Large-conductance calcium-dependent potassium channels prevent dendritic excitability in neocortical pyramidal neurons

Large conductance voltage-and calcium-activated K+ (BK) channel in health and disease

Large Conductance Voltage- and Calcium-activated K+ (BK) channels are transmembrane pore-forming proteins that regulate cell excitability and are also expressed in non-excitable cells. They play a role in regulating vascular tone, neuronal excitability, neurotransmitter release, and muscle contraction. Dysfunction of the BK channel can lead to arterial hypertension, hearing disorders, epilepsy, and ataxia. Here, we provide an overview of BK channel functioning and the implications of its abnormal functioning in various diseases. Understanding the function of BK channels is crucial for comprehending the mechanisms involved in regulating vital physiological processes, both in normal and pathological conditions, controlled by BK. This understanding may lead to the development of therapeutic interventions to address BK channelopathies.

Silences, spikes and bursts: Three-part knot of the neural code

When a neuron breaks silence, it can emit action potentials in a number of patterns. Some responses are so sudden and intense that electrophysiologists felt the need to single them out, labelling action potentials emitted at a particularly high frequency with a metonym - bursts. Is there more to bursts than a figure of speech? After all, sudden bouts of high-frequency firing are expected to occur whenever inputs surge. The burst coding hypothesis advances that the neural code has three syllables: silences, spikes and bursts. We review evidence supporting this ternary code in terms of devoted mechanisms for burst generation, synaptic transmission and synaptic plasticity. We also review the learning and attention theories for which such a triad is beneficial.

Ca2+- and Voltage-Activated K+ (BK) Channels in the Nervous System: One Gene, a Myriad of Physiological Functions

BK channels are large conductance potassium channels characterized by four pore-forming α subunits, often co-assembled with auxiliary β and γ subunits to regulate Ca2+ sensitivity, voltage dependence and gating properties. BK channels are abundantly expressed throughout the brain and in different compartments within a single neuron, including axons, synaptic terminals, dendritic arbors, and spines. Their activation produces a massive efflux of K+ ions that hyperpolarizes the cellular membrane. Together with their ability to detect changes in intracellular Ca2+ concentration, BK channels control neuronal excitability and synaptic communication through diverse mechanisms. Moreover, increasing evidence indicates that dysfunction of BK channel-mediated effects on neuronal excitability and synaptic function has been implicated in several neurological disorders, including epilepsy, fragile X syndrome, mental retardation, and autism, as well as in motor and cognitive behavior. Here, we discuss current evidence highlighting the physiological importance of this ubiquitous channel in regulating brain function and its role in the pathophysiology of different neurological disorders.

Potassium channelopathies associated with epilepsy-related syndromes and directions for therapeutic intervention

A number of mutations to members of several CNS potassium (K) channel families have been identified which result in rare forms of neonatal onset epilepsy, or syndromes of which one prominent characteristic is a form of epilepsy. Benign Familial Neonatal Convulsions or Seizures (BFNC or BFNS), also referred to as Self-Limited Familial Neonatal Epilepsy (SeLNE), results from mutations in 2 members of the K_V7 family (KCNQ) of K channels; while generally self-resolving by about 15 weeks of age, these mutations significantly increase the probability of generalized seizure disorders in the adult, in some cases they result in more severe developmental syndromes. Epilepsy of Infancy with Migrating Focal Seizures (EIMSF), or Migrating Partial Seizures of Infancy (MMPSI), is a rare severe form of epilepsy linked primarily to gain of function mutations in a member of the sodium-dependent K channel family, KCNT1 or SLACK. Finally, KCNMA1 channelopathies, including Liang-Wang syndrome (LIWAS), are rare combinations of neurological symptoms including seizure, movement abnormalities, delayed development and intellectual disabilities, with Liang-Wang syndrome an extremely serious polymalformative syndrome with a number of neurological sequelae including epilepsy. These are caused by mutations in the pore-forming subunit of the large-conductance calcium-activated K channel (BK channel) KCNMA1. The identification of these rare but significant channelopathies has resulted in a resurgence of interest in their treatment by direct pharmacological or genetic modulation. We will briefly review the genetics, biophysics and pharmacology of these K channels, their linkage with the 3 syndromes described above, and efforts to more effectively target these syndromes.

