Noise of the slowly inactivating Na current in suprachiasmatic nucleus neurons

Rapidly constructed scaffold-free embryonic stem cell sheets for ocular surface reconstruction

This study is an extension of our previously published work demonstrating the generation of corneal epithelial sheets using optimal centrifugation procedure. In this study, we investigated the therapeutic potential of embryonic stem cells (ESCs) for ocular surface reconstruction in rabbits with limbal stem cell deficiency (LSCD) using the centrifugation method. Rabbits with LSCD were assigned to two groups: 30 LSCD rabbits were treated with scaffold-free embryonic stem cells sheets (SESCS) and six LSCD rabbits received no treatment (control group). The two groups were followed up for 15 days using slit lamp observation. Cytokeratin K3, mucin 5AC, and OCT-4 were used to evaluate the re-epithelialization of the cornea. Fluorescent DiIC18(3)-DS dye was used to trace the transplanted cells. Atomic force microscopy (AFM) was used to investigate the cultured epithelial cells of the SESCS-treated group. The SESCS transplant facilitated the reconstruction of the ocular surface in 75% of the treated animals. Conjunctivalization and neovascularization were observed in the control group. The SESCS group was K3 positive and MUC5AC negative, and OCT-4 was observed on the re-epithelialized corneal epithelium. Labeled SESCS cells were detected in vivo at 15 days post-transplant. Using AFM, three different types of cultured cells were identified in the rabbit corneal epithelium of the SESCS treatment group. The SESCS were demonstrated to reconstruct the ocular surface in rabbits with LSCD. ES cells differentiated into corneal epithelial cells when in direct contact with the stroma and thus can serve as a cell source in corneal tissue engineering.

Light exposure induces short- and long-term changes in the excitability of retinorecipient neurons in suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) is the locus of a hypothalamic circadian clock that synchronizes physiological and behavioral responses to the daily light-dark cycle. The nucleus is composed of functionally and peptidergically diverse populations of cells for which distinct electrochemical properties are largely unstudied. SCN neurons containing gastrin-releasing peptide (GRP) receive direct retinal input via the retinohypothalamic tract. We targeted GRP neurons with a green fluorescent protein (GFP) marker for whole cell patch-clamping. In these neurons, we studied short (0.5-1.5 h)- and long-term (2-6 h) effects of a 1-h light pulse (LP) given 2 h after lights off [Zeitgeber time (ZT) 14:00-15:00] on membrane potential and spike firing. In brain slices taken from light-exposed animals, cells were depolarized, and spike firing rate increased between ZT 15:30 and 16:30. During a subsequent 4-h period beginning around ZT 17:00, GRP neurons from light-exposed animals were hyperpolarized by ∼15 mV. None of these effects was observed in GRP neurons from animals not exposed to light or in immediately adjacent non-GRP neurons whether or not exposed to light. Depolarization of GRP neurons was associated with a reduction in GABA(A)-dependent synaptic noise, whereas hyperpolarization was accompanied both by a loss of GABA(A) drive and suppression of a TTX-resistant leakage current carried primarily by Na. This suggests that, in the SCN, exposure to light may induce a short-term increase in GRP neuron excitability mediated by retinal neurotransmitters and neuropeptides, followed by long-term membrane hyperpolarization resulting from suppression of a leakage current, possibly resulting from genomic signals.

Enhanced functional properties of corneal epithelial cells by coculture with embryonic stem cells via the integrin β1-FAK-PI3K/Akt pathway

Adult stem cells are important cell sources in regenerative medicine, but isolating them is technically challenging. This study employed a novel strategy to generate stem-like corneal epithelial cells and promote the functional properties of these cells by coculture with embryonic stem cells. The primary corneal epithelial cells were labelled with GFP and cocultured with embryonic stem cells in a transwell or by direct cell-cell contact. The embryonic stem cells were pre-transfected with HSV-tk-puro plasmids and became sensitive to ganciclovir. After 10 days of coculture, the corneal epithelial cells were isolated by treating the cultures with ganciclovir to kill the embryonic stem cells. The expression of stem cell-associated markers (ABCG2, p63) increased whereas the differentiation mark (Keratin 3) decreased in corneal epithelial cells isolated from the cocultures as evaluated by RT-PCR and flow cytometry. Their functional properties of corneal epithelial cells, including cell adhesion, migration and proliferation, were also enhanced. These cells could regenerate a functional stratified corneal epithelial equivalent but did not form tumors. Integrin β1, phosphorylated focal adhesion kinase and Akt were significantly upregulated in corneal epithelial cells. FAK Inhibitor 14 that suppressed the expression of phosphorylated focal adhesion kinase and Akt inhibited cell adhesion, migration and proliferation. LY294002 that suppressed phosphorylated Akt but not phosphorylated focal adhesion kinase inhibited cell proliferation but had no effect on cell adhesion or migration. These findings demonstrated that the functional properties of stem-like corneal epithelial cells were enhanced by cocultured embryonic stem cells via activation of the integrin β1-FAK-PI3K/Akt signalling pathway.

