Calcium release from internal stores is required for the generation of spontaneous hyperpolarizations in dopaminergic neurons of neonatal rats

Blunting neuroinflammation with resolvin D1 prevents early pathology in a rat model of Parkinson's disease

Neuroinflammation is one of the hallmarks of Parkinson’s disease (PD) and may contribute to midbrain dopamine (DA) neuron degeneration. Recent studies link chronic inflammation with failure to resolve early inflammation, a process operated by specialized pro-resolving mediators, including resolvins. However, the effects of stimulating the resolution of inflammation in PD – to modulate disease progression – still remain unexplored. Here we show that rats overexpressing human α-synuclein (Syn) display altered DA neuron properties, reduced striatal DA outflow and motor deficits prior to nigral degeneration. These early alterations are coupled with microglia activation and perturbations of inflammatory and pro-resolving mediators, namely IFN-γ and resolvin D1 (RvD1). Chronic and early RvD1 administration in Syn rats prevents central and peripheral inflammation, as well as neuronal dysfunction and motor deficits. We also show that endogenous RvD1 is decreased in human patients with early-PD. Our results suggest there is an imbalance between neuroinflammatory and pro-resolving processes in PD.

Resolvins are endogenous lipids with pro-resolving activity. Here the authors find that rats overexpressing human α-synuclein show defects in dopamine signalling before dopamine cell loss, and that this is associated with low Resolvin D1 levels and neuroinflammation.

Non-linear developmental trajectory of electrical phenotype in rat substantia nigra pars compacta dopaminergic neurons

Neurons have complex electrophysiological properties, however, it is often difficult to determine which properties are the most relevant to neuronal function. By combining current-clamp measurements of electrophysiological properties with multi-variate analysis (hierarchical clustering, principal component analysis), we were able to characterize the postnatal development of substantia nigra dopaminergic neurons' electrical phenotype in an unbiased manner, such that subtle changes in phenotype could be analyzed. We show that the intrinsic electrical phenotype of these neurons follows a non-linear trajectory reaching maturity by postnatal day 14, with two developmental transitions occurring between postnatal days 3-5 and 9-11. This approach also predicted which parameters play a critical role in phenotypic variation, enabling us to determine (using pharmacology, dynamic-clamp) that changes in the leak, sodium and calcium-activated potassium currents are central to these two developmental transitions. This analysis enables an unbiased definition of neuronal type/phenotype that is applicable to a range of research questions.

Mechanism of the medium-duration afterhyperpolarization in rat serotonergic neurons

Most serotonergic neurons display a prominent medium-duration afterhyperpolarization (mAHP), which is mediated by small-conductance Ca(2+) -activated K(+) (SK) channels. Recent ex vivo and in vivo experiments have suggested that SK channel blockade increases the firing rate and/or bursting in these neurons. The purpose of this study was therefore to characterize the source of Ca(2+) which activates the mAHP channels in serotonergic neurons. In voltage-clamp experiments, an outward current was recorded at -60 mV after a depolarizing pulse to +100 mV. A supramaximal concentration of the SK channel blockers apamin or (-)-bicuculline methiodide blocked this outward current. This current was also sensitive to the broad Ca(2+) channel blocker Co(2+) and was partially blocked by both ω-conotoxin and mibefradil, which are blockers of N-type and T-type Ca(2+) channels, respectively. Neither blockers of other voltage-gated Ca(2+) channels nor DBHQ, an inhibitor of Ca(2+)-induced Ca(2+) release, had any effect on the SK current. In current-clamp experiments, mAHPs following action potentials were only blocked by ω-conotoxin and were unaffected by mibefradil. This was observed in slices from both juvenile and adult rats. Finally, when these neurons were induced to fire in an in vivo-like pacemaker rate, only ω-conotoxin was able to increase their firing rate (by ~30%), an effect identical to the one previously reported for apamin. Our results demonstrate that N-type Ca(2+) channels are the only source of Ca(2+) which activates the SK channels underlying the mAHP. T-type Ca(2+) channels may also activate SK channels under different circumstances.

