GABA-independent GABAA receptor openings maintain tonic currents.

Activation of GABA(A) receptors (GABA(A)Rs) produces two forms of inhibition: phasic inhibition generated by the rapid, transient activation of synaptic GABA(A)Rs by presynaptic GABA release, and tonic inhibition generated by the persistent activation of perisynaptic or extrasynaptic GABA(A)Rs, which can detect extracellular GABA. Such tonic GABA(A)R-mediated currents are particularly evident in dentate granule cells in which they play a major role in regulating cell excitability. Here we show that in rat dentate granule cells in ex vivo hippocampal slices, tonic currents are predominantly generated by GABA-independent GABA(A) receptor openings. This tonic GABA(A)R conductance is resistant to the competitive GABA(A)R antagonist SR95531 (gabazine), which at high concentrations acts as a partial agonist, but can be blocked by an open channel blocker, picrotoxin. When slices are perfused with 200 nm GABA, a concentration that is comparable to CSF concentrations but is twice that measured by us in the hippocampus in vivo using zero-net-flux microdialysis, negligible GABA is detected by dentate granule cells. Spontaneously opening GABA(A)Rs, therefore, maintain dentate granule cell tonic currents in the face of low extracellular GABA concentrations.

Neural oscillations during non-rapid eye movement sleep as biomarkers of circuit dysfunction in schizophrenia.

The neurophysiology of non-rapid eye movement sleep is characterized by the occurrence of neural network oscillations with distinct origins and frequencies, which act in concert to support sleep-dependent information processing. Thalamocortical circuits generate slow (0.25-4 Hz) oscillations reflecting synchronized temporal windows of cortical activity, whereas concurrent waxing and waning spindle oscillations (8-15 Hz) act to facilitate cortical plasticity. Meanwhile, fast (140-200 Hz) and brief (< 200 ms) hippocampal ripple oscillations are associated with the reactivation of neural assemblies recruited during prior wakefulness. The extent of the forebrain areas engaged by these oscillations, and the variety of cellular and synaptic mechanisms involved, make them sensitive assays of distributed network function. Each of these three oscillations makes crucial contributions to the offline memory consolidation processes supported by non-rapid eye movement sleep. Slow, spindle and ripple oscillations are therefore potential surrogates of cognitive function and may be used as diagnostic measures in a range of brain diseases. We review the evidence for disrupted slow, spindle and ripple oscillations in schizophrenia, linking pathophysiological mechanisms to the functional impact of these neurophysiological changes and drawing links with the cognitive symptoms that accompany this condition. Finally, we discuss potential therapies that may normalize the coordinated activity of these three oscillations in order to restore healthy cognitive function.

Long-lasting behavioral responses to stress involve a direct interaction of glucocorticoid receptors with ERK1/2-MSK1-Elk-1 signaling.

Stressful events are known to have a long-term impact on future behavioral stress responses. Previous studies suggested that both glucocorticoid hormones and glutamate acting via glucocorticoid receptors (GRs) and N-methyl D-aspartate (NMDA) receptors, respectively, are of critical importance for the consolidation of these long-lasting behavioral responses at the dentate gyrus, the gateway of the hippocampal formation. We found that an acute psychologically stressful event resulted in ERK1/2 phosphorylation (pERK1/2), which within 15 min led to the activation of the nuclear kinases MSK1 and Elk-1 in granule neurons of the dentate gyrus. Next, MSK1 and Elk-1 activation evoked serine-10 phosphorylation and lysine-14 acetylation in histone H3, resulting in the induction of the neuroplasticity-associated immediate-early genes c-Fos and Egr-1 in these neurons. The pERK1/2-mediated activation of MSK1 and Elk-1 required a rapid protein-protein interaction between pERK1/2 and activated GRs. This is a unique nongenomic mechanism of glucocorticoid hormone action in dentate gyrus granule neurons on long-lasting behavioral responses to stress involving direct cross-talk of GRs with ERK1/2-MSK1-Elk-1 signaling to the nucleus.

