Sensitization of human and rat nociceptors by low dose morphine is toll-like receptor 4-dependent

While opioids remain amongst the most effective treatments for moderate-to-severe pain, their substantial side effect profile remains a major limitation to broader clinical use. One such side effect is opioid-induced hyperalgesia (OIH), which includes a transition from opioid-induced analgesia to pain enhancement. Evidence in rodents supports the suggestion that OIH may be produced by the action of opioids at Toll-like Receptor 4 (TLR4) either on immune cells that, in turn, produce pronociceptive mediators to act on nociceptors, or by a direct action at nociceptor TLR4. And, sub-analgesic doses of several opioids have been shown to induce hyperalgesia in rodents by their action as TLR4 agonists. In the present in vitro patch-clamp electrophysiology experiments, we demonstrate that low dose morphine directly sensitizes human as well as rodent dorsal root ganglion (DRG) neurons, an effect of this opioid analgesic that is antagonized by LPS-RS Ultrapure, a selective TLR4 antagonist. We found that low concentration (100 nM) of morphine reduced rheobase in human (by 36%) and rat (by 26%) putative C-type nociceptors, an effect of morphine that was markedly attenuated by preincubation with LPS-RS Ultrapure. Our findings support the suggestion that in humans, as in rodents, OIH is mediated by the direct action of opioids at TLR4 on nociceptors.

Structural dynamics of GluK2 kainate receptors in apo and partial agonist bound states

Kainate receptors (KARs) belong to the family of ionotropic glutamate receptors (iGluRs) and are tetrameric ligand-gated ion channels that regulate neurotransmitter release and excitatory synaptic transmission in the central nervous system. While KARs share overall architectures with other iGluR subfamilies, their dynamics are significantly different from those of other iGluRs. KARs are activated by both full and partial agonists. While there is less efficacy with partial agonists than with full agonists, the detailed mechanism has remained elusive. Here, we used cryo-electron microscopy to determine the structures of homomeric rat GluK2 KARs in the absence of ligands (apo) and in complex with a partial agonist. Intriguingly, the apo state KARs were captured in desensitized conformation. This structure confirms the KAR desensitization prior to activation. Structures of KARs complexed to the partial agonist domoate populate in domoate bound desensitized and non-active/non-desensitized states. These previously unseen intermediate structures highlight the molecular mechanism of partial agonism in KARs. Additionally, we show how N-glycans stabilized the ligand-binding domain dimer via cation/anion binding and modulated receptor gating properties using electrophysiology. Our findings provide vital structural and functional insights into the unique KAR gating mechanisms.

Mechanistic Insight into the Early Stages of Toroidal Pore Formation by the Antimicrobial Peptide Smp24

The antimicrobial peptide Smp24, originally derived from the venom of Scorpio maurus palmatus, is a promising candidate for further drug development. However, before doing so, greater insight into the mechanism of action is needed to construct a reliable structure-activity relationship. The aim of this study was to specifically investigate the critical early stages of peptide-induced membrane disruption. Single-channel current traces were obtained via planar patch-clamp electrophysiology, with multiple types of pore-forming events observed, unlike those expected from the traditional, more rigid mechanistic models. To better understand the molecular-level structures of the peptide-pore assemblies underlying these observed conductance events, molecular dynamics simulations were used to investigate the peptide structure and orientation both before and during pore formation. The transition of the peptides to transmembrane-like states within disordered toroidal pores occurred due to a peptide-induced bilayer-leaflet asymmetry, explaining why pore stabilization does not always follow pore nucleation in the experimental observations. To fully grasp the structure-activity relationship of antimicrobial peptides, a more nuanced view of the complex and dynamic mechanistic behaviour must be adopted.

Motor cortex analogue neurons in songbirds utilize Kv3 channels to generate ultranarrow spikes

Complex motor skills in vertebrates require specialized upper motor neurons with precise action potential (AP) firing. To examine how diverse populations of upper motor neurons subserve distinct functions and the specific repertoire of ion channels involved, we conducted a thorough study of the excitability of upper motor neurons controlling somatic motor function in the zebra finch. We found that robustus arcopallialis projection neurons (RAPNs), key command neurons for song production, exhibit ultranarrow spikes and higher firing rates compared to neurons controlling non-vocal somatic motor functions (dorsal intermediate arcopallium [AId] neurons). Pharmacological and molecular data indicate that this striking difference is associated with the higher expression in RAPNs of high threshold, fast-activating voltage-gated Kv3 channels, that likely contain Kv3.1 (KCNC1) subunits. The spike waveform and Kv3.1 expression in RAPNs mirror properties of Betz cells, specialized upper motor neurons involved in fine digit control in humans and other primates but absent in rodents. Our study thus provides evidence that songbirds and primates have convergently evolved the use of Kv3.1 to ensure precise, rapid AP firing in upper motor neurons controlling fast and complex motor skills.

High-Fat Diet Modulates the Excitability of Neurons within the Brain-Liver Pathway

Stimulation of hepatic sympathetic nerves increases glucose production and glycogenolysis. Activity of pre-sympathetic neurons in the paraventricular nucleus (PVN) of the hypothalamus and in the ventrolateral and ventromedial medulla (VLM/VMM) largely influence the sympathetic output. Increased activity of the sympathetic nervous system (SNS) plays a role in the development and progression of metabolic diseases; however, despite the importance of the central circuits, the excitability of pre-sympathetic liver-related neurons remains to be determined. Here, we tested the hypothesis that the activity of liver-related neurons in the PVN and VLM/VMM is altered in diet-induced obese mice, as well as their response to insulin. Patch-clamp recordings were conducted from liver-related PVN neurons, VLM-projecting PVN neurons, and pre-sympathetic liver-related neurons in the ventral brainstem. Our data demonstrate that the excitability of liver-related PVN neurons increased in high-fat diet (HFD)-fed mice compared to mice fed with control diet. Insulin receptor expression was detected in a population of liver-related neurons, and insulin suppressed the firing activity of liver-related PVN and pre-sympathetic VLM/VMM neurons in HFD mice; however, it did not affect VLM-projecting liver-related PVN neurons. These findings further suggest that HFD alters the excitability of pre-autonomic neurons as well as their response to insulin.

Studying Synaptic Connectivity and Strength with Optogenetics and Patch-Clamp Electrophysiology

Over the last two decades the combination of brain slice patch clamp electrophysiology with optogenetic stimulation has proven to be a powerful approach to analyze the architecture of neural circuits and (experience-dependent) synaptic plasticity in such networks. Using this combination of methods, originally termed channelrhodopsin-assisted circuit mapping (CRACM), a multitude of measures of synaptic functioning can be taken. The current review discusses their rationale, current applications in the field, and their associated caveats. Specifically, the review addresses: (1) How to assess the presence of synaptic connections, both in terms of ionotropic versus metabotropic receptor signaling, and in terms of mono- versus polysynaptic connectivity. (2) How to acquire and interpret measures for synaptic strength and function, like AMPAR/NMDAR, AMPAR rectification, paired-pulse ratio (PPR), coefficient of variance and input-specific quantal sizes. We also address how synaptic modulation by G protein-coupled receptors can be studied with pharmacological approaches and advanced technology. (3) Finally, we elaborate on advances on the use of dual color optogenetics in concurrent investigation of multiple synaptic pathways. Overall, with this review we seek to provide practical insights into the methods used to study neural circuits and synapses, by combining optogenetics and patch-clamp electrophysiology.

Alcohol Dependence Induces CRF Sensitivity in Female Central Amygdala GABA Synapses

Alcohol use disorder (AUD) is a chronically relapsing disease characterized by loss of control in seeking and consuming alcohol (ethanol) driven by the recruitment of brain stress systems. However, AUD differs among the sexes: men are more likely to develop AUD, but women progress from casual to binge drinking and heavy alcohol use more quickly. The central amygdala (CeA) is a hub of stress and anxiety, with corticotropin-releasing factor (CRF)-CRF1 receptor and Gamma-Aminobutyric Acid (GABA)-ergic signaling dysregulation occurring in alcohol-dependent male rodents. However, we recently showed that GABAergic synapses in female rats are less sensitive to the acute effects of ethanol. Here, we used patch-clamp electrophysiology to examine the effects of alcohol dependence on the CRF modulation of rat CeA GABAergic transmission of both sexes. We found that GABAergic synapses of naïve female rats were unresponsive to CRF application compared to males, although alcohol dependence induced a similar CRF responsivity in both sexes. In situ hybridization revealed that females had fewer CeA neurons containing mRNA for the CRF1 receptor (Crhr1) than males, but in dependence, the percentage of Crhr1-expressing neurons in females increased, unlike in males. Overall, our data provide evidence for sexually dimorphic CeA CRF system effects on GABAergic synapses in dependence.

A calibrated functional patch-clamp assay to enhance clinical variant interpretation in KCNH2-related long QT syndrome

Modern sequencing technologies have revolutionized our detection of gene variants. However, in most genes, including KCNH2, the majority of missense variants are currently classified as variants of uncertain significance (VUSs). The aim of this study was to investigate the utility of an automated patch-clamp assay for aiding clinical variant classification in KCNH2. The assay was designed according to recommendations proposed by the Clinical Genome Sequence Variant Interpretation Working Group. Thirty-one variants (17 pathogenic/likely pathogenic, 14 benign/likely benign) were classified internally as variant controls. They were heterozygously expressed in Flp-In HEK293 cells for assessing the effects of variants on current density and channel gating in order to determine the sensitivity and specificity of the assay. All 17 pathogenic variant controls had reduced current density, and 13 of 14 benign variant controls had normal current density, which enabled determination of normal and abnormal ranges for applying evidence of moderate or supporting strength for VUS reclassification. Inclusion of functional assay evidence enabled us to reclassify 6 out of 44 KCNH2 VUSs as likely pathogenic. The high-throughput patch-clamp assay can provide moderate-strength evidence for clinical interpretation of clinical KCNH2 variants and demonstrates the value of developing automated patch-clamp assays for functional characterization of ion channel gene variants.