Protective Roles for Potassium SK/K(Ca)2 Channels in Microglia and Neurons

New concepts on potassium channel function in neuroinflammation suggest that they regulate mechanisms of microglial activation, including intracellular calcium homeostasis, morphological alterations, pro-inflammatory cytokine release, antigen presentation, and phagocytosis. Although little is known about voltage independent potassium channels in microglia, special attention emerges on small (SK/KCNN1-3/K(Ca)2) and intermediate (IK/KCNN4/K(Ca)3.1)-conductance calcium-activated potassium channels as regulators of microglial activation in the field of research on neuroinflammation and neurodegeneration. In particular, recent findings suggested that SK/K(Ca)2 channels, by regulating calcium homeostasis, may elicit a dual mechanism of action with protective properties in neurons and inhibition of inflammatory responses in microglia. Thus, modulating SK/K(Ca)2 channels and calcium signaling may provide novel therapeutic strategies in neurological disorders, where neuronal cell death and inflammatory responses concomitantly contribute to disease progression. Here, we review the particular role of SK/K(Ca)2 channels for [Ca(2+)](i) regulation in microglia and neurons, and we discuss the potential impact for further experimental approaches addressing novel therapeutic strategies in neurological diseases, where neuronal cell death and neuroinflammatory processes are prominent.

Ca(2+)-activated ion currents triggered by ryanodine receptor-mediated Ca(2+) release control firing of inhibitory neurons in the prepositus hypoglossi nucleus

Spontaneous miniature outward currents (SMOCs) are known to exist in smooth muscles and peripheral neurons, and evidence for the presence of SMOCs in central neurons has been accumulating. SMOCs in central neurons are induced through Ca(2+)-activated K(+) (K(Ca)) channels, which are activated through Ca(2+)-induced Ca(2+) release from the endoplasmic reticulum via ryanodine receptors (RyRs). Previously, we found that some neurons in the prepositus hypoglossi nucleus (PHN) showed spontaneous outward currents (SOCs). In the present study, we used whole cell recordings in slice preparations of the rat brain stem to investigate the following: 1) the ionic mechanisms of SOCs, 2) the types of neurons exhibiting frequent SOCs, and 3) the effect of Ca(2+)-activated conductance on neuronal firing. Pharmacological analyses revealed that SOCs were induced via the activation of small-conductance-type K(Ca) (SK) channels and RyRs, indicating that SOCs correspond to SMOCs. An analysis of the voltage responses to current pulses of the fluorescence-expressing inhibitory neurons of transgenic rats revealed that inhibitory neurons frequently exhibited SOCs. Abolition of SOCs via blockade of SK channels enhanced the frequency of spontaneous firing of inhibitory PHN neurons. However, abolition of SOCs via blockade of RyRs reduced the firing frequency and hyperpolarized the membrane potential. Similar reductions in firing frequency and hyperpolarization were also observed when Ca(2+)-activated nonselective cation (CAN) channels were blocked. These results suggest that, in inhibitory neurons in the PHN, Ca(2+) release via RyRs activates SK and CAN channels, and these channels regulate spontaneous firing in a complementary manner.

