Action Potential Firing Properties Research Articles

Neuronal Subtypes and Connectivity of the Adult Mouse Paralaminar Amygdala.

The paralaminar nucleus of the amygdala (PL) comprises neurons that exhibit delayed maturation. PL neurons are born during gestation but mature during adolescent ages, differentiating into excitatory neurons. These late-maturing PL neurons contribute to the increase in size and cell number of the amygdala between birth and adulthood. However, the function of the PL upon maturation is unknown, as the region has only recently begun to be characterized in detail. In this study, we investigated key defining features of the adult mouse PL; the intrinsic morpho-electric properties of its neurons, and its input and output circuit connectivity. We identify two subtypes of excitatory neurons in the PL based on unsupervised clustering of electrophysiological properties. These subtypes are defined by differential action potential firing properties and dendritic architecture, suggesting divergent functional roles. We further uncover major axonal inputs to the adult PL from the main olfactory network and basolateral amygdala. We also find that axonal outputs from the PL project reciprocally to these inputs and to diverse targets including the amygdala, frontal cortex, hippocampus, hypothalamus, and brainstem. Thus, the adult mouse PL is centrally placed to play a major role in the integration of olfactory sensory information, to coordinate affective and autonomic behavioral responses to salient odor stimuli.

Unblock the Block! Preventing Inhibitory Failure to Maintain Inhibitory Restraint.

Disrupting Epileptiform Activity by Preventing Parvalbumin Interneuron Depolarization Block Călin A, Ilie AS, Akerman CJ. J Neurosci. 2021;41(45):9452-9465. doi:10.1523/JNEUROSCI.1002-20.2021. Epub 2021 Oct 5.Inhibitory synaptic mechanisms oppose epileptic network activity in the brain. The breakdown in this inhibitory restraint and propagation of seizure activity has been linked to the overwhelming of feedforward inhibition, which is provided in large part by parvalbumin-expressing (PV) interneurons in the cortex. The underlying cellular processes therefore represent potential targets for understanding and preventing the propagation of seizure activity. Here we use an optogenetic strategy to test the hypothesis that depolarization block in PV interneurons is a significant factor during the loss of inhibitory restraint. Depolarization block results from the inactivation of voltage-gated sodium channels and leads to impaired action potential firing. We used focal NMDA stimulation to elicit reproducible epileptiform discharges in hippocampal organotypic brain slices from male and female mice and combined this with targeted recordings from defined neuronal populations. Simultaneous patch-clamp recordings from PV interneurons and pyramidal neurons revealed epileptiform activity that was associated with an overwhelming of inhibitory synaptic mechanisms and the emergence of a partial, and then complete, depolarization block in PV interneurons. To counteract this depolarization block, we developed protocols for eliciting pulsed membrane hyperpolarization via the inhibitory opsin, archaerhodopsin. This optical approach was effective in counteracting cumulative inactivation of voltage-gated channels, maintaining PV interneuron action potential firing properties during the inhibitory restraint period, and reducing the probability of initiating epileptiform activity. These experiments support the idea that depolarization block is a point of weakness in feedforward inhibitory synaptic mechanisms and represents a target for preventing the initiation and spread of seizure activity.

Disrupting Epileptiform Activity by Preventing Parvalbumin Interneuron Depolarization Block

