M(1)-like muscarinic acetylcholine receptors regulate fast-spiking interneuron excitability in rat dentate gyrus

Increased excitability of dentate gyrus mossy cells occurs early in life in the Tg2576 model of Alzheimer's disease

INTRODUCTION

Hyperexcitability in Alzheimer's disease (AD) emerge early and contribute to disease progression. The dentate gyrus (DG) is implicated in hyperexcitability in AD. We hypothesized that mossy cells (MCs), regulators of DG excitability, contribute to early hyperexcitability in AD. Indeed, MCs generate hyperexcitability in epilepsy.

METHODS

Using the Tg2576 model and WT mice (∼1month-old), we compared MCs electrophysiologically, assessed c-Fos activity marker, Aβ expression and mice performance in a hippocampal-dependent memory task.

RESULTS

Tg2576 MCs exhibit increased spontaneous excitatory events and decreased inhibitory currents, increasing the charge transfer excitation/inhibition ratio. Tg2576 MC intrinsic excitability was enhanced, and showed higher c-Fos, intracellular Aβ expression, and axon sprouting. Granule cells only showed changes in synaptic properties, without intrinsic changes. The effects occurred before a memory task is affected.

DISCUSSION

Early electrophysiological and morphological alterations in Tg2576 MCs are consistent with enhanced excitability, suggesting an early role in DG hyperexcitability and AD pathophysiology.

HIGHLIGHTS

∘ MCs from 1 month-old Tg2576 mice had increased spontaneous excitatory synaptic input. ∘ Tg2576 MCs had reduced spontaneous inhibitory synaptic input. ∘ Several intrinsic properties were abnormal in Tg2576 MCs. ∘ Tg2576 GCs had enhanced synaptic excitation but no changes in intrinsic properties. ∘ Tg2576 MCs exhibited high c-Fos expression, soluble Aβ and axonal sprouting.

Oxidative Stress and Endoplasmic Reticulum Stress Contributes to Arecoline and Its Secondary Metabolites-Induced Dyskinesia in Zebrafish Embryos

Areca nut has been listed as one of the most addictive substances, along with tobacco, alcohol and caffeine. Areca nut contains seven psychoactive alkaloids; however, the effects of these alkaloids on embryonic development and motor behavior are rarely addressed in zebrafish embryo-larvae. Herein, we investigated the effects of exposure to three alkaloids (arecoline and secondary metabolites—arecaidine and arecoline N-oxide) on the developmental parameters, locomotive behavior, oxidative stress and transcriptome of zebrafish embryos. Zebrafish embryos exposed to different concentrations (0, 0.1, 1, 10, 100 and 1000 μM) of arecoline, arecaidine and arecoline N-oxide showed no changes in mortality and hatchability rates, but the malformation rate of zebrafish larvae was significantly increased in a dose-dependent manner and accompanied by changes in body length. Moreover, the swimming activity of zebrafish larvae decreased, which may be due to the increase in reactive oxygen species and the imbalance between oxidation and antioxidation. Meanwhile, transcriptome analysis showed that endoplasmic reticulum stress and the apoptosis p53 signaling pathway were significantly enriched after exposure to arecoline and arecoline N-oxide. However, arecaidine exposure focuses on protein synthesis and transport. These findings provide an important reference for risk assessment and early warning of areca nut alkaloid exposure.

Molecular and electrophysiological features of GABAergic neurons in the dentate gyrus reveal limited homology with cortical interneurons

GABAergic interneurons tend to diversify into similar classes across telencephalic regions. However, it remains unclear whether the electrophysiological and molecular properties commonly used to define these classes are discriminant in the hilus of the dentate gyrus. Here, using patch-clamp combined with single cell RT-PCR, we compare the relevance of commonly used electrophysiological and molecular features for the clustering of GABAergic interneurons sampled from the mouse hilus and primary sensory cortex. While unsupervised clustering groups cortical interneurons into well-established classes, it fails to provide a convincing partition of hilar interneurons. Statistical analysis based on resampling indicates that hilar and cortical GABAergic interneurons share limited homology. While our results do not invalidate the use of classical molecular marker in the hilus, they indicate that classes of hilar interneurons defined by the expression of molecular markers do not exhibit strongly discriminating electrophysiological properties.