Selective activation of BK channels in small-headed dendritic spines suppresses excitatory postsynaptic potentials

Dendritic spines are the main receptacles of excitatory information in the brain. Their particular morphology, with a small head connected to the dendrite by a slender neck, has inspired theoretical and experimental work to understand how these structural features affect the processing, storage and integration of synaptic inputs in pyramidal neurons (PNs). The activation of glutamate receptors in spines triggers a large voltage change as well as calcium signals at the spine head. Thus, voltage-gated and calcium-activated potassium channels located in the spine head likely play a key role in synaptic transmission. Here we study the presence and function of large conductance calcium-activated potassium (BK) channels in spines from layer 5 PNs. We found that BK channels are localized to dendrites and spines regardless of their size, but their activity can only be detected in spines with small head volumes (≤0.09 μm³ ), which reduces the amplitude of two-photon uncaging excitatory postsynaptic potentials recorded at the soma. In addition, we found that calcium signals in spines with small head volumes are significantly larger than those observed in spines with larger head volumes. In accordance with our experimental data, numerical simulations predict that synaptic inputs impinging onto spines with small head volumes generate voltage responses and calcium signals within the spine head itself that are significantly larger than those observed in spines with larger head volumes, which are sufficient to activate spine BK channels. These results show that BK channels are selectively activated in small-headed spines, suggesting a new level of dendritic spine-mediated regulation of synaptic processing, integration and plasticity in cortical PNs. KEY POINTS: BK channels are expressed in the visual cortex and layer 5 pyramidal neuron somata, dendrites and spines regardless of their size. BK channels are selectively activated in small-headed spines (≤0.09 μm³ ), which reduces the amplitude of two-photon (2P) uncaging excitatory postsynaptic potentials (EPSPs) recorded at the soma. Two-photon imaging revealed that intracellular calcium responses in the head of 2P-activated spines are significantly larger in small-headed spines (≤0.09 μm³ ) than in spines with larger head volumes. In accordance with our experimental data, numerical simulations showed that synaptic inputs impinging onto spines with small head volumes (≤0.09 μm³ ) generate voltage responses and calcium signals within the spine head itself that are significantly larger than those observed in spines with larger head volumes, sufficient to activate spine BK channels and suppress EPSPs.

Trophic factors as potential therapies for treatment of major mental disorders

Notwithstanding major advances in psychotherapeutics, their efficacy and specificity remain limited. The slow onset of beneficial outcomes and numerous adverse effects of widely used medications remain of chief concern, warranting in-depth studies. The majority of frontline therapies are thought to enhance the endogenous monoaminergic drive, to initiate a cascade of molecular events leading to lasting functional and structural plasticity. They also involve alterations in trophic factor signalling, including brain-derived neurotrophic factor (BDNF), VGF (non-acronymic), vascular endothelial growth factor (VEGF), fibroblast growth factor 2 (FGF2), glial cell-derived neurotrophic factor (GDNF), and others. In several major mental disorders, emerging data suggest protective and restorative effects of trophic factors in preclinical models, when applied on their own. Antidepressant outcomes of VGF and FGF2, for instance, were shown in experimental animals, while BDNF and GDNF prove useful in the treatment of addiction, schizophrenia, and autism spectrum disorders. The main challenge with the effective translation of these and other findings in the clinic is the knowledge gap in action mechanisms with potential risks, as well as the lack of effective platforms for validation under clinical settings. Herein, we review the state-of-the-art and advances in the therapeutic use of trophic factors in several major neuropsychiatric disorders.

