Mild neonatal hypoxia disrupts adult hippocampal learning and memory and is associated with CK2-mediated dysregulation of synaptic calcium-activated potassium channel KCNN2.

Objective: Although nearly half of preterm survivors display persistent neurobehavioral dysfunction including memory impairment without overt gray matter injury, the underlying mechanisms of neuronal or glial dysfunction, and their relationship to commonly observed cerebral white matter injury are unclear. We developed a mouse model to test the hypothesis that mild hypoxia during preterm equivalence is sufficient to persistently disrupt hippocampal neuronal maturation related to adult cellular mechanisms of learning and memory. Methods: Neonatal (P2) mice were exposed to mild hypoxia (8%O 2 ) for 30 min and evaluated for acute injury responses or survived until adulthood for assessment of learning and memory and hippocampal neurodevelopment. Results: Neonatal mild hypoxia resulted in clinically relevant oxygen desaturation and tachycardia without bradycardia and was not accompanied by cerebral gray or white matter injury. Neonatal hypoxia exposure was sufficient to cause hippocampal learning and memory deficits and abnormal maturation of CA1 neurons that persisted into adulthood. This was accompanied by reduced hippocampal CA3-CA1 synaptic strength and LTP and reduced synaptic activity of calcium-sensitive SK2 channels, key regulators of spike timing dependent neuroplasticity, including LTP. Structural illumination microscopy revealed reduced synaptic density, but intact SK2 localization at the synapse. Persistent loss of SK2 activity was mediated by altered casein kinase 2 (CK2) signaling. Interpretation: Clinically relevant mild hypoxic exposure in the neonatal mouse is sufficient to produce morphometric and functional disturbances in hippocampal neuronal maturation independently of white matter injury. Additionally, we describe a novel persistent mechanism of potassium channel dysregulation after neonatal hypoxia. Collectively our findings suggest an unexplored explanation for the broad spectrum of neurobehavioral, cognitive and learning disabilities that paradoxically persist into adulthood without overt gray matter injury after preterm birth.

Anti-Müllerian Hormone Regulation of Synaptic Transmission in the Hippocampus Requires MAPK Signaling and Kv4.2 Potassium Channel Activity.

Anti-Müllerian hormone (AMH) is a paracrine factor generated peripherally by the gonads to regulate gonadal function in adult mammals. We recently reported that AMH and AMH-specific receptor Anti-Müllerian hormone receptor 2 (AMHR2) are expressed in the hippocampus, and exogenous AMH protein rapidly increased synaptic transmission and long-term synaptic plasticity at the CA3-CA1 synapses. Here we examined the cell-specific expression of AMHR2 and the cellular mechanism of rapid boosting effect of AMH on synaptic transmission in mouse hippocampus. Immunofluorescence staining showed that AMHR2 was specifically expressed in the soma and dendrites of hippocampal pyramidal neurons, but not glial cells. Electrophysiological recordings on acute hippocampal slices showed that AMH did not affect AMPAR-mediated or N-Methyl-D-aspartic acid receptor (NMDAR)-mediated excitatory postsynaptic currents at the CA3-CA1 synapses. The small-conductance Ca2+-activated K+ channel (SK2) and A-type K+ channel (Kv4.2) contribute to shaping excitatory postsynaptic potentials (EPSPs) at the CA3-CA1 synapses. Bath application of apamin to block SK2 did not alter AMH effect on increasing EPSPs, whereas blocking Kv4.2 channel with 4-aminopyridine, or chelating internal Ca2+ with BAPTA occluded the action of AMH on boosting EPSPs. Kv4.2 activity is regulated by p38 mitogen-activated kinase (MAPK). Blocking p38 MAPK with SB203580 occluded the effect of AMH on increasing EPSPs. These results show that Kv4.2 channel contributes to the rapid action of AMH on boosting synaptic transmission in a Ca2+- and p38 MAPK-dependent manner. Our findings provide functional evidence that AMH enhances synaptic transmission through Kv4.2 channel in the hippocampus, suggesting a possible role of Kv4.2 channel in AMH-regulated neuronal process underlying learning and memory.

