Imaging stochastic spatial variability of active channel clusters during excitation of single neurons

An old hypothesis and new tools: Alfred Fessard's approach to the problem of consciousness

In 1954, a symposium was held in Canada on "Brain Mechanisms and Consciousness." It was a time for the promotion of international and interdisciplinary scientific cooperation, of new technological expectation, and of speculating about complex human behavior. Alfred Fessard's lecture on "Mechanisms of Nervous Integration and Conscious Experience" was one of the outstanding presentations, rich in critical analysis of the then available experimental data and in working hypothesis proposals. Reading the concept expressed by Fessard, it was found that several of his ideas had anticipated data obtained in modern research with new technologies.

Tonic excitation or inhibition is set by GABA(A) conductance in hippocampal interneurons

Inhibition is a physiological process that decreases the probability of a neuron generating an action potential. The two main mechanisms that have been proposed for inhibition are hyperpolarization and shunting. Shunting results from increased membrane conductance, and it reduces the neuron-firing probability. Here we show that ambient GABA, the main inhibitory neurotransmitter in the brain, can excite adult hippocampal interneurons. In these cells, the GABA(A) current reversal potential is depolarizing, making baseline tonic GABA(A) conductance excitatory. Increasing the tonic conductance enhances shunting-mediated inhibition, which eventually overpowers the excitation. Such a biphasic change in interneuron firing leads to corresponding changes in the GABA(A)-mediated synaptic signalling. The described phenomenon suggests that the excitatory or inhibitory actions of the current are set not only by the reversal potential, but also by the conductance.

Probability distributions of Markovian sodium channel states during propagating axonal impulses with or without recovery supernormality

This study addressed a macroscopic neurophysiological phenomenon - supernormality during the recovery phase of propagating axonal impulses - in explicit chemical terms. Excitation was reconstructed numerically using the kinetic scheme of multiple-state probabilistic transitions within a population of voltage-dependent sodium channels (NaCh) derived by Vandenberg and Bezanilla ("PC" scheme). Each NaCh transition was characterized as a reversible Markov process with voltage-dependent rate constants associated with each respective directional transition. While recovery reconstructed with the Hodgkin-Huxley formalism included a supernormal period, the PC scheme did not. The present analysis showed that the occurrence and degree of supernormality with the PC scheme was determined by the relative speed of the transitions within the closed loop of the kinetic scheme; supernormality was promoted by speeding these kinetics. The analysis also showed that concurrent with supernormality, the faster loop kinetics caused (1) an elevation in the C(1) --> C(2) transitions, and (2) a reduction in the I(4) --> I(5) transitions. Thus, macroscopic functionality in information processing could be expressed in terms of probabilistic interstate transitions among a population of NaCh molecules.

Parameter sensitivity of distributed transfer properties of neuronal dendrites: a passive cable approximation

Geometry and membrane properties of the dendrites crucially determine input-output relations in neurons. Unlike geometry often available in detail from computer reconstruction, the membrane resistivity is fragmentarily known if at all. Moreover, it varies during ongoing activity. In this study we address the question: what is the impact of the variation in membrane resistivity on the transfer properties of dendrites? Following a standard approach of the control system theory, we derive and explore the sensitivity functions complementary to the transfer functions of the passive dendrites with arbitrary geometrical parameters (length and diameter) and boundary conditions. We use the location-dependent somatopetal current transfer ratio (the reciprocal of the somatofugal voltage) as the transfer function, and its membrane resistivity derivatives, as the sensitivity functions. In the dendrites, at every path distance from the origin, the sensitivity function in a common form relates the transfer function, membrane resistivity, characteristic input conductance of semi-infinite cable and directional somatofugal input conductances at the given internal site and origin, and the length. Plotted in membrane resistivity versus path distance coordinates, the sensitivity functions display common features: along any coordinate there are low and high ranges, in which the sensitivity, respectively, increases and decreases. The ranges and corresponding rates depend on morphology and boundary conditions in a characteristic manner. These features predict existence of the geometry-dependent range of membrane resistivity (the earlier unattended mid-conductance state), such that the dendrites with a given metrical asymmetry are most distinguished in their transfer properties and electrical states if membrane resistivity is within the range and are not otherwise.