Paradoxical Excitatory Impact of SK Channels on Dendritic Excitability

Dendritic excitability regulates how neurons integrate synaptic inputs and thereby influences neuronal output. As active dendritic events are associated with significant calcium influx they are likely to be modulated by calcium-dependent processes, such as calcium-activated potassium channels. Here we investigate the impact of small conductance calcium-activated potassium channels (SK channels) on dendritic excitability in male and female rat cortical pyramidal neurons in vitro and in vivo Using local applications of the SK channel antagonist apamin in vitro, we show that blocking somatic SK channels enhances action potential output, whereas blocking dendritic SK channels paradoxically reduces the generation of dendritic calcium spikes and associated somatic burst firing. Opposite effects were observed using the SK channel enhancer NS309. The effect of apamin on dendritic SK channels was occluded when R-type calcium channels were blocked, indicating that the inhibitory impact of apamin on dendritic calcium spikes involved R-type calcium channels. Comparable effects were observed in vivo Intracellular application of apamin via the somatic whole-cell recording pipette reduced the medium afterhyperpolarization and increased action potential output during UP states. In contrast, extracellular application of apamin to the cortical surface to block dendritic SK channels shifted the distribution of action potentials within UP states from an initial burst to a more distributed firing pattern, while having no impact on overall action potential firing frequency or UP and DOWN states. These data indicate that somatic and dendritic SK channels have opposite effects on neuronal excitability, with dendritic SK channels counter-intuitively promoting rather than suppressing neuronal output.SIGNIFICANCE STATEMENT Neurons typically receive input from other neurons onto processes called dendrites, and use electrical events such as action potentials for signaling. As electrical events in neurons are usually associated with calcium influx they can be regulated by calcium-dependent processes. One such process is through the activation of calcium-dependent potassium channels, which usually act to reduce action potential signaling. Although this is the case for calcium-dependent potassium channels found at the cell body, we show here that calcium-dependent potassium channels in dendrites of cortical pyramidal neurons counter-intuitively promote rather than suppress action potential output.

Double-Nanodomain Coupling of Calcium Channels, Ryanodine Receptors, and BK Channels Controls the Generation of Burst Firing

Action potentials clustered into high-frequency bursts play distinct roles in neural computations. However, little is known about ionic currents that control the duration and probability of these bursts. We found that, in cartwheel inhibitory interneurons of the dorsal cochlear nucleus, the likelihood of bursts and the interval between their spikelets were controlled by Ca²⁺ acting across two nanodomains, one between plasma membrane P/Q Ca²⁺ channels and endoplasmic reticulum (ER) ryanodine receptors and another between ryanodine receptors and large-conductance, voltage- and Ca²⁺-activated K⁺ (BK) channels. Each spike triggered Ca²⁺-induced Ca²⁺ release (CICR) from the ER immediately beneath somatic, but not axonal or dendritic, plasma membrane. Moreover, immunolabeling demonstrated close apposition of ryanodine receptors and BK channels. Double-nanodomain coupling between somatic plasma membrane and hypolemmal ER cisterns provides a unique mechanism for rapid control of action potentials on the millisecond timescale.

The Impact of BK Channels on Cellular Excitability Depends on their Subcellular Location

Large conductance calcium-activated potassium channels (or BK channels) fulfil a multitude of roles in the central nervous system. At the soma of many neuronal cell types they control the speed of action potential (AP) repolarization and therefore they can have an impact on neuronal excitability. Due to their presence in nerve terminals they also regulate transmitter release. BK channels have also been shown to be present in the dendrites of some neurons where they can regulate the magnitude and duration of dendritic spikes. Here, we investigate the impact of modulating the activation of BK channels at different locations on the cellular excitability of cortical layer 5 pyramidal neurons. We find that while somatic BK channels help to repolarize APs at the soma and mediate the fast after-hyperpolarization, dendritic BK channels are responsible for repolarization of dendritic calcium spikes and thereby regulate somatic AP burst firing. We found no evidence for a role of dendritic BK channels in the regulation of backpropagating AP amplitude or duration. These experiments highlight the diverse roles of BK channels in regulating neuronal excitability and indicate that their functional impact depends on their subcellular location.

BK Channels in the Central Nervous System

Large conductance Ca(2+)- and voltage-activated K(+) (BK) channels are widely distributed in the postnatal central nervous system (CNS). BK channels play a pleiotropic role in regulating the activity of brain and spinal cord neural circuits by providing a negative feedback mechanism for local increases in intracellular Ca(2+) concentrations. In neurons, they regulate the timing and duration of K(+) influx such that they can either increase or decrease firing depending on the cellular context, and they can suppress neurotransmitter release from presynaptic terminals. In addition, BK channels located in astrocytes and arterial myocytes modulate cerebral blood flow. Not surprisingly, both loss and gain of BK channel function have been associated with CNS disorders such as epilepsy, ataxia, mental retardation, and chronic pain. On the other hand, the neuroprotective role played by BK channels in a number of pathological situations could potentially be leveraged to correct neurological dysfunction.

