Arjen B. Brussaard

Plasticity in Fast Synaptic Inhibition of Adult Oxytocin Neurons Caused by Switch in GABAA Receptor Subunit Expression

We found that magnocellular oxytocin neurons in adult female rats exhibit an endogenous GABA(A) receptor subunit switch around parturition: a decrease in alpha1:alpha2 subunit mRNA ratio correlated with a decrease in allopregnanolone potentiation and increase in decay time constant of the GABA(A) receptor-mediated IPSCs in these cells. The causal relationship between changes in alpha1:alpha2 mRNA ratio and the ion channel kinetics was confirmed using in vitro antisense deletion. Further, GABA(A) receptors exhibited a tonic inhibitory influence upon oxytocin release in vivo, and allopregnanolone helped to restrain oxytocin neuron in vitro firing only before parturition, when the alpha1:alpha2 subunit mRNA ratio was still high. Such observations provide evidence for the physiological significance of GABA(A) receptor subunit heterogeneity and plasticity in the adult brain.

Distributed network actions by nicotine increase the threshold for spike-timing-dependent plasticity in prefrontal cortex.

Nicotine enhances attention and working memory by activating nicotinic acetylcholine receptors (nAChRs). The prefrontal cortex (PFC) is critical for these cognitive functions and is also rich in nAChR expression. Specific cellular and synaptic mechanisms underlying nicotines effects on cognition remain elusive. Here we show that nicotine exposure increases the threshold for synaptic spike-timing-dependent potentiation (STDP) in layer V pyramidal neurons of the mouse PFC. During coincident presynaptic and postsynaptic activity, nicotine reduces dendritic calcium signals associated with action potential propagation by enhancing GABAergic transmission. This results from a series of presynaptic actions involving different PFC interneurons and multiple nAChR subtypes. Pharmacological block of nAChRs or GABA(A) receptors prevented nicotines actions and restored STDP, as did increasing dendritic calcium signals with stronger postsynaptic activity. Thus, by activating nAChRs distributed throughout the PFC neuronal network, nicotine affects PFC information processing and storage by increasing the amount of postsynaptic activity necessary to induce STDP.

Nicotinic modulation of neuronal networks: from receptors to cognition

RationaleNicotine affects many aspects of human cognition, including attention and memory. Activation of nicotinic acetylcholine receptors (nAChRs) in neuronal networks modulates activity and information processing during cognitive tasks, which can be observed in electroencephalograms (EEGs) and functional magnetic resonance imaging studies.ObjectivesIn this review, we will address aspects of nAChR functioning as well as synaptic and cellular modulation important for nicotinic impact on neuronal networks that ultimately underlie its effects on cognition. Although we will focus on general mechanisms, an emphasis will be put on attention behavior and nicotinic modulation of prefrontal cortex. In addition, we will discuss how nicotinic effects at the neuronal level could be related to its effects on the cognitive level through the study of electrical oscillations as observed in EEGs and brain slices.Results/ConclusionsVery little is known about mechanisms of how nAChR activation leads to a modification of electrical oscillation frequencies in EEGs. The results of studies using pharmacological interventions and transgenic animals implicate some nAChR types in aspects of cognition, but neuronal mechanisms are only poorly understood. We are only beginning to understand how nAChR distribution in neuronal networks impacts network functioning. Unveiling receptor and neuronal mechanisms important for nicotinic modulation of cognition will be instrumental for treatments of human disorders in which cholinergic signaling have been implicated, such as schizophrenia, attention deficit/hyperactivity disorder, and addiction.

Altered temporal correlations in parietal alpha and prefrontal theta oscillations in early-stage Alzheimer disease

Encoding and retention of information in memory are associated with a sustained increase in the amplitude of neuronal oscillations for up to several seconds. We reasoned that coordination of oscillatory activity over time might be important for memory and, therefore, that the amplitude modulation of oscillations may be abnormal in Alzheimer disease (AD). To test this hypothesis, we measured magnetoencephalography (MEG) during eyes-closed rest in 19 patients diagnosed with early-stage AD and 16 age-matched control subjects and characterized the autocorrelation structure of ongoing oscillations using detrended fluctuation analysis and an analysis of the life- and waiting-time statistics of oscillation bursts. We found that Alzheimers patients had a strongly reduced incidence of alpha-band oscillation bursts with long life- or waiting-times (< 1 s) over temporo-parietal regions and markedly weaker autocorrelations on long time scales (1–25 seconds). Interestingly, the life- and waiting-times of theta oscillations over medial prefrontal regions were greatly increased. Whereas both temporo-parietal alpha and medial prefrontal theta oscillations are associated with retrieval and retention of information, metabolic and structural deficits in early-stage AD are observed primarily in temporo-parietal areas, suggesting that the enhanced oscillations in medial prefrontal cortex reflect a compensatory mechanism. Together, our results suggest that amplitude modulation of neuronal oscillations is important for cognition and that indices of amplitude dynamics of oscillations may prove useful as neuroimaging biomarkers of early-stage AD.

