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Input Resistance (input + resistance)
Selected AbstractsIntramuscular AAV delivery of NT-3 alters synaptic transmission to motoneurons in adult ratsEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 6 2010Jeffrey C. Petruska Abstract We examined whether elevating levels of neurotrophin-3 (NT-3) in the spinal cord and dorsal root ganglion (DRG) would alter connections made by muscle spindle afferent fibers on motoneurons. Adeno-associated virus (AAV) serotypes AAV1, AAV2 and AAV5, selected for their tropism profile, were engineered with the NT-3 gene and administered to the medial gastrocnemius muscle in adult rats. ELISA studies in muscle, DRG and spinal cord revealed that NT-3 concentration in all tissues peaked about 3 months after a single viral injection; after 6 months NT-3 concentration returned to normal values. Intracellular recording in triceps surae motoneurons revealed complex electrophysiological changes. Moderate elevation in cord NT-3 resulted in diminished segmental excitatory postsynaptic potential (EPSP) amplitude, perhaps as a result of the observed decrease in motoneuron input resistance. With further elevation in NT-3 expression, the decline in EPSP amplitude was reversed, indicating that NT-3 at higher concentration could increase EPSP amplitude. No correlation was observed between EPSP amplitude and NT-3 concentration in the DRG. Treatment with control viruses could elevate NT-3 levels minimally resulting in measurable electrophysiological effects, perhaps as a result of inflammation associated with injection. EPSPs elicited by stimulation of the ventrolateral funiculus underwent a consistent decline in amplitude independent of NT-3 level. These novel correlations between modified NT-3 expression and single-cell electrophysiological parameters indicate that intramuscular administration of AAV(NT-3) can exert long-lasting effects on synaptic transmission to motoneurons. This approach to neurotrophin delivery could be useful in modifying spinal function after injury. [source] Developmental expression of Na+ currents in mouse Purkinje neuronsEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 9 2006Mark Fry Abstract As Purkinje neurons mature during postnatal development, they change from electrically quiescent to active and exhibit high frequency spontaneous action potentials. This change in electrical activity is determined by both alteration in ion channel expression and the acquisition of synaptic input. To gain a better understanding of the development the intrinsic electrical properties of these neurons, acutely isolated Purkinje neurons from mice aged postnatal day 4 (P4) to P18 were examined. This included recording action potential frequency, threshold, height and slope, and input resistance and capacitance. Changes in a number of these properties were observed, suggesting significant changes in voltage-gated Na+ currents. Because voltage-gated Na+ currents, including the transient, resurgent and persistent currents, are known to play important roles in generating spontaneous action potentials, the developmental changes in these currents were examined. A large increase in the density of transient current, resurgent current and persistent current was observed at times corresponding with changes in action potential properties. Interestingly, the developmental up-regulation of the persistent current and resurgent current occurred at rate which was faster than the up-regulation of the transient current. Moreover, the relative amplitudes of the persistent and resurgent currents increased in parallel, suggesting that they share a common basis. The data indicate that developmental up-regulation of Na+ currents plays a key role in the acquisition of Purkinje neuron excitability. [source] Postsynaptic calcium contributes to reinforcement in a three-neuron network exhibiting associative plasticityEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 2 2004P. M. Balaban Abstract We show that activation of a single serotonergic cell is sufficient to trigger long-term associative enhancement of synaptic input to the withdrawal interneuron in a simple network consisting of three interconnected identified cells in the nervous system of terrestrial snail Helix. 