Functional Voltage-Gated Sodium Channels Are Present in the Human B Cell Membrane

B cells express various ion channels, but the presence of voltage-gated sodium (Na_V) channels has not been confirmed in the plasma membrane yet. In this study, we have identified several Na_V channels, which are expressed in the human B cell membrane, by electrophysiological and molecular biology methods. The sensitivity of the detected sodium current to tetrodotoxin was between the values published for TTX-sensitive and TTX-insensitive channels, which suggests the co-existence of multiple Na_V1 subtypes in the B cell membrane. This was confirmed by RT-qPCR results, which showed high expression of TTX-sensitive channels along with the lower expression of TTX-insensitive Na_V1 channels. The biophysical characteristics of the currents also supported the expression of multiple Na_V channels. In addition, we investigated the potential functional role of Na_V channels by membrane potential measurements. Removal of Na⁺ from the extracellular solution caused a reversible hyperpolarization, supporting the role of Na_V channels in shaping and maintaining the resting membrane potential. As this study was mainly limited to electrophysiological properties, we cannot exclude the possible non-canonical functions of these channels. This work concludes that the presence of voltage-gated sodium channels in the plasma membrane of human B cells should be recognized and accounted for in the future.

Teaching bioelectricity and neurophysiology to medical students using LabAXON simulations

Patch-clamp electrophysiological recordings of neuronal activity require a large amount of space and equipment. The technique is difficult to master and not conducive to demonstration to more than a few medical students. Therefore, neurophysiological education is mostly limited to classroom-based pedagogies such as lectures. However, the demonstration of concepts such as changes in membrane potential and ion channel activity is best achieved with hands-on approaches. This article details an in silico activity suitable for large groups of medical students that demonstrates the key concepts in neurophysiology using the LabAXON simulation software. Learning activities in our practical include 1) measurements of voltage and time parameters of the neuronal action potential and its relationship to the Nernst potentials of Na⁺ and K⁺; 2) determination of the stimulus threshold to evoke action potentials; 3) demonstration of the refractory period of an action potential; and 4) voltage-clamp experiments to determine the current-voltage relationship of voltage-gated Na⁺ and K⁺ channels and the voltage dependence of, and recovery from, inactivation of voltage-gated Na⁺ channels. We emphasized the accuracy of quantitative measurements as well as the correct use of units. The level of difficulty of the activity can be altered through different multiple choice questions relating to material introduced in the associated lectures. This practical activity is suitable for different class sizes and is adaptable for delivery with online platforms. Student feedback showed that the students felt the activity helped them consolidate their understanding of the lecture material.

Pharmacological Inhibition of Wee1 Kinase Selectively Modulates the Voltage-Gated Na+ Channel 1.2 Macromolecular Complex

Voltage-gated Na+ (Nav) channels are a primary molecular determinant of the action potential (AP). Despite the canonical role of the pore-forming α subunit in conferring this function, protein-protein interactions (PPI) between the Nav channel α subunit and its auxiliary proteins are necessary to reconstitute the full physiological activity of the channel and to fine-tune neuronal excitability. In the brain, the Nav channel isoforms 1.2 (Nav1.2) and 1.6 (Nav1.6) are enriched, and their activities are differentially regulated by the Nav channel auxiliary protein fibroblast growth factor 14 (FGF14). Despite the known regulation of neuronal Nav channel activity by FGF14, less is known about cellular signaling molecules that might modulate these regulatory effects of FGF14. To that end, and building upon our previous investigations suggesting that neuronal Nav channel activity is regulated by a kinase network involving GSK3, AKT, and Wee1, we interrogate in our current investigation how pharmacological inhibition of Wee1 kinase, a serine/threonine and tyrosine kinase that is a crucial component of the G2-M cell cycle checkpoint, affects the Nav1.2 and Nav1.6 channel macromolecular complexes. Our results show that the highly selective inhibitor of Wee1 kinase, called Wee1 inhibitor II, modulates FGF14:Nav1.2 complex assembly, but does not significantly affect FGF14:Nav1.6 complex assembly. These results are functionally recapitulated, as Wee1 inhibitor II entirely alters FGF14-mediated regulation of the Nav1.2 channel, but displays no effects on the Nav1.6 channel. At the molecular level, these effects of Wee1 inhibitor II on FGF14:Nav1.2 complex assembly and FGF14-mediated regulation of Nav1.2-mediated Na+ currents are shown to be dependent upon the presence of Y158 of FGF14, a residue known to be a prominent site for phosphorylation-mediated regulation of the protein. Overall, our data suggest that pharmacological inhibition of Wee1 confers selective modulatory effects on Nav1.2 channel activity, which has important implications for unraveling cellular signaling pathways that fine-tune neuronal excitability.

Nucleus Reuniens Afferents in Hippocampus Modulate CA1 Network Function via Monosynaptic Excitation and Polysynaptic Inhibition

The thalamic midline nucleus reuniens modulates hippocampal CA1 and subiculum function via dense projections to the stratum lacunosum-moleculare (SLM). Previously, anatomical data has shown that reuniens inputs in the SLM form synapses with dendrites of both CA1 principal cells and inhibitory interneurons. However, the ability of thalamic inputs to excite the CA1 principal cells remains controversial. In addition, nothing is known about the impact of reuniens inputs on diverse subpopulations of interneurons in CA1. Therefore, using whole cell patch-clamp electrophysiology in ex vivo hippocampal slices of wild-type and transgenic mice, we measured synaptic responses in different CA1 neuronal subtypes to optogenetic stimulation of reuniens afferents. Our data shows that reuniens inputs mediate both excitation and inhibition of the CA1 principal cells. However, the optogenetic excitation of the reuniens inputs failed to drive action potential firing in the majority of the principal cells. While the excitatory postsynaptic currents were mediated via direct monosynaptic activation of the CA1 principal cells, the inhibitory postsynaptic currents were generated polysynaptically via activation of local GABAergic interneurons. Moreover, we demonstrate that optogenetic stimulation of reuniens inputs differentially recruit at least two distinct and non-overlapping subpopulations of local GABAergic interneurons in CA1. We show that neurogliaform cells located in SLM, and calretinin-containing interneuron-selective interneurons at the SLM/stratum radiatum border can be excited by stimulation of reuniens inputs. Together, our data demonstrate that optogenetic stimulation of reuniens afferents can mediate excitation, feedforward inhibition, and disinhibition of the postsynaptic CA1 principal cells via multiple direct and indirect mechanisms.

Differential Modulation of the Voltage-Gated Na+ Channel 1.6 by Peptides Derived From Fibroblast Growth Factor 14

The voltage-gated Na⁺ (Nav) channel is a primary molecular determinant of the initiation and propagation of the action potential. Despite the central role of the pore-forming α subunit in conferring this functionality, protein:protein interactions (PPI) between the α subunit and auxiliary proteins are necessary for the full physiological activity of Nav channels. In the central nervous system (CNS), one such PPI occurs between the C-terminal domain of the Nav1.6 channel and fibroblast growth factor 14 (FGF14). Given the primacy of this PPI in regulating the excitability of neurons in clinically relevant brain regions, peptides targeting the FGF14:Nav1.6 PPI interface could be of pre-clinical value. In this work, we pharmacologically evaluated peptides derived from FGF14 that correspond to residues that are at FGF14's PPI interface with the CTD of Nav1.6. These peptides, Pro-Leu-Glu-Val (PLEV) and Glu-Tyr-Tyr-Val (EYYV), which correspond to residues of the β12 sheet and β8-β9 loop of FGF14, respectively, were shown to inhibit FGF14:Nav1.6 complex assembly. In functional studies using whole-cell patch-clamp electrophysiology, PLEV and EYYV were shown to confer differential modulation of Nav1.6-mediated currents through mechanisms dependent upon the presence of FGF14. Crucially, these FGF14-dependent effects of PLEV and EYYV on Nav1.6-mediated currents were further shown to be dependent on the N-terminal domain of FGF14. Overall, these data suggest that the PLEV and EYYV peptides represent scaffolds to interrogate the Nav1.6 channel macromolecular complex in an effort to develop targeted pharmacological modulators.

Ion-channel degeneracy: Multiple ion channels heterogeneously regulate intrinsic physiology of rat hippocampal granule cells

Degeneracy, the ability of multiple structural components to elicit the same characteristic functional properties, constitutes an elegant mechanism for achieving biological robustness. In this study, we sought electrophysiological signatures for the expression of ion-channel degeneracy in the emergence of intrinsic properties of rat hippocampal granule cells. We measured the impact of four different ion-channel subtypes-hyperpolarization-activated cyclic-nucleotide-gated (HCN), barium-sensitive inward rectifier potassium (Kir ), tertiapin-Q-sensitive inward rectifier potassium, and persistent sodium (NaP) channels-on 21 functional measurements employing pharmacological agents, and report electrophysiological data on two characteristic signatures for the expression of ion-channel degeneracy in granule cells. First, the blockade of a specific ion-channel subtype altered several, but not all, functional measurements. Furthermore, any given functional measurement was altered by the blockade of many, but not all, ion-channel subtypes. Second, the impact of blocking each ion-channel subtype manifested neuron-to-neuron variability in the quantum of changes in the electrophysiological measurements. Specifically, we found that blocking HCN or Ba-sensitive Kir channels enhanced action potential firing rate, but blockade of NaP channels reduced firing rate of granule cells. Subthreshold measures of granule cell intrinsic excitability (input resistance, temporal summation, and impedance amplitude) were enhanced by blockade of HCN or Ba-sensitive Kir channels, but were not significantly altered by NaP channel blockade. We confirmed that the HCN and Ba-sensitive Kir channels independently altered sub- and suprathreshold properties of granule cells through sequential application of pharmacological agents that blocked these channels. Finally, we found that none of the sub- or suprathreshold measurements of granule cells were significantly altered upon treatment with tertiapin-Q. Together, the heterogeneous many-to-many mapping between ion channels and single-neuron intrinsic properties emphasizes the need to account for ion-channel degeneracy in cellular- and network-scale physiology.

