Ephaptic interactions within a chemical synapse: hemichannel-mediated ephaptic inhibition in the retina

The two best-known types of cell-cell communication are chemical synapses and electrical synapses, which are formed by gap junctions. A third, less well known, form of communication is ephaptic transmission, in which electric fields generated by a specific neuron alter the excitability of neighboring neurons as a result of their anatomical and electrical proximity. Ephaptic communication can be present in a variety of forms, each with their specific features and functional implications. One of these is ephaptic modulation within a chemical synapse. This type of communication has recently been proposed for the cone-horizontal cell synapse in the vertebrate retina. Evidence indicates that the extracellular potential in the synaptic terminal of photoreceptors is modulated by current flowing through connexin hemichannels at the tips of the horizontal cell dendrites, mediating negative feedback from horizontal cells to cones. This example can be added to the growing list of cases of ephaptic communication in the central nervous system.     

Neurophylogeny: Architecture of the nervous system and a fresh view on arthropod phyologeny

The phylogenetic relationships within the Arthropoda have beencontroversial for more than a century. Today, comparative studieson the structure and development of the nervous system contributeimportant arguments to this discussion, so that the term "neurophylogeny"was coined for this discipline. The large number of recent studieson the nervous system in various nonmodel arthropods indicatesthat we are far advanced in the process of analyzing the cellulararchitecture of the arthropod nervous system in a depth thatwill ultimately provide characters at a level of resolutionequal or even superior to that of characters traditionally usedin morphological phylogenetic studies. This article sets outto summarize the current state of the discussion on arthropodphylogeny and briefly evaluates the morphological charactersthat have been used as arguments in favor of the traditionalTracheata hypothesis. Then, a thorough overview is given ofcharacters derived from structure and development of the arthropodbrain and the ventral nerve cord from the cellular level tothe level of larger neuropil systems. These characters supportthe new Tetraconata hypothesis suggested by Dohle and provideevidence for a clade that unites malacostracan and remipedecrustaceans with the Hexapoda.     

The potential coding utility of intercell cross-correlations in the retina

The action potentials (impulses) produced by pairs of neighboring retinal ganglion cells often show a tendency either to fire in close temporal synchrony or to avoid temporal synchrony. This cross-correlation (a rate of coincidences that differs from that expected by chance) has been exploited as a window into retinal processing, but its possible functional significance has proven elusive. Previous work has failed to show that the coincidences serve as a direct code for visual stimuli. In this analysis it is shown that the coincidences serve neither as a key for reducing variability nor as a key for improving the coding by the individual cells. The residual impulse trains (trains with coincidences deleted) are more variable than the raw impulse trains and provide an inferior coding to that of the raw impulse trains. There is negative correlation between the firing rate of the residual impulse trains and that of the coincidence impulse trains, which is consistent with the lower variance of the raw impulse trains. There is no consistent cross-correlation between the rates of residual impulse trains of cells in pairs showing cross-correlation; however, it is found that this observation does not discriminate among models for generating coincidences.     

Colour coding in the primate retina: diverse cell types and cone-specific circuitry

How is the trichromatic cone mosaic of Old World primates sampled by retinal circuits to create wavelength opponency? Red-green (L versus M cone) opponency appears to be mediated largely by the segregation of L versus M cone signals to the centre versus the surround of the midget ganglion cell receptive field, implying a complex cone type-specific wiring, the basis of which remains mysterious. Blue-yellow (S versus L+M cone) opponency is mediated by a growing family of low-density ganglion types that receive either excitatory or inhibitory input from S cones. Thus, the retinal circuits that underlie colour signalling in primates may be both more complex and more diverse then previously appreciated.     

Olfactory coding: inhibition reshapes odor responses

Olfactory information is dramatically restructured as it makes its way through the brain. Recent work using a remarkable experimental preparation has revealed how this transformation is achieved.     