Electrophysiological characteristics of dopamine neurons: a 35-year update

This chapter consists of four sections. The first section provides a general description of the electrophysiological characteristics of dopamine (DA) neurons in both the substantia nigra and ventral tegmental area. Emphasis is placed on the differences between DA and neighboring non-DA neurons. The second section discusses the ionic mechanisms underlying the generation of action potential in DA cells. Evidence is provided to suggest that these mechanisms differ not only between DA and non-DA neurons but also between DA cells located in different areas, with different projection sites and at different developmental stages. Some of the differences may play a critical role in the vulnerability of a DA neuron to cell death. The third section describes the firing patterns of DA cells. Data are presented to show that the current "80/160 ms" criteria for burst identification need to be revised and that the burst firing, originally described by Bunney et al., can be described as slow oscillations in firing rate. In the ventral tegmental area, the slow oscillations are, at least partially, derived from the prefrontal cortex and part of prefrontal information is transferred to DA cells indirectly through inhibitory neurons. The final section focuses on the feedback regulation of DA cells. New evidence suggests that DA autoreceptors are coupled to multiple effectors, and both D1 and D2-like receptors are involved in long-loop feedback control of DA neurons. Because of the presence of multiple feedback and nonfeedback pathways, the effect of a drug on a DA neuron can be far more complex than an inhibition or excitation. A better understanding of the intrinsic properties of DA neurons and their regulation by afferent input will, in time, help to point to the way to more effective and safer treatments for disorders including schizophrenia, drug addiction, and Parkinson's disease.

Molecular and cellular basis of small--and intermediate-conductance, calcium-activated potassium channel function in the brain

Small conductance calcium-activated potassium (SK or K_Ca2) channels link intracellular calcium transients to membrane potential changes. SK channel subtypes present different pharmacology and distribution in the nervous system. The selective blocker apamin, SK enhancers and mice lacking specific SK channel subunits have revealed multifaceted functions of these channels in neurons, glia and cerebral blood vessels. SK channels regulate neuronal firing by contributing to the afterhyperpolarization following action potentials and mediating I_AHP, and partake in a calcium-mediated feedback loop with NMDA receptors, controlling the threshold for induction of hippocampal long-term potentiation. The function of distinct SK channel subtypes in different neurons often results from their specific coupling to different calcium sources. The prominent role of SK channels in the modulation of excitability and synaptic function of limbic, dopaminergic and cerebellar neurons hints at their possible involvement in neuronal dysfunction, either as part of the causal mechanism or as potential therapeutic targets.

Computational model predicts a role for ERG current in repolarizing plateau potentials in dopamine neurons: implications for modulation of neuronal activity

Blocking the small-conductance (SK) calcium-activated potassium channel promotes burst firing in dopamine neurons both in vivo and in vitro. In vitro, the bursting is unusual in that spiking persists during the hyperpolarized trough and frequently terminates by depolarization block during the plateau. We focus on the underlying plateau potential oscillation generated in the presence of both apamin and TTX, so that action potentials are not considered. We find that although the plateau potentials are mediated by a voltage-gated Ca(2+) current, they do not depend on the accumulation of cytosolic Ca(2+), then use a computational model to test the hypothesis that the slowly voltage-activated ether-a-go-go-related gene (ERG) potassium current repolarizes the plateaus. The model, which includes a material balance on calcium, is able to reproduce the time course of both membrane potential and somatic calcium concentration, and can also mimic the induction of plateau potentials by the calcium chelator BAPTA. The principle of separation of timescales was used to gain insight into the mechanisms of oscillation and its modulation using nullclines in the phase space. The model predicts that the plateau will be elongated and ultimately result in a persistent depolarization as the ERG current is reduced. This study suggests that the ERG current may play a role in burst termination and the relief of depolarization block in vivo.

Ca2+ release-dependent hyperpolarizations modulate the firing pattern of juvenile GABA neurons in mouse substantia nigra pars reticulata in vitro