1,2-bis (2-aminophenoxy) Ethane- N,N,N,,N,-tetraacetic acid (BAPTA) injection in the postsynaptic neuron abolishes the pairing-specific enhancement of synaptic input. Activation of a single modulatory cell that we used to reinforce the synaptic input induced an increase of the intracellular [Ca2+] in the ipsilateral withdrawal interneuron without any changes of its membrane potential or input resistance. Similar changes in intracellular [Ca2+] were observed in the same withdrawal interneuron under bath application of 10,5 m serotonin. Responses to repeated glutamate applications to the soma of synaptically isolated withdrawal interneurons increased after 10 min of serotonin or thapsigargin bath application, but were absent in conditions of preliminary BAPTA intracellular injection, significantly decreased under heparin injection. Thus, activity of a single modulatory cell may mediate reinforcement via an increase of [Ca2+] in the postsynaptic cell in a simple network consisting of neurons with defined behavioural roles. [source] Kv1 currents mediate a gradient of principal neuron excitability across the tonotopic axis in the rat lateral superior oliveEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 2 2004Margaret Barnes-Davies Abstract Principal neurons of the lateral superior olive (LSO) detect interaural intensity differences by integration of excitatory projections from ipsilateral bushy cells and inhibitory inputs from the medial nucleus of the trapezoid body. The intrinsic membrane currents active around firing threshold will form an important component of this binaural computation. Whole cell patch recording in an in vitro brain slice preparation was employed to study conductances regulating action potential (AP) firing in principal neurons. Current-clamp recordings from different neurons showed two types of firing pattern on depolarization, one group fired only a single initial AP and had low input resistance while the second group fired multiple APs and had a high input resistance. Under voltage-clamp, single-spiking neurons showed significantly higher levels of a dendrotoxin-sensitive, low threshold potassium current (ILT). Block of ILT by dendrotoxin-I allowed single-spiking cells to fire multiple APs and indicated that this current was mediated by Kv1 channels. Both neuronal types were morphologically similar and possessed similar amounts of the hyperpolarization-activated nonspecific cation conductance (Ih). However, single-spiking cells predominated in the lateral limb of the LSO (receiving low frequency sound inputs) while multiple-firing cells dominated the medial limb. This functional gradient was mirrored by a medio-lateral distribution of Kv1.1 immunolabelling. We conclude that Kv1 channels underlie the gradient of LSO principal neuron firing properties. The properties of single-spiking neurons would render them particularly suited to preserving timing information. [source] Differential maturation of motoneurons innervating ankle flexor and extensor muscles in the neonatal ratEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 12 2000L Vinay Abstract The first postnatal week is a critical period for the development of posture in the rat. The use of ankle extensor muscles in postural reactions increases during this period. Changes in excitability of motoneurons are probably an important factor underlying this maturation. The aim of this study was to identify whether variations in the maturation exist between motor pools innervating antagonistic muscles. Intracellular recordings in the in vitro brain stem,spinal cord preparation of neonatal rats (from postnatal day 0,5) were used to examine the developmental changes in excitability of motoneurons innervating the ankle flexors (F-MNs) and the antigravity ankle extensors (E-MNs). No significant difference in resting potential, action potential threshold, input resistance or rheobase was observed at birth. The age-related increase in rheobase was more pronounced for F-MNs than for E-MNs. The development of discharge properties of E-MNs lagged behind that of F-MNs. More F-MNs than E-MNs were able to fire repetitively in response to current injection at birth. F-MNs discharged at a higher frequency than E-MNs at all ages. Differences in the duration of action potential afterhyperpolarization accounted, at least partly, for the differences in