Although neurons are known to exhibit a broad array of intrinsic properties that impact critically on the computations they perform, very few studies have quantified such biophysical diversity and ...its functional consequences. Using in vivo and in vitro whole-cell recordings here we show that mitral cells are extremely heterogeneous in their expression of a rebound depolarization (sag) at hyperpolarized potentials that is mediated by a ZD7288-sensitive current with properties typical of hyperpolarization-activated cyclic nucleotide gated (HCN) channels. The variability in sag expression reflects a functionally diverse population of mitral cells. For example, those cells with large amplitude sag exhibit more membrane noise, a lower rheobase and fire action potentials more regularly than cells where sag is absent. Thus, cell-to-cell variability in sag potential amplitude reflects diversity in the integrative properties of mitral cells that ensures a broad dynamic range for odor representation across these principal neurons.
Although oscillations in membrane potential are a prominent feature of sensory, motor, and cognitive function, their precise role in signal processing remains elusive. Here we show, using a ...combination of in vivo, in vitro, and theoretical approaches, that both synaptically and intrinsically generated membrane potential oscillations dramatically improve action potential (AP) precision by removing the membrane potential variance associated with jitter-accumulating trains of APs. This increased AP precision occurred irrespective of cell type and--at oscillation frequencies ranging from 3 to 65 Hz--permitted accurate discernment of up to 1,000 different stimuli. At low oscillation frequencies, stimulus discrimination showed a clear phase dependence whereby inputs arriving during the trough and the early rising phase of an oscillation cycle were most robustly discriminated. Thus, by ensuring AP precision, membrane potential oscillations dramatically enhance the discriminatory capabilities of individual neurons and networks of cells and provide one attractive explanation for their abundance in neurophysiological systems.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Key points
•
Granule cells are the main source of inhibition in the olfactory bulb (i.e. the first station of odour processing in the mammalian brain), but very little is known about the inhibition ...that acts upon them.
•
Using in vivo whole cell patch clamp recordings in anaesthetized mice we report the following new findings:
•
We found odour‐evoked responses to be rare (seen only in 18% of the odour presentations, and only in cells that showed also evoked excitatory responses to odours).
•
We report for the first time the presence of tonic inhibition in the olfactory bulb.
•
We show that tonic inhibition dominates over phasic synaptic inhibition evoked by odours, thereby being the key regulator shaping the granule cells spike output.
•
Preliminary (in vivo) evidence suggests that sensory evoked phasic inhibition onto granule cells is provided by deep short axon cells in the olfactory bulb.
GABAergic granule cells (GCs) regulate, via mitral cells, the final output from the olfactory bulb to piriform cortex and are central for the speed and accuracy of odour discrimination. However, little is known about the local circuits in which GCs are embedded and how GCs respond during functional network activity. We recorded inhibitory and excitatory currents evoked during a single sniff‐like odour presentation in GCs in vivo. We found that synaptic excitation was extensively activated across cells, whereas phasic inhibition was rare. Furthermore, our analysis indicates that GCs are innervated by a persistent firing of deep short axon cells that mediated the inhibitory evoked responses. Blockade of GABAergic synaptic input onto GCs revealed a tonic inhibitory current mediated by furosemide‐sensitive GABAA receptors. The average current associated with this tonic GABAergic conductance was 3‐fold larger than that of phasic inhibitory postsynaptic currents. We show that the pharmacological blockage of tonic inhibition markedly increased the occurrence of supra‐threshold responses during an odour‐stimulated sniff. Our findings suggest that GCs mediate recurrent or lateral inhibition, depending on the ambient level of extracellular GABA.