Inhibitory synaptic mechanisms oppose epileptic network activity in the brain. The breakdown in this inhibitory restraint and propagation of seizure activity has been linked to the overwhelming of feedforward inhibition, which is provided in large part by parvalbumin-expressing (PV) interneurons in the cortex. The underlying cellular processes therefore represent potential targets for understanding and preventing the propagation of seizure activity. Here we use an optogenetic strategy to test the hypothesis that depolarization block in PV interneurons is a significant factor during the loss of inhibitory restraint. Depolarization block results from the inactivation of voltage-gated sodium channels and leads to impaired action potential firing. We used focal NMDA stimulation to elicit reproducible epileptiform discharges in hippocampal organotypic brain slices from male and female mice and combined this with targeted recordings from defined neuronal populations. Simultaneous patch-clamp recordings from PV interneurons and pyramidal neurons revealed epileptiform activity that was associated with an overwhelming of inhibitory synaptic mechanisms and the emergence of a partial, and then complete, depolarization block in PV interneurons. To counteract this depolarization block, we developed protocols for eliciting pulsed membrane hyperpolarization via the inhibitory opsin, archaerhodopsin. This optical approach was effective in counteracting cumulative inactivation of voltage-gated channels, maintaining PV interneuron action potential firing properties during the inhibitory restraint period, and reducing the probability of initiating epileptiform activity. These experiments support the idea that depolarization block is a point of weakness in feedforward inhibitory synaptic mechanisms and represents a target for preventing the initiation and spread of seizure activity.SIGNIFICANCE STATEMENT GABAA receptor-mediated synaptic transmission opposes seizure activity by establishing an inhibitory restraint against spreading excitation. Parvalbumin-expressing (PV) interneurons contribute significantly to this inhibitory restraint, but it has been suggested that these cells are overwhelmed as they enter a state of "depolarization block." Here we test the importance of this process by devising an optogenetic strategy to selectively relieve depolarization block in PV interneurons. By inducing brief membrane hyperpolarization, we show that it is possible to reduce depolarization block in PV interneurons, maintain their action potential firing in the face of strong excitation, and disrupt epileptiform activity in an in vitro model. This represents a proof of principle that targeting rate-limiting processes can strengthen the inhibitory restraint of epileptiform activity.

Neuron-specific responses to acetylcholine within the spinal dorsal horn circuits of rodent and primate

Excitatory and inhibitory neurotransmission within the spinal dorsal horn is tightly controlled to regulate transmission of nociceptive signals to the brain. One aspect of this control is modulation of neuronal activity through cholinergic signaling. Nociceptive neurons in the dorsal horn express both nicotinic and muscarinic cholinergic receptors and activation of these receptors reduces pain in humans, while inhibition leads to nociceptive hypersensitivity. At a cellular level, acetylcholine (ACh) has diverse effects on excitability which is dependent on the receptor and neuronal subtypes involved. In the present study we sought to characterize the electrophysiological responses of specific subsets of lamina II interneurons from rat and marmoset spinal cord. Neurons were grouped by morphology and by action potential firing properties. Whole-cell voltage-clamp recordings from lamina II dorsal horn neurons of adult rats showed that bath applied acetylcholine increased, decreased or had no effect on spontaneous synaptic current activity in a cell-type specific manner. ACh modulated inhibitory synaptic activity in 80% of neurons, whereas excitatory synaptic activity was affected in less than 50% of neurons. In whole-cell current clamp recordings, brief somatic application of ACh induced cell-type specific responses in 79% of rat lamina II neurons, which included: depolarization and action potential firing, subthreshold membrane depolarization, biphasic responses characterized by transient depolarization followed by hyperpolarization and membrane hyperpolarization alone. Similar responses were seen in marmoset lamina II neurons and the properties of each neuron group were consistent across species. ACh-induced hyperpolarization was blocked by the muscarinic antagonist atropine and all forms of acetylcholine-induced depolarization were blocked by the nicotinic antagonist mecamylamine. The cholinergic system plays an important role in regulating nociception and this study contributes to our understanding of how circuit activity is controlled by ACh at a cellular level in primate and rodent spinal cord.

The transcription factor ZEB1 regulates stem cell self-renewal and cell fate in the adult hippocampus.

