Na+ currents near and away from endplates on human fast and slow twitch muscle fibers

Fast and slow twitch muscle fibers have distinct contractile properties. Here we determined that membrane excitability also varies with fiber type. Na⁺ currents (I_NA) were studied with the loose-patch voltage clamp technique on 29 histochemically classified human intercostal skeletal muscle fibers at the endplate border and <200 μm from the endplate (extrajunctional). Fast and slow twitch fibers showed slow inactivation of endplate border and extrajunctional I_NA and had increased I_NA at the endplate border compared to extrajunctional membrane. The voltage dependencies of I_NA were similar on the endplate border and extrajunctional membrane, which suggests thatboth regions have physiclogically similar channels. Fast twitch fibers had larger I_NA on the endplate border and extrajunctional membrane and manifest fast and slow inactivation of I_NA at more negative potentials than slow twitch fibers. For normal muscle, the differences between I_NA on fast and slow twitch fibers might: (1) enable fast twitch fibers to operate at high firing frequencies for brief periods; and (2) enable slow twitch fibers to operate at low firing frequencies for prolonged times. Disorders of skeletal membrane excitability, such as the periodic paralyses and myotonias, may impact fast and slow twitch fibers differently due to the distinctive Na⁺ channel properties of each fiber type. © 1993 John Wiley & Sons, Inc.     

Metabotropic glutamate receptor 1 activity generates persistent, N-methyl-d-aspartate receptor-dependent depression of hippocampal pyramidal cell excitability

Metabotropic glutamate receptors (mGluRs) are involved in many forms of neuronal plasticity. In the hippocampus, they have well‐defined roles in long‐lasting forms of both synaptic and intrinsic plasticity. Here, we describe a novel form of long‐lasting intrinsic plasticity that we call (S)‐3,5‐dihydroxyphenylglycine (DHPG)‐mediated long‐term depression of excitability (DHPG‐LDE), and which is generated following transient pharmacological activation of group I mGluRs. In extracellular recordings from hippocampal slices, DHPG‐LDE was expressed as a long‐lasting depression of antidromic compound action potentials (cAPs) in CA1 or CA3 cells following a 4‐min exposure to the group I mGluR agonist (S)‐DHPG. A similar phenomenon was also seen for orthodromic fibre volleys evoked in CA3 axons. In single‐cell recordings from CA1 pyramids, DHPG‐LDE was manifest as persistent failures in antidromic action potential generation. DHPG‐LDE was blocked by (S)‐(+)‐a‐amino‐4‐carboxy‐2‐methylbenzeneacetic acid (LY367385), an antagonist of mGluR1, but not 2‐methyl‐6‐(phenylethynyl)pyridine hydrochloride (MPEP), an mGluR5 inhibitor. Although insensitive to antagonists of α‐amino‐3‐hydroxyl‐5‐methyl‐4‐isoxazole‐propionate/kainate and γ‐aminobutyric acid_A receptors, DHPG‐LDE was blocked by antagonists of N‐methyl‐d ‐aspartate (NMDA) receptors. Similarly, in single‐cell recordings, DHPG‐mediated antidromic spike failures were eliminated by NMDA receptor antagonism. Long after (S)‐DHPG washout, DHPG‐LDE was reversed by mGluR1 antagonism. A 4‐min application of (S)‐DHPG also produced an NMDA receptor‐dependent persistent depolarization of CA1 pyramidal cells. This depolarization was not solely responsible for DHPG‐LDE, because a similar level of depolarization elicited by raising extracellular K⁺ increased the amplitude of the cAP. DHPG‐LDE did not involve HCN channels or protein synthesis, but was eliminated by blockers of protein kinase C or tyrosine phosphatases.     