Rapid actions of anti-Müllerian hormone in regulating synaptic transmission and long-term synaptic plasticity in the hippocampus.

Anti-Müllerian hormone (Amh) is a peptide factor that is known to regulate sexual differentiation and gonadal function in mammals. Although Amh is also suggested to be associated with cognitive development and function in the postnatal brain, little is known about its expression or direct effects on neuronal activities in the hippocampus. Therefore, we assessed Amh and its receptor expression in the hippocampus of male and female mice using PCR, Western blot, and immunofluorescence staining. While Amh-specific receptor expression was comparable between males and females, mRNA and protein levels of Amh were higher in females than those of males. Electrophysiological recordings on acute hippocampal slices showed that exogenous Amh protein addition increased synaptic transmission and long-term synaptic plasticity at the Cornu Ammonis (CA) 3-CA1 synapses. Amh exposure also increased the excitatory postsynaptic potential at CA1 synapses. Our findings support direct and rapid actions of Amh as a paracrine and/or autocrine factor in regulating hippocampal neuronal activities. Data provide functional evidence of Amh-mediated postsynaptic modulation of synaptic transmission and Amh-regulated long-term synaptic plasticity in the hippocampus. These results suggest a potential role of Amh in learning and memory, and a possible cause of the sex differences in cognitive development and function.

Transient Hypoxemia Disrupts Anatomical and Functional Maturation of Preterm Fetal Ovine CA1 Pyramidal Neurons.

Children who survive premature birth often exhibit reductions in hippocampal volumes and deficits in working memory. However, it is unclear whether synaptic plasticity and cellular mechanisms of learning and memory can be elicited or disrupted in the preterm fetal hippocampus. CA1 hippocampal neurons were exposed to two common insults to preterm brain: transient hypoxia-ischemia (HI) and hypoxia (Hx). We used a preterm fetal sheep model using both sexes in twin 0.65 gestation fetuses that reproduces the spectrum of injury and abnormal growth in preterm infants. Using Cavalieri measurements, hippocampal volumes were reduced in both Hx and HI fetuses compared with controls. This volume loss was not the result of neuronal cell death. Instead, morphometrics revealed alterations in both basal and apical dendritic arborization that were significantly associated with the level of systemic hypoxemia and metabolic stress regardless of etiology. Anatomical alterations of CA1 neurons were accompanied by reductions in probability of presynaptic glutamate release, long-term synaptic plasticity and intrinsic excitability. The reduction in intrinsic excitability was in part due to increased activity of the channels underlying the fast and slow component of the afterhyperpolarization in Hx and HI. Our studies suggest that even a single brief episode of hypoxemia can markedly disrupt hippocampal maturation. Hypoxemia may contribute to long-term working memory disturbances in preterm survivors by disrupting neuronal maturation with resultant functional disturbances in hippocampal action potential throughput. Strategies directed at limiting the duration or severity of hypoxemia during brain development may mitigate disturbances in hippocampal maturation.SIGNIFICANCE STATEMENT Premature infants commonly sustain hypoxia-ischemia, which results in reduced hippocampal growth and life-long disturbances in learning and memory. We demonstrate that the circuitry related to synaptic plasticity and cellular mechanisms of learning and memory (LTP) are already functional in the fetal hippocampus. Unlike adults, the fetal hippocampus is surprisingly resistant to cell death from hypoxia-ischemia. However, the hippocampus sustains robust structural and functional disturbances in the dendritic maturation of CA1 neurons that are significantly associated with the magnitude of a brief hypoxic stress. Since transient hypoxic episodes occur commonly in preterm survivors, our findings suggest that the learning problems that ensue may be related to the unique susceptibility of the hippocampus to brief episodes of hypoxemia.

