Differential regulation of nimodipine-sensitive and -insensitive Ca2+ influx by the Na+/Ca2+ exchanger and mitochondria in the rat suprachiasmatic nucleus neurons

Afterhyperpolarization potential modulated by local [K+]o in K+ diffusion-restricted extracellular space in the central clock of suprachiasmatic nucleus

BACKGROUND

Intercellular coupling is essential for the suprachiasmatic nucleus (SCN) to serve as a coherent central clock. Synaptic release of neurotransmitters and neuropeptides is critical for synchronizing SCN neurons. However, intercellular coupling via non-synaptic mechanisms has also been demonstrated. In particular, the abundant perikaryal appositions with morphological specializations in the narrow extracellular space (ECS) may hinder molecular diffusion to allow for ion accumulation or depletion.

METHODS

The SCN neurons were recorded in the whole-cell current-clamp mode, with pipette filled with high (26 mM)-Na+ or low (6 mM)-Na+ solution.

RESULTS

Cells recorded with high-Na+ pipette solution could fire spontaneous action potentials (AP) with peak AHP more negative than the calculated value of K+ equilibrium potential (EK) and with peak AP more positive than calculated ENa. Cells recorded with low-Na+ pipette solution could also have peak AHP more negative than calculated EK. In contrast, the resting membrane potential (RMP) was always less negative to calculated EK. The distribution and the averaged amplitude of peak AHP, peak AP, or RMP was similar between cells recorded with high-Na+ and low-Na+ solution pipette. In a number of cells, the peak AHP could increase from more positive to become more negative than calculated EK spontaneously or after treatments to hyperpolarize the RMP. TTX blocked the Na+ -dependent APs and tetraethylammonium (TEA), but not Ba2+ or Cd2+, markedly reduced the peak AHP. Perforated-patch cells could also but rarely fire APs with peak AHP more negative than calculated EK.

CONCLUSION

The result of peak AHP negative to calculated EK indicates that local [K+]o sensed by the TEA-sensitive AHP K+ channels must be lower than bulk [K+]o, most likely due to K+ clearance from K+ diffusion-restricted ECS by the Na+/K+-ATPase. The K+ diffusion-restricted ECS may allow for K+-mediated ionic interactions among neurons to regulate SCN excitability.

Clinical evaluation of Alprostadil combined with Nimodipine in treatment of Cerebral Vasospasm after Subarachnoid Hemorrhage in elderly patients

Objective

To analyze the clinical efficacy of alprostadil combined with nimodipine in the treatment of cerebral vasospasm (CVS) after subarachnoid hemorrhage (SAH) in elderly patients.

Methods

This is a retrospective study. According to different treatment methods, the elderly 100 patients with CVS after SAH hospitalized in Baoding First Central Hospital from March 2020 to May 2021 were randomly divided into control group and observation group, with 50 patients in each group. The control group was treated with nimodipine, while the observation group was additionally combined with alprostadil. The levels of inflammatory factors and hemorheological indexes were measured before and after treatment. The clinical efficacy was compared and the adverse reactions were observed of the two groups.

Results

The overall clinical efficacy in the observation group (95.00%) was significantly higher than that in the control group (74.00%) (p<0.05). After treatment, serum tumor necrosis factor-α (TNF-α), interleukin-8 (IL-8), high-sensitivity C-reactive protein (hs-CRP) and hemorheological indexes such as plasma viscosity, whole blood viscosity at high shear, whole blood viscosity at low shear, hematocrit and platelet adhesion decreased significantly compared with those before treatment (p<0.05), which were more obvious in the observation group (p<0.05). During treatment, the rate of adverse reactions in the observation group was 12.00%, and that in the control group was 8.00%, without statistically significant difference between the two groups (p> 0.05).

Conclusion

Alprostadil combined with nimodipine is markedly effective in the treatment of CVS after SAH in elderly patients. It can effectively reduce inflammatory factor levels and improve hemorheological indexes in patients, which is conducive to the repair of neurological function.

