CPT1C is required for synaptic plasticity and oscillatory activity that supports motor, associative and non-associative learning.

Carnitine palmitoyltransferase 1c (CPT1C) is a neuron-specific protein widely distributed throughout the CNS and highly expressed in discrete brain areas including the hypothalamus, hippocampus, amygdala and different motor regions. Its deficiency has recently been shown to disrupt dendritic spine maturation and AMPA receptor synthesis and trafficking in the hippocampus, but its contribution to synaptic plasticity and cognitive learning and memory processes remains mostly unknown. Here, we aimed to explore the molecular, synaptic, neural network and behavioural role of CPT1C in cognition-related functions by using CPT1C knockout (KO) mice. CPT1C-deficient mice showed extensive learning and memory deficits. The CPT1C KO animals exhibited impaired motor and instrumental learning that seemed to be related, in part, to locomotor deficits and muscle weakness but not to mood alterations. In addition, CPT1C KO mice showed detrimental hippocampus-dependent spatial and habituation memory, most probably attributable to inefficient dendritic spine maturation, impairments in long-term plasticity at the CA3-CA1 synapse and aberrant cortical oscillatory activity. In conclusion, our results reveal that CPT1C is not only crucial for motor function, coordination and energy homeostasis, but also has a crucial role in the maintenance of learning and memory cognitive functions. KEY POINTS: CPT1C, a neuron-specific interactor protein involved in AMPA receptor synthesis and trafficking, was found to be highly expressed in the hippocampus, amygdala and various motor regions. CPT1C-deficient animals exhibited energy deficits and impaired locomotion, but no mood changes were found. CPT1C deficiency disrupts hippocampal dendritic spine maturation and long-term synaptic plasticity and reduces cortical γ oscillations. CPT1C was found to be crucial for motor, associative and non-associative learning and memory.

Recognition Memory Induces Natural LTP-like Hippocampal Synaptic Excitation and Inhibition.

Synaptic plasticity is a cellular process involved in learning and memory by which specific patterns of neural activity adapt the synaptic strength and efficacy of the synaptic transmission. Its induction is governed by fine tuning between excitatory/inhibitory synaptic transmission. In experimental conditions, synaptic plasticity can be artificially evoked at hippocampal CA1 pyramidal neurons by repeated stimulation of Schaffer collaterals. However, long-lasting synaptic modifications studies during memory formation in physiological conditions in freely moving animals are very scarce. Here, to study synaptic plasticity phenomena during recognition memory in the dorsal hippocampus, field postsynaptic potentials (fPSPs) evoked at the CA3-CA1 synapse were recorded in freely moving mice during object-recognition task performance. Paired pulse stimuli were applied to Schaffer collaterals at the moment that the animal explored a new or a familiar object along different phases of the test. Stimulation evoked a complex synaptic response composed of an ionotropic excitatory glutamatergic fEPSP, followed by two inhibitory responses, an ionotropic, GABAA-mediated fIPSP and a metabotropic, G-protein-gated inwardly rectifying potassium (GirK) channel-mediated fIPSP. Our data showed the induction of LTP-like enhancements for both the glutamatergic and GirK-dependent components of the dorsal hippocampal CA3-CA1 synapse during the exploration of novel but not familiar objects. These results support the contention that synaptic plasticity processes that underlie hippocampal-dependent memory are sustained by fine tuning mechanisms that control excitatory and inhibitory neurotransmission balance.

G-Protein-Gated Inwardly Rectifying Potassium (Kir3/GIRK) Channels Govern Synaptic Plasticity That Supports Hippocampal-Dependent Cognitive Functions in Male Mice.

