Developmental expression of the small-conductance Ca(2+)-activated potassium channel SK2 in the rat retina

Small-conductance Ca(2+)-activated potassium (SK) channels are present in most central neurons, where they mediate the afterhyperpolarizations (AHPs) following action potentials. SK channels integrate changes in intracellular Ca(2+) concentration with membrane potential and thus play an important role in controlling firing pattern and excitability. Here, we characterize the expression pattern of the apamin-sensitive SK subunits, SK2 and SK3, in the developing and adult rat retina using in situ hybridization and immunohistochemistry. The SK2 subunit showed a distinct and developmentally regulated pattern of expression. It appeared during the first postnatal week and located to retinal ganglion cells and to subpopulations of neurons in the inner nuclear layer. These neurons were identified as horizontal cells and dopaminergic amacrine cells by specific markers. In contrast to SK2, the SK3 subunit was detected neither in the developing nor in the adult retina. These results show cell-specific expression of the SK2 subunit in the retina and suggest that this channel underlies the apamin-sensitive AHP currents described in retinal ganglion cells.     

Activation of SK channels inhibits epileptiform bursting in hippocampal CA3 neurons

The role of calcium-activated potassium channels in the regulation of neuronal hyperexcitability, as in epilepsy, is unclear. To examine this issue, we have used the acute hippocampal slice model of epileptiform activity to investigate the effects of an enhancer of SK channel activity, 1-ethyl-benzimidazolinone (EBIO). That EBIO is an SK channel modulator was confirmed by its potentiation of hSK1, hSK2, hSK3 and hIK currents (EC(50) values in the range of 130-870 microM) and its apamin (1 microM) sensitive reduction of the number of action potentials fired in CA3 pyramidal neurons in response to a depolarizing current step. In addition, while EBIO did not significantly affect electrically evoked glutamatergic synaptic transmission, it did inhibit epileptiform activity (IC(50) values in the range of 150-325 microM) induced by (1) modifying the extracellular ionic environment by removing extracellular Mg(2+) or elevating extracellular K(+) from 3.0 to 8.5 mM and (2) disinhibiting the slice using 3 mM pentylenetetrazol or combined application of 10 microM gabazine and 10 microM CGP55845. Furthermore, its inhibitory effect in the full disinhibition model of epileptiform activity (10 microM gabazine + 10 microM CGP55845) was occluded by the SK channel blocker apamin (300 nM-1 microM) which in its own right increased the duration and reduced the frequency of individual epileptiform bursts. In conclusion, compounds that enhance the activation of small conductance Ca(2+) -activated K(+) channels are effective inhibitors of epileptiform activity in vitro.     

Suppression of a slow post-spike afterhyperpolarization by calcineurin inhibitors

A subset of myenteric neurons in the intestine (AH neurons) generate prolonged (>5 s) post-spike afterhyperpolarizations (slow AHPs) that are insensitive to apamin and tetraethylammonium. Generation of slow AHPs depends critically on Ca(2+) entry and intracellular release of Ca(2+) from stores, which then leads to the activation of a K(+) conductance that underlies the slow AHP (g(sAHP)). Slow AHPs are inhibited by stimulation of the cAMP/protein kinase A (PKA) pathway, suggesting that phosphorylation of the K(+)-channels that mediate the g(sAHP) (K(sAHP)-channels) is responsible for suppression of slow AHPs and possibly for the repolarization phase of slow AHPs. In the present study, we investigated the possibility that the rising phase of the slow AHP is mediated by dephosphorylation of K(sAHP)-channels by calcineurin (CaN), a Ca(2+)-calmodulin-dependent protein phosphatase, leading to an increase in g(sAHP) and activation of the associated current I(sAHP). Slow AHPs and I(sAHP) were recorded using conventional recording techniques, and we tested the actions of two inhibitors of CaN, FK506 and cyclosporin A, and also the effect of the CaN autoinhibitory peptide applied intracellularly, on these events. We report here that all three treatments inhibited the slow AHP and I(sAHP) (>70%) without significantly affecting the ability of neurons to fire action potentials. In addition, the slow AHP and I(sAHP) were suppressed by okadaic acid, an inhibitor of protein phosphatases 1 and 2A. Our results indicate that activation of the g(sAHP) that underlies the post-depolarization slow AHPs in AH neurons is mediated by the actions CaN and non-Ca(2+)-dependent phosphatases.     

