Distinct Ca2+ channels mediate transmitter release at excitatory synapses displaying different dynamic properties in rat neocortex

Inter and intralaminar excitation of parvalbumin interneurons in mouse barrel cortex

Parvalbumin (PV) interneurons are inhibitory fast-spiking cells with essential roles in directing the flow of information through cortical circuits. These neurons set the balance between excitation and inhibition and control rhythmic activity. PV interneurons differ between cortical layers in their morphology, circuitry, and function, but how their electrophysiological properties vary has received little attention. Here we investigate responses of PV interneurons in different layers of primary somatosensory barrel cortex (BC) to different excitatory inputs. With the genetically-encoded hybrid voltage sensor, hVOS, we recorded voltage changes in many L2/3 and L4 PV interneurons simultaneously, with stimulation applied to either L2/3 or L4. A semi-automated procedure was developed to identify small regions of interest corresponding to single responsive PV interneurons. Amplitude, half-width, and rise-time were greater for PV interneurons residing in L2/3 compared to L4. Stimulation in L2/3 elicited responses in both L2/3 and L4 with longer latency compared to stimulation in L4. These differences in latency between layers could influence their windows for temporal integration. Thus, PV interneurons in different cortical layers of BC respond in a layer specific and input specific manner, and these differences have potential roles in cortical computations.

Inter and Intralaminar Excitation of Parvalbumin Interneurons in Mouse Barrel Cortex

Parvalbumin (PV) interneurons are inhibitory fast-spiking cells with essential roles in directing the flow of information through cortical circuits. These neurons set the balance between excitation and inhibition, control rhythmic activity, and have been linked to disorders including autism spectrum and schizophrenia. PV interneurons differ between cortical layers in their morphology, circuitry, and function, but how their electrophysiological properties vary has received little attention. Here we investigate responses of PV interneurons in different layers of primary somatosensory barrel cortex (BC) to different excitatory inputs. With the genetically-encoded hybrid voltage sensor, hVOS, we recorded voltage changes simultaneously in many L2/3 and L4 PV interneurons to stimulation in either L2/3 or L4. Decay-times were consistent across L2/3 and L4. Amplitude, half-width, and rise-time were greater for PV interneurons residing in L2/3 compared to L4. Stimulation in L2/3 elicited responses in both L2/3 and L4 with longer latency compared to stimulation in L4. These differences in latency between layers could influence their windows for temporal integration. Thus PV interneurons in different cortical layers of BC show differences in response properties with potential roles in cortical computations.

A neurodevelopmental disorder caused by a dysfunctional CACNA1A allele

P/Q-type Ca²⁺ flux into nerve terminals via Ca_V2.1 channels is essential for neurotransmitter release at neuromuscular junctions and nearly all central synapses. Mutations in CACNA1A, the gene encoding Ca_V2.1, cause a spectrum of pediatric neurological disorders. We have identified a patient harboring an autosomal-dominant de novo frameshift-causing nucleotide duplication in CACNA1A (c.5018dupG). The duplicated guanine precipitated 43 residues of altered amino acid sequence beginning with a glutamine to serine substitution in Ca_V2.1 at position 1674 ending with a premature stop codon (Ca_V2.1 p.Gln1674Serfs*43). The patient presented with episodic downbeat vertical nystagmus, hypotonia, ataxia, developmental delay and febrile seizures. In patch-clamp experiments, no Ba²⁺ current was observed in tsA-201 cells expressing Ca_V2.1 p.Gln1674Serfs*43 with β₄ and α₂δ-1 auxiliary subunits. The ablation of divalent flux in response to depolarization was likely attributable to the inability of Ca_V2.1 p.Gln1674Serfs*43 to form a complete channel pore. Our results suggest that the pathology resulting from this frameshift-inducing nucleotide duplication is a consequence of an effective haploinsufficiency.

Spontaneous and action potential-evoked Ca2+ release from endoplasmic reticulum in neocortical synaptic boutons

Although the endoplasmic reticulum (ER) is present throughout axons, and IP3 and ryanodine receptors are widely expressed in nerve terminals, whether Ca2+ release from presynaptic stores contributes to action potential (AP)-evoked Ca2+ transients remains controversial. We investigated the release of Ca2+ from ER stores in boutons en passant of neocortical layer 5 pyramidal neurons. A hallmark of these stores is that they spontaneously release Ca2+ at a low frequency. Using a high-affinity Ca2+ indicator, we documented and characterised such spontaneous Ca2+ transients (sCaTs), which occurred at a rate of ~0.2 per min and raised the intracellular Ca2+ concentration ([Ca2+]i) by ~2 µM in the absence of exogenous buffers. Caffeine increased the average frequency of sCaTs by 90%, without affecting their amplitude and decay kinetics. Therefore, presynaptic ryanodine receptors were likely involved. To determine if presynaptic ER stores contribute to intracellular Ca2+ accumulation during repetitive stimulation, we measured [Ca2+]i during 2 s long trains of APs evoked at 10-50 Hz. We found that for frequencies <20 Hz, [Ca2+]i reached a steady state within ~500 ms after stimulation onset. However, for higher frequencies, [Ca2+]i continued to increase with AP number, suggesting that the rate of Ca2+ entry exceeded the rate of clearance. Comparison between measured and predicted values indicates supralinear summation of Ca2+. Block of the sarco/endoplasmic reticulum Ca2+-ATPase reduced the supralinearity of summation, without reducing the amplitude of a single AP-evoked Ca2+ transient. Together, our results implicate presynaptic ER stores as a source of Ca2+ during repetitive stimulation.

Selective Modulation of α5 GABAA Receptors Exacerbates Aberrant Inhibition at Key Hippocampal Neuronal Circuits in APP Mouse Model of Alzheimer's Disease

Selective negative allosteric modulators (NAMs), targeting α5 subunit-containing GABA_A receptors (GABA_ARs) as potential therapeutic targets for disorders associated with cognitive deficits, including Alzheimer's disease (AD), continually fail clinical trials. We investigated whether this was due to the change in the expression of α5 GABA_ARs, consequently altering synaptic function during AD pathogenesis. Using medicinal chemistry and computational modeling, we developed aqueous soluble hybrids of 6,6-dimethyl-3-(2-hydroxyethyl) thio-1-(thiazol-2-yl)-6,7-dihydro-2-benzothiophene-4(5H)-one, that demonstrated selective binding and high negative allosteric modulation, specifically for the α5 GABA_AR subtypes in constructed HEK293 stable cell-lines. Using a knock-in mouse model of AD (APP ^NL-F/NL-F), which expresses a mutant form of human amyloid-β (Aβ), we performed immunofluorescence studies combined with electrophysiological whole-cell recordings to investigate the effects of our key molecule, α5-SOP002 in the hippocampal CA1 region. In aged APP ^NL-F/NL-F mice, selective preservation of α5 GABA_ARs was observed in, calretinin- (CR), cholecystokinin- (CCK), somatostatin- (SST) expressing interneurons, and pyramidal cells. Previously, we reported that CR dis-inhibitory interneurons, specialized in regulating other interneurons displayed abnormally high levels of synaptic inhibition in the APP ^NL-F/NL-F mouse model, here we show that this excessive inhibition was "normalized" to control values with bath-applied α5-SOP002 (1 μM). However, α5-SOP002, further impaired inhibition onto CCK and pyramidal cells that were already largely compromised by exhibiting a deficit of inhibition in the AD model. In summary, using a multi-disciplinary approach, we show that exposure to α5 GABA_AR NAMs may further compromise aberrant synapses in AD. We, therefore, suggest that the α5 GABA_AR is not a suitable therapeutic target for the treatment of AD or other cognitive deficits due to the widespread neuronal-networks that use α5 GABA_ARs.

Gyroxin, a toxin from Crotalus durissus terrificus snake venom, induces a calcium dependent increase in glutamate release in mice brain cortical synaptosomes

Gyroxin is a thrombin-like toxin obtained from the venom of the South American rattlesnake, Crotalus durissus terrificus. Literature has reported "gyroxin syndrome" characterized, in mice, as series of aberrant motor behavior, known as barrel rotation, mainly after intraperitoneal administration. Despites several studies, a physiological mechanism of "gyroxin syndrome" are still not completely understood. In this context, alterations on the central nervous system (CNS), especially causing neurotoxic events, are pointed out as likely candidates. Then, we decided to investigate whether gyroxin induces alterations in glutamate release, one of the most important neurotransmitter involved in neurotoxicity. For that, we performed all experiments, in vitro, using a model of mice brain cortical synaptosomes. Notably, our results indicate that the administration of gyroxin on purified presynaptic brain cortical terminals resulted in an extracellular Ca²⁺- dependent raise in glutamate release. Indeed, our results also showed that gyroxin increases intrasynaptosomal calcium (Ca²⁺) levels through acting on voltage gated calcium channels (VGCC), specifically N and P/Q subtypes. Moreover, our data show that gyroxin increases exocytosis rate. Interestingly, these data suggest that gyroxin might induce neurotoxicity by increasing glutamate levels. However, future investigations are needed in order to elucidate the nature of the following events.