Functional characterization of the H-current in SCN neurons in subjective day and night: a whole-cell patch-clamp study in acutely prepared brain slices

Neurons of the rat suprachiasmatic nucleus (SCN) exhibit a circadian rhythm in spontaneous firing rate. In this whole-cell patch-clamp study in slices, we examined the possibility that H-current (I_H) contributes to the spontaneous firing rate of SCN neurons. Most of our experiments were performed during the subjective day, because this is the time epoch during which one would expect the largest excitatory effect of I_H if it were to fluctuate in a circadian rhythm. Current-clamp experiments showed that blockade of I_H by Cs⁺ (1 mM) did not influence the spontaneous firing rate and resting membrane potential. Voltage-clamp experiments revealed that I_H, when activated at the resting membrane potential, is probably too small in magnitude and too slow in activation to make a significant contribution to the spontaneous firing rate. Both results suggest that I_H does not significantly contribute to the spontaneous firing of SCN neurons. In addition, we investigated whether the kinetics and voltage dependence of I_H were modulated in a circadian manner. However, no substantial day–night differences in I_H were found. We conclude that I_H, as recorded in whole-cell mode, does not contribute significantly to spontaneous firing in most SCN neurons and that this current, is more likely to be involved in `rescuing' SCN neurons from large and long-lasting hyperpolarizations by depolarizing the membrane.     

Electrophysiological properties and their modulation by norepinephrine in the ambiguus neurons of the guinea pig

Electrophysiological properties of guinea pig ambiguus (AMB) neurons were studied in a brainstem slice preparation. During subthreshold depolarization AMB neurons displayed an early slow depolarization and a late outward rectification both of which were blocked by replacing Ca²⁺ with Co²⁺ in the extracellular solution. AMB neurons showed hyperpolarizing inward rectification which was blocked by extracellular Cs⁺ and is likely caused by the activation of Ih. In 58% (n = 49) of AMB neurons spike firing was restricted to the early phase of a long-lasting depolarizing current injection (phasic firing). The remaining AMB neurons showed repetitive firing throughout the depolarization (tonic firing). A Ca²⁺-mediated K⁺ current (I_K(Ca)) caused an afterhyperpolarization that followed both single and repetitive spike firing. I_K(Ca) also controlled the firing pattern in both types of firing, especially in the phasic firing. Norepinephrine (NE) blocked both the hyperpolarizing inward rectification and the Ca²⁺-dependent AHP. These effects of NE were antagonized by propranolol. It is proposed that the blockade of I_K(Ca) and Ih contribute to the improvement of the ‘signal-to-noise ratio’ by NE in AMB neurons.     

Functional analysis of the inhibitory neurotransmitter transporters GlyT1, GAT‐1, and GAT‐3 in astrocytes of the lateral superior olive

Neurotransmitter clearance from the synaptic cleft is a major function of astrocytes and requires neurotransmitter transporters. In the rodent lateral superior olive (LSO), a conspicuous auditory brainstem center, both glycine and GABA mediate synaptic inhibition. However, the main inhibitory input from the medial nucleus of the trapezoid body (MNTB) appears to be glycinergic by postnatal day (P) 14, when circuit maturation is almost accomplished. Using whole‐cell patch‐clamp recordings at P3‐20, we analyzed glycine transporters (GlyT1) and GABA transporters (GAT‐1, GAT‐3) in mouse LSO astrocytes, emphasizing on their developmental regulation. Application of glycine or GABA induced a dose‐ and age‐dependent inward current and a respective depolarization. The GlyT1‐specific inhibitor sarcosine reduced the maximal glycine‐induced current (I_{Gly (max)}) by about 60%. The GAT‐1 and GAT‐3 antagonists NO711 and SNAP5114, respectively, reduced the maximal GABA‐induced current (I_{GABA (max)}) by about 35%. Furthermore, [Cl^?]_o reduction decreased I_{Gly (max)} and I_{GABA (max)} by about 85 to 95%, showing the Cl^? dependence of GlyT and GAT. I_{GABA (max)} was stronger than I_{Gly (max)}, and the ratio increased developmentally from 1.6‐fold to 3.7‐fold. Together, our results demonstrate the functional presence of the three inhibitory neurotransmitter transporters GlyT1, GAT‐1, and GAT‐3 in LSO astrocytes. Furthermore, the uptake capability for GABA was higher than for glycine, pointing toward eminent GABAergic signaling in the LSO. GABA may originate from another source than the MNTB‐LSO synapses, namely from another projection or from reversal of astrocytic GATs. Thus, neuronal signaling in the LSO appears to be more versatile than previously thought. GLIA 2014;62:1992–2003     