SummaryRadial glia-like (RGL) stem cells persist in the adult mammalian hippocampus, where they generate new neurons and astrocytes throughout life. The process of adult neurogenesis is well documented, but cell-autonomous factors regulating neuronal and astroglial differentiation are incompletely understood. Here, we evaluate the functions of the transcription factor zinc-finger E-box binding homeobox 1 (ZEB1) in adult hippocampal RGL cells using a conditional-inducible mouse model. We find that ZEB1 is necessary for self-renewal of active RGL cells. Genetic deletion of Zeb1 causes a shift toward symmetric cell division that consumes the RGL cell and generates pro-neuronal progenies, resulting in an increase of newborn neurons and a decrease of newly generated astrocytes. We identify ZEB1 as positive regulator of the ets-domain transcription factor ETV5 that is critical for asymmetric division.

Fibroblast growth factor 13 stabilizes microtubules to promote Na+ channel function in nociceptive DRG neurons and modulates inflammatory pain

IntroductionFibroblast growth factor homologous factors (FHFs), among other fibroblast growth factors, are increasingly found to be important regulators of ion channel functions. Although FHFs have been link to several neuronal diseases and arrhythmia, its role in inflammatory pain still remains unclear. ObjectivesThis study aimed to investigate the role and mechanism of FGF13 in inflammatory pain. MethodsFgf13 conditional knockout mice were generated and CFA-induced chronic inflammatory pain model was established to measure the pain threshold. Immunostaining, western blot and quantitative real-time reverse transcription PCR (qRT-PCR) were performed to detect the expression of FGF13 in CFA-induced inflammatory pain. Whole-cell patch clamp recording was used to record the action potential firing properties and sodium currents of DRG neurons. ResultsConditional knockout of Fgf13 in dorsal root ganglion (DRG) neurons (Fgf13-/Y) led to attenuated pain responses induced by complete Freund's adjuvant (CFA). FGF13 was expressed predominantly in small-diameter DRG neurons. CFA treatment resulted in an increased expression of FGF13 proteins as well as an increased excitability in nociceptive DRG neurons which was inhibited when FGF13 was absent. The role of FGF13 in neuronal excitability of DRG was linked to its modulation of voltage-gated Na+ channels mediated by microtubules. Overexpression of FGF13, but not FGF13 mutant which lacks the ability to bind and stabilize microtubules, rescued the decreased neuronal excitability and Na+ current density in DRG neurons of Fgf13-/Y mice. ConclusionThis study revealed that FGF13 could stabilize microtubules to modulate sodium channel function in DRG neurons and modulate inflammatory pain. This study provides a novel mechanism for FGF13 modulation of sodium channel function and suggests that FGF13 might be a novel target for inflammatory pain treatment.

ON-Type Retinal Ganglion Cells are Preferentially Affected in STZ-Induced Diabetic Mice.

We investigate morphologic and physiologic alterations of ganglion cells (GCs) in a streptozocin (STZ)-induced diabetic mouse model. Experiments were conducted in flat-mount retinas of mice 3 months after the induction of diabetes. Changes in morphology of four subtypes of GCs (ON-type RGA2 [ON-RGA2], OFF-type RGA2 [OFF-RGA2], ON-type RGC1 [ON-RGC1], and ON-OFF type RGD2 [ON-OFF RGD2]) were characterized in Thy1-YFP transgenic mice. Using whole-cell patch-clamp recording, passive membrane properties and action potential (AP) firing properties were further investigated in transient ON- and OFF-RGA2 cells. Morphologic parameters were significantly altered in the dendrites branching in the ON sublamina of the inner plexiform layer (IPL) for ON-RGA2 cells and ON-OFF RGD2 cells. Much less significant changes, if any, were seen in those arborizing in the OFF sublamina of the IPL for OFF-RGA2 and ON-OFF RGD2 cells. No detectable changes in morphology were seen in RGC1 cells. Electrophysiologically, increased resting membrane potentials and decreased membrane capacitance were found in transient ON-RGA2 cells, but not in transient OFF-RGA2 cells. Similar alterations in AP firing properties, such as an increase in AP width and reduction in maximum spiking rate, were shared by these two subtypes. Furthermore, in response to depolarizing current injections, both cells generated more APs suggesting an enhanced excitability of these cells in diabetic conditions. These differential changes in morphology and electrophysiology in subtypes of GCs may be responsible for reduced contrast sensitivity known to occur during the early stage of diabetic retinopathy.

