A model of cholinergic suppression of hippocampal ripples through disruption of balanced excitation/inhibition

Sharp wave‐ripples (140–220 Hz) are patterns of brain activity observed in the local field potential of the hippocampus which are present during memory consolidation. As rodents switch from memory consolidation to memory encoding behaviors, cholinergic inputs to the hippocampus from neurons in the medial septum‐diagonal band of Broca cause a marked reduction in ripple incidence. The mechanism for this disruption in ripple power is not fully understood. In isolated neurons, the major effect of cholinergic input on hippocampal neurons is depolarization of the membrane potential, which affects both hippocampal pyramidal neurons and inhibitory interneurons. Using an existing model of ripple‐frequency oscillations that includes both pyramidal neurons and interneurons, we investigated the mechanism whereby depolarizing inputs to these neurons can affect ripple power and frequency. We observed that ripple power and frequency are maintained, as long as inputs to pyramidal neurons and interneurons are balanced. Preferential drive to pyramidal neurons or interneurons, however, affects ripple power and can disrupt ripple oscillations by pushing ripple frequency higher or lower. Thus, an imbalance in drive to pyramidal neurons and interneurons provides a means whereby cholinergic input can suppress hippocampal ripples.     

Human cerebrospinal fluid promotes spontaneous gamma oscillations in the hippocampus in vitro

Gamma oscillations (30–80 Hz) are fast network activity patterns frequently linked to cognition. They are commonly studied in hippocampal brain slices in vitro, where they can be evoked via pharmacological activation of various receptor families. One limitation of this approach is that neuronal activity is studied in a highly artificial extracellular fluid environment, as provided by artificial cerebrospinal fluid (aCSF). Here, we examine the influence of human cerebrospinal fluid (hCSF) on kainate‐evoked and spontaneous gamma oscillations in mouse hippocampus. We show that hCSF, as compared to aCSF of matched electrolyte and glucose composition, increases the power of kainate‐evoked gamma oscillations and induces spontaneous gamma activity in areas CA3 and CA1 that is reversed by washout. Bath application of atropine entirely abolished hCSF‐induced gamma oscillations, indicating critical contribution from muscarinic acetylcholine receptor‐mediated signaling. In separate whole‐cell patch clamp recordings from rat hippocampus, hCSF increased theta resonance frequency and strength in pyramidal cells along with enhancement of h‐current (I_h) amplitude. We found no evidence of intrinsic gamma frequency resonance at baseline (aCSF) among fast‐spiking interneurons, and this was not altered by hCSF. However, hCSF increased the excitability of fast‐spiking interneurons, which likely contributed to gamma rhythmogenesis. Our findings show that hCSF promotes network gamma oscillations in the hippocampus in vitro and suggest that neuromodulators distributed in CSF could have significant influence on neuronal network activity in vivo.     

Comparison between spontaneous and kainate-induced gamma oscillations in the mouse hippocampus in vitro

Neuronal synchronization at gamma frequency, implicated in cognition, can be evoked in hippocampal slices by pharmacological activation. We characterized spontaneous small‐amplitude gamma oscillations (SγO) recorded in area CA3 of mouse hippocampal slices and compared it with kainate‐induced gamma oscillations (KγO). SγO had a lower peak frequency, a more sinusoidal waveform and was spatially less coherent than KγO, irrespective of oscillation amplitude. CA3a had the smallest oscillation power, phase‐led CA3c by ～4 ms and had the highest SγO frequency in isolated subslices. During SγO CA3c neurons fired at the rebound of inhibitory postsynaptic potentials (IPSPs) that were associated with a current source in stratum lucidum, whereas CA3a neurons often fired from spikelets, 3–4 ms earlier in the cycle, and had smaller IPSPs. Kainate induced faster/larger IPSPs that were associated with an earlier current source in stratum pyramidale. SγO and KγO power were dependent on α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA) receptors, gap junctions and γ‐aminobutyric acid (GABA)_A receptors. SγO was suppressed by elevating extracellular KCl, blocking N‐methyl‐d ‐aspartate (NMDA) receptors or muscarinic receptors, or activating GluR5‐containing kainate receptors. SγO was not affected by blocking metabotropic glutamate receptors or hyperpolarization‐activated currents. The adenosine A₁ receptor antagonist 8‐cyclopentyl‐1,3‐dimethoxyxanthine (8‐CPT) and the CB1 cannabinoid receptor antagonist N‐(piperidin‐1‐yl)‐5‐(4‐iodophenyl)‐1‐(2,4‐dichlorophenyl)‐4‐methyl‐1H‐pyrazole‐3‐carboxamide (AM251) increased SγO power, indicating that endogenous adenosine and/or endocannabinoids suppress or prevent SγO in vitro. SγO emerges from a similar basic network as KγO, but differs in involvement of somatically projecting interneurons and pharmacological modulation profile. These observations advocate the use of SγO as a natural model for hippocampal gamma oscillations, particularly during less activated behavioural states.     

