Dependence of spontaneous neuronal firing and depolarisation block on astroglial membrane transport mechanisms

Exposed to a sufficiently high extracellular potassium concentration ([K^?+?]_o), the neuron can fire spontaneous discharges or even become inactivated due to membrane depolarisation (??depolarisation block??). Since these phenomena likely are related to the maintenance and propagation of seizure discharges, it is of considerable importance to understand the conditions under which excess [K^?+?]_o causes them. To address the putative effect of glial buffering on neuronal activity under elevated [K^?+?]_o conditions, we combined a recently developed dynamical model of glial membrane ion and water transport with a Hodgkin?CHuxley type neuron model. In this interconnected glia-neuron model we investigated the effects of natural heterogeneity or pathological changes in glial membrane transporter density by considering a large set of models with different, yet empirically plausible, sets of model parameters. We observed both the high [K^?+?]_o-induced duration of spontaneous neuronal firing and the prevalence of depolarisation block to increase when reducing the magnitudes of the glial transport mechanisms. Further, in some parameter regions an oscillatory bursting spiking pattern due to the dynamical coupling of neurons and glia was observed. Bifurcation analyses of the neuron model and of a simplified version of the neuron-glia model revealed further insights about the underlying mechanism behind these phenomena. The above insights emphasise the importance of combining neuron models with detailed astroglial models when addressing phenomena suspected to be influenced by the astroglia-neuron interaction. To facilitate the use of our neuron-glia model, a CellML version of it is made publicly available.     

Molecular mechanism of modulation of nociceptive neuron membrane excitability by a tripeptide

Using the whole-cell patch-clamp method, the ability of arginine-containing tripeptide Ac-RER-NH₂, dipeptide Ac-RR-NH₂, and free Arg molecule to modulate the membrane excitability of nociceptors was studied. Extracellular Ac-RER-NH₂ upon interaction with the outer membrane of the nociceptive neuron decreases the Z_eff value of the activation gating system of Na_v1.8 channels. Thus, the tripeptide Ac-RER-NH₂ can be considered as a new effective and safe analgesic.     

Mechanisms of membrane potential oscillation in bursting neurons on the snail,Helix pomatia

• 1.1. The mechanism of generation of membrane potential (MP) oscillations was studied in identified bursting neurons from the snail Helix pomatia.
• 2.2. Long-lasting stimulation of an identified peptidergic interneuron produced a persistent bursting activity in a non-active burster.
• 3.3. External application of calcium channel blockers (1 mM Cd²⁺ or 5 mM La²⁺) resulted in a transient increase in the slow-wave amplitude and subsequent prevention of pacemaker activity generation in bursting neurons. Application of these blockers together with endogenous neuropeptide initiating bursting activity generation, increased MP wave amplitude without prevention of bursting activity generation.
• 4.4. Replacement of all NaCl in normal Ringer's solution with isoosmotic CaCl₂, glucose or Tris-HCl produced a reversible block of bursting activity generation. Stationary current-voltage relation (CVR) of bursting neuron membrane has a region of negative resistance (NRR) and does not intersect the potential axis in threshold region for action potential (AP) generation in normal Ringer's solution. In Na-free solution stationary CVR is linear and intersects the potential axis near — 52 mV.
• 5.5. Novel potential- and time-dependent outward (E_rev = − 58 mV) current, I_B, activated by hyperpolarization was found in the bursting neuron membrane. Having achieved a maximal value, this current decayed with a time constant of about 1 sec. Hyperpolarization inactivated maximal conductance, g_B, responsible for I_B, and depolarization abolished inactivation of g_B.
• 6.6. Short-lasting (0.01 sec) hyperpolarization of the bursting neuron membrane by inward current pulse induced the development of prolonged hyperpolarization wave lasting up to 10 sec.
• 7.7. These results suggest that: (a) persistent bursting activity of RPal neuron in the snail Helix pomatia is not endogenous but is due to a constant activation of peptidergic synaptic inputs of these neurons; (b) Ca²⁺ ions do not play a pivotal role in the ionic mechanism of MP oscillations but play a determining role in the process of secretion of a peptide initiating bursting activity by the interneuron presynaptic terminal; (c) depolarizing phase of the MP wave is due to specific properties of stationary CVR and hyperpolarization phase is due to regenerative properties of hyperpolarization-activated outward current I_B. The minimal mathematical version of MP oscillations based on the experimental data is presented.

