Serotonin modulates oscillations of the membrane potential in isolated spinal neurons from lampreys.

Studies were performed on spinal neurons from lampreys isolated by an enzymatic/mechanical method using pronase. The effects of 100 microM serotonin (5-HT) on membrane potential oscillations induced by a variety of excitatory amino acids were studied. 5-HT was found to depolarize branched cells (presumptive motoneurons and interneurons) by 2-6 mV without inducing membrane potential oscillations. However, when oscillations were already present because of an excitatory amino acid, 5-HT changed the parameters of these oscillations, increasing the amplitudes of all types of oscillations, increasing the frequency of irregular oscillations, and increasing the duration of the depolarization plateaus accompanied by action potentials. Serotonin modulation of the effects of excitatory amino acids and the electrical activity of cells in the neural locomotor network facilitates motor activity and leads to increases in the contraction of truncal muscles and more intense movements by the animal. The possible mechanisms of receptor coactivation are discussed, along with increases in action potential frequency and changes in the parameters of the locomotor rhythm.

The effects of serotonin on functionally diverse isolated lamprey spinal cord neurons.

The experiments reported here showed that application of serotonin (5-hydroxytryptamine, 5-HT) (100 microM) did not induce any significant current through the membranes of any of the spinal neurons studied (n = 62). At the same time, the membranes of most motoneurons and interneurons (15 of 18) underwent slight depolarization (2-6 mV) in the presence of 5-HT, which was not accompanied by any change in the input resistance of the cells. Depolarization to 10-20 mV occurred in some cells (3 of 18) of these functional groups, this being accompanied by 20-60% decreases in input resistance. The same concentration of 5-HT induced transient low-amplitude depolarization of most sensory spinal neurons (dorsal sensory cells), this changing smoothly to long-term hyperpolarization by 2-7 mV. The input resistance of the cell membranes in these cases showed no significant change (n = 8). Data were obtained which provided a better understanding of the mechanism by which 5-HT modulates the activity of spinal neurons. Thus, 5-HT facilitates chemoreceptive currents induced by application of NMDA to motoneurons and interneurons, while the NMDA responses of dorsal sensory cells were decreased by 5-HT. 5-HT affected the post-spike afterresponses of neurons. 5-HT significantly decreased the amplitude of afterhyperpolarization arising at the end of the descending phase of action potentials in motoneurons and interneurons and increased the amplitude of afterdepolarization in these types of cells. In sensory spinal neurons, 5-HT had no great effect on post-spike afterresponses. The results obtained here support the suggestion that 5-HT significantly modulates the activity of spinal neurons of different functional types. 5-HT facilitates excitation induced by subthreshold depolarization in motoneurons and some interneurons, facilitating the generation of rhythmic discharges by decreasing afterhyperpolarization. In sensory cells, 5-HT enhances inhibition due to hyperpolarization, suppressing NMDA currents. The differences in the effects of 5-HT on functionally diverse neurons are presumed to be associated with the combination of different types of 5-HT receptors on the membranes of these types of spinal neurons.

[Serotonin modulates oscillation parameters of the membrane potential in isolated spinal neurons in the lamprey]. / Serotonin moduliruet parametry ostsilliatsii v izolirovannykh spinal'nykh neironakh minogi.

The 5-HT was shown to depolarize branch cells (supposedly motoneurones and interneurones) by 2-6 mV, inducing, however, no MP oscillations. In case the MP oscillations were present (induced by the NMDA, for instance), the 5-HT altered their parameters: increased the amplitude of all types of oscillations, frequency of irregular oscillations, and duration of the depolarising plateau with the AP discharges. This modulation of the induced oscillations may enhance activity of neuronal locomotor network and thus reinforce muscle contractions and increase the intensity of the animal's movements. Possible mechanisms of the receptor modulation, of the AP enhancement, and of the changes in locomotor rhythm parameters, are discussed.

[Effect of serotonin on isolated cells with the various functionality from the lamprey spinal cord]. / Vliianie serotonina na izolirovannye funktsional'no razlichnye kletki spinnogo mosga minogi.

