Cav1.2 calcium channels modulate the spiking pattern of hippocampal pyramidal cells

Selective blockade of Cav1.2 (α1C) versus Cav1.3 (α1D) L-type calcium channels by the black mamba toxin calciseptine

L-type voltage-gated calcium channels are involved in multiple physiological functions. Currently available antagonists do not discriminate between L-type channel isoforms. Importantly, no selective blocker is available to dissect the role of L-type isoforms Cav1.2 and Cav1.3 that are concomitantly co-expressed in the heart, neuroendocrine and neuronal cells. Here we show that calciseptine, a snake toxin purified from mamba venom, selectively blocks Cav1.2 -mediated L-type calcium currents (ICaL) at concentrations leaving Cav1.3-mediated ICaL unaffected in both native cardiac myocytes and HEK-293T cells expressing recombinant Cav1.2 and Cav1.3 channels. Functionally, calciseptine potently inhibits cardiac contraction without altering the pacemaker activity in sino-atrial node cells, underscoring differential roles of Cav1.2- and Cav1.3 in cardiac contractility and automaticity. In summary, calciseptine is a selective L-type Cav1.2 Ca2+ channel blocker and should be a valuable tool to dissect the role of these L-channel isoforms.

Voltage-gated calcium channels in genetic epilepsies

Voltage-gated calcium channels (VGCC) are abundant in the central nervous system and serve a broad spectrum of functions, either directly in cellular excitability or indirectly to regulate Ca2+ homeostasis. Ca2+ ions act as one of the main connections in excitation-transcription coupling, muscle contraction and excitation-exocytosis coupling, including synaptic transmission. In recent years, many genes encoding VGCCs main α or additional auxiliary subunits have been associated with epilepsy. This review sums up the current state of knowledge on disease mechanisms and provides guidance on disease-specific therapies where applicable.

Bipolar‐associated miR ‐499‐5p controls neuroplasticity by downregulating the Cav1.2 subunit CACNB2

Bipolar disorder (BD) is a chronic mood disorder characterized by manic and depressive episodes. Dysregulation of neuroplasticity and calcium homeostasis are frequently observed in BD patients, but the underlying molecular mechanisms are largely unknown. Here, we show that miR‐499‐5p regulates dendritogenesis and cognitive function by downregulating the BD risk gene CACNB2. miR‐499‐5p expression is increased in peripheral blood of BD patients, as well as in the hippocampus of rats which underwent juvenile social isolation. In rat hippocampal neurons, miR‐499‐5p impairs dendritogenesis and reduces surface expression and activity of the L‐type calcium channel Cav1.2. We further identified CACNB2, which encodes a regulatory β‐subunit of Cav1.2, as a direct functional target of miR‐499‐5p in neurons. miR‐499‐5p overexpression in the hippocampus in vivo induces short‐term memory impairments selectively in rats haploinsufficient for the Cav1.2 pore forming subunit Cacna1c. In humans, miR‐499‐5p expression is negatively associated with gray matter volumes of the left superior temporal gyrus, a region implicated in auditory and emotional processing. We propose that stress‐induced miR‐499‐5p overexpression contributes to dendritic impairments, deregulated calcium homeostasis, and neurocognitive dysfunction in BD.

Cellular Effects of Rhynchophylline and Relevance to Sleep Regulation

Uncaria rhynchophylla is a plant highly used in the traditional Chinese and Japanese medicines. It has numerous health benefits, which are often attributed to its alkaloid components. Recent studies in humans show that drugs containing Uncaria ameliorate sleep quality and increase sleep time, both in physiological and pathological conditions. Rhynchophylline (Rhy) is one of the principal alkaloids in Uncaria species. Although treatment with Rhy alone has not been tested in humans, observations in rodents show that Rhy increases sleep time. However, the mechanisms by which Rhy could modulate sleep have not been comprehensively described. In this review, we are highlighting cellular pathways that are shown to be targeted by Rhy and which are also known for their implications in the regulation of wakefulness and sleep. We conclude that Rhy can impact sleep through mechanisms involving ion channels, N-methyl-d-aspartate (NMDA) receptors, tyrosine kinase receptors, extracellular signal-regulated kinases (ERK)/mitogen-activated protein kinases (MAPK), phosphoinositide 3-kinase (PI3K)/RAC serine/threonine-protein kinase (AKT), and nuclear factor-kappa B (NF-κB) pathways. In modulating multiple cellular responses, Rhy impacts neuronal communication in a way that could have substantial effects on sleep phenotypes. Thus, understanding the mechanisms of action of Rhy will have implications for sleep pharmacology.

