Niflumic acid reduces the hyperpolarization-activated current (I(h)) in rod photoreceptor cells.

We examined the effects of niflumic acid (NFA), a chloride channel blocker, on the hyperpolarization-activated current (I_h) in newt rod photoreceptors. At 100 μM, NFA delayed the activation of I_h induced by hyperpolarizing voltage pulses to −83 mV from a holding potential of −43 mV, and reduced the steady-state current. However, reduction by NFA was weakened when I_h was activated by hyperpolarizing steps to −123 mV, suggesting that these effects were voltage-dependent. The suppressive effects of NFA on I_h were accompanied by a negative shift in activation voltage. NFA also delayed the relaxation of I_h tail currents, showing that this drug also inhibited deactivation of the current. The reversal potential and the fully activated conductance were not affected. These observations suggest that NFA reduces I_h by modifying the gating kinetics of the underlying channels. The suppressive actions of NFA remained when intracellular Ca²⁺ was strongly chelated, and the failure of suppression by NFA in inside-out patches suggests that the agent may act on the I_h channel from the extracellular side. These results, obtained in rod photoreceptors, are consistent with similar effects of NFA on I_f in cardiac myocytes, suggesting that both currents share similar pharmacological properties.     

Acetylcholine receptors in dissociated nucleus basalis of Meynert neurons of the rat

Electrical and pharmacological properties of acetylcholine (ACh)-induced currents in neurons dissociated from the nucleus basalis of Meynert (nBM) of immature (2-week-old) rats were investigated with the whole-cell mode of the patch-clamp technique. At a holding potential (V_H) of −50 mV, ACh (10⁻⁴M) evoked a transient inward current mimicked by nicotine (I_nACh), followed by a sustained outward current mimicked by carbamylcholine (I_mACh). The K_D values were 1.2 × 10⁻⁴ M for I_nACh) and 8.7 × 10⁻⁷ M for I_mACh. The reversal potenial of I_mACh was close to E_K. The I_mACh was determined to be elicited via the M₂ muscarinic receptor, based on the differences in sensitivity to muscarinic antagonists such as pirenzepine and AF-DX-116.     

Dihydropyridine-sensitive calcium currents in bipolar cells of salamander retina are inhibited by reductions in extracellular chloride

Dihydropyridine-sensitive calcium currents (I_Ca) in photoreceptors are unusual in that they can be inhibited by reductions in extracellular chloride. The present study examined whether I_Ca in retinal bipolar cells, which as in photoreceptors mediates sustained neurotransmission, is also inhibited by reductions in chloride. Nystatin-perforated patch, whole cell recordings were obtained from bipolar cells in a retinal slice preparation of larval tiger salamander. In the presence of Ba²⁺, voltage steps above −40 mV evoked sustained inward currents, which were enhanced by the dihydropyridine, (−)BayK8644, and inhibited by nisoldipine. Similar to photoreceptors, replacing Cl⁻ with gluconate or CH₃SO₄ inhibited bipolar cell I_Ca and produced a negative shift in the current/voltage relationship. Thus, sensitivity to Cl⁻ may be a more general property of L-type Ca²⁺ channel subtypes that mediate sustained neurotransmission.     

Inhibitory responses of rat basolateral amygdaloid neurons recorded in vitro

The purpose of the present study was to characterize the ionic and pharmacological basis of the actions of synaptically released and exogenously applied GABA in basolateral amygdaloid pyramidal cells in vitro. Stimulation of forebrain afferents to pyramidal neurons in the basolateral amygdala evoked an excitatory postsynaptic potential followed by early and late inhibitory postsynaptic potentials. The early inhibitory postsynaptic potential had a reversal potential near −70 mV, was sensitive to changes in the chloride gradient across the membrane and was blocked by the GABA_A antagonists picrotoxin and bicuculline methiodide but not by the GABA_B antagonists phaclofen or 2-hydroxysaclofen. In contrast, the late inhibitory postsynaptic potential had a reversal potential of approximately −95 mV and was markedly reduced or abolished by GABA_B antagonists.