Functional changes in hippocampal synaptic signaling in offspring survivors of a mouse model of intrauterine inflammation.

BACKGROUND: Recent evidence suggests that exposure to intrauterine inflammation causes acute fetal brain injury and is linked to a spectrum of neurobehavioral disorders. In a rodent model of intrauterine inflammation induced by lipopolysaccharide (LPS) exposure in utero, activated microglia can be detected in the hippocampus of offspring survivors, as late as 60 days postnatal (DPN). Given that the hippocampus is important for learning and memory, these results suggest that in utero inflammation underlies long-term cognitive deficits observed in children/survivors. METHODS: An established mouse model of LPS-induced intrauterine inflammation was used to study hippocampal function from offspring at 44-59 DPN. Microgliosis was examined at 45 DPN. Extracellular field recordings of synaptic transmission were performed on acute hippocampal slices. RESULTS: LPS offspring mice displayed persistent microglial activation and increased CA3-CA1 excitatory synaptic strength, which can be explained in part by an increase in the probability of glutamate release, and reduced long-term synaptic potentiation compared to control mice. CONCLUSIONS: These results offer a mechanistic explanation for the cognitive and behavioral deficits observed in survivors of preterm birth caused by intrauterine inflammation.

Assessment of mutations in KCNN2 and ZNF135 to patient neurological symptoms.

Exome sequencing from a patient with neurological and developmental symptoms revealed two mutations in separate genes. One was a homozygous transition mutation that results in an in-frame, premature translational stop codon in the ZNF135 gene predicted to encode a transcriptional repressor. Another mutation was heterozygous, a single nucleotide duplication in the KCNN2 gene that encodes a Ca-activated K channel, SK2, and leads to a translational frame shift and a premature stop codon. Heterologous expression studies, brain slice recordings, and coordination tests from a transgenic mouse line carrying the SK2 mutation suggest that it does not contribute to the patient's symptoms. ZNF135 is expressed in human brain and it is likely that the homozygous mutation underlies the human phenotype.

Endothelial SK3 channel-associated Ca2+ microdomains modulate blood pressure.

Activation of vascular endothelial small- (KCa2.3, SK3) or intermediate- (KCa3.1, IK1) conductance Ca(2+)-activated potassium channels induces vasorelaxation via an endothelium-derived hyperpolarization (EDH) pathway. Although the activation of SK3 and IK1 channels converges on EDH, their subcellular effects on signal transduction are different and not completely clear. In this study, a novel endothelium-specific SK3 knockout (SK3(-/-)) mouse model was utilized to specifically examine the contribution of SK3 channels to mesenteric artery vasorelaxation, endothelial Ca(2+) dynamics, and blood pressure. The absence of SK3 expression was confirmed using real-time quantitative PCR and Western blot analysis. Functional studies showed impaired EDH-mediated vasorelaxation in SK3(-/-) small mesenteric arteries. Immunostaining results from SK3(-/-) vessels confirmed the absence of SK3 and further showed altered distribution of transient receptor potential channels, type 4 (TRPV4). Electrophysiological recordings showed a lack of SK3 channel activity, while TRPV4-IK1 channel coupling remained intact in SK3(-/-) endothelial cells. Moreover, Ca(2+) imaging studies in SK3(-/-) endothelium showed increased Ca(2+) transients with reduced amplitude and duration under basal conditions. Importantly, SK3(-/-) endothelium lacked a distinct type of Ca(2+) dynamic that is sensitive to TRPV4 activation. Blood pressure measurements showed that the SK3(-/-) mice were hypertensive, and the blood pressure increase was further enhanced during the 12-h dark cycle when animals are most active. Taken together, our results reveal a previously unappreciated SK3 signaling microdomain that modulates endothelial Ca(2+) dynamics, vascular tone, and blood pressure.

Membrane palmitoylated protein 2 is a synaptic scaffold protein required for synaptic SK2-containing channel function.

