Experiments on neuromuscular preparations of frog skin- thoracic muscle and sartorius muscle, using extracellular recording and two-electrode clamping of the muscle fiber membrane potential, were used to study the effects of the nitric oxide donor sodium nitroprusside on endplate currents. At a concentration of 100 microM, sodium nitroprusside sharply decreased the amplitude and quantum composition of the endplate currents, and also decreased the miniature endplate current frequency. The amplitude-time characteristics of miniature endplate currents, the voltage-dependent amplitude, and the decay time constant of miniature endplate currents did not change as compared with controls. However, unlike the situation with other secretion inhibitors, the decrease in endplate current amplitude was not accompanied by increased facilitation in response to rhythmic stimulation or changes in postsynaptic potentiation in conditions of application of pairs of stimuli to muscles. The suppression of acetylcholine secretion was not seen with inactivated sodium nitroprusside solution. These results provide evidence that nitric oxide can be a powerful inhibitor of both spontaneous and evoked transmitter secretion in the neuromuscular synapse, and that this is accompanied by decreases in the efficiency of presynaptic forms of short-term plasticity, while the functional characteristics of the postsynaptic membrane remain unchanged.

Extracellular recording experiments using neuromuscular skin/chest muscle preparations from lake frogs were performed at low extracellular Ca2+ ion concentrations to study the effect of L-arginine (the substrate for nitric oxide synthesis) and N(G)-nitro-L-arginine methyl ester (a blocker of NO synthase) on the parameters of evoked transmitter secretion and ion currents in motor nerve endings. L-arginine at a concentration of 100 microM decreased the amplitude of endplate currents as well as their quantum composition, and also increased the amplitude of the third phase of the evoked nerve ending response, which reflects the kinetics of potassium influx currents. N(G)-nitro-L-arginine methyl ester at a condition of 100 microM led to increases in the amplitude and quantum composition of endplate currents and decreased the amplitude of the third phase of the evoked nerve ending response. It is suggested that endogenous nitric oxide is produced in frog neuromuscular synapses, which in normal conditions suppresses transmitter secretion and modulates the function of potassium channels in the nerve ending.

Extracellular recording was used to study the effect of sodium nitroprusside, a donor of NO, on parameters of action potential and ionic currents in single sciatic nerve fibers and unmyelinated nerve terminals in the sternal muscle in frogs. Sodium nitroprusside significantly decreased the duration of action potential in Ranvier node and the amplitude of afterdepolarization. In motor nerve terminals bathed in low Ca(2+) saline, sodium nitroprusside increased phase III amplitude of the nerve terminal response corresponding to outward potassium currents. Blockade of voltage-dependent potassium channels with 4-aminopyridine abolished the effects of NO. These data indicate that exogenous NO reduced the duration of action potential and afterdepolarization through enhancement of voltage-dependent potassium currents.

Extracellular recording was used to study the effect of sodium nitroprusside, a donor of NO, on endplate transmitter release and ionic currents in frog cutaneous pectoris muscle. Exogenous NO inhibits induced transmitter secretion, and this effect is antagonized by extracellular Ca 2+. Exogenous NO increases potential-dependent outward potassium current and inhibits Ca2+-activated potassium current in the motor nerve terminals.Key Words: synapse; nitric oxide; nerve terminal ionic currents; transmitter secretion Nitric oxide (NO) is a key regulator of various physiological processes. Of particular importance is the role of NO in nerve cells and synapses in the central and peripheral nervous systems. NO acts as a second messenger of intracellular signaling, transmitter in cell-cell signal transmission, activity modulator in nerve cell neurotransmitter systems, etc. [5,6,8,12,14]. One of the methods to reveal the effects of NO and decipher the mechanism of its action is to increase the level of exogenous NO by using NO donors, i.e. the substances releasing NO in aqueous solutions [10]. Experiments with sodium nitroprusside (SN) as the NO donor showed that exogenous NO decreases the amplitudes of endplate potentials (EPP) evoked by motor nerve stimulation and frequency of miniature endplate potentials (MEPP) in frog neuromuscular synapse, without affecting their amplitude and dependence on membrane potential. These data suggest that exogenous NO has a presynaptic effect that moderates quantal secretion of the transmitter from motor terminals without modifying the function of chemosensitive ionic channels in the postsynaptic membrane [4,10]. However, the mechanisms of the presynaptic effects of NO are poorly understood.Our aim was to study the effects of exogenous NO on transmitter secretion and ionic currents in frog motor nerve terminals.Depa~rnent of Normal Physiology, Medical University, Kazan' MATERIALS AND METHODS Experiments were carried out on isolated neuromus. cular preparations of cutaneous pectoris muscle o:Rana ridibunda at room temperature. In most experiments we used a low-calcium physiological solutior (pH 7.2-7.4) containing (in mM): 115 NaCI, 2.5 KCI 2.4 NaHCO3, 0.2-0.4 CaCI2, and 2 MgC12. Some experiments were carried out with a standard solution (in mM): 115 NaCI, 2.5 KCI, 2.4 NaHCO3, and 1.8 CaCI 2. Blockade of action potentials and muscle contraction, as well as attenuation of endplate ionic currents were performed by adding of tubocurarine (0.02 mM) into the perfusion solution. The evoked responses of nerve terminals, EPP, and MEPP were recorded with extracellular glass microelectrodes (tip diameter 2-5 la) filled with 1 M NaCl [1][2][3][4]. Quantal composition of the endplate currents was calculated by falling (probability) analysis [9]: m--In n/no, where n is the number of stimuli and n o is the number of realizations.The motor nerve was stimulated by suprathreshold stimuli at a rate of 1 pulse per second.The synaptic signals were recorded with the help of an L-1230 digitizer incorporated in ...