Early changes in synaptic and intrinsic properties of dentate gyrus granule cells in a mouse model of Alzheimer's disease neuropathology and atypical effects of the cholinergic antagonist atropine

It has been reported that hyperexcitability occurs in a subset of patients with Alzheimer's disease (AD) and hyperexcitability could contribute to the disease. Several studies have suggested that the hippocampal dentate gyrus (DG) may be an important area where hyperexcitability occurs. Therefore, we tested the hypothesis that the principal DG cell type, granule cells (GCs), would exhibit changes at the single-cell level which would be consistent with hyperexcitability and might help explain it. We used the Tg2576 mouse, where it has been shown that hyperexcitability is robust at 2-3 months of age. GCs from 2 to 3-month-old Tg2576 mice were compared to age-matched wild type (WT) mice. Effects of muscarinic cholinergic antagonism were tested because previously we found that Tg2576 mice exhibited hyperexcitability in vivo that was reduced by the muscarinic cholinergic antagonist atropine, counter to the dogma that in AD one needs to boost cholinergic function. The results showed that GCs from Tg2576 mice exhibited increased frequency of spontaneous excitatory postsynaptic potentials/currents (sEPSP/Cs) and reduced frequency of spontaneous inhibitory synaptic events (sIPSCs) relative to WT, increasing the excitation:inhibition (E:I) ratio. There was an inward NMDA receptor-dependent current that we defined here as a novel synaptic current (nsC) in Tg2576 mice because it was very weak in WT mice. Intrinsic properties were distinct in Tg2576 GCs relative to WT. In summary, GCs of the Tg2576 mouse exhibit early electrophysiological alterations that are consistent with increased synaptic excitation, reduced inhibition, and muscarinic cholinergic dysregulation. The data support previous suggestions that the DG contributes to hyperexcitability and there is cholinergic dysfunction early in life in AD mouse models.

Gq-Coupled Muscarinic Receptor Enhancement of KCNQ2/3 Channels and Activation of TRPC Channels in Multimodal Control of Excitability in Dentate Gyrus Granule Cells

KCNQ (Kv7, "M-type") K⁺ channels and TRPC (transient receptor potential, "canonical") cation channels are coupled to neuronal discharge properties and are regulated via G_q/11-protein-mediated signals. Stimulation of G_q/11-coupled receptors both consumes phosphatidylinositol 4,5-bisphosphate (PIP₂) via phosphalipase Cβ hydrolysis and stimulates PIP₂ synthesis via rises in Ca²⁺ _i and other signals. Using brain-slice electrophysiology and Ca²⁺ imaging from male and female mice, we characterized threshold K⁺ currents in dentate gyrus granule cells (DGGCs) and CA1 pyramidal cells, the effects of G_q/11-coupled muscarinic M₁ acetylcholine (M₁R) stimulation on M current and on neuronal discharge properties, and elucidated the intracellular signaling mechanisms involved. We observed disparate signaling cascades between DGGCs and CA1 neurons. DGGCs displayed M₁R enhancement of M-current, rather than suppression, due to stimulation of PIP₂ synthesis, which was paralleled by increased PIP₂-gated G-protein coupled inwardly rectifying K⁺ currents as well. Deficiency of KCNQ2-containing M-channels ablated the M₁R-induced enhancement of M-current in DGGCs. Simultaneously, M₁R stimulation in DGGCs induced robust increases in [Ca²⁺]_i, mostly due to TRPC currents, consistent with, and contributing to, neuronal depolarization and hyperexcitability. CA1 neurons did not display such multimodal signaling, but rather M current was suppressed by M₁R stimulation in these cells, similar to the previously described actions of M₁R stimulation on M-current in peripheral ganglia that mostly involves PIP₂ depletion. Therefore, these results point to a pleiotropic network of cholinergic signals that direct cell-type-specific, precise control of hippocampal function with strong implications for hyperexcitability and epilepsy.SIGNIFICANCE STATEMENT At the neuronal membrane, protein signaling cascades consisting of ion channels and metabotropic receptors govern the electrical properties and neurotransmission of neuronal networks. Muscarinic acetylcholine receptors are G-protein-coupled metabotropic receptors that control the excitability of neurons through regulating ion channels, intracellular Ca²⁺ signals, and other second-messenger cascades. We have illuminated previously unknown actions of muscarinic stimulation on the excitability of hippocampal principal neurons that include M channels, TRPC (transient receptor potential, "canonical") cation channels, and powerful regulation of lipid metabolism. Our results show that these signaling pathways, and mechanisms of excitability, are starkly distinct between peripheral ganglia and brain, and even between different principal neurons in the hippocampus.