     

Interval analysis of bursting discharges in aplysia neurons

Time intervals of 12 records of bursting discharges in Aplysia neurons were analysed by digital computer to determine the interrelations between the burst period, the interburst interval and the burst duration. The effects of membrane potential changes on the parameters of bursting discharges were examined also. A low correlation was found between burst duration and burst period in the majority of cases, and this was interpreted as an indication of probable independence between the mechanisms governing these parameters. Also, a specific temporal organization of interspike intervals seems to be present in each type of neuron. The results suggest that the mechanism governing the burst period is characterized by a slow membrane potential oscillation resembling that observed in bursting neurons when actions potentials are blocked by tetrodotoxin. The burst duration would be determined by the response of the neuron to suprathreshold depolarization.     

Multiple modes of a conditional neural oscillator

We present a model for a conditional bursting neuron consisting of five conductances: Hodgkin-Huxley type time- and voltage-dependent Na⁺ and K⁺ conductances, a calcium activated voltage-dependent K⁺ conductance, a calcium-inhibited time- and voltage-dependent Ca⁺⁺ conductance, and a leakage Cl(_– conductance. With an initial set of parameters (versionS), the model shows a hyperpolarized steady-state membrane potential at which the neuron is silent. Increasingg _Na and decreasingg _Cl, whereg _i , is the maximal conductance for speciesi, produces bursts of action potentials (BursterN). Alternatively, an increase ing _Ca produces a different bursting state (BursterC). The two bursting states differ in the periods and amplitudes of their bursting pacemaker potentials. They show different steady-stateI–V curves under simulated voltage-clamp conditions; in simulations that mimic a steady-stateI–V curve taken under experimental conditions only BursterN shows a negative slope resistance region. ModelC continues to burst in the presence of TTX, while bursting in ModelN is suppressed in TTX. Hybrid models show a smooth transition between the two states.     

Nonlinear dynamics and information processing in pacemaker neurons

The paper treats some nonlinear dynamic phenomena in oscillatory activity of a single nerve cell. Based on experiments with CNS bursting pacemaker neurons ofHelix pomatia snail, a mathematical model was studied. The model demonstrates the majority of experimentally observable phenomena and allows one to investigate the role of its separate components. The phenomena demonstrated by model neuron (chaotic behavior, bistability, and sensitivity to parameter variations, initial conditions, and stimuli) may be relevant to information processing in nerve cells. The complexity of [Ca²⁺] _in −V phase diagrams of initial conditions depends on parameters. Transient synaptic impulse produces stable parameter-independent changes in activity of model neuron. These results indicate that a single bursting neuron can work in the neuronal ensemble as a dynamic switch. The sensitivity of this switch is regulated by a neurotransmitter.     

Mechanism of action of suprofen, a new peripheral analgesic, as demonstrated by its effects on several nociceptive mediators

Suprofen is a new potent, orally effective non-narcotic analgesic agent having a potent inhibitory action on prostaglandin (PG) biosynthesis. Recent experiments have shown that suprofen inhibits uterine hyperactivity induced by the physiological substances, arachidonic acid, bradykinin (BK) and PGF_2α. The present study explores the possibility that the analgesic activity of suprofen may involve multiple mechanisms of interaction with PGs, inhibiting synthesis at low doses and with higher doses possibly directly interacting with PGs and other physiological mediators of nociception at a common site. Experiments in mice have shown that suprofen antagonizes abdominal stretching induced by the physiological precursor of PG release, arachidonic acid (ED₅₀ = 0.07 mg/kg, p.p.), and by the nociceptive agents acetylcholine (ACh) (ED₅₀ = 1.7 mg/kg, p.o), BK (ED₅₀ = 65 mg/kg, p.o.) acetic acid (HAC) (H⁺ ion; ED₅₀ = mg/kg, p.o), and PGE₂, itself (ED₅₀ = 20.2 mg/kg, i.p.). In rabbits, i.a. administered suprofen (ED₅₀ = 0.98 mg/kg) blocked the reflex discharge of spinal sensory neurons evoked by BK (2 to 8 μg, i.a). The analgesic activity of suprofen may involve multiple mechanism of interaction with PGs and other mediators, including BK; suprofen blocks the nociceptive actions of PGs by inhibiting their formation, via the cyclooxygenase pathway, and possibly at PG sites of action, probably at peripheral nerve endings.     