SK channel blockade promotes burst firing in dorsal raphe serotonergic neurons

Previous in vivo studies have shown that blockade of small-conductance Ca(2+)-activated potassium (SK) channels enhances burst firing in dopaminergic neurons. As bursting has been found to be physiologically relevant for the synaptic release of serotonin (5-HT), we investigated the possible role of SK channels in the control of this firing pattern in 5-HT neurons of the dorsal raphe nucleus. In these cells, bursts are usually composed of doublets consisting of action potentials separated by a small interval (< 20 ms). Both in vivo and in vitro extracellular recordings were performed, using anesthetized rats and rat brain slices, respectively. In vivo, the specific SK blocker UCL 1684 (200 microm) iontophoresed onto presumed 5-HT neurons significantly increased the production of bursts in 13 out of 25 cells. Furthermore, the effect of UCL 1684 persisted in the presence of both the GABA(A) antagonist SR 95531 (10 mm) and the GABA(B) antagonist CGP 35348 (10 mm), whereas these agents by themselves did not significantly influence the neuronal firing pattern. In vitro, bath superfusion of the SK channel blocker apamin (300 nm) induced bursting in only three out of 18 neurons, although it increased the coefficient of variation of the interspike intervals in all the other cells. Our results suggest that SK channel blockade promotes bursting activity in 5-HT neurons via a direct action. An input which is present only in vivo seems to be important for the induction of this firing pattern in these cells.     

Paraoxon suppresses Ca(2+) spike and afterhyperpolarization in snail neurons: Relevance to the hyperexcitability induction

The effects of organophosphate (OP) paraoxon, active metabolite of parathion, were studied on the Ca(2+) and Ba(2+) spikes and on the excitability of the neuronal soma membranes of land snail (Caucasotachea atrolabiata). Paraoxon (0.3 muM) reversibly decreased the duration and amplitude of Ca(2+) and Ba(2+) spikes. It also reduced the duration and the amplitude of the afterhyperpolarization (AHP) that follows spikes, leading to a significant increase in the frequency of Ca(2+) spikes. Pretreatment with atropine and hexamethonium, selective blockers of muscarinic and nicotinic receptors, respectively, did not prevent the effects of paraoxon on Ca(2+) spikes. Intracellular injection of the calcium chelator BAPTA dramatically decreased the duration and amplitude of AHP and increased the duration and frequency of Ca(2+) spikes. In the presence of BAPTA, paraoxon decreased the duration of the Ca(2+) spikes without affecting their frequency. Apamin, a neurotoxin from bee venom, known to selectively block small conductance of calcium-activated potassium channels (SK), significantly decreased the duration and amplitude of the AHP, an effect that was associated with an increase in spike frequency. In the presence of apamin, bath application of paraoxon reduced the duration of Ca(2+) spike and AHP and increased the firing frequency of nerve cells. In summary, these data suggest that exposure to submicromolar concentration of paraoxon may directly affect membrane excitability. Suppression of Ca(2+) entry during the action potential would down regulate Ca(2+)-activated K(+) channels leading to a reduction of the AHP and an increase in cell firing.     

Effect of charybdotoxin and leiurotoxin I on potassium currents in bullfrog sympathetic ganglion and hippocampal neurons.

The effects of charybdotoxin and leiurotoxin I were examined on several classes of K+ currents in bullfrog sympathetic ganglion and hippocampal CA1 pyramidal neurons. Highly purified preparations of charybdotoxin selectively blocked a large voltage- and Ca(2+)-dependent K+ current (IC) responsible for action potential repolarization (IC50 = 6 nM) while leiurotoxin I selectively blocked a small Ca(2+)-dependent K+ conductance (IAHP) responsible for the slow afterhyperpolarization following an action potential (IC50 = 7.5 nM) in bullfrog sympathetic ganglion neurons. Neither of the toxins had significant effects on other K+ currents (M-current [IM], A-current [IA] and the delayed rectifier [IK]) present in these cells. Leiurotoxin I at a concentration of 20 nM had no detectable effect on currents in hippocampal CA1 pyramidal neurons. This lack of effect on IAHP in central neurons suggests that the channels underlying slow AHPs in those neurons are pharmacologically distinct from analogous channels in peripheral neurons.     