Modulation of Hippocampal Circuits by Muscarinic and Nicotinic Receptors

This article provides a review of the effects of activation of muscarinic and nicotinic receptors on the physiological properties of circuits in the hippocampal formation. Previous articles have described detailed computational hypotheses about the role of cholinergic neuromodulation in enhancing the dynamics for encoding in cortical structures and the role of reduced cholinergic modulation in allowing consolidation of previously encoded information. This article will focus on addressing the broad scope of different modulatory effects observed within hippocampal circuits, highlighting the heterogeneity of cholinergic modulation in terms of the physiological effects of activation of muscarinic and nicotinic receptors and the heterogeneity of effects on different subclasses of neurons.

Differential surface density and modulatory effects of presynaptic GABAB receptors in hippocampal cholecystokinin and parvalbumin basket cells

The perisomatic domain of cortical neurons is under the control of two major GABAergic inhibitory interneuron types: regular-spiking cholecystokinin (CCK) basket cells (BCs) and fast-spiking parvalbumin (PV) BCs. CCK and PV BCs are different not only in their intrinsic physiological, anatomical and molecular characteristics, but also in their presynaptic modulation of their synaptic output. Most GABAergic terminals are known to contain GABA_B receptors (GABA_BR), but their role in presynaptic inhibition and surface expression have not been comparatively characterized in the two BC types. To address this, we performed whole-cell recordings from CCK and PV BCs and postsynaptic pyramidal cells (PCs), as well as freeze-fracture replica-based quantitative immunogold electron microscopy of their synapses in the rat hippocampal CA1 area. Our results demonstrate that while both CCK and PV BCs contain functional presynaptic GABA_BRs, their modulatory effects and relative abundance are markedly different at these two synapses: GABA release is dramatically inhibited by the agonist baclofen at CCK BC synapses, whereas a moderate reduction in inhibitory transmission is observed at PV BC synapses. Furthermore, GABA_BR activation has divergent effects on synaptic dynamics: paired-pulse depression (PPD) is enhanced at CCK BC synapses, but abolished at PV BC synapses. Consistent with the quantitative differences in presynaptic inhibition, virtually all CCK BC terminals were found to contain GABA_BRs at high densities, but only 40% of PV BC axon terminals contain GABA_BRs at detectable levels. These findings add to an increasing list of differences between these two interneuron types, with implications for their network functions.

M1 muscarinic activation induces long-lasting increase in intrinsic excitability of striatal projection neurons

The dorsolateral striatum is critically involved in movement control and motor learning. Striatal function is regulated by a variety of neuromodulators including acetylcholine. Previous studies have shown that cholinergic activation excites striatal principal projection neurons, medium spiny neurons (MSNs), and this action is mediated by muscarinic acetylcholine subtype 1 receptors (M₁) through modulating multiple potassium channels. In the present study, we used electrophysiology techniques in conjunction with optogenetic and pharmacological tools to determine the long-term effects of striatal cholinergic activation on MSN intrinsic excitability. A transient increase in acetylcholine release in the striatum by optogenetic stimulation resulted in a long-lasting increase in excitability of MSNs, which was associated with hyperpolarizing shift of action potential threshold and decrease in afterhyperpolarization (AHP) amplitude, leading to an increase in probability of EPSP-action potential coupling. The M₁ selective antagonist VU0255035 prevented, while the M₁ selective positive allosteric modulator (PAM) VU0453595 potentiated the cholinergic activation-induced persistent increase in MSN intrinsic excitability, suggesting that M₁ receptors are critically involved in the induction of this long-lasting response. This M₁ receptor-dependent long-lasting change in MSN intrinsic excitability could have significant impact on striatal processing and might provide a novel mechanism underlying cholinergic regulation of the striatum-dependent motor learning and cognitive function. Consistent with this, behavioral studies indicate that potentiation of M₁ receptor signaling by VU0453595 enhanced performance of mice in cue-dependent water-based T-maze, a dorsolateral striatum-dependent learning task.

