Frequency Of Action Potential Discharge Research Articles

Circular RNA-GRIN2B Suppresses Neuropathic Pain by Targeting the NF-κB/SLICK Pathway.

The role of circular RNAs (circRNAs) in neuropathic pain is linked to the fundamental physiological mechanisms involved. However, the exact function of circRNAs in the context of neuropathic pain is still not fully understood. The functional impact of circGRIN2B on the excitability of dorsal root ganglion (DRG) neurons was investigated using siRNA or overexpression technology in conjunction with fluorescence in situ hybridization and whole-cell patch-clamp technology. The therapeutic efficacy of circGRIN2B in treating neuropathic pain was confirmed by assessing the pain threshold in a chronic constrictive injury (CCI) model. The interaction between circGRIN2B and NF-κB was examined through RNA pulldown, RIP, and mass spectrometry assays. CircGRIN2B knockdown significantly affected the action potential discharge frequency and the sodium-dependent potassium current flux (SLICK) in DRG neurons. Furthermore, knockdown of circGRIN2B dramatically reduced the SLICK channel protein and mRNA expression in vivo and in vitro. Our research confirmed the interaction between circGRIN2B and NF-κB. These findings demonstrated that circGRIN2B promotes the transcription of the SLICK gene by binding to NF-κB. In CCI rat models, the overexpression of circGRIN2B has been shown to hinder the progression of neuropathic pain, particularly by reducing mechanical and thermal hyperalgesia. Additionally, this upregulation significantly diminished the levels of the inflammatory cytokines IL-1β, IL-6, and TNF-α in the DRG. Upon reviewing these findings, it was determined that circGRIN2B may mitigate the onset of neuropathic pain by modulating the NF-κB/SLICK pathway.

Mechanosensitivity of Murine Lung Slowly Adapting Receptors: Minimal Impact of Chemosensory, Serotonergic, and Purinergic Signaling.

Murine slowly adapting receptors (SARs) within airway smooth muscle provide volume-related feedback; however, their mechanosensitivity and morphology are incompletely characterized. We explored two aspects of SAR physiology: their inherent static mechanosensitivity and a potential link to pulmonary neuroepithelial bodies (NEBs). SAR mechanosensitivity displays a rate sensitivity linked to speed of inflation; however, to what extent static SAR mechanosensitivity is tuned for the very rapid breathing frequency (Bf) of small mammals (e.g., mouse) is unclear. NEB-associated, morphologically described smooth muscle-associated receptors (SMARs) may be a structural analog for functionally characterized SARs, suggesting functional linkages between SARs and NEBs. We addressed the hypotheses that: (1) rapid murine Bf is associated with enhanced in vivo SAR static sensitivity; (2) if SARs and NEBs are functionally linked, stimuli reported to impact NEB function would alter SAR mechanosensitivity. We measured SAR action potential discharge frequency (AP f, action potentials/s) during quasi-static inflation [0–20 cmH2O trans-respiratory pressure (PTR)] in NEB-relevant conditions of hypoxia (FIO2 = 0.1), hypercarbia (FICO2 = 0.1), and pharmacologic intervention (serotonergic 5-HT3 receptor antagonist, Tropisetron, 4.5 mg/kg; P2 purinergic receptor antagonist, Suramin, 50 mg/kg). In all protocols, we obtained: (1) AP f vs. PTR; (2) PTR threshold; and (3) AP f onset at PTR threshold. The murine AP f vs. PTR response comprises high AP f (average maximum AP f: 236.1 ± 11.1 AP/s at 20 cmH2O), a low PTR threshold (mean 2.0 ± 0.1 cmH2O), and a plateau in AP f between 15 and 20 cmH2O. Murine SAR mechanosensitivity (AP f vs. PTR) is up to 60% greater than that reported for larger mammals. Even the maximum difference between intervention and control conditions was minimally impacted by NEB-related alterations: Tropisetron −7.6 ± 1.8% (p = 0.005); Suramin −10.6 ± 1.5% (p = 0.01); hypoxia +9.3 ± 1.9% (p < 0.001); and hypercarbia −6.2 ± 0.9% (p < 0.001). We conclude that the high sensitivity of murine SARs to inflation provides enhanced resolution of operating lung volume, which is aligned with the rapid Bf of the mouse. We found minimal evidence supporting a functional link between SARs and NEBs and speculate that the <10% change in SAR mechanosensitivity during altered NEB-related stimuli is not consistent with a meaningful physiologic role.

Acetylcholine receptors in the equatorial region of intrafusal muscle fibres modulate mouse muscle spindle sensitivity.