Remodeled cortical inhibition prevents motor seizures in generalized epilepsy

OBJECTIVE

Deletions of CACNA1A, encoding the α1 subunit of Ca_V 2.1 channels, cause epilepsy with ataxia in humans. Whereas the deletion of Cacna1a in γ-aminobutyric acidergic (GABAergic) interneurons (INs) derived from the medial ganglionic eminence (MGE) impairs cortical inhibition and causes generalized seizures in Nkx2.1^Cre ;Cacna1a^c/c mice, the targeted deletion of Cacna1a in somatostatin-expressing INs (SOM-INs), a subset of MGE-derived INs, does not result in seizures, indicating a crucial role of parvalbumin-expressing (PV) INs. Here we identify the cellular and network consequences of Cacna1a deletion specifically in PV-INs.

METHODS

We generated PV^Cre ;Cacna1a^c/c mutant mice carrying a conditional Cacna1a deletion in PV neurons and evaluated the cortical cellular and network outcomes of this mutation by combining immunohistochemical assays, in vitro electrophysiology, 2-photon imaging, and in vivo video-electroencephalographic recordings.

RESULTS

PV^Cre ;Cacna1a^c/c mice display reduced cortical perisomatic inhibition and frequent absences but only rare motor seizures. Compared to Nkx2.1^Cre ;Cacna1a^c/c mice, PV^Cre ;Cacna1a^c/c mice have a net increase in cortical inhibition, with a gain of dendritic inhibition through sprouting of SOM-IN axons, largely preventing motor seizures. This beneficial compensatory remodeling of cortical GABAergic innervation is mTORC1-dependent and its inhibition with rapamycin leads to a striking increase in motor seizures. Furthermore, we show that a direct chemogenic activation of cortical SOM-INs prevents motor seizures in a model of kainate-induced seizures.

INTERPRETATION

Our findings provide novel evidence suggesting that the remodeling of cortical inhibition, with an mTOR-dependent gain of dendritic inhibition, determines the seizure phenotype in generalized epilepsy and that mTOR inhibition can be detrimental in epilepsies not primarily due to mTOR hyperactivation. Ann Neurol 2018;84:436-451.

Opposite Roles in Short-Term Plasticity for N-Type and P/Q-Type Voltage-Dependent Calcium Channels in GABAergic Neuronal Connections in the Rat Cerebral Cortex

Neurotransmitter release is triggered by Ca²⁺ influx through voltage-dependent Ca²⁺ channels (VDCCs). Distinct expression patterns of VDCC subtypes localized on the synaptic terminal affect intracellular Ca²⁺ dynamics induced by action potential-triggered Ca²⁺ influx. However, it has been unknown whether the expression pattern of VDCC subtypes depends on each axon terminal or neuronal subtype. Furthermore, little information is available on how these VDCC subtypes regulate the release probability of neurotransmitters. To address these questions, we performed multiple whole-cell patch-clamp recordings from GABAergic neurons in the insular cortex of either the male or the female rat. The paired-pulse ratio (PPR; 50 ms interstimulus interval) varied widely among inhibitory connections between GABAergic neurons. The PPR of unitary IPSCs was enhanced by ω-conotoxin GVIA (CgTx; 3 μm), an N-type VDCC blocker, whereas blockade of P/Q-type VDCCs by ω-agatoxin IVA (AgTx, 200 nm) decreased the PPR. In the presence of CgTx, application of 4 mm [Ca²⁺]_o or of roscovitine, a P/Q-type activator, increased the PPR. These results suggest that the recruitment of P/Q-type VDCCs increases the PPR, whereas N-type VDCCs suppress the PPR. Furthermore, we found that charybdotoxin or apamin, blockers of Ca²⁺-dependent K⁺ channels, with AgTx increased the PPR, suggesting that Ca²⁺-dependent K⁺ channels are coupled to N-type VDCCs and suppress the PPR in GABAergic neuronal terminals. Variance-mean analysis with changing [Ca²⁺]_o showed a negative correlation between the PPR and release probability in GABAergic synapses. These results suggest that GABAergic neurons differentially express N-type and/or P/Q-type VDCCs and that these VDCCs regulate the GABA release probability in distinct manners.SIGNIFICANCE STATEMENT GABAergic neuronal axons target multiple neurons and release GABA triggered by Ca²⁺ influx via voltage-dependent Ca²⁺ channels (VDCCs), including N-type and P/Q-type channels. Little is known about VDCC expression patterns in GABAergic synaptic terminals and their role in short-term plasticity. We focused on inhibitory synaptic connections between GABAergic neurons in the cerebral cortex using multiple whole-cell patch-clamp recordings and found different expression patterns of VDCCs in the synaptic terminals branched from a single presynaptic neuron. Furthermore, we observed facilitative and depressive short-term plasticity of IPSCs mediated by P/Q-type and N-type VDCCs, respectively. These results suggest that VDCC expression patterns regulate distinctive types of synaptic transmission in each GABAergic axon terminal even though they are branched from a common presynaptic neuron.

Posterior Thalamic Nucleus Modulation of Tactile Stimuli Processing in Rat Motor and Primary Somatosensory Cortices

Rodents move rhythmically their facial whiskers and compute differences between signals predicted and those resulting from the movement to infer information about objects near their head. These computations are carried out by a large network of forebrain structures that includes the thalamus and the primary somatosensory (S1BF) and motor (M1wk) cortices. Spatially and temporally precise mechanorreceptive whisker information reaches the S1BF cortex via the ventroposterior medial thalamic nucleus (VPM). Other whisker-related information may reach both M1wk and S1BF via the axons from the posterior thalamic nucleus (Po). However, Po axons may convey, in addition to direct sensory signals, the dynamic output of computations between whisker signals and descending motor commands. It has been proposed that this input may be relevant for adjusting cortical responses to predicted vs. unpredicted whisker signals, but the effects of Po input on M1wk and S1BF function have not been directly tested or compared in vivo. Here, using electrophysiology, optogenetics and pharmacological tools, we compared in adult rats M1wk and S1BF in vivo responses in the whisker areas of the motor and primary somatosensory cortices to passive multi-whisker deflection, their dependence on Po activity, and their changes after a brief intense activation of Po axons. We report that the latencies of the first component of tactile-evoked local field potentials in M1wk and S1BF are similar. The evoked potentials decrease markedly in M1wk, but not in S1BF, by injection in Po of the GABA_A agonist muscimol. A brief high-frequency electrical stimulation of Po decreases the responsivity of M1wk and S1BF cells to subsequent whisker stimulation. This effect is prevented by the local application of omega-agatoxin, suggesting that it may in part depend on GABA release by fast-spiking parvalbumin (PV)-expressing cortical interneurons. Local optogenetic activation of Po synapses in different cortical layers also diminishes M1wk and S1BF responses. This effect is most pronounced in the superficial layers of both areas, known to be the main source and target of their reciprocal cortico-cortical connections.

Isolated P/Q Calcium Channel Deletion in Layer VI Corticothalamic Neurons Generates Absence Epilepsy

Generalized spike-wave seizures involving abnormal synchronization of cortical and underlying thalamic circuitry represent a major category of childhood epilepsy. Inborn errors of Cacna1a, the P/Q-type voltage-gated calcium channel α subunit gene, expressed throughout the brain destabilize corticothalamic rhythmicity and produce this phenotype. To determine the minimal cellular lesion required for this network disturbance, we used neurotensin receptor 1 (Ntsr1) cre-driver mice to ablate floxed Cacna1a in layer VI pyramidal neurons, which supply the sole descending cortical synaptic input to thalamocortical relay cells and reticular interneurons and activate intrathalamic circuits. Targeted Cacna1a ablation in layer VI cells resulted in mice that display a robust spontaneous spike-wave absence seizure phenotype accompanied by behavioral arrest and inhibited by ethosuximide. To verify the selectivity of the molecular lesion, we determined that P/Q subunit proteins were reduced in corticothalamic relay neuron terminal zones, and confirmed that P/Q-mediated glutamate release was reduced at these synapses. Spike-triggered exocytosis was preserved by N-type calcium channel rescue, demonstrating that evoked release at layer VI terminals relies on both P/Q and N-type channels. Whereas intrinsic excitability of the P/Q channel depleted layer VI neurons was unaltered, T-type calcium currents in the postsynaptic thalamic relay and reticular cells were dramatically elevated, favoring rebound bursting and seizure generation. We find that an early P/Q-type release defect, limited to synapses of a single cell-type within the thalamocortical circuit, is sufficient to remodel synchronized firing behavior and produce a stable generalized epilepsy phenotype.