A unified view of theta-phase coding in the entorhinal-hippocampal system

The discovery of theta-rhythm-dependent firing of rodent hippocampal neurons highlighted the functional significance of temporal encoding in hippocampal memory. However, earlier theoretical studies on this topic seem divergent and experimental implications are invariably complicated. To obtain a unified understanding of neural dynamics in the hippocampal memory, we here review recent developments in computational models and experimental discoveries on the 'theta-phase precession' of hippocampal place cells and entorhinal grid cells. We identify a theoretical hypothesis that is well supported by experimental facts; this model reveals a significant contribution of theta-phase coding to the on-line real-time operation of episodic events, through highly parallel representation of spatiotemporal information.     

Synaptic limitations to contrast coding in the retina of the blowfly Calliphora

We investigate the effects of synaptic transmission on early visual processing by examining the passage of signals from photoreceptors to second order neurons (LMCS). We concentrate on the roles played by three properties of synaptic transmission: (1) the shape of the characteristic curve, relating pre- and postsynaptic signal amplitudes, (2) the dynamics of synaptic transmission and (3) the noise introduced during transmission. The characteristic curve is sigmoidal and follows a simple model of synaptic transmission (Appendix) in which transmitter release rises exponentially with presynaptic potential. According to this model a presynaptic depolarization of 1.50-1.86 mV produces an e-fold increase in postsynaptic conductance. The characteristic curve generates a sigmoidal relation between postsynaptic (LMC) response amplitude and stimulus contrast. The shape and slope of the characteristic curve is unaffected by the state of light adaptation. Retinal antagonism adjusts the characteristic curve to keep it centred on the mean level of receptor response generated by the background. Thus the photoreceptor synapses operate in the mid-region of the curve, where the slope or gain is highest and equals approximately 6. The dynamics of transmission of a signal from photoreceptor to second-order neuron approximates to the sum of two processes with exponential time courses. A momentary receptor depolarization generates a postsynaptic hyperpolarization of time constant 0.5-1.0 ms, followed by a slower and weaker depolarization. Light adaptation increases the relative amplitude of the depolarizing process and reduces its time constant from 80 ms to 1.5 ms. The hyperpolarizing process is too rapid to bandlimit receptor signals. The noise introduced during the passage of the signal from receptor to second-order neuron is measured by comparing signal:noise ratios and noise power spectra in the two cell types. Under daylight conditions from 50 to 70% of the total noise power is generated by events associated with the transmission of photoreceptor signals and the generation of LMC responses. According to the exponential model of transmitter release, the effects of synaptic noise are minimized when synaptic gain is maximized. Moreover, both retinal antagonism and the sigmoidal shape of the characteristic curve promote synaptic gain. We conclude that retinal antagonism and nonlinear synaptic amplification act in concert to protect receptor signals from contamination by synaptic noise. This action may explain the widespread occurrence of these processes in early visual processing.     

Rates and rhythms: a synergistic view of frequency and temporal coding in neuronal networks

In the CNS, activity of individual neurons has a small but quantifiable relationship to sensory representations and motor outputs. Coactivation of a few 10s to 100s of neurons can code sensory inputs and behavioral task performance within psychophysical limits. However, in a sea of sensory inputs and demand for complex motor outputs how is the activity of such small subpopulations of neurons organized? Two theories dominate in this respect: increases in spike rate (rate coding) and sharpening of the coincidence of spiking in active neurons (temporal coding). Both have computational advantages and are far from mutually exclusive. Here, we review evidence for a bias in neuronal circuits toward temporal coding and the coexistence of rate and temporal coding during population rhythm generation. The coincident expression of multiple types of gamma rhythm in sensory cortex suggests a mechanistic substrate for combining rate and temporal codes?on the basis of stimulus strength.     

A fresh view of the cell biology of copper in enterobacteria

Copper ions are essential but also very toxic. Copper resistance in bacteria is based on export of the toxic ion, oxidation from Cu(I) to Cu(II), and sequestration by copper‐binding metal chaperones, which deliver copper ions to efflux systems or metal‐binding sites of copper‐requiring proteins. In their publication in this issue, Osman et al. ( 2013 ) demonstrate how tightly copper resistance, homeostasis and delivery pathways are interwoven in Salmonella enterica sv. Typhimurium. Copper is transported from the cytoplasm by the two P‐type ATPases CopA and GolT to the periplasm and transferred to SodCII by CueP, a periplasmic copper chaperone. When copper levels are higher, SodCII is also able to bind copper without the help of CueP. This scheme raises the question as to why copper ions present in the growth medium have to make the detour through the cytoplasm. The data presented in the publication by Osman et al. ( 2013 ) change our view of the cell biology of copper in enterobacteria.     