Tonically Active α5GABAA Receptors Reduce Motoneuron Excitability and Decrease the Monosynaptic Reflex.

Motoneurons, the final common path of the Central Nervous System (CNS), are under a complex control of its excitability in order to precisely translate the interneuronal pattern of activity into skeletal muscle contraction and relaxation. To fulfill this relevant function, motoneurons are provided with a vast repertoire of receptors and channels, including the extrasynaptic GABAA receptors which have been poorly investigated. Here, we confirmed that extrasynaptic α5 subunit-containing GABAA receptors localize with choline acetyltransferase (ChAT) positive cells, suggesting that these receptors are expressed in turtle motoneurons as previously reported in rodents. In these cells, α5GABAA receptors are activated by ambient GABA, producing a tonic shunt that reduces motoneurons’ membrane resistance and affects their action potential firing properties. In addition, α5GABAA receptors shunted the synaptic excitatory inputs depressing the monosynaptic reflex (MSR) induced by activation of primary afferents. Therefore, our results suggest that α5GABAA receptors may play a relevant physiological role in motor control.

Regulating excitability of peripheral afferents: emerging ion channel targets

The transmission and processing of pain signals relies critically on the activities of ion channels that are expressed in afferent pain fibers. This includes voltage-gated channels, as well as background (or leak) channels that collectively regulate resting membrane potential and action potential firing properties. Dysregulated ion channel expression in response to nerve injury and inflammation results in enhanced neuronal excitability that underlies chronic neuropathic and inflammatory pain. Pharmacological modulators of ion channels, particularly those that target channels on peripheral neurons, are being pursued as possible analgesics. Over the past few years, a number of different types of ion channels have been implicated in afferent pain signaling. Here we give an overview of recent advances on sodium, calcium, potassium and chloride channels that are emerging as especially attractive targets for the treatment of pain.

Thermal sensitivity of voltage‐gated Na+ channels and A‐type K+ channels contributes to somatosensory neuron excitability at cooling temperatures

Cooling temperatures may modify action potential firing properties to alter sensory modalities. Herein, we investigated how cooling temperatures modify action potential firing properties in two groups of rat dorsal root ganglion (DRG) neurons, tetrodotoxin-sensitive (TTXs) Na(+) channel-expressing neurons and tetrodotoxin-resistant (TTXr) Na(+) channel-expressing neurons. We found that multiple action potential firing in response to membrane depolarization was suppressed in TTXs neurons but maintained or facilitated in TTXr neurons at cooling temperatures. We showed that cooling temperatures strongly inhibited A-type K(+) currents (IA) and TTXs Na(+) channels but had fewer inhibitory effects on TTXr Na(+) channels and non-inactivating K(+) currents (IK). We demonstrated that the sensitivity of A-type K(+) channels and voltage-gated Na(+) channels to cooling temperatures and their interplay determine somatosensory neuron excitability at cooling temperatures. Our results provide a putative mechanism by which cooling temperatures modify different sensory modalities including pain.

Glycinergic and GABAergic tonic inhibition fine tune inhibitory control in regionally distinct subpopulations of dorsal horn neurons