Impact of calcium-activated potassium channels on NMDA spikes in cortical layer 5 pyramidal neurons

Active electrical events play an important role in shaping signal processing in dendrites. As these events are usually associated with an increase in intracellular calcium, they are likely to be under the control of calcium-activated potassium channels. Here, we investigate the impact of calcium-activated potassium channels onN-methyl-d-aspartate (NMDA) receptor-dependent spikes, or NMDA spikes, evoked by glutamate iontophoresis onto basal dendrites of cortical layer 5 pyramidal neurons. We found that small-conductance calcium-activated potassium channels (SK channels) act to reduce NMDA spike amplitude but at the same time, also decrease the iontophoretic current required for their generation. This SK-mediated decrease in NMDA spike threshold was dependent on R-type voltage-gated calcium channels and indicates a counterintuitive, excitatory effect of SK channels on NMDA spike generation, whereas the capacity of SK channels to suppress NMDA spike amplitude is in line with the expected inhibitory action of potassium channels on dendritic excitability. Large-conductance calcium-activated potassium channels had no significant impact on NMDA spikes, indicating that these channels are either absent from basal dendrites or not activated by NMDA spikes. These experiments reveal complex and opposing interactions among NMDA receptors, SK channels, and voltage-gated calcium channels in basal dendrites of cortical layer 5 pyramidal neurons during NMDA spike generation, which are likely to play an important role in regulating the way these neurons integrate the thousands of synaptic inputs they receive.

Transient BK outward current enhances motoneurone firing rates during Drosophila larval locomotion

KEY POINTS

We combine in situ electrophysiology with genetic manipulation in Drosophila larvae aiming to investigate the role of fast calcium-activated potassium currents for motoneurone firing patterns during locomotion. We first demonstrate that slowpoke channels underlie fast calcium-activated potassium currents in these motoneurones. By conducting recordings in semi-intact animals that produce crawling-like movements, we show that slowpoke channels are required specifically in motoneurones for maximum firing rates during locomotion. Such enhancement of maximum firing rates occurs because slowpoke channels prevent depolarization block by limiting the amplitude of motoneurone depolarization in response to synaptic drive. In addition, slowpoke channels mediate a fast afterhyperpolarization that ensures the efficient recovery of sodium channels from inactivation during high frequency firing. The results of the present study provide new insights into the mechanisms by which outward conductances facilitate neuronal excitability and also provide direct confirmation of the functional relevance of precisely regulated slowpoke channel properties in motor control.

ABSTRACT

A large number of voltage-gated ion channels, their interactions with accessory subunits, and their post-transcriptional modifications generate an immense functional diversity of neurones. Therefore, a key challenge is to understand the genetic basis and precise function of specific ionic conductances for neuronal firing properties in the context of behaviour. The present study identifies slowpoke (slo) as exclusively mediating fast activating, fast inactivating BK current (ICF ) in larval Drosophila crawling motoneurones. Combining in vivo patch clamp recordings during larval crawling with pharmacology and targeted genetic manipulations reveals that ICF acts specifically in motoneurones to sculpt their firing patterns in response to a given input from the central pattern generating (CPG) networks. First, ICF curtails motoneurone postsynaptic depolarizations during rhythmical CPG drive. Second, ICF is activated during the rising phase of the action potential and mediates a fast afterhyperpolarization. Consequently, ICF is required for maximal intraburst firing rates during locomotion, probably by allowing recovery from inactivation of fast sodium channels and decreased potassium channel activation. This contrasts the common view that outward conductances oppose excitability but is in accordance with reports on transient BK and Kv3 channel function in multiple types of vertebrate neurones. Therefore, our finding that ICF enhances firing rates specifically during bursting patterns relevant to behaviour is probably of relevance to all brains.