Glutamate-Evoked Ca2+ Responses in the Rat Suprachiasmatic Nucleus: Involvement of Na+/K+-ATPase and Na+/Ca2+-Exchanger

Glutamate mediates photic entrainment of the central clock in the suprachiasmatic nucleus (SCN) by evoking intracellular Ca2+ signaling mechanisms. However, the detailed mechanisms of glutamate-evoked Ca2+ signals are not entirely clear. Here, we used a ratiometric Ca2+ and Na+ imaging technique to investigate glutamate-evoked Ca2+ responses. The comparison of Ca2+ responses to glutamate (100 μM) and high (20 mM) K+ solution indicated slower Ca2+ clearance, along with rebound Ca2+ suppression for glutamate-evoked Ca2+ transients. Increasing the length of exposure time in glutamate, but not in 20 mM K+, slowed Ca2+ clearance and increased rebound Ca2+ suppression, a result correlated with glutamate-induced Na+ loads. The rebound Ca2+ suppression was abolished by ouabain, monensin, Na+-free solution, or nimodipine, suggesting an origin of activated Na+/K+-ATPase (NKA) by glutamate-induced Na+ loads. Ouabain or Na+-free solution also slowed Ca2+ clearance, apparently by retarding Na+/Ca2+-exchanger (NCX)-mediated Ca2+ extrusion. Together, our results indicated the involvement of glutamate-induced Na+ loads, NKA, and NCX in shaping the Ca2+ response to glutamate. Nevertheless, in the absence of external Na+ (NMDG substituted), Ca2+ clearance was still slower for the Ca2+ response to glutamate than for 20 mM K+, suggesting participation of additional Ca2+ handlers to the slower Ca2+ clearance under this condition.

Contributions of CaV1.3 Channels to Ca2+ Current and Ca2+-Activated BK Current in the Suprachiasmatic Nucleus

Daily regulation of Ca²⁺– and voltage-activated BK K⁺ channel activity is required for action potential rhythmicity in the suprachiasmatic nucleus (SCN) of the hypothalamus, the brain's circadian clock. In SCN neurons, BK activation is dependent upon multiple types of Ca²⁺ channels in a circadian manner. Daytime BK current predominantly requires Ca²⁺ influx through L-type Ca²⁺ channels (LTCCs), a time when BK channels are closely coupled with their Ca²⁺ source. Here we show that daytime BK current is resistant to the Ca²⁺ chelator BAPTA. However, at night when LTCCs contribute little to BK activation, BK current decreases by a third in BAPTA compared to control EGTA conditions. In phase with this time-of-day specific effect on BK current activation, LTCC current is larger during the day. The specific Ca²⁺ channel subtypes underlying the LTCC current in SCN, as well as the subtypes contributing the Ca²⁺ influx relevant for BK current activation, have not been identified. SCN neurons express two LTCC subtypes, Ca_V1.2 and Ca_V1.3. While a role for Ca_V1.2 channels has been identified during the night, Ca_V1.3 channel modulation has also been suggested to contribute to daytime SCN action potential activity, as well as subthreshold Ca²⁺ oscillations. Here we characterize the role of Ca_V1.3 channels in LTCC and BK current activation in SCN neurons using a global deletion of CACNA1D in mouse (Ca_V1.3 KO). Ca_V1.3 KO SCN neurons had a 50% reduction in the daytime LTCC current, but not total Ca²⁺ current, with no difference in Ca²⁺ current levels at night. During the day, Ca_V1.3 KO neurons exhibited oscillations in membrane potential, and most neurons, although not all, also had BK currents. Changes in BK current activation were only detectable at the highest voltage tested. These data show that while Ca_V1.3 channels contribute to the daytime Ca²⁺ current, this does not translate into a major effect on the daytime BK current. These data suggest that BK current activation does not absolutely require Ca_V1.3 channels and may therefore also depend on other LTCC subtypes, such as Ca_V1.2.