Mechanisms of Generation of Pacemaker Activity by Identified Helix Neurons

We studied the mechanisms of generation of pacemaker activity in identified neurons of Helix pomatia. For this purpose, we isolated the PPa2 and PPa7 neurons generating spontaneous rhythmic monomodal activity and PPa1 neuron with bursting activity. It was demonstrated that isolated PPa2 and PPa7 cells produce endogenous rhythmic activity that was not considerably modified by external application of 1 mM CdCl₂. Sometimes, only low-amplitude dendritic action potentials (AP) were observed instead of generation of full-amplitude somatic AP. In contrast, isolation of the PPa1 neuron eliminated its bursting activity, but subsequent application of oxytocin on this neuron recovered such activity. This finding shows that the bursting activity of the PPa1 neuron is of an exogenous nature. Application of 1 mM CdCl₂ suppressed this bursting activity, but when Cd²⁺ was applied against the background of superfusion of the neuron with Ringer solution containing a bursting activity-initiating neuropeptide obtained from the molluscan CNS, this blocker was incapable of suppressing the bursting activity. A blocker of the hyperpolarization-activated current (I _h , H current), Cs⁺ (10 mM) exerted no noticeable effect on the activity of the studied neurons. Our findings allow us to conclude that the pacemaker activity is initiated within the dendritic tree of a cell and is then electrotonically spread to the soma, where full-amplitude AP are generated. It seems probable that Ca²⁺ ions and H current are not directly involved in generation of the pacemaker activity in the studied snail neurons.     

A unified model for two modes of bursting in GnRH neurons

Gonadotropin-releasing hormone (GnRH) neurons exhibit at least two intrinsic modes of action potential burst firing, referred to as parabolic and irregular bursting. Parabolic bursting is characterized by a slow wave in membrane potential that can underlie periodic clusters of action potentials with increased interspike interval at the beginning and at the end of each cluster. Irregular bursting is characterized by clusters of action potentials that are separated by varying durations of interburst intervals and a relatively stable baseline potential. Based on recent studies of isolated ionic currents, a stochastic Hodgkin-Huxley (HH)-like model for the GnRH neuron is developed to reproduce each mode of burst firing with an appropriate set of conductances. Model outcomes for bursting are in agreement with the experimental recordings in terms of interburst interval, interspike interval, active phase duration, and other quantitative properties specific to each mode of bursting. The model also shows similar outcomes in membrane potential to those seen experimentally when tetrodotoxin (TTX) is used to block action potentials during bursting, and when estradiol transitions cells exhibiting slow oscillations to irregular bursting mode in vitro. Based on the parameter values used to reproduce each mode of bursting, the model suggests that GnRH neurons can switch between the two through changes in the maximum conductance of certain ionic currents, notably the slow inward Ca²⁺ current I _s, and the Ca²⁺ -activated K⁺ current I _KCa. Bifurcation analysis of the model shows that both modes of bursting are similar from a dynamical systems perspective despite differences in burst characteristics.     

A class of novel conjugates of substituted purine and Gly-AA-OBzl: Synthesis and evaluation of orally analgesic activity

Aimed at the chemotherapy of chronic pain two kinds of analgesic pharmacophores, substituted purine and Gly-AA-OBzl, were coupled via a five-step-reaction procedure and 19 novel conjugates N-[2-chloro-9-(tetrahydropyran-2-yl)-9H-purin-6-yl]-N-cyclopropylglycylamino acid benzylesters were provided. On mouse-tail flick model their in vivo analgesic activities were assayed. The results indicate that introducing Gly-OC₂H₅ into the 6-position of the substituted purine leads to ambiguous increase of the analgesic activity, while introducing Gly-AA-OBzl into this position leads to significant increase of the analgesic activity.     