Neuromodulation of the Feedforward Dentate Gyrus-CA3 Microcircuit

The feedforward dentate gyrus-CA3 microcircuit in the hippocampus is thought to activate ensembles of CA3 pyramidal cells and interneurons to encode and retrieve episodic memories. The creation of these CA3 ensembles depends on neuromodulatory input and synaptic plasticity within this microcircuit. Here we review the mechanisms by which the neuromodulators aceylcholine, noradrenaline, dopamine, and serotonin reconfigure this microcircuit and thereby infer the net effect of these modulators on the processes of episodic memory encoding and retrieval.

A novel muscarinic receptor-independent mechanism of KCNQ2/3 potassium channel blockade by Oxotremorine-M

Inhibition of KCNQ (Kv7) potassium channels by activation of muscarinic acetylcholine receptors has been well established, and the ion currents through these channels have been long known as M-currents. We found that this cross-talk can be reconstituted in Xenopus oocytes by co-transfection of human recombinant muscarinic M1 receptors and KCNQ2/3 potassium channels. Application of the muscarinic acetylcholine receptor agonist Oxotremorine-methiodide (Oxo-M) between voltage pulses to activate KCNQ2/3 channels caused inhibition of the subsequent KCNQ2/3 responses. This effect of Oxo-M was blocked by the muscarinic acetylcholine receptor antagonist atropine. We also found that KCNQ2/3 currents were inhibited when Oxo-M was applied during an ongoing KCNQ2/3 response, an effect that was not blocked by atropine, suggesting that Oxo-M inhibits KCNQ2/3 channels directly. Indeed, also in oocytes that were transfected with only KCNQ2/3 channels, but not with muscarinic M1 receptors, Oxo-M inhibited the KCNQ2/3 response. These results show that besides the usual muscarinic acetylcholine receptor-mediated inhibition, Oxo-M also inhibits KCNQ2/3 channels by a direct mechanism. We subsequently tested xanomeline, which is a chemically distinct muscarinic acetylcholine receptor agonist, and oxotremorine, which is a close analogue of Oxo-M. Both compounds inhibited KCNQ2/3 currents via activation of M1 muscarinic acetylcholine receptors but, in contrast to Oxo-M, they did not directly inhibit KCNQ2/3 channels. Xanomeline and oxotremorine do not contain a positively charged trimethylammonium moiety that is present in Oxo-M, suggesting that such a charged moiety could be a crucial component mediating this newly described direct inhibition of KCNQ2/3 channels.

Effects of Hypocretin/Orexin and Major Transmitters of Arousal on Fast Spiking Neurons in Mouse Cortical Layer 6B

Fast spiking (FS) GABAergic neurons are thought to be involved in the generation of high-frequency cortical rhythms during the waking state. We previously showed that cortical layer 6b (L6b) was a specific target for the wake-promoting transmitter, hypocretin/orexin (hcrt/orx). Here, we have investigated whether L6b FS cells were sensitive to hcrt/orx and other transmitters associated with cortical activation. Recordings were thus made from L6b FS cells in either wild-type mice or in transgenic mice in which GFP-positive GABAergic cells are parvalbumin positive. Whereas in a control condition hcrt/orx induced a strong increase in the frequency, but not amplitude, of spontaneous synaptic currents, in the presence of TTX, it had no effect at all on miniature synaptic currents. Hcrt/orx effect was thus presynaptic although not by an action on glutamatergic terminals but rather on neighboring cells. In contrast, noradrenaline and acetylcholine depolarized and excited these cells through a direct postsynaptic action. Neurotensin, which is colocalized in hcrt/orx neurons, also depolarized and excited these cells but the effect was indirect. Morphologically, these cells exhibited basket-like features. These results suggest that hcrt/orx, noradrenaline, acetylcholine, and neurotensin could contribute to high-frequency cortical activity through an action on L6b GABAergic FS cells.