Blockade of Na+/H+ exchanger type 3 causes intracellular acidification and hyperexcitability via inhibition of pH-sensitive K+ channels in chemosensitive respiratory neurons of the dorsal vagal nucleus in rats

Extracellular pH (pH_e) and intracellular pH (pH_i) are important factors for the excitability of chemosensitive central respiratory neurons that play an important role in respiration and obstructive sleep apnea. It has been proposed that inhibition of central Na⁺/ H⁺ exchanger 3 (NHE-3), a key pHi regulator in the brainstem, decreases the pH_i, leading to membrane depolarization for the maintenance of respiration. However, how intracellular pH affects the neuronal excitability of respiratory neurons remains largely unknown. In this study, we showed that NHE-3 mRNA is widely distributed in respiration-related neurons of the rat brainstem, including the dorsal vagal nucleus (DVN). Whole-cell patch clamp recordings from DVN neurons in brain slices revealed that the standing outward current (I _so) through pH-sensitive K⁺ channels was inhibited in the presence of the specific NHE-3 inhibitor AVE0657 that decreased the pHi. Exposure of DVN neurons to an acidified pH_e and AVE0657 (5 μmol/L) resulted in a stronger effect on firing rate and I _so than acidified pH_e alone. Taken together, our results showed that intracellular acidification by blocking NHE-3 results in inhibition of a pHsensitive K⁺ current, leading to synergistic excitation of chemosensitive DVN neurons for the regulation of respiration.     

AMPA and metabotropic glutamate receptors cooperatively generate inspiratory-like depolarization in mouse respiratory neurons in vitro

Excitatory transmission mediated by AMPA receptors is critical for respiratory rhythm generation. However, the role of AMPA receptors has not been fully explored. Here we tested the functional role of AMPA receptors in inspiratory neurons of the neonatal mouse preBötzinger complex (preBötC) using an in vitro slice model that retains active respiratory function. Immediately before and during inspiration, preBötC neurons displayed envelopes of depolarization, dubbed inspiratory drive potentials, that required AMPA receptors but largely depended on the Ca²⁺-activated non-specific cation current (I_CAN). We showed that AMPA receptor-mediated depolarization opened voltage-gated Ca²⁺ channels to directly evoke I_CAN. Inositol 1,4,5-trisphosphate receptor-mediated intracellular Ca²⁺ release also evoked I_CAN. Inositol 1,4,5-trisphosphate receptors acted downstream of group I metabotropic glutamate receptor activity but, here too, AMPA receptor-mediated Ca²⁺ influx was essential to trigger the metabotropic glutamate receptor contribution to inspiratory drive potential generation. This study helps to elucidate the role of excitatory transmission in respiratory rhythm generation in vitro. AMPA receptors in preBötC neurons initiate convergent signaling pathways that evoke post-synaptic I_CAN, which underlies inspiratory drive potentials. The coupling of AMPA receptors with I_CAN suggests that latent burst-generating intrinsic conductances are recruited by excitatory synaptic interactions among preBötC neurons in the context of respiratory network activity in vitro, exemplifying a rhythmogenic mechanism based on emergent properties of the network.     

Distinct modes of modulation of GABAergic transmission by Group I metabotropic glutamate receptors in rat entorhinal cortex