Activation of large-conductance Ca(2+)-activated K(+) channels inhibits glutamate-induced oxidative stress through attenuating ER stress and mitochondrial dysfunction

Large-conductance Ca(2+)-activated K(+) channels (BK channels) are widely expressed throughout the vertebrate nervous system, and are involved in the regulation of neurotransmitter release and neuronal excitability. Here, the neuroprotective effects of NS11021, a selective and chemically unrelated BK channel activator, and potential molecular mechanism involved have been studied in rat cortical neurons exposed to glutamate in vitro. Pretreatment with NS11021 significantly inhibited the loss of neuronal viability, LDH release and neuronal apoptosis in a dose-dependent manner. All these protective effects were fully antagonized by the BK-channel inhibitor paxilline. NS11021-induced neuroprotection was associated with reduced oxidative stress, as evidenced by decreased reactive oxygen species (ROS) generation, lipid peroxidation and preserved activity of antioxidant enzymes. Moreover, NS11021 significantly attenuated the glutamate-induced endoplasmic reticulum (ER) calcium release and activation of ER stress markers, including glucose-regulated protein 78 (GRP78), C/EBP homologous protein (CHOP) and caspase-12. Pretreatment with NS11021 also mitigated the mitochondrial membrane potential (MMP) collapse, cytochrome c release, and preserved mitochondrial Ca(2+) buffering capacity and ATP synthesis after glutamate exposure. Taken together, these results suggest that activation of BK channels via NS11021 protects cortical neurons against glutamate-induced excitatory damage, which may be dependent on the inhibition of ER stress and preservation of mitochondrial dysfunction.

Anatomy and physiology of the thick-tufted layer 5 pyramidal neuron

The thick-tufted layer 5 (TTL5) pyramidal neuron is one of the most extensively studied neuron types in the mammalian neocortex and has become a benchmark for understanding information processing in excitatory neurons. By virtue of having the widest local axonal and dendritic arborization, the TTL5 neuron encompasses various local neocortical neurons and thereby defines the dimensions of neocortical microcircuitry. The TTL5 neuron integrates input across all neocortical layers and is the principal output pathway funneling information flow to subcortical structures. Several studies over the past decades have investigated the anatomy, physiology, synaptology, and pathophysiology of the TTL5 neuron. This review summarizes key discoveries and identifies potential avenues of research to facilitate an integrated and unifying understanding on the role of a central neuron in the neocortex.

Ethanol modulation of mammalian BK channels in excitable tissues: molecular targets and their possible contribution to alcohol-induced altered behavior

In most tissues, the function of Ca²⁺- and voltage-gated K⁺ (BK) channels is modified in response to ethanol concentrations reached in human blood during alcohol intoxication. In general, modification of BK current from ethanol-naïve preparations in response to brief ethanol exposure results from changes in channel open probability without modification of unitary conductance or change in BK protein levels in the membrane. Protracted and/or repeated ethanol exposure, however, may evoke changes in BK expression. The final ethanol effect on BK open probability leading to either BK current potentiation or BK current reduction is determined by an orchestration of molecular factors, including levels of activating ligand (Ca²⁺_i), BK subunit composition and post-translational modifications, and the channel's lipid microenvironment. These factors seem to allosterically regulate a direct interaction between ethanol and a recognition pocket of discrete dimensions recently mapped to the channel-forming (slo1) subunit. Type of ethanol exposure also plays a role in the final BK response to the drug: in several central nervous system regions (e.g., striatum, primary sensory neurons, and supraoptic nucleus), acute exposure to ethanol reduces neuronal excitability by enhancing BK activity. In contrast, protracted or repetitive ethanol administration may alter BK subunit composition and membrane expression, rendering the BK complex insensitive to further ethanol exposure. In neurohypophyseal axon terminals, ethanol potentiation of BK channel activity leads to a reduction in neuropeptide release. In vascular smooth muscle, however, ethanol inhibition of BK current leads to cell contraction and vascular constriction.

Dendritic channelopathies contribute to neocortical and sensory hyperexcitability in Fmr1(-/y) mice

Hypersensitivity in response to sensory stimuli and neocortical hyperexcitability are prominent features of Fragile X Syndrome (FXS) and autism spectrum disorders, but little is known about the dendritic mechanisms underlying these phenomena. We found that the primary somatosensory neocortex (S1) was hyperexcited in response to tactile sensory stimulation in Fmr1(-/y) mice. This correlated with neuronal and dendritic hyperexcitability of S1 pyramidal neurons, which affect all major aspects of neuronal computation, from the integration of synaptic input to the generation of action potential output. Using dendritic electrophysiological recordings, calcium imaging, pharmacology, biochemistry and a computer model, we found that this defect was, at least in part, attributable to the reduction and dysfunction of dendritic h- and BKCa channels. We pharmacologically rescued several core hyperexcitability phenomena by targeting BKCa channels. Our results provide strong evidence pointing to the utility of BKCa channel openers for the treatment of the sensory hypersensitivity aspects of FXS.