Exposure to Gulf War Illness chemicals induces functional muscarinic receptor maladaptations in muscle nociceptors

Chronic pain is a component of the multisymptom disease known as Gulf War Illness (GWI). There is evidence that pain symptoms could have been a consequence of prolonged and/or excessive exposure to anticholinesterases and other GW chemicals. We previously reported that rats exposed, for 8 weeks, to a mixture of anticholinesterases (pyridostigmine bromide, chlorpyrifos) and a Nav (voltage activated Na(+) channel) deactivation-inhibiting pyrethroid, permethrin, exhibited a behavior pattern that was consistent with a delayed myalgia. This myalgia-like behavior was accompanied by persistent changes to Kv (voltage activated K(+)) channel physiology in muscle nociceptors (Kv7, KDR). In the present study, we examined how exposure to the above agents altered the reactivity of Kv channels to a muscarinic receptor (mAChR) agonist (oxotremorine-M). Comparisons between muscle nociceptors harvested from vehicle and GW chemical-exposed rats revealed that mAChR suppression of Kv7 activity was enhanced in exposed rats. Yet in these same muscle nociceptors, a Stromatoxin-insensitive component of the KDR (voltage activated delayed rectifier K(+) channel) exhibited decreased sensitivity to activation of mAChR. We have previously shown that a unique mAChR-induced depolarization and burst discharge (MDBD) was exaggerated in muscle nociceptors of rats exposed to GW chemicals. We now provide evidence that both muscle and vascular nociceptors of naïve rats exhibit MDBD. Examination of the molecular basis of the MDBD in naïve animals revealed that while the mAChR depolarization was independent of Kv7, the action potential burst was modulated by Kv7 status. mAChR depolarizations were shown to be dependent, in part, on TRPA1. We argue that dysfunction of the MDBD could be a functional convergence point for maladapted ion channels and receptors consequent to exposure to GW chemicals.

Presynaptic cholinergic neuromodulation alters the temporal dynamics of short-term depression at parvalbumin-positive basket cell synapses from juvenile CA1 mouse hippocampus

Parvalbumin-positive basket cells (PV BCs) of the CA1 hippocampus are active participants in theta (5-12 Hz) and gamma (20-80 Hz) oscillations in vivo. When PV BCs are driven at these frequencies in vitro, inhibitory postsynaptic currents (IPSCs) in synaptically connected CA1 pyramidal cells exhibit paired-pulse depression (PPD) and multiple-pulse depression (MPD). Moreover, PV BCs express presynaptic muscarinic acetylcholine receptors (mAChRs) that may be activated by synaptically released acetylcholine during learning behaviors in vivo. Using acute hippocampal slices from the CA1 hippocampus of juvenile PV-GFP mice, we performed whole cell recordings from synaptically connected PV BC-CA1 pyramidal cell pairs to investigate how bath application of 10 μM muscarine impacts PPD and MPD at CA1 PV BC-pyramidal cell synapses. In accordance with previous studies, PPD and MPD magnitude increased with stimulation frequency. mAChR activation reduced IPSC amplitude and transiently reduced PPD, but MPD was largely maintained. Consistent with a reduction in release probability (pr), MPD and mAChR activation increased both the coefficient of variation of IPSC amplitudes and the fraction of failures. Using variance-mean analysis, we converted MPD trains to pr functions and developed a kinetic model that optimally fit six distinct pr conditions. The model revealed that vesicular depletion caused MPD and that recovery from depression was dependent on calcium. mAChR activation reduced the presynaptic calcium transient fourfold and initial pr twofold, thereby reducing PPD. However, mAChR activation slowed calcium-dependent recovery from depression during sustained repetitive activity, thereby preserving MPD. Thus the activation of presynaptic mAChRs optimally protects PV BCs from vesicular depletion during short bursts of high-frequency activity.

