A model of signal processing at a mammalian sympathetic neurone

Changes in cationic selectivity of the nicotinic channel at the rat ganglionic synapse: a role for chloride ions?

The permeability of the nicotinic channel (nAChR) at the ganglionic synapse has been examined, in the intact rat superior cervical ganglion in vitro, by fitting the Goldman current equation to the synaptic current (EPSC) I–V relationship. Subsynaptic nAChRs, activated by neurally-released acetylcholine (ACh), were thus analyzed in an intact environment as natively expressed by the mature sympathetic neuron. Postsynaptic neuron hyperpolarization (from −40 to −90 mV) resulted in a change of the synaptic potassium/sodium permeability ratio (P_K/P_Na) from 1.40 to 0.92, corresponding to a reversible shift of the apparent acetylcholine equilibrium potential, E_ACh, by about +10 mV. The effect was accompanied by a decrease of the peak synaptic conductance (g_syn) and of the EPSC decay time constant. Reduction of [Cl⁻]_o to 18 mM resulted in a change of P_K/P_Na from 1.57 (control) to 2.26, associated with a reversible shift of E_ACh by about −10 mV. Application of 200 nM αBgTx evoked P_K/P_Na and g_syn modifications similar to those observed in reduced [Cl⁻]_o. The two treatments were overlapping and complementary, as if the same site/mechanism were involved. The difference current before and after chloride reduction or toxin application exhibited a strongly positive equilibrium potential, which could not be explained by the block of a calcium component of the EPSC. Observations under current-clamp conditions suggest that the driving force modification of the EPSC due to P_K/P_Na changes represent an additional powerful integrative mechanism of neuron behavior. A possible role for chloride ions is suggested: the nAChR selectivity was actually reduced by increased chloride gradient (membrane hyperpolarization), while it was increased, moving towards a channel preferentially permeable for potassium, when the chloride gradient was reduced.

A software for simulating steady-state properties of passive dendrites based on the cable theory

In this study, a computer software, CableTeo, is introduced for simulating the steady-state electrical properties of passive dendrite based on the cable theory. The cable theory for dendritic neurons addresses to current-voltage relations in a continuous passive dendritic tree. It is briefly summarized that the cable theory related to passive cables and dendrites, which is a useful approximation and an important reference for excitable cases. The proposed software can be used to construct user-defined dendritic tree model. The user can define the model in detail, display the constructed dendritic tree, and examine the basic electrical properties of the dendritic tree using transfer impedance approach. The software addresses to ones who want to run simple simulations of the cable theory without need to any programming language skills or expensive software. It can also be used for educational purposes.

Biophysical properties of the silent and activated rat sympathetic neuron following denervation

A biophysical description of the denervated rat sympathetic neuron is reported, obtained by the two-electrode voltage-clamp technique in mature intact superior cervical ganglia in vitro. At membrane potential values negative to -50 mV, the normal, quiescent neuron displays voltage-dependent K and Cl conductances; following direct or synaptic stimulation (15Hz for 10 s), the neuron moves to a new resting state characterized by increased amplitude and voltage dependence of Cl conductance. Denervation produces two main effects: 1) resting Cl conductance gradually increases while its voltage-dependence decreases; by 30 days a high-conductance resting state prevails, almost independent of membrane potential in the -50/-110 mV range; 2) the increase in amplitude and voltage-dependence of Cl conductance, produced by direct stimulation in control neurons, is less marked in denervated neurons, and is observed over an increasingly small range of membrane potentials. Thirty days after denervation, the prevailing high-conductance resting state appears virtually insensitive to changes in membrane potential and stimulation. Voltage-dependent potassium currents involved in spike electrogenesis (the delayed compound potassium current and the fast transient potassium current) exhibit an early drastic decrease in peak amplitude in the denervated neuron; the effect is largely reversed after 6 days. Remarkable changes in fast transient potassium current kinetics occur following denervation: the steady-state inactivation curve shifts by up to +15 mV toward positive potential and voltage sensitivity of inactivation removal becomes more steep. A comprehensive mathematical model of the denervated neuron is presented that fits the neuron behavior under current-clamp conditions. It confirms that neuronal excitability is tuned by the conductances (mostly chloride conductance) that control the resting membrane potential level, and by fast transient potassium current. Impairment of the latter reduces both inward threshold charge for firing and spike repolarization rate, and fast transient potassium current failure cancels the voltage dependence of both processes.

A computer software for simulating single-compartmental model of neurons

In this paper, a new computer software package, Yalzer, is introduced for simulating single-compartmental model of neurons. Passive or excitable membranes with voltage-gated ion channels can be modeled, and current clamp and voltage clamp experiments can be simulated. In the Yalzer, first-order differential equations used to define the dynamics of the gate variables and the membrane potential are solved by two separate integration methods with variable time steps: forward Euler and exponential Euler methods. Outputs of the simulation are shown on a spreadsheet template for allowing flexible data manipulation and can be graphically displayed. The user can define the model in detail, and examine the excitability of the model and the dynamics of voltage-gated ion channels. The software package addresses to ones who want to run simple simulations of neurons without need to any programming language skills or expensive software. It can also be used for educational purposes.

Voltage- and activity-dependent chloride conductance controls the resting status of the intact rat sympathetic neuron

Remarkable activity dependence was uncovered in the chloride conductance that operates in the subthreshold region of membrane potential, by using the two-microelectrode voltage-clamp technique in the mature and intact rat sympathetic neuron. Both direct and synaptic neuron tetanization (15 Hz, 10-s duration to saturate the response) resulted in a long-lasting (not less than 15 min) increase of cell input conductance (+70-150% 10 min after tetanus), accompanied by the onset of an inward current with the same time course. Both processes developed with similar properties in the postganglionic neuron when presynaptic stimulation was performed under current- or voltage-clamp conditions and were unaffected by external calcium on direct stimulation. The posttetanic effects were sustained by gCl increase because both conductance and current modifications were blocked by 0.5 mM Anthracene-9-carboxylic acid (a chloride channel blocker) but were unaffected by TEACl or cesium chloride treatments. The chloride channel properties were modified by stimulation: their voltage sensitivity and rate of closure in response to hyperpolarization strongly increased. The voltage dependence of the three major conductances governing the cell subthreshold status (gCl, gK, and gL) was evaluated over the -40/-110 mV membrane potential range in unstimulated neurons and compared with previous results in stimulated neurons. A drastic difference between the voltage-conductance profiles was observed, exclusively sustained by gCl increase. The chloride channel thus hosts an intrinsic mechanism, a memory of previous neuron activity, which makes the chloride current a likely candidate for natural controller of the balance between opposite resting currents and thus of membrane potential level.