Mouse CA1 pyramidal neurons express apamin-sensitive SK2-containing channels in the post-synaptic membrane, positioned close to NMDA-type (N-methyl-D-aspartate) glutamate receptors. Activated by synaptically evoked NMDAR-dependent Ca(2+) influx, the synaptic SK2-containing channels modulate excitatory post-synaptic responses and the induction of synaptic plasticity. In addition, their activity- and protein kinase A-dependent trafficking contributes to expression of long-term potentiation (LTP). We have identified a novel synaptic scaffold, MPP2 (membrane palmitoylated protein 2; p55), a member of the membrane-associated guanylate kinase (MAGUK) family that interacts with SK2-containing channels. MPP2 and SK2 co-immunopurified from mouse brain, and co-immunoprecipitated when they were co-expressed in HEK293 cells. MPP2 is highly expressed in the post-synaptic density of dendritic spines on CA1 pyramidal neurons. Knocking down MPP2 expression selectively abolished the SK2-containing channel contribution to synaptic responses and decreased LTP. Thus, MPP2 is a novel synaptic scaffold that is required for proper synaptic localization and function of SK2-containing channels.

Apamin Boosting of Synaptic Potentials in CaV2.3 R-Type Ca2+ Channel Null Mice.

SK2- and KV4.2-containing K+ channels modulate evoked synaptic potentials in CA1 pyramidal neurons. Each is coupled to a distinct Ca2+ source that provides Ca2+-dependent feedback regulation to limit AMPA receptor (AMPAR)- and NMDA receptor (NMDAR)-mediated postsynaptic depolarization. SK2-containing channels are activated by Ca2+ entry through NMDARs, whereas KV4.2-containing channel availability is increased by Ca2+ entry through SNX-482 (SNX) sensitive CaV2.3 R-type Ca2+ channels. Recent studies have challenged the functional coupling between NMDARs and SK2-containing channels, suggesting that synaptic SK2-containing channels are instead activated by Ca2+ entry through R-type Ca2+ channels. Furthermore, SNX has been implicated to have off target affects, which would challenge the proposed coupling between R-type Ca2+ channels and KV4.2-containing K+ channels. To reconcile these conflicting results, we evaluated the effect of SK channel blocker apamin and R-type Ca2+ channel blocker SNX on evoked excitatory postsynaptic potentials (EPSPs) in CA1 pyramidal neurons from CaV2.3 null mice. The results show that in the absence of CaV2.3 channels, apamin application still boosted EPSPs. The boosting effect of CaV2.3 channel blockers on EPSPs observed in neurons from wild type mice was not observed in neurons from CaV2.3 null mice. These data are consistent with a model in which SK2-containing channels are functionally coupled to NMDARs and KV4.2-containing channels to CaV2.3 channels to provide negative feedback regulation of EPSPs in the spines of CA1 pyramidal neurons.

Distinct Ca2+ sources in dendritic spines of hippocampal CA1 neurons couple to SK and Kv4 channels.

Small conductance Ca(2+)-activated K(+) (SK) channels and voltage-gated A-type Kv4 channels shape dendritic excitatory postsynaptic potentials (EPSPs) in hippocampal CA1 pyramidal neurons. Synaptically evoked Ca(2+) influx through N-methyl-D-aspartate receptors (NMDARs) activates spine SK channels, reducing EPSPs and the associated spine head Ca(2+) transient. However, results using glutamate uncaging implicated Ca(2+) influx through SNX-482-sensitive (SNX-sensitive) Cav2.3 (R-type) Ca(2+) channels as the Ca(2+) source for SK channel activation. The present findings show that, using Schaffer collateral stimulation, the effects of SNX and apamin are not mutually exclusive and SNX increases EPSPs independent of SK channel activity. Dialysis with 1,2-bis(o-aminophenoxy)ethane-N'N'N'-tetraacetic acid (BAPTA), application of 4-Aminopyridine (4-AP), expression of a Kv4.2 dominant negative subunit, and dialysis with a KChIPs antibody occluded the SNX-induced increase of EPSPs. The results suggest two distinct Ca(2+) signaling pathways within dendritic spines that link Ca(2+) influx through NMDARs to SK channels and Ca(2+) influx through R-type Ca(2+) channels to Kv4.2-containing channels.