SIGNIFICANCE STATEMENT

This study dissects a critical component of the corticothalamic circuit in spike-wave epilepsy and identifies the developmental importance of P/Q-type calcium channel-mediated presynaptic glutamate release at layer VI pyramidal neuron terminals. Genetic ablation of Cacna1a in layer VI neurons produced synchronous spike-wave discharges in the cortex and thalamus that were inhibited by ethosuximide. These mice also displayed N-type calcium channel compensation at descending thalamic synapses, and consistent with other spike-wave models increased low-threshold T-type calcium currents within postsynaptic thalamic relay and reticular neurons. These results demonstrate, for the first time, that preventing the developmental homeostatic switch from loose to tightly coupled synaptic release at a single class of deep layer cortical excitatory output neurons results in generalized spike-wave epilepsy.

CACNA1A haploinsufficiency causes cognitive impairment, autism and epileptic encephalopathy with mild cerebellar symptoms

CACNA1A loss-of-function mutations classically present as episodic ataxia type 2 (EA2), with brief episodes of ataxia and nystagmus, or with progressive spinocerebellar ataxia (SCA6). A minority of patients carrying CACNA1A mutations develops epilepsy. Non-motor symptoms associated with these mutations are often overlooked. In this study, we report 16 affected individuals from four unrelated families presenting with a spectrum of cognitive impairment including intellectual deficiency, executive dysfunction, ADHD and/or autism, as well as childhood-onset epileptic encephalopathy with refractory absence epilepsy, febrile seizures, downbeat nystagmus and episodic ataxia. Sequencing revealed one CACNA1A gene deletion, two deleterious CACNA1A point mutations including one known stop-gain and one new frameshift variant and a new splice-site variant. This report illustrates the phenotypic heterogeneity of CACNA1A loss-of-function mutations and stresses the cognitive and epileptic manifestations caused by the loss of CaV2.1 channels function, presumably affecting cerebellar, cortical and limbic networks.

Melanocortin 4 receptor activation inhibits presynaptic N-type calcium channels in amygdaloid complex neurons

The melanocortin 4 receptor (MC4R) is a G protein-coupled receptor involved in food intake and energy expenditure regulation. MC4R activation modifies neuronal activity but the molecular mechanisms by which this regulation occurs remain unclear. Here, we tested the hypothesis that MC4R activation regulates the activity of voltage-gated calcium channels and, as a consequence, synaptic activity. We also tested whether the proposed effect occurs in the amygdala, a brain area known to mediate the anorexigenic actions of MC4R signaling. Using the patch-clamp technique, we found that the activation of MC4R with its agonist melanotan II specifically inhibited 34.5 ± 1.5% of N-type calcium currents in transiently transfected HEK293 cells. This inhibition was concentration-dependent, voltage-independent and occluded by the Gαs pathway inhibitor cholera toxin. Moreover, we found that melanotan II specifically inhibited 25.9 ± 2.0% of native N-type calcium currents and 55.4 ± 14.4% of evoked inhibitory postsynaptic currents in mouse cultured amygdala neurons. In vivo, we found that the MC4R agonist RO27-3225 increased the marker of cellular activity c-Fos in several components of the amygdala, whereas the N-type channel blocker ω conotoxin GVIA increased c-Fos expression exclusively in the central subdivision of the amygdala. Thus, MC4R specifically inhibited the presynaptic N-type channel subtype, and this inhibition may be important for the effects of melanocortin in the central subdivision of the amygdala.

Mechanism underlying unaltered cortical inhibitory synaptic transmission in contrast with enhanced excitatory transmission in CaV2.1 knockin migraine mice

Familial hemiplegic migraine type 1 (FHM1), a monogenic subtype of migraine with aura, is caused by gain-of-function mutations in Ca_V2.1 (P/Q-type) calcium channels. In FHM1 knockin mice, excitatory neurotransmission at cortical pyramidal cell synapses is enhanced, but inhibitory neurotransmission at connected pairs of fast-spiking (FS) interneurons and pyramidal cells is unaltered, despite being initiated by Ca_V2.1 channels. The mechanism underlying the unaltered GABA release at cortical FS interneuron synapses remains unknown. Here, we show that the FHM1 R192Q mutation does not affect inhibitory transmission at autapses of cortical FS and other types of multipolar interneurons in microculture from R192Q knockin mice, and investigate the underlying mechanism. Lowering the extracellular [Ca²⁺] did not reveal gain-of-function of evoked transmission neither in control nor after prolongation of the action potential (AP) with tetraethylammonium, indicating unaltered AP-evoked presynaptic calcium influx at inhibitory autapses in FHM1 KI mice. Neither saturation of the presynaptic calcium sensor nor short duration of the AP can explain the unaltered inhibitory transmission in the mutant mice. Recordings of the P/Q-type calcium current in multipolar interneurons in microculture revealed that the current density and the gating properties of the Ca_V2.1 channels expressed in these interneurons are barely affected by the FHM1 mutation, in contrast with the enhanced current density and left-shifted activation gating of mutant Ca_V2.1 channels in cortical pyramidal cells. Our findings suggest that expression of specific Ca_V2.1 channels differentially sensitive to modulation by FHM1 mutations in inhibitory and excitatory cortical neurons underlies the gain-of-function of excitatory but unaltered inhibitory synaptic transmission and the likely consequent dysregulation of the cortical excitatory–inhibitory balance in FHM1.

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Multipolar interneuron autaptic transmission is unaltered in FHM1 knockin mice.

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This is due to unaltered action potential (AP)-evoked presynaptic Ca influx.

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Unaltered AP-evoked Ca influx is not due to short duration of interneuron APs.

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Ca_V2.1 channels of multipolar interneurons are barely affected by the FHM1 mutation.

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This may explain unaltered inhibitory neurotransmission in FHM1.

Isoproterenol facilitates GABAergic autapses in fast-spiking cells of rat insular cortex

In the cerebral cortex, fast-spiking (FS) cells are the principal GABAergic interneurons and potently suppress neural activity in targeting neurons. Some FS neurons make synaptic contacts with themselves. Such synapses are called autapses and contribute to self-inhibition of FS neural activity. β-Adrenoceptors have a crucial role in regulating GABAergic synaptic inputs from FS cells to pyramidal (Pyr) cells; however, the β-adrenergic functions on FS autapses are unknown. To determine how the β-adrenoceptor agonist isoproterenol modulates inhibitory synaptic transmission in the autapses of FS cells, paired whole-cell patch-clamp recordings were obtained from FS and Pyr cells in layer V of rat insular cortex. Previous studies found that isoproterenol (100 μM) had pleiotropic effects on unitary inhibitory postsynaptic currents (uIPSCs) in FS→Pyr connections, whereas autapses in FS cells were always facilitated by isoproterenol. Facilitation of autapses by isoproterenol was accompanied by decreases in the paired-pulse ratio of second to first uIPSC amplitudes and the coefficient of variation of the uIPSC amplitude, which suggests that β-adrenergic facilitation is likely mediated by presynaptic mechanisms. The discrepancy between isoproterenol-induced modulation of uIPSCs in FS autapses and in FS→Pyr connections may reflect the presence of different presynaptic mechanisms of GABA release in each synapse.

Calcium channels and synaptic transmission in familial hemiplegic migraine type 1 animal models

One of the outstanding developments in clinical neurology has been the identification of ion channel mutations as the origin of a wide variety of inherited disorders like migraine, epilepsy, and ataxia. The study of several channelopathies has provided crucial insights into the molecular mechanisms, pathogenesis, and therapeutic approaches to complex neurological diseases. This review addresses the mutations underlying familial hemiplegic migraine (FHM) with particular interest in Cav2.1 (i.e., P/Q-type) voltage-activated Ca²⁺ channel FHM type-1 mutations (FHM1). Transgenic mice harboring the human pathogenic FHM1 mutation R192Q or S218L (KI) have been used as models to study neurotransmission at several central and peripheral synapses. FHM1 KI mice are a powerful tool to explore presynaptic regulation associated with expression of Cav2.1 channels. FHM1 Cav2.1 channels activate at more hyperpolarizing potentials and show an increased open probability. These biophysical alterations may lead to a gain-of-function on synaptic transmission depending upon factors such as action potential waveform and/or Cav2.1 splice variants and auxiliary subunits. Analysis of FHM knock-in mouse models has demonstrated a deficient regulation of the cortical excitation/inhibition (E/I) balance. The resulting excessive increases in cortical excitation may be the mechanisms that underlie abnormal sensory processing together with an increase in the susceptibility to cortical spreading depression (CSD). Increasing evidence from FHM KI animal studies support the idea that CSD, the underlying mechanism of aura, can activate trigeminal nociception, and thus trigger the headache mechanisms.