Acetylcholine receptors are aggregated in the central regions of intrafusal muscle fibres. Single unit muscle spindle afferent responses from isolated mouse extensor digitorum longus muscle were recorded in the absence of fusimotor input to ramp and hold stretches as well as to sinusoidal vibrations in the presence and absence of the acetylcholine receptor blockers d-tubocurarine andα-bungarotoxin. Proprioceptive afferent responses to both types of stretch were enhanced in the presence of either blocker. Blocking acetylcholine uptake and vesicular acetylcholine release by hemicholinium-3 also enhanced stretch-evoked responses. These results represent the first evidence that acetylcholine receptors negatively modulate muscle spindle responses to stretch. The data support the hypothesis that the sensory nerve terminal is able to release vesicles to fine-tune proprioceptive afferent sensitivity. Muscle spindles are complex stretch-sensitive mechanoreceptors. They consist of specialized skeletal muscle fibres, called intrafusal fibres, which are innervated in the central (equatorial) region by afferent sensory axons and in both polar regions by efferent γ-motoneurons. Previously it was shown that acetylcholine receptors (AChR) are concentrated in the equatorial region at the contact site between the sensory neuron and the intrafusal muscle fibre. To address the function of these AChRs, single unit sensory afferents were recorded from an isolated mouse extensor digitorum longus muscle in the absence of γ-motoneuron activity. Specifically, we investigated the responses of individual sensory neurons to ramp-and-hold stretches and sinusoidal vibrations before and after the addition of the competitive and non-competitive AChR blockers d-tubocurarine and α-bungarotoxin, respectively. The presence of either drug did not affect the resting action potential discharge frequency. However, the action potential frequencies in response to stretch were increased. In particular, frequencies of the dynamic peak and dynamic index to ramp-and-hold stretches were significantly higher in the presence of either drug. Treatment of muscle spindle afferents with the high-affinity choline transporter antagonist hemicholinium-3 similarly increased muscle spindle afferent firing frequencies during stretch. Moreover, the firing rate during sinusoidal vibration stimuli at low amplitudes was higher in the presence of α-bungarotoxin compared to control spindles also indicating an increased sensitivity to stretch. Collectively these data suggest a modulation of the muscle spindle afferent response to stretch by AChRs in the central region of intrafusal fibres possibly fine-tuning muscle spindle sensitivity.

Presynaptic CaMKIIα modulates dopamine D3 receptor activation in striatonigral terminals of the rat brain in a Ca2+ dependent manner

CaMKIIα is expressed at high density in the nucleus accumbens where it binds to postsynaptic D3 receptors inhibiting their effects. In striatonigral projections, activation of presynaptic D3 receptors potentiates D1 receptor-induced stimulation of cAMP production and GABA release. In this study we examined whether the presynaptic effects of D3 receptor stimulation in the substantia nigra reticulata (SNr) are modulated by Ca2+ activation of CaMKIIα. In SNr synaptosomes two procedures that increase cytoplasmic Ca2+, ionomycin and K+-depolarization, blocked the additional stimulation of cAMP accumulation produced by coactivating D3 and D1 dopamine receptors. The selective CaMKIIα inhibitor KN-62 reversed the blockade produced by ionomycin and K+-depolarization. Incubation in either Ca2 -free solutions or with the selective Ca2+ blocker nifedipine, also reversed the blocking effects of K+-depolarization. Immunoblot studies showed that K+-depolarization increased CaMKIIα phosphorylation in a KN-62 sensitive manner and promoted CaMKIIα binding to D3 receptors. In K+-depolarized tissues, D3 receptors potentiated D1 receptor-induced stimulation of [3H]GABA release only when CaMKIIα was blocked with KN-62. In the presence of this inhibitor, the selective D3 agonist PD 128,907 reduced the ED50 for the D1 agonist SKF 38393 from 56 to 4 nM. KN-62 also enhanced the effects of dopamine on depolarization induced [3H]GABA release. KN-62 changed ED50 for dopamine from 584 to 56 nM. KN-62 did not affect D1 and D4 receptor responses. These experiments show that in striatonigral projections, CaMKIIα inhibits the action of D3 receptors in a Ca2+ dependent manner blocking their modulatory effects on GABA release. These findings suggest a mechanism through which the frequency of action potential discharge in presynaptic terminals regulates dopamine effects.