A kinetic model for the frequency dependence of cholinergic modulation at hippocampal GABAergic synapses

In this paper we use a simple model of presynaptic neuromodulation of GABA signaling to decipher paired whole-cell recordings of frequency dependent cholinergic neuromodulation at CA1 parvalbumin-containing basket cell (PV BC)-pyramidal cell synapses. Variance-mean analysis is employed to normalize the data, which is then used to estimate parameters in the mathematical model. Various parameterizations and hidden parameter dependencies are investigated using Markov Chain Monte Carlo (MCMC) parameter estimation techniques. This analysis reveals that frequency dependence of cholinergic modulation requires both calcium-dependent recovery from depression and mAChR-induced inhibition of presynaptic calcium entry. A reduction in calcium entry into the presynaptic terminal in the kinetic model accounted for the frequency-dependent effects of mAChR activation.

Cholinergic immunotoxin 192 IgG-SAPORIN alters subicular theta-gamma activity and impairs spatial learning in rats

Subiculum is an important structure of hippocampal formation and is a part of intra hippocampal network involved in spatial information processing. However, relatively very few studies are available in literature demonstrating the explicit role of subiculum in spatial information processing. The present study investigated the cholinergic modulation of subicular theta-gamma activity on spatial learning and memory functions in rats. The cholinergic projections to ventral subiculum were selectively eliminated using 192 IgG-SAPORIN. Eliminations of cholinergic inputs to ventral subiculum significantly reduced the subicular theta and enhanced the gamma activity during active wake and REM sleep states. In addition, the spatial learning was severely impaired following cholinergic elimination of ventral subiculum. The ChAT immunocytochemical studies showed sparse distribution of cholinergic fibers in the ventral subiculum confirming the cholinergic elimination to ventral subiculum. Cholinotoxic infusions to ventral subiculum did not alter the hippocampal cholinergic innervations and retained the hippocampal theta and gamma activities. The present findings support that cholinergic modulation of subicular theta-gamma oscillations is crucial for spatial information processing.

Direct excitation of parvalbumin-positive interneurons by M1 muscarinic acetylcholine receptors: roles in cellular excitability, inhibitory transmission and cognition

Parvalbumin-containing (PV) neurons, a major class of GABAergic interneurons, are essential circuit elements of learning networks. As levels of acetylcholine rise during active learning tasks, PV neurons become increasingly engaged in network dynamics. Conversely, impairment of either cholinergic or PV interneuron function induces learning deficits. Here, we examined PV interneurons in hippocampus (HC) and prefrontal cortex (PFC) and their modulation by muscarinic acetylcholine receptors (mAChRs). HC PV cells, visualized by crossing PV-CRE mice with Rosa26YFP mice, were anatomically identified as basket cells and PV bistratified cells in the stratum pyramidale; in stratum oriens, HC PV cells were electrophysiologically distinct from somatostatin-containing cells. With glutamatergic transmission pharmacologically blocked, mAChR activation enhanced PV cell excitability in both CA1 HC and PFC; however, CA1 HC PV cells exhibited a stronger postsynaptic depolarization than PFC PV cells. To delete M1 mAChRs genetically from PV interneurons, we created PV-M1 knockout mice by crossing PV-CRE and floxed M1 mice. The elimination of M1 mAChRs from PV cells diminished M1 mAChR immunoreactivity and muscarinic excitation of HC PV cells. Selective cholinergic activation of HC PV interneurons using Designer Receptors Exclusively Activated by Designer Drugs technology enhanced the frequency and amplitude of inhibitory synaptic currents in CA1 pyramidal cells. Finally, relative to wild-type controls, PV-M1 knockout mice exhibited impaired novel object recognition and, to a lesser extent, impaired spatial working memory, but reference memory remained intact. Therefore, the direct activation of M1 mAChRs on PV cells contributes to some forms of learning and memory.