Tuning the excitability of midbrain dopamine neurons by modulating the Ca2+ sensitivity of SK channels

Small conductance Ca²⁺ -activated K⁺ (SK) channels play a prominent role in modulating the spontaneous activity of dopamine (DA) neurons as well as their response to synaptically-released glutamate. SK channel gating is dependent on Ca²⁺ binding to constitutively bound calmodulin, which itself is subject to endogenous and exogenous modulation. In the present study, patch-clamp recording techniques were used to examine the relationship between the apparent Ca²⁺ affinity of cloned SK3 channels expressed in cultured human embryonic kidney 293 cells and the excitability of DA neurons in slices from rat substantia nigra using the positive and negative SK channel modulators, 6,7-dichloro-1 H -indole-2,3-dione-3-oxime and R- N -(benzimidazol-2-yl)-1,2,3,4-tetrohydro-1-naphtylamine. Increasing the apparent Ca²⁺ affinity of SK channels decreased the responsiveness of DA neurons to depolarizing current pulses, enhanced spike frequency adaptation and slowed spontaneous firing, effects attributable to an increase in the amplitude and duration of an apamin-sensitive afterhyperpolarization. In contrast, decreasing the apparent Ca²⁺ affinity of SK channels enhanced DA neuronal excitability and changed the firing pattern from a pacemaker to an irregular or bursting discharge. Both the reduction in apparent Ca²⁺ affinity and the bursting associated with negative SK channel modulation were gradually surmounted by co-application of the positive SK channel modulator. These results underscore the importance of SK channels in 'tuning' the excitability of DA neurons and demonstrate that gating modulation, in a manner analogous to physiological regulation of SK channels in vivo , represents a means of altering the response of DA neurons to membrane depolarization.     

SK channels modulate the excitability and firing precision of projection neurons in the robust nucleus of the arcopallium in adult male zebra finches

Objective Motor control is encoded by neuronal activity. Small conductance Ca2+-activated K+ channels (SK channels) maintain the regularity and precision of firing by contributing to the afterhyperpolarization (AHP) of the action potential in mammals. However, it is not clear how SK channels regulate the output of the vocal motor system in songbirds. The premotor robust nucleus of the arcopallium (RA) in the zebra finch is responsible for the output of song information. The temporal pattern of spike bursts in RA projection neurons is associated with the timing of the acoustic features of birdsong. Methods The firing properties of RA projection neurons were analyzed using patch clamp whole-cell and cell-attached recording techniques. Results SK channel blockade by apamin decreased the AHP amplitude and increased the evoked firing rate in RA projection neurons. It also caused reductions in the regularity and precision of firing. RA projection neurons displayed regular spontaneous action potentials, while apamin caused irregular spontaneous firing but had no effect on the firing rate. In the absence of synaptic inputs, RA projection neurons still had spontaneous firing, and apamin had an evident effect on the firing rate, but caused no significant change in the firing regularity, compared with apamin application in the presence of synaptic inputs. Conclusion SK channels contribute to the maintenance of firing regularity in RA projection neurons which requires synaptic activity, and consequently ensures the precision of song encoding.     

Learning-induced modulation of SK channels-mediated effect on synaptic transmission

Although small conductance (SK)-mediated calcium-dependent potassium currents are usually mostly thought to modulate neuronal adaptation by suppressing repetitive spike firing, recent evidence suggests that these channels also modulate synaptic transmission. SK2 channels were shown to be activated in dendritic spines following calcium entry via N-methyl-d-aspartate (NMDA) receptor. Such activation of potassium currents terminates the NMDA-dependent postsynaptic potential (PSP). Synaptic potentials in pyramidal neurons in the piriform cortex from olfactory-discrimination-trained rats have enhanced rise time 3 days after learning, and their dendritic spines are significantly smaller at this time. In the present study we examined whether the SK channel-mediated effect on PSPs is modified after learning. The SK channels inhibitor, apamin, that selectively blocks the SK channels-mediated potassium currents enhanced the width of the PSP in neurons from trained rats only. This effect is abolished in the presence of the NMDA-channel blocker, APV. The learning-induced reduction in paired-pulse facilitation was not affected by apamin. Although the effect of the SK channels is increased after learning, the protein expression level of the SK2 channels, the type located in dendritic spines, was decreased after learning. The protein expression level of the SK3 channel, suggested to be located mainly in axon terminals, was not modified by learning. We suggest that the enhanced effect of the SK channels on NMDA-mediated synaptic transmission is the result of the reduction in the spine volume after learning. Moreover, these data indicate that spines are more excitable after learning, and are thus more predisposed to activity-dependent modifications.     