Conformational changes in erythrocyte membranes by prostaglandins as measured by circular dichroism

Membranes were prepared from fresh, washed human erythrocytes by hemolysis and washing with 5 mm sodium phosphate buffer (pH 7.4). The mean residue ellipticity, [θ], of erythrocyte membrane circular dichroism was altered by prostaglandin E₁ or prostaglandin F_2α at 37 °C when observed from 250 nm to 190 nm. The decrease in negativity of [θ] with 10^?6m prostaglandin E₁ was 12.7% at 222 nm and 17.7% at 208 nm, and with 10^?6m prostaglandin F_2α 22.5% and 34.2%, respectively (P < 0.01). Similar changes in [θ] were observed at lower concentrations of prostaglandins. No strict relationship between amount of change of [θ] and prostaglandin concentrations of 3 × 10^?5m to 3 × 10^?12m was evident. A persistent alteration of [θ] with prostaglandin was observed at 37 °C. Transient change of [θ] occurred at 25 °C with prostaglandin. No change of [θ] was observed at 15 or 20 °C. Buffer or palmitic acid were without effect on membrane [θ]. Phosphatidyl inositol or methyl arachidonate caused an increase in negativity of membrane spectra. The observed alterations of membrane [θ] did not arise from changes in light scattering as the OD₇₀₀–OD₂₀₀ of membranes was not changed by prostaglandin. Effects of prostaglandin were not dependent on light path length. The prostaglandin E₁ antagonist, 7-oxa-13-prostynoic acid, at 10^?7m produced no change of [θ] of membrane spectra and prevented the otherwise demonstrable effects of 10^?10m prostaglandin E₁ on [θ]. The decrease in negativity of [θ] at 222 nm is indicative of a decrease in ellipticity of membrane protein. These studies suggest that prostaglandins may act by inducing a conformational change in membrane protein.     

Antinociceptive potency of a fluorinated cyclopeptide Dmt-c[D-Lys-Phe-p-CF3-Phe-Asp]NH2

Opioid peptides and opiate drugs such as morphine, mediate their analgesic effects, but also undesired side effects, mostly through activation of the mu opioid receptor. However, delta- and kappa-opioid receptors can also contribute to the analgesic effects of opioids. Recent findings showed that simultaneous activation of multiple opioid receptors may result in additional analgesia with fewer side effects. Here, we evaluated the pharmacological profile of our formerly developed mixed mu/kappa-opioid receptor ligands, Dmt-c[D-Lys-Phe-Phe-Asp]NH₂ (C-36) and Dmt-c[D-Lys-Phe-p-CF₃-Phe-Asp]NH₂ (F-81). The ability of these peptides to cross the blood–brain barrier was tested in the parallel artificial membrane permeability (PAMPA) assay. On the basis of the hot-plate test in mice after central and peripheral administration, analog F-81 was selected for the anti-nociceptive and anti-inflammatory activity assessment after peripheral administration.     

Electrical connection between two bursting neurons inHelix pomatia

In some preparations of the CNS ofHelix pomatia, two neurons with bursting activity may be present in the right parietal ganglion, where usually there is only one bursting neuron RPal. If electrical activity of these neurons is recorded simultaneously, fluctuations of membrane potential are almost completely synchronized. Artificial depolarization and hyperpolarization of the membrane of one neuron caused depolarization or hyperpolarization of the other neuron. During long-term recording of the activity of both neurons synchronous modulation of their bursting activity was observed. Modulating factor (a peptide fraction obtained from the water-soluble part of snail brain homogenate) led to potentiation of the bursting activity of both neurons. It is concluded from the results of these experiments that two bursting RPal neurons, connected electrically with one another, may exist in the snail nervous system. In cases when the parameters of pacemaker activity of these two neurons are closely similar, electrical connection guarantees synchronization of their bursting activity and ensures a common frequency of changes in their membrane potential.     