Balanced synaptic currents underlie low‐frequency oscillations in the subiculum

Numerous synaptic and intrinsic membrane mechanisms have been proposed for generating oscillatory activity in the hippocampus. Few studies, however, have directly measured synaptic conductances and membrane properties during oscillations. The time course and relative contribution of excitatory and inhibitory synaptic conductances, as well as the role of intrinsic membrane properties in amplifying synaptic inputs, remains unclear. To address this issue, we used an isolated whole hippocampal preparation that generates autonomous low‐frequency oscillations near the theta range. Using 2‐photon microscopy and expression of genetically encoded fluorophores, we obtained on‐cell and whole‐cell patch recordings of pyramidal cells and fast‐firing interneurons in the distal subiculum. Pyramidal cell and interneuron spiking shared similar phase‐locking to local field potential oscillations. In pyramidal cells, spiking resulted from a concomitant and balanced increase in excitatory and inhibitory synaptic currents. In contrast, interneuron spiking was driven almost exclusively by excitatory synaptic current. Thus, similar to tightly balanced networks underlying hippocampal gamma oscillations and ripples, balanced synaptic inputs in the whole hippocampal preparation drive highly phase‐locked spiking at the peak of slower network oscillations.     

Action of tachykinins in the rat hippocampus: modulation of inhibitory synaptic transmission

Substance P and other neuropeptides of the tachykinin family can powerfully excite CA1 hippocampal interneurons present in the CA1 region. In the present work we show that, by exciting hippocampal interneurons, tachykinins can indirectly inhibit pyramidal neurons. We found that tachykinins caused a decrease in the inhibitory synaptic current interval and an increase in the inhibitory synaptic current amplitude in almost all pyramidal neurons tested. This effect was tetrodotoxin sensitive. Tachykinins did not alter the frequency or amplitude of miniature inhibitory synaptic currents and were without effect on evoked inhibitory synaptic currents. Thus, these neuropeptides acted at the somatodendritic membrane of GABAergic interneurons, rather than at their axon terminals. The effect of substance P on spontaneous inhibitory synaptic currents could be mimicked by a selective agonist of NK1 receptors, but not by selective agonists of NK2 and NK3 receptors. It was suppressed by an NK1 receptor antagonist. In CA1 interneurons located in stratum radiatum, substance P generated a sustained tetrodotoxin-insensitive inward current or induced membrane depolarization and action potential firing. This direct excitatory action was mediated by NK1 receptors. Current-voltage relationships indicate that the net tachykinin-evoked current reversed in polarity at or near the K+ equilibrium potential, suggesting that a suppression of a resting K+ conductance was involved. By increasing the excitability of CA1 GABAergic interneurons, tachykinins can powerfully facilitate the inhibitory synaptic input to pyramidal neurons. This indirect inhibition could play a role in regulating short-term and/or long-term synaptic plasticity, promoting neuronal circuit synchronization or, in some physiopathological situations, influencing epileptogenesis.     

Oxytocin receptor agonists enhance inhibitory synaptic transmission in the rat hippocampus by activating interneurons in stratum pyramidale