Monogenic models of absence epilepsy: windows into the complex balance between inhibition and excitation in thalamocortical microcircuits

Absence epilepsy is a common disorder that arises in childhood and can be refractory to medical treatment. Single genetic mutations in mice, at times found in patients with absence epilepsy, provide the unique opportunity to bridge the gap between dysfunction at the genetic level and pathological oscillations within the thalamocortical circuit. Interestingly, unlike other forms of epilepsy, only genes related to ion channels have so far been linked to absence phenotypes. Here, we delineate a paradigm which attempts to unify the various monogenic models based on decades of research. While reviewing the particular impact of these individual mutations, we posit a framework involving fast feedforward disinhibition as one common mechanism that can lead to increased tonic inhibition in the cortex and/or thalamus. Enhanced tonic inhibition hyperpolarizes principal cells, deinactivates T-type calcium channels, and leads to reciprocal burst firing within the thalamocortical loop. We also review data from pharmacologic and polygenic models in light of this paradigm. Ultimately, many questions remain unanswered regarding the pathogenesis of absence epilepsy.

Functional properties and short-term dynamics of unidirectional and reciprocal synaptic connections between layer 2/3 pyramidal cells and fast-spiking interneurons in juvenile rat prefrontal cortex

The interactions between inhibitory fast-spiking (FS) interneurons and excitatory pyramidal neurons contribute to the fundamental properties of cortical networks. An important role for FS interneurons in mediating rapid inhibition in local sensory and motor cortex microcircuits and processing thalamic inputs to the cortex has been shown in multiple reports; however, studies in the prefrontal cortex, a key neocortical region supporting working memory, are less numerous. In the present work, connections between layer 2/3 pyramidal cells and FS interneurons were studied with paired whole-cell recordings in acute neocortical slices of the medial prefrontal cortex from juvenile rats. The connection rate between FS interneurons and pyramidal neurons was about 40% in each direction with 16% of pairs connected reciprocally. Excitatory and inhibitory connections had a high efficacy and a low neurotransmission failure rate. Sustained presynaptic activity decreased the amplitude of responses and increased the failure rate more in excitatory connections than in inhibitory connections. In the reciprocal connections between the FS and pyramidal neurons, inhibitory and excitatory neurotransmission was more efficient and had a lower failure rate than in the unidirectional connections; the differences increased during the train stimulation. These results suggest the presence of distinct preferential subnetworks between FS interneurons and pyramidal cells in the rat prefrontal cortex that might be specific for this cortical area.

Stable respiratory activity requires both P/Q-type and N-type voltage-gated calcium channels

P/Q-type voltage-gated calcium channels (Ca(v)2.1) play critical presynaptic and postsynaptic roles throughout the nervous system and have been implicated in a variety of neurological disorders. Here we report that mice with a genetic ablation of the Ca(v)2.1 pore-forming α(1A) subunit (α(1A)⁻/⁻) encoded by CACNA1a (Jun et al., 1999) suffer during postnatal development from increasing breathing disturbances that lead ultimately to death. Breathing abnormalities include decreased minute ventilation and a specific loss of sighs, which was associated with lung atelectasis. Similar respiratory alterations were preserved in the isolated in vitro brainstem slice preparation containing the pre-Bötzinger complex. The loss of Ca(v)2.1 was associated with an alteration in the functional dependency on N-type calcium channels (Ca(v)2.2). Blocking N-type calcium channels with conotoxin GVIA had only minor effects on respiratory activity in slices from control (CT) littermates, but abolished respiratory activity in all slices from α(1A)⁻/⁻ mice. The amplitude of evoked EPSPs was smaller in inspiratory neurons from α(1A)⁻/⁻ mice compared with CTs. Conotoxin GVIA abolished all EPSPs in inspiratory neurons from α(1A)⁻/⁻ mice, while the EPSP amplitude was reduced by only 30% in CT mice. Moreover, neuromodulation was significantly altered as muscarine abolished respiratory network activity in α(1A)⁻/⁻ mice but not in CT mice. We conclude that excitatory synaptic transmission dependent on N-type and P/Q-type calcium channels is required for stable breathing and sighing. In the absence of P/Q-type calcium channels, breathing, sighing, and neuromodulation are severely compromised, leading to early mortality.

Calcium channels and short-term synaptic plasticity

Voltage-gated Ca(2+) channels in presynaptic nerve terminals initiate neurotransmitter release in response to depolarization by action potentials from the nerve axon. The strength of synaptic transmission is dependent on the third to fourth power of Ca(2+) entry, placing the Ca(2+) channels in a unique position for regulation of synaptic strength. Short-term synaptic plasticity regulates the strength of neurotransmission through facilitation and depression on the millisecond time scale and plays a key role in encoding information in the nervous system. Ca(V)2.1 channels are the major source of Ca(2+) entry for neurotransmission in the central nervous system. They are tightly regulated by Ca(2+), calmodulin, and related Ca(2+) sensor proteins, which cause facilitation and inactivation of channel activity. Emerging evidence reviewed here points to this mode of regulation of Ca(V)2.1 channels as a major contributor to short-term synaptic plasticity of neurotransmission and its diversity among synapses.

Calcium channels and migraine

Missense mutations in CACNA1A, the gene that encodes the pore-forming α1 subunit of human voltage-gated Ca(V)2.1 (P/Q-type) calcium channels, cause a rare form of migraine with aura (familial hemiplegic migraine type 1: FHM1). Migraine is a common disabling brain disorder whose key manifestations are recurrent attacks of unilateral headache that may be preceded by transient neurological aura symptoms. This review, first, briefly summarizes current understanding of the pathophysiological mechanisms that are believed to underlie migraine headache, migraine aura and the onset of a migraine attack, and briefly describes the localization and function of neuronal Ca(V)2.1 channels in the brain regions that have been implicated in migraine pathogenesis. Then, the review describes and discusses i) the functional consequences of FHM1 mutations on the biophysical properties of recombinant human Ca(V)2.1 channels and native Ca(V)2.1 channels in neurons of knockin mouse models carrying the mild R192Q or severe S218L mutations in the orthologous gene, and ii) the functional consequences of these mutations on neurophysiological processes in the cerebral cortex and trigeminovascular system thought to be involved in the pathophysiology of migraine, and the insights into migraine mechanisms obtained from the functional analysis of these processes in FHM1 knockin mice. This article is part of a Special Issue entitled: Calcium channels.

Parvalbumin-producing cortical interneurons receive inhibitory inputs on proximal portions and cortical excitatory inputs on distal dendrites

To examine inputs to parvalbumin (PV)-producing interneurons, we generated transgenic mice expressing somatodendritic membrane-targeted green fluorescent protein specifically in the interneurons, and completely visualized their dendrites and somata. Using immunolabeling for vesicular glutamate transporter (VGluT)1, VGluT2, and vesicular GABA transporter, we found that VGluT1-positive terminals made contacts 4- and 3.1-fold more frequently with PV-producing interneurons than VGluT2-positive and GABAergic terminals, respectively, in the primary somatosensory cortex. Even in layer 4, where VGluT2-positive terminals were most densely distributed, VGluT1-positive inputs to PV-producing interneurons were 2.4-fold more frequent than VGluT2-positive inputs. Furthermore, although GABAergic inputs to PV-producing interneurons were as numerous as VGluT2-positive inputs in most cortical layers, GABAergic inputs clearly preferred the proximal dendrites and somata of the interneurons, indicating that the sites of GABAergic inputs were more optimized than those of VGluT2-positive inputs. Simulation analysis with a PV-producing interneuron model compatible with the present morphological data revealed a plausible reason for this observation, by showing that GABAergic and glutamatergic postsynaptic potentials evoked by inputs to distal dendrites were attenuated to 60 and 87%, respectively, of those evoked by somatic inputs. As VGluT1-positive and VGluT2-positive axon terminals were presumed to be cortical and thalamic glutamatergic inputs, respectively, cortical excitatory inputs to PV-producing interneurons outnumbered the thalamic excitatory and intrinsic inhibitory inputs more than two-fold in any cortical layer. Although thalamic inputs are known to evoke about two-fold larger unitary excitatory postsynaptic potentials than cortical ones, the present results suggest that cortical inputs control PV-producing interneurons at least as strongly as thalamic inputs.

