Small-conductance Ca2+-activated K+ channels modulate action potential-induced Ca2+ transients in hippocampal neurons

Long-term depression induction and maintenance across regions of the apical branch of CA1 dendrites

Well known as the center for learning and memory, hippocampus is the crucial brain region to study synaptic plasticity in the context of cellular fundamental mechanisms such as long-term depression (LTD) and long-term potentiation (LTP). However, despite years of extensive research, the key to our LTD queries and their induction mechanisms has not been fully understood. Previously, we reported the induction of late-LTD (L-LTD) in the distally located synapses of apical branch of hippocampal CA1 dendrites using strong low-frequency stimulation (SLFS). In contrast synapses at the proximal site could not express L-LTD. Thus, in the present study, we wanted to investigate whether or not synapses of apical dendritic branch at the proximal location could induce and maintain LTD and its related properties in in vitro rat hippocampal slices. Results indicated that the SLFS in the distal and proximal region triggered the plasticity related proteins (PRP) synthesis in both regions, as evident by the induction and maintenance of L-LTD in the distal region by virtue of synaptic and cross-tagging. In addition, the application of emetine at the time of proximal input stimulation prevented the transition of early-LTD (E-LTD) into L-LTD at the distal region, proving PRP synthesis at the proximal site. Further, it was observed that weak low-frequency stimulation (WLFS) could induce E-LTD in the proximal region along with LTD-specific tag-setting at the synapses. In conclusion, the current study suggests unique findings that the synaptic and cross-tagging mediate L-LTD expression is maintained in the proximal location of hippocampus apical CA1 dendrites.

Positive Modulation of SK Channel Impedes Neuron-Specific Cytoskeletal Organization and Maturation

N-methyl-D-aspartate receptor (NMDAR) modulates the structural plasticity of dendritic spines by impacting cytoskeletal organization and kinase signaling. In the developing nervous system, activation of NMDAR is pertinent for neuronal migration, neurite differentiation, and cellular organization. Given that small conductance potassium channels (SK2/3) repress NMDAR ionotropic signaling, this study highlights the impact of neonatal SK channel potentiation on adult cortical and hippocampal organization. Neonatal SK channel potentiation was performed by one injection of SK2/3 agonist (CyPPA) into the pallium of mice on postnatal day 2 (P2). When the animals reached adulthood (P55), the hippocampus and cortex were examined to assess neuronal maturation, lamination, and the distribution of synaptic cytoskeletal proteins. Immunodetection of neuronal markers in the brain of P2-treated P55 mice revealed the presence of immature neurons in the upper cortical layers (layers II-IV) and CA1 (hippocampus). Also, layer-dependent cortical-cell density was attenuated due to the ectopic localization of mature (NeuN+) and immature (Doublecortin+ [DCX+]) neurons in cortical layers II-IV. Similarly, the decreased count of NeuN+ neurons in the CA1 is accompanied by an increase in the number of immature DCX+ neurons. Ectopic localization of neurons in the upper cortex and CA1 caused the dramatic expression of neuron-specific cytoskeletal proteins. In line with this, structural deformity of neuronal projections and the loss of postsynaptic densities suggests that postsynaptic integrity is compromised in the SK2/3+ brain. From these results, we deduced that SK channel activity in the developing brain likely impacts neuronal maturation through its effects on cytoskeletal formation.

Intracellular Calcium Responses Encode Action Potential Firing in Spinal Cord Lamina I Neurons

