Serine/threonine protein phosphatases and synaptic inhibition regulate the expression of cholinergic-dependent plateau potentials

Switching between persistent firing and depolarization block in individual rat CA1 pyramidal neurons

The hippocampal formation plays a role in mnemonic tasks and epileptic discharges in vivo. In vitro, these functions and malfunctions may relate to persistent firing (PF) and depolarization block (DB), respectively. Pyramidal neurons of the CA1 field have previously been reported to engage in either PF or DB during cholinergic stimulation. However, it is unknown whether these cells constitute disparate populations of neurons. Furthermore, it is unclear which cell-specific peculiarities may mediate their diverse response properties. However, it has not been shown whether individual CA1 pyramidal neurons can switch between PF and DB states. Here, we used whole cell patch clamp in the current clamp mode on in vitro CA1 pyramidal neurons from acutely sliced rat tissue to test various intrinsic properties which may provoke individual cells to switch between PF and DB. We found that individual cells could switch from PF to DB, in a cholinergic agonist concentration dependent manner and depending on the parameters of stimulation. We also demonstrate involvement of TRPC and potassium channels in this switching. Finally, we report that the probability for DB was more pronounced in the proximal than in the distal half of CA1. These findings offer a potential mechanism for the stronger spatial modulation in proximal, compared to distal CA1, as place field formation was shown to be affected by DB. Taken together, our results suggest that PF and DB are not mutually exclusive response properties of individual neurons. Rather, a cell's response mode depends on a variety of intrinsic properties, and modulation of these properties enables switching between PF and DB.

The CAN-In network: A biologically inspired model for self-sustained theta oscillations and memory maintenance in the hippocampus

During working memory tasks, the hippocampus exhibits synchronous theta-band activity, which is thought to be correlated with the short-term memory maintenance of salient stimuli. Recent studies indicate that the hippocampus contains the necessary circuitry allowing it to generate and sustain theta oscillations without the need of extrinsic drive. However, the cellular and network mechanisms supporting synchronous rhythmic activity are far from being fully understood. Based on electrophysiological recordings from hippocampal pyramidal CA1 cells, we present a possible mechanism for the maintenance of such rhythmic theta-band activity in the isolated hippocampus. Our model network, based on the Hodgkin-Huxley formalism, comprising pyramidal neurons equipped with calcium-activated nonspecific cationic (CAN) ion channels, is able to generate and sustain synchronized theta oscillations (4-12 Hz), following a transient stimulation. The synchronous network activity is maintained by an intrinsic CAN current (I_CAN ), in the absence of constant external input. When connecting the pyramidal-CAN network to fast-spiking inhibitory interneurons, the dynamics of the model reveal that feedback inhibition improves the robustness of fast theta oscillations, by tightening the synchronization of the pyramidal CAN neurons. The frequency and power of the theta oscillations are both modulated by the intensity of the I_CAN , which allows for a wide range of oscillation rates within the theta band. This biologically plausible mechanism for the maintenance of synchronous theta oscillations in the hippocampus aims at extending the traditional models of septum-driven hippocampal rhythmic activity. © 2017 Wiley Periodicals, Inc.

Review: Cav2.3 R-type Voltage-Gated Ca2+ Channels - Functional Implications in Convulsive and Non-convulsive Seizure Activity

Background:

Researchers have gained substantial insight into mechanisms of synaptic transmission, hyperexcitability, excitotoxicity and neurodegeneration within the last decades. Voltage-gated Ca²⁺ channels are of central relevance in these processes. In particular, they are key elements in the etiopathogenesis of numerous seizure types and epilepsies. Earlier studies predominantly targeted on Ca_v2.1 P/Q-type and Ca_v3.2 T-type Ca²⁺ channels relevant for absence epileptogenesis. Recent findings bring other channels entities more into focus such as the Ca_v2.3 R-type Ca²⁺ channel which exhibits an intriguing role in ictogenesis and seizure propagation. Ca_v2.3 R-type voltage gated Ca²⁺ channels (VGCC) emerged to be important factors in the pathogenesis of absence epilepsy, human juvenile myoclonic epilepsy (JME), and cellular epileptiform activity, e.g. in CA1 neurons. They also serve as potential target for various antiepileptic drugs, such as lamotrigine and topiramate.

Objective:

This review provides a summary of structure, function and pharmacology of VGCCs and their fundamental role in cellular Ca²⁺ homeostasis. We elaborate the unique modulatory properties of Ca_v2.3 R-type Ca²⁺ channels and point to recent findings in the proictogenic and proneuroapoptotic role of Ca_v2.3 R-type VGCCs in generalized convulsive tonic–clonic and complex-partial hippocampal seizures and its role in non-convulsive absence like seizure activity.

Conclusion:

Development of novel Ca_v2.3 specific modulators can be effective in the pharmacological treatment of epilepsies and other neurological disorders.