discharge frequency between E-MNs and F-MNs at birth, and for the age-related increase in firing rate. These results suggest that E-MNs are more immature at birth than F-MNs and that there is a differential development of motoneurons innervating antagonistic muscles. This may be a critical factor in the development of posture and locomotion. [source] Cholinergic suppression of excitatory synaptic responses in layer II of the medial entorhinal cortexHIPPOCAMPUS, Issue 2 2007Bassam N. Hamam Abstract Theta-frequency (4,12 Hz) electroencephalographic activity is thought to play a role in mechanisms mediating sensory and mnemonic processing in the entorhinal cortex and hippocampus, but the effects of acetylcholine on excitatory synaptic inputs to the entorhinal cortex are not well understood. Field excitatory postsynaptic potentials (fEPSPs) evoked by stimulation of the piriform (olfactory) cortex were recorded in the medial entorhinal cortex during behaviors associated with theta activity (active mobility) and were compared with those recorded during nontheta behaviors (awake immobility and slow wave sleep). Synaptic responses were smaller during behavioral activity than during awake immobility and sleep, and responses recorded during movement were largest during the negative phase of the theta rhythm. Systemic administration of cholinergic agonists reduced the amplitude of fEPSPs, and the muscarinic receptor blocker scopolamine strongly enhanced fEPSPs, suggesting that the theta-related suppression of fEPSPs is mediated in part by cholinergic inputs. The reduction in fEPSPs was investigated using in vitro intracellular recordings of EPSPs in Layer II neurons evoked by stimulation of Layer I afferents. Constant bath application of the muscarinic agonist carbachol depolarized membrane potential and suppressed EPSP amplitude in Layer II neurons. The suppression of EPSPs was not associated with a substantial change in input resistance, and could not be accounted for by a depolarization-induced reduction in driving force on the EPSP. The GABAA receptor-blocker bicuculline (50 ,M) did not prevent the cholinergic suppression of EPSPs, suggesting that the suppression is not dependent on inhibitory mechanisms. Paired-pulse facilitation of field and intracellular EPSPs were enhanced by carbachol, indicating that the suppression is likely due to inhibition of presynaptic glutamate release. These results indicate that, in addition to well known effects on postsynaptic conductances that increase cellular excitability, cholinergic activation in the entorhinal cortex results in a strong reduction in strength of excitatory synaptic inputs from the piriform cortex. © 2006 Wiley-Liss, Inc. [source] The essential oil of Croton nepetaefolius selectively blocks histamine-augmented neuronal excitability in guinea-pig celiac ganglionJOURNAL OF PHARMACY AND PHARMACOLOGY: AN INTERNATI ONAL JOURNAL OF PHARMACEUTICAL SCIENCE, Issue 8 2010José Henrique Leal-Cardoso Abstract Objectives,Croton nepetaefolius is a medicinal plant useful against intestinal disorders. In this study, we elucidate the effects of its essential oil (EOCN) on sympathetic neurons, with emphasis on the interaction of EOCN- and histamine-induced effects. Methods, The effects of EOCN and histamine were studied in guinea-pig celiac ganglion in vitro. Key findings, Histamine significantly altered the resting potential (Em) and the input resistance (Ri) of phasic neurons (from ,56.6 ± 1.78 mV and 88.6 ± 11.43 M,, to ,52.9 ± 1.96 mV and 108.6 ± 11.00 M,, respectively). Em, Ri and the histamine-induced alterations of these parameters were not affected by 200 µg/ml EOCN. The number of action potentials produced by a 1-s (two-times threshold) depolarising current and the current threshold (Ith) for eliciting action potentials (rheobase) were evaluated. Number of action potentials and Ith were altered by histamine (from 2.6 ± 0.43 action potentials and 105.4 ± 11.15 pA to 6.2 ± 1.16 action potentials and 67.3 ± 8.21 pA, respectively). EOCN alone did not affect number of action potentials and Ith but it fully blocked the histamine-induced modifications of number of action potentials and Ith. All the effects produced by histamine were abolished by pyrilamine. Conclusions, EOCN selectively blocked histamine-induced modulation of active membrane