Regulation of persistent activity in hippocampal mossy cells by inhibitory synaptic potentials

The hippocampal formation receives strong cholinergic input from the septal/diagonal band complex. Although the functional effects of cholinergic activation have been extensively studied in pyramidal neurons within the hippocampus and entorhinal cortex, less is known about the role of cholinergic receptors on dentate gyrus neurons. Using intracellular recordings from rat dentate hilar neurons, we find that activation of m1-type muscarinic receptors selectively increases the excitability of glutamatergic mossy cells but not of hilar interneurons. Following brief stimuli, cholinergic modulation reveals a latent afterdepolarization response in mossy cells that can extend the duration of stimulus-evoked depolarization by >100 msec. Depolarizing stimuli also could trigger persistent firing in mossy cells exposed to carbachol or an m1 receptor agonist. Evoked IPSPs attenuated the ADP response in mossy cells. The functional effect of IPSPs was amplified during ADP responses triggered in the presence of cholinergic receptor agonists but not during slowly decaying simulated ADPs, suggesting that modulation of ADP responses by IPSPs arises from destabilization of the intrinsic currents underlying the ADP. Evoked IPSPs also could halt persistent firing triggered by depolarizing stimuli. These results show that through intrinsic properties modulated by muscarinic receptors, mossy cells can prolong depolarizing responses to excitatory input and extend the time window where multiple synaptic inputs can summate. By actively regulating the intrinsic response to synaptic input, inhibitory synaptic input can dynamically control the integration window that enables detection of coincident inputs and shape the spatial pattern of hilar cell activity.

Early-life cocaine interferes with BDNF-mediated behavioral plasticity

An important aspect of goal-directed action selection is differentiating between actions that are more or less likely to be reinforced. With repeated performance or psychostimulant exposure, however, actions can assume stimulus-elicited-or "habitual"-qualities that are resistant to change. We show that selective knockdown of prelimbic prefrontal cortical Brain-derived neurotrophic factor (Bdnf) increases sensitivity to response-outcome associations, blocking habit-like behavioral inflexibility. A history of adolescent cocaine exposure, however, occludes the "beneficial" effects of Bdnf knockdown. This finding highlights a challenge in treating addiction-that drugs of abuse may bias decision-making toward habit systems even in individuals with putative neurobiological resiliencies.

Synaptic muscarinic response types in hippocampal CA1 interneurons depend on different levels of presynaptic activity and different muscarinic receptor subtypes

Depolarizing, hyperpolarizing and biphasic muscarinic responses have been described in hippocampal inhibitory interneurons, but the receptor subtypes and activity patterns required to synaptically activate muscarinic responses in interneurons have not been completely characterized. Using optogenetics combined with whole cell patch clamp recordings in acute slices, we measured muscarinic responses produced by endogenously released acetylcholine (ACh) from cholinergic medial septum/diagonal bands of Broca inputs in hippocampal CA1. We found that depolarizing responses required more cholinergic terminal stimulation than hyperpolarizing ones. Furthermore, elevating extracellular ACh with the acetylcholinesterase inhibitor physostigmine had a larger effect on depolarizing versus hyperpolarizing responses. Another subpopulation of interneurons responded biphasically, and periodic release of ACh entrained some of these interneurons to rhythmically burst. M4 receptors mediated hyperpolarizing responses by activating inwardly rectifying K(+) channels, whereas the depolarizing responses were inhibited by the nonselective muscarinic antagonist atropine but were unaffected by M1, M4 or M5 receptor modulators. In addition, activation of M4 receptors significantly altered biphasic interneuron firing patterns. Anatomically, interneuron soma location appeared predictive of muscarinic response types but response types did not correlate with interneuron morphological subclasses. Together these observations suggest that the hippocampal CA1 interneuron network will be differentially affected by cholinergic input activity levels. Low levels of cholinergic activity will preferentially suppress some interneurons via hyperpolarization and increased activity will recruit other interneurons to depolarize, possibly because of elevated extracellular ACh concentrations. These data provide important information for understanding how cholinergic therapies will affect hippocampal network function in the treatment of some neurodegenerative diseases.