Translocation of cytidine 5′-monophosphosialic acid across golgi apparatus membranes

Golgi apparatus, isolated from rat liver, incorporate [¹⁴C]sialic acid from CMP[¹⁴C]sialic acid into endogenous glycolipid and glycoprotein acceptors. Incorporation of [¹⁴C]sialic acid into endogenous glycoprotein acceptors was stimulated an average of 3-fold by Triton X-100 at an optimal concentration of 0.05% and was inhibited at higher concentrations. Incorporation of [¹⁴C]sialic acid into endogenous glycolipid acceptors was not stimulated by detergent. The major glycolipid product was identified by thin-layer chromatography as the ganglioside G_d3. SDS-polyacrylamide gel electrophoresis of the glycoprotein products demonstrated incorporation of [¹⁴C]sialic acid into 6–7 major bands. Neuraminidase studies determined that approximately 60% of the [¹⁴C]sialic acid incorporated into endogenous acceptors in the absence of detergent had a luminal orientation. Furthermore, electron microscopy studies showed that the isolated Golgi apparatus fraction consisted of intact membrane cisternae. Our results demonstrate that sialylation of cisternal acceptors located on the inside of the membrane occurs in the absence of detergent. They are consistent with carrier-mediated transport as a mechanism to allow CMPsialic acid to traverse the Golgi apparatus membrane and to be used to glycosylate endogenous glycoprotein and glycolipid acceptors.     

Ethanol-induced decreases in membrane long-chain unsaturated fatty acids correlate with impaired chick brain development

Exposure to ethanol at 0 days of development induced changes in total membrane fatty acid composition at 18 days of development. When exposed to ethanol concentrations ranging from 0–743.27μm/kg egg wt, decreased levels of long-chain, unsaturated membrane fatty acids and increased levels of short-chain, saturated membrane fatty acids were observed in embryonic chick brains at 18 days of development. The ratios of unsaturated membrane/saturated membrane fatty acids correlated with an ethanol-induced reduction in neuron densities within the cerebral hemispheres and three different regions of the optic lobes with correlation coefficients (r) ranging from 0.44 [F = (1, 32) 7.84; P ≤ 0.009] to 0.59 [F = (1, 32) 17.38; P ≤ 0.0002]. The ratios of long-chain/short-chain membrane fatty acids also correlated with an ethanol-induced reduction in neuron densities within the cerebral hemispheres and three different regions of the optic lobes with correlation coefficients (r) ranging from 0.51 [F = (1, 32) 11.27; P≤ 0.002] to 0.66 [F = (1, 32) 24.40; P ≤ 0.0001]. Cell fractionation studies indicated that the ethanol-induced changes in brain membrane fatty acid composition were restricted to microsomal membranes.     

Seizure-like afterdischarges simulated in a model neuron

To explore non-synaptic mechanisms in paroxysmal discharges, we used a computer model of a simplified hippocampal pyramidal cell, surrounded by interstitial space and a “glial-endothelial” buffer system. Ion channels for Na⁺, K⁺, Ca²⁺ and Cl⁻ _, ion antiport 3Na/Ca, and “active” ion pumps were represented in the neuron membrane. The glia had “leak” conductances and an ion pump. Fluxes, concentration changes and cell swelling were computed. The neuron was stimulated by injecting current. Afterdischarge (AD) followed stimulation if depolarization due to rising interstitial K⁺ concentration ([K⁺]_o) activated persistent Na⁺ current (I _Na,P). AD was either simple or self-regenerating; either regular (tonic) or burst-type (clonic); and always self-limiting. Self-regenerating AD required sufficient I _Na,P to ensure re-excitation. Burst firing depended on activation of dendritic Ca²⁺ currents and Ca-dependent K⁺ current. Varying glial buffer function influenced [K⁺]_o accumulation and afterdischarge duration. Variations in Na⁺ and K⁺ currents influenced the threshold and the duration of AD. The data show that high [K⁺]_o and intrinsic membrane currents can produce the feedback of self-regenerating afterdischarges without synaptic input. The simulated discharge resembles neuron behavior during paroxysmal firing in living brain tissue. Action Editor: David Terman     

Avermectin B1a: an irreversible activator of the gamma-aminobutyric acid-benzodiazepine-chloride-ionophore receptor complex