Neuronal autoantibodies excite guinea pig myenteric neurons

The role of IgG in effecting gastrointestinal (G.I.) dysmotility associated with neurologic autoimmunity (idiopathic or paraneoplastic) remains poorly defined. Serum IgG prepared from patients with anti-neuronal nuclear autoantibody type 1 (ANNA-1 [aka anti-Hu], n=7) or ganglionic nicotinic acetylcholine receptor (α3AChR, n=6) autoantibody were tested on myenteric neurons in longitudinal muscle myenteric plexus (LMMP) preparations from the ileum of 38 guinea pigs. IgG prepared from healthy subjects or neurologic patients (n=7) without G.I. symptoms served as controls. Action potential discharges in all neurons of a ganglion in response to acute application of IgG were monitored by fast imaging using a voltage-sensitive dye. Data were analysed as% neurons per ganglion responding and frequency of action potential discharge. Only 0.7±1.2% of 708 neurons (26 ganglia) responded to 2 of 7 control IgG samples (mean action potential frequency 0.5±0.9Hz). Six of 7 samples containing ANNA-1-IgG evoked a response in 7.4±5.7% of 1266 neurons (45 ganglia) (P&lt;0.05) with a significantly higher action potential discharge (1.8±1.1Hz, P&lt;0.05). We observed no correlation between ANNA-1 titer and the percentage of responding neurons or action potential frequency. Four of 6 samples containing α3AChR-IgG evoked a response in a smaller number of neurons (6.5±9.9% of 612 neurons (32 ganglia)) with an action potential frequency of 1.9±1.5Hz.

INCREASED EXCITABILITY OF RVLM‐PROJECTING HYPOTHALAMIC PVN NEURONS IN ANGIOTENSIN II‐SALT HYPERTENSIVE RATS.

The contribution of the hypothalamic PVN to maintenance of SNA and high blood pressure (BP) in rats with Ang II‐salt HT was investigated in anesthetized rats by microinjecting the GABA‐A receptor agonist muscimol bilaterally into the PVN. Muscimol significantly (P&lt;0.05) reduced renal SNA (51±5%) and MAP (27±10 mmHg) in rats with Ang II‐salt HT (n=3). In contrast, muscimol had no effect on renal SNA and produced a blunted fall of MAP (−15±5 mmHg, P&lt;0.05) in normotensive rats (n=6). These data indicate an enhanced role for PVN neuronal activity in maintaining renal SNA and BP in rats with Ang II‐salt HT. Consistent with this interpretation, whole‐cell patch‐clamp recordings from brain slices revealed that excitability of RVLM‐projecting pre‐sympathetic PVN neurons from rats with Ang II‐salt HT was enhanced compared to neurons from normotensive controls. Indeed, graded current injections (50, 100, 150, 200 pA; 800 ms) evoked graded increases in action potential discharge frequency among neurons of both groups. Maximum discharge was evoked by 200 pA injections and averaged 47 ± 5 Hz in neurons from rats with Ang II‐salt HT (n=9). This response was significantly higher (P&lt;0.05) than that of neurons from normotensive controls, which averaged 25 ± 3 Hz (n=12). We conclude that increased excitability of PVN‐RVLM neurons may contribute to sympathetic activation and maintenance of Ang II‐salt HT. Support: HL076312 (NCDC)

Structural and Single-Channel Results Indicate That the Rates of Ligand Binding Domain Closing and Opening Directly Impact AMPA Receptor Gating

At most excitatory central synapses, glutamate is released from presynaptic terminals and binds to postsynaptic AMPA receptors, initiating a series of conformational changes that result in ion channel opening. Efficient transmission at these synapses requires that glutamate binding to AMPA receptors results in rapid and near-synchronous opening of postsynaptic receptor channels. In addition, if the information encoded in the frequency of action potential discharge is to be transmitted faithfully, glutamate must dissociate from the receptor quickly, enabling the synapse to discriminate presynaptic action potentials that are spaced closely in time. The current view is that the efficacy of agonists is directly related to the extent to which ligand binding results in closure of the binding domain. For glutamate to dissociate from the receptor, however, the binding domain must open. Previously, we showed that mutations in glutamate receptor subunit 2 that should destabilize the closed conformation not only sped deactivation but also altered the relative efficacy of glutamate and quisqualate. Here we present x-ray crystallographic and single-channel data that support the conclusions that binding domain closing necessarily precedes channel opening and that the kinetics of conformational changes at the level of the binding domain importantly influence ion channel gating. Our findings suggest that the stability of the closed-cleft conformation has been tuned during evolution so that glutamate dissociates from the receptor as rapidly as possible but remains an efficacious agonist.

