R. A. Giniatullin

Pre- and postsynaptic effects of the calcium channel blocker verapamil at neuromuscular junctions.

Experiments on the frog sartorius muscle were used to study the effects of the L-type calcium channel blocker verapamil on endplate currents. Verapamil had no effect on the amplitudes of miniature and multiple-quantum endplate currents, the synchronicity of transmitter secretion, or repeat activity in nerve endings. Verapamil had no effect on the decay of miniature currents, but accelerated that of multiple-quantum currents. This effect was sharply increased after inhibition of cholinesterase activity. In conditions of inhibited cholinesterase activity, verapamil depressed currents during rhythmic stimulation. This depression was more marked in synapses with high quantal compositions and in conditions of membrane depolarization. Thus, the sensitivity of neuromuscular junction calcium channels to verapamil was unrelated to the release of transmitter from the motor nerve ending either at physiological levels of secretion or when secretion was potentiated by potassium channel blockers. At the postsynaptic level, the effect of verapamil was insignificant in relation to cholinoreceptors in the resting and active states, though verapamil could cooperatively enhance the transition of postsynaptic receptors into the desensitized state in conditions of prolonged transmitter action.

Mechanisms of the Effect of 1,1-Dimethyl-3-Hydroxybutyl Phosphonic Acid Derivatives on Synaptic Transmission in Neuromuscular Junction

We studied the effect of homologues derivatives of 1,1-dimethyl-3-hydroxybutyl phosphonic acid on synaptic transmission in frog neuromuscular junction. Here we reviewed general mechanisms of inhibition of the postsynaptic current.

Effects of carbachol and adenosine on neurotransmitter secretion induced by potassium chloride, ionomycin, and sucrose.

We compared the effects of adenosine and cholinergic agonist carbachol on spontaneous secretion during local application of K+, ionomycin, and sucrose increasing Ca2+ concentration in the nerve terminal. Adenosine and carbachol had no effect on Ca2+ entry, but modulated later stages of exocytosis.

Effects of Glucocorticoids and Epinephrine on the Modulatory Action of Purines in the Frog Neuromuscular Junction

The effect of corticoids (hydrocortisone and dexamethasone) and epinephrine on the presynaptic action of purines was studied at the neuromuscular junction of the frog under two-electrode voltage-clamp conditions. Daily administration of hydrocortisone/dexamethasone (100 mg/kg into the lymphatic system) increased initially and later depressed the amplitude of multiquantum end-plate currents evoked by motor nerve stimulation. An initial facilitatory phase of the hormone action was accompanied by removal of the presynaptic action of ATP (for hydrocortisone only). Within the later phase (2 weeks of hydrocortisone administration), the inhibitory action of ATP was restored once again. The counteraction of ATP effect was reproduced under superfusion of the isolated muscle by a physiological solution containing hydrocortisone (not dexamethasone), indicating the nongenomic nature of the action of the hormone on presynaptic P2 receptors. This proved to be true in experiments on animals, which were stressed 30 min prior to the beginning of the experiment by electrical stimulation in a special cage. Independently of acute or chronic administration of hydrocortisone, the presynaptic action of another purine, adenosine, was preserved. Epinephrine only partially abolished the inhibitory effect of purines, which is indicative of the difference in the paths of incorporation of the biological effects of these agents. We suggest that prevention of the inhibitory action of ATP might be one of the components of a facilitatory acute stress reaction, while such an inhibitory feedback action is missing under chronic stress conditions.

Effect of hydrocortisone on ATP receptors in the neuromuscular junction of the frog

The effect of a hormone, hydrocortisone, on the presynaptic action of ATP was analyzed in experiments on frog neuromuscular junctions under voltage-clamp conditions. ATP reduced the amplitude of end-plate currents (EPC) due to decreased evoked transmitter release. Hydrocortisone increased the amplitude and slowed the decay of EPC but did not change the rise-time of the currents. The presynaptic action of ATP in hydrocorti-

Dynamics of desensitization and recovery of proton-activated ion channels in pheochromocytoma cells

Proton-activated ion channels of the ASIC 1a subtype in pheochromocytoma PC12 cells were inactivated under the long-lasting influence of an acidic milieu (pH 6.0) because of desensitization developing in a monoexponential mode with τ = 1.7 ± 0.3 sec. The recovery of currents after desensitization was also monoexponential, with τ = 4.9 ± 0.8 sec. Approximation of the dose-effect curve by the Hill equation showed that activation of a channel of this type needs, most probably, binding of three hydrogen ions.

Mechanisms of the Effect of Dimephosphone on Synaptic Transmission in the Frog Neuromuscular Junction

We studied the influence of dimephosphone, an organophosphorus drug with a broad spectrum of therapeutic effects on the peripheral and central nervous systems, on postsynaptic end-plate currents (EPC) in the frog neuromuscular junction. Dimephosphone was demonstrated to decrease in a voltage-independent manner the EPC amplitude and to prolong the EPC decay. These effects are not related to inhibition of acetylcholinesterase. We propose a theoretical interpretation of the observed phenomena based on the model of blockade of an open ion channel of the acetylcholine receptor and conclude that postsynaptic receptors are one of the most probable targets for the action of dimephosphone.

Interaction between the Cholinergic and Purinergic Control of Transmitter Release in the Neuromuscular Junction

The interaction between the cholinergic and purinergic receptors in the frog neuromuscular junction was studied using a standard microelectrode technique. The inhibitory action of an acetylcholine analog, carbachol, on transmitter release virtually disappeared when the releasing machinery was initially blocked by adenosine, indicating the existence of a functional cross-talk between the purinergic and cholinergic receptors.

Role of Adenosine Receptors in Modulation of Synaptic Transmission

Mechanisms of the effect of adenosine on transmitter release were investigated. Exogenous adenosine reduced transmitter release via P1 receptors. This reduction was not affected by changes in the [Ca2+], but was abolished by a P1 blocker, theophylline, and Ba2+. The effects of Ba2+ ions may be due to the involvement of A2areceptors. Blockers of the A-type (catechol and 4-aminopyridine) and BK-type (iberiotoxin) potassium channels enhanced transmitter release and abolished the effect of adenosine. The findings suggest a possibility of A2a channels to be coupled to BK and A potassium channels.

Localization of the Repetitive Firing Generation Site Induced by 4-Aminopyridine at the Frog Neuromuscular Junction

In the neuromuscular junction, blockade of potassium channels can produce multiple discharges after single nerve stimulation. Multielectrode recording from the nerve trunk and myelinated and non-myelinated parts of the nerve ending demonstrated that repetitive presynaptic spikes elicited by 4-aminopyridine arise earliest within the part of the axon proximal to the motor nerve ending.