A phasic activation of small-conductance Ca(2+)-dependent K(+) channels (SK channels) underlies spike-afterhyperpolarizations and spike-independent, transient hyperpolarizations in juvenile substantia nigra neurons. Outward current pulses that cause the spike-independent hyperpolarizations result from ryanodine receptor-mediated Ca(2+) release from intracellular stores. To study the modulation of excitability by the outward current pulses, we recorded from GABAergic pars reticulata neurons of mice at postnatal days 12-16. We induced a prolongation of SK channel open states by 1-ethyl-2-benzimidazolinone (1-EBIO). In addition to a prolongation of spike-afterhyperpolarizations, 1-EBIO (200 microm) potentiated outward current pulses by increasing their duration. Neurons were manipulated by current injection to display continuous or discontinuous discharge. Despite the prolongation of the outward current pulses by 1-EBIO, continuous action potential discharge became more regular, although its frequency declined. Durations of silent periods (periods of >2x average interspike interval) increased. Caffeine (1 mm) further increased the duration of such silent periods. Caffeine, however, had no effect at short interspike intervals (<600 ms). Cyclopiazonic acid (10 microm) silenced discharge in 1-EBIO, but discharge reappeared with the depletion of Ca(2+) stores. We conclude that the modulation of excitability by an activation of SK channels through ryanodine receptor-mediated release of Ca(2+) critically depends on the frequency of discharge. Outward current pulses occur only if interspike intervals exceed the duration of spike-afterhyperpolarizations. In this instance, the phasic, spike-independent activation of SK channels supports pauses to interrupt autonomous discharge in juvenile GABAergic pars reticulata neurons.

SK Ca2+-activated K+ channel ligands alter the firing pattern of dopamine-containing neurons in vivo

Apamin-sensitive, SK channels play an important role in generating the rhythmic firing patterns exhibited by midbrain dopamine neurons in vitro. However, their contribution to the firing properties of these cells in intact animals has yet to be determined. In the present series of experiments, extracellular single unit recording techniques were used to assess the central effects of prototypical SK channel ligands on the firing pattern of dopamine neurons in the substantia nigra of the chloral hydrate anesthetized rat. I.v. administration of the SK channel blocker apamin (0.4 mg/kg), increased bursting activity in approximately 50% of the dopamine neurons tested without altering average firing rate. The majority of these cells responded slowly to the effects of apamin, gradually transitioning from an irregular single spike to a phasic discharge composed of the same relative proportion of long (>or=three spike) and short (two spike) bursts as "natural" bursting activity recorded in drug naive animals. Local administration of apamin increased bursting activity in all cells tested. Systemic administration of the SK channel opener, 1-ethyl-2-benzimidazolinone (5-25 mg/kg) also had no effect on average firing rate but suppressed bursting activity and increased the precision of firing. The effects of 1-ethyl-2-benzimidazolinon on firing pattern were abolished when recording electrodes contained apamin (125 microM). These results suggest that SK channels actively contribute to the spontaneous firing patterns exhibited by dopamine neurons in vivo and provide additional support for the proposition that this channel could serve as a useful target for modifying their activity.

Spontaneous opening of T-type Ca2+ channels contributes to the irregular firing of dopamine neurons in neonatal rats

During early postnatal development, midbrain dopamine (DA) neurons display anomalous firing patterns and amphetamine response. Spontaneous miniature hyperpolarizations (SMHs) are observed in DA neurons during the same period but not in adults. These hyperpolarizations have been shown to be dependent on the release of Ca2+ from internal stores and the subsequent activation of Ca2+-sensitive K+ channels. However, the triggering mechanism and the functional significance of SMHs remain poorly understood. To address these issues, using brain slices, we recorded spontaneous miniature outward currents (SMOCs) in DA neurons of neonatal rats. Two types of SMOCs were identified based on the peak amplitude. Both types were suppressed by intracellular dialysis of ruthenium red, a ryanodine receptor (RyR) antagonist, yet none of the known Ca2+-releasing messengers were involved. T-type Ca2+ channel blockers (Ni2+ and mibefradil) inhibited large-amplitude SMOCs without affecting the small-amplitude ones. The voltage dependence of SMOCs displayed a peak of approximately -50 mV, consistent with the involvement of low-threshold T-type Ca2+ channels. Blockade of SMOCs with cyclopiazonic acid or ryanodine converted the irregular firing of DA neurons in neonatal rats into an adult-like pacemaker pattern. This effect was reversed by the injection of artificial currents mimicking SMOCs. Finally, amphetamine inhibited SMOCs and transformed the irregular firing pattern into a more regular one. These data demonstrate that Ca2+ influx through T-type Ca2+ channels, followed by Ca2+-induced Ca2+ release via RyRs, contributes to the generation of SMOCs. We propose that SMOCs-SMHs may underlie the anomalous firing and amphetamine response of DA neurons during the postnatal developmental period.