Mast cell-mediated long-lasting increases in excitability of vagal C fibers in guinea pig esophagus

Several esophageal pathologies are associated with an increased number of mast cells in the esophageal wall. We addressed the hypothesis that activation of esophageal mast cells leads to an increase in the excitability of local sensory C fibers. Guinea pigs were actively sensitized to ovalbumin. The mast cells in the esophagus were selectively activated ex vivo by superfusion with ovalbumin. Action potential discharge in guinea pig vagal nodose esophageal C-fiber nerve endings was monitored in the isolated (ex vivo) vagally innervated esophagus by extracellular recordings. Ovalbumin activated esophageal mast cells, leading to the rapid release of approximately 20% of the tissue histamine stores. This was associated with a consistent and significant increase in excitability of the nodose C fibers as reflected in a two- to threefold increase in action potential discharge frequency evoked by mechanical (increases in intraluminal pressure) stimulation. The increase in excitability persisted unchanged for at least 90 min (longest time period tested) after ovalbumin was washed from the tissue. This effect could be prevented by the histamine H1 receptor antagonist pyrilamine, but once the increase in excitability occurred, it persisted in the nominal absence of histamine and could not be reversed even with large concentrations of the histamine receptor antagonist. In conclusion, activation of esophageal mast cells leads to a pronounced and long-lived increase in nociceptive C-fiber excitability such that any sensation or reflex evoked via the vagal nociceptors will likely be enhanced. The effect is initiated by histamine acting via H1 receptor activation and maintained in the absence of the initiating stimulus.

Pinch‐current injection defines two discharge profiles in mouse superficial dorsal horn neurones, in vitro

Neurones in the superficial dorsal horn (SDH) are a major target for nociceptive afferents and play an important role in pain processing. One approach to understanding the role of SDH neurones has been to study their action potential (AP) discharge in spinal cord slices during injection of depolarizing step-currents. Four or five neurone subpopulations are typically identified based on AP discharge, with various roles proposed for each in pain processing. During noxious peripheral stimulation in vivo, however, SDH neurones are activated via synaptic inputs. This produces a conductance change with different somato-dendritic distributions and temporal characteristics to that provided by a somatic step-current injection. Here we introduce an alternative approach to studying SDH neurone discharge under in vitro conditions. We recorded voltage-clamp responses in SDH neurones, in vivo, during noxious mechanical stimulation of the hindpaw (1 s pinch, approximately 100 g mm(-2)). From these recordings a representative 'pinch-current' was selected and subsequently injected into SDH neurones in spinal cord slices (recording temperature 32 degrees C). Pinch-current-evoked discharge was compared to that evoked by rectangular step-current injections. Pinch- and step-current-evoked AP discharge frequency was highly correlated (r2 = 0.61). This was also true for rheobase current comparisons (r2 = 0.61). Conversely, latency to discharge and discharge duration were not correlated when step- and pinch-current responses were compared. When neurones were grouped according to step-current-evoked discharge, five distinct patterns were apparent (tonic firing, initial bursting, delayed firing, single spiking, and reluctant firing). In contrast, pinch-current responses separated into two clear patterns of activity (robust and resistant firing). During pinch-current injection, tonic-firing and initial-bursting neurones exhibited robust AP discharge with similar characteristics. In contrast, single-spiking and reluctant-firing neurones were resistant to AP discharge. Delayed-firing neurones exhibited pinch-current responses that were transitional between those of tonic-firing/initial-bursting and single-spiking/reluctant-firing neurones. Injection of digitally filtered pinch-currents indicated that transient current fluctuations are necessary for robust repetitive discharge in initial-bursting neurones. These data suggest the functional significance of the diverse step-current-evoked firing patterns, previously reported in SDH neurones remains to be fully understood. When a 'facsimile' current profile or pinch-current is used in place of step-currents, AP discharge diversity is much reduced.

Chronic LPS exposure produces changes in intrinsic membrane properties and a sustained IL-β-dependent increase in GABAergic inhibition in hippocampal CA1 pyramidal neurons

Chronic inflammation has been reported to be a significant factor in the induction and progression of a number of chronic neurological disorders including Alzheimer's disease and Down syndrome. It is believed that inflammation may promote synaptic dysfunction, an effect that is mediated in part by pro-inflammatory cytokines such as interleukin-1beta (IL-1beta). However, the role of IL-1beta and other cytokines in synaptic transmission is still poorly understood. In this study, we have investigated how synaptic transmission and neuronal excitability in hippocampal pyramidal neurons are affected by chronic inflammation induced by exposing organotypic slices to the bacterial cell-wall product lipopolysaccharide (LPS). We report that CA1 pyramidal neurons recorded in whole cell from slices previously exposed to LPS for 7 days had resting membrane potential and action potential properties similar to those of the controls. However, they had significantly lower membrane resistance and a more elevated action potential threshold, and displayed a slower frequency of action potential discharge. Moreover, the amplitude of pharmacologically isolated postsynaptic gamma-aminobutyric acid (GABA)ergic potentials, but not excitatory glutamatergic postsynaptic potentials, was significantly larger following chronic LPS exposure. Interestingly, co-incubation of the IL-1 receptor antagonist (IL-1Ra) concurrently with LPS prevented the increase in GABAergic transmission, but not the reduction in intrinsic neuronal excitability. Finally, we confirmed that LPS dramatically increased IL-1beta, and IL-1beta-dependent IL-6 levels in the culture medium for 2 days before returning to baseline. We conclude that CA1 pyramidal neurons in slices chronically exposed to LPS show a persistent decrease in excitability due to a combined decrease in intrinsic membrane excitability and an enhancement in synaptic GABAergic input, the latter being dependent on IL-1beta. Therefore, chronic inflammation in hippocampus produces IL-1beta-dependent and -independent effects in neuronal and synaptic function that could contribute significantly to cognitive disturbances.