Maladaptive plasticity of neurons in lamina I of the spinal cord is a lynchpin for the development of chronic pain, and is critically dependent on intracellular calcium signaling. However, the relationship between neuronal activity and intracellular calcium in these neurons is unknown. Here we combined two-photon calcium imaging with whole-cell electrophysiology to determine how action potential firing drives calcium responses within subcellular compartments of male rat spinal cord lamina I neurons. We found that single action potentials generated at the soma increase calcium concentration in the somatic cytosol and nucleus, and these calcium responses invade dendrites and dendritic spines by active backpropagation. Calcium responses in each compartment were dependent on voltage-gated calcium channels, and somatic and nuclear calcium responses were amplified by release of calcium from ryanodine-sensitive intracellular stores. Grouping single action potential-evoked calcium responses by neuron type demonstrated their presence in all defined types, as well as a high degree of similarity in calcium responses between neuron types. With bursts of action potentials, we found that calcium responses have the capacity to encode action potential frequency and number in all compartments, with action potential number being preferentially encoded. Together, these findings indicate that intracellular calcium serves as a readout of neuronal activity within lamina I neurons, providing a unifying mechanism through which activity may regulate plasticity, including that seen in chronic pain.SIGNIFICANCE STATEMENT Despite their critical role in both acute pain sensation and chronic pain, little is known of the fundamental physiology of spinal cord lamina I neurons. This is especially the case with respect to calcium dynamics within these neurons, which could regulate maladaptive plasticity observed in chronic pain. By combining two-photon calcium imaging and patch-clamp electrophysiological recordings from lamina I neurons, we found that action potential firing induces calcium responses within the somatic cytosol, nucleus, dendrites, and dendritic spines of lamina I neurons. Our findings demonstrate the presence of actively backpropagating action potentials, shifting our understanding of how these neurons process information, such that calcium provides a mechanism for lamina I neurons to track their own activity.

Physiological Roles and Therapeutic Potential of Ca2+ Activated Potassium Channels in the Nervous System

Within the potassium ion channel family, calcium activated potassium (K_Ca) channels are unique in their ability to couple intracellular Ca²⁺ signals to membrane potential variations. K_Ca channels are diversely distributed throughout the central nervous system and play fundamental roles ranging from regulating neuronal excitability to controlling neurotransmitter release. The physiological versatility of K_Ca channels is enhanced by alternative splicing and co-assembly with auxiliary subunits, leading to fundamental differences in distribution, subunit composition and pharmacological profiles. Thus, understanding specific K_Ca channels’ mechanisms in neuronal function is challenging. Based on their single channel conductance, K_Ca channels are divided into three subtypes: small (SK, 4–14 pS), intermediate (IK, 32–39 pS) and big potassium (BK, 200–300 pS) channels. This review describes the biophysical characteristics of these K_Ca channels, as well as their physiological roles and pathological implications. In addition, we also discuss the current pharmacological strategies and challenges to target K_Ca channels for the treatment of various neurological and psychiatric disorders.

Genes regulated by SATB2 during neurodevelopment contribute to schizophrenia and educational attainment

SATB2 is associated with schizophrenia and is an important transcription factor regulating neocortical organization and circuitry. Rare mutations in SATB2 cause a syndrome that includes developmental delay, and mouse studies identify an important role for SATB2 in learning and memory. Interacting partners BCL11B and GATAD2A are also schizophrenia risk genes indicating that other genes interacting with or are regulated by SATB2 are making a contribution to schizophrenia and cognition. We used data from Satb2 mouse models to generate three gene-sets that contain genes either functionally related to SATB2 or targeted by SATB2 at different stages of development. Each was tested for enrichment using the largest available genome-wide association studies (GWAS) datasets for schizophrenia and educational attainment (EA) and enrichment analysis was also performed for schizophrenia and other neurodevelopmental disorders using data from rare variant sequencing studies. These SATB2 gene-sets were enriched for genes containing common variants associated with schizophrenia and EA, and were enriched for genes containing rare variants reported in studies of schizophrenia, autism and intellectual disability. In the developing cortex, genes targeted by SATB2 based on ChIP-seq data, and functionally affected when SATB2 is not expressed based on differential expression analysis using RNA-seq data, show strong enrichment for genes associated with EA. For genes expressed in the hippocampus or at the synapse, those targeted by SATB2 are more strongly enriched for genes associated EA than gene-sets not targeted by SATB2. This study demonstrates that single gene findings from GWAS can provide important insights to pathobiological processes. In this case we find evidence that genes influenced by SATB2 and involved in synaptic transmission, axon guidance and formation of the corpus callosum are contributing to schizophrenia and cognition.

Schizophrenia is a complex disorder caused by many genes. Using new gene discoveries to understand pathobiology is a foundation for development of new treatments. Current drugs for schizophrenia are only partially effective and do not treat cognitive deficits, which are key factors for explaining disability, leading to unemployment, homelessness and social isolation. Genome-wide association studies (GWAS) of schizophrenia have been effective at identifying individual SNPs and genes that contribute to risk but have struggled to immediately uncover the bigger picture of the underlying biology of the disorder. Here we take an individual gene identified in a schizophrenia GWAS called SATB2, which on its own is a very important regulator of brain development. We use functional genomics data from mouse studies to identify sets of others genes that are influenced by SATB2 during development. We show that these gene sets are enriched for common variants associated with schizophrenia and educational attainment (used as a proxy for cognition), and for rare variants that increase risk of various neurodevelopmental disorders. This study provides evidence that the molecular mechanisms that underpin schizophrenia and cognitive function include disruption of biological processes influenced by SATB2 as the brain is being organized and wired during development.