The differential actions of 5-hydroxytryptamine (5-HT) (100 microM) were investigated on isolated motoneurons, interneurons, and primary sensory neurons from the lamprey spinal cord using patch-clamp techniques. Application of 5-HT did not evoke membrane currents in any of the spinal neurons tested (n = 62). However, in most motoneurons and interneurons (15 of 18), 5-HT produced a small depolarization (2-6 mV), which was not accompanied by a change in input resistance. In the remaining motoneurons and interneurons (3 of 18), 5-HT induced a large depolarization (up to 10-20 mV) and a decrease in input resistance of 20-60%. In most sensory neurons (dorsal sensory cells, DSCs), 5-HT evoked a short-lasting, low-amplitude depolarization, followed by a long-lasting hyperpolarization of 2-7 mV. The DSCs showed no significant change in input resistance to 5-HT application (n = 8). Spike afterpolarization were also differentially modulated by 5-HT. In motoneurons and interneurons, 5-HT decreased the amplitude of the afterhyperpolarization following the action potential while increasing the amplitude of the after depolarization. In the DSCs, no significant effect of 5-HT on spike afterpolarization was observed. 5-HT differentially modulated the current induced by application of N-methyl-D-aspartate (NMDA). In motoneurons and interneurons, 5-HT enhanced NMDA-evoked current, while in DSCs, 5-HT decreased this current. These results demonstrate that 5-HT differentially modulates the activity of functionally different groups of spinal neurons. In motoneurons and interneurons, 5-HT enhances excitation by inducing depolarization and decreasing the afterhyperpolatization, while NMDA currents are enhanced. These effects facilitate the appearance of rhythmic discharges in these cells in the presence of NMDA. In primary dorsal sensory cells, 5-HT enhances inhibition by hyperpolarizing the cells and depressing NMDA currents. These differential effects are presumably mediated by different types of 5-HT receptors on these classes of spinal neurons.

[Inhibition in the lamprey spinal cord]. / Tormozhenie v spinnom mozge minogi.

Glycine and GABA play the role of inhibitory transmitters in the lamprey spinal cord. The mechanisms of action of both amino acids to the membrane receptors producing the postsynaptic inhibition as well as role and mechanism of GABA action producing the presynaptic inhibition are considered in this paper. The data concerned with morphological substrates of both type inhibitions are discussed.

Calcium currents and GABAB receptors in the dorsal sensory cells of the lamprey spinal cord.

Patch-clamp studies were performed on the isolated dorsal sensory cells of the spinal cords of three species of lamprey, Ichthyomyzon unicuspis, Petromyzon marinus, and Lampetra fluviatilis, to measure changes in the amplitudes of calcium current induced by GABA and its specific antagonists and agonists. The experiments showed that GABA (4 mM) reduced the peak amplitude of the calcium current by 28.5 +/- 4.9%, with subsequent recovery to 96.2 +/- 9.2% of control (n = 45). The GABAB agonist baclofen had similar effects. The GABAA agonists glycine and taurine had no effect on the Ca2+ current. The inhibitory effect of GABA was blocked by 2-hydroxysaclofen (a GABAB antagonist), but persisted in the presence of bicuculline (a GABAA antagonist). These results are evidence that the membranes of dorsal sensory cells contain GABAB receptors, which significantly increases our understanding of the mechanisms of presynaptic inhibition in the spinal cords of the cyclostomata.

The effects of baclofen on calcium channel currents in dorsal sensory cells of the spinal cord in the lamprey.

Dorsal sensory cells isolated from the spinal cord of the lamprey species Ichthyomyzon unicuspis and Lampetra fluviatilis were used for whole-cell patch-clamp studies of the effects of baclofen on calcium channel currents, evoked in conditions in which Na+, K+ currents were blocked, by depolarizing membranes from constant holding potentials of -100 or -80 mV to +30 mV. Ba ions were used as carriers of currents through calcium channels. These studies demonstrated that baclofen (0.5 mM) decreased the peak amplitude of the Ba2+ current by an average of 22.5 +/- 4.2% (n = 12) in dorsal sensory cells of the lamprey Ichthyomyzon unicuspis and by 28.4 +/- 3.3% in the dorsal sensory cells of Lampetra fluviatilis (n = 25). The conductivity of dorsal sensory cell membranes in the presence of baclofen (and GABA) did not change. The blocking action of baclofen persisted in the presence of bicuculline (100 microM) and was lifted by addition of delta-aminovaleric acid and 2-hydroxysaclofen to the perfusing solution. These results are interpreted as evidence for the presence of GABAB receptors in dorsal sensory cell membranes. The data were compared with published results, and the question of the functional significance of GABAB receptors in the dorsal sensory cells (primary afferent cells) of cyclostomata is discussed.