Two pathways for the activation of small-conductance potassium channels in neurons of substantia nigra pars reticulata

Neurons in substantia nigra pars reticulata express the messenger RNA for SK2 but not for SK3 subunits that form small-conductance, Ca2+-dependent K+ channels in dopamine neurons. To determine pathways for the activation of small-conductance, Ca2+-dependent K+ channels in substantia nigra pars reticulata neurons of rats and mice, we studied effects of the selective blocker of small-conductance, Ca2+-dependent K+ channels, apamin (0.01 or 0.3 microM). Apamin diminished the afterhyperpolarization following each action potential and induced burst discharges in substantia nigra pars reticulata neurons. Apamin had a robust effect already at a low (10 nM) concentration consistent with the expression of the SK2 subunit. Afterhyperpolarizations were also reduced by the Ca2+ channel blockers Ni2+ (100 microM) and omega-conotoxin GVIA (1 microM). Depletion of intracellular Ca2+ stores did not change the afterhyperpolarization. However, we observed outward current pulses that occurred independently from action potentials and were abrogated by apamin. Apart from a faster time course, they shared all properties with spontaneous hyperpolarizations or outward currents that ryanodine receptor-mediated Ca2+ release from intracellular stores induces in juvenile dopamine neurons. Sensitization of ryanodine receptors by caffeine silenced substantia nigra pars reticulata neurons. This effect was abolished by the depletion of intracellular Ca2+ stores. We conclude that SK2 channels in substantia nigra pars reticulata neurons are activated by Ca2+ influx through at least two types of Ca2+ channels in the membrane and by ryanodine receptor-mediated Ca2+ release from intracellular stores. Ryanodine receptors do not amplify small-conductance, Ca2+-dependent K+ channel activation by the Ca2+ influx during a single spike. Yet, ryanodine receptor-mediated Ca2+ release and, thereby, an activation of small-conductance, Ca2+-dependent K+ channels by intracellular Ca2+ are available for excitability modulation in these output neurons of the basal ganglia system.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

A Ca2+-independent slow afterhyperpolarization in substantia nigra compacta neurons

The discharge properties of dopaminergic neurons in substantia nigra are influenced by slow adaptive responses, which have not been fully identified. The present study describes, in a slice preparation from the rat, a complex afterhyperpolarization (AHP), elicited by action potential trains. The AHP could be subdivided into a fast component (AHP(f)), which was generated near action potential threshold, relaxed within approximately 1 s, and had highest amplitude when evoked by short-lasting (0.1 s) depolarizations, and a slow component (AHP(s)), which lasted several seconds, was evoked from subthreshold potentials, and required prolonged depolarizing stimuli (>0.1 s). A large proportion of the AHP(f) was sensitive to (i) 0.1 microM apamin, (ii) the Ca(2+) antagonists, Cd(2+) (0.2 mM) and Ni(2+) (0.3 mM), (iii) low (0.2 mM) extracellular Ca(2+) concentration, and (iv), Ca(2+) chelation with intracellular EGTA. The AHP(s) was resistant to the above treatments, and it was insensitive to 25 microM dantrolene or prolonged exposure to 1 microM thapsigargin. The reversal potential of the AHP(s) (-97 mV) was close to the K(+) equilibrium potential. It was significantly inhibited by 5 mM 4-aminopyridine, 5 microM haloperidol, 10 microM terfenadine, or high extracellular Mg(2+) (10 mM), but not by 30 mM tetraethylammonium chloride, 50 microM carbachol, 0.5 microM glipizide, 2 microM (-)sulpiride, 100 microM N-allyl-normetazocine, or 100 microM pentazocine. Haloperidol reduced the post-stimulus inhibitory period seen during spontaneous discharge, but had no detectable effect on spike frequency adaptation. It is concluded that the SK-type Ca(2+)-activated K(+) channels underlies a major component of the AHP(f), whereas the AHP(s) is Ca(2+)-independent and relies, in part, on a voltage-dependent K(+) current with properties resembling the ether-a-go-go-related gene K(+) channel. The latter component exerts a slow, spike-independent, inhibitory influence on repetitive discharge and contributes to the prolonged decrease in excitability following sustained depolarizing stimuli.

