltaF/F(0) for the [Ca(2+)](res)-subtracted response to the second of paired stimuli, resulting primarily from a slowing of the decay time constant. In addition to the role of presynaptic [Ca(2+)](res) in PPF, we observed a decrease in EC(50) and a greater maximum for Hill function fits to fEPSP versus DeltaF/F(0) during the second of paired responses. The enhancement of fEPSP PPF by PREGS did not result from an increase of DeltaF/F(0). The data presented here support a PREGS-induced increase in presynaptic glutamate release from the second, but not the first, of a pair of stimuli for a given presynaptic [Ca(2+)] because: (a) there is actually a decrease in the integralDeltaF/F(0) of the [Ca(2+)](res)-subtracted second response over that seen in ACSF; (b) PREGS causes no change in presynaptic Ca(2+) buffering; and (c) there is a decrease in EC(50) and an increase of y(max) in the Hill function fits to DeltaF/F(0) versus fEPSP data. We hypothesize that PREGS enhances short-term facilitation by acting on the Ca(2+)-dependent vesicle release machinery and that this mechanism plays a role in the cognitive effects of this sulphated neurosteroid.

Taurine potentiates presynaptic NMDA receptors in hippocampal Schaffer collateral axons

We have previously shown that activation of presynaptic N-methyl-d-aspartate (NMDA) receptors (NMDAR) enhances the amplitude of the presynaptic fibre volley (FV) evoked in Schaffer collateral axons of rat hippocampal slices, by a mechanism independent of extracellular Ca(2+). Here we compared the pharmacological characteristics of presynaptic NMDARs affecting axon excitability (activated by 10-300 microM NMDA for 10 min), with those mediating field excitatory postsynaptic potentials (NMDA-fEPSP). We found that NMDA-induced potentiation was completely inhibited by NVP-AAM077, an antagonist of NR2A-containing NMDAR, but not by ifenprodil, an NR2B-selective antagonist. The inhibitor of the glycine-binding site in NMDARs, 7-clorokynurenic acid (7-CK), was more potent against NMDA-fEPSP (IC(50) = 6.3 +/- 1.3 microM) than against the NMDA-induced FV potentiation (IC(50) = 26.5 +/- 1.3 microM). Moreover, both post- and presynaptic NMDAR-mediated phenomena were enhanced by glycine and d-serine, but taurine, an endogenous analogue of glycine, only enhanced the latter (EC(50) = 19 microM). Taurine was able to block the inhibitory effect of low doses of 7-CK on NMDA-induced FV potentiation, while glycine and d-serine only reduced the effects of higher concentrations of this drug. Surprisingly, the enhancing effect of taurine on NMDA-induced FV potentiation was blocked when it was co-applied with glycine. Furthermore, the glutamate released synaptically with a train of stimuli also increased FV amplitude by a mechanism dependent on NMDARs; this was potentiated by taurine but not by co-application of taurine and glycine. These results reveal that presynaptic NMDARs have unique properties that mediate the facilitation of axon excitability.

Novel presynaptic mechanisms for coincidence detection in synaptic plasticity

Long-term plasticity typically relies on postsynaptic NMDA receptors to detect the coincidence of pre- and postsynaptic activity. Recent studies, however, have revealed forms of plasticity that depend on coincidence detection by presynaptic NMDA receptors. In the amygdala, cortical afferent associative presynaptic long-term potentiation (LTP) requires activation of presynaptic NMDA receptors by simultaneous thalamic and cortical afferents. Surprisingly, both types of afferent can also undergo postsynaptically induced NMDA-receptor-dependent LTP. In the neocortex, spike-timing-dependent long-term depression (LTD) requires simultaneous activation of presynaptic NMDA autoreceptors and retrograde signalling by endocannabinoids. In cerebellar LTD, presynaptic NMDA receptor activation suggests that similar presynaptic mechanisms may exist. Recent studies also indicate the existence of presynaptic coincidence detection that is independent of NMDA receptors, suggesting that such mechanisms have a widespread role in plasticity.

Alcohol increases efficacy of immature synapses in a neurosteroid-dependent manner

Fetal ethanol exposure persistently affects hippocampal circuits leading to learning and memory disabilities. Although the mechanisms responsible for these effects are not fully understood, several studies implicate neurosteroids as mediators of the actions of ethanol. A neurosteroid that appears to be critical for the fetal actions of ethanol is pregnenolone sulfate (PREGS). We found that chronic prenatal ethanol exposure increases PREGS levels in the fetal brain and that an endogenous PREGS-like neurosteroid strengthens excitatory transmission in the neonatal hippocampus. Therefore, we hypothesized that ethanol could affect synaptic transmission in the developing hippocampus in a PREGS-dependent manner. We used patch-clamp electrophysiological techniques and found that 50 mm ethanol strengthens AMPA receptor-mediated transmission in the CA1 region by reducing the failure rate of low-efficacy synapses. This effect was age-dependent and was occluded by application of exogenous PREGS. An anti-PREGS antibody scavenger and blockade of PREGS synthesis prevented the effect of ethanol. These data indicate that the deleterious effects of ethanol on hippocampal development are mediated in part by alterations in neurosteroid production, which results in premature stabilization of excitatory synapses.