Comparative Ca2+ channel contributions to intracellular Ca2+ levels in the circadian clock

Circadian rhythms in mammals are coordinated by the central clock in the brain, located in the suprachiasmatic nucleus (SCN). Multiple molecular and cellular signals display a circadian variation within SCN neurons, including intracellular Ca2+, but the mechanisms are not definitively established. SCN cytosolic Ca2+ levels exhibit a peak during the day, when both action potential firing and Ca2+ channel activity are increased, and are decreased at night, correlating with a reduction in firing rate. In this study, we employ a single-color fluorescence anisotropy reporter (FLARE), Venus FLARE-Cameleon, and polarization inverted selective-plane illumination microscopy to measure rhythmic changes in cytosolic Ca2+ in SCN neurons. Using this technique, the Ca2+ channel subtypes contributing to intracellular Ca2+ at the peak and trough of the circadian cycle were assessed using a pharmacological approach with Ca2+ channel inhibitors. Peak (218 ± 16 nM) and trough (172 ± 13 nM) Ca2+ levels were quantified, indicating a 1.3-fold circadian variance in Ca2+ concentration. Inhibition of ryanodine-receptor-mediated Ca2+ release produced a larger relative decrease in cytosolic Ca2+ at both time points compared to voltage-gated Ca2+channels. These results support the hypothesis that circadian Ca2+ rhythms in SCN neurons are predominantly driven by intracellular Ca2+ channels, although not exclusively so. The study provides a foundation for future experiments to probe Ca2+ signaling in a dynamic biological context using FLAREs.

Glycolytic metabolism and activation of Na+ pumping contribute to extracellular acidification in the central clock of the suprachiasmatic nucleus: Differential glucose sensitivity and utilization between oxidative and non-oxidative glycolytic pathways

BACKGROUND

The central clock of the suprachiasmatic nucleus (SCN) controls the metabolism of glucose and is sensitive to glucose shortage. However, it is only beginning to be understood how metabolic signals such as glucose availability regulate the SCN physiology. We previously showed that the ATP-sensitive K⁺ channel plays a glucose-sensing role in regulating SCN neuronal firing at times of glucose shortage. Nevertheless, it is unknown whether the energy-demanding Na⁺/K⁺-ATPase (NKA) is also sensitive to glucose availability. Furthermore, we recently showed that the metabolically active SCN constantly extrudes H⁺ to acidify extracellular pH (pHe). This study investigated whether the standing acidification is associated with Na⁺ pumping activity, energy metabolism, and glucose utilization, and whether glycolysis- and mitochondria-fueled NKAs may be differentially sensitive to glucose shortage.

METHODS

Double-barreled pH-selective microelectrodes were used to determine the pHe in the SCN in hypothalamic slices.

RESULTS

NKA inhibition with K⁺-free (0-K⁺) solution rapidly and reversibly alkalinized the pHe, an effect abolished by ouabain. Mitochondrial inhibition with cyanide acidified the pHe but did not inhibit 0-K⁺-induced alkalinization. Glycolytic inhibition with iodoacetate alkalinized the pHe, completely blocked cyanide-induced acidification, and nearly completely blocked 0-K⁺-induced alkalinization. The results indicate that glycolytic metabolism and activation of Na⁺ pumping contribute to the standing acidification. Glucoprivation also alkalinized the pHe, nearly completely eliminated cyanide-induced acidification, but only partially reduced 0-K⁺-induced alkalinization. In contrast, hypoglycemia preferentially and partially blocked cyanide-induced acidification. The result indicates sensitivity to glucose shortage for the mitochondria-associated oxidative glycolytic pathway.

CONCLUSION

Glycolytic metabolism and activation of glycolysis-fueled NKA Na⁺ pumping activity contribute to the standing acidification in the SCN. Furthermore, the oxidative and non-oxidative glycolytic pathways differ in their glucose sensitivity and utilization, with the oxidative glycolytic pathway susceptible to glucose shortage, and the non-oxidative glycolytic pathway able to maintain Na⁺ pumping even in glucoprivation.