Angiotensin II Excites Paraventricular Nucleus Neurons That Innervate the Rostral Ventrolateral Medulla: An In Vitro Patch-Clamp Study in Brain Slices

Neurons of the hypothalamic paraventricular nucleus (PVN) are key controllers of sympathetic nerve activity and receive input from angiotensin II (ANG II)-containing neurons in the forebrain. This study determined the effect of ANG II on PVN neurons that innervate in the rostral ventrolateral medulla (RVLM)-a brain stem site critical for maintaining sympathetic outflow and arterial pressure. Using an in vitro brain slice preparation, whole cell patch-clamp recordings were made from PVN neurons retrogradely labeled from the ipsilateral RVLM of rats. Of 71 neurons tested, 62 (87%) responded to ANG II. In current-clamp mode, bath-applied ANG II (2 muM) significantly (P < 0.05) depolarized membrane potential from -58.5 +/- 2.5 to -54.5 +/- 2.0 mV and increased the frequency of action potential discharge from 0.7 +/- 0.3 to 2.8 +/- 0.8 Hz (n = 4). Local application of ANG II by low-pressure ejection from a glass pipette (2 pmol, 0.4 nl, 5 s) also elicited rapid and reproducible excitation in 17 of 20 cells. In this group, membrane potential depolarization averaged 21.5 +/- 4.1 mV, and spike activity increased from 0.7 +/- 0.4 to 21.3 +/- 3.3 Hz. In voltage-clamp mode, 41 of 47 neurons responded to pressure-ejected ANG II with a dose-dependent inward current that averaged -54.7 +/- 3.9 pA at a maximally effective dose of 2.0 pmol. Blockade of ANG II AT1 receptors significantly reduced discharge (P < 0.001, n = 5), depolarization (P < 0.05, n = 3), and inward current (P < 0.01, n = 11) responses to locally applied ANG II. In six of six cells tested, membrane input conductance increased (P < 0.001) during local application of ANG II (2 pmol), suggesting influx of cations. The ANG II current reversed polarity at +2.2 +/- 2.2 mV (n = 9) and was blocked (P < 0.01) by bath perfusion with gadolinium (Gd(3+), 100 muM, n = 8), suggesting that ANG II activates membrane channels that are nonselectively permeable to cations. These findings indicate that ANG II excites PVN neurons that innervate the ipsilateral RVLM by a mechanism that depends on activation of AT1 receptors and gating of one or more classes of ion channels that result in a mixed cation current.

Mechanosensory transduction of vagal and baroreceptor afferents revealed by study of isolated nodose neurons in culture

Changes in arterial pressure and blood volume are sensed by baroreceptor and vagal afferent nerves innervating aorta and heart with soma in nodose ganglia. The inability to measure membrane potential at the nerve terminals has limited our understanding of mechanosensory transduction. Goals of the present study were to: (1) Characterize membrane potential and action potential responses to mechanical stimulation of isolated nodose sensory neurons in culture; and (2) Determine whether the degenerin/epithelial sodium channel (DEG/ENaC) blocker amiloride selectively blocks mechanically induced depolarization without suppressing membrane excitability. Membrane potential of isolated rat nodose neurons was measured with sharp microelectrodes. Mechanical stimulation with buffer ejected from a micropipette (5, 10, 20 psi) depolarized 6 of 10 nodose neurons (60%) in an intensity-dependent manner. The depolarization evoked action potentials in 4 of the 6 neurons. Amiloride (1 μM) essentially abolished mechanically induced depolarization (15±4 mV during control vs. 1±2 mV during amiloride with 20-psi stimulation, n=6) and action potential discharge. In contrast, amiloride did not inhibit the frequency of action potential discharge in response to depolarizing current injection ( n=6). In summary, mechanical stimulation depolarizes and triggers action potentials in a subpopulation of nodose sensory neurons in culture. The DEG/ENaC blocker amiloride at a concentration of 1 μM inhibits responses to mechanical stimulation without suppressing membrane excitability. The results support the hypothesis that DEG/ENaC subunits are components of mechanosensitive ion channels on vagal afferent and baroreceptor neurons.