The G-protein-gated inwardly rectifying potassium (Kir3/GIRK) channel is the effector of many G-protein-coupled receptors (GPCRs). Its dysfunction has been linked to the pathophysiology of Down syndrome, Alzheimer's and Parkinson's diseases, psychiatric disorders, epilepsy, drug addiction, or alcoholism. In the hippocampus, GIRK channels decrease excitability of the cells and contribute to resting membrane potential and inhibitory neurotransmission. Here, to elucidate the role of GIRK channels activity in the maintenance of hippocampal-dependent cognitive functions, their involvement in controlling neuronal excitability at different levels of complexity was examined in C57BL/6 male mice. For that purpose, GIRK activity in the dorsal hippocampus CA3-CA1 synapse was pharmacologically modulated by two drugs: ML297, a GIRK channel opener, and Tertiapin-Q (TQ), a GIRK channel blocker. Ex vivo, using dorsal hippocampal slices, we studied the effect of pharmacological GIRK modulation on synaptic plasticity processes induced in CA1 by Schaffer collateral stimulation. In vivo, we performed acute intracerebroventricular (i.c.v.) injections of the two GIRK modulators to study their contribution to electrophysiological properties and synaptic plasticity of dorsal hippocampal CA3-CA1 synapse, and to learning and memory capabilities during hippocampal-dependent tasks. We found that pharmacological disruption of GIRK channel activity by i.c.v. injections, causing either function gain or function loss, induced learning and memory deficits by a mechanism involving neural excitability impairments and alterations in the induction and maintenance of long-term synaptic plasticity processes. These results support the contention that an accurate control of GIRK activity must take place in the hippocampus to sustain cognitive functions.SIGNIFICANCE STATEMENT Cognitive processes of learning and memory that rely on hippocampal synaptic plasticity processes are critically ruled by a finely tuned neural excitability. G-protein-gated inwardly rectifying K+ (GIRK) channels play a key role in maintaining resting membrane potential, cell excitability and inhibitory neurotransmission. Here, we demonstrate that modulation of GIRK channels activity, causing either function gain or function loss, transforms high-frequency stimulation (HFS)-induced long-term potentiation (LTP) into long-term depression (LTD), inducing deficits in hippocampal-dependent learning and memory. Together, our data show a crucial GIRK-activity-mediated mechanism that governs synaptic plasticity direction and modulates subsequent hippocampal-dependent cognitive functions.

Therapeutic potential of targeting G protein-gated inwardly rectifying potassium (GIRK) channels in the central nervous system.

G protein-gated inwardly rectifying potassium channels (Kir3/GirK) are important for maintaining resting membrane potential, cell excitability and inhibitory neurotransmission. Coupled to numerous G protein-coupled receptors (GPCRs), they mediate the effects of many neurotransmitters, neuromodulators and hormones contributing to the general homeostasis and particular synaptic plasticity processes, learning, memory and pain signaling. A growing number of behavioral and genetic studies suggest a critical role for the appropriate functioning of the central nervous system, as well as their involvement in many neurologic and psychiatric conditions, such as neurodegenerative diseases, mood disorders, attention deficit hyperactivity disorder, schizophrenia, epilepsy, alcoholism and drug addiction. Hence, GirK channels emerge as a very promising tool to be targeted in the current scenario where these conditions already are or will become a global public health problem. This review examines recent findings on the physiology, function, dysfunction, and pharmacology of GirK channels in the central nervous system and highlights the relevance of GirK channels as a worthful potential target to improve therapies for related diseases.

Impairments of Synaptic Plasticity Induction Threshold and Network Oscillatory Activity in the Hippocampus Underlie Memory Deficits in a Non-Transgenic Mouse Model of Amyloidosis.

In early Alzheimer disease (AD) models synaptic failures and upstreaming aberrant patterns of network synchronous activity result in hippocampal-dependent memory deficits. In such initial stage, soluble forms of Amyloid-ß (Aß) peptides have been shown to play a causal role. Among different Aß species, Aß25-35 has been identified as the biologically active fragment, as induces major neuropathological signs related to early AD stages. Consequently, it has been extensively used to acutely explore the pathophysiological events related with neuronal dysfunction induced by soluble Aß forms. However, the synaptic mechanisms underlying its toxic effects on hippocampal-dependent memory remain unresolved. Here, in an in vivo model of amyloidosis generated by intracerebroventricular injections of Aß25-35 we studied the synaptic dysfunction mechanisms underlying hippocampal cognitive deficits. At the synaptic level, long-term potentiation (LTP) of synaptic excitation and inhibition was induced in CA1 region by high frequency simulation (HFS) applied to Schaffer collaterals. Aß25-35 was found to alter metaplastic mechanisms of plasticity, facilitating long-term depression (LTD) of both types of LTP. In addition, aberrant synchronization of hippocampal network activity was found while at the behavioral level, deficits in hippocampal-dependent habituation and recognition memories emerged. Together, our results provide a substrate for synaptic disruption mechanism underlying hippocampal cognitive deficits present in Aß25-35 amyloidosis model.

Publisher Correction: Activation of G-protein-gated inwardly rectifying potassium (Kir3/GirK) channels rescues hippocampal functions in a mouse model of early amyloid-ß pathology.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Hippocampal long-term synaptic depression and memory deficits induced in early amyloidopathy are prevented by enhancing G-protein-gated inwardly rectifying potassium channel activity.