Short-term plasticity of unitary inhibitory-to-inhibitory synapses depends on the presynaptic interneuron subtype

Excitatory-to-inhibitory cortical synapses exhibit either short-term facilitation or depression, depending on the subtype identity of the postsynaptic interneuron, while the short-term plasticity (STP) of inhibitory-to-excitatory synapses depends on the presynaptic interneuron. However, the rules governing STP of inhibitory-to-inhibitory synapses have not yet been determined. We recorded 109 unitary connections made by the two major inhibitory interneuron subtypes in layer 4 of mouse somatosensory cortex, fast-spiking (FS) and somatostatin-containing (SOM) interneurons, on each other and on excitatory, regular-spiking (RS) neurons. In all pairs, we measured dynamic changes in the postsynaptic response to a 20 Hz train of presynaptic action potentials. In half of our dataset, we also measured kinetic properties of the unitary IPSC: latency, rise time, and decay time constant. We found a pronounced dependency of STP on the presynaptic, but not the postsynaptic, identity: FS interneurons made strongly depressing connections on FS, SOM, and RS targets, while in synapses made by SOM interneurons on FS and RS targets, weak early depression was followed by weak late facilitation. IPSC latency and rise time were also strongly dependent on the presynaptic interneuron subtype, being 1.5-2× slower in output synapses of SOM compared with FS interneurons. In contrast, the IPSC decay time constant depended only on the postsynaptic class, with 1.5× slower decay on excitatory compared with inhibitory targets. The properties of the inhibitory outputs of FS and SOM interneurons reciprocate the properties of their excitatory inputs and imply a dynamic spatiotemporal division of labor between these two major inhibitory subsystems.

Development of NMDA NR2 subunits and their roles in critical period maturation of neocortical GABAergic interneurons

The goals of this research are to (1) determine the changes in the composition of NMDA receptor (NMDAR) subunits in GABAergic interneurons during critical period (CP); and (2) test the effect of chronic blockage of specific NR2 subunits on the maturation of specific GABAergic interneurons. Our data demonstrate that: (1) The amplitude of NMDAR mediated EPSCs (EPSCs(NMDAR) ) was significantly larger in the postCP group. (2) The coefficient of variation (CV), τ(decay) and half-width of EPSCs(NMDAR) were significantly larger in the preCP group. (3) A leftward shift in the half-activation voltages in the postCP vs. preCP group. (4) Using subunit-specific antagonists, we found a postnatal shift in NR2 composition towards more NR2A mediated EPSCs(NMDAR) . These changes occurred within a two-day narrow window of CP and were similar between fast-spiking (FS) and regular spiking (RSNP) interneurons. (5) Chronic blockage of NR2A, but not NR2B, decreased the expression of parvalbumin (PV), but not other calcium binding proteins in layer 2/3 and 4 of barrel cortex. (6) Chronic blockage of NR2A selectively affected the maturation of IPSCs mediated by FS cells. In summary, we have reported, for the first time, developmental changes in the molecular composition of NMDA NR2 subunits in interneurons during CP, and the effects of chronic blockage of NR2A but not NR2B on PV expression and inhibitory synaptic transmission from FS cells. These results support an important role of NR2A subunits in developmental plasticity of fast-spiking GABAergic circuits during CP.

Interneuronal calcium channel abnormalities in posttraumatic epileptogenic neocortex

Decreased release probability (Pr) and increased failure rate for monosynaptic inhibitory postsynaptic currents (IPSCs) indicate abnormalities in presynaptic inhibitory terminals on pyramidal (Pyr) neurons of the undercut (UC) model of posttraumatic epileptogenesis. These indices of inhibition are normalized in high [Ca++] ACSF, suggesting dysfunction of Ca2+ channels in GABAergic terminals. We tested this hypothesis using selective blockers of P/Q and N-type Ca2+ channels whose activation underlies transmitter release in cortical inhibitory terminals. Pharmacologically isolated monosynaptic IPSCs were evoked in layer V Pyr cells by extracellular stimuli in adult rat sensorimotor cortical slices. Local perfusion of 0.2/1 μM ω-agatoxin IVa and/or 1 μM ω-conotoxin GVIA was used to block P/Q and N-type calcium channels, respectively. In control layer V Pyr cells, peak amplitude of eIPSCs was decreased by ~50% after treatment with either 1 μM ω-conotoxin GVIA or 1 μM ω-agatoxin IVa. In contrast, there was a lack of sensitivity to 1 μM ω-conotoxin GVIA in UCs. Immunocytochemical results confirmed decreased perisomatic density of N-channels on Pyr cells in UCs. We suggest that decreased calcium influx via N-type channels in presynaptic GABAergic terminals is a mechanism contributing to decreased inhibitory input onto layer V Pyr cells in this model of cortical posttraumatic epileptogenesis.

CaV2.1 voltage activated calcium channels and synaptic transmission in familial hemiplegic migraine pathogenesis

Studies on the genetic forms of epilepsy, chronic pain, and migraine caused by mutations in ion channels have given crucial insights into the molecular mechanisms, pathogenesis, and therapeutic approaches to complex neurological disorders. In this review we focus on the role of mutated CaV2.1 (i.e., P/Q-type) voltage-activated Ca2+ channels, and on the ultimate consequences that mutations causing familial hemiplegic migraine type-1 (FHM1) have in neurotransmitter release. Transgenic mice harboring the human pathogenic FHM1 mutation R192Q or S218L (KI) have been used as models to study neurotransmission at several central and peripheral synapses. FHM1 KI mice are a powerful tool to explore presynaptic regulation associated with expression of CaV2.1 channels. Mutated CaV2.1 channels activate at more hyperpolarizing potentials and lead to a gain-of-function in synaptic transmission. This gain-of-function might underlie alterations in the excitatory/ inhibitory balance of synaptic transmission, favoring a persistent state of hyperexcitability in cortical neurons that would increase the susceptibility for cortical spreading depression (CSD), a mechanism believed to initiate the attacks of migraine with aura.

A key mechanism underlying sensory experience-dependent maturation of neocortical GABAergic circuits in vivo

Mechanisms underlying experience-dependent refinement of cortical connections, especially GABAergic inhibitory circuits, are unknown. By using a line of mutant mice that lack activity-dependent BDNF expression (bdnf-KIV), we show that experience regulation of cortical GABAergic network is mediated by activity-driven BDNF expression. Levels of endogenous BDNF protein in the barrel cortex are strongly regulated by sensory inputs from whiskers. There is a severe alteration of excitation and inhibition balance in the barrel cortex of bdnf-KIV mice as a result of reduced inhibitory but not excitatory conductance. Within the inhibitory circuits, the mutant barrel cortex exhibits significantly reduced levels of GABA release only from the parvalbumin-expressing fast-spiking (FS) interneurons, but not other interneuron subtypes. Postnatal deprivation of sensory inputs markedly decreased perisomatic inhibition selectively from FS cells in wild-type but not bdnf-KIV mice. These results suggest that postnatal experience, through activity-driven BDNF expression, controls cortical development by regulating FS cell-mediated perisomatic inhibition in vivo.

CB1 modulation of temporally distinct synaptic facilitation among local circuit interneurons mediated by N-type calcium channels in CA1

One of the critical factors in determining network behavior of neurons is the influence of local circuit connections among interneurons. The short-term synaptic plasticity and the subtype of presynaptic calcium channels used at local circuit connections of inhibitory interneurons in CA1 were investigated using dual whole-cell recordings combined with biocytin and double immunofluorescence labeling in acute slices of P18- to 21-day-old rat stratum radiatum (SR) and stratum lacunosum molecular (SLM). Two forms of temporally distinct synaptic facilitation were observed among interneuron connections involving presynaptic cholecystokinin (CCK)-positive cells in SR, frequency-dependent facilitation, and a delayed onset of release (45-80 ms) with subsequent facilitation (DORF). Inhibition at both these synapses was under tonic cannabinoid-type 1 (CB1) receptor activity. DORF synapses did not display conventional release-dependent properties; however, blocking CB1 receptors with antagonist AM-251 (10 μM) altered the synaptic transmission to frequency-dependent depression with a fast onset of release (2-4 ms). Presynaptic CCK-negative interneurons in SLM elicited inhibitory postsynaptic potentials (IPSPs) insensitive to CB1 receptor pharmacology displayed frequency-dependent depression. Release of GABA at facilitating synapses was solely mediated via N-type presynaptic calcium channels, whereas depressing synapses utilized P/Q-type channels. These data reveal two distinct models of neurotransmitter release patterns among interneuron circuits that correlate with the subtype of presynaptic calcium channel. These data suggest that endocannabinoids act via CB1 receptors to selectively modulate N-type calcium channels to alter signal transmission.

