Photopic vision in eels: evidences of color discrimination.

Several classes of second-order retinal neurons have been studied electrophysiologically in European eel (Anguilla anguilla) from two different localities, Lake Seliger in Russia and the coastal waters of the Adriatic Sea in Montenegro. The majority of L-horizontal cells (68 explored) had both rod and cone inputs, an uncommon phenomenon among teleosts. Pronounced color-opponent properties, often taken as pointing to the capacity of color vision, were identified in one amacrine cell, apparently of the "blue/yellow" (or "blue/green") type.

Differences in amplitude-voltage relations between minimal and composite mossy fibre responses of rat CA3 hippocampal neurons support the existence of intrasynaptic ephaptic feedback in large synapses.

Computer simulations and electrophysiological experiments have been performed to test the hypothesis on the existence of an ephaptic interaction in purely chemical synapses. According to this hypothesis, the excitatory postsynaptic current would depolarize the presynaptic release site and further increase transmitter release, thus creating an intrasynaptic positive feedback. For synapses with the ephaptic feedback, computer simulations predicted non-linear amplitude-voltage relations and voltage dependence of paired-pulse facilitation.

[Electrical feedback in the chemical synapses].

Electrical feedback in chemical synapses and the efficacy of synaptic transmission grow with the increase in the gap resistance, so they should be higher in invaginated synapses than in the flat ones. So the plastic changes in the invagination depth may provide a morphological basis for long-term changes in synaptic efficacy: long-term potentiation (LTP) in brain and retinal synapses. In retinal photoreceptor triad synapses, the electrical feedback can provide an "operational" (instantaneous) control of synaptic transmission.

Photoreceptor coupling and boundary detection.

Electrical coupling between photoreceptors results in the extensive spreading of output potentials along the syncytium of photoreceptor terminals. This smoothing of output potentials seems to make spatial resolution worse. However, the photoreceptor noise that is considered to be non-correlated both in space and time is smoothed to the greater extent than the correlated potential difference across the boundaries between areas of different brightness.

Horizontal cell dynamics: what are the main factors?

The factors potentially determining the dynamics of horizontal cell (HC) responses are: (1) the rate of transmitter release (including its transient component) and removal; (2) the voltage non-linearity of HC non-synaptic membrane combined with its capacitance; and (3) the dynamics of feedback from HCs to photoreceptors. Using, in consecutive order, the models of an isolated HC, a HC with one or two synaptic inputs and a HC of chromatic type, we have analysed the relative importance of three factors in shaping HC responses to the light and electrical current. The most prominent effect on the shape of HC ON responses derives from the voltage-dependency of the non-synaptic membrane.

Low-calcium-induced enhancement of chemical synaptic transmission from photoreceptors to horizontal cells in the vertebrate retina.

According to the classical calcium hypothesis of synaptic transmission, the release of neurotransmitter from presynaptic terminals occurs through an exocytotic process triggered by depolarization-induced presynaptic calcium influx. However, evidence has been accumulating in the last two decades indicating that, in many preparations, synaptic transmitter release can persist or even increase when calcium is omitted from the perfusing saline, leading to the notion of a "calcium-independent release" mechanism. Our study shows that the enhancement of synaptic transmission between photoreceptors and horizontal cells of the vertebrate retina induced by low-calcium media is caused by an increase of calcium influx into presynaptic terminals.

Neurophysiological analysis of long-term potentiation in mammalian brain.

Long-term potentiation (LTP) is a persistent increase in postsynaptic response following a high-frequency presynaptic activation. Characteristic LTP features, including input specificity and associativity, make it a popular model to study memory mechanisms. Mechanisms of LTP induction and maintenance are briefly reviewed.

Spectral characteristics of photoreceptors and horizontal cells in the retina of the Siberian sturgeon Acipenser baeri Brandt.

In the retina of Siberian sturgeon, three spectral classes of photoreceptors were identified by microspectrophotometry. These were rods, oil drop-containing and oil drop-free cones possessing P549, and oil drop-containing cones possessing P613 and P465. With intracellular recordings, rod-driven, cone-driven, and mixed horizontal cells of luminosity type were found, as well as color-opponent horizontal cells of, at least, 6 kinds.

[Voltage dependence of the cobalt-induced blockage of synaptic transmission between the photoreceptors and horizontal cells of the retina].

Synaptic transmission between photoreceptors and horizontal cells (HC) in the turtle retina blocked by Co2+ ions can be restored by constant radial current passed through the retina and depolarizing presynaptic receptor terminals. This result is not related with the action of current on horizontal cells themselves because their depolarization through the intracellular microelectrode did not restore the response to light. The restoring effect of presynaptic depolarization consists of two components: the opening of additional Ca-channels not blocked by Co2+ in the presynaptic membrane, and the voltage-dependence of Co2+ blockade.