Activation of metabotropic glutamate receptors (mGluRs) modulates synaptic transmission, whereas the roles of mGluRs in GABAergic transmission in the entorhinal cortex (EC) are elusive. Here, we examined the effects of mGluRs on GABAergic transmission onto the principal neurons in the superficial layers of the EC. Bath application of DHPG, a selective Group I mGluR agonist, increased the frequency and amplitude of spontaneous IPSCs (sIPSCs) whereas application of DCG‐IV, an agonist for Group II mGluRs or L‐AP4, an agonist for Group III mGluRs failed to change significantly sIPSC frequency and amplitude. Bath application of DHPG failed to change significantly the frequency and amplitude of miniature IPSCs (mIPSCs) recorded in the presence of tetradotoxin but significantly reduced the amplitude of IPSCs evoked by extracellular field stimulation or in synaptically connected interneuron‐pyramidal neuron pairs in layer III of the EC. DHPG increased the frequency but reduced the amplitude of APs recorded from entorhinal interneurons. Bath application of DHPG generated membrane depolarization and increased the input resistance of GABAergic interneurons. DHPG‐mediated depolarization of GABAergic interneurons was mediated by inhibition of background K⁺ channels which are insensitive to extracellular Cs⁺, TEA, 4‐AP, and Ba²⁺. DHPG‐induced facilitation of sIPSCs was mediated by mGluR₅ and required the function of Gαq but was independent of phospholipase C activity. Elevation of synaptic glutamate concentration by bath application of glutamate transporter inhibitors significantly increased sIPSC frequency and amplitude demonstrating a physiological role of mGluRs in GABAergic transmission. Our results provide a cellular and molecular mechanism to explain the physiological and pathological roles of mGluRs in the EC. © 2009 Wiley‐Liss, Inc.     

Nitric oxide as intracellular modulator: internal production of NO increases neuronal excitability via modulation of several ionic conductances

Nitric oxide (NO) has been shown to regulate neuronal excitability in the nervous system, but little is known as to whether NO, which is synthesized in certain neurons, also serves functional roles within NO‐producing neurons themselves. We investigated this possibility by using a nitric oxide synthase (NOS)‐expressing neuron, and studied the role of intrinsic NO production on neuronal firing properties in single‐cell culture. B5 neurons of the pond snail Helisoma trivolvis fire spontaneous action potentials (APs), but once the intrinsic activity of NOS was inhibited, neurons became hyperpolarized and were unable to fire evoked APs. These striking long‐term effects could be attributed to intrinsic NO acting on three types of conductances, a persistent sodium current (I_NaP), voltage‐gated Ca currents (I_Ca) and small‐conductance calcium‐activated potassium (SK) channels. We show that NOS inhibitors 7‐nitroindazole and S‐methyl‐l ‐thiocitrulline resulted in a decrease in I_NaP, and that their hyperpolarizing and inhibiting effects on spontaneous spiking were mimicked by the inhibitor of I_NaP, riluzole. Moreover, inhibition of NOS, soluble guanylate cyclase (sGC) or protein kinase G (PKG) attenuated I_Ca, and blocked spontaneous and depolarization‐induced spiking, suggesting that intrinsic NO controlled I_Ca via the sGC/PKG pathway. The SK channel inhibitor apamin partially prevented the hyperpolarization observed after inhibition of NOS, suggesting a downregulation of SK channels by intrinsic NO. Taken together, we describe a novel mechanism by which neurons utilize their self‐produced NO as an intrinsic modulator of neuronal excitability. In B5 neurons, intrinsic NO production is necessary to maintain spontaneous tonic and evoked spiking activity.     

Potassium currents and excitability in second-order auditory and vestibular neurons

Potassium channels are involved in the control of neuronal excitability by fixing the membrane potential, shaping the action potential, and setting firing rates. Recently, attention has been focused on identifying the factors influencing excitability in second-order auditory and vestibular neurons. Located in the brainstem, second-order auditory and vestibular neurons are sites for convergence of inputs from first-order auditory or vestibular ganglionic cells with other sensory systems and also motor areas. Typically, second-order auditory neurons exhibit two distinct firing patterns in response to depolarization: tonic, with a repetitive firing of action potentials, and phasic, characterized by only one or a few action potentials. In contrast, all mature vestibular second-order neurons fire tonically on depolarization. Already, certain fundamental roles have emerged for potassium currents in these neurons. In mature auditory and vestibular neurons, I_K, the delayed rectifier, is required for the fast repolarization of action potentials. In tonically firing auditory neurons, I_A, the transient outward rectifier, defines the discharge pattern. I_DS, a delayed rectifier-like current distinguished by its low threshold of activation, is found in phasically firing auditory and some developing vestibular neurons where it limits firing to one or a few spikes, and also may contribute to forming short-duration excitatory postsynaptic potential (EPSPs). Also, I_DS sets the threshold for action potential generation rather high, which may prevent spontaneous discharge in phasically firing cells. During development, there is a gradual acquisition and loss of some potassium conductances, suggesting developmental regulation. As there are similarities in membrane properties of second-order auditory and vestibular neurons, investigations on firing pattern and its underlying mechanisms in one system should help to uncover fundamental properties of the other. J. Neurosci. Res. 53:511–520, 1998. © 1998 Wiley-Liss, Inc.     