Avermectin B_1a, an antihelminthic macrocyclic lactone, has been previously shown to reduce muscle membrane resistance by stimulating γ-aminobutyric acid-mediated chloride conductance. Since the benzodiazepine receptor is coupled to a receptor for γ-aminobutyric acid and related chloride ionophore, the effects of Avermectin B_1a on [³H]diazepam binding to the benzodiazepine receptor were studied. In well-washed membrane fragments from rat cerebral cortex, Avermectin B_1a markedly increased the binding of [³H]diazepam to benzodiazepine receptors. This effect was qualitatively similar to that observed with either γ-aminobutyric acid or chloride ion and was partially reversed by the γ-aminobutyric acid receptor antagonist, bicuculline. In contrast to the effects of γ-aminobutyric acid and chloride, the enhanced binding of [³H]benzodiazepine elicited by Avermectin B_1a was $\text{not}$ reversed by extensive washing of the membrane preparation. Avermectin B_1a appears to irreversibly modify benzodiazepine receptors at a γ-aminobutyric acid-chloride recognition site and may be valuable in biochemical studies of the regulation of benzodiazepine receptor function.     

A computational study of the effects of serotonin on a molluscan burster neuron

A mathematical model of burster neuron R₁₅ from the abdominal ganglion of Aplysia is presented. This is an improvement over earlier models in that the bursting mechanism is more accurately represented. The improved model allows for simulated application of the neurotransmitter serotonin, which has been reported to have profound effects on the voltage waveform produced by R₁₅. Computational analysis indicates that the serotonin-induced modulation of the waveform can be explained in terms of competition between stationary, bursting, and beating attractors. Analysis also indicates that, as a result of this competition, serotonin increases the sensitivity of the neuron to synaptic perturbations. This may have important consequences with regard to water balance in the Aplysia, particularly during egg laying.     

Intrinsic bursting activity in the pre-Bötzinger Complex: Role of persistent sodium and potassium currents

Computational models of single pacemaker neuron and neural population in the pre-Bötzinger Complex (pBC) were developed based on the previous models by Butera et al. (1999a,b). Our modeling study focused on the conditions that could define endogenous bursting vs. tonic activity in single pacemaker neurons and population bursting vs. asynchronous firing in populations of pacemaker neurons. We show that both bursting activity in single pacemaker neurons and population bursting activity may be released or suppressed depending on the expression of persistent sodium (I_NaP) and delayed-rectifier potassium (I_K) currents. Specifically, a transition from asynchronous firing to population bursting could be induced by a reduction of I_K via a direct suppression of the potassium conductance or through an elevation of extracellular potassium concentration. Similar population bursting activity could be triggered by an augmentation of I_NaP. These findings are discussed in the context of the possible role of population bursting activity in the pBC in the respiratory rhythm generation in vivo vs. in vitro and during normal breathing in vivo vs. gasping.     

Transduction and transmission properties of primary nociceptive afferents.

The prototypical primary nociceptive afferent is the polymodal C-fiber nociceptor, which responds to noxious thermal, mechanical, and chemical stimuli. C-fiber nociceptors are peripheral terminals of small neurons in the dorsal root ganglia (DRG). DRG neurons must therefore supply their peripheral terminals with the molecular machinery for the encoding of noxious stimuli into trains of action potentials. The following phenomena are known for this encoding process in vivo: 1) adaptation: for a constant stimulus intensity the action potential discharge decreases slowly within 2-3 seconds, 2) fatigue: recovery from adaptation may take ten minutes or more, 3) sensitization: preceding tissue damage enhances the response, particularly to heat stimuli. Recent studies in vitro have provided important clues about the molecular mechanisms underlying these phenomena. Several membrane receptors and channels are specifically expressed in small nociceptive neurons, such as vanilloid receptors (VR1), purinergic receptors (P2X3), acid sensing ion channels (ASIC), and TTX-resistant Na-channels. In the near future, we may therefore expect major advances in our understanding of the transduction of noxious stimuli into generator potentials and transformation into trains of action potentials. Along the axon that leads from the innervated tissue to the spinal cord, primary nociceptive afferents have a limited capacity to transmit high impulse rates, suggesting a different composition of voltage-gated channels than in other primary afferents (low-threshold mechanoreceptors and thermoreceptors). Finally, the DRG neuron also supplies its central terminals with the molecular machinery for synaptic transmission and its presynaptic modulation. Progress in understanding the cellular mechanisms at both ends of the primary nociceptive neuron promises to lead to new analgesic treatment modalities for both acute and chronic pain.