Intracellular calcium level is an important factor influencing ion channel modulations by PLC-coupled metabotropic receptors in hippocampal neurons

Signaling pathways involving phospholipase C (PLC) are involved in various neural functions. Understanding how these pathways are regulated will lead to a better understanding of their roles in neural functions. Previous studies demonstrated that receptor-driven PLCβ activation depends on intracellular Ca(2+) concentration ([Ca(2+)]i), suggesting the possibility that PLCβ-dependent cellular responses are basically Ca(2+) dependent. To test this possibility, we examined whether modulations of ion channels driven by PLC-coupled metabotropic receptors are sensitive to [Ca(2+)]i using cultured hippocampal neurons. Muscarinic activation triggered an inward current at -100 mV (the equilibrium potential for K(+)) in a subpopulation of neurons. This current response was suppressed by pirenzepine (an M1-preferring antagonist), PLC inhibitor, non-selective cation channel blocker, and lowering [Ca(2+)]i. Using the neurons showing no response at -100 mV, effects of muscarinic activation on K(+) channels were examined at -40 mV. Muscarinic activation induced a transient decrease of the holding outward current. This current response was mimicked and occluded by XE991, an M-current K(+) channel blocker, suppressed by pirenzepine, PLC inhibitor and lowering [Ca(2+)]i, and enhanced by elevating [Ca(2+)]i. Similar results were obtained when group I metabotropic glutamate receptors were activated instead of muscarinic receptors. These results clearly show that ion channel modulations driven by PLC-coupled metabotropic receptors are dependent on [Ca(2+)]i, supporting the hypothesis that cellular responses induced by receptor-driven PLCβ activation are basically Ca(2+) dependent.

Dysregulation of neural calcium signaling in Alzheimer disease, bipolar disorder and schizophrenia

Neurons have highly developed Ca(2+) signaling systems responsible for regulating a large number of neural functions such as the control of brain rhythms, information processing and the changes in synaptic plasticity that underpin learning and memory. The tonic excitatory drive, which is activated by the ascending arousal system, is particularly important for processes such as sensory perception, cognition and consciousness. The Ca(2+) signaling pathway is a key component of this arousal system that regulates the neuronal excitability responsible for controlling the neural brain rhythms required for information processing and cognition. Dysregulation of the Ca(2+) signaling pathway responsible for many of these neuronal processes has been implicated in the development of some of the major neural diseases in man such as Alzheimer disease, bipolar disorder and schizophrenia. Various treatments, which are known to act by reducing the activity of Ca(2+) signaling, have proved successful in alleviating the symptoms of some of these neural diseases.

Layer-specific processing of excitatory signals in CA1 interneurons depends on postsynaptic M₂ muscarinic receptors

The hippocampus receives a diffuse cholinergic innervation which acts on pre- and postsynaptic sites to modulate neurotransmission and excitability of pyramidal cells and interneurons in an intricate fashion. As one missing piece in this puzzle, we explored how muscarinic receptor activation modulates the somatodendritic processing of glutamatergic input in CA1 interneurons. We performed whole-cell recordings from visually identified interneurons of stratum radiatum (SR) and stratum oriens (SO) and examined the effects of the cholinergic agonist carbachol (CCh) on EPSP-like waveforms evoked by brief glutamate pulses onto their proximal dendrites. In SO interneurons, CCh consistently reduced glutamate-induced postsynaptic potentials (GPSPs) in control rat and mice, but not in M₂ muscarinic receptor knockout mice. By contrast, the overwhelming majority of interneurons recorded in SR of control and M₂ receptor-deficient hippocampi exhibited muscarinic enhancement of GPSPs. Interestingly, the non-responding interneurons were strictly confined to the SR subfield closest to the subiculum. Our data suggest that postsynaptic modulation by acetylcholine of excitatory input onto CA1 interneurons occurs in a stratum-specific fashion, which is determined by the absence or presence of M₂ receptors in their (somato-)dendritic compartments. Thus cholinergic projections might be capable of recalibrating synaptic weights in different inhibitory circuits of the CA1 region.