Ibogaine and a Total Alkaloidal Extract of Voacanga africana Modulate Neuronal Excitability and Synaptic Transmission in the Rat Parabrachial Nucleus In Vitro

Ibogaine is a natural alkaloid of Voacanga africana that is effective in the treatment of withdrawal symptoms and craving in drug addicts. As the synaptic and cellular basis of ibogaine’s actions are not well understood, this study tested the hypothesis that ibogaine and Voacanga africana extract modulate neuronal excitability and synaptic transmission in the parabrachial nucleus using the nystatin perforated patch-recording technique. Ibogaine and Voacanga africana extract dose dependently, reversibly, and consistently attenuate evoked excitatory synaptic currents recorded in parabrachial neurons. The ED₅₀ of ibogaine’s effect is 5 μM, while that of Voacanga africana extract is 170 μg/ml. At higher concentrations, ibogaine and Voacanga africana extract induce inward currents or depolarization that are accompanied by increases in evoked and spontaneous firing rate. The depolarization or inward current is also accompanied by an increase in input resistance and reverses polarity around 0 mV. The depolarization and synaptic depression were blocked by the dopamine receptor antagonist haloperidol. These results indicate that ibogaine and Voacanga africana extract 1) depolarize parabrachial neurons with increased excitability and firing rate; 2) depress non-NMDA receptor-mediated fast synaptic transmission; 3) involve dopamine receptor activation in their actions. These results further reveal that the Voacanga africana extract has one-hundredth the activity of ibogaine in depressing synaptic responses. Thus, ibogaine and Voacanga africana extract may produce their central effects by altering dopaminergic and glutamatergic processes.     

A Study of the Barium-sensitive and -insensitive Components of the Action of Thyrotropin-releasing Hormone on Lumbar Motoneurons of the Rat Isolated Spinal Cord

The electrophysiological action of thyrotropin-releasing hormone (TRH) on rat spinal motoneurons was studied in vitro using single-electrode voltage- and current-clamp techniques. In current-clamp conditions TRH elicited a slowly developing depolarization, associated with a large input resistance increase and sustained neuronal firing; the primary metabolites of TRH were ineffective. Under voltage-clamp conditions in the presence of tetrodotoxin, TRH evoked a large inward current (I_TRH; peaking at approximately –40 mV) associated with a large input conductance fall. Only 44% of cells displayed I_TRH reversal; when the chord conductance values of these cells were plotted against membrane potential, a bell-shaped relation occurred, indicating voltage-dependent block by TRH of a persistent conductance active over a wide range of membrane potentials. I_TRH reversal values were shifted to more positive levels in high K⁺ solution in Nernstian fashion; hence a large proportion of the TRH response is suggested to be mediated by the block of a K⁺ conductance (I_K(T)). I_K(T) (and its voltage-dependent block by TRH) was resistant to certain K⁺ channel antagonists (tetraethylammonium, Cs⁺, 4-aminopyridine or apamin), but was depressed by Ba²⁺. The Ba²⁺-resistant fraction of I_TRH was attenuated by Cd²⁺, Mn²⁺ or Co²⁺, indicating that it probably involved a Ca²⁺-sensitive inward current. Concomitant application of Ba²⁺ and Cd²⁺ induced a near-total block of the response to TRH. It is suggested that suppression of I_K(T), associated with the onset of a Ca²⁺-sensitive current, can explain the excitatory effect of TRH on rat spinal motoneurons.     