The isolation of the peptide inhibitor of M-type K+ current, BeKm-1, from the venom of the Central Asian scorpion Buthus eupeus has been described previously (Fillipov A. K., Kozlov, S. A., ...Pluzhnikov, K. A., Grishin, E. V., and Brown, D. A. (1996) FEBS Lett. 384, 277–280). Here we report the cloning, expression, and selectivity of BeKm-1. A full-length cDNA of 365 nucleotides encoding the precursor of BeKm-1 was isolated using the rapid amplification of cDNA ends polymerase chain reaction technique from mRNA obtained from scorpion telsons. Sequence analysis of the cDNA revealed that the precursor contains a signal peptide of 21 amino acid residues. The mature toxin consists of 36 amino acid residues. BeKm-1 belongs to the family of scorpion venom potassium channel blockers and represents a new subgroup of these toxins. The recombinant BeKm-1 was produced as a Protein A fusion product in the periplasm of Escherichia coli. After cleavage and high performance liquid chromatography purification, recombinant BeKm-1 displayed the same properties as the native toxin. Three BeKm-1 mutants (R27K, F32K, and R27K/F32K) were generated, purified, and characterized. Recombinant wild-type BeKm-1 and the three mutants partly inhibited the native M-like current in NG108-15 at 100 nm. The effect of the recombinant BeKm-1 on different K+ channels was also studied. BeKm-1 inhibited hERG1 channels with an IC50 of 3.3 nm, but had no effect at 100 nm on hEAG, hSK1, rSK2, hIK, hBK, KCNQ1/KCNE1, KCNQ2/KCNQ3, KCNQ4 channels, and minimal effect on rELK1. Thus, BeKm-1 was shown to be a novel specific blocker of hERG1 potassium channels.
AF276623
1 Division of Cellular and Molecular Physiology, Department
of Medical Physiology, University of Copenhagen, DK-2200 Copenhagen
N; and 2 NeuroSearch A/S, DK-2750 Ballerup, Denmark
Human cloned ...KCNQ4 channels were stably
expressed in HEK-293 cells and characterized with respect to function
and pharmacology. Patch-clamp measurements showed that the KCNQ4
channels conducted slowly activating currents at potentials more
positive than 60 mV. From the Boltzmann function fitted to the
activation curve, a half-activation potential of 32 mV and an
equivalent gating charge of 1.4 elementary charges was determined. The
instantaneous current-voltage relationship revealed strong inward
rectification. The KCNQ4 channels were blocked in a voltage-independent
manner by the memory-enhancing M current blockers XE-991 and
linopirdine with IC 50 values of 5.5 and 14 µM,
respectively. The antiarrhythmic KCNQ1 channel blocker bepridil
inhibited KCNQ4 with an IC 50 value of 9.4 µM, whereas
clofilium was without significant effect at 100 µM. The
KCNQ4-expressing cells exhibited average resting membrane potentials of
56 mV in contrast to 12 mV recorded in the nontransfected cells. In
conclusion, the activation and pharmacology of KCNQ4 channels resemble
those of M currents, and it is likely that the function of the KCNQ4
channel is to regulate the subthreshold electrical activity of
excitable cells.
XE-991; linopirdine; bepridil; M current
*
R. Søgaard and T. Ljungstrøm contributed equally to this work.
The scorpion toxin BeKm-1 is unique among a variety of known short scorpion toxins affecting potassium channels in its selective
action on ether-a-go-go-related gene (ERG)-type channels. BeKm-1 ...shares the common molecular scaffold with other short scorpion
toxins. The toxin spatial structure resolved by NMR consists of a short α-helix and a triple-stranded antiparallel β-sheet.
By toxin mutagenesis study we identified the residues that are important for the binding of BeKm-1 to the human ERG K + (HERG) channel. The most critical residues (Tyr-11, Lys-18, Arg-20, Lys-23) are located in the α-helix and following loop
whereas the âtraditionalâ functional site of other short scorpion toxins is formed by residues from the β-sheet. Thus the
unique location of the binding site of BeKm-1 provides its specificity toward the HERG channel.