Pyeloureteral motility and ureteral peristalsis: essential role of sensory nerves and endogenous prostaglandins.

The cellular mechanisms that underlie the initiation and propagation of the peristaltic contractions, which transport urine from the kidney to the bladder for storage, remain little understood. Extracellular and intracellular microelectrode recordings have identified two populations of smooth muscle cells as well as a population of renal interstitial cells (RICs) that all display spontaneous electrical activity. By analogy with the heart it has been proposed that atypical smooth muscle cells, preferentially located in the very proximal regions of the renal pelvis, generate the essential pacemaker signal. These pacemaker potentials propagate to neighbouring typical smooth muscle cells or RICs to trigger action potential discharge. These action potentials then propagate distally to trigger other bundles of typical smooth muscle cells. The frequency of action potential discharge and contraction decreases as the relative number of RICs and atypical smooth muscle cells compared to typical smooth muscle cells decreases with distance from the renal fornix. It is clear that functional capsaicin-sensitive sensory afferents and the endogenous release of both tachykinins and prostaglandins are essential in the maintenance of normal peristalsis, as well as in monitoring and responding to any chemical or mechanical stimulation. However, the cellular mechanisms underlying the action of these endogenously-released agents remain to be elucidated.

Model of gamma frequency burst discharge generated by conditional backpropagation.

Pyramidal cells of the electrosensory lateral line lobe (ELL) of the weakly electric fish Apteronotus leptorhynchus have been shown to produce oscillatory burst discharge in the gamma-frequency range (20-80 Hz) in response to constant depolarizing stimuli. Previous in vitro studies have shown that these bursts arise through a recurring spike backpropagation from soma to apical dendrites that is conditional on the frequency of action potential discharge ("conditional backpropagation"). Spike bursts are characterized by a progressive decrease in inter-spike intervals (ISIs), and an increase of dendritic spike duration and the amplitude of a somatic depolarizing afterpotential (DAP). The bursts are terminated when a high-frequency somatic spike doublet exceeds the dendritic spike refractory period, preventing spike backpropagation. We present a detailed multi-compartmental model of an ELL basilar pyramidal cell to simulate somatic and dendritic spike discharge and test the conditions necessary to produce a burst output. The model ionic channels are described by modified Hodgkin-Huxley equations and distributed over both soma and dendrites under the constraint of available immunocytochemical and electrophysiological data. The currents modeled are somatic and dendritic sodium and potassium involved in action potential generation, somatic and proximal apical dendritic persistent sodium, and K(V)3.3 and fast transient A-like potassium channels distributed over the entire model cell. The core model produces realistic somatic and dendritic spikes, differential spike refractory periods, and a somatic DAP. However, the core model does not produce oscillatory spike bursts with constant depolarizing stimuli. We find that a cumulative inactivation of potassium channels underlying dendritic spike repolarization is a necessary condition for the model to produce a sustained gamma-frequency burst pattern matching experimental results. This cumulative inactivation accounts for a frequency-dependent broadening of dendritic spikes and results in a conditional failure of backpropagation when the intraburst ISI exceeds dendritic spike refractory period, terminating the burst. These findings implicate ion channels involved in repolarizing dendritic spikes as being central to the process of conditional backpropagation and oscillatory burst discharge in this principal sensory output neuron of the ELL.

Effects of nerve stimulation on spontaneously active preparations of the guinea pig ureter.

The effects of intrinsic nerve stimulation on the spontaneous electrical activity of the smooth muscle cells of the guinea pig ureter still attached to its renal pelvis were investigated using standard intracellular microelectrode techniques. Action potentials discharged spontaneously at a frequency of 3.3 +/- 0.2 min(-1) (n = 67) and consisted of an initial rapidly rising spike, followed by a variable period (0.2-5 s) of membrane potential oscillation and a quiescent plateau phase which was terminated by an abrupt repolarisation and after-hyperpolarisation to -66 mV. Transmural electrical stimulation (20-50 Hz for 2 s) transiently decreased the frequency of action potential discharge; the half-amplitude duration of the following action potentials, however, was transiently increased to 156 +/- 12% of control. Substance P (1 microM applied for 2 min) or neurokinin A (100 nM for 2 min) transiently increased the frequency of action potential discharge to 155 +/- 19% and 142 +/- 21%, respectively, of control. The excitatory actions of nerve stimulation or agonist application were reduced by the tachykinin antagonist, MEN 10,627 (1-3 microM), while the inhibitory actions of nerve stimulation were enhanced by MEN 10,627 (1 microM) or thiorphan (1 microM). Capsaicin (10 microM for 10-15 min) also evoked a transient increase in the frequency and half-amplitude duration of the ureteric action potentials, in a manner blocked by MEN 10,627 (3 microM), which was followed by a long period of membrane potential quiescence. Human calcitonin gene related peptide (hCGRP) (100 nM applied for 2-5 min) induced a time-dependent decrease in the frequency amplitude and duration of the spontaneous action potentials, in a manner blocked by glibenclamide (1 microM). It was concluded that the nerve-evoked excitatory and inhibitory changes in the parameters of the spontaneous ureteric action potentials arise from the release of the sensory neuropeptides, tachykinins and CGRP, respectively.