Long-lasting small-amplitude TRP-mediated dendritic depolarizations in CA1 pyramidal neurons are intrinsically stable and originate from distal tuft regions

In several regions of the nervous system, neurons display bi- or multistable intrinsic properties. Such stable states may be subthreshold and long-lasting, and can appear as a sustained afterdepolarization. In hippocampal CA1 pyramidal neurons, small-amplitude (1 mV) long-lasting (seconds) afterdepolarizations have been reported and are thought to depend on calcium-activated nonselective (CAN) currents recently identified as transient receptor potential (TRP) channels. Continuing our previous experimental and computational work on synaptically metabotropic glutamate receptor (mGluR)-activated TRP currents, we here explore small-amplitude long-lasting depolarizations in a detailed multicompartmental model of a CA1 pyramidal neuron. We confirm a previous hypothesis suggesting that the depolarization results from an interplay of TRP and voltage-gated calcium channels, and contribute to the understanding of the depolarization in several ways. Specifically, we show that: (i) the long-lasting depolarization may be intrinsically stable to weak excitatory and inhibitory input, (ii) the phenomenon is essentially located in distal apical dendrites, (iii) induction is facilitated if simultaneous input arrives at several dendritic branches, and if calcium- and/or mGluR-evoked signals undergo summation, suggesting that both spatial and temporal synaptic summation might be required for the depolarization to occur and (iv) we also show that the integration of inputs to oblique dendrites is strongly modulated by the presence of small-amplitude long-lasting depolarizations in distal tuft dendrites. To conclude, we suggest that small-amplitude long-lasting dendritic depolarizations may contribute to sustaining neural information during behavioural tasks in cases where information is separated in time, as in trace conditioning and delay tasks.

Regulation of cation channel voltage and Ca2+ dependence by multiple modulators

Ion channel regulation is key to controlling neuronal excitability. However, the extent that modulators and gating factors interact to regulate channels is less clear. For Aplysia, a nonselective cation channel plays an essential role in reproduction by driving an afterdischarge in the bag cell neurons to elicit egg-laying hormone secretion. We examined the regulation of cation channel voltage and Ca2+ dependence by protein kinase C (PKC) and inositol trisphosphate (IP3)-two prominent afterdischarge signals. In excised, inside-out patches, the channel remained open longer and reopened more often with depolarization from -90 to +30 mV. As previously reported, PKC could closely associate with the channel and increase activity at -60 mV. We now show that, following the effects of PKC, voltage dependence was shifted to the left (essentially enhanced), particularly at more negative voltages. Conversely, the voltage dependence of channels lacking PKC was shifted to the right (essentially suppressed). Predictably, activity was increased at all Ca2+ concentrations following the effects of PKC; nevertheless, Ca2+ dependence was actually shifted to the right. Moreover, whereas IP3 did not alter activity at -60 mV, it drastically shifted Ca2+ dependence to the right-an outcome largely reversed by PKC. With respect to the afterdischarge, these data suggest PKC initially upregulates the channel by direct gating and shifting voltage dependence to the left. Subsequently, PKC and IP3 attenuate the channel by suppressing Ca2+ dependence. This ensures hormone delivery by allowing afterdischarge initiation and maintenance but also prevents interminable bursting. Similar regulatory interactions may be used by other neurons to achieve diverse outputs.

Topiramate Inhibits the Initiation of Plateau Potentials in CA1 Neurons by Depressing R-type Calcium Channels

PURPOSE

Cholinergic-dependent plateau potentials (PPs) are intrinsically generated conductances that can elicit ictal-type seizure activity. The aim of this study was to investigate the actions of topiramate (TPM) on the generation of PPs.

METHODS

We used whole-cell patch-clamp recordings from CA1 pyramidal neurons in rat hippocampal slices to examine the effects of TPM on the PPs.

RESULTS

In current-clamp mode, action potentials evoked PPs after cholinergic receptor stimulation. Therapeutically relevant concentrations of TPM (50 microM) depressed the PPs evoked by action potentials. Surprisingly, in voltage-clamp mode, we discovered that the cyclic nucleotide-gated (CNG) current that underlies PP generation (denoted as I(tail)) was not depressed. However, significantly longer depolarizing voltage steps were required to elicit I(tail). This suggested that the calcium entry trigger for evoking PPs was depressed by TPM and not I(tail) itself. TPM had no effect on calcium spikes in control conditions; however, TPM did reduce calcium spikes after cholinergic-receptor stimulation. We recently found that R-type calcium spikes are enhanced by cholinergic-receptor stimulation. Therefore we isolated R-type calcium spikes with a cocktail containing tetrodotoxin, omega-conotoxin MVIIC, omega-conotoxin-GVIA, omega-agatoxin IVA, and nifedipine. R-type calcium spikes were significantly depressed by TPM. We also examined the effects of TPM on recombinant Ca(V)2.3 calcium channels expressed in tsA-201 cells. TPM depressed currents mediated by Ca(V)2.3 subunits by a hyperpolarizing shift in steady-state inactivation.

CONCLUSIONS

We have found that TPM reduces ictal-like activity in CA1 hippocampal neurons through a novel inhibitory action of R-type calcium channels.