A functional role for both -aminobutyric acid (GABA) transporter-1 and GABA transporter-3 in the modulation of extracellular GABA and GABAergic tonic conductances in the rat hippocampus.

Tonic γ-aminobutyric acid (GABA)A receptor-mediated signalling controls neuronal network excitability in the hippocampus. Although the extracellular concentration of GABA (e[GABA]) is critical in determining tonic conductances, knowledge on how e[GABA] is regulated by different GABA transporters (GATs) in vivo is limited. Therefore, we studied the role of GATs in the regulation of hippocampal e[GABA] using in vivo microdialysis in freely moving rats. Here we show that GAT-1, which is predominantly presynaptically located, is the major GABA transporter under baseline, quiescent conditions. Furthermore, a significant contribution of GAT-3 in regulating e[GABA] was revealed by administration of the GAT-3 inhibitor SNAP-5114 during simultaneous blockade of GAT-1 by NNC-711. Thus, the GABA transporting activity of GAT-3 (the expression of which is confined to astrocytes) is apparent under conditions in which GAT-1 is blocked. However, sustained neuronal activation by K(+)-induced depolarization caused a profound spillover of GABA into the extrasynaptic space and this increase in e[GABA] was significantly potentiated by sole blockade of GAT-3 (i.e. even when uptake of GAT-1 is intact). Furthermore, experiments using tetrodotoxin to block action potentials revealed that GAT-3 regulates extrasynaptic GABA levels from action potential-independent sources when GAT-1 is blocked. Importantly, changes in e[GABA] resulting from both GAT-1 and GAT-3 inhibition directly precipitate changes in tonic conductances in dentate granule cells as measured by whole-cell patch-clamp recording. Thus, astrocytic GAT-3 contributes to the regulation of e[GABA] in the hippocampus in vivo and may play an important role in controlling the excitability of hippocampal cells when network activity is increased.

The GABAA receptor modulator zolpidem augments hippocampal-prefrontal coupling during non-REM sleep.

Benzodiazepines and 'Z-drugs' (including zolpidem and zopiclone) are GABAA receptor (GABAAR) positive modulators commonly prescribed as hypnotics to treat insomnia and/or anxiety. However, alongside sedation, augmenting GABAAR function may also alter coordinated neuronal activity during sleep, thereby influencing sleep-dependent processes including memory consolidation. We used simultaneous recordings of neural population activity from the medial prelimbic cortex (PrL) and CA1 of the dorsal hippocampus (dCA1) of naturally sleeping rats to detail the effects of zolpidem on network activity during the cardinal oscillations of non-REM sleep. For comparison, we also characterized the effects of diazepam and 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol (THIP/gaboxadol), which acts predominantly at extra-synaptic GABAARs. Zolpidem and THIP significantly increased the amplitudes of slow-waves, which were attenuated by diazepam. Zolpidem increased hippocampal ripple density whereas diazepam decreased both ripple density and intrinsic frequency. While none of the drugs affected thalamocortical spindles in isolation, zolpidem augmented the temporal coordination between slow-waves and spindles. At the cellular level, analyses of spiking activity from 523 PrL and 579 dCA1 neurons revealed that zolpidem significantly enhanced synchronized pauses in cortical firing during slow-wave down states, while increasing correlated activity within and between dCA1 and PrL populations. Of the drugs compared here, zolpidem was unique in augmenting coordinated activity within and between hippocampus and neocortex during non-REM sleep. Zolpidem's enhancement of hippocampal-prefrontal coupling may reflect the cellular basis of its potential to modulate offline memory processing.

Anti-opioid activities of NPFF1 receptors in a SH-SY5Y model.

In order to elucidate the mechanisms of the neuronal anti-opioid activity of Neuropeptide FF, we have transfected the SH-SY5Y neuroblastoma cell line, which expresses mu- and delta-opioid receptors, with the human NPFF1 receptor. The SH1-C7 clone expresses high affinity NPFF1 receptors in the same range order of density as opioid receptors. Similarly to the opioids, acute stimulation with the NPFF1 agonist NPVF inhibits adenylyl cyclase activity and voltage-gated (N-type) Ca2+ currents and enhances the intracellular Ca2+ release triggered by muscarinic receptors activation. In contrast, preincubation of cells with NPVF decreases the response to opioids on both calcium signaling, thus reproducing the cellular anti-opioid activity described in neurons. SH1-C7 cells are therefore a suitable model to investigate the interactions between NPFF and opioid receptors.