In vivo activation of the SK channel in the spinal cord reduces the NMDA receptor antagonist dose needed to produce antinociception in an inflammatory pain model

N-methyl-D-aspartate receptor (NMDAR) antagonists have been shown to reduce mechanical hypersensitivity in animal models of inflammatory pain. However, their clinical use is associated with significant dose-limiting side effects. Small-conductance Ca-activated K channels (SK) have been shown to modulate NMDAR activity in the brain. We demonstrate that in vivo activation of SK channels in the spinal cord can alleviate mechanical hypersensitivity in a rat model of inflammatory pain. Intrathecal (i.t.) administration of the SK channel activator, 6,7-dichloro-1H-indole-2,3-dione 3-oxime (NS309), attenuates complete Freund adjuvant (CFA)-induced mechanical hypersensitivity in a dose-dependent manner. Postsynaptic expression of the SK channel subunit, SK3, and apamin-sensitive SK channel-mediated currents recorded from superficial laminae are significantly reduced in the dorsal horn (DH) after CFA. Complete Freund adjuvant-induced decrease in SK-mediated currents can be reversed in vitro by bath application of NS309. In addition, immunostaining for the SK3 subunit indicates that SK3-containing channels within DH neurons can have both somatic and dendritic localization. Double immunostaining shows coexpression of SK3 and NMDAR subunit, NR1, compatible with functional interaction. Moreover, we demonstrate that i.t. coadministration of NS309 with an NMDAR antagonist reduces the dose of NMDAR antagonist, DL-2-amino-5-phosphonopentanoic acid (DL-AP5), required to produce antinociceptive effects in the CFA model. This reduction could attenuate the unwanted side effects associated with NMDAR antagonists, giving this combination potential clinical implications.

Functional coupling of diverse voltage-gated Ca(2+) channels underlies high fidelity of fast dendritic Ca(2+) signals during burst firing

KEY POINTS

In neurons, the Ca(2+) signal associated with the dendritic back-propagating action potential codes a chemical message to the different dendritic sites, playing a crucial role in electrical signalling, synaptic transmission and synaptic plasticity. The study of the underlying Ca(2+) current, mediated by different types of voltage-gated Ca(2+) channels, cannot be achieved by using the patch clamp technique. In this article, we used a recently developed cutting-edge optical technique to investigate the physiological behaviour of local Ca(2+) currents along the apical dendrite of CA1 hippocampal pyramidal neurons. We directly measure, for the first time, the synergistic activation and deactivation of the diverse dendritic voltage-gated Ca(2+) channels operating during bursts of back-propagating action potentials to precisely control the Ca(2+) signal. We demonstrate that the Ca(2+) loss via high-voltage-activated channels is compensated by the Ca(2+) entry via the other channels translating in high fidelity of Ca(2+) signalling.

ABSTRACT

In CA1 hippocampal pyramidal neurons, the dendritic Ca(2+) signal associated with somatic firing represents a fundamental activation code for several proteins. This signal, mediated by voltage-gated Ca(2+) channels (VGCCs), varies along the dendrites. In this study, using a recent optical technique based on the low-affinity indicator Oregon Green 488 BAPTA-5N, we analysed how activation and deactivation of VGCCs produced by back-propagating action potentials (bAPs) along the apical dendrite shape the Ca(2+) signal at different locations in CA1 hippocampal pyramidal neurons of the mouse. We measured, at multiple dendritic sites, the Ca(2+) transients and the changes in membrane potential associated with bAPs at 50 μs temporal resolution and we estimated the kinetics of the Ca(2+) current. We found that during somatic bursts, the bAPs decrease in amplitude along the apical dendrite but the amplitude of the associated Ca(2+) signal in the initial 200 μm dendritic segment does not change. Using a detailed pharmacological analysis, we demonstrate that this effect is due to the perfect compensation of the loss of Ca(2+) via high-voltage-activated (HVA) VGCCs by a larger Ca(2+) component via low-voltage-activated (LVA) VGCCs, revealing a mechanism coupling the two VGCC families of K(+) channels. More distally, where the bAP does not activate HVA-VGCCs, the Ca(2+) signal is variable during the burst. Thus, we demonstrate that HVA- and LVA-VGCCs operate synergistically to stabilise Ca(2+) signals associated with bAPs in the most proximal 200 μm dendritic segment.