SK channels are necessary but not sufficient for denervation-induced hyperexcitability

Skeletal muscle hyperexcitability is characteristically associated with denervation. Expression of SK3, a small conductance Ca(2+)-activated K(+) channel (SK channel) in skeletal muscle is induced by denervation, and direct application of apamin, a peptide blocker of SK channels, dramatically reduces hyperexcitability. To investigate the role of SK3 channels in denervation- induced hyperexcitability, SK3 expression was manipulated using a transgenic mouse that harbors a tetracycline-regulated SK3 gene. Electromyographic (EMG) recordings from anterior tibial (AT) muscle showed that denervated muscle from transgenic or wild-type animals had equivalent hyperexcitability that was blocked by apamin. In contrast, denervated skeletal muscle from SK3tTA mice lacking SK3 channels showed little or no hyperexcitability, similar to results from wild-type innervated skeletal muscle. However, innervated skeletal muscle from SK3tTA mice containing SK3 channels did not show hyperexcitability. The results demonstrate that SK3 channels are necessary but not sufficient for denervation-induced skeletal muscle hyperexcitability.     

Differential distribution of SK channel subtypes in the brain of the weakly electric fish Apteronotus leptorhynchus

Calcium signals in vertebrate neurons can induce hyperpolarizing membrane responses through the activation of Ca(2+)-activated potassium channels. Of these, small conductance (SK) channels regulate neuronal responses through the generation of the medium after-hyperpolarization (mAHP). We have previously shown that an SK channel (AptSK2) contributes to signal processing in the electrosensory system of Apteronotus leptorhynchus. It was shown that for pyramidal neurons in the electrosensory lateral line lobe (ELL), AptSK2 expression selectively decreases responses to low-frequency signals. The localization of all the SK subunits throughout the brain of Apteronotus then became of substantial interest. We have now cloned two additional SK channel subunits from Apteronotus and determined the expression patterns of all three AptSK subunits throughout the brain. In situ hybridization experiments have revealed that, as in mammalian systems, the AptSK1 and 2 channels showed a partially overlapping expression pattern, whereas the AptSK3 channel was expressed in different brain areas. The AptSK1 and 2 channels were the primary subunits found in the major electrosensory processing areas. Immunohistochemistry further revealed distinct compartmentalization of the AptSK1 and 2 channels in the ELL. AptSK1 was localized to the apical dendrites of pyramidal neurons, whereas AptSK2 channels are primarily somatic. The distinct expression patterns of all three AptSK channels may reflect subtype-specific contributions to neuronal function, and the high homology between subtypes from a number of species suggests that the functional roles for each channel subtype are conserved from early vertebrate evolution.     

mGluR1 agonists elicit a Ca 2+ signal and membrane hyperpolarization mediated by apamin-sensitive potassium channels in immature rat purkinje neurons

The type 1 metabotropic glutamate receptor (mGluR1) plays an import role in the synaptic physiology and development of cerebellar Purkinje neurons. mGluR1 expression occurs early in the developmental program of Purkinje neurons, at an age that precedes expression of the dendritic structure. Few studies have investigated the physiological response produced by mGluR1 activation in early-developing Purkinje neurons. To address this question, simultaneous recording of membrane potential and intracellular Ca(2+) was performed in immature cultured Purkinje neurons coupled with exogenous application of mGluR1 agonists. Membrane potential was measured using the perforated patch method of whole-cell recording, and intracellular Ca(2+) was measured using fura-2-based Ca(2+) imaging. Brief, 1-sec micropressure application of the group I mGluR-selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) evoked a prominent Ca(2+) signal and coincident fast hyperpolarization in the immature neurons. The mGluR1-selective antagonist 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester blocked the Ca(2+) signal and fast hyperpolarization, confirming the involvement of mGluR1s. Amplitude of the fast hyperpolarization varied as a function of membrane potential and intracellular Ca(2+) and was blocked by apamin, an antagonist of the small-conductance Ca(2+)-activated K(+) channel (SK), identifying this K(+) channel as an underlying mechanism. In similar experiments with mature cultured Purkinje neurons, DHPG elicited a Ca(2+) signal, but fast membrane hyperpolarization was not evident. These results suggest that mGluR1 activation and the resulting release of Ca(2+) from intracellular stores and activation of SK channels may be a mechanism through which mGluR1 can modulate neuronal excitability of Purkinje neurons during early development.     