Identification of CO2Chemoreceptors inHelix pomatia

SYNOPSIS. Gas exchange in pulmonate snails of the family Helicidae occurs through a highly vascularized diffusion lung known as the mantle. The extent of ventilation of the mantle depends upon the duration and size of opening of an occlusible pore known as the pneumostome. In Helix aspersa and Helix pomatia , pneumostomal size and frequency of opening are exquisitely sensitive to CO2. Respiratory CO2 chemosensitivity resides in a discrete region of the subesophageal ganglia. The discharge pattern of many neurons in the chemoreceptor area changes during stimulation with CO2. However, the electrophysiological response to CO, stimulation alone does not discriminate between CO2 chemoreceptor cells and CO2-insensitive neurons active in the pneumostomal response to CO2. We identified a subset of CO2-sensitive neurons from the larger population of neurons active during CO2 stimulation. The action potential discharge frequency of CO2 chemosensory neurons increased in response to CO2 stimulation. An increased discharge frequency of CO2-sensitive neurons was associated with increased pneumostomal opening, and both the size and the frequency of pneumostomal opening increased during CO2 stimulation. Injecting depolarizing current into individual chemosensory neurons elicited opening of the pneumostome in the absence of CO2. Action potential generation in response to CO2 was independent of synaptic transmission. Removal of individual CO2-sensitive cells or inhibition of action potential generation in CO2-sensitive cells reduced or eliminated pneumostomal responses to CO2. CO2 sensitivity in chemoreceptor cells required extracellular calcium, but not sodium. Substituting barium for calcium supported chemoreceptor activity. In summary, we have identified respiratory related, chemosensory neurons that are CO2 sensitive in the absence of synaptic input.

The Effects of K + Channel Blockers on the Spontaneous Electrical and Contractile Activity in the Proximal Renal Pelvis of the Guinea Pig

Purpose The role of various K sup + channel populations in determining the time course and amplitude of the spontaneous action potentials and contractions in the smooth muscle cells of the guinea pig renal pelvis were investigated by standard electrophysiological and tension recording techniques. Materials and Methods Electrical recordings in the proximal renal pelvis were used to demonstrate the presence of 3 cell populations based on the waveform of their Ca 2+ −dependent action potentials which we have termed “pacemaker,” “intermediate” and “driven” action potentials. Results Blockade of large conductance (BK) or small conductance (SK) Ca sup 2+ −activated K sup + channels with charybdotoxin (30 to 60 nM.) and tetraethylammonium (TEA) (0.5 to 2 mM.) or apamin (200 nM.) increased the duration of the action potentials recorded in “driven” cells. Tetraethylammonium (2 mM.) and 4-aminopyridine (2 mM.) increased the frequency of action potential discharge in both “driven” and “pacemaker” cells, as well as having a positive inotropic and chronotropic effect on the spontaneous contractions of the renal pelvis. In contrast, glibenclamide (0.1 to 1 micromolar), Cs 2+ (1 mM.) and Ba 2+ (100 micromolar) had little effect on either the contractile or electrical activity of the renal pelvis. Conclusion It was concluded that the frequency of contraction in the proximal renal pelvis is determined by the frequency of action potential discharge in “pacemaker” cells. On the other hand, increases in the amplitude of pelvic contractions may well be correlated with membrane depolarization and/or increases in the duration of the action potentials recorded in “driven” cells.

On the mechanisms underlying hypoxia-induced membrane depolarization in striatal neurons.

