Electroregulated potassium channels of the somatic membrane of mollusk neuron membranes

The delayed potassium current in mollusc neurons was separated into two components: a noninactivating component and a transient (inactivating) component. A noninactivating potassium current is not affected by changes in temperature, whereas an inactivating component decreases substantially under cooling. Internal Na+ can block the delayed outward potassium current. The blockade of the inactivating potassium current is strongly voltage-dependent. The voltage dependence of the blockade of the noninactivating potassium current is not so clear. These results are consistent with the hypothesis that there are two forms of the delayed potassium current channel. The effect of temperature and internal Na+ on the fast outward current is similar to that on the inactivating component of the delayed potassium current.     

Calcium channels in the somatic membrane of the rat dorsal root ganglion neurons, effect of cAMP

Isolated rat dorsal root ganglion neurons have been perfused with potassium-free solutions containing cAMP, ATP and Mg2+ ions. In these conditions stable inward calcium currents can be recorded in the somatic membrane of all investigated cells. The kinetics of these currents can be approximated by a modified Hodgkin-Huxley equation using a square power of the m-variable; its inactivation is extremely slow. The corresponding channels pass Ba2+ ions about twice more effective than Ca2+.     

Functional characteristics of potassium channels in the somatic membrane of neurons of Lymnaea stagnalis of various ages

It has been found that in old Lymnaea stagnalis neurons the maximal amplitude of the delayed potassium current and the maximal potassium conductance decreases and the kinetics of inactivation of the delayed outward current slows down. It is suggested that these changes may be associated with the age peculiarities of the phospholipid composition of membranes and not related to shifts in the surface area of the nerve cell during aging. The age dependence of the function of potassium channels may underlie the changes in the neuronal function during aging.     

Neurotransmitter modulation of calcium channels in rat sympathetic neurons.

Adrenergic, cholinergic, and a variety of peptide neurotransmitters are known to modulate Ca currents in peripheral neurons. Using a protocol that allows for simultaneous assessment of effects on dihydropyridine (DHP)-sensitive and DHP-insensitive current components, we compared the actions of norepinephrine (NE), bethanechol (BeCh), and neuropeptide Y (NPY) on Ca currents in neonatal rat superior cervical ganglion neurons. Here, we show that these transmitters selectively depress the activity of DHP-insensitive Ca channels. Intracellular application of GTP-gamma-S, an activator of GTP-binding proteins, also exclusively affected the DHP-insensitive current, whereas 1,2-oleoylacetylglycerol (OAG), a protein kinase C (PKC) activator, depressed both the DHP-sensitive and DHP-insensitive currents. Pertussis toxin interrupted the coupling between NE and its effector, whereas three different inhibitors of PKC did not. Thus, we confirmed that the selective actions of the transmitters on Ca current appear to be mediated via GTP-binding proteins, but we found no evidence for direct involvement of PKC and conclude that the observed actions of OAG are distinct from those mediated by the neurotransmitters studied.     

Development of excitable membrane properties in mammalian sympathetic neurons

Using the whole-cell patch-clamp recording technique, resting membrane potentials (RPs), action potential (AP) waveforms, and the properties of voltage-activated inward Na+ (lNa) and Ca2+ (lCa) currents and outward K+ (lA,lK) currents were examined in embryonic and neonatal rat superior cervical ganglion (SCG) neurons as a function of time during development in vivo and in vitro. The passive and active membrane properties of neonatal SCG cells examined less than or equal to 24 hr after isolation were similar to those described previously for adult SCG neurons and for neonatal SCG cells maintained for several weeks in culture. Since recordings were obtained within hours of cell dissociations, it is assumed that the results reflect the membrane properties of neonatal SCG neurons in vivo at the time of isolation. When neonatal cells were examined as a function of time (up to approximately 2-3 weeks) in vitro, neither RPs nor AP waveforms varied measurably. Although absolute (inward and outward) current amplitudes increased in cells maintained in vitro, in parallel with increases in cell size, no changes in the time- or voltage-dependent properties of the currents were observed. Similar results were obtained for cells isolated on or after embryonic day 18.5 (greater than or equal to E 18.5). The membrane properties of E 14.5-16.5 SCG cells examined less than or equal to 24 hr after isolation, in contrast, were significantly different: mean lCa density was higher, and APs were broader than in greater than or equal to E 18.5 cells, and, in addition, lA was absent in these cells. When E 14.5-16.5 cells were examined after approximately 1 week in vitro, lCa densities and AP waveforms were indistinguishable from those in greater than or equal to E 18.5 SCG neurons, and lA was present. These studies reveal that rat SCG neurons are electrophysiologically mature early in development. Even lA, which seems to be the last voltage-gated current to develop, appears well before birth. As the appearance of lA correlates with decreased membrane excitability and AP shortening, it seems likely that the development of lA either reflects or regulates a marked change in the overall input and output properties of developing sympathetic neurons.     