Pressure application of GABA to the surface of the slice typically elicited a triphasic response in basolateral amygdaloid pyramidal neurons consisting of a short-latency hyperpolarization that preceded or was superimposed on a membrane depolarization followed by a longer latency hyperpolarization. Each of the responses was associated with an increase in membrane conductance. Determinations of the reversal potential, ionic dependency and sensitivity to pharmacological blockade of each component of the GABA-induced response revealed that the initial hyperpolarizing (E_rev approximately −70 mV) and depolarizing (E_rev approximately −55 mV) responses were mediated by a GABA_A-mediated increase in chloride conductance, whereas the late hyperpolarizing response (E_rev approximately −82 mV) to GABA arose from a GABA_B-mediated increase in potassium conductance. Experiments in which GABA was applied at various locations on the cell suggested that the short-latency hyperpolarization resulted from activation of somatic GABA receptors, whereas the depolarizing and late hyperpolarizing responses were generated primarily in the dendrites. In contrast to the complex membrane response profile elicited by GABA, pressure ejection of the GABA_B agonist baclofen produced only membrane hyperpolarizations.

Taken together, these results suggest that inhibitory responses that are recorded in basolateral amygdaloid pyramidal cells are mediated by activation of both GABA_A and GABA_B receptors. Consistent with findings elsewhere in the CNS, the early inhibitory postsynaptic potential and initial hyperpolarization and depolarizing response to local GABA application appear to involve a GABA_A-mediated increase in chloride conductance, whereas the late inhibitory postsynaptic potential and the late hyperpolarizing response to GABA arise from a GABA_B-mediated increase in potassium conductance.     

Molecular identification of 13 new enterovirus types, EV79-88, EV97, and EV100-101, members of the species Human Enterovirus B

Paired PBMCs and plasma samples from 34 HIV-infected patients were studied to verify the relationship between coreceptor use based on genotyping of V3 region of HIV-1 envelope gp120 and biological phenotype with virus isolation and subsequent correlation to clinical characteristics. The “11/25” rule, geno2pheno and PSSM were compared. All SI patients were HIV-1 subtype B (p = 0.04) and had a lower CD4 count than NSI patients (p = 0.01), while no differences were observed in mean HIV-RNA _log (p = 0.6). SI phenotype was not associated with AIDS-defining events (p = 0.1) or with concurrent antiretroviral therapy (p = 0.4). With geno2pheno, which shows the highest sensibility (83%), an X4 or X4/R5 genotype in PBMC DNA was also associated to B-subtype and lower CD4 count (p = 0.01) compared to R5 isolates. Based on plasma RNA sequences, the predicted coreceptor usage agreed with PBMC DNA in 79% of cases with the “11/25” rule, 82% with geno2pheno, and 82% with PSSM. A X4 virus in plasma (but not in PBMCs) was significantly associated with HAART in all three methods (p = 0.01 for “11/25” rule, p = 0.01 for geno2pheno and p = 0.03 for PSSM). Due to viral mixtures and/or difficulties in genotype interpretation, current V3 sequence-based methods cannot accurately predict HIV-1 coreceptor use.     

The effects of ouabain on resting membrane potential and hyperpolarization-activated current in neonatal rat nodose ganglion neurons

To determine whether the responses of resting membrane potential (RMP) and hyperpolarization-activated current (IH) are altered by the application of ouabain, one of the Na+-K+ pump inhibitors, in neonatal rat small-diameter (<30microm) nodose ganglion (NG) neurons, we examined the effects of 1microM ouabain on those responses using perforated patch-clamp techniques. In current-clamp mode, the RMP was 40.2+/-1.6mV (n=31). Twenty of 31 cells tested were depolarized by ouabain application, and these responses were associated with an increase in the cell input resistance. In the remaining 11 cells studied, 3 showed hyperpolarization in response to ouabain and 8 showed no effect on RMP. In voltage-clamp mode, 1muM ouabain application enhanced the IH in all of 10 neurons examined. These results suggest that ouabain application at 1microM is capable of setting both the RMP level and the neuronal excitability in small-diameter NG neurons.     