Bilateral histone deacetylase 1 and 2 activity and enrichment at unique genes following induction of long-term potentiation in vivo

Long-term potentiation (LTP) is a synaptic plasticity mechanism critical to long-term memory. LTP induced in vivo is characterized by altered transcriptional activity, including a period of upregulation of gene expression which is followed by a later dominant downregulation. This temporal shift to downregulated gene expression is predicted to be partly mediated by epigenetic inhibitors of gene expression, such as histone deacetylases (HDACs). Further, pharmacological inhibitors of HDAC activity have previously been shown to enhance LTP persistence in vitro. To explore the contribution of HDACs to the persistence of LTP in vivo, we examined HDAC1 and HDAC2 activity over a 24 hr period following unilateral LTP induction in the dentate gyrus of freely moving rats. Surprisingly, we found significant changes in HDAC1 and HDAC2 activity in both the stimulated as well as the unstimulated hemispheres, with the largest increase in activity occurring bilaterally, 20 min after LTP stimulation. During this time point of heightened activity, chromatin immunoprecipitation assays showed that both HDAC1 and HDAC2 were enriched at distinct sets of genes within each hemispheres. Further, the HDAC inhibitor Trichostatin A enhanced an intermediate phase of LTP lasting days, which has not previously been associated with altered transcription. The inhibitor had no effect on the persistence of LTP lasting weeks. Together, these data suggest that HDAC activity early after the induction of LTP may negatively regulate plasticity-related gene expression that is involved in the initial stabilization of LTP, but not its long-term maintenance.

Delta-opioid receptor-mediated modulation of excitability of individual hippocampal neurons: mechanisms involved

BACKGROUND

Delta-opioid receptor (DOR)-mediated modulation of hippocampal neural networks is involved in emotions, cognition, and in pathophysiology and treatment of mood disorders. In this study, we examined the effects of DOR agonist (SNC80) and antagonist (naltrindole) on the excitability of individual hippocampal neurons.

METHODS

Primary neuronal cultures were prepared from hippocampi of newborn rats and cultivated in vitro for 8-14 days (DIV8-14). The effects of SNC80 naltrindole on evoked and spontaneous action potentials (APs) were measured at DIV8-9 and DIV13-14, respectively.

RESULTS

SNC80 (100 µM) potentiated spontaneous AP firing and stimulated sodium current; naltrindole had opposite effects. The stimulatory effect of 100 µM of SNC80 was revoked by pre-administration of 1 µM of naltrindole. SNC80 and naltrindole induced similar inhibitory effects on the evoked AP firing and on the calcium current. Further, SNC80 inhibited both peak and sustained potassium currents. Naltrindole had no effect on potassium currents.

CONCLUSION

We suggest that the effects of naltrindole and high concentration of SNC80 on the sodium currents are mediated via DORs and underlying the changes in spontaneous activity. The inhibitory effects of SNC80 on calcium and potassium currents might also be DOR-dependent; these currents might mediate SNC80 effect on the evoked AP firing. The inhibitory effects of naltrindole on calcium and of low doses of SNC80 on sodium currents might be however DOR independent. The behavioral effects of SNC80 and naltrindole, observed in previous studies, might be mediated, at least in part, via the modulatory effect of these ligands on the excitability of hippocampal neurons.