Anti-opioid effects of neuropeptide FF receptors in the ventral tegmental area.

The present study investigates the modulatory effects of neuropeptide FF (NPFF) receptors on the mesolimbic dopaminergic pathway controlled by opioid receptors. A stable NPFF(2) receptor agonist, dNPA, was injected into the ventral tegmental area (VTA) and the release of dopamine and serotonin within the nucleus accumbens (NAc), induced by intraperitoneal injection of morphine, was monitored using the brain microdialysis, in non-constrained rat. dNPA decreased systemic morphine-induced elevation of dopamine and serotonin metabolites within the NAc. Furthermore, co-injected with morphine into the VTA, NPFF inhibited morphine-induced stereotypy 60-120min after the injection. This neurochemical and behavioural anti-opioid effect mediated by NPFF(2) receptors at the level of VTA suggests the involvement of NPFF in the rewarding effects of opiates on the mesolimbic dopamine system.

A rapid release of corticosteroid-binding globulin from the liver restrains the glucocorticoid hormone response to acute stress.

A strict control of glucocorticoid hormone responses to stress is essential for health. In blood, glucocorticoid hormones are for the largest part bound to corticosteroid-binding globulin (CBG), and just a minor fraction of hormone is free. Only free glucocorticoid hormone is able to exert biological effects, but little is known about its regulation during stress. We found, using a dual-probe in vivo microdialysis method, that in rats, the forced-swim stress-induced rise in free corticosterone (its major glucocorticoid hormone) is strikingly similar in the blood and in target compartments such as the subcutaneous tissue and the brain. However, in all compartments, the free corticosterone response was delayed by 20-30 min as compared with the total corticosterone response in the blood. We discovered that CBG is the key player in this delay. Swim stress evoked a fast (within 5 min) and profound rise in CBG protein and binding capacity in the blood through a release of the protein from the liver. Thus, the increase in circulating CBG levels after stress restrains the rise in free corticosterone concentrations for approximately 20 min in the face of mounting total hormone levels in the circulation. The stress-induced increase in CBG seems to be specific for moderate and strong stressors. Both restraint stress and forced swimming caused an increase in circulating CBG, whereas its levels were not affected by mild novelty stress. Our data uncover a new, highly dynamic role for CBG in the regulation of glucocorticoid hormone physiology after acute stress.

Modulation by neuropeptide FF of the interaction of mu-opioid (MOP) receptor with G-proteins.

The Neuropeptide FF (NPFF) system is known to modulate the effects of opioids in vivo and in vitro. In the present study, we have investigated the effect of NPFF agonists on the coupling of the Mu-opioid (MOP) receptor to G-proteins in a model of SH-SY5Y cells transfected with NPFF(2) receptor, in which the neuronal anti-opioid activity of NPFF was previously reproduced. Activation of G-proteins was monitored by [(35)S]GTPgammaS binding assay and analysis of G-protein subunits associated with MOP receptors was performed by Western blotting after immunoprecipitation of the receptor. The results demonstrate that concentrations of NPFF agonists that produce a cellular anti-opioid effect, did not affect the ability of the opioid agonist DAMGO to activate G-proteins. However, at saturating concentration of agonist or when expression of receptor was high, opioid and NPFF agonists did not stimulate [(35)S]GTPgammaS binding in an additive manner, indicating that both receptors share a common fraction of a G-protein pool. In addition, stimulation of NPFF receptors in living cells modified the G-protein environment of MOP receptor by favoring its interaction with alpha(s), alpha(i2) and beta subunits. This change in G-protein coupling to MOP receptor might participate in the mechanism by which NPFF agonists reduce the inhibitory activity of opioids.