Inhibition of SARS CoV Envelope Protein by Flavonoids and Classical Viroporin Inhibitors

Severe acute respiratory syndrome coronavirus (SARS-CoV), an enveloped single-stranded positive-sense RNA virus, is a member of the genus Betacoronavirus, family Coronaviridae. The SARS-CoV envelope protein E is a small (∼8.4 kDa) channel-forming membrane protein whose sequence is highly conserved between SARS-CoV and SARS-CoV-2. As a viroporin, it is involved in various aspects of the virus life cycle including assembly, budding, envelope formation, virus release, and inflammasome activation. Here, SARS-CoV E protein was recombinantly expressed in HEK293 cells and channel activity and the effects of viroporin inhibitors studied using patch-clamp electrophysiology and a cell viability assay. We introduced a membrane-directing signal peptide to ensure transfer of recombinant E protein to the plasma membrane. E protein expression induced transmembrane currents that were blocked by various inhibitors. In an ion-reduced buffer system, currents were proton-dependent and blocked by viroporin inhibitors rimantadine and amantadine. I-V relationships of recombinant E protein were not pH-dependent in a classical buffer system with high extracellular Na⁺ and high intracellular K⁺. E-protein mediated currents were inhibited by amantadine and rimantadine, as well as 5-(N,N-hexamethylene)amiloride (HMA). We tested a total of 10 flavonoids, finding inhibitory activity of varying potency. Epigallocatechin and quercetin were most effective, with IC₅₀ values of 1.5 ± 0.1 and 3.7 ± 0.2 nM, respectively, similar to the potency of rimantadine (IC₅₀ = 1.7 ± 0.6 nM). Patch-clamp results were independently verified using a modified cell viability assay for viroporin inhibitors. These results contribute to the development of novel antiviral drugs that suppress virus activity and proliferation.

VGluT1 Deficiency Impairs Visual Attention and Reduces the Dynamic Range of Short-Term Plasticity at Corticothalamic Synapses

The most common excitatory neurotransmitter in the central nervous system, glutamate, is loaded into synaptic vesicles by vesicular glutamate transporters (VGluTs). The primary isoforms, VGluT1 and 2, are expressed in complementary patterns throughout the brain and correlate with short-term synaptic plasticity. VGluT1 deficiency is observed in certain neurological disorders, and hemizygous (VGluT1^+/−) mice display increased anxiety and depression, altered sensorimotor gating, and impairments in learning and memory. The synaptic mechanisms underlying these behavioral deficits are unknown. Here, we show that VGluT1^+/− mice had decreased visual processing speeds during a sustained visual-spatial attention task. Furthermore, in vitro recordings of corticothalamic (CT) synapses revealed dramatic reductions in short-term facilitation, increased initial release probability, and earlier synaptic depression in VGluT1^+/− mice. Our electron microscopy results show that VGluT1 concentration is reduced at CT synapses of hemizygous mice, but other features (such as vesicle number and active zone size) are unchanged. We conclude that VGluT1-haploinsuficiency decreases the dynamic range of gain modulation provided by CT feedback to the thalamus, and this deficiency contributes to the observed attentional processing deficit. We further hypothesize that VGluT1 concentration regulates release probability by applying a “brake” to an unidentified presynaptic protein that typically acts as a positive regulator of release.

Reduced Expression of GABA A Receptor Alpha2 Subunit Is Associated With Disinhibition of DYT-THAP1 Dystonia Patient-Derived Striatal Medium Spiny Neurons

DYT-THAP1 dystonia (formerly DYT6) is an adolescent-onset dystonia characterized by involuntary muscle contractions usually involving the upper body. It is caused by mutations in the gene THAP1 encoding for the transcription factor Thanatos-associated protein (THAP) domain containing apoptosis-associated protein 1 and inherited in an autosomal-dominant manner with reduced penetrance. Alterations in the development of striatal neuronal projections and synaptic function are known from transgenic mice models. To investigate pathogenetic mechanisms, human induced pluripotent stem cell (iPSC)-derived medium spiny neurons (MSNs) from two patients and one family member with reduced penetrance carrying a mutation in the gene THAP1 (c.474delA and c.38G > A) were functionally characterized in comparison to healthy controls. Calcium imaging and quantitative PCR analysis revealed significantly lower Ca²⁺ amplitudes upon GABA applications and a marked downregulation of the gene encoding the GABA_A receptor alpha2 subunit in THAP1 MSNs indicating a decreased GABAergic transmission. Whole-cell patch-clamp recordings showed a significantly lower frequency of miniature postsynaptic currents (mPSCs), whereas the frequency of spontaneous action potentials (APs) was elevated in THAP1 MSNs suggesting that decreased synaptic activity might have resulted in enhanced generation of APs. Our molecular and functional data indicate that a reduced expression of GABA_A receptor alpha2 subunit could eventually lead to limited GABAergic synaptic transmission, neuronal disinhibition, and hyperexcitability of THAP1 MSNs. These data give pathophysiological insight and may contribute to the development of novel treatment strategies for DYT-THAP1 dystonia.

Functional and Molecular Properties of DYT-SGCE Myoclonus-Dystonia Patient-Derived Striatal Medium Spiny Neurons

Myoclonus-dystonia (DYT-SGCE, formerly DYT11) is characterized by alcohol-sensitive, myoclonic-like appearance of fast dystonic movements. It is caused by mutations in the SGCE gene encoding ε-sarcoglycan leading to a dysfunction of this transmembrane protein, alterations in the cerebello-thalamic pathway and impaired striatal plasticity. To elucidate underlying pathogenic mechanisms, we investigated induced pluripotent stem cell (iPSC)-derived striatal medium spiny neurons (MSNs) from two myoclonus-dystonia patients carrying a heterozygous mutation in the SGCE gene (c.298T>G and c.304C>T with protein changes W100G and R102X) in comparison to two matched healthy control lines. Calcium imaging showed significantly elevated basal intracellular Ca2+ content and lower frequency of spontaneous Ca2+ signals in SGCE MSNs. Blocking of voltage-gated Ca2+ channels by verapamil was less efficient in suppressing KCl-induced Ca2+ peaks of SGCE MSNs. Ca2+ amplitudes upon glycine and acetylcholine applications were increased in SGCE MSNs, but not after GABA or glutamate applications. Expression of voltage-gated Ca2+ channels and most ionotropic receptor subunits was not altered. SGCE MSNs showed significantly reduced GABAergic synaptic density. Whole-cell patch-clamp recordings displayed elevated amplitudes of miniature postsynaptic currents and action potentials in SGCE MSNs. Our data contribute to a better understanding of the pathophysiology and the development of novel therapeutic strategies for myoclonus-dystonia.

Cell viability assay as a tool to study activity and inhibition of hepatitis C p7 channels

The p7 viroporin of the hepatitis C virus (HCV) forms an intracellular proton-conducting transmembrane channel in virus-infected cells, shunting the pH of intracellular compartments and thus helping virus assembly and release. This activity is essential for virus infectivity, making viroporins an attractive target for drug development. The protein sequence and drug sensitivity of p7 vary between the seven major genotypes of the hepatitis C virus, but the essential channel activity is preserved. Here, we investigated the effect of several inhibitors on recombinant HCV p7 channels corresponding to genotypes 1a-b, 2a-b, 3a and 4a using patch-clamp electrophysiology and cell-based assays. We established a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT)-based cell viability assay for recombinant p7 expressed in HEK293 cells to assess channel activity and its sensitivity to inhibitors. The results from the cell viability assay were consistent with control measurements using established assays of haemadsorption and intracellular pH, and agreed with data from patch-clamp electrophysiology. Hexamethylene amiloride (HMA) was the most potent inhibitor of p7 activity, but possessed cytotoxic activity at higher concentrations. Rimantadine was active against p7 of all genotypes, while amantadine activity was genotype-dependent. The alkyl-chain iminosugars NB-DNJ, NN-DNJ and NN-DGJ were tested and their activity was found to be genotype-specific. In the current study, we introduce cell viability assays as a rapid and cost-efficient technique to assess viroporin activity and identify channel inhibitors as potential novel antiviral drugs.