L-type calcium channel contributions to intrinsic excitability and synaptic activity during basolateral amygdala postnatal development

The amygdala contributes toward emotional processes such as fear, anxiety, and social cognition. Furthermore, evidence suggests that increased excitability of basolateral amygdala (BLA) principal neurons underlie certain neuropsychiatric disorders. Gain-of-function mutations in neuronal L-type calcium channels (LTCCs) are linked to neurodevelopmental diseases, including autism spectrum disorders (ASDs). While LTCCs are expressed throughout the BLA, direct evidence for increased LTCC activity affecting BLA excitability and potentially contributing to disease pathophysiology is lacking. In this study, we utilized a pharmacological approach to examine the contributions of LTCCs to BLA principal cell excitability and synaptic activity at immature (postnatal day 7, P7) and juvenile (P21) developmental stages. Acute upregulation of LTCC activity in brain slices by application of the agonist (S)-Bay K 8644 resulted in increased intrinsic excitability properties including firing frequency response, plateau potential, and spike-frequency adaptation selectively in P7 neurons. Contrastingly, for P21 neurons, the main effect of (S)-Bay K 8644 was to enhance burst firing. (S)-Bay K 8644 increased spontaneous inhibitory synaptic currents at both P7 and P21 but did not affect evoked synaptic currents at either stage. (S)-Bay K 8644 did not alter P7 spontaneous excitatory synaptic currents, although it increased current amplitude in P21 neurons. Overall, the results provide support for the notion that alteration of LTCC activity at specific periods of early brain development may lead to functional alterations to neuronal network activity and subsequently contribute to underlying mechanisms of amygdala-related neurological disorders.NEW & NOTEWORTHY The role of L-type calcium channels (LTCCs) in regulating neuronal electrophysiological properties during development remains unclear. We show that in basolateral amygdala principal neurons, an increase of LTCC activity alters both neuronal excitability and synaptic activity. The results also provide evidence for the distinct contributions of LTCCs at different stages of neurodevelopment and shed insight into our understanding of LTCC dysfunction in amygdala-related neurological disorders.

Kv2.1 mediates spatial and functional coupling of L-type calcium channels and ryanodine receptors in mammalian neurons

The voltage-gated K⁺ channel Kv2.1 serves a major structural role in the soma and proximal dendrites of mammalian brain neurons, tethering the plasma membrane (PM) to endoplasmic reticulum (ER). Although Kv2.1 clustering at neuronal ER-PM junctions (EPJs) is tightly regulated and highly conserved, its function remains unclear. By identifying and evaluating proteins in close spatial proximity to Kv2.1-containing EPJs, we discovered that a significant role of Kv2.1 at EPJs is to promote the clustering and functional coupling of PM L-type Ca²⁺ channels (LTCCs) to ryanodine receptor (RyR) ER Ca²⁺ release channels. Kv2.1 clustering also unexpectedly enhanced LTCC opening at polarized membrane potentials. This enabled Kv2.1-LTCC-RyR triads to generate localized Ca²⁺ release events (i.e., Ca²⁺ sparks) independently of action potentials. Together, these findings uncover a novel mode of LTCC regulation and establish a unique mechanism whereby Kv2.1-associated EPJs provide a molecular platform for localized somatodendritic Ca²⁺ signals in mammalian brain neurons.

Vitamin D deficiency: infertility and neurodevelopmental diseases (attention deficit hyperactivity disorder, autism, and schizophrenia)

The process of development depends on a number of signaling systems that regulates the progressive sequence of developmental events. Infertility and neurodevelopmental diseases, such as attention deficit hyperactivity disorder, autism spectrum disorders, and schizophrenia, are caused by specific alterations in these signaling processes. Calcium signaling plays a prominent role throughout development beginning at fertilization and continuing through early development, implantation, and organ differentiation such as heart and brain development. Vitamin D plays a major role in regulating these signaling processes that control development. There is an increase in infertility and an onset of neurodevelopmental diseases when vitamin D is deficient. The way in which vitamin D deficiency acts to alter development is a major feature of this review. One of the primary functions of vitamin D is to maintain the phenotypic stability of both the Ca²⁺ and redox signaling pathways that play such a key role throughout development.