Interdependence of PKC-Dependent and PKC-Independent Pathways for Presynaptic Plasticity

Diacylglycerol (DAG) is a prominent endogenous modulator of synaptic transmission. Recent studies proposed two apparently incompatible pathways, via protein kinase C (PKC) and via Munc13. Here we show how these two pathways converge. First, we confirm that DAG analogs indeed continue to potentiate transmission after PKC inhibition (the Munc13 pathway), but only in neurons that previously experienced DAG analogs, before PKC inhibition started. Second, we identify an essential PKC pathway by expressing a PKC-insensitive Munc18-1 mutant in munc18-1 null mutant neurons. This mutant supported basic transmission, but not DAG-induced potentiation and vesicle redistribution. Moreover, synaptic depression was increased, but not Ca2+-independent release evoked by hypertonic solutions. These data show that activation of both PKC-dependent and -independent pathways (via Munc13) are required for DAG-induced potentiation. Munc18-1 is an essential downstream target in the PKC pathway. This pathway is of general importance for presynaptic plasticity.

Changes in properties and neurosteroid regulation of GABAergic synapses in the supraoptic nucleus during the mammalian female reproductive cycle

1 GABAA receptor‐mediated synaptic innervation of oxytocin neurones in the supraoptic nucleus (SON) was analysed in adult female rats going through their first reproductive cycle by recording the spontaneous inhibitory postsynaptic currents (sIPSCs) at six stages of female reproduction. 2 During pregnancy we observed a reduction in the interval between monoquantal sIPSCs. The synaptic current amplitude, current decay and neurosteroid sensitivity of postsynaptic GABAA receptors observed at this stage were not distinguishable from those measured in virgin stage SON. 3 Upon parturition an increase in monoquantal synaptic current decay occurred, whereas potentiation by the progesterone metabolite allopregnanolone (3α‐OH‐DHP) was suppressed. 4 Throughout a substantial part of the lactation period the decay of synaptic currents remained attenuated, whilst the potentiation by 3α‐OH‐DHP remained suppressed. 5 Several weeks after the end of lactation sIPSC intervals, their current decay velocity as well as the potentiation by 3α‐OH‐DHP were restored to pre‐pregnancy levels, which is indicative of the cyclical nature of synaptic plasticity in the adult SON. 6 Competitive polymerase chain reaction (PCR) analysis showed that virgin animals expressed α1 and α2 GABAA receptor subunit mRNA at a relative ratio of 2 : 1 compared with β‐actin. After pregnancy both α1 and α2 subunit mRNA levels were transiently increased, although at a relative ratio of 1 : 4, in line with the hypothesis that α2 plays a large role in postsynaptic receptor functioning. During post‐lactation both α subunits were downregulated. 7 We propose that synaptic remodelling in the SON during pregnancy includes changes in the putative number of GABA release sites per neurone. At parturition, and during the two consecutive weeks of lactation, a subtype of postsynaptic GABAA receptors was observed, distinct from the one being expressed before and during pregnancy. Synaptic current densities, calculated in order to compare the impact of synaptic inhibition, showed that, in particular, the differences in 3α‐OH‐DHP potentiation of these two distinct GABAA receptor subtypes produce robust shifts in the impact of synaptic inhibition of oxytocin neurones at the different stages of female reproduction.

Oxytocin Regulates Neurosteroid Modulation of GABA A Receptors in Supraoptic Nucleus around Parturition

In this study, we investigate how neurosteroid sensitivity of GABAA receptors (GABAARs) is regulated. We examined this issue in neurons of the supraoptic nucleus (SON) of the rat and found that, during parturition, the GABAARs become insensitive to the neurosteroid allopregnanolone attributable to a shift in the balance between the activities of endogenous Ser/Thr phosphatase and PKC. In particular, a constitutive endogenous tone of oxytocin within the SON after parturition suppressed neurosteroid sensitivity of GABAARs via activation of PKC. Vice versa before parturition, during late pregnancy, application of exogenous oxytocin brings the GABAARs from a neurosteroid-sensitive mode toward a condition in which the receptors are not sensitive. This indicates that there may be an inverse causal relationship between the extent to which the GABAAR or one of its interacting proteins is phosphorylated and the neurosteroid sensitivity of the GABAAR. Neurosteroid sensitivity was not affected by changes in subunit composition of GABAARs known to occur concurrently in these cells.