Transient cerebral ischemia increases CA1 pyramidal neuron excitability

In human and experimental animals, the hippocampal CA1 region is one of the most vulnerable areas of the brain to ischemia. Pyramidal neurons in this region die 2–3 days after transient cerebral ischemia whereas other neurons in the same region remain intact. The mechanisms underlying the selective and delayed neuronal death are unclear. We tested the hypothesis that there is an increase in post-synaptic intrinsic excitability of CA1 pyramidal neurons after ischemia that exacerbates glutamatergic excitotoxicity. We performed whole-cell patch-clamp recordings in brain slices obtained 24 h after in vivo transient cerebral ischemia. We found that the input resistance and membrane time constant of the CA1 pyramidal neurons were significantly increased after ischemia, indicating an increase in neuronal excitability. This increase was associated with a decrease in voltage sag, suggesting a reduction of the hyperpolarization-activated non-selective cationic current (I_h). Moreover, after blocking I_h with ZD7288, the input resistance of the control neurons increased to that of the post-ischemia neurons, suggesting that a decrease in I_h contributes to increased excitability after ischemia. Finally, when lamotrigine, an enhancer of dendritic I_h, was applied immediately after ischemia, there was a significant attenuation of CA1 cell loss. These data suggest that an increase in CA1 pyramidal neuron excitability after ischemia may exacerbate cell loss. Moreover, this dendritic channelopathy may be amenable to treatment.     

Blockade of bepridil on IA and IK in acutely isolated hippocampal CA1 neurons

The effects of bepridil, an antianginal agent with antiarrhythmic action, on voltage-dependent K⁺ currents in the CA1 pyramidal neurons acutely isolated from rat hippocampus were studied by means of whole-cell patch clamp techniques. Current recordings were made in the presence of TTX to block Na⁺ current. Depolarizing test pulses activated two components of outward K⁺ currents: a rapidly activating and inactivating component, I_A; and a delayed component, I_K. Results showed that bepridil reduced the amplitude of I_A and I_K, and exerted its inhibitory action in time- and dose-dependent manner. Half-blocking concentrations (IC₅₀) of bepridil on I_A and I_K were 17.8 μM and 1.7 μM, respectively. 10 μM bepridil suppressed I_A and I_K by 46.7% and 77.1% at +30 mV of depolarization, respectively. When I_K was activated nearly uncontaminated with I_A by holding at −50 mV, 10 μM bepridil inhibited I_K by 71.6% at +30 mV of depolarization; 10 μM bepridil positively shifted the voltage-dependent of activation curves of I_A and I_K 12.1 mV and 28.7 mV, respectively. These results suggested that blockade on K⁺ currents by bepridil is preferential for I_K, and contributes to the protection brain against ischemic damage.     

Muscarinic modulation of TREK currents in mouse sympathetic superior cervical ganglion neurons

Muscarinic receptors play a key role in the control of neurotransmission in the autonomic ganglia, which has mainly been ascribed to the regulation of potassium M‐currents and voltage‐dependent calcium currents. Muscarinic agonists provoke depolarization of the membrane potential and a reduction in spike frequency adaptation in postganglionic neurons, effects that may be explained by M‐current inhibition. Here, we report the presence of a riluzole‐activated current (I_RIL) that flows through the TREK‐2 channels, and that is also inhibited by muscarinic agonists in neurons of the mouse superior cervical ganglion (mSCG). The muscarinic agonist oxotremorine‐M (Oxo‐M) inhibited the I_RIL by 50%, an effect that was abolished by pretreatment with atropine or pirenzepine, but was unaffected in the presence of himbacine. Moreover, these antagonists had similar effects on single‐channel TREK‐2 currents. I_RIL inhibition was unaffected by pretreatment with pertussis toxin. The protein kinase C blocker bisindolylmaleimide did not have an effect, and neither did the inositol triphosphate antagonist 2‐aminoethoxydiphenylborane. Nevertheless, the I_RIL was markedly attenuated by the phospholipase C (PLC) inhibitor ET‐18‐OCH3. Finally, the phosphatidylinositol‐3‐kinase/phosphatidylinositol‐4‐kinase inhibitor wortmannin strongly attenuated the I_RIL, whereas blocking phosphatidylinositol 4,5‐bisphosphate (PIP₂) depletion consistently prevented I_RIL inhibition by Oxo‐M. These results demonstrate that TREK‐2 currents in mSCG neurons are inhibited by muscarinic agonists that activate M1 muscarinic receptors, reducing PIP₂ levels via a PLC‐dependent pathway. The similarities between the signaling pathways regulating the I_RIL and the M‐current in the same neurons reflect an important role of this new pathway in the control of autonomic ganglia excitability.     