Chronic Ethanol Exposure Potentiates Cholinergic Neurotransmission in the Basolateral Amygdala

Chronic intermittent ethanol (CIE) exposure dysregulates glutamatergic and GABAergic neurotransmission, facilitating basolateral amygdala (BLA) pyramidal neuron hyperexcitability and the expression of anxiety during withdrawal. It is unknown whether ethanol-induced alterations in nucleus basalis magnocellularis (NBM) cholinergic projections to the BLA mediate anxiety-related behaviors through direct modulation of GABA and glutamate afferents. Following 10 days of CIE exposure and 24 h of withdrawal, we recorded GABAergic and glutamatergic synaptic responses in BLA pyramidal neurons with electrophysiology, assessed total protein expression of cholinergic markers, and quantified acetylcholine and choline concentrations using a colorimetric assay. We measured α₇ nicotinic acetylcholine receptor (nAChR) dependent modulation of presynaptic function at distinct inputs in AIR- and CIE-exposed BLA coronal slices as a functional read-out of cholinergic neurotransmission. CIE/withdrawal upregulates the endogenous activity of α₇ nAChRs, facilitating release at both GABAergic' local' interneuron and glutamatergic synaptic responses to stria terminalis (ST) stimulation, with no effect at GABAergic lateral paracapsular cells (LPCs). CIE caused a three-fold increase in BLA acetylcholine concentration, with no changes in α₇ nAChR or cholinergic marker expression. These data illustrate that α₇ nAChR-dependent changes in presynaptic function serve as a proxy for CIE-dependent alterations in synaptic acetylcholine levels. Thus, cholinergic projections appear to mediate CIE-induced alterations at GABA/glutamate inputs.

GABA receptor inhibition and severe hypoxia induce a paroxysmal depolarization shift in goldfish neurons

Mammalian neurons undergo rapid excitotoxic cell death when deprived of oxygen; however, the common goldfish (Carassius auratus) has the unique ability of surviving in oxygen-free waters, under anoxia. This organism utilizes γ-amino butyric acid (GABA) signaling to suppress excitatory glutamatergic activity during anoxic periods. Although GABA_A receptor antagonists are not deleterious to the cellular survival, coinhibition of GABA_A and GABA_B receptors is detrimental by abolishing anoxia-induced neuroprotective mechanisms. Here we show that blocking the anoxic GABAergic neurotransmission induces seizure-like activity (SLA) analogous to a paroxysmal depolarization shift (PDS), with hyperpolarization of action potential (AP) threshold and elevation of threshold currents. The observed PDS was attributed to an increase in excitatory postsynaptic currents (EPSCs) that are normally attenuated with decreasing oxygen levels. Furthermore, for the first time, we show that in addition to PDS, some neurons undergo depolarization block and do not generate AP despite a suprathreshold membrane potential. In conclusion, our results indicate that with severe hypoxia and absence of GABA receptor activity, telencephalic neurons of C. auratus manifest a paroxysmal depolarization shift, a key feature of epileptic discharge.NEW & NOTEWORTHY This work shows that the combination of anoxia and inhibition of GABA receptors induces seizure-like activities in goldfish telencephalic pyramidal and stellate neurons. Importantly, to prevent seizure-like activity, an intact GABA-mediated inhibitory pathway is required.

Direct Conversion of Human Stem Cell-Derived Glial Progenitor Cells into GABAergic Interneurons

Glial progenitor cells are widely distributed in brain parenchyma and represent a suitable target for future therapeutic interventions that generate new neurons via in situ reprogramming. Previous studies have shown successful reprogramming of mouse glia into neurons whereas the conversion of human glial cells remains challenging due to the limited accessibility of human brain tissue. Here, we have used a recently developed stem cell-based model of human glia progenitor cells (hGPCs) for direct neural reprogramming by overexpressing a set of transcription factors involved in GABAergic interneuron fate specification. GABAergic interneurons play a key role in balancing excitatory and inhibitory neural circuitry in the brain and loss or dysfunction of these have been implicated in several neurological disorders such as epilepsy, schizophrenia, and autism. Our results demonstrate that hGPCs successfully convert into functional induced neurons with postsynaptic activity within a month. The induced neurons have properties of GABAergic neurons, express subtype-specific interneuron markers (e.g. parvalbumin) and exhibit a complex neuronal morphology with extensive dendritic trees. The possibility of inducing GABAergic interneurons from a renewable in vitro hGPC system could provide a foundation for the development of therapies for interneuron pathologies.

Charged pore-lining residues are required for normal channel kinetics in the eukaryotic mechanosensitive ion channel MSL1

Mechanosensitive (MS) ion channels are widespread mechanisms for cellular mechanosensation that can be directly activated by increasing membrane tension. The well-studied MscS family of MS ion channels is found in bacteria, archaea, and plants. MscS-Like (MSL)1 is localized to the inner mitochondrial membrane of Arabidopsis thaliana, where it is required for normal mitochondrial responses to oxidative stress. Like Escherichia coli MscS, MSL1 has a pore-lining helix that is kinked. However, in MSL1 this kink is comprised of two charged pore-lining residues, R326 and D327. Using single-channel patch-clamp electrophysiology in E. coli, we show that altering the size and charge of R326 and D327 leads to dramatic changes in channel kinetics. Modest changes in gating pressure were also observed while no effects on channel rectification or conductance were detected. MSL1 channel variants had differing physiological function in E. coli hypoosmotic shock assays, without clear correlation between function and particular channel characteristics. Taken together, these results demonstrate that altering pore-lining residue charge and size disrupts normal channel state stability and gating transitions, and led us to propose the “sweet spot” model. In this model, the transition to the closed state is facilitated by attraction between R326 and D327 and repulsion between R326 residues of neighboring monomers. In the open state, expansion of the channel reduces inter-monomeric repulsion, rendering open state stability influenced mainly by attractive forces. This work provides insight into how unique charge-charge interactions can be combined with an otherwise conserved structural feature to help modulate MS channel function.

Synthesis and Pharmacological Characterization of 2-Aminoethyl Diphenylborinate (2-APB) Derivatives for Inhibition of Store-Operated Calcium Entry (SOCE) in MDA-MB-231 Breast Cancer Cells

Calcium ions regulate a wide array of physiological functions including cell differentiation, proliferation, muscle contraction, neurotransmission, and fertilization. The endoplasmic reticulum (ER) is the major intracellular Ca²⁺ store and cellular events that induce ER store depletion (e.g., activation of inositol 1,4,5-triphosphate (IP₃) receptors) trigger a refilling process known as store-operated calcium entry (SOCE). It requires the intricate interaction between the Ca²⁺ sensing stromal interaction molecules (STIM) located in the ER membrane and the channel forming Orai proteins in the plasma membrane (PM). The resulting active STIM/Orai complexes form highly selective Ca²⁺ channels that facilitate a measurable Ca²⁺ influx into the cytosol followed by successive refilling of the ER by the sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA). STIM and Orai have attracted significant therapeutic interest, as enhanced SOCE has been associated with several cancers, and mutations in STIM and Orai have been linked to immunodeficiency, autoimmune, and muscular diseases. 2-Aminoethyl diphenylborinate (2-APB) is a known modulator and depending on its concentration can inhibit or enhance SOCE. We have synthesized several novel derivatives of 2-APB, introducing halogen and other small substituents systematically on each position of one of the phenyl rings. Using a fluorometric imaging plate reader (FLIPR) Tetra-based calcium imaging assay we have studied how these structural changes of 2-APB affect the SOCE modulation activity at different compound concentrations in MDA-MB-231 breast cancer cells. We have discovered 2-APB derivatives that block SOCE at low concentrations, at which 2-APB usually enhances SOCE.

Chronic Ethanol Differentially Modulates Glutamate Release from Dorsal and Ventral Prefrontal Cortical Inputs onto Rat Basolateral Amygdala Principal Neurons

The medial prefrontal cortex (mPFC) and the basolateral amygdala (BLA) have strong reciprocal connectivity. Projections from the BLA to the mPFC can drive innate, anxiety-related behaviors, but it is unclear whether reciprocal projections from the mPFC to BLA have similar roles. Here, we use optogenetics and chemogenetics to characterize the neurophysiological and behavioral alterations produced by chronic ethanol exposure and withdrawal on dorsal mPFC (dmPFC) and ventral mPFC (vmPFC) medial prefrontal cortical terminals in the BLA. We exposed adult male Sprague Dawley rats to chronic intermittent ethanol (CIE) using vapor chambers, measured anxiety-like behavior on the elevated zero maze, and used electrophysiology to record glutamatergic and GABAergic responses in BLA principal neurons. We found that withdrawal from a 7 d CIE exposure produced opposing effects at dmPFC (increased glutamate release) and vmPFC (decreased glutamate release) terminals in the BLA. Chemogenetic inhibition of dmPFC terminals in the BLA attenuated the increased anxiety-like behavior we observed during withdrawal. These data demonstrate that chronic ethanol exposure and withdrawal strengthen the synaptic connections between the dmPFC and BLA but weakens the vmPFC–BLA pathway. Moreover, facilitation of the dmPFC–BLA pathway during withdrawal contributes to anxiety-like behavior. Given the opposing roles of dmPFC–BLA and vmPFC–BLA pathways in fear conditioning, our results suggest that chronic ethanol exposure simultaneously facilitates circuits involved in the acquisition of and diminishes circuits involved with the extinction of withdrawal-related aversive behaviors.

A stepwise approach to resolving small ionic currents in vascular tissue

Arterial membrane potential (V_m) is set by an active interplay among ion channels whose principal function is to set contractility through the gating of voltage-operated Ca²⁺ channels. To garner an understanding of this electrical parameter, the activity of each channel must be established under near-physiological conditions, a significant challenge given their small magnitude. The inward rectifying K⁺ (K_IR) channel is illustrative of the problem, as its outward "physiological" component is almost undetectable. This study describes a stepwise approach to dissect small ionic currents at physiological V_m using endothelial and smooth muscle cells freshly isolated from rat cerebral arteries. We highlight three critical steps, beginning with the voltage clamping of vascular cells bathed in physiological solutions while maintaining a giga-ohm seal. K_IR channels are then inhibited (micromolar Ba²⁺) so that a difference current can be created, once Ba²⁺ traces are corrected for the changing seal resistance and subtle instrument drift, pulling the reversal potential rightward. The latter is a new procedure and entails the alignment of whole cell current traces at a voltage where K_IR is silent and other channels exhibit limited activity. We subsequently introduced corrected and uncorrected currents into computer models of the arterial wall to show how these subtle adjustments markedly impact the importance of K_IR in V_m and arterial tone regulation. We argue that this refined approach can be used on an array of vascular ion channels to build a complete picture of how they dynamically interact to set arterial tone in key organs like the brain.NEW & NOTEWORTHY This work describes a stepwise approach to resolve small ionic currents involved in controlling V_m in resistance arteries. Using this new methodology, we particularly resolved the outward component of the K_IR current in native vascular cells, voltage clamped in near-physiological conditions. This novel approach can be applied to any other vascular currents and used to better interpret how vascular ion channels cooperate to control arterial tone.