Oxytocin probably plays a role as a neurotransmitter/neuromodulator in the hippocampus of the rat. Oxytocin binding sites are present in the subiculum and CA1 region and oxytocin can excite a class of CA1 nonpyramidal neurons. In the present work we characterized the effect of oxytocin on hippocampal synaptic transmission. Whole-cell recordings were obtained from pyramidal neurons, in conditions of nearly symmetrical chloride concentrations. The selective oxytocin receptor agonist, [Thr4,Gly7]-oxytocin (TGOT), caused an increase in the frequency and amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) in virtually all neurons. These peptide-enhanced IPSCs were blocked by bicuculline, but not by strychnine, and reversed near 0 mV, indicating that they were mediated by gamma-aminobutyric acid (GABA)A receptors. On average, TGOT caused a nearly threefold increase in the frequency and almost a doubling in the amplitude of spontaneous IPSCs. TGOT did not influence the frequency and the amplitude of miniature IPSCs or spontaneous excitatory postsynaptic currents (EPSCs), and had no effect on evoked IPSCs. The peptide did not affect the basic membrane properties of pyramidal neurons or their GABA sensitivity. Thus, TGOT facilitated inhibitory transmission by exerting an excitatory action on the soma and/or dendrites of GABAergic interneurons. Extracellular recordings were performed in interneurons located in various hippocampal strata. Their sensitivity to TGOT was compared to that of substance P (SP). Interneurons in stratum pyramidale were excited both by TGOT and by SP. By contrast, stratum radiatum interneurons responded to SP but not to TGOT. In stratum oriens, half of the interneurons responded to SP, but only a minority to TGOT. Thus, oxytocin-responsive interneurons appear to be preferentially located in close vicinity of pyramidal neurons.     

Voltage clamp analysis of lamprey neurons — role of N-methyl-d-aspartate receptors in fictive locomotion

Spinal neurons in the lamprey have been subjected to a voltage clamp analysis of the excitatory currents generated during fictive locomotion with particular reference to the phasic activation of voltage dependent N-methyl-D-aspartate (NMDA) receptors. Voltage-clamped neurons observed during NMDA-induced fictive swimming show excitatory and inhibitory synaptic currents in phase with the ipsilateral and contralateral ventral root discharges, respectively. The excitatory synaptic currents showed a marked voltage dependence suggesting that potential sensitive conductances such as the NMDA ionophore are involved in the synaptic events underlying rhythmic locomotor activity. The effect of NMDA receptor activation during application of tetrodotoxin has also been analyzed during NMDA-induced pacemaker-like oscillations. Such NMDA-induced oscillations are essentially abolished during the voltage clamp. In the presence of NMDA current voltage plots reveal a negative slope conductance in the potential range of the inherent oscillations. The addition of tetraethyl ammonium (TEA) to NMDA solution enhanced a net steady state inward current by more than 10-fold due to a partial block of the outward currents. A kinetic analysis was done with a frequency domain technique using a white noise stimulus to linearly perturb the membrane potential over a wide range of frequencies. The analysis revealed that the induced negative conductance leads to a response which is nearly 180 degrees out of phase with the stimulus at low frequencies. This is an unstable condition which leads to the depolarizing phase of the induced oscillations.     

Modulation of AMPA receptor-mediated ion current by pituitary adenylate cyclase-activating polypeptide (PACAP) in CA1 pyramidal neurons from rat hippocampus

Pituitary adenylate cyclase-activating polypeptide (PACAP), a neurotrophic and neuromodulatory peptide, was recently shown to enhance NMDA receptor-mediated currents in the hippocampus (Macdonald, et al. 2005. J Neurosci 25:11374-11384). To check if PACAP might also modulate AMPA receptor function, we tested its effects on AMPA receptor-mediated synaptic currents on CA1 pyramidal neurons, using the patch clamp technique on hippocampal slices. In the presence of the NMDA antagonist D-AP5, PACAP (10 nM) reduced the amplitude of excitatory postsynaptic currents (EPSCs) evoked in CA1 pyramidal neurons by stimulation of Schaffer collaterals. Following a paired-pulse stimulation protocol, the paired-pulse ratio was unaffected in most neurons, suggesting that the AMPA-mediated EPSC was modulated by PACAP mainly at a postsynaptic level. PACAP also modulated the currents induced on CA1 pyramidal neurons by applications of either glutamate or AMPA. The effects of PACAP were dose-dependent: at a 0.5 nM dose, PACAP increased AMPA-mediated current; such effect was blocked by PACAP 6-38, a selective antagonist of PAC1 receptors. The enhancement of AMPA-mediated current by PACAP 0.5 nM was abolished when cAMPS-Rp, a PKA inhibitor, was added to the intracellular solution. At a 10 nM concentration, PACAP reduced AMPA-mediated current; such effect was not blocked by PACAP 6-38. The inhibitory effect of 10 nM PACAP was mimicked by Bay 55-9837 (a selective agonist of VPAC2 receptors), persisted in the presence of intracellular BAPTA and was abolished by intracellular cAMPS-Rp. Stimulation-evoked EPSCs in CA1 neurons were significantly reduced following application of the PAC1 antagonist PACAP 6-38; this result indicates that PAC1 receptors in the CA1 region are tonically activated by endogenous PACAP and enhance CA3-CA1 synaptic transmission. Our results show that PACAP differentially modulates AMPA receptor-mediated current in CA1 pyramidal neurons by activation of PAC1 and VPAC2 receptors, both involving the cAMP/PKA pathway; the functional significance will be discussed in light of the multiple effects exerted by PACAP on the CA3-CA1 synapse at different levels.     