Effects of caffeine on the excitability and intracellular Ca(2+) transients of neonatal rat hypoglossal motoneurons in vitro

Since constitutively-high intracellular Ca(2+) ([Ca(2+)](i)) may confer hypoglossal motoneurons special vulnerability to excitoxic damage, we investigated the spatiotemporal dynamics of [Ca(2+)](i) and its relation to spike firing of rat hypoglossal motoneurons recorded under whole-cell patch clamp coupled with high resolution [Ca(2+)](i) imaging. A rise in [Ca(2+)](i), appearing in conjunction with single action potentials and becoming larger during spike trains, was first detected immediately beneath the cell membrane area, peaked 10-20 ms after each spike, and propagated to the cell core with slow decay time. Depletion of ryanodine-sensitive [Ca(2+)](i) stores by caffeine increased background [Ca(2+)](i), augmented the spike medium afterhyperpolarization, slowed down the action potential firing rate and depolarized cells (after an early hyperpolarization). The decay time constant of [Ca(2+)](i) transients was more than doubled by caffeine, although peak [Ca(2+)](i) remained unchanged. It is suggested that the main role of caffeine-sensitive stores was to buffer [Ca(2+)](i) elevated by sustained firing and to control spike accommodation.

Glutamate-mediated [Ca2+]c dynamics in spontaneously firing dopamine neurons of the rat substantia nigra pars compacta

The mechanism by which glutamate regulates the cytosolic free Ca2+ concentration ([Ca2+]c) in spontaneously firing dopamine neurons is not clear. Thus we have investigated the glutamate-mediated [Ca2+]c dynamics in the acutely isolated dopamine neurons from the rat substantia nigra pars compacta by measuring [Ca2+]c and spontaneously occurring action potentials (SAPs). The freshly isolated dopamine neurons showed tetrodotoxin (TTX)-sensitive spontaneous firing of 2-3 Hz and the resting [Ca2+]c decreased with abolition of the SAPs. The level of [Ca2+]c was affected by the spontaneous firing rate. In the presence of the Na+ channel antagonist, TTX (0.5 microM), glutamate increased [Ca2+]c by activating different glutamate receptors depending on the glutamate concentration used. Addition of glutamate at low concentrations (<3 microM) raised [Ca2+]c mainly by activating metabotropic glutamate receptors (mGluR), whereas at high concentrations (>10 microM) it raised [Ca2+]c mainly by activating AMPA/kainate receptors. The contribution of NMDA receptors to the glutamate-mediated [Ca2+]c rises was largest at intermediate concentrations of glutamate. Activation of mGluR elicited a Ca2+ release from intracellular Ca2+ stores and continuous Ca2+ influx out of the cell. The spontaneous firing activities were highly enhanced by submicromolar levels of glutamate and abolished at levels above 10 microM. From these results, we conclude that at low glutamate concentrations the [Ca2+]c in the dopamine neurons is mainly governed by mGluR and the firing activities, whose rate is regulated at submicromolar glutamate concentrations, but at higher glutamate concentrations [Ca2+]c is dominantly affected by AMPA/kainate receptors.