[Calcium current and GABA(B) receptors in dorsal sensory cells of the lamprey spinal cord]. / Kal'tsievyi tok i GAMK(B) retseptory v dorsal'nykh chuvstvitel'nykh kletkakh spinnogo mozga minogi.

GABA and GABAB receptor agonists were shown to reduce the peak calcium current amplitude with its subsequent recovery, whereas glycine and taurine, the GABAA receptor agonists, did not modify the current. The findings suggest that the GABAB receptors mediate a presynaptic inhibition by suppression of the Calcium currents in the cyclostome spinal cord.

[Effect of baclofen on calcium channel currents in dorsal sensory cells of the lamprey spinal cord]. / Vliianie baklofena na tok kal'tsievykh kanalov v dorsal'nykh chuvstvitel'nykh kletkakh spinnogo mozga minogi.

delta-Baclofen reduced the peak amplitude of the barium current in the DSCs from Lunicuspis and L.fluviatilis. The membrane conductance was not altered either by baclofen or GABA. The reducing effect could be abolished with delta-aminovaleric acid or 2(OH)saclofen. The data obtained suggest presence of the GABAB receptors in the DSC membranes. Functional role of the GABAB receptors in primary afferent cells of cyclostomes, is discussed.

[Potential-gated currents in isolated spinal cord neurons of the river lamprey Lampetra fluviatilis]. / Issledovanie potentsial-aktiviruemykh tokov v izolirovannykh neironakh spinnogo mozga rechnoi minogi Lampetra fluviatilis.

Potential-gated currents in the membranes of enzymatically isolated neurons from the lamprey spinal cord were investigated using the whole cell variant of the patch-clamp technique. As it was revealed the main currents underlying action potential (AP) in the dorsal sensory cells as well as in branched cells are inward Na(+)- and outward K(+)-currents. Sodium current has the duration 4-9 ms, it was rapidly activated and inactivated and blocked by TTX. Its activation and inactivation processes can be fit to modified Bolzmann equations. Outward potassium current can be subdivided into at least two components: 1) early transient rapidly inactivated component which was sensitive to 4-AP and 2) later noninactivated more longer one which was sensitive to TEA. Their function as in the other neurons may be to repolarize membranes after AP generation and to provide the interval between APs during rhythmic activity. Ca2(+)-current was investigated using Ba2+ as a divalent carrier. Judging on two different values of the threshold there are two types of the Ca2(+)-channels in the membranes of the lamprey spinal cells. Ba2(+)-current depend not only on the voltage but on the intracellular enzyme systems particularly on the G-proteins, ATP and CP-CPK. Under normal conditions Ca2(+)-current activated during AP is small but it increase up to 5-10 nA when the other voltage-gated currents are blocked. We conclude that lamprey spinal neurons retain after isolation the electrical properties of neurons in the spinal cord. The obtained results have testified that the kinetic and pharmacological properties of the basic currents in the cells under study are close to those in the other vertebrate neurons and these cells are suitable for investigation of neurotransmitter, neuromodulator and pharmacological agents action on voltage- and agonist-gated membrane currents in vertebrate neurons.

[The pathways of the stabilization of the amplitude of postsynaptic potentials in the interneuronal synapses of vertebrates]. / Puti stabilizatsii amplitudy postsinapticheskikh potentsialov v mezhneironnykh sinapsakh pozvonochnikh.