Quantitative expression analysis of the small conductance calcium-activated potassium channels, SK1, SK2 and SK3, in human brain

Small conductance calcium-activated potassium (SK) channels are important in controlling neuronal excitability and three SK channels have been identified to date. In the present study, we report the first quantitative analysis of SK1, SK2 and SK3 expression in human brain using TaqMan RT-PCR on a range of human brain and peripheral tissue samples. SK1 expression is restricted to the brain whereas SK2 and SK3 are more widely expressed.     

Role of L-type Ca2+ channels in neural stem/progenitor cell differentiation

Ca(2+) influx through voltage-gated Ca(2+) channels, especially the L-type (Ca(v)1), activates downstream signaling to the nucleus that affects gene expression and, consequently, cell fate. We hypothesized that these Ca(2+) signals may also influence the neuronal differentiation of neural stem/progenitor cells (NSCs) derived from the brain cortex of postnatal mice. We first studied Ca(2+) transients induced by membrane depolarization in Fluo 4-AM-loaded NSCs using confocal microscopy. Undifferentiated cells (nestin(+)) exhibited no detectable Ca(2+) signals whereas, during 12 days of fetal bovine serum-induced differentiation, neurons (beta-III-tubulin(+)/MAP2(+)) displayed time-dependent increases in intracellular Ca(2+) transients, with DeltaF/F ratios ranging from 0.4 on day 3 to 3.3 on day 12. Patch-clamp experiments revealed similar correlation between NSC differentiation and macroscopic Ba(2+) current density. These currents were markedly reduced (-77%) by Ca(v)1 channel blockade with 5 microm nifedipine. To determine the influence of Ca(v)1-mediated Ca(2+) influx on NSC differentiation, cells were cultured in differentiative medium with either nifedipine (5 microm) or the L-channel activator Bay K 8644 (10 microm). The latter treatment significantly increased the percentage of beta-III-tubulin(+)/MAP2(+) cells whereas nifedipine produced opposite effects. Pretreatment with nifedipine also inhibited the functional maturation of neurons, which responded to membrane depolarization with weak Ca(2+) signals. Conversely, Bay K 8644 pretreatment significantly enhanced the percentage of responsive cells and the amplitudes of Ca(2+) transients. These data suggest that NSC differentiation is strongly correlated with the expression of voltage-gated Ca(2+) channels, especially the Ca(v)1, and that Ca(2+) influx through these channels plays a key role in promoting neuronal differentiation.     

Calcium activates two types of potassium channels in rat hippocampal neurons in culture

Several calcium-dependent potassium currents can contribute to the electrophysiological properties of neurons. In hippocampal pyramidal cells, 2 afterhyperpolarizations (AHPs) are mediated by different calcium-activated potassium currents. First, a rapidly activated current contributes to action-potential repolarization and the fast AHP following individual action potentials. In addition, a slowly developing current underlies the slow AHP, which occurs after a burst of action potentials and contributes substantially to the spike-frequency accommodation observed in these cells during a prolonged depolarizing current pulse. In order to investigate the single Ca2(+)-dependent channels that might underlie these currents, we performed patch-clamp experiments on hippocampal neurons in primary culture. When excised inside-out patches were exposed to 1 microM Ca2+, 2 types of channel activity were observed. In symmetrical bathing solutions containing 140 mM K+, the channels had conductances of 19 pS and 220 pS, and both were permeable mainly to potassium ions. The properties of these 2 channels differed in a number of ways. At negative membrane potentials, the small-conductance channels were more sensitive to Ca2+ than the large channels. At positive potentials, the small-conductance channels displayed a flickery block by Mg2+ ions on the cytoplasmic face of the membrane. Low concentrations of tetraethylammonium (TEA) on the extracellular face of the membrane specifically caused an apparent reduction of the large-channel conductance. The properties of the large- and small-conductance channels are in accord with those of the fast and slow AHP, respectively.     