Effects of insulin-like growth factor 1 on synaptic excitability in cultured rat hippocampal neurons

Insulin-like growth factor 1 (IGF-1) has important functions in the brain, including metabolic, neurotrophic, neuromodulatory and neuroendocrine actions, and it also prevents beta amyloid-induced death of hippocampal neurons. However, its functions in the synaptic excitability remain uncertain. Here we investigated the effects of IGF-1 on synaptic excitability in cultured rat hippocampal neurons using whole-cell patch clamp recordings. Incubation the hippocampal neurons with different concentrations of IGF-1 for 24 h or 30 min significantly increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs), but had no effect on the frequency of miniature EPSCs (mEPSCs) and spontaneous inhibitory postsynaptic currents (sIPSCs). The mean amplitudes, rise, and decay kinetics of sEPSCs, mEPSCs, and sIPSCs were not significantly affected by IGF-1, indicating that IGF-1 increased the probability of neurotransmitter release but did not modulate postsynaptic receptors. The effects of IGF-1 were mediated by mitogen-activated protein kinase (MAPK). IGF-1 activated the ERK1/2 signaling pathway in cultured hippocampal neurons, and the inhibitor PD98059 blocked the enhancement of sEPSCs induced by IGF-1. These results demonstrated the regulatory function of IGF-1 on synaptic excitability in hippocampal neurons and its underlying signaling mechanism.     

Excitatory amino acid-evoked membrane currents and excitatory synaptic transmission in lamprey reticulospinal neurons

The characteristics of excitatory amino acid-evoked currents and of excitatory synaptic events have been examined in lamprey Müller neurons using voltage clamp and current clamp recording techniques. Application of glutamate evoked depolarizations associated with a decrease in input resistance. The reversal potential of the responses was -35 mV. Under voltage clamp conditions, a series of excitatory amino acid agonists evoked inward currents associated with little or no increase in baseline current noise. The order of potency of the excitatory amino acid agonists was quisqualate greater than kainate greater than glutamate greater than aspartate, while N-methyl-D-aspartic acid (NMDA) was inactive. Inward currents evoked by glutamate, as well as by kainate and quisqualate were attenuated reversibly by 1 mM kynurenic acid (KYN). In contrast, glutamate-evoked currents were not affected by 100 microM D(-)-2-amino-5-phosphonovaleric acid (APV), a selective NMDA antagonist. Spontaneously occurring and stimulus-evoked excitatory postsynaptic events were antagonized reversibly by 1 mM KYN. At this concentration, KYN had no effect on membrane potential, input resistance, or excitability of the cells. In contrast, excitatory postsynaptic currents were unaffected by APV. It is concluded that both glutamate responses and excitatory synaptic transmission in lamprey Müller neurons are mediated by non-NMDA-type receptors and that these receptors are associated with ionic channels with a low elementary conductance. The combined pharmacological and biophysical characteristics of these responses are therefore different from those previously reported in other preparations. Spontaneous (but not stimulus-evoked) inhibitory synaptic events in Müller neurons were blocked reversibly by 1 mM KYN but not by 100 microM APV, suggesting that excitation of interneurons inhibitory to Müller cells was also mediated by non-NMDA receptors.     