Inhibition mediated by glycine and GABA in the spinal cord dorsal horn is essential for controlling sensitivity to painful stimuli. Loss of inhibition results in hyperalgesia, a sensitized response to a painful stimulus, and allodynia, a pain-like response to an innocuous stimulus like touch. The latter is due, in part, to disinhibition of an excitatory polysynaptic pathway linking low threshold touch input to pain projection neurons. This critical impact of disinhibition raises the issue of what regulates the activity of inhibitory interneurons in the dorsal horn under non-pathological conditions. We have found that inhibitory neurons throughout lamina I-III, identified by the GAD67 promoter-driven EGFP, are tonically inhibited by glycine or GABA in a regionally distinct way that is mirrored by their inhibitory synaptic input. This tonic inhibition strongly modifies action potential firing properties. Surprisingly, we found that inhibitory neurons at the lamina II/III border are under tonic glycinergic control and receive synapses that are predominantly glycinergic. Futhermore, this tonic glycinergic inhibition remains strong as the mice mature postnatally. Interestingly, GlyT1, the glial glycine transporter, regulates the strength of tonic glycinergic inhibition of these glycine-dominant neurons. The more dorsal lamina I and IIo inhibitory neurons are mainly under control by tonic GABA action and receive synapses that are predominantly GABAergic. Our work supports the hypothesis that tonic glycine inhibition controls the inhibitory circuitry deep in lamina II that is likely to be responsible for separating low threshold input from high threshold output neurons of lamina I.

A Potassium Channel Expressed in the Olfactory Bulb Subserves as a Metabolic Sensor

Event Abstract Back to Event A Potassium Channel Expressed in the Olfactory Bulb Subserves as a Metabolic Sensor Kristal Tucker1*, Melissa Cavallin1, Patrick Jean-Baptiste2, Michael Overton3 and Debra A. Fadool1, 2, 4 1 Florida State University, Program in Neuroscience, United States 2 Florida State University, Department of Biological Science, United States 3 Florida State University, College of Medicine, United States 4 Florida State University, Institute of Molecular Biophysics, United States Naturally occurring modulators of the Kv1.3 ion channel expressed in mitral cells located in the olfactory bulb (OB), include the insulin receptor and the TrkB neurotrophin receptor - both of which have been implicated in other parts of the CNS to modulate metabolism. We now demonstrate via OB slice electrophysiology that mitral cell action potential firing properties are sensitive to another metabolically important substance, glucose. Like the hypothalamus, we have found the olfactory bulb contains two populations of glucose-sensitive mitral cells; glucose excited and glucose inhibited. Gene-targeted deletion or acute suppression of Kv1.3 results in an overall increased mitral cell sensitivity via increased-current evoked spiking frequency, decreased latency to first spike, and a more depolarized resting membrane potential. To explore the correlation between Kv1.3, metabolism, and olfaction, 11 week old mice were maintained on a moderately high fat diet (MHF, 32% fat) for 26 weeks and then systems physiological parameters of body weight, oxygen consumption, locomotion, and ingestive behaviors were quantified in a custom designed, computer interfaced, metabolic chamber. Diet-induced obese (DIO) mice exhibited a 47% increase in body weight, a 32% increase in serum insulin, and a loss of 52% of M72-expressing olfactory sensory neurons (OSNs). Kv1.3-null mice were resistant to DIO with a weight gain of only 10% and no change in adiposity as a result of a significant increase in basal metabolic rate linked to the MHF challenge. Bilateral olfactory bulbectomy (OBX) in a Kv1.3-null background yielded mice that were no longer resistant to DIO. The mice exhibited a 30% increase in body weight by preventing the increase in basal metabolic rate in response to the MHF challenge and decreased activity-dependent metabolism resulting in decreased total energy expenditure. Conference: 2010 South East Nerve Net (SENN) and Georgia/South Carolina Neuroscience Consortium (GASCNC) conferences, Atlanta , United States, 5 Mar - 7 Mar, 2010. Presentation Type: Poster Presentation Topic: Posters Citation: Tucker K, Cavallin M, Jean-Baptiste P, Overton M and Fadool DA (2010). A Potassium Channel Expressed in the Olfactory Bulb Subserves as a Metabolic Sensor. Front. Neurosci. Conference Abstract: 2010 South East Nerve Net (SENN) and Georgia/South Carolina Neuroscience Consortium (GASCNC) conferences. doi: 10.3389/conf.fnins.2010.04.00084 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 18 Mar 2010; Published Online: 18 Mar 2010. * Correspondence: Kristal Tucker, Florida State University, Program in Neuroscience, Tallahassee, United States, tucker@neuro.fsu.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Kristal Tucker Melissa Cavallin Patrick Jean-Baptiste Michael Overton Debra A Fadool Google Kristal Tucker Melissa Cavallin Patrick Jean-Baptiste Michael Overton Debra A Fadool Google Scholar Kristal Tucker Melissa Cavallin Patrick Jean-Baptiste Michael Overton Debra A Fadool PubMed Kristal Tucker Melissa Cavallin Patrick Jean-Baptiste Michael Overton Debra A Fadool Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