Postsynaptic Cell Type–Dependent Cholinergic Regulation of GABAergic Synaptic Transmission in Rat Insular Cortex

The cerebral cortex consists of multiple neuron subtypes whose electrophysiological properties exhibit diverse modulation patterns in response to neurotransmitters, including noradrenaline and acetylcholine (ACh). We performed multiple whole cell patch-clamp recording from layer V GABAergic interneurons and pyramidal cells of rat insular cortex (IC) to examine whether cholinergic effects on unitary inhibitory postsynaptic currents (uIPSCs) are differentially regulated by ACh receptors, depending on their presynaptic and postsynaptic cell subtypes. In fast-spiking (FS) to pyramidal cell synapses, carbachol (10 μM) invariably decreased uIPSC amplitude by 51.0%, accompanied by increases in paired-pulse ratio (PPR) of the second to first uIPSC amplitude, coefficient of variation (CV) of the first uIPSC amplitude, and failure rate. Carbachol-induced uIPSC suppression was dose dependent and blocked by atropine, a muscarinic ACh receptor antagonist. Similar cholinergic suppression was observed in non-FS to pyramidal cell synapses. In contrast, FS to FS/non-FS cell synapses showed heterogeneous effects on uIPSC amplitude by carbachol. In roughly 40% of pairs, carbachol suppressed uIPSCs by 35.8%, whereas in a similar percentage of pairs uIPSCs were increased by 34.8%. Non-FS to FS/non-FS cell synapses also showed carbachol-induced uIPSC facilitation by 29.2% in about half of the pairs, whereas nearly 40% of pairs showed carbachol-induced suppression of uIPSCs by 40.3%. Carbachol tended to increase uIPSC amplitude in interneuron-to-interneuron synapses with higher PPR, suggesting that carbachol facilitates GABA release in interneuron synapses with lower release probability. These results suggest that carbachol-induced effects on uIPSCs are not homogeneous but preiotropic: i.e., cholinergic modulation of GABAergic synaptic transmission is differentially regulated depending on postsynaptic neuron subtypes.

Effects of T-type calcium channel blockers on cocaine-induced hyperlocomotion and thalamocortical GABAergic abnormalities in mice

RATIONALE

Repetitive cocaine exposure has been shown to induce GABAergic thalamic alterations. Given the key role of T-type (Ca(V)3) calcium channels in thalamocortical physiology, the direct involvement of these calcium channels in cocaine-mediated effects needs to be further explored.

OBJECTIVE

The objective of this study was to investigate the effect of T-type calcium channel blockers on acute and repetitive cocaine administration that mediates thalamocortical alterations in mice using three different T-type blockers: 2-octanol, nickel, and mibefradil.

METHODS

During in vitro experiments, whole-cell patch-clamp recordings were conducted in ventrobasal (VB) thalamic neurons from mice treated with acute repetitive cocaine administration (3 x 15 mg/kg, i.p., 1 h apart), under bath application of mibefradil (10 μM), 2-octanol (50 μM), or nickel (200 μM). After systemic administration of T-type calcium channel blockers, we evaluated locomotor activity and also recorded GABAergic neurotransmission onto VB neurons in vitro.

RESULTS

Bath-applied mibefradil, 2-octanol, or nickel significantly reduced both GABAergic neurotransmission and T-type currents of VB neurons in cocaine-treated mice. In vivo i.p. pre-administration of either mibefradil (20 mg/kg and 5 mg/kg) or 2-octanol (0.5 mg/kg and 0.07 mg/kg) significantly reduced GABAergic mini frequencies onto VB neurons. Moreover, both mibefradil and 2-octanol were able to decrease cocaine-induced hyperlocomotion.

CONCLUSION

The results shown in this study strongly suggest that T-type calcium channels play a key role in cocaine-mediated GABAergic thalamocortical alterations, and further propose T-type channel blockers as potential targets for future pharmacological strategies aimed at treating cocaine's deleterious effects on physiology and behavior.

Insights into migraine mechanisms and CaV2.1 calcium channel function from mouse models of familial hemiplegic migraine

Migraine is a very common disabling brain disorder with unclear pathogenesis. A subtype of migraine with aura (familial hemiplegic migraine type 1: FHM1) is caused by mutations in CaV2.1 (P/Q-type) Ca2+ channels. This review describes the functional consequences of FHM1 mutations in knockin mouse models carrying the mild R192Q or severe S218L mutations in the orthologous gene. The FHM1 knockin mice show allele dosage-dependent gain-of-function of neuronal P/Q-type Ca2+ current, reflecting activation of mutant channels at lower voltages, and allele dosage- and sex-dependent facilitation of induction and propagation of cortical spreading depression (CSD), the phenomenon that underlies migraine aura. Gain-of-function of neuronal Ca2+ current, facilitation of CSD and post-CSD motor deficits were larger in S218L than R192Q knockin mice, in correlation with the more severe human S218L phenotype. Enhanced cortical excitatory neurotransmission, due to increased action potential-evoked Ca2+ influx and increased probability of glutamate release at pyramidal cell synapses, but unaltered inhibitory neurotransmission at fast-spiking interneuron synapses, were demonstrated in R192Q knockin mice. Evidence for a causative link between enhanced glutamate release and CSD facilitation was obtained. The data from FHM1 mice strengthen the view of CSD as a key player in the pathogenesis of migraine, give insight into CSD mechanisms and point to episodic disruption of excitation-inhibition balance and neuronal hyperactivity as the basis for vulnerability to CSD ignition in migraine.

Presynaptic Interneuron Subtype- and Age-Dependent Modulation of GABAergic Synaptic Transmission by β-Adrenoceptors in Rat Insular Cortex

β-Adrenoceptors play a crucial role in the regulation of taste aversion learning in the insular cortex (IC). However, β-adrenergic effects on inhibitory synaptic transmission mediated by γ-aminobutyric acid (GABA) remain unknown. To elucidate the mechanisms of β-adrenergic modulation of inhibitory synaptic transmission, we performed paired whole cell patch-clamp recordings from layer V GABAergic interneurons and pyramidal cells of rat IC aged from postnatal day 17 (PD17) to PD46 and examined the effects of isoproterenol, a β-adrenoceptor agonist, on unitary inhibitory postsynaptic currents (uIPSCs). Isoproterenol (100 μM) induced facilitating effects on uIPSCs in 33.3% of cell pairs accompanied by decreases in coefficient of variation (CV) of the first uIPSC amplitude and paired-pulse ratio (PPR) of the second to first uIPSC amplitude, whereas 35.9% of pairs showed suppressive effects of isoproterenol on uIPSC amplitude obtained from fast spiking (FS) to pyramidal cell pairs. Facilitatory effects of isoproterenol were frequently observed in FS–pyramidal cell pairs at ≥PD24. On the other hand, isoproterenol suppressed uIPSC amplitude by 52.3 and 39.8% in low-threshold spike (LTS)–pyramidal and late spiking (LS)–pyramidal cell pairs, respectively, with increases in CV and PPR. The isoproterenol-induced suppressive effects were blocked by preapplication of 100 μM propranolol, a β-adrenoceptor antagonist. There was no significant correlation between age and changes of uIPSCs in LTS–/LS–pyramidal cell pairs. These results suggest the presence of differential mechanisms in presynaptic GABA release and/or postsynaptic GABAA receptor-related assemblies among interneuron subtypes. Age- and interneuron subtype-specific β-adrenergic modulation of IPSCs may contribute to experience-dependent plasticity in the IC.

CaV2.1 channelopathies

Mutations in the CACNA1A gene that encodes the pore-forming alpha1 subunit of human voltage-gated CaV2.1 (P/Q-type) Ca2+ channels cause several autosomal-dominant neurologic disorders, including familial hemiplegic migraine type 1 (FHM1), episodic ataxia type 2, and spinocerebellar ataxia type 6 (SCA6). For each channelopathy, the review describes the disease phenotype as well as the functional consequences of the disease-causing mutations on recombinant human CaV2.1 channels and, in the case of FHM1 and SCA6, on neuronal CaV2.1 channels expressed at the endogenous physiological level in knockin mouse models. The effects of FHM1 mutations on cortical spreading depression, the phenomenon underlying migraine aura, and on cortical excitatory and inhibitory synaptic transmission in FHM1 knockin mice are also described, and their implications for the disease mechanism discussed. Moreover, the review describes different ataxic spontaneous cacna1a mouse mutants and the important insights into the cerebellar mechanisms underlying motor dysfunction caused by mutant CaV2.1 channels that were obtained from their functional characterization.