Spatial profiles of store-dependent calcium release in motoneurones of the nucleus hypoglossus from newborn mouse

Hypoglossal motoneurones (HMN) are selectively damaged in both human amyotrophic lateral sclerosis (ALS) and corresponding mouse models of this neurodegenerative disease, a process which has been linked to their low endogenous Ca2+ buffering capacity and an exceptional vulnerability to Ca2+-mediated excitotoxic events. In this report, we investigated local Ca2+ profiles in low buffered HMNs by utilizing multiphoton microscopy, CCD imaging and patch clamp recordings in slice preparations. Bath application of caffeine induced highly localized Ca2+ release events, which displayed an initial peak followed by a slow 'shoulder' lasting several seconds. Peak amplitudes were paralleled by Ca2+-activated, apamin-sensitive K+ currents (IKCa), demonstrating a functional link between Ca2+ stores and HMN excitability. The potential involvement of mitochondria was investigated by bath application of CCCP, which collapses the electrochemical potential across the inner mitochondrial membrane. CCCP reduced peak amplitudes of caffeine responses and consequently IKCa, indicating that functionally intact mitochondria were critical for store-dependent modulation of HMN excitability. Taken together, our results indicate localized Ca2+ release profiles in HMNs, where low buffering capacities enhance the role of Ca2+-regulating organelles as local determinants of [Ca2+]i. This might expose HMN to exceptional risks during pathophysiological organelle disruptions and other ALS-related, cellular disturbances.

Selective coupling of T-type calcium channels to SK potassium channels prevents intrinsic bursting in dopaminergic midbrain neurons

Dopaminergic midbrain (DA) neurons display two principal activity patterns in vivo, single-spike and burst firing, the latter coding for reward-related events. We have shown recently that the small-conductance calcium-activated potassium channel SK3 controls pacemaker frequency and precision in DA neurons of the substantia nigra (SN), and previous studies have implicated SK channels in the transition to burst firing. To identify the upstream calcium sources for SK channel activation in DA SN neurons, we studied the sensitivity of SK channel-mediated afterhyperpolarization (AHP) currents to inhibitors of different types of voltage-gated calcium channels in perforated patch-clamp recordings. Cobalt-sensitive AHP currents were not affected by L-type and P/Q-type calcium channel inhibitors and were reduced slightly (26%) by the N-type channel inhibitor omega-conotoxin-GVIA. In contrast, AHP currents were blocked substantially (85-94%) by micromolar concentrations of nickel (IC50, 33.75 microm) and mibefradil (IC50, 4.83 microm), indistinguishable from the nickel and mibefradil sensitivities of T-type calcium currents (IC50 values, 33.86 and 4.59 microm, respectively). These results indicate that SK channels are activated selectively via T-type calcium channels in DA SN neurons. Consequently, SK currents displayed use-dependent inactivation with similar time constants when compared with those of T-type calcium currents and generated a transient rebound inhibition. Both SK and T-type channels were essential for the stability of spontaneous pacemaker activity, and, in some DA SN neurons, T-type channel inhibition was sufficient to induce intrinsic burst firing. The functional coupling of SK to T-type channels has important implications for the temporal integration of synaptic input and might help to understand how DA neurons switch between pacemaker and burst-firing modes in vivo.

Effects of dantrolene and its derivatives on Ca(2+) release from the sarcoplasmic reticulum of mouse skeletal muscle fibres