Densin-180 Controls the Trafficking and Signaling of L-Type Voltage-Gated Cav1.2 Ca2+ Channels at Excitatory Synapses

Voltage-gated Ca_v1.2 and Ca_v1.3 (L-type) Ca²⁺ channels regulate neuronal excitability, synaptic plasticity, and learning and memory. Densin-180 (densin) is an excitatory synaptic protein that promotes Ca²⁺-dependent facilitation of voltage-gated Ca_v1.3 Ca²⁺ channels in transfected cells. Mice lacking densin (densin KO) exhibit defects in synaptic plasticity, spatial memory, and increased anxiety-related behaviors-phenotypes that more closely match those in mice lacking Ca_v1.2 than Ca_v1.3. Therefore, we investigated the functional impact of densin on Ca_v1.2. We report that densin is an essential regulator of Ca_v1.2 in neurons, but has distinct modulatory effects compared with its regulation of Ca_v1.3. Densin binds to the N-terminal domain of Ca_v1.2, but not that of Ca_v1.3, and increases Ca_v1.2 currents in transfected cells and in neurons. In transfected cells, densin accelerates the forward trafficking of Ca_v1.2 channels without affecting their endocytosis. Consistent with a role for densin in increasing the number of postsynaptic Ca_v1.2 channels, overexpression of densin increases the clustering of Ca_v1.2 in dendrites of hippocampal neurons in culture. Compared with wild-type mice, the cell surface levels of Ca_v1.2 in the brain, as well as Ca_v1.2 current density and signaling to the nucleus, are reduced in neurons from densin KO mice. We conclude that densin is an essential regulator of neuronal Ca_v1 channels and ensures efficient Ca_v1.2 Ca²⁺ signaling at excitatory synapses.SIGNIFICANCE STATEMENT The number and localization of voltage-gated Ca_v Ca²⁺ channels are crucial determinants of neuronal excitability and synaptic transmission. We report that the protein densin-180 is highly enriched at excitatory synapses in the brain and enhances the cell surface trafficking and postsynaptic localization of Ca_v1.2 L-type Ca²⁺ channels in neurons. This interaction promotes coupling of Ca_v1.2 channels to activity-dependent gene transcription. Our results reveal a mechanism that may contribute to the roles of Ca_v1.2 in regulating cognition and mood.

TGFβ1 downregulates neurite outgrowth, expression of Ca2+ transporters, and mitochondrial dynamics of in vitro cerebellar granule cells

Acute injury to central nervous system (CNS) triggers neurodegenerative processes that can result in serious damage or complete loss of function. After injury, production of transforming growth factor β1 (TGFβ1) increases and initiates creation of a fibrotic scar that prevents normal growth, plasticity, and recovery of damaged neurons. Administration of TGFβ1 antagonists can prevent its pathological effects. To define consequences of increased TGFβ1 release on calcium signaling, neuronal plasticity, excitability, and mitochondrial dynamics in CNS neurons we directly exposed a rat primary culture of cerebellar granule neurons to TGFβ1. We focused on changes in expression of intracellular calcium transporters, especially inositol-1,4,5-trisphosphate receptor (IP3R) type 1, mitochondrial dynamics, and membrane excitability. TGFβ1 significantly decreased the gene and protein expression of inositol-1,4,5-trisphosphate receptor type 1 and the gene expression of additional intracellular Ca transporters such as IP3R2, ryanodine receptor type 1 (RyR1), RyR2, and SERCA2. Altered calcium signaling suppressed neurite outgrowth and significantly decreased the length of the mitochondria and the frequency of mitochondrial fusion. The resting membrane potential of cerebellar granule neurons was hyperpolarized and slow after depolarization of single action potential was suppressed. LY364947, a blocker of TGFβ1 receptor I, prevented these effects, and IP3 receptor blocker 2-aminoethoxydiphenyl borate (2APB) mimicked them. After CNS injury TGFβ1 downregulates intracellular Ca levels and alters Ca signaling within injured neurons. We suggest that in our model TGFβ1 may trigger both neurodegenerative and neuroprotective events through IP3-induced Ca signaling.

Cav1.2 and Cav1.3 L-type calcium channels operate in a similar voltage range but show different coupling to Ca(2+)-dependent conductances in hippocampal neurons

