Cholinergic signaling through synaptic SK channels: it's a protein kinase but which one?

Muscarinic Modulation of SK2-Type K+ Channels Promotes Intrinsic Plasticity in L2/3 Pyramidal Neurons of the Mouse Primary Somatosensory Cortex

Muscarinic acetylcholine receptors (mAChRs) inhibit small-conductance calcium-activated K+ channels (SK channels) and enhance synaptic weight via this mechanism. SK channels are also involved in activity-dependent plasticity of membrane excitability ("intrinsic plasticity"). Here, we investigate whether mAChR activation can drive SK channel-dependent intrinsic plasticity in L2/3 cortical pyramidal neurons. Using whole-cell patch-clamp recordings from these neurons in slices prepared from mouse primary somatosensory cortex (S1), we find that brief bath application of the mAChR agonist oxotremorine-m (oxo-m) causes long-term enhancement of excitability in wild-type mice that is not observed in mice deficient of SK channels of the SK2 isoform. Similarly, repeated injection of depolarizing current pulses into the soma triggers intrinsic plasticity that is absent from SK2 null mice. Intrinsic plasticity lowers spike frequency adaptation and attenuation of spike firing upon prolonged activation, consistent with SK channel modulation. Depolarization-induced plasticity is prevented by bath application of the protein kinase A (PKA) inhibitor H89, and the casein kinase 2 (CK2) inhibitor TBB, respectively. These findings point toward a recruitment of two known signaling pathways in SK2 regulation: SK channel trafficking (PKA) and reduction of the calcium sensitivity (CK2). Using mice with an inactivation of CaMKII (T305D mice), we show that intrinsic plasticity does not require CaMKII. Finally, we demonstrate that repeated injection of depolarizing pulses in the presence of oxo-m causes intrinsic plasticity that surpasses the plasticity amplitude reached by either manipulation alone. Our findings show that muscarinic activation enhances membrane excitability in L2/3 pyramidal neurons via a downregulation of SK2 channels.

Cholinergic Grb2-Associated-Binding Protein 1 Regulates Cognitive Function

Grb2-associated-binding protein 1 (Gab1) is a docking/scaffolding molecule known to play an important role in cell growth and survival. Here, we report that Gab1 is decreased in cholinergic neurons in Alzheimer's disease (AD) patients and in a mouse model of AD. In mice, selective ablation of Gab1 in cholinergic neurons in the medial septum impaired learning and memory and hippocampal long-term potentiation. Gab1 ablation also inhibited SK channels, leading to an increase in firing in septal cholinergic neurons. Gab1 overexpression, on the other hand, improved cognitive function and restored hippocampal CaMKII autorphosphorylation in AD mice. These results suggest that Gab1 plays an important role in the pathophysiology of AD and may represent a novel therapeutic target for diseases involving cholinergic dysfunction.

Molecular mechanism for muscarinic M1 receptor-mediated endocytosis of TWIK-related acid-sensitive K+ 1 channels in rat adrenal medullary cells

KEY POINTS

The muscarinic acetylcholine receptor (mAChR)-mediated increase in excitability in rat adrenal medullary cells is at least in part due to inhibition of TWIK (tandem of P domains in a weak inwardly rectifying K⁺ channel)-related acid-sensitive K⁺ (TASK)1 channels. In this study we focused on the molecular mechanism of mAChR-mediated inhibition of TASK1 channels. Exposure to muscarine resulted in a clathrin-dependent endocytosis of TASK1 channels following activation of the muscarinic M₁ receptor (M₁ R). This muscarinic signal for the endocytosis was mediated in sequence by phospholipase C (PLC), protein kinase C (PKC), and then the non-receptor tyrosine kinase Src with the consequent tyrosine phosphorylation of TASK1. The present results establish that TASK1 channels are tyrosine phosphorylated and internalized in a clathrin-dependent manner in response to M₁ R stimulation and this translocation is at least in part responsible for muscarinic inhibition of TASK1 channels in rat AM cells.

ABSTRACT

Activation of muscarinic receptor (mAChR) in rat adrenal medullary (AM) cells induces depolarization through the inhibition of TWIK-related acid-sensitive K⁺ (TASK)1 channels. Here, pharmacological and immunological approaches were used to elucidate the molecular mechanism for this mAChR-mediated inhibition. TASK1-like immunoreactive (IR) material was mainly located at the cell periphery in dissociated rat AM cells, and its majority was internalized in response to muscarine. The muscarine-induced inward current and translocation of TASK1 were suppressed by dynasore, a dynamin inhibitor. The muscarinic translocation was suppressed by MT7, a specific M₁ antagonist, and the dose-response curves for muscarinic agonist-induced translocation were similar to those for the muscarinic inhibition of TASK1 currents. The muscarine-induced inward current and/or translocation of TASK1 were suppressed by inhibitors for phospholipase C (PLC), protein kinase C (PKC), and/or Src. TASK1 channels in AM cells and PC12 cells were transiently associated with Src and were tyrosine phosphorylated in response to muscarinic stimulation. After internalization, TASK1 channels were quickly dephosphorylated even while they remained in the cytoplasm. The cytoplasmic TASK1-like IR material quickly recycled back to the cell periphery after muscarine stimulation for 0.5 min, but not 10 min. We conclude that M₁ R stimulation results in internalization of TASK1 channels through the PLC-PKC-Src pathway with the consequent phosphorylation of tyrosine and that this M₁ R-mediated internalization is at least in part responsible for muscarinic inhibition of TASK1 channels in rat AM cells.