CGRP Selectively Activates a Subpopulation of Nodose Sensory Neurons with Tonic Action Potential Firing Properties

Calcitonin gene‐related peptide (CGRP) and its receptors are highly expressed in subpopulations of sensory C‐fibers innervating heart, viscera, and blood vessels including carotid sinuses. While CGRP release and its vasodilator activity have been studied extensively, little is known regarding its effects on the activity of vagal afferent and baroreceptor neurons in nodose ganglia. We hypothesized that CGRP may selectively activate a subpopulation of nodose sensory neurons. Resting membrane potential (RMP) and action potential (AP) responses to depolarizing current injection (0.1‐1.0nA for 1s) were recorded by intracellular microelectrode from neurons isolated from nodose ganglia of C57BL/6 mice. Two types of neurons were studied: 'tonic' neurons that repetitively fire APs during sustained depolarization, and 'phasic' neurons that fire only a single AP at the beginning of the period of depolarization. CGRP (500 nM) depolarized 8 of 10 tonic neurons from ‐52±2 to ‐47±2 mV and increased AP discharge from 17±2 to 29±2 spikes during the period of current injection (n=8, P<0.01). These excitatory effects were fully reversed upon drug washout. In contrast, CGRP did not change RMP or AP discharge in phasic neurons (n=6). Consistent with these findings, CGRP increased cytosolic Ca2+ concentration (Fluo‐4) in a subset of rat nodose neurons. We conclude that CGRP selectively activates the majority of tonic nodose sensory neurons without affecting RMP or AP discharge in phasic neurons. The selectivity of the response likely reflects co‐expression of CGRP receptors with ion channels enabling repetitive spike firing in tonic neurons. (HL14388, VA)

Multiple Kv Channel-interacting Proteins Contain an N-terminal Transmembrane Domain That Regulates Kv4 Channel Trafficking and Gating

Kv channel-interacting proteins (KChIPs) are auxiliary subunits of the heteromultimeric channel complexes that underlie neuronal I(SA), the subthreshold transient K(+) current that dynamically regulates membrane excitability, action potential firing properties, and long term potentiation. KChIPs form cytoplasmic associations with the principal pore-forming Kv4 subunits and typically mediate enhanced surface expression and accelerated recovery from depolarization-induced inactivation. An exception is KChIP4a, which dramatically suppresses Kv4 inactivation while promoting neither surface expression nor recovery. These unusual properties are attributed to the effects of a K channel inactivation suppressor domain (KISD) encoded within the variable N terminus of KChIP4a. Here, we have functionally and biochemically characterized two brain KChIP isoforms, KChIP2x and KChIP3x (also known as KChIP3b) and show that they also contain a functional KISD. Like KChIP4a and in contrast with non-KISD-containing KChIPs, both KChIP2x and KChIP3x strongly suppress inactivation and slow activation and inhibit the typical increases in surface expression of Kv4.2 channels. We then examined the properties of the KISD to determine potential mechanisms for its action. Subcellular fractionation shows that KChIP4a, KChIP2x, and KChIP3x are highly associated with the membrane fraction. Fluorescent confocal imaging of enhanced green fluorescent proteins (eGFP) N-terminally fused with KISD in HEK293T cells indicates that KISDs of KChIP4a, KChIP2x, and KChIP3x all autonomously target eGFP to intracellular membranes. Cell surface biotinylation experiments on KChIP4a indicate that the N terminus is exposed extracellularly, consistent with a transmembrane KISD. In summary, KChIP4a, KChIP2x, and KChIP3x comprise a novel class of KChIP isoforms characterized by an unusual transmembrane domain at their N termini that modulates Kv4 channel gating and trafficking.