In the central nervous system, L-type voltage-gated calcium channels (LTCCs) come in two isoforms, namely Cav1.2 and Cav1.3 channels. It has been shown previously that these channels differ in biophysical properties, in subcellular localization, and in the coupling to the gene transcription machinery. In previous work on rat hippocampal neurons we have identified an excitatory cation conductance and an inhibitory potassium conductance as important LTCC coupling partners. Notably, a stimulus-dependent interplay of LTCC-mediated Ca(2+) influx and activation of these Ca(2+)-dependent conductances was found to give rise to characteristic voltage responses. However, the contribution of Cav1.2 and Cav1.3 to these voltage responses remained unknown. Hence, the relative contribution of the LTCC isoforms therein was the focus of the current study on hippocampal neurons derived from genetically modified mice, which either lack a LTCC isoform (Cav1.3 knockout mice) or express a dihydropyridine-insensitive LTCC isoform (Cav1.2DHP(-)-knockin mice). We identified common and alternate ion channel couplings of Cav1.2 and Cav1.3 channels. Whereas hyperpolarizing Ca(2+)-dependent conductances were coupled to both Cav1.2 and Cav1.3 channels, an afterdepolarizing potential was only induced by the activity of Cav1.2 channels. Unexpectedly, the activity of Cav1.2 channels was found at relatively hyperpolarized membrane voltages. Our data add important information about the differences between Cav1.2 and Cav1.3 channels that furthers our understanding of the physiological and pathophysiological neuronal roles of these calcium channels. Moreover, our findings suggest that Cav1.3 knockout mice together with Cav1.2DHP(-)-knockin mice provide valuable models for future investigation of hippocampal LTCC-dependent afterdepolarizations.

Calcium signalling and psychiatric disease: bipolar disorder and schizophrenia

Neurons have highly developed Ca(2+) signalling systems responsible for regulating many neural functions such as the generation of brain rhythms, information processing and the changes in synaptic plasticity that underpins learning and memory. The signalling mechanisms that regulate neuronal excitability are particularly important for processes such as sensory perception, cognition and consciousness. The Ca(2+) signalling pathway is a key component of the mechanisms responsible for regulating neuronal excitability, information processing and cognition. Alterations in gene transcription are particularly important as they result in subtle alterations in the neuronal signalling mechanisms that have been implicated in many neural diseases. In particular, dysregulation of the Ca(2+) signalling pathway has been implicated in the development of some of the major psychiatric diseases such as bipolar disorder (BPD) and schizophrenia.

L-type Ca2+ channels in heart and brain

L-type calcium channels (Cav1) represent one of the three major classes (Cav1–3) of voltage-gated calcium channels. They were identified as the target of clinically used calcium channel blockers (CCBs; so-called calcium antagonists) and were the first class accessible to biochemical characterization. Four of the 10 known α1 subunits (Cav1.1–Cav1.4) form the pore of L-type calcium channels (LTCCs) and contain the high-affinity drug-binding sites for dihydropyridines and other chemical classes of organic CCBs. In essentially all electrically excitable cells one or more of these LTCC isoforms is expressed, and therefore it is not surprising that many body functions including muscle, brain, endocrine, and sensory function depend on proper LTCC activity. Gene knockouts and inherited human diseases have allowed detailed insight into the physiological and pathophysiological role of these channels. Genome-wide association studies and analysis of human genomes are currently providing even more hints that even small changes of channel expression or activity may be associated with disease, such as psychiatric disease or cardiac arrhythmias. Therefore, it is important to understand the structure–function relationship of LTCC isoforms, their differential contribution to physiological function, as well as their fine-tuning by modulatory cellular processes.

The impact of a CACNA1C gene polymorphism on learning and hippocampal formation in healthy individuals: a diffusion tensor imaging study

BACKGROUND

Genome-wide association studies have identified the CACNA1C single nucleotide polymorphism (SNP) rs1006737 as one of the most consistent genetic findings as susceptibility locus for major psychiatric disorders. Furthermore, animal and genetic imaging studies have reported strong functional evidence for the association of CACNA1C with learning, memory, neural plasticity, and its association with the hippocampal formation. In the present study we investigated the impact of the CACNA1C SNP rs1006737 on the fractional anisotropy (FA) in the hippocampal formation as well as on verbal learning and memory in healthy individuals.

METHODS

118 healthy individuals (72 males, 46 females, age 18-56years) initially underwent diffusion tensor imaging (DTI), 100 of them were included in the final analysis. We used Tract-Based Spatial Statistics (TBSS) to examine the impact of the CACNA1C SNP rs1006737 on the hippocampal formation as predefined region of interest (ROI). Furthermore, all participants completed the Verbal Learning and Memory Test (VLMT).