Thyrotropin-Releasing-Hormone-Synthesizing Neurons of the Hypothalamic Paraventricular Nucleus Are Inhibited by Glycinergic Inputs

Background: Glycine is a classical neurotransmitter that has role in both inhibitory and excitatory synapses. To understand whether glycinergic inputs are involved in the regulation of the hypophysiotropic thyrotropin-releasing hormone (TRH) neurons, the central controllers of the hypothalamic-pituitary-thyroid axis, the glycinergic innervation of the TRH neurons was studied in the hypothalamic paraventricular nucleus (PVN). Methods: Double-labeling immunocytochemistry and patch-clamp electrophysiology were used to determine the role of glycinergic neurons in the regulation of TRH neurons in the PVN. Anterograde and retrograde tracing methods were used to determine the sources of the glycinergic input of TRH neurons. Results: Glycine transporter-2 (GLYT2), a marker of glycinergic neurons, containing axons were found to establish symmetric type of synapses on TRH neurons in the PVN. Furthermore, glycine receptor immunoreactivity was observed in these TRH neurons. The raphe magnus (RMg) and the ventrolateral periaqueductal gray (VLPAG) were found to be the exclusive sources of the glycinergic innervation of the TRH neurons within the PVN. Patch-clamp electrophysiology using sections of TRH-IRES-tdTomato mice showed that glycine hyperpolarized the TRH neurons and completely blocked the firing of these neurons. Glycine also markedly hyperpolarized the TRH neurons in the presence of tetrodotoxin demonstrating the direct effect of glycine. In more than 60% of the TRH neurons, spontaneous inhibitory postsynaptic currents (sIPSCs) were observed, even after the pharmacological inhibition of glutamatergic and GABAergic neuronal transmission. The glycine antagonist, strychnine, almost completely abolished these sIPSCs, demonstrating the inhibitory nature of the glycinergic input of TRH neurons. Conclusions: These data demonstrate that TRH neurons in the PVN receive glycinergic inputs from the RMg and the VLPAG. The symmetric type of synaptic connection and the results of the electrophysiological experiments demonstrate the inhibitory nature of these inputs.

Electroconvulsive Shock Enhances Responsive Motility and Purinergic Currents in Microglia in the Mouse Hippocampus

Microglia are in a privileged position to both affect and be affected by neuroinflammation, neuronal activity and injury, which are all hallmarks of seizures and the epilepsies. Hippocampal microglia become activated after prolonged, damaging seizures known as status epilepticus (SE). However, since SE causes both hyperactivity and injury of neurons, the mechanisms triggering this activation remain unclear, as does the relevance of the microglial activation to the ensuing epileptogenic processes. In this study, we use electroconvulsive shock (ECS) to study the effect of neuronal hyperactivity without neuronal degeneration on mouse hippocampal microglia. Unlike SE, ECS did not alter hippocampal CA1 microglial density, morphology, or baseline motility. In contrast, both ECS and SE produced a similar increase in ATP-directed microglial process motility in acute slices, and similarly upregulated expression of the chemokine C-C motif chemokine ligand 2 (CCL2). Whole-cell patch-clamp recordings of hippocampal CA1sr microglia showed that ECS enhanced purinergic currents mediated by P2X7 receptors in the absence of changes in passive properties or voltage-gated currents, or changes in receptor expression. This differs from previously described alterations in intrinsic characteristics which coincided with enhanced purinergic currents following SE. These ECS-induced effects point to a "seizure signature" in hippocampal microglia characterized by altered purinergic signaling. These data demonstrate that ictal activity per se can drive alterations in microglial physiology without neuronal injury. These physiological changes, which up until now have been associated with prolonged and damaging seizures, are of added interest as they may be relevant to electroconvulsive therapy (ECT), which remains a gold-standard treatment for depression.

Physiological Profile of Neuropeptide Y-Expressing Neurons in Bed Nucleus of Stria Terminalis in Mice: State of High Excitability

Both, the anterior bed nucleus of the stria terminalis (BNST) and the neuropeptide Y (NPY) system are involved in shaping fear and defensive responses that adapt the organism to potentially life-threatening conditions. NPY is expressed in the BNST but NPY-expressing neurons in this critical hub in the stress response network have not been addressed before. Therefore, we performed whole-cell patch-clamp recordings in acute slices of anterior BNST from Npy-hrGFP transgenic mice to identify and characterize NPY-expressing neurons. We show that NPY-positive and NPY-negative neurons in anterior BNST match the previous classification scheme of type I (Regular Spiking), type II (Low-Threshold Bursting), and type III (fast Inward Rectifying) cells, although the proportion of these physiological phenotypes was similar within both neuronal subpopulations. However, NPY-positive and NPY-negative neurons possessed distinct intrinsic electrophysiological properties. NPY-positive neurons displayed higher input resistance and lower membrane capacitance, corresponding to small cell bodies and shorter less ramified dendrites, as compared to their NPY-negative counterparts. Furthermore, NPY-positive neurons generated higher frequent series of action potentials upon membrane depolarization and displayed significantly lower GABA_A receptor-mediated synaptic responsiveness during evoked, spontaneous, and elementary synaptic activity. Taken together, these properties indicate an overall state of high excitability in NPY-positive neurons in anterior BNST. In view of the role of the anterior BNST in anxiety- and stress-related behaviors, these findings suggest a scenario where NPY-positive neurons are preferentially active and responsive to afferent inputs, thereby contributing to adaptation of the organism to stressful environmental encounters.

α-Neurexins Together with α2δ-1 Auxiliary Subunits Regulate Ca2+ Influx through Cav2.1 Channels

Action potential-evoked neurotransmitter release is impaired in knock-out neurons lacking synaptic cell-adhesion molecules α-neurexins (αNrxns), the extracellularly longer variants of the three vertebrate Nrxn genes. Ca²⁺ influx through presynaptic high-voltage gated calcium channels like the ubiquitous P/Q-type (Ca_V2.1) triggers release of fusion-ready vesicles at many boutons. α2δ Auxiliary subunits regulate trafficking and kinetic properties of Ca_V2.1 pore-forming subunits but it has remained unclear if this involves αNrxns. Using live cell imaging with Ca²⁺ indicators, we report here that the total presynaptic Ca²⁺ influx in primary hippocampal neurons of αNrxn triple knock-out mice of both sexes is reduced and involved lower Ca_V2.1-mediated transients. This defect is accompanied by lower vesicle release, reduced synaptic abundance of Ca_V2.1 pore-forming subunits, and elevated surface mobility of α2δ-1 on axons. Overexpression of Nrxn1α in αNrxn triple knock-out neurons is sufficient to restore normal presynaptic Ca²⁺ influx and synaptic vesicle release. Moreover, coexpression of Nrxn1α together with α2δ-1 subunits facilitates Ca²⁺ influx further but causes little augmentation together with a different subunit, α2δ-3, suggesting remarkable specificity. Expression of defined recombinant Ca_V2.1 channels in heterologous cells validates and extends the findings from neurons. Whole-cell patch-clamp recordings show that Nrxn1α in combination with α2δ-1, but not with α2δ-3, facilitates Ca²⁺ currents of recombinant Ca_V2.1 without altering channel kinetics. These results suggest that presynaptic Nrxn1α acts as a positive regulator of Ca²⁺ influx through Ca_V2.1 channels containing α2δ-1 subunits. We propose that this regulation represents an important way for neurons to adjust synaptic strength.SIGNIFICANCE STATEMENT Synaptic transmission between neurons depends on the fusion of neurotransmitter-filled vesicles with the presynaptic membrane, which subsequently activates postsynaptic receptors. Influx of calcium ions into the presynaptic terminal is the key step to trigger vesicle release and involves different subtypes of voltage-gated calcium channels. We study the regulation of calcium channels by neurexins, a family of synaptic cell-adhesion molecules that are essential for many synapse properties. Using optical measurements of calcium influx in cultured neurons and electrophysiological recordings of calcium currents from recombinant channels, we show that a major neurexin variant facilitates calcium influx through P/Q-type channels by interacting with their α2δ-1 auxiliary subunits. These results propose a novel way how neurons can modulate the strength of distinct synapses.

Multiple genetic variations in sodium channel subunits in a case of sudden infant death syndrome

BACKGROUND

Dysfunction of Na_V 1.5 encoded by SCN5A accounts for approximately half of the channelopathic SIDS cases. We investigated the functional effect of two gene variants identified in the same patient, one in SCN5A and one in SCN1Bb. The aim of the study was to risk stratify the proband's family.

METHODS

The family was referred for cardiovascular genetic evaluation to assess familial risk of cardiac disease. Functional analysis of the identified variants was performed with patch-clamp electrophysiology in HEK293 cells.