Sodium background currents in endocrine/neuroendocrine cells: Towards unraveling channel identity and contribution in hormone secretion

In endocrine/neuroendocrine tissues, excitability of secretory cells is patterned by the repertoire of ion channels and there is clear evidence that extracellular sodium (Na⁺) ions contribute to hormone secretion. While voltage-gated channels involved in action potential generation are well-described, the background 'leak' channels operating near the resting membrane potential are much less known, and in particular the channels supporting a background entry of Na⁺ ions. These background Na⁺ currents (called here 'I_Nab') have the ability to modulate the resting membrane potential and subsequently affect action potential firing. Here we compile and analyze the data collected from three endocrine/neuroendocrine tissues: the anterior pituitary gland, the adrenal medulla and the endocrine pancreas. We also model how I_Nab can be functionally involved in cellular excitability. Finally, towards deciphering the physiological role of I_Nab in endocrine/neuroendocrine cells, its implication in hormone release is also discussed.     

Temporal tuning of odor responses in pheromone-responsive projection neurons in the brain of the sphinx moth Manduca sexta

By means of intracellular recording and staining, we studied the ability of several distinct classes of projection (output) neurons, which innervate the sexually dimorphic macroglomerular complex (MGC-PNs) in the antennal lobe of the male moth Manduca sexta, to encode naturally intermittent sex pheromonal stimuli. In many MGC-PNs, antennal stimulation with a blend of the two essential pheromone components evoked a characteristic triphasic response consisting of a brief, hyperpolarizing inhibitory potential (I₁) followed by depolarization with firing of action potentials and then a delayed period of hyperpolarization (I₂). MGC-PNs described in this study resolved pulsed pheromonal stimuli, up to about five pulses/second, with a distinct burst of action potentials for each pulse of odor. The larger the amplitude of I₁, the higher the pulse rate an MGC-PN could follow, illustrating the importance of inhibitory synaptic input in shaping the temporal firing properties of these glomerular output neurons. In some MGC-PNs, triphasic responses were evoked by antennal stimulation with only one of the two key pheromone components. Again, the maximal pulse rate that an MGC-PN could follow with that pheromone component as sole stimulus was high in MGC-PNs that responded with a strong I₁. These component-specific MGC-PNs innervated only one of the two principal glomeruli of the MGC, while MGC-PNs that were primarily excited by antennal stimulation with either key pheromone component had arborizations in both major MGC glomeruli. These observations therefore suggest that the population of antennal olfactory receptor cells responding to a single pheromone component is functionally heterogeneous: a subset of these sensory cells activates the excitatory drive to many uniglomerular MGC-PNs, while others feed onto inhibitory circuits that hyperpolarize the same PNs. This convergence of opposing inputs is a circuit property common to uniglomerular MGC-PNs branching in either of the major MGC glomeruli, and it enhances the ability of these glomerular output neurons to resolve intermittent olfactory input. Synaptic integration at the uniglomerular PN level thus contributes to the transmission of behaviorally important temporal information about each key pheromone component to higher centers in the brain. J. Comp. Neurol. 409:1–12, 1999. © 1999 Wiley-Liss, Inc.     

Enhancement of dendritic persistent Na<Superscript>+</Superscript> currents by mGluR5 leads to an advancement of spike timing with an increase in temporal precision