properties. [source] Extracellular ATP inhibits chloride channels in mature mammalian skeletal muscle by activating P2Y1 receptorsTHE JOURNAL OF PHYSIOLOGY, Issue 23 2009Andrew A. VossArticle first published online: 30 NOV 200 ATP is released from skeletal muscle during exercise, a discovery dating back to 1969. Surprisingly, few studies have examined the effects of extracellular ATP on mature mammalian skeletal muscle. This electrophysiological study examined the effects of extracellular ATP on fully innervated rat levator auris longus using two intracellular microelectrodes. The effects of ATP were determined by measuring the relative changes of miniature endplate potentials (mEPPs) and voltage responses to step current pulses in individual muscle fibres. Exposure to ATP (20 ,m) prolonged the mEPP falling phase by 31 ± 7.5% (values ±s.d., n= 3 fibres). Concurrently, the input resistance increased by 31 ± 2.0% and the time course of the voltage responses increased by 59 ± 3.0%. Analogous effects were observed using 2 and 5 ,mATP, and on regions distal from the neuromuscular junction, indicating that physiologically relevant levels of ATP enhanced electrical signalling over the entire muscle fibre. The effects of extracellular ATP were blocked by 200 ,manthracene-9-carboxylic acid, a chloride channel inhibitor, and reduced concentrations of extracellular chloride, indicating that ATP inhibited chloride channels. A high affinity agonist for P2Y receptors, 2-methylthioadenosine-5,- O -diphosphate (2MeSADP), induced similar effects to ATP with an EC50 of 160 ± 30 nm. The effects of 250 nm2MeSADP were blocked by 500 nmMRS2179, a specific P2Y1 receptor inhibitor, suggesting that ATP acts on P2Y1 receptors to inhibit chloride channels. The inhibition of chloride channels by extracellular ATP has implications for muscle excitability and fatigue, and the pathophysiology of myotonias. [source] Functional role of cyclic nucleotide-gated channels in rat medial vestibular nucleus neuronsTHE JOURNAL OF PHYSIOLOGY, Issue 3 2008Maria Vittoria Podda Although cyclic nucleotide-gated (CNG) channels are expressed in numerous brain areas, little information is available on their functions in CNS neurons. The aim of the present study was to define the distribution of CNG channels in the rat medial vestibular nucleus (MVN) and their possible involvement in regulating MVN neuron (MVNn) excitability. The majority of MVNn expressed both CNG1 and CNG2 A subunits. In whole-cell current-clamp experiments carried out on brainstem slices containing the MVNn, the membrane-permeant analogues of cyclic nucleotides, 8-Br-cGMP and 8-Br-cAMP (1 mm), induced membrane depolarizations (8.9 ± 0.8 and 9.2 ± 1.0 mV, respectively) that were protein kinase independent. The cGMP-induced depolarization was associated with a significant decrease in the membrane input resistance. The effects of cGMP on membrane potential were almost completely abolished by the CNG channel blockers, Cd2+ and l - cis -diltiazem, but they were unaffected by blockade of hyperpolarization-activated cyclic nucleotide-gated channels. In voltage-clamp experiments, 8-Br-cGMP induced non-inactivating inward currents (,22.2 ± 3.9 pA) with an estimated reversal potential near 0 mV, which were markedly inhibited by reduction of extracellular Na+ and Ca2+ concentrations. Membrane depolarization induced by CNG channel activation increased the firing rate of MVNn without changing the action potential shape. Collectively, these findings provide novel evidence that CNG channels affect membrane potential and excitability of MVNn. Such action should have a significant impact on the function of these neurons in sensory,motor integration processes. More generally, it might represent a broad mechanism for regulating the excitability of different CNS neurons. [source] Intrinsic properties and mechanisms of spontaneous firing in mouse cerebellar unipolar brush cellsTHE JOURNAL OF PHYSIOLOGY, Issue 2 2007Marco J. Russo Neuronal firing patterns are determined by the cell's intrinsic electrical and morphological properties and are regulated by synaptic interactions. While the properties of cerebellar neurons have generally been studied in much detail, little is known about the unipolar brush cells (UBCs), a type of glutamatergic interneuron that is enriched in the