BK channels mediate dopamine inhibition of firing in a subpopulation of core nucleus accumbens medium spiny neurons

Dopamine, a key neurotransmitter mediating the rewarding properties of drugs of abuse, is widely believed to exert some of its effects by modulating neuronal activity of nucleus accumbens (NAcc) medium spiny neurons (MSNs). Although its effects on synaptic transmission have been well documented, its regulation of intrinsic neuronal excitability is less understood. In this study, we examined the cellular mechanisms of acute dopamine effects on core accumbens MSNs evoked firing. We found that 0.5 µM A-77636 and 10 µM quinpirole, dopamine D1 (DR1s) and D2 receptor (D2Rs) agonists, respectively, markedly inhibited MSN evoked action potentials. This effect, observed only in about 25% of all neurons, was associated with spike-timing-dependent (STDP) long-term potentiation (tLTP), but not long-term depression (tLTD). Dopamine inhibits evoked firing by compromising subthreshold depolarization, not by altering action potentials themselves. Recordings in voltage-clamp mode revealed that all MSNs expressed fast (IA), slowly inactivating delayed rectifier (Idr), and large conductance voltage- and calcium-activated potassium (BKs) channels. Although A-77636 and quinpirole enhanced IA, its selective blockade by 0.5 µM phrixotoxin-1 had no effect on evoked firing. In contrast, exposing tissue to low TEA concentrations and to 10 µM paxilline, a selective BK channel blocker, prevented D1R agonist from inhibiting MSN firing. This result indicates that dopamine inhibits MSN firing through BK channels in a subpopulation of core accumbens MSNs exclusively associated with spike-timing-dependent long-term potentiation.

Properties of BK-type Ca(+) (+)-dependent K(+) channel currents in medial prefrontal cortex pyramidal neurons in rats of different ages

The medial prefrontal cortex (PFC) is involved in cognitive functions, which undergo profound changes during adolescence. This alteration of the PFC function derives from neuron activity, which, in turn, may depend on age-dependent properties and the expression of neuronal ion channels. BK-type channels are involved in controlling both the Ca(+) (+) ion concentration in the cell interior and cell excitability. The purpose of this study was to test the properties of BK currents in the medial PFC pyramidal neurons of young (18- to 22-day-old), adolescent (38- to 42-day-old), and adult (60- to 65-day-old) rats. Whole-cell currents evoked by depolarizing voltage steps were recorded from dispersed medial PFC pyramidal neurons. A selective BK channel blocker - paxilline (10 μM) - irreversibly decreased the non-inactivating K(+) current in neurons that were isolated from the young and adult rats. This current was not significantly affected by paxilline in the neurons obtained from adolescent rats. The properties of single-channel K(+) currents were recorded from the soma of dispersed medial PFC pyramidal neurons in the cell-attached configuration. Of the K(+) channel currents that were recorded, ~90% were BK and leak channel currents. The BK-type channel currents were dependent on the Ca(+) (+) concentration and the voltage and were inhibited by paxilline. The biophysical properties of the BK channel currents did not differ among the pyramidal neurons isolated from young, adolescent, and adult rats. Among all of the recorded K(+) channel currents, 38.9, 12.7, and 21.1% were BK-type channel currents in the neurons isolated from the young, adolescent, and adult rats, respectively. Furthermore, application of paxilline effectively prolonged the half-width of the action potential in pyramidal neurons in slices isolated from young and adult rats but not in neurons isolated from adolescent rats. We conclude that the availability of BK channel currents decreases in medial PFC pyramidal neurons of adolescent rats compared with those in the neurons of young and adult rats while their properties did not change across ages.