Early NMDA Receptor Ablation in Interneurons Causes an Activity-Dependent E/I Imbalance in vivo in Prefrontal Cortex Pyramidal Neurons of a Mouse Model Useful for the Study of Schizophrenia

Altered Excitatory/Inhibitory (E/I) balance of cortical synaptic inputs has been proposed as a central pathophysiological factor for psychiatric neurodevelopmental disorders, including schizophrenia (SZ). However, direct measurement of E/I synaptic balance have not been assessed in vivo for any validated SZ animal model. Using a mouse model useful for the study of SZ we show that a selective ablation of NMDA receptors (NMDAr) in cortical and hippocampal interneurons during early postnatal development results in an E/I imbalance in vivo, with synaptic inputs to pyramidal neurons shifted towards excitation in the adult mutant medial prefrontal cortex (mPFC). Remarkably, this imbalance depends on the cortical state, only emerging when theta and gamma oscillations are predominant in the network. Additional brain slice recordings and subsequent 3D morphological reconstruction showed that E/I imbalance emerges after adolescence concomitantly with significant dendritic retraction and dendritic spine re-localization in pyramidal neurons. Therefore, early postnatal ablation of NMDAr in cortical and hippocampal interneurons developmentally impacts on E/I imbalance in vivo in an activity-dependent manner.     

成年小鼠癫痫发作早期突触内、外N-甲基-D-天冬氨酸受体介导的电流变化

在癫痫疾病的发生和反复发作的过程中,N-甲基-D-天冬氨酸（NMDA）型谷氨酸受体（NMDA受体）起着重要的作用。近年来的研究发现,突触内和突触外的NMDA受体在包括突触可塑性和细胞死亡的信号通路中起着不同的甚至是截然相反的作用。因此,我们在本研究中探讨了突触内、外NMDA受体介导的兴奋性突触后电流在癫痫发病的病理过程中的变化。我们利用氯化锂联合匹罗卡品（pilocarpine, PILO）诱导了成年癫痫小鼠模型,并在癫痫发作24小时后制作了急性海马切片,利用膜片钳全细胞记录法对CA1区锥体神经元的突触内、外NMDA受体电流进行了记录。我们发现,1）突触内NMDA受体电流的上升时间及衰减时间与对照组比较均无统计学差异;2）突触外NMDA受体电流的兴奋性电流峰值、面积峰值比、以及上升时间亦无统计学差异;3）但突触外电流的衰减时程相对于对照组加快。以上结果提示突触外NMDA受体可能参与癫痫的发病机制。     

3‐Hz subthreshold oscillations of CA2 neurons In vivo

The CA2 region is unique in the hippocampus; it receives direct synaptic innervations from several hypothalamic nuclei and expresses various receptors of neuromodulators, including adenosine, vasopressin, and oxytocin. Furthermore, the CA2 region may have distinct brain functions, such as the control of instinctive and social behaviors; however, little is known about the dynamics of the subthreshold membrane potentials of CA2 neurons in vivo. We conducted whole‐cell current‐clamp recordings from CA2 pyramidal cells in urethane‐anesthetized mice and monitored the intrinsic fluctuations in their membrane potentials. The CA2 pyramidal cells emitted spontaneous action potentials at mean firing rates of ～0.8 Hz. In approximately half of the neurons, the subthreshold membrane potential oscillated at ～3 Hz. In two neurons, we obtained simultaneous recordings of local field potentials from the CA1 stratum radiatum and demonstrated that the 3‐Hz oscillations of CA2 neurons were not correlated with CA1 field potentials. In tetrodotoxin‐perfused acute hippocampal slices, the membrane potentials of CA2 pyramidal cells were not preferentially entrained to 3‐Hz sinusoidal current inputs, which suggest that intracellular 3‐Hz oscillations reflect the neuronal dynamics of the surrounding networks. © 2016 Wiley Periodicals, Inc.     

Effects of Thyrotropin-releasing Hormone on GABAergic Synaptic Transmission of the Rat Hippocampus