Properties of glutamatergic synapses in immature layer Vb pyramidal neurons: coupling of pre- and postsynaptic maturational states

Following initial contact formation, glutamatergic synapses in cortical neurons undergo pronounced functional maturation. These maturational events, occurring both pre- and postsynaptically, have been well described in the developing hippocampus. In this paper, we characterized glutamatergic synapses in immature layer Vb pyramidal neurons of the mouse somatosensory cortex during early postnatal development. At postnatal day 7, a significant subpopulation of glutamatergic synapses exhibited a low release probability that was accompanied by strong paired-pulse facilitation of AMPA EPSCs (paired-pulse ratio C > or = 2). Increasing extracellular Ca(2+) concentration increased release probability and led to paired-pulse depression. During further postnatal development, these functionally immature synapses disappeared. As shown pharmacologically,these synapses expressed postsynaptic NMDA receptors containing NR2B subunits, while NMDA receptors with NR2A subunits were lacking. Taken together, a low release probability presynaptically was coupled to postsynaptic NR2B signaling. This subpopulation of neocortical synapses thus differed from the majority of synapses in the developing hippocampus, where high release probability is coupled to NR2B signaling. The novel type of functionally immature glutamatergic synapse described here might play an important role in early developmental synapse elimination and in the activity-dependent refinement of the neocortical synaptic microcircuitry.

Developmental alterations in the functional properties of excitatory neocortical synapses

In the neocortex, most excitatory, glutamatergic synapses are established during the first 4-5 weeks after birth. During this period profound changes in the properties of synaptic transmission occur. Excitatory postsynaptic potentials (EPSPs) at immature synaptic connections are profoundly and progressively reduced in response to moderate to high frequency (5-100 Hz) stimulation. With maturation, this frequency-dependent depression becomes progressively weaker and may eventually transform into a weak to moderate EPSP facilitation. In parallel to changes in the short-term plasticity, a reduction in the synaptic reliability occurs at most glutamatergic neocortical synapses: immature synapses show a high probability of neurotransmitter release as indicated by their low failure rate and small EPSP amplitude variation. This high reliability is reduced in mature synapses, which show considerably higher failure rates and more variable EPSP amplitudes. During early neocortical development synaptic vesicle pools are not yet fully differentiated and their replenishment may be slow, thus resulting in EPSP amplitude depression. The decrease in the probability of neurotransmitter release may be the result of an altered Ca(2+) control in the presynaptic terminal with a reduced Ca(2+) influx and/or a higher Ca(2+) buffering capacity. This may lead to a lower synaptic reliability and a weaker short-term synaptic depression with maturation.

Conditional bursting enhances resonant firing in neocortical layer 2-3 pyramidal neurons

The frequency response properties of neurons are critical for signal transmission and control of network oscillations. At subthreshold membrane potential, some neurons show resonance caused by voltage-gated channels. During action potential firing, resonance of the spike output may arise from subthreshold mechanisms and/or spike-dependent currents that cause afterhyperpolarizations (AHPs) and afterdepolarizations (ADPs). Layer 2-3 pyramidal neurons (L2-3 PNs) have a fast ADP that can trigger bursts. The present study investigated what stimuli elicit bursting in these cells and whether bursts transmit specific frequency components of the synaptic input, leading to resonance at particular frequencies. We found that two-spike bursts are triggered by step onsets, sine waves in two frequency bands, and noise. Using noise adjusted to elicit firing at approximately 10 Hz, we measured the gain for modulation of the time-varying firing rate as a function of stimulus frequency, finding a primary peak (7-16 Hz) and a high-frequency resonance (250-450 Hz). Gain was also measured separately for single and burst spikes. For a given spike rate, bursts provided higher gain at the primary peak and lower gain at intermediate frequencies, sharpening the high-frequency resonance. Suppression of bursting using automated current feedback weakened the primary and high-frequency resonances. The primary resonance was also influenced by the SK channel-mediated medium AHP (mAHP), because the SK blocker apamin reduced the sharpness of the primary peak. Our results suggest that resonance in L2-3 PNs depends on burst firing and the mAHP. Bursting enhances resonance in two distinct frequency bands.

Maturation of intrinsic and synaptic properties of layer 2/3 pyramidal neurons in mouse auditory cortex

We investigated the development of L2/3 pyramidal cell (PC) circuitry in juvenile mice from postnatal day 10 (P10) to P29. Using whole cell recordings in an in vitro thalamocortical slice preparation, we examined the connection architecture and intrinsic and synaptic properties of PCs. The excitatory connections between PCs were highly localized: the probability of connection between PCs declined with intersomatic distance from 0.18 to about 0.05 over 150 microm, but did not vary with age. However, the mean and variance of the intrinsic and synaptic properties of PCs changed dramatically between P10 and P29. The input resistance, membrane time constant, and resting membrane potential decreased, leading to reduced neural excitability in older animals. Likewise, there were age-dependent decreases in the amplitude and decay time of the excitatory postsynaptic potentials as well as short-term synaptic depression. Both the intrinsic and synaptic properties underwent a transitional period between P10 and P18 prior to reaching steady state at P19-P29. We show that these properties combine to produce age-related differential synaptic responses to low- and high-frequency synaptic input that may contribute to differences in auditory processing during development.

Cholinergic modulation of local pyramid-interneuron synapses exhibiting divergent short-term dynamics in rat sensory cortex

Acetylcholine (ACh) influences attention, short-term memory, and sleep/waking transitions, through its modulatory influence on cortical neurons. It has been proposed that behavioral state changes mediated by ACh result from its selective effects on the intrinsic membrane properties of diverse cortical inhibitory interneuron classes. ACh has been widely shown to reduce the strength of excitatory (glutamatergic) synapses. But past studies using extracellular stimulation have not been able to examine the effects of ACh on local cortical connections important for shaping sensory processing. Here, using dual intracellular recording in slices of rat somatosensory cortex, we show that reduction of local excitatory input to inhibitory neurons by ACh is coupled to differences in the underlying short-term synaptic plasticity (STP). In synapses with short-term depression, where successive evoked excitatory postsynaptic potentials (EPSPs; >5 Hz) usually diminish in strength (short-term depression), cholinergic agonist (5-10 microM carbachol (CCh)) reduced the amplitude of the first EPSP in an evoked train, but CCh's net effect on subsequent EPSPs rapidly diminished. In synapses where successive EPSPs increased in strength (facilitation), the effect of CCh on later EPSPs in an evoked train became progressively greater. The effect of CCh on both depressing and facilitating synapses was blocked by the muscarinic antagonist, 1-5 microM atropine. It is suggested that selective influence on STP contributes fundamentally to cholinergic "switching" between cortical rhythms that underlie different behavioral states.

Gamma-band deficiency and abnormal thalamocortical activity in P/Q-type channel mutant mice

Thalamocortical in vivo and in vitro function was studied in mice lacking P/Q-type calcium channels (Cav2.1), in which N-type calcium channels (Cav2.2) supported central synaptic transmission. Unexpectedly, in vitro patch recordings from thalamic neurons demonstrated no gamma-band subthreshold oscillation, and voltage-sensitive dye imaging demonstrated an absence of cortical gamma-band-dependent columnar activation involving cortical inhibitory interneuron activity. In vivo electroencephalogram recordings showed persistent absence status and a dramatic reduction of gamma-band activity. Pharmacological block of T-type calcium channels (Cav3), although not noticeably affecting normal control animals, left the knockout mice in a coma-like state. Hence, although N-type calcium channels can rescue P/Q-dependent synaptic transmission, P/Q calcium channels are essential in the generation of gamma-band activity and resultant cognitive function.

Presynaptic Inhibition of GABAA Receptor-Mediated Unitary IPSPs by Cannabinoid Receptors at Synapses Between CCK-Positive Interneurons in Rat Hippocampus

There is growing evidence to link cholecystokinin (CCK)-positive interneurons and anxiety disorders. Despite this, little is known about the physiology and pharmacology of synaptic interactions between CCK-positive interneurons. This study aims to investigate the local circuit connections among CCK-positive Schaffer collateral associated (SCA) interneurons in stratum radiatum (SR) and their modulatory interactions using paired whole cell recordings combined with biocytin and double immunofluorescence labeling in slices of rat hippocampus. The cell bodies of SCA interneurons were located in SR, and their sparsely spiny dendrites projected toward s. pyramidale (SP) and along SR. Their axons innervated SR, SP, and s. oriens (SO) with predominant ramification in SR. These cells were immunopositive for CCK and immunonegative for parvalbumin (PV). SCA interneurons often displayed an accommodating firing pattern with or without a “sag” in response to hyperpolarizing current injection. Pairs of these cells exhibited electrical coupling and reciprocal chemical connections in which inhibitory postsynaptic potentials (IPSPs) displayed powerful frequency-dependent facilitation and augmentation. The synaptic connections were modulated by the endogenous cannabinoid receptor (CB) agonist, anandamide and by depolarization-induced suppression of inhibition (DSI), both of which reduced the amplitude of unitary IPSPs to 50% of control and increased the number of apparent failures of transmission. These effects were blocked by the CB1 receptor antagonist, AM-251. I suggest that synaptic facilitation between CCK-positive SCA interneurons may modify the onset of CB1 receptor-mediated regulation of inhibition, thereby affecting spike timing, and that this process could influence the expression of anxiety.