Effects of manipulating slowpoke calcium-dependent potassium channel expression on rhythmic locomotor activity in Drosophila larvae

Rhythmic motor behaviors are generated by networks of neurons. The sequence and timing of muscle contractions depends on both synaptic connections between neurons and the neurons' intrinsic properties. In particular, motor neuron ion currents may contribute significantly to motor output. Large conductance Ca(2+)-dependent K(+) (BK) currents play a role in action potential repolarization, interspike interval, repetitive and burst firing, burst termination and interburst interval in neurons. Mutations in slowpoke (slo) genes encoding BK channels result in motor disturbances. This study examined the effects of manipulating slo channel expression on rhythmic motor activity using Drosophila larva as a model system. Dual intracellular recordings from adjacent body wall muscles were made during spontaneous crawling-related activity in larvae expressing a slo mutation or a slo RNA interference construct. The incidence and duration of rhythmic activity in slo mutants were similar to wild-type control animals, while the timing of the motor pattern was altered. slo mutants showed decreased burst durations, cycle durations, and quiescence intervals, and increased duty cycles, relative to wild-type. Expressing slo RNAi in identified motor neurons phenocopied many of the effects observed in the mutant, including decreases in quiescence interval and cycle duration. Overall, these results show that altering slo expression in the whole larva, and specifically in motor neurons, changes the frequency of crawling activity. These results suggest an important role for motor neuron intrinsic properties in shaping the timing of motor output.

BK channels reveal novel phosphate sensitivity in SNr neurons

Whether large conductance Ca²⁺-activated potassium (BK) channels are present in the substantia nigra pars reticulata (SNr) is a matter of debate. Using the patch-clamp technique, we examined the functional expression of BK channels in neurons of the SNr and showed that the channels were activated or inhibited by internal high-energy phosphates (IHEPs) at positive and negative membrane potentials, respectively. SNr neurons showed membrane potential hyperpolarization under glucose-deprivation conditions which was attenuated by paxilline, a specific BK channel blocker. In addition, Fluo-3 fluorescence recording detected an increase in the level of internal free calcium ([Ca²⁺]_i) during ischemic hyperpolarization. These results confirm that BK channels are present in SNr neurons and indicate that their unique IHEP sensitivity is requisite in neuronal ischemic responses. Bearing in mind that the K_ATP channel blocker tolbutamide also attenuated the hyperpolarization, we suggest that BK channels may play a protective role in the basal ganglia by modulating the excitability of SNr neurons along with K_ATP channels under ischemic stresses.

P/Q and N channels control baseline and spike-triggered calcium levels in neocortical axons and synaptic boutons

Cortical axons contain a diverse range of voltage-activated ion channels, including Ca(2+) currents. Interestingly, Ca(2+) channels are not only located at presynaptic terminals, but also in the axon initial segment (AIS), suggesting a potentially important role in the regulation of action potential generation and neuronal excitability. Here, using two-photon microscopy and whole-cell patch-clamp recording, we examined the properties and role of calcium channels located in the AIS and presynaptic terminals of ferret layer 5 prefrontal cortical pyramidal cells in vitro. Subthreshold depolarization of the soma resulted in an increase in baseline and spike-triggered calcium concentration in both the AIS and nearby synaptic terminals. The increase in baseline calcium concentration rose with depolarization and fell with hyperpolarization with a time constant of approximately 1 s and was blocked by removal of Ca(2+) from the bathing medium. The increases in calcium concentration at the AIS evoked by subthreshold or suprathreshold depolarization of the soma were blocked by the P/Q-channel antagonist omega-agatoxin IVA or the N-channel antagonist omega-conotoxin GVIA or both. The presence of these channels in the AIS pyramidal cells was confirmed with immunochemistry. Block of these channels slowed axonal action potential repolarization, apparently from reduction of the activation of a Ca(2+)-activated K(+) current, and increased neuronal excitability. These results demonstrate novel mechanisms by which calcium currents may control the electrophysiological properties of axonal spike generation and neurotransmitter release in the neocortex.