Further characterization of muscarinic agonist-induced epileptiform bursting activity in immature rat piriform cortex, in vitro

The characteristics of muscarinic acetylcholine receptor agonist-induced epileptiform bursting seen in immature rat piriform cortex slices in vitro were further investigated using intracellular recording, with particular focus on its postnatal age-dependence (P+14-P+30), pharmacology, site(s) of origin and the likely contribution of the muscarinic acetylcholine receptor agonist-induced post-stimulus slow afterdepolarization and gap junction functionality toward its generation. The muscarinic agonist, oxotremorine-M (10 microM), induced rhythmic bursting only in immature piriform cortex slices; however, paroxysmal depolarizing shift amplitude, burst duration and burst incidence were inversely related to postnatal age. No significant age-dependent changes in neuronal membrane properties or postsynaptic muscarinic responsiveness accounted for this decline. Burst incidence was higher when recorded in anterior and posterior regions of the immature piriform cortex. In adult and immature neurones, oxotremorine-M effects were abolished by M1-, but not M2-muscarinic acetylcholine receptor-selective antagonists. Rostrocaudal lesions, between piriform cortex layers I and II, or layer III and endopiriform nucleus in adult or immature slices did not influence oxotremorine-M effects; however, the slow afterdepolarization in adult (but not immature) lesioned slices was abolished. Gap junction blockers (carbenoxolone or octanol) disrupted muscarinic bursting and diminished the slow afterdepolarization in immature slices, suggesting that gap junction connectivity was important for bursting. Our data show that neural networks within layers II-III function as primary oscillatory circuits for burst initiation in immature rat piriform cortex during persistent muscarinic receptor activation. Furthermore, we propose that muscarinic slow afterdepolarization induction and gap junction communication could contribute towards the increased epileptiform susceptibility of this brain area.

Systemic adrenocorticotropic hormone administration down-regulates the expression of corticotropin-releasing hormone (CRH) and CRH-binding protein in infant rat hippocampus

Systemic adrenocorticotropic hormone (ACTH) administration is a first-line therapy for the treatment of infantile spasms, an age-specific seizure disorder of infancy. It is proposed that exogenous ACTH acts via negative feedback to suppress the synthesis of corticotropin-releasing hormone (CRH), a possible endogenous convulsant in infant brain tissue. The aim of this study was to determine whether systemic ACTH treatment in infant rats down-regulates the hippocampal CRH system, including CRH, CRH-binding protein (CRH-BP), and CRH receptors (CRH-R1 and CRH-R2). Daily i.p. injection of ACTH for 7 consecutive days (postnatal days 3-9) elevated serum corticosterone levels 20-fold measured on postnatal day 10, indicating systemic absorption and circulation of the ACTH. Semiquantitative reverse transcriptase-PCR demonstrated that both CRH and CRH-BP mRNA obtained from the hippocampi of ACTH-injected infant rats was significantly depressed relative to saline-injected animals. Comparable reductions in both CRH and CRH-BP synthesis were further demonstrated with radioimmunoassay. In contrast, neither CRH-R1 nor CRH-R2 mRNA was altered by ACTH treatment, relative to saline-injected rats. This latter finding was confirmed electrophysiologically by measuring the enhancement of hippocampal population spikes by exogenous CRH, also showing no differences between ACTH- and saline-injected rats. The results of this study support the proposal that systemic ACTH treatment down-regulates CRH expression in infant brain, perhaps contributing to the therapeutic efficacy observed during treatment of infantile spasms.

Cyclic nucleotide-gated channels contribute to the cholinergic plateau potential in hippocampal CA1 pyramidal neurons

Plateau potentials are prolonged membrane depolarizations that are observed in hippocampal pyramidal neurons when spiking and Ca(2+) entry occur in combination with muscarinic receptor activation. In this study, we used whole-cell voltage clamping to study the current underlying the plateau potential and to determine the cellular signaling pathways contributing to this current. When combined with muscarinic stimulation, depolarizing command potentials that evoked Ca(2+) influx elicited a prolonged tail current (I(tail)) that had an extrapolated reversal potential of -20 mV. I(tail) was not observed when intracellular Ca(2+) levels were chelated with 10 mm intracellular BAPTA, and I(tail) was reversibly depressed in low external sodium. When I(tail) was evoked at intervals >3 min, current amplitudes were stable for up to 1 hr. However, at shorter intervals, I(tail) was refractory, with a time constant of recovery of 43.5 sec. The inhibitors of soluble guanylate cyclase 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one and 6-anilino-5,8-quinolinequinone depressed I(tail) and zaprinast, which blocks cGMP-specific phosphodiesterase, enhanced I(tail), suggesting that a component of I(tail) was activated by cGMP. The inhibitors of cyclic nucleotide-gated (CNG) channels l-cis-diltiazem and 2',4'-dichlorobenzamil reversibly depressed I(tail). However, protein kinase G inhibition had no effect. Therefore, these results indicate that a component of I(tail) is attributable to activation of CNG channels. We conclude that Ca(2+) influx when combined with muscarinic receptor activation activates soluble guanylate cyclase and increases cGMP levels. The increased cGMP activates CNG channels and leads to prolonged depolarization. The cation conductance of the CNG channel contributes to the prolonged depolarization of the plateau potential.