Regulation of stimulatory adenylyl cyclase signaling during forskolin-induced differentiation of mouse neuroblastoma × rat glioma (NG108–15) cells

Chronic exposure of neuroblastoma × glioma (NG108–15) cells to substances that elevate intracellular cAMP levels results in morphological differentiation into a more neuronal-like phenotype. Here we report that forskolin-induced differentiation is accompanied by a biphasic regulation of stimulatory adenylyl cyclase (AC) signaling. While 1 day of forskolin exposure produces an initial increase in basal, [AIF₄]⁻-, and prostaglandin E₁ (PGE₁)-stimulated AC activities, stimulatory signal transduction is substantially reduced after complete differentiation of the cells (6 days). Western blot analysis revealed that these functional changes correlate well with changes in the quantity of G_s, the stimulatory component of AC. Additional forskolin-induced adaptations were found for PGE₁ receptors, inhibitory G proteins and AC. These data demonstrate that neuronal differentiation of NG108–15 cells is associated with complex regulatory changes within the stimulatory PGE₁ receptor system.     

Effects of noradrenaline on some potassium currents in CA1 neurones in rat hippocampal slices

Pyramidal (CA1) cells in rat hippocampal slices were voltage clamped using a single electrode voltage clamp. In the presence of tetrodotoxin (TTX), depolarizing pulses from holding potentials of −60 to −70 mV elicited a slow inward calcium (Ca²⁺) current and two outward potassium (K⁺) currents: an A current and a slower, Ca²⁺-dependent K⁺ current. Noradrenaline (NA) (20 μM) depressed the amplitude of the K⁺ currents without affecting the Ca²⁺ current. The effect of NA could be blocked with propranolol and was mimicked by isoprenaline, suggesting that NA depresses the K⁺ currents by binding to β-receptors.     

Organ-specific and differential requirement of TYK2 and IFNAR1 for LPS-induced iNOS expression in vivo

Lipopolysaccharide (LPS) is an integral structural component of the outer membrane of Gram-negative bacteria and the principal active agent in the pathogenesis of endotoxin shock. LPS is a potent inducer of a variety of cytokines and inflammatory agents that lead to a profound alteration of gene expression patterns in cells and organs. The gene coding for the inducible nitric oxide synthase (iNOS) is highly responsive to LPS in vitro and in vivo and accounts for the production of nitric oxide (NO). The Janus kinase (JAK) family member tyrosine kinase 2 (TYK2) is a constituent of the interferon (IFN) type I response pathway and an important effector in the progression of endotoxin shock. Macrophages deficient for IFNβ receptor chain 1 (IFNAR1) or TYK2 were shown to have an impaired LPS-induced iNOS expression. Here we determined the contribution of IFNAR1 and TYK2 to iNOS expression in vivo in a lethal LPS challenge model. TYK2 and IFNAR1 were found to be crucial for the LPS-induced iNOS mRNA and protein expression in spleen and lung that could be attributed to the Mac3-positive population. In liver LPS-induced iNOS mRNA expression was only partially impaired in TYK2-deficient mice and was unimpaired in IFNAR1-deficient mice, indicating organ specificity. TYK2^−/− and IFNAR1^−/− mice also differ with respect to IFNγ production upon LPS challenge in that TYK2^−/− mice show a defect while IFNAR1^−/− mice do not. Our data suggest that iNOS is induced through IFNAR1 and TYK2 in Mac3-positive cells which are the main source of iNOS in spleen and lung. The LPS-induced iNOS expression in liver is independent of IFNAR1 and partially dependent on TYK2, which is most likely due to the lack of IFNγ production in the absence of TYK2.     

Bm-CPI-2, a cystatin from Brugia malayi nematode parasites, differs from Caenorhabditis elegans cystatins in a specific site mediating inhibition of the antigen-processing enzyme AEP

Nucleotide-sensitive chloride current regulating proteins (ICln's) of the chloride channels have been characterized from man and animals. An ICln of Fasciola hepatica (ICln-Fh) consisting of 231 amino acids revealed high similarities to both consensus domain of ICln's and two acidic residue-abundant patches in its C-terminus. Native ICln-Fh protein was confirmed present in F. hepatica soluble extract by immunoblotting. The recombinant ICln-Fh protein expressed in collagenase-defolliculated Xenopus oocytes induced fast rising and outward rectifying Cl⁻ currents (I_Cln-Fh). The recombinant ICln-Fh protein, however, did not trigger cell swelling-induced Cl⁻ currents (I_Cl-swell). The I_Cln-Fh currents were significantly reduced by substituting external Cl⁻ with gluconic acid and by externally adding cAMP. Collectively, these results suggest that ICln-Fh protein is an inducer of Cl⁻ currents in F. hepatica.     