RESULTS

In the VLMT genotype was significantly associated with learning performance. Bonferroni corrected post-hoc tests indicated a diminished performance at the beginning of the learning curve in risk allele carriers compared to non-risk allele carriers. The TBSS ROI analysis revealed one cluster of reduced FA in risk allele carriers compared to non-risk allele carriers located in the right hippocampal formation. Moreover, an association between the initial learning performance and FA values was found.

DISCUSSION

These findings demonstrate that genetic variation in the CACNA1C SNP rs1006737 is associated with FA reduction in the hippocampal formation as well as with differences in learning performance in healthy individuals.

Activation of CRH receptor type 1 expressed on glutamatergic neurons increases excitability of CA1 pyramidal neurons by the modulation of voltage-gated ion channels

Corticotropin-releasing hormone (CRH) plays an important role in a substantial number of patients with stress-related mental disorders, such as anxiety disorders and depression. CRH has been shown to increase neuronal excitability in the hippocampus, but the underlying mechanisms are poorly understood. The effects of CRH on neuronal excitability were investigated in acute hippocampal brain slices. Population spikes (PS) and field excitatory postsynaptic potentials (fEPSP) were evoked by stimulating Schaffer-collaterals and recorded simultaneously from the somatic and dendritic region of CA1 pyramidal neurons. CRH was found to increase PS amplitudes (mean ± Standard error of the mean; 231.8 ± 31.2% of control; n = 10) while neither affecting fEPSPs (104.3 ± 4.2%; n = 10) nor long-term potentiation (LTP). However, when Schaffer-collaterals were excited via action potentials (APs) generated by stimulation of CA3 pyramidal neurons, CRH increased fEPSP amplitudes (119.8 ± 3.6%; n = 8) and the magnitude of LTP in the CA1 region. Experiments in slices from transgenic mice revealed that the effect on PS amplitude is mediated exclusively by CRH receptor 1 (CRHR1) expressed on glutamatergic neurons. The effects of CRH on PS were dependent on phosphatase-2B, L- and T-type calcium channels and voltage-gated potassium channels but independent on intracellular Ca²⁺-elevation. In patch-clamp experiments, CRH increased the frequency and decay times of APs and decreased currents through A-type and delayed-rectifier potassium channels. These results suggest that CRH does not affect synaptic transmission per se, but modulates voltage-gated ion currents important for the generation of APs and hence elevates by this route overall neuronal activity.

CACNA1C (Cav1.2) in the pathophysiology of psychiatric disease

One of the most consistent genetic findings to have emerged from bipolar disorder genome wide association studies (GWAS) is with CACNA1C, a gene that codes for the α(1C) subunit of the Ca(v)1.2 voltage-dependent L-type calcium channel (LTCC). Genetic variation in CACNA1C have also been associated with depression, schizophrenia, autism spectrum disorders, as well as changes in brain function and structure in control subjects who have no diagnosable psychiatric illness. These data are consistent with a continuum of shared neurobiological vulnerability between diverse-Diagnostic and Statistical Manual (DSM) defined-neuropsychiatric diseases. While involved in numerous cellular functions, Ca(v)1.2 is most frequently implicated in coupling of cell membrane depolarization to transient increase of the membrane permeability for calcium, leading to activation and, potentially, changes in intracellular signaling pathway activity, gene transcription, and synaptic plasticity. Ca(v)1.2 is involved in the proper function of numerous neurological circuits including those involving the hippocampus, amygdala, and mesolimbic reward system, which are strongly implicated in psychiatric disease pathophysiology. A number of behavioral effects of LTCC inhibitors have been described including antidepressant-like behavioral actions in rodent models. Clinical studies suggest possible treatment effects in a subset of patients with mood disorders. We review the genetic structure and variation of CACNA1C, discussing relevant human genetic and clinical findings, as well as the biological actions of Ca(v)1.2 that are most relevant to psychiatric illness.