Ovarian hormone loss impairs excitatory synaptic transmission at hippocampal CA3-CA1 synapses.

Premature and long-term ovarian hormone loss following ovariectomy (OVX) is associated with cognitive impairment. This condition is prevented by estradiol (E2) therapy when initiated shortly following OVX but not after substantial delay. To determine whether these clinical findings are correlated with changes in synaptic functions, we used adult OVX rats to evaluate the consequences of short-term (7-10 d, OVXControl) and long-term (∼5 months, OVXLT) ovarian hormone loss, as well as subsequent in vivo E2 treatment, on excitatory synaptic transmission at the hippocampal CA3-CA1 synapses important for learning and memory. The results show that ovarian hormone loss was associated with a marked decrease in synaptic strength. E2 treatment increased synaptic strength in OVXControl but not OVXLT rats, demonstrating a change in the efficacy for E2 5 months following OVX. E2 also had a more rapid effect: within minutes of bath application, E2 acutely increased synaptic strength in all groups except OVXLT rats that did not receive in vivo E2 treatment. E2's acute effect was mediated postsynaptically, and required Ca(2+) influx through the voltage-gated Ca(2+) channels. Despite E2's acute effect, synaptic strength of OVXLT rats remained significantly lower than that of OVXControl rats. Thus, changes in CA3-CA1 synaptic transmission associated with ovarian hormone loss cannot be fully reversed with delayed E2 treatment. Given that synaptic strength at CA3-CA1 synapses is related to the ability to learn hippocampus-dependent tasks, these findings provide additional insights for understanding cognitive impairment-associated long-term ovarian hormone loss and ineffectiveness for delayed E2 treatment to maintain cognitive functions.

Modulation of endothelial SK3 channel activity by Ca²+dependent caveolar trafficking.

Small- and intermediate-conductance Ca(2+)-activated K(+) channels (SK3/Kcnn3 and IK1/Kcnn4) are expressed in vascular endothelium. Their activities play important roles in regulating vascular tone through their modulation of intracellular concentration ([Ca(2+)](i)) required for the production of endothelium-derived vasoactive agents. Activation of endothelial IK1 or SK3 channels hyperpolarizes endothelial cell membrane potential, increases Ca(2+) influx, and leads to the release of vasoactive factors, thereby impacting blood pressure. To examine the distinct roles of IK1 and SK3 channels, we used electrophysiological recordings to investigate IK1 and SK3 channel trafficking in acutely dissociated endothelial cells from mouse aorta. The results show that SK3 channels undergo Ca(2+)-dependent cycling between the plasma membrane and intracellular organelles; disrupting Ca(2+)-dependent endothelial caveolae cycling abolishes SK3 channel trafficking. Moreover, transmitter-induced changes in SK3 channel activity and surface expression modulate endothelial membrane potential. In contrast, IK1 channels do not undergo rapid trafficking and their activity remains unchanged when either exo- or endocytosis is block. Thus modulation of SK3 surface expression may play an important role in regulating endothelial membrane potential in a Ca(2+)-dependent manner.

Small-conductance Ca2+-activated K+ channels: form and function.

Small-conductance Ca(2+)-activated K(+) channels (SK channels) are widely expressed throughout the central nervous system. These channels are activated solely by increases in intracellular Ca(2+). SK channels are stable macromolecular complexes of the ion pore-forming subunits with calmodulin, which serves as the intrinsic Ca(2+) gating subunit, as well as with protein kinase CK2 and protein phosphatase 2A, which modulate Ca(2+) sensitivity. Well-known for their roles in regulating somatic excitability in central neurons, SK channels are also expressed in the postsynaptic membrane of glutamatergic synapses, where their activation and regulated trafficking modulate synaptic transmission and the induction and expression of synaptic plasticity, thereby affecting learning and memory. In this review we discuss the molecular and functional properties of SK channels and their physiological roles in central neurons.