Role of smooth muscle cell membrane potential in neointima formation in arteries of spontaneously hypertensive rats

Based on observations that vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats (SHR) have altered resting potentials as well as abnormal cell proliferation rates, neointima formation after controlled balloon injury was compared in arteries from SHR and Wistar Kyoto rats (WKY). SHR aortic VSMC showed hyperpolarized resting membrane potentials (−93±8 mV) when compared to those from WKY (−61±6 mV). Histomorphometric analysis of cross sections from aortic segments submitted to balloon injury showed reduced neointima formation in SHR (neointima/media ratio: 0.04±0.03) as compared to WKY (0.2±0.1). On the other hand, in injured carotid arteries, neointima formation was more extensive in SHR (neointima/media ratio 5.0±0.9) than in WKY (0.8±0.7), leading in most cases to luminal occlusion. Measurements of VSMC resting potential showed that carotid artery cells from SHR were depolarized with respect to those from WKY (−46±4 vs. −69±5 mV, respectively). The results demonstrate an inverse relationship between VSMC membrane polarization and neointima formation in SHR arteries, suggesting that genetic modifications in SHR determine a dysfunctional cellular physiology that may influence cell proliferation subsequent to injury.     

Ionic mechanisms underlying excitatory effects of serotonin on embryonic rat motoneurons in long-term culture

The actions of serotonin were investigated on motoneurons isolated from embryonic day 14 rat spinal cord and enriched by metrizamide density gradient centrifugation. Trophic support was provided by a spinal cord glial monolayer, ciliary neurotrophic factor and heat-inactivated serum. Cultures were maintained for 17–83 days and investigated using whole-cell patch-clamp recording. Serotonin evoked slow depolarizations (6.2±0.7 or 9.3±1.3 mV in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione and strychnine, ₅₀ 8.2 nM), which were reversibly blocked by 0.1 μM ketanserin. Serotonin generated synaptic potentials in motoneurons, lowered the threshold for repetitive firing and changed the slope of the current intensity–firing frequency relationship. The inward current evoked by serotonin (−147±15.2 pA) was ascribed to a complex ionic mechanism, which varied amongst neurons in the sampled population. It was due to closure of barium-sensitive potassium channels, effects on I_h and increase in a separate mixed cation current which comprised both transient voltage-sensitive and sustained components.

We conclude that serotonergic responses develop in motoneurons cultured under these conditions in the absence of serotonergic input, sensory neurons or many interneurons.     

Calcium currents in rat cerebellar purkinje cells maintained in culture

Calcium permeabilities were examined in large cerebellar neurons maintained in culture, and morphologically identified as Purkinje cells. When cells were supplied with a Dulbecco Minimum Eagle's Medium with 10% horse serum added (5–10 days), somatic recordings revealed complex spikes and these were shown to be generated by Na and Ca components, the Na one being tetrodotoxin-sensitive. At the dendritic level, Ca currents were better resolved than at the soma. In dendrites, Ca entry was shown to occur through at least two distinct currents. The first was a low-threshold transient current (elicited above −60 mV from a holding potential of −80 mV) which was reduced by almost 30% by 50μ M cadmium. The second was a high-threshold current (above −20 mV) which gave rise to (1) a transient component exhibiting a steady-state inactivation and so requiring holding potentials at −80 mV, and (2) a sustained component. Both components were suppressed by 50 μm cadmium. We measured a total Ca current at the dendritic level reaching values of up to 1 nA.

In another culture medium (Leibovitz medium) known to allow expression of three types of calcium currents in nodose cells we observed the development of the dendritic tree of Purkinje cells but with no simultaneous expression of the high-threshold Ca current.     