granular layer of the mammalian vestibulocerebellum and participates in the representation of head orientation in space. Here we show that UBCs can be distinguished from adjacent granule cells on the basis of differences in membrane capacitance, input resistance and response to hyperpolarizing current injection. We also show that UBCs are intrinsically firing neurons. Using action potential clamp experiments and whole-cell recordings we demonstrate that two currents contribute to this property: a persistent TTX-sensitive sodium current and a ruthenium red-sensitive, TRP-like cationic current, both of which are active during interspike intervals and have reversal potentials positive to threshold. Interestingly, although UBCs are also endowed with a large Ih current, this current is not involved in their intrinsic firing, perhaps because it activates at voltages that are more hyperpolarized than those associated with autonomous activity. [source] A voltage-dependent K+ current contributes to membrane potential of acutely isolated canine articular chondrocytesTHE JOURNAL OF PHYSIOLOGY, Issue 1 2004Jim R. Wilson The electrophysiological properties of acutely isolated canine articular chondrocytes have been characterized using patch-clamp methods. The ,steady-state' current,voltage relationship (I,V) of single chondrocytes over the range of potentials from ,100 to +40 mV was highly non-linear, showing strong outward rectification positive to the zero-current potential. Currents activated at membrane potentials negative to ,50 mV were time independent, and the I,V from ,100 to ,60 mV was linear, corresponding to an apparent input resistance of 9.3 ± 1.4 G, (n= 23). The outwardly rectifying current was sensitive to the K+ channel blocking ion tetraethylammonium (TEA), which had a 50% blocking concentration of 0.66 mm (at +50 mV). The ,TEA-sensitive' component of the outwardly rectifying current had time- and membrane potential-dependent properties, activated near ,45 mV and was half-activated at ,25 mV. The reversal potential of the ,TEA-sensitive' current with external K+ concentration of 5 mm and internal concentration of 145 mm, was ,84 mV, indicating that the current was primarily carried by K+ ions. The resting membrane potential of isolated chondrocytes (,38.1 ± 1.4 mV; n= 19) was depolarized by 14.8 ± 0.9 mV by 25 mm TEA, which completely blocked the K+ current of these cells. These data suggest that this voltage-sensitive K+ channel has an important role in regulating the membrane potential of canine articular chondrocytes. [source] Flufenamic acid blocks depolarizing afterpotentials and phasic firing in rat supraoptic neuronesTHE JOURNAL OF PHYSIOLOGY, Issue 2 2002Masoud Ghamari-Langroudi Depolarizing afterpotentials (DAPs) that follow action potentials in magnocellular neurosecretory cells (MNCs) are thought to underlie the generation of phasic firing, a pattern that optimizes vasopressin release from the neurohypophysis. Previous work has suggested that the DAP may result from the Ca2+ -dependent reduction of a resting K+ conductance. Here we examined the effects of flufenamic acid (FFA), a blocker of Ca2+ -dependent non-selective cation (CAN) channels, on DAPs and phasic firing using intracellular recordings from supraoptic MNCs in superfused explants of rat hypothalamus. Application of FFA, but not solvent (0.1 % DMSO), reversibly inhibited (IC50+ 13.8 ,m; R+ 0.97) DAPs and phasic firing with a similar time course, but had no significant effects (P > 0.05) on membrane potential, spike threshold and input resistance, nor on the frequency and amplitude of spontaneous synaptic potentials. Moreover, FFA did not affect (P > 0.05) the amplitude, duration, undershoot, or frequency-dependent broadening of action potentials elicited during the spike trains used to evoke DAPs. These findings suggest that FFA inhibits the DAP by directly blocking the channels responsible for its production, rather than by interfering with Ca2+ influx. They also support a role for DAPs in the generation of phasic firing in MNCs. Finally, the absence of a depolarization and increased membrane resistance upon application of FFA suggests that the DAP in MNCs may not be due to the inhibition of resting K+ current, but to the activation of CAN channels. [source] A modelling study of locomotion-induced