Developmental profile of SK2 channel expression and function in CA1 neurons.

We investigated the temporal and spatial expression of SK2 in the developing mouse hippocampus using molecular and biochemical techniques, quantitative immunogold electron microscopy, and electrophysiology. The mRNA encoding SK2 was expressed in the developing and adult hippocampus. Western blotting and immunohistochemistry showed that SK2 protein increased with age. This was accompanied by a shift in subcellular localization. Early in development (P5), SK2 was predominantly localized to the endoplasmic reticulum in the pyramidal cell layer. But by P30 SK2 was almost exclusively expressed in the dendrites and spines. The level of SK2 at the postsynaptic density (PSD) also increased during development. In the adult, SK2 expression on the spine plasma membrane showed a proximal-to-distal gradient. Consistent with this redistribution and gradient of SK2, the selective SK channel blocker apamin increased evoked excitatory postsynaptic potentials (EPSPs) only in CA1 pyramidal neurons from mice older than P15. However, the effect of apamin on EPSPs was not different between synapses in proximal or distal stratum radiatum or stratum lacunosum-moleculare in adult. These results show a developmental increase and gradient in SK2-containing channel surface expression that underlie their influence on neurotransmission, and that may contribute to increased memory acquisition during early development.

SK2 channels are neuroprotective for ischemia-induced neuronal cell death.

In mouse hippocampal CA1 pyramidal neurons, the activity of synaptic small-conductance Ca(2+)-activated K(+) channels type 2 (SK2 channels) provides a negative feedback on N-methyl-D-aspartate receptors (NMDARs), reestablishing Mg(2+) block that reduces Ca(2+) influx. The well-established role of NMDARs in ischemia-induced excitotoxicity led us to test the neuroprotective effect of modulating SK2 channel activity following cerebral ischemia induced by cardiac arrest and cardiopulmonary resuscitation (CA/CPR). Administration of the SK channel positive modulator, 1-ethyl-benzimidazolinone (1-EBIO), significantly reduced CA1 neuron cell death and improved CA/CPR-induced cognitive outcome. Electrophysiological recordings showed that CA/CPR-induced ischemia caused delayed and sustained reduction of synaptic SK channel activity, and immunoelectron microscopy showed that this is associated with internalization of synaptic SK2 channels, which was prevented by 1-EBIO treatment. These results suggest that increasing SK2 channel activity, or preventing ischemia-induced loss of synaptic SK2 channels, are promising and novel approaches to neuroprotection following cerebral ischemia.

The SK2-long isoform directs synaptic localization and function of SK2-containing channels.

SK2-containing channels are expressed in the postsynaptic density (PSD) of dendritic spines on mouse hippocampal area CA1 pyramidal neurons and influence synaptic responses, plasticity and learning. The Sk2 gene (also known as Kcnn2) encodes two isoforms that differ only in the length of their N-terminal domains. SK2-long (SK2-L) and SK2-short (SK2-S) are coexpressed in CA1 pyramidal neurons and likely form heteromeric channels. In mice lacking SK2-L (SK2-S only mice), SK2-S-containing channels were expressed in the extrasynaptic membrane, but were excluded from the PSD. The SK channel contribution to excitatory postsynaptic potentials was absent in SK2-S only mice and was restored by SK2-L re-expression. Blocking SK channels increased the amount of long-term potentiation induced in area CA1 in slices from wild-type mice but had no effect in slices from SK2-S only mice. Furthermore, SK2-S only mice outperformed wild-type mice in the novel object recognition task. These results indicate that SK2-L directs synaptic SK2-containing channel expression and is important for normal synaptic signaling, plasticity and learning.