Nicotinic ACh receptors in the hippocampal circuit; functional expression and role in synaptic plasticity

Acetylcholine (ACh) can regulate neuronal excitability in the hippocampus, an important area in the brain for learning and memory, by acting on both nicotinic (nAChRs) and muscarinic ACh receptors. The primary cholinergic input to the hippocampus arises from the medial septum and diagonal band of Broca (MS-DBB), and we investigated how their activation regulated hippocampal synaptic plasticity. We found that activation of these endogenous cholinergic inputs can directly induce different forms of hippocampal synaptic plasticity with a timing precision in the millisecond range. Furthermore, we observed a prolonged enhancement of excitability both pre- and postsynaptically. Lastly we found that the presence of the α7 nAChR subtype to both pre- and postsynaptic sites appeared to be required to induce this plasticity. We propose that α7 nAChRs coordinate pre- and postsynaptic activities to induce glutamatergic synaptic plasticity, and thus provide a novel mechanism underlying physiological neuronal communication that could lead to timing-dependent synaptic plasticity in the hippocampus.

Gain control of synaptic response function in cerebellar nuclear neurons by a calcium-activated potassium conductance

Small conductance Ca(2+)-activated potassium (SK) current provides an important modulator of excitatory synaptic transmission, which undergoes plastic regulation via multiple mechanisms. We examined whether inhibitory input processing is also dependent on SK current in the cerebellar nuclei (CN) where inhibition provides the only route of information transfer from the cerebellar cortical Purkinje cells. We employed dynamic clamping in conjunction with computer simulations to address this question. We found that SK current plays a critical role in the inhibitory synaptic control of spiking output. Specifically, regulation of SK current density resulted in a gain control of spiking output, such that low SK current promoted large output signaling for large inhibitory cell input fluctuations due to Purkinje cell synchronization. In contrast, smaller nonsynchronized Purkinje cell input fluctuations were not amplified. Regulation of SK density in the CN therefore would likely lead to important consequences for the transmission of synchronized Purkinje cell activity to the motor system.

Cholinergic receptors: functional role of nicotinic ACh receptors in brain circuits and disease

The neurotransmitter acetylcholine (ACh) can regulate neuronal excitability throughout the nervous system by acting on both the cys-loop ligand-gated nicotinic ACh receptor channels (nAChRs) and the G protein-coupled muscarinic ACh receptors (mAChRs). The hippocampus is an important area in the brain for learning and memory, where both nAChRs and mAChRs are expressed. The primary cholinergic input to the hippocampus arises from the medial septum and diagonal band of Broca, the activation of which can activate both nAChRs and mAChRs in the hippocampus and regulate synaptic communication and induce oscillations that are thought to be important for cognitive function. Dysfunction in the hippocampal cholinergic system has been linked with cognitive deficits and a variety of neurological disorders and diseases, including Alzheimer's disease and schizophrenia. My lab has focused on the role of the nAChRs in regulating hippocampal function, from understanding the expression and functional properties of the various subtypes of nAChRs, and what role these receptors may be playing in regulating synaptic plasticity. Here, I will briefly review this work, and where we are going in our attempts to further understand the role of these receptors in learning and memory, as well as in disease and neuroprotection.

Nicotinic ACh receptors in the hippocampus: role in excitability and plasticity

INTRODUCTION

The nicotinic ACh receptors (nAChRs) are in the cys-loop family of ligand-gated ion channels. They are widely expressed throughout the brain, including in the hippocampus where they are thought to be involved in regulating excitability, plasticity, and cognitive function. In addition, dysfunction in hippocampal nAChRs has been linked to a variety of neurological disorders and diseases, including Alzheimer's disease, schizophrenia, and epilepsy. In order to understand how to treat nAChR-related disorders and diseases, it is critical to understand how these receptors participate in normal brain function; this entails not only understanding the biophysical properties of ion channel function and their pattern of expression but also how these receptors are regulating excitability and circuit behavior.

DISCUSSION

The primary cholinergic input to the hippocampus comes from the medial septum and diagonal band of Broca; however, the mechanistic details are unknown of how activation of cholinergic receptors, either through exogenous nAChR ligands or the activation of endogenous acetylcholine release, regulates hippocampal network activity. This entails direct study of the excitatory and inhibitory neuronal networks, as well as the role of nonneuronal cells, in regulating hippocampal function.

CONCLUSIONS

Here, I will review the latest work from my laboratory in which we have attempted to do just that, with the overall goal of learning more about the role of the hippocampal nAChR in synaptic plasticity.

Timing-dependent septal cholinergic induction of dynamic hippocampal synaptic plasticity

Cholinergic modulation of hippocampal synaptic plasticity has been studied extensively by applying receptor agonists or blockers; however, the effect of rapid physiological cholinergic stimuli on plasticity is largely unknown. Here, we report that septal cholinergic input, activated either by electrical stimulation or via an optogenetic approach, induced different types of hippocampal Schaffer collateral (SC) to CA1 synaptic plasticity, depending on the timing of cholinergic input relative to the SC input. When the cholinergic input was activated 100 or 10 ms prior to SC stimulation, it resulted in α7 nAChR-dependent long-term potentiation (LTP) or short-term depression, respectively. When the cholinergic stimulation was delayed until 10 ms after the SC stimulation, a muscarinic AChR-dependent LTP was induced. Moreover, these various forms of plasticity were disrupted by Aβ exposure. These results have revealed the remarkable temporal precision of cholinergic functions, providing a novel mechanism for information processing in cholinergic-dependent higher cognitive functions.