Timing and temporal precision of action potential generation are thought to be important for encoding of information in the brain. The ability of single neurons to transform their input into output action potential is primarily determined by intrinsic excitability. Particularly, plastic changes in intrinsic excitability represent the cellular substrate for spatial memory formation in CA1 pyramidal neurons (CA1-PNs). Here, we report that synaptically activated mGluR5-signaling can modulate the intrinsic excitability of CA1-PNs. Specifically, high-frequency stimulation at CA3-CA1 synapses increased firing rate and advanced spike onset with an improvement of temporal precision. These changes are mediated by mGluR5 activation that induces cADPR/RyR-dependent Ca²⁺ release in the dendrites of CA1-PNs, which in turn causes an increase in persistent Na⁺ currents (I_Na,P) in the dendrites. When group I mGluRs in CA1-PNs are globally activated pharmacologically, afterdepolarization (ADP) generation as well as increased firing rate are observed. These effects are abolished by inhibiting mGluR5/cADPR/RyR-dependent Ca²⁺ release. However, the increase in firing rate, but not the generation of ADP is affected by inhibiting I_Na,P. The differences between local and global activation of mGluR5-signaling in CA1-PNs indicates that mGluR5-dependent modulation of intrinsic excitability is highly compartmentalized and a variety of ion channels are recruited upon their differential subcellular localizations. As mGluR5 activation is induced by physiologically plausible brief high-frequency stimulation at CA3-CA1 synapses, our results suggest that mGluR5-induced enhancement of dendritic I_Na,P in CA1-PNs may provide important implications for our understanding about place field formation in the hippocampus.     

Development of resurgent and persistent sodium currents in mesencephalic trigeminal neurons

Sodium channels play multiple roles in the formation of neural membrane properties in mesencephalic trigeminal (Mes V) neurons and in other neural systems. Mes V neurons exhibit conditional robust high‐frequency spike discharges. As previously reported, resurgent and persistent sodium currents (I_NaR and I_NaP, respectively) may carry small currents at subthreshold voltages that contribute to generation of spike firing. These currents play an important role in maintaining and allowing high‐frequency spike discharge during a burst. In the present study, we investigated the developmental changes in tetrodotoxin‐sensitive I_NaR and I_NaP underlying high‐frequency spike discharges in Mes V neurons. Whole‐cell patch‐clamp recordings showed that both current densities increased one and a half times from postnatal day (P) 0–6 neurons to P7–14 neurons. Although these neurons do not exhibit subthreshold oscillations or burst discharges with high‐frequency firing, I_NaR and I_NaP do exist in Mes V neurons at P0–6. When the spike frequency at rheobase was examined in firing Mes V neurons, the developmental change in firing frequency among P7–14 neurons was significant. I_NaR and I_NaP density at ?40 mV also increased significantly among P7–14 neurons. The change to an increase in excitability in the P7–14 group could result from this quantitative change in I_NaP. In neurons older than P7 that exhibit repetitive firing, quantitative increases in I_NaR and I_NaP density may be major factors that facilitate and promote high‐frequency firing as a function of age in Mes V neurons.     

Glial cell line-derived neurotrophic factor acutely modulates the excitability of rat small-diameter trigeminal ganglion neurons innervating facial skin

Glial cell line-derived neurotrophic factor (GDNF) plays an important role in adult sensory neuron function. However, the acute effects of GDNF on primary sensory neuron excitability remain to be elucidated. The aim of the present study was to investigate whether GDNF acutely modulates the excitability of adult rat trigeminal ganglion (TRG) neurons that innervate the facial skin by using perforated-patch clamping, retrograde-labeling and immunohistochemistry techniques. Fluorogold (FG) retrograde labeling was used to identify the TRG neurons innervating the facial skin. The FG-labeled small- and medium-diameter GDNF immunoreactive TRG neurons, and most of these neurons also expressed the GDNF family receptor α-1 (GFRα-1). In whole-cell voltage-clamp mode, GDNF application significantly inhibited voltage-gated K⁺ transient (I_A) and sustained (I_K) currents in most dissociated FG-labeled small-diameter TRG neurons. This effect was concentration-dependent and was abolished by co-application of the protein tyrosine kinase inhibitor, K252b. Under current-clamp conditions, the repetitive firing during a depolarizing pulse were significantly increased by GDNF application. GDNF application also increased the duration of the repolarization phase and decreased the duration of the depolarization phase of the action potential, and these characteristic effects were also abolished by co-application of K252b. These results suggest that acute application of GDNF enhances the neuronal excitability of adult rat small-diameter TRG neurons innervating the facial skin, via activation of GDNF-induced intracellular signaling pathway. We therefore conclude that a local release of GDNF from TRG neuronal soma and/or nerve terminals may regulate normal sensory function, including nociception.