Intrinsic membrane potential oscillations in hippocampal neurons in vitro

Membrane potential oscillations (MPOs) of 2-10 Hz and up to 6 mV were found in almost all stable hippocampal CA1 and CA3 neurons in the in vitro slice preparation. MPOs were prominent for pyramidal cells but less pronounced in putative interneurons. MPOs were activated at threshold depolarizations that evoked a spike and the frequency of the MPOs increased with the level of depolarization. MPOs were distinct from and seemed to regulate spiking, with a spike often riding near the top of a depolarizing MPO wave. Analysis of the periodicity of the oscillations indicate that the period of MPOs did not depend on the afterhyperpolarization (AHP) following a single spike. MPOs persisted in low (0-0.1 mM) Ca2+ medium, with or without Cd2+ (0.2 mM), when synaptic transmission was blocked. Choline-substituted low-Na+ (0-26 mM) medium, 3 microM tetrodotoxin (TTX) or intracellular injection of QX-314 reduced or abolished the fast Na(+)-spike and reduced inward anomalous rectification. About 40% of CA1 neurons had no MPOs after Na+ currents were blocked, suggesting that these MPOs were Na(+)-dependent. In about 60% of the cells, a large depolarization activated Ca(2+)-dependent MPOs and slow spikes. MPOs were not critically affected by extracellular Ba2+ or Cs2+, or by 0.2 mM 4-aminopyridine, with or without 2 mM tetraethylammonium (TEA). However, in 5-10 mM TEA medium, MPOs were mostly replaced by 0.2-3 Hz spontaneous bursts of wide-duration spikes followed by large AHPs. Low Ca2+, Cd2+ medium greatly reduced the spike width but not the spike-bursts. In conclusion, each cycle of an MPO in normal medium probably consists of a depolarization phase mediated by Na+ currents, possibly mixed with Ca2+ currents activated at a higher depolarization. The repolarization/hyperpolarization phase may be mediated by Na+/Ca2+ current inactivation and partly by TEA-sensitive, possibly the delayed rectifier, K+ currents. The presence of prominent intrinsic, low-threshold MPOs in all hippocampal pyramidal neurons suggests that MPOs may play an important role in information processing in the hippocampus.     

SK channel function regulates the dopamine phenotype of neurons in the substantia nigra pars compacta

Parkinson's disease (PD) is characterized by loss of dopaminergic (DAergic) neurons in the substantia nigra pars compacta (SNc). It is widely believed that replacing lost SNc DA neurons is a key to longer-term effective treatment of PD motor symptoms, but generating new SNc DA neurons in PD patients has proven difficult. Following loss of tyrosine hydroxylase-positive (TH+) SNc neurons in the rodent 6-hydroxy-DA (6-OHDA) model of PD, the number of TH+ neurons partially recovers and there is evidence this occurs via phenotype “shift” from TH− to TH+ cells. Understanding how this putative phenotype shift occurs may help increase SNc DAergic neurons in PD patients. In this study we characterize the electrophysiology of SNc TH− and TH+ cells during recovery from 6-OHDA in mice. Three distinct phenotypes were observed: (1) TH− were fast discharging with a short duration action potential (AP), short afterhyperpolarization (AHP) and no small conductance Ca²⁺-activated K⁺ (SK) current; (2) TH+ were slow discharging with a long AP, long AHP and prominent SK current; and (3) cells with features “intermediate” between these TH− and TH+ phenotypes. The same 3 phenotypes were present also in the normal and D2 DA receptor knock-out SNc suggesting they are more closely related to the biology of TH expression than recovery from 6-OHDA. Acute inhibition of SK channel function shifted the electrophysiological phenotype of TH+ neurons toward TH− and chronic (2 weeks) inhibition of SK channel function in normal mice shifted the neurochemical phenotype of SNc from TH+ to TH− (i.e. decreased TH+ and increased TH− cell numbers). Importantly, chronic facilitation of SK channel function shifted the neurochemical phenotype of SNc from TH− to TH+ (i.e. increased TH+ and decreased TH− cell numbers). We conclude that SK channel function bidirectionally regulates the DA phenotype of SNc cells and facilitation of SK channels may be a novel way to increase the number of SNc DAergic neurons in PD patients.