Ovarian hormone deficiency reduces intrinsic excitability and abolishes acute estrogen sensitivity in hippocampal CA1 pyramidal neurons.

Premature and uncompensated loss of ovarian hormones following ovariectomy (OVX) elevates the risks of cognitive impairment and dementia. These risks are prevented with estrogen (E(2))-containing hormone replacement therapy initiated shortly following OVX but not after substantial delay. Currently, the cellular bases underlying these clinical findings are unknown. At the cellular level, intrinsic membrane properties regulate the efficiency of synaptic inputs to initiate output action potentials (APs), thereby affecting neuronal communication, hence cognitive processing. This study tested the hypothesis that in CA1 pyramidal neurons, intrinsic membrane properties and their acute regulation by E(2) require ovarian hormones for maintenance. Whole-cell current-clamp recordings were performed on neurons from ∼ 7-month-old OVX rats that experienced either short-term (10 d, control OVX) or long-term (5 months, OVX(LT)) ovarian hormone deficiency. The results reveal that long-term hormone deficiency reduced intrinsic membrane excitability (IE) as measured by the number of evoked APs and firing duration for a given current injection. This was accompanied by AP broadening, an increased slow afterhyperpolarization (sAHP), and faster accumulation of Na(V) channel inactivation during repetitive firing. In the control OVX neurons, E(2) acutely increased IE and reduced the sAHP. In contrast, acute regulation of IE by E(2) was absent in the OVX(LT) neurons. Since the degree of IE of hippocampal pyramidal neurons is positively related with hippocampus-dependent learning ability, and modulation of IE is observed following successful learning, these findings provide a framework for understanding hormone deficiency-related cognitive impairment and the critical window for therapy initiation.

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

Cholinergic signaling modulates synaptic responses and influences cognition. In this issue of Neuron, two groups (Buchanan et al. and Giessel and Sabatini) present evidence that cholinergic signaling enhances postsynaptic responses in CA1 neurons by decreasing synaptic SK channel activity. However, they come to different conclusions about the protein kinases involved in this process.

Coupled activity-dependent trafficking of synaptic SK2 channels and AMPA receptors.

Small conductance Ca(2+)-activated K(+) type 2 (SK2) channels are expressed in the postsynaptic density of CA1 neurons where they are activated by synaptically evoked Ca(2+) influx to limit the size of EPSPs and spine Ca(2+) transients. At Schaffer collateral synapses, the induction of long-term potentiation (LTP) increases the alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor (AMPAR)-mediated contribution to synaptic transmission and decreases the synaptic SK2 channel contribution through protein kinase A-dependent channel endocytosis. Using a combination of electrophysiology and immunoelectron microscopy in mice, the relationship between the dynamics of spine SK2 channels and AMPARs was investigated. Unlike AMPARs, synaptic SK2 channels under basal conditions do not rapidly recycle. Furthermore, SK2 channels occupy a distinct population of endosomes separate from AMPARs. However, blocking vesicular exocytosis or the delivery of synaptic GluA1-containing AMPARs during the induction of LTP blocks SK2 channel endocytosis. By approximately 2 h after the induction of LTP, synaptic SK2 channel expression and function are restored. Thus, LTP-dependent endocytosis of SK2 is coupled to LTP-dependent AMPA exocytosis, and the approximately 2 h window after the induction of LTP during which synaptic SK2 activity is absent may be important for consolidating the later phases of LTP.

New sites of action for GIRK and SK channels.

It was recently discovered that two different types of voltage-insensitive K+ channels, G protein-coupled inwardly rectifying K+ (GIRK) and small-conductance Ca2+-activated K+ (SK) channels, are located on dendritic branches, spines and shafts in the postsynaptic densities of excitatory synapses in many central neurons. Together with increases in our knowledge of how these channels are regulated through stable protein-protein interactions in multi-protein complexes, this has added another layer of complexity to our understanding of synaptic transmission and plasticity.