P/Q-Type, But Not N-Type, Calcium Channels Mediate GABA Release From Fast-Spiking Interneurons to Pyramidal Cells in Rat Prefrontal Cortex

The Cav2.1 (P/Q-) and Cav2.2 (N-type) voltage-gated calcium channels (VGCCs) play a predominant role in neurotransmitter release at central synapses, but their distribution is not uniform across different types of synapses. Although the functional significance of the differential distribution of N- and P/Q-type VGCCs is poorly understood, distinct types of VGCCs appear to differentially affect synaptic properties. For example, P/Q-type VGCCs are located closer to release sites and are less affected by G-protein-mediated inhibition than are N-type VGCCs. Thus P/Q-type VGCCs might be beneficial at synapses with high probability of release and precise timing of neurotransmission, such as the inhibitory inputs from parvalbumin-containing fast-spiking (FS) interneurons to pyramidal cells (PCs) in the neocortex. To determine whether VGCCs types predominate at synapses from FS interneurons to PCs in rat prefrontal cortex, whole cell paired recordings ( n = 14) combined with intracellular labeling and fluorescence immunohistochemistry for parvalbumin were performed in acute slices. Bath application of the specific N-type VGCC blocker ω-conotoxin-GVIa (1 μM) did not alter inhibitory postsynaptic potential amplitude, failure rate, or synaptic dynamics; in contrast, application of P/Q-type VGCC blocker ω-agatoxin-IVa (0.5 μM) completely and irreversibly blocked neurotransmission. These results indicate that P/Q-type VGCCs mediate the GABA release from parvalbumin-positive FS interneurons to PCs in the rat neocortex.

Reliable long-lasting depression interacts with variable short-term facilitation to determine corticostriatal paired-pulse plasticity in young rats

Synaptic plasticity at corticostraital synapses is proposed to fine tune movment and improve motor skills. We found paired-pulse plasticity at corticostriatal synapses reflected variably expressed short-term facilitation blended with a consistent background of longer-lasting depression. Presynaptic modulation via neuotransmitter receptor activation was ruled out as a mechanism for long-lasting paired-pulse depression by examining the effect of selective receptor antagonists. EPSC amplitude and paired-pulse plasticity, however, was influenced by block of D2 dopamine receptors. Block of glutamate transport with l-transdicarboxylic acid (PDC) reduced EPSCs, possibly through a mechanism of AMPA receptor desensitization. Removal of AMPA receptor desensitization with cyclothiazide reduced the paired-pulse depression at long-duration interstimulus intervals (ISIs), indicating that AMPA receptor desensitization participates in corticostriatal paired-pulse plasticity. The low-affinity glutamate receptor antagonist cis-2,3-piperidine dicarboxylic acid (PDA) increased paired-pulse depression, suggesting that a presynaptic component also exists for long-lasting paired-pulse depression. Low Ca(2+)-high Mg(2+) or BAPTA-AM dramatically reduced the amplitude of corticostriatal EPSCs and both manipulations increased the expression of facilitation and, to a lesser extent, they reduced long-lasting paired-pulse depression. EGTA-AM produced a smaller reduction in EPSC amplitude and it did not alter paired-pulse facilitation, but in contrast to low Ca(2+) and BAPTA-AM, EGTA-AM increased long-lasting paired-pulse depression. These experiments suggest that facilitation and depression are sensitive to vesicle depletion, which is dependent upon changes in peak Ca(2+) (i.e. low Ca(2+)-high Mg(2+) or BAPTA-AM). In addition, the action of EGTA-AM suggests that basal Ca(2+) regulates the recovery from long-lasting paired-pulse depression, possibly thourgh a Ca(2+)-sensitive process of vesicle delivery.

Robust correlations between action potential duration and the properties of synaptic connections in layer 4 interneurones in neocortical slices from juvenile rats and adult rat and cat

Many studies of cortical interneurones use immature rodent tissue, while many recordings in vivo are made in adult cats. To determine the extent to which interneuronal circuitry studied with one approach can transfer to another, we compared layer 4 interneurones and their local connections across two age groups and two species and with similar connections in layers 3 and 5, using two common recording techniques: dual whole cell recordings at 20 degrees C and dual sharp electrode recordings at 35 degrees C. In each group, a range of morphological and electrophysiological characteristics was observed. In all groups, however, positive correlations were found between the width of the action potential and rise times and widths at half-amplitude of EPSPs and IPSPs and the EPSP paired pulse ratio. Multipolar interneurones with narrow spikes generated the fastest IPSPs in pyramidal cells and received the briefest, most strongly depressing EPSPs, while bitufted interneurones with broader spikes and adapting and burst firing patterns activated the broadest IPSPs and received the slowest, most strongly facilitating/augmenting EPSPs. Correlations were similar in all groups, with no significant differences between adult rat and cat, or between layers, but events were four times slower in juveniles at 20 degrees C. Comparisons with previous studies indicate that this is due in part to age, but in large part to temperature. Studies in adults were extended with detailed analysis of synaptic dynamics, which appeared to decay more rapidly than at juvenile connections. EPSPs exhibited the complexity in time course of facilitation, augmentation and depression previously described in other adult neocortical connections. That is, the time course of recovery from facilitation or depression rarely followed a simple smooth exponential decay. Facilitation and depression were not always maximal at the shortest interspike intervals, and recovery was often interrupted by peaks and troughs in mean EPSP amplitude with a periodicity around 80 Hz.

Short-term synaptic plasticity in layer II/III of the rat anterior cingulate cortex

Recent in vivo electrophysiological studies in our laboratory demonstrated medial thalamus (MT) induced short-term facilitation in the middle layers of the anterior cingulate cortex (ACC). The aim of the present study was to investigate different forms of short-term plasticity (STP) in layer II/III of the ACC in an in vitro slice preparation. Extracellular field potentials in layer II/III consisting of an early component (fAP) and a late component (fPSP) were activated by electrical stimulation of the deep layers. The fPSP and intracellularly recorded excitatory post-synaptic potential (EPSP) could be facilitated by paired-pulse stimulation at a low frequency (0.033Hz, pulse interval 20-400ms). An initial facilitation and subsequent depression were obtained when high frequency (12.5, 25 and 50Hz) tetanus stimulations were applied to the ACC slice. A post-tetanic augmentation 30s in duration was also observed. The effects of tetanic stimulation were altered in the presence of an increased or a decreased calcium concentration. Application of omega-conotoxin GVIA (CTX) in normal calcium concentration conditions decreased overall responses during tetanic stimulation similar to reducing calcium exposure. However CTX application did not increase paired-pulse facilitation (PPF) as is seen under low calcium conditions. These results indicate that calcium is involved in the formation of certain features of STP in layer II/III of the ACC and that N-type calcium channels contribute to some, but not all, components of these plastic changes. Two-site electrical stimulation testing showed that two separate presynaptic inputs can produce short-term facilitation. Our findings implicate a post-synaptic mechanism in STP in layer II/III of the ACC.

GABAergic synaptic inhibition is reduced before seizure onset in a genetic model of cortical malformation

Malformations of the neocortex are a common cause of human epilepsy; however, the critical issue of how disturbances in cortical organization render neurons epileptogenic remains controversial. The present study addressed this issue by studying inhibitory structure and function before seizure onset in the telencephalic internal structural heterotopia (tish) rat, which is a genetic model of heightened seizure susceptibility associated with a prominent neocortical malformation. Both normally positioned (normotopic) and misplaced (heterotopic) pyramidal neurons in the tish neocortex exhibited lower resting membrane potentials and a tendency toward higher input resistance compared with pyramidal neurons from control brains. GABAergic synaptic transmission was attenuated in the tish cortex, characterized by significant reductions in the frequency of spontaneous IPSCs (sIPSCs) and miniature IPSCs recorded from pyramidal neurons. In addition, the amplitudes of sIPSCs were reduced in the tish neocortex, an effect that was more profound in the normotopic cells. Immunohistochemical assessment of presynaptic GABAergic terminals showed a reduction in terminals surrounding pyramidal cell somata in normotopic and heterotopic tish neocortex. The attenuation of inhibitory innervation was more prominent for normotopic neurons and was associated with a reduction in a subset of GABAergic interneurons expressing the calcium-binding protein parvalbumin. Together, these findings indicate that key facets of inhibitory GABAergic neurotransmission are disturbed before seizure onset in a brain predisposed to developing seizures. Such alterations represent a rational substrate for reduced seizure thresholds associated with certain cortical malformations.