Differential modulation by monoamine membrane receptor agonists of reticulospinal input to lamina VIII feline spinal commissural interneurons

Noradrenaline and serotonin have previously been demonstrated to facilitate the transmission between descending reticulospinal tracts fibres and commissural interneurons coordinating left-right hindlimb muscle activity. The aim of the present study was to investigate the contribution of subclasses of monoaminergic membrane receptors to this facilitation. The neurons were located in Rexed lamina VIII in midlumbar segments and identified by their projections to the contralateral gastrocnemius-soleus motor nuclei and by lack of projections rostral to the lumbosacral enlargement. The effects of ionophoretically applied membrane receptor agonists [phenylephrine (noradrenergic alpha(1)), clonidine (noradrenergic alpha(2)), 8-OH-DPAT (5-HT(1A), 5-HT(7)), 2-me-5-HT (5-HT(3)), 5-me-5-HT (5-HT(2)) and alpha-me-5-HT (5-HT(2))] were examined on extracellularly recorded spikes evoked monosynaptically by electric stimulation of descending reticulospinal fibres in the medial longitudinal fascicle. Application of alpha(1) and 5-HT(2) agonists resulted in a facilitation of responses in all investigated neurons while application of alpha(2), 5-HT(1A/7) and 5-HT(3) agonists resulted in a depression. These opposite modulatory effects of different agonists suggest that the facilitatory actions of noradrenaline and serotonin on responses of commissural interneurons reported previously following ionophoretic application are the net outcome of the activation of different subclasses of monoaminergic membrane receptors. As these receptors may be distributed predominantly, or even selectively, at either pre- or postsynaptic sites their differential modulatory actions could be compatible with a presynaptically induced depression and a postsynaptically evoked enhancement of synaptic transmission between reticulospinal neurons and commissural interneurons.

Neural membrane microdomains as computational systems: Toward molecular modeling in the study of neural disease

Several studies indicate that the lipid biological membrane contains discrete regions known as rafts or microdomains. These structures range in size from approximately 50 to 70nm to nearly a mum and play important roles in cell signaling. In the neuron, computational models suggest that transiently polarized microdomain ethenes may regulate ion-channel dynamics and control impulse propagation. Thus the microdomain is nominated as the fundamental unit of nervous system signaling. Based on this model, the article proposes a first-approximation design for a supported-membrane device which would mimic microdomain properties. The basic architecture would consist of an electrically addressable biotemplated nanowire crossing an artificial membrane corralled in a vertical carbon nanofiber barrier. Advantages and disadvantages of model components are discussed at length. It is proposed that artificial devices of this type would be medically useful in simulating membrane states correlated with neural disease. This possibility is examined with reference to the A-current potassium channel, implicated in epilepsy.

Spatial reconfiguration of charge transfer effectiveness in active bistable dendritic arborizations

The aim of this work was to explore the electrical spatial profile of the dendritic arborization during membrane potential oscillations of a bistable motoneuron. Computational simulations provided the spatial counterparts of the temporal dynamics of bistability and allowed simultaneous depiction the electrical states of any sites in the arborization. We assumed that the dendritic membrane had homogeneously distributed specific electrical properties and was equipped with a cocktail of passive extrasynaptic and NMDA synaptic conductances. The electrical conditions for evoking bistability in a single isopotential compartment and in a whole dendritic arborization were computed and showed differences, revealing a crucial effect of dendritic geometry. Snapshots of the whole arborization during bistability revealed the spatial distribution of the density of the transmembrane current generated at the synapses and the effectiveness of the current transfer from any dendritic site to the soma. These functional maps changed dynamically according to the phase of the oscillatory cycle. In the low depolarization state, the current density was low in the proximal dendrites and higher in the distal parts of the arborization while the transfer effectiveness varied in a narrow range with small differences between proximal and distal dendritic segments. When the neuron switched to high depolarization state, the current density was high in the proximal dendrites and low in the distal branches while a large domain of the dendritic field became electrically disconnected beyond 200 micro m from the soma with a null transfer efficiency. These spatial reconfigurations affected dynamically the size and shape of the functional dendritic field and were strongly geometry-dependent.

Dendritic spatial flicker of local membrane potential due to channel noise and probabilistic firing of hippocampal neurons in culture

Whole-cell recordings and imaging of dissociated hippocampal neurons stained with voltage sensitive dye provide a new microscopic picture of neuronal excitation. This is the first attempt to combine imaging of active channel clusters on the geometry of live neurons and a theoretical approach. During single somatic action potentials and the back-invasion into the neurites, local mean potentials are generated at sites of active channel clusters which are unevenly distributed in the neuronal membrane. Similar mean membrane potentials are observed in the neurites and at the soma. Identical action potentials produce different spatial patterns of mean membrane potentials from trial to trial. This spatial variability is explained by the stochastic behavior of the channels in the clusters. When hippocampal neurons are excited by synaptic inputs, their evoked responses are probabilistic and generate variable spatial patterns of mean membrane potential trial after trial. Our stochastic model reproduces this random behavior by assuming that the voltage fluctuations generated by channel noise are added to the synaptic potentials reaching the soma. We demonstrate that the probability of action potential initiation depends on the strength of the synaptic input, the diameter of the dendrites and the relative positions of the channel clusters, of the synapse and of the soma.