The effect of thyrotropin-releasing hormone (TRH), a neuropeptide physiologically present in the mammalian hippocampus, on spontaneous, miniature and evoked GABAergic postsynaptic currents was investigated using whole-cell patch-clamp recording from pyramidal cells and interneurons of the rat hippocampal thin slice preparation. Bath application of 10 μM TRH induced an increase in the frequency of spontaneous postsynaptic currents from 1.07 ± 0.68 to 3.16 ± 0.73 Hz in pyramidal neurons and interneurons of the stratum lacunosum-moleculare (SL-M). In tetrodotoxin solution TRH did not change miniature postsynaptic currents. Application of TRH to minislices containing only the CA1 region still produced an increase in the spontaneous postsynaptic current frequency, indicative of an action by TRH upon a local GABAergic circuitry. Paired recordings from one pyramidal cell and one stratum lacunosum moleculare interneuron displayed synchronous events whose frequency rose after TRH application, suggestive of a common, TRH-sensitive input. In a small subset of cells TRH induced the appearance of highly rhythmic large postsynaptic currents at a frequency of ∼2 Hz, as confirmed by autocorrelation analysis. Postsynaptic currents electrically evoked by focal stimulation of stratum lacunosum-moleculare were depressed from 90 ± 27 to 44 ± 15 pA after application of TRH. This phenomenon was solely due to an increase in the number of synaptic failures. It is proposed that the effect of TRH on the GABAergic system was primarily exerted on a subset of interneurons controlling the activity of pyramidal cells as well as stratum lacunosum-moleculare interneurons.     

Acetylcholine modulates gamma frequency oscillations in the hippocampus by activation of muscarinic M1 receptors

Modulation of gamma oscillations is important for the processing of information and the disruption of gamma oscillations is a prominent feature of schizophrenia and Alzheimer's disease. Gamma oscillations are generated by the interaction of excitatory and inhibitory neurons where their precise frequency and amplitude are controlled by the balance of excitation and inhibition. Acetylcholine enhances the intrinsic excitability of pyramidal neurons and suppresses both excitatory and inhibitory synaptic transmission, but the net modulatory effect on gamma oscillations is not known. Here, we find that the power, but not frequency, of optogenetically induced gamma oscillations in the CA3 region of mouse hippocampal slices is enhanced by low concentrations of the broad‐spectrum cholinergic agonist carbachol but reduced at higher concentrations. This bidirectional modulation of gamma oscillations is replicated within a mathematical model by neuronal depolarisation, but not by reducing synaptic conductances, mimicking the effects of muscarinic M1 receptor activation. The predicted role for M1 receptors was supported experimentally; bidirectional modulation of gamma oscillations by acetylcholine was replicated by a selective M1 receptor agonist and prevented by genetic deletion of M1 receptors. These results reveal that acetylcholine release in CA3 of the hippocampus modulates gamma oscillation power but not frequency in a bidirectional and dose‐dependent manner by acting primarily through muscarinic M1 receptors.     

Nicotinic activity layer specifically modulates synaptic potentiation in the mouse insular cortex

Nicotinic acetylcholine receptors (nAChRs) in the insular cortex play an important role in nicotine addiction, but its cellular and synaptic mechanisms underlying nicotine addiction still remain unresolved. In layer 5 pyramidal neurons of the mouse insular cortex, activation of nAChRs suppresses synaptic potentiation through enhancing GABAergic synaptic transmission via activation of β2‐containing nAChRs in non‐fast‐spiking (non‐FS) interneurons. However, it has not been addressed whether and how activation of nAChRs modulates synaptic plasticity in layers 3 and 6 pyramidal neurons of the insular cortex. In this study, I demonstrate that activation of nAChRs oppositely modulates synaptic potentiation in layers 3 and 6 pyramidal neurons of the insular cortex. In layer 3 pyramidal neurons, activation of nAChRs depressed synaptic potentiation induced by combination of presynaptic stimulation with postsynaptic depolarization (paired training) through enhancing GABAergic synaptic transmission via activation of β2‐containing nAChRs in non‐FS interneurons. By contrast, in layer 6 pyramidal neurons, activation of nAChRs enhanced synaptic potentiation through activating postsynaptic β2‐containing nAChRs. These results indicate, in different layers of the mouse insular cortex, paired training‐induced synaptic potentiation is oppositely regulated by activation of nAChRs which are located on GABAergic interneurons (layer 3) and on pyramidal neurons (layer 6). Thus, layer‐specific modulation of synaptic potentiation may be involved in cellular and synaptic mechanisms of insular cortical changes in nicotine addiction.     

