E. A. Tsvetkov

Physiological and morphological correlates of presynaptic inhibition in primary afferents of the lamprey spinal cord.

Patch-clamp recordings in a whole-cell mode were performed on dorsal sensory cells enzymatically isolated from the spinal cord of two lamprey species, Ichthyomyzon unicuspis and Lampetra fluviatilis. The voltage-activated currents through calcium channels were analysed. GABA and the specific GABA(B) receptor agonist baclofen reduced the peak amplitude of inward Ba2+ current, as a robust alternate charge carrier through voltage-dependent Ca2+ channels. These effects were dose-dependent and reversible. GABA(B) receptor antagonists, 2-hydroxysaclofen and delta-amino-n-valeric acid, blocked the reduction of Ba2+ currents by GABA and baclofen, while bicuculline, a GABA(A) receptor antagonist, had no blocking action. GABA and baclofen did not modify the dorsal sensory cell membrane conductance, indicating that they did not activate ligand-gated channels. However, GABA, but not baclofen, considerably increased membrane conductance and induced Cl- currents in isolated multipolar neurons (presumably interneurons and/or motoneurons). These findings suggest that GABA and baclofen action on lamprey dorsal sensory cells is mediated by GABA(B) receptors. We concluded that GABA-mediated presynaptic inhibition of lamprey dorsal sensory cell fibers results from GABA(B) receptor activation followed by a decrease of inward voltage-activated calcium currents. Appositions of GABA-immunoreactive boutons to horseradish peroxidase-labeled fibers from the dorsal root were observed at the ultrastructural level in the dorsal column using postembedding immunogold cytochemistry. It seems likely that these appositions represent the morphological substrate of dorsal sensory cell fiber presynaptic inhibition. In very rare cases, ultrastructural features were observed which could be interpreted as synaptic specializations between the GABA-immunoreactive boutons and the primary afferent fibers. The extrasynaptic action of GABA as a basis of presynaptic inhibition of this population of primary afferent neurons is discussed.

[Role of long-term potentiation in mechanism of the conditioned learning].

The review analyzes the fundamental problem of study of the neuronal mechanisms underlying processes of learning and memory. As a neuronal model of these phenomena there was considered one of the cellular phenomena that has characteristics similar with those in the process of “memorizing”—such as the long-term potentiation (LTP). LTP is easily reproduced in certain synapses of the central nervous system, specifically in synapses of hippocampus and amygdala. As the behavioral model of learning, there was used the conditioned learning, in frames of which production of the context-dependent/independent conditioned reaction was considered. Analysis of literature data showed that various stages of LTP produced on synapses of hippocampus or amygdala can be comparable with certain phases of the process of learning. Based on the exposed material the authors conclude that plastic changes of synapses of hippocampus and amygdala can represent the morphological substrate of some kinds of learning and memory.

Gap Junctions: Structure, Functions, and Regulation

The review presents data on the role of gap junctions in intercellular communication. The review includes information on history of the appearance of this problem. Data are presented on ultrastructure and function of gap junctions as well as on the mechanisms providing for their activity. A part of the review deals with the problem of regulation of intercellular communication realized by the gap junctions.

Effect of antagonists of serotonin receptors on modulation with serotonin of synaptic activity of projectional neurons of rat amygdala dorsolateral nucleus

The work studies role of different receptor types of serotonin (5-hydroxytryptamine; 5-HT) in the process of synaptic activity modulation with 5-HT of rat dorsolateral amygdala projection neurons. The selective antagonist of 5-HT1,2 receptors methylsergid maleate was shown to suppress the 5-HT inhibitory action on amplitude of the postsynaptic currents evoked by glutamate and GABA, whereas the antagonist of 5-HT3,4 receptors SDZ202-557 produced no effect on the above-mentioned 5-HT action. The obtained action indicates that the 5-HT modulatory effect on the projectional neuron synaptic inputs is mediated by 5-HT receptors of the 1 and 2 types.

[Transmembranous currents of isolated spinal cord neurons of ammocete--larva of the lamprey Lampetra fluviatilis].

The work is carried out with aid of the patch-clamp method in the “whole cell” modification. There were studied potential-activated and chemosensitive currents of isolated spinal neurons of the lamprey larva-ammocete. The described properties of the currents indicate their similarities with those in adult lampreys.