Developmental mapping of small-conductance calcium-activated potassium channel expression in the rat nervous system

Early electrical activity and calcium influx regulate crucial aspects of neuronal development. Small-conductance calcium-activated potassium (SK) channels regulate action potential firing and shape calcium influx through feedback regulation in mature neurons. These functions, observed in the adult nervous system, make them ideal candidates to regulate activity-and calcium-dependent processes in neurodevelopment. However, to date little is known about the onset of expression and regions expressing SK channel subunits in the embryonic and postnatal development of the central nervous system (CNS). To allow studies on the contribution of SK channels to different phases of development of single neurons and networks, we have performed a detailed in situ hybridization mapping study, providing comprehensive distribution profiles of all three SK subunits (SK1, SK2, and SK3) in the rat CNS during embryonic and postnatal development. SK channel transcripts are expressed at early stages of prenatal CNS development. The three SK channel subunits display different developmental expression gradients in distinct CNS regions, with time points of expression and up-or downregulation that can be associated with a range of diverse developmental events. Their early expression in embryonic development suggests an involvement of SK channels in the regulation of developmental processes. Additionally, this study shows how the postnatal ontogenetic patterns lead to the adult expression map for each SK channel subunit and how their coexpression in the same regions or neurons varies throughout development. J. Comp. Neurol. 522:1072–1101, 2014. © 2013 Wiley Periodicals, Inc.

Role of small conductance Ca²⁺-activated K⁺ channels in controlling CA1 pyramidal cell excitability

Small-conductance Ca(2+)-activated K(+) (SK or K(Ca)2) channels are widely expressed in the CNS. In several types of neurons, these channels were shown to become activated during repetitive firing, causing early spike frequency adaptation. In CA1 pyramidal cells, SK channels in dendritic spines were shown to regulate synaptic transmission. However, the presence of functional SK channels in the somata and their role in controlling the intrinsic firing of these neurons has been controversial. Using whole-cell voltage-clamp and current-clamp recordings in acute hippocampal slices and focal applications of irreversible and reversible SK channel blockers, we provide evidence that functional SK channels are expressed in the somata and proximal dendrites of adult rat CA1 pyramidal cells. Although these channels can generate a medium duration afterhyperpolarizing current, they play only an auxiliary role in controlling the intrinsic excitability of these neurons, secondary to the low voltage-activating, noninactivating K(V)7/M channels. As long as K(V)7/M channels are operative, activation of SK channels during repetitive firing does not notably affect the spike output of CA1 pyramidal cells. However, when K(V)7/M channel activity is compromised, SK channel activation significantly and uniquely reduces spike output of these neurons. Therefore, proximal SK channels provide a "second line of defense" against intrinsic hyperexcitability, which may play a role in multiple conditions in which K(V)7/M channels activity is compromised, such as hyposmolarity.

GluN2A and GluN2B subunit-containing NMDA receptors in hippocampal plasticity

N-Methyl-d-aspartate receptor (NMDAR)-dependent synaptic plasticity is a strong candidate to mediate learning and memory processes that require the hippocampus. This plasticity is bidirectional, and how the same receptor can mediate opposite changes in synaptic weights remains a conundrum. It has been suggested that the NMDAR subunit composition could be involved. Specifically, one subunit composition of NMDARs would be responsible for the induction of long-term potentiation (LTP), whereas NMDARs with a different subunit composition would be engaged in the induction of long-term depression (LTD). Unfortunately, the results from studies that have investigated this hypothesis are contradictory, particularly in relation to LTD. Nevertheless, current evidence does suggest that the GluN2B subunit might be particularly important for plasticity and may make a synapse bidirectionally malleable. In particular, we conclude that the presence of GluN2B subunit-containing NMDARs at the postsynaptic density might be a necessary, though not a sufficient, condition for the strengthening of individual synapses. This is owing to the interaction of GluN2B with calcium/calmodulin-dependent protein kinase II (CaMKII) and is distinct from its contribution as an ion channel.