Synantocytes: the fifth element

Classic studies have recognized neurons and three glial elements in the central nervous system (CNS) - astrocytes, oligodendrocytes and microglia. The identification of novel glia that specifically express the NG2 chondroitin sulphate proteoglycan (CSPG) raises the possibility of a fifth element. Until recently, all NG2-expressing glia were considered to be oligodendrocyte precursor cells (OPCs) that persist in the adult CNS to generate oligodendrocytes throughout life. However, this narrow view of the function of 'NG2-glia' is being challenged. The majority of NG2-expressing glia in the adult CNS are a distinct class of cells that we have called 'synantocytes' (from the Greek synanto for contact). Synantocytes are stellate cells, with large process arborizations, and are exquisitely related to neurons. Individual cells traverse white and grey matter and form multiple contacts with neurons, astrocytes, oligodendrocytes and myelin. Synantocytes are an integral component of the 'tetrapartite' synapse, and provide a potential integrative neuron-glial communications pathway. Neuronal activity, glutamate and adenosine triphosphate (ATP) act on synantocyte receptors and evoke raised intracellular calcium. It remains to be seen whether this serves a physiological function, but synantocytes may be specialized to monitor signals from neurons and glia, and to respond to changes in the integrity of the CNS via their specific contacts and ion channel and receptor profiles. The general consequences of synantocyte activation are proliferation and phenotypic changes, resulting in glial scar formation, or regeneration of oligodendrocytes, and possibly neurons.     

Ion channels in spinal cord astrocytes in vitro. I. Transient expression of high levels of Na+ and K+ channels.

1. Na+ and K+ channel expression was studied in cultured astrocytes derived from P--0 rat spinal cord using whole cell patch-clamp recording techniques. Two subtypes of astrocytes, pancake and stellate, were differentiated morphologically. Both astrocyte types showed Na+ channels and up to three forms of K+ channels at certain stages of in vitro development. 2. Both astrocyte types showed pronounced K+ currents immediately after plating. Stellate but not pancake astrocytes additionally showed tetrodotoxin (TTX)-sensitive inward Na+ currents, which displayed properties similar to neuronal Na+ currents. 3. Within 4-5 days in vitro (DIV), pancake astrocytes lost K(+)-current expression almost completely, but acquired Na+ currents in high densities (estimated channel density approximately 2-8 channels/microns2). Na+ channel expression in these astrocytes is approximately 10- to 100-fold higher than previously reported for glial cells. Concomitant with the loss of K+ channels, pancake astrocytes showed significantly depolarized membrane potentials (-28.1 +/- 15.4 mV, mean +/- SD), compared with stellate astrocytes (-62.5 +/- 11.9 mV, mean +/- SD). 4. Pancake astrocytes were capable of generating action-potential (AP)-like responses under current clamp, when clamp potential was more negative than resting potential. Both depolarizing and hyperpolarizing current injections elicited overshooting responses, provided that cells were current clamped to membrane potentials more negative than -70 mV. Anode-break spikes were evoked by large hyperpolarizations (less than -150 mV). AP-like responses in these hyperpolarized astrocytes showed a time course similar to neuronal APs under conditions of low K+ conductance. 5. In stellate astrocytes, AP-like responses were not observed, because the K+ conductance always exceeded Na+ conductance by at least a factor of 3. Thus stellate spinal cord astrocyte membranes are stabilized close to EK as previously reported for hippocampal astrocytes. 6. It is concluded that spinal cord pancake astrocytes are capable of synthesizing Na+ channels at densities that can, under some conditions, support electrogenesis. In vivo, however, AP-like responses are unlikely to occur because the cells' resting potential is too depolarized to allow current activation. Thus the absence of electrogenesis in astrocytes may be explained by two mechanisms: 1) a low Na-to-K conductance ratio, as in stellate spinal cord astrocytes and in other previously studied astrocyte preparations; or, 2) as described in detail in the companion paper, a mismatch between the h infinity curve and resting potential, which results in Na+ current inactivation in spinal cord pancake astrocytes.     