1. We analysed the effect of dantrolene (Dan) and five newly synthesized derivatives (GIFs) on Ca(2+) release from the sarcoplasmic reticulum (SR) of mouse skeletal muscle. 2. In intact muscles, GIF-0185 reduced the size of twitch contraction induced by electrical stimulation to the same extent as Dan. GIF-0082, an azido-functionalized Dan derivative, also inhibited twitch contraction, although the extent of inhibition was less than that of Dan and of GIF-0185. 3. In skinned fibres, Dan inhibited Ca(2+)-induced Ca(2+) release (CICR) under Mg(2+)-free conditions at room temperature. In contrast, GIF-0082 and GIF-0185 showed no inhibitory effect on CICR under the same conditions. 4. Dan-induced inhibition of CICR was not affected by the presence of GIF-0082, whereas it was diminished in the presence of GIF-0185. 5. GIF-0082 and GIF-0185 significantly inhibited clofibric acid (Clof)-induced Ca(2+) release, as did Dan. 6. Several Dan derivatives other than GIF-0082 and GIF-0185 showed an inhibitory effect on twitch tension but not on the CICR mechanism. All of these derivatives inhibited Clof-induced Ca(2+) release. 7. The magnitudes of inhibition of Clof-induced Ca(2+) release by all Dan derivatives were well correlated with those of twitch inhibition. This supports the notion that the mode of Clof-induced opening of the RyR-Ca(2+) release channel may be similar to that of physiological Ca(2+) release (PCR). 8. These results indicate that the difference in opening modes of the RyR-Ca(2+) release channel is recognized by certain Dan derivatives.

Spontaneous miniature outward currents in mechanically dissociated rat Meynert neurons

1. Spontaneous miniature outward currents (SMOCs) were observed in mechanically dissociated rat Meynert neurons using nystatin perforated patch recordings under voltage-clamp conditions. 2. SMOCs were blocked by apamin, a selective blocker of small conductance Ca(2+)-activated K(+) (SK) channels, but not by blockers for other types of Ca(2+)-activated K(+) channel. 3. Ryanodine (10-100 microM) reduced both the amplitude and frequency of SMOCs. Caffeine (1 mM) increased the SMOC frequency. Blockers of the sarco/endoplasmic reticulum Ca(2+)-ATPase completely abolished SMOCs, indicating a requirement for functioning sarco/endoplasmic reticulum (SR/ER) Ca(2+) stores. 4. Both Cd(2+)-containing and Ca(2+)-free solutions partially inhibited SMOC frequency, a result which suggests that Ca(2+) influx contributes to, but is not essential for, SMOC generation. 5. Thus, SMOCs are SK currents linked to ryanodine- and caffeine-sensitive SR/ER Ca(2+) stores, and are only indirectly influenced by extracellular Ca(2+) influx. The development of this new, minimally invasive mechanical dissociation method has revealed that SMOCs are common in native CNS neurons.

Differential expression of the small-conductance, calcium-activated potassium channel SK3 is critical for pacemaker control in dopaminergic midbrain neurons

The physiological activity of dopaminergic midbrain (DA) neurons is important for movement, cognition, and reward. Altered activity of DA neurons is a key finding in schizophrenia, but the cellular mechanisms have not been identified. Recently, KCNN3, a gene that encodes a member (SK3) of the small-conductance, calcium-activated potassium (SK) channels, has been proposed as a candidate gene for schizophrenia. However, the functional role of SK3 channels in DA neurons is unclear. We combined patch-clamp recordings with single-cell RT-PCR and confocal immunohistochemistry in mouse midbrain slices to study the function of molecularly defined SK channels in DA neurons. Biophysical and pharmacological analysis, single-cell mRNA, and protein expression profiling strongly suggest that SK3 channels mediate the calcium-dependent afterhyperpolarization in DA neurons. Perforated patch recordings of DA neurons in the substantia nigra (SN) demonstrated that SK3 channels dynamically control the frequency of spontaneous firing. In addition, SK3 channel activity was essential to maintain the high precision of the intrinsic pacemaker of DA SN neurons. In contrast, in the ventral tegmental area, DA neurons displayed significantly smaller SK currents and lower SK3 protein expression. In these DA neurons, SK3 channels were not involved in pacemaker control. Accordingly, they discharged in a more irregular manner compared with DA SN neurons. Thus, our study shows that differential SK3 channel expression is a critical molecular mechanism in DA neurons to control neuronal activity. This provides a cellular framework to understand the functional consequences of altered SK3 expression, a candidate disease mechanism for schizophrenia.