Ion Channels Controlling Circadian Rhythms in Suprachiasmatic Nucleus Excitability

Animals synchronize to the environmental day-night cycle by means of an internal circadian clock in the brain. In mammals, this timekeeping mechanism is housed in the suprachiasmatic nucleus (SCN) of the hypothalamus and is entrained by light input from the retina. One output of the SCN is a neural code for circadian time, which arises from the collective activity of neurons within the SCN circuit and comprises two fundamental components: 1) periodic alterations in the spontaneous excitability of individual neurons that result in higher firing rates during the day and lower firing rates at night, and 2) synchronization of these cellular oscillations throughout the SCN. In this review, we summarize current evidence for the identity of ion channels in SCN neurons and the mechanisms by which they set the rhythmic parameters of the time code. During the day, voltage-dependent and independent Na⁺ and Ca²⁺ currents, as well as several K⁺ currents, contribute to increased membrane excitability and therefore higher firing frequency. At night, an increase in different K⁺ currents, including Ca²⁺-activated BK currents, contribute to membrane hyperpolarization and decreased firing. Layered on top of these intrinsically regulated changes in membrane excitability, more than a dozen neuromodulators influence action potential activity and rhythmicity in SCN neurons, facilitating both synchronization and plasticity of the neural code.

Role of Intracellular Na+ in the Regulation of [Ca2+]i in the Rat Suprachiasmatic Nucleus Neurons

Transmembrane Ca²⁺ influx is essential to the proper functioning of the central clock in the suprachiasmatic nucleus (SCN). In the rat SCN neurons, the clearance of somatic Ca²⁺ following depolarization-induced Ca²⁺ transients involves Ca²⁺ extrusion via Na⁺/Ca²⁺ exchanger (NCX) and mitochondrial Ca²⁺ buffering. Here we show an important role of intracellular Na⁺ in the regulation of [Ca²⁺]_i in these neurons. The effect of Na⁺ loading on [Ca²⁺]_i was determined with the Na⁺ ionophore monensin and the cardiac glycoside ouabain to block Na⁺/K⁺-ATPase (NKA). Ratiometric Na⁺ and Ca²⁺ imaging was used to measure the change in [Na⁺]_i and [Ca²⁺]_i, and cell-attached recordings to investigate the effects of monensin and ouabain on spontaneous firing. Our results show that in spite of opposite effects on spontaneous firing and basal [Ca²⁺], both monensin and ouabain induced Na⁺ loading, and increased the peak amplitude, slowed the fast decay rate, and enhanced the slow decay phase of 20 mM K⁺-evoked Ca²⁺ transients. Furthermore, both ouabain and monensin preferentially enhanced nimodipine-insensitive Ca²⁺ transients. Together, our results indicate that in the SCN neurons the NKA plays an important role in regulating [Ca²⁺]_i, in particular, associated with nimodipine-insensitive Ca²⁺ channels.

The Na+/H+-Exchanger NHE1 Regulates Extra- and Intracellular pH and Nimodipine-sensitive [Ca2+]i in the Suprachiasmatic Nucleus

The central clock in the suprachiasmatic nucleus (SCN) has higher metabolic activity than extra-SCN areas in the anterior hypothalamus. Here we investigated whether the Na⁺/H⁺ exchanger (NHE) may regulate extracellular pH (pHe), intracellular pH (pHi) and [Ca²⁺]_i in the SCN. In hypothalamic slices bathed in HEPES-buffered solution a standing acidification of ~0.3 pH units was recorded with pH-sensitive microelectrodes in the SCN but not extra-SCN areas. The NHE blocker amiloride alkalinised the pHe. RT-PCR revealed mRNA for plasmalemmal-type NHE1, NHE4, and NHE5 isoforms, whereas the NHE1-specific antagonist cariporide alkalinised the pHe. Real-time PCR and western blotting failed to detect day-night variation in NHE1 mRNA and protein levels. Cariporide induced intracellular acidosis, increased basal [Ca²⁺]_i, and decreased depolarisation-induced Ca²⁺ rise, with the latter two effects being abolished with nimodipine blocking the L-type Ca²⁺ channels. Immunofluorescent staining revealed high levels of punctate colocalisation of NHE1 with serotonin transporter (SERT) or CaV1.2, as well as triple staining of NHE1, CaV1.2, and SERT or the presynaptic marker Bassoon. Our results indicate that NHE1 actively extrudes H⁺ to regulate pHi and nimodipine-sensitive [Ca²⁺]_i in the soma, and along with CaV1.2 may also regulate presynaptic Ca²⁺ levels and, perhaps at least serotonergic, neurotransmission in the SCN.