Effects of glutamate receptor activation on NG2-glia in the rat optic nerve

NG2‐glia are a substantial population of cells in the central nervous system (CNS) that can be identified by their specific expression of the NG2 chondroitin sulphate (CSPG). NG2‐glia can generate oligodendrocytes, but it is unlikely this is their only function; indeed, they may be multipotent neural stem cells. Moreover, NG2‐glia are a highly reactive cell type and a major function is to help form the axon growth inhibitory glial scar in response to CNS injury. The factors that regulate these diverse behaviours of NG2‐glia are not fully resolved, but NG2‐glia express receptors to the neurotransmitter glutamate, which has known potent effects on other glia. Here, we have examined the actions of glutamate receptor activation on NG2‐glia in the rat optic nerve, a typical CNS white matter tract that does not contain neuronal cell bodies. Glutamate induces an increase in [Ca²⁺]_i in immuno‐identified NG2‐glia in situ and in vitro. In addition, we examined the effects of glutamate receptor activation in vivo by focal injection of the glutamate receptor agonist kainate into the optic nerve; saline was injected in controls. Changes in glial and axonal function were determined at 7 days post injection (dpi), by immunohistochemistry and electrophysiological measurement of the compound action potential (CAP). Injection of kainate resulted in a highly localized ‘injury response’ in NG2‐glia, marked by dense labelling for NG2 at the lesion site, as compared to astrocytes, which displayed a more extensive reactive astrogliosis. Furthermore, injection of kainate resulted in an axonal conduction block. These glial and axonal changes were not observed following injection of saline vehicle. In addition, we provide evidence that endogenous glutamate induces calcium‐dependent phosphorylation of extracellular signal‐regulated kinases (ERK1/2), which may provide a potential mechanism by which glutamate‐mediated changes in raised intracellular calcium could regulate the observed gliosis. The results provide evidence that activation of AMPA‐kainate type ionotropic glutamate receptors evoke raised calcium in NG2‐glia and induces an injury response in NG2‐glia.     

Pyramidal cells of rodent presubiculum express a tetrodotoxin-insensitive Na+ current

Presubicular neurons are activated physiologically by a specific preferred head direction. Here we show that firing in these neurones is characterized by action potentials with a large overshoot and a reduced firing frequency adaptation during repetitive firing. We found that a component of the sodium current of presubicular cells was not abolished by tetrodotoxin (TTX, 10 μ m ) and was activated at more depolarized voltages than TTX-sensitive currents. This inward current was completely abolished by the removal of external sodium, suggesting that sodium is the charge carrier of this TTX-insensitive (TTX-I) current. The channels responsible for the TTX-I sodium current seemed to be expressed at sites distant from the soma, giving rise to a voltage-dependent delay in current activation. The voltage required for half-maximal activation was −21 mV, and −36 mV for inactivation, which is similar to that reported for Na_V1.8 sodium channels. However, the kinetics were considerably slower, with a time constant of current decay of 1.4 s. The current was not abolished in pyramidal cells from animals lacking either the Na_V1.8 or the Na_V1.9 subunit. This, possibly novel, TTX-I sodium current could contribute to the coding functions of presubicular neurons, specifically the maintained firing associated with signalling of a stable head position.     

Ion channels in spinal cord astrocytes in vitro. II. Biophysical and pharmacological analysis of two Na+ current types.