hyperpolarization of voltage threshold in cat lumbar motoneuronesTHE JOURNAL OF PHYSIOLOGY, Issue 2 2002Yue Dai During fictive locomotion the excitability of adult cat lumbar motoneurones is increased by a reduction (a mean hyperpolarization of ,6.0 mV) of voltage threshold (Vth) for action potential (AP) initiation that is accompanied by only small changes in AP height and width. Further examination of the experimental data in the present study confirms that Vth lowering is present to a similar degree in both the hyperpolarized and depolarized portions of the locomotor step cycle. This indicates that Vth reduction is a modulation of motoneurone membrane currents throughout the locomotor state rather than being related to the phasic synaptic input within the locomotor cycle. Potential ionic mechanisms of this locomotor-state-dependent increase in excitability were examined using three five-compartment models of the motoneurone innervating slow, fast fatigue resistant and fast fatigable muscle fibres. Passive and active membrane conductances were set to produce input resistance, rheobase, afterhyperpolarization (AHP) and membrane time constant values similar to those measured in adult cat motoneurones in non-locomoting conditions. The parameters of 10 membrane conductances were then individually altered in an attempt to replicate the hyperpolarization of Vth that occurs in decerebrate cats during fictive locomotion. The goal was to find conductance changes that could produce a greater than 3 mV hyperpolarization of Vth with only small changes in AP height (< 3 mV) and width (< 1.2 ms). Vth reduction without large changes in AP shape could be produced either by increasing fast sodium current or by reducing delayed rectifier potassium current. The most effective Vth reductions were achieved by either increasing the conductance of fast sodium channels or by hyperpolarizing the voltage dependency of their activation. These changes were particularly effective when localized to the initial segment. Reducing the conductance of delayed rectifier channels or depolarizing their activation produced similar but smaller changes in Vth. Changes in current underlying the AHP, the persistent Na+ current, three Ca2+ currents, the ,h' mixed cation current, the ,A' potassium current and the leak current were either ineffective in reducing Vth or also produced gross changes in the AP. It is suggested that the increased excitability of motoneurones during locomotion could be readily accomplished by hyperpolarizing the voltage dependency of fast sodium channels in the axon hillock by a hitherto unknown neuromodulatory action. [source] Von Neuronen zu Netzwerken.BIOLOGIE IN UNSERER ZEIT (BIUZ), Issue 6 2009Mathematische Gehirnmodelle Abstract Obwohl unser Verständnis des Nervensystems große Fortschritte macht, stellen Netzwerke aus Milliarden von Neuronen die Neurobiologie vor eine praktisch unlösbare Aufgabe: Die Aktivität eines Gehirns möglichst vollständig zu erfassen und das Beobachtete detailliert zu verstehen. Die "Computational Neuroscience" versucht Brücken zwischen den Konzepten der Teildisziplinen zu schlagen. Die mathematische Beschreibung von Nervenzellen und neuronalen Netzwerken, sowie die Simulation dieser Systeme in Form von Computermodellen, erlaubt Phänomene zu ergründen, die in biologischen Gehirnen nur unter größten Schwierigkeiten messbar sind. Jüngste Studien konnten unter anderem zeigen, dass erregungsabhängige Veränderungen der elektrischen Leitfähigkeit in Neuronen ein Netzwerk davor bewahren, dass räumlich begrenzte Erregung sich aufschaukelt und als Folge die Aktivität im gesamten Netzwerk zum Erliegen bringt. Dieselbe Eigenschaft führt außerdem dazu, dass ein Netzwerk auch ohne äußere Anregung aktiv bleiben kann , eine wichtige Grundeigenschaft von Gehirnen, deren neuronale Funktionsmechanismen bis heute weitgehend unverstanden sind. Our understanding of the nervous system has made great leaps forward. Yet still, the study of networks of billions of neurons poses an almost insolvable challenge to empirical neurobiology: to capture the activity of a brain as a whole, and to make sense of the observations in detail. Here, "Computational Neuroscience" attempts to build bridges between the concepts of the involved disciplines. The mathematical description of neurons and