The influence of various factors on the degree of stabilization of postsynaptic potential amplitude has been studied by mathematical modelling. Increasing of transmitter release probability in single boutons, spatial non-uniformity of these probabilities, interaction of release sites, non-linear summation of potentials lead to amplitude stabilization. Temporal fluctuations of the release probability, failures of responses from contact groups and activation of a variable number of the fibers have the opposite influence. Comparison of synaptic transmission parameters in sensorimotor synapses of Cyclostomata, amphibians and mammals showed that during evolution of these synapses two different pathways of amplitude stabilization had been realized. Formation of highly effective chemical contacts is probably the most progressive pathway among them.

[Effect of penicillin on the synaptic activity of isolated spinal cord motor neurons in the lamprey]. / Vliianie penitsillina na sinapticheskuiu aktivnost' motoneironov izolirovannogo spinnogo mozga minogi.

Penicillin (PCN) has been studied for its effect on the membrane potential (MP) and synaptic activity of lamprey spinal cord motoneurons using intracellular recording in the in vitro spinal cord-notochord preparation. In one group of motoneurons with relative low MP (58.7 +/- 5.2 mV, n = 28) PCN induced depolarization, enhancement and prolongation (up to 80-220%) of the initial amplitude of excitatory postsynaptic potentials (EPSPs) evoked by the stimulation of dorsal roots spinal tracts. If the MPs (in the other group of motoneurons) were high (70.0 +/- 5.7 mV, n = 20) depolarization was not observed and the potentiation of EPSPs did not exceed 25-70% of the initial value. These effects of PCN can be eliminated by a preliminary addition of excitatory or inhibitory amino acid antagonists in the superfusion solution. The obtained results allow suggesting the presence of two different acceptor sites for PCN in membranes of lamprey spinal cord motoneurons.

[The effect of glycine and gamma-aminobutyric acid on the evoked activity of the spinal cord motor neurons in the lamprey]. / Vliianie glitsina i gamma-aminomaslianoi kisloty na vyzvannuiu aktivnost' motoneironov spinnogo mozga minogi.

The influence of bath application of glycine (10(-5)-10(-3) mol/l) and gamma-aminobutyric acid (10(-5)-10(-2) mol/l) on the monosynaptic EPSPs evoked in motoneurons by stimulation of a descending tract and individual Müller axons was studied in spinal cord-notochord preparation of lamprey (Lampetra fluviatilis). Both amino acids hyperpolarized the motoneuron membrane and depressed the evoked synaptic activity. But the inhibitory effect of glycine was stronger and it was revealed at lower concentrations as compared to that of GABA. It is concluded that glycine is more effective in blocking the postsynaptic motoneuron activity in comparison with GABA in the spinal cord of lamprey.

[The effect of glycine and gamma-aminobutyric acid on the excitatory postsynaptic potentials of the spinal motor neurons in the lamprey in the presence of antagonists]. / Vliianie glitsina i gamma-aminomaslianoi kisloty na vozbuzhdaiushchie postsinapticheskie potentsialy spinal'nykh motoneironov minogi v prisutstvii antagonistov.

Effect of bath application of the inhibitory amino acids (glycine and GABA) on motoneurons of EPSPs was studied in the normal physiological solutions and after preliminary administration of antagonists: strychnine (10(-6) mol/l), bicuculline (10(-4) mol/l) or picrotoxin (10(-4) mol/l). All these antagonists diminished the depression of monosynaptic EPSPs which were elicited by both amino acids (glycine and GABA). Data obtained in this study and previously reported ones permit concluding that motoneuron membranes in the spinal cord of lamprey possess the unit receptor channel complex sensitive to both amino acids.

Peculiarities of receptor-channel complexes for inhibitory mediators in the membranes of lamprey spinal cord neurones.

Effects of inhibitory mediators on the membranes of isolated lamprey spinal cord neurones were investigated by means of whole-cell recording and concentration clamp techniques. Glycine and gamma-aminobutyric acid (GABA) applications evoked desensitizing chloride currents. The concentrations at half-maximum effects were 16 microM for glycine- and 1.5 mM for GABA-activated currents. Ionic responses to applications of both amino acids demonstrated full cross-desensitization. Strychnine and bicuculline suppressed both glycine- and GABA-activated conductances to the same degree. We suggest that in the membranes of lamprey spinal cord neurones glycine and GABA act on the same receptor-channel complexes.