Manipulating Kv4.2 identifies a specific component of hippocampal pyramidal neuron A‐current that depends upon Kv4.2 expression

The somatodendritic A-current, I(SA), in hippocampal CA1 pyramidal neurons regulates the processing of synaptic inputs and the amplitude of back propagating action potentials into the dendritic tree, as well as the action potential firing properties at the soma. In this study, we have used RNA interference and over-expression to show that expression of the Kv4.2 gene specifically regulates the I(SA) component of A-current in these neurons. In dissociated hippocampal pyramidal neuron cultures, or organotypic cultured CA1 pyramidal neurons, the expression level of Kv4.2 is such that the I(SA) channels are maintained in the population at a peak conductance of approximately 950 pS/pF. Suppression of Kv4.2 transcripts in hippocampal pyramidal neurons using an RNA interference vector suppresses I(SA) current by 60% in 2 days, similar to the effect of expressing dominant-negative Kv4 channel constructs. Increasing the expression of Kv4.2 in these neurons increases the level of I(SA) to 170% of the normal set point without altering the biophysical properties. Our results establish a specific role for native Kv4.2 transcripts in forming and maintaining I(SA) current at characteristic levels in hippocampal pyramidal neurons.

Whole-cell recording from honeybee olfactory receptor neurons: ionic currents, membrane excitability and odourant response in developing workerbee and drone.

Whole-cell recording techniques were used to characterize ionic membrane currents and odourant responses in honeybee olfactory receptor neurons (ORNs) in primary cell culture. ORNs of workerbee (female) and drone (male) were isolated at an early stage of development before sensory axons connect to their target in the antennal lobe. The results collectively indicate that honeybee ORNs have electrical properties similar, but not necessarily identical to, those currently envisaged for ORNs of other species. Under voltage clamp at least four ionic currents could be distinguished. Inward currents were made of a fast transient, tetrodotoxin-sensitive sodium current. In some ORNs a cadmium-sensitive calcium current was detected. ORNs showed heterogeneity in their outward currents: either outward currents were made of a delayed rectifier type potassium current, which was partially blocked by tetraethyl ammonium or quinidine, or were composed of a delayed rectifier type and a transient calcium-dependent potassium current, which was cadmium-sensitive and abolished by removal of external calcium. The proportion of each of the two outward currents, however, was different within the ORNs of the two sexes suggesting a gender-specific functional heterogeneity. ORNs showed heterogeneity in action potential firing properties: depolarizing current steps elicited either one action potential or, as in most of the cells, it led to repetitive spiking. Action potentials were tetrodotoxin-sensitive suggesting they are carried by sodium. Odourant stimulation with different mixtures and pure substances evoked depolarizing receptor potentials with superimposed action potentials when spike threshold was reached. In summary, honeybee ORNs are remarkably mature at early stages in their development.