neuronal networks, as well as the simulation of these systems as computer models, allows fathoming phenomena that could be measured in biological brains only under severe difficulties. In particular, recent studies showed that activity dependent changes of neuronal input resistance can prevent a network from local "explosions" of activity, which otherwise could lead to a complete breakdown of network operation. The same property of neurons also causes a network to remain active when external excitation is switched off. This is an important property of brains, the neuronal mechanisms of which are still widely unknown. [source] Altered functional properties of satellite glial cells in compressed spinal gangliaGLIA, Issue 15 2009Haijun Zhang Abstract The cell bodies of sensory neurons in the dorsal root ganglion (DRG) are enveloped by satellite glial cells (SGCs). In an animal model of intervertebral foraminal stenosis and low-back pain, a chronic compression of the DRG (CCD) increases the excitability of neuronal cell bodies in the compressed ganglion. The morphological and electrophysiological properties of SGCs were investigated in both CCD and uninjured, control lumbar DRGs. SGCs responded within 12 h of the onset of CCD as indicated by an increased expression of glial fibrillary acidic protein (GFAP) in the compressed DRG but to lesser extent in neighboring or contralateral DRGs. Within 1 week, coupling through gap junctions between SGCs was significantly enhanced in the compressed ganglion. Under whole-cell patch clamp recordings, inward and outward potassium currents, but not sodium currents, were detected in individual SGCs. SGCs enveloping differently sized neurons had similar electrophysiological properties. SGCs in the compressed vs. control DRG exhibited significantly reduced inwardly rectifying potassium currents (Kir), increased input resistances and positively shifted resting membrane potentials. The reduction in Kir was greater for nociceptive medium-sized neurons compared to non-nociceptive neurons. Kir currents of SGCs around spontaneously active neurons were significantly reduced 1 day after compression but recovered by 7 days. These data demonstrate rapid alterations in glial membrane currents and GFAP expression in close temporal association with the development of neuronal hyperexcitability in the CCD model of neuropathic pain. However, these alterations are not fully sustained and suggest other mechanisms for the maintenance of the hyperexcitable state. © 2009 Wiley-Liss, Inc. [source] Fractional contribution of major ions to the membrane potential of Drosophila melanogaster oocytesARCHIVES OF INSECT BIOCHEMISTRY AND PHYSIOLOGY (ELECTRONIC), Issue 4 2009Susan M. Munley Abstract In ovarian follicles of Drosophila melanogaster, ion substitution experiments revealed that K+ is the greatest contributor (68%) in setting oocyte steady-state potential (Em), while Mg2+ and a metabolic component account for the rest. Because of the intense use made of Drosophila ovarian follicles in many lines of research, it is important to know how changes in the surrounding medium, particularly in major diffusible ions, may affect the physiology of the cells. The contributions made to the Drosophila oocyte membrane potential (Em) by [Na+]o, [K+]o, [Mg2+]o, [Ca2+]o, [Cl,]o, and pH (protons) were determined by substitutions made to the composition of the incubation medium. Only K+ and Mg2+ were found to participate in setting the level of Em. In follicles subjected to changes in external pH from the normal 7.3 to either pH 6 or pH 8, Em changed rapidly by about 6,mV, but within 8,min had returned to the original Em. Approximately half of all follicles exposed to reduced [Cl,]o showed no change in Em, and these all had input resistances of 330,k, or greater. The remaining follicles had smaller input resistances, and these first depolarized by about 5,mV. Over several minutes, their input resistances increased and they repolarized to a value more electronegative than their value prior to reduction in [Cl,]o. Together, K+ and Mg2+ accounted for up to 87% of measured steady-state potential. Treatment with sodium azide, ammonium vanadate, or chilling revealed a metabolically driven component that could account for the remaining 13%. © 2009 Wiley Periodicals, Inc. [source] | ||||||||||