Differential regulation of spontaneous and evoked inhibitory synaptic transmission in somatosensory cortex by retinoic acid

Retinoic acid (RA), a developmental morphogen, has emerged in recent studies as a novel synaptic signaling molecule that acts in mature hippocampal neurons to modulate excitatory and inhibitory synaptic transmission in the context of homeostatic synaptic plasticity. However, it is unclear whether RA is capable of modulating neural circuits outside of the hippocampus, and if so, whether the mode of RA's action at synapses is similar to that within the hippocampal network. Here we explore for the first time RA's synaptic function outside the hippocampus and uncover a novel function of all‐trans retinoic acid at inhibitory synapses. Acute RA treatment increases spontaneous inhibitory synaptic transmission in L2/3 pyramidal neurons of the somatosensory cortex, and this effect requires expression of RA's receptor RARα both pre‐ and post‐synaptically. Intriguingly, RA does not seem to affect evoked inhibitory transmission assayed with either extracellular stimulation or direct activation of action potentials in presynaptic interneurons at connected pairs of interneurons and pyramidal neurons. Taken together, these results suggest that RA's action at synapses is not monotonous, but is diverse depending on the type of synaptic connection (excitatory versus inhibitory) and circuit (hippocampal versus cortical). Thus, synaptic signaling of RA may mediate multi‐faceted regulation of synaptic plasticity. In addition to its classic roles in brain development, retinoic acid (RA) has recently been shown to regulate excitatory and inhibitory transmission in the adult brain. Here, the authors show that in layer 2/3 (L2/3) of the somatosensory cortex (S1), acute RA induces increases in spontaneous but not action‐potential evoked transmission, and that this requires retinoic acid receptor (RARα) both in presynaptic PV‐positive interneurons and postsynaptic pyramidal (PN) neurons.     

Frequency‐dependent regulation of intrinsic excitability by voltage‐activated membrane conductances,computational modeling and dynamic clamp

As one of the most unique properties of nerve cells, their intrinsic excitability allows them to transform synaptic inputs into action potentials. This process reflects a complex interplay between the synaptic inputs and the voltage‐dependent membrane currents of the postsynaptic neuron. While neurons in natural conditions mostly fire under the action of intense synaptic bombardment and receive fluctuating patterns of excitation and inhibition, conventional techniques to characterize intrinsic excitability mainly utilize static means of stimulation. Recently, we have shown that voltage‐gated membrane currents regulate the firing responses under current step stimulation and under physiologically more realistic inputs in a differential manner. At the same time, a multitude of neuron types have been shown to exhibit some form of subthreshold resonance that potentially allows them to respond to synaptic inputs in a frequency‐selective manner. In this study, we performed virtual experiments in computational models of neurons to examine how specific voltage‐gated currents regulate their excitability under simulated frequency‐modulated synaptic inputs. The model simulations and subsequent dynamic clamp experiments on mouse hippocampal pyramidal neurons revealed that the impact of voltage‐gated currents in regulating the firing output is strongly frequency‐dependent and mostly affecting the synaptic integration at theta frequencies. Notably, robust frequency‐dependent regulation of intrinsic excitability was observed even when conventional analysis of membrane impedance suggested no such tendency. Consequently, plastic or homeostatic regulation of intrinsic membrane properties can tune the frequency selectivity of neuron populations in a way that is not readily expected from subthreshold impedance measurements.     

Excitatory synaptic inputs to pyramidal neurons of the lateral amygdala

Whole-cell patch clamp recordings were made from pyramidal neurons in the rat lateral amygdala (LA). Synaptic currents were evoked by stimulating in either the external capsule (ec), internal capsule (ic) or basolateral nucleus (BLA). Stimulation of either the ic, ec or BLA evoked a glutamatergic excitatory synaptic current (EPSC) which was mediated by both non-NMDA and NMDA (N-methyl-d-aspartic acid) receptors. The ratio of the amplitude of the NMDA receptor-mediated component measured at +40 mV to the amplitude of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) component measured at -60 mV was similar regardless of whether EPSCs were evoked in the ec, ic or BLA. At resting membrane potentials, excitatory synaptic potentials evoked from either the ec or putative thalamic inputs were unaffected by application of the NMDA receptor antagonist APV. Spontaneous glutamatergic currents had two components to their decay phase. The slow component was selectively blocked by the NMDA receptor antagonist D-APV, indicating that AMPA and NMDA receptors are colocalized in spiny neurons. We conclude that pyramidal cells of the LA receive convergent inputs from the cortex, thalamus and basal nuclei. At all inputs, both AMPA/kainate and NMDA-type receptors are active and colocalized in the postsynaptic density.