Retrieval of context-associated memory is dependent on the Ca(v)3.2 T-type calcium channel

Among all voltage-gated calcium channels, the T-type Ca²⁺ channels encoded by the Ca_v3.2 genes are highly expressed in the hippocampus, which is associated with contextual, temporal and spatial learning and memory. However, the specific involvement of the Ca_v3.2 T-type Ca²⁺ channel in these hippocampus-dependent types of learning and memory remains unclear. To investigate the functional role of this channel in learning and memory, we subjected Ca_v3.2 homozygous and heterozygous knockout mice and their wild-type littermates to hippocampus-dependent behavioral tasks, including trace fear conditioning, the Morris water-maze and passive avoidance. The Ca_v3.2 ^−/− mice performed normally in the Morris water-maze and auditory trace fear conditioning tasks but were impaired in the context-cued trace fear conditioning, step-down and step-through passive avoidance tasks. Furthermore, long-term potentiation (LTP) could be induced for 180 minutes in hippocampal slices of WTs and Ca_v3.2 ^+/− mice, whereas LTP persisted for only 120 minutes in Ca_v3.2 ^−/− mice. To determine whether the hippocampal formation is responsible for the impaired behavioral phenotypes, we next performed experiments to knock down local function of the Ca_v3.2 T-type Ca²⁺ channel in the hippocampus. Wild-type mice infused with mibefradil, a T-type channel blocker, exhibited similar behaviors as homozygous knockouts. Taken together, our results demonstrate that retrieval of context-associated memory is dependent on the Ca_v3.2 T-type Ca²⁺ channel.

Calcium channel blocking as a therapeutic strategy for Alzheimer's disease: the case for isradipine

Alzheimer's disease is the most devastating neurodegenerative disorder in the elderly, yet treatment options are severely limited. The drug development effort to modify Alzheimer's disease pathology by intervention at beta amyloid production sites has been largely ineffective or inconclusive. The greatest challenge has been to identify and define downstream mechanisms reliably predictive of clinical symptoms. Beta amyloid accumulation leads to dysregulation of intracellular calcium by plasma membrane L-type calcium channels located on neuronal somatodendrites and axons in the hippocampus and cortex. Paradoxically, L-type calcium channel subtype Ca(v)1.2 also promotes synaptic plasticity and spatial memory. Increased intracellular calcium modulates amyloid precursor protein processing and affects multiple downstream pathways including increased hyperphosphorylated tau and suppression of autophagy. Isradipine is a Federal Drug Administration-approved dihydropyridine calcium channel blocker that binds selectively to Ca(v)1.2 in the hippocampus. Our studies have shown that isradipine in vitro attenuates beta amyloid oligomer toxicity by suppressing calcium influx into cytoplasm and by suppressing Ca(v)1.2 expression. We have previously shown that administration of isradipine to triple transgenic animal model for Alzheimer's disease was well-tolerated. Our results further suggest that isradipine became bioavailable, lowered tau burden, and improved autophagy function in the brain. A better understanding of brain pharmacokinetics of calcium channel blockers will be critical for designing new experiments with appropriate drug doses in any future clinical trials for Alzheimer's disease. This review highlights the importance of Ca(v)1.2 channel overexpression, the accumulation of hyperphosphorylated tau and suppression of autophagy in Alzheimer's disease and modulation of this pathway by isradipine.

HCN2 channels in local inhibitory interneurons constrain LTP in the hippocampal direct perforant path

Neuronal hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are known to modulate spontaneous activity, resting membrane potential, input resistance, afterpotential, rebound activity, and dendritic integration. To evaluate the role of HCN2 for hippocampal synaptic plasticity, we recorded long-term potentiation (LTP) in the direct perforant path (PP) to CA1 pyramidal cells. LTP was enhanced in mice carrying a global deletion of the channel (HCN2(-/-)) but not in a pyramidal neuron-restricted knockout. This precludes an influence of HCN2 located in postsynaptic pyramidal neurons. Additionally, the selective HCN blocker zatebradine reduced the activity of oriens-lacunosum moleculare interneurons in wild-type but not HCN2(-/-) mice and decreased the frequency of spontaneous inhibitory currents in postsynaptic CA1 pyramidal cells. Finally, we found amplified LTP in the PP of mice carrying an interneuron-specific deletion of HCN2. We conclude that HCN2 channels in inhibitory interneurons modulate synaptic plasticity in the PP by facilitating the GABAergic output onto pyramidal neurons.