Developmentally regulated actions of alcohol on hippocampal glutamatergic transmission

Ethanol exposure during fetal development is a leading cause of learning disabilities. Studies suggest that it alters learning and memory by permanently damaging the hippocampus. It is generally assumed that this is mediated, in part, via alterations in glutamatergic transmission. Although NMDA receptors are presumed to be the most sensitive targets of ethanol in immature neurons, this issue has not been explored in the developing hippocampus. We performed whole-cell patch-clamp recordings in hippocampal slices from neonatal rats. Unexpectedly, we found that acute ethanol (10-50 mM) exposure depresses inward currents elicited by local application of exogenous AMPA, but not NMDA, in CA3 pyramidal neurons. These findings revealed a direct effect of ethanol on postsynaptic AMPA receptors. Ethanol significantly decreased the amplitude of both AMPA and NMDA receptor-mediated EPSCs evoked by electrical stimulation. This effect was associated with an increase in the paired-pulse ratio and a decrease in the frequency of miniature EPSCs driven by depolarization of axonal terminals. These findings demonstrate that ethanol also acts at the presynaptic level. Omega-conotoxin-GVIA occluded the effect of ethanol on NMDA EPSCs, indicating that ethanol decreases glutamate release via inhibition of N-type voltage-gated Ca2+ channels. In more mature rats, ethanol did not affect the probability of glutamate release or postsynaptic AMPA receptor-mediated currents, but it did inhibit NMDA-mediated currents. We conclude that the mechanism by which ethanol inhibits glutamatergic transmission is age dependent and challenge the view that postsynaptic NMDA receptors are the primary targets of ethanol early in development.

Identification of neuroactive steroids and their precursors and metabolites in adult male rat brain

Steroids in the brain arise both from local synthesis and from peripheral sources and have a variety of effects on neuronal function. However, there is little direct chemical evidence for the range of steroids present in brain or of the pathways for their synthesis and inactivation. This information is a prerequisite for understanding the regulation and function of brain steroids. After extraction from adult male rat brain, we have fractionated free steroids and their sulfate esters and then converted them to heptafluorobutyrate or methyloxime-trimethylsilyl ether derivatives for unequivocal identification and assay by gas chromatography analysis and selected ion monitoring mass spectrometry. In the free steroid fraction, corticosterone, 3alpha,5alpha-tetrahydrodeoxycorticosterone, testosterone, and dehydroepiandrosterone were found in the absence of detectable precursors usually found in endocrine glands, indicating peripheral sources and/or alternative synthetic pathways in brain. Conversely, the potent neuroactive steroid 3alpha,5alpha-tetrahydroprogesterone (allopregnanolone) was found in the presence of its precursors pregnenolone, progesterone, and 5alpha-dihydroprogesterone. Furthermore, the presence of 3beta-, 11beta-, 17alpha-, and 20alpha-hydroxylated metabolites of 3alpha,5alpha-tetrahydroprogesterone implicated possible inactivation pathways for this steroid. The 20alpha-reduced metabolites could also be found for pregnenolone, progesterone, and 5alpha-dihydroprogesterone, introducing a possible regulatory diversion from the production of 3alpha,5alpha-tetrahydroprogesterone. In the steroid sulfate fraction, dehydroepiandrostrone sulfate was identified but not pregnenolone sulfate. Although pharmacologically active, identification of the latter appears to be an earlier methodological artifact, and the compound is thus of doubtful physiological significance in the adult brain. Our results provide a basis for elucidating the origins and regulation of brain steroids.

Pregnenolone sulfate acts through a G-protein-coupled sigma1-like receptor to enhance short term facilitation in adult hippocampal neurons

Neurosteroids have been linked to cognitive performance, and their levels are altered in neuropsychiatric diseases. These neuromodulators are produced in the brain where they have important effects on synaptic transmission at postsynaptic gamma-amino-butyric acid receptors and N-methyl-D-aspartate receptors and at presynaptic sites. We previously found, in cultured neonatal hippocampal neurons, that the neurosteroid, pregnenolone sulfate, acts presynaptically through a sigma1-like receptor to modulate basal glutamate release. The present study was designed to test whether pregnenolone sulfate acts through a similar presynaptic receptor in adult hippocampal neurons. The sigma1-receptor agonist, 2-(4-morpholino)ethyl-1-phenylcyclohexane-1-carboxylate, enhanced paired-pulse facilitation (PPF) by a similar extent to that which we had previously reported for pregnenolone sulfate. The sigma1-receptor antagonists, 1-(4-Iodophenyl)-3-(2-adamantyl)guanidine and 1[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine, blocked the pregnenolone sulfate enhancement of PPF as did pretreatment of slices in pertussis toxin. We conclude that pregnenolone sulfate acts through a Gi/o-coupled sigma1-like receptor to enhance short-term presynaptic facilitation onto adult hippocampal CA1 neurons.

Presynaptic modulation of synaptic transmission by pregnenolone sulfate as studied by optical recordings

The effects of pregnenolone sulfate (PREGS), a putative neurosteroid, on the transmission of perforant path-granule cell synapses were investigated with an optical recording technique in rat hippocampal slices stained with voltage-sensitive dyes. Application of PREGS to the bath solution resulted in an acute augmentation of EPSP in a dose-dependent manner. The PREGS effect was dependent on the extracellular Ca(2+) concentration ([Ca(2+)](o)), but independent of NMDA receptor activation. PREGS caused a decrease in paired-pulse facilitation, which implies that PREGS positively modulates presynaptic neurotransmitter releases. Firmer support for this mechanism was that PREGS augmented the synaptically induced glial depolarization (SIGD) that reflects the activity of electrogenic glutamate transporters in glial cells during the uptake of released glutamate. The selective alpha7nAChR antagonist alpha-BGT or MLA prevented the SIGD increase by PREGS. Furthermore DMXB, a selective alpha7nAChR agonist, mimicked the PREGS effect on SIGD and antagonized the effect of PREGS. The presynaptic effect of PREGS was partially attenuated by the L-type Ca(2+) channel (VGCC) blocker nifedipine. Based on these findings, we proposed a novel mechanism underlying the facilitated synaptic transmission by PREGS: this neurosteroid sensitizes presynaptic alpha7nAChR that is followed by an activation of L-type VGCC to increase the presynaptic glutamate release.