Back-Propagation of Physiological Action Potential Output in Dendrites of Slender-Tufted L5A Pyramidal Neurons

Pyramidal neurons of layer 5A are a major neocortical output type and clearly distinguished from layer 5B pyramidal neurons with respect to morphology, in vivo firing patterns, and connectivity; yet knowledge of their dendritic properties is scant. We used a combination of whole-cell recordings and Ca²⁺ imaging techniques in vitro to explore the specific dendritic signaling role of physiological action potential patterns recorded in vivo in layer 5A pyramidal neurons of the whisker-related ‘barrel cortex’. Our data provide evidence that the temporal structure of physiological action potential patterns is crucial for an effective invasion of the main apical dendrites up to the major branch point. Both the critical frequency enabling action potential trains to invade efficiently and the dendritic calcium profile changed during postnatal development. In contrast to the main apical dendrite, the more passive properties of the short basal and apical tuft dendrites prevented an efficient back-propagation. Various Ca²⁺ channel types contributed to the enhanced calcium signals during high-frequency firing activity, whereas A-type K⁺ and BK_Ca channels strongly suppressed it. Our data support models in which the interaction of synaptic input with action potential output is a function of the timing, rate and pattern of action potentials, and dendritic location.

BK channel and alcohol, a complicated affair

Alcohol is a fast acting molecule that alters behavior within a few minutes of absorption. Its rapid behavioral impact suggests early action on ion channels. Of all voltage-gated potassium ion channels, BK channels, a subcategory of potassium channels characterized by their large unitary conductance, and by their capacity of being activated synergistically by membrane potential and intracellular free calcium, are unique due to their high sensitivity to alcohol. In this review, we discuss BK channels structure and function, and how they help us understand the various ways BK channel mediates alcohol's effects on neuronal function and on behavior in the striatum.

The role of microRNAs in drug addiction: a big lesson from tiny molecules

Alcoholism is a multifactorial disease of unclear molecular underpinnings. Currently, we are witnessing a major shift in our understanding of the functional elements of the genome, which could help us to discover novel insights into the nature of alcoholism. In humans, the vast majority of the genome encodes non-protein-coding DNA with unclear function. Recent research has started to unveil this mystery by describing the functional relevance of microRNAs, and examining which genes are regulated by non-protein-coding DNA. Here, I describe alcohol regulation of microRNAs and provide examples of microRNAs that control the expression of alcohol-relevant genes. Emphasis is put on the potential of microRNAs in explaining the polygenic nature of alcoholism and prospects of microRNA research and future directions of this burgeoning field.

BK Channels: mediators and models for alcohol tolerance

Enhanced acute tolerance predicts alcohol abuse. We describe work on the role of the calcium- and voltage-gated BK channel in alcohol tolerance, highlighting the lipid environment, BK protein isoform selection and auxiliary BK channel proteins. We show how ethanol, which had the reputation of a nonspecific membrane perturbant, is now being examined at realistic concentrations with cutting-edge techniques, providing novel molecular targets for therapeutic approaches to alcoholism. Addictive disorders impact our emotional, physical and financial status, and burden our healthcare system. Although alcohol is the focus of this review, it is highly probable, given the common neural and biochemical pathways used by drugs of abuse, that the findings described here will also apply to other drugs.

Experimentally guided modelling of dendritic excitability in rat neocortical pyramidal neurones

Constructing physiologically relevant compartmental models of neurones is critical for understanding neuronal activity and function. We recently suggested that measurements from multiple locations along the soma, dendrites and axon are necessary as a data set when using a genetic optimization algorithm to constrain the parameters of a compartmental model of an entire neurone. However, recordings from L5 pyramidal neurones can routinely be performed simultaneously from only two locations. Now we show that a data set recorded from the soma and apical dendrite combined with a parameter peeling procedure is sufficient to constrain a compartmental model for the apical dendrite of L5 pyramidal neurones. The peeling procedure was tested on several compartmental models showing that it avoids local minima in parameter space. Based on the requirements of this analysis procedure, we designed and performed simultaneous whole-cell recordings from the soma and apical dendrite of rat L5 pyramidal neurones. The data set obtained from these recordings allowed constraining a simplified compartmental model for the apical dendrite of L5 pyramidal neurones containing four voltage-gated conductances. In agreement with experimental findings, the optimized model predicts that the conductance density gradients of voltage-gated K(+) conductances taper rapidly proximal to the soma, while the density gradient of the voltage-gated Na(+) conductance tapers slowly along the apical dendrite. The model reproduced the back-propagation of the action potential and the modulation of the resting membrane potential along the apical dendrite. Furthermore, the optimized model provided a mechanistic explanation for the back-propagation of the action potential into the apical dendrite and the generation of dendritic Na(+) spikes.