Исследовано влияние тритерпеновых гликозидов на пейсмекерную активность нейронов моллюска. Монодесмозидные гликозиды в концентрациях 2·10-3 – 10-4 М, введенные в наружный раствор, вызывают устойчивую гиперполяризацию нейронов. Установлено, что данные вещества увеличивают амплитуду выходящего калиевого тока. Действие гликозидов является неспецифическим и развивается в течение 1 – 2 минут. Высказано предположение, что влияние тритерпеновых гликозидов в высоких концентрациях связано с образованием неселективных пор в мембранах нейронов.
Ключевые слова: нейроны моллюсков, пейсмекерная активность, аксодендритное дерево, протонуправляемый ток, тритерпеновые гликозиды.
Kostyuchenko O.V. Investigation of the pacemaker activity of molluscan neurons and influence of triterpene saponins on it. – Manuscript.
Thesis for a candidate's degree by specialty 03.00.13 – physiology of man and animals. – National Taurida V. I. Vernadsky University, Simferopol, 2002.
The dissertation is devoted to investigation of pacemaker activity mechanisms in neurons of snail, and to investigation of the influence of plant triterpene saponins (TS) on this activity. It has been shown that isolated RPa2 and Rpa7 neurons display endogenous pacemaker activity. Superfusion of the ganglion with CdCl2 at concentration of 1mM, at which it blocks all voltage-gated Ca2+ channels, did not significantly affect pacemaker activity of these neurons. In contrast, isolation of bursting RPa1 neuron abolished its bursting activity, whereas subsequent application of oxytocin to the isolated neuron resulted in its transient reappearance. These data indicate exogenous nature of bursting activity in RPa1 neuron. Extracellular application of 1mM CdCl2 to the intact RPa1 neuron resulted in inhibition of its activity. However, CdCl2 applied simultaneously with oxytocin did not prevent its bursting activity. Cs+ (10mM), which inhibits Ih channels (activated by hyperpolarization, known to participate in pacemaker activity of neurons in some preparations), did not influence rhythmic activity of all neurons in study. Rhythmic activity of these neurons revealed spikes of smaller than somatic action potentials (APs), considered by us as dendritic APs. From our data one may conclude that rhythmic activity has its origin in dendritic tree. Then it electrotonically propagates to the neuronal soma resulting in generation of somatic APs.
A novel type of proton-gated current in ganglia of water mollusc Lymnaea stagnalis strikingly different from those provided by acid-sensitive ion channels types observed in mammalian neurons was discovered. It was found that extracellular pH drop from 8.1 to 5.3 elicited a rapidly activated transient inward current. The peak amplitude of the pH-response was proportional to the cell diameter. Replacing of extracellular Na+ by Tris ions did not affect the response. Manipulations with the extracellular Ca2+ concentration yielded rather complicated effects: at high Ca2+ concentrations (10mM), the current-voltage relationship was shifted to positive voltages, while the amplitudes somewhat decreased. Decreasing of the Ca2+ concentration from 2 to 0.5 mM resulted in a 10-20% potentiation. Probably, Ca2+ plays a dual role in regulating the conductance, combining the roles of a current carrier and a partial voltage-dependent channel blocker. Thus, it was shown that Ca2+ current is much less Na+-selective (if provided by the Na+ permeability at all) and it is highly voltage-dependent, being activated at positive voltages.
The effect of plant triterpene saponins on Helix pomatia neurons was investigated. It was found that monodesmosidic TS being perfused to the extracellular solution at concentrations 2Ч10-3 – 10-4 M induced persistent hyperpolarization of the neurons. By using the patch-clamp technique it was obtained that these substances increased the amplitude of the outward potassium current. At lower concentrations (10-5 – 10-7 M) TS either led to the transient reversible oppression of the impulse activity, or didn't change the background activity.