One defining characteristic of the mammalian brain is its neuronal diversity. For a given region, substructure, layer or even cell type, variability in neuronal morphology and connectivity persists. ...Although it is well known that such cellular properties vary considerably according to neuronal type, the substantial biophysical diversity of neurons of the same morphological class is typically averaged out and ignored. Here we show that the amplitude of hyperpolarization-evoked sag of membrane potential recorded in olfactory bulb mitral cells is an emergent, homotypic property of local networks and sensory information processing. Simultaneous whole-cell recordings from pairs of cells show that the amount of hyperpolarization-evoked sag potential and current (Ih) is stereotypic for mitral cells belonging to the same glomerular circuit. This is corroborated by a mosaic, glomerulus-based pattern of expression of the HCN2 (hyperpolarization-activated cyclic nucleotide-gated channel 2) subunit of the Ih channel. Furthermore, inter-glomerular differences in both membrane potential sag and HCN2 protein are diminished when sensory input to glomeruli is genetically and globally altered so that only one type of odorant receptor is universally expressed. Population diversity in this intrinsic property therefore reflects differential expression between local mitral cell networks processing distinct odour-related information.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The hyperpolarization-activated cation current
I
h
exhibits a steep gradient of channel density in dendrites of pyramidal neurons, which is associated with location independence of temporal summation ...of EPSPs at the soma. In striking contrast, here we show by using dendritic patch-clamp recordings that in cerebellar Purkinje cells, the principal neurons of the cerebellar cortex,
I
h
exhibits a uniform dendritic density, while location independence of EPSP summation is observed. Using compartmental modeling in realistic and simplified dendritic geometries, we demonstrate that the dendritic distribution of
I
h
only weakly affects the degree of temporal summation at the soma, while having an impact at the dendritic input location. We further analyze the effect of
I
h
on temporal summation using cable theory and derive bounds for temporal summation for any spatial distribution of
I
h
. We show that the total number of
I
h
channels, not their distribution, governs the degree of temporal summation of EPSPs. Our findings explain the effect of
I
h
on EPSP shape and temporal summation, and suggest that neurons are provided with two independent degrees of freedom for different functions: the total amount of
I
h
(controlling the degree of temporal summation of dendritic inputs at the soma) and the dendritic spatial distribution of
I
h
(regulating local dendritic processing).
The hyperpolarization-activated cation current
I
h
exhibits a steep gradient of channel density in dendrites of pyramidal neurons, which is associated with location independence of temporal summation ...of EPSPs at the soma. In striking contrast, here we show by using dendritic patch-clamp recordings that in cerebellar Purkinje cells, the principal neurons of the cerebellar cortex,
I
h
exhibits a uniform dendritic density, while location independence of EPSP summation is observed. Using compartmental modeling in realistic and simplified dendritic geometries, we demonstrate that the dendritic distribution of
I
h
only weakly affects the degree of temporal summation at the soma, while having an impact at the dendritic input location. We further analyze the effect of
I
h
on temporal summation using cable theory and derive bounds for temporal summation for any spatial distribution of
I
h
. We show that the total number of
I
h
channels, not their distribution, governs the degree of temporal summation of EPSPs. Our findings explain the effect of
I
h
on EPSP shape and temporal summation, and suggest that neurons are provided with two independent degrees of freedom for different functions: the total amount of
I
h
(controlling the degree of temporal summation of dendritic inputs at the soma) and the dendritic spatial distribution of
I
h
(regulating local dendritic processing).
The function of the KCNE5 (KCNE1-like) protein has not previously been described. Here we show that KCNE5 induces both a time- and voltage-dependent modulation of the KCNQ1 current. Interaction of ...the KCNQ1 channel with KCNE5 shifted the voltage activation curve of KCNQ1 by more than 140
mV in the positive direction. The activation threshold of the KCNQ1+KCNE5 complex was +40
mV and the midpoint of activation was +116
mV. The KCNQ1+KCNE5 current activated slowly and deactivated rapidly as compared to the KCNQ1+KCNE1 at 22°C; however, at physiological temperature, the activation time constant of the KCNQ1+KCNE5 current decreased fivefold, thus exceeding the activation rate of the KCNQ1+KCNE1 current. The KCNE5 subunit is specific for the KCNQ1 channel, as none of other members of the KCNQ-family or the human
ether a-go-go related channel (hERG1) was affected by KCNE5. Four residues in the transmembrane domain of the KCNE5 protein were found to be important for the control of the voltage-dependent activation of the KCNQ1 current. We speculate that since KCNE5 is expressed in cardiac tissue it may here along with the KCNE1
β-subunit regulate KCNQ1 channels. It is possible that KCNE5 shapes the
I
Ks current in certain parts of the mammalian heart.
PDF