Ionic currents of drosophila embryonic neurons derived from selectively cultured CNS midline precursors

In order to investigate the electrogenesis of defined cell populations, we applied an in vitro system that allows the selective culturing of individual Drosophila CNS precursors under different conditions. CNS midline (ML) precursors prepared from gastrula stage embryos gave rise to progeny cells with neuronal and glial morphology that expressed specific markers. Using whole-cell patch-clamp recordings, a detailed description of ionic currents present in this defined cell population is provided. Most ionic currents of cultured ML neurons were similar to other cultured Drosophila neurons, even though their embryonic origin is different. They displayed at least two voltage-gated potassium currents, a voltage-gated sodium, two voltage-gated calcium currents, and responded to the neurotransmitters ACh and GABA. They showed homogeneity in action potential firing properties, generating only a single spike even upon sustained depolarization. Interestingly, although the expression of the voltage-gated potassium currents appeared to be highly cell autonomous, for all other currents significant changes were observed in the presence of fiber contacts. © 2000 John Wiley & Sons, Inc. J Neurobiol 44: 392–413, 2000

Ionic currents ofdrosophila embryonic neurons derived from selectively cultured CNS midline precursors

In order to investigate the electrogenesis of defined cell populations, we applied an in vitro system that allows the selective culturing of individual Drosophila CNS precursors under different conditions. CNS midline (ML) precursors prepared from gastrula stage embryos gave rise to progeny cells with neuronal and glial morphology that expressed specific markers. Using whole-cell patch-clamp recordings, a detailed description of ionic currents present in this defined cell population is provided. Most ionic currents of cultured ML neurons were similar to other cultured Drosophila neurons, even though their embryonic origin is different. They displayed at least two voltage-gated potassium currents, a voltage-gated sodium, two voltage-gated calcium currents, and responded to the neurotransmitters ACh and GABA. They showed homogeneity in action potential firing properties, generating only a single spike even upon sustained depolarization. Interestingly, although the expression of the voltage-gated potassium currents appeared to be highly cell autonomous, for all other currents significant changes were observed in the presence of fiber contacts.

Influence of the mucosa on the excitability of myenteric neurons.

Intracellular microelectrodes were used to examine the active and passive membrane properties of neurons in the myenteric plexus of the guinea-pig small intestine. Neurons of two types were examined: S neurons, which have prominent fast excitatory postsynaptic potentials and in which action potentials are not followed by long-lasting afterhyperpolarizations, and AH neurons, which have long-lasting afterhyperpolarizations following soma action potentials. In preparations in which the myenteric ganglia and longitudinal muscle, but no mucosa, were present, most S neurons (59/64) responded to intracellular depolarizing current with brief bursts of action potentials. Regardless of the strength of a depolarizing current of 500-ms duration, these neurons never fired action potentials beyond the first 250 ms. S neurons in this state were called rapidly accommodating. In contrast, within 600 microm circumferential to the intact mucosa, 26/58 S neurons fired action potentials for most or all of the period of a 500-ms insightful depolarizing pulse. S neurons in this state were called slowly accommodating. Depolarization of S neurons in the rapidly accommodating state caused a rapidly developing reduction in membrane resistance (outward rectification; onset about 7 ms). This rectification was absent from S neurons in the slowly accommodating state. Tetraethylammonium blocked the early rectification and the changed neuronal state from rapidly accommodating to slowly accommodating. Application of tetrodotoxin to neurons in the slowly accommodating state revealed the early rectification, indicating that its absence from these neurons before tetrodotoxin was applied had been due to ongoing activity in axons providing synaptic input to the neurons. After the mucosa was disconnected from the other layers and laid back in its original position, all S neurons close to the mucosa were in the rapidly accommodating state (17/17). Slow excitatory postsynaptic potentials, evoked by electrical stimulation of nerve tracts, converted 17 of 43 S neurons from rapidly accommodating to slowly accommodating and eliminated the early outward rectification in these neurons. These results indicate that the action potential firing properties of S neurons can be changed by external influences, including the activity of synaptic inputs that release a slowly acting transmitter. Spontaneous antidromic action potentials were recorded in 8/62 AH neurons within 600 microm circumferential to the intact mucosa. It is concluded that, when the mucosa is intact, a background firing of sensory neurons occurs which leads to a state change in many S neurons innervated by the active sensory neurons. We conclude that this state change is caused by the block of a voltage-sensitive outward rectification.