RESULTS

A 16-month-old healthy boy died suddenly in the context of nonspecific illness and possible fever. Postmortem genetic testing revealed variants in the SCN5A and SCN1Bb genes. The proband's father carries the same variants but is asymptomatic. Electrophysiological analysis of the Na_V 1.5_1281X truncation revealed complete loss-of-function of the channel. Coexpression of Na_V 1.5 with Na_V β1b significantly increased I_Na density when compared to Na_V 1.5 alone. The Na_V β1b _V268I variant abolished this I_Na density increase. Moreover, it shifted the activation curve toward more depolarized potentials.

CONCLUSIONS

Genetic variation of both sodium channel and its modifiers may contribute to sudden unexplained death in childhood. However, the asymptomatic father suggests that genetic variation of these genes is not sufficient to cause sudden death or clinically detectable SCN5A phenotypes.

CALHM3 Is Essential for Rapid Ion Channel-Mediated Purinergic Neurotransmission of GPCR-Mediated Tastes

Binding of sweet, umami, and bitter tastants to G protein-coupled receptors (GPCRs) in apical membranes of type II taste bud cells (TBCs) triggers action potentials that activate a voltage-gated nonselective ion channel to release ATP to gustatory nerves mediating taste perception. Although calcium homeostasis modulator 1 (CALHM1) is necessary for ATP release, the molecular identification of the channel complex that provides the conductive ATP-release mechanism suitable for action potential-dependent neurotransmission remains to be determined. Here we show that CALHM3 interacts with CALHM1 as a pore-forming subunit in a CALHM1/CALHM3 hexameric channel, endowing it with fast voltage-activated gating identical to that of the ATP-release channel in vivo. Calhm3 is co-expressed with Calhm1 exclusively in type II TBCs, and its genetic deletion abolishes taste-evoked ATP release from taste buds and GPCR-mediated taste perception. Thus, CALHM3, together with CALHM1, is essential to form the fast voltage-gated ATP-release channel in type II TBCs required for GPCR-mediated tastes.

Neuronal Dysfunction in iPSC-Derived Medium Spiny Neurons from Chorea-Acanthocytosis Patients Is Reversed by Src Kinase Inhibition and F-Actin Stabilization

Chorea-acanthocytosis (ChAc) is a fatal neurological disorder characterized by red blood cell acanthocytes and striatal neurodegeneration. Recently, severe cell membrane disturbances based on depolymerized cortical actin and an elevated Lyn kinase activity in erythrocytes from ChAc patients were identified. How this contributes to the mechanism of neurodegeneration is still unknown. To gain insight into the pathophysiology, we established a ChAc patient-derived induced pluripotent stem cell model and an efficient differentiation protocol providing a large population of human striatal medium spiny neurons (MSNs), the main target of neurodegeneration in ChAc. Patient-derived MSNs displayed enhanced neurite outgrowth and ramification, whereas synaptic density was similar to controls. Electrophysiological analysis revealed a pathologically elevated synaptic activity in ChAc MSNs. Treatment with the F-actin stabilizer phallacidin or the Src kinase inhibitor PP2 resulted in the significant reduction of disinhibited synaptic currents to healthy control levels, suggesting a Src kinase- and actin-dependent mechanism. This was underlined by increased G/F-actin ratios and elevated Lyn kinase activity in patient-derived MSNs. These data indicate that F-actin stabilization and Src kinase inhibition represent potential therapeutic targets in ChAc that may restore neuronal function.

SIGNIFICANCE STATEMENT

Chorea-acanthocytosis (ChAc) is a fatal neurodegenerative disease without a known cure. To gain pathophysiological insight, we newly established a human in vitro model using skin biopsies from ChAc patients to generate disease-specific induced pluripotent stem cells (iPSCs) and developed an efficient iPSC differentiation protocol providing striatal medium spiny neurons. Using patch-clamp electrophysiology, we detected a pathologically enhanced synaptic activity in ChAc neurons. Healthy control levels of synaptic activity could be restored by treatment of ChAc neurons with the F-actin stabilizer phallacidin and the Src kinase inhibitor PP2. Because Src kinases are involved in bridging the membrane to the actin cytoskeleton by membrane protein phosphorylation, our data suggest an actin-dependent mechanism of this dysfunctional phenotype and potential treatment targets in ChAc.

Ancestral Reconstruction Approach to Acetylcholine Receptor Structure and Function

Acetylcholine receptors (AChRs) are members of a superfamily of proteins called pentameric ligand-gated ion channels, which are found in almost all forms of life and thus have a rich evolutionary history. Muscle-type AChRs are heteropentameric complexes assembled from four related subunits (α, β, δ, and ɛ). Here we reconstruct the amino acid sequence of a β subunit ancestor shared by humans and cartilaginous fishes (i.e., Torpedo). Then, by resurrecting this ancestral β subunit and co-expressing it with human α, δ, and ɛ subunits, we show that despite 132 substitutions, the ancestral subunit is capable of forming human/ancestral hybrid AChRs. Whole-cell currents demonstrate that the agonist acetylcholine has reduced potency for hybrid receptors, while single-channel recordings reveal that hybrid receptors display reduced conductance and open probability. Our results outline a promising strategy for studies of AChR evolution aimed at identifying the amino acid origins of AChR structure and function.

The proinflammatory cytokine tumor necrosis factor-α excites subfornical organ neurons

Tumor necrosis factor-α (TNF-α) is a proinflammatory cytokine implicated in cardiovascular and autonomic regulation via actions in the central nervous system. TNF-α^-/- mice do not develop angiotensin II (ANG II)-induced hypertension, and administration of TNF-α into the bloodstream of rats increases blood pressure and sympathetic tone. Recent studies have shown that lesion of the subfornical organ (SFO) attenuates the hypertensive and autonomic effects of TNF-α, while direct administration of TNF-α into the SFO increases blood pressure, suggesting the SFO to be a key site for the actions of TNF-α. Therefore, we used patch-clamp techniques to examine both acute and long-term effects of TNF-α on the excitability of Sprague-Dawley rat SFO neurons. It was observed that acute bath application of TNF-α depolarized SFO neurons and subsequently increased action potential firing rate. Furthermore, the magnitude of depolarization and the proportion of depolarized SFO neurons were concentration dependent. Interestingly, following 24-h incubation with TNF-α, the basal firing rate of the SFO neurons was increased and the rheobase was decreased, suggesting that TNF-α elevates SFO neuron excitability. This effect was likely mediated by the transient sodium current, as TNF-α increased the magnitude of the current and lowered its threshold of activation. In contrast, TNF-α did not appear to modulate either the delayed rectifier potassium current or the transient potassium current. These data suggest that acute and long-term TNF-α exposure elevates SFO neuron activity, providing a basis for TNF-α hypertensive and sympathetic effects.NEW & NOTEWORTHY Considerable recent evidence has suggested important links between inflammation and the pathological mechanisms underlying hypertension. The present study describes cellular mechanisms through which acute and long-term exposure of tumor necrosis factor-α (TNF-α) influences the activity of subfornical organ neurons by modulating the voltage-gated transient Na⁺ current. This provides critical new information regarding the specific pathological mechanisms through which inflammation and TNF-α in particular may result in the development of hypertension.

Poring over furrows

Cryo-electron microscopy reveals the structure of a chloride channel that is closely related to a protein that transports lipids.

A comparative analysis of the physiological properties of neurons in the anterolateral bed nucleus of the stria terminalis in the Mus musculus, Rattus norvegicus, and Macaca mulatta

The anterolateral group of the bed nucleus of the stria terminalis (BNST_ALG ) is a critical modulator of a variety of rodent and primate behaviors spanning anxiety behavior and drug addiction. Three distinct neuronal cell types have been previously defined in the rat BNST_ALG based on differences in the voltage-response to hyperpolarizing and depolarizing current injection. Differences in genetic expression profile between these three cell types suggest electrophysiological cell type may be an indicator for functional differences in the circuit of the rat BNST_ALG . Although the behavioral role of the BNST is conserved across species, it is unknown if the same electrophysiological cell types exist in the BNST_ALG of the mouse and nonhuman primate. Here, we used whole-cell patch clamp electrophysiology and neuronal reconstructions of biocytin-filled neurons to compare and contrast the electrophysiological and morphological properties of neurons in the BNST_ALG from the mouse, rat, and rhesus macaque. We provide evidence that the BNST_ALG of all three species contains neurons that match the three defined cell types found in the rat; however, there are intriguing differences in the relative frequency of these cell types as well as electrophysiological and morphological properties of the BNST_ALG neurons across species. This study suggests that the overall landscape of the BNST_ALG in the primate and mouse may be similar to that of the rat in some aspects but perhaps significantly different in others.

Novel regulatory mechanism in human urinary bladder: central role of transient receptor potential melastatin 4 channels in detrusor smooth muscle function

Transient receptor potential melastatin 4 (TRPM4) channels are Ca(2+)-activated nonselective cation channels that have been recently identified as regulators of detrusor smooth muscle (DSM) function in rodents. However, their expression and function in human DSM remain unexplored. We provide insights into the functional role of TRPM4 channels in human DSM under physiological conditions. We used a multidisciplinary experimental approach, including RT-PCR, Western blotting, immunohistochemistry and immunocytochemistry, patch-clamp electrophysiology, and functional studies of DSM contractility. DSM samples were obtained from patients without preoperative overactive bladder symptoms. RT-PCR detected mRNA transcripts for TRPM4 channels in human DSM whole tissue and freshly isolated single cells. Western blotting and immunohistochemistry with confocal microscopy revealed TRPM4 protein expression in human DSM. Immunocytochemistry further detected TRPM4 protein expression in DSM single cells. Patch-clamp experiments showed that 9-phenanthrol, a selective TRPM4 channel inhibitor, significantly decreased the transient inward cation currents and voltage step-induced whole cell currents in freshly isolated human DSM cells. In current-clamp mode, 9-phenanthrol hyperpolarized the human DSM cell membrane potential. Furthermore, 9-phenanthrol attenuated the spontaneous phasic, carbachol-induced and nerve-evoked contractions in human DSM isolated strips. Significant species-related differences in TRPM4 channel activity between human, rat, and guinea pig DSM were revealed, suggesting a more prominent physiological role for the TRPM4 channel in the regulation of DSM function in humans than in rodents. In conclusion, TRPM4 channels regulate human DSM excitability and contractility and are critical determinants of human urinary bladder function. Thus, TRPM4 channels could represent promising novel targets for the pharmacological or genetic control of overactive bladder.