Two types of lateral inhibition in the frog retina

Two independent types of lateral inhibition were distinguished in experiments on the frog eye in which the effects of pharmacological agents on the electroretinogram were studied: proximal or picrotoxin-sensitive, and distal or strychnine-sensitive. Distal lateral inhibition (at distances up to 1.5 mm) is the familiar type already well investigated and based on a spike mechanism of transmission of the inhibitory signal. The proximal (up to 400 µ) picrotoxin-sensitive inhibition has a different mechanism of transmission, not dependent on spikes. The localization of the two types of lateral inhibition in the synaptic layers of the frog retina is discussed.Institute for Problems in Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 13, No. 5, pp. 549–555, September–October, 1981.     

Caspase activation, inhibition, and reactivation: a mechanistic view

Caspases, a unique family of cysteine proteases, execute programmed cell death (apoptosis). Caspases exist as inactive zymogens in cells and undergo a cascade of catalytic activation at the onset of apoptosis. The activated caspases are subject to inhibition by the inhibitor-of-apoptosis (IAP) family of proteins. This inhibition can be effectively removed by diverse proteins that share an IAP-binding tetrapeptide motif. Recent structural and biochemical studies have revealed the underlying molecular mechanisms for these processes in mammals and in Drosophila. This paper reviews these latest advances.     

Trichromacy, opponent colours coding and optimum colour information transmission in the retina

This paper presents a systematic analysis of the role of opponent type processing in colour vision and the relation between opponent type colour transformations and the initial three colour mechanisms. It is shown that efficient information transmission is achieved by a transformation of the initial three colour mechanisms into an achromatic and two opponent chromatic channels. The derivation of the transformation is dependent solely on criteria from information theory. Thus it provides a logical rationale reconciling opponent type processing as an optimal necessary step after the initial three colour mechanisms, unifying respectively the Hering and Young-Helmholtz approaches to colour vision. The effects of chromatic adaptation on the spectral response of the achromatic and two chromatic channels are discussed from the point of view of information theory. It is argued that adaptation serves as a dynamic readjustment of these responses, necessary to meet criteria of efficient colour information transmission. The results are confronted with empirical observations to test the principles of the theory and the relation to other theories is discussed. Within the same framework the issue of trichromacy is discussed. It is argued that a broad class of typical colour spectra can effectively be represented by three significant degrees of freedom that make up a trichromatic system.     

Crossover inhibition in the retina: circuitry that compensates for nonlinear rectifying synaptic transmission

In the mammalian retina, complementary ON and OFF visual streams are formed at the bipolar cell dendrites, then carried to amacrine and ganglion cells via nonlinear excitatory synapses from bipolar cells. Bipolar, amacrine and ganglion cells also receive a nonlinear inhibitory input from amacrine cells. The most common form of such inhibition crosses over from the opposite visual stream: Amacrine cells carry ON inhibition to the OFF cells and carry OFF inhibition to the ON cells (”crossover inhibition”). Although these synapses are predominantly nonlinear, linear signal processing is required for computing many properties of the visual world such as average intensity across a receptive field. Linear signaling is also necessary for maintaining the distinction between brightness and contrast. It has long been known that a subset of retinal outputs provide exactly this sort of linear representation of the world; we show here that rectifying (nonlinear) synaptic currents, when combined thorough crossover inhibition can generate this linear signaling. Using simple mathematical models we show that for a large set of cases, repeated rounds of synaptic rectification without crossover inhibition can destroy information carried by those synapses. A similar circuit motif is employed in the electronics industry to compensate for transistor nonlinearities in analog circuits.