缩宫素对大鼠背根神经节神经元胞体膜的作用

应用细胞内记录技术,观察了缩宫素(OXT,10 -8～10-5 mol/L)对2～3周龄大鼠背根神经节神经元的作用.在受检的92个细胞中,有71个神经元滴加缩宫素产生明显的超极化反应.在滴加10-5mol/L缩宫素后, 膜电导由平均的3.68×10-7S增加约21.43%.灌流平衡液中的NaCl以氯化胆碱置代或用C d2+阻断Ca2+通道后,OXT引起超极化反应的幅值无明显变化.当灌流平衡液中含10-2 mol/L四乙基碘化铵后,OXT引起的超极化反应幅值明显减小.     

缩宫素对大鼠背根神经节神经元胞体膜的作用

应用细胞内记录技术 ,观察了缩宫素 ( OXT,1 0 -8～ 1 0 -5mol/ L)对 2～ 3周龄大鼠背根神经节神经元的作用。在受检的 92个细胞中 ,有 71个神经元滴加缩宫素产生明显的超极化反应。在滴加 1 0 -5mol/ L缩宫素后 ,膜电导由平均的 3.68× 1 0 -7S增加约 2 1 .43%。灌流平衡液中的 Na Cl以氯化胆碱置代或用 Cd2 阻断Ca2 通道后 ,OXT引起超极化反应的幅值无明显变化。当灌流平衡液中含 1 0 -2 mol/ L四乙基碘化铵后 ,OXT引起的超极化反应幅值明显减小。     

Effect of vasopressin on the somatic membrane of spinal ganglion neurons

The application of vasopressin (VP) in the isolated perfused dorsal root ganglia of 22-36 days old rats was studied by means of intracellular technique. 86.76% of cells have responded to the VP application. Depolarization was observed in 67.8% responded cells, the mixed response--in 16.95% cells, hyperpolarization--in 15.25% cells. All responses were dose-dependent and reversible. Input resistance (Rm) of the cell membrane decreased during depolarization and increased during hyperpolarization. The VP-evoked depolarization was accompanied by an increase in the action potential (AP) duration and decrease in the AP amplitude and after-hyperpolarization. Neurons with slow conduction velocity, high Rm and prolonged AP (small cells) had the lowest threshold of the sensitivity to VP (1.10(-11) M) and prolonged high-amplitude responses. Cells with the rapid conduction velocity, low Rm and rapid AP (large cells) responded to 1.10(-8) M, but sometimes even 1.10(-6) M had no effect. Depolarization in these neurons had smaller duration and low amplitude: sometimes hyperpolarization was observed. These results confirm the possibility that VP has effect on small neurons predominantly.     

Propylparaben reduces the excitability of hippocampal neurons by blocking sodium channels

Propylparaben (PPB) is an antimicrobial preservative widely used in food, cosmetics, and pharmaceutics. Virtual screening methodologies predicted anticonvulsant activity of PPB that was confirmed in vivo. Thus, we explored the effects of PPB on the excitability of hippocampal neurons by using standard patch clamp techniques. Bath perfusion of PPB reduced the fast-inactivating sodium current (I_Na) amplitude, causing a hyperpolarizing shift in the inactivation curve of the I_Na, and markedly delayed the sodium channel recovery from the inactivation state. Also, PPB effectively suppressed the riluzole-sensitive, persistent sodium current (I_NaP). PPB perfusion also modified the action potential kinetics, and higher concentrations of PPB suppressed the spike activity. Nevertheless, the modulatory effects of PPB did not occur when PPB was internally applied by whole-cell dialysis. These results indicate that PPB reduces the excitability of CA1 pyramidal neurons by modulating voltage-dependent sodium channels. The mechanistic basis of this effect is a marked delay in the recovery from inactivation state of the voltage-sensitive sodium channels. Our results indicate that similar to local anesthetics and anticonvulsant drugs that act on sodium channels, PPB acts in a use-dependent manner.     

2 calcium currents in the somatic membrane of neurons of Helix pomatia

Two different calcium currents were revealed in the somatic membrane of Helix pomatia neurons. In addition to the main current described in literature, depolarizing the membrane from the holding potential level (-120 divided by -100 mV) an additional calcium current was observed. It was activated at depolarizations to -80 divided by -40 mV. Contrary to the main calcium current it did not deteriorate during intracellular perfusion by solutions containing fluoride. Time-dependence of this current could be described in the framework of the Hodgkin-Huxley model with time constants for activation and inactivation equal to tau m = 6-8 ms and tau h = 300-600 ms, respectively. The amplitude of this current increased with increase of extracellular Ca2+ concentration and decreased after addition of Co2+, Ni2+, Cd2+, nifedipine and verapamil. Dissociation constants of these substances with corresponding channels determined for the maximum of current-voltage relationship were 2 (Ca2+), 3 (Co2+), 0.06 (nifedipine) and 0.2 mmol/l (verapamil). Properties of the fluoride-insensitive calcium current and data obtained for other calcium channels are compared. Its possible functional role is also discussed.     