Positive Modulation of the Glycine Receptor by Means of Glycine Receptor-Binding Aptamers

According to the gate control theory of pain, the glycine receptors (GlyRs) are putative targets for development of therapeutic analgesics. A possible approach for novel analgesics is to develop a positive modulator of the glycine-activated Cl⁻ channels. Unfortunately, there has been limited success in developing drug-like small molecules to study the impact of agonists or positive modulators on GlyRs. Eight RNA aptamers with low nanomolar affinity to GlyRα1 were generated, and their pharmacological properties analyzed. Cytochemistry using fluorescein-labeled aptamers demonstrated GlyRα1-dependent binding to the plasma membrane but also intracellular binding. Using a fluorescent membrane potential assay, we could identify five aptamers to be positive modulators. The positive modulation of one of the aptamers was confirmed by patch-clamp electrophysiology on L(tk) cells expressing GlyRα1 and/or GlyRα1β. This aptamer potentiated whole-cell Cl⁻ currents in the presence of low concentrations of glycine. To our knowledge, this is the first demonstration ever of RNA aptamers acting as positive modulators for an ion channel. We believe that these aptamers are unique and valuable tools for further studies of GlyR biology and possibly also as tools for assay development in identifying small-molecule agonists and positive modulators.

KCa3.1/IK1 Channel Regulation by cGMP-Dependent Protein Kinase (PKG) via Reactive Oxygen Species and CaMKII in Microglia: An Immune Modulating Feedback System?

The intermediate conductance Ca²⁺-activated K⁺ channel, KCa3.1 (IK1/SK4/KCNN4) is widely expressed in the innate and adaptive immune system. KCa3.1 contributes to proliferation of activated T lymphocytes, and in CNS-resident microglia, it contributes to Ca²⁺ signaling, migration, and production of pro-inflammatory mediators (e.g., reactive oxygen species, ROS). KCa3.1 is under investigation as a therapeutic target for CNS disorders that involve microglial activation and T cells. However, KCa3.1 is post-translationally regulated, and this will determine when and how much it can contribute to cell functions. We previously found that KCa3.1 trafficking and gating require calmodulin (CaM) binding, and this is inhibited by cAMP kinase (PKA) acting at a single phosphorylation site. The same site is potentially phosphorylated by cGMP kinase (PKG), and in some cells, PKG can increase Ca²⁺, CaM activation, and ROS. Here, we addressed KCa3.1 regulation through PKG-dependent pathways in primary rat microglia and the MLS-9 microglia cell line, using perforated-patch recordings to preserve intracellular signaling. Elevating cGMP increased both the KCa3.1 current and intracellular ROS production, and both were prevented by the selective PKG inhibitor, KT5823. The cGMP/PKG-evoked increase in KCa3.1 current in intact MLS-9 microglia was mediated by ROS, mimicked by applying hydrogen peroxide (H₂O₂), inhibited by a ROS scavenger (MGP), and prevented by a selective CaMKII inhibitor (mAIP). Similar results were seen in alternative-activated primary rat microglia; their KCa3.1 current required PKG, ROS, and CaMKII, and they had increased ROS production that required KCa3.1 activity. The increase in current apparently did not result from direct effects on the channel open probability (P_o) or Ca²⁺ dependence because, in inside-out patches from transfected HEK293 cells, single-channel activity was not affected by cGMP, PKG, H₂O₂ at normal or elevated intracellular Ca²⁺. The regulation pathway we have identified in intact microglia and MLS-9 cells is expected to have broad implications because KCa3.1 plays important roles in numerous cells and tissues.

Expressing and characterizing mechanosensitive channels in Xenopus oocytes

The oocytes of the African clawed frog (Xenopus laevis) comprise one of the most widely used membrane protein expression systems. While frequently used for studies of transporters and ion channels, the application of this system to the study of mechanosensitive ion channels has been overlooked, perhaps due to a relative abundance of native expression systems. Recent advances, however, have illustrated the advantages of the oocyte system for studying plant and bacterial mechanosensitive channels. Here we describe in detail the methods used for heterologous expression and characterization of bacterial and plant mechanosensitive channels in Xenopus oocytes.

Infralimbic BDNF/TrkB enhancement of GluN2B currents facilitates extinction of a cocaine-conditioned place preference

Brain-derived neurotrophic factor (BDNF) regulates synaptic activity and behavioral flexibility, and reduction of BDNF strongly predicts psychiatric disorders and cognitive dysfunction. Restoration of BDNF-dependent activity could alleviate these impairments, but BDNF has limited clinical utility due to its pharmacokinetics. Here we demonstrate that activation of a primary BDNF target, the tropomyosin-related kinase B (TrkB) receptor, enhances the amplitude and prolongs the decay kinetics of N-methyl-d-aspartate receptor (NMDAR) currents in male rat infralimbic prefrontal pyramidal neurons. Moreover, these effects were prevented and reversed by blockade of NMDARs containing the GluN2B subunit. Our results show that this signaling cascade bidirectionally regulates extinction of a cocaine-induced conditioned place preference (CPP), a task that requires behavioral flexibility. Blockade of infralimbic TrkB receptors or GluN2B-containing NMDARs disrupted consolidation of extinction of the CPP. In contrast, extinction was strengthened by potentiation of TrkB receptor activity with infralimbic infusions of BDNF or systemic injections of 7,8 dihydroxyflavone (7,8DHF), the newly synthesized TrkB receptor agonist. The 7,8DHF-induced enhancement of extinction was prevented by infralimbic infusions of a GluN2B-specific receptor antagonist, demonstrating that TrkB receptor activation enhances extinction of cocaine-CPP via GluN2B-containing NMDARs. Together, infralimbic TrkB receptor activation strengthens GluN2B-containing NMDAR currents to support extinction learning. TrkB receptor agonists would therefore be useful as pharmacological adjuncts for extinction-based therapies for treatment of psychiatric disorders associated with reduced BDNF activity.

An alcohol-sensing site in the calcium- and voltage-gated, large conductance potassium (BK) channel

Ethanol alters BK (slo1) channel function leading to perturbation of physiology and behavior. Site(s) and mechanism(s) of ethanol-BK channel interaction are unknown. We demonstrate that ethanol docks onto a water-accessible site that is strategically positioned between the slo1 calcium-sensors and gate. Ethanol only accesses this site in presence of calcium, the BK channel's physiological agonist. Within the site, ethanol hydrogen-bonds with K361. Moreover, substitutions that hamper hydrogen bond formation or prevent ethanol from accessing K361 abolish alcohol action without altering basal channel function. Alcohol interacting site dimensions are approximately 10.7 × 8.6 × 7.1 Å, accommodating effective (ethanol-heptanol) but not ineffective (octanol, nonanol) channel activators. This study presents: (i) to our knowledge, the first identification and characterization of an n-alkanol recognition site in a member of the voltage-gated TM6 channel superfamily; (ii) structural insights on ethanol allosteric interactions with ligand-gated ion channels; and (iii) a first step for designing agents that antagonize BK channel-mediated alcohol actions without perturbing basal channel function.

Functional effects of KCNQ K(+) channels in airway smooth muscle

KCNQ (K_v7) channels underlie a voltage-gated K⁺ current best known for control of neuronal excitability, and its inhibition by G_q/11-coupled, muscarinic signaling. Studies have indicated expression of KCNQ channels in airway smooth muscle (ASM), a tissue that is predominantly regulated by muscarinic receptor signaling. Therefore, we investigated the function of KCNQ channels in rodent ASM and their interplay with G_q/11-coupled M₃ muscarinic receptors. Perforated-patch clamp of dissociated ASM cells detected a K⁺ current inhibited by the KCNQ antagonist, XE991, and augmented by the specific agonist, flupirtine. KCNQ channels begin to activate at voltages near resting potentials for ASM cells, and indeed XE991 depolarized resting membrane potentials. Muscarinic receptor activation inhibited KCNQ current weakly (~20%) at concentrations half-maximal for contractions. Thus, we were surprised to see that KCNQ had no affect on membrane voltage or muscle contractility following muscarinic activation. Further, M₃ receptor-specific antagonist J104129 fumarate alone did not reveal KCNQ effects on muscarinic evoked depolarization or contractility. However, a role for KCNQ channels was revealed when BK-K⁺ channel activities are reduced. While KCNQ channels do control resting potentials, they appear to play a redundant role with BK calcium-activated K⁺ channels during ASM muscarinic signaling. In contrast to effect of antagonist, we observe that KCNQ agonist flupirtine caused a significant hyperpolarization and reduced contraction in vitro irrespective of muscarinic activation. Using non-invasive whole animal plethysmography, the clinically approved KCNQ agonist retigabine caused a transient reduction in indexes of airway resistance in both wild type and BK β1 knockout (KO) mice treated with the muscarinic agonist. These findings indicate that KCNQ channels can be recruited via agonists to oppose muscarinic evoked contractions and may be of therapeutic value as bronchodilators.