Hippocampal synaptic plasticity disruption by amyloid-ß (Aß) peptides + thought to be responsible for learning and memory impairments in Alzheimer's disease (AD) early stage. Failures in neuronal excitability maintenance seems to be an underlying mechanism. G-protein-gated inwardly rectifying potassium (GirK) channels control neural excitability by hyperpolarization in response to many G-protein-coupled receptors activation. Here, in early in vitro and in vivo amyloidosis mouse models, we study whether GirK channels take part of the hippocampal synaptic plasticity impairments generated by Aß1-42 . In vitro electrophysiological recordings from slices showed that Aß1-42 alters synaptic plasticity by switching high-frequency stimulation (HFS) induced long-term potentiation (LTP) to long-term depression (LTD), which led to in vivo hippocampal-dependent memory deficits. Remarkably, selective pharmacological activation of GirK channels with ML297 rescued both HFS-induced LTP and habituation memory from Aß1-42 action. Moreover, when GirK channels were specifically blocked by Tertiapin-Q, their activation with ML297 failed to rescue LTP from the HFS-dependent LTD induced by Aß1-42 . On the other hand, the molecular analysis of the recorded slices by western blot showed that the expression of GIRK1/2 subunits, which form the prototypical GirK channel in the hippocampus, was not significantly regulated by Aß1-42 . However, immunohistochemical examination of our in vivo amyloidosis model showed Aß1-42 to down-regulate hippocampal GIRK1 subunit expression. Together, our results describe an Aß-mediated deleterious synaptic mechanism that modifies the induction threshold for hippocampal LTP/LTD and underlies memory alterations observed in amyloidosis models. In this scenario, GirK activation assures memory formation by preventing the transformation of HFS-induced LTP into LTD.

Role of GirK Channels in Long-Term Potentiation of Synaptic Inhibition in an In Vivo Mouse Model of Early Amyloid-ß Pathology.

Imbalances of excitatory/inhibitory synaptic transmission occur early in the pathogenesis of Alzheimer's disease (AD), leading to hippocampal hyperexcitability and causing synaptic, network, and cognitive dysfunctions. G-protein-gated potassium (GirK) channels play a key role in the control of neuronal excitability, contributing to inhibitory signaling. Here, we evaluate the relationship between GirK channel activity and inhibitory hippocampal functionality in vivo. In a non-transgenic mouse model of AD, field postsynaptic potentials (fPSPs) from the CA3⁻CA1 synapse in the dorsal hippocampus were recorded in freely moving mice. Intracerebroventricular (ICV) injections of amyloid-ß (Aß) or GirK channel modulators impaired ionotropic (GABAA-mediated fPSPs) and metabotropic (GirK-mediated fPSPs) inhibitory signaling and disrupted the potentiation of synaptic inhibition. However, the activation of GirK channels prevented Aß-induced changes in GABAA components. Our data shows, for the first time, the presence of long-term potentiation (LTP) for both the GABAA and GirK-mediated inhibitory postsynaptic responses in vivo. In addition, our results support the importance of an accurate level of GirK-dependent signaling for dorsal hippocampal performance in early amyloid pathology models by controlling the excess of excitation that disrupts synaptic plasticity processes.

Activation of G-protein-gated inwardly rectifying potassium (Kir3/GirK) channels rescues hippocampal functions in a mouse model of early amyloid-ß pathology.

The hippocampus plays a critical role in learning and memory. Its correct performance relies on excitatory/inhibitory synaptic transmission balance. In early stages of Alzheimer's disease (AD), neuronal hyperexcitability leads to network dysfunction observed in cortical regions such as the hippocampus. G-protein-gated potassium (GirK) channels induce neurons to hyperpolarize, contribute to the resting membrane potential and could compensate any excesses of excitation. Here, we have studied the relationship between GirK channels and hippocampal function in a mouse model of early AD pathology. Intracerebroventricular injections of amyloid-ß (Aß 1-42) peptide-which have a causal role in AD pathogenesis-were performed to evaluate CA3-CA1 hippocampal synapse functionality in behaving mice. Aß increased the excitability of the CA3-CA1 synapse, impaired long-term potentiation (LTP) and hippocampal oscillatory activity, and induced deficits in novel object recognition (NOR) tests. Injection of ML297 alone, a selective GirK activator, was also translated in LTP and NOR deficits. However, increasing GirK activity rescued all hippocampal deficits induced by Aß due to the restoration of excitability values in the CA3-CA1 synapse. Our results show a synaptic mechanism, through GirK channel modulation, for the prevention of the hyperexcitability that causally contributes to synaptic, network, and cognitive deficits found in early AD pathogenesis.