Two modes of activity of nicotinic acetylcholine receptor channels in sympathetic neurons

Spectral analysis of acetylcholine (ACh) noise was performed in voltage-clamped neurons of the isolated rabbit superior cervical ganglion at 34–37°C and at membrane potential −80 mV. Two modes of activity were found in the ionic channels of nicotinic ACh receptors, with mean channel life-times of τ_f = 1.1 ± 0.1 ms for fast-operating channels and τ_s = 5.6 ± 0.6 ms for slow-operating channels. Excitatory postsynaptic current (EPSC) decays exponentially with a time constant which is very close to τ_s, indicating that the slow-operating channel activity determines the duration of EPSC. The mean value of conductance of single nicotinic ACh-receptor channel is 36 ± 3 pS.     

Ionic basis of the resting membrane potential in cultured rat sympathetic neurons

The conductances which determine the resting membrane potential of rat superior cervical ganglia (SCG) neurons were investigated using perforated voltage- and current-clamp whole-cell techniques. The resting potential of SCG cells varied from -47 to -80 mV (-58.3 +/- 0.8 mV, n = 55). Blockade of M and h currents induced a depolarisation (7.4 +/- 0.7 mV, n = 22) and a hyperpolarisation (7.2 +/- 0.7 mV, n = 20) respectively; however, no correlation between the amplitude of these currents and the resting potential was found. The inhibition of the Na/K pump also induced membrane depolarisation (3.2 +/- 0.2 mV, n = 8). Inhibition of voltage-gated currents unmasked a voltage-independent resting conductance reversing at -50 mV. The reversal potential of the voltage-independent conductance, which included the electrogenic contribution of the Na/K pump, was strongly correlated with the resting potential (R = 0.87, p < 0.0001, n = 30). Ionic substitution experiments confirmed the existence of a voltage-independent conductance (leakage) with four components, a main potassium conductance, two minor sodium and chloride conductances and a small contribution of the Na/K pump. It is concluded that the resting potential of SCG cells strongly depends on the reversal potential of the voltage-independent conductance, with voltage-activated M and h currents playing a prominent stabilising role.     

Roles of somatic A-type K+ channels in the synaptic plasticity of hippocampal neurons

In the mammalian brain, information encoding and storage have been explained by revealing the cellular and molecular mechanisms of synaptic plasticity at various levels in the central nervous system, including the hippocampus and the cerebral cortices. The modulatory mechanisms of synaptic excitability that are correlated with neuronal tasks are fundamental factors for synaptic plasticity, and they are dependent on intracellular Ca²⁺-mediated signaling. In the present review, the A-type K⁺ (I _A) channel, one of the voltage-dependent cation channels, is considered as a key player in the modulation of Ca²⁺ influx through synaptic NMDA receptors and their correlated signaling pathways. The cellular functions of I _A channels indicate that they possibly play as integral parts of synaptic and somatic complexes, completing the initiation and stabilization of memory.     

Low-threshold area of action potential generation in the somatic membrane of mollusk neurons

The excitability of somatic membrane of Helix lucorum neurons was studied by means of extracellular electric current of various direction (frequency 0.1 Hz). Inhomogenous excitability of somatic membrane was shown and the existence of the local low-threshold area of spike generation in the soma was demonstrated.     

Single channels of low and high-threshold calcium channels of the membrane of sensory neurons in the mouse

Single calcium channels of cultured dorsal root ganglion cells from mouse embryos were studied using patch clamp method in its cell-attached configuration. Two types of activity of unitary calcium channels were found. The first one which arose at membrane potentials near--50-40 mV was characterized by unitary current amplitude of 0.37 +/- 0.04 pA with 40 mmol/l Ca2+ in the pipette solution, mean open time of 0.6 ms and intraburst mean shut time of 1.2 ms. It displayed voltage- and time-dependent inactivation. The corresponding values for the second one which required much more positive depolarization to be activated (approximately 0 mV) and did not express noticeable inactivation were: 0.53 +/- 0.04 pA, 0.8 ms and 0.8 ms. It is concluded that the recorded types of unitary activity are associated respectively with low- and high-threshold calcium currents which have been found earlier while studying whole cell currents.     

The potential dependence of acetylcholine-activated membrane conductance in sympathetic ganglion neurons

Membrane conductance activated by acetylcholine (ACh-conductance) was studied in rat isolated superior cervical ganglion neurons by means of the patch-clamp method in the whole-cell recording mode. It was found that ACh-conductance was increased or decreased with membrane hyper- or depolarization, respectively. The decrease in ACh-conductance was not associated with the reversal of ACh-current or with the presence of Ca2+ ions in external solution. The time constant of voltage-jump relaxation of ACh-current revealed e-fold increase with membrane hyperpolarization by 70 mV, which corresponded to the voltage dependence of ACh-conductance. Basing upon these results it was concluded that the voltage dependence of ACh-conductance is mostly determined by the voltage dependence of nicotinic receptor channel gating kinetics.