Ethanol reduces neuronal excitability of lateral orbitofrontal cortex neurons via a glycine receptor dependent mechanism

Trauma-induced damage to the orbitofrontal cortex (OFC) often results in behavioral inflexibility and impaired judgment. Human alcoholics exhibit similar cognitive deficits suggesting that OFC neurons are susceptible to alcohol-induced dysfunction. A previous study from this laboratory examined OFC mediated cognitive behaviors in mice and showed that behavioral flexibility during a reversal learning discrimination task was reduced in alcohol-dependent mice. Despite these intriguing findings, the actions of alcohol on OFC neuron function are unknown. To address this issue, slices containing the lateral OFC (lOFC) were prepared from adult C57BL/6J mice and whole-cell patch clamp electrophysiology was used to characterize the effects of ethanol (EtOH) on neuronal function. EtOH (66 mM) had no effect on AMPA-mediated EPSCs but decreased those mediated by NMDA receptors. EtOH (11-66 mM) also decreased current-evoked spike firing and this was accompanied by a decrease in input resistance and a modest hyperpolarization. EtOH inhibition of spike firing was prevented by the GABAA antagonist picrotoxin, but EtOH had no effect on evoked or spontaneous GABA IPSCs. EtOH increased the holding current of voltage-clamped neurons and this action was blocked by picrotoxin but not the more selective GABAA antagonist biccuculine. The glycine receptor antagonist strychnine also prevented EtOH's effect on holding current and spike firing, and western blotting revealed the presence of glycine receptors in lOFC. Overall, these results suggest that acutely, EtOH may reduce lOFC function via a glycine receptor dependent process and this may trigger neuroadaptive mechanisms that contribute to the impairment of OFC-dependent behaviors in alcohol-dependent subjects.

Hypoxia enhances high-voltage-activated calcium currents in rat primary cortical neurons via calcineurin

Hypoxia regulates neuronal ion channels, sometimes resulting in seizures. We evaluated the effects of brief sustained hypoxia (1% O(2), 4h) on voltage-gated calcium channels (VGCCs) in cultured rat primary cortical neurons. High-voltage activated (HVA) Ca(2+) currents were acquired immediately after hypoxic exposure or after 48h recovery in 95% air/5% CO(2). Maximal Ca(2+) current density increased 1.5-fold immediately after hypoxia, but reverted to baseline after 48h normoxia. This enhancement was primarily due to an increase in L-type VGCC activity, since nimodipine-insensitive residual Ca(2+) currents were unchanged. The half-maximal potentials of activation and steady-state inactivation were unchanged. The calcineurin inhibitors FK-506 (in the recording pipette) or cyclosporine A (during hypoxia) prevented the post-hypoxic increase in HVA Ca(2+) currents, while rapamycin and okadaic acid did not. L-type VGCCs were the source of Ca(2+) for calcineurin activation, as nimodipine during hypoxia prevented post-hypoxic enhancement. Hypoxia transiently potentiated L-type VGCC currents via calcineurin, suggesting a positive feedback loop to amplify neuronal calcium signaling that may contribute to seizure generation.

ATP-sensitive potassium channels induced in liver cells after transfection with insulin cDNA and the GLUT 2 transporter regulate glucose-stimulated insulin secretion

As part of our research into the liver-directed gene therapy of Type I diabetes, we have engineered a human hepatoma cell line (HEPG2ins/g cells) to store and secrete insulin to a glucose stimulus. The aim of the present study was to determine whether HEPG2ins/g cells respond to glucose via signaling pathways that depend on ATP-sensitive potassium channels (KATP). Using patch-clamp electrophysiology with symmetrical KCl solutions, the single-channel conductance of KATP was 61pS. KATP was inhibited by ATP (1 mM) or cAMP (50 microM) applied to the cytosolic side of the membrane. Single KATP channels and macroscopic whole-cell currents were inhibited by glucose (20 mM) and glibenclamide (20 microM) and were activated by diazoxide (150 microM). Immunoprecipitation and Western blot analysis confirmed the presence of Kir6.2 KATP channel subunit protein in HEPG2ins/g and HEPG2ins cells. Using radioimmunoassay techniques, we report that exposure of the cells to tolbutamide (100 microM) resulted in an increase in insulin secretion from 0.3 +/- 0.05 to 1.8 +/- 0.2 pmol insulin/10(6) cells and glibenclamide (20 microM) from 0.4 +/- 0.06 to 2.1 +/- 0.3 (n=4), similar to what is seen on glucose (20 mM) stimulation. Diazoxide (150 microM) completely inhibited glucose-stimulated insulin release. Glucose 20 mM and glibenclamide 100 microM increased intracellular Ca2+ level in the HEPG2ins/g cells. However, glucose 20 mM did not stimulate a rise in intracellular Ca2+ in the un-transfected parent cell-line HEPG2. We used confocal microscopy to confirm that glucose (20 mM) stimulated the release of insulin from the fluorescently labeled secretion granules in the cells. Furthermore, glibenclamide (20 microM) also stimulated the release of insulin from fluorescently labeled secretion granules, and diazoxide (150 microM) blocked that stimulated release of insulin. Our results suggest that HEPG2ins/g cells respond to glucose via signaling pathways that depend on KATP, similar to a normal pancreatic beta cell.

Intrinsic light responses of retinal ganglion cells projecting to the circadian system

In mammals, light entrainment of the circadian clock, located in the suprachiasmatic nuclei (SCN), requires retinal input. Traditional rod and cone photoreceptors, however, are not required. Instead, the SCN-projecting retinal ganglion cells (RGCs) function as autonomous photoreceptors and exhibit light responses independent of rod- and cone-driven input. Using whole-cell patch-clamp recording techniques, we have investigated the morphological and electrophysiological properties of this unique class of RGCs. Although SCN-projecting RGCs resemble Type III cells in form, they display strikingly different physiological properties from these neurons. First, in response to the injection of a sustained depolarizing current, SCN-projecting cells fired in a transient fashion, in contrast to most RGCs which fired robust trains of action potentials. Second, in response to light, SCN-projecting RGCs exhibited an intensity-dependent transient depolarization in the absence of rod and cone input. This depolarization reached a peak within 5 s and generated increased spiking activity before decaying to a plateau. Voltage-clamp recordings were used to characterize the light-activated conductance which generated this depolarization. In response to varying light intensities, SCN-projecting RGCs exhibited a graded transient inward current which peaked within 5 s and decayed to a plateau. The voltage dependence of the light-activated current was obtained by subtracting currents elicited by a voltage ramp before and during illumination. The light-activated current displayed both inward and outward rectification and was largely unaffected by substitution of extracellular Na+ with choline. In both respects, the intrinsic light-activated current observed in SCN-projecting RGCs resembles currents carried by ion channels of the transient receptor potential (trp) family, which are known to mediate the light response of invertebrate photoreceptors.

Electrophysiological characterization of voltage-gated currents in defined taste cell types of mice

Despite extensive immunological characterization of the cells within taste buds, little is known about the functional significance of the different cell types. In this study, we use taste cells isolated from mouse vallate and foliate papillae to characterize voltage-gated currents in the three principal elongate types of taste cells: type I, II, and III. Cell types are identified by using antibodies to external epitopes [antigen H for type I cells, antigen A for type II cells, and neural cell adhesion molecule (NCAM) for type III cells]. In addition, we identify the subset of type II cells that contains alpha-gustducin, a G-protein involved in bitter transduction, by using transgenic mice expressing green fluorescent protein under the control of the gustducin promoter. Our results indicate that antigen H-immunoreactive (-IR) cells and many of the antigen A-IR cells have small voltage-gated inward Na(+) and outward K(+) currents but no voltage-gated Ca(2+) currents. In contrast, a subset of antigen A-IR cells and all NCAM-IR cells have large inward Na(+) and outward K(+) currents as well as voltage-gated Ca(2+) currents. Unexpectedly, all gustducin-expressing cells lacked voltage-gated Ca(2+) currents, suggesting that these cells use mechanisms other than classical synapses to communicate signals to the brain.

Heterogeneity in the basic membrane properties of postnatal gonadotropin-releasing hormone neurons in the mouse

The electrophysiological characteristics of unmodified, postnatal gonadotropin-releasing hormone (GnRH) neurons in the female mouse were studied using whole-cell recordings and single-cell RT-PCR methodology. The GnRH neurons of adult animals fired action potentials and exhibited distinguishable voltage-current relationships in response to hyperpolarizing and depolarizing current injections. On the basis of their patterns of inward rectification, rebound depolarization, and ability to fire repetitively, GnRH neurons in intact adult females were categorized into four cell types (type I, 48%; type II, 36%; type III, 11%; type IV, 5%). The GnRH neurons of juvenile animals (15-22 d) exhibited passive membrane properties similar to those of adult GnRH neurons, although only type I (61%) and type II (7%) cells were encountered, in addition to a group of "silent-type" GnRH neurons (32%) that were unable to fire action potentials. A massive, action potential-independent tonic GABA input, signaling through the GABA(A) receptor, was present at all ages. Afterdepolarization and afterhyperpolarization potentials (AHPs) were observed after single action potentials in subpopulations of each GnRH neuron type. Tetrodotoxin (TTX)-independent calcium spikes, as well as AHPs, were encountered more frequently in juvenile GnRH neurons compared with adults. These observations demonstrate the existence of multiple layers of functional heterogeneity in the firing properties of GnRH neurons. Together with pharmacological experiments, these findings suggest that potassium and calcium channels are expressed in a differential manner within the GnRH phenotype. This heterogeneity occurs in a development-specific manner and may underlie the functional maturation and diversity of this unique neuronal phenotype.