Peculiarities of dopamine receptors on the membrane of multipolar spinal cord neurons of the brook lamprey Lampetra planeri

AbstractsOn isolated multipolar neurons of spinal cord of amniocoete (larva of the brook lamprey Lampetra planeri) by the patch-clamp method in configuration “the whole cell,” a modulating effect of dopamine on potential-activated Na+ currents was studied. Application of dopamine (10 μM) was shown to produce a complex action on the sodium current amplitude. In some cases a decrease of the amplitude, on average, by 13.5 ± 2.2% was found, while in others—an increase, on average, by 8.6 ± 6.1%. The modulation dopamine effect was not accompanied by any changes either of the threshold of the current appearance or of resistance of neuronal cell membranes. Pharmacological analysis with use of dopamine agonist has shown that the agonist of D1-receptors (−)-SKF-38393 (10 μM) decreases the Na+ current amplitude, whereas the agonist of D2-receptors (−)-quinpirole (10 μM) can produce in different cells both an increase, by 30.7 ± 17.0%, and a decrease, by 13.2 ± 3.1%, of the Na+ current amplitude. The obtained data indicate the existence of D1-and D2-receptors on the membrane of multipolar spinal neurons of the amniocoete (larva of the brook lamprey). Study of action of antagonists has shown that the antagonist of D1-receptors (+)-SCH-23390 (10 μM) does not affect action of the agonist of D1-receptors (−)-SKF-38393 (10 μM); the antagonist of D2-receptors (−)-sulpiride (10 μM) blocks completely effects both of the agonist of D1-receptors (−)-SKF-38393 (10 μM) and of the agonist of D2-receptors (−)-quinpirole (10 μM). The antagonist of D1-receptors (+)-SCH-23390 (10 μM) produced no effect on action of the agonist of D1-receptors (−)-SKF-38393 (10 μM). The obtained data indicate peculiarities of dopamine receptors of Cyclostomata as compared with those in mammals.

Transmitter Sensitivity of Primary Afferent Cells of the Lamprey Lampetra fluviatilis Spinal Cord

Using dorsal sensory cells (primary afferents) of the spinal cord of the lamprey Lampetra fluviatilis, isolated by an enzymatic-mechanical method, their responses have been studied to application of excitatory amino acids (EAA), glutamate, aspartate, kainate, N-methyl-D-aspartate (NMDA), and of inhibitory amino acids, GABA and glycine, as well as of neuromodulator serotonin (5-hydroxytryptamine, 5-HT). The patch-clamp method was applied to fix either potential or current through membranes of studied cells. It was shown that of all the above substances, only NMDA and kainate could produce in dorsal cells depolarization, input current with maximal amplitude up to 16 nA, and action potential. The main attention in this work was paid to NMDA-current properties. The dose–effect curves were obtained; a blocking effect of specific antagonist of NMDA-receptors, 2-amino-5-phosphonovalerian acid (APV), was shown; passage of Ba ions through Ca2+-channels of dorsal sensory cell membranes at NMDA application was proven. It has been established that in a half of the studied cells under effect of NMDA there is an increase of the maximal amplitude of the potential-activated current through Ca2+-channels, on average, by 22.5 ± 10.5% (n = 21). It was suggested that this variation might be essential for an increase of amount of the transmitter released in synapses formed by processes dorsal sensory cells, while NMDA could be considered a modulator facilitating synaptic activity of these cells.

Membrane Potential Oscillations Produced by Excitatory Amino Acids in Isolated Spinal Neurons of the Lamprey Lampetra fluviatilis

Membrane potential (MP) oscillations produced by excitatory amino acids (EAA) have been studied in branching neurons isolated by an enzymatic-mechanical method from the lamprey spinal cord. It was shown that (1) all studied EAA (glutamate, kainate, NMDA, aspartate, and quisqualate) evoke an ion current and a short-term reversible depolarization in studied cells; (2) EAA added to perfusion solution may produce MP oscillations, with kinetic parameters and duration of the oscillation depending on the amino acid used (the most effective are kainate and NMDA, the least effective, quisqualate); (3) oscillations can be irregular (of the type of a synaptic noise or of a long-term plateau of depolarization with action potentials—AP) or regular, with frequency of 0.5–1.5 Hz. Amplitude of both oscillation types depends on MP level, frequency is more steady for each cell and less depends on MP. In 68 out of 128 studied cells, oscillations could be evoked, which indicates that a significant part of lamprey spinal neurons have intrinsic capability for MP oscillations and probably pacemaker properties. The functional role of oscillations can be different. They can take cells out from the profound inhibition state, synchronize activity of rhythm generation neurons and/or be the base for trigger signals (AP firing) sent by locomotor neuronal circuits to trunk muscles.