Electrical feedback mechanism in the processing of signals in the outer plexiform layer of the retina.

In any chemical synapse there is an electrical feedback initiated by the electrical current generated by the postsynaptic neurone. We argue that this feedback can be rather effective in the case of invaginating synapses in the retina. A model of a dyad synapse (cone-horizontal cell-bipolar cell) is developed, in which the transfer function between the cone and bipolar cell is modulated by the synaptic current of the horizontal cell.

[Effect of changes in blood glucose concentration on the standing potential of the rabbit eye].

The paper deals with the study of the influence of changes of glucose blood concentration in rabbits on the standing potential of the eye, which reflects the functioning of the retinal pigment epithelium and photoreceptors. Recordings of the electrooculogram (EOG), determination of glucose blood level and ophthalmoscopic and histological examinations of the eye were performed in 48 rabbits. Short-term increase of glucose concentration was accompanied by the increase in the standing potential and Arden EOG ratio, with hypoglycemia evoking the opposite effect.

[Local nonuniformities in the syncytium of the horizontal cells of the retina in the turtle].

Electrical coupling between horizontal cells of the turtle retina was investigated by means of two microelectrodes (current and recording ones) penetrating neighbouring cells at a fixed distance from each other. The morphological coupling was revealed by means of fluorescent dye Lucifer Yellow. The electrical coupling was confirmed between elements of similar type (L1--axonal terminals, or L2--cell bodies, or R/G type cells) and no coupling was found between elements of different types, though L1 and L2 are directly connected through thin axons.

Spread of potentials along the network of horizontal cells in the retina of the turtle.

The spread of potentials along electrically-coupled horizontal cell networks in the turtle retina was studied at different levels of illumination. The mean values of the stationary space constant, lambda st, measured by the steady-state responses to a bar of light moved along the retina, is about 0.13 mm for the LII horizontal cell response and 0.

[Electrical response of frog lingual epithelium to stimulation of the gustatory nerve].

Electrical stimulation of the gustatory nerve evoked a positive electrical response of the outer surface of the tongue (relative to its inner layers). The amplitude and the rate of rise of the response grew and the latency fell with increased frequency of stimulation. The response originated in the superficial epithelial layer (30-60 micrometers) and was produced by a decrease of "resting potential" (negative-outside) of this layer.

[Intracellular recording of potentials of cells of isolated cerebellum in the frog].

Intracellular potentials of granular, Purkinje and glial cells were recorded in an isolated frog cerebellum in response to electrical stimulation of cerebellar peduncle. The stimuli inhibited strongly the activity of most granular cells. "Slow" (20-50 ms) and "fast" (1-2 ms) spikes were recorded in Purkinje cells.

[Ionic metabolism in the horizontal cells of the retina].

According to the experimental data, the membrane potential of L-type horizontal cells in the bright light (i.e. when the input synapses are completely closed) is close to the potassium equilibrium potential.

[Ionic mechanisms of the nonlinearity of retinal horizontal cell membranes].

Changes in ionic conductivity underlying the nonlinearity a voltage-current curve of the nonsynaptic membrane of horizontal cells were investigated in experiment on goldfish and turtle retina. The measurements were made under conditions when conductivity of a subsynaptic membrane was minimal, i.e.

[Nature of the impulse activity in the anterior portions of the fish olfactory bulb].

The impulse activity of anterior portions of the fish olfactory bulb depends on coming of signals from the receptors as shown by the fact that action of novokain on the receptors entirely suppressed the activity. A direct application of CoCl2 solution (1--1.5 mM) had no effect on either spontaneous or evoked by adequate stimulation impulse activity in these areas which indicates its presynaptic, i.

[Model of a neuron-regulator of the effectiveness of synaptic transmission].

In diad and triad synapses of vertebrate retina the transmitter released from one and the same presynaptic membrane can act simultaneously on two (or even three) postsynaptic neurones. The model of such a diad synapse is proposed, in which the positive electrical feed-back is caried out between one of postsynaptic and presynaptic neurones. The feed-back appears because of potential drop across the longitudinal resistance of intercellular gap (RIII) near the activated synapse (see [1]).

[Model of electrical feedback mechanisms through chemical synapses].

The model of excitatory chemical synapse is proposed in which the feed-back between the post- and presynaptic neurones is carried out by means of synaptic current generated by postsynaptic neurone. In the model, the potential drop generated by this current along the intercellular gap near the activated synapse (RIII) evokes additional depolarization of presynaptic membrane and thus stimulates the additional release of transmitter (fig. 1, A and B).