1. Na+ currents expressed in astrocytes cultured from spinal cord were studied by whole cell patch-clamp recording. Two subtypes of astrocytes, pancake and stellate cells, were morphologically differentiated and showed expression of Na+ channels at densities that are unusually high for glial cells (2-8 channels/microns2) and comparable to cultured neurons. 2. Na+ currents in stellate and pancake astrocytes were comparable to neuronal Na+ currents with regard to Na(+)-current activation (tau m) and inactivation (tau h) time constants, which were equally fast in both astrocyte types. However, they differed with respect to voltage dependence of activation, and current-voltage (I-V) curves were approximately 10 mV more positive in stellate cells (-11.1 +/- 5.6 mV, mean +/- SD) than in pancake cells (19.7 +/- 4.5 mV). Steady-state activation (m infinity curves) was 16 mV more negative in pancake (mean V1/2 = -48.8 mV) than in stellate cells (mean V1/2 = -32.7 mV). 3. Steady-state inactivation (h infinity curves) of Na+ currents was distinctly different in the two astrocyte types. In stellate astrocytes h infinity curves had midpoints close to -65 mV (-64.6 +/- 6.5 mV), similar to most cultured neurons. In pancake astrocytes h infinity-curves were approximately 25 mV more negative, with midpoints close to -85 mV (84.5 +/- 9.5 mV). 4. The two forms of Na+ currents were additionally distinguishable by their sensitivity to tetrodotoxin (TTX). Na+ currents in stellate astrocytes were highly TTX sensitive [half-maximal inhibition (Kd) = 5.7 nM] whereas Na+ currents in pancake astrocytes were relatively TTX resistant, requiring 100- to 1,000-fold higher concentrations for blockage (Kd = 1,007 nM). 5. Na+ currents were fit by the Hodgkin-Huxley (HH) model. In pancake astrocytes, as in squid gigant axons, Na(+)-current kinetics could be well described with an m3h model, whereas in stellate astrocytes Na+ currents were better described with higher-order power terms for activation (m). On average, best fits were obtained using an m4h model. 6. Pancake astrocytes were capable of generating action-potential (AP)-like responses under current clamp whereas stellate astrocytes were not. The h infinity curve for APs shows that membrane potentials more negative than -70 mV are required to allow these responses to occur.(ABSTRACT TRUNCATED AT 400 WORDS)     

Development of GLAST(+) astrocytes and NG2(+) glia in rat hippocampus CA1: mature astrocytes are electrophysiologically passive

Glia show marked heterogeneity in terms of electrophysiology in the developing brain, and two major types can be identified based on GFAP or NG2 expression. However, it remains to be determined if such an electrophysiological diversity holds for the adult brain and how GFAP and NG2 lineage glia are associated with different electrophysiological phenotypes during the course of development. To address these fundamental questions, we performed in situ whole cell recording from morphologically identified glia from the rat hippocampal CA1 region from postnatal (P) days 1-106 and double-stained postrecorded cells with GLAST and NG2 antibodies. We found glia express mostly voltage-gated outward K(+) currents and also have inward Na(+) currents in the newborn (P1-P3), but these are no longer present after P22. They consist equally of GLAST(+) and NG2(+) cells in the newborn, but are mainly NG2(+) in juvenile animals (P4-P21). Glia showing voltage-gated outward and inward K(+) currents are also present at P1, peak at P5 and decline to a stationary level of approximately 10% in the adult. They are GLAST(+) astrocytes from newborn to juvenile but NG2(+) glia in the adult. Electrophysiologically passive glia first appear at P4 and increase to 91% in adults, of which 85% are GLAST(+). These results indicate that glial electrophysiological diversity occurs predominantly in the developing brain. While most glia in the NG2 lineage preserve a certain amount of voltage-gated ion conductances, mature GLAST(+) astrocytes are electrophysiologically passive.     

Lithium discriminates between muscarinic receptor subtypes on guinea pig hippocampal neurons in vitro

In CA3 pyramidal neurons of guinea pig hippocampal slices an outward current activated by the GABA_B agonist, baclofen (0.3 μM, I_bac) was reduced by low concentrations of carbachol (Cch, 0.1–0.3 μM). The effect of Cch desensitized suggesting that the receptor subtype involved in this muscarinic effect of Cch was of the M₁ subtype. The receptor subtype was also characterized by its equilibrium dissociation constant for pirenzepine (10 nM) as an M₁ receptor. Li⁺ applied extracellularly (1 mM) or intracellularly blocked the suppression of I_bac by Cch without affecting the Cch blockade of a current termed I_AHP, which is mediated by M₂ receptors. While the effect of intracellular Li⁺ application was immediate, it developed very slowly with extracellular application. Since Li⁺-salts are used effectively in the treatment of mania and depression, the selective effect of Li⁺ on M₁-mediated muscarinic neurotransmission might be important for the cholinergic hypothesis of affective disorders.