Modeling the electrophysiology of suprachiasmatic nucleus neurons

Neurons in the SCN act as the central circadian (approximately 24-h) pacemaker in mammals. Using measurements of the ionic currents in SCN neurons, the authors fit a Hodgkin-Huxley-type model that accurately reproduces slow (approximately 28 Hz) neural firing as well as the contributions of ionic currents during an action potential. When inputs of other SCN neurons are considered, the model accurately predicts the fractal nature of firing rates and the appearance of random bursting. In agreement with experimental data, the molecular clock within these neurons modulates the firing rate through small changes in the concentration of internal calcium, calcium channels, or potassium channels. Predictions are made on how signals from other neurons can start, stop, speed up, or slow down firing. Only a slow sodium inactivation variable and voltage do not reach equilibrium during the interval between action potentials, and based on this finding, a reduced model is formulated.

Lamotrigine and phenytoin, but not amiodarone, impair peripheral chemoreceptor responses to hypoxia

Amiodarone, lamotrigine, and phenytoin, common antiarrhythmic and antiepileptic drugs, inhibit a persistent sodium current in neurons (I(NaP)). Previous results from our laboratory suggested that I(NaP) is critical for functionality of peripheral chemoreceptors. In this study, we determined the effects of therapeutic levels of amiodarone, lamotrigine, and phenytoin on peripheral chemoreceptor and ventilatory responses to hypoxia. Action potentials (APs) of single chemoreceptor afferents were recorded using suction electrodes advanced into the petrosal ganglion of an in vitro rat peripheral chemoreceptor complex. AP frequency (at Po(2) approximately 150 Torr and Po(2) approximately 90 Torr), conduction time, duration, and amplitude were measured before and during perfusion with therapeutic dosages of the drug or vehicle. Hypoxia-induced catecholamine secretion within the carotid body was measured using amperometry. With the use of whole body plethysmography, respiration was measured in unanesthesized rats while breathing room air, 12% O(2), and 5% CO(2), before and after intraperitoneal administration of amiodarone, lamotrigine, phenytoin, or vehicle. Lamotrigine (10 microM) and phenytoin (5 microM), but not amiodarone (5 microM), decreased chemoreceptor AP frequency without affecting other AP parameters or magnitude of catecholamine secretion. Similarly, lamotrigine (5 mg/kg) and phenytoin (10 mg/kg) blunted the hypoxic but not the hypercapnic ventilatory response. In contrast, amiodarone (2.5 mg/kg) did not alter the ventilatory response to hypoxia or hypercapnia. We conclude that lamotrigine and phenytoin at therapeutic levels impair peripheral chemoreceptor function and ventilatory response to acute hypoxia. These are consistent with I(NaP) serving an important function in AP generation and may be clinically important in the care of patients using these drugs.

Persistent calcium current in rat suprachiasmatic nucleus neurons

The suprachiasmatic nuclei contain the primary circadian clock, and suprachiasmatic nuclei neurons exhibit a diurnal modulation of spontaneous firing rate. The present study examined the voltage-gated persistent Ca(2+) current, in acutely isolated rat suprachiasmatic nuclei neurons using a ramp-type voltage-clamp protocol. Slow triangular voltage-clamp commands from a holding potential of -85 mV to +5 mV elicited inward current (100-400 pA) that was completely blocked by Cd(2+). This current showed little or no hysteresis, and was identified as persistent Ca(2+) current. The threshold for persistent Ca(2+) current ranged between -60 and -45 mV, and it was maximal at about -8 mV. Nifedipine at 10-20 microM blocked 80-100%. To assess the role of persistent Ca(2+) current in the generation of spontaneous action potentials in both acutely isolated and intact suprachiasmatic nuclei neurons, the effect of Cd(2+) and nifedipine on firing rate was studied using on-cell recording. Application of Cd(2+) exerted a weak excitatory effect and nifedipine had no significant effect on the spontaneous firing rate of isolated suprachiasmatic nuclei neurons. In all intact suprachiasmatic nuclei neurons in slice preparations (n=15), Cd(2+) slowly inhibited spontaneous firing; in high-frequency firing cells (four of 15), a transient increase of firing rate preceded inhibition. No significant effect of nifedipine on firing rate of intact suprachiasmatic nuclei neurons was found. Therefore, persistent Ca(2+) current itself (as carrier of charge) does not appear to contribute significantly to spontaneous firing of suprachiasmatic nuclei neurons. A slowly developing inhibitory effect of Cd(2+) on spontaneous firing of intact suprachiasmatic nuclei neurons in slice preparations may be due to penetration of Cd(2+) through Ca(2+) channels, and its subsequent effect on intracellular mechanisms, while the transient increase of firing rate in high-frequency firing neurons is probably due to inhibition of Ca(2+)-activated K(+) current.