Postsynaptic mechanism of depression of GABAergic synapses by oxytocin in the supraoptic nucleus of immature rat.

1. Oxytocin is known to act on autoreceptors of oxytocin neurones in the supraoptic nucleus (SON). We investigated whether oxytocin modulates putative oxytocin neurones by suppressing the GABAA receptor‐mediated synaptic inputs on these cells. 2. GABAergic inhibitory postsynaptic currents (IPSCs) were recorded from SON neurones in hypothalamic slices from young rats. Oxytocin specifically reduced the amplitude of both spontaneous and evoked IPSCs, without altering their current kinetics. 3. The effect of oxytocin was observed in 70% of the magnocellular neurones recorded from the dorsomedial part of the SON. d(CH2)5OVT, a specific antagonist of oxytocin receptors, blocked the effect of oxytocin on the IPSCs. Vasopressin had no effect on oxytocin‐sensitive SON neurones. 4. The intervals between spontaneous IPSCs were not affected by oxytocin. This suggested that oxytocin had a postsynaptic effect on SON neurones. 5. This postsynaptic origin was further substantiated by application of TTX, which blocked all evoked release but did not prevent the suppressive effect of oxytocin on the amplitude of the spontaneous IPSCs still present in the recording. The selective effect of oxytocin on IPSC amplitude was also maintained in nominally zero extracellular calcium. 6. Intracellular perfusion of SON neurones with GTP gamma S mimicked the effect of oxytocin on IPSCs, while GDP beta S, similarly applied, abolished the effect of oxytocin. 7. Application of calcium mobilizers such as thapsigargin and caffeine also reduced the amplitude of spontaneous IPSCs without significantly altering the frequency at which IPSCs occurred. 8. Thus, oxytocin depresses GABAergic synapses in the SON via modulation of the postsynaptic GABAA receptors. This would lead to disinhibition of SON neurones sensitive to oxytocin and could, therefore, be a powerful means of controlling the firing of oxytocin neurones.

Neonatal development of the rat visual cortex: synaptic function of GABAa receptor α subunits

Each GABAA receptor consists of two α and three other subunits. The spatial and temporal distribution of different α subunit isomeres expressed by the CNS is highly regulated. Here we study changes in functional contribution of different α subunits during neonatal development in rat visual cortex. First, we characterized postsynaptic α subunit expression in layer II‐III neurons, using subunit‐specific pharmacology combined with electrophysiological recordings in acutely prepared brain slices. This showed clear developmental downregulation of the effects of bretazenil (1 μm) and marked upregulation of the effect of 100 nm of zolpidem on the decay of spontaneous inhibitory postsynaptic currents (sIPSCs). Given the concentrations used we interpret this as downregulation of the synaptic α3 and upregulation of α1 subunit. Furthermore, the effect of furosemide, being indicative of the functional contribution of α4, was increased between postnatal days 6 and 21. Our second aim was to study the effects of plasticity in α subunit expression on decay properties of GABAergic IPSCs. We found that bretazenil‐sensitive IPSCs have the longest decay time constant in juvenile neurons. In mature neurons, zolpidem‐ and furosemide‐sensitive IPSCs have relatively fast decay kinetics, whereas bretazenil‐sensitive IPSCs decay relatively slowly. Analysis of α1 deficient mice and α1 antisense oligonucleotide deletion in rat explants showed similar results to those obtained by zolpidem application. Thus, distinct α subunit contributions create heterogeneity in developmental acceleration of IPSC decay in neocortex.

Munc18-1 expression levels control synapse recovery by regulating readily releasable pool size

Prompt recovery after intense activity is an essential feature of most mammalian synapses. Here we show that synapses with reduced expression of the presynaptic gene munc18-1 suffer from increased depression during intense stimulation at glutamatergic, GABAergic, and neuromuscular synapses. Conversely, munc18-1 overexpression makes these synapses recover faster. Concomitant changes in the readily releasable vesicle pool and its refill kinetics were found. The number of vesicles docked at the active zone and the total number of vesicles per terminal correlated with both munc18-1 expression levels and the size of the releasable vesicle pool. These data show that varying expression of a single gene controls synaptic recovery by modulating the number of docked, release-ready vesicles and thereby replenishment of the secretion capacity.