Clinical and experimental evidence has shown that the striatal neurons are particularly vulnerable to hypoxia and ischaemia. An excessive excitatory action of glutamate, released by the corticostriatal terminals, has been implicated in this peculiar vulnerability of striatal neurons. We have studied the effects of hypoxia on the membrane properties of striatal neurons intracellularly recorded from a corticostriatal slice preparation. Brief (2-10 min) periods of hypoxia produced reversible membrane depolarizations. During the initial phase of the hypoxia-induced depolarization the frequency of action potential discharge was transiently increased; 2-3 min after the onset of hypoxia the firing activity was fully abolished. Brief periods of hypoxia also caused a reversible reduction of the amplitude of the excitatory postsynaptic potentials (EPSPs) evoked by cortical stimulation. Longer period of hypoxia (12-20 min) produced irreversible membrane depolarizations. In voltage-clamp experiments hypoxia caused an inward current coupled with an increased membrane conductance. Tetrodotoxin (TTX) or low calcium (Ca2+)-high magnesium containing solutions blocked synaptic transmission, but they did not reduce the hypoxia-induced electrical changes. Antagonists of excitatory amino acid receptors failed to affect the electrical effects caused by oxygen deprivation. Hypoxia-induced inward currents were reduced either by the potassium (K+) channel blockers, barium and tetraethyl ammonium (TEA) cations, or by lowering external sodium (Na+) concentration. Blockade of ATP-dependent Na(+)-K+ pump by both ouabain and strophanthidin enhanced hypoxia-induced membrane depolarization/inward current. Our findings indicate that the release of excitatory amino acids does not seem to be required for the acute hypoxia-induced electrical changes in striatal neurons. Moreover, TTX-resistant Na+ influx and K+ currents seem to play an important role in the generation of hypoxia-induced electrical changes. These data also suggest that the selective vulnerability of striatal neurons to oxygen deprivation may be caused by their peculiar sensitivity to energy metabolism failure.

Activation of C fibers by metabolic perturbations associated with tourniquet ischemia.

Peripheral A-delta and C fibers are activated during the production of ischemic or tourniquet pain; however, individual metabolic or molecular factors responsible for neural activation are not known. To elucidate these mechanisms the in vitro corneal nerve preparation was used. Electrophysiologic effects of individual metabolic perturbations associated with ischemia (hypoxia, hypoglycemia, lactic acid, and decreased pH) were investigated on A-delta and C fiber nociceptors. Increased tonic action potential activity occurred in C fibers but not in A-delta fibers after ischemia. The conduction velocity of C fibers was 0.85 +/- 0.2 m/s (mean +/- SD). Under control conditions (n = 43) there was very little fluctuation in the baseline action potential frequency (+/- 3.2%). Hypoxia (n = 12) resulted in a 213 +/- 3.4% (mean +/- SD) increase in C fiber action potential frequency relative to control (P less than 0.001, ANOVA). L-glucose substitution for D-glucose (n = 8) increased C fiber discharge frequency by 653 +/- 28% relative to control (P less than 0.001) as did the combination of hypoxia and L-glucose substitution (n = 6) by 671 +/- 14%. Comparison of hypoxia versus hypoxia and hypoglycemia conditions did not show them to be statistically different (P greater than 0.5). Lactate (10-1000 micrograms/ml) at a pH of 6.9 or 7.4 did not alter the action potential discharge frequency in corneal C fibers (n = 5, P greater than 0.5).(ABSTRACT TRUNCATED AT 250 WORDS)

Repetitive firing properties in subpopulations of the chick Edinger Westphal nucleus.

The avian Edinger Westphal nucleus, through the ciliary ganglion, controls accommodation, iris constriction, and blood flow through the choroid. In live brainstem slices, the nucleus is easily identifiable as an olive-shaped cluster of neurons dorsal to the oculomotor nerve and nucleus. Intracellular recordings from neurons in the nucleus identified two classes of responses to sustained (300 to 500 ms) injections of depolarizing current. One set of cells fired action potentials for the duration of the pulse while a second set of cells fired action potentials only transiently, during the first 50 to 100 ms, after which they remained silent regardless of the size of the depolarization. Intracellular recordings followed by injections of the fluorescent dye lucifer yellow revealed that repetitively firing cells were located in the lateral half of the nucleus while non-repetitively or transiently firing cells were located in the medial half. These locations correspond to different Edinger Westphal subdivisions which have distinct inputs and target populations. The varying firing patterns are discussed with reference to the known functions of the subdivisions in which they occur. Replacement of calcium by magnesium in the extracellular medium had no effect on the number of action potentials fired by non-repetitively firing cells, suggesting that a calcium-activated potassium current is not responsible for suppressing repetitive firing in these cells. In contrast, in repetitively firing cells removal of extracellular calcium increased the frequency of action potential discharge and decreased the amplitude of afterhyperpolarizations following single action potentials. Addition of cadmium